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INTERNATIONAL STANDARD
©ISO/IEC
ISO/IEC9899:2017

Programming languages — C

(cover sheet to be replaced by ISO)

This is a working document of SC22/WG14
This version of the document is intended to be the version that is to go into ballot for C17.

A brief explanation of the changes could still be added to the foreword.

Document conventions
This document classifies identifiers into different categories. This categorization is important to
produce a correct index.
The classes are

The classification is not always unique, we have identifiers that can refer to a library function in
math.h or to a type generic macro in tgmath.h. Currently we give preference to the fact of being a
library function, e.g we have sqrt.

Abstract

(Cover sheet to be provided by ISO Secretariat.)

This International Standard specifies the form and establishes the interpretation of programs
expressed in the programming language C. Its purpose is to promote portability, reliability, main-
tainability, and efficient execution of C language programs on a variety of computing systems.

Clauses are included that detail the C language itself and the contents of the C language
execution library. Annexes summarize aspects of both of them, and enumerate factors that
influence the portability of C programs.

Although this International Standard is intended to guide knowledgeable C language pro-
grammers as well as implementors of C language translation systems, the document itself is not
designed to serve as a tutorial.

Recipients of this draft are invited to submit, with their comments, notification of any relevant
patent rights of which they are aware and to provide supporting documentation.

Contents

Foreword

ISO (the International Organization for Standardization) and IEC (the International Electrotechnical
Commission) form the specialized system for worldwide standardization. National bodies that
are member of ISO or IEC participate in the development of International Standards through
technical committees established by the respective organization to deal with particular fields of
technical activity. ISO and IEC technical committees collaborate in fields of mutual interest. Other
international organizations, governmental and non-governmental, in liaison with ISO and IEC, also
take part in the work. In the field of information technology, ISO and IEC have established a joint
technical committee, ISO/IEC JTC 1.

The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for
the different types of document should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see http://www.iso.org/directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject
of patent rights. ISO and IEC shall not be held responsible for identifying any or all such patent
rights. Details of any patent rights identified during the development of the document will be in
the Introduction and/or on the ISO list of patent declarations received (see http://www.iso.org/patents).

Any trade name used in this document is information given for the convenience of users and does
not constitute an endorsement.

For an explanation on the meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO’s adherence to the WTO principles in the Techni-
cal Barriers to Trade (TBT), see the following URL: http://www.iso.org/iso/home/standards_development/resources-for-technical-work/foreword.htm.

The committee responsible for this document is ISO/IEC JTC 1, Information technology, Subcommittee
SC 22, Programming languages, their environments and system software interfaces.

This fourth edition cancels and replaces the third edition, ISO/IEC 9899:2011, which has been
technically revised. It also incorporates the Technical Corrigendum ISO/IEC 9899:2011/Cor 1:2012.
There are no major changes in this edition, only technical corrections and clarifications.

Major changes in the third edition included:

Major changes in the second edition included:

Introduction

With the introduction of new devices and extended character sets, new features may be added to
this International Standard. Subclauses in the language and library clauses warn implementors and
programmers of usages which, though valid in themselves, may conflict with future additions.

Certain features are obsolescent, which means that they may be considered for withdrawal in future
revisions of this International Standard. They are retained because of their widespread use, but their
use in new implementations (for implementation features) or new programs (for language [6.11] or
library features [7.31]) is discouraged.

This International Standard is divided into four major subdivisions:

Examples are provided to illustrate possible forms of the constructions described. Footnotes are
provided to emphasize consequences of the rules described in that subclause or elsewhere in this
International Standard. References are used to refer to other related subclauses. Recommendations
are provided to give advice or guidance to implementors. Annexes provide additional information
and summarize the information contained in this International Standard. A bibliography lists
documents that were referred to during the preparation of the standard.

The language clause (clause 6) is derived from “The C Reference Manual”.

The library clause (clause 7) is based on the 1984 /usr/group Standard.

The Working Group responsible for this standard (WG 14) maintains a site on the World Wide Web at
http://www.open-std.org/JTC1/SC22/WG14/ containing additional information relevant to this
standard such as a Rationale for many of the decisions made during its preparation and a log of
Defect Reports and Responses.

Programming languages — C

1. Scope

This International Standard specifies the form and establishes the interpretation of programs written
in the C programming language.1) It specifies

This International Standard does not specify

2. Normative references

The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any amendments)
applies.

ISO/IEC 2382:2015, Information technology — Vocabulary. Available from the ISO online browsing
platform at http://www.iso.org/obp.

ISO 4217, Codes for the representation of currencies and funds.

ISO 8601, Data elements and interchange formats — Information interchange — Representation of dates and
times.

ISO/IEC 10646, Information technology — Universal Coded Character Set (UCS). Available from the
ISO/IEC Information Technology Task Force (ITTF) web site at http://isotc.iso.org/livelink/livelink/fetch/2000/2489/Ittf_Home/PubliclyAvailableStandards.htm.

IEC 60559:1989, Binary floating-point arithmetic for microprocessor systems (previously designated
IEC 559:1989).

ISO 80000–2, Quantities and units — Part 2: Mathematical signs and symbols to be used in the natural
sciences and technology.

3. Terms, definitions, and symbols

For the purposes of this document, the following terms and definitions apply. Other terms are
defined where they appear in italic type or on the left side of a syntax rule. Terms explicitly defined
in this International Standard are not to be presumed to refer implicitly to similar terms defined
elsewhere. Terms not defined in this International Standard are to be interpreted according to
ISO/IEC 2382. Mathematical symbols not defined in this International Standard are to be interpreted
according to ISO 80000–2.

3.1

access
hexecution-time actioni to read or modify the value of an object

Note 1 to entry: Where only one of these two actions is meant, “read” or “modify” is used.

Note 2 to entry: “Modify” includes the case where the new value being stored is the same as the previous value.

Note 3 to entry: Expressions that are not evaluated do not access objects.

3.2

alignment
requirement that objects of a particular type be located on storage boundaries with addresses that
are particular multiples of a byte address

3.3

argument
actual argument
actual parameter (deprecated)
expression in the comma-separated list bounded by the parentheses in a function call expression, or
a sequence of preprocessing tokens in the comma-separated list bounded by the parentheses in a
function-like macro invocation

3.4

behavior
external appearance or action

3.4.1

implementation-defined behavior
unspecified behavior where each implementation documents how the choice is made

EXAMPLE An example of implementation-defined behavior is the propagation of the high-order bit when a signed integer
is shifted right.

3.4.2

locale-specific behavior
behavior that depends on local conventions of nationality, culture, and language that each implemen-
tation documents

EXAMPLE An example of locale-specific behavior is whether the islower function returns true for characters other than
the 26 lowercase Latin letters.

3.4.3

undefined behavior
behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which

this International Standard imposes no requirements

Note 1 to entry: Possible undefined behavior ranges from ignoring the situation completely with unpredictable results,
to behaving during translation or program execution in a documented manner characteristic of the environment (with or
without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic
message).

EXAMPLE An example of undefined behavior is the behavior on integer overflow.

3.4.4

unspecified behavior
use of an unspecified value, or other behavior where this International Standard provides two or
more possibilities and imposes no further requirements on which is chosen in any instance

EXAMPLE An example of unspecified behavior is the order in which the arguments to a function are evaluated.

3.5

bit
unit of data storage in the execution environment large enough to hold an object that may have one
of two values

Note 1 to entry: It need not be possible to express the address of each individual bit of an object.

3.6

byte
addressable unit of data storage large enough to hold any member of the basic character set of the
execution environment

Note 1 to entry: It is possible to express the address of each individual byte of an object uniquely.

Note 2 to entry: A byte is composed of a contiguous sequence of bits, the number of which is implementation-defined. The
least significant bit is called the low-order bit; the most significant bit is called the high-order bit.

3.7

character
habstracti member of a set of elements used for the organization, control, or representation of data

3.7.1

character
single-byte character
hCi bit representation that fits in a byte

3.7.2

multibyte character
sequence of one or more bytes representing a member of the extended character set of either the
source or the execution environment

Note 1 to entry: The extended character set is a superset of the basic character set.

3.7.3

wide character
value representable by an object of type wchar_t, capable of representing any character in the
current locale

3.8

constraint
restriction, either syntactic or semantic, by which the exposition of language elements is to be
interpreted

3.9

correctly rounded result
representation in the result format that is nearest in value, subject to the current rounding mode, to
what the result would be given unlimited range and precision

3.10

diagnostic message
message belonging to an implementation-defined subset of the implementation’s message output

3.11

forward reference
reference to a later subclause of this International Standard that contains additional information
relevant to this subclause

3.12

implementation
particular set of software, running in a particular translation environment under particular con-
trol options, that performs translation of programs for, and supports execution of functions in, a
particular execution environment

3.13

implementation limit
restriction imposed upon programs by the implementation

3.14

memory location
either an object of scalar type, or a maximal sequence of adjacent bit-fields all having nonzero width

Note 1 to entry: Two threads of execution can update and access separate memory locations without interfering with each
other.

Note 2 to entry: A bit-field and an adjacent non-bit-field member are in separate memory locations. The same applies to
two bit-fields, if one is declared inside a nested structure declaration and the other is not, or if the two are separated by a
zero-length bit-field declaration, or if they are separated by a non-bit-field member declaration. It is not safe to concurrently
update two non-atomic bit-fields in the same structure if all members declared between them are also (nonzero-length)
bit-fields, no matter what the sizes of those intervening bit-fields happen to be.

EXAMPLE A structure declared as

    struct {
          char a;
          int b:5, c:11,:0, d:8;
          struct { int ee:8; } e;
    }
  

contains four separate memory locations: The member a, and bit-fields d and e.ee are each separate memory locations,
and can be modified concurrently without interfering with each other. The bit-fields b and c together constitute the fourth
memory location. The bit-fields b and c cannot be concurrently modified, but b and a, for example, can be.

3.15

object
region of data storage in the execution environment, the contents of which can represent values

Note 1 to entry: When referenced, an object may be interpreted as having a particular type; see 6.3.2.1.

3.16

parameter
formal parameter

formal argument (deprecated)
object declared as part of a function declaration or definition that acquires a value on entry to the
function, or an identifier from the comma-separated list bounded by the parentheses immediately
following the macro name in a function-like macro definition

3.17

recommended practice
specification that is strongly recommended as being in keeping with the intent of the standard, but
that may be impractical for some implementations

3.18

runtime-constraint
requirement on a program when calling a library function

Note 1 to entry: Despite the similar terms, a runtime-constraint is not a kind of constraint as defined by 3.8, and need not be
diagnosed at translation time.

Note 2 to entry: Implementations that support the extensions in annex K are required to verify that the runtime-constraints
for a library function are not violated by the program; see K.3.1.4.

3.19

value
precise meaning of the contents of an object when interpreted as having a specific type

3.19.1

implementation-defined value
unspecified value where each implementation documents how the choice is made

3.19.2

indeterminate value
either an unspecified value or a trap representation

3.19.3

unspecified value
valid value of the relevant type where this International Standard imposes no requirements on
which value is chosen in any instance

Note 1 to entry: An unspecified value cannot be a trap representation.

3.19.4

trap representation
an object representation that need not represent a value of the object type

3.19.5

perform a trap
interrupt execution of the program such that no further operations are performed

Note 1 to entry: In this International Standard, when the word “trap” is not immediately followed by “representation”, this
is the intended usage.2)

3.20

dxe
ceiling of x: the least integer greater than or equal to x

EXAMPLE d2.4e is 3, d−2.4e is −2.

3.21

bxc
floor of x: the greatest integer less than or equal to x

EXAMPLE b2.4c is 2, b−2.4c is −3.

4. Conformance

In this International Standard, “shall” is to be interpreted as a requirement on an implementation or
on a program; conversely, “shall not” is to be interpreted as a prohibition.

If a “shall” or “shall not” requirement that appears outside of a constraint or runtime-constraint is
violated, the behavior is undefined. Undefined behavior is otherwise indicated in this International
Standard by the words “undefined behavior” or by the omission of any explicit definition of behavior.
There is no difference in emphasis among these three; they all describe “behavior that is undefined”.

A program that is correct in all other aspects, operating on correct data, containing unspecified
behavior shall be a correct program and act in accordance with 5.1.2.3.

The implementation shall not successfully translate a preprocessing translation unit containing a
#error preprocessing directive unless it is part of a group skipped by conditional inclusion.

A strictly conforming program shall use only those features of the language and library specified in
this International Standard.3) It shall not produce output dependent on any unspecified, undefined,
or implementation-defined behavior, and shall not exceed any minimum implementation limit.

The two forms of conforming implementation are hosted and freestanding. A conforming hosted
implementation shall accept any strictly conforming program. A conforming freestanding implemen-
tation shall accept any strictly conforming program in which the use of the features specified
in the library clause (clause 7) is confined to the contents of the standard headers <float.h>,
<iso646.h>, <limits.h>, <stdalign.h>, <stdarg.h>, <stdbool.h>, <stddef.h>, <stdint.h>,
and <stdnoreturn.h>. A conforming implementation may have extensions (including additional
library functions), provided they do not alter the behavior of any strictly conforming program.4)

A conforming program is one that is acceptable to a conforming implementation.5)

An implementation shall be accompanied by a document that defines all implementation-defined
and locale-specific characteristics and all extensions.

Forward references: conditional inclusion (6.10.1), error directive (6.10.5), characteristics of floating
types <float.h> (7.7), alternative spellings <iso646.h> (7.9), sizes of integer types <limits.h>
(7.10), alignment <stdalign.h> (7.15), variable arguments <stdarg.h> (7.16), boolean type and
values <stdbool.h> (7.18), common definitions <stddef.h> (7.19), integer types <stdint.h> (7.20),
<stdnoreturn.h> (7.23).

5. Environment

An implementation translates C source files and executes C programs in two data-processing-system
environments, which will be called the translation environment and the execution environment in
this International Standard. Their characteristics define and constrain the results of executing
conforming C programs constructed according to the syntactic and semantic rules for conforming
implementations.

Forward references: In this clause, only a few of many possible forward references have been
noted.

5.1 Conceptual models

5.1.1 Translation environment

5.1.1.1 Program structure

A C program need not all be translated at the same time. The text of the program is kept in units
called source files, (or preprocessing files) in this International Standard. A source file together with
all the headers and source files included via the preprocessing directive #include is known as
a preprocessing translation unit. After preprocessing, a preprocessing translation unit is called a
translation unit. Previously translated translation units may be preserved individually or in libraries.
The separate translation units of a program communicate by (for example) calls to functions whose
identifiers have external linkage, manipulation of objects whose identifiers have external linkage, or
manipulation of data files. Translation units may be separately translated and then later linked to
produce an executable program.

Forward references: linkages of identifiers (6.2.2), external definitions (6.9), preprocessing direc-
tives (6.10).

5.1.1.2 Translation phases

The precedence among the syntax rules of translation is specified by the following phases.6)

  1. Physical source file multibyte characters are mapped, in an implementation-defined manner, to the source character set (introducing new-line characters for end-of-line indicators) if necessary. Trigraph sequences are replaced by corresponding single-character internal representations.
  2. Each instance of a backslash character (\ ) immediately followed by a new-line character is
    deleted, splicing physical source lines to form logical source lines. Only the last backslash on
    any physical source line shall be eligible for being part of such a splice. A source file that is
    not empty shall end in a new-line character, which shall not be immediately preceded by a
    backslash character before any such splicing takes place.
  3. The source file is decomposed into preprocessing tokens7) and sequences of white-space
    characters (including comments). A source file shall not end in a partial preprocessing token or
    in a partial comment. Each comment is replaced by one space character. New-line characters
    are retained. Whether each nonempty sequence of white-space characters other than new-line
    is retained or replaced by one space character is implementation-defined.
  4. Preprocessing directives are executed, macro invocations are expanded, and _Pragma unary
    operator expressions are executed. If a character sequence that matches the syntax of a univer-
    sal character name is produced by token concatenation (6.10.3.3), the behavior is undefined. A
    #include preprocessing directive causes the named header or source file to be processed from
    phase 1 through phase 4, recursively. All preprocessing directives are then deleted.
  5. Each source character set member and escape sequence in character constants and string
    literals is converted to the corresponding member of the execution character set; if there is no
    corresponding member, it is converted to an implementation-defined member other than the
    null (wide) character.8)
  6. Adjacent string literal tokens are concatenated.
  7. White-space characters separating tokens are no longer significant. Each preprocessing token
    is converted into a token. The resulting tokens are syntactically and semantically analyzed
    and translated as a translation unit.
  8. All external object and function references are resolved. Library components are linked to
    satisfy external references to functions and objects not defined in the current translation. All
    such translator output is collected into a program image which contains information needed
    for execution in its execution environment.

Forward references: universal character names (6.4.3), lexical elements (6.4), preprocessing direc-
tives (6.10), trigraph sequences (5.2.1.1), external definitions (6.9).

5.1.1.3 Diagnostics

A conforming implementation shall produce at least one diagnostic message (identified in an imple-
mentation-defined manner) if a preprocessing translation unit or translation unit contains a violation
of any syntax rule or constraint, even if the behavior is also explicitly specified as undefined or
implementation-defined. Diagnostic messages need not be produced in other circumstances.9)

EXAMPLE An implementation is required to issue a diagnostic for the translation unit:

    char i;
    int i;
  

because in those cases where wording in this International Standard describes the behavior for a construct as being both a
constraint error and resulting in undefined behavior, the constraint error shall be diagnosed.

5.1.2 Execution environments

Two execution environments are defined: freestanding and hosted. In both cases, program startup
occurs when a designated C function is called by the execution environment. All objects with static
storage duration shall be initialized (set to their initial values) before program startup. The manner
and timing of such initialization are otherwise unspecified. Program termination returns control to
the execution environment.

Forward references: storage durations of objects (6.2.4), initialization (6.7.9).

5.1.2.1 Freestanding environment

In a freestanding environment (in which C program execution may take place without any benefit
of an operating system), the name and type of the function called at program startup are implemen-
tation-defined. Any library facilities available to a freestanding program, other than the minimal set
required by clause 4, are implementation-defined.

The effect of program termination in a freestanding environment is implementation-defined.

5.1.2.2 Hosted environment

A hosted environment need not be provided, but shall conform to the following specifications if
present.

5.1.2.2.1 Program startup

The function called at program startup is named main. The implementation declares no prototype
for this function. It shall be defined with a return type of int and with no parameters:

    int main(void) { /* ... */ }
  

or with two parameters (referred to here as argc and argv, though any names may be used, as they
are local to the function in which they are declared):

    int main(int argc, char *argv[]) { /* ... */ }
  

or equivalent;10) or in some other implementation-defined manner.

If they are declared, the parameters to the main function shall obey the following constraints:

5.1.2.2.2 Program execution

In a hosted environment, a program may use all the functions, macros, type definitions, and objects
described in the library clause (clause 7).

5.1.2.2.3 Program termination

If the return type of the main function is a type compatible with int, a return from the initial call
to the main function is equivalent to calling the exit function with the value returned by the main
function as its argument;11) reaching the } that terminates the main function returns a value of 0. If
the return type is not compatible with int, the termination status returned to the host environment
is unspecified.

Forward references: definition of terms (7.1.1), the exit function (7.22.4.4).

5.1.2.3 Program execution

The semantic descriptions in this International Standard describe the behavior of an abstract machine
in which issues of optimization are irrelevant.

Accessing a volatile object, modifying an object, modifying a file, or calling a function that does any
of those operations are all side effects,12) which are changes in the state of the execution environment.
Evaluation of an expression in general includes both value computations and initiation of side effects.

Value computation for an lvalue expression includes determining the identity of the designated
object.

Sequenced before is an asymmetric, transitive, pair-wise relation between evaluations executed by a
single thread, which induces a partial order among those evaluations. Given any two evaluations
A and B, if A is sequenced before B, then the execution of A shall precede the execution of B.
(Conversely, if A is sequenced before B, then B is sequenced after A.) If A is not sequenced before or
after B, then A and B are unsequenced. Evaluations A and B are indeterminately sequenced when A is
sequenced either before or after B, but it is unspecified which.13) The presence of a sequence point
between the evaluation of expressions A and B implies that every value computation and side effect
associated with A is sequenced before every value computation and side effect associated with B. (A
summary of the sequence points is given in annex C.)

In the abstract machine, all expressions are evaluated as specified by the semantics. An actual
implementation need not evaluate part of an expression if it can deduce that its value is not used and
that no needed side effects are produced (including any caused by calling a function or accessing a
volatile object).

When the processing of the abstract machine is interrupted by receipt of a signal, the values of
objects that are neither lock-free atomic objects nor of type volatile sig_atomic_t are unspecified,
as is the state of the floating-point environment. The value of any object modified by the handler
that is neither a lock-free atomic object nor of type volatile sig_atomic_t becomes indeterminate
when the handler exits, as does the state of the floating-point environment if it is modified by the
handler and not restored to its original state.

The least requirements on a conforming implementation are:

This is the observable behavior of the program.

What constitutes an interactive device is implementation-defined.

More stringent correspondences between abstract and actual semantics may be defined by each
implementation.

EXAMPLE 1 An implementation might define a one-to-one correspondence between abstract and actual semantics: at every
sequence point, the values of the actual objects would agree with those specified by the abstract semantics. The keyword
volatile would then be redundant.

Alternatively, an implementation might perform various optimizations within each translation unit, such that the actual
semantics would agree with the abstract semantics only when making function calls across translation unit boundaries. In
such an implementation, at the time of each function entry and function return where the calling function and the called
function are in different translation units, the values of all externally linked objects and of all objects accessible via pointers
therein would agree with the abstract semantics. Furthermore, at the time of each such function entry the values of the
parameters of the called function and of all objects accessible via pointers therein would agree with the abstract semantics. In
this type of implementation, objects referred to by interrupt service routines activated by the signal function would require
explicit specification of volatile storage, as well as other implementation-defined restrictions.

EXAMPLE 2 In executing the fragment

    char c1, c2;
    /* ... */
    c1 = c1 + c2;
  

the “integer promotions” require that the abstract machine promote the value of each variable to int size and then add
the two ints and truncate the sum. Provided the addition of two chars can be done without overflow, or with overflow
wrapping silently to produce the correct result, the actual execution need only produce the same result, possibly omitting the
promotions.

EXAMPLE 3 Similarly, in the fragment

    float f1, f2;
    double d;
    /* ... */
    f1 = f2 * d;
  

the multiplication may be executed using single-precision arithmetic if the implementation can ascertain that the result would
be the same as if it were executed using double-precision arithmetic (for example, if d were replaced by the constant 2.0,
which has type double).

EXAMPLE 4 Implementations employing wide registers have to take care to honor appropriate semantics. Values are
independent of whether they are represented in a register or in memory. For example, an implicit spilling of a register is
not permitted to alter the value. Also, an explicit store and load is required to round to the precision of the storage type. In
particular, casts and assignments are required to perform their specified conversion. For the fragment

    double d1, d2;
    float f;
    d1 = f = expression;
    d2 = (float) expression;
  

the values assigned to d1 and d2 are required to have been converted to float.

EXAMPLE 5 Rearrangement for floating-point expressions is often restricted because of limitations in precision as well as
range. The implementation cannot generally apply the mathematical associative rules for addition or multiplication, nor
the distributive rule, because of roundoff error, even in the absence of overflow and underflow. Likewise, implementations
cannot generally replace decimal constants in order to rearrange expressions. In the following fragment, rearrangements
suggested by mathematical rules for real numbers are often not valid (see F.9).

    double x, y, z;
    /* ... */
    x = (x * y) * z;        //   not   equivalent     to   x   *= y * z;
    z = (x - y) + y;        //   not   equivalent     to   z   = x;
    z = x + x * y;          //   not   equivalent     to   z   = x * (1.0 + y);
    y = x / 5.0;            //   not   equivalent     to   y   = x * 0.2;
  

EXAMPLE 6 To illustrate the grouping behavior of expressions, in the following fragment

    int a, b;
    /* ... */
    a = a + 32760 + b + 5;
  

the expression statement behaves exactly the same as

    a = (((a + 32760) + b) + 5);
  

due to the associativity and precedence of these operators. Thus, the result of the sum (a + 32760) is next added to b, and
that result is then added to 5 which results in the value assigned to a. On a machine in which overflows produce an explicit
trap and in which the range of values representable by an int is [−32768, +32767], the implementation cannot rewrite this
expression as

    a = ((a + b) + 32765);
  

since if the values for a and b were, respectively, −32754 and −15, the sum a + b would produce a trap while the original
expression would not; nor can the expression be rewritten either as

    a = ((a + 32765) + b);
  

or

    a = (a + (b + 32765));
  

since the values for a and b might have been, respectively, 4 and −8 or −17 and 12. However, on a machine in which
overflow silently generates some value and where positive and negative overflows cancel, the above expression statement
can be rewritten by the implementation in any of the above ways because the same result will occur.

EXAMPLE 7 The grouping of an expression does not completely determine its evaluation. In the following fragment

    #include <stdio.h>
    int sum;
    char *p;
    /* ... */
    sum = sum * 10 - ’0’ + (*p++ = getchar());
  

the expression statement is grouped as if it were written as

    sum = (((sum * 10) - ’0’) + ((*(p++)) = (getchar())));
  

but the actual increment of p can occur at any time between the previous sequence point and the next sequence point (the ;),
and the call to getchar can occur at any point prior to the need of its returned value.

Forward references: expressions (6.5), type qualifiers (6.7.3), statements (6.8), floating-point envi-
ronment <fenv.h> (7.6), the signal function (7.14), files (7.21.3).

5.1.2.4 Multi-threaded executions and data races

Under a hosted implementation, a program can have more than one thread of execution (or thread)
running concurrently. The execution of each thread proceeds as defined by the remainder of this
standard. The execution of the entire program consists of an execution of all of its threads.14) Under
a freestanding implementation, it is implementation-defined whether a program can have more
than one thread of execution.

The value of an object visible to a thread T at a particular point is the initial value of the object, a
value stored in the object by T , or a value stored in the object by another thread, according to the
rules below.

NOTE 1 In some cases, there may instead be undefined behavior. Much of this section is motivated by the desire to support
atomic operations with explicit and detailed visibility constraints. However, it also implicitly supports a simpler view for
more restricted programs.

Two expression evaluations conflict if one of them modifies a memory location and the other one
reads or modifies the same memory location.

The library defines a number of atomic operations (7.17) and operations on mutexes (7.26.4) that are
specially identified as synchronization operations. These operations play a special role in making
assignments in one thread visible to another. A synchronization operation on one or more memory
locations is either an acquire operation, a release operation, both an acquire and release operation, or a
consume operation. A synchronization operation without an associated memory location is a fence and
can be either an acquire fence, a release fence, or both an acquire and release fence. In addition, there
are relaxed atomic operations, which are not synchronization operations, and atomic read-modify-write
operations, which have special characteristics.

NOTE 2 For example, a call that acquires a mutex will perform an acquire operation on the locations composing the mutex.
Correspondingly, a call that releases the same mutex will perform a release operation on those same locations. Informally,
performing a release operation on A forces prior side effects on other memory locations to become visible to other threads
that later perform an acquire or consume operation on A. We do not include relaxed atomic operations as synchronization
operations although, like synchronization operations, they cannot contribute to data races.

All modifications to a particular atomic object M occur in some particular total order, called the
modification order of M . If A and B are modifications of an atomic object M , and A happens before B,
then A shall precede B in the modification order of M , which is defined below.

NOTE 3 This states that the modification orders must respect the “happens before” relation.

NOTE 4 There is a separate order for each atomic object. There is no requirement that these can be combined into a single
total order for all objects. In general this will be impossible since different threads may observe modifications to different
variables in inconsistent orders.

A release sequence headed by a release operation A on an atomic object M is a maximal contiguous
sub-sequence of side effects in the modification order of M , where the first operation is A and every
subsequent operation either is performed by the same thread that performed the release or is an
atomic read-modify-write operation.

Certain library calls synchronize with other library calls performed by another thread. In particular,
an atomic operation A that performs a release operation on an object M synchronizes with an atomic
operation B that performs an acquire operation on M and reads a value written by any side effect in
the release sequence headed by A.

NOTE 5 Except in the specified cases, reading a later value does not necessarily ensure visibility as described below. Such a
requirement would sometimes interfere with efficient implementation.

NOTE 6 The specifications of the synchronization operations define when one reads the value written by another. For atomic
variables, the definition is clear. All operations on a given mutex occur in a single total order. Each mutex acquisition “reads
the value written” by the last mutex release.

An evaluation A carries a dependency15) to an evaluation B if:

An evaluation A is dependency-ordered before16) an evaluation B if:

An evaluation A inter-thread happens before an evaluation B if A synchronizes with B, A is
dependency-ordered before B, or, for some evaluation X:

NOTE 7 The “inter-thread happens before” relation describes arbitrary concatenations of “sequenced before”, “synchronizes
with”, and “dependency-ordered before” relationships, with two exceptions. The first exception is that a concatenation is
not permitted to end with “dependency-ordered before” followed by “sequenced before”. The reason for this limitation is
that a consume operation participating in a “dependency-ordered before” relationship provides ordering only with respect
to operations to which this consume operation actually carries a dependency. The reason that this limitation applies only
to the end of such a concatenation is that any subsequent release operation will provide the required ordering for a prior
consume operation. The second exception is that a concatenation is not permitted to consist entirely of “sequenced before”.
The reasons for this limitation are (1) to permit “inter-thread happens before” to be transitively closed and (2) the “happens
before” relation, defined below, provides for relationships consisting entirely of “sequenced before”.

An evaluation A happens before an evaluation B if A is sequenced before B or A inter-thread happens
before B. The implementation shall ensure that no program execution demonstrates a cycle in the
“happens before” relation.

NOTE 8 This cycle would otherwise be possible only through the use of consume operations.

A visible side effect A on an object M with respect to a value computation B of M satisfies the
conditions:

The value of a non-atomic scalar object M , as determined by evaluation B, shall be the value stored
by the visible side effect A.

NOTE 9 If there is ambiguity about which side effect to a non-atomic object is visible, then there is a data race and the
behavior is undefined.

NOTE 10 This states that operations on ordinary variables are not visibly reordered. This is not actually detectable without
data races, but it is necessary to ensure that data races, as defined here, and with suitable restrictions on the use of atomics,
correspond to data races in a simple interleaved (sequentially consistent) execution.

The value of an atomic object M , as determined by evaluation B, shall be the value stored by some
side effect A that modifies M , where B does not happen before A.

NOTE 11 The set of side effects from which a given evaluation might take its value is also restricted by the rest of the rules
described here, and in particular, by the coherence requirements below.

If an operation A that modifies an atomic object M happens before an operation B that modifies M ,
then A shall be earlier than B in the modification order of M .

NOTE 12 The requirement above is known as “write-write coherence”.

If a value computation A of an atomic object M happens before a value computation B of M , and A
takes its value from a side effect X on M , then the value computed by B shall either be the value
stored by X or the value stored by a side effect Y on M , where Y follows X in the modification
order of M .

NOTE 13 The requirement above is known as “read-read coherence”.

If a value computation A of an atomic object M happens before an operation B on M , then A shall
take its value from a side effect X on M , where X precedes B in the modification order of M .

NOTE 14 The requirement above is known as “read-write coherence”.

If a side effect X on an atomic object M happens before a value computation B of M , then the
evaluation B shall take its value from X or from a side effect Y that follows X in the modification
order of M .

NOTE 15 The requirement above is known as “write-read coherence”.

NOTE 16 This effectively disallows compiler reordering of atomic operations to a single object, even if both operations are
“relaxed” loads. By doing so, we effectively make the “cache coherence” guarantee provided by most hardware available to C
atomic operations.

NOTE 17 The value observed by a load of an atomic object depends on the “happens before” relation, which in turn depends
on the values observed by loads of atomic objects. The intended reading is that there must exist an association of atomic loads
with modifications they observe that, together with suitably chosen modification orders and the “happens before” relation
derived as described above, satisfy the resulting constraints as imposed here.

The execution of a program contains a data race if it contains two conflicting actions in different
threads, at least one of which is not atomic, and neither happens before the other. Any such data
race results in undefined behavior.

NOTE 18 It can be shown that programs that correctly use simple mutexes and memory_order_seq_cst operations to
prevent all data races, and use no other synchronization operations, behave as though the operations executed by their
constituent threads were simply interleaved, with each value computation of an object being the last value stored in that
interleaving. This is normally referred to as “sequential consistency”. However, this applies only to data-race-free programs,
and data-race-free programs cannot observe most program transformations that do not change single-threaded program
semantics. In fact, most single-threaded program transformations continue to be allowed, since any program that behaves
differently as a result must contain undefined behavior.

NOTE 19 Compiler transformations that introduce assignments to a potentially shared memory location that would not

be modified by the abstract machine are generally precluded by this standard, since such an assignment might overwrite
another assignment by a different thread in cases in which an abstract machine execution would not have encountered a
data race. This includes implementations of data member assignment that overwrite adjacent members in separate memory
locations. Reordering of atomic loads in cases in which the atomics in question may alias is also generally precluded, since
this may violate the coherence requirements.

NOTE 20 Transformations that introduce a speculative read of a potentially shared memory location may not preserve the
semantics of the program as defined in this standard, since they potentially introduce a data race. However, they are typically
valid in the context of an optimizing compiler that targets a specific machine with well-defined semantics for data races. They
would be invalid for a hypothetical machine that is not tolerant of races or provides hardware race detection.

5.2 Environmental considerations

5.2.1 Character sets

Two sets of characters and their associated collating sequences shall be defined: the set in which
source files are written (the source character set), and the set interpreted in the execution environment
(the execution character set). Each set is further divided into a basic character set, whose contents are
given by this subclause, and a set of zero or more locale-specific members (which are not members
of the basic character set) called extended characters. The combined set is also called the extended
character set. The values of the members of the execution character set are implementation-defined.

In a character constant or string literal, members of the execution character set shall be represented by
corresponding members of the source character set or by escape sequences consisting of the backslash
\ followed by one or more characters. A byte with all bits set to 0, called the null character, shall exist
in the basic execution character set; it is used to terminate a character string.
Both the basic source and basic execution character sets shall have the following members: the 26
uppercase letters of the Latin alphabet

    A   B    C   D   E   F    G   H   I    J   K   L   M
    N   O    P   Q   R   S    T   U   V    W   X   Y   Z
  

the 26 lowercase letters of the Latin alphabet

    a   b    c   d   e   f    g   h   i    j   k   l   m
    n   o    p   q   r   s    t   u   v    w   x   y   z
  

the 10 decimal digits

    0   1    2   3   4   5    6   7   8    9
  

the following 29 graphic characters

    !   "    #   %   &   ’    (   )   *    +   ,   -   .    /   :
    ;   <    =   >   ?   [    \   ]   ^    _   {   |   }    ~
  

the space character, and control characters representing horizontal tab, vertical tab, and form feed.
The representation of each member of the source and execution basic character sets shall fit in a byte.
In both the source and execution basic character sets, the value of each character after 0 in the above
list of decimal digits shall be one greater than the value of the previous. In source files, there shall
be some way of indicating the end of each line of text; this International Standard treats such an
end-of-line indicator as if it were a single new-line character. In the basic execution character set,
there shall be control characters representing alert, backspace, carriage return, and new line. If any
other characters are encountered in a source file (except in an identifier, a character constant, a string
literal, a header name, a comment, or a preprocessing token that is never converted to a token), the
behavior is undefined.

A letter is an uppercase letter or a lowercase letter as defined above; in this International Standard
the term does not include other characters that are letters in other alphabets.

The universal character name construct provides a way to name other characters.

Forward references: universal character names (6.4.3), character constants (6.4.4.4), preprocessing
directives (6.10), string literals (6.4.5), comments (6.4.9), string (7.1.1).

5.2.1.1 Trigraph sequences

Before any other processing takes place, each occurrence of one of the following sequences of three
characters (called trigraph sequences)17) is replaced with the corresponding single character.

    ??=     #                    ??) ]                           ??! |
    ??(     [                    ??’ ^                           ??> }
    ??/     \                    ??< {                           ??- ~
  

No other trigraph sequences exist. Each ? that does not begin one of the trigraphs listed above is not
changed.

EXAMPLE 1

    ??=define arraycheck(a, b) a??(b??) ??!??! b??(a??)
  

becomes

    #define arraycheck(a, b) a[b] || b[a]
  

EXAMPLE 2 The following source line

    printf("Eh???/n");
  

becomes (after replacement of the trigraph sequence ??/)

    printf("Eh?\n");
  

5.2.1.2 Multibyte characters

The source character set may contain multibyte characters, used to represent members of the
extended character set. The execution character set may also contain multibyte characters, which
need not have the same encoding as for the source character set. For both character sets, the following
shall hold:

For source files, the following shall hold:

5.2.2 Character display semantics

The active position is that location on a display device where the next character output by the
fputc function would appear. The intent of writing a printing character (as defined by the isprint
function) to a display device is to display a graphic representation of that character at the active

position and then advance the active position to the next position on the current line. The direction
of writing is locale-specific. If the active position is at the final position of a line (if there is one), the
behavior of the display device is unspecified.

Alphabetic escape sequences representing nongraphic characters in the execution character set are
intended to produce actions on display devices as follows:

\a (alert) Produces an audible or visible alert without changing the active position.

\b (backspace) Moves the active position to the previous position on the current line. If the active
position is at the initial position of a line, the behavior of the display device is unspecified.

\f (form feed) Moves the active position to the initial position at the start of the next logical page.

\n (new line) Moves the active position to the initial position of the next line.

\r (carriage return) Moves the active position to the initial position of the current line.

\t (horizontal tab) Moves the active position to the next horizontal tabulation position on the current
line. If the active position is at or past the last defined horizontal tabulation position, the behavior
of the display device is unspecified.

\v (vertical tab) Moves the active position to the initial position of the next vertical tabulation
position. If the active position is at or past the last defined vertical tabulation position, the
behavior of the display device is unspecified.

Each of these escape sequences shall produce a unique implementation-defined value which can be
stored in a single char object. The external representations in a text file need not be identical to the
internal representations, and are outside the scope of this International Standard.

Forward references: the isprint function (7.4.1.8), the fputc function (7.21.7.3).

5.2.3 Signals and interrupts

Functions shall be implemented such that they may be interrupted at any time by a signal, or may be
called by a signal handler, or both, with no alteration to earlier, but still active, invocations’ control
flow (after the interruption), function return values, or objects with automatic storage duration.
All such objects shall be maintained outside the function image (the instructions that compose the
executable representation of a function) on a per-invocation basis.

5.2.4 Environmental limits

Both the translation and execution environments constrain the implementation of language trans-
lators and libraries. The following summarizes the language-related environmental limits on a
conforming implementation; the library-related limits are discussed in clause 7.

5.2.4.1 Translation limits

The implementation shall be able to translate and execute at least one program that contains at least
one instance of every one of the following limits:18)

5.2.4.2 Numerical limits

An implementation is required to document all the limits specified in this subclause, which are
specified in the headers <limits.h> and <float.h>. Additional limits are specified in <stdint.h>.

Forward references: integer types <stdint.h> (7.20).

5.2.4.2.1 Sizes of integer types <limits.h>

The values given below shall be replaced by constant expressions suitable for use in #if preprocess-
ing directives.
Moreover, except for CHAR_BIT and MB_LEN_MAX, the following shall be replaced by expressions that
have the same type as would an expression that is an object of the corresponding type converted
according to the integer promotions. Their implementation-defined values shall be equal or greater
in magnitude (absolute value) to those shown, with the same sign.

If an object of type char can hold negative values, the value of CHAR_MIN shall be the same as that of
SCHAR_MIN and the value of CHAR_MAX shall be the same as that of SCHAR_MAX. Otherwise, the value
of CHAR_MIN shall be 0 and the value of CHAR_MAX shall be the same as that of UCHAR_MAX.20) The

    _
  

value UCHAR_MAX shall equal 2CHAR BIT − 1.

Forward references: representations of types (6.2.6), conditional inclusion (6.10.1).

5.2.4.2.2 Characteristics of floating types <float.h>

The characteristics of floating types are defined in terms of a model that describes a representation
of floating-point numbers and values that provide information about an implementation’s floating-
point arithmetic.21) The following parameters are used to define the model for each floating-point
type:

    s      sign (±1)
    b      base or radix of exponent representation (an integer > 1)
    e      exponent (an integer between a minimum emin and a maximum emax )
    p      precision (the number of base-b digits in the significand)
    fk     nonnegative integers less than b (the significand digits)
  

A floating-point number (x) is defined by the following model:

               p
    x = sbe         fk b−k ,
               P
                               emin ≤ e ≤ emax
              k=1
  

In addition to normalized floating-point numbers (f1 > 0 if x6=0), floating types may be able to
contain other kinds of floating-point numbers, such as subnormal floating-point numbers (x 6= 0,
e = emin , f1 = 0) and unnormalized floating-point numbers (x 6= 0, e > emin , f1 = 0), and values
that are not floating-point numbers, such as infinities and NaNs. A NaN is an encoding signifying
Not-a-Number. A quiet NaN propagates through almost every arithmetic operation without raising a
floating-point exception; a signaling NaN generally raises a floating-point exception when occurring
as an arithmetic operand.22)

An implementation may give zero and values that are not floating-point numbers (such as infinities
and NaNs) a sign or may leave them unsigned. Wherever such values are unsigned, any requirement
in this International Standard to retrieve the sign shall produce an unspecified sign, and any
requirement to set the sign shall be ignored.

The minimum range of representable values for a floating type is the most negative finite floating-
point number representable in that type through the most positive finite floating-point number
representable in that type. In addition, if negative infinity is representable in a type, the range of

that type is extended to all negative real numbers; likewise, if positive infinity is representable in a
type, the range of that type is extended to all positive real numbers.

The accuracy of the floating-point operations (+ ,- , * , / ) and of the library functions in <math.h>
and <complex.h> that return floating-point results is implementation-defined, as is the accuracy of
the conversion between floating-point internal representations and string representations performed
by the library functions in <stdio.h>, <stdlib.h>, and <wchar.h>. The implementation may state
that the accuracy is unknown.

All integer values in the <float.h> header, except FLT_ROUNDS, shall be constant expressions
suitable for use in #if preprocessing directives; all floating values shall be constant expressions. All
except DECIMAL_DIG, FLT_EVAL_METHOD, FLT_RADIX, and FLT_ROUNDS have separate names for all
three floating-point types. The floating-point model representation is provided for all values except
FLT_EVAL_METHOD and FLT_ROUNDS.

The rounding mode for floating-point addition is characterized by the implementation-defined
value of FLT_ROUNDS:23)

-1
indeterminable
0
toward zero
1
to nearest
2
toward positive infinity
3
toward negative infinity

All other values for FLT_ROUNDS characterize implementation-defined rounding behavior.

Except for assignment and cast (which remove all extra range and precision), the values yielded
by operators with floating operands and values subject to the usual arithmetic conversions and of
floating constants are evaluated to a format whose range and precision may be greater than required
by the type. The use of evaluation formats is characterized by the implementation-defined value of
FLT_EVAL_METHOD:24)

-1
indeterminable;
0
evaluate all operations and constants just to the range and precision of the type;
1
evaluate operations and constants of type float and double to the range and precision of
the double type, evaluate long double operations and constants to the range and precision
of the long double type;
2
evaluate all operations and constants to the range and precision of the long double type.

All other negative values for FLT_EVAL_METHOD characterize implementation-defined behavior.

The presence or absence of subnormal numbers is characterized by the implementation-defined
values of FLT_HAS_SUBNORM, DBL_HAS_SUBNORM, and LDBL_HAS_SUBNORM:

-1
indeterminable25)
0
absent (type does not support subnormal numbers)26)
1
present (type does support subnormal numbers)

The values given in the following list shall be replaced by constant expressions with implementa-
tion-defined values that are greater or equal in magnitude (absolute value) to those shown, with the
same sign:

The values given in the following list shall be replaced by constant expressions with implementa-
tion-defined values that are greater than or equal to those shown:

The values given in the following list shall be replaced by constant expressions with implementa-
tion-defined (positive) values that are less than or equal to those shown:

Recommended practice

Conversion from (at least) double to decimal with DECIMAL_DIG digits and back should be the
identity function.

EXAMPLE 1 The following describes an artificial floating-point representation that meets the minimum requirements of this
International Standard, and the appropriate values in a <float.h> header for type float:

               6
    x = s16e         fk 16−k ,
               P
                                 −31 ≤ e ≤ +32
               k=1
  
    FLT_RADIX                                   16
    FLT_MANT_DIG                                 6
    FLT_EPSILON                    9.53674316E-07F
    FLT_DECIMAL_DIG                              9
    FLT_DIG                                      6
    FLT_MIN_EXP                                -31
    FLT_MIN                        2.93873588E-39F
    FLT_MIN_10_EXP                             -38
    FLT_MAX_EXP                                +32
    FLT_MAX                        3.40282347E+38F
    FLT_MAX_10_EXP                             +38
  

EXAMPLE 2 The following describes floating-point representations that also meet the requirements for single-precision and
double-precision numbers in IEC 60559,28) and the appropriate values in a <float.h> header for types float and double:

               24
    xf = s2e         fk 2−k ,
               P
                                 −125 ≤ e ≤ +128
               k=1
  
               53
    xd = s2e         fk 2−k ,
               P
                                 −1021 ≤ e ≤ +1024
               k=1
  
    FLT_RADIX                          2
    DECIMAL_DIG                       17
    FLT_MANT_DIG                      24
    FLT_EPSILON          1.19209290E-07F                // decimal constant
    FLT_EPSILON                 0X1P-23F                // hex constant
    FLT_DECIMAL_DIG                    9
    FLT_DIG                            6
    FLT_MIN_EXP                     -125
    FLT_MIN              1.17549435E-38F                //   decimal constant
    FLT_MIN                    0X1P-126F                //   hex constant
    FLT_TRUE_MIN         1.40129846E-45F                //   decimal constant
    FLT_TRUE_MIN               0X1P-149F                //   hex constant
    FLT_HAS_SUBNORM                    1
    FLT_MIN_10_EXP                   -37
    FLT_MAX_EXP                     +128
    FLT_MAX              3.40282347E+38F                // decimal constant
    FLT_MAX              0X1.fffffeP127F                // hex constant
    FLT_MAX_10_EXP                   +38
    DBL_MANT_DIG                      53
    DBL_EPSILON   2.2204460492503131E-16                // decimal constant
    DBL_EPSILON                  0X1P-52                // hex constant
    DBL_DECIMAL_DIG                   17
    DBL_DIG                           15
    DBL_MIN_EXP                    -1021
    DBL_MIN      2.2250738585072014E-308                //   decimal constant
    DBL_MIN                    0X1P-1022                //   hex constant
    DBL_TRUE_MIN 4.9406564584124654E-324                //   decimal constant
    DBL_TRUE_MIN               0X1P-1074                //   hex constant
    DBL_HAS_SUBNORM                    1
  
    DBL_MIN_10_EXP                  -307
    DBL_MAX_EXP                    +1024
    DBL_MAX      1.7976931348623157E+308 // decimal constant
    DBL_MAX       0X1.fffffffffffffP1023 // hex constant
    DBL_MAX_10_EXP                  +308
  

If a type wider than double were supported, then DECIMAL_DIG would be greater than 17. For example, if the widest type
were to use the minimal-width IEC 60559 double-extended format (64 bits of precision), then DECIMAL_DIG would be 21.

Forward references: conditional inclusion (6.10.1), complex arithmetic <complex.h> (7.3), ex-
tended multibyte and wide character utilities <wchar.h> (7.29), floating-point environment
<fenv.h> (7.6), general utilities <stdlib.h> (7.22), input/output <stdio.h> (7.21), mathematics
<math.h> (7.12).

6. Language

6.1 Notation

In the syntax notation used in this clause, syntactic categories (nonterminals) are indicated by italic
type, and literal words and character set members (terminals) by bold type. A colon (:) following
a nonterminal introduces its definition. Alternative definitions are listed on separate lines, except
when prefaced by the words “one of”. An optional symbol is indicated by the subscript “opt”, so
that

    { expressionopt }
  

indicates an optional expression enclosed in braces.

When syntactic categories are referred to in the main text, they are not italicized and words are
separated by spaces instead of hyphens.

A summary of the language syntax is given in annex A.

6.2 Concepts

6.2.1 Scopes of identifiers

An identifier can denote an object; a function; a tag or a member of a structure, union, or enumeration;
a typedef name; a label name; a macro name; or a macro parameter. The same identifier can denote
different entities at different points in the program. A member of an enumeration is called an
enumeration constant. Macro names and macro parameters are not considered further here, because
prior to the semantic phase of program translation any occurrences of macro names in the source file
are replaced by the preprocessing token sequences that constitute their macro definitions.

For each different entity that an identifier designates, the identifier is visible (i.e., can be used) only
within a region of program text called its scope. Different entities designated by the same identifier
either have different scopes, or are in different name spaces. There are four kinds of scopes: function,
file, block, and function prototype. (A function prototype is a declaration of a function that declares
the types of its parameters.)

A label name is the only kind of identifier that has function scope. It can be used (in a goto statement)
anywhere in the function in which it appears, and is declared implicitly by its syntactic appearance
(followed by a : and a statement).

Every other identifier has scope determined by the placement of its declaration (in a declarator or
type specifier). If the declarator or type specifier that declares the identifier appears outside of any
block or list of parameters, the identifier has file scope, which terminates at the end of the translation
unit. If the declarator or type specifier that declares the identifier appears inside a block or within the
list of parameter declarations in a function definition, the identifier has block scope, which terminates
at the end of the associated block. If the declarator or type specifier that declares the identifier
appears within the list of parameter declarations in a function prototype (not part of a function
definition), the identifier has function prototype scope, which terminates at the end of the function
declarator. If an identifier designates two different entities in the same name space, the scopes might
overlap. If so, the scope of one entity (the inner scope) will end strictly before the scope of the other
entity (the outer scope). Within the inner scope, the identifier designates the entity declared in the
inner scope; the entity declared in the outer scope is hidden (and not visible) within the inner scope.

Unless explicitly stated otherwise, where this International Standard uses the term “identifier” to
refer to some entity (as opposed to the syntactic construct), it refers to the entity in the relevant name
space whose declaration is visible at the point the identifier occurs.

Two identifiers have the same scope if and only if their scopes terminate at the same point.

Structure, union, and enumeration tags have scope that begins just after the appearance of the tag in
a type specifier that declares the tag. Each enumeration constant has scope that begins just after the
appearance of its defining enumerator in an enumerator list. Any other identifier has scope that

begins just after the completion of its declarator.

As a special case, a type name (which is not a declaration of an identifier) is considered to have
a scope that begins just after the place within the type name where the omitted identifier would
appear were it not omitted.

Forward references: declarations (6.7), function calls (6.5.2.2), function definitions (6.9.1), identifiers
(6.4.2), macro replacement (6.10.3), name spaces of identifiers (6.2.3), source file inclusion (6.10.2),
statements and blocks (6.8).

6.2.2 Linkages of identifiers

An identifier declared in different scopes or in the same scope more than once can be made to refer to
the same object or function by a process called linkage.29) There are three kinds of linkage: external,
internal, and none.

In the set of translation units and libraries that constitutes an entire program, each declaration of a
particular identifier with external linkage denotes the same object or function. Within one translation
unit, each declaration of an identifier with internal linkage denotes the same object or function. Each
declaration of an identifier with no linkage denotes a unique entity.

If the declaration of a file scope identifier for an object or a function contains the storage-class
specifier static, the identifier has internal linkage.30)

For an identifier declared with the storage-class specifier extern in a scope in which a prior dec-
laration of that identifier is visible,31) if the prior declaration specifies internal or external linkage,
the linkage of the identifier at the later declaration is the same as the linkage specified at the prior
declaration. If no prior declaration is visible, or if the prior declaration specifies no linkage, then the
identifier has external linkage.

If the declaration of an identifier for a function has no storage-class specifier, its linkage is determined
exactly as if it were declared with the storage-class specifier extern. If the declaration of an identifier
for an object has file scope and no storage-class specifier, its linkage is external.

The following identifiers have no linkage: an identifier declared to be anything other than an object
or a function; an identifier declared to be a function parameter; a block scope identifier for an object
declared without the storage-class specifier extern.

If, within a translation unit, the same identifier appears with both internal and external linkage, the
behavior is undefined.

Forward references: declarations (6.7), expressions (6.5), external definitions (6.9), statements (6.8).

6.2.3 Name spaces of identifiers

If more than one declaration of a particular identifier is visible at any point in a translation unit, the
syntactic context disambiguates uses that refer to different entities. Thus, there are separate name
spaces for various categories of identifiers, as follows:

Forward references: enumeration specifiers (6.7.2.2), labeled statements (6.8.1), structure and union
specifiers (6.7.2.1), structure and union members (6.5.2.3), tags (6.7.2.3), the goto statement (6.8.6.1).

6.2.4 Storage durations of objects

An object has a storage duration that determines its lifetime. There are four storage durations: static,
thread, automatic, and allocated. Allocated storage is described in 7.22.3.

The lifetime of an object is the portion of program execution during which storage is guaranteed
to be reserved for it. An object exists, has a constant address,33) and retains its last-stored value
throughout its lifetime.34) If an object is referred to outside of its lifetime, the behavior is undefined.
The value of a pointer becomes indeterminate when the object it points to (or just past) reaches the
end of its lifetime.

An object whose identifier is declared without the storage-class specifier _Thread_local , and either
with external or internal linkage or with the storage-class specifier static, has static storage duration.
Its lifetime is the entire execution of the program and its stored value is initialized only once, prior
to program startup.

An object whose identifier is declared with the storage-class specifier _Thread_local has thread
storage duration. Its lifetime is the entire execution of the thread for which it is created, and its
stored value is initialized when the thread is started. There is a distinct object per thread, and use of
the declared name in an expression refers to the object associated with the thread evaluating the
expression. The result of attempting to indirectly access an object with thread storage duration from
a thread other than the one with which the object is associated is implementation-defined.

An object whose identifier is declared with no linkage and without the storage-class specifier static
has automatic storage duration, as do some compound literals. The result of attempting to indirectly
access an object with automatic storage duration from a thread other than the one with which the
object is associated is implementation-defined.

For such an object that does not have a variable length array type, its lifetime extends from entry
into the block with which it is associated until execution of that block ends in any way. (Entering an
enclosed block or calling a function suspends, but does not end, execution of the current block.) If
the block is entered recursively, a new instance of the object is created each time. The initial value of
the object is indeterminate. If an initialization is specified for the object, it is performed each time
the declaration or compound literal is reached in the execution of the block; otherwise, the value
becomes indeterminate each time the declaration is reached.

For such an object that does have a variable length array type, its lifetime extends from the declaration
of the object until execution of the program leaves the scope of the declaration.35) If the scope is
entered recursively, a new instance of the object is created each time. The initial value of the object is
indeterminate.

A non-lvalue expression with structure or union type, where the structure or union contains a
member with array type (including, recursively, members of all contained structures and unions)
refers to an object with automatic storage duration and temporary lifetime.36) Its lifetime begins
when the expression is evaluated and its initial value is the value of the expression. Its lifetime ends
when the evaluation of the containing full expression ends. Any attempt to modify an object with
temporary lifetime results in undefined behavior. An object with temporary lifetime behaves as if it
were declared with the type of its value for the purposes of effective type. Such an object need not
have a unique address.

Forward references: array declarators (6.7.6.2), compound literals (6.5.2.5), declarators (6.7.6),
function calls (6.5.2.2), initialization (6.7.9), statements (6.8), effective type (6.5).

6.2.5 Types

The meaning of a value stored in an object or returned by a function is determined by the type of the
expression used to access it. (An identifier declared to be an object is the simplest such expression;
the type is specified in the declaration of the identifier.) Types are partitioned into object types (types
that describe objects) and function types (types that describe functions). At various points within a
translation unit an object type may be incomplete (lacking sufficient information to determine the
size of objects of that type) or complete (having sufficient information).37)

An object declared as type _Bool is large enough to store the values 0 and 1.

An object declared as type char is large enough to store any member of the basic execution character
set. If a member of the basic execution character set is stored in a char object, its value is guaranteed
to be nonnegative. If any other character is stored in a char object, the resulting value is implemen-
tation-defined but shall be within the range of values that can be represented in that type.

There are five standard signed integer types, designated as signed char, short int, int, long int,
and long long int. (These and other types may be designated in several additional ways, as
described in 6.7.2.) There may also be implementation-defined extended signed integer types.38) The
standard and extended signed integer types are collectively called signed integer types.39)

An object declared as type signed char occupies the same amount of storage as a “plain” char
object. A “plain” int object has the natural size suggested by the architecture of the execution
environment (large enough to contain any value in the range INT_MIN to INT_MAX as defined in the
header <limits.h>).

For each of the signed integer types, there is a corresponding (but different) unsigned integer type
(designated with the keyword unsigned) that uses the same amount of storage (including sign
information) and has the same alignment requirements. The type _Bool and the unsigned integer
types that correspond to the standard signed integer types are the standard unsigned integer types.
The unsigned integer types that correspond to the extended signed integer types are the extended
unsigned integer types. The standard and extended unsigned integer types are collectively called
unsigned integer types.40)

The standard signed integer types and standard unsigned integer types are collectively called the
standard integer types; the extended signed integer types and extended unsigned integer types are
collectively called the extended integer types.

For any two integer types with the same signedness and different integer conversion rank (see
6.3.1.1), the range of values of the type with smaller integer conversion rank is a subrange of the
values of the other type.

The range of nonnegative values of a signed integer type is a subrange of the corresponding unsigned
integer type, and the representation of the same value in each type is the same.41) A computation
involving unsigned operands can never overflow, because a result that cannot be represented by the
resulting unsigned integer type is reduced modulo the number that is one greater than the largest
value that can be represented by the resulting type.

There are three real floating types, designated as float, double, and long double.42) The set of
values of the type float is a subset of the set of values of the type double; the set of values of the
type double is a subset of the set of values of the type long double.

There are three complex types, designated as float _Complex, double _Complex, and long double
_Complex .43) (Complex types are a conditional feature that implementations need not support; see

6.10.8.3.) The real floating and complex types are collectively called the floating types.

For each floating type there is a corresponding real type, which is always a real floating type. For real
floating types, it is the same type. For complex types, it is the type given by deleting the keyword
_Complex from the type name.

Each complex type has the same representation and alignment requirements as an array type
containing exactly two elements of the corresponding real type; the first element is equal to the real
part, and the second element to the imaginary part, of the complex number.

The type char, the signed and unsigned integer types, and the floating types are collectively called
the basic types. The basic types are complete object types. Even if the implementation defines two or
more basic types to have the same representation, they are nevertheless different types.44)

The three types char, signed char, and unsigned char are collectively called the character types.
The implementation shall define char to have the same range, representation, and behavior as either
signed char or unsigned char.45)

An enumeration comprises a set of named integer constant values. Each distinct enumeration
constitutes a different enumerated type.

The type char, the signed and unsigned integer types, and the enumerated types are collectively
called integer types. The integer and real floating types are collectively called real types.

Integer and floating types are collectively called arithmetic types. Each arithmetic type belongs to
one type domain: the real type domain comprises the real types, the complex type domain comprises the
complex types.

The void type comprises an empty set of values; it is an incomplete object type that cannot be
completed.

Any number of derived types can be constructed from the object and function types, as follows:

These methods of constructing derived types can be applied recursively.

Arithmetic types and pointer types are collectively called scalar types. Array and structure types are
collectively called aggregate types.46)

An array type of unknown size is an incomplete type. It is completed, for an identifier of that type,
by specifying the size in a later declaration (with internal or external linkage). A structure or union
type of unknown content (as described in 6.7.2.3) is an incomplete type. It is completed, for all
declarations of that type, by declaring the same structure or union tag with its defining content later
in the same scope.

A type has known constant size if the type is not incomplete and is not a variable length array type.

Array, function, and pointer types are collectively called derived declarator types. A declarator type
derivation from a type T is the construction of a derived declarator type from T by the application of
an array-type, a function-type, or a pointer-type derivation to T.

A type is characterized by its type category, which is either the outermost derivation of a derived
type (as noted above in the construction of derived types), or the type itself if the type consists of no
derived types.

Any type so far mentioned is an unqualified type. Each unqualified type has several qualified versions
of its type,47) corresponding to the combinations of one, two, or all three of the const, volatile,
and restrict qualifiers. The qualified or unqualified versions of a type are distinct types that
belong to the same type category and have the same representation and alignment requirements.48)
A derived type is not qualified by the qualifiers (if any) of the type from which it is derived.

Further, there is the _Atomic qualifier. The presence of the _Atomic qualifier designates an atomic
type. The size, representation, and alignment of an atomic type need not be the same as those of
the corresponding unqualified type. Therefore, this Standard explicitly uses the phrase “atomic,
qualified or unqualified type” whenever the atomic version of a type is permitted along with the
other qualified versions of a type. The phrase “qualified or unqualified type”, without specific
mention of atomic, does not include the atomic types.

A pointer to void shall have the same representation and alignment requirements as a pointer to a
character type.48) Similarly, pointers to qualified or unqualified versions of compatible types shall
have the same representation and alignment requirements. All pointers to structure types shall have
the same representation and alignment requirements as each other. All pointers to union types shall
have the same representation and alignment requirements as each other. Pointers to other types
need not have the same representation or alignment requirements.

EXAMPLE 1 The type designated as “float *” has type “pointer to float”. Its type category is pointer, not a floating type.
The const-qualified version of this type is designated as “float * const” whereas the type designated as “const float *”
is not a qualified type — its type is “pointer to const-qualified float” and is a pointer to a qualified type.

EXAMPLE 2 The type designated as “struct tag (*[5])(float)” has type “array of pointer to function returning
struct tag”. The array has length five and the function has a single parameter of type float. Its type category is array.

Forward references: compatible type and composite type (6.2.7), declarations (6.7).

6.2.6 Representations of types

6.2.6.1 General

The representations of all types are unspecified except as stated in this subclause.

Except for bit-fields, objects are composed of contiguous sequences of one or more bytes, the number,
order, and encoding of which are either explicitly specified or implementation-defined.

Values stored in unsigned bit-fields and objects of type unsigned char shall be represented using a
pure binary notation.49)

Values stored in non-bit-field objects of any other object type consist of n × CHAR_BIT bits, where
n is the size of an object of that type, in bytes. The value may be copied into an object of type
unsigned char [n] (e.g., by memcpy); the resulting set of bytes is called the object representation of
the value. Values stored in bit-fields consist of m bits, where m is the size specified for the bit-field.
The object representation is the set of m bits the bit-field comprises in the addressable storage unit
holding it. Two values (other than NaNs) with the same object representation compare equal, but
values that compare equal may have different object representations.

Certain object representations need not represent a value of the object type. If the stored value of an
object has such a representation and is read by an lvalue expression that does not have character
type, the behavior is undefined. If such a representation is produced by a side effect that modifies
all or any part of the object by an lvalue expression that does not have character type, the behavior
is undefined.50) Such a representation is called a trap representation.

When a value is stored in an object of structure or union type, including in a member object, the
bytes of the object representation that correspond to any padding bytes take unspecified values.51)
The value of a structure or union object is never a trap representation, even though the value of a
member of the structure or union object may be a trap representation.

When a value is stored in a member of an object of union type, the bytes of the object representation
that do not correspond to that member but do correspond to other members take unspecified values.

Where an operator is applied to a value that has more than one object representation, which object
representation is used shall not affect the value of the result.52) Where a value is stored in an object
using a type that has more than one object representation for that value, it is unspecified which
representation is used, but a trap representation shall not be generated.

Loads and stores of objects with atomic types are done with memory_order_seq_cst semantics.

Forward references: declarations (6.7), expressions (6.5), lvalues, arrays, and function designators
(6.3.2.1), order and consistency (7.17.3).

6.2.6.2 Integer types

For unsigned integer types other than unsigned char, the bits of the object representation shall be
divided into two groups: value bits and padding bits (there need not be any of the latter). If there are
N value bits, each bit shall represent a different power of 2 between 1 and 2N −1 , so that objects of
that type shall be capable of representing values from 0 to 2N − 1 using a pure binary representation;
this shall be known as the value representation. The values of any padding bits are unspecified.53)

For signed integer types, the bits of the object representation shall be divided into three groups:
value bits, padding bits, and the sign bit. There need not be any padding bits; signed char shall
not have any padding bits. There shall be exactly one sign bit. Each bit that is a value bit shall have
the same value as the same bit in the object representation of the corresponding unsigned type (if
there are M value bits in the signed type and N in the unsigned type, then M ≤ N ). If the sign bit is
zero, it shall not affect the resulting value. If the sign bit is one, the value shall be modified in one of

the following ways:

Which of these applies is implementation-defined, as is whether the value with sign bit 1 and all
value bits zero (for the first two), or with sign bit and all value bits 1 (for ones’ complement), is a
trap representation or a normal value. In the case of sign and magnitude and ones’ complement, if
this representation is a normal value it is called a negative zero.

If the implementation supports negative zeros, they shall be generated only by:

It is unspecified whether these cases actually generate a negative zero or a normal zero, and whether
a negative zero becomes a normal zero when stored in an object.

If the implementation does not support negative zeros, the behavior of the &, |, ^, ~ , << , and >>
operators with operands that would produce such a value is undefined.

The values of any padding bits are unspecified.54) A valid (non-trap) object representation of a
signed integer type where the sign bit is zero is a valid object representation of the corresponding
unsigned type, and shall represent the same value. For any integer type, the object representation
where all the bits are zero shall be a representation of the value zero in that type.

The precision of an integer type is the number of bits it uses to represent values, excluding any sign
and padding bits. The width of an integer type is the same but including any sign bit; thus for
unsigned integer types the two values are the same, while for signed integer types the width is one
greater than the precision.

6.2.7 Compatible type and composite type

Two types have compatible type if their types are the same. Additional rules for determining whether
two types are compatible are described in 6.7.2 for type specifiers, in 6.7.3 for type qualifiers, and in
6.7.6 for declarators.55) Moreover, two structure, union, or enumerated types declared in separate
translation units are compatible if their tags and members satisfy the following requirements: If
one is declared with a tag, the other shall be declared with the same tag. If both are completed
anywhere within their respective translation units, then the following additional requirements
apply: there shall be a one-to-one correspondence between their members such that each pair of
corresponding members are declared with compatible types; if one member of the pair is declared
with an alignment specifier, the other is declared with an equivalent alignment specifier; and if
one member of the pair is declared with a name, the other is declared with the same name. For
two structures, corresponding members shall be declared in the same order. For two structures or
unions, corresponding bit-fields shall have the same widths. For two enumerations, corresponding
members shall have the same values.

All declarations that refer to the same object or function shall have compatible type; otherwise, the
behavior is undefined.

A composite type can be constructed from two types that are compatible; it is a type that is compatible
with both of the two types and satisfies the following conditions:

These rules apply recursively to the types from which the two types are derived.

For an identifier with internal or external linkage declared in a scope in which a prior declaration of
that identifier is visible,56) if the prior declaration specifies internal or external linkage, the type of
the identifier at the later declaration becomes the composite type.

Forward references: array declarators (6.7.6.2).

EXAMPLE Given the following two file scope declarations:

    int f(int (*)(), double (*)[3]);
    int f(int (*)(char *), double (*)[]);
  

The resulting composite type for the function is:

    int f(int (*)(char *), double (*)[3]);
  

6.2.8 Alignment of objects

Complete object types have alignment requirements which place restrictions on the addresses at
which objects of that type may be allocated. An alignment is an implementation-defined integer
value representing the number of bytes between successive addresses at which a given object can be
allocated. An object type imposes an alignment requirement on every object of that type: stricter
alignment can be requested using the _Alignas keyword.

A fundamental alignment is a valid alignment less than or equal to _Alignof (max_align_t) . Fun-
damental alignments shall be supported by the implementation for objects of all storage durations.
The alignment requirements of the following types shall be fundamental alignments:

An extended alignment is represented by an alignment greater than _Alignof (max_align_t) . It is
implementation-defined whether any extended alignments are supported and the storage durations
for which they are supported. A type having an extended alignment requirement is an over-aligned
type.57)

Alignments are represented as values of the type size_t. Valid alignments include only fundamental
alignments, plus an additional implementation-defined set of values, which may be empty. Every
valid alignment value shall be a nonnegative integral power of two.

Alignments have an order from weaker to stronger or stricter alignments. Stricter alignments have
larger alignment values. An address that satisfies an alignment requirement also satisfies any weaker
valid alignment requirement.

The alignment requirement of a complete type can be queried using an _Alignof expression. The
types char, signed char, and unsigned char shall have the weakest alignment requirement.

Comparing alignments is meaningful and provides the obvious results:

6.3 Conversions

Several operators convert operand values from one type to another automatically. This subclause
specifies the result required from such an implicit conversion, as well as those that result from a cast
operation (an explicit conversion). The list in 6.3.1.8 summarizes the conversions performed by most
ordinary operators; it is supplemented as required by the discussion of each operator in 6.5.

Unless explicitly stated otherwise, conversion of an operand value to a compatible type causes no
change to the value or the representation.

Forward references: cast operators (6.5.4).

6.3.1 Arithmetic operands

6.3.1.1 Boolean, characters, and integers

Every integer type has an integer conversion rank defined as follows:

The following may be used in an expression wherever an int or unsigned int may be used:

If an int can represent all values of the original type (as restricted by the width, for a bit-field), the
value is converted to an int; otherwise, it is converted to an unsigned int. These are called the
integer promotions.58) All other types are unchanged by the integer promotions.

The integer promotions preserve value including sign. As discussed earlier, whether a “plain” char
can hold negative values is implementation-defined.

Forward references: enumeration specifiers (6.7.2.2), structure and union specifiers (6.7.2.1).

6.3.1.2 Boolean type

When any scalar value is converted to _Bool , the result is 0 if the value compares equal to 0;
otherwise, the result is 1.59)

6.3.1.3 Signed and unsigned integers

When a value with integer type is converted to another integer type other than _Bool , if the value
can be represented by the new type, it is unchanged.

Otherwise, if the new type is unsigned, the value is converted by repeatedly adding or subtracting
one more than the maximum value that can be represented in the new type until the value is in the
range of the new type.60)

Otherwise, the new type is signed and the value cannot be represented in it; either the result is
implementation-defined or an implementation-defined signal is raised.

6.3.1.4 Real floating and integer

When a finite value of real floating type is converted to an integer type other than _Bool , the
fractional part is discarded (i.e., the value is truncated toward zero). If the value of the integral part
cannot be represented by the integer type, the behavior is undefined.61)

When a value of integer type is converted to a real floating type, if the value being converted can
be represented exactly in the new type, it is unchanged. If the value being converted is in the
range of values that can be represented but cannot be represented exactly, the result is either the
nearest higher or nearest lower representable value, chosen in an implementation-defined manner.

If the value being converted is outside the range of values that can be represented, the behavior is
undefined. Results of some implicit conversions may be represented in greater range and precision
than that required by the new type (see 6.3.1.8 and 6.8.6.4).

6.3.1.5 Real floating types

When a value of real floating type is converted to a real floating type, if the value being converted
can be represented exactly in the new type, it is unchanged. If the value being converted is in the
range of values that can be represented but cannot be represented exactly, the result is either the
nearest higher or nearest lower representable value, chosen in an implementation-defined manner.
If the value being converted is outside the range of values that can be represented, the behavior is
undefined. Results of some implicit conversions may be represented in greater range and precision
than that required by the new type (see 6.3.1.8 and 6.8.6.4).

6.3.1.6 Complex types

When a value of complex type is converted to another complex type, both the real and imaginary
parts follow the conversion rules for the corresponding real types.

6.3.1.7 Real and complex

When a value of real type is converted to a complex type, the real part of the complex result value is
determined by the rules of conversion to the corresponding real type and the imaginary part of the
complex result value is a positive zero or an unsigned zero.

When a value of complex type is converted to a real type other than _Bool ,62) the imaginary part of
the complex value is discarded and the value of the real part is converted according to the conversion
rules for the corresponding real type.

6.3.1.8 Usual arithmetic conversions

Many operators that expect operands of arithmetic type cause conversions and yield result types in
a similar way. The purpose is to determine a common real type for the operands and result. For the
specified operands, each operand is converted, without change of type domain, to a type whose
corresponding real type is the common real type. Unless explicitly stated otherwise, the common
real type is also the corresponding real type of the result, whose type domain is the type domain of
the operands if they are the same, and complex otherwise. This pattern is called the usual arithmetic
conversions:

    First, if the corresponding real type of either operand is long double, the other operand
    is converted, without change of type domain, to a type whose corresponding real type is
    long double.
  
    Otherwise, if the corresponding real type of either operand is double, the other operand is
    converted, without change of type domain, to a type whose corresponding real type is double.
  
    Otherwise, if the corresponding real type of either operand is float, the other operand is
    converted, without change of type domain, to a type whose corresponding real type is float.63)
  
    Otherwise, the integer promotions are performed on both operands. Then the following rules
    are applied to the promoted operands:
  
    If both operands have the same type, then no further conversion is needed.
    Otherwise, if both operands have signed integer types or both have unsigned integer
    types, the operand with the type of lesser integer conversion rank is converted to the type
    of the operand with greater rank.
    Otherwise, if the operand that has unsigned integer type has rank greater or equal to
    the rank of the type of the other operand, then the operand with signed integer type is
    converted to the type of the operand with unsigned integer type.
  
    Otherwise, if the type of the operand with signed integer type can represent all of the
    values of the type of the operand with unsigned integer type, then the operand with
    unsigned integer type is converted to the type of the operand with signed integer type.
  
    Otherwise, both operands are converted to the unsigned integer type corresponding to
    the type of the operand with signed integer type.
  

The values of floating operands and of the results of floating expressions may be represented in
greater range and precision than that required by the type; the types are not changed thereby.64)

6.3.2 Other operands

6.3.2.1 Lvalues, arrays, and function designators

An lvalue is an expression (with an object type other than void) that potentially designates an
object;65) if an lvalue does not designate an object when it is evaluated, the behavior is undefined.
When an object is said to have a particular type, the type is specified by the lvalue used to designate
the object. A modifiable lvalue is an lvalue that does not have array type, does not have an incomplete
type, does not have a const-qualified type, and if it is a structure or union, does not have any
member (including, recursively, any member or element of all contained aggregates or unions) with
a const-qualified type.

Except when it is the operand of the sizeof operator, the unary & operator, the ++ operator, the--
operator, or the left operand of the . operator or an assignment operator, an lvalue that does not
have array type is converted to the value stored in the designated object (and is no longer an lvalue);
this is called lvalue conversion. If the lvalue has qualified type, the value has the unqualified version
of the type of the lvalue; additionally, if the lvalue has atomic type, the value has the non-atomic
version of the type of the lvalue; otherwise, the value has the type of the lvalue. If the lvalue has an
incomplete type and does not have array type, the behavior is undefined. If the lvalue designates an
object of automatic storage duration that could have been declared with the register storage class
(never had its address taken), and that object is uninitialized (not declared with an initializer and no
assignment to it has been performed prior to use), the behavior is undefined.

Except when it is the operand of the sizeof operator, or the unary & operator, or is a string literal
used to initialize an array, an expression that has type “array of type” is converted to an expression
with type “pointer to type” that points to the initial element of the array object and is not an lvalue.
If the array object has register storage class, the behavior is undefined.

A function designator is an expression that has function type. Except when it is the operand of the
sizeof operator,66) or the unary & operator, a function designator with type “function returning
type” is converted to an expression that has type “pointer to function returning type”.

Forward references: address and indirection operators (6.5.3.2), assignment operators (6.5.16),
common definitions <stddef.h> (7.19), initialization (6.7.9), postfix increment and decrement
operators (6.5.2.4), prefix increment and decrement operators (6.5.3.1), the sizeof and _Alignof
operators (6.5.3.4), structure and union members (6.5.2.3).

6.3.2.2 void

The (nonexistent) value of a void expression (an expression that has type void) shall not be used in any
way, and implicit or explicit conversions (except to void) shall not be applied to such an expression.
If an expression of any other type is evaluated as a void expression, its value or designator is
discarded. (A void expression is evaluated for its side effects.)

6.3.2.3 Pointers

A pointer to void may be converted to or from a pointer to any object type. A pointer to any object
type may be converted to a pointer to void and back again; the result shall compare equal to the
original pointer.

For any qualifier q, a pointer to a non-q-qualified type may be converted to a pointer to the q-qualified
version of the type; the values stored in the original and converted pointers shall compare equal.

An integer constant expression with the value 0, or such an expression cast to type void *, is called
a null pointer constant.67) If a null pointer constant is converted to a pointer type, the resulting
pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.

Conversion of a null pointer to another pointer type yields a null pointer of that type. Any two null
pointers shall compare equal.

An integer may be converted to any pointer type. Except as previously specified, the result is imple-
mentation-defined, might not be correctly aligned, might not point to an entity of the referenced
type, and might be a trap representation.68)

Any pointer type may be converted to an integer type. Except as previously specified, the result
is implementation-defined. If the result cannot be represented in the integer type, the behavior is
undefined. The result need not be in the range of values of any integer type.

A pointer to an object type may be converted to a pointer to a different object type. If the resulting
pointer is not correctly aligned69) for the referenced type, the behavior is undefined. Otherwise,
when converted back again, the result shall compare equal to the original pointer. When a pointer to
an object is converted to a pointer to a character type, the result points to the lowest addressed byte
of the object. Successive increments of the result, up to the size of the object, yield pointers to the
remaining bytes of the object.

A pointer to a function of one type may be converted to a pointer to a function of another type and
back again; the result shall compare equal to the original pointer. If a converted pointer is used to
call a function whose type is not compatible with the referenced type, the behavior is undefined.

Forward references: cast operators (6.5.4), equality operators (6.5.9), integer types capable of
holding object pointers (7.20.1.4), simple assignment (6.5.16.1).

6.4 Lexical elements

Syntax

token:

    keyword
    identifier
    constant
    string-literal
    punctuator
  

preprocessing-token:

     header-name
     identifier
     pp-number
     character-constant
     string-literal
     punctuator
    each non-white-space character that cannot be one of the above
  

Constraints

Each preprocessing token that is converted to a token shall have the lexical form of a keyword, an
identifier, a constant, a string literal, or a punctuator.

Semantics

A token is the minimal lexical element of the language in translation phases 7 and 8. The categories of
tokens are: keywords, identifiers, constants, string literals, and punctuators. A preprocessing token
is the minimal lexical element of the language in translation phases 3 through 6. The categories of
preprocessing tokens are: header names, identifiers, preprocessing numbers, character constants,
string literals, punctuators, and single non-white-space characters that do not lexically match the
other preprocessing token categories.70) If a ’ or a " character matches the last category, the behavior
is undefined. Preprocessing tokens can be separated by white space; this consists of comments
(described later), or white-space characters (space, horizontal tab, new-line, vertical tab, and form-
feed), or both. As described in 6.10, in certain circumstances during translation phase 4, white
space (or the absence thereof) serves as more than preprocessing token separation. White space
may appear within a preprocessing token only as part of a header name or between the quotation
characters in a character constant or string literal.

If the input stream has been parsed into preprocessing tokens up to a given character, the next
preprocessing token is the longest sequence of characters that could constitute a preprocessing
token. There is one exception to this rule: header name preprocessing tokens are recognized only
within #include preprocessing directives and in implementation-defined locations within #pragma
directives. In such contexts, a sequence of characters that could be either a header name or a string
literal is recognized as the former.

EXAMPLE 1 The program fragment 1Ex is parsed as a preprocessing number token (one that is not a valid floating or integer
constant token), even though a parse as the pair of preprocessing tokens 1 and Ex might produce a valid expression (for
example, if Ex were a macro defined as +1 ). Similarly, the program fragment 1E1 is parsed as a preprocessing number (one
that is a valid floating constant token), whether or not E is a macro name.

EXAMPLE 2 The program fragment x+++++y is parsed as x ++ ++ + y, which violates a constraint on increment operators,
even though the parse x ++ + ++ y might yield a correct expression.

Forward references: character constants (6.4.4.4), comments (6.4.9), expressions (6.5), floating
constants (6.4.4.2), header names (6.4.7), macro replacement (6.10.3), postfix increment and decrement
operators (6.5.2.4), prefix increment and decrement operators (6.5.3.1), preprocessing directives (6.10),
preprocessing numbers (6.4.8), string literals (6.4.5).

6.4.1 Keywords

Syntax

keyword: one of

    auto                       extern                    short                      while
    break                      float                     signed                     _Alignas
    case                       for                       sizeof                     _Alignof
    char                       goto                      static                     _Atomic
    const                      if                        struct                     _Bool
    continue                   inline                    switch                     _Complex
    default                    int                       typedef                    _Generic
    do                         long                      union                      _Imaginary
    double                     register                  unsigned                   _Noreturn
    else                       restrict                  void                       _Static_assert
    enum                       return                    volatile                   _Thread_local
  

Semantics

The above tokens (case sensitive) are reserved (in translation phases 7 and 8) for use as keywords,
and shall not be used otherwise. The keyword _Imaginary is reserved for specifying imaginary

types.71)

6.4.2 Identifiers

6.4.2.1 General

Syntax

identifier:

    identifier-nondigit
    identifier identifier-nondigit
    identifier digit
  

identifier-nondigit:

    nondigit
    universal-character-name
    other implementation-defined characters
  

nondigit: one of

    _ a   b   c   d   e   f   g   h   i   j   k   l   m
      n   o   p   q   r   s   t   u   v   w   x   y   z
      A   B   C   D   E   F   G   H   I   J   K   L   M
      N   O   P   Q   R   S   T   U   V   W   X   Y   Z
  

digit: one of

    0 1 2 3 4 5 6 7 8 9
  

Semantics

An identifier is a sequence of nondigit characters (including the underscore _ , the lowercase and
uppercase Latin letters, and other characters) and digits, which designates one or more entities as
described in 6.2.1. Lowercase and uppercase letters are distinct. There is no specific limit on the
maximum length of an identifier.

Each universal character name in an identifier shall designate a character whose encoding in
ISO/IEC 10646 falls into one of the ranges specified in D.1.72) The initial character shall not be
a universal character name designating a character whose encoding falls into one of the ranges
specified in D.2. An implementation may allow multibyte characters that are not part of the basic
source character set to appear in identifiers; which characters and their correspondence to universal
character names is implementation-defined.

When preprocessing tokens are converted to tokens during translation phase 7, if a preprocessing
token could be converted to either a keyword or an identifier, it is converted to a keyword.

Implementation limits

As discussed in 5.2.4.1, an implementation may limit the number of significant initial characters
in an identifier; the limit for an external name (an identifier that has external linkage) may be more
restrictive than that for an internal name (a macro name or an identifier that does not have external
linkage). The number of significant characters in an identifier is implementation-defined.

Any identifiers that differ in a significant character are different identifiers. If two identifiers differ
only in nonsignificant characters, the behavior is undefined.

Forward references: universal character names (6.4.3), macro replacement (6.10.3).

6.4.2.2 Predefined identifiers

Semantics

The identifier __func__ shall be implicitly declared by the translator as if, immediately following
the opening brace of each function definition, the declaration

    static const char __func__[] = "function-name";
  

appeared, where function-name is the name of the lexically-enclosing function.73)

This name is encoded as if the implicit declaration had been written in the source character set and
then translated into the execution character set as indicated in translation phase 5.

EXAMPLE Consider the code fragment:

    #include <stdio.h>
    void myfunc(void)
    {
          printf("%s\n", __func__);
          /* ... */
    }
  

Each time the function is called, it will print to the standard output stream:

    myfunc
  

Forward references: function definitions (6.9.1).

6.4.3 Universal character names

Syntax

universal-character-name:

    \u hex-quad
    \U hex-quad hex-quad
  

hex-quad:

    hexadecimal-digit hexadecimal-digit hexadecimal-digit hexadecimal-digit
  

Constraints

A universal character name shall not specify a character whose short identifier is less than 00A0
other than 0024 ($), 0040 (@), or 0060 (‘), nor one in the range D800 through DFFF inclusive.74)

Description

Universal character names may be used in identifiers, character constants, and string literals to
designate characters that are not in the basic character set.

Semantics

The universal character name \U nnnnnnnn designates the character whose eight-digit short identifier
(as specified by ISO/IEC 10646) is nnnnnnnn.75) Similarly, the universal character name \u nnnn
designates the character whose four-digit short identifier is nnnn (and whose eight-digit short
identifier is 0000nnnn).

6.4.4 Constants

Syntax

constant:

    integer-constant
    floating-constant
    enumeration-constant
    character-constant
  

Constraints

Each constant shall have a type and the value of a constant shall be in the range of representable
values for its type.

Semantics

Each constant has a type, determined by its form and value, as detailed later.

6.4.4.1 Integer constants

Syntax

integer-constant:

    decimal-constant integer-suffixopt
    octal-constant integer-suffixopt
    hexadecimal-constant integer-suffixopt
  

decimal-constant:

    nonzero-digit
    decimal-constant digit
  

octal-constant:

    0
    octal-constant octal-digit
  

hexadecimal-constant:

    hexadecimal-prefix hexadecimal-digit
    hexadecimal-constant hexadecimal-digit
  

hexadecimal-prefix: one of

    0X 0X
  

nonzero-digit: one of

    1 2 3 4 5 6 7 8 9
  

octal-digit: one of

    0 1 2 3 4 5 6 7
  

hexadecimal-digit: one of

    0 1 2 3 4 5 6 7 8 9
    a b c d e f
    A B C D E F
  

integer-suffix:

    unsigned-suffix long-suffixopt
    unsigned-suffix long-long-suffix
    long-suffix unsigned-suffixopt
    long-long-suffix unsigned-suffixopt
  

unsigned-suffix: one of

    u U
  

long-suffix: one of

    l L
  

long-long-suffix: one of

    ll LL
  

Description

An integer constant begins with a digit, but has no period or exponent part. It may have a prefix
that specifies its base and a suffix that specifies its type.

A decimal constant begins with a nonzero digit and consists of a sequence of decimal digits. An
octal constant consists of the prefix 0 optionally followed by a sequence of the digits 0 through 7
only. A hexadecimal constant consists of the prefix 0x or 0X followed by a sequence of the decimal
digits and the letters a (or A) through f (or F) with values 10 through 15 respectively.

Semantics

The value of a decimal constant is computed base 10; that of an octal constant, base 8; that of a
hexadecimal constant, base 16. The lexically first digit is the most significant.

The type of an integer constant is the first of the corresponding list in which its value can be
represented.

                                                       Octal or Hexadecimal
    Suffix                 Decimal Constant            Constant
    none                   int                         int
                           long int                    unsigned int
                           long long int               long int
                                                       unsigned long      int
                                                       long long int
                                                       unsigned long      long int
     u or U                unsigned int                unsigned int
                           unsigned long int           unsigned long      int
                           unsigned long long int      unsigned long      long int
     l or L                long int                    long int
                           long long int               unsigned long      int
                                                       long long int
                                                       unsigned long      long int
    Both u or U            unsigned long int           unsigned long      int
    and l or L             unsigned long long int      unsigned long      long int
    ll or LL               long long int               long long int
                                                       unsigned long      long int
    Both u or U            unsigned long long int      unsigned long      long int
    and ll or LL
  

If an integer constant cannot be represented by any type in its list, it may have an extended integer
type, if the extended integer type can represent its value. If all of the types in the list for the constant
are signed, the extended integer type shall be signed. If all of the types in the list for the constant
are unsigned, the extended integer type shall be unsigned. If the list contains both signed and
unsigned types, the extended integer type may be signed or unsigned. If an integer constant cannot
be represented by any type in its list and has no extended integer type, then the integer constant has
no type.

6.4.4.2 Floating constants

Syntax

floating-constant:

    decimal-floating-constant
    hexadecimal-floating-constant
  

decimal-floating-constant:

    fractional-constant exponent-partopt floating-suffixopt
    digit-sequence exponent-part floating-suffixopt
  

hexadecimal-floating-constant:

    hexadecimal-prefix hexadecimal-fractional-constant
                       binary-exponent-part floating-suffixopt
    hexadecimal-prefix hexadecimal-digit-sequence
                       binary-exponent-part floating-suffixopt
  

fractional-constant:

    digit-sequenceopt . digit-sequence
    digit-sequence .
  

exponent-part:

    e signopt digit-sequence
    E signopt digit-sequence
  

sign: one of

    + -
  

digit-sequence:

    digit
    digit-sequence digit
  

hexadecimal-fractional-constant:

    hexadecimal-digit-sequenceopt .
                       hexadecimal-digit-sequence
    hexadecimal-digit-sequence .
  

binary-exponent-part:

    p signopt digit-sequence
    P signopt digit-sequence
  

hexadecimal-digit-sequence:

    hexadecimal-digit
    hexadecimal-digit-sequence hexadecimal-digit
  

floating-suffix: one of

    f l F L
  

Description

A floating constant has a significand part that may be followed by an exponent part and a suffix that
specifies its type. The components of the significand part may include a digit sequence representing
the whole-number part, followed by a period ( .), followed by a digit sequence representing the
fraction part. The components of the exponent part are an e, E, p, or P followed by an exponent
consisting of an optionally signed digit sequence. Either the whole-number part or the fraction part
has to be present; for decimal floating constants, either the period or the exponent part has to be
present.

Semantics

The significand part is interpreted as a (decimal or hexadecimal) rational number; the digit sequence
in the exponent part is interpreted as a decimal integer. For decimal floating constants, the exponent
indicates the power of 10 by which the significand part is to be scaled. For hexadecimal floating
constants, the exponent indicates the power of 2 by which the significand part is to be scaled. For
decimal floating constants, and also for hexadecimal floating constants when FLT_RADIX is not a
power of 2, the result is either the nearest representable value, or the larger or smaller representable
value immediately adjacent to the nearest representable value, chosen in an implementation-defined
manner. For hexadecimal floating constants when FLT_RADIX is a power of 2, the result is correctly
rounded.

An unsuffixed floating constant has type double. If suffixed by the letter f or F, it has type float.
If suffixed by the letter l or L, it has type long double.

Floating constants are converted to internal format as if at translation-time. The conversion of a
floating constant shall not raise an exceptional condition or a floating-point exception at execution
time. All floating constants of the same source form76) shall convert to the same internal format with
the same value.

Recommended practice

The implementation should produce a diagnostic message if a hexadecimal constant cannot be
represented exactly in its evaluation format; the implementation should then proceed with the
translation of the program.

The translation-time conversion of floating constants should match the execution-time conversion
of character strings by library functions, such as strtod, given matching inputs suitable for both
conversions, the same result format, and default execution-time rounding.77)

6.4.4.3 Enumeration constants

Syntax

enumeration-constant:

    identifier
  

Semantics

An identifier declared as an enumeration constant has type int.

Forward references: enumeration specifiers (6.7.2.2).

6.4.4.4 Character constants

Syntax

character-constant:

    ’ c-char-sequence ’
      L’ c-char-sequence ’
      u’ c-char-sequence ’
      U’ c-char-sequence ’
  

c-char-sequence:

    c-char
    c-char-sequence c-char
  

c-char:

    any member of the source character set except
                        the single-quote ’, backslash \ , or new-line character
      escape-sequence
  

escape-sequence:

    simple-escape-sequence
    octal-escape-sequence
    hexadecimal-escape-sequence
    universal-character-name
  

simple-escape-sequence: one of

    \’ \" \? \\
    \a \b \f \n \r \t \v
  

octal-escape-sequence:

    \ octal-digit
    \ octal-digit octal-digit
    \ octal-digit octal-digit octal-digit
  

hexadecimal-escape-sequence:

    \x hexadecimal-digit
     hexadecimal-escape-sequence hexadecimal-digit
  

Description

An integer character constant is a sequence of one or more multibyte characters enclosed in single-
quotes, as in ’x’ . A wide character constant is the same, except prefixed by the letter L, u, or U. With
a few exceptions detailed later, the elements of the sequence are any members of the source character
set; they are mapped in an implementation-defined manner to members of the execution character
set.

The single-quote ’, the double-quote ", the question-mark ?, the backslash \, and arbitrary integer
values are representable according to the following table of escape sequences:

    single quote ’                   \’
    double quote "                   \"
    question mark ?                  \?
    backslash \                      \\
    octal character                  \ octal digits
    hexadecimal character            \x hexadecimal digits
  

The double-quote " and question-mark ? are representable either by themselves or by the escape
sequences \" and \?, respectively, but the single-quote ’ and the backslash \ shall be represented,
respectively, by the escape sequences \’ and \\ .

The octal digits that follow the backslash in an octal escape sequence are taken to be part of the
construction of a single character for an integer character constant or of a single wide character for a
wide character constant. The numerical value of the octal integer so formed specifies the value of
the desired character or wide character.

The hexadecimal digits that follow the backslash and the letter x in a hexadecimal escape sequence
are taken to be part of the construction of a single character for an integer character constant or of a
single wide character for a wide character constant. The numerical value of the hexadecimal integer
so formed specifies the value of the desired character or wide character.

Each octal or hexadecimal escape sequence is the longest sequence of characters that can constitute
the escape sequence.

In addition, characters not in the basic character set are representable by universal character names
and certain nongraphic characters are representable by escape sequences consisting of the backslash \
followed by a lowercase letter: \a, \b, \f, \n, \r, \t, and \v.78)

Constraints

The value of an octal or hexadecimal escape sequence shall be in the range of representable values
for the corresponding type:

    Prefix     Corresponding Type
    none       unsigned char
    L          the unsigned type corresponding to wchar_t
    u          char16_t
    U          char32_t
  

Semantics

An integer character constant has type int. The value of an integer character constant containing
a single character that maps to a single-byte execution character is the numerical value of the
representation of the mapped character interpreted as an integer. The value of an integer character
constant containing more than one character (e.g., ’ab’ ), or containing a character or escape sequence
that does not map to a single-byte execution character, is implementation-defined. If an integer
character constant contains a single character or escape sequence, its value is the one that results
when an object with type char whose value is that of the single character or escape sequence is
converted to type int.

A wide character constant prefixed by the letter L has type wchar_t, an integer type defined in the
<stddef.h> header; a wide character constant prefixed by the letter u or U has type char16_t or
char32_t, respectively, unsigned integer types defined in the <uchar.h> header. The value of a
wide character constant containing a single multibyte character that maps to a single member of the
extended execution character set is the wide character corresponding to that multibyte character,
as defined by the mbtowc, mbrtoc16, or mbrtoc32 function as appropriate for its type, with an
implementation-defined current locale. The value of a wide character constant containing more
than one multibyte character or a single multibyte character that maps to multiple members of
the extended execution character set, or containing a multibyte character or escape sequence not
represented in the extended execution character set, is implementation-defined.

EXAMPLE 1 The construction ’\0’ is commonly used to represent the null character.

EXAMPLE 2 Consider implementations that use two’s complement representation for integers and eight bits for objects
that have type char. In an implementation in which type char has the same range of values as signed char, the integer
character constant ’\xFF’ has the value −1; if type char has the same range of values as unsigned char, the character
constant ’\xFF’ has the value +255.

EXAMPLE 3 Even if eight bits are used for objects that have type char, the construction ’\x123’ specifies an integer character
constant containing only one character, since a hexadecimal escape sequence is terminated only by a non-hexadecimal
character. To specify an integer character constant containing the two characters whose values are ’\x12’ and ’3’ , the
construction ’\0223’ may be used, since an octal escape sequence is terminated after three octal digits. (The value of this
two-character integer character constant is implementation-defined.)

EXAMPLE 4 Even if 12 or more bits are used for objects that have type wchar_t, the construction L’\1234’ specifies the
implementation-defined value that results from the combination of the values 0123 and ’4’ .

Forward references: common definitions <stddef.h> (7.19), the mbtowc function (7.22.7.2), Uni-
code utilities <uchar.h> (7.28).

6.4.5 String literals

Syntax

string-literal:

    encoding-prefixopt " s-char-sequenceopt "
  

encoding-prefix:

    u8
    u
    U
    L
  

s-char-sequence:

    s-char
    s-char-sequence s-char
  

s-char:

    any member of the source character set except
                     the double-quote ", backslash \, or new-line character
     escape-sequence
  

Constraints

A sequence of adjacent string literal tokens shall not include both a wide string literal and a UTF–8
string literal.

Description

A character string literal is a sequence of zero or more multibyte characters enclosed in double-quotes,
as in "xyz". A UTF–8 string literal is the same, except prefixed by u8. A wide string literal is the same,
except prefixed by the letter L, u, or U.

The same considerations apply to each element of the sequence in a string literal as if it were in an
integer character constant (for a character or UTF–8 string literal) or a wide character constant (for a
wide string literal), except that the single-quote ’ is representable either by itself or by the escape
sequence \’, but the double-quote " shall be represented by the escape sequence \".

Semantics

In translation phase 6, the multibyte character sequences specified by any sequence of adjacent
character and identically-prefixed string literal tokens are concatenated into a single multibyte
character sequence. If any of the tokens has an encoding prefix, the resulting multibyte character
sequence is treated as having the same prefix; otherwise, it is treated as a character string literal.
Whether differently-prefixed wide string literal tokens can be concatenated and, if so, the treatment
of the resulting multibyte character sequence are implementation-defined.

In translation phase 7, a byte or code of value zero is appended to each multibyte character sequence
that results from a string literal or literals.79) The multibyte character sequence is then used to
initialize an array of static storage duration and length just sufficient to contain the sequence. For
character string literals, the array elements have type char, and are initialized with the individual
bytes of the multibyte character sequence. For UTF–8 string literals, the array elements have type
char, and are initialized with the characters of the multibyte character sequence, as encoded in
UTF–8. For wide string literals prefixed by the letter L, the array elements have type wchar_t
and are initialized with the sequence of wide characters corresponding to the multibyte character
sequence, as defined by the mbstowcs function with an implementation-defined current locale.
For wide string literals prefixed by the letter u or U, the array elements have type char16_t or
char32_t, respectively, and are initialized with the sequence of wide characters corresponding
to the multibyte character sequence, as defined by successive calls to the mbrtoc16, or mbrtoc32
function as appropriate for its type, with an implementation-defined current locale. The value of a
string literal containing a multibyte character or escape sequence not represented in the execution
character set is implementation-defined.

It is unspecified whether these arrays are distinct provided their elements have the appropriate
values. If the program attempts to modify such an array, the behavior is undefined.

EXAMPLE 1 This pair of adjacent character string literals

    "\x12" "3"
  

produces a single character string literal containing the two characters whose values are ’\x12’ and ’3’ , because escape
sequences are converted into single members of the execution character set just prior to adjacent string literal concatenation.

EXAMPLE 2 Each of the sequences of adjacent string literal tokens

    "a" "b" L"c"
    "a" L"b" "c"
    L"a" "b" L"c"
    L"a" L"b" L"c"
  

is equivalent to the string literal

    L"abc"
  

Likewise, each of the sequences

    "a" "b" u"c"
    "a" u"b" "c"
    u"a" "b" u"c"
    u"a" u"b" u"c"
  

is equivalent to

    u"abc"
  

Forward references: common definitions <stddef.h> (7.19), the mbstowcs function (7.22.8.1),
Unicode utilities <uchar.h> (7.28).

6.4.6 Punctuators

Syntax

punctuator: one of

    [    ] ( ) { } . ->
    ++    --  & * + - ~ !
    /    % << >> < > <= >=                    ==    !=     ^    |   &&    ||
    ?    : ; ...
    =    *= /= %= += -= <<=                   >>=     &=       ^=   |=
    ,    # ##
    <:     :> <% %> %: %:%:
  

Semantics

A punctuator is a symbol that has independent syntactic and semantic significance. Depending on
context, it may specify an operation to be performed (which in turn may yield a value or a function
designator, produce a side effect, or some combination thereof) in which case it is known as an
operator (other forms of operator also exist in some contexts). An operand is an entity on which an
operator acts.

In all aspects of the language, the six tokens80)

    <:    :>    <%     %>    %:    %:%:
  

behave, respectively, the same as the six tokens

    [     ]     {     }      #     ##
                       81)
  

except for their spelling.

Forward references: expressions (6.5), declarations (6.7), preprocessing directives (6.10), statements
(6.8).

6.4.7 Header names

Syntax

header-name:

    < h-char-sequence >
    " q-char-sequence "
  

h-char-sequence:

    h-char
    h-char-sequence h-char
  

h-char:

    any member of the source character set except
                    the new-line character and >
  

q-char-sequence:

    q-char
    q-char-sequence q-char
  

q-char:

    any member of the source character set except
                    the new-line character and "
  

Semantics

The sequences in both forms of header names are mapped in an implementation-defined manner to
headers or external source file names as specified in 6.10.2.

If the characters ’ , \ , ", // , or /* occur in the sequence between the < and > delimiters, the behavior
is undefined. Similarly, if the characters ’ , \ , // , or /* occur in the sequence between the "
delimiters, the behavior is undefined.82) Header name preprocessing tokens are recognized only
within #include preprocessing directives and in implementation-defined locations within #pragma
directives.83)

EXAMPLE The following sequence of characters:

    0x3<1/a.h>1e2
    #include <1/a.h>
    #define const.member@$
  

forms the following sequence of preprocessing tokens (with each individual preprocessing token delimited by a { on the left
and a } on the right).

    {0x3}{<}{1}{/}{a}{.}{h}{>}{1e2}
    {#}{include} {<1/a.h>}
    {#}{define} {const}{.}{member}{@}{$}
  

Forward references: source file inclusion (6.10.2).

6.4.8 Preprocessing numbers

Syntax

pp-number:

    digit
  
    . digit
    pp-number      digit
    pp-number      identifier-nondigit
    pp-number      e sign
    pp-number      E sign
    pp-number      p sign
    pp-number      P sign
    pp-number      .
  

Description

A preprocessing number begins with a digit optionally preceded by a period (.) and may be followed
by valid identifier characters and the character sequences e+, e-, E+, E-, p+, p-, P+, or P-.

Preprocessing number tokens lexically include all floating and integer constant tokens.

Semantics

A preprocessing number does not have type or a value; it acquires both after a successful conversion
(as part of translation phase 7) to a floating constant token or an integer constant token.

6.4.9 Comments

Except within a character constant, a string literal, or a comment, the characters /* introduce a
comment. The contents of such a comment are examined only to identify multibyte characters and
to find the characters */ that terminate it.84)

Except within a character constant, a string literal, or a comment, the characters // introduce a
comment that includes all multibyte characters up to, but not including, the next new-line character.
The contents of such a comment are examined only to identify multibyte characters and to find the
terminating new-line character.

EXAMPLE

    "a//b"                                 //   four-character string literal
    #include "//e"                         //   undefined behavior
    // */                                  //   comment, not syntax error
    f = g/**//h;                           //   equivalent to f = g / h;
    //\
    i();                                   // part of a two-line comment
    /\
    / j();                                 // part of a two-line comment
    #define glue(x,y) x##y
    glue(/,/) k();                         // syntax error, not comment
    /*//*/ l();                            // equivalent to l();
    m = n//**/o
      + p;                                 // equivalent to m = n + p;
  

6.5 Expressions

An expression is a sequence of operators and operands that specifies computation of a value, or that
designates an object or a function, or that generates side effects, or that performs a combination
thereof. The value computations of the operands of an operator are sequenced before the value
computation of the result of the operator.

If a side effect on a scalar object is unsequenced relative to either a different side effect on the
same scalar object or a value computation using the value of the same scalar object, the behavior
is undefined. If there are multiple allowable orderings of the subexpressions of an expression, the
behavior is undefined if such an unsequenced side effect occurs in any of the orderings.85)

The grouping of operators and operands is indicated by the syntax.86) Except as specified later, side
effects and value computations of subexpressions are unsequenced.87)

Some operators (the unary operator ~ , and the binary operators << , >> , &, ^, and |, collectively
described as bitwise operators) are required to have operands that have integer type. These operators
yield values that depend on the internal representations of integers, and have implementation-
defined and undefined aspects for signed types.

If an exceptional condition occurs during the evaluation of an expression (that is, if the result is not
mathematically defined or not in the range of representable values for its type), the behavior is
undefined.

The effective type of an object for an access to its stored value is the declared type of the object, if
any.88) If a value is stored into an object having no declared type through an lvalue having a type
that is not a character type, then the type of the lvalue becomes the effective type of the object for
that access and for subsequent accesses that do not modify the stored value. If a value is copied into
an object having no declared type using memcpy or memmove, or is copied as an array of character
type, then the effective type of the modified object for that access and for subsequent accesses that
do not modify the value is the effective type of the object from which the value is copied, if it has
one. For all other accesses to an object having no declared type, the effective type of the object is
simply the type of the lvalue used for the access.

An object shall have its stored value accessed only by an lvalue expression that has one of the
following types:89)

A floating expression may be contracted, that is, evaluated as though it were a single opera-
tion, thereby omitting rounding errors implied by the source code and the expression evalua-
tion method.90) The FP_CONTRACT pragma in <math.h> provides a way to disallow contracted
expressions. Otherwise, whether and how expressions are contracted is implementation-defined.91)

Forward references: the FP_CONTRACT pragma (7.12.2), copying functions (7.24.2).

6.5.1 Primary expressions

Syntax

primary-expression:

    identifier
    constant
    string-literal
    ( expression )
    generic-selection
  

Semantics

An identifier is a primary expression, provided it has been declared as designating an object (in
which case it is an lvalue) or a function (in which case it is a function designator).92)

A constant is a primary expression. Its type depends on its form and value, as detailed in 6.4.4.

A string literal is a primary expression. It is an lvalue with type as detailed in 6.4.5.

A parenthesized expression is a primary expression. Its type and value are identical to those of
the unparenthesized expression. It is an lvalue, a function designator, or a void expression if the
unparenthesized expression is, respectively, an lvalue, a function designator, or a void expression.

A generic selection is a primary expression. Its type and value depend on the selected generic
association, as detailed in the following subclause.

Forward references: declarations (6.7).

6.5.1.1 Generic selection

Syntax

generic-selection:

    _Generic ( assignment-expression , generic-assoc-list )
  

generic-assoc-list:

    generic-association
    generic-assoc-list , generic-association
  

generic-association:

    type-name : assignment-expression
    default : assignment-expression
  

Constraints

A generic selection shall have no more than one default generic association. The type name in a
generic association shall specify a complete object type other than a variably modified type. No two
generic associations in the same generic selection shall specify compatible types. The type of the
controlling expression is the type of the expression as if it had undergone an lvalue conversion,93)
array to pointer conversion, or function to pointer conversion. That type shall be compatible with at
most one of the types named in the generic association list. If a generic selection has no default
generic association, its controlling expression shall have type compatible with exactly one of the
types named in its generic association list.

Semantics

The controlling expression of a generic selection is not evaluated. If a generic selection has a generic
association with a type name that is compatible with the type of the controlling expression, then the
result expression of the generic selection is the expression in that generic association. Otherwise, the
result expression of the generic selection is the expression in the default generic association. None
of the expressions from any other generic association of the generic selection is evaluated.

The type and value of a generic selection are identical to those of its result expression. It is an
lvalue, a function designator, or a void expression if its result expression is, respectively, an lvalue, a
function designator, or a void expression.

EXAMPLE The cbrt type-generic macro could be implemented as follows:

    #define cbrt(X) _Generic((X),                              \
                            long double: cbrtl,                \
                            default: cbrt,                     \
                            float: cbrtf                       \
                            )(X)
  

6.5.2 Postfix operators

Syntax

postfix-expression:

    primary-expression
    postfix-expression [ expression ]
    postfix-expression ( argument-expression-listopt )
    postfix-expression . identifier
    postfix-expression -> identifier
    postfix-expression ++
    postfix-expression -
    ( type-name ) { initializer-list }
    ( type-name ) { initializer-list , }
  

argument-expression-list:

    assignment-expression
    argument-expression-list , assignment-expression
  

6.5.2.1 Array subscripting

Constraints

One of the expressions shall have type “pointer to complete object type”, the other expression shall
have integer type, and the result has type “type”.

Semantics

A postfix expression followed by an expression in square brackets [] is a subscripted designation of
an element of an array object. The definition of the subscript operator [] is that E1[E2] is identical

to (*((E1)+(E2))) . Because of the conversion rules that apply to the binary + operator, if E1 is an
array object (equivalently, a pointer to the initial element of an array object) and E2 is an integer,
E1[E2] designates the E2 -th element of E1 (counting from zero).

Successive subscript operators designate an element of a multidimensional array object. If E is an
n-dimensional array (n ≥ 2) with dimensions i × j × · · · × k, then E (used as other than an lvalue) is
converted to a pointer to an (n − 1)-dimensional array with dimensions j × · · · × k. If the unary *
operator is applied to this pointer explicitly, or implicitly as a result of subscripting, the result is the
referenced (n − 1)-dimensional array, which itself is converted into a pointer if used as other than an
lvalue. It follows from this that arrays are stored in row-major order (last subscript varies fastest).

EXAMPLE Consider the array object defined by the declaration

    int x[3][5];
  

Here x
is a 3 × 5 array of
int s; more precisely, x is an array of three element objects, each of which is an array of five int s. In the expression x[i],
which is equivalent to (*((x)+(i))) , x is first converted to a pointer to the initial array of five int s. Then i is adjusted
according to the type of x, which conceptually entails multiplying i by the size of the object to which the pointer points,
namely an array of five int objects. The results are added and indirection is applied to yield an array of five int s. When
used in the expression x[i][j], that array is in turn converted to a pointer to the first of the int s, so x[i][j] yields an int.

Forward references: additive operators (6.5.6), address and indirection operators (6.5.3.2), array
declarators (6.7.6.2).

6.5.2.2 Function calls

Constraints

The expression that denotes the called function94) shall have type pointer to function returning void
or returning a complete object type other than an array type.

If the expression that denotes the called function has a type that includes a prototype, the number of
arguments shall agree with the number of parameters. Each argument shall have a type such that its
value may be assigned to an object with the unqualified version of the type of its corresponding
parameter.

Semantics

A postfix expression followed by parentheses () containing a possibly empty, comma-separated
list of expressions is a function call. The postfix expression denotes the called function. The list of
expressions specifies the arguments to the function.

An argument may be an expression of any complete object type. In preparing for the call to a
function, the arguments are evaluated, and each parameter is assigned the value of the corresponding
argument.95)

If the expression that denotes the called function has type pointer to function returning an object
type, the function call expression has the same type as that object type, and has the value determined
as specified in 6.8.6.4. Otherwise, the function call has type void.

If the expression that denotes the called function has a type that does not include a prototype, the
integer promotions are performed on each argument, and arguments that have type float are
promoted to double. These are called the default argument promotions. If the number of arguments
does not equal the number of parameters, the behavior is undefined. If the function is defined with
a type that includes a prototype, and either the prototype ends with an ellipsis (, ...) or the types
of the arguments after promotion are not compatible with the types of the parameters, the behavior
is undefined. If the function is defined with a type that does not include a prototype, and the types
of the arguments after promotion are not compatible with those of the parameters after promotion,
the behavior is undefined, except for the following cases:

If the expression that denotes the called function has a type that does include a prototype, the
arguments are implicitly converted, as if by assignment, to the types of the corresponding parameters,
taking the type of each parameter to be the unqualified version of its declared type. The ellipsis
notation in a function prototype declarator causes argument type conversion to stop after the last
declared parameter. The default argument promotions are performed on trailing arguments.

No other conversions are performed implicitly; in particular, the number and types of arguments are
not compared with those of the parameters in a function definition that does not include a function
prototype declarator.

If the function is defined with a type that is not compatible with the type (of the expression) pointed
to by the expression that denotes the called function, the behavior is undefined.

There is a sequence point after the evaluations of the function designator and the actual arguments
but before the actual call. Every evaluation in the calling function (including other function calls)
that is not otherwise specifically sequenced before or after the execution of the body of the called
function is indeterminately sequenced with respect to the execution of the called function.96)

Recursive function calls shall be permitted, both directly and indirectly through any chain of other
functions.

EXAMPLE In the function call

    (*pf[f1()]) (f2(), f3() + f4())
  

the functions f1, f2, f3, and f4 may be called in any order. All side effects have to be completed before the function pointed
to by pf[f1()] is called.

Forward references: function declarators (including prototypes) (6.7.6.3), function definitions
(6.9.1), the return statement (6.8.6.4), simple assignment (6.5.16.1).

6.5.2.3 Structure and union members

Constraints

The first operand of the . operator shall have an atomic, qualified, or unqualified structure or union
type, and the second operand shall name a member of that type.

The first operand of the-> operator shall have type “pointer to atomic, qualified, or unqualified
structure” or “pointer to atomic, qualified, or unqualified union”, and the second operand shall
name a member of the type pointed to.

Semantics

A postfix expression followed by the . operator and an identifier designates a member of a structure
or union object. The value is that of the named member,97) and is an lvalue if the first expression is
an lvalue. If the first expression has qualified type, the result has the so-qualified version of the type
of the designated member.

A postfix expression followed by the-> operator and an identifier designates a member of a structure
or union object. The value is that of the named member of the object to which the first expression
points, and is an lvalue.98) If the first expression is a pointer to a qualified type, the result has the
so-qualified version of the type of the designated member.

Accessing a member of an atomic structure or union object results in undefined behavior.99)

One special guarantee is made in order to simplify the use of unions: if a union contains several
structures that share a common initial sequence (see below), and if the union object currently contains
one of these structures, it is permitted to inspect the common initial part of any of them anywhere
that a declaration of the completed type of the union is visible. Two structures share a common initial
sequence if corresponding members have compatible types (and, for bit-fields, the same widths) for a
sequence of one or more initial members.

EXAMPLE 1 If f is a function returning a structure or union, and x is a member of that structure or union, f().x is a valid
postfix expression but is not an lvalue.

EXAMPLE 2 In:

    struct s { int i; const int ci; };
    struct s s;
    const struct s cs;
    volatile struct s vs;
  

the various members have the types:
s.i int
s.ci const int
cs.i const int
cs.ci const int
vs.i volatile int
vs.ci volatile const int

EXAMPLE 3 The following is a valid fragment:

    union {
          struct {
                int    alltypes;
          } n;
          struct {
                int    type;
                int    intnode;
          } ni;
          struct {
                int    type;
                double doublenode;
          } nf;
    } u;
    u.nf.type = 1;
    u.nf.doublenode = 3.14;
    /* ... */
    if (u.n.alltypes == 1)
          if (sin(u.nf.doublenode) == 0.0)
                /* ... */
  

The following is not a valid fragment (because the union type is not visible within function f):

    struct t1 { int m; };
    struct t2 { int m; };
    int f(struct t1 *p1, struct t2 *p2)
    {
          if (p1->m < 0)
                p2->m = -p2->m;
          return p1->m;
    }
    int g()
    {
          union {
                struct t1 s1;
                struct t2 s2;
  

member from another thread, where at least one access is a modification. Members can be safely accessed using a non-atomic
object which is assigned to or from the atomic object.

           } u;
           /* ... */
           return f(&u.s1, &u.s2);
    }
  

Forward references: address and indirection operators (6.5.3.2), structure and union specifiers
(6.7.2.1).

6.5.2.4 Postfix increment and decrement operators

Constraints

The operand of the postfix increment or decrement operator shall have atomic, qualified, or unquali-
fied real or pointer type, and shall be a modifiable lvalue.

Semantics

The result of the postfix ++ operator is the value of the operand. As a side effect, the value of the
operand object is incremented (that is, the value 1 of the appropriate type is added to it). See the
discussions of additive operators and compound assignment for information on constraints, types,
and conversions and the effects of operations on pointers. The value computation of the result is
sequenced before the side effect of updating the stored value of the operand. With respect to an
indeterminately-sequenced function call, the operation of postfix ++ is a single evaluation. Postfix
++ on an object with atomic type is a read-modify-write operation with memory_order_seq_cst
memory order semantics.100)

The postfix-- operator is analogous to the postfix ++ operator, except that the value of the operand
is decremented (that is, the value 1 of the appropriate type is subtracted from it).

Forward references: additive operators (6.5.6), compound assignment (6.5.16.2).

6.5.2.5 Compound literals

Constraints

The type name shall specify a complete object type or an array of unknown size, but not a variable
length array type.

All the constraints for initializer lists in 6.7.9 also apply to compound literals.

Semantics

A postfix expression that consists of a parenthesized type name followed by a brace-enclosed list of
initializers is a compound literal. It provides an unnamed object whose value is given by the initializer
list.101)

If the type name specifies an array of unknown size, the size is determined by the initializer list as
specified in 6.7.9, and the type of the compound literal is that of the completed array type. Otherwise
(when the type name specifies an object type), the type of the compound literal is that specified by
the type name. In either case, the result is an lvalue.

The value of the compound literal is that of an unnamed object initialized by the initializer list. If
the compound literal occurs outside the body of a function, the object has static storage duration;

otherwise, it has automatic storage duration associated with the enclosing block.

All the semantic rules for initializer lists in 6.7.9 also apply to compound literals.102)

String literals, and compound literals with const-qualified types, need not designate distinct ob-
jects.103)

EXAMPLE 1 The file scope definition

    int *p = (int []){2, 4};
  

initializes p to point to the first element of an array of two ints, the first having the value two and the second, four. The
expressions in this compound literal are required to be constant. The unnamed object has static storage duration.

EXAMPLE 2 In contrast, in

    void f(void)
    {
          int *p;
          /*...*/
          p = (int [2]){*p};
          /*...*/
    }
  

p is assigned the address of the first element of an array of two ints, the first having the value previously pointed to by p and
the second, zero. The expressions in this compound literal need not be constant. The unnamed object has automatic storage
duration.

EXAMPLE 3 Initializers with designations can be combined with compound literals. Structure objects created using
compound literals can be passed to functions without depending on member order:

    drawline((struct point){.x=1, .y=1},
          (struct point){.x=3, .y=4});
  

Or, if drawline instead expected pointers to struct point:

    drawline(&(struct point){.x=1, .y=1},
          &(struct point){.x=3, .y=4});
  

EXAMPLE 4 A read-only compound literal can be specified through constructions like:

    (const float []){1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6}
  

EXAMPLE 5 The following three expressions have different meanings:

    "/tmp/fileXXXXXX"
    (char []){"/tmp/fileXXXXXX"}
    (const char []){"/tmp/fileXXXXXX"}
  

The first always has static storage duration and has type array of char, but need not be modifiable; the last two have
automatic storage duration when they occur within the body of a function, and the first of these two is modifiable.

EXAMPLE 6 Like string literals, const-qualified compound literals can be placed into read-only memory and can even be
shared. For example,

    (const char []){"abc"} == "abc"
  

might yield 1 if the literals’ storage is shared.

EXAMPLE 7 Since compound literals are unnamed, a single compound literal cannot specify a circularly linked object. For
example, there is no way to write a self-referential compound literal that could be used as the function argument in place of
the named object endless_zeros below:

    struct int_list { int car; struct int_list *cdr; };
  
    struct int_list endless_zeros = {0, &endless_zeros};
    eval(endless_zeros);
  

EXAMPLE 8 Each compound literal creates only a single object in a given scope:

    struct s { int i; };
  
    int f (void)
    {
          struct s *p = 0, *q;
          int j = 0;
  
    again:
          q = p, p = &((struct s){ j++ });
          if (j < 2) goto again;
  
           return p == q && q->i == 1;
    }
  

The function f() always returns the value 1.

Note that if an iteration statement were used instead of an explicit goto and a labeled statement, the lifetime of the unnamed
object would be the body of the loop only, and on entry next time around p would have an indeterminate value, which would
result in undefined behavior.

Forward references: type names (6.7.7), initialization (6.7.9).

6.5.3 Unary operators

Syntax

unary-expression:

    postfix-expression
    ++ unary-expression
    - unary-expression
    unary-operator cast-expression
    sizeof unary-expression
    sizeof ( type-name )
    _Alignof ( type-name )
  

unary-operator: one of

    &    *   +    - ˜ !
  

6.5.3.1 Prefix increment and decrement operators

Constraints

The operand of the prefix increment or decrement operator shall have atomic, qualified, or unquali-
fied real or pointer type, and shall be a modifiable lvalue.

Semantics

The value of the operand of the prefix ++ operator is incremented. The result is the new value of the
operand after incrementation. The expression ++E is equivalent to (E+=1) . See the discussions of
additive operators and compound assignment for information on constraints, types, side effects,
and conversions and the effects of operations on pointers.

The prefix-- operator is analogous to the prefix ++ operator, except that the value of the operand is
decremented.

Forward references: additive operators (6.5.6), compound assignment (6.5.16.2).

6.5.3.2 Address and indirection operators

Constraints

The operand of the unary & operator shall be either a function designator, the result of a [] or unary
* operator, or an lvalue that designates an object that is not a bit-field and is not declared with the
register storage-class specifier.

The operand of the unary * operator shall have pointer type.

Semantics

The unary & operator yields the address of its operand. If the operand has type “type”, the result has
type “pointer to type”. If the operand is the result of a unary * operator, neither that operator nor
the & operator is evaluated and the result is as if both were omitted, except that the constraints on
the operators still apply and the result is not an lvalue. Similarly, if the operand is the result of a []
operator, neither the & operator nor the unary * that is implied by the [] is evaluated and the result
is as if the & operator were removed and the [] operator were changed to a + operator. Otherwise,
the result is a pointer to the object or function designated by its operand.

The unary * operator denotes indirection. If the operand points to a function, the result is a function
designator; if it points to an object, the result is an lvalue designating the object. If the operand has
type “pointer to type”, the result has type “type”. If an invalid value has been assigned to the pointer,
the behavior of the unary * operator is undefined.104)

Forward references: storage-class specifiers (6.7.1), structure and union specifiers (6.7.2.1).

6.5.3.3 Unary arithmetic operators

Constraints

The operand of the unary + or- operator shall have arithmetic type; of the ~ operator, integer type;
of the ! operator, scalar type.

Semantics

The result of the unary + operator is the value of its (promoted) operand. The integer promotions
are performed on the operand, and the result has the promoted type.

The result of the unary- operator is the negative of its (promoted) operand. The integer promotions
are performed on the operand, and the result has the promoted type.

The result of the ~ operator is the bitwise complement of its (promoted) operand (that is, each bit in
the result is set if and only if the corresponding bit in the converted operand is not set). The integer
promotions are performed on the operand, and the result has the promoted type. If the promoted
type is an unsigned type, the expression ~E is equivalent to the maximum value representable in
that type minus E.

The result of the logical negation operator ! is 0 if the value of its operand compares unequal to
0, 1 if the value of its operand compares equal to 0. The result has type int. The expression !E is
equivalent to (0==E) .

6.5.3.4 The sizeof and _Alignof operators

Constraints

The sizeof operator shall not be applied to an expression that has function type or an incomplete
type, to the parenthesized name of such a type, or to an expression that designates a bit-field member.
The _Alignof operator shall not be applied to a function type or an incomplete type.

Semantics

The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the
parenthesized name of a type. The size is determined from the type of the operand. The result
is an integer. If the type of the operand is a variable length array type, the operand is evaluated;
otherwise, the operand is not evaluated and the result is an integer constant.

The _Alignof operator yields the alignment requirement of its operand type. The operand is not
evaluated and the result is an integer constant. When applied to an array type, the result is the
alignment requirement of the element type.

When sizeof is applied to an operand that has type char, unsigned char, or signed char, (or
a qualified version thereof) the result is 1. When applied to an operand that has array type, the
result is the total number of bytes in the array.105) When applied to an operand that has structure or
union type, the result is the total number of bytes in such an object, including internal and trailing
padding.

The value of the result of both operators is implementation-defined, and its type (an unsigned
integer type) is size_t, defined in <stddef.h> (and other headers).

EXAMPLE 1 A principal use of the sizeof operator is in communication with routines such as storage allocators and I/O
systems. A storage-allocation function might accept a size (in bytes) of an object to allocate and return a pointer to void. For
example:

    extern void *alloc(size_t);
    double *dp = alloc(sizeof *dp);
  

The implementation of the alloc function should ensure that its return value is aligned suitably for conversion to a pointer
to double.

EXAMPLE 2 Another use of the sizeof operator is to compute the number of elements in an array:

    sizeof array / sizeof array[0]
  

EXAMPLE 3 In this example, the size of a variable length array is computed and returned from a function:

    #include <stddef.h>
  
    size_t fsize3(int n)
    {
          char b[n+3];                  // variable length array
          return sizeof b;              // execution time sizeof
    }
  
    int main()
    {
          size_t size;
          size = fsize3(10); // fsize3 returns 13
          return 0;
    }
  

Forward references: common definitions <stddef.h> (7.19), declarations (6.7), structure and union
specifiers (6.7.2.1), type names (6.7.7), array declarators (6.7.6.2).

6.5.4 Cast operators

Syntax

cast-expression:

    unary-expression
    ( type-name ) cast-expression
  

Constraints

Unless the type name specifies a void type, the type name shall specify atomic, qualified, or
unqualified scalar type, and the operand shall have scalar type.

Conversions that involve pointers, other than where permitted by the constraints of 6.5.16.1, shall be
specified by means of an explicit cast.

A pointer type shall not be converted to any floating type. A floating type shall not be converted to
any pointer type.

Semantics

Preceding an expression by a parenthesized type name converts the value of the expression to the
unqualified version of the named type. This construction is called a cast.106) A cast that specifies no
conversion has no effect on the type or value of an expression.

If the value of the expression is represented with greater range or precision than required by the type
named by the cast (6.3.1.8), then the cast specifies a conversion even if the type of the expression is
the same as the named type and removes any extra range and precision.

Forward references: equality operators (6.5.9), function declarators (including prototypes) (6.7.6.3),
simple assignment (6.5.16.1), type names (6.7.7).

6.5.5 Multiplicative operators

Syntax

multiplicative-expression:

    cast-expression
    multiplicative-expression * cast-expression
    multiplicative-expression / cast-expression
    multiplicative-expression % cast-expression
  

Constraints

Each of the operands shall have arithmetic type. The operands of the % operator shall have integer
type.

Semantics

The usual arithmetic conversions are performed on the operands.

The result of the binary * operator is the product of the operands.

The result of the / operator is the quotient from the division of the first operand by the second; the
result of the % operator is the remainder. In both operations, if the value of the second operand is
zero, the behavior is undefined.

When integers are divided, the result of the / operator is the algebraic quotient with any fractional
part discarded.107) If the quotient a/b is representable, the expression (a/b)*b + a%b shall equal a;
otherwise, the behavior of both a/b and a%b is undefined.

6.5.6 Additive operators

Syntax

additive-expression:

    multiplicative-expression
    additive-expression + multiplicative-expression
    additive-expression - multiplicative-expression
  

Constraints

For addition, either both operands shall have arithmetic type, or one operand shall be a pointer to a
complete object type and the other shall have integer type. (Incrementing is equivalent to adding 1.)

For subtraction, one of the following shall hold:

(Decrementing is equivalent to subtracting 1.)

Semantics

If both operands have arithmetic type, the usual arithmetic conversions are performed on them.

The result of the binary + operator is the sum of the operands.

The result of the binary- operator is the difference resulting from the subtraction of the second
operand from the first.

For the purposes of these operators, a pointer to an object that is not an element of an array behaves
the same as a pointer to the first element of an array of length one with the type of the object as its
element type.

When an expression that has integer type is added to or subtracted from a pointer, the result has the
type of the pointer operand. If the pointer operand points to an element of an array object, and the
array is large enough, the result points to an element offset from the original element such that the
difference of the subscripts of the resulting and original array elements equals the integer expression.
In other words, if the expression P points to the i-th element of an array object, the expressions
(P)+N (equivalently, N+(P)) and (P)-N (where N has the value n) point to, respectively, the i + n-th
and i − n-th elements of the array object, provided they exist. Moreover, if the expression P points to
the last element of an array object, the expression (P)+1 points one past the last element of the array
object, and if the expression Q points one past the last element of an array object, the expression
(Q)-1 points to the last element of the array object. If both the pointer operand and the result point
to elements of the same array object, or one past the last element of the array object, the evaluation
shall not produce an overflow; otherwise, the behavior is undefined. If the result points one past
the last element of the array object, it shall not be used as the operand of a unary * operator that is
evaluated.

When two pointers are subtracted, both shall point to elements of the same array object, or one past
the last element of the array object; the result is the difference of the subscripts of the two array
elements. The size of the result is implementation-defined, and its type (a signed integer type) is
ptrdiff_t defined in the <stddef.h> header. If the result is not representable in an object of that
type, the behavior is undefined. In other words, if the expressions P and Q point to, respectively, the
i-th and j-th elements of an array object, the expression (P)-(Q) has the value i − j provided the
value fits in an object of type ptrdiff_t. Moreover, if the expression P points either to an element of
an array object or one past the last element of an array object, and the expression Q points to the last
element of the same array object, the expression ((Q)+1)-(P) has the same value as ((Q)-(P))+1
and as-((P)-((Q)+1)) , and has the value zero if the expression P points one past the last element
of the array object, even though the expression (Q)+1 does not point to an element of the array
object.108)

EXAMPLE Pointer arithmetic is well defined with pointers to variable length array types.

    {
            int n = 4, m = 3;
            int a[n][m];
            int (*p)[m] = a;        //   p == &a[0]
            p += 1;                 //   p == &a[1]
            (*p)[2] = 99;           //   a[1][2] == 99
            n = p - a;              //   n == 1
    }
  

If array a in the above example were declared to be an array of known constant size, and pointer p were declared to be a
pointer to an array of the same known constant size (pointing to a), the results would be the same.

Forward references: array declarators (6.7.6.2), common definitions <stddef.h> (7.19).

6.5.7 Bitwise shift operators

Syntax

shift-expression:

    additive-expression
    shift-expression « additive-expression
    shift-expression » additive-expression
  

Constraints

Each of the operands shall have integer type.

Semantics

The integer promotions are performed on each of the operands. The type of the result is that of the
promoted left operand. If the value of the right operand is negative or is greater than or equal to the
width of the promoted left operand, the behavior is undefined.

The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits are filled with zeros. If E1 has
an unsigned type, the value of the result is E1 × 2E2 , reduced modulo one more than the maximum
value representable in the result type. If E1 has a signed type and nonnegative value, and E1 × 2E2 is
representable in the result type, then that is the resulting value; otherwise, the behavior is undefined.

The result of E1 >> E2 is E1 right-shifted E2 bit positions. If E1 has an unsigned type or if E1 has a
signed type and a nonnegative value, the value of the result is the integral part of the quotient of
E1/2E2 . If E1 has a signed type and a negative value, the resulting value is implementation-defined.

6.5.8 Relational operators

Syntax

relational-expression:

    shift-expression
    relational-expression     < shift-expression
    relational-expression     > shift-expression
    relational-expression     <= shift-expression
    relational-expression     >= shift-expression
  

Constraints

One of the following shall hold:

Semantics

If both of the operands have arithmetic type, the usual arithmetic conversions are performed.

For the purposes of these operators, a pointer to an object that is not an element of an array behaves
the same as a pointer to the first element of an array of length one with the type of the object as its
element type.

When two pointers are compared, the result depends on the relative locations in the address space
of the objects pointed to. If two pointers to object types both point to the same object, or both point
one past the last element of the same array object, they compare equal. If the objects pointed to
are members of the same aggregate object, pointers to structure members declared later compare
greater than pointers to members declared earlier in the structure, and pointers to array elements
with larger subscript values compare greater than pointers to elements of the same array with lower
subscript values. All pointers to members of the same union object compare equal. If the expression
P points to an element of an array object and the expression Q points to the last element of the same
array object, the pointer expression Q+1 compares greater than P. In all other cases, the behavior is
undefined.

Each of the operators < (less than), > (greater than), <= (less than or equal to), and >= (greater than or
equal to) shall yield 1 if the specified relation is true and 0 if it is false.109) The result has type int.

6.5.9 Equality operators

Syntax

equality-expression:

    relational-expression
    equality-expression == relational-expression
    equality-expression != relational-expression
  

Constraints

One of the following shall hold:

Semantics

The == (equal to) and != (not equal to) operators are analogous to the relational operators except for
their lower precedence.110) Each of the operators yields 1 if the specified relation is true and 0 if it is
false. The result has type int. For any pair of operands, exactly one of the relations is true.

If both of the operands have arithmetic type, the usual arithmetic conversions are performed. Values
of complex types are equal if and only if both their real parts are equal and also their imaginary parts
are equal. Any two values of arithmetic types from different type domains are equal if and only
if the results of their conversions to the (complex) result type determined by the usual arithmetic
conversions are equal.

Otherwise, at least one operand is a pointer. If one operand is a pointer and the other is a null
pointer constant, the null pointer constant is converted to the type of the pointer. If one operand is a
pointer to an object type and the other is a pointer to a qualified or unqualified version of void, the
former is converted to the type of the latter.

Two pointers compare equal if and only if both are null pointers, both are pointers to the same object
(including a pointer to an object and a subobject at its beginning) or function, both are pointers to

one past the last element of the same array object, or one is a pointer to one past the end of one array
object and the other is a pointer to the start of a different array object that happens to immediately
follow the first array object in the address space.111)

For the purposes of these operators, a pointer to an object that is not an element of an array behaves
the same as a pointer to the first element of an array of length one with the type of the object as its
element type.

6.5.10 Bitwise AND operator

Syntax

AND-expression:

    equality-expression
    AND-expression & equality-expression
  

Constraints

Each of the operands shall have integer type.

Semantics

The usual arithmetic conversions are performed on the operands.

The result of the binary & operator is the bitwise AND of the operands (that is, each bit in the result
is set if and only if each of the corresponding bits in the converted operands is set).

6.5.11 Bitwise exclusive OR operator

Syntax

exclusive-OR-expression:

    AND-expression
    exclusive-OR-expression ^ AND-expression
  

Constraints

Each of the operands shall have integer type.

Semantics

The usual arithmetic conversions are performed on the operands.

The result of the ^ operator is the bitwise exclusive OR of the operands (that is, each bit in the result
is set if and only if exactly one of the corresponding bits in the converted operands is set).

6.5.12 Bitwise inclusive OR operator

Syntax

inclusive-OR-expression:

    exclusive-OR-expression
    inclusive-OR-expression | exclusive-OR-expression
  

Constraints

Each of the operands shall have integer type.

Semantics

The usual arithmetic conversions are performed on the operands.

The result of the | operator is the bitwise inclusive OR of the operands (that is, each bit in the result

is set if and only if at least one of the corresponding bits in the converted operands is set).

6.5.13 Logical AND operator

Syntax

logical-AND-expression:

    inclusive-OR-expression
    logical-AND-expression && inclusive-OR-expression
  

Constraints

Each of the operands shall have scalar type.

Semantics

The && operator shall yield 1 if both of its operands compare unequal to 0; otherwise, it yields 0. The
result has type int.

Unlike the bitwise binary & operator, the && operator guarantees left-to-right evaluation; if the
second operand is evaluated, there is a sequence point between the evaluations of the first and
second operands. If the first operand compares equal to 0, the second operand is not evaluated.

6.5.14 Logical OR operator

Syntax

logical-OR-expression:

    logical-AND-expression
    logical-OR-expression || logical-AND-expression
  

Constraints

Each of the operands shall have scalar type.

Semantics

The || operator shall yield 1 if either of its operands compare unequal to 0; otherwise, it yields 0.
The result has type int.

Unlike the bitwise | operator, the || operator guarantees left-to-right evaluation; if the second
operand is evaluated, there is a sequence point between the evaluations of the first and second
operands. If the first operand compares unequal to 0, the second operand is not evaluated.

6.5.15 Conditional operator

Syntax

conditional-expression:

    logical-OR-expression
    logical-OR-expression ? expression : conditional-expression
  

Constraints

The first operand shall have scalar type.

One of the following shall hold for the second and third operands:

Semantics

The first operand is evaluated; there is a sequence point between its evaluation and the evaluation
of the second or third operand (whichever is evaluated). The second operand is evaluated only if
the first compares unequal to 0; the third operand is evaluated only if the first compares equal to 0;
the result is the value of the second or third operand (whichever is evaluated), converted to the type
described below.112)

If both the second and third operands have arithmetic type, the result type that would be determined
by the usual arithmetic conversions, were they applied to those two operands, is the type of the
result. If both the operands have structure or union type, the result has that type. If both operands
have void type, the result has void type.

If both the second and third operands are pointers or one is a null pointer constant and the other
is a pointer, the result type is a pointer to a type qualified with all the type qualifiers of the types
referenced by both operands. Furthermore, if both operands are pointers to compatible types or to
differently qualified versions of compatible types, the result type is a pointer to an appropriately
qualified version of the composite type; if one operand is a null pointer constant, the result has the
type of the other operand; otherwise, one operand is a pointer to void or a qualified version of void,
in which case the result type is a pointer to an appropriately qualified version of void.

EXAMPLE The common type that results when the second and third operands are pointers is determined in two independent
stages. The appropriate qualifiers, for example, do not depend on whether the two pointers have compatible types.

Given the declarations

    const void *c_vp;
    void *vp;
    const int *c_ip;
    volatile int *v_ip;
    int *ip;
    const char *c_cp;
  

the third column in the following table is the common type that is the result of a conditional expression in which the first two
columns are the second and third operands (in either order):

    c_vp    c_ip    const void *
    v_ip    0       volatile int *
    c_ip    v_ip    const volatile int *
    vp      c_cp    const void *
    ip      c_ip    const int *
    vp      ip      void *
  

6.5.16 Assignment operators

Syntax

assignment-expression:

    conditional-expression
    unary-expression assignment-operator assignment-expression
  

assignment-operator: one of

    =   *=     /=    %=    +=    -=      <<=   >>=     &=     ^=    |=
  

Constraints

An assignment operator shall have a modifiable lvalue as its left operand.

Semantics

An assignment operator stores a value in the object designated by the left operand. An assignment
expression has the value of the left operand after the assignment,113) but is not an lvalue. The type of
an assignment expression is the type the left operand would have after lvalue conversion. The side
effect of updating the stored value of the left operand is sequenced after the value computations of
the left and right operands. The evaluations of the operands are unsequenced.

6.5.16.1 Simple assignment

Constraints

One of the following shall hold:114)

Semantics

In simple assignment (=), the value of the right operand is converted to the type of the assignment
expression and replaces the value stored in the object designated by the left operand.

If the value being stored in an object is read from another object that overlaps in any way the
storage of the first object, then the overlap shall be exact and the two objects shall have qualified or
unqualified versions of a compatible type; otherwise, the behavior is undefined.

EXAMPLE 1 In the program fragment

    int f(void);
    char c;
    /* ... */
    if ((c = f()) == -1)
          /* ... */
  

the int value returned by the function may be truncated when stored in the char, and then converted back to int width
prior to the comparison. In an implementation in which “plain” char has the same range of values as unsigned char (and
char is narrower than int), the result of the conversion cannot be negative, so the operands of the comparison can never
compare equal. Therefore, for full portability, the variable c should be declared as int.

EXAMPLE 2 In the fragment:

    char c;
    int i;
  
    long l;
  
    l = (c = i);
  

the value of i is converted to the type of the assignment expression c = i, that is, char type. The value of the expression
enclosed in parentheses is then converted to the type of the outer assignment expression, that is, long int type.

EXAMPLE 3 Consider the fragment:

    const char **cpp;
    char *p;
    const char c = ’A’;
  
    cpp = &p;              // constraint violation
    *cpp = &c;             // valid
    *p = 0;                // valid
  

The first assignment is unsafe because it would allow the following valid code to attempt to change the value of the const
object c.

6.5.16.2 Compound assignment

Constraints

For the operators += and-= only, either the left operand shall be an atomic, qualified, or unqualified
pointer to a complete object type, and the right shall have integer type; or the left operand shall have
atomic, qualified, or unqualified arithmetic type, and the right shall have arithmetic type.

For the other operators, the left operand shall have atomic, qualified, or unqualified arithmetic type,
and (considering the type the left operand would have after lvalue conversion) each operand shall
have arithmetic type consistent with those allowed by the corresponding binary operator.

Semantics

A compound assignment of the form E1 op= E2 is equivalent to the simple assignment expression
E1 = E1 op (E2) , except that the lvalue E1 is evaluated only once, and with respect to an inde-
terminately-sequenced function call, the operation of a compound assignment is a single evalu-
ation. If E1 has an atomic type, compound assignment is a read-modify-write operation with
memory_order_seq_cst memory order semantics.115)

6.5.17 Comma operator

Syntax

expression:

    assignment-expression
    expression , assignment-expression
  

Semantics

The left operand of a comma operator is evaluated as a void expression; there is a sequence point
between its evaluation and that of the right operand. Then the right operand is evaluated; the result
has its type and value.116)

EXAMPLE As indicated by the syntax, the comma operator (as described in this subclause) cannot appear in contexts where
a comma is used to separate items in a list (such as arguments to functions or lists of initializers). On the other hand, it can be
used within a parenthesized expression or within the second expression of a conditional operator in such contexts. In the
function call

    f(a, (t=3, t+2), c)
  

the function has three arguments, the second of which has the value 5.

Forward references: initialization (6.7.9).

    for (;;) {
          new = old op val;
          if (atomic_compare_exchange_strong(addr, &old, new))
                      break;
          feclearexcept(FE_ALL_EXCEPT);
    }
    feupdateenv(&fenv);
  

If FLT_EVAL_METHOD is not 0, then T2 must be a type with the range and precision to which E2 is evaluated in order to satisfy
the equivalence.

6.6 Constant expressions

Syntax

constant-expression:

    conditional-expression
  

Description

A constant expression can be evaluated during translation rather than runtime, and accordingly
may be used in any place that a constant may be.

Constraints

Constant expressions shall not contain assignment, increment, decrement, function-call, or comma
operators, except when they are contained within a subexpression that is not evaluated.117)

Each constant expression shall evaluate to a constant that is in the range of representable values for
its type.

Semantics

An expression that evaluates to a constant is required in several contexts. If a floating expression
is evaluated in the translation environment, the arithmetic range and precision shall be at least as
great as if the expression were being evaluated in the execution environment.118)

An integer constant expression119) shall have integer type and shall only have operands that are integer
constants, enumeration constants, character constants, sizeof expressions whose results are integer
constants, _Alignof expressions, and floating constants that are the immediate operands of casts.
Cast operators in an integer constant expression shall only convert arithmetic types to integer types,
except as part of an operand to the sizeof or _Alignof operator.

More latitude is permitted for constant expressions in initializers. Such a constant expression shall
be, or evaluate to, one of the following:

An arithmetic constant expression shall have arithmetic type and shall only have operands that are
integer constants, floating constants, enumeration constants, character constants, sizeof expressions
whose results are integer constants, and _Alignof expressions. Cast operators in an arithmetic
constant expression shall only convert arithmetic types to arithmetic types, except as part of an
operand to a sizeof or _Alignof operator.

An address constant is a null pointer, a pointer to an lvalue designating an object of static storage
duration, or a pointer to a function designator; it shall be created explicitly using the unary &
operator or an integer constant cast to pointer type, or implicitly by the use of an expression of array
or function type. The array-subscript [] and member-access . and-> operators, the address & and
indirection * unary operators, and pointer casts may be used in the creation of an address constant,
but the value of an object shall not be accessed by use of these operators.

An implementation may accept other forms of constant expressions.

The semantic rules for the evaluation of a constant expression are the same as for nonconstant
expressions.120)

Forward references: array declarators (6.7.6.2), initialization (6.7.9).

6.7 Declarations

Syntax

declaration:

    declaration-specifiers init-declarator-listopt ;
    static_assert-declaration
  

declaration-specifiers:

    storage-class-specifier declaration-specifiersopt
    type-specifier declaration-specifiersopt
    type-qualifier declaration-specifiersopt
    function-specifier declaration-specifiersopt
    alignment-specifier declaration-specifiersopt
  

init-declarator-list:

    init-declarator
    init-declarator-list , init-declarator
  

init-declarator:

    declarator
    declarator = initializer
  

Constraints

A declaration other than a static_assert declaration shall declare at least a declarator (other than
the parameters of a function or the members of a structure or union), a tag, or the members of an
enumeration.

If an identifier has no linkage, there shall be no more than one declaration of the identifier (in a
declarator or type specifier) with the same scope and in the same name space, except that:

All declarations in the same scope that refer to the same object or function shall specify compatible
types.

Semantics

A declaration specifies the interpretation and attributes of a set of identifiers. A definition of an
identifier is a declaration for that identifier that:

The declaration specifiers consist of a sequence of specifiers that indicate the linkage, storage
duration, and part of the type of the entities that the declarators denote. The init-declarator-list is a
comma-separated sequence of declarators, each of which may have additional type information, or
an initializer, or both. The declarators contain the identifiers (if any) being declared.

If an identifier for an object is declared with no linkage, the type for the object shall be complete
by the end of its declarator, or by the end of its init-declarator if it has an initializer; in the case of
function parameters (including in prototypes), it is the adjusted type (see 6.7.6.3) that is required to
be complete.

Forward references: declarators (6.7.6), enumeration specifiers (6.7.2.2), initialization (6.7.9), type
names (6.7.7), type qualifiers (6.7.3).

6.7.1 Storage-class specifiers

Syntax

storage-class-specifier:

    typedef
    extern
    static
    _Thread_local
    auto
    register
  

Constraints

At most, one storage-class specifier may be given in the declaration specifiers in a declaration, except
that _Thread_local may appear with static or extern.122)

In the declaration of an object with block scope, if the declaration specifiers include _Thread_local ,
they shall also include either static or extern. If _Thread_local appears in any declaration of an
object, it shall be present in every declaration of that object.

_Thread_local shall not appear in the declaration specifiers of a function declaration.

Semantics

The typedef specifier is called a “storage-class specifier” for syntactic convenience only; it is
discussed in 6.7.8. The meanings of the various linkages and storage durations were discussed in
6.2.2 and 6.2.4.

A declaration of an identifier for an object with storage-class specifier register suggests that
access to the object be as fast as possible. The extent to which such suggestions are effective is
implementation-defined.123)

The declaration of an identifier for a function that has block scope shall have no explicit storage-class
specifier other than extern.

If an aggregate or union object is declared with a storage-class specifier other than typedef, the
properties resulting from the storage-class specifier, except with respect to linkage, also apply to the
members of the object, and so on recursively for any aggregate or union member objects.

Forward references: type definitions (6.7.8).

6.7.2 Type specifiers

Syntax

type-specifier:

    void
    char
    short
    int
    long
    float
    double
    signed
    unsigned
    _Bool
    _Complex
    atomic-type-specifier
  
    struct-or-union-specifier
    enum-specifier
    typedef-name
  

Constraints

At least one type specifier shall be given in the declaration specifiers in each declaration, and in
the specifier-qualifier list in each struct declaration and type name. Each list of type specifiers shall
be one of the following multisets (delimited by commas, when there is more than one multiset per
item); the type specifiers may occur in any order, possibly intermixed with the other declaration
specifiers.

The type specifier _Complex shall not be used if the implementation does not support complex types
(see 6.10.8.3).

Semantics

Specifiers for structures, unions, enumerations, and atomic types are discussed in 6.7.2.1 through
6.7.2.4. Declarations of typedef names are discussed in 6.7.8. The characteristics of the other types
are discussed in 6.2.5.

Each of the comma-separated multisets designates the same type, except that for bit-fields, it is
implementation-defined whether the specifier int designates the same type as signed int or the
same type as unsigned int.

Forward references: atomic type specifiers (6.7.2.4), enumeration specifiers (6.7.2.2), structure and
union specifiers (6.7.2.1), tags (6.7.2.3), type definitions (6.7.8).

6.7.2.1 Structure and union specifiers

Syntax

struct-or-union-specifier:

    struct-or-union identifieropt { struct-declaration-list }
    struct-or-union identifier
  

struct-or-union:

    struct
    union
  

struct-declaration-list:

    struct-declaration
    struct-declaration-list struct-declaration
  

struct-declaration:

    specifier-qualifier-list struct-declarator-listopt ;
    static_assert-declaration
  

specifier-qualifier-list:

    type-specifier specifier-qualifier-listopt
    type-qualifier specifier-qualifier-listopt
    alignment-specifier specifier-qualifier-listopt
  

struct-declarator-list:

    struct-declarator
    struct-declarator-list , struct-declarator
  

struct-declarator:

    declarator
    declaratoropt : constant-expression
  

Constraints

A struct-declaration that does not declare an anonymous structure or anonymous union shall contain
a struct-declarator-list.

A structure or union shall not contain a member with incomplete or function type (hence, a structure
shall not contain an instance of itself, but may contain a pointer to an instance of itself), except that
the last member of a structure with more than one named member may have incomplete array type;
such a structure (and any union containing, possibly recursively, a member that is such a structure)
shall not be a member of a structure or an element of an array.

The expression that specifies the width of a bit-field shall be an integer constant expression with a
nonnegative value that does not exceed the width of an object of the type that would be specified
were the colon and expression omitted.124) If the value is zero, the declaration shall have no
declarator.

A bit-field shall have a type that is a qualified or unqualified version of _Bool , signed int,
unsigned int, or some other implementation-defined type. It is implementation-defined whether
atomic types are permitted.

Semantics

As discussed in 6.2.5, a structure is a type consisting of a sequence of members, whose storage is
allocated in an ordered sequence, and a union is a type consisting of a sequence of members whose
storage overlap.

Structure and union specifiers have the same form. The keywords struct and union indicate that
the type being specified is, respectively, a structure type or a union type.

The presence of a struct-declaration-list in a struct-or-union-specifier declares a new type, within
a translation unit. The struct-declaration-list is a sequence of declarations for the members of
the structure or union. If the struct-declaration-list does not contain any named members, either
directly or via an anonymous structure or anonymous union, the behavior is undefined. The type is

incomplete until immediately after the } that terminates the list, and complete thereafter.

A member of a structure or union may have any complete object type other than a variably modified
type.125) In addition, a member may be declared to consist of a specified number of bits (including
a sign bit, if any). Such a member is called a bit-field;126) its width is preceded by a colon.

A bit-field is interpreted as having a signed or unsigned integer type consisting of the specified
number of bits.127) If the value 0 or 1 is stored into a nonzero-width bit-field of type _Bool , the
value of the bit-field shall compare equal to the value stored; a _Bool bit-field has the semantics of a
_Bool .

An implementation may allocate any addressable storage unit large enough to hold a bit-field. If
enough space remains, a bit-field that immediately follows another bit-field in a structure shall be
packed into adjacent bits of the same unit. If insufficient space remains, whether a bit-field that
does not fit is put into the next unit or overlaps adjacent units is implementation-defined. The
order of allocation of bit-fields within a unit (high-order to low-order or low-order to high-order) is
implementation-defined. The alignment of the addressable storage unit is unspecified.

A bit-field declaration with no declarator, but only a colon and a width, indicates an unnamed
bit-field.128) As a special case, a bit-field structure member with a width of 0 indicates that no
further bit-field is to be packed into the unit in which the previous bit-field, if any, was placed.

An unnamed member whose type specifier is a structure specifier with no tag is called an anonymous
structure; an unnamed member whose type specifier is a union specifier with no tag is called an
anonymous union. The members of an anonymous structure or union are considered to be members
of the containing structure or union. This applies recursively if the containing structure or union is
also anonymous.

Each non-bit-field member of a structure or union object is aligned in an implementation-defined
manner appropriate to its type.

Within a structure object, the non-bit-field members and the units in which bit-fields reside have
addresses that increase in the order in which they are declared. A pointer to a structure object,
suitably converted, points to its initial member (or if that member is a bit-field, then to the unit in
which it resides), and vice versa. There may be unnamed padding within a structure object, but not
at its beginning.

The size of a union is sufficient to contain the largest of its members. The value of at most one of the
members can be stored in a union object at any time. A pointer to a union object, suitably converted,
points to each of its members (or if a member is a bit-field, then to the unit in which it resides), and
vice versa.

There may be unnamed padding at the end of a structure or union.

As a special case, the last element of a structure with more than one named member may have an
incomplete array type; this is called a flexible array member. In most situations, the flexible array
member is ignored. In particular, the size of the structure is as if the flexible array member were
omitted except that it may have more trailing padding than the omission would imply. However,
when a . (or-> ) operator has a left operand that is (a pointer to) a structure with a flexible array
member and the right operand names that member, it behaves as if that member were replaced with
the longest array (with the same element type) that would not make the structure larger than the
object being accessed; the offset of the array shall remain that of the flexible array member, even if
this would differ from that of the replacement array. If this array would have no elements, it behaves
as if it had one element but the behavior is undefined if any attempt is made to access that element
or to generate a pointer one past it.

EXAMPLE 1 The following illustrates anonymous structures and unions:

    struct v {
          union {     // anonymous union
                struct { int i, j; };   // anonymous structure
                struct { long k, l; } w;
          };
          int m;
    } v1;
  
    v1.i = 2;   // valid
    v1.k = 3;   // invalid:          inner structure is not anonymous
    v1.w.k = 5; // valid
  

EXAMPLE 2 After the declaration:

    struct s { int n; double d[]; };
  

the structure struct s has a flexible array member d. A typical way to use this is:

    int m = /* some value */;
    struct s *p = malloc(sizeof (struct s) + sizeof (double [m]));
  

and assuming that the call to malloc succeeds, the object pointed to by p behaves, for most purposes, as if p had been
declared as:

    struct { int n; double d[m]; } *p;
  

(there are circumstances in which this equivalence is broken; in particular, the offsets of member d might not be the same).

Following the above declaration:

    struct s t1 = { 0 };                    //   valid
    struct s t2 = { 1, { 4.2 }};            //   invalid
    t1.n = 4;                               //   valid
    t1.d[0] = 4.2;                          //   might be undefined behavior
  

The initialization of t2 is invalid (and violates a constraint) because struct s is treated as if it did not contain member d.
The assignment to t1.d[0] is probably undefined behavior, but it is possible that

    sizeof (struct s) >= offsetof(struct s, d) + sizeof (double)
  

in which case the assignment would be legitimate. Nevertheless, it cannot appear in strictly conforming code.

After the further declaration:

    struct ss { int n; };
  

the expressions:

    sizeof (struct s) >= sizeof (struct ss)
    sizeof (struct s) >= offsetof(struct s, d)
  

are always equal to 1.

If sizeof (double) is 8, then after the following code is executed:

    struct s *s1;
    struct s *s2;
    s1 = malloc(sizeof (struct s) + 64);
    s2 = malloc(sizeof (struct s) + 46);
  

and assuming that the calls to malloc succeed, the objects pointed to by s1 and s2 behave, for most purposes, as if the
identifiers had been declared as:

    struct { int n; double d[8]; } *s1;
    struct { int n; double d[5]; } *s2;
  

Following the further successful assignments:

    s1 = malloc(sizeof (struct s) + 10);
    s2 = malloc(sizeof (struct s) + 6);
  

they then behave as if the declarations were:

    struct { int n; double d[1]; } *s1, *s2;
  

and:

    double *dp;
    dp = &(s1->d[0]);        //   valid
    *dp = 42;                //   valid
    dp = &(s2->d[0]);        //   valid
    *dp = 42;                //   undefined behavior
  

The assignment:

    *s1 = *s2;
  

only copies the member n; if any of the array elements are within the first sizeof (struct s) bytes of the structure, they
might be copied or simply overwritten with indeterminate values.

EXAMPLE 3 Because members of anonymous structures and unions are considered to be members of the containing
structure or union, struct s in the following example has more than one named member and thus the use of a flexible array
member is valid:

    struct s {
          struct { int i; };
          int a[];
    };
  

Forward references: declarators (6.7.6), tags (6.7.2.3).

6.7.2.2 Enumeration specifiers

Syntax

enum-specifier:

    enum identifieropt { enumerator-list }
    enum identifieropt { enumerator-list , }
    enum identifier
  

enumerator-list:

    enumerator
    enumerator-list , enumerator
  

enumerator:

    enumeration-constant
    enumeration-constant = constant-expression
  

Constraints

The expression that defines the value of an enumeration constant shall be an integer constant
expression that has a value representable as an int.

Semantics

The identifiers in an enumerator list are declared as constants that have type int and may appear
wherever such are permitted.129) An enumerator with = defines its enumeration constant as the
value of the constant expression. If the first enumerator has no =, the value of its enumeration
constant is 0. Each subsequent enumerator with no = defines its enumeration constant as the value
of the constant expression obtained by adding 1 to the value of the previous enumeration constant.
(The use of enumerators with = may produce enumeration constants with values that duplicate
other values in the same enumeration.) The enumerators of an enumeration are also known as its
members.

Each enumerated type shall be compatible with char, a signed integer type, or an unsigned integer
type. The choice of type is implementation-defined,130) but shall be capable of representing the
values of all the members of the enumeration. The enumerated type is incomplete until immediately
after the } that terminates the list of enumerator declarations, and complete thereafter.

EXAMPLE The following fragment:

    enum hue { chartreuse, burgundy, claret=20, winedark };
    enum hue col, *cp;
    col = claret;
    cp = &col;
    if (*cp != burgundy)
          /* ... */
  

makes hue the tag of an enumeration, and then declares col as an object that has that type and cp as a pointer to an object
that has that type. The enumerated values are in the set {0, 1, 20, 21}.

Forward references: tags (6.7.2.3).

6.7.2.3 Tags

Constraints

A specific type shall have its content defined at most once.

Where two declarations that use the same tag declare the same type, they shall both use the same
choice of struct, union, or enum.

A type specifier of the form

    enum identifier
  

without an enumerator list shall only appear after the type it specifies is complete.

Semantics

All declarations of structure, union, or enumerated types that have the same scope and use the same
tag declare the same type. Irrespective of whether there is a tag or what other declarations of the
type are in the same translation unit, the type is incomplete131) until immediately after the closing
brace of the list defining the content, and complete thereafter.

Two declarations of structure, union, or enumerated types which are in different scopes or use
different tags declare distinct types. Each declaration of a structure, union, or enumerated type
which does not include a tag declares a distinct type.

A type specifier of the form

    struct-or-union identifieropt { struct-declaration-list }
  

or

    enum identifieropt { enumerator-list }
  

or

    enum identifieropt { enumerator-list , }
  

declares a structure, union, or enumerated type. The list defines the structure content, union content,
or enumeration content. If an identifier is provided,132) the type specifier also declares the identifier to
be the tag of that type.

A declaration of the form

    struct-or-union identifier ;
  

specifies a structure or union type and declares the identifier as a tag of that type.133)

If a type specifier of the form

    struct-or-union identifier
  

occurs other than as part of one of the above forms, and no other declaration of the identifier as a
tag is visible, then it declares an incomplete structure or union type, and declares the identifier as
the tag of that type.133)

If a type specifier of the form

    struct-or-union identifier
  

or

    enum identifier
  

occurs other than as part of one of the above forms, and a declaration of the identifier as a tag is
visible, then it specifies the same type as that other declaration, and does not redeclare the tag.

EXAMPLE 1 This mechanism allows declaration of a self-referential structure.

    struct tnode {
          int count;
          struct tnode *left, *right;
    };
  

specifies a structure that contains an integer and two pointers to objects of the same type. Once this declaration has been
given, the declaration

    struct tnode s, *sp;
  

declares s to be an object of the given type and sp to be a pointer to an object of the given type. With these declarations, the
expression sp->left refers to the left struct tnode pointer of the object to which sp points; the expression s.right->count
designates the count member of the right struct tnode pointed to from s.

The following alternative formulation uses the typedef mechanism:

    typedef struct tnode TNODE;
    struct tnode {
          int count;
          TNODE *left, *right;
    };
    TNODE s, *sp;
  

EXAMPLE 2 To illustrate the use of prior declaration of a tag to specify a pair of mutually referential structures, the
declarations

    struct s1 { struct s2 *s2p; /* ... */ }; // D1
    struct s2 { struct s1 *s1p; /* ... */ }; // D2
  

specify a pair of structures that contain pointers to each other. Note, however, that if s2 were already declared as a tag in an
enclosing scope, the declaration D1 would refer to it, not to the tag s2 declared in D2. To eliminate this context sensitivity, the
declaration

    struct s2;
  

may be inserted ahead of D1. This declares a new tag s2 in the inner scope; the declaration D2 then completes the specification
of the new type.

Forward references: declarators (6.7.6), type definitions (6.7.8).

6.7.2.4 Atomic type specifiers

Syntax

atomic-type-specifier:

    _Atomic ( type-name )
  

Constraints

Atomic type specifiers shall not be used if the implementation does not support atomic types (see
6.10.8.3).

The type name in an atomic type specifier shall not refer to an array type, a function type, an atomic
type, or a qualified type.

Semantics

The properties associated with atomic types are meaningful only for expressions that are lvalues.
If the _Atomic keyword is immediately followed by a left parenthesis, it is interpreted as a type
specifier (with a type name), not as a type qualifier.

6.7.3 Type qualifiers

Syntax

type-qualifier:

    const
    restrict
    volatile
    _Atomic
  

Constraints

Types other than pointer types whose referenced type is an object type shall not be restrict-qualified.

The _Atomic qualifier shall not be used if the implementation does not support atomic types
(see 6.10.8.3).

The type modified by the _Atomic qualifier shall not be an array type or a function type.

Semantics

The properties associated with qualified types are meaningful only for expressions that are lval-
ues.134)

If the same qualifier appears more than once in the same specifier-qualifier-list or as declaration-specifiers,
either directly or via one or more typedefs, the behavior is the same as if it appeared only once. If
other qualifiers appear along with the _Atomic qualifier the resulting type is the so-qualified atomic
type.

If an attempt is made to modify an object defined with a const-qualified type through use of an
lvalue with non-const-qualified type, the behavior is undefined. If an attempt is made to refer to an
object defined with a volatile-qualified type through use of an lvalue with non-volatile-qualified

type, the behavior is undefined.135)

An object that has volatile-qualified type may be modified in ways unknown to the implementation
or have other unknown side effects. Therefore any expression referring to such an object shall be
evaluated strictly according to the rules of the abstract machine, as described in 5.1.2.3. Furthermore,
at every sequence point the value last stored in the object shall agree with that prescribed by the
abstract machine, except as modified by the unknown factors mentioned previously.136) What
constitutes an access to an object that has volatile-qualified type is implementation-defined.

An object that is accessed through a restrict-qualified pointer has a special association with that
pointer. This association, defined in 6.7.3.1 below, requires that all accesses to that object use, directly
or indirectly, the value of that particular pointer.137) The intended use of the restrict qualifier (like
the register storage class) is to promote optimization, and deleting all instances of the qualifier
from all preprocessing translation units composing a conforming program does not change its
meaning (i.e., observable behavior).

If the specification of an array type includes any type qualifiers, the element type is so-qualified, not
the array type. If the specification of a function type includes any type qualifiers, the behavior is
undefined.138)

For two qualified types to be compatible, both shall have the identically qualified version of a
compatible type; the order of type qualifiers within a list of specifiers or qualifiers does not affect the
specified type.

EXAMPLE 1 An object declared

    extern const volatile int real_time_clock;
  

may be modifiable by hardware, but cannot be assigned to, incremented, or decremented.

EXAMPLE 2 The following declarations and expressions illustrate the behavior when type qualifiers modify an aggregate
type:

    const struct s { int mem; } cs = { 1 };
    struct s ncs; // the object ncs is modifiable
    typedef int A[2][3];
    const A a = {{4, 5, 6}, {7, 8, 9}}; // array of array of const int
    int *pi;
    const int *pci;
  
    ncs = cs;             //   valid
    cs = ncs;             //   violates modifiable lvalue constraint for =
    pi = &ncs.mem;        //   valid
    pi = &cs.mem;         //   violates type constraints for =
    pci = &cs.mem;        //   valid
    pi = a[0];            //   invalid: a[0] has type “const int *”
  

EXAMPLE 3 The declaration

    _Atomic volatile int *p;
  

specifies that p has the type “pointer to volatile atomic int”, a pointer to a volatile-qualified atomic type.

6.7.3.1 Formal definition of restrict

Let D be a declaration of an ordinary identifier that provides a means of designating an object P as a
restrict-qualified pointer to type T.

If D appears inside a block and does not have storage class extern, let B denote the block. If D
appears in the list of parameter declarations of a function definition, let B denote the associated block.
Otherwise, let B denote the block of main (or the block of whatever function is called at program
startup in a freestanding environment).

In what follows, a pointer expression E is said to be based on object P if (at some sequence point in
the execution of B prior to the evaluation of E) modifying P to point to a copy of the array object into
which it formerly pointed would change the value of E.139) Note that “based” is defined only for
expressions with pointer types.

During each execution of B, let L be any lvalue that has &L based on P. If L is used to access the
value of the object X that it designates, and X is also modified (by any means), then the following
requirements apply: T shall not be const-qualified. Every other lvalue used to access the value of
X shall also have its address based on P. Every access that modifies X shall be considered also to
modify P, for the purposes of this subclause. If P is assigned the value of a pointer expression E that
is based on another restricted pointer object P2, associated with block B2, then either the execution
of B2 shall begin before the execution of B, or the execution of B2 shall end prior to the assignment.
If these requirements are not met, then the behavior is undefined.

Here an execution of B means that portion of the execution of the program that would correspond to
the lifetime of an object with scalar type and automatic storage duration associated with B.

A translator is free to ignore any or all aliasing implications of uses of restrict.

EXAMPLE 1 The file scope declarations

    int * restrict a;
    int * restrict b;
    extern int c[];
  

assert that if an object is accessed using one of a, b, or c, and that object is modified anywhere in the program, then it is never
accessed using either of the other two.

EXAMPLE 2 The function parameter declarations in the following example

    void f(int n, int * restrict p, int * restrict q)
    {
          while (n-- > 0)
                *p++ = *q++;
    }
  

assert that, during each execution of the function, if an object is accessed through one of the pointer parameters, then it is not
also accessed through the other.

The benefit of the restrict qualifiers is that they enable a translator to make an effective dependence analysis of function f
without examining any of the calls of f in the program. The cost is that the programmer has to examine all of those calls to
ensure that none give undefined behavior. For example, the second call of f in g has undefined behavior because each of
d[1] through d[49] is accessed through both p and q.

    void g(void)
    {
          extern int d[100];
          f(50, d + 50, d); // valid
          f(50, d + 1, d); // undefined behavior
    }
  

EXAMPLE 3 The function parameter declarations

    void h(int n, int * restrict p, int * restrict q, int * restrict r)
    {
          int i;
          for (i = 0; i < n; i++)
                p[i] = q[i] + r[i];
    }
  

illustrate how an unmodified object can be aliased through two restricted pointers. In particular, if a and b are disjoint arrays,
a call of the form h(100, a, b, b) has defined behavior, because array b is not modified within function h.

EXAMPLE 4 The rule limiting assignments between restricted pointers does not distinguish between a function call and
an equivalent nested block. With one exception, only “outer-to-inner” assignments between restricted pointers declared in
nested blocks have defined behavior.

    {
            int * restrict p1;
            int * restrict q1;
            p1 = q1; // undefined behavior
            {
                  int * restrict p2 = p1; //               valid
                  int * restrict q2 = q1; //               valid
                  p1 = q2;                //               undefined behavior
                  p2 = q2;                //               undefined behavior
            }
    }
  

The one exception allows the value of a restricted pointer to be carried out of the block in which it (or, more precisely, the
ordinary identifier used to designate it) is declared when that block finishes execution. For example, this permits new_vector
to return a vector.

    typedef struct { int n; float * restrict v; } vector;
    vector new_vector(int n)
    {
          vector t;
          t.n = n;
          t.v = malloc(n * sizeof (float));
          return t;
    }
  

6.7.4 Function specifiers

Syntax

function-specifier:

    inline
    _Noreturn
  

Constraints

Function specifiers shall be used only in the declaration of an identifier for a function.

An inline definition of a function with external linkage shall not contain a definition of a modifiable
object with static or thread storage duration, and shall not contain a reference to an identifier with
internal linkage.

In a hosted environment, no function specifier(s) shall appear in a declaration of main.

Semantics

A function specifier may appear more than once; the behavior is the same as if it appeared only
once.

A function declared with an inline function specifier is an inline function. Making a function an
inline function suggests that calls to the function be as fast as possible.140) The extent to which such

suggestions are effective is implementation-defined.141)

Any function with internal linkage can be an inline function. For a function with external linkage,
the following restrictions apply: If a function is declared with an inline function specifier, then it
shall also be defined in the same translation unit. If all of the file scope declarations for a function in
a translation unit include the inline function specifier without extern, then the definition in that
translation unit is an inline definition. An inline definition does not provide an external definition
for the function, and does not forbid an external definition in another translation unit. An inline
definition provides an alternative to an external definition, which a translator may use to implement
any call to the function in the same translation unit. It is unspecified whether a call to the function
uses the inline definition or the external definition.142)

A function declared with a _Noreturn function specifier shall not return to its caller.

Recommended practice

The implementation should produce a diagnostic message for a function declared with a _Noreturn
function specifier that appears to be capable of returning to its caller.

EXAMPLE 1 The declaration of an inline function with external linkage can result in either an external definition, or a
definition available for use only within the translation unit. A file scope declaration with extern creates an external definition.
The following example shows an entire translation unit.

    inline double fahr(double t)
    {
          return (9.0 * t) / 5.0 + 32.0;
    }
  
    inline double cels(double t)
    {
          return (5.0 * (t - 32.0)) / 9.0;
    }
  
    extern double fahr(double);                // creates an external definition
  
    double convert(int is_fahr, double temp)
    {
          /* A translator may perform inline substitutions */
          return is_fahr ? cels(temp): fahr(temp);
    }
  

Note that the definition of fahr is an external definition because fahr is also declared with extern, but the definition of cels
is an inline definition. Because cels has external linkage and is referenced, an external definition has to appear in another
translation unit (see 6.9); the inline definition and the external definition are distinct and either may be used for the call.

EXAMPLE 2

    _Noreturn void f () {
          abort(); // ok
    }
  
    _Noreturn void g (int i) { // causes undefined behavior if i <= 0
          if (i > 0) abort();
    }
  

substitution is not textual substitution, nor does it create a new function. Therefore, for example, the expansion of a macro
used within the body of the function uses the definition it had at the point the function body appears, and not where the
function is called; and identifiers refer to the declarations in scope where the body occurs. Likewise, the function has a single
address, regardless of the number of inline definitions that occur in addition to the external definition.

Forward references: function definitions (6.9.1).

6.7.5 Alignment specifier

Syntax

alignment-specifier:

    _Alignas ( type-name )
    _Alignas ( constant-expression )
  

Constraints

An alignment specifier shall appear only in the declaration specifiers of a declaration, or in the
specifier-qualifier list of a member declaration, or in the type name of a compound literal. An
alignment specifier shall not be used in conjunction with either of the storage-class specifiers
typedef or register, nor in a declaration of a function or bit-field.

The constant expression shall be an integer constant expression. It shall evaluate to a valid funda-
mental alignment, or to a valid extended alignment supported by the implementation for an object
of the storage duration (if any) being declared, or to zero.

An object shall not be declared with an over-aligned type with an extended alignment requirement
not supported by the implementation for an object of that storage duration.

The combined effect of all alignment specifiers in a declaration shall not specify an alignment that is
less strict than the alignment that would otherwise be required for the type of the object or member
being declared.

Semantics

The first form is equivalent to _Alignas(_Alignof( type-name)).

The alignment requirement of the declared object or member is taken to be the specified alignment.
An alignment specification of zero has no effect.143) When multiple alignment specifiers occur in a
declaration, the effective alignment requirement is the strictest specified alignment.

If the definition of an object has an alignment specifier, any other declaration of that object shall
either specify equivalent alignment or have no alignment specifier. If the definition of an object does
not have an alignment specifier, any other declaration of that object shall also have no alignment
specifier. If declarations of an object in different translation units have different alignment specifiers,
the behavior is undefined.

6.7.6 Declarators

Syntax

declarator:

    pointeropt direct-declarator
  

direct-declarator:

    identifier
    ( declarator )
    direct-declarator   [   type-qualifier-listopt assignment-expressionopt ]
    direct-declarator   [     static type-qualifier-listopt assignment-expression ]
    direct-declarator   [    type-qualifier-list static assignment-expression ]
    direct-declarator   [   type-qualifier-listopt * ]
    direct-declarator   (    parameter-type-list )
    direct-declarator   (   identifier-listopt )
  

pointer:

    * type-qualifier-listopt
  
    * type-qualifier-listopt pointer
  

type-qualifier-list:

    type-qualifier
    type-qualifier-list type-qualifier
  

parameter-type-list:

    parameter-list
    parameter-list , ...
  

parameter-list:

    parameter-declaration
    parameter-list , parameter-declaration
  

parameter-declaration:

    declaration-specifiers declarator
    declaration-specifiers abstract-declaratoropt
  

identifier-list:

    identifier
    identifier-list , identifier
  

Semantics

Each declarator declares one identifier, and asserts that when an operand of the same form as
the declarator appears in an expression, it designates a function or object with the scope, storage
duration, and type indicated by the declaration specifiers.

A full declarator is a declarator that is not part of another declarator. If, in the nested sequence of
declarators in a full declarator, there is a declarator specifying a variable length array type, the type
specified by the full declarator is said to be variably modified. Furthermore, any type derived by
declarator type derivation from a variably modified type is itself variably modified.

In the following subclauses, consider a declaration

    T D1
  

where T contains the declaration specifiers that specify a type T (such as int) and D1 is a declarator
that contains an identifier ident. The type specified for the identifier ident in the various forms of
declarator is described inductively using this notation.

If, in the declaration “T D1”, D1 has the form

    identifier
  

then the type specified for ident is T.

If, in the declaration “T D1”, D1 has the form

    (D )
  

then ident has the type specified by the declaration “T D”. Thus, a declarator in parentheses is
identical to the unparenthesized declarator, but the binding of complicated declarators may be
altered by parentheses.

Implementation limits

As discussed in 5.2.4.1, an implementation may limit the number of pointer, array, and function
declarators that modify an arithmetic, structure, union, or void type, either directly or via one or
more typedef s.

Forward references: array declarators (6.7.6.2), type definitions (6.7.8).

6.7.6.1 Pointer declarators

Semantics

If, in the declaration “T D1”, D1 has the form

    * type-qualifier-listopt D
  

and the type specified for ident in the declaration “T D” is “derived-declarator-type-list T”, then the

type specified for ident is “derived-declarator-type-list type-qualifier-list pointer to T”. For each type
qualifier in the list, ident is a so-qualified pointer.

For two pointer types to be compatible, both shall be identically qualified and both shall be pointers
to compatible types.

EXAMPLE The following pair of declarations demonstrates the difference between a “variable pointer to a constant value”
and a “constant pointer to a variable value”.

    const int *ptr_to_constant;
    int *const constant_ptr;
  

The contents of any object pointed to by ptr_to_constant shall not be modified through that pointer, but ptr_to_constant
itself may be changed to point to another object. Similarly, the contents of the int pointed to by constant_ptr may be
modified, but constant_ptr itself shall always point to the same location.

The declaration of the constant pointer constant_ptr may be clarified by including a definition for the type “pointer to int”.

    typedef int *int_ptr;
    const int_ptr constant_ptr;
  

declares constant_ptr as an object that has type “const-qualified pointer to int”.

6.7.6.2 Array declarators

Constraints

In addition to optional type qualifiers and the keyword static, the [ and ] may delimit an expres-
sion or * . If they delimit an expression (which specifies the size of an array), the expression shall
have an integer type. If the expression is a constant expression, it shall have a value greater than
zero. The element type shall not be an incomplete or function type. The optional type qualifiers and
the keyword static shall appear only in a declaration of a function parameter with an array type,
and then only in the outermost array type derivation.

If an identifier is declared as having a variably modified type, it shall be an ordinary identifier (as
defined in 6.2.3), have no linkage, and have either block scope or function prototype scope. If an
identifier is declared to be an object with static or thread storage duration, it shall not have a variable
length array type.

Semantics

If, in the declaration “T D1”, D1 has one of the forms:

    D   [   type-qualifier-listopt assignment-expressionopt ]
    D   [   type-qualifier-listopt assignment-expression ]
    D   [   type-qualifier-list static assignment-expression                   ]
    D   [   type-qualifier-listopt ]
  

and the type specified for ident in the declaration “T D” is “derived-declarator-type-list T”, then the
type specified for ident is “derived-declarator-type-list array of T”.144) (See 6.7.6.3 for the meaning of
the optional type qualifiers and the keyword static.)

If the size is not present, the array type is an incomplete type. If the size is * instead of being an
expression, the array type is a variable length array type of unspecified size, which can only be used in
declarations or type names with function prototype scope;145) such arrays are nonetheless complete
types. If the size is an integer constant expression and the element type has a known constant size,
the array type is not a variable length array type; otherwise, the array type is a variable length array
type. (Variable length arrays are a conditional feature that implementations need not support; see
6.10.8.3.)

If the size is an expression that is not an integer constant expression: if it occurs in a declaration at
function prototype scope, it is treated as if it were replaced by * ; otherwise, each time it is evaluated
it shall have a value greater than zero. The size of each instance of a variable length array type
does not change during its lifetime. Where a size expression is part of the operand of a sizeof

operator and changing the value of the size expression would not affect the result of the operator, it
is unspecified whether or not the size expression is evaluated.

For two array types to be compatible, both shall have compatible element types, and if both size
specifiers are present, and are integer constant expressions, then both size specifiers shall have
the same constant value. If the two array types are used in a context which requires them to be
compatible, it is undefined behavior if the two size specifiers evaluate to unequal values.

EXAMPLE 1

    float fa[11], *afp[17];
  

declares an array of float numbers and an array of pointers to float numbers.

EXAMPLE 2 Note the distinction between the declarations

    extern int *x;
    extern int y[];
  

The first declares x to be a pointer to int; the second declares y to be an array of int of unspecified size (an incomplete type),
the storage for which is defined elsewhere.

EXAMPLE 3 The following declarations demonstrate the compatibility rules for variably modified types.

    extern int n;
    extern int m;
  
    void fcompat(void)
    {
          int a[n][6][m];
          int (*p)[4][n+1];
          int c[n][n][6][m];
          int (*r)[n][n][n+1];
          p = a;      // invalid: not compatible because 4 != 6
          r = c;      // compatible, but defined behavior only if
                      // n == 6 and m == n+1
    }
  

EXAMPLE 4 All declarations of variably modified (VM) types have to be at either block scope or function prototype scope.
Array objects declared with the _Thread_local , static, or extern storage-class specifier cannot have a variable length
array (VLA) type. However, an object declared with the static storage-class specifier can have a VM type (that is, a pointer
to a VLA type). Finally, all identifiers declared with a VM type have to be ordinary identifiers and cannot, therefore, be
members of structures or unions.

    extern int n;
    int A[n];                                          // invalid: file scope VLA
    extern int (*p2)[n];                               // invalid: file scope VM
    int B[100];                                        // valid: file scope but not VM
  
    void fvla(int m, int C[m][m]);                     // valid:       VLA with prototype scope
  
    void fvla(int m, int C[m][m])                      // valid:       adjusted to auto pointer to VLA
    {
          typedef int VLA[m][m];                       // valid:       block scope typedef VLA
  
    struct tag {
          int (*y)[n];                          // invalid:        y not ordinary identifier
          int z[n];                             // invalid:        z not ordinary identifier
    };
    int D[m];                                   //   valid: auto VLA
    static int E[m];                            //   invalid: static block scope VLA
    extern int F[m];                            //   invalid: F has linkage and is VLA
    int (*s)[m];                                //   valid: auto pointer to VLA
    extern int (*r)[m];                         //   invalid: r has linkage and points to VLA
    static int (*q)[m] = &B;                    //   valid: q is a static block pointer to VLA
  
    }
  

Forward references: function declarators (6.7.6.3), function definitions (6.9.1), initialization (6.7.9).

6.7.6.3 Function declarators (including prototypes)

Constraints

A function declarator shall not specify a return type that is a function type or an array type.

The only storage-class specifier that shall occur in a parameter declaration is register.

An identifier list in a function declarator that is not part of a definition of that function shall be
empty.

After adjustment, the parameters in a parameter type list in a function declarator that is part of a
definition of that function shall not have incomplete type.

Semantics

If, in the declaration “T D1”, D1 has the form

    D ( parameter-type-list )
  

or

    D ( identifier-listopt )
  

and the type specified for ident in the declaration “T D” is “derived-declarator-type-list T”, then the
type specified for ident is “derived-declarator-type-list function returning the unqualified version of T”.

A parameter type list specifies the types of, and may declare identifiers for, the parameters of the
function.

A declaration of a parameter as “array of type” shall be adjusted to “qualified pointer to type”, where
the type qualifiers (if any) are those specified within the [ and ] of the array type derivation. If the
keyword static also appears within the [ and ] of the array type derivation, then for each call to
the function, the value of the corresponding actual argument shall provide access to the first element
of an array with at least as many elements as specified by the size expression.

A declaration of a parameter as “function returning type” shall be adjusted to “pointer to function
returning type”, as in 6.3.2.1.

If the list terminates with an ellipsis (, ...), no information about the number or types of the
parameters after the comma is supplied.146)

The special case of an unnamed parameter of type void as the only item in the list specifies that the
function has no parameters.

If, in a parameter declaration, an identifier can be treated either as a typedef name or as a parameter
name, it shall be taken as a typedef name.

If the function declarator is not part of a definition of that function, parameters may have incomplete
type and may use the [*] notation in their sequences of declarator specifiers to specify variable
length array types.

The storage-class specifier in the declaration specifiers for a parameter declaration, if present, is
ignored unless the declared parameter is one of the members of the parameter type list for a function
definition.

An identifier list declares only the identifiers of the parameters of the function. An empty list in
a function declarator that is part of a definition of that function specifies that the function has no
parameters. The empty list in a function declarator that is not part of a definition of that function
specifies that no information about the number or types of the parameters is supplied.147)

For two function types to be compatible, both shall specify compatible return types.148) Moreover,

the parameter type lists, if both are present, shall agree in the number of parameters and in use
of the ellipsis terminator; corresponding parameters shall have compatible types. If one type has
a parameter type list and the other type is specified by a function declarator that is not part of a
function definition and that contains an empty identifier list, the parameter list shall not have an
ellipsis terminator and the type of each parameter shall be compatible with the type that results
from the application of the default argument promotions. If one type has a parameter type list and
the other type is specified by a function definition that contains a (possibly empty) identifier list,
both shall agree in the number of parameters, and the type of each prototype parameter shall be
compatible with the type that results from the application of the default argument promotions to the
type of the corresponding identifier. (In the determination of type compatibility and of a composite
type, each parameter declared with function or array type is taken as having the adjusted type
and each parameter declared with qualified type is taken as having the unqualified version of its
declared type.)

EXAMPLE 1 The declaration

    int f(void), *fip(), (*pfi)();
  

declares a function f with no parameters returning an int, a function fip with no parameter specification returning a pointer
to an int, and a pointer pfi to a function with no parameter specification returning an int. It is especially useful to compare
the last two. The binding of *fip() is *(fip()) , so that the declaration suggests, and the same construction in an expression
requires, the calling of a function fip, and then using indirection through the pointer result to yield an int. In the declarator
(*pfi)() , the extra parentheses are necessary to indicate that indirection through a pointer to a function yields a function
designator, which is then used to call the function; it returns an int.

If the declaration occurs outside of any function, the identifiers have file scope and external linkage. If the declaration
occurs inside a function, the identifiers of the functions f and fip have block scope and either internal or external linkage
(depending on what file scope declarations for these identifiers are visible), and the identifier of the pointer pfi has block
scope and no linkage.

EXAMPLE 2 The declaration

    int (*apfi[3])(int *x, int *y);
  

declares an array apfi of three pointers to functions returning int. Each of these functions has two parameters that are
pointers to int. The identifiers x and y are declared for descriptive purposes only and go out of scope at the end of the
declaration of apfi.

EXAMPLE 3 The declaration

    int (*fpfi(int (*)(long), int))(int, ...);
  

declares a function fpfi that returns a pointer to a function returning an int. The function fpfi has two parameters: a
pointer to a function returning an int (with one parameter of type long int), and an int. The pointer returned by fpfi
points to a function that has one int parameter and accepts zero or more additional arguments of any type.

EXAMPLE 4 The following prototype has a variably modified parameter.

    void addscalar(int n, int m,
          double a[n][n*m+300], double x);
  
    int main()
    {
          double b[4][308];
          addscalar(4, 2, b, 2.17);
          return 0;
    }
  
    void addscalar(int n, int m,
          double a[n][n*m+300], double x)
    {
          for (int i = 0; i < n; i++)
                for (int j = 0, k = n*m+300; j < k; j++)
                      // a is a pointer to a VLA with n*m+300 elements
                      a[i][j] += x;
    }
  

EXAMPLE 5 The following are all compatible function prototype declarators.

    double    maximum(int       n,   int   m,   double   a[n][m]);
    double    maximum(int       n,   int   m,   double   a[*][*]);
    double    maximum(int       n,   int   m,   double   a[ ][*]);
    double    maximum(int       n,   int   m,   double   a[ ][m]);
  

as are:

    void   f(double     (* restrict a)[5]);
    void   f(double     a[restrict][5]);
    void   f(double     a[restrict 3][5]);
    void   f(double     a[restrict static 3][5]);
  

(Note that the last declaration also specifies that the argument corresponding to a in any call to f must be a non-null pointer
to the first of at least three arrays of 5 doubles, which the others do not.)

Forward references: function definitions (6.9.1), type names (6.7.7).

6.7.7 Type names

Syntax

type-name:

    specifier-qualifier-list abstract-declaratoropt
  

abstract-declarator:

    pointer
    pointeropt direct-abstract-declarator
  

direct-abstract-declarator:

    ( abstract-declarator )
    direct-abstract-declaratoropt [ type-qualifier-listopt
                          assignment-expressionopt ]
    direct-abstract-declaratoropt [ static type-qualifier-listopt
                          assignment-expression ]
    direct-abstract-declaratoropt [ type-qualifier-list static
                          assignment-expression ]
    direct-abstract-declaratoropt [ * ]
    direct-abstract-declaratoropt ( parameter-type-listopt )
  

Semantics

In several contexts, it is necessary to specify a type. This is accomplished using a type name, which is
syntactically a declaration for a function or an object of that type that omits the identifier.149)

EXAMPLE The constructions

    (a)           int
    (b)           int   *
    (c)           int   *[3]
    (d)           int   (*)[3]
    (e)           int   (*)[*]
    (f)           int   *()
    (g)           int   (*)(void)
    (h)           int   (*const [])(unsigned int, ...)
  

name respectively the types (a) int, (b) pointer to int, (c) array of three pointers to int, (d) pointer to an array of three int s,
(e) pointer to a variable length array of an unspecified number of int s, (f) function with no parameter specification returning
a pointer to int, (g) pointer to function with no parameters returning an int, and (h) array of an unspecified number of
constant pointers to functions, each with one parameter that has type unsigned int and an unspecified number of other
parameters, returning an int.

6.7.8 Type definitions

Syntax

typedef-name:

    identifier
  

Constraints

If a typedef name specifies a variably modified type then it shall have block scope.

Semantics

In a declaration whose storage-class specifier is typedef, each declarator defines an identifier to
be a typedef name that denotes the type specified for the identifier in the way described in 6.7.6.
Any array size expressions associated with variable length array declarators are evaluated each time
the declaration of the typedef name is reached in the order of execution. A typedef declaration
does not introduce a new type, only a synonym for the type so specified. That is, in the following
declarations:

    typedef T type_ident;
    type_ident D;
  

type_ident is defined as a typedef name with the type specified by the declaration specifiers in T
(known as T), and the identifier in D has the type “derived-declarator-type-list T” where the derived-
declarator-type-list is specified by the declarators of D. A typedef name shares the same name space
as other identifiers declared in ordinary declarators.

EXAMPLE 1 After

    typedef int MILES, KLICKSP();
    typedef struct { double hi, lo; } range;
  

the constructions

    MILES distance;
    extern KLICKSP *metricp;
    range x;
    range z, *zp;
  

are all valid declarations. The type of distance is int, that of metricp is “pointer to function with no parameter specification
returning int”, and that of x and z is the specified structure; zp is a pointer to such a structure. The object distance has a
type compatible with any other int object.

EXAMPLE 2 After the declarations

    typedef struct s1 { int x; } t1, *tp1;
    typedef struct s2 { int x; } t2, *tp2;
  

type t1 and the type pointed to by tp1 are compatible. Type t1 is also compatible with type struct s1, but not compatible
with the types struct s2, t2, the type pointed to by tp2, or int.

EXAMPLE 3 The following obscure constructions

    typedef signed int t;
    typedef int plain;
    struct tag {
          unsigned t:4;
          const t:5;
          plain r:5;
    };
  

declare a typedef name t with type signed int, a typedef name plain with type int, and a structure with three bit-field
members, one named t that contains values in the range [0, 15], an unnamed const-qualified bit-field which (if it could

be accessed) would contain values in either the range [−15, +15] or [−16, +15], and one named r that contains values in
one of the ranges [0, 31], [−15, +15], or [−16, +15]. (The choice of range is implementation-defined.) The first two bit-field
declarations differ in that unsigned is a type specifier (which forces t to be the name of a structure member), while const is
a type qualifier (which modifies t which is still visible as a typedef name). If these declarations are followed in an inner scope
by

    t f(t (t));
    long t;
  

then a function f is declared with type “function returning signed int with one unnamed parameter with type pointer
to function returning signed int with one unnamed parameter with type signed int”, and an identifier t with type
long int.

EXAMPLE 4 On the other hand, typedef names can be used to improve code readability. All three of the following
declarations of the signal function specify exactly the same type, the first without making use of any typedef names.

    typedef void fv(int), (*pfv)(int);
  
    void (*signal(int, void (*)(int)))(int);
    fv *signal(int, fv *);
    pfv signal(int, pfv);
  

EXAMPLE 5 If a typedef name denotes a variable length array type, the length of the array is fixed at the time the typedef
name is defined, not each time it is used:

    void copyt(int n)
    {
          typedef int B[n];   // B                is n ints, n evaluated now
          n += 1;
          B a;                // a                is n ints, n without += 1
          int b[n];           // a                and b are different sizes
          for (int i = 1; i < n;                  i++)
                a[i-1] = b[i];
    }
  

6.7.9 Initialization

Syntax

initializer:

    assignment-expression
    { initializer-list }
    { initializer-list , }
  

initializer-list:

    designationopt initializer
    initializer-list , designationopt initializer
  

designation:

    designator-list =
  

designator-list:

    designator
    designator-list designator
  

designator:

    [ constant-expression ]
    . identifier
  

Constraints

No initializer shall attempt to provide a value for an object not contained within the entity being
initialized.

The type of the entity to be initialized shall be an array of unknown size or a complete object type
that is not a variable length array type.

All the expressions in an initializer for an object that has static or thread storage duration shall be
constant expressions or string literals.

If the declaration of an identifier has block scope, and the identifier has external or internal linkage,
the declaration shall have no initializer for the identifier.

If a designator has the form

    [ constant-expression ]
  

then the current object (defined below) shall have array type and the expression shall be an integer
constant expression. If the array is of unknown size, any nonnegative value is valid.

If a designator has the form

    . identifier
  

then the current object (defined below) shall have structure or union type and the identifier shall be
the name of a member of that type.

Semantics

An initializer specifies the initial value stored in an object.

Except where explicitly stated otherwise, for the purposes of this subclause unnamed members
of objects of structure and union type do not participate in initialization. Unnamed members of
structure objects have indeterminate value even after initialization.

If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate.
If an object that has static or thread storage duration is not initialized explicitly, then:

The initializer for a scalar shall be a single expression, optionally enclosed in braces. The initial value
of the object is that of the expression (after conversion); the same type constraints and conversions
as for simple assignment apply, taking the type of the scalar to be the unqualified version of its
declared type.

The rest of this subclause deals with initializers for objects that have aggregate or union type.

The initializer for a structure or union object that has automatic storage duration shall be either
an initializer list as described below, or a single expression that has compatible structure or union
type. In the latter case, the initial value of the object, including unnamed members, is that of the
expression.

An array of character type may be initialized by a character string literal or UTF–8 string literal,
optionally enclosed in braces. Successive bytes of the string literal (including the terminating null
character if there is room or if the array is of unknown size) initialize the elements of the array.

An array with element type compatible with a qualified or unqualified version of wchar_t, char16_t,
or char32_t may be initialized by a wide string literal with the corresponding encoding prefix (L,
u, or U, respectively), optionally enclosed in braces. Successive wide characters of the wide string
literal (including the terminating null wide character if there is room or if the array is of unknown
size) initialize the elements of the array.

Otherwise, the initializer for an object that has aggregate or union type shall be a brace-enclosed list
of initializers for the elements or named members.

Each brace-enclosed initializer list has an associated current object. When no designations are present,
subobjects of the current object are initialized in order according to the type of the current object:
array elements in increasing subscript order, structure members in declaration order, and the first
named member of a union.150) In contrast, a designation causes the following initializer to begin
initialization of the subobject described by the designator. Initialization then continues forward in
order, beginning with the next subobject after that described by the designator.151)

Each designator list begins its description with the current object associated with the closest sur-
rounding brace pair. Each item in the designator list (in order) specifies a particular member of its
current object and changes the current object for the next designator (if any) to be that member.152)
The current object that results at the end of the designator list is the subobject to be initialized by the
following initializer.

The initialization shall occur in initializer list order, each initializer provided for a particular subobject
overriding any previously listed initializer for the same subobject;153) all subobjects that are not
initialized explicitly shall be initialized implicitly the same as objects that have static storage duration.

If the aggregate or union contains elements or members that are aggregates or unions, these rules
apply recursively to the subaggregates or contained unions. If the initializer of a subaggregate or
contained union begins with a left brace, the initializers enclosed by that brace and its matching right
brace initialize the elements or members of the subaggregate or the contained union. Otherwise, only
enough initializers from the list are taken to account for the elements or members of the subaggregate
or the first member of the contained union; any remaining initializers are left to initialize the next
element or member of the aggregate of which the current subaggregate or contained union is a part.

If there are fewer initializers in a brace-enclosed list than there are elements or members of an
aggregate, or fewer characters in a string literal used to initialize an array of known size than there
are elements in the array, the remainder of the aggregate shall be initialized implicitly the same as
objects that have static storage duration.

If an array of unknown size is initialized, its size is determined by the largest indexed element with
an explicit initializer. The array type is completed at the end of its initializer list.

The evaluations of the initialization list expressions are indeterminately sequenced with respect to
one another and thus the order in which any side effects occur is unspecified.154)

EXAMPLE 1 Provided that <complex.h> has been #included, the declarations

    int i = 3.5;
    double complex c = 5 + 3 * I;
  

define and initialize i with the value 3 and c with the value 5.0 + i3.0.

EXAMPLE 2 The declaration

    int x[] = { 1, 3, 5 };
  

defines and initializes x as a one-dimensional array object that has three elements, as no size was specified and there are three
initializers.

EXAMPLE 3 The declaration

    int y[4][3] = {
          { 1, 3, 5 },
          { 2, 4, 6 },
  
             { 3, 5, 7 },
    };
  

is a definition with a fully bracketed initialization: 1, 3, and 5 initialize the first row of y (the array object y[0]), namely
y[0][0], y[0][1], and y[0][2]. Likewise the next two lines initialize y[1] and y[2]. The initializer ends early, so y[3] is
initialized with zeros. Precisely the same effect could have been achieved by

    int y[4][3] = {
          1, 3, 5, 2, 4, 6, 3, 5, 7
    };
  

The initializer for y[0] does not begin with a left brace, so three items from the list are used. Likewise the next three are
taken successively for y[1] and y[2].

EXAMPLE 4 The declaration

    int z[4][3] = {
          { 1 }, { 2 }, { 3 }, { 4 }
    };
  

initializes the first column of z as specified and initializes the rest with zeros.

EXAMPLE 5 The declaration

    struct { int a[3], b; } w[] = { { 1 }, 2 };
  

is a definition with an inconsistently bracketed initialization. It defines an array with two element structures: w[0].a[0] is 1
and w[1].a[0] is 2; all the other elements are zero.

EXAMPLE 6 The declaration

    short q[4][3][2] = {
          { 1 },
          { 2, 3 },
          { 4, 5, 6 }
    };
  

contains an incompletely but consistently bracketed initialization. It defines a three-dimensional array object: q[0][0][0] is
1, q[1][0][0] is 2, q[1][0][1] is 3, and 4, 5, and 6 initialize q[2][0][0], q[2][0][1], and q[2][1][0], respectively; all
the rest are zero. The initializer for q[0][0] does not begin with a left brace, so up to six items from the current list may be
used. There is only one, so the values for the remaining five elements are initialized with zero. Likewise, the initializers for
q[1][0] and q[2][0] do not begin with a left brace, so each uses up to six items, initializing their respective two-dimensional
subaggregates. If there had been more than six items in any of the lists, a diagnostic message would have been issued. The
same initialization result could have been achieved by:

    short q[4][3][2] = {
          1, 0, 0, 0, 0, 0,
          2, 3, 0, 0, 0, 0,
          4, 5, 6
    };
  

or by:

    short q[4][3][2] = {
          {
                { 1 },
          },
          {
                { 2, 3 },
          },
          {
                { 4, 5 },
                { 6 },
          }
    };
  

in a fully bracketed form.

Note that the fully bracketed and minimally bracketed forms of initialization are, in general, less likely to cause confusion.

EXAMPLE 7 One form of initialization that completes array types involves typedef names. Given the declaration

    typedef int A[];         // OK - declared with block scope
  

the declaration

    A a = { 1, 2 }, b = { 3, 4, 5 };
  

is identical to

    int a[] = { 1, 2 }, b[] = { 3, 4, 5 };
  

due to the rules for incomplete types.

EXAMPLE 8 The declaration

    char s[] = "abc", t[3] = "abc";
  

defines “plain” char array objects s and t whose elements are initialized with character string literals. This declaration is
identical to

    char s[] = { ’a’, ’b’, ’c’, ’\0’ },
         t[] = { ’a’, ’b’, ’c’ };
  

The contents of the arrays are modifiable. On the other hand, the declaration

    char *p = "abc";
  

defines p with type “pointer to char” and initializes it to point to an object with type “array of char” with length 4 whose
elements are initialized with a character string literal. If an attempt is made to use p to modify the contents of the array, the
behavior is undefined.

EXAMPLE 9 Arrays can be initialized to correspond to the elements of an enumeration by using designators:

    enum { member_one,         member_two };
    const char *nm[] =         {
          [member_two]         = "member two",
          [member_one]         = "member one",
    };
  

EXAMPLE 10 Structure members can be initialized to nonzero values without depending on their order:

    div_t answer = {.quot = 2, .rem = -1 };
  

EXAMPLE 11 Designators can be used to provide explicit initialization when unadorned initializer lists might be misunder-
stood:

    struct { int a[3], b; } w[] =
          { [0].a = {1}, [1].a[0] = 2 };
  

EXAMPLE 12

    struct T {
          int k;
          int l;
    };
  
    struct S {
          int i;
          struct T t;
    };
  
    struct T x = {.l = 43, .k = 42, };
  
    void f(void)
    {
          struct S l = { 1, .t = x, .t.l = 41, };
    }
  

The value of l.t.k is 42, because implicit initialization does not override explicit initialization.

EXAMPLE 13 Space can be “allocated” from both ends of an array by using a single designator:

    int a[MAX] = {
          1, 3, 5, 7, 9, [MAX-5] = 8, 6, 4, 2, 0
    };
  

In the above, if MAX is greater than ten, there will be some zero-valued elements in the middle; if it is less than ten, some of
the values provided by the first five initializers will be overridden by the second five.

EXAMPLE 14 Any member of a union can be initialized:

    union { /* ... */ } u = {.any_member = 42 };
  

Forward references: common definitions <stddef.h> (7.19).

6.7.10 Static assertions

Syntax

static_assert-declaration:

    _Static_assert ( constant-expression , string-literal ) ;
  

Constraints

The constant expression shall compare unequal to 0.

Semantics

The constant expression shall be an integer constant expression. If the value of the constant expres-
sion compares unequal to 0, the declaration has no effect. Otherwise, the constraint is violated and
the implementation shall produce a diagnostic message that includes the text of the string literal,
except that characters not in the basic source character set are not required to appear in the message.

Forward references: diagnostics (7.2).

6.8 Statements and blocks

Syntax

statement:

    labeled-statement
    compound-statement
    expression-statement
    selection-statement
    iteration-statement
    jump-statement
  

Semantics

A statement specifies an action to be performed. Except as indicated, statements are executed in
sequence.

A block allows a set of declarations and statements to be grouped into one syntactic unit. The
initializers of objects that have automatic storage duration, and the variable length array declarators
of ordinary identifiers with block scope, are evaluated and the values are stored in the objects
(including storing an indeterminate value in objects without an initializer) each time the declaration
is reached in the order of execution, as if it were a statement, and within each declaration in the
order that declarators appear.

A full expression is an expression that is not part of another expression, nor part of a declarator
or abstract declarator. There is also an implicit full expression in which the non-constant size
expressions for a variably modified type are evaluated; within that full expression, the evaluation of
different size expressions are unsequenced with respect to one another. There is a sequence point
between the evaluation of a full expression and the evaluation of the next full expression to be
evaluated.

NOTE Each of the following is a full expression:

While a constant expression satisfies the definition of a full expression, evaluating it does not depend on nor produce any
side effects, so the sequencing implications of being a full expression are not relevant to a constant expression.

Forward references: expression and null statements (6.8.3), selection statements (6.8.4), iteration
statements (6.8.5), the return statement (6.8.6.4).

6.8.1 Labeled statements

Syntax

labeled-statement:

    identifier : statement
    case constant-expression : statement
    default : statement
  

Constraints

A case or default label shall appear only in a switch statement. Further constraints on such labels
are discussed under the switch statement.

Label names shall be unique within a function.

Semantics

Any statement may be preceded by a prefix that declares an identifier as a label name. Labels in
themselves do not alter the flow of control, which continues unimpeded across them.

Forward references: the goto statement (6.8.6.1), the switch statement (6.8.4.2).

6.8.2 Compound statement

Syntax

compound-statement:

    { block-item-listopt }
  

block-item-list:

    block-item
    block-item-list block-item
  

block-item:

    declaration
    statement
  

Semantics

A compound statement is a block.

6.8.3 Expression and null statements

Syntax

expression-statement:

    expressionopt ;
  

Semantics

The expression in an expression statement is evaluated as a void expression for its side effects.155)

A null statement (consisting of just a semicolon) performs no operations.

EXAMPLE 1 If a function call is evaluated as an expression statement for its side effects only, the discarding of its value may
be made explicit by converting the expression to a void expression by means of a cast:

    int p(int);
    /* ... */
    (void)p(0);
  

EXAMPLE 2 In the program fragment

    char *s;
    /* ... */
    while (*s++ != ’\0’)
          ;
  

a null statement is used to supply an empty loop body to the iteration statement.

EXAMPLE 3 A null statement may also be used to carry a label just before the closing } of a compound statement.

    while (loop1) {
          /* ... */
          while (loop2) {
                /* ... */
                if (want_out)
                      goto end_loop1;
                /* ... */
          }
          /* ... */
    end_loop1:;
    }
  

Forward references: iteration statements (6.8.5).

6.8.4 Selection statements

Syntax

selection-statement:

    if ( expression ) statement
    if ( expression ) statement else statement
    switch ( expression ) statement
  

Semantics

A selection statement selects among a set of statements depending on the value of a controlling
expression.

A selection statement is a block whose scope is a strict subset of the scope of its enclosing block. Each
associated substatement is also a block whose scope is a strict subset of the scope of the selection
statement.

6.8.4.1 The if statement

Constraints

The controlling expression of an if statement shall have scalar type.

Semantics

In both forms, the first substatement is executed if the expression compares unequal to 0. In the
else form, the second substatement is executed if the expression compares equal to 0. If the first
substatement is reached via a label, the second substatement is not executed.

An else is associated with the lexically nearest preceding if that is allowed by the syntax.

6.8.4.2 The switch statement

Constraints

The controlling expression of a switch statement shall have integer type.

If a switch statement has an associated case or default label within the scope of an identifier with
a variably modified type, the entire switch statement shall be within the scope of that identifier.156)

The expression of each case label shall be an integer constant expression and no two of the case
constant expressions in the same switch statement shall have the same value after conversion.
There may be at most one default label in a switch statement. (Any enclosed switch statement
may have a default label or case constant expressions with values that duplicate case constant
expressions in the enclosing switch statement.)

Semantics

A switch statement causes control to jump to, into, or past the statement that is the switch body,
depending on the value of a controlling expression, and on the presence of a default label and the
values of any case labels on or in the switch body. A case or default label is accessible only within
the closest enclosing switch statement.

The integer promotions are performed on the controlling expression. The constant expression in
each case label is converted to the promoted type of the controlling expression. If a converted value
matches that of the promoted controlling expression, control jumps to the statement following the
matched case label. Otherwise, if there is a default label, control jumps to the labeled statement. If
no converted case constant expression matches and there is no default label, no part of the switch
body is executed.

Implementation limits

As discussed in 5.2.4.1, the implementation may limit the number of case values in a switch
statement.

EXAMPLE In the artificial program fragment

    switch (expr)
    {
          int i = 4;
          f(i);
    case 0:
          i = 17;
          /* falls through into default code */
    default:
          printf("%d\n", i);
    }
  

the object whose identifier is i exists with automatic storage duration (within the block) but is never initialized, and thus if
the controlling expression has a nonzero value, the call to the printf function will access an indeterminate value. Similarly,
the call to the function f cannot be reached.

6.8.5 Iteration statements

Syntax

iteration-statement:

    while ( expression ) statement
    do statement while ( expression ) ;
    for ( expressionopt ; expressionopt ; expressionopt ) statement
    for ( declaration expressionopt ; expressionopt ) statement
  

Constraints

The controlling expression of an iteration statement shall have scalar type.

The declaration part of a for statement shall only declare identifiers for objects having storage class
auto or register.

Semantics

An iteration statement causes a statement called the loop body to be executed repeatedly until the
controlling expression compares equal to 0. The repetition occurs regardless of whether the loop
body is entered from the iteration statement or by a jump.157)

An iteration statement is a block whose scope is a strict subset of the scope of its enclosing block.
The loop body is also a block whose scope is a strict subset of the scope of the iteration statement.

An iteration statement may be assumed by the implementation to terminate if its controlling
expression is not a constant expression,158) and none of the following operations are performed in its
body, controlling expression or (in the case of a for statement) its expression-3:159)

6.8.5.1 The while statement

The evaluation of the controlling expression takes place before each execution of the loop body.

6.8.5.2 The do statement

The evaluation of the controlling expression takes place after each execution of the loop body.

6.8.5.3 The for statement

The statement

    for (clause-1; expression-2; expression-3) statement
  

behaves as follows: The expression expression-2 is the controlling expression that is evaluated before
each execution of the loop body. The expression expression-3 is evaluated as a void expression after
each execution of the loop body. If clause-1 is a declaration, the scope of any identifiers it declares
is the remainder of the declaration and the entire loop, including the other two expressions; it is
reached in the order of execution before the first evaluation of the controlling expression. If clause-1
is an expression, it is evaluated as a void expression before the first evaluation of the controlling
expression.160)

Both clause-1 and expression-3 can be omitted. An omitted expression-2 is replaced by a nonzero
constant.

6.8.6 Jump statements

Syntax

jump-statement:

    goto identifier ;
    continue ;
    break ;
    return expressionopt ;
  

Semantics

A jump statement causes an unconditional jump to another place.

6.8.6.1 The goto statement

Constraints

The identifier in a goto statement shall name a label located somewhere in the enclosing function. A
goto statement shall not jump from outside the scope of an identifier having a variably modified
type to inside the scope of that identifier.

Semantics

A goto statement causes an unconditional jump to the statement prefixed by the named label in the
enclosing function.

EXAMPLE 1 It is sometimes convenient to jump into the middle of a complicated set of statements. The following outline
presents one possible approach to a problem based on these three assumptions:

  1. The general initialization code accesses objects only visible to the current function.
  2. The general initialization code is too large to warrant duplication.
  3. The code to determine the next operation is at the head of the loop. (To allow it to be reached by continue statements,
    for example.)
            /* ... */
            goto first_time;
            for (;;) {
                  // determine next operation
                  /* ... */
                  if (need to reinitialize) {
                        // reinitialize-only code
                        /* ... */
                  first_time:
                        // general initialization code
          
                             /* ... */
                             continue;
                    }
                    // handle other operations
                    /* ... */
            }
          

EXAMPLE 2 A goto statement is not allowed to jump past any declarations of objects with variably modified types. A jump
within the scope, however, is permitted.

    goto lab3;                        // invalid:      going INTO scope of VLA.
    {
          double a[n];
          a[j] = 4.4;
    lab3:
          a[j] = 3.3;
          goto lab4;                  // valid:      going WITHIN scope of VLA.
          a[j] = 5.5;
    lab4:
          a[j] = 6.6;
    }
    goto lab4;                        // invalid:      going INTO scope of VLA.
  

6.8.6.2 The continue statement

Constraints

A continue statement shall appear only in or as a loop body.

Semantics

A continue statement causes a jump to the loop-continuation portion of the smallest enclosing
iteration statement; that is, to the end of the loop body. More precisely, in each of the statements

    while (/* ...          */) {             do {                           for (/* ... */) {
       /* ... */                                /* ... */                      /* ... */
       continue;                                continue;                      continue;
       /* ... */                                /* ... */                      /* ... */
    contin:;                                 contin:;                       contin:;
    }                                        } while (/* ...        */);    }
  

unless the continue statement shown is in an enclosed iteration statement (in which case it is
interpreted within that statement), it is equivalent to goto contin;.161)

6.8.6.3 The break statement

Constraints

A break statement shall appear only in or as a switch body or loop body.

Semantics

A break statement terminates execution of the smallest enclosing switch or iteration statement.

6.8.6.4 The return statement

Constraints

A return statement with an expression shall not appear in a function whose return type is void. A
return statement without an expression shall only appear in a function whose return type is void .

Semantics

A return statement terminates execution of the current function and returns control to its caller. A
function may have any number of return statements.

If a return statement with an expression is executed, the value of the expression is returned to the
caller as the value of the function call expression. If the expression has a type different from the
return type of the function in which it appears, the value is converted as if by assignment to an
object having the return type of the function.162)

EXAMPLE In:

    struct s { double i; } f(void);
    union {
          struct {
                int f1;
                struct s f2;
          } u1;
          struct {
                struct s f3;
                int f4;
          } u2;
    } g;
  
    struct s f(void)
    {
          return g.u1.f2;
    }
  
    /* ... */
    g.u2.f3 = f();
  

there is no undefined behavior, although there would be if the assignment were done directly (without using a function call
to fetch the value).

6.9 External definitions

Syntax

translation-unit:

    external-declaration
    translation-unit external-declaration
  

external-declaration:

    function-definition
    declaration
  

Constraints

The storage-class specifiers auto and register shall not appear in the declaration specifiers in an
external declaration.

There shall be no more than one external definition for each identifier declared with internal linkage
in a translation unit. Moreover, if an identifier declared with internal linkage is used in an expression
(other than as a part of the operand of a sizeof or _Alignof operator whose result is an integer
constant), there shall be exactly one external definition for the identifier in the translation unit.

Semantics

As discussed in 5.1.1.1, the unit of program text after preprocessing is a translation unit, which
consists of a sequence of external declarations. These are described as “external” because they
appear outside any function (and hence have file scope). As discussed in 6.7, a declaration that also
causes storage to be reserved for an object or a function named by the identifier is a definition.

An external definition is an external declaration that is also a definition of a function (other than an
inline definition) or an object. If an identifier declared with external linkage is used in an expression
(other than as part of the operand of a sizeof or _Alignof operator whose result is an integer
constant), somewhere in the entire program there shall be exactly one external definition for the
identifier; otherwise, there shall be no more than one.163)

6.9.1 Function definitions

Syntax

function-definition:

    declaration-specifiers declarator declaration-listopt compound-statement
  

declaration-list:

    declaration
    declaration-list declaration
  

Constraints

The identifier declared in a function definition (which is the name of the function) shall have a
function type, as specified by the declarator portion of the function definition.164)

The return type of a function shall be void or a complete object type other than array type.

The storage-class specifier, if any, in the declaration specifiers shall be either extern or static.

If the declarator includes a parameter type list, the declaration of each parameter shall include an
identifier, except for the special case of a parameter list consisting of a single parameter of type void,
in which case there shall not be an identifier. No declaration list shall follow.

If the declarator includes an identifier list, each declaration in the declaration list shall have at least

one declarator, those declarators shall declare only identifiers from the identifier list, and every
identifier in the identifier list shall be declared. An identifier declared as a typedef name shall not
be redeclared as a parameter. The declarations in the declaration list shall contain no storage-class
specifier other than register and no initializations.

Semantics

The declarator in a function definition specifies the name of the function being defined and the
identifiers of its parameters. If the declarator includes a parameter type list, the list also specifies the
types of all the parameters; such a declarator also serves as a function prototype for later calls to the
same function in the same translation unit. If the declarator includes an identifier list,165) the types
of the parameters shall be declared in a following declaration list. In either case, the type of each
parameter is adjusted as described in 6.7.6.3 for a parameter type list; the resulting type shall be a
complete object type.

If a function that accepts a variable number of arguments is defined without a parameter type list
that ends with the ellipsis notation, the behavior is undefined.

Each parameter has automatic storage duration; its identifier is an lvalue.166) The layout of the
storage for parameters is unspecified.

On entry to the function, the size expressions of each variably modified parameter are evaluated
and the value of each argument expression is converted to the type of the corresponding parameter
as if by assignment. (Array expressions and function designators as arguments were converted to
pointers before the call.)

After all parameters have been assigned, the compound statement that constitutes the body of the
function definition is executed.

If the } that terminates a function is reached, and the value of the function call is used by the caller,
the behavior is undefined.

EXAMPLE 1 In the following:

    extern int max(int a, int b)
    {
          return a > b ? a: b;
    }
  

extern is the storage-class specifier and int is the type specifier; max(int a, int b) is the function declarator; and

    { return a       >    b ? a: b; }
  

is the function body. The following similar definition uses the identifier-list form for the parameter declarations:

    extern int max(a, b)
    int a, b;
    {
          return a > b ? a: b;
    }
  

Here int a, b; is the declaration list for the parameters. The difference between these two definitions is that the first form acts
as a prototype declaration that forces conversion of the arguments of subsequent calls to the function, whereas the second
form does not.

EXAMPLE 2 To pass one function to another, one might say

    int f(void);
    /* ... */
    g(f);
  

Then the definition of g might read

    void g(int (*funcp)(void))
    {
          /* ... */
          (*funcp)(); /* or funcp(); ...*/
    }
  

or, equivalently,

    void g(int func(void))
    {
          /* ... */
          func(); /* or (*func)(); ...*/
    }
  

6.9.2 External object definitions

Semantics

If the declaration of an identifier for an object has file scope and an initializer, the declaration is an
external definition for the identifier.

A declaration of an identifier for an object that has file scope without an initializer, and without a
storage-class specifier or with the storage-class specifier static, constitutes a tentative definition. If a
translation unit contains one or more tentative definitions for an identifier, and the translation unit
contains no external definition for that identifier, then the behavior is exactly as if the translation
unit contains a file scope declaration of that identifier, with the composite type as of the end of the
translation unit, with an initializer equal to 0.

If the declaration of an identifier for an object is a tentative definition and has internal linkage, the
declared type shall not be an incomplete type.

EXAMPLE 1

    int i1 = 1;                   //   definition, external linkage
    static int i2 = 2;            //   definition, internal linkage
    extern int i3 = 3;            //   definition, external linkage
    int i4;                       //   tentative definition, external linkage
    static int i5;                //   tentative definition, internal linkage
  
    int   i1;                     //   valid   tentative definition, refers to previous
    int   i2;                     //   6.2.2   renders undefined, linkage disagreement
    int   i3;                     //   valid   tentative definition, refers to previous
    int   i4;                     //   valid   tentative definition, refers to previous
    int   i5;                     //   6.2.2   renders undefined, linkage disagreement
  
    extern int i1;                // refers to previous, whose linkage is external
  
    extern    int   i2;         //   refers   to   previous,     whose   linkage     is   internal
    extern    int   i3;         //   refers   to   previous,     whose   linkage     is   external
    extern    int   i4;         //   refers   to   previous,     whose   linkage     is   external
    extern    int   i5;         //   refers   to   previous,     whose   linkage     is   internal
  

EXAMPLE 2 If at the end of the translation unit containing

    int i[];
  

the array i still has incomplete type, the implicit initializer causes it to have one element, which is set to zero on program
startup.

6.10 Preprocessing directives

Syntax

preprocessing-file:

    groupopt
  

group:

    group-part
    group group-part
  

group-part:

    if-section
    control-line
    text-line
    # non-directive
  

if-section:

    if-group elif-groupsopt else-groupopt endif-line
  

if-group:

    # if constant-expression new-line groupopt
    # ifdef identifier new-line groupopt
    # ifndef identifier new-line groupopt
  

elif-groups:

    elif-group
    elif-groups elif-group
  

elif-group:

    # elif constant-expression new-line groupopt
  

else-group:

    # else new-line groupopt
  

endif-line:

    # endif new-line
  

control-line:

    # include pp-tokens new-line
    # define identifier replacement-list new-line
    # define identifier lparen identifier-listopt )
                                                       replacement-list new-line
    # define identifier lparen ... ) replacement-list new-line
    # define identifier lparen identifier-list , ... )
                                                       replacement-list new-line
    # undef identifier new-line
    # line pp-tokens new-line
    # error pp-tokensopt new-line
    # pragma pp-tokensopt new-line
    # new-line
  

text-line:

    pp-tokensopt new-line
  

non-directive:

    pp-tokens new-line
  

lparen:

    a ( character not immediately preceded by white-space
  

replacement-list:

    pp-tokensopt
  

pp-tokens:

    preprocessing-token
    pp-tokens preprocessing-token
  

new-line:

    the new-line character
  

Description

A preprocessing directive consists of a sequence of preprocessing tokens that satisfies the following
constraints: The first token in the sequence is a # preprocessing token that (at the start of translation
phase 4) is either the first character in the source file (optionally after white space containing no
new-line characters) or that follows white space containing at least one new-line character. The last
token in the sequence is the first new-line character that follows the first token in the sequence.167)
A new-line character ends the preprocessing directive even if it occurs within what would otherwise
be an invocation of a function-like macro.

A text line shall not begin with a # preprocessing token. A non-directive shall not begin with any of
the directive names appearing in the syntax.

When in a group that is skipped (6.10.1), the directive syntax is relaxed to allow any sequence of
preprocessing tokens to occur between the directive name and the following new-line character.

Constraints

The only white-space characters that shall appear between preprocessing tokens within a prepro-
cessing directive (from just after the introducing # preprocessing token through just before the
terminating new-line character) are space and horizontal-tab (including spaces that have replaced
comments or possibly other white-space characters in translation phase 3).

Semantics

The implementation can process and skip sections of source files conditionally, include other source
files, and replace macros. These capabilities are called preprocessing, because conceptually they occur
before translation of the resulting translation unit.

The preprocessing tokens within a preprocessing directive are not subject to macro expansion unless
otherwise stated.

EXAMPLE In:

    #define EMPTY
    EMPTY # include <file.h>
  

the sequence of preprocessing tokens on the second line is not a preprocessing directive, because it does not begin with a # at
the start of translation phase 4, even though it will do so after the macro EMPTY has been replaced.

The execution of a non-directive preprocessing directive results in undefined behavior.

6.10.1 Conditional inclusion

Constraints

The expression that controls conditional inclusion shall be an integer constant expression except that:
identifiers (including those lexically identical to keywords) are interpreted as described below;168)
and it may contain unary operator expressions of the form

    defined identifier
  

or

    defined ( identifier )
  

which evaluate to 1 if the identifier is currently defined as a macro name (that is, if it is predefined
or if it has been the subject of a #define preprocessing directive without an intervening #undef
directive with the same subject identifier), 0 if it is not.

Each preprocessing token that remains (in the list of preprocessing tokens that will become the
controlling expression) after all macro replacements have occurred shall be in the lexical form of a
token (6.4).

Semantics

Preprocessing directives of the forms

    # if   constant-expression new-line groupopt
    # elif constant-expression new-line groupopt
  

check whether the controlling constant expression evaluates to nonzero.

Prior to evaluation, macro invocations in the list of preprocessing tokens that will become the control-
ling constant expression are replaced (except for those macro names modified by the defined unary
operator), just as in normal text. If the token defined is generated as a result of this replacement
process or use of the defined unary operator does not match one of the two specified forms prior to
macro replacement, the behavior is undefined. After all replacements due to macro expansion and
the defined unary operator have been performed, all remaining identifiers (including those lexically
identical to keywords) are replaced with the pp-number 0, and then each preprocessing token is
converted into a token. The resulting tokens compose the controlling constant expression which is
evaluated according to the rules of 6.6. For the purposes of this token conversion and evaluation,
all signed integer types and all unsigned integer types act as if they have the same representation
as, respectively, the types intmax_t and uintmax_t defined in the header <stdint.h>.169) This
includes interpreting character constants, which may involve converting escape sequences into
execution character set members. Whether the numeric value for these character constants matches
the value obtained when an identical character constant occurs in an expression (other than within a
#if or #elif directive) is implementation-defined.170) Also, whether a single-character character
constant may have a negative value is implementation-defined.

Preprocessing directives of the forms

    # ifdef identifier new-line groupopt
    # ifndef identifier new-line groupopt
  

check whether the identifier is or is not currently defined as a macro name. Their conditions are
equivalent to #if defined identifier and #if !defined identifier respectively.

Each directive’s condition is checked in order. If it evaluates to false (zero), the group that it
controls is skipped: directives are processed only through the name that determines the directive
in order to keep track of the level of nested conditionals; the rest of the directives’ preprocessing
tokens are ignored, as are the other preprocessing tokens in the group. Only the first group whose
control condition evaluates to true (nonzero) is processed; any following groups are skipped and
their controlling directives are processed as if they were in a group that is skipped. If none of the
conditions evaluates to true, and there is a #else directive, the group controlled by the #else is
processed; lacking a #else directive, all the groups until the #endif are skipped.171)

Forward references: macro replacement (6.10.3), source file inclusion (6.10.2), largest integer types
(7.20.1.5).

6.10.2 Source file inclusion

Constraints

A #include directive shall identify a header or source file that can be processed by the implementa-
tion.

Semantics

A preprocessing directive of the form

    # include < h-char-sequence > new-line
  

searches a sequence of implementation-defined places for a header identified uniquely by the
specified sequence between the < and > delimiters, and causes the replacement of that directive
by the entire contents of the header. How the places are specified or the header identified is
implementation-defined.

A preprocessing directive of the form

    # include " q-char-sequence " new-line
  

causes the replacement of that directive by the entire contents of the source file identified by the
specified sequence between the " delimiters. The named source file is searched for in an implementa-
tion-defined manner. If this search is not supported, or if the search fails, the directive is reprocessed
as if it read

    # include < h-char-sequence > new-line
  

with the identical contained sequence (including > characters, if any) from the original directive.

A preprocessing directive of the form

    # include pp-tokens new-line
  

(that does not match one of the two previous forms) is permitted. The preprocessing tokens after
include in the directive are processed just as in normal text. (Each identifier currently defined as a
macro name is replaced by its replacement list of preprocessing tokens.) The directive resulting after
all replacements shall match one of the two previous forms.172) The method by which a sequence
of preprocessing tokens between a < and a > preprocessing token pair or a pair of " characters is
combined into a single header name preprocessing token is implementation-defined.

The implementation shall provide unique mappings for sequences consisting of one or more nondig-
its or digits (6.4.2.1) followed by a period (.) and a single nondigit. The first character shall not be a
digit. The implementation may ignore distinctions of alphabetical case and restrict the mapping to
eight significant characters before the period.

A #include preprocessing directive may appear in a source file that has been read because of a
#include directive in another file, up to an implementation-defined nesting limit (see 5.2.4.1).

EXAMPLE 1 The most common uses of #include preprocessing directives are as in the following:

    #include <stdio.h>
    #include "myprog.h"
  

EXAMPLE 2 This illustrates macro-replaced #include directives:

    #if VERSION == 1
          #define INCFILE             "vers1.h"
    #elif VERSION == 2
          #define INCFILE             "vers2.h"         // and so on
    #else
          #define INCFILE             "versN.h"
    #endif
    #include INCFILE
  

Forward references: macro replacement (6.10.3).

6.10.3 Macro replacement

Constraints

Two replacement lists are identical if and only if the preprocessing tokens in both have the same
number, ordering, spelling, and white-space separation, where all white-space separations are
considered identical.

An identifier currently defined as an object-like macro shall not be redefined by another #define
preprocessing directive unless the second definition is an object-like macro definition and the two
replacement lists are identical. Likewise, an identifier currently defined as a function-like macro
shall not be redefined by another #define preprocessing directive unless the second definition is a
function-like macro definition that has the same number and spelling of parameters, and the two
replacement lists are identical.

There shall be white-space between the identifier and the replacement list in the definition of an
object-like macro.

If the identifier-list in the macro definition does not end with an ellipsis, the number of arguments
(including those arguments consisting of no preprocessing tokens) in an invocation of a function-like
macro shall equal the number of parameters in the macro definition. Otherwise, there shall be more
arguments in the invocation than there are parameters in the macro definition (excluding the ...).
There shall exist a ) preprocessing token that terminates the invocation.

The identifier __VA_ARGS__ shall occur only in the replacement-list of a function-like macro that
uses the ellipsis notation in the parameters.

A parameter identifier in a function-like macro shall be uniquely declared within its scope.

Semantics

The identifier immediately following the define is called the macro name. There is one name
space for macro names. Any white-space characters preceding or following the replacement list of
preprocessing tokens are not considered part of the replacement list for either form of macro.

If a # preprocessing token, followed by an identifier, occurs lexically at the point at which a prepro-
cessing directive could begin, the identifier is not subject to macro replacement.

A preprocessing directive of the form

    # define identifier replacement-list new-line
  

defines an object-like macro that causes each subsequent instance of the macro name173) to be replaced
by the replacement list of preprocessing tokens that constitute the remainder of the directive. The
replacement list is then rescanned for more macro names as specified below.

A preprocessing directive of the form

    # define identifier lparen identifier-listopt ) replacement-list new-line
    # define identifier lparen ... ) replacement-list new-line
    # define identifier lparen identifier-list , ... ) replacement-list new-line
  

defines a function-like macro with parameters, whose use is similar syntactically to a function call. The
parameters are specified by the optional list of identifiers, whose scope extends from their declaration
in the identifier list until the new-line character that terminates the #define preprocessing directive.
Each subsequent instance of the function-like macro name followed by a ( as the next preprocessing
token introduces the sequence of preprocessing tokens that is replaced by the replacement list
in the definition (an invocation of the macro). The replaced sequence of preprocessing tokens is
terminated by the matching ) preprocessing token, skipping intervening matched pairs of left and
right parenthesis preprocessing tokens. Within the sequence of preprocessing tokens making up an
invocation of a function-like macro, new-line is considered a normal white-space character.

The sequence of preprocessing tokens bounded by the outside-most matching parentheses forms

the list of arguments for the function-like macro. The individual arguments within the list are
separated by comma preprocessing tokens, but comma preprocessing tokens between matching
inner parentheses do not separate arguments. If there are sequences of preprocessing tokens within
the list of arguments that would otherwise act as preprocessing directives,174) the behavior is
undefined.

If there is a ... in the identifier-list in the macro definition, then the trailing arguments, including
any separating comma preprocessing tokens, are merged to form a single item: the variable arguments.
The number of arguments so combined is such that, following merger, the number of arguments is
one more than the number of parameters in the macro definition (excluding the ...).

6.10.3.1 Argument substitution

After the arguments for the invocation of a function-like macro have been identified, argument
substitution takes place. A parameter in the replacement list, unless preceded by a # or ## prepro-
cessing token or followed by a ## preprocessing token (see below), is replaced by the corresponding
argument after all macros contained therein have been expanded. Before being substituted, each
argument’s preprocessing tokens are completely macro replaced as if they formed the rest of the
preprocessing file; no other preprocessing tokens are available.

An identifier __VA_ARGS__ that occurs in the replacement list shall be treated as if it were a parameter,
and the variable arguments shall form the preprocessing tokens used to replace it.

6.10.3.2 The # operator

Constraints

Each # preprocessing token in the replacement list for a function-like macro shall be followed by a
parameter as the next preprocessing token in the replacement list.

Semantics

If, in the replacement list, a parameter is immediately preceded by a # preprocessing token, both
are replaced by a single character string literal preprocessing token that contains the spelling of
the preprocessing token sequence for the corresponding argument. Each occurrence of white space
between the argument’s preprocessing tokens becomes a single space character in the character
string literal. White space before the first preprocessing token and after the last preprocessing token
composing the argument is deleted. Otherwise, the original spelling of each preprocessing token in
the argument is retained in the character string literal, except for special handling for producing
the spelling of string literals and character constants: a \ character is inserted before each " and \
character of a character constant or string literal (including the delimiting " characters), except that
it is implementation-defined whether a \ character is inserted before the \ character beginning a
universal character name. If the replacement that results is not a valid character string literal, the
behavior is undefined. The character string literal corresponding to an empty argument is "". The
order of evaluation of # and ## operators is unspecified.

6.10.3.3 The ## operator

Constraints

A ## preprocessing token shall not occur at the beginning or at the end of a replacement list for
either form of macro definition.

Semantics

If, in the replacement list of a function-like macro, a parameter is immediately preceded or followed
by a ## preprocessing token, the parameter is replaced by the corresponding argument’s preprocess-
ing token sequence; however, if an argument consists of no preprocessing tokens, the parameter is
replaced by a placemarker preprocessing token instead.175)

For both object-like and function-like macro invocations, before the replacement list is reexamined
for more macro names to replace, each instance of a ## preprocessing token in the replacement list

(not from an argument) is deleted and the preceding preprocessing token is concatenated with the
following preprocessing token. Placemarker preprocessing tokens are handled specially: concatena-
tion of two placemarkers results in a single placemarker preprocessing token, and concatenation
of a placemarker with a non-placemarker preprocessing token results in the non-placemarker pre-
processing token. If the result is not a valid preprocessing token, the behavior is undefined. The
resulting token is available for further macro replacement. The order of evaluation of ## operators is
unspecified.

EXAMPLE In the following fragment:

    #define   hash_hash # ## #
    #define   mkstr(a) # a
    #define   in_between(a) mkstr(a)
    #define   join(c, d) in_between(c hash_hash d)
  
    char p[] = join(x, y); // equivalent to
                           // char p[] = "x ## y";
  

The expansion produces, at various stages:

    join(x, y)
  
    in_between(x hash_hash y)
  
    in_between(x ## y)
  
    mkstr(x ## y)
  
    "x ## y"
  

In other words, expanding hash_hash produces a new token, consisting of two adjacent sharp signs, but this new token is
not the ## operator.

6.10.3.4 Rescanning and further replacement

After all parameters in the replacement list have been substituted and # and ## processing has
taken place, all placemarker preprocessing tokens are removed. The resulting preprocessing token
sequence is then rescanned, along with all subsequent preprocessing tokens of the source file, for
more macro names to replace.

If the name of the macro being replaced is found during this scan of the replacement list (not
including the rest of the source file’s preprocessing tokens), it is not replaced. Furthermore, if any
nested replacements encounter the name of the macro being replaced, it is not replaced. These
nonreplaced macro name preprocessing tokens are no longer available for further replacement even
if they are later (re)examined in contexts in which that macro name preprocessing token would
otherwise have been replaced.

The resulting completely macro-replaced preprocessing token sequence is not processed as a prepro-
cessing directive even if it resembles one, but all pragma unary operator expressions within it are
then processed as specified in 6.10.9 below.

EXAMPLE There are cases where it is not clear whether a replacement is nested or not. For example, given the following
macro definitions:

    #define f(a) a*g
    #define g(a) f(a)
  

the invocation

    f(2)(9)
  

may expand to either

    2*f(9)
  

or

    2*9*g
  

Strictly conforming programs are not permitted to depend on such unspecified behavior.

6.10.3.5 Scope of macro definitions

A macro definition lasts (independent of block structure) until a corresponding #undef directive is
encountered or (if none is encountered) until the end of the preprocessing translation unit. Macro
definitions have no significance after translation phase 4.

A preprocessing directive of the form

    # undef identifier new-line
  

causes the specified identifier no longer to be defined as a macro name. It is ignored if the specified
identifier is not currently defined as a macro name.

EXAMPLE 1 The simplest use of this facility is to define a “manifest constant”, as in

    #define TABSIZE 100
  
    int table[TABSIZE];
  

EXAMPLE 2 The following defines a function-like macro whose value is the maximum of its arguments. It has the advantages
of working for any compatible types of the arguments and of generating in-line code without the overhead of function calling.
It has the disadvantages of evaluating one or the other of its arguments a second time (including side effects) and generating
more code than a function if invoked several times. It also cannot have its address taken, as it has none.

    #define max(a, b) ((a)         >   (b) ? (a): (b))
  

The parentheses ensure that the arguments and the resulting expression are bound properly.

EXAMPLE 3 To illustrate the rules for redefinition and reexamination, the sequence

    #define   x        3
    #define   f(a)     f(x * (a))
    #undef    x
    #define   x        2
    #define   g        f
    #define   z        z[0]
    #define   h        g(\~{ }
    #define   m(a)     a(w)
    #define   w        0,1
    #define   t(a)     a
    #define   p()      int
    #define   q(x)     x
    #define   r(x,y)   x ## y
    #define   str(x)   # x
  
    f(y+1) + f(f(z)) % t(t(g)(0) + t)(1);
    g(x+(3,4)-w) | h 5) & m
          (f)^m(m);
    p() i[q()] = { q(1), r(2,3), r(4,), r(,5), r(,) };
    char c[2][6] = { str(hello), str() };
  

results in

    f(2 * (y+1)) + f(2 * (f(2 * (z[0])))) % f(2 * (0)) + t(1);
    f(2 * (2+(3,4)-0,1)) | f(2 * (\~{ } 5)) & f(2 * (0,1))^m(0,1);
    int i[] = { 1, 23, 4, 5, };
    char c[2][6] = { "hello", "" };
  

EXAMPLE 4 To illustrate the rules for creating character string literals and concatenating tokens, the sequence

    #define str(s)      # s
    #define xstr(s)     str(s)
    #define debug(s, t) printf("x" # s "= %d, x" # t "= %s", \
                            x ## s, x ## t)
    #define INCFILE(n) vers ## n
    #define glue(a, b) a ## b
    #define xglue(a, b) glue(a, b)
    #define HIGHLOW     "hello"
    #define LOW         LOW ", world"
  
    debug(1, 2);
    fputs(str(strncmp("abc\0d", "abc", ’\4’) // this goes away
          == 0) str(: @\n), s);
    #include xstr(INCFILE(2).h)
    glue(HIGH, LOW);
    xglue(HIGH, LOW)
  

results in

    printf("x" "1" "= %d, x" "2" "= %s", x1, x2);
    fputs(
      "strncmp(\"abc\\0d\", \"abc\", ’\\4’) == 0" ": @\n",
      s);
    #include "vers2.h"    (after macro replacement, before file access)
    "hello";
    "hello" ", world"
  

or, after concatenation of the character string literals,

    printf("x1= %d, x2= %s", x1, x2);
    fputs(
      "strncmp(\"abc\\0d\", \"abc\", ’\\4’) == 0: @\n",
      s);
    #include "vers2.h"    (after macro replacement, before file access)
    "hello";
    "hello, world"
  

Space around the # and ## tokens in the macro definition is optional.

EXAMPLE 5 To illustrate the rules for placemarker preprocessing tokens, the sequence

    #define t(x,y,z) x ## y ## z
    int j[] = { t(1,2,3), t(,4,5), t(6,,7), t(8,9,),
               t(10,,), t(,11,), t(,,12), t(,,) };
  

results in

    int j[] = { 123, 45, 67, 89,
                10, 11, 12, };
  

EXAMPLE 6 To demonstrate the redefinition rules, the following sequence is valid.

    #define   OBJ_LIKE      (1-1)
    #define   OBJ_LIKE      /* white space */ (1-1) /* other */
    #define   FUNC_LIKE(a)   (a)
    #define   FUNC_LIKE(a)(   /* note the white space */ \
                              a /* other stuff on this line
                                 */)
  

But the following redefinitions are invalid:

    #define   OBJ_LIKE    (0)     // different             token sequence
    #define   OBJ_LIKE    (1 - 1) // different             white space
    #define   FUNC_LIKE(b) (a)    // different             parameter usage
    #define   FUNC_LIKE(b) (b)    // different             parameter spelling
  

EXAMPLE 7 Finally, to show the variable argument list macro facilities:

    #define debug(...)      fprintf(stderr, __VA_ARGS__)
    #define showlist(...)   puts(#__VA_ARGS__)
    #define report(test, ...) ((test)?puts(#test):\
                printf(__VA_ARGS__))
    debug("Flag");
    debug("X = %d\n", x);
    showlist(The first, second, and third items.);
    report(x>y, "x is %d but y is %d", x, y);
  

results in

    fprintf(stderr, "Flag");
    fprintf(stderr, "X = %d\n", x);
    puts("The first, second, and third items.");
    ((x>y)?puts("x>y"):
                printf("x is %d but y is %d", x, y));
  

6.10.4 Line control

Constraints

The string literal of a #line directive, if present, shall be a character string literal.

Semantics

The line number of the current source line is one greater than the number of new-line characters read
or introduced in translation phase 1 (5.1.1.2) while processing the source file to the current token.

A preprocessing directive of the form

    # line digit-sequence new-line
  

causes the implementation to behave as if the following sequence of source lines begins with a source
line that has a line number as specified by the digit sequence (interpreted as a decimal integer). The
digit sequence shall not specify zero, nor a number greater than 2147483647.

A preprocessing directive of the form

    # line digit-sequence " s-char-sequenceopt " new-line
  

sets the presumed line number similarly and changes the presumed name of the source file to be the
contents of the character string literal.

A preprocessing directive of the form

    # line pp-tokens new-line
  

(that does not match one of the two previous forms) is permitted. The preprocessing tokens after
line on the directive are processed just as in normal text (each identifier currently defined as a
macro name is replaced by its replacement list of preprocessing tokens). The directive resulting after
all replacements shall match one of the two previous forms and is then processed as appropriate.176)

6.10.5 Error directive

Semantics

A preprocessing directive of the form

    # error pp-tokensopt new-line
  

causes the implementation to produce a diagnostic message that includes the specified sequence of
preprocessing tokens.

6.10.6 Pragma directive

Semantics

A preprocessing directive of the form

    # pragma pp-tokensopt new-line
  

where the preprocessing token STDC does not immediately follow pragma in the directive (prior to
any macro replacement)177) causes the implementation to behave in an implementation-defined man-
ner. The behavior might cause translation to fail or cause the translator or the resulting program to
behave in a non-conforming manner. Any such pragma that is not recognized by the implementation
is ignored.

If the preprocessing token STDC does immediately follow pragma in the directive (prior to any macro
replacement), then no macro replacement is performed on the directive, and the directive shall have
one of the following forms178) whose meanings are described elsewhere:

    # pragma STDC FP_CONTRACT on-off-switch
    # pragma STDC FENV_ACCESS on-off-switch
    # pragma STDC CX_LIMITED_RANGE on-off-switch
  

on-off-switch: one of

    ON        OFF        DEFAULT
  

Forward references: the FP_CONTRACT pragma (7.12.2), the FENV_ACCESS pragma (7.6.1), the
CX_LIMITED_RANGE pragma (7.3.4).

6.10.7 Null directive

Semantics

A preprocessing directive of the form

    # new-line
  

has no effect.

6.10.8 Predefined macro names

The values of the predefined macros listed in the following subclauses179) (except for __FILE__ and

__LINE__
) remain constant throughout the translation unit.

None of these macro names, nor the identifier defined, shall be the subject of a #define or a #undef
preprocessing directive. Any other predefined macro names shall begin with a leading underscore
followed by an uppercase letter or a second underscore.

The implementation shall not predefine the macro __cplusplus , nor shall it define it in any standard
header.

Forward references: standard headers (7.1.2).

6.10.8.1 Mandatory macros

The following macro names shall be defined by the implementation:

__DATE__
The date of translation of the preprocessing translation unit: a character string literal of
the form "Mmm dd yyyy", where the names of the months are the same as those generated
by the asctime function, and the first character of dd is a space character if the value is
less than 10. If the date of translation is not available, an implementation-defined valid
date shall be supplied.
__FILE__
The presumed name of the current source file (a character string literal).180)
__LINE__
The presumed line number (within the current source file) of the current source line (an
integer constant).180)
__STDC__
The integer constant 1 , intended to indicate a conforming implementation.
__STDC_HOSTED__
The integer constant 1 if the implementation is a hosted implementation or the
integer constant 0 if it is not.
__STDC_VERSION__
The integer constant 201710L.181)
__TIME__
The time of translation of the preprocessing translation unit: a character string literal of
the form "hh:mm:ss" as in the time generated by the asctime function. If the time of
translation is not available, an implementation-defined valid time shall be supplied.

Forward references: the asctime function (7.27.3.1).

6.10.8.2 Environment macros

The following macro names are conditionally defined by the implementation:

__STDC_ISO_10646__
An integer constant of the form yyyymmL (for example, 199712L ). If this
symbol is defined, then every character in the Unicode required set, when stored in an
object of type wchar_t, has the same value as the short identifier of that character. The
Unicode required set consists of all the characters that are defined by ISO/IEC 10646, along
with all amendments and technical corrigenda, as of the specified year and month. If
some other encoding is used, the macro shall not be defined and the actual encoding
used is implementation-defined.
__STDC_MB_MIGHT_NEQ_WC__
The integer constant 1 , intended to indicate that, in the encoding for
wchar_t, a member of the basic character set need not have a code value equal to its
value when used as the lone character in an integer character constant.
__STDC_UTF_16__
The integer constant 1 , intended to indicate that values of type char16_t are
UTF–16 encoded. If some other encoding is used, the macro shall not be defined and the
actual encoding used is implementation-defined.
__STDC_UTF_32__
The integer constant 1 , intended to indicate that values of type char32_t are
UTF–32 encoded. If some other encoding is used, the macro shall not be defined and the
actual encoding used is implementation-defined.

Forward references: common definitions (7.19), unicode utilities (7.28).

6.10.8.3 Conditional feature macros

The following macro names are conditionally defined by the implementation:

__STDC_ANALYZABLE__
The integer constant 1 , intended to indicate conformance to the specifica-
tions in annex L (Analyzability).
__STDC_IEC_559__
The integer constant 1 , intended to indicate conformance to the specifications
in annex F (IEC 60559 floating-point arithmetic).
__STDC_IEC_559_COMPLEX__
The integer constant 1 , intended to indicate adherence to the specifi-
cations in annex G (IEC 60559 compatible complex arithmetic).
__STDC_LIB_EXT1__
The integer constant 201710L, intended to indicate support for the extensions
defined in annex K (Bounds-checking interfaces).182)
__STDC_NO_ATOMICS__
The integer constant 1, intended to indicate that the implementation does
not support atomic types (including the _Atomic type qualifier) and the <stdatomic.h>
header.
__STDC_NO_COMPLEX__
The integer constant 1, intended to indicate that the implementation does
not support complex types or the <complex.h> header.
__STDC_NO_THREADS__
The integer constant 1, intended to indicate that the implementation does
not support the <threads.h> header.
__STDC_NO_VLA__
The integer constant 1 , intended to indicate that the implementation does not
support variable length arrays or variably modified types.

An implementation that defines __STDC_NO_COMPLEX__ shall not define __STDC_IEC_559_COMPLEX__ .

6.10.9 Pragma operator

Semantics

A unary operator expression of the form:

    _Pragma ( string-literal )
  

is processed as follows: The string literal is destringized by deleting any encoding prefix, deleting
the leading and trailing double-quotes, replacing each escape sequence \" by a double-quote, and
replacing each escape sequence \\ by a single backslash. The resulting sequence of characters
is processed through translation phase 3 to produce preprocessing tokens that are executed as if
they were the pp-tokens in a pragma directive. The original four preprocessing tokens in the unary
operator expression are removed.

EXAMPLE A directive of the form:

    #pragma listing on "..\listing.dir"
  

can also be expressed as:

    _Pragma ("listing on \"..\\listing.dir\"")
  

The latter form is processed in the same way whether it appears literally as shown, or results from macro replacement, as in:

    #define LISTING(x) PRAGMA(listing on #x)
    #define PRAGMA(x) _Pragma(#x)
  
    LISTING (..\listing.dir)
  

6.11 Future language directions

6.11.1 Floating types

Future standardization may include additional floating-point types, including those with greater
range, precision, or both than long double.

6.11.2 Linkages of identifiers

Declaring an identifier with internal linkage at file scope without the static storage-class specifier
is an obsolescent feature.

6.11.3 External names

Restriction of the significance of an external name to fewer than 255 characters (considering each
universal character name or extended source character as a single character) is an obsolescent feature
that is a concession to existing implementations.

6.11.4 Character escape sequences

Lowercase letters as escape sequences are reserved for future standardization. Other characters may
be used in extensions.

6.11.5 Storage-class specifiers

The placement of a storage-class specifier other than at the beginning of the declaration specifiers in
a declaration is an obsolescent feature.

6.11.6 Function declarators

The use of function declarators with empty parentheses (not prototype-format parameter type
declarators) is an obsolescent feature.

6.11.7 Function definitions

The use of function definitions with separate parameter identifier and declaration lists (not prototype-
format parameter type and identifier declarators) is an obsolescent feature.

6.11.8 Pragma directives

Pragmas whose first preprocessing token is STDC are reserved for future standardization.

6.11.9 Predefined macro names

Macro names beginning with __STDC_ are reserved for future standardization.

7. Library

7.1 Introduction

7.1.1 Definitions of terms

A string is a contiguous sequence of characters terminated by and including the first null character.
The term multibyte string is sometimes used instead to emphasize special processing given to
multibyte characters contained in the string or to avoid confusion with a wide string. A pointer to
a string is a pointer to its initial (lowest addressed) character. The length of a string is the number
of bytes preceding the null character and the value of a string is the sequence of the values of the
contained characters, in order.

The decimal-point character is the character used by functions that convert floating-point numbers
to or from character sequences to denote the beginning of the fractional part of such character
sequences.183) It is represented in the text and examples by a period, but may be changed by the
setlocale function.

A null wide character is a wide character with code value zero.

A wide string is a contiguous sequence of wide characters terminated by and including the first null
wide character. A pointer to a wide string is a pointer to its initial (lowest addressed) wide character.
The length of a wide string is the number of wide characters preceding the null wide character and the
value of a wide string is the sequence of code values of the contained wide characters, in order.

A shift sequence is a contiguous sequence of bytes within a multibyte string that (potentially) causes
a change in shift state (see 5.2.1.2). A shift sequence shall not have a corresponding wide character;
it is instead taken to be an adjunct to an adjacent multibyte character.184)

Forward references: character handling (7.4), the setlocale function (7.11.1.1).

7.1.2 Standard headers

Each library function is declared, with a type that includes a prototype, in a header,185) whose contents
are made available by the #include preprocessing directive. The header declares a set of related
functions, plus any necessary types and additional macros needed to facilitate their use. Declarations
of types described in this clause shall not include type qualifiers, unless explicitly stated otherwise.

The standard headers are186)

<assert.h> <math.h> <stdlib.h>
<complex.h> <setjmp.h> <stdnoreturn.h>
<ctype.h> <signal.h> <string.h>
<errno.h> <stdalign.h> <tgmath.h>
<fenv.h> <stdarg.h> <threads.h>
<float.h> <stdatomic.h> <time.h>
<inttypes.h> <stdbool.h> <uchar.h>
<iso646.h> <stddef.h> <wchar.h>
<limits.h> <stdint.h> <wctype.h>
<locale.h> <stdio.h>

If a file with the same name as one of the above < and > delimited sequences, not provided as part of

the implementation, is placed in any of the standard places that are searched for included source
files, the behavior is undefined.

Standard headers may be included in any order; each may be included more than once in a given
scope, with no effect different from being included only once, except that the effect of including
<assert.h> depends on the definition of NDEBUG (see 7.2). If used, a header shall be included outside
of any external declaration or definition, and it shall first be included before the first reference to
any of the functions or objects it declares, or to any of the types or macros it defines. However, if
an identifier is declared or defined in more than one header, the second and subsequent associated
headers may be included after the initial reference to the identifier. The program shall not have any
macros with names lexically identical to keywords currently defined prior to the inclusion of the
header or when any macro defined in the header is expanded.

Any definition of an object-like macro described in this clause shall expand to code that is fully
protected by parentheses where necessary, so that it groups in an arbitrary expression as if it were a
single identifier.

Any declaration of a library function shall have external linkage.

A summary of the contents of the standard headers is given in annex B.

Forward references: diagnostics (7.2).

7.1.3 Reserved identifiers

Each header declares or defines all identifiers listed in its associated subclause, and optionally
declares or defines identifiers listed in its associated future library directions subclause and identifiers
which are always reserved either for any use or for use as file scope identifiers.

No other identifiers are reserved. If the program declares or defines an identifier in a context in
which it is reserved (other than as allowed by 7.1.4), or defines a reserved identifier as a macro name,
the behavior is undefined.

If the program removes (with #undef) any macro definition of an identifier in the first group listed
above, the behavior is undefined.

7.1.4 Use of library functions

Each of the following statements applies unless explicitly stated otherwise in the detailed descrip-
tions that follow: If an argument to a function has an invalid value (such as a value outside the
domain of the function, or a pointer outside the address space of the program, or a null pointer, or a
pointer to non-modifiable storage when the corresponding parameter is not const-qualified) or a

type (after promotion) not expected by a function with variable number of arguments, the behavior
is undefined. If a function argument is described as being an array, the pointer actually passed
to the function shall have a value such that all address computations and accesses to objects (that
would be valid if the pointer did point to the first element of such an array) are in fact valid. Any
function declared in a header may be additionally implemented as a function-like macro defined
in the header, so if a library function is declared explicitly when its header is included, one of the
techniques shown below can be used to ensure the declaration is not affected by such a macro. Any
macro definition of a function can be suppressed locally by enclosing the name of the function in
parentheses, because the name is then not followed by the left parenthesis that indicates expansion of
a macro function name. For the same syntactic reason, it is permitted to take the address of a library
function even if it is also defined as a macro.189) The use of #undef to remove any macro definition
will also ensure that an actual function is referred to. Any invocation of a library function that is
implemented as a macro shall expand to code that evaluates each of its arguments exactly once,
fully protected by parentheses where necessary, so it is generally safe to use arbitrary expressions as
arguments.190) Likewise, those function-like macros described in the following subclauses may be
invoked in an expression anywhere a function with a compatible return type could be called.191) All
object-like macros listed as expanding to integer constant expressions shall additionally be suitable
for use in #if preprocessing directives.

Provided that a library function can be declared without reference to any type defined in a header, it
is also permissible to declare the function and use it without including its associated header.

There is a sequence point immediately before a library function returns.

The functions in the standard library are not guaranteed to be reentrant and may modify objects
with static or thread storage duration.192)

Unless explicitly stated otherwise in the detailed descriptions that follow, library functions shall
prevent data races as follows: A library function shall not directly or indirectly access objects
accessible by threads other than the current thread unless the objects are accessed directly or
indirectly via the function’s arguments. A library function shall not directly or indirectly modify
objects accessible by threads other than the current thread unless the objects are accessed directly
or indirectly via the function’s non-const arguments.193) Implementations may share their own
internal objects between threads if the objects are not visible to users and are protected against data
races.

Unless otherwise specified, library functions shall perform all operations solely within the current
thread if those operations have effects that are visible to users.194)

EXAMPLE The function atoi may be used in any of several ways:

or

    #include <stdlib.h>
    const char *str;
    /* ... */
    i = (atoi)(str);
  

7.2 Diagnostics <assert.h>

The header <assert.h> defines the assert and static_assert macros and refers to another
macro,

    NDEBUG
  

which is not defined by <assert.h>. If NDEBUG is defined as a macro name at the point in the source
file where <assert.h> is included, the assert macro is defined simply as

    #define assert(ignore) ((void)0)
  

The assert macro is redefined according to the current state of NDEBUG each time that <assert.h>
is included.

The assert macro shall be implemented as a macro, not as an actual function. If the macro definition
is suppressed in order to access an actual function, the behavior is undefined.

The macro

    static_assert
  

expands to _Static_assert .

7.2.1 Program diagnostics

7.2.1.1 The assert macro

Synopsis

      #include <assert.h>
      void assert(scalar expression);
    

Description

The assert macro puts diagnostic tests into programs; it expands to a void expression. When it
is executed, if expression (which shall have a scalar type) is false (that is, compares equal to 0),
the assert macro writes information about the particular call that failed (including the text of the
argument, the name of the source file, the source line number, and the name of the enclosing function

Returns

The assert macro returns no value.

Forward references: the abort function (7.22.4.1).

7.3 Complex arithmetic <complex.h>

7.3.1 Introduction

The header <complex.h> defines macros and declares functions that support complex arithmetic.196)

Implementations that define the macro __STDC_NO_COMPLEX__ need not provide this header nor
support any of its facilities.

Each synopsis other than for the CMPLX macros specifies a family of functions consisting of a principal
function with one or more double complex parameters and a double complex or double return
value; and other functions with the same name but with f and l suffixes which are corresponding
functions with float and long double parameters and return values.

The macro

    complex
  

expands to _Complex ; the macro

    _Complex_I
  

expands to a constant expression of type const float _Complex, with the value of the imaginary
unit.197)

The macros

    imaginary
  

and

    _Imaginary_I
  

are defined if and only if the implementation supports imaginary types;198) if defined, they expand
to _Imaginary and a constant expression of type const float _Imaginary with the value of the
imaginary unit.

The macro

    I
  

expands to either _Imaginary_I or _Complex_I . If _Imaginary_I is not defined, I shall expand to
_Complex_I .

Notwithstanding the provisions of 7.1.3, a program may undefine and perhaps then redefine the
macros complex, imaginary, and I.

Forward references: the CMPLX macros (7.3.9.3), IEC 60559-compatible complex arithmetic (an-
nex G).

7.3.2 Conventions

Values are interpreted as radians, not degrees. An implementation may set errno but is not required
to.

7.3.3 Branch cuts

Some of the functions below have branch cuts, across which the function is discontinuous. For
implementations with a signed zero (including all IEC 60559 implementations) that follow the
specifications of annex G, the sign of zero distinguishes one side of a cut from another so the function
is continuous (except for format limitations) as the cut is approached from either side. For example,

for the square root function, which has a branch cut along the negative real axis, the top of the
cut, with imaginary part +0 , maps to the positive imaginary axis, and the bottom of the cut, with
imaginary part-0 , maps to the negative imaginary axis.

Implementations that do not support a signed zero (see annex F) cannot distinguish the sides of
branch cuts. These implementations shall map a cut so the function is continuous as the cut is
approached coming around the finite endpoint of the cut in a counter clockwise direction. (Branch
cuts for the functions specified here have just one finite endpoint.) For example, for the square root
function, coming counter clockwise around the finite endpoint of the cut along the negative real axis
approaches the cut from above, so the cut maps to the positive imaginary axis.

7.3.4 The CX_LIMITED_RANGE pragma

Synopsis

      #include <complex.h>
      #pragma STDC CX_LIMITED_RANGE on-off-switch
    

Description

The usual mathematical formulas for complex multiply, divide, and absolute value are problem-
atic because of their treatment of infinities and because of undue overflow and underflow. The
CX_LIMITED_RANGE pragma can be used to inform the implementation that (where the state is “on”)
the usual mathematical formulas are acceptable.199) The pragma can occur either outside external
declarations or preceding all explicit declarations and statements inside a compound statement.
When outside external declarations, the pragma takes effect from its occurrence until another
CX_LIMITED_RANGE pragma is encountered, or until the end of the translation unit. When inside a
compound statement, the pragma takes effect from its occurrence until another CX_LIMITED_RANGE
pragma is encountered (including within a nested compound statement), or until the end of the
compound statement; at the end of a compound statement the state for the pragma is restored to
its condition just before the compound statement. If this pragma is used in any other context, the
behavior is undefined. The default state for the pragma is “off”.

7.3.5 Trigonometric functions

7.3.5.1 The cacos functions

Synopsis

      #include <complex.h>
      double complex cacos(double complex z);
      float complex cacosf(float complex z);
      long double complex cacosl(long double complex z);
    

Description

The cacos functions compute the complex arc cosine of z, with branch cuts outside the interval
[−1, +1] along the real axis.

Returns

The cacos functions return the complex arc cosine value, in the range of a strip mathematically
unbounded along the imaginary axis and in the interval [0, π] along the real axis.

7.3.5.2 The casin functions

Synopsis

      #include <complex.h>
      double complex casin(double complex z);
      float complex casinf(float complex z);
      long double complex casinl(long double complex z);
    

Description

The casin functions compute the complex arc sine of z, with branch cuts outside the interval
[−1, +1] along the real axis.

Returns

The casin functions return the complex arc sine value, in the range of a strip mathematically
unbounded along the imaginary axis and in the interval [−π/2, +π/2] along the real axis.

7.3.5.3 The catan functions

Synopsis

      #include <complex.h>
      double complex catan(double complex z);
      float complex catanf(float complex z);
      long double complex catanl(long double complex z);
    

Description

The catan functions compute the complex arc tangent of z, with branch cuts outside the interval
[−i, +i] along the imaginary axis.

Returns

The catan functions return the complex arc tangent value, in the range of a strip mathematically
unbounded along the imaginary axis and in the interval [−π/2, +π/2] along the real axis.

7.3.5.4 The ccos functions

Synopsis

      #include <complex.h>
      double complex ccos(double complex z);
      float complex ccosf(float complex z);
      long double complex ccosl(long double complex z);
    

Description

The ccos functions compute the complex cosine of z.

Returns

The ccos functions return the complex cosine value.

7.3.5.5 The csin functions

Synopsis

      #include <complex.h>
      double complex csin(double complex z);
      float complex csinf(float complex z);
      long double complex csinl(long double complex z);
    

Description

The csin functions compute the complex sine of z.

Returns

The csin functions return the complex sine value.

7.3.5.6 The ctan functions

Synopsis

      #include <complex.h>
      double complex ctan(double complex z);
      float complex ctanf(float complex z);
      long double complex ctanl(long double complex z);
    

Description

The ctan functions compute the complex tangent of z.

Returns

The ctan functions return the complex tangent value.

7.3.6 Hyperbolic functions

7.3.6.1 The cacosh functions

Synopsis

      #include <complex.h>
      double complex cacosh(double complex z);
      float complex cacoshf(float complex z);
      long double complex cacoshl(long double complex z);
    

Description

The cacosh functions compute the complex arc hyperbolic cosine of z, with a branch cut at values
less than 1 along the real axis.

Returns

The cacosh functions return the complex arc hyperbolic cosine value, in the range of a half-strip of
nonnegative values along the real axis and in the interval [−iπ, +iπ] along the imaginary axis.

7.3.6.2 The casinh functions

Synopsis

      #include <complex.h>
      double complex casinh(double complex z);
      float complex casinhf(float complex z);
      long double complex casinhl(long double complex z);
    

Description

The casinh functions compute the complex arc hyperbolic sine of z, with branch cuts outside the
interval [−i, +i] along the imaginary axis.

Returns

The casinh functions return the complex arc hyperbolic sine value, in the range of a strip math-
ematically unbounded along the real axis and in the interval [−iπ/2, +iπ/2] along the imaginary
axis.

7.3.6.3 The catanh functions

Synopsis

      #include <complex.h>
      double complex catanh(double complex z);
      float complex catanhf(float complex z);
      long double complex catanhl(long double complex z);
    

Description

The catanh functions compute the complex arc hyperbolic tangent of z, with branch cuts outside
the interval [−1, +1] along the real axis.

Returns

The catanh functions return the complex arc hyperbolic tangent value, in the range of a strip
mathematically unbounded along the real axis and in the interval [−iπ/2, +iπ/2] along the imaginary
axis.

7.3.6.4 The ccosh functions

Synopsis

      #include <complex.h>
      double complex ccosh(double complex z);
      float complex ccoshf(float complex z);
      long double complex ccoshl(long double complex z);
    

Description

The ccosh functions compute the complex hyperbolic cosine of z.

Returns

The ccosh functions return the complex hyperbolic cosine value.

7.3.6.5 The csinh functions

Synopsis

      #include <complex.h>
      double complex csinh(double complex z);
      float complex csinhf(float complex z);
      long double complex csinhl(long double complex z);
    

Description

The csinh functions compute the complex hyperbolic sine of z.

Returns

The csinh functions return the complex hyperbolic sine value.

7.3.6.6 The ctanh functions

Synopsis

      #include <complex.h>
      double complex ctanh(double complex z);
      float complex ctanhf(float complex z);
      long double complex ctanhl(long double complex z);
    

Description

The ctanh functions compute the complex hyperbolic tangent of z.

Returns

The ctanh functions return the complex hyperbolic tangent value.

7.3.7 Exponential and logarithmic functions

7.3.7.1 The cexp functions

Synopsis

      #include <complex.h>
      double complex cexp(double complex z);
      float complex cexpf(float complex z);
    
    long double complex cexpl(long double complex z);
  

Description

The cexp functions compute the complex base-e exponential of z.

Returns

The cexp functions return the complex base-e exponential value.

7.3.7.2 The clog functions

Synopsis

      #include <complex.h>
      double complex clog(double complex z);
      float complex clogf(float complex z);
      long double complex clogl(long double complex z);
    

Description

The clog functions compute the complex natural (base-e) logarithm of z, with a branch cut along
the negative real axis.

Returns

The clog functions return the complex natural logarithm value, in the range of a strip mathematically
unbounded along the real axis and in the interval [−iπ, +iπ] along the imaginary axis.

7.3.8 Power and absolute-value functions

7.3.8.1 The cabs functions

Synopsis

      #include <complex.h>
      double cabs(double complex z);
      float cabsf(float complex z);
      long double cabsl(long double complex z);
    

Description

The cabs functions compute the complex absolute value (also called norm, modulus, or magnitude)
of z.

Returns

The cabs functions return the complex absolute value.

7.3.8.2 The cpow functions

Synopsis

      #include <complex.h>
      double complex cpow(double complex x, double complex y);
      float complex cpowf(float complex x, float complex y);
      long double complex cpowl(long double complex x,
            long double complex y);
    

Description

The cpow functions compute the complex power function xy , with a branch cut for the first parameter
along the negative real axis.

Returns

The cpow functions return the complex power function value.

7.3.8.3 The csqrt functions

Synopsis

      #include <complex.h>
      double complex csqrt(double complex z);
      float complex csqrtf(float complex z);
      long double complex csqrtl(long double complex z);
    

Description

The csqrt functions compute the complex square root of z, with a branch cut along the negative
real axis.

Returns

The csqrt functions return the complex square root value, in the range of the right half-plane
(including the imaginary axis).

7.3.9 Manipulation functions

7.3.9.1 The carg functions

Synopsis

      #include <complex.h>
      double carg(double complex z);
      float cargf(float complex z);
      long double cargl(long double complex z);
    

Description

The carg functions compute the argument (also called phase angle) of z, with a branch cut along
the negative real axis.

Returns

The carg functions return the value of the argument in the interval [−π, +π].

7.3.9.2 The cimag functions

Synopsis

      #include <complex.h>
      double cimag(double complex z);
      float cimagf(float complex z);
      long double cimagl(long double complex z);
    

Description

The cimag functions compute the imaginary part of z.200)

Returns

The cimag functions return the imaginary part value (as a real).

7.3.9.3 The CMPLX macros

Synopsis

      #include <complex.h>
      double complex CMPLX(double x, double y);
      float complex CMPLXF(float x, float y);
      long double complex CMPLXL(long double x, long double y);
    

Description

The CMPLX macros expand to an expression of the specified complex type, with the real part having
the (converted) value of x and the imaginary part having the (converted) value of y. The resulting
expression shall be suitable for use as an initializer for an object with static or thread storage duration,
provided both arguments are likewise suitable.

Returns

The CMPLX macros return the complex value x + iy.

NOTE These macros act as if the implementation supported imaginary types and the definitions were:

#define CMPLX(x, y) ((double complex)((double)(x) + \

    _Imaginary_I * (double)(y)))
  

#define CMPLXF(x, y) ((float complex)((float)(x) + \

    _Imaginary_I * (float)(y)))
  

#define CMPLXL(x, y) ((long double complex)((long double)(x) + \

    _Imaginary_I * (long double)(y)))
  

7.3.9.4 The conj functions

Synopsis

      #include <complex.h>
      double complex conj(double complex z);
      float complex conjf(float complex z);
      long double complex conjl(long double complex z);
    

Description

The conj functions compute the complex conjugate of z, by reversing the sign of its imaginary part.

Returns

The conj functions return the complex conjugate value.

7.3.9.5 The cproj functions

Synopsis

      #include <complex.h>
      double complex cproj(double complex z);
      float complex cprojf(float complex z);
      long double complex cprojl(long double complex z);
    

Description

The cproj functions compute a projection of z onto the Riemann sphere: z projects to z except that
all complex infinities (even those with one infinite part and one NaN part) project to positive infinity
on the real axis. If z has an infinite part, then cproj(z) is equivalent to

    INFINITY + I * copysign(0.0, cimag(z))
  

Returns

The cproj functions return the value of the projection onto the Riemann sphere.

7.3.9.6 The creal functions

Synopsis

      #include <complex.h>
      double creal(double complex z);
      float crealf(float complex z);
      long double creall(long double complex z);
    

Description

The creal functions compute the real part of z.201)

Returns

The creal functions return the real part value.

7.4 Character handling <ctype.h>

The header <ctype.h> declares several functions useful for classifying and mapping characters.202)
In all cases the argument is an int, the value of which shall be representable as an unsigned char
or shall equal the value of the macro EOF. If the argument has any other value, the behavior is
undefined.

The behavior of these functions is affected by the current locale. Those functions that have locale-
specific aspects only when not in the "C" locale are noted below.

The term printing character refers to a member of a locale-specific set of characters, each of which
occupies one printing position on a display device; the term control character refers to a member of a
locale-specific set of characters that are not printing characters.203) All letters and digits are printing
characters.

Forward references: EOF (7.21.1), localization (7.11).

7.4.1 Character classification functions

The functions in this subclause return nonzero (true) if and only if the value of the argument c
conforms to that in the description of the function.

7.4.1.1 The isalnum function

Synopsis

      #include <ctype.h>
      int isalnum(int c);
    

Description

The isalnum function tests for any character for which isalpha or isdigit is true.

7.4.1.2 The isalpha function

Synopsis

      #include <ctype.h>
      int isalpha(int c);
    

Description

The isalpha function tests for any character for which isupper or islower is true, or any character
that is one of a locale-specific set of alphabetic characters for which none of iscntrl, isdigit,
ispunct, or isspace is true.204) In the "C" locale, isalpha returns true only for the characters for
which isupper or islower is true.

7.4.1.3 The isblank function

Synopsis

      #include <ctype.h>
      int isblank(int c);
    

Description

The isblank function tests for any character that is a standard blank character or is one of a locale-
specific set of characters for which isspace is true and that is used to separate words within a line
of text. The standard blank characters are the following: space (’ ’ ), and horizontal tab (’\t’ ). In
the "C" locale, isblank returns true only for the standard blank characters.

7.4.1.4 The iscntrl function

Synopsis

      #include <ctype.h>
      int iscntrl(int c);
    

Description

The iscntrl function tests for any control character.

7.4.1.5 The isdigit function

Synopsis

      #include <ctype.h>
      int isdigit(int c);
    

Description

The isdigit function tests for any decimal-digit character (as defined in 5.2.1).

7.4.1.6 The isgraph function

Synopsis

      #include <ctype.h>
      int isgraph(int c);
    

Description

The isgraph function tests for any printing character except space (’ ’ ).

7.4.1.7 The islower function

Synopsis

      #include <ctype.h>
      int islower(int c);
    

Description

The islower function tests for any character that is a lowercase letter or is one of a locale-specific set
of characters for which none of iscntrl, isdigit, ispunct, or isspace is true. In the "C" locale,
islower returns true only for the lowercase letters (as defined in 5.2.1).

7.4.1.8 The isprint function

Synopsis

      #include <ctype.h>
      int isprint(int c);
    

Description

The isprint function tests for any printing character including space (’ ’ ).

7.4.1.9 The ispunct function

Synopsis

      #include <ctype.h>
      int ispunct(int c);
    

Description

The ispunct function tests for any printing character that is one of a locale-specific set of punctuation
characters for which neither isspace nor isalnum is true. In the "C" locale, ispunct returns true
for every printing character for which neither isspace nor isalnum is true.

7.4.1.10 The isspace function

Synopsis

      #include <ctype.h>
      int isspace(int c);
    

Description

The isspace function tests for any character that is a standard white-space character or is one of
a locale-specific set of characters for which isalnum is false. The standard white-space characters
are the following: space (’ ’ ), form feed (’\f’ ), new-line (’\n’ ), carriage return (’\r’ ), horizontal
tab (’\t’ ), and vertical tab (’\v’ ). In the "C" locale, isspace returns true only for the standard
white-space characters.

7.4.1.11 The isupper function

Synopsis

      #include <ctype.h>
      int isupper(int c);
    

Description

The isupper function tests for any character that is an uppercase letter or is one of a locale-specific
set of characters for which none of iscntrl, isdigit, ispunct, or isspace is true. In the "C" locale,
isupper returns true only for the uppercase letters (as defined in 5.2.1).

7.4.1.12 The isxdigit function

Synopsis

      #include <ctype.h>
      int isxdigit(int c);
    

Description

The isxdigit function tests for any hexadecimal-digit character (as defined in 6.4.4.1).

7.4.2 Character case mapping functions

7.4.2.1 The tolower function

Synopsis

      #include <ctype.h>
      int tolower(int c);
    

Description

The tolower function converts an uppercase letter to a corresponding lowercase letter.

Returns

If the argument is a character for which isupper is true and there are one or more corresponding
characters, as specified by the current locale, for which islower is true, the tolower function returns
one of the corresponding characters (always the same one for any given locale); otherwise, the
argument is returned unchanged.

7.4.2.2 The toupper function

Synopsis

      #include <ctype.h>
      int toupper(int c);
    

Description

The toupper function converts a lowercase letter to a corresponding uppercase letter.

Returns

If the argument is a character for which islower is true and there are one or more corresponding
characters, as specified by the current locale, for which isupper is true, the toupper function returns
one of the corresponding characters (always the same one for any given locale); otherwise, the
argument is returned unchanged.

7.5 Errors <errno.h>

The header <errno.h> defines several macros, all relating to the reporting of error conditions.

The macros are

    EDOM
    EILSEQ
    ERANGE
  

which expand to integer constant expressions with type int, distinct positive values, and which are
suitable for use in #if preprocessing directives; and

    errno
  

which expands to a modifiable lvalue205) that has type int and thread local storage duration, the
value of which is set to a positive error number by several library functions. If a macro definition is
suppressed in order to access an actual object, or a program defines an identifier with the name
errno, the behavior is undefined.

The value of errno in the initial thread is zero at program startup (the initial value of errno in other
threads is an indeterminate value), but is never set to zero by any library function.206) The value of
errno may be set to nonzero by a library function call whether or not there is an error, provided the
use of errno is not documented in the description of the function in this International Standard.

Additional macro definitions, beginning with E and a digit or E and an uppercase letter,207) may also
be specified by the implementation.

7.6 Floating-point environment <fenv.h>

The header <fenv.h> defines several macros, and declares types and functions that provide access to
the floating-point environment. The floating-point environment refers collectively to any floating-point
status flags and control modes supported by the implementation.208) A floating-point status flag is a
system variable whose value is set (but never cleared) when a floating-point exception is raised, which
occurs as a side effect of exceptional floating-point arithmetic to provide auxiliary information.209)
A floating-point control mode is a system variable whose value may be set by the user to affect the
subsequent behavior of floating-point arithmetic.

The floating-point environment has thread storage duration. The initial state for a thread’s floating-
point environment is the current state of the floating-point environment of the thread that creates it
at the time of creation.

Certain programming conventions support the intended model of use for the floating-point environ-
ment:210)

The type

    fenv_t
  

represents the entire floating-point environment.

The type

    fexcept_t
  

represents the floating-point status flags collectively, including any status the implementation
associates with the flags.

Each of the macros

    FE_DIVBYZERO
    FE_INEXACT
    FE_INVALID
    FE_OVERFLOW
    FE_UNDERFLOW
  

is defined if and only if the implementation supports the floating-point exception by means of
the functions in 7.6.2.211) Additional implementation-defined floating-point exceptions, with
macro definitions beginning with FE_ and an uppercase letter,212) may also be specified by the

implementation. The defined macros expand to integer constant expressions with values such that
bitwise ORs of all combinations of the macros result in distinct values, and furthermore, bitwise
ANDs of all combinations of the macros result in zero.213)

The macro

    FE_ALL_EXCEPT
  

is simply the bitwise OR of all floating-point exception macros defined by the implementation. If no
such macros are defined, FE_ALL_EXCEPT shall be defined as 0.

Each of the macros

    FE_DOWNWARD
    FE_TONEAREST
    FE_TOWARDZERO
    FE_UPWARD
  

is defined if and only if the implementation supports getting and setting the represented rounding
direction by means of the fegetround and fesetround functions. Additional implementation-
defined rounding directions, with macro definitions beginning with FE_ and an uppercase letter,214)
may also be specified by the implementation. The defined macros expand to integer constant
expressions whose values are distinct nonnegative values.215)

The macro

    FE_DFL_ENV
  

represents the default floating-point environment — the one installed at program startup — and has
type “pointer to const-qualified fenv_t”. It can be used as an argument to <fenv.h> functions that
manage the floating-point environment.

Additional implementation-defined environments, with macro definitions beginning with FE_ and
an uppercase letter,216) and having type “pointer to const-qualified fenv_t”, may also be specified
by the implementation.

7.6.1 The FENV_ACCESS pragma

Synopsis

      #include <fenv.h>
      #pragma STDC FENV_ACCESS on-off-switch
    

Description

The FENV_ACCESS pragma provides a means to inform the implementation when a program might
access the floating-point environment to test floating-point status flags or run under non-default
floating-point control modes.217) The pragma shall occur either outside external declarations or
preceding all explicit declarations and statements inside a compound statement. When outside
external declarations, the pragma takes effect from its occurrence until another FENV_ACCESS pragma
is encountered, or until the end of the translation unit. When inside a compound statement, the
pragma takes effect from its occurrence until another FENV_ACCESS pragma is encountered (including
within a nested compound statement), or until the end of the compound statement; at the end of a
compound statement the state for the pragma is restored to its condition just before the compound

statement. If this pragma is used in any other context, the behavior is undefined. If part of a
program tests floating-point status flags, sets floating-point control modes, or runs under non-
default mode settings, but was translated with the state for the FENV_ACCESS pragma “off”, the
behavior is undefined. The default state (“on” or “off”) for the pragma is implementation-defined.
(When execution passes from a part of the program translated with FENV_ACCESS “off” to a part
translated with FENV_ACCESS “on”, the state of the floating-point status flags is unspecified and the
floating-point control modes have their default settings.)

EXAMPLE

    #include <fenv.h>
    void f(double x)
    {
          #pragma STDC FENV_ACCESS ON
          void g(double);
          void h(double);
          /* ... */
          g(x + 1);
          h(x + 1);
          /* ... */
    }
  

If the function g might depend on status flags set as a side effect of the first x + 1, or if the second x + 1 might depend on
control modes set as a side effect of the call to function g, then the program shall contain an appropriately placed invocation
of #pragma STDC FENV_ACCESS ON.218)

7.6.2 Floating-point exceptions

The following functions provide access to the floating-point status flags.219) The int input argument
for the functions represents a subset of floating-point exceptions, and can be zero or the bitwise
OR of one or more floating-point exception macros, for example FE_OVERFLOW | FE_INEXACT. For
other argument values, the behavior of these functions is undefined.

7.6.2.1 The feclearexcept function

Synopsis

      #include <fenv.h>
      int feclearexcept(int excepts);
    

Description

The feclearexcept function attempts to clear the supported floating-point exceptions represented
by its argument.

Returns

The feclearexcept function returns zero if the excepts argument is zero or if all the specified
exceptions were successfully cleared. Otherwise, it returns a nonzero value.

7.6.2.2 The fegetexceptflag function

Synopsis

      #include <fenv.h>
      int fegetexceptflag(fexcept_t *flagp,
            int excepts);
    

Description

The fegetexceptflag function attempts to store an implementation-defined representation of the
states of the floating-point status flags indicated by the argument excepts in the object pointed to
by the argument flagp.

Returns

The fegetexceptflag function returns zero if the representation was successfully stored. Otherwise,
it returns a nonzero value.

7.6.2.3 The feraiseexcept function

Synopsis

      #include <fenv.h>
      int feraiseexcept(int excepts);
    

Description

The feraiseexcept function attempts to raise the supported floating-point exceptions represented
by its argument.220) The order in which these floating-point exceptions are raised is unspecified,
except as stated in F.8.6. Whether the feraiseexcept function additionally raises the “inexact”
floating-point exception whenever it raises the “overflow” or “underflow” floating-point exception
is implementation-defined.

Returns

The feraiseexcept function returns zero if the excepts argument is zero or if all the specified
exceptions were successfully raised. Otherwise, it returns a nonzero value.

7.6.2.4 The fesetexceptflag function

Synopsis

      #include <fenv.h>
      int fesetexceptflag(const fexcept_t *flagp,
            int excepts);
    

Description

The fesetexceptflag function attempts to set the floating-point status flags indicated by the
argument excepts to the states stored in the object pointed to by flagp. The value of *flagp shall
have been set by a previous call to fegetexceptflag whose second argument represented at least
those floating-point exceptions represented by the argument excepts. This function does not raise
floating-point exceptions, but only sets the state of the flags.

Returns

The fesetexceptflag function returns zero if the excepts argument is zero or if all the specified
flags were successfully set to the appropriate state. Otherwise, it returns a nonzero value.

7.6.2.5 The fetestexcept function

Synopsis

      #include <fenv.h>
      int fetestexcept(int excepts);
    

Description

The fetestexcept function determines which of a specified subset of the floating-point excep-
tion flags are currently set. The excepts argument specifies the floating-point status flags to be
queried.221)

Returns

The fetestexcept function returns the value of the bitwise OR of the floating-point exception
macros corresponding to the currently set floating-point exceptions included in excepts.

EXAMPLE Call f if “invalid” is set, then g if “overflow” is set:

    #include <fenv.h>
    /* ... */
    {
          #pragma STDC FENV_ACCESS ON
          int set_excepts;
          feclearexcept(FE_INVALID | FE_OVERFLOW);
          // maybe raise exceptions
          set_excepts = fetestexcept(FE_INVALID | FE_OVERFLOW);
          if (set_excepts & FE_INVALID) f();
          if (set_excepts & FE_OVERFLOW) g();
          /* ... */
    }
  

7.6.3 Rounding

The fegetround and fesetround functions provide control of rounding direction modes.

7.6.3.1 The fegetround function

Synopsis

      #include <fenv.h>
      int fegetround(void);
    

Description

The fegetround function gets the current rounding direction.

Returns

The fegetround function returns the value of the rounding direction macro representing the current
rounding direction or a negative value if there is no such rounding direction macro or the current
rounding direction is not determinable.

7.6.3.2 The fesetround function

Synopsis

      #include <fenv.h>
      int fesetround(int round);
    

Description

The fesetround function establishes the rounding direction represented by its argument round. If
the argument is not equal to the value of a rounding direction macro, the rounding direction is not
changed.

Returns

The fesetround function returns zero if and only if the requested rounding direction was estab-
lished.

EXAMPLE Save, set, and restore the rounding direction. Report an error and abort if setting the rounding direction fails.

    #include <fenv.h>
    #include <assert.h>
  
    void f(int round_dir)
    {
          #pragma STDC FENV_ACCESS ON
          int save_round;
  
          int setround_ok;
          save_round = fegetround();
          setround_ok = fesetround(round_dir);
          assert(setround_ok == 0);
          /* ... */
          fesetround(save_round);
          /* ... */
    }
  

7.6.4 Environment

The functions in this section manage the floating-point environment — status flags and control
modes — as one entity.

7.6.4.1 The fegetenv function

Synopsis

      #include <fenv.h>
      int fegetenv(fenv_t *envp);
    

Description

The fegetenv function attempts to store the current floating-point environment in the object pointed
to by envp.

Returns

The fegetenv function returns zero if the environment was successfully stored. Otherwise, it returns
a nonzero value.

7.6.4.2 The feholdexcept function

Synopsis

      #include <fenv.h>
      int feholdexcept(fenv_t *envp);
    

Description

The feholdexcept function saves the current floating-point environment in the object pointed to by
envp, clears the floating-point status flags, and then installs a non-stop (continue on floating-point
exceptions) mode, if available, for all floating-point exceptions.222)

Returns

The feholdexcept function returns zero if and only if non-stop floating-point exception handling
was successfully installed.

7.6.4.3 The fesetenv function

Synopsis

      #include <fenv.h>
      int fesetenv(const fenv_t *envp);
    

Description

The fesetenv function attempts to establish the floating-point environment represented by the
object pointed to by envp. The argument envp shall point to an object set by a call to fegetenv or
feholdexcept, or equal a floating-point environment macro. Note that fesetenv merely installs

the state of the floating-point status flags represented through its argument, and does not raise these
floating-point exceptions.

Returns

The fesetenv function returns zero if the environment was successfully established. Otherwise, it
returns a nonzero value.

7.6.4.4 The feupdateenv function

Synopsis

      #include <fenv.h>
      int feupdateenv(const fenv_t *envp);
    

Description

The feupdateenv function attempts to save the currently raised floating-point exceptions in its
automatic storage, install the floating-point environment represented by the object pointed to by
envp, and then raise the saved floating-point exceptions. The argument envp shall point to an object
set by a call to feholdexcept or fegetenv, or equal a floating-point environment macro.

Returns

The feupdateenv function returns zero if all the actions were successfully carried out. Otherwise, it
returns a nonzero value.

EXAMPLE Hide spurious underflow floating-point exceptions:

    #include <fenv.h>
    double f(double x)
    {
          #pragma STDC FENV_ACCESS ON
          double result;
          fenv_t save_env;
          if (feholdexcept(&save_env))
                return /* indication of an environmental problem */;
          // compute result
          if (/* test spurious underflow */)
                if (feclearexcept(FE_UNDERFLOW))
                      return /* indication of an environmental problem */;
          if (feupdateenv(&save_env))
                return /* indication of an environmental problem */;
          return result;
    }
  

7.7 Characteristics of floating types <float.h>

The header <float.h> defines several macros that expand to various limits and parameters of the
standard floating-point types.

The macros, their meanings, and the constraints (or restrictions) on their values are listed in 5.2.4.2.2.

7.8 Format conversion of integer types <inttypes.h>

The header <inttypes.h> includes the header <stdint.h> and extends it with additional facilities
provided by hosted implementations.

It declares functions for manipulating greatest-width integers and converting numeric character
strings to greatest-width integers, and it declares the type

    imaxdiv_t
  

which is a structure type that is the type of the value returned by the imaxdiv function. For each
type declared in <stdint.h>, it defines corresponding macros for conversion specifiers for use with
the formatted input/output functions.223)

Forward references: integer types <stdint.h> (7.20), formatted input/output functions (7.21.6),
formatted wide character input/output functions (7.29.2).

7.8.1 Macros for format specifiers

Each of the following object-like macros expands to a character string literal containing a conversion
specifier, possibly modified by a length modifier, suitable for use within the format argument of a
formatted input/output function when converting the corresponding integer type. These macro
names have the general form of PRI (character string literals for the fprintf and fwprintf family)
or SCN (character string literals for the fscanf and fwscanf family),224) followed by the conversion
specifier, followed by a name corresponding to a similar type name in 7.20.1. In these names, N
represents the width of the type as described in 7.20.1. For example, PRIdFAST32 can be used in a
format string to print the value of an integer of type int_fast32_t.

The fprintf macros for signed integers are:

    PRIdN     PRIdLEASTN         PRIdFASTN        PRIdMAX       PRIdPTR
    PRIiN     PRIiLEASTN         PRIiFASTN        PRIiMAX       PRIiPTR
  

The fprintf macros for unsigned integers are:

    PRIoN     PRIoLEASTN         PRIoFASTN        PRIoMAX       PRIoPTR
    PRIuN     PRIuLEASTN         PRIuFASTN        PRIuMAX       PRIuPTR
    PRIxN     PRIxLEASTN         PRIxFASTN        PRIxMAX       PRIxPTR
    PRIXN     PRIXLEASTN         PRIXFASTN        PRIXMAX       PRIXPTR
  

The fscanf macros for signed integers are:

    SCNdN     SCNdLEASTN         SCNdFASTN        SCNdMAX       SCNdPTR
    SCNiN     SCNiLEASTN         SCNiFASTN        SCNiMAX       SCNiPTR
  

The fscanf macros for unsigned integers are:

    SCNoN     SCNoLEASTN         SCNoFASTN        SCNoMAX       SCNoPTR
    SCNuN     SCNuLEASTN         SCNuFASTN        SCNuMAX       SCNuPTR
    SCNxN     SCNxLEASTN         SCNxFASTN        SCNxMAX       SCNxPTR
  

For each type that the implementation provides in <stdint.h>, the corresponding fprintf macros
shall be defined and the corresponding fscanf macros shall be defined unless the implementation
does not have a suitable fscanf length modifier for the type.

EXAMPLE

    #include <inttypes.h>
    #include <wchar.h>
    int main(void)
    {
          uintmax_t i = UINTMAX_MAX;    // this type always exists
          wprintf(L"The largest integer value is %020"
  
                  PRIxMAX "\n", i);
            return 0;
    }
  

7.8.2 Functions for greatest-width integer types

7.8.2.1 The imaxabs function

Synopsis

      #include <inttypes.h>
      intmax_t imaxabs(intmax_t j);
    

Description

The imaxabs function computes the absolute value of an integer j. If the result cannot be represented,
the behavior is undefined.225)

Returns

The imaxabs function returns the absolute value.

7.8.2.2 The imaxdiv function

Synopsis

      #include <inttypes.h>
      imaxdiv_t imaxdiv(intmax_t numer, intmax_t denom);
    

Description

The imaxdiv function computes numer / denom and numer % denom in a single operation.

Returns

The imaxdiv function returns a structure of type imaxdiv_t comprising both the quotient and the
remainder. The structure shall contain (in either order) the members quot (the quotient) and rem
(the remainder), each of which has type intmax_t. If either part of the result cannot be represented,
the behavior is undefined.

7.8.2.3 The strtoimax and strtoumax functions

Synopsis

      #include <inttypes.h>
      intmax_t strtoimax(const char * restrict nptr,
            char ** restrict endptr, int base);
      uintmax_t strtoumax(const char * restrict nptr,
            char ** restrict endptr, int base);
    

Description

The strtoimax and strtoumax functions are equivalent to the strtol, strtoll, strtoul, and
strtoull functions, except that the initial portion of the string is converted to intmax_t and
uintmax_t representation, respectively.

Returns

The strtoimax and strtoumax functions return the converted value, if any. If no conversion could
be performed, zero is returned. If the correct value is outside the range of representable values,
INTMAX_MAX, INTMAX_MIN, or UINTMAX_MAX is returned (according to the return type and sign of the
value, if any), and the value of the macro ERANGE is stored in errno.

Forward references: the strtol, strtoll, strtoul, and strtoull functions (7.22.1.4).

7.8.2.4 The wcstoimax and wcstoumax functions

Synopsis

      #include <stddef.h>            // for wchar_t
      #include <inttypes.h>
      intmax_t wcstoimax(const wchar_t * restrict nptr,
            wchar_t ** restrict endptr, int base);
      uintmax_t wcstoumax(const wchar_t * restrict nptr,
            wchar_t ** restrict endptr, int base);
    

Description

The wcstoimax and wcstoumax functions are equivalent to the wcstol, wcstoll, wcstoul, and
wcstoull functions except that the initial portion of the wide string is converted to intmax_t and
uintmax_t representation, respectively.

Returns

The wcstoimax function returns the converted value, if any. If no conversion could be performed,
zero is returned. If the correct value is outside the range of representable values, INTMAX_MAX,
INTMAX_MIN, or UINTMAX_MAX is returned (according to the return type and sign of the value, if any),
and the value of the macro ERANGE is stored in errno.

Forward references: the wcstol, wcstoll, wcstoul, and wcstoull functions (7.29.4.1.2).

7.9 Alternative spellings <iso646.h>

The header <iso646.h> defines the following eleven macros (on the left) that expand to the corre-
sponding tokens (on the right):

    and      &&
    and_eq   &=
    bitand   &
    bitor    |
    compl    ~
    not      !
    not_eq   !=
    or       ||
    or_eq    |=
    xor      ^
    xor_eq   ^=
  

7.10 Sizes of integer types <limits.h>

The header <limits.h> defines several macros that expand to various limits and parameters of the
standard integer types.

The macros, their meanings, and the constraints (or restrictions) on their values are listed in 5.2.4.2.1.

7.11 Localization <locale.h>

The header <locale.h> declares two functions, one type, and defines several macros.

The type is

    struct lconv
  

which contains members related to the formatting of numeric values. The structure shall contain
at least the following members, in any order. The semantics of the members and their normal
ranges are explained in 7.11.2.1. In the "C" locale, the members shall have the values specified in the
comments.

    char   *decimal_point;               //   "."
    char   *thousands_sep;               //   ""
    char   *grouping;                    //   ""
    char   *mon_decimal_point;           //   ""
    char   *mon_thousands_sep;           //   ""
    char   *mon_grouping;                //   ""
    char   *positive_sign;               //   ""
    char   *negative_sign;               //   ""
    char   *currency_symbol;             //   ""
    char   frac_digits;                  //   CHAR_MAX
    char   p_cs_precedes;                //   CHAR_MAX
    char   n_cs_precedes;                //   CHAR_MAX
    char   p_sep_by_space;               //   CHAR_MAX
    char   n_sep_by_space;               //   CHAR_MAX
    char   p_sign_posn;                  //   CHAR_MAX
    char   n_sign_posn;                  //   CHAR_MAX
    char   *int_curr_symbol;             //   ""
    char   int_frac_digits;              //   CHAR_MAX
    char   int_p_cs_precedes;            //   CHAR_MAX
    char   int_n_cs_precedes;            //   CHAR_MAX
    char   int_p_sep_by_space;           //   CHAR_MAX
    char   int_n_sep_by_space;           //   CHAR_MAX
    char   int_p_sign_posn;              //   CHAR_MAX
    char   int_n_sign_posn;              //   CHAR_MAX
  

The macros defined are NULL (described in 7.19); and

    LC_ALL
    LC_COLLATE
    LC_CTYPE
    LC_MONETARY
    LC_NUMERIC
    LC_TIME
  

which expand to integer constant expressions with distinct values, suitable for use as the first argu-
ment to the setlocale function.226) Additional macro definitions, beginning with the characters
LC_ and an uppercase letter,227) may also be specified by the implementation.

7.11.1 Locale control

7.11.1.1 The setlocale function

Synopsis

      #include <locale.h>
      char *setlocale(int category, const char *locale);
    

Description

The setlocale function selects the appropriate portion of the program’s locale as specified by
the category and locale arguments. The setlocale function may be used to change or query
the program’s entire current locale or portions thereof. The value LC_ALL for category names
the program’s entire locale; the other values for category name only a portion of the program’s
locale. LC_COLLATE affects the behavior of the strcoll and strxfrm functions. LC_CTYPE affects
the behavior of the character handling functions228) and the multibyte and wide character functions.
LC_MONETARY affects the monetary formatting information returned by the localeconv function.
LC_NUMERIC affects the decimal-point character for the formatted input/output functions and the
string conversion functions, as well as the nonmonetary formatting information returned by the
localeconv function. LC_TIME affects the behavior of the strftime and wcsftime functions.

A value of "C" for locale specifies the minimal environment for C translation; a value of "" for
locale specifies the locale-specific native environment. Other implementation-defined strings may
be passed as the second argument to setlocale.

At program startup, the equivalent of

    setlocale(LC_ALL, "C");
  

is executed.

A call to the setlocale function may introduce a data race with other calls to the setlocale
function or with calls to functions that are affected by the current locale. The implementation shall
behave as if no library function calls the setlocale function.

Returns

If a pointer to a string is given for locale and the selection can be honored, the setlocale function
returns a pointer to the string associated with the specified category for the new locale. If the
selection cannot be honored, the setlocale function returns a null pointer and the program’s locale
is not changed.

A null pointer for locale causes the setlocale function to return a pointer to the string associated
with the category for the program’s current locale; the program’s locale is not changed.229)

The pointer to string returned by the setlocale function is such that a subsequent call with that
string value and its associated category will restore that part of the program’s locale. The string
pointed to shall not be modified by the program, but may be overwritten by a subsequent call to the
setlocale function.

Forward references: formatted input/output functions (7.21.6), multibyte/wide character conver-
sion functions (7.22.7), multibyte/wide string conversion functions (7.22.8), numeric conversion
functions (7.22.1), the strcoll function (7.24.4.3), the strftime function (7.27.3.5), the strxfrm
function (7.24.4.5).

7.11.2 Numeric formatting convention inquiry

7.11.2.1 The localeconv function

Synopsis

      #include <locale.h>
      struct lconv *localeconv(void);
    

Description

The localeconv function sets the components of an object with type struct lconv with values
appropriate for the formatting of numeric quantities (monetary and otherwise) according to the
rules of the current locale.

The members of the structure with type char * are pointers to strings, any of which (except
decimal_point) can point to "", to indicate that the value is not available in the current locale or is
of zero length. Apart from grouping and mon_grouping, the strings shall start and end in the initial
shift state. The members with type char are nonnegative numbers, any of which can be CHAR_MAX
to indicate that the value is not available in the current locale. The members include the following:

char *decimal_point

    The decimal-point character used to format nonmonetary quantities.
  

char *thousands_sep

    The character used to separate groups of digits before the decimal-point character in
    formatted nonmonetary quantities.
  

char *grouping

    A string whose elements indicate the size of each group of digits in formatted nonmon-
    etary quantities.
  

char *mon_decimal_point

    The decimal-point used to format monetary quantities.
  

char *mon_thousands_sep

    The separator for groups of digits before the decimal-point in formatted monetary
    quantities.
  

char *mon_grouping

    A string whose elements indicate the size of each group of digits in formatted monetary
    quantities.
  

char *positive_sign

    The string used to indicate a nonnegative-valued formatted monetary quantity.
  

char *negative_sign

    The string used to indicate a negative-valued formatted monetary quantity.
  

char *currency_symbol

    The local currency symbol applicable to the current locale.
  

char frac_digits

    The number of fractional digits (those after the decimal-point) to be displayed in a
    locally formatted monetary quantity.
  

char p_cs_precedes

    Set to 1 or 0 if the currency_symbol respectively precedes or succeeds the value for a
    nonnegative locally formatted monetary quantity.
  

char n_cs_precedes

    Set to 1 or 0 if the currency_symbol respectively precedes or succeeds the value for a
    negative locally formatted monetary quantity.
  

char p_sep_by_space

    Set to a value indicating the separation of the currency_symbol, the sign string, and
    the value for a nonnegative locally formatted monetary quantity.
  

char n_sep_by_space

    Set to a value indicating the separation of the currency_symbol, the sign string, and
    the value for a negative locally formatted monetary quantity.
  

char p_sign_posn

    Set to a value indicating the positioning of the positive_sign for a nonnegative locally
    formatted monetary quantity.
  

char n_sign_posn

    Set to a value indicating the positioning of the negative_sign for a negative locally
    formatted monetary quantity.
  

char *int_curr_symbol

    The international currency symbol applicable to the current locale. The first three
    characters contain the alphabetic international currency symbol in accordance with
    those specified in ISO 4217. The fourth character (immediately preceding the null
    character) is the character used to separate the international currency symbol from the
    monetary quantity.
  

char int_frac_digits

    The number of fractional digits (those after the decimal-point) to be displayed in an
    internationally formatted monetary quantity.
  

char int_p_cs_precedes

    Set to 1 or 0 if the int_curr_symbol respectively precedes or succeeds the value for a
    nonnegative internationally formatted monetary quantity.
  

char int_n_cs_precedes

    Set to 1 or 0 if the int_curr_symbol respectively precedes or succeeds the value for a
    negative internationally formatted monetary quantity.
  

char int_p_sep_by_space

    Set to a value indicating the separation of the int_curr_symbol, the sign string, and
    the value for a nonnegative internationally formatted monetary quantity.
  

char int_n_sep_by_space

    Set to a value indicating the separation of the int_curr_symbol, the sign string, and
    the value for a negative internationally formatted monetary quantity.
  

char int_p_sign_posn

    Set to a value indicating the positioning of the positive_sign for a nonnegative
    internationally formatted monetary quantity.
  

char int_n_sign_posn

    Set to a value indicating the positioning of the negative_sign for a negative interna-
    tionally formatted monetary quantity.
  

The elements of grouping and mon_grouping are interpreted according to the following:

CHAR_MAX
No further grouping is to be performed.
0
The previous element is to be repeatedly used for the remainder of the digits.
other
The integer value is the number of digits that compose the current group. The next
element is examined to determine the size of the next group of digits before the current
group.

The values of p_sep_by_space, n_sep_by_space, int_p_sep_by_space, and
_ _ _ _
int n sep by space are interpreted according to the following:

0
No space separates the currency symbol and value.
1
If the currency symbol and sign string are adjacent, a space separates them from the value;
otherwise, a space separates the currency symbol from the value.
2
If the currency symbol and sign string are adjacent, a space separates them; otherwise, a space
separates the sign string from the value.

For int_p_sep_by_space and int_n_sep_by_space, the fourth character of int_curr_symbol is
used instead of a space.

The values of p_sign_posn, n_sign_posn, int_p_sign_posn, and int_n_sign_posn are inter-
preted according to the following:

0
Parentheses surround the quantity and currency symbol.
1
The sign string precedes the quantity and currency symbol.
2
The sign string succeeds the quantity and currency symbol.
3
The sign string immediately precedes the currency symbol.
4
The sign string immediately succeeds the currency symbol.

The implementation shall behave as if no library function calls the localeconv function.

Returns

The localeconv function returns a pointer to the filled-in object. The structure pointed to by the
return value shall not be modified by the program, but may be overwritten by a subsequent call
to the localeconv function. In addition, calls to the setlocale function with categories LC_ALL,
LC_MONETARY, or LC_NUMERIC may overwrite the contents of the structure.

EXAMPLE 1 The following table illustrates rules which may well be used by four countries to format monetary quantities.

                             Local format                       International format
    Country       Positive           Negative            Positive          Negative
    Country1      1.234,56 mk        -1.234,56 mk        FIM   1.234,56    FIM -1.234,56
    Country2      L.1.234            -L.1.234            ITL   1.234       -ITL 1.234
    Country3      ƒ 1.234,56         ƒ -1.234,56         NLG   1.234,56    NLG -1.234,56
    Country4      SFrs.1,234.56      SFrs.1,234.56C      CHF   1,234.56    CHF 1,234.56C
  

For these four countries, the respective values for the monetary members of the structure returned by localeconv could be:

                              Country1     Country2     Country3     Country4
    mon_decimal_point         ","          ""           ","          "."
    mon_thousands_sep         "."          "."          "."          ","
    mon_grouping              "\3"         "\3"         "\3"         "\3"
    positive_sign             ""           ""           ""           ""
    negative_sign             "-"          "-"          "-"          "C"
    currency_symbol           "mk"         "L."         "\u0192"     "SFrs."
    frac_digits               2            0            2            2
    p_cs_precedes             0            1            1            1
    n_cs_precedes             0            1            1            1
    p_sep_by_space            1            0            1            0
    n_sep_by_space            1            0            2            0
    p_sign_posn               1            1            1            1
    n_sign_posn               1            1            4            2
    int_curr_symbol           "FIM "       "ITL "       "NLG "       "CHF "
    int_frac_digits           2            0            2            2
    int_p_cs_precedes         1            1            1            1
    int_n_cs_precedes         1            1            1            1
    int_p_sep_by_space        1            1            1            1
    int_n_sep_by_space        2            1            2            1
    int_p_sign_posn           1            1            1            1
    int_n_sign_posn           4            1            4            2
  

EXAMPLE 2 The following table illustrates how the cs_precedes, sep_by_space, and sign_posn members affect the
formatted value.

                                           p_sep_by_space
    p_cs_precedes   p_sign_posn     0         1          2
                0              0    (1.25$)   (1.25 $)      (1.25$)
                               1    +1.25$    +1.25 $       + 1.25$
                               2    1.25$+    1.25 $+       1.25$ +
                               3    1.25+$    1.25 +$       1.25+ $
                               4    1.25$+    1.25 $+       1.25$ +
                1              0    ($1.25)   ($ 1.25)      ($1.25)
                               1    +$1.25    +$ 1.25       + $1.25
                               2    $1.25+    $ 1.25+       $1.25 +
                               3    +$1.25    +$ 1.25       + $1.25
                               4    $+1.25    $+ 1.25       $ +1.25
  

7.12 Mathematics <math.h>

The header <math.h> declares two types and many mathematical functions and defines several
macros. Most synopses specify a family of functions consisting of a principal function with one
or more double parameters, a double return value, or both; and other functions with the same
name but with f and l suffixes, which are corresponding functions with float and long double
parameters, return values, or both.230) Integer arithmetic functions and conversion functions are
discussed later.

The types

    float_t
    double_t
  

are floating types at least as wide as float and double, respectively, and such that double_t is
at least as wide as float_t. If FLT_EVAL_METHOD equals 0, float_t and double_t are float and
double, respectively; if FLT_EVAL_METHOD equals 1, they are both double; if FLT_EVAL_METHOD
equals 2, they are both long double; and for other values of FLT_EVAL_METHOD, they are otherwise
implementation-defined.231)

The macro

    HUGE_VAL
  

expands to a positive double constant expression, not necessarily representable as a float. The
macros

    HUGE_VALF
    HUGE_VALL
  

are respectively float and long double analogs of HUGE_VAL.232)

The macro

    INFINITY
  

expands to a constant expression of type float representing positive or unsigned infinity, if available;
else to a positive constant of type float that overflows at translation time.233)

The macro

    NAN
  

is defined if and only if the implementation supports quiet NaNs for the float type. It expands to a
constant expression of type float representing a quiet NaN.

The number classification macros

    FP_INFINITE
    FP_NAN
    FP_NORMAL
    FP_SUBNORMAL
    FP_ZERO
  

represent the mutually exclusive kinds of floating-point values. They expand to integer constant
expressions with distinct values. Additional implementation-defined floating-point classifications,
with macro definitions beginning with FP_ and an uppercase letter, may also be specified by the
implementation.

The macro

    FP_FAST_FMA
  

is optionally defined. If defined, it indicates that the fma function generally executes about as fast as,
or faster than, a multiply and an add of double operands.234) The macros

    FP_FAST_FMAF
    FP_FAST_FMAL
  

are, respectively, float and long double analogs of FP_FAST_FMA. If defined, these macros expand
to the integer constant 1.

The macros

    FP_ILOGB0
    FP_ILOGBNAN
  

expand to integer constant expressions whose values are returned by ilogb(x) if x is zero or
NaN, respectively. The value of FP_ILOGB0 shall be either INT_MIN or -INT_MAX . The value of
FP_ILOGBNAN shall be either INT_MAX or INT_MIN.

The macros

    MATH_ERRNO
    MATH_ERREXCEPT
  

expand to the integer constants 1 and 2, respectively; the macro

    math_errhandling
  

expands to an expression that has type int and the value MATH_ERRNO, MATH_ERREXCEPT, or the
bitwise OR of both. The value of math_errhandling is constant for the duration of the program. It is
unspecified whether math_errhandling is a macro or an identifier with external linkage. If a macro
definition is suppressed or a program defines an identifier with the name math_errhandling, the
behavior is undefined. If the expression math_errhandling & MATH_ERREXCEPT can be nonzero,
the implementation shall define the macros FE_DIVBYZERO, FE_INVALID, and FE_OVERFLOW in
<fenv.h>.

7.12.1 Treatment of error conditions

The behavior of each of the functions in <math.h> is specified for all representable values of its
input arguments, except where stated otherwise. Each function shall execute as if it were a single
operation without raising SIGFPE and without generating any of the floating-point exceptions
“invalid”, “divide-by-zero”, or “overflow” except to reflect the result of the function.

For all functions, a domain error occurs if and only if an input argument is outside the do-
main over which the mathematical function is defined. The description of each function lists
any required domain errors; an implementation may define additional domain errors, provided
that such errors are consistent with the mathematical definition of the function.235) On a do-
main error, the function returns an implementation-defined value; if the integer expression

math_errhandling & MATH_ERRNO is nonzero, the integer expression errno acquires the value
EDOM; if the integer expression
math_errhandling & MATH_ERREXCEPT is nonzero, the “invalid” floating-point exception is raised.

Similarly, a pole error (also known as a singularity or infinitary) occurs if and only if the mathematical
function has an exact infinite result as the finite input argument(s) are approached in the limit (for ex-
ample, log(0.0)). The description of each function lists any required pole errors; an implementation
may define additional pole errors, provided that such errors are consistent with the mathematical
definition of the function. On a pole error, the function returns an implementation-defined value;
if the integer expression math_errhandling & MATH_ERRNO is nonzero, the integer expression
errno acquires the value ERANGE; if the integer expression math_errhandling & MATH_ERREXCEPT
is nonzero, the “divide-by-zero” floating-point exception is raised.

Likewise, a range error occurs if and only if the mathematical result of the function cannot be
represented in an object of the specified type, due to extreme magnitude. The description of each
function lists any required range errors; an implementation may define additional range errors,
provided that such errors are consistent with the mathematical definition of the function and are the
result of either overflow or underflow.

A floating result overflows if the magnitude of the mathematical result is finite but so large that
the mathematical result cannot be represented without extraordinary roundoff error in an object
of the specified type. If a floating result overflows and default rounding is in effect, then the
function returns the value of the macro HUGE_VAL, HUGE_VALF, or HUGE_VALL according to the
return type, with the same sign as the correct value of the function; if the integer expression
math_errhandling & MATH_ERRNO is nonzero, the integer expression errno acquires the value
ERANGE; if the integer expression math_errhandling & MATH_ERREXCEPT is nonzero, the “overflow”
floating-point exception is raised.

The result underflows if the magnitude of the mathematical result is so small that the mathematical re-
sult cannot be represented, without extraordinary roundoff error, in an object of the specified type.236)
If the result underflows, the function returns an implementation-defined value whose magnitude
is no greater than the smallest normalized positive number in the specified type; if the integer ex-
pression math_errhandling & MATH_ERRNO is nonzero, whether errno acquires the value ERANGE
is implementation-defined; if the integer expression math_errhandling & MATH_ERREXCEPT is
nonzero, whether the “underflow” floating-point exception is raised is implementation-defined.

If a domain, pole, or range error occurs and the integer expression math_errhandling & MATH_ERRNO
is zero,237) then errno shall either be set to the value corresponding to the error or left unmodified. If
no such error occurs, errno shall be left unmodified regardless of the setting of math_errhandling.

7.12.2 The FP_CONTRACT pragma

Synopsis

      #include <math.h>
      #pragma STDC FP_CONTRACT on-off-switch
    

Description

The FP_CONTRACT pragma can be used to allow (if the state is “on”) or disallow (if the state is
“off”) the implementation to contract expressions (6.5). Each pragma can occur either outside
external declarations or preceding all explicit declarations and statements inside a compound
statement. When outside external declarations, the pragma takes effect from its occurrence until
another FP_CONTRACT pragma is encountered, or until the end of the translation unit. When inside
a compound statement, the pragma takes effect from its occurrence until another FP_CONTRACT
pragma is encountered (including within a nested compound statement), or until the end of the
compound statement; at the end of a compound statement the state for the pragma is restored to
its condition just before the compound statement. If this pragma is used in any other context, the
behavior is undefined. The default state (“on” or “off”) for the pragma is implementation-defined.

7.12.3 Classification macros

In the synopses in this subclause, real-floating indicates that the argument shall be an expression of
real floating type.

7.12.3.1 The fpclassify macro

Synopsis

      #include <math.h>
      int fpclassify(real-floating x);
    

Description

The fpclassify macro classifies its argument value as NaN, infinite, normal, subnormal, zero, or
into another implementation-defined category. First, an argument represented in a format wider
than its semantic type is converted to its semantic type. Then classification is based on the type of
the argument.238)

Returns

The fpclassify macro returns the value of the number classification macro appropriate to the value
of its argument.

7.12.3.2 The isfinite macro

Synopsis

      #include <math.h>
      int isfinite(real-floating x);
    

Description

The isfinite macro determines whether its argument has a finite value (zero, subnormal, or
normal, and not infinite or NaN). First, an argument represented in a format wider than its semantic
type is converted to its semantic type. Then determination is based on the type of the argument.

Returns

The isfinite macro returns a nonzero value if and only if its argument has a finite value.

7.12.3.3 The isinf macro

Synopsis

      #include <math.h>
      int isinf(real-floating x);
    

Description

The isinf macro determines whether its argument value is an infinity (positive or negative). First,
an argument represented in a format wider than its semantic type is converted to its semantic type.
Then determination is based on the type of the argument.

Returns

The isinf macro returns a nonzero value if and only if its argument has an infinite value.

7.12.3.4 The isnan macro

Synopsis

      #include <math.h>
      int isnan(real-floating x);
    

Description

The isnan macro determines whether its argument value is a NaN. First, an argument represented
in a format wider than its semantic type is converted to its semantic type. Then determination is
based on the type of the argument.239)

Returns

The isnan macro returns a nonzero value if and only if its argument has a NaN value.

7.12.3.5 The isnormal macro

Synopsis

      #include <math.h>
      int isnormal(real-floating x);
    

Description

The isnormal macro determines whether its argument value is normal (neither zero, subnormal,
infinite, nor NaN). First, an argument represented in a format wider than its semantic type is
converted to its semantic type. Then determination is based on the type of the argument.

Returns

The isnormal macro returns a nonzero value if and only if its argument has a normal value.

7.12.3.6 The signbit macro

Synopsis

      #include <math.h>
      int signbit(real-floating x);
    

Description

The signbit macro determines whether the sign of its argument value is negative.240)

Returns

The signbit macro returns a nonzero value if and only if the sign of its argument value is negative.

7.12.4 Trigonometric functions

7.12.4.1 The acos functions

Synopsis

      #include <math.h>
      double acos(double x);
      float acosf(float x);
      long double acosl(long double x);
    

Description

The acos functions compute the principal value of the arc cosine of x. A domain error occurs for
arguments not in the interval [−1, +1].

Returns

The acos functions return arccos x in the interval [0, π] radians.

7.12.4.2 The asin functions

Synopsis

      #include <math.h>
      double asin(double x);
      float asinf(float x);
      long double asinl(long double x);
    

Description

The asin functions compute the principal value of the arc sine of x. A domain error occurs for
arguments not in the interval [−1, +1].

Returns

The asin functions return arcsin x in the interval [−π/2, +π/2] radians.

7.12.4.3 The atan functions

Synopsis

      #include <math.h>
      double atan(double x);
      float atanf(float x);
      long double atanl(long double x);
    

Description

The atan functions compute the principal value of the arc tangent of x.

Returns

The atan functions return arctan x in the interval [−π/2, +π/2] radians.

7.12.4.4 The atan2 functions

Synopsis

      #include <math.h>
      double atan2(double y, double x);
      float atan2f(float y, float x);
      long double atan2l(long double y, long double x);
    

Description

The atan2 functions compute the value of the arc tangent of y/x, using the signs of both arguments
to determine the quadrant of the return value. A domain error may occur if both arguments are zero.

Returns

The atan2 functions return arctan y/x in the interval [−π, +π] radians.

7.12.4.5 The cos functions

Synopsis

      #include <math.h>
      double cos(double x);
      float cosf(float x);
      long double cosl(long double x);
    

Description

The cos functions compute the cosine of x (measured in radians).

Returns

The cos functions return cos x.

7.12.4.6 The sin functions

Synopsis

      #include <math.h>
      double sin(double x);
      float sinf(float x);
      long double sinl(long double x);
    

Description

The sin functions compute the sine of x (measured in radians).

Returns

The sin functions return sin x.

7.12.4.7 The tan functions

Synopsis

      #include <math.h>
      double tan(double x);
      float tanf(float x);
      long double tanl(long double x);
    

Description

The tan functions return the tangent of x (measured in radians).

Returns

The tan functions return tan x.

7.12.5 Hyperbolic functions

7.12.5.1 The acosh functions

Synopsis

      #include <math.h>
      double acosh(double x);
      float acoshf(float x);
      long double acoshl(long double x);
    

Description

The acosh functions compute the (nonnegative) arc hyperbolic cosine of x. A domain error occurs
for arguments less than 1.

Returns

The acosh functions return arcosh x in the interval [0, +∞].

7.12.5.2 The asinh functions

Synopsis

      #include <math.h>
      double asinh(double x);
      float asinhf(float x);
      long double asinhl(long double x);
    

Description

The asinh functions compute the arc hyperbolic sine of x.

Returns

The asinh functions return arsinh x.

7.12.5.3 The atanh functions

Synopsis

      #include <math.h>
      double atanh(double x);
      float atanhf(float x);
      long double atanhl(long double x);
    

Description

The atanh functions compute the arc hyperbolic tangent of x. A domain error occurs for arguments
not in the interval [−1, +1]. A pole error may occur if the argument equals-1 or +1 .

Returns

The atanh functions return artanh x.

7.12.5.4 The cosh functions

Synopsis

      #include <math.h>
      double cosh(double x);
      float coshf(float x);
      long double coshl(long double x);
    

Description

The cosh functions compute the hyperbolic cosine of x. A range error occurs if the magnitude of x
is too large.

Returns

The cosh functions return cosh x.

7.12.5.5 The sinh functions

Synopsis

      #include <math.h>
      double sinh(double x);
      float sinhf(float x);
      long double sinhl(long double x);
    

Description

The sinh functions compute the hyperbolic sine of x. A range error occurs if the magnitude of x is
too large.

Returns

The sinh functions return sinh x.

7.12.5.6 The tanh functions

Synopsis

      #include <math.h>
      double tanh(double x);
      float tanhf(float x);
      long double tanhl(long double x);
    

Description

The tanh functions compute the hyperbolic tangent of x.

Returns

The tanh functions return tanh x.

7.12.6 Exponential and logarithmic functions

7.12.6.1 The exp functions

Synopsis

      #include <math.h>
      double exp(double x);
      float expf(float x);
      long double expl(long double x);
    

Description

The exp functions compute the base-e exponential of x. A range error occurs if the magnitude of x is
too large.

Returns

The exp functions return ex .

7.12.6.2 The exp2 functions

Synopsis

      #include <math.h>
      double exp2(double x);
      float exp2f(float x);
      long double exp2l(long double x);
    

Description

The exp2 functions compute the base-2 exponential of x. A range error occurs if the magnitude of x
is too large.

Returns

The exp2 functions return 2x .

7.12.6.3 The expm1 functions

Synopsis

      #include <math.h>
      double expm1(double x);
      float expm1f(float x);
      long double expm1l(long double x);
    

Description

The expm1 functions compute the base-e exponential of the argument, minus 1. A range error occurs
if positive x is too large.241)

Returns

The expm1 functions return ex − 1.

7.12.6.4 The frexp functions

Synopsis

      #include <math.h>
      double frexp(double value, int *exp);
      float frexpf(float value, int *exp);
      long double frexpl(long double value, int *exp);
    

Description

The frexp functions break a floating-point number into a normalized fraction and an integral power
of 2. They store the integer in the int object pointed to by exp.

Returns

If value is not a floating-point number or if the integral power of 2 is outside the range of int,
the results are unspecified. Otherwise, the frexp functions return the value x, such that x has a
magnitude in the interval [1/2, 1) or zero, and value equals x × 2*exp . If value is zero, both parts of
the result are zero.

7.12.6.5 The ilogb functions

Synopsis

      #include <math.h>
      int ilogb(double x);
      int ilogbf(float x);
      int ilogbl(long double x);
    

Description

The ilogb functions extract the exponent of x as a signed int value. If x is zero they compute the
value FP_ILOGB0; if x is infinite they compute the value INT_MAX; if x is a NaN they compute the
value FP_ILOGBNAN; otherwise, they are equivalent to calling the corresponding logb function and
casting the returned value to type int. A domain error or range error may occur if x is zero, infinite,
or NaN. If the correct value is outside the range of the return type, the numeric result is unspecified
and a domain error or range error may occur.

Returns

The ilogb functions return the exponent of x as a signed int value.

Forward references: the logb functions (7.12.6.11).

7.12.6.6 The ldexp functions

Synopsis

      #include <math.h>
      double ldexp(double x, int exp);
      float ldexpf(float x, int exp);
      long double ldexpl(long double x, int exp);
    

Description

The ldexp functions multiply a floating-point number by an integral power of 2. A range error may
occur.

Returns

The ldexp functions return x × 2exp .

7.12.6.7 The log functions

Synopsis

      #include <math.h>
      double log(double x);
      float logf(float x);
      long double logl(long double x);
    

Description

The log functions compute the base-e (natural) logarithm of x. A domain error occurs if the
argument is negative. A pole error may occur if the argument is zero.

Returns

The log functions return loge x.

7.12.6.8 The log10 functions

Synopsis

      #include <math.h>
      double log10(double x);
      float log10f(float x);
      long double log10l(long double x);
    

Description

The log10 functions compute the base-10 (common) logarithm of x. A domain error occurs if the
argument is negative. A pole error may occur if the argument is zero.

Returns

The log10 functions return log10 x.

7.12.6.9 The log1p functions

Synopsis

      #include <math.h>
      double log1p(double x);
      float log1pf(float x);
      long double log1pl(long double x);
    

Description

The log1p functions compute the base-e (natural) logarithm of 1 plus the argument.242) A domain
error occurs if the argument is less than −1. A pole error may occur if the argument equals −1.

Returns

The log1p functions return loge (1 + x).

7.12.6.10 The log2 functions

Synopsis

      #include <math.h>
      double log2(double x);
      float log2f(float x);
      long double log2l(long double x);
    

Description

The log2 functions compute the base-2 logarithm of x. A domain error occurs if the argument is less
than zero. A pole error may occur if the argument is zero.

Returns

The log2 functions return log2 x.

7.12.6.11 The logb functions

Synopsis

      #include <math.h>
      double logb(double x);
      float logbf(float x);
      long double logbl(long double x);
    

Description

The logb functions extract the exponent of x, as a signed integer value in floating-point format. If x
is subnormal it is treated as though it were normalized; thus, for positive finite x,

1 ≤ x × FLT_RADIX−logb(x) < FLT_RADIX

A domain error or pole error may occur if the argument is zero.

Returns

The logb functions return the signed exponent of x.

7.12.6.12 The modf functions

Synopsis

      #include <math.h>
      double modf(double value, double *iptr);
      float modff(float value, float *iptr);
      long double modfl(long double value, long double *iptr);
    

Description

The modf functions break the argument value into integral and fractional parts, each of which has
the same type and sign as the argument. They store the integral part (in floating-point format) in the
object pointed to by iptr.

Returns

The modf functions return the signed fractional part of value.

7.12.6.13 The scalbn and scalbln functions

Synopsis

      #include <math.h>
      double scalbn(double x, int n);
      float scalbnf(float x, int n);
      long double scalbnl(long double x, int n);
      double scalbln(double x, long int n);
      float scalblnf(float x, long int n);
      long double scalblnl(long double x, long int n);
    

Description

The scalbn and scalbln functions compute x × FLT_RADIXn efficiently, not normally by computing
FLT_RADIXn explicitly. A range error may occur.

Returns

The scalbn and scalbln functions return x × FLT_RADIXn .

7.12.7 Power and absolute-value functions

7.12.7.1 The cbrt functions

Synopsis

      #include <math.h>
      double cbrt(double x);
      float cbrtf(float x);
      long double cbrtl(long double x);
    

Description

The cbrt functions compute the real cube root of x.

Returns

The cbrt functions return x1/3 .

7.12.7.2 The fabs functions

Synopsis

      #include <math.h>
      double fabs(double x);
      float fabsf(float x);
      long double fabsl(long double x);
    

Description

The fabs functions compute the absolute value of a floating-point number x.

Returns

The fabs functions return |x|.

7.12.7.3 The hypot functions

Synopsis

      #include <math.h>
      double hypot(double x, double y);
      float hypotf(float x, float y);
      long double hypotl(long double x, long double y);
    

Description

The hypot functions compute the square root of the sum of the squares of x and y, without undue
overflow or underflow. A range error may occur.

Returns

    p
  

The hypot functions return x 2 + y2 .

7.12.7.4 The pow functions

Synopsis

      #include <math.h>
      double pow(double x, double y);
      float powf(float x, float y);
      long double powl(long double x, long double y);
    

Description

The pow functions compute x raised to the power y. A domain error occurs if x is finite and negative
and y is finite and not an integer value. A range error may occur. A domain error may occur if x is
zero and y is zero. A domain error or pole error may occur if x is zero and y is less than zero.

Returns

The pow functions return xy .

7.12.7.5 The sqrt functions

Synopsis

      #include <math.h>
      double sqrt(double x);
      float sqrtf(float x);
      long double sqrtl(long double x);
    

Description

The sqrt functions compute the nonnegative square root of x. A domain error occurs if the argument
is less than zero.

Returns

The sqrt functions return x.

7.12.8 Error and gamma functions

7.12.8.1 The erf functions

Synopsis

      #include <math.h>
      double erf(double x);
      float erff(float x);
      long double erfl(long double x);
    

Description

The erf functions compute the error function of x.

Returns

    Rx      2
  

The erf functions return erf x = √2

    π
             e−t dt.
        0
  

7.12.8.2 The erfc functions

Synopsis

      #include <math.h>
      double erfc(double x);
      float erfcf(float x);
      long double erfcl(long double x);
    

Description

The erfc functions compute the complementary error function of x. A range error occurs if positive
x is too large.

Returns

    R∞      2
  

The erfc functions return erfc x = 1 − erf x = √2

    π
             e−t dt.
        x
  

7.12.8.3 The lgamma functions

Synopsis

      #include <math.h>
      double lgamma(double x);
      float lgammaf(float x);
      long double lgammal(long double x);
    

Description

The lgamma functions compute the natural logarithm of the absolute value of gamma of x. A range
error occurs if positive x is too large. A pole error may occur if x is a negative integer or zero.

Returns

The lgamma functions return loge |Γ(x)|.

7.12.8.4 The tgamma functions

Synopsis

      #include <math.h>
      double tgamma(double x);
      float tgammaf(float x);
      long double tgammal(long double x);
    

Description

The tgamma functions compute the gamma function of x. A domain error or pole error may occur if
x is a negative integer or zero. A range error occurs if the magnitude of x is too large and may occur
if the magnitude of x is too small.

Returns

The tgamma functions return Γ(x).

7.12.9 Nearest integer functions

7.12.9.1 The ceil functions

Synopsis

      #include <math.h>
      double ceil(double x);
      float ceilf(float x);
      long double ceill(long double x);
    

Description

The ceil functions compute the smallest integer value not less than x.

Returns

The ceil functions return dxe, expressed as a floating-point number.

7.12.9.2 The floor functions

Synopsis

      #include <math.h>
      double floor(double x);
      float floorf(float x);
      long double floorl(long double x);
    

Description

The floor functions compute the largest integer value not greater than x.

Returns

The floor functions return bxc, expressed as a floating-point number.

7.12.9.3 The nearbyint functions

Synopsis

      #include <math.h>
      double nearbyint(double x);
      float nearbyintf(float x);
      long double nearbyintl(long double x);
    

Description

The nearbyint functions round their argument to an integer value in floating-point format, using
the current rounding direction and without raising the “inexact” floating-point exception.

Returns

The nearbyint functions return the rounded integer value.

7.12.9.4 The rint functions

Synopsis

      #include <math.h>
      double rint(double x);
      float rintf(float x);
      long double rintl(long double x);
    

Description

The rint functions differ from the nearbyint functions (7.12.9.3) only in that the rint functions
may raise the “inexact” floating-point exception if the result differs in value from the argument.

Returns

The rint functions return the rounded integer value.

7.12.9.5 The lrint and llrint functions

Synopsis

      #include <math.h>
      long int lrint(double x);
      long int lrintf(float x);
      long int lrintl(long double x);
      long long int llrint(double x);
      long long int llrintf(float x);
      long long int llrintl(long double x);
    

Description

The lrint and llrint functions round their argument to the nearest integer value, rounding
according to the current rounding direction. If the rounded value is outside the range of the return
type, the numeric result is unspecified and a domain error or range error may occur.

Returns

The lrint and llrint functions return the rounded integer value.

7.12.9.6 The round functions

Synopsis

      #include <math.h>
      double round(double x);
      float roundf(float x);
      long double roundl(long double x);
    

Description

The round functions round their argument to the nearest integer value in floating-point format,
rounding halfway cases away from zero, regardless of the current rounding direction.

Returns

The round functions return the rounded integer value.

7.12.9.7 The lround and llround functions

Synopsis

      #include <math.h>
      long int lround(double x);
      long int lroundf(float x);
    
    long   int lroundl(long double x);
    long   long int llround(double x);
    long   long int llroundf(float x);
    long   long int llroundl(long double x);
  

Description

The lround and llround functions round their argument to the nearest integer value, rounding
halfway cases away from zero, regardless of the current rounding direction. If the rounded value is
outside the range of the return type, the numeric result is unspecified and a domain error or range
error may occur.

Returns

The lround and llround functions return the rounded integer value.

7.12.9.8 The trunc functions

Synopsis

      #include <math.h>
      double trunc(double x);
      float truncf(float x);
      long double truncl(long double x);
    

Description

The trunc functions round their argument to the integer value, in floating format, nearest to but no
larger in magnitude than the argument.

Returns

The trunc functions return the truncated integer value.

7.12.10 Remainder functions

7.12.10.1 The fmod functions

Synopsis

      #include <math.h>
      double fmod(double x, double y);
      float fmodf(float x, float y);
      long double fmodl(long double x, long double y);
    

Description

The fmod functions compute the floating-point remainder of x/y.

Returns

The fmod functions return the value x − ny, for some integer n such that, if y is nonzero, the result
has the same sign as x and magnitude less than the magnitude of y. If y is zero, whether a domain
error occurs or the fmod functions return zero is implementation-defined.

7.12.10.2 The remainder functions

Synopsis

      #include <math.h>
      double remainder(double x, double y);
      float remainderf(float x, float y);
      long double remainderl(long double x, long double y);
    

Description

The remainder functions compute the remainder x REM y required by IEC 60559.243)

Returns

The remainder functions return x REM y. If y is zero, whether a domain error occurs or the functions
return zero is implementation defined.

7.12.10.3 The remquo functions

Synopsis

      #include <math.h>
      double remquo(double x, double y, int *quo);
      float remquof(float x, float y, int *quo);
      long double remquol(long double x, long double y,
            int *quo);
    

Description

The remquo functions compute the same remainder as the remainder functions. In the object pointed
to by quo they store a value whose sign is the sign of x/y and whose magnitude is congruent modulo
2n to the magnitude of the integral quotient of x/y, where n is an implementation-defined integer
greater than or equal to 3.

Returns

The remquo functions return x REM y. If y is zero, the value stored in the object pointed to by quo
is unspecified and whether a domain error occurs or the functions return zero is implementation
defined.

7.12.11 Manipulation functions

7.12.11.1 The copysign functions

Synopsis

      #include <math.h>
      double copysign(double x, double y);
      float copysignf(float x, float y);
      long double copysignl(long double x, long double y);
    

Description

The copysign functions produce a value with the magnitude of x and the sign of y. They produce a
NaN (with the sign of y) if x is a NaN. On implementations that represent a signed zero but do not
treat negative zero consistently in arithmetic operations, the copysign functions regard the sign of
zero as positive.

Returns

The copysign functions return a value with the magnitude of x and the sign of y.

7.12.11.2 The nan functions

Synopsis

      #include <math.h>
      double nan(const char *tagp);
      float nanf(const char *tagp);
      long double nanl(const char *tagp);
    

Description

The nan, nanf, and nanl functions convert the string pointed to by tagp according to the
following rules. The call nan("n-char-sequence") is equivalent to strtod("NAN(n-char-sequence)",
(char**)NULL) ; the call nan("") is equivalent to strtod("NAN()",(char**)NULL). If
tagp does not point to an n-char sequence or an empty string, the call is equivalent to
strtod("NAN",(char**)NULL). Calls to nanf and nanl are equivalent to the corresponding calls
to strtof and strtold.

Returns

The nan functions return a quiet NaN, if available, with content indicated through tagp. If the
implementation does not support quiet NaNs, the functions return zero.

Forward references: the strtod, strtof, and strtold functions (7.22.1.3).

7.12.11.3 The nextafter functions

Synopsis

      #include <math.h>
      double nextafter(double x, double y);
      float nextafterf(float x, float y);
      long double nextafterl(long double x, long double y);
    

Description

The nextafter functions determine the next representable value, in the type of the function, after x
in the direction of y, where x and y are first converted to the type of the function.244) The nextafter
functions return y if x equals y. A range error may occur if the magnitude of x is the largest finite
value representable in the type and the result is infinite or not representable in the type.

Returns

The nextafter functions return the next representable value in the specified format after x in the
direction of y.

7.12.11.4 The nexttoward functions

Synopsis

      #include <math.h>
      double nexttoward(double x, long double y);
      float nexttowardf(float x, long double y);
      long double nexttowardl(long double x, long double y);
    

Description

The nexttoward functions are equivalent to the nextafter functions except that the second pa-
rameter has type long double and the functions return y converted to the type of the function if x
equals y.245)

7.12.12 Maximum, minimum, and positive difference functions

7.12.12.1 The fdim functions

Synopsis

      #include <math.h>
      double fdim(double x, double y);
      float fdimf(float x, float y);
      long double fdiml(long double x, long double y);
    

Description

The fdim functions determine the positive difference between their arguments:
(

    x − y if x > y
    +0     if x ≤ y
  

A range error may occur.

Returns

The fdim functions return the positive difference value.

7.12.12.2 The fmax functions

Synopsis

      #include <math.h>
      double fmax(double x, double y);
      float fmaxf(float x, float y);
      long double fmaxl(long double x, long double y);
    

Description

The fmax functions determine the maximum numeric value of their arguments.246)

Returns

The fmax functions return the maximum numeric value of their arguments.

7.12.12.3 The fmin functions

Synopsis

      #include <math.h>
      double fmin(double x, double y);
      float fminf(float x, float y);
      long double fminl(long double x, long double y);
    

Description

The fmin functions determine the minimum numeric value of their arguments.247)

Returns

The fmin functions return the minimum numeric value of their arguments.

7.12.13 Floating multiply-add

7.12.13.1 The fma functions

Synopsis

      #include <math.h>
      double fma(double x, double y, double z);
      float fmaf(float x, float y, float z);
      long double fmal(long double x, long double y,
            long double z);
    

Description

The fma functions compute (x × y) + z, rounded as one ternary operation: they compute the value
(as if) to infinite precision and round once to the result format, according to the current rounding
mode. A range error may occur.

Returns

The fma functions return (x × y) + z, rounded as one ternary operation.

7.12.14 Comparison macros

The relational and equality operators support the usual mathematical relationships between numeric
values. For any ordered pair of numeric values exactly one of the relationships — less, greater, and
equal — is true. Relational operators may raise the “invalid” floating-point exception when argument
values are NaNs. For a NaN and a numeric value, or for two NaNs, just the unordered relationship
is true.248) The following subclauses provide macros that are quiet (non floating-point exception
raising) versions of the relational operators, and other comparison macros that facilitate writing
efficient code that accounts for NaNs without suffering the “invalid” floating-point exception. In
the synopses in this subclause, real-floating indicates that the argument shall be an expression of real
floating type249) (both arguments need not have the same type).250)

7.12.14.1 The isgreater macro

Synopsis

      #include <math.h>
      int isgreater(real-floating x, real-floating y);
    

Description

The isgreater macro determines whether its first argument is greater than its second argu-
ment. The value of isgreater(x,y) is always equal to (x)> (y) ; however, unlike (x)> (y) ,
isgreater(x,y) does not raise the “invalid” floating-point exception when x and y are unordered.

Returns

The isgreater macro returns the value of (x)> (y) .

7.12.14.2 The isgreaterequal macro

Synopsis

      #include <math.h>
      int isgreaterequal(real-floating x, real-floating y);
    

Description

The isgreaterequal macro determines whether its first argument is greater than or equal to its
second argument. The value of isgreaterequal(x,y) is always equal to (x)>= (y) ; however,
unlike (x)>= (y) , isgreaterequal(x,y) does not raise the “invalid” floating-point exception
when x and y are unordered.

Returns

The isgreaterequal macro returns the value of (x)>= (y) .

7.12.14.3 The isless macro

Synopsis

      #include <math.h>
      int isless(real-floating x, real-floating y);
    

Description

The isless macro determines whether its first argument is less than its second argument. The value
of isless(x,y) is always equal to (x)< (y) ; however, unlike (x)< (y) , isless(x,y) does not

raise the “invalid” floating-point exception when x and y are unordered.

Returns

The isless macro returns the value of (x) < (y).

7.12.14.4 The islessequal macro

Synopsis

      #include <math.h>
      int islessequal(real-floating x, real-floating y);
    

Description

The islessequal macro determines whether its first argument is less than or equal to its sec-
ond argument. The value of islessequal(x,y) is always equal to (x)<= (y) ; however, unlike
(x)<= (y) , islessequal(x,y) does not raise the “invalid” floating-point exception when x and y
are unordered.

Returns

The islessequal macro returns the value of (x)<= (y) .

7.12.14.5 The islessgreater macro

Synopsis

      #include <math.h>
      int islessgreater(real-floating x, real-floating y);
    

Description

The islessgreater macro determines whether its first argument is less than or greater than its
second argument. The islessgreater(x,y) macro is similar to (x)< (y)|| (x)> (y) ; however,
islessgreater(x,y) does not raise the “invalid” floating-point exception when x and y are un-
ordered (nor does it evaluate x and y twice).

Returns

The islessgreater macro returns the value of (x)< (y)|| (x)> (y) .

7.12.14.6 The isunordered macro

Synopsis

      #include <math.h>
      int isunordered(real-floating x, real-floating y);
    

Description

The isunordered macro determines whether its arguments are unordered.

Returns

The isunordered macro returns 1 if its arguments are unordered and 0 otherwise.

7.13 Nonlocal jumps <setjmp.h>

The header <setjmp.h> defines the macro setjmp, and declares one function and one type, for
bypassing the normal function call and return discipline.251)

The type declared is

    jmp_buf
  

which is an array type suitable for holding the information needed to restore a calling environment.
The environment of a call to the setjmp macro consists of information sufficient for a call to the
longjmp function to return execution to the correct block and invocation of that block, were it called
recursively. It does not include the state of the floating-point status flags, of open files, or of any
other component of the abstract machine.

It is unspecified whether setjmp is a macro or an identifier declared with external linkage. If a
macro definition is suppressed in order to access an actual function, or a program defines an external
identifier with the name setjmp, the behavior is undefined.

7.13.1 Save calling environment

7.13.1.1 The setjmp macro

Synopsis

      #include <setjmp.h>
      int setjmp(jmp_buf env);
    

Description

The setjmp macro saves its calling environment in its jmp_buf argument for later use by the
longjmp function.

Returns

If the return is from a direct invocation, the setjmp macro returns the value zero. If the return is
from a call to the longjmp function, the setjmp macro returns a nonzero value.

Environmental limits

An invocation of the setjmp macro shall appear only in one of the following contexts:

If the invocation appears in any other context, the behavior is undefined.

7.13.2 Restore calling environment

7.13.2.1 The longjmp function

Synopsis

      #include <setjmp.h>
      _Noreturn void longjmp(jmp_buf env, int val);
    

Description

The longjmp function restores the environment saved by the most recent invocation of the setjmp
macro in the same invocation of the program with the corresponding jmp_buf argument. If there
has been no such invocation, or if the invocation was from another thread of execution, or if the
function containing the invocation of the setjmp macro has terminated execution252) in the interim,
or if the invocation of the setjmp macro was within the scope of an identifier with variably modified
type and execution has left that scope in the interim, the behavior is undefined.

All accessible objects have values, and all other components of the abstract machine253) have state,
as of the time the longjmp function was called, except that the values of objects of automatic storage
duration that are local to the function containing the invocation of the corresponding setjmp macro
that do not have volatile-qualified type and have been changed between the setjmp invocation and
longjmp call are indeterminate.

Returns

After longjmp is completed, thread execution continues as if the corresponding invocation of the
setjmp macro had just returned the value specified by val. The longjmp function cannot cause the
setjmp macro to return the value 0; if val is 0, the setjmp macro returns the value 1.

EXAMPLE The longjmp function that returns control back to the point of the setjmp invocation might cause memory
associated with a variable length array object to be squandered.

    #include <setjmp.h>
    jmp_buf buf;
    void g(int n);
    void h(int n);
    int n = 6;
  
    void f(void)
    {
          int x[n];                // valid:     f is not terminated
          setjmp(buf);
          g(n);
    }
  
    void g(int n)
    {
          int a[n];                // a may remain allocated
          h(n);
    }
  
    void h(int n)
    {
          int b[n];                // b may remain allocated
          longjmp(buf, 2);         // might cause memory loss
    }
  

7.14 Signal handling <signal.h>

The header <signal.h> declares a type and two functions and defines several macros, for handling
various signals (conditions that may be reported during program execution).

The type defined is

    sig_atomic_t
  

which is the (possibly volatile-qualified) integer type of an object that can be accessed as an atomic
entity, even in the presence of asynchronous interrupts.

The macros defined are

    SIG_DFL
    SIG_ERR
    SIG_IGN
  

which expand to constant expressions with distinct values that have type compatible with the second
argument to, and the return value of, the signal function, and whose values compare unequal to
the address of any declarable function; and the following, which expand to positive integer constant
expressions with type int and distinct values that are the signal numbers, each corresponding to
the specified condition:

SIGABRT abnormal termination, such as is initiated by the abort function

SIGFPE
an erroneous arithmetic operation, such as zero divide or an operation resulting in
overflow
SIGILL
detection of an invalid function image, such as an invalid instruction
SIGINT
receipt of an interactive attention signal

SIGSEGV an invalid access to storage

SIGTERM a termination request sent to the program

An implementation need not generate any of these signals, except as a result of explicit calls to the
raise function. Additional signals and pointers to undeclarable functions, with macro definitions
beginning, respectively, with the letters SIG and an uppercase letter or with SIG_ and an uppercase
letter,254) may also be specified by the implementation. The complete set of signals, their semantics,
and their default handling is implementation-defined; all signal numbers shall be positive.

7.14.1 Specify signal handling

7.14.1.1 The signal function

Synopsis

      #include <signal.h>
      void (*signal(int sig, void (*func)(int)))(int);
    

Description

The signal function chooses one of three ways in which receipt of the signal number sig is to
be subsequently handled. If the value of func is SIG_DFL, default handling for that signal will
occur. If the value of func is SIG_IGN, the signal will be ignored. Otherwise, func shall point to a
function to be called when that signal occurs. An invocation of such a function because of a signal, or
(recursively) of any further functions called by that invocation (other than functions in the standard
library),255) is called a signal handler.

When a signal occurs and func points to a function, it is implementation-defined whether the
equivalent of signal(sig, SIG_DFL); is executed or the implementation prevents some imple-
mentation-defined set of signals (at least including sig) from occurring until the current signal
handling has completed; in the case of SIGILL, the implementation may alternatively define that
no action is taken. Then the equivalent of (*func)(sig); is executed. If and when the function
returns, if the value of sig is SIGFPE, SIGILL, SIGSEGV, or any other implementation-defined value
corresponding to a computational exception, the behavior is undefined; otherwise the program will
resume execution at the point it was interrupted.

If the signal occurs as the result of calling the abort or raise function, the signal handler shall not
call the raise function.

If the signal occurs other than as the result of calling the abort or raise function, the behavior is
undefined if the signal handler refers to any object with static or thread storage duration that is
not a lock-free atomic object other than by assigning a value to an object declared as volatile
sig_atomic_t, or the signal handler calls any function in the standard library other than

At program startup, the equivalent of

    signal(sig, SIG_IGN);
  

may be executed for some signals selected in an implementation-defined manner; the equivalent of

    signal(sig, SIG_DFL);
  

is executed for all other signals defined by the implementation.

Use of this function in a multi-threaded program results in undefined behavior. The implementation
shall behave as if no library function calls the signal function.

Returns

If the request can be honored, the signal function returns the value of func for the most recent
successful call to signal for the specified signal sig. Otherwise, a value of SIG_ERR is returned and
a positive value is stored in errno.

Forward references: the abort function (7.22.4.1), the exit function (7.22.4.4), the _Exit function
(7.22.4.5), the quick_exit function (7.22.4.7).

7.14.2 Send signal

7.14.2.1 The raise function

Synopsis

      #include <signal.h>
      int raise(int sig);
    

Description

The raise function carries out the actions described in 7.14.1.1 for the signal sig. If a signal handler
is called, the raise function shall not return until after the signal handler does.

Returns

The raise function returns zero if successful, nonzero if unsuccessful.

7.15 Alignment <stdalign.h>

The header <stdalign.h> defines four macros.

The macro

    alignas
  

expands to _Alignas ; the macro

    alignof
  

expands to _Alignof .

The remaining macros are suitable for use in #if preprocessing directives. They are

    __alignas_is_defined
  

and

    __alignof_is_defined
  

which both expand to the integer constant 1.

7.16 Variable arguments <stdarg.h>

The header <stdarg.h> declares a type and defines four macros, for advancing through a list of
arguments whose number and types are not known to the called function when it is translated.

A function may be called with a variable number of arguments of varying types. As described in
6.9.1, its parameter list contains one or more parameters. The rightmost parameter plays a special
role in the access mechanism, and will be designated parmN in this description.

The type declared is

    va_list
  

which is a complete object type suitable for holding information needed by the macros va_start,
va_arg, va_end, and va_copy . If access to the varying arguments is desired, the called function
shall declare an object (generally referred to as ap in this subclause) having type va_list. The object
ap may be passed as an argument to another function; if that function invokes the va_arg macro
with parameter ap, the value of ap in the calling function is indeterminate and shall be passed to the
va_end macro prior to any further reference to ap.257)

7.16.1 Variable argument list access macros

The va_start and va_arg macros described in this subclause shall be implemented as macros,
not functions. It is unspecified whether va_copy and va_end are macros or identifiers declared
with external linkage. If a macro definition is suppressed in order to access an actual function,
or a program defines an external identifier with the same name, the behavior is undefined. Each
invocation of the va_start and va_copy macros shall be matched by a corresponding invocation of
the va_end macro in the same function.

7.16.1.1 The va_arg macro

Synopsis

      #include <stdarg.h>
      type va_arg(va_list ap, type);
    

Description

The va_arg macro expands to an expression that has the specified type and the value of the next
argument in the call. The parameter ap shall have been initialized by the va_start or va_copy
macro (without an intervening invocation of the va_end macro for the same ap). Each invocation of
the va_arg macro modifies ap so that the values of successive arguments are returned in turn. The
parameter type shall be a type name specified such that the type of a pointer to an object that has the
specified type can be obtained simply by postfixing a * to type. If there is no actual next argument,
or if type is not compatible with the type of the actual next argument (as promoted according to the
default argument promotions), the behavior is undefined, except for the following cases:

Returns

The first invocation of the va_arg macro after that of the va_start macro returns the value of the
argument after that specified by parmN. Successive invocations return the values of the remaining
arguments in succession.

7.16.1.2 The va_copy macro

Synopsis

      #include <stdarg.h>
      void va_copy(va_list dest, va_list src);
    

Description

The va_copy macro initializes dest as a copy of src, as if the va_start macro had been applied
to dest followed by the same sequence of uses of the va_arg macro as had previously been used
to reach the present state of src. Neither the va_copy nor va_start macro shall be invoked to
reinitialize dest without an intervening invocation of the va_end macro for the same dest.

Returns

The va_copy macro returns no value.

7.16.1.3 The va_end macro

Synopsis

      #include <stdarg.h>
      void va_end(va_list ap);
    

Description

The va_end macro facilitates a normal return from the function whose variable argument list was
referred to by the expansion of the va_start macro, or the function containing the expansion of
the va_copy macro, that initialized the va_list ap. The va_end macro may modify ap so that it
is no longer usable (without being reinitialized by the va_start or va_copy macro). If there is no
corresponding invocation of the va_start or va_copy macro, or if the va_end macro is not invoked
before the return, the behavior is undefined.

Returns

The va_end macro returns no value.

7.16.1.4 The va_start macro

Synopsis

      #include <stdarg.h>
      void va_start(va_list ap, parmN);
    

Description

The va_start macro shall be invoked before any access to the unnamed arguments.

The va_start macro initializes ap for subsequent use by the va_arg and va_end macros. Neither the
va_start nor va_copy macro shall be invoked to reinitialize ap without an intervening invocation
of the va_end macro for the same ap.

The parameter parmN is the identifier of the rightmost parameter in the variable parameter list in
the function definition (the one just before the , ...). If the parameter parmN is declared with the
register storage class, with a function or array type, or with a type that is not compatible with the
type that results after application of the default argument promotions, the behavior is undefined.

Returns

The va_start macro returns no value.

EXAMPLE 1 The function f1 gathers into an array a list of arguments that are pointers to strings (but not more than MAXARGS
arguments), then passes the array as a single argument to function f2. The number of pointers is specified by the first
argument to f1.

    #include <stdarg.h>
    #define MAXARGS   31
  
    void f1(int n_ptrs, ...)
    {
          va_list ap;
          char *array[MAXARGS];
          int ptr_no = 0;
  
             if (n_ptrs > MAXARGS)
                   n_ptrs = MAXARGS;
             va_start(ap, n_ptrs);
             while (ptr_no < n_ptrs)
                   array[ptr_no++] = va_arg(ap, char *);
             va_end(ap);
             f2(n_ptrs, array);
    }
  

Each call to f1 is required to have visible the definition of the function or a declaration such as

    void f1(int, ...);
  

EXAMPLE 2 The function f3 is similar, but saves the status of the variable argument list after the indicated number of
arguments; after f2 has been called once with the whole list, the trailing part of the list is gathered again and passed to
function f4.

    #include <stdarg.h>
    #define MAXARGS 31
  
    void f3(int n_ptrs, int f4_after, ...)
    {
          va_list ap, ap_save;
          char *array[MAXARGS];
          int ptr_no = 0;
          if (n_ptrs > MAXARGS)
                n_ptrs = MAXARGS;
          va start(ap, f4_after);
            _
          while (ptr_no < n_ptrs) {
                array[ptr_no++] = va_arg(ap, char *);
                if (ptr_no == f4_after)
                      va_copy(ap_save, ap);
          }
          va_end(ap);
          f2(n_ptrs, array);
  
    // Now process the saved copy.
  
             n_ptrs -= f4_after;
             ptr_no = 0;
             while (ptr_no < n_ptrs)
                   array[ptr_no++] = va_arg(ap_save, char *);
             va_end(ap_save);
             f4(n_ptrs, array);
    }
  

7.17 Atomics <stdatomic.h>

7.17.1 Introduction

The header <stdatomic.h> defines several macros and declares several types and functions for
performing atomic operations on data shared between threads.258)

Implementations that define the macro __STDC_NO_ATOMICS__ need not provide this header nor
support any of its facilities.

The macros defined are the atomic lock-free macros

    ATOMIC_BOOL_LOCK_FREE
    ATOMIC_CHAR_LOCK_FREE
    ATOMIC_CHAR16_T_LOCK_FREE
    ATOMIC_CHAR32_T_LOCK_FREE
    ATOMIC_WCHAR_T_LOCK_FREE
    ATOMIC_SHORT_LOCK_FREE
    ATOMIC_INT_LOCK_FREE
    ATOMIC_LONG_LOCK_FREE
    ATOMIC_LLONG_LOCK_FREE
    ATOMIC_POINTER_LOCK_FREE
  

which expand to constant expressions suitable for use in #if preprocessing directives and which
indicate the lock-free property of the corresponding atomic types (both signed and unsigned); and

    ATOMIC_FLAG_INIT
  

which expands to an initializer for an object of type atomic_flag.

The types include

    memory_order
  

which is an enumerated type whose enumerators identify memory ordering constraints;

    atomic_flag
  

which is a structure type representing a lock-free, primitive atomic flag; and several atomic analogs
of integer types.

In the following synopses:

It is unspecified whether any generic function declared in <stdatomic.h> is a macro or an identifier
declared with external linkage. If a macro definition is suppressed in order to access an actual
function, or a program defines an external identifier with the name of a generic function, the
behavior is undefined.

NOTE Many operations are volatile-qualified. The “volatile as device register” semantics have not changed in the standard.
This qualification means that volatility is preserved when applying these operations to volatile objects.

7.17.2 Initialization

7.17.2.1 The ATOMIC_VAR_INIT macro

Synopsis

      #include <stdatomic.h>
      #define ATOMIC_VAR_INIT(C value)
    

Description

The ATOMIC_VAR_INIT macro expands to a token sequence suitable for initializing an atomic object
of a type that is initialization-compatible with value. An atomic object with automatic storage
duration that is not explicitly initialized is initially in an indeterminate state; however, the default
(zero) initialization for objects with static or thread-local storage duration is guaranteed to produce
a valid state.

Concurrent access to the variable being initialized, even via an atomic operation, constitutes a data
race.

EXAMPLE

    atomic_int guide = ATOMIC_VAR_INIT(42);
  

7.17.2.2 The atomic_init generic function

Synopsis

      #include <stdatomic.h>
      void atomic_init(volatile A *obj, C value);
    

Description

The atomic_init generic function initializes the atomic object pointed to by obj to the value value,
while also initializing any additional state that the implementation might need to carry for the
atomic object.

Although this function initializes an atomic object, it does not avoid data races; concurrent access to
the variable being initialized, even via an atomic operation, constitutes a data race.

If a signal occurs other than as the result of calling the abort or raise functions, the behavior is
undefined if the signal handler calls the atomic_init generic function.

Returns

The atomic_init generic function returns no value.

EXAMPLE

    atomic_int guide;
    atomic_init(&guide, 42);
  

7.17.3 Order and consistency

The enumerated type memory_order specifies the detailed regular (non-atomic) memory synchro-
nization operations as defined in 5.1.2.4 and may provide for operation ordering. Its enumeration
constants are as follows:259)

    memory_order_relaxed
    memory_order_consume
    memory_order_acquire
    memory_order_release
    memory_order_acq_rel
    memory_order_seq_cst
  

For memory_order_relaxed, no operation orders memory.

For memory_order_release, memory_order_acq_rel, and memory_order_seq_cst, a store opera-
tion performs a release operation on the affected memory location.

For memory_order_acquire, memory_order_acq_rel, and memory_order_seq_cst, a load opera-
tion performs an acquire operation on the affected memory location.

For memory_order_consume, a load operation performs a consume operation on the affected mem-
ory location.

There shall be a single total order S on all memory_order_seq_cst operations, consistent with the
“happens before” order and modification orders for all affected locations, such that each
memory_order_seq_cst operation B that loads a value from an atomic object M observes one of
the following values:

NOTE 1 Although it is not explicitly required that S include lock operations, it can always be extended to an order that does
include lock and unlock operations, since the ordering between those is already included in the “happens before” ordering.

NOTE 2 Atomic operations specifying memory_order_relaxed are relaxed only with respect to memory ordering. Imple-
mentations must still guarantee that any given atomic access to a particular atomic object be indivisible with respect to all
other atomic accesses to that object.

For an atomic operation B that reads the value of an atomic object M , if there is a
memory_order_seq_cst fence X sequenced before B, then B observes either the last
memory_order_seq_cst modification of M preceding X in the total order S or a later mod-
ification of M in its modification order.

For atomic operations A and B on an atomic object M , where A modifies M and B takes its value, if
there is a memory_order_seq_cst fence X such that A is sequenced before X and B follows X in S,
then B observes either the effects of A or a later modification of M in its modification order.

For atomic modifications A and B of an atomic object M , B occurs later than A in the modification
order of M if:

Atomic read-modify-write operations shall always read the last value (in the modification order)
stored before the write associated with the read-modify-write operation.

An atomic store shall only store a value that has been computed from constants and program input
values by a finite sequence of program evaluations, such that each evaluation observes the values of
variables as computed by the last prior assignment in the sequence. The ordering of evaluations in
this sequence shall be such that

NOTE 3 The second requirement disallows “out-of-thin-air”, or “speculative” stores of atomics when relaxed atomics are
used. Since unordered operations are involved, evaluations may appear in this sequence out of thread order. For example,
with x and y initially zero,

    // Thread 1:
    r1 = atomic_load_explicit(&y, memory_order_relaxed);
    atomic_store_explicit(&x, r1, memory_order_relaxed);
  
    // Thread 2:
    r2 = atomic_load_explicit(&x, memory_order_relaxed);
    atomic_store_explicit(&y, 42, memory_order_relaxed);
  

is allowed to produce r1 == 42 && r2 == 42. The sequence of evaluations justifying this consists of:

    atomic_store_explicit(&y, 42,           memory_order_relaxed);
    r1 = atomic_load_explicit(&y,           memory_order_relaxed);
    atomic_store_explicit(&x, r1,           memory_order_relaxed);
    r2 = atomic_load_explicit(&x,           memory_order_relaxed);
  

On the other hand,

    // Thread 1:
    r1 = atomic_load_explicit(&y, memory_order_relaxed);
    atomic_store_explicit(&x, r1, memory_order_relaxed);
  
    // Thread 2:
    r2 = atomic_load_explicit(&x, memory_order_relaxed);
    atomic_store_explicit(&y, r2, memory_order_relaxed);
  

is not allowed to produce r1 == 42 && r2 = 42, since there is no sequence of evaluations that results in the computation
of 42. In the absence of “relaxed” operations and read-modify-write operations with weaker than memory_order_acq_rel
ordering, the second requirement has no impact.

Recommended practice

The requirements do not forbid r1 == 42 && r2 == 42 in the following example, with x and y
initially zero:

    // Thread 1:
    r1 = atomic_load_explicit(&x, memory_order_relaxed);
    if (r1 == 42)
          atomic_store_explicit(&y, r1, memory_order_relaxed);
  
    // Thread 2:
    r2 = atomic_load_explicit(&y, memory_order_relaxed);
    if (r2 == 42)
          atomic_store_explicit(&x, 42, memory_order_relaxed);
  

However, this is not useful behavior, and implementations should not allow it.

Implementations should make atomic stores visible to atomic loads within a reasonable amount of
time.

7.17.3.1 The kill_dependency macro

Synopsis

      #include <stdatomic.h>
      type kill_dependency(type y);
    

Description

The kill_dependency macro terminates a dependency chain; the argument does not carry a depen-
dency to the return value.

Returns

The kill_dependency macro returns the value of y.

7.17.4 Fences

This subclause introduces synchronization primitives called fences. Fences can have acquire seman-
tics, release semantics, or both. A fence with acquire semantics is called an acquire fence; a fence with
release semantics is called a release fence.

A release fence A synchronizes with an acquire fence B if there exist atomic operations X and Y ,
both operating on some atomic object M , such that A is sequenced before X, X modifies M , Y is
sequenced before B, and Y reads the value written by X or a value written by any side effect in the
hypothetical release sequence X would head if it were a release operation.

A release fence A synchronizes with an atomic operation B that performs an acquire operation on an
atomic object M if there exists an atomic operation X such that A is sequenced before X, X modifies
M , and B reads the value written by X or a value written by any side effect in the hypothetical
release sequence X would head if it were a release operation.

An atomic operation A that is a release operation on an atomic object M synchronizes with an
acquire fence B if there exists some atomic operation X on M such that X is sequenced before B
and reads the value written by A or a value written by any side effect in the release sequence headed
by A.

7.17.4.1 The atomic_thread_fence function

Synopsis

      #include <stdatomic.h>
      void atomic_thread_fence(memory_order order);
    

Description

Depending on the value of order, this operation:

Returns

The atomic_thread_fence function returns no value.

7.17.4.2 The atomic_signal_fence function

Synopsis

      #include <stdatomic.h>
      void atomic_signal_fence(memory_order order);
    

Description

Equivalent to atomic_thread_fence(order), except that the resulting ordering constraints are
established only between a thread and a signal handler executed in the same thread.

NOTE 1 The atomic_signal_fence function can be used to specify the order in which actions performed by the thread
become visible to the signal handler.

NOTE 2 Compiler optimizations and reorderings of loads and stores are inhibited in the same way as with
atomic_thread_fence, but the hardware fence instructions that atomic_thread_fence would have inserted are not
emitted.

Returns

The atomic_signal_fence function returns no value.

7.17.5 Lock-free property

The atomic lock-free macros indicate the lock-free property of integer and address atomic types. A
value of 0 indicates that the type is never lock-free; a value of 1 indicates that the type is sometimes
lock-free; a value of 2 indicates that the type is always lock-free.

NOTE Operations that are lock-free should also be address-free. That is, atomic operations on the same memory location via
two different addresses will communicate atomically. The implementation should not depend on any per-process state. This
restriction enables communication via memory mapped into a process more than once and memory shared between two
processes.

7.17.5.1 The atomic_is_lock_free generic function

Synopsis

      #include <stdatomic.h>
      _Bool atomic_is_lock_free(const volatile A *obj);
    

Description

The atomic_is_lock_free generic function indicates whether or not atomic operations on objects
of the type pointed to by obj are lock-free.

Returns

The atomic_is_lock_free generic function returns nonzero (true) if and only if atomic operations
on objects of the type pointed to by the argument are lock-free. In any given program execution, the
result of the lock-free query shall be consistent for all pointers of the same type.260)

7.17.6 Atomic integer types

For each line in the following table,261) the atomic type name is declared as a type that has the same
representation and alignment requirements as the corresponding direct type.262)

    Atomic type name                   Direct type
    atomic_bool                        _Atomic     _Bool
    atomic_char                        _Atomic     char
    atomic_schar                       _Atomic     signed char
    atomic_uchar                       _Atomic     unsigned char
    atomic_short                       _Atomic     short
    atomic_ushort                      _Atomic     unsigned short
    atomic_int                         _Atomic     int
    atomic_uint                        _Atomic     unsigned int
    atomic_long                        _Atomic     long
    atomic_ulong                       _Atomic     unsigned long
    atomic_llong                       _Atomic     long long
    atomic_ullong                      _Atomic     unsigned long long
    atomic_char16_t                    _Atomic     char16_t
    atomic_char32_t                    _Atomic     char32_t
    atomic_wchar_t                     _Atomic     wchar_t
    atomic_int_least8_t                _Atomic     int_least8_t
    atomic_uint_least8_t               _Atomic     uint_least8_t
    atomic_int_least16_t               _Atomic     int_least16_t
    atomic_uint_least16_t              _Atomic     uint_least16_t
    atomic_int_least32_t               _Atomic     int_least32_t
    atomic_uint_least32_t              _Atomic     uint_least32_t
  
    Atomic type name                  Direct type
    atomic_int_least64_t              _Atomic    int_least64_t
    atomic_uint_least64_t             _Atomic    uint_least64_t
    atomic_int_fast8_t                _Atomic    int_fast8_t
    atomic_uint_fast8_t               _Atomic    uint_fast8_t
    atomic_int_fast16_t               _Atomic    int_fast16_t
    atomic_uint_fast16_t              _Atomic    uint_fast16_t
    atomic_int_fast32_t               _Atomic    int_fast32_t
    atomic_uint_fast32_t              _Atomic    uint_fast32_t
    atomic_int_fast64_t               _Atomic    int_fast64_t
    atomic_uint_fast64_t              _Atomic    uint_fast64_t
    atomic_intptr_t                   _Atomic    intptr_t
    atomic_uintptr_t                  _Atomic    uintptr_t
    atomic_size_t                     _Atomic    size_t
    atomic_ptrdiff_t                  _Atomic    ptrdiff_t
    atomic_intmax_t                   _Atomic    intmax_t
    atomic_uintmax_t                  _Atomic    uintmax_t
  

NOTE The representation of atomic integer types need not have the same size as their corresponding regular types. They
should have the same size whenever possible, as it eases effort required to port existing code.

7.17.7 Operations on atomic types

There are only a few kinds of operations on atomic types, though there are many instances of those
kinds. This subclause specifies each general kind.

7.17.7.1 The atomic_store generic functions

Synopsis

      #include <stdatomic.h>
      void atomic_store(volatile A *object, C desired);
      void atomic_store_explicit(volatile A *object,
            C desired, memory_order order);
    

Description

The order argument shall not be memory_order_acquire, memory_order_consume, nor
memory_order_acq_rel. Atomically replace the value pointed to by object with the value of
desired. Memory is affected according to the value of order.

Returns

The atomic_store generic functions return no value.

7.17.7.2 The atomic_load generic functions

Synopsis

      #include <stdatomic.h>
      C atomic_load(const volatile A *object);
      C atomic_load_explicit(const volatile A *object,
            memory_order order);
    

Description

The order argument shall not be memory_order_release nor memory_order_acq_rel. Memory is
affected according to the value of order.

Returns
Atomically returns the value pointed to by object.

7.17.7.3 The atomic_exchange generic functions

Synopsis

      #include <stdatomic.h>
      C atomic_exchange(volatile A *object, C desired);
      C atomic_exchange_explicit(volatile A *object,
            C desired, memory_order order);
    

Description

Atomically replace the value pointed to by object with desired. Memory is affected according to
the value of order. These operations are read-modify-write operations (5.1.2.4).

Returns

Atomically returns the value pointed to by object immediately before the effects.

7.17.7.4 The atomic_compare_exchange generic functions

Synopsis

      #include <stdatomic.h>
      _Bool atomic_compare_exchange_strong(volatile A *object,
            C *expected, C desired);
      _Bool atomic_compare_exchange_strong_explicit(
            volatile A *object, C *expected, C desired,
            memory_order success, memory_order failure);
      _Bool atomic_compare_exchange_weak(volatile A *object,
            C *expected, C desired);
      _Bool atomic_compare_exchange_weak_explicit(
            volatile A *object, C *expected, C desired,
            memory_order success, memory_order failure);
    

Description

The failure argument shall not be memory_order_release nor memory_order_acq_rel. The
failure argument shall be no stronger than the success argument.

Atomically, compares the contents of the memory pointed to by object for equality with that
pointed to by expected, and if true, replaces the contents of the memory pointed to by object
with desired, and if false, updates the contents of the memory pointed to by expected with that
pointed to by object. Further, if the comparison is true, memory is affected according to the value
of success, and if the comparison is false, memory is affected according to the value of failure.
These operations are atomic read-modify-write operations (5.1.2.4).

NOTE 1 For example, the effect of atomic_compare_exchange_strong is

    if (memcmp(object, expected, sizeof (*object)) == 0)
          memcpy(object, &desired, sizeof (*object));
    else
          memcpy(expected, object, sizeof (*object));
  

A weak compare-and-exchange operation may fail spuriously. That is, even when the contents
of memory referred to by expected and object are equal, it may return zero and store back to
expected the same memory contents that were originally there.

NOTE 2 This spurious failure enables implementation of compare-and-exchange on a broader class of machines, e.g.
load-locked store-conditional machines.

EXAMPLE A consequence of spurious failure is that nearly all uses of weak compare-and-exchange will be in a loop.

    exp = atomic_load(&cur);
    do {
          des = function(exp);
    } while (!atomic_compare_exchange_weak(&cur, &exp, des));
  

When a compare-and-exchange is in a loop, the weak version will yield better performance on some platforms. When a weak
compare-and-exchange would require a loop and a strong one would not, the strong one is preferable.

Returns

The result of the comparison.

7.17.7.5 The atomic_fetch and modify generic functions

The following operations perform arithmetic and bitwise computations. All of these operations
are applicable to an object of any atomic integer type. None of these operations is applicable to
atomic_bool. The key, operator, and computation correspondence is:

key op computation
add + addition
sub - subtraction
or | bitwise inclusive or
xor ^ bitwise exclusive or
and & bitwise and

Synopsis

      #include <stdatomic.h>
      C atomic_fetch_key(volatile A *object, M operand);
      C atomic_fetch_key_explicit(volatile A *object,
            M operand, memory_order order);
    

Description

Atomically replaces the value pointed to by object with the result of the computation applied to
the value pointed to by object and the given operand. Memory is affected according to the value of
order. These operations are atomic read-modify-write operations (5.1.2.4). For signed integer types,
arithmetic is defined to use two’s complement representation with silent wrap-around on overflow;
there are no undefined results. For address types, the result may be an undefined address, but the
operations otherwise have no undefined behavior.

Returns

Atomically, the value pointed to by object immediately before the effects.

NOTE The operation of the atomic_fetch and modify generic functions are nearly equivalent to the operation of the
corresponding op= compound assignment operators. The only differences are that the compound assignment operators are
not guaranteed to operate atomically, and the value yielded by a compound assignment operator is the updated value of the
object, whereas the value returned by the atomic_fetch and modify generic functions is the previous value of the atomic
object.

7.17.8 Atomic flag type and operations

The atomic_flag type provides the classic test-and-set functionality. It has two states, set and clear.

Operations on an object of type atomic_flag shall be lock free.

NOTE Hence the operations should also be address-free. No other type requires lock-free operations, so the atomic_flag
type is the minimum hardware-implemented type needed to conform to this International standard. The remaining types can
be emulated with atomic_flag, though with less than ideal properties.

The macro ATOMIC_FLAG_INIT may be used to initialize an atomic_flag to the clear state. An
atomic_flag that is not explicitly initialized with ATOMIC_FLAG_INIT is initially in an indeterminate
state.

EXAMPLE

    atomic_flag guard = ATOMIC_FLAG_INIT;
  

7.17.8.1 The atomic_flag_test_and_set functions

Synopsis

      #include <stdatomic.h>
      _Bool atomic_flag_test_and_set(
            volatile atomic_flag *object);
      _Bool atomic_flag_test_and_set_explicit(
            volatile atomic_flag *object, memory_order order);
    

Description

Atomically places the atomic flag pointed to by object in the set state and returns the value
corresponding to the immediately preceding state. Memory is affected according to the value of
order. These operations are atomic read-modify-write operations (5.1.2.4).

Returns

The atomic_flag_test_and_set functions return the value that corresponds to the state of the
atomic flag immediately before the effects. The return value true corresponds to the set state and the
return value false corresponds to the clear state.

7.17.8.2 The atomic_flag_clear functions

Synopsis

      #include <stdatomic.h>
      void atomic_flag_clear(volatile atomic_flag *object);
      void atomic_flag_clear_explicit(
            volatile atomic_flag *object, memory_order order);
    

Description

The order argument shall not be memory_order_acquire nor memory_order_acq_rel. Atomically
places the atomic flag pointed to by object into the clear state. Memory is affected according to the
value of order.

Returns

The atomic_flag_clear functions return no value.

7.18 Boolean type and values <stdbool.h>

The header <stdbool.h> defines four macros.

The macro

    bool
  

expands to _Bool .

The remaining three macros are suitable for use in #if preprocessing directives. They are

    true
  

which expands to the integer constant 1,

    false
  

which expands to the integer constant 0, and

    __bool_true_false_are_defined
  

which expands to the integer constant 1.

Notwithstanding the provisions of 7.1.3, a program may undefine and perhaps then redefine the
macros bool, true, and false.263)

7.19 Common definitions <stddef.h>

The header <stddef.h> defines the following macros and declares the following types. Some are
also defined in other headers, as noted in their respective subclauses.

The types are

    ptrdiff_t
  

which is the signed integer type of the result of subtracting two pointers;

    size_t
  

which is the unsigned integer type of the result of the sizeof operator;

    max_align_t
  

which is an object type whose alignment is the greatest fundamental alignment; and

    wchar_t
  

which is an integer type whose range of values can represent distinct codes for all members of the
largest extended character set specified among the supported locales; the null character shall have
the code value zero. Each member of the basic character set shall have a code value equal to its
value when used as the lone character in an integer character constant if an implementation does
not define __STDC_MB_MIGHT_NEQ_WC__ .

The macros are

    NULL
  

which expands to an implementation-defined null pointer constant; and

    offsetof(type, member-designator)
  

which expands to an integer constant expression that has type size_t, the value of which is the
offset in bytes, to the structure member (designated by member-designator), from the beginning of its
structure (designated by type). The type and member designator shall be such that given

    static type t;
  

then the expression &(t. member-designator) evaluates to an address constant. (If the specified
member is a bit-field, the behavior is undefined.)

Recommended practice

The types used for size_t and ptrdiff_t should not have an integer conversion rank greater than
that of signed long int unless the implementation supports objects large enough to make this
necessary.

7.20 Integer types <stdint.h>

The header <stdint.h> declares sets of integer types having specified widths, and defines corre-
sponding sets of macros.264) It also defines macros that specify limits of integer types corresponding
to types defined in other standard headers.

Types are defined in the following categories:

(Some of these types may denote the same type.)

Corresponding macros specify limits of the declared types and construct suitable constants.

For each type described herein that the implementation provides,265) <stdint.h> shall declare that
typedef name and define the associated macros. Conversely, for each type described herein that
the implementation does not provide, <stdint.h> shall not declare that typedef name nor shall it
define the associated macros. An implementation shall provide those types described as “required”,
but need not provide any of the others (described as “optional”).

7.20.1 Integer types

When typedef names differing only in the absence or presence of the initial u are defined, they shall
denote corresponding signed and unsigned types as described in 6.2.5; an implementation providing
one of these corresponding types shall also provide the other.

In the following descriptions, the symbol N represents an unsigned decimal integer with no leading
zeros (e.g., 8 or 24, but not 04 or 048).

7.20.1.1 Exact-width integer types

The typedef name intN_t designates a signed integer type with width N, no padding bits, and a
two’s complement representation. Thus, int8_t denotes such a signed integer type with a width of
exactly 8 bits.

The typedef name uintN_t designates an unsigned integer type with width N and no padding bits.
Thus, uint24_t denotes such an unsigned integer type with a width of exactly 24 bits.

These types are optional. However, if an implementation provides integer types with widths of
8, 16, 32, or 64 bits, no padding bits, and (for the signed types) that have a two’s complement
representation, it shall define the corresponding typedef names.

7.20.1.2 Minimum-width integer types

The typedef name int_leastN_t designates a signed integer type with a width of at least N, such
that no signed integer type with lesser size has at least the specified width. Thus, int_least32_t
denotes a signed integer type with a width of at least 32 bits.

The typedef name uint_leastN_t designates an unsigned integer type with a width of at least
N, such that no unsigned integer type with lesser size has at least the specified width. Thus,
uint_least16_t denotes an unsigned integer type with a width of at least 16 bits.

The following types are required:

int_least8_t uint_least8_t
int_least16_t uint_least16_t
int_least32_t uint_least32_t
int_least64_t uint_least64_t

All other types of this form are optional.

7.20.1.3 Fastest minimum-width integer types

Each of the following types designates an integer type that is usually fastest266) to operate with
among all integer types that have at least the specified width.

The typedef name int_fastN_t designates the fastest signed integer type with a width of at least
N. The typedef name uint_fastN_t designates the fastest unsigned integer type with a width of at
least N.

The following types are required:

int_fast8_t uint_fast8_t
int_fast16_t uint_fast16_t
int_fast32_t uint_fast32_t
int_fast64_t uint_fast64_t

All other types of this form are optional.

7.20.1.4 Integer types capable of holding object pointers

The following type designates a signed integer type with the property that any valid pointer to void
can be converted to this type, then converted back to pointer to void, and the result will compare
equal to the original pointer:

    intptr_t
  

The following type designates an unsigned integer type with the property that any valid pointer
to void can be converted to this type, then converted back to pointer to void, and the result will
compare equal to the original pointer:

    uintptr_t
  

These types are optional.

7.20.1.5 Greatest-width integer types

The following type designates a signed integer type capable of representing any value of any signed
integer type:

    intmax_t
  

The following type designates an unsigned integer type capable of representing any value of any
unsigned integer type:

    uintmax_t
  

These types are required.

7.20.2 Limits of specified-width integer types

The following object-like macros specify the minimum and maximum limits of the types declared in
<stdint.h>. Each macro name corresponds to a similar type name in 7.20.1.

Each instance of any defined macro shall be replaced by a constant expression suitable for use in
#if preprocessing directives, and this expression shall have the same type as would an expression
that is an object of the corresponding type converted according to the integer promotions. Its
implementation-defined value shall be equal to or greater in magnitude (absolute value) than the
corresponding value given below, with the same sign, except where stated to be exactly the given
value.

7.20.2.1 Limits of exact-width integer types

— minimum values of exact-width signed integer types

    INTN_MIN                                    exactly −(2N −1 )
  

7.20.2.2 Limits of minimum-width integer types

— minimum values of minimum-width signed integer types

    INT_LEASTN_MIN                                       −(2N −1 − 1)
  

7.20.2.3 Limits of fastest minimum-width integer types

— minimum values of fastest minimum-width signed integer types

    INT_FASTN_MIN                                        −(2N −1 − 1)
  

7.20.2.4 Limits of integer types capable of holding object pointers

— minimum value of pointer-holding signed integer type

    INTPTR_MIN                                −(215 − 1)
  

7.20.2.5 Limits of greatest-width integer types

— minimum value of greatest-width signed integer type

    INTMAX_MIN                                       −(263 − 1)
  

7.20.3 Limits of other integer types

The following object-like macros specify the minimum and maximum limits of integer types corre-
sponding to types defined in other standard headers.

Each instance of these macros shall be replaced by a constant expression suitable for use in #if
preprocessing directives, and this expression shall have the same type as would an expression
that is an object of the corresponding type converted according to the integer promotions. Its
implementation-defined value shall be equal to or greater in magnitude (absolute value) than the
corresponding value given below, with the same sign. An implementation shall define only the
macros corresponding to those typedef names it actually provides.267)

If sig_atomic_t (see 7.14) is defined as a signed integer type, the value of SIG_ATOMIC_MIN shall
be no greater than −127 and the value of SIG_ATOMIC_MAX shall be no less than 127; otherwise,
sig_atomic_t is defined as an unsigned integer type, and the value of SIG_ATOMIC_MIN shall be 0
and the value of SIG_ATOMIC_MAX shall be no less than 255.

If wchar_t (see 7.19) is defined as a signed integer type, the value of WCHAR_MIN shall be no greater
than −127 and the value of WCHAR_MAX shall be no less than 127; otherwise, wchar_t is defined as
an unsigned integer type, and the value of WCHAR_MIN shall be 0 and the value of WCHAR_MAX shall
be no less than 255.268)

If wint_t (see 7.29) is defined as a signed integer type, the value of WINT_MIN shall be no greater
than −32767 and the value of WINT_MAX shall be no less than 32767; otherwise, wint_t is defined as
an unsigned integer type, and the value of WINT_MIN shall be 0 and the value of WINT_MAX shall be
no less than 65535.

7.20.4 Macros for integer constants

The following function-like macros expand to integer constants suitable for initializing objects that
have integer types corresponding to types defined in <stdint.h>. Each macro name corresponds to
a similar type name in 7.20.1.2 or 7.20.1.5.

The argument in any instance of these macros shall be an unsuffixed integer constant (as defined in
6.4.4.1) with a value that does not exceed the limits for the corresponding type.

Each invocation of one of these macros shall expand to an integer constant expression suitable for
use in #if preprocessing directives. The type of the expression shall have the same type as would
an expression of the corresponding type converted according to the integer promotions. The value
of the expression shall be that of the argument.

7.20.4.1 Macros for minimum-width integer constants

The macro INTN_C( value) shall expand to an integer constant expression corresponding to the
type int_leastN_t . The macro UINTN_C( value) shall expand to an integer constant expression
corresponding to the type uint_leastN_t . For example, if uint_least64_t is a name for the type
unsigned long long int, then UINT64_C(0x123) might expand to the integer constant 0x123ULL.

7.20.4.2 Macros for greatest-width integer constants

The following macro expands to an integer constant expression having the value specified by its
argument and the type intmax_t:

    INTMAX_C(value)
  

The following macro expands to an integer constant expression having the value specified by its
argument and the type uintmax_t:

    UINTMAX_C(value)
  

7.21 Input/output <stdio.h>

7.21.1 Introduction

The header <stdio.h> defines several macros, and declares three types and many functions for
performing input and output.

The types declared are size_t (described in 7.19);

    FILE
  

which is an object type capable of recording all the information needed to control a stream, including
its file position indicator, a pointer to its associated buffer (if any), an error indicator that records
whether a read/write error has occurred, and an end-of-file indicator that records whether the end of
the file has been reached; and

    fpos_t
  

which is a complete object type other than an array type capable of recording all the information
needed to specify uniquely every position within a file.

The macros are NULL (described in 7.19);

    _IOFBF
    _IOLBF
    _IONBF
  

which expand to integer constant expressions with distinct values, suitable for use as the third
argument to the setvbuf function;

    BUFSIZ
  

which expands to an integer constant expression that is the size of the buffer used by the setbuf
function;

    EOF
  

which expands to an integer constant expression, with type int and a negative value, that is returned
by several functions to indicate end-of-file, that is, no more input from a stream;

    FOPEN_MAX
  

which expands to an integer constant expression that is the minimum number of files that the
implementation guarantees can be open simultaneously;

    FILENAME_MAX
  

which expands to an integer constant expression that is the size needed for an array of char large
enough to hold the longest file name string that the implementation guarantees can be opened;269)

    L_tmpnam
  

which expands to an integer constant expression that is the size needed for an array of char large
enough to hold a temporary file name string generated by the tmpnam function;

    SEEK_CUR
    SEEK_END
    SEEK_SET
  

which expand to integer constant expressions with distinct values, suitable for use as the third
argument to the fseek function;

    TMP_MAX
  

which expands to an integer constant expression that is the minimum number of unique file names
that can be generated by the tmpnam function;

    stderr
    stdin
    stdout
  

which are expressions of type “pointer to FILE” that point to the FILE objects associated, respectively,
with the standard error, input, and output streams.

The header <wchar.h> declares a number of functions useful for wide character input and output.
The wide character input/output functions described in that subclause provide operations analogous
to most of those described here, except that the fundamental units internal to the program are
wide characters. The external representation (in the file) is a sequence of “generalized” multibyte
characters, as described further in 7.21.3.

The input/output functions are given the following collective terms:

Forward references: files (7.21.3), the fseek function (7.21.9.2), streams (7.21.2), the tmpnam func-
tion (7.21.4.4), <wchar.h> (7.29).

7.21.2 Streams

Input and output, whether to or from physical devices such as terminals and tape drives, or whether
to or from files supported on structured storage devices, are mapped into logical data streams, whose
properties are more uniform than their various inputs and outputs. Two forms of mapping are
supported, for text streams and for binary streams.270)

A text stream is an ordered sequence of characters composed into lines, each line consisting of
zero or more characters plus a terminating new-line character. Whether the last line requires a
terminating new-line character is implementation-defined. Characters may have to be added, altered,
or deleted on input and output to conform to differing conventions for representing text in the host
environment. Thus, there need not be a one-to-one correspondence between the characters in a

stream and those in the external representation. Data read in from a text stream will necessarily
compare equal to the data that were earlier written out to that stream only if: the data consist only
of printing characters and the control characters horizontal tab and new-line; no new-line character
is immediately preceded by space characters; and the last character is a new-line character. Whether
space characters that are written out immediately before a new-line character appear when read in
is implementation-defined.

A binary stream is an ordered sequence of characters that can transparently record internal data.
Data read in from a binary stream shall compare equal to the data that were earlier written out to
that stream, under the same implementation. Such a stream may, however, have an implementation-
defined number of null characters appended to the end of the stream.

Each stream has an orientation. After a stream is associated with an external file, but before any
operations are performed on it, the stream is without orientation. Once a wide character input/out-
put function has been applied to a stream without orientation, the stream becomes a wide-oriented
stream. Similarly, once a byte input/output function has been applied to a stream without orien-
tation, the stream becomes a byte-oriented stream. Only a call to the freopen function or the fwide
function can otherwise alter the orientation of a stream. (A successful call to freopen removes any
orientation.)271)

Byte input/output functions shall not be applied to a wide-oriented stream and wide character
input/output functions shall not be applied to a byte-oriented stream. The remaining stream
operations do not affect, and are not affected by, a stream’s orientation, except for the following
additional restrictions:

Each wide-oriented stream has an associated mbstate_t object that stores the current parse state
of the stream. A successful call to fgetpos stores a representation of the value of this mbstate_t
object as part of the value of the fpos_t object. A later successful call to fsetpos using the same
stored fpos_t value restores the value of the associated mbstate_t object as well as the position
within the controlled stream.

Each stream has an associated lock that is used to prevent data races when multiple threads of
execution access a stream, and to restrict the interleaving of stream operations performed by multiple
threads. Only one thread may hold this lock at a time. The lock is reentrant: a single thread may
hold the lock multiple times at a given time.

All functions that read, write, position, or query the position of a stream lock the stream before
accessing it. They release the lock associated with the stream when the access is complete.

Environmental limits

An implementation shall support text files with lines containing at least 254 characters, including
the terminating new-line character. The value of the macro BUFSIZ shall be at least 256.

Forward references: the freopen function (7.21.5.4), the fwide function (7.29.3.5), mbstate_t
(7.30.1), the fgetpos function (7.21.9.1), the fsetpos function (7.21.9.3).

7.21.3 Files

A stream is associated with an external file (which may be a physical device) by opening a file, which
may involve creating a new file. Creating an existing file causes its former contents to be discarded,
if necessary. If a file can support positioning requests (such as a disk file, as opposed to a terminal),
then a file position indicator associated with the stream is positioned at the start (character number

zero) of the file, unless the file is opened with append mode in which case it is implementation-
defined whether the file position indicator is initially positioned at the beginning or the end of the
file. The file position indicator is maintained by subsequent reads, writes, and positioning requests,
to facilitate an orderly progression through the file.

Binary files are not truncated, except as defined in 7.21.5.3. Whether a write on a text stream causes
the associated file to be truncated beyond that point is implementation-defined.

When a stream is unbuffered, characters are intended to appear from the source or at the destination
as soon as possible. Otherwise characters may be accumulated and transmitted to or from the host
environment as a block. When a stream is fully buffered, characters are intended to be transmitted
to or from the host environment as a block when a buffer is filled. When a stream is line buffered,
characters are intended to be transmitted to or from the host environment as a block when a new-line
character is encountered. Furthermore, characters are intended to be transmitted as a block to the
host environment when a buffer is filled, when input is requested on an unbuffered stream, or when
input is requested on a line buffered stream that requires the transmission of characters from the
host environment. Support for these characteristics is implementation-defined, and may be affected
via the setbuf and setvbuf functions.

A file may be disassociated from a controlling stream by closing the file. Output streams are
flushed (any unwritten buffer contents are transmitted to the host environment) before the stream
is disassociated from the file. The value of a pointer to a FILE object is indeterminate after the
associated file is closed (including the standard text streams). Whether a file of zero length (on which
no characters have been written by an output stream) actually exists is implementation-defined.

The file may be subsequently reopened, by the same or another program execution, and its contents
reclaimed or modified (if it can be repositioned at its start). If the main function returns to its original
caller, or if the exit function is called, all open files are closed (hence all output streams are flushed)
before program termination. Other paths to program termination, such as calling the abort function,
need not close all files properly.

The address of the FILE object used to control a stream may be significant; a copy of a FILE object
need not serve in place of the original.

At program startup, three text streams are predefined and need not be opened explicitly — standard
input (for reading conventional input), standard output (for writing conventional output), and standard
error (for writing diagnostic output). As initially opened, the standard error stream is not fully
buffered; the standard input and standard output streams are fully buffered if and only if the stream
can be determined not to refer to an interactive device.

Functions that open additional (nontemporary) files require a file name, which is a string. The
rules for composing valid file names are implementation-defined. Whether the same file can be
simultaneously open multiple times is also implementation-defined.

Although both text and binary wide-oriented streams are conceptually sequences of wide characters,
the external file associated with a wide-oriented stream is a sequence of multibyte characters,
generalized as follows:

Moreover, the encodings used for multibyte characters may differ among files. Both the nature and
choice of such encodings are implementation-defined.

The wide character input functions read multibyte characters from the stream and convert them
to wide characters as if they were read by successive calls to the fgetwc function. Each conversion
occurs as if by a call to the mbrtowc function, with the conversion state described by the stream’s

own mbstate_t object. The byte input functions read characters from the stream as if by successive
calls to the fgetc function.

The wide character output functions convert wide characters to multibyte characters and write them
to the stream as if they were written by successive calls to the fputwc function. Each conversion
occurs as if by a call to the wcrtomb function, with the conversion state described by the stream’s
own mbstate_t object. The byte output functions write characters to the stream as if by successive
calls to the fputc function.

In some cases, some of the byte input/output functions also perform conversions between multibyte
characters and wide characters. These conversions also occur as if by calls to the mbrtowc and
wcrtomb functions.

An encoding error occurs if the character sequence presented to the underlying mbrtowc function
does not form a valid (generalized) multibyte character, or if the code value passed to the underlying
wcrtomb does not correspond to a valid (generalized) multibyte character. The wide character
input/output functions and the byte input/output functions store the value of the macro EILSEQ in
errno if and only if an encoding error occurs.

Environmental limits

The value of FOPEN_MAX shall be at least eight, including the three standard text streams.

Forward references: the exit function (7.22.4.4), the fgetc function (7.21.7.1), the fopen function
(7.21.5.3), the fputc function (7.21.7.3), the setbuf function (7.21.5.5), the setvbuf function (7.21.5.6),
the fgetwc function (7.29.3.1), the fputwc function (7.29.3.3), conversion state (7.29.6), the mbrtowc
function (7.29.6.3.2), the wcrtomb function (7.29.6.3.3).

7.21.4 Operations on files

7.21.4.1 The remove function

Synopsis

      #include <stdio.h>
      int remove(const char *filename);
    

Description

The remove function causes the file whose name is the string pointed to by filename to be no longer
accessible by that name. A subsequent attempt to open that file using that name will fail, unless it is
created anew. If the file is open, the behavior of the remove function is implementation-defined.

Returns

The remove function returns zero if the operation succeeds, nonzero if it fails.

7.21.4.2 The rename function

Synopsis

      #include <stdio.h>
      int rename(const char *old, const char *new);
    

Description

The rename function causes the file whose name is the string pointed to by old to be henceforth
known by the name given by the string pointed to by new. The file named old is no longer accessible
by that name. If a file named by the string pointed to by new exists prior to the call to the rename
function, the behavior is implementation-defined.

Returns

The rename function returns zero if the operation succeeds, nonzero if it fails,273) in which case if the
file existed previously it is still known by its original name.

7.21.4.3 The tmpfile function

Synopsis

      #include <stdio.h>
      FILE *tmpfile(void);
    

Description

The tmpfile function creates a temporary binary file that is different from any other existing file
and that will automatically be removed when it is closed or at program termination. If the program
terminates abnormally, whether an open temporary file is removed is implementation-defined. The
file is opened for update with "wb+" mode.

Recommended practice

It should be possible to open at least TMP_MAX temporary files during the lifetime of the program
(this limit may be shared with tmpnam) and there should be no limit on the number simultaneously
open other than this limit and any limit on the number of open files (FOPEN_MAX).

Returns

The tmpfile function returns a pointer to the stream of the file that it created. If the file cannot be
created, the tmpfile function returns a null pointer.

Forward references: the fopen function (7.21.5.3).

7.21.4.4 The tmpnam function

Synopsis

      #include <stdio.h>
      char *tmpnam(char *s);
    

Description

The tmpnam function generates a string that is a valid file name and that is not the same as the name
of an existing file.274) The function is potentially capable of generating at least TMP_MAX different
strings, but any or all of them may already be in use by existing files and thus not be suitable return
values.

The tmpnam function generates a different string each time it is called.

Calls to the tmpnam function with a null pointer argument may introduce data races with each other.
The implementation shall behave as if no library function calls the tmpnam function.

Returns

If no suitable string can be generated, the tmpnam function returns a null pointer. Otherwise, if
the argument is a null pointer, the tmpnam function leaves its result in an internal static object and
returns a pointer to that object (subsequent calls to the tmpnam function may modify the same object).
If the argument is not a null pointer, it is assumed to point to an array of at least L_tmpnam chars;
the tmpnam function writes its result in that array and returns the argument as its value.

Environmental limits

The value of the macro TMP_MAX shall be at least 25.

7.21.5 File access functions

7.21.5.1 The fclose function

Synopsis

      #include <stdio.h>
      int fclose(FILE *stream);
    

Description

A successful call to the fclose function causes the stream pointed to by stream to be flushed and
the associated file to be closed. Any unwritten buffered data for the stream are delivered to the host
environment to be written to the file; any unread buffered data are discarded. Whether or not the
call succeeds, the stream is disassociated from the file and any buffer set by the setbuf or setvbuf
function is disassociated from the stream (and deallocated if it was automatically allocated).

Returns

The fclose function returns zero if the stream was successfully closed, or EOF if any errors were
detected.

7.21.5.2 The fflush function

Synopsis

      #include <stdio.h>
      int fflush(FILE *stream);
    

Description

If stream points to an output stream or an update stream in which the most recent operation was
not input, the fflush function causes any unwritten data for that stream to be delivered to the host
environment to be written to the file; otherwise, the behavior is undefined.

If stream is a null pointer, the fflush function performs this flushing action on all streams for which
the behavior is defined above.

Returns

The fflush function sets the error indicator for the stream and returns EOF if a write error occurs,
otherwise it returns zero.

Forward references: the fopen function (7.21.5.3).

7.21.5.3 The fopen function

Synopsis

      #include <stdio.h>
      FILE *fopen(const char * restrict filename,
            const char * restrict mode);
    

Description

The fopen function opens the file whose name is the string pointed to by filename, and associates
a stream with it.

The argument mode points to a string. If the string is one of the following, the file is open in the
indicated mode. Otherwise, the behavior is undefined.275)

r open text file for reading
w truncate to zero length or create text file for writing
wx create text file for writing
a append; open or create text file for writing at end-of-file
rb open binary file for reading
wb truncate to zero length or create binary file for writing
wbx create binary file for writing
ab append; open or create binary file for writing at end-of-file
r+ open text file for update (reading and writing)

w+ truncate to zero length or create text file for update
w+x create text file for update
a+ append; open or create text file for update, writing at end-of-file
r+b or rb+ open binary file for update (reading and writing)
w+b or wb+ truncate to zero length or create binary file for update
w+bx or wb+x create binary file for update
a+b or ab+ append; open or create binary file for update, writing at end-of-file

Opening a file with read mode (’r’ as the first character in the mode argument) fails if the file does
not exist or cannot be read.

Opening a file with exclusive mode (’x’ as the last character in the mode argument) fails if the file
already exists or cannot be created. Otherwise, the file is created with exclusive (also known as
non-shared) access to the extent that the underlying system supports exclusive access.

Opening a file with append mode (’a’ as the first character in the mode argument) causes all
subsequent writes to the file to be forced to the then current end-of-file, regardless of intervening
calls to the fseek function. In some implementations, opening a binary file with append mode (’b’
as the second or third character in the above list of mode argument values) may initially position the
file position indicator for the stream beyond the last data written, because of null character padding.

When a file is opened with update mode (’+’ as the second or third character in the above list
of mode argument values), both input and output may be performed on the associated stream.
However, output shall not be directly followed by input without an intervening call to the fflush
function or to a file positioning function (fseek, fsetpos, or rewind), and input shall not be directly
followed by output without an intervening call to a file positioning function, unless the input
operation encounters end-of-file. Opening (or creating) a text file with update mode may instead
open (or create) a binary stream in some implementations.

When opened, a stream is fully buffered if and only if it can be determined not to refer to an
interactive device. The error and end-of-file indicators for the stream are cleared.

Returns

The fopen function returns a pointer to the object controlling the stream. If the open operation fails,
fopen returns a null pointer.

Forward references: file positioning functions (7.21.9).

7.21.5.4 The freopen function

Synopsis

      #include <stdio.h>
      FILE *freopen(const char * restrict filename,
            const char * restrict mode,
            FILE * restrict stream);
    

Description

The freopen function opens the file whose name is the string pointed to by filename and associates
the stream pointed to by stream with it. The mode argument is used just as in the fopen function.276)

If filename is a null pointer, the freopen function attempts to change the mode of the stream to
that specified by mode, as if the name of the file currently associated with the stream had been
used. It is implementation-defined which changes of mode are permitted (if any), and under what
circumstances.

The freopen function first attempts to close any file that is associated with the specified stream.
Failure to close the file is ignored. The error and end-of-file indicators for the stream are cleared.

Returns

The freopen function returns a null pointer if the open operation fails. Otherwise, freopen returns
the value of stream.

7.21.5.5 The setbuf function

Synopsis

      #include <stdio.h>
      void setbuf(FILE * restrict stream,
            char * restrict buf);
    

Description

Except that it returns no value, the setbuf function is equivalent to the setvbuf function invoked
with the values _IOFBF for mode and BUFSIZ for size, or (if buf is a null pointer), with the value
_IONBF for mode.

Returns

The setbuf function returns no value.

Forward references: the setvbuf function (7.21.5.6).

7.21.5.6 The setvbuf function

Synopsis

      #include <stdio.h>
      int setvbuf(FILE * restrict stream,
            char * restrict buf,
            int mode, size_t size);
    

Description

The setvbuf function may be used only after the stream pointed to by stream has been associated
with an open file and before any other operation (other than an unsuccessful call to setvbuf) is
performed on the stream. The argument mode determines how stream will be buffered, as follows:
_IOFBF causes input/output to be fully buffered; _IOLBF causes input/output to be line buffered;
_IONBF causes input/output to be unbuffered. If buf is not a null pointer, the array it points to may
be used instead of a buffer allocated by the setvbuf function277) and the argument size specifies
the size of the array; otherwise, size may determine the size of a buffer allocated by the setvbuf
function. The contents of the array at any time are indeterminate.

Returns

The setvbuf function returns zero on success, or nonzero if an invalid value is given for mode or if
the request cannot be honored.

7.21.6 Formatted input/output functions

The formatted input/output functions shall behave as if there is a sequence point after the actions
associated with each specifier.278)

7.21.6.1 The fprintf function

Synopsis

      #include <stdio.h>
      int fprintf(FILE * restrict stream,
            const char * restrict format, ...);
    

Description

The fprintf function writes output to the stream pointed to by stream, under control of the string
pointed to by format that specifies how subsequent arguments are converted for output. If there are
insufficient arguments for the format, the behavior is undefined. If the format is exhausted while
arguments remain, the excess arguments are evaluated (as always) but are otherwise ignored. The
fprintf function returns when the end of the format string is encountered.

The format shall be a multibyte character sequence, beginning and ending in its initial shift state.
The format is composed of zero or more directives: ordinary multibyte characters (not %), which
are copied unchanged to the output stream; and conversion specifications, each of which results
in fetching zero or more subsequent arguments, converting them, if applicable, according to the
corresponding conversion specifier, and then writing the result to the output stream.

Each conversion specification is introduced by the character %. After the %, the following appear in
sequence:

As noted above, a field width, or precision, or both, may be indicated by an asterisk. In this case,
an int argument supplies the field width or precision. The arguments specifying field width, or
precision, or both, shall appear (in that order) before the argument (if any) to be converted. A
negative field width argument is taken as a - flag followed by a positive field width. A negative
precision argument is taken as if the precision were omitted.

The flag characters and their meanings are:

-
The result of the conversion is left-justified within the field. (It is right-justified if this flag is
not specified.)
+
The result of a signed conversion always begins with a plus or minus sign. (It begins with a
sign only when a negative value is converted if this flag is not specified.)280)

space If the first character of a signed conversion is not a sign, or if a signed conversion results in
no characters, a space is prefixed to the result. If the space and + flags both appear, the space
flag is ignored.

#
The result is converted to an “alternative form”. For o conversion, it increases the precision, if
and only if necessary, to force the first digit of the result to be a zero (if the value and precision
are both 0, a single 0 is printed). For x (or X) conversion, a nonzero result has 0x (or 0X)
prefixed to it. For a, A, e, E, f, F, g, and G conversions, the result of converting a floating-point
number always contains a decimal-point character, even if no digits follow it. (Normally, a
    decimal-point character appears in the result of these conversions only if a digit follows it.)
    For g and G conversions, trailing zeros are not removed from the result. For other conversions,
    the behavior is undefined.
  
0
For d, i, o, u, x, X, a, A, e, E, f, F, g, and G conversions, leading zeros (following any indication
of sign or base) are used to pad to the field width rather than performing space padding,
except when converting an infinity or NaN. If the 0 and - flags both appear, the 0 flag is
ignored. For d, i, o, u, x, and X conversions, if a precision is specified, the 0 flag is ignored.
For other conversions, the behavior is undefined.

The length modifiers and their meanings are:

hh
Specifies that a following d, i, o, u, x, or X conversion specifier applies to a signed char
or unsigned char argument (the argument will have been promoted according to the
integer promotions, but its value shall be converted to signed char or unsigned char
before printing); or that a following n conversion specifier applies to a pointer to a
signed char argument.
h
Specifies that a following d, i, o, u, x, or X conversion specifier applies to a short int
or unsigned short int argument (the argument will have been promoted accord-
ing to the integer promotions, but its value shall be converted to short int or
unsigned short int before printing); or that a following n conversion specifier applies
to a pointer to a short int argument.

l (ell) Specifies that a following d, i, o, u, x, or X conversion specifier applies to a long int

    or unsigned long int argument; that a following n conversion specifier applies to
    a pointer to a long int argument; that a following c conversion specifier applies to
    a wint_t argument; that a following s conversion specifier applies to a pointer to a
    wchar_t argument; or has no effect on a following a, A, e, E, f, F, g, or G conversion
    specifier.
  

ll (ell-ell) Specifies that a following d, i, o, u, x, or X conversion specifier applies to a

    long long int or unsigned long long int argument; or that a following n con-
    version specifier applies to a pointer to a long long int argument.
  
j
Specifies that a following d, i, o, u, x, or X conversion specifier applies to an intmax_t
or uintmax_t argument; or that a following n conversion specifier applies to a pointer
to an intmax_t argument.
z
Specifies that a following d, i, o, u, x, or X conversion specifier applies to a size_t or the
corresponding signed integer type argument; or that a following n conversion specifier
applies to a pointer to a signed integer type corresponding to size_t argument.
t
Specifies that a following d, i, o, u, x, or X conversion specifier applies to a ptrdiff_t
or the corresponding unsigned integer type argument; or that a following n conversion
specifier applies to a pointer to a ptrdiff_t argument.
L
Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to a
long double argument.

If a length modifier appears with any conversion specifier other than as specified above, the behavior
is undefined.

The conversion specifiers and their meanings are:

d,i
The int argument is converted to signed decimal in the style [-]dddd. The precision
specifies the minimum number of digits to appear; if the value being converted can be
represented in fewer digits, it is expanded with leading zeros. The default precision is 1.
The result of converting a zero value with a precision of zero is no characters.

o,u,x,X The unsigned int argument is converted to unsigned octal (o), unsigned decimal (u), or

    unsigned hexadecimal notation (x or X) in the style dddd; the letters abcdef are used for x
    conversion and the letters ABCDEF for X conversion. The precision specifies the minimum
        number of digits to appear; if the value being converted can be represented in fewer digits,
        it is expanded with leading zeros. The default precision is 1. The result of converting a
        zero value with a precision of zero is no characters.
  
f,F
A double argument representing a floating-point number is converted to decimal notation
in the style [-]ddd.ddd, where the number of digits after the decimal-point character is
equal to the precision specification. If the precision is missing, it is taken as 6; if the
precision is zero and the # flag is not specified, no decimal-point character appears. If a
decimal-point character appears, at least one digit appears before it. The value is rounded
to the appropriate number of digits.
A double argument representing an infinity is converted in one of the styles [-]inf or
[-]infinity — which style is implementation-defined. A double argument representing a
NaN is converted in one of the styles [-]nan or [-]nan(n-char-sequence) — which style, and
the meaning of any n-char-sequence, is implementation-defined. The F conversion specifier
produces INF, INFINITY, or NAN instead of inf, infinity, or nan, respectively.281)
e,E
A double argument representing a floating-point number is converted in the style
[-]d.ddde±dd, where there is one digit (which is nonzero if the argument is nonzero) before
the decimal-point character and the number of digits after it is equal to the precision; if the
precision is missing, it is taken as 6; if the precision is zero and the # flag is not specified,
no decimal-point character appears. The value is rounded to the appropriate number of
digits. The E conversion specifier produces a number with E instead of e introducing the
exponent. The exponent always contains at least two digits, and only as many more digits
as necessary to represent the exponent. If the value is zero, the exponent is zero.
A double argument representing an infinity or NaN is converted in the style of an f or F
conversion specifier.
g,G
A double argument representing a floating-point number is converted in style f or e (or
in style F or E in the case of a G conversion specifier), depending on the value converted
and the precision. Let P equal the precision if nonzero, 6 if the precision is omitted, or 1 if
the precision is zero. Then, if a conversion with style E would have an exponent of X:
    if P > X ≥ −4, the conversion is with style f (or F) and precision P − (X + 1).
    otherwise, the conversion is with style e (or E) and precision P − 1.
  
    Finally, unless the # flag is used, any trailing zeros are removed from the fractional portion
    of the result and the decimal-point character is removed if there is no fractional portion
    remaining.
    A double argument representing an infinity or NaN is converted in the style of an f or F
    conversion specifier.
  
a,A
A double argument representing a floating-point number is converted in the style
[-]0xh.hhhhp±d, where there is one hexadecimal digit (which is nonzero if the argument is a
normalized floating-point number and is otherwise unspecified) before the decimal-point
character282) and the number of hexadecimal digits after it is equal to the precision; if the
precision is missing and FLT_RADIX is a power of 2, then the precision is sufficient for an
exact representation of the value; if the precision is missing and FLT_RADIX is not a power
of 2, then the precision is sufficient to distinguish283) values of type double, except that
trailing zeros may be omitted; if the precision is zero and the # flag is not specified, no
    decimal-point character appears. The letters abcdef are used for a conversion and the
    letters ABCDEF for A conversion. The A conversion specifier produces a number with X and
    P instead of x and p. The exponent always contains at least one digit, and only as many
    more digits as necessary to represent the decimal exponent of 2. If the value is zero, the
    exponent is zero.
    A double argument representing an infinity or NaN is converted in the style of an f or F
    conversion specifier.
  
c
If no l length modifier is present, the int argument is converted to an unsigned char,and
the resulting character is written.
If an l length modifier is present, the wint_t argument is converted as if by an ls
conversion specification with no precision and an argument that points to the initial
element of a two-element array of wchar_t, the first element containing the wint_t
argument to the lc conversion specification and the second a null wide character.
s
If no l length modifier is present, the argument shall be a pointer to the initial element
of an array of character type.284) Characters from the array arewritten up to (but not
including) the terminating null character. If the precision is specified, no more than that
many bytes are written. If the precision is not specified or is greater than the size of the
array, the array shall contain a null character.
If an l length modifier is present, the argument shall be a pointer to the initial element
of an array of wchar_t type. Wide characters from the array are converted to multibyte
characters (each as if by a call to the wcrtomb function, with the conversion state described
by an mbstate_t object initialized to zero before the first wide character is converted) up
to and including a terminating null wide character. The resulting multibyte characters are
written up to (but not including) the terminating null character (byte). If no precision is
specified, the array shall contain a null wide character. If a precision is specified, no more
than that many bytes are written (including shift sequences, if any), and the array shall
contain a null wide character if, to equal the multibyte character sequence length given by
the precision, the function would need to access a wide character one past the end of the
array. In no case is a partial multibyte character written.285)
p
The argument shall be a pointer to void. The value of the pointer is converted to a
sequence of printing characters, in an implementation-defined manner.
n
The argument shall be a pointer to signed integer into which is written the number of
characters written to the output stream so far by this call to fprintf. No argument is
converted, but one is consumed. If the conversion specification includes any flags, a field
width, or a precision, the behavior is undefined.
%
A % character is written. No argument is converted. The complete conversion specification
shall be %%.

If a conversion specification is invalid, the behavior is undefined.286) If any argument is not the
correct type for the corresponding conversion specification, the behavior is undefined.

In no case does a nonexistent or small field width cause truncation of a field; if the result of a
conversion is wider than the field width, the field is expanded to contain the conversion result.

For a and A conversions, if FLT_RADIX is a power of 2, the value is correctly rounded to a hexadecimal
floating number with the given precision.

Recommended practice

For a and A conversions, if FLT_RADIX is not a power of 2 and the result is not exactly representable
in the given precision, the result should be one of the two adjacent numbers in hexadecimal floating
style with the given precision, with the extra stipulation that the error should have a correct sign for
the current rounding direction.

For e, E, f, F, g, and G conversions, if the number of significant decimal digits is at most DECIMAL_DIG,
then the result should be correctly rounded.287) If the number of significant decimal digits is more
than DECIMAL_DIG but the source value is exactly representable with DECIMAL_DIG digits, then the
result should be an exact representation with trailing zeros. Otherwise, the source value is bounded
by two adjacent decimal strings L < U, both having DECIMAL_DIG significant digits; the value of the
resultant decimal string D should satisfy L ≤ D ≤ U, with the extra stipulation that the error should
have a correct sign for the current rounding direction.

Returns

The fprintf function returns the number of characters transmitted, or a negative value if an output
or encoding error occurred.

Environmental limits

The number of characters that can be produced by any single conversion shall be at least 4095.

EXAMPLE 1 To print a date and time in the form “Sunday, July 3, 10:02” followed by π to five decimal places:

    #include <math.h>
    #include <stdio.h>
    /* ... */
    char *weekday, *month;    // pointers to strings
    int day, hour, min;
    fprintf(stdout, "%s, %s %d, %.2d:%.2d\n",
          weekday, month, day, hour, min);
    fprintf(stdout, "pi = %.5f\n", 4 * atan(1.0));
  

EXAMPLE 2 In this example, multibyte characters do not have a state-dependent encoding, and the members of the extended
character set that consist of more than one byte each consist of exactly two bytes, the first of which is denoted here by a □ and
the second by an uppercase letter.

Given the following wide string with length seven,

    static wchar_t wstr[] = L"□X□Yabc□Z□W";
  

the seven calls

    fprintf(stdout,        "|1234567890123|\n");
    fprintf(stdout,        "|%13ls|\n", wstr);
    fprintf(stdout,        "|%-13.9ls|\n", wstr);
    fprintf(stdout,        "|%13.10ls|\n", wstr);
    fprintf(stdout,        "|%13.11ls|\n", wstr);
    fprintf(stdout,        "|%13.15ls|\n", &wstr[2]);
    fprintf(stdout,        "|%13lc|\n", (wint_t) wstr[5]);
  

will print the following seven lines:

    |1234567890123|
    | □X□Yabc□Z□W|
    |□X□Yabc□Z    |
    |    □X□Yabc□Z|
    | □X□Yabc□Z□W|
    |      abc□Z□W|
    |           □Z|
  

Forward references: conversion state (7.29.6), the wcrtomb function (7.29.6.3.3).

7.21.6.2 The fscanf function

Synopsis

      #include <stdio.h>
      int fscanf(FILE * restrict stream,
            const char * restrict format, ...);
    

Description

The fscanf function reads input from the stream pointed to by stream, under control of the string
pointed to by format that specifies the admissible input sequences and how they are to be converted
for assignment, using subsequent arguments as pointers to the objects to receive the converted
input. If there are insufficient arguments for the format, the behavior is undefined. If the format
is exhausted while arguments remain, the excess arguments are evaluated (as always) but are
otherwise ignored.

The format shall be a multibyte character sequence, beginning and ending in its initial shift state.The
format is composed of zero or more directives: one or more white-space characters, an ordinary
multibyte character (neither % nor a white-space character), or a conversion specification. Each
conversion specification is introduced by the character %. After the %, the following appear in
sequence:

The fscanf function executes each directive of the format in turn. When all directives have been
executed, or if a directive fails (as detailed below), the function returns. Failures are described as
input failures (due to the occurrence of an encoding error or the unavailability of input characters),
or matching failures (due to inappropriate input).

A directive composed of white-space character(s) is executed by reading input up to the first non-
white-space character (which remains unread), or until no more characters can be read. The directive
never fails.

A directive that is an ordinary multibyte character is executed by reading the next characters of the
stream. If any of those characters differ from the ones composing the directive,the directive fails and
the differing and subsequent characters remain unread. Similarly, if end-of-file, an encoding error,
or a read error prevents a character from being read, the directive fails.

A directive that is a conversion specification defines a set of matching input sequences, as described
below for each specifier. A conversion specification is executed in the following steps:

Input white-space characters (as specified by the isspace function) are skipped, unless the specifi-
cation includes a [, c, or n specifier.288)

An input item is read from the stream, unless the specification includes an n specifier. An input
item is defined as the longest sequence of input characters which does not exceed any specified
field width and which is, or is a prefix of, a matching input sequence.289) The first character, if any,
after the input item remains unread. If the length of the input item is zero, the execution of the
directive fails; this condition is a matching failure unless end-of-file, an encoding error, or a read
error prevented input from the stream, in which case it is an input failure.

Except in the case of a % specifier, the input item (or, in the case of a %n directive, the count of input
characters) is converted to a type appropriate to the conversion specifier. If the input item is not a

matching sequence, the execution of the directive fails: this condition is a matching failure. Unless
assignment suppression was indicated by a *, the result of the conversion is placed in the object
pointed to by the first argument following the format argument that has not already received a
conversion result. If this object does not have an appropriate type, or if the result of the conversion
cannot be represented in the object, the behavior is undefined.

The length modifiers and their meanings are:

hh Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument

    with type pointer to signed char or unsigned char.
  

h Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument

    with type pointer to short int or unsigned short int.
  

l (ell) Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument

    with type pointer to long int or unsigned long int; that a following a, A, e, E, f, F,
    g, or G conversion specifier applies to an argument with type pointer to double; or that
    a following c, s, or [ conversion specifier applies to an argument with type pointer to
    wchar_t .
  

ll (ell-ell) Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument

    with type pointer to long long int or unsigned long long int.
  

j Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument

    with type pointer to intmax_t or uintmax_t.
  

z Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument

    with type pointer to size_t or the corresponding signed integer type.
  

t Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument

    with type pointer to ptrdiff_t or the corresponding unsigned integer type.
  

L Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to an argument

    with type pointer to long double.
  

If a length modifier appears with any conversion specifier other than as specified above, the behavior
is undefined.

The conversion specifiers and their meanings are:

d Matches an optionally signed decimal integer, whose format is the same as expected for

    the subject sequence of the strtol function with the value 10 for the base argument. The
    corresponding argument shall be a pointer to signed integer.
  

i Matches an optionally signed integer, whose format is the same as expected for the

    subject sequence of the strtol function with the value 0 for the base argument. The
    corresponding argument shall be a pointer to signed integer.
  

o Matches an optionally signed octal integer, whose format is the same as expected for the

    subject sequence of the strtoul function with the value 8 for the base argument. The
    corresponding argument shall be a pointer to unsigned integer.
  

u Matches an optionally signed decimal integer, whose format is the same as expected for

    the subject sequence of the strtoul function with the value 10 for the base argument.
    The corresponding argument shall be a pointer to unsigned integer.
  

x Matches an optionally signed hexadecimal integer, whose format is the same as expected

    for the subject sequence of the strtoul function with the value 16 for the base argument.
    The corresponding argument shall be a pointer to unsigned integer.
  

a,e,f,g Matches an optionally signed floating-point number, infinity, or NaN, whose format is

    the same as expected for the subject sequence of the strtod function. The corresponding
      argument shall be a pointer to floating.
  
c
Matches a sequence of characters of exactly the number specified by the field width (1 if
no field width is present in the directive).290)
If no l length modifier is present, thecorresponding argument shall be a pointer to the
initial element of a character array large enough to accept the sequence. No null character
is added.
If an l length modifier is present, the input shall be a sequence of multibyte characters that
begins in the initial shift state. Each multibyte character in the sequence is converted to a
wide character as if by a call to the mbrtowc function, with the conversion state described
by an mbstate_t object initialized to zero before the first multibyte character is converted.
Thecorresponding argument shall be a pointer to the initial element of an array of wchar_t
large enough to accept the resulting sequence of wide characters.No null wide character is
added.
s
Matches a sequence of non-white-space characters.290)
If no l length modifier is present, thecorresponding argument shall be a pointer to the
initial element of a character array large enough to accept the sequence and a terminating
null character, which will be added automatically.
If an l length modifier is present, the input shall be a sequence of multibyte characters
that begins in the initial shift state. Each multibyte character is converted to a wide
character as if by a call to the mbrtowc function, with the conversion state described by
an mbstate_t object initialized to zero before the first multibyte character is converted.
Thecorresponding argument shall be a pointer to the initial element of an array of wchar_t
large enough to accept the sequence and the terminating null wide character, which will
be added automatically.
[
Matches a nonempty sequence of characters from a set of expected characters (the
scanset).290)
If no l length modifier is present, thecorresponding argument shall be a pointer to the
initial element of a character array large enough to accept the sequence and a terminating
null character, which will be added automatically.
If an l length modifier is present, the input shall be a sequence of multibyte characters
that begins in the initial shift state. Each multibyte character is converted to a wide
character as if by a call to the mbrtowc function, with the conversion state described by
an mbstate_t object initialized to zero before the first multibyte character is converted.
Thecorresponding argument shall be a pointer to the initial element of an array of wchar_t
large enough to accept the sequence and the terminating null wide character, which will
be added automatically.
The conversion specifier includes all subsequent characters in the format string, up to
and including the matching right bracket (]). The characters between the brackets (the
scanlist) compose the scanset, unless the character after the left bracket is a circumflex (^),
in which case the scanset contains all characters that do not appear in the scanlist between
the circumflex and the right bracket. If the conversion specifier begins with [] or [^], the
right bracket character is in the scanlist and the next following right bracket character is
the matching right bracket that ends the specification; otherwise the first following right
bracket character is the one that ends the specification. If a - character is in the scanlist
and is not the first, nor the second where the first character is a ^, nor the last character,
the behavior is implementation-defined.
p
Matches an implementation-defined set of sequences, which should be the same as the
set of sequences that may be produced by the %p conversion of the fprintf function.
The corresponding argument shall be a pointer to a pointer to void. The input item is
converted to a pointer value in an implementation-defined manner. If the input item is a
    value converted earlier during the same program execution, the pointer that results shall
    compare equal to that value; otherwise the behavior of the %p conversion is undefined.
  
n
No input is consumed. The corresponding argument shall be a pointer to signed integer
into which is to be written the number of characters read from the input stream so far
by this call to the fscanf function. Execution of a %n directive does not increment the
assignment count returned at the completion of execution of the fscanf function. No
argument is converted, but one is consumed. If the conversion specification includes an
assignment-suppressing character or a field width, the behavior is undefined.
%
Matches a single % character; no conversion or assignment occurs. The complete conversion
specification shall be %%.

If a conversion specification is invalid, the behavior is undefined.291)

The conversion specifiers A, E, F, G, and X are also valid and behave the same as, respectively, a, e, f,
g, and x.

Trailing white space (including new-line characters) is left unread unless matched by a directive.
The success of literal matches and suppressed assignments is not directly determinable other than
via the %n directive.

Returns

The fscanf function returns the value of the macro EOF if an input failure occurs before the first
conversion (if any) has completed. Otherwise, the function returns the number of input items
assigned, which can be fewer than provided for, or even zero, in the event of an early matching
failure.

EXAMPLE 1 The call:

    #include <stdio.h>
    /* ... */
    int n, i; float x; char name[50];
    n = fscanf(stdin, "%d%f%s", &i, &x, name);
  

with the input line:

    25 54.32E-1 thompson
  

will assign to n the value 3, to i the value 25, to x the value 5.432, and to name the sequence thompson\0.

EXAMPLE 2 The call:

    #include <stdio.h>
    /* ... */
    int i; float x; char name[50];
    fscanf(stdin, "%2d%f%*d %[0123456789]", &i, &x, name);
  

with input:

    56789 0123 56a72
  

will assign to i the value 56 and to x the value 789.0, will skip 0123, and will assign to name the sequence 56\0. The next
character read from the input stream will be a.

EXAMPLE 3 To accept repeatedly from stdin a quantity, a unit of measure, and an item name:

    #include <stdio.h>
    /* ... */
    int count; float quant; char units[21], item[21];
    do {
          count = fscanf(stdin, "%f%20s of %20s", &quant, units, item);
          fscanf(stdin,"%*[^\n]");
  
    } while (!feof(stdin) && !ferror(stdin));
  

If the stdin stream contains the following lines:

    2 quarts of oil
    -12.8degrees Celsius
    lots of luck
    10.0LBS     of
    dirt
    100ergs of energy
  

the execution of the above example will be analogous to the following assignments:

    quant   =   2; strcpy(units, "quarts"); strcpy(item, "oil");
    count   =   3;
    quant   =   -12.8; strcpy(units, "degrees");
    count   =   2; // "C" fails to match "o"
    count   =   0; // "l" fails to match "%f"
    quant   =   10.0; strcpy(units, "LBS"); strcpy(item, "dirt");
    count   =   3;
    count   =   0; // "100e" fails to match "%f"
    count   =   EOF;
  

EXAMPLE 4 In:

    #include <stdio.h>
    /* ... */
    int d1, d2, n1, n2, i;
    i = sscanf("123", "%d%n%n%d", &d1, &n1, &n2, &d2);
  

the value 123 is assigned to d1 and the value 3 to n1. Because %n can never get an input failure, the value of 3 is also assigned
to n2. The value of d2 is not affected. The value 1 is assigned to i.

EXAMPLE 5 The call:

    #include <stdio.h>
    /* ... */
    int n, i;
    n = sscanf("foo %bar             42", "foo%%bar%d", &i);
  

will assign to n the value 1 and to i the value 42 because input white-space characters are skipped for both the % and d
conversion specifiers.

EXAMPLE 6 In these examples, multibyte characters do have a state-dependent encoding, and the members of the extended
character set that consist of more than one byte each consist of exactly two bytes, the first of which is denoted here by a □ and
the second by an uppercase letter, but are only recognized as such when in the alternate shift state. The shift sequences are
denoted by ↑ and ↓, in which the first causes entry into the alternate shift state.

After the call:

    #include <stdio.h>
    /* ... */
    char str[50];
    fscanf(stdin, "a%s", str);
  

with the input line:

    a↑□X□Y↓ bc
  

str will contain ↑□X□Y↓\0 assuming that none of the bytes of the shift sequences (or of the multibyte characters, in the more
general case) appears to be a single-byte white-space character.

In contrast, after the call:

    #include <stdio.h>
    #include <stddef.h>
    /* ... */
    wchar_t wstr[50];
    fscanf(stdin, "a%ls", wstr);
  

with the same input line, wstr will contain the two wide characters that correspond to □X and □Y and a terminating null
wide character.

However, the call:

    #include <stdio.h>
    #include <stddef.h>
    /* ... */
    wchar_t wstr[50];
    fscanf(stdin, "a↑□X↓%ls", wstr);
  

with the same input line will return zero due to a matching failure against the ↓ sequence in the format string.

Assuming that the first byte of the multibyte character □X is the same as the first byte of the multibyte character □Y, after the
call:

    #include <stdio.h>
    #include <stddef.h>
    /* ... */
    wchar_t wstr[50];
    fscanf(stdin, "a↑□Y↓%ls", wstr);
  

with the same input line, zero will again be returned, but stdin will be left with a partially consumed multibyte character.

Forward references: the strtod, strtof, and strtold functions (7.22.1.3), the strtol, strtoll,
strtoul, and strtoull functions (7.22.1.4), conversion state (7.29.6), the wcrtomb function
(7.29.6.3.3).

7.21.6.3 The printf function

Synopsis

      #include <stdio.h>
      int printf(const char * restrict format, ...);
    

Description

The printf function is equivalent to fprintf with the argument stdout interposed before the
arguments to printf.

Returns

The printf function returns the number of characters transmitted, or a negative value if an output
or encoding error occurred.

7.21.6.4 The scanf function

Synopsis

      #include <stdio.h>
      int scanf(const char * restrict format, ...);
    

Description

The scanf function is equivalent to fscanf with the argument stdin interposed before the argu-
ments to scanf.

Returns

The scanf function returns the value of the macro EOF if an input failure occurs before the first
conversion (if any) has completed. Otherwise, the scanf function returns the number of input items
assigned, which can be fewer than provided for, or even zero, in the event of an early matching
failure.

7.21.6.5 The snprintf function

Synopsis

      #include <stdio.h>
      int snprintf(char * restrict s, size_t n,
            const char * restrict format, ...);
    

Description

The snprintf function is equivalent to fprintf, except that the output is written into an array
(specified by argument s) rather than to a stream. If n is zero, nothing is written, and s may be a
null pointer. Otherwise, output characters beyond the n-1st are discarded rather than being written
to the array, and a null character is written at the end of the characters actually written into the array.
If copying takes place between objects that overlap, the behavior is undefined.

Returns

The snprintf function returns the number of characters that would have been written had n been
sufficiently large, not counting the terminating null character, or a negative value if an encoding
error occurred. Thus, the null-terminated output has been completely written if and only if the
returned value is nonnegative and less than n.

7.21.6.6 The sprintf function

Synopsis

      #include <stdio.h>
      int sprintf(char * restrict s,
            const char * restrict format, ...);
    

Description

The sprintf function is equivalent to fprintf, except that the output is written into an array
(specified by the argument s) rather than to a stream. A null character is written at the end of the
characters written; it is not counted as part of the returned value. If copying takes place between
objects that overlap, the behavior is undefined.

Returns

The sprintf function returns the number of characters written in the array, not counting the
terminating null character, or a negative value if an encoding error occurred.

7.21.6.7 The sscanf function

Synopsis

      #include <stdio.h>
      int sscanf(const char * restrict s,
            const char * restrict format, ...);
    

Description

The sscanf function is equivalent to fscanf, except that input is obtained from a string (specified
by the argument s) rather than from a stream. Reaching the end of the string is equivalent to
encountering end-of-file for the fscanf function. If copying takes place between objects that overlap,
the behavior is undefined.

Returns

The sscanf function returns the value of the macro EOF if an input failure occurs before the first
conversion (if any) has completed. Otherwise, the sscanf function returns the number of input
items assigned, which can be fewer than provided for, or even zero, in the event of an early matching
failure.

7.21.6.8 The vfprintf function

Synopsis

      #include <stdarg.h>
      #include <stdio.h>
      int vfprintf(FILE * restrict stream,
            const char * restrict format,
            va_list arg);
    

Description

The vfprintf function is equivalent to fprintf, with the variable argument list replaced by arg,
which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls).
The vfprintf function does not invoke the va_end macro.292)

Returns

The vfprintf function returns the number of characters transmitted, or a negative value if an
output or encoding error occurred.

EXAMPLE The following shows the use of the vfprintf function in a general error-reporting routine.

    #include <stdarg.h>
    #include <stdio.h>
  
    void error(char *function_name, char *format, ...)
    {
          va_list args;
  
            va_start(args, format);
            // print out name of function causing error
            fprintf(stderr, "ERROR in %s: ", function_name);
            // print out remainder of message
            vfprintf(stderr, format, args);
            va_end(args);
    }
  

7.21.6.9 The vfscanf function

Synopsis

      #include <stdarg.h>
      #include <stdio.h>
      int vfscanf(FILE * restrict stream,
            const char * restrict format,
            va_list arg);
    

Description

The vfscanf function is equivalent to fscanf, with the variable argument list replaced by arg,
which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls).
The vfscanf function does not invoke the va_end macro.292)

Returns

The vfscanf function returns the value of the macro EOF if an input failure occurs before the first
conversion (if any) has completed. Otherwise, the vfscanf function returns the number of input
items assigned, which can be fewer than provided for, or even zero, in the event of an early matching
failure.

7.21.6.10 The vprintf function

Synopsis

      #include <stdarg.h>
      #include <stdio.h>
      int vprintf(const char * restrict format,
            va_list arg);
    

Description

The vprintf function is equivalent to printf, with the variable argument list replaced by arg,
which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls).
The vprintf function does not invoke the va_end macro.292)

Returns

The vprintf function returns the number of characters transmitted, or a negative value if an output
or encoding error occurred.

7.21.6.11 The vscanf function

Synopsis

      #include <stdarg.h>
      #include <stdio.h>
      int vscanf(const char * restrict format,
            va_list arg);
    

Description

The vscanf function is equivalent to scanf, with the variable argument list replaced by arg, which
shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The
vscanf function does not invoke the va_end macro.292)

Returns

The vscanf function returns the value of the macro EOF if an input failure occurs before the first
conversion (if any) has completed. Otherwise, the vscanf function returns the number of input
items assigned, which can be fewer than provided for, or even zero, in the event of an early matching
failure.

7.21.6.12 The vsnprintf function

Synopsis

      #include <stdarg.h>
      #include <stdio.h>
      int vsnprintf(char * restrict s, size_t n,
            const char * restrict format,
            va_list arg);
    

Description

The vsnprintf function is equivalent to snprintf, with the variable argument list replaced by arg,
which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls).
The vsnprintf function does not invoke the va_end macro.292) If copying takes place between
objects that overlap, the behavior is undefined.

Returns

The vsnprintf function returns the number of characters that would have been written had n been
sufficiently large, not counting the terminating null character, or a negative value if an encoding
error occurred. Thus, the null-terminated output has been completely written if and only if the
returned value is nonnegative and less than n.

7.21.6.13 The vsprintf function

Synopsis

      #include <stdarg.h>
      #include <stdio.h>
      int vsprintf(char * restrict s,
            const char * restrict format,
            va_list arg);
    

Description

The vsprintf function is equivalent to sprintf, with the variable argument list replaced by arg,
which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls).
The vsprintf function does not invoke the va_end macro.292) If copying takes place between objects
that overlap, the behavior is undefined.

Returns

The vsprintf function returns the number of characters written in the array, not counting the
terminating null character, or a negative value if an encoding error occurred.

7.21.6.14 The vsscanf function

Synopsis

      #include <stdarg.h>
      #include <stdio.h>
      int vsscanf(const char * restrict s,
            const char * restrict format,
            va_list arg);
    

Description

The vsscanf function is equivalent to sscanf, with the variable argument list replaced by arg,
which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls).
The vsscanf function does not invoke the va_end macro.292)

Returns

The vsscanf function returns the value of the macro EOF if an input failure occurs before the first
conversion (if any) has completed. Otherwise, the vsscanf function returns the number of input
items assigned, which can be fewer than provided for, or even zero, in the event of an early matching
failure.

7.21.7 Character input/output functions

7.21.7.1 The fgetc function

Synopsis

      #include <stdio.h>
      int fgetc(FILE *stream);
    

Description

If the end-of-file indicator for the input stream pointed to by stream is not set and a next character
is present, the fgetc function obtains that character as an unsigned char converted to an int and
advances the associated file position indicator for the stream (if defined).

Returns

If the end-of-file indicator for the stream is set, or if the stream is at end-of-file, the end-of-file
indicator for the stream is set and the fgetc function returns EOF. Otherwise, the fgetc function
returns the next character from the input stream pointed to by stream. If a read error occurs, the
error indicator for the stream is set and the fgetc function returns EOF.293)

7.21.7.2 The fgets function

Synopsis

      #include <stdio.h>
      char *fgets(char * restrict s, int n,
            FILE * restrict stream);
    

Description

The fgets function reads at most one less than the number of characters specified by n from the
stream pointed to by stream into the array pointed to by s. No additional characters are read after a
new-line character (which is retained) or after end-of-file. A null character is written immediately
after the last character read into the array.

Returns

The fgets function returns s if successful. If end-of-file is encountered and no characters have been
read into the array, the contents of the array remain unchanged and a null pointer is returned. If a
read error occurs during the operation, the array contents are indeterminate and a null pointer is
returned.

7.21.7.3 The fputc function

Synopsis

      #include <stdio.h>
      int fputc(int c, FILE *stream);
    

Description

The fputc function writes the character specified by c (converted to an unsigned char) to the
output stream pointed to by stream, at the position indicated by the associated file position indicator
for the stream (if defined), and advances the indicator appropriately. If the file cannot support
positioning requests, or if the stream was opened with append mode, the character is appended to
the output stream.

Returns

The fputc function returns the character written. If a write error occurs, the error indicator for the
stream is set and fputc returns EOF.

7.21.7.4 The fputs function

Synopsis

      #include <stdio.h>
      int fputs(const char * restrict s,
            FILE * restrict stream);
    

Description

The fputs function writes the string pointed to by s to the stream pointed to by stream. The
terminating null character is not written.

Returns

The fputs function returns EOF if a write error occurs; otherwise it returns a nonnegative value.

7.21.7.5 The getc function

Synopsis

      #include <stdio.h>
      int getc(FILE *stream);
    

Description

The getc function is equivalent to fgetc, except that if it is implemented as a macro, it may evaluate
stream more than once, so the argument should never be an expression with side effects.

Returns

The getc function returns the next character from the input stream pointed to by stream. If the
stream is at end-of-file, the end-of-file indicator for the stream is set and getc returns EOF. If a read
error occurs, the error indicator for the stream is set and getc returns EOF.

7.21.7.6 The getchar function

Synopsis

      #include <stdio.h>
      int getchar(void);
    

Description

The getchar function is equivalent to getc with the argument stdin.

Returns

The getchar function returns the next character from the input stream pointed to by stdin. If the
stream is at end-of-file, the end-of-file indicator for the stream is set and getchar returns EOF. If a
read error occurs, the error indicator for the stream is set and getchar returns EOF.

7.21.7.7 The putc function

Synopsis

      #include <stdio.h>
      int putc(int c, FILE *stream);
    

Description

The putc function is equivalent to fputc, except that if it is implemented as a macro, it may evaluate
stream more than once, so that argument should never be an expression with side effects.

Returns

The putc function returns the character written. If a write error occurs, the error indicator for the
stream is set and putc returns EOF.

7.21.7.8 The putchar function

Synopsis

      #include <stdio.h>
      int putchar(int c);
    

Description

The putchar function is equivalent to putc with the second argument stdout.

Returns

The putchar function returns the character written. If a write error occurs, the error indicator for
the stream is set and putchar returns EOF.

7.21.7.9 The puts function

Synopsis

      #include <stdio.h>
      int puts(const char *s);
    

Description

The puts function writes the string pointed to by s to the stream pointed to by stdout, and appends
a new-line character to the output. The terminating null character is not written.

Returns

The puts function returns EOF if a write error occurs; otherwise it returns a nonnegative value.

7.21.7.10 The ungetc function

Synopsis

      #include <stdio.h>
      int ungetc(int c, FILE *stream);
    

Description

The ungetc function pushes the character specified by c (converted to an unsigned char) back
onto the input stream pointed to by stream. Pushed-back characters will be returned by subsequent
reads on that stream in the reverse order of their pushing. A successful intervening call (with the
stream pointed to by stream) to a file positioning function (fseek, fsetpos, or rewind) discards
any pushed-back characters for the stream. The external storage corresponding to the stream is
unchanged.

One character of pushback is guaranteed. If the ungetc function is called too many times on the
same stream without an intervening read or file positioning operation on that stream, the operation
may fail.

If the value of c equals that of the macro EOF, the operation fails and the input stream is unchanged.

A successful call to the ungetc function clears the end-of-file indicator for the stream. The value
of the file position indicator for the stream after reading or discarding all pushed-back characters
shall be the same as it was before the characters were pushed back.294) For a text stream, the value
of its file position indicator after a successful call to the ungetc function is unspecified until all
pushed-back characters are read or discarded. For a binary stream, its file position indicator is
decremented by each successful call to the ungetc function; if its value was zero before a call, it is
indeterminate after the call.295)

Returns

The ungetc function returns the character pushed back after conversion, or EOF if the operation
fails.

Forward references: file positioning functions (7.21.9).

7.21.8 Direct input/output functions

7.21.8.1 The fread function

Synopsis

      #include <stdio.h>
      size_t fread(void * restrict ptr,
            size_t size, size_t nmemb,
            FILE * restrict stream);
    

Description

The fread function reads, into the array pointed to by ptr, up to nmemb elements whose size is
specified by size, from the stream pointed to by stream. For each object, size calls are made to
the fgetc function and the results stored, in the order read, in an array of unsigned char exactly
overlaying the object. The file position indicator for the stream (if defined) is advanced by the
number of characters successfully read. If an error occurs, the resulting value of the file position
indicator for the stream is indeterminate. If a partial element is read, its value is indeterminate.

Returns

The fread function returns the number of elements successfully read, which may be less than nmemb
if a read error or end-of-file is encountered. If size or nmemb is zero, fread returns zero and the
contents of the array and the state of the stream remain unchanged.

7.21.8.2 The fwrite function

Synopsis

      #include <stdio.h>
      size_t fwrite(const void * restrict ptr,
            size_t size, size_t nmemb,
            FILE * restrict stream);
    

Description

The fwrite function writes, from the array pointed to by ptr, up to nmemb elements whose size is
specified by size, to the stream pointed to by stream. For each object, size calls are made to the
fputc function, taking the values (in order) from an array of unsigned char exactly overlaying the
object. The file position indicator for the stream (if defined) is advanced by the number of characters
successfully written. If an error occurs, the resulting value of the file position indicator for the stream
is indeterminate.

Returns

The fwrite function returns the number of elements successfully written, which will be less than
nmemb only if a write error is encountered. If size or nmemb is zero, fwrite returns zero and the
state of the stream remains unchanged.

7.21.9 File positioning functions

7.21.9.1 The fgetpos function

Synopsis

      #include <stdio.h>
      int fgetpos(FILE * restrict stream,
            fpos_t * restrict pos);
    

Description

The fgetpos function stores the current values of the parse state (if any) and file position indicator
for the stream pointed to by stream in the object pointed to by pos. The values stored contain
unspecified information usable by the fsetpos function for repositioning the stream to its position
at the time of the call to the fgetpos function.

Returns

If successful, the fgetpos function returns zero; on failure, the fgetpos function returns nonzero
and stores an implementation-defined positive value in errno.

Forward references: the fsetpos function (7.21.9.3).

7.21.9.2 The fseek function

Synopsis

      #include <stdio.h>
      int fseek(FILE *stream, long int offset, int whence);
    

Description

The fseek function sets the file position indicator for the stream pointed to by stream. If a read or
write error occurs, the error indicator for the stream is set and fseek fails.

For a binary stream, the new position, measured in characters from the beginning of the file, is
obtained by adding offset to the position specified by whence. The specified position is the
beginning of the file if whence is SEEK_SET, the current value of the file position indicator if
SEEK_CUR, or end-of-file if SEEK_END. A binary stream need not meaningfully support fseek calls
with a whence value of SEEK_END.

For a text stream, either offset shall be zero, or offset shall be a value returned by an earlier
successful call to the ftell function on a stream associated with the same file and whence shall be
SEEK_SET.

After determining the new position, a successful call to the fseek function undoes any effects of the
ungetc function on the stream, clears the end-of-file indicator for the stream, and then establishes
the new position. After a successful fseek call, the next operation on an update stream may be
either input or output.

Returns

The fseek function returns nonzero only for a request that cannot be satisfied.

Forward references: the ftell function (7.21.9.4).

7.21.9.3 The fsetpos function

Synopsis

      #include <stdio.h>
      int fsetpos(FILE *stream, const fpos_t *pos);
    

Description

The fsetpos function sets the mbstate_t object (if any) and file position indicator for the stream
pointed to by stream according to the value of the object pointed to by pos, which shall be a value
obtained from an earlier successful call to the fgetpos function on a stream associated with the
same file. If a read or write error occurs, the error indicator for the stream is set and fsetpos fails.

A successful call to the fsetpos function undoes any effects of the ungetc function on the stream,
clears the end-of-file indicator for the stream, and then establishes the new parse state and position.
After a successful fsetpos call, the next operation on an update stream may be either input or
output.

Returns

If successful, the fsetpos function returns zero; on failure, the fsetpos function returns nonzero
and stores an implementation-defined positive value in errno.

7.21.9.4 The ftell function

Synopsis

      #include <stdio.h>
      long int ftell(FILE *stream);
    

Description

The ftell function obtains the current value of the file position indicator for the stream pointed to
by stream. For a binary stream, the value is the number of characters from the beginning of the file.

For a text stream, its file position indicator contains unspecified information, usable by the fseek
function for returning the file position indicator for the stream to its position at the time of the ftell
call; the difference between two such return values is not necessarily a meaningful measure of the
number of characters written or read.

Returns

If successful, the ftell function returns the current value of the file position indicator for the stream.
On failure, the ftell function returns −1L and stores an implementation-defined positive value in
errno.

7.21.9.5 The rewind function

Synopsis

      #include <stdio.h>
      void rewind(FILE *stream);
    

Description

The rewind function sets the file position indicator for the stream pointed to by stream to the
beginning of the file. It is equivalent to

    (void)fseek(stream, 0L, SEEK_SET)
  

except that the error indicator for the stream is also cleared.

Returns

The rewind function returns no value.

7.21.10 Error-handling functions

7.21.10.1 The clearerr function

Synopsis

      #include <stdio.h>
      void clearerr(FILE *stream);
    

Description

The clearerr function clears the end-of-file and error indicators for the stream pointed to by
stream.

Returns

The clearerr function returns no value.

7.21.10.2 The feof function

Synopsis

      #include <stdio.h>
      int feof(FILE *stream);
    

Description

The feof function tests the end-of-file indicator for the stream pointed to by stream.

Returns

The feof function returns nonzero if and only if the end-of-file indicator is set for stream.

7.21.10.3 The ferror function

Synopsis

      #include <stdio.h>
      int ferror(FILE *stream);
    

Description

The ferror function tests the error indicator for the stream pointed to by stream.

Returns

The ferror function returns nonzero if and only if the error indicator is set for stream.

7.21.10.4 The perror function

Synopsis

      #include <stdio.h>
      void perror(const char *s);
    

Description

The perror function maps the error number in the integer expression errno to an error message.
It writes a sequence of characters to the standard error stream thus: first (if s is not a null pointer
and the character pointed to by s is not the null character), the string pointed to by s followed by a
colon (:) and a space; then an appropriate error message string followed by a new-line character.
The contents of the error message strings are the same as those returned by the strerror function
with argument errno.

Returns

The perror function returns no value.

Forward references: the strerror function (7.24.6.2).

7.22 General utilities <stdlib.h>

The header <stdlib.h> declares five types and several functions of general utility, and defines
several macros.296)

The types declared are size_t and wchar_t (both described in 7.19),

    div_t
  

which is a structure type that is the type of the value returned by the div function,

    ldiv_t
  

which is a structure type that is the type of the value returned by the ldiv function, and

    lldiv_t
  

which is a structure type that is the type of the value returned by the lldiv function.

The macros defined are NULL (described in 7.19);

    EXIT_FAILURE
  

and

    EXIT_SUCCESS
  

which expand to integer constant expressions that can be used as the argument to the exit function
to return unsuccessful or successful termination status, respectively, to the host environment;

    RAND_MAX
  

which expands to an integer constant expression that is the maximum value returned by the rand
function; and

    MB_CUR_MAX
  

which expands to a positive integer expression with type size_t that is the maximum number of
bytes in a multibyte character for the extended character set specified by the current locale (category
LC_CTYPE), which is never greater than MB_LEN_MAX.

7.22.1 Numeric conversion functions

The functions atof, atoi, atol, and atoll need not affect the value of the integer expression errno
on an error. If the value of the result cannot be represented, the behavior is undefined.

7.22.1.1 The atof function

Synopsis

      #include <stdlib.h>
      double atof(const char *nptr);
    

Description

The atof function converts the initial portion of the string pointed to by nptr to double representa-
tion. Except for the behavior on error, it is equivalent to

    strtod(nptr, (char **)NULL)
  

Returns

The atof function returns the converted value.

Forward references: the strtod, strtof, and strtold functions (7.22.1.3).

7.22.1.2 The atoi, atol, and atoll functions

Synopsis

      #include <stdlib.h>
      int atoi(const char *nptr);
      long int atol(const char *nptr);
      long long int atoll(const char *nptr);
    

Description

The atoi, atol, and atoll functions convert the initial portion of the string pointed to by nptr to
int, long int, and long long int representation, respectively. Except for the behavior on error,
they are equivalent to

    atoi: (int)strtol(nptr, (char **)NULL, 10)
    atol: strtol(nptr, (char **)NULL, 10)
    atoll: strtoll(nptr, (char **)NULL, 10)
  

Returns

The atoi, atol, and atoll functions return the converted value.

Forward references: the strtol, strtoll, strtoul, and strtoull functions (7.22.1.4).

7.22.1.3 The strtod, strtof, and strtold functions

Synopsis

      #include <stdlib.h>
      double strtod(const char * restrict nptr,
            char ** restrict endptr);
      float strtof(const char * restrict nptr,
            char ** restrict endptr);
      long double strtold(const char * restrict nptr,
            char ** restrict endptr);
    

Description

The strtod, strtof, and strtold functions convert the initial portion of the string pointed to
by nptr to double, float, and long double representation, respectively. First, they decompose
the input string into three parts: an initial, possibly empty, sequence of white-space characters
(as specified by the isspace function), a subject sequence resembling a floating-point constant or
representing an infinity or NaN; and a final string of one or more unrecognized characters, including
the terminating null character of the input string. Then, they attempt to convert the subject sequence
to a floating-point number, and return the result.

The expected form of the subject sequence is an optional plus or minus sign, then one of the
following:

The subject sequence is defined as the longest initial subsequence of the input string, starting with
the first non-white-space character, that is of the expected form. The subject sequence contains no
characters if the input string is not of the expected form.

If the subject sequence has the expected form for a floating-point number, the sequence of characters
starting with the first digit or the decimal-point character (whichever occurs first) is interpreted as a
floating constant according to the rules of 6.4.4.2, except that the decimal-point character is used
in place of a period, and that if neither an exponent part nor a decimal-point character appears in
a decimal floating point number, or if a binary exponent part does not appear in a hexadecimal
floating point number, an exponent part of the appropriate type with value zero is assumed to
follow the last digit in the string. If the subject sequence begins with a minus sign, the sequence
is interpreted as negated.297) A character sequence INF or INFINITY is interpreted as an infinity,
if representable in the return type, else like a floating constant that is too large for the range of the
return type. A character sequence NAN or NAN(n-char-sequenceopt ) is interpreted as a quiet NaN, if
supported in the return type, else like a subject sequence part that does not have the expected form;
the meaning of the n-char sequence is implementation-defined.298) A pointer to the final string is
stored in the object pointed to by endptr, provided that endptr is not a null pointer.

If the subject sequence has the hexadecimal form and FLT_RADIX is a power of 2, the value resulting
from the conversion is correctly rounded.

In other than the "C" locale, additional locale-specific subject sequence forms may be accepted.

If the subject sequence is empty or does not have the expected form, no conversion is performed; the
value of nptr is stored in the object pointed to by endptr, provided that endptr is not a null pointer.

Recommended practice

If the subject sequence has the hexadecimal form, FLT_RADIX is not a power of 2, and the result is
not exactly representable, the result should be one of the two numbers in the appropriate internal
format that are adjacent to the hexadecimal floating source value, with the extra stipulation that the
error should have a correct sign for the current rounding direction.

If the subject sequence has the decimal form and at most DECIMAL_DIG (defined in <float.h>)
significant digits, the result should be correctly rounded. If the subject sequence D has the decimal
form and more than DECIMAL_DIG significant digits, consider the two bounding, adjacent decimal
strings L and U, both having DECIMAL_DIG significant digits, such that the values of L, D, and U
satisfy L ≤ D ≤ U. The result should be one of the (equal or adjacent) values that would be obtained
by correctly rounding L and U according to the current rounding direction, with the extra stipulation
that the error with respect to D should have a correct sign for the current rounding direction.299)

Returns

The functions return the converted value, if any. If no conversion could be performed, zero is
returned. If the correct value overflows and default rounding is in effect (7.12.1), plus or minus
HUGE_VAL, HUGE_VALF, or HUGE_VALL is returned (according to the return type and sign of the value),
and the value of the macro ERANGE is stored in errno. If the result underflows (7.12.1), the functions

return a value whose magnitude is no greater than the smallest normalized positive number in the
return type; whether errno acquires the value ERANGE is implementation-defined.

7.22.1.4 The strtol, strtoll, strtoul, and strtoull functions

Synopsis

      #include <stdlib.h>
      long int strtol(
            const char * restrict nptr,
            char ** restrict endptr,
            int base);
      long long int strtoll(
            const char * restrict nptr,
            char ** restrict endptr,
            int base);
      unsigned long int strtoul(
            const char * restrict nptr,
            char ** restrict endptr,
            int base);
      unsigned long long int strtoull(
            const char * restrict nptr,
            char ** restrict endptr,
            int base);
    

Description

The strtol, strtoll, strtoul, and strtoull functions convert the initial portion of
the string pointed to by nptr to long int, long long int, unsigned long int, and
unsigned long long int representation, respectively. First, they decompose the input
string into three parts: an initial, possibly empty, sequence of white-space characters (as specified
by the isspace function), a subject sequence resembling an integer represented in some radix
determined by the value of base, and a final string of one or more unrecognized characters,
including the terminating null character of the input string. Then, they attempt to convert the
subject sequence to an integer, and return the result.

If the value of base is zero, the expected form of the subject sequence is that of an integer constant as
described in 6.4.4.1, optionally preceded by a plus or minus sign, but not including an integer suffix.
If the value of base is between 2 and 36 (inclusive), the expected form of the subject sequence is a
sequence of letters and digits representing an integer with the radix specified by base, optionally
preceded by a plus or minus sign, but not including an integer suffix. The letters from a (or A)
through z (or Z) are ascribed the values 10 through 35; only letters and digits whose ascribed values
are less than that of base are permitted. If the value of base is 16, the characters 0x or 0X may
optionally precede the sequence of letters and digits, following the sign if present.

The subject sequence is defined as the longest initial subsequence of the input string, starting with
the first non-white-space character, that is of the expected form. The subject sequence contains no
characters if the input string is empty or consists entirely of white space, or if the first non-white-
space character is other than a sign or a permissible letter or digit.

If the subject sequence has the expected form and the value of base is zero, the sequence of characters
starting with the first digit is interpreted as an integer constant according to the rules of 6.4.4.1. If
the subject sequence has the expected form and the value of base is between 2 and 36, it is used as
the base for conversion, ascribing to each letter its value as given above. If the subject sequence
begins with a minus sign, the value resulting from the conversion is negated (in the return type). A
pointer to the final string is stored in the object pointed to by endptr, provided that endptr is not a
null pointer.

In other than the "C" locale, additional locale-specific subject sequence forms may be accepted.

If the subject sequence is empty or does not have the expected form, no conversion is performed; the
value of nptr is stored in the object pointed to by endptr, provided that endptr is not a null pointer.

Returns

The strtol, strtoll, strtoul, and strtoull functions return the converted value, if any. If
no conversion could be performed, zero is returned. If the correct value is outside the range of
representable values, LONG_MIN, LONG_MAX, LLONG_MIN, LLONG_MAX, ULONG_MAX, or ULLONG_MAX is
returned (according to the return type and sign of the value, if any), and the value of the macro
ERANGE is stored in errno.

7.22.2 Pseudo-random sequence generation functions

7.22.2.1 The rand function

Synopsis

      #include <stdlib.h>
      int rand(void);
    

Description

The rand function computes a sequence of pseudo-random integers in the range 0 to RAND_MAX.300)

The rand function is not required to avoid data races with other calls to pseudo-random sequence
generation functions. The implementation shall behave as if no library function calls the rand
function.

Returns

The rand function returns a pseudo-random integer.

Environmental limits

The value of the RAND_MAX macro shall be at least 32767.

7.22.2.2 The srand function

Synopsis

      #include <stdlib.h>
      void srand(unsigned int seed);
    

Description

The srand function uses the argument as a seed for a new sequence of pseudo-random numbers
to be returned by subsequent calls to rand. If srand is then called with the same seed value, the
sequence of pseudo-random numbers shall be repeated. If rand is called before any calls to srand
have been made, the same sequence shall be generated as when srand is first called with a seed
value of 1.

The srand function is not required to avoid data races with other calls to pseudo-random sequence
generation functions. The implementation shall behave as if no library function calls the srand
function.

Returns

The srand function returns no value.

EXAMPLE The following functions define a portable implementation of rand and srand.

    static unsigned long int next = 1;
  
    int rand(void)   // RAND_MAX assumed to be 32767
    {
          next = next * 1103515245 + 12345;
          return (unsigned int)(next/65536) % 32768;
    }
  
    void srand(unsigned int seed)
    {
          next = seed;
    }
  

7.22.3 Memory management functions

The order and contiguity of storage allocated by successive calls to the aligned_alloc, calloc,
malloc, and realloc functions is unspecified. The pointer returned if the allocation succeeds is
suitably aligned so that it may be assigned to a pointer to any type of object with a fundamental
alignment requirement and then used to access such an object or an array of such objects in the space
allocated (until the space is explicitly deallocated). The lifetime of an allocated object extends from
the allocation until the deallocation. Each such allocation shall yield a pointer to an object disjoint
from any other object. The pointer returned points to the start (lowest byte address) of the allocated
space. If the space cannot be allocated, a null pointer is returned. If the size of the space requested is
zero, the behavior is implementation-defined: either a null pointer is returned to indicate an error,
or the behavior is as if the size were some nonzero value, except that the returned pointer shall not
be used to access an object.

For purposes of determining the existence of a data race, memory allocation functions behave as
though they accessed only memory locations accessible through their arguments and not other
static duration storage. These functions may, however, visibly modify the storage that they allocate
or deallocate. Calls to these functions that allocate or deallocate a particular region of memory
shall occur in a single total order, and each such deallocation call shall synchronize with the next
allocation (if any) in this order.

7.22.3.1 The aligned_alloc function

Synopsis

      #include <stdlib.h>
      void *aligned_alloc(size_t alignment, size_t size);
    

Description

The aligned_alloc function allocates space for an object whose alignment is specified by
alignment, whose size is specified by size, and whose value is indeterminate. If the value of
alignment is not a valid alignment supported by the implementation the function shall fail by
returning a null pointer.

Returns

The aligned_alloc function returns either a null pointer or a pointer to the allocated space.

7.22.3.2 The calloc function

Synopsis

      #include <stdlib.h>
      void *calloc(size_t nmemb, size_t size);
    

Description

The calloc function allocates space for an array of nmemb objects, each of whose size is size. The
space is initialized to all bits zero.301)

Returns

The calloc function returns either a null pointer or a pointer to the allocated space.

7.22.3.3 The free function

Synopsis

      #include <stdlib.h>
      void free(void *ptr);
    

Description

The free function causes the space pointed to by ptr to be deallocated, that is, made available
for further allocation. If ptr is a null pointer, no action occurs. Otherwise, if the argument does
not match a pointer earlier returned by a memory management function, or if the space has been
deallocated by a call to free or realloc, the behavior is undefined.

Returns

The free function returns no value.

7.22.3.4 The malloc function

Synopsis

      #include <stdlib.h>
      void *malloc(size_t size);
    

Description

The malloc function allocates space for an object whose size is specified by size and whose value
is indeterminate.

Returns

The malloc function returns either a null pointer or a pointer to the allocated space.

7.22.3.5 The realloc function

Synopsis

      #include <stdlib.h>
      void *realloc(void *ptr, size_t size);
    

Description

The realloc function deallocates the old object pointed to by ptr and returns a pointer to a new
object that has the size specified by size. The contents of the new object shall be the same as that of
the old object prior to deallocation, up to the lesser of the new and old sizes. Any bytes in the new
object beyond the size of the old object have indeterminate values.

If ptr is a null pointer, the realloc function behaves like the malloc function for the specified size.
Otherwise, if ptr does not match a pointer earlier returned by a memory management function, or if
the space has been deallocated by a call to the free or realloc function, the behavior is undefined.
If size is nonzero and memory for the new object is not allocated, the old object is not deallocated.
If size is zero and memory for the new object is not allocated, it is implementation-defined whether
the old object is deallocated. If the old object is not deallocated, its value shall be unchanged.

Returns

The realloc function returns a pointer to the new object (which may have the same value as a
pointer to the old object), or a null pointer if the new object has not been allocated.

7.22.4 Communication with the environment

7.22.4.1 The abort function

Synopsis

      #include <stdlib.h>
      _Noreturn void abort(void);
    

Description

The abort function causes abnormal program termination to occur, unless the signal SIGABRT
is being caught and the signal handler does not return. Whether open streams with unwritten
buffered data are flushed, open streams are closed, or temporary files are removed is implementa-
tion-defined. An implementation-defined form of the status unsuccessful termination is returned to
the host environment by means of the function call raise(SIGABRT).

Returns

The abort function does not return to its caller.

7.22.4.2 The atexit function

Synopsis

      #include <stdlib.h>
      int atexit(void (*func)(void));
    

Description

The atexit function registers the function pointed to by func, to be called without arguments at
normal program termination.302) It is unspecified whether a call to the atexit function that does
not happen before the exit function is called will succeed.

Environmental limits

The implementation shall support the registration of at least 32 functions.

Returns

The atexit function returns zero if the registration succeeds, nonzero if it fails.

Forward references: the at_quick_exit function (7.22.4.3), the exit function (7.22.4.4).

7.22.4.3 The at_quick_exit function

Synopsis

      #include <stdlib.h>
      int at_quick_exit(void (*func)(void));
    

Description

The at_quick_exit function registers the function pointed to by func, to be called without argu-
ments should quick_exit be called.303) It is unspecified whether a call to the at_quick_exit
function that does not happen before the quick_exit function is called will succeed.

Environmental limits

The implementation shall support the registration of at least 32 functions.

Returns

The at_quick_exit function returns zero if the registration succeeds, nonzero if it fails.

Forward references: the quick_exit function (7.22.4.7).

7.22.4.4 The exit function

Synopsis

      #include <stdlib.h>
      _Noreturn void exit(int status);
    

Description

The exit function causes normal program termination to occur. No functions registered by the
at_quick_exit function are called. If a program calls the exit function more than once, or calls the
quick_exit function in addition to the exit function, the behavior is undefined.

First, all functions registered by the atexit function are called, in the reverse order of their registra-
tion,304) except that a function is called after any previously registered functions that had already
been called at the time it was registered. If, during the call to any such function, a call to the longjmp
function is made that would terminate the call to the registered function, the behavior is undefined.

Next, all open streams with unwritten buffered data are flushed, all open streams are closed, and all
files created by the tmpfile function are removed.

Finally, control is returned to the host environment. If the value of status is zero or EXIT_SUCCESS,
an implementation-defined form of the status successful termination is returned. If the value of
status is EXIT_FAILURE, an implementation-defined form of the status unsuccessful termination is
returned. Otherwise the status returned is implementation-defined.

Returns

The exit function cannot return to its caller.

7.22.4.5 The _Exit function

Synopsis

      #include <stdlib.h>
      _Noreturn void _Exit(int status);
    

Description

The _Exit function causes normal program termination to occur and control to be returned to the
host environment. No functions registered by the atexit function, the at_quick_exit function,
or signal handlers registered by the signal function are called. The status returned to the host
environment is determined in the same way as for the exit function (7.22.4.4). Whether open
streams with unwritten buffered data are flushed, open streams are closed, or temporary files are
removed is implementation-defined.

Returns

The _Exit function cannot return to its caller.

7.22.4.6 The getenv function

Synopsis

      #include <stdlib.h>
      char *getenv(const char *name);
    

Description

The getenv function searches an environment list, provided by the host environment, for a string that
matches the string pointed to by name. The set of environment names and the method for altering
the environment list are implementation-defined. The getenv function need not avoid data races
with other threads of execution that modify the environment list.305)

The implementation shall behave as if no library function calls the getenv function.

Returns

The getenv function returns a pointer to a string associated with the matched list member. The
string pointed to shall not be modified by the program, but may be overwritten by a subsequent call
to the getenv function. If the specified name cannot be found, a null pointer is returned.

7.22.4.7 The quick_exit function

Synopsis

      #include <stdlib.h>
      _Noreturn void quick_exit(int status);
    

Description

The quick_exit function causes normal program termination to occur. No functions registered by
the atexit function or signal handlers registered by the signal function are called. If a program calls
the quick_exit function more than once, or calls the exit function in addition to the quick_exit
function, the behavior is undefined. If a signal is raised while the quick_exit function is executing,
the behavior is undefined.

The quick_exit function first calls all functions registered by the at_quick_exit function, in the
reverse order of their registration,306) except that a function is called after any previously registered
functions that had already been called at the time it was registered. If, during the call to any such
function, a call to the longjmp function is made that would terminate the call to the registered
function, the behavior is undefined.

Then control is returned to the host environment by means of the function call _Exit(status) .

Returns

The quick_exit function cannot return to its caller.

7.22.4.8 The system function

Synopsis

      #include <stdlib.h>
      int system(const char *string);
    

Description

If string is a null pointer, the system function determines whether the host environment has a
command processor. If string is not a null pointer, the system function passes the string pointed to
by string to that command processor to be executed in a manner which the implementation shall
document; this might then cause the program calling system to behave in a non-conforming manner
or to terminate.

Returns

If the argument is a null pointer, the system function returns nonzero only if a command processor
is available. If the argument is not a null pointer, and the system function does return, it returns an
implementation-defined value.

7.22.5 Searching and sorting utilities

These utilities make use of a comparison function to search or sort arrays of unspecified type. Where
an argument declared as size_t nmemb specifies the length of the array for a function, nmemb can
have the value zero on a call to that function; the comparison function is not called, a search finds no
matching element, and sorting performs no rearrangement. Pointer arguments on such a call shall
still have valid values, as described in 7.1.4.

The implementation shall ensure that the second argument of the comparison function (when called
from bsearch), or both arguments (when called from qsort), are pointers to elements of the array.307)
The first argument when called from bsearch shall equal key.

The comparison function shall not alter the contents of the array. The implementation may reorder
elements of the array between calls to the comparison function, but shall not alter the contents of
any individual element.

When the same objects (consisting of size bytes, irrespective of their current positions in the array)
are passed more than once to the comparison function, the results shall be consistent with one
another. That is, for qsort they shall define a total ordering on the array, and for bsearch the same
object shall always compare the same way with the key.

A sequence point occurs immediately before and immediately after each call to the comparison
function, and also between any call to the comparison function and any movement of the objects
passed as arguments to that call.

7.22.5.1 The bsearch function

Synopsis

      #include <stdlib.h>
      void *bsearch(const void *key, const void *base,
            size_t nmemb, size_t size,
            int (*compar)(const void *, const void *));
    

Description

The bsearch function searches an array of nmemb objects, the initial element of which is pointed to
by base, for an element that matches the object pointed to by key. The size of each element of the
array is specified by size.

The comparison function pointed to by compar is called with two arguments that point to the key
object and to an array element, in that order. The function shall return an integer less than, equal to,
or greater than zero if the key object is considered, respectively, to be less than, to match, or to be
greater than the array element. The array shall consist of: all the elements that compare less than, all
the elements that compare equal to, and all the elements that compare greater than the key object, in
that order.308)

Returns

The bsearch function returns a pointer to a matching element of the array, or a null pointer if no
match is found. If two elements compare as equal, which element is matched is unspecified.

7.22.5.2 The qsort function

Synopsis

      #include <stdlib.h>
      void qsort(void *base, size_t nmemb, size_t size,
            int (*compar)(const void *, const void *));
    

Description

The qsort function sorts an array of nmemb objects, the initial element of which is pointed to by
base. The size of each object is specified by size.

The contents of the array are sorted into ascending order according to a comparison function pointed
to by compar, which is called with two arguments that point to the objects being compared. The
function shall return an integer less than, equal to, or greater than zero if the first argument is
considered to be respectively less than, equal to, or greater than the second.

If two elements compare as equal, their order in the resulting sorted array is unspecified.

Returns

The qsort function returns no value.

7.22.6 Integer arithmetic functions

7.22.6.1 The abs, labs and dlabs functions

Synopsis

      #include <stdlib.h>
      int abs(int j);
      long int labs(long int j);
      long long int llabs(long long int j);
    

Description

The abs, labs, and llabs functions compute the absolute value of an integer j. If the result cannot
be represented, the behavior is undefined.309)

Returns

The abs, labs, and llabs, functions return the absolute value.

7.22.6.2 The div, ldiv, and lldiv functions

Synopsis

      #include <stdlib.h>
      div_t div(int numer, int denom);
      ldiv_t ldiv(long int numer, long int denom);
      lldiv_t lldiv(long long int numer, long long int denom);
    

Description

The div, ldiv, and lldiv, functions compute numer/denom and numer%denom in a single operation.

Returns

The div, ldiv, and lldiv functions return a structure of type div_t, ldiv_t, and lldiv_t, respec-
tively, comprising both the quotient and the remainder. The structures shall contain (in either order)
the members quot (the quotient) and rem (the remainder), each of which has the same type as
the arguments numer and denom. If either part of the result cannot be represented, the behavior is
undefined.

7.22.7 Multibyte/wide character conversion functions

The behavior of the multibyte character functions is affected by the LC_CTYPE category of the current
locale. For a state-dependent encoding, each function is placed into its initial conversion state at
program startup and can be returned to that state by a call for which its character pointer argument,
s, is a null pointer. Subsequent calls with s as other than a null pointer cause the internal conversion
state of the function to be altered as necessary. A call with s as a null pointer causes these functions
to return a nonzero value if encodings have state dependency, and zero otherwise.310) Changing the
LC_CTYPE category causes the conversion state of these functions to be indeterminate.

7.22.7.1 The mblen function

Synopsis

      #include <stdlib.h>
      int mblen(const char *s, size_t n);
    

Description

If s is not a null pointer, the mblen function determines the number of bytes contained in the
multibyte character pointed to by s. Except that the conversion state of the mbtowc function is not
affected, it is equivalent to

    mbtowc((wchar_t *)0, (const char *)0, 0);
    mbtowc((wchar_t *)0, s, n);
  

The implementation shall behave as if no library function calls the mblen function.

Returns

If s is a null pointer, the mblen function returns a nonzero or zero value, if multibyte character
encodings, respectively, do or do not have state-dependent encodings. If s is not a null pointer, the
mblen function either returns 0 (if s points to the null character), or returns the number of bytes
that are contained in the multibyte character (if the next n or fewer bytes form a valid multibyte
character), or returns-1 (if they do not form a valid multibyte character).

Forward references: the mbtowc function (7.22.7.2).

7.22.7.2 The mbtowc function

Synopsis

      #include <stdlib.h>
      int mbtowc(wchar_t * restrict pwc,
            const char * restrict s,
            size_t n);
    

Description

If s is not a null pointer, the mbtowc function inspects at most n bytes beginning with the byte
pointed to by s to determine the number of bytes needed to complete the next multibyte character
(including any shift sequences). If the function determines that the next multibyte character is
complete and valid, it determines the value of the corresponding wide character and then, if pwc
is not a null pointer, stores that value in the object pointed to by pwc. If the corresponding wide
character is the null wide character, the function is left in the initial conversion state.

The implementation shall behave as if no library function calls the mbtowc function.

Returns

If s is a null pointer, the mbtowc function returns a nonzero or zero value, if multibyte character
encodings, respectively, do or do not have state-dependent encodings. If s is not a null pointer, the
mbtowc function either returns 0 (if s points to the null character), or returns the number of bytes
that are contained in the converted multibyte character (if the next n or fewer bytes form a valid
multibyte character), or returns-1 (if they do not form a valid multibyte character).

In no case will the value returned be greater than n or the value of the MB_CUR_MAX macro.

7.22.7.3 The wctomb function

Synopsis

      #include <stdlib.h>
      int wctomb(char *s, wchar_t wc);
    

Description

The wctomb function determines the number of bytes needed to represent the multibyte character
corresponding to the wide character given by wc (including any shift sequences), and stores the
multibyte character representation in the array whose first element is pointed to by s (if s is not a
null pointer). At most MB_CUR_MAX characters are stored. If wc is a null wide character, a null byte is
stored, preceded by any shift sequence needed to restore the initial shift state, and the function is
left in the initial conversion state.

The implementation shall behave as if no library function calls the wctomb function.

Returns

If s is a null pointer, the wctomb function returns a nonzero or zero value, if multibyte character
encodings, respectively, do or do not have state-dependent encodings. If s is not a null pointer, the
wctomb function returns-1 if the value of wc does not correspond to a valid multibyte character, or
returns the number of bytes that are contained in the multibyte character corresponding to the value
of wc.

In no case will the value returned be greater than the value of the MB_CUR_MAX macro.

7.22.8 Multibyte/wide string conversion functions

The behavior of the multibyte string functions is affected by the LC_CTYPE category of the current
locale.

7.22.8.1 The mbstowcs function

Synopsis

      #include <stdlib.h>
      size_t mbstowcs(wchar_t * restrict pwcs,
            const char * restrict s,
            size_t n);
    

Description

The mbstowcs function converts a sequence of multibyte characters that begins in the initial shift
state from the array pointed to by s into a sequence of corresponding wide characters and stores not
more than n wide characters into the array pointed to by pwcs. No multibyte characters that follow
a null character (which is converted into a null wide character) will be examined or converted. Each
multibyte character is converted as if by a call to the mbtowc function, except that the conversion
state of the mbtowc function is not affected.

No more than n elements will be modified in the array pointed to by pwcs. If copying takes place
between objects that overlap, the behavior is undefined.

Returns

If an invalid multibyte character is encountered, the mbstowcs function returns (size_t)(-1) .
Otherwise, the mbstowcs function returns the number of array elements modified, not including a
terminating null wide character, if any.311)

7.22.8.2 The wcstombs function

Synopsis

      #include <stdlib.h>
      size_t wcstombs(char * restrict s,
            const wchar_t * restrict pwcs,
            size_t n);
    

Description

The wcstombs function converts a sequence of wide characters from the array pointed to by pwcs
into a sequence of corresponding multibyte characters that begins in the initial shift state, and stores
these multibyte characters into the array pointed to by s, stopping if a multibyte character would
exceed the limit of n total bytes or if a null character is stored. Each wide character is converted
as if by a call to the wctomb function, except that the conversion state of the wctomb function is not
affected.

No more than n bytes will be modified in the array pointed to by s. If copying takes place between
objects that overlap, the behavior is undefined.

Returns

If a wide character is encountered that does not correspond to a valid multibyte character, the
wcstombs function returns (size_t)(-1) . Otherwise, the wcstombs function returns the number
of bytes modified, not including a terminating null character, if any.311)

7.23 _Noreturn <stdnoreturn.h>

The header <stdnoreturn.h> defines the macro

    noreturn
  

which expands to _Noreturn .

7.24 String handling <string.h>

7.24.1 String function conventions

The header <string.h> declares one type and several functions, and defines one macro useful
for manipulating arrays of character type and other objects treated as arrays of character type.312)
The type is size_t and the macro is NULL (both described in 7.19). Various methods are used for
determining the lengths of the arrays, but in all cases a char * or void * argument points to the
initial (lowest addressed) character of the array. If an array is accessed beyond the end of an object,
the behavior is undefined.

Where an argument declared as size_t n specifies the length of the array for a function, n can have
the value zero on a call to that function. Unless explicitly stated otherwise in the description of a
particular function in this subclause, pointer arguments on such a call shall still have valid values, as
described in 7.1.4. On such a call, a function that locates a character finds no occurrence, a function
that compares two character sequences returns zero, and a function that copies characters copies
zero characters.

For all functions in this subclause, each character shall be interpreted as if it had the type
unsigned char (and therefore every possible object representation is valid and has a different
value).

7.24.2 Copying functions

7.24.2.1 The memcpy function

Synopsis

      #include <string.h>
      void *memcpy(void * restrict s1,
            const void * restrict s2,
            size_t n);
    

Description

The memcpy function copies n characters from the object pointed to by s2 into the object pointed to
by s1. If copying takes place between objects that overlap, the behavior is undefined.

Returns

The memcpy function returns the value of s1.

7.24.2.2 The memmove function

Synopsis

      #include <string.h>
      void *memmove(void *s1, const void *s2, size_t n);
    

Description

The memmove function copies n characters from the object pointed to by s2 into the object pointed to
by s1. Copying takes place as if the n characters from the object pointed to by s2 are first copied
into a temporary array of n characters that does not overlap the objects pointed to by s1 and s2, and
then the n characters from the temporary array are copied into the object pointed to by s1.

Returns

The memmove function returns the value of s1.

7.24.2.3 The strcpy function

Synopsis

      #include <string.h>
      char *strcpy(char * restrict s1,
            const char * restrict s2);
    

Description

The strcpy function copies the string pointed to by s2 (including the terminating null character)
into the array pointed to by s1. If copying takes place between objects that overlap, the behavior is
undefined.

Returns

The strcpy function returns the value of s1.

7.24.2.4 The strncpy function

Synopsis

      #include <string.h>
      char *strncpy(char * restrict s1,
            const char * restrict s2,
            size_t n);
    

Description

The strncpy function copies not more than n characters (characters that follow a null character are
not copied) from the array pointed to by s2 to the array pointed to by s1.313) If copying takes place
between objects that overlap, the behavior is undefined.

If the array pointed to by s2 is a string that is shorter than n characters, null characters are appended
to the copy in the array pointed to by s1, until n characters in all have been written.

Returns

The strncpy function returns the value of s1.

7.24.3 Concatenation functions

7.24.3.1 The strcat function

Synopsis

      #include <string.h>
      char *strcat(char * restrict s1,
            const char * restrict s2);
    

Description

The strcat function appends a copy of the string pointed to by s2 (including the terminating null
character) to the end of the string pointed to by s1. The initial character of s2 overwrites the null
character at the end of s1. If copying takes place between objects that overlap, the behavior is
undefined.

Returns

The strcat function returns the value of s1.

7.24.3.2 The strncat function

Synopsis

      #include <string.h>
      char *strncat(char * restrict s1,
            const char * restrict s2,
            size_t n);
    

Description

The strncat function appends not more than n characters (a null character and characters that
follow it are not appended) from the array pointed to by s2 to the end of the string pointed to by

s1. The initial character of s2 overwrites the null character at the end of s1. A terminating null
character is always appended to the result.314) If copying takes place between objects that overlap,
the behavior is undefined.

Returns

The strncat function returns the value of s1.

Forward references: the strlen function (7.24.6.3).

7.24.4 Comparison functions

The sign of a nonzero value returned by the comparison functions memcmp, strcmp, and strncmp
is determined by the sign of the difference between the values of the first pair of characters (both
interpreted as unsigned char) that differ in the objects being compared.

7.24.4.1 The memcmp function

Synopsis

      #include <string.h>
      int memcmp(const void *s1, const void *s2, size_t n);
    

Description

The memcmp function compares the first n characters of the object pointed to by s1 to the first n
characters of the object pointed to by s2.315)

Returns

The memcmp function returns an integer greater than, equal to, or less than zero, accordingly as the
object pointed to by s1 is greater than, equal to, or less than the object pointed to by s2.

7.24.4.2 The strcmp function

Synopsis

      #include <string.h>
      int strcmp(const char *s1, const char *s2);
    

Description

The strcmp function compares the string pointed to by s1 to the string pointed to by s2.

Returns

The strcmp function returns an integer greater than, equal to, or less than zero, accordingly as the
string pointed to by s1 is greater than, equal to, or less than the string pointed to by s2.

7.24.4.3 The strcoll function

Synopsis

      #include <string.h>
      int strcoll(const char *s1, const char *s2);
    

Description

The strcoll function compares the string pointed to by s1 to the string pointed to by s2, both
interpreted as appropriate to the LC_COLLATE category of the current locale.

Returns

The strcoll function returns an integer greater than, equal to, or less than zero, accordingly as the
string pointed to by s1 is greater than, equal to, or less than the string pointed to by s2 when both
are interpreted as appropriate to the current locale.

7.24.4.4 The strncmp function

Synopsis

      #include <string.h>
      int strncmp(const char *s1, const char *s2, size_t n);
    

Description

The strncmp function compares not more than n characters (characters that follow a null character
are not compared) from the array pointed to by s1 to the array pointed to by s2.

Returns

The strncmp function returns an integer greater than, equal to, or less than zero, accordingly as the
possibly null-terminated array pointed to by s1 is greater than, equal to, or less than the possibly
null-terminated array pointed to by s2.

7.24.4.5 The strxfrm function

Synopsis

      #include <string.h>
      size_t strxfrm(char * restrict s1,
            const char * restrict s2,
            size_t n);
    

Description

The strxfrm function transforms the string pointed to by s2 and places the resulting string into
the array pointed to by s1. The transformation is such that if the strcmp function is applied to two
transformed strings, it returns a value greater than, equal to, or less than zero, corresponding to the
result of the strcoll function applied to the same two original strings. No more than n characters
are placed into the resulting array pointed to by s1, including the terminating null character. If n is
zero, s1 is permitted to be a null pointer. If copying takes place between objects that overlap, the
behavior is undefined.

Returns

The strxfrm function returns the length of the transformed string (not including the terminating
null character). If the value returned is n or more, the contents of the array pointed to by s1 are
indeterminate.

EXAMPLE The value of the following expression is the size of the array needed to hold the transformation of the string
pointed to by s.

    1 + strxfrm(NULL, s, 0)
  

7.24.5 Search functions

7.24.5.1 The memchr function

Synopsis

      #include <string.h>
      void *memchr(const void *s, int c, size_t n);
    

Description

The memchr function locates the first occurrence of c (converted to an unsigned char) in the initial
n characters (each interpreted as unsigned char) of the object pointed to by s. The implementation
shall behave as if it reads the characters sequentially and stops as soon as a matching character is
found.

Returns

The memchr function returns a pointer to the located character, or a null pointer if the character does
not occur in the object.

7.24.5.2 The strchr function

Synopsis

      #include <string.h>
      char *strchr(const char *s, int c);
    

Description

The strchr function locates the first occurrence of c (converted to a char) in the string pointed to
by s. The terminating null character is considered to be part of the string.

Returns

The strchr function returns a pointer to the located character, or a null pointer if the character does
not occur in the string.

7.24.5.3 The strcspn function

Synopsis

      #include <string.h>
      size_t strcspn(const char *s1, const char *s2);
    

Description

The strcspn function computes the length of the maximum initial segment of the string pointed to
by s1 which consists entirely of characters not from the string pointed to by s2.

Returns

The strcspn function returns the length of the segment.

7.24.5.4 The strpbrk function

Synopsis

      #include <string.h>
      char *strpbrk(const char *s1, const char *s2);
    

Description

The strpbrk function locates the first occurrence in the string pointed to by s1 of any character
from the string pointed to by s2.

Returns

The strpbrk function returns a pointer to the character, or a null pointer if no character from s2
occurs in s1.

7.24.5.5 The strrchr function

Synopsis

      #include <string.h>
      char *strrchr(const char *s, int c);
    

Description

The strrchr function locates the last occurrence of c (converted to a char) in the string pointed to
by s. The terminating null character is considered to be part of the string.

Returns

The strrchr function returns a pointer to the character, or a null pointer if c does not occur in the
string.

7.24.5.6 The strspn function

Synopsis

      #include <string.h>
      size_t strspn(const char *s1, const char *s2);
    

Description

The strspn function computes the length of the maximum initial segment of the string pointed to
by s1 which consists entirely of characters from the string pointed to by s2.

Returns

The strspn function returns the length of the segment.

7.24.5.7 The strstr function

Synopsis

      #include <string.h>
      char *strstr(const char *s1, const char *s2);
    

Description

The strstr function locates the first occurrence in the string pointed to by s1 of the sequence of
characters (excluding the terminating null character) in the string pointed to by s2.

Returns

The strstr function returns a pointer to the located string, or a null pointer if the string is not found.
If s2 points to a string with zero length, the function returns s1.

7.24.5.8 The strtok function

Synopsis

      #include <string.h>
      char *strtok(char * restrict s1,
            const char * restrict s2);
    

Description

A sequence of calls to the strtok function breaks the string pointed to by s1 into a sequence of
tokens, each of which is delimited by a character from the string pointed to by s2. The first call
in the sequence has a non-null first argument; subsequent calls in the sequence have a null first
argument. The separator string pointed to by s2 may be different from call to call.

The first call in the sequence searches the string pointed to by s1 for the first character that is not
contained in the current separator string pointed to by s2. If no such character is found, then there
are no tokens in the string pointed to by s1 and the strtok function returns a null pointer. If such a
character is found, it is the start of the first token.

The strtok function then searches from there for a character that is contained in the current separator
string. If no such character is found, the current token extends to the end of the string pointed to by
s1, and subsequent searches for a token will return a null pointer. If such a character is found, it is
overwritten by a null character, which terminates the current token. The strtok function saves a
pointer to the following character, from which the next search for a token will start.

Each subsequent call, with a null pointer as the value of the first argument, starts searching from the
saved pointer and behaves as described above.

The strtok function is not required to avoid data races with other calls to the strtok function.316)
The implementation shall behave as if no library function calls the strtok function.

Returns

The strtok function returns a pointer to the first character of a token, or a null pointer if there is no
token.

EXAMPLE

    #include <string.h>
    static char str[] = "?a???b,,,#c";
    char *t;
  
    t   =   strtok(str, "?");          //   t   points to   the token "a"
    t   =   strtok(NULL, ",");         //   t   points to   the token "??b"
    t   =   strtok(NULL, "#,");        //   t   points to   the token "c"
    t   =   strtok(NULL, "?");         //   t   is a null   pointer
  

Forward references: The strtok_s function (K.3.7.3.1).

7.24.6 Miscellaneous functions

7.24.6.1 The memset function

Synopsis

      #include <string.h>
      void *memset(void *s, int c, size_t n);
    

Description

The memset function copies the value of c (converted to an unsigned char) into each of the first n
characters of the object pointed to by s.

Returns

The memset function returns the value of s.

7.24.6.2 The strerror function

Synopsis

      #include <string.h>
      char *strerror(int errnum);
    

Description

The strerror function maps the number in errnum to a message string. Typically, the values for
errnum come from errno, but strerror shall map any value of type int to a message.

The strerror function is not required to avoid data races with other calls to the strerror func-
tion.317) The implementation shall behave as if no library function calls the strerror function.

Returns

The strerror function returns a pointer to the string, the contents of which are locale-specific. The
array pointed to shall not be modified by the program, but may be overwritten by a subsequent call
to the strerror function.

Forward references: The strerror_s function (K.3.7.4.2).

7.24.6.3 The strlen function

Synopsis

      #include <string.h>
      size_t strlen(const char *s);
    

Description

The strlen function computes the length of the string pointed to by s.

Returns

The strlen function returns the number of characters that precede the terminating null character.

7.25 Type-generic math <tgmath.h>

The header <tgmath.h> includes the headers <math.h> and <complex.h> and defines several
type-generic macros.

Of the <math.h> and <complex.h> functions without an f (float) or l (long double) suffix, several
have one or more parameters whose corresponding real type is double. For each such function,
except modf, there is a corresponding type-generic macro.318) The parameters whose corresponding
real type is double in the function synopsis are generic parameters. Use of the macro invokes a
function whose corresponding real type and type domain are determined by the arguments for the
generic parameters.319)

Use of the macro invokes a function whose generic parameters have the corresponding real type
determined as follows:

For each unsuffixed function in <math.h> for which there is a function in <complex.h> with the
same name except for a c prefix, the corresponding type-generic macro (for both functions) has the
same name as the function in <math.h>. The corresponding type-generic macro for fabs and cabs
is fabs.
<math.h> <complex.h> type-generic
function function macro

    acos            cacos                acos
    asin            casin                asin
    atan            catan                atan
    acosh           cacosh               acosh
    asinh           casinh               asinh
    atanh           catanh               atanh
    cos             ccos                 cos
    sin             csin                 sin
    tan             ctan                 tan
    cosh            ccosh                cosh
    sinh            csinh                sinh
    tanh            ctanh                tanh
    exp             cexp                 exp
    log             clog                 log
    pow             cpow                 pow
    sqrt            csqrt                sqrt
    fabs            cabs                 fabs
  

If at least one argument for a generic parameter is complex, then use of the macro invokes a complex
function; otherwise, use of the macro invokes a real function.

For each unsuffixed function in <math.h> without a c -prefixed counterpart in <complex.h> (except
modf), the corresponding type-generic macro has the same name as the function. These type-generic
macros are:

atan2 fdim ilogb logb rint
cbrt floor ldexp lrint round
ceil fma lgamma lround scalbn
copysign fmax llrint nearbyint scalbln
erf fmin llround nextafter tgamma
erfc fmod log10 nexttoward trunc
exp2 frexp log1p remainder
expm1 hypot log2 remquo

If all arguments for generic parameters are real, then use of the macro invokes a real function;
otherwise, use of the macro results in undefined behavior.

For each unsuffixed function in <complex.h> that is not a c -prefixed counterpart to a function
in <math.h>, the corresponding type-generic macro has the same name as the function. These
type-generic macros are:

carg cimag conj cproj creal

Use of the macro with any real or complex argument invokes a complex function.

EXAMPLE With the declarations

    #include <tgmath.h>
    int n;
    float f;
    double d;
    long double ld;
    float complex fc;
    double complex dc;
    long double complex ldc;
  

functions invoked by use of type-generic macros are shown in the following table:

    macro use              invokes
  
    exp(n)                 exp(n) , the function
    acosh(f)               acoshf(f)
    sin(d)                 sin(d) , the function
    atan(ld)               atanl(ld)
    log(fc)                clogf(fc)
    sqrt(dc)               csqrt(dc)
    pow(ldc, f)            cpowl(ldc, f)
    remainder(n, n)        remainder(n, n) , the function
    nextafter(d, f)        nextafter(d, f) , the function
    nexttoward(f, ld)      nexttowardf(f, ld)
    copysign(n, ld)        copysignl(n, ld)
    ceil(fc)               undefined behavior
    rint(dc)               undefined behavior
    fmax(ldc, ld)          undefined behavior
    carg(n)                carg(n) , the function
    cproj(f)               cprojf(f)
    creal(d)               creal(d) , the function
    cimag(ld)              cimagl(ld)
    fabs(fc)               cabsf(fc)
    carg(dc)               carg(dc) , the function
    cproj(ldc)             cprojl(ldc)
  

7.26 Threads <threads.h>

7.26.1 Introduction

The header <threads.h> includes the header <time.h>, defines macros, and declares types, enu-
meration constants, and functions that support multiple threads of execution.320)

Implementations that define the macro __STDC_NO_THREADS__ need not provide this header nor
support any of its facilities.

The macros are

    thread_local
  

which expands to _Thread_local ;

    ONCE_FLAG_INIT
  

which expands to a value that can be used to initialize an object of type once_flag; and

    TSS_DTOR_ITERATIONS
  

which expands to an integer constant expression representing the maximum number of times that
destructors will be called when a thread terminates.

The types are

    cnd_t
  

which is a complete object type that holds an identifier for a condition variable;

    thrd_t
  

which is a complete object type that holds an identifier for a thread;

    tss_t
  

which is a complete object type that holds an identifier for a thread-specific storage pointer;

    mtx_t
  

which is a complete object type that holds an identifier for a mutex;

    tss_dtor_t
  

which is the function pointer type void (*)(void*), used for a destructor for a thread-specific
storage pointer;

    thrd_start_t
  

which is the function pointer type int (*)(void*) that is passed to thrd_create to create a new
thread; and

    once_flag
  

which is a complete object type that holds a flag for use by call_once.

The enumeration constants are

    mtx_plain
  

which is passed to mtx_init to create a mutex object that does not support timeout;

    mtx_recursive
  

which is passed to mtx_init to create a mutex object that supports recursive locking;

    mtx_timed
  

which is passed to mtx_init to create a mutex object that supports timeout;

    thrd_timedout
  

which is returned by a timed wait function to indicate that the time specified in the call was reached
without acquiring the requested resource;

    thrd_success
  

which is returned by a function to indicate that the requested operation succeeded;

    thrd_busy
  

which is returned by a function to indicate that the requested operation failed because a resource
requested by a test and return function is already in use;

    thrd_error
  

which is returned by a function to indicate that the requested operation failed; and

    thrd_nomem
  

which is returned by a function to indicate that the requested operation failed because it was unable
to allocate memory.

Forward references: date and time (7.27).

7.26.2 Initialization functions

7.26.2.1 The call_once function

Synopsis

      #include <threads.h>
      void call_once(once_flag *flag, void (*func)(void));
    

Description

The call_once function uses the once_flag pointed to by flag to ensure that func is called exactly
once, the first time the call_once function is called with that value of flag. Completion of an
effective call to the call_once function synchronizes with all subsequent calls to the call_once
function with the same value of flag.

Returns

The call_once function returns no value.

7.26.3 Condition variable functions

7.26.3.1 The cnd_broadcast function

Synopsis

      #include <threads.h>
      int cnd_broadcast(cnd_t *cond);
    

Description

The cnd_broadcast function unblocks all of the threads that are blocked on the condition variable
pointed to by cond at the time of the call. If no threads are blocked on the condition variable pointed
to by cond at the time of the call, the function does nothing.

Returns

The cnd_broadcast function returns thrd_success on success, or thrd_error if the request could
not be honored.

7.26.3.2 The cnd_destroy function

Synopsis

      #include <threads.h>
      void cnd_destroy(cnd_t *cond);
    

Description

The cnd_destroy function releases all resources used by the condition variable pointed to by cond.
The cnd_destroy function requires that no threads be blocked waiting for the condition variable
pointed to by cond.
Returns

The cnd_destroy function returns no value.

7.26.3.3 The cnd_init function

Synopsis

      #include <threads.h>
      int cnd_init(cnd_t *cond);
    

Description

The cnd_init function creates a condition variable. If it succeeds it sets the variable pointed to by
cond to a value that uniquely identifies the newly created condition variable. A thread that calls
cnd_wait on a newly created condition variable will block.
Returns

The cnd_init function returns thrd_success on success, or thrd_nomem if no memory could be
allocated for the newly created condition, or thrd_error if the request could not be honored.

7.26.3.4 The cnd_signal function

Synopsis

      #include <threads.h>
      int cnd_signal(cnd_t *cond);
    

Description

The cnd_signal function unblocks one of the threads that are blocked on the condition variable
pointed to by cond at the time of the call. If no threads are blocked on the condition variable at the
time of the call, the function does nothing and returns success.
Returns

The cnd_signal function returns thrd_success on success or thrd_error if the request could not
be honored.

7.26.3.5 The cnd_timedwait function

Synopsis

      #include <threads.h>
      int cnd_timedwait(cnd_t *restrict cond, mtx_t *restrict mtx,
            const struct timespec *restrict ts);
    

Description

The cnd_timedwait function atomically unlocks the mutex pointed to by mtx and blocks until the
condition variable pointed to by cond is signaled by a call to cnd_signal or to cnd_broadcast, or
until after the TIME_UTC-based calendar time pointed to by ts, or until it is unblocked due to an
unspecified reason. When the calling thread becomes unblocked it locks the variable pointed to by
mtx before it returns. The cnd_timedwait function requires that the mutex pointed to by mtx be
locked by the calling thread.

Returns

The cnd_timedwait function returns thrd_success upon success, or thrd_timedout if the time
specified in the call was reached without acquiring the requested resource, or thrd_error if the
request could not be honored.

7.26.3.6 The cnd_wait function

Synopsis

      #include <threads.h>
      int cnd_wait(cnd_t *cond, mtx_t *mtx);
    

Description

The cnd_wait function atomically unlocks the mutex pointed to by mtx and blocks until the condi-
tion variable pointed to by cond is signaled by a call to cnd_signal or to cnd_broadcast, or until it
is unblocked due to an unspecified reason. When the calling thread becomes unblocked it locks the
mutex pointed to by mtx before it returns. The cnd_wait function requires that the mutex pointed
to by mtx be locked by the calling thread.

Returns

The cnd_wait function returns thrd_success on success or thrd_error if the request could not be
honored.

7.26.4 Mutex functions

For purposes of determining the existence of a data race, lock and unlock operations behave as
atomic operations. All lock and unlock operations on a particular mutex occur in some particular
total order.

NOTE This total order can be viewed as the modification order of the mutex.

7.26.4.1 The mtx_destroy function

Synopsis

      #include <threads.h>
      void mtx_destroy(mtx_t *mtx);
    

Description

The mtx_destroy function releases any resources used by the mutex pointed to by mtx. No threads
can be blocked waiting for the mutex pointed to by mtx.

Returns

The mtx_destroy function returns no value.

7.26.4.2 The mtx_init function

Synopsis

      #include <threads.h>
      int mtx_init(mtx_t *mtx, int type);
    

Description

The mtx_init function creates a mutex object with properties indicated by type, which must have
one of these values:

mtx_plain for a simple non-recursive mutex,

mtx_timed for a non-recursive mutex that supports timeout,

mtx_plain | mtx_recursive for a simple recursive mutex, or

mtx_timed | mtx_recursive for a recursive mutex that supports timeout.

If the mtx_init function succeeds, it sets the mutex pointed to by mtx to a value that uniquely
identifies the newly created mutex.

Returns

The mtx_init function returns thrd_success on success, or thrd_error if the request could not
be honored.

7.26.4.3 The mtx_lock function

Synopsis

      #include <threads.h>
      int mtx_lock(mtx_t *mtx);
    

Description

The mtx_lock function blocks until it locks the mutex pointed to by mtx. If the mutex is non-
recursive, it shall not be locked by the calling thread. Prior calls to mtx_unlock on the same mutex
synchronize with this operation.

Returns

The mtx_lock function returns thrd_success on success, or thrd_error if the request could not
be honored.

7.26.4.4 The mtx_timedlock function

Synopsis

      #include <threads.h>
      int mtx_timedlock(mtx_t *restrict mtx, const struct timespec *restrict ts);
    

Description

The mtx_timedlock function endeavors to block until it locks the mutex pointed to by mtx or
until after the TIME_UTC-based calendar time pointed to by ts. The specified mutex shall support
timeout. If the operation succeeds, prior calls to mtx_unlock on the same mutex synchronize with
this operation.

Returns

The mtx_timedlock function returns thrd_success on success, or thrd_timedout if the time
specified was reached without acquiring the requested resource, or thrd_error if the request could
not be honored.

7.26.4.5 The mtx_trylock function

Synopsis

      #include <threads.h>
      int mtx_trylock(mtx_t *mtx);
    

Description

The mtx_trylock function endeavors to lock the mutex pointed to by mtx. If the mutex is already
locked, the function returns without blocking. If the operation succeeds, prior calls to mtx_unlock
on the same mutex synchronize with this operation.

Returns

The mtx_trylock function returns thrd_success on success, or thrd_busy if the resource requested
is already in use, or thrd_error if the request could not be honored. mtx_trylock may spuriously
fail to lock an unused resource, in which case it returns thrd_busy.

7.26.4.6 The mtx_unlock function

Synopsis

      #include <threads.h>
      int mtx_unlock(mtx_t *mtx);
    

Description

The mtx_unlock function unlocks the mutex pointed to by mtx. The mutex pointed to by mtx shall
be locked by the calling thread.

Returns

The mtx_unlock function returns thrd_success on success or thrd_error if the request could not
be honored.

7.26.5 Thread functions

7.26.5.1 The thrd_create function

Synopsis

      #include <threads.h>
      int thrd_create(thrd_t *thr, thrd_start_t func, void *arg);
    

Description

The thrd_create function creates a new thread executing func(arg). If the thrd_create function
succeeds, it sets the object pointed to by thr to the identifier of the newly created thread. (A thread’s
identifier may be reused for a different thread once the original thread has exited and either been
detached or joined to another thread.) The completion of the thrd_create function synchronizes
with the beginning of the execution of the new thread.

Returning from func has the same behavior as invoking thrd_exit with the value returned from
func.

Returns

The thrd_create function returns thrd_success on success, or thrd_nomem if no memory could
be allocated for the thread requested, or thrd_error if the request could not be honored.

7.26.5.2 The thrd_current function

Synopsis

      #include <threads.h>
      thrd_t thrd_current(void);
    

Description

The thrd_current function identifies the thread that called it.

Returns