(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.
(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.
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:
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.
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
This International Standard is designed to promote the portability of C programs among a variety of data-processing
systems. It is intended for use by implementors and programmers.
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.
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.
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.
alignment
requirement that objects of a particular type be located on storage boundaries with addresses that
are particular multiples of a byte address
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
behavior
external appearance or action
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.
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.
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.
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.
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.
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.
character
habstracti member of a set of elements used for the organization, control, or representation of data
character
single-byte character
hCi bit representation that fits in a byte
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.
wide character
value representable by an object of type wchar_t, capable of representing any character in the
current locale
constraint
restriction, either syntactic or semantic, by which the exposition of language elements is to be
interpreted
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
diagnostic message
message belonging to an implementation-defined subset of the implementation’s message output
forward reference
reference to a later subclause of this International Standard that contains additional information
relevant to this subclause
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
implementation limit
restriction imposed upon programs by the implementation
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.
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.
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
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
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.
value
precise meaning of the contents of an object when interpreted as having a specific type
implementation-defined value
unspecified value where each implementation documents how the choice is made
indeterminate value
either an unspecified value or a trap representation
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.
trap representation
an object representation that need not represent a value of the object type
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)
For example, “Trapping or stopping (if supported) is disabled . . . ” (F.8.2). Note that fetching a trap representation might
perform a trap but is not required to (see 6.2.6.1).
dxe
ceiling of x: the least integer greater than or equal to x
EXAMPLE d2.4e is 3, d−2.4e is −2.
bxc
floor of x: the greatest integer less than or equal to x
EXAMPLE b2.4c is 2, b−2.4c is −3.
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).
A strictly conforming program can use conditional features (see 6.10.8.3) provided the use is guarded by an appropriate
conditional inclusion preprocessing directive using the related macro. For example:
#ifdef __STDC_IEC_559__ /* FE_UPWARD defined */ /* ... */ fesetround(FE_UPWARD); /* ... */ #endif
This implies that a conforming implementation reserves no identifiers other than those explicitly reserved in this
International Standard.
Strictly conforming programs are intended to be maximally portable among conforming implementations. Conforming
programs may depend upon nonportable features of a conforming implementation.
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.
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).
The precedence among the syntax rules of translation is specified by the following phases.6)
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).
Implementations shall behave as if these separate phases occur, even though many are typically folded together in
practice. Source files, translation units, and translated translation units need not necessarily be stored as files, nor need there
be any one-to-one correspondence between these entities and any external representation. The description is conceptual only,
and does not specify any particular implementation.
As described in 6.4, the process of dividing a source file’s characters into preprocessing tokens is context-dependent. For
example, see the handling of < within a #include preprocessing directive.
An implementation need not convert all non-corresponding source characters to the same execution character.
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.
The intent is that an implementation should identify the nature of, and where possible localize, each violation. Of course,
an implementation is free to produce any number of diagnostics as long as a valid program is still correctly translated. It may
also successfully translate an invalid program.
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).
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.
A hosted environment need not be provided, but shall conform to the following specifications if
present.
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:
Thus, int can be replaced by a typedef name defined as int, or the type of argv can be written as char
** argv, and so
on.
In a hosted environment, a program may use all the functions, macros, type definitions, and objects
described in the library clause (clause 7).
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).
In accordance with 6.2.4, the lifetimes of objects with automatic storage duration declared in main will have ended in the
former case, even where they would not have in the latter.
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).
The IEC 60559 standard for binary floating-point arithmetic requires certain user-accessible status flags and control
modes. Floating-point operations implicitly set the status flags; modes affect result values of floating-point operations.
Implementations that support such floating-point state are required to regard changes to it as side effects — see annex F for
details. The floating-point environment library <fenv.h> provides a programming facility for indicating when these side
effects matter, freeing the implementations in other cases.
The executions of unsequenced evaluations can interleave. Indeterminately sequenced evaluations cannot interleave, but
can be executed in any order.
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:
or
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.
The execution can usually be viewed as an interleaving of all of the threads. However, some kinds of atomic operations,
for example, allow executions inconsistent with a simple interleaving as described below.
The “carries a dependency” relation is a subset of the “sequenced before” relation, and is similarly strictly intra-thread.
The “dependency-ordered before” relation is analogous to the “synchronizes with” relation, but uses release/consume in
place of release/acquire.
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).
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");
The trigraph sequences enable the input of characters that are not defined in the Invariant Code Set as described in
ISO/IEC 646, which is a subset of the seven-bit US ASCII code set.
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:
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).
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.
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.
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)
Implementations should avoid imposing fixed translation limits whenever possible.
See “future language directions” (6.11.3).
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).
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.
CHAR_BIT 8
SCHAR_MIN -127 // −(27 − 1)
SCHAR_MAX +127 // 27 − 1
UCHAR_MAX 255 // 28 − 1
CHAR_MIN see below
CHAR_MAX see below
MB_LEN_MAX 1
SHRT_MIN -32767 // −(215 − 1)
SHRT_MAX +32767 // 215 − 1
USHRT_MAX 65535 // 216 − 1
INT_MIN -32767 // −(215 − 1)
INT_MAX +32767 // 215 − 1
UINT_MAX 65535 // 216 − 1
LONG_MIN -2147483647 // −(231 − 1)
LONG_MAX +2147483647 // 231 − 1
ULONG_MAX 4294967295 // 232 − 1
LLONG_MIN -9223372036854775807 // −(263 − 1)
LLONG_MAX +9223372036854775807 // 263 − 1
ULLONG_MAX 18446744073709551615 // 264 − 1
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).
See 6.2.5.
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)
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)
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:
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:
FLT_RADIX 2
FLT_MANT_DIG DBL_MANT_DIG LDBL_MANT_DIG
( p log10 b if b is a power of 10 d1 + p log10 be otherwise
FLT_DECIMAL_DIG 6 DBL_DECIMAL_DIG 10 LDBL_DECIMAL_DIG 10
( pmax log10 b if b is a power of 10 d1 + pmax log10 be otherwise
DECIMAL_DIG 10
( p log10 b if b is a power of 10 b(p − 1) log10 bc otherwise
FLT_DIG 6 DBL_DIG 10 LDBL_DIG 10
FLT_MIN_EXP DBL_MIN_EXP LDBL_MIN_EXP
such that10 raised to that power is in the range of normalized floating-point numbers, log10 bemin −1
FLT_MIN_10_EXP -37 DBL_MIN_10_EXP -37 LDBL_MIN_10_EXP -37
FLT_MAX_EXP DBL_MAX_EXP LDBL_MAX_EXP
FLT_MAX_10_EXP +37 DBL_MAX_10_EXP +37 LDBL_MAX_10_EXP +37
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:
FLT_MAX 1E+37 DBL_MAX 1E+37 LDBL_MAX 1E+37
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:
FLT_EPSILON 1E-5 DBL_EPSILON 1E-9 LDBL_EPSILON 1E-9
FLT_MIN 1E-37 DBL_MIN 1E-37 LDBL_MIN 1E-37
FLT_TRUE_MIN 1E-37 DBL_TRUE_MIN 1E-37 LDBL_TRUE_MIN 1E-37
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).
The floating-point model is intended to clarify the description of each floating-point characteristic and does not require
the floating-point arithmetic of the implementation to be identical.
IEC 60559:1989 specifies quiet and signaling NaNs. For implementations that do not support IEC 60559:1989, the terms
quiet NaN and signaling NaN are intended to apply to encodings with similar behavior.
Evaluation of FLT_ROUNDS correctly reflects any execution-time change of rounding mode through the function
fesetround in <fenv.h>.
The evaluation method determines evaluation formats of expressions involving all floating types, not just real
types. For example, if FLT_EVAL_METHOD is 1, then the product of two float _Complex operands is represented in the
double _Complex format, and its parts are evaluated to double.
Characterization as indeterminable is intended if floating-point operations do not consistently interpret subnormal
representations as zero, nor as nonzero.
Characterization as absent is intended if no floating-point operations produce subnormal results from non-subnormal
inputs, even if the type format includes representations of subnormal numbers.
If the presence or absence of subnormal numbers is indeterminable, then the value is intended to be a positive number
no greater than the minimum normalized positive number for the type.
The floating-point model in that standard sums powers of b from zero, so the values of the exponent limits are one less
than shown here.
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.
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).
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).
There is no linkage between different identifiers.
A function declaration can contain the storage-class specifier static only if it is at file scope; see 6.7.1.
As specified in 6.2.1, the later declaration might hide the prior declaration.
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).
There is only one name space for tags even though three are possible.
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).
The term “constant address” means that two pointers to the object constructed at possibly different times will compare
equal. The address may be different during two different executions of the same program.
In the case of a volatile object, the last store need not be explicit in the program.
Leaving the innermost block containing the declaration, or jumping to a point in that block or an embedded block prior
to the declaration, leaves the scope of the declaration.
The address of such an object is taken implicitly when an array member is accessed.
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).
A type may be incomplete or complete throughout an entire translation unit, or it may change states at different points
within a translation unit.
Implementation-defined keywords shall have the form of an identifier reserved for any use as described in 7.1.3.
Therefore, any statement in this Standard about signed integer types also applies to the extended signed integer types.
Therefore, any statement in this Standard about unsigned integer types also applies to the extended unsigned integer
types.
The same representation and alignment requirements are meant to imply interchangeability as arguments to functions,
return values from functions, and members of unions.
See “future language directions” (6.11.1).
A specification for imaginary types is in annex G.
An implementation may define new keywords that provide alternative ways to designate a basic (or any other) type; this
does not violate the requirement that all basic types be different. Implementation-defined keywords shall have the form of an
identifier reserved for any use as described in 7.1.3.
CHAR_MIN, defined in <limits.h>, will have one of the values 0 or SCHAR_MIN, and this can be used to distinguish the
two options. Irrespective of the choice made, char is a separate type from the other two and is not compatible with either.
Note that aggregate type does not include union type because an object with union type can only contain one member at
a time.
See 6.7.3 regarding qualified array and function types.
The same representation and alignment requirements are meant to imply interchangeability as arguments to functions,
return values from functions, and members of unions.
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).
A positional representation for integers that uses the binary digits 0 and 1, in which the values represented by successive
bits are additive, begin with 1, and are multiplied by successive integral powers of 2, except perhaps the bit with the highest
position. (Adapted from the American National Dictionary for Information Processing Systems.) A byte contains CHAR_BIT bits,
_
and the values of type unsigned char range from 0 to 2CHAR BIT − 1.
Thus, an automatic variable can be initialized to a trap representation without causing undefined behavior, but the value
of the variable cannot be used until a proper value is stored in it.
Thus, for example, structure assignment need not copy any padding bits.
It is possible for objects x and y with the same effective type T to have the same value when they are accessed as objects
of type T, but to have different values in other contexts. In particular, if == is defined for type T, then x == y does not imply
that memcmp(&x, &y, sizeof (T))== 0. Furthermore, x == y does not necessarily imply that x and y have the same value;
other operations on values of type T may distinguish between them.
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.
Some combinations of padding bits might generate trap representations, for example, if one padding bit is a parity bit.
Regardless, no arithmetic operation on valid values can generate a trap representation other than as part of an exceptional
condition such as an overflow, and this cannot occur with unsigned types. All other combinations of padding bits are
alternative object representations of the value specified by the value bits.
Some combinations of padding bits might generate trap representations, for example, if one padding bit is a parity bit.
Regardless, no arithmetic operation on valid values can generate a trap representation other than as part of an exceptional
condition such as an overflow. All other combinations of padding bits are alternative object representations of the value
specified by the value bits.
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:
The element type of the composite type is the composite type of the two element types.
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]);
Two types need not be identical to be compatible.
As specified in 6.2.1, the later declaration might hide the prior declaration.
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:
Every over-aligned type is, or contains, a structure or union type with a member to which an extended alignment has
been applied.
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).
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).
The integer promotions are applied only: as part of the usual arithmetic conversions, to certain argument expressions, to
the operands of the unary + ,- , and ~ operators, and to both operands of the shift operators, as specified by their respective
subclauses.
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)
NaNs do not compare equal to 0 and thus convert to 1.
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.
The rules describe arithmetic on the mathematical value, not the value of a given type of expression.
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).
The remaindering operation performed when a value of integer type is converted to unsigned type need not be
performed when a value of real floating type is converted to unsigned type. Thus, the range of portable real floating values is
(−1, Utype_MAX + 1).
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).
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.
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.
See 6.3.1.2.
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)
For example, addition of a double _Complex and a float entails just the conversion of the float operand to double
(and yields a double _Complex result).
The cast and assignment operators are still required to remove extra range and precision.
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).
The name “lvalue” comes originally from the assignment expression E1 = E2, in which the left operand E1 is required to
be a (modifiable) lvalue. It is perhaps better considered as representing an object “locator value”. What is sometimes called
“rvalue” is in this International Standard described as the “value of an expression”.
An obvious example of an lvalue is an identifier of an object. As a further example, if E is a unary expression that is a
pointer to an object, *E is an lvalue that designates the object to which E points.
Because this conversion does not occur, the operand of the sizeof operator remains a function designator and violates
the constraints in 6.5.3.4.
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.)
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).
The macro NULL is defined in <stddef.h> (and other headers) as a null pointer constant; see 7.19.
The mapping functions for converting a pointer to an integer or an integer to a pointer are intended to be consistent with
the addressing structure of the execution environment.
In general, the concept “correctly aligned” is transitive: if a pointer to type A is correctly aligned for a pointer to type B,
which in turn is correctly aligned for a pointer to type C, then a pointer to type A is correctly aligned for a pointer to type C.
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).
An additional category, placemarkers, is used internally in translation phase 4 (see 6.10.3.3); it cannot occur in source
files.
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)
One possible specification for imaginary types appears in annex G.
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).
On systems in which linkers cannot accept extended characters, an encoding of the universal character name may be
used in forming valid external identifiers. For example, some otherwise unused character or sequence of characters may be
used to encode the \u in a universal character name. Extended characters may produce a long external identifier.
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).
Since the name __func__ is reserved for any use by the implementation (7.1.3), if any other identifier is explicitly declared
using the name __func__ , the behavior is undefined.
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).
The disallowed characters are the characters in the basic character set and the code positions reserved by ISO/IEC 10646
for control characters, the character DELETE, and the S-zone (reserved for use by UTF–16).
Short identifiers for characters were first specified in ISO/IEC 10646–1:1993/Amd.9:1997.
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.
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.
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)
1.23 , 1.230 , 123e-2 , 123e-02 , and 1.23L are all different source forms and thus need not convert to the same internal
format and value.
The specification for the library functions recommends more accurate conversion than required for floating constants
(see 7.22.1.3).
Syntax
enumeration-constant:
identifier
Semantics
An identifier declared as an enumeration constant has type int.
Forward references: enumeration specifiers (6.7.2.2).
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).
The semantics of these characters were discussed in 5.2.2. If any other character follows a backslash, the result is not a
token and a diagnostic is required. See “future language directions” (6.11.4).
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).
A string literal need not be a string (see 7.1.1), because a null character may be embedded in it by a \0 escape sequence.
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).
These tokens are sometimes called “digraphs”.
Thus [ and <: behave differently when “stringized” (see 6.10.3.2), but can otherwise be freely interchanged.
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).
Thus, sequences of characters that resemble escape sequences cause undefined behavior.
For an example of a header name preprocessing token used in a #pragma directive, see 6.10.9.
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.
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;
Thus, /
* . . . */ comments do not nest.
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).
This paragraph renders undefined statement expressions such as
i = ++i + 1; a[i++] = i;
while allowing
i = i + 1; a[i] = i;
The syntax specifies the precedence of operators in the evaluation of an expression, which is the same as the order of the
major subclauses of this subclause, highest precedence first. Thus, for example, the expressions allowed as the operands
of the binary + operator (6.5.6) are those expressions defined in 6.5.1 through 6.5.6. The exceptions are cast expressions
(6.5.4) as operands of unary operators (6.5.3), and an operand contained between any of the following pairs of operators:
grouping parentheses () (6.5.1), subscripting brackets [] (6.5.2.1), function-call parentheses () (6.5.2.2), and the conditional
operator ?: (6.5.15).
Within each major subclause, the operators have the same precedence. Left- or right-associativity is indicated in each
subclause by the syntax for the expressions discussed therein.
In an expression that is evaluated more than once during the execution of a program, unsequenced and indeterminately
sequenced evaluations of its subexpressions need not be performed consistently in different evaluations.
Allocated objects have no declared type.
The intent of this list is to specify those circumstances in which an object may or may not be aliased.
The intermediate operations in the contracted expression are evaluated as if to infinite range and precision, while the
final operation is rounded to the format determined by the expression evaluation method. A contracted expression might
also omit the raising of floating-point exceptions.
This license is specifically intended to allow implementations to exploit fast machine instructions that combine multiple
C operators. As contractions potentially undermine predictability, and can even decrease accuracy for containing expressions,
their use needs to be well-defined and clearly documented.
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).
Thus, an undeclared identifier is a violation of the syntax.
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)
lvalue conversion drops type qualifiers.
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
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).
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).
Most often, this is the result of converting an identifier that is a function designator.
A function may change the values of its parameters, but these changes cannot affect the values of the arguments. On the
other hand, it is possible to pass a pointer to an object, and the function may change the value of the object pointed to. A
parameter declared to have array or function type is adjusted to have a pointer type as described in 6.9.1.
In other words, function executions do not “interleave” with each other.
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; } 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).
If the member used to read the contents of a union object is not the same as the member last used to store a value in the
object, the appropriate part of the object representation of the value is reinterpreted as an object representation in the new
type as described in 6.2.6 (a process sometimes called “type punning”). This might be a trap representation.
If &E is a valid pointer expression (where & is the “address-of” operator, which generates a pointer to its operand), the
expression (&E)->MOS is the same as E.MOS.
For example, a data race would occur if access to the entire structure or union in one thread conflicts with access to a
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.
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).
Where a pointer to an atomic object can be formed and E has integer type, E++ is equivalent to the following code
sequence where T is the type of E:
T *addr = &E; T old = *addr; T new; do { new = old + 1; } while (!atomic_compare_exchange_strong(addr, &old, new));
with old being the result of the operation.
Special care must be taken if E has floating type; see 6.5.16.2.
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).
Note that this differs from a cast expression. For example, a cast specifies a conversion to scalar types or void only, and
the result of a cast expression is not an lvalue.
For example, subobjects without explicit initializers are initialized to zero.
This allows implementations to share storage for string literals and constant compound literals with the same or
overlapping representations.
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
& * + - ˜ !
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).
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).
Thus, & E is equivalent to E (even if E is a null pointer), and &(E1[E2]) to ((E1)+(E2)) . It is always true that if E is a
*
function designator or an lvalue that is a valid operand of the unary & operator, *&E is a function designator or an lvalue
equal to E. If *P is an lvalue and T is the name of an object pointer type, *(T)P is an lvalue that has a type compatible with
that to which T points.
Among the invalid values for dereferencing a pointer by the unary * operator are a null pointer, an address inappropriately
aligned for the type of object pointed to, and the address of an object after the end of its lifetime.
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) .
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).
When applied to a parameter declared to have array or function type, the sizeof operator yields the size of the adjusted
(pointer) type (see 6.9.1).
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).
A cast does not yield an lvalue.
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.
This is often called “truncation toward zero”.
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).
Another way to approach pointer arithmetic is first to convert the pointer(s) to character pointer(s): In this scheme the
integer expression added to or subtracted from the converted pointer is first multiplied by the size of the object originally
pointed to, and the resulting pointer is converted back to the original type. For pointer subtraction, the result of the difference
between the character pointers is similarly divided by the size of the object originally pointed to.
When viewed in this way, an implementation need only provide one extra byte (which may overlap another object in the
program) just after the end of the object in order to satisfy the “one past the last element” requirements.
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.
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.
The expression a<b<c is not interpreted as in ordinary mathematics. As the syntax indicates, it means (a<b)<c ; in other
words, “if a is less than b, compare 1 to c; otherwise, compare 0 to c”.
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.
Because of the precedences, a<b == c<d is 1 whenever a<b and c<d have the same truth-value.
Two objects may be adjacent in memory because they are adjacent elements of a larger array or adjacent members
of a structure with no padding between them, or because the implementation chose to place them so, even though they
are unrelated. If prior invalid pointer operations (such as accesses outside array bounds) produced undefined behavior,
subsequent comparisons also produce undefined behavior.
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).
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).
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).
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.
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.
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 *
A conditional expression does not yield an lvalue.
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.
The implementation is permitted to read the object to determine the value but is not required to, even when the object
has volatile-qualified type.
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.
The asymmetric appearance of these constraints with respect to type qualifiers is due to the conversion (specified in
6.3.2.1) that changes lvalues to “the value of the expression” and thus removes any type qualifiers that were applied to the
type category of the expression (for example, it removes const but not volatile from the type int volatile * const).
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)
Where a pointer to an atomic object can be formed and E1 and E2 have integer type, this is equivalent to the following
code sequence where T1 is the type of E1 and T2 is the type of E2:
T1 *addr = &E1; T2 val = (E2); T1 old = *addr; T1 new; do { new = old op val; } while (!atomic_compare_exchange_strong(addr, &old, new));
with new being the result of the operation.
If E1 or E2 has floating type, then exceptional conditions or floating-point exceptions encountered during discarded
evaluations of new should also be discarded in order to satisfy the equivalence of E1 op= E2 and E1 = E1 op (E2) . For
example, if annex F is in effect, the floating types involved have IEC 60559 formats, and FLT_EVAL_METHOD is 0, the equivalent
code would be:
#include <fenv.h> #pragma STDC FENV_ACCESS ON /* ... */ fenv_t fenv; T1 *addr = &E1; T2 val = E2; T1 old = *addr; T1 new; feholdexcept(&fenv);
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.
A comma operator does not yield an lvalue.
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).
The operand of a sizeof or _Alignof operator is usually not evaluated (6.5.3.4).
The use of evaluation formats as characterized by FLT_EVAL_METHOD also applies to evaluation in the translation
environment.
An integer constant expression is required in a number of contexts such as the size of a bit-field member of a structure,
the value of an enumeration constant, and the size of a non-variable length array. Further constraints that apply to the integer
constant expressions used in conditional-inclusion preprocessing directives are discussed in 6.10.1.
Thus, in the following initialization,
static int i = 2 || 1 / 0;
the expression is a valid integer constant expression with value one.
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).
Function definitions have a different syntax, described in 6.9.1.
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).
See “future language directions” (6.11.5).
The implementation may treat any register declaration simply as an auto declaration. However, whether or not
addressable storage is actually used, the address of any part of an object declared with storage-class specifier register
cannot be computed, either explicitly (by use of the unary & operator as discussed in 6.5.3.2) or implicitly (by converting
an array name to a pointer as discussed in 6.3.2.1). Thus, the only operator that can be applied to an array declared with
storage-class specifier register is sizeof.
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).
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).
While the number of bits in a _Bool object is at least CHAR_BIT, the width (number of sign and value bits) of a _Bool
may be just 1 bit.
A structure or union cannot contain a member with a variably modified type because member names are not ordinary
identifiers as defined in 6.2.3.
The unary & (address-of) operator cannot be applied to a bit-field object; thus, there are no pointers to or arrays of bit-field
objects.
As specified in 6.7.2 above, if the actual type specifier used is int or a typedef-name defined as int, then it is implemen-
tation-defined whether the bit-field is signed or unsigned.
An unnamed bit-field structure member is useful for padding to conform to externally imposed layouts.
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).
Thus, the identifiers of enumeration constants declared in the same scope shall all be distinct from each other and from
other identifiers declared in ordinary declarators.
An implementation may delay the choice of which integer type until all enumeration constants have been seen.
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).
An incomplete type may only by used when the size of an object of that type is not needed. It is not needed, for example,
when a typedef name is declared to be a specifier for a structure or union, or when a pointer to or a function returning a
structure or union is being declared. (See incomplete types in 6.2.5.) The specification has to be complete before such a
function is called or defined.
If there is no identifier, the type can, within the translation unit, only be referred to by the declaration of which it is a part.
Of course, when the declaration is of a typedef name, subsequent declarations can make use of that typedef name to declare
objects having the specified structure, union, or enumerated type.
A similar construction with enum does not exist.
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.
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.
The implementation may place a const object that is not volatile in a read-only region of storage. Moreover, the
implementation need not allocate storage for such an object if its address is never used.
This applies to those objects that behave as if they were defined with qualified types, even if they are never actually
defined as objects in the program (such as an object at a memory-mapped input/output address).
A volatile declaration may be used to describe an object corresponding to a memory-mapped input/output port or an
object accessed by an asynchronously interrupting function. Actions on objects so declared shall not be “optimized out” by
an implementation or reordered except as permitted by the rules for evaluating expressions.
For example, a statement that assigns a value returned by malloc to a single pointer establishes this association between
the allocated object and the pointer.
Both of these can occur through the use of typedef s.
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; }
In other words, E depends on the value of P itself rather than on the value of an object referenced indirectly through P.
For example, if identifier p has type (int **restrict) , then the pointer expressions p and p+1 are based on the restricted
pointer object designated by p, but the pointer expressions *p and p[1] are not.
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).
By using, for example, an alternative to the usual function call mechanism, such as “inline substitution”. Inline
For example, an implementation might never perform inline substitution, or might only perform inline substitutions to
calls in the scope of an inline declaration.
Since an inline definition is distinct from the corresponding external definition and from any other corresponding inline
definitions in other translation units, all corresponding objects with static storage duration are also distinct in each of the
definitions.
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.
An alignment specification of zero also does not affect other alignment specifications in the same declaration.
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).
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”.
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).
When several “array of” specifications are adjacent, a multidimensional array is declared.
Thus, * can be used only in function declarations that are not definitions (see 6.7.6.3).
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).
The macros defined in the <stdarg.h> header (7.16) may be used to access arguments that correspond to the ellipsis.
See “future language directions” (6.11.6).
If both function types are “old style”, parameter types are not compared.
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.
As indicated by the syntax, empty parentheses in a type name are interpreted as “function with no parameter specifica-
tion”, rather than redundant parentheses around the omitted identifier.
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]; }
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).
If the initializer list for a subaggregate or contained union does not begin with a left brace, its subobjects are initialized as
usual, but the subaggregate or contained union does not become the current object: current objects are associated only with
brace-enclosed initializer lists.
After a union member is initialized, the next object is not the next member of the union; instead, it is the next subobject of
an object containing the union.
Thus, a designator can only specify a strict subobject of the aggregate or union that is associated with the surrounding
brace pair. Note, too, that each separate designator list is independent.
Any initializer for the subobject which is overridden and so not used to initialize that subobject might not be evaluated at
all.
In particular, the evaluation order need not be the same as the order of subobject initialization.
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).
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).
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).
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.
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).
Such as assignments, and function calls which have side effects.
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.
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.
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.
That is, the declaration either precedes the switch statement, or it follows the last case or default label associated with
the switch that is in the block containing the declaration.
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)
Code jumped over is not executed. In particular, the controlling expression of a for or while statement is not evaluated
before entering the loop body, nor is clause-1 of a for statement.
An omitted controlling expression is replaced by a nonzero constant, which is a constant expression.
This is intended to allow compiler transformations such as removal of empty loops even when termination cannot be
proven.
The evaluation of the controlling expression takes place before each execution of the loop body.
The evaluation of the controlling expression takes place after each execution of the loop body.
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.
Thus, clause-1 specifies initialization for the loop, possibly declaring one or more variables for use in the loop; the
controlling expression, expression-2, specifies an evaluation made before each iteration, such that execution of the loop
continues until the expression compares equal to 0; and expression-3 specifies an operation (such as incrementing) that is
performed after each iteration.
Syntax
jump-statement:
goto identifier ; continue ; break ; return expressionopt ;
Semantics
A jump statement causes an unconditional jump to another place.
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:
/* ... */ 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.
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)
Following the contin: label is a null 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.
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).
The return statement is not an assignment. The overlap restriction of subclause 6.5.16.1 does not apply to the case of
function return. The representation of floating-point values may have wider range or precision than implied by the type; a
cast may be used to remove this extra range and precision.
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)
Thus, if an identifier declared with external linkage is not used in an expression, there need be no external definition for
it.
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)(); ...*/ }
The intent is that the type category in a function definition cannot be inherited from a typedef:
typedef int F(void); // type F is “function with no parameters // returning int” F f, g; // f and g both have type compatible with F F f { /* ... */ } // WRONG: syntax/constraint error F g() { /* ... */ } // WRONG: declares that g returns a function int f(void) { /* ... */ } // RIGHT: f has type compatible with F int g() { /* ... */ } // RIGHT: g has type compatible with F F *e(void) { /* ... */ } // e returns a pointer to a function F *((e))(void) { /* ... */ } // same: parentheses irrelevant int (*fp)(void); // fp points to a function that has type F F *Fp; // Fp points to a function that has type F
See “future language directions” (6.11.7).
A parameter identifier cannot be redeclared in the function body except in an enclosed block.
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.
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.
Thus, preprocessing directives are commonly called “lines”. These “lines” have no other syntactic significance, as all
white space is equivalent except in certain situations during preprocessing (see the # character string literal creation operator
in 6.10.3.2, for example).
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).
Because the controlling constant expression is evaluated during translation phase 4, all identifiers either are or are not
macro names — there simply are no keywords, enumeration constants, etc.
Thus, on an implementation where INT_MAX is 0x7FFF and UINT_MAX is 0xFFFF, the constant 0x8000 is signed and
positive within a #if expression even though it would be unsigned in translation phase 7.
Thus, the constant expression in the following #if directive and if statement is not guaranteed to evaluate to the same
value in these two contexts.
#if ’z’ - ’a’ == 25
if (’z’ - ’a’ == 25)
As indicated by the syntax, a preprocessing token shall not follow a #else or #endif directive before the terminating
new-line character. However, comments may appear anywhere in a source file, including within a preprocessing directive.
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).
Note that adjacent string literals are not concatenated into a single string literal (see the translation phases in 5.1.1.2);
thus, an expansion that results in two string literals is an invalid directive.
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 ...).
Since, by macro-replacement time, all character constants and string literals are preprocessing tokens, not sequences
possibly containing identifier-like subsequences (see 5.1.1.2, translation phases), they are never scanned for macro names or
parameters.
Despite the name, a non-directive is a preprocessing directive.
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.
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.
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.
Placemarker preprocessing tokens do not appear in the syntax because they are temporary entities that exist only within
translation phase 4.
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.
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));
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)
Because a new-line is explicitly included as part of the #line directive, the number of new-line characters read while
processing to the first pp-token may be different depending on whether or not the implementation uses a one-pass preprocessor.
Therefore, there are two possible values for the line number following a directive of the form #line __LINE__ new-line.
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.
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).
An implementation is not required to perform macro replacement in pragmas, but it is permitted except for in standard
pragmas (where STDC immediately follows pragma). If the result of macro replacement in a non-standard pragma has the
same form as a standard pragma, the behavior is still implementation-defined; an implementation is permitted to behave as
if it were the standard pragma, but is not required to.
See “future language directions” (6.11.8).
Semantics
A preprocessing directive of the form
# new-line
has no effect.
The values of the predefined macros listed in the following subclauses179) (except for __FILE__ and
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).
See “future language directions” (6.11.9).
The following macro names shall be defined by the implementation:
Forward references: the asctime function (7.27.3.1).
The presumed source file name and line number can be changed by the #line directive.
This macro was not specified in ISO/IEC 9899:1990 and was specified as 199409L in ISO/IEC 9899:1990/Amd.1:1995, as
199901L in ISO/IEC 9899:1999 and as 201112L in ISO/IEC 9899:2011/Amd.1:2012. The intention is that this will remain an
integer constant of type long int that is increased with each revision of this International Standard.
The following macro names are conditionally defined by the implementation:
Forward references: common definitions (7.19), unicode utilities (7.28).
The following macro names are conditionally defined by the implementation:
An implementation that defines __STDC_NO_COMPLEX__ shall not define __STDC_IEC_559_COMPLEX__ .
The intention is that this will remain an integer constant of type long int that is increased with each revision of this
International Standard.
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)
Future standardization may include additional floating-point types, including those with greater
range, precision, or both than long double.
Declaring an identifier with internal linkage at file scope without the static storage-class specifier
is an obsolescent feature.
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.
Lowercase letters as escape sequences are reserved for future standardization. Other characters may
be used in extensions.
The placement of a storage-class specifier other than at the beginning of the declaration specifiers in
a declaration is an obsolescent feature.
The use of function declarators with empty parentheses (not prototype-format parameter type
declarators) is an obsolescent feature.
The use of function definitions with separate parameter identifier and declaration lists (not prototype-
format parameter type and identifier declarators) is an obsolescent feature.
Pragmas whose first preprocessing token is STDC are reserved for future standardization.
Macro names beginning with __STDC_ are reserved for future standardization.
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).
The functions that make use of the decimal-point character are the numeric conversion functions (7.22.1, 7.29.4.1) and the
formatted input/output functions (7.21.6, 7.29.2).
For state-dependent encodings, the values for MB_CUR_MAX and MB_LEN_MAX shall thus be large enough to count all the
bytes in any complete multibyte character plus at least one adjacent shift sequence of maximum length. Whether these counts
provide for more than one shift sequence is the implementation’s choice.
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).
A header is not necessarily a source file, nor are the < and > delimited sequences in header names necessarily valid source
file names.
The headers <complex.h>, <stdatomic.h>, and <threads.h> are conditional features that implementations need not
support; see 6.10.8.3.
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.
Allows identifiers spelled with a leading underscore followed by an uppercase letter that match the spelling of a keyword
to be used as macro names by the program.
The list of reserved identifiers with external linkage includes math_errhandling, setjmp, va_copy , and va_end .
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:
#include <stdlib.h> const char *str; /* ... */ i = atoi(str);
#include <stdlib.h> #undef atoi const char *str; /* ... */ i = atoi(str);
or
#include <stdlib.h> const char *str; /* ... */ i = (atoi)(str);
extern int atoi(const char *); const char *str; /* ... */ i = atoi(str);
This means that an implementation shall provide an actual function for each library function, even if it also provides a
macro for that function.
Such macros might not contain the sequence points that the corresponding function calls do.
Because external identifiers and some macro names beginning with an underscore are reserved, implementations may
provide special semantics for such names. For example, the identifier _BUILTIN_abs could be used to indicate generation of
in-line code for the abs function. Thus, the appropriate header could specify
#define abs(x) _BUILTIN_abs(x)
for a compiler whose code generator will accept it.
In this manner, a user desiring to guarantee that a given library function such as abs will be a genuine function may write
#undef abs
whether the implementation’s header provides a macro implementation of abs or a built-in implementation. The prototype
for the function, which precedes and is hidden by any macro definition, is thereby revealed also.
Thus, a signal handler cannot, in general, call standard library functions.
This means, for example, that an implementation is not permitted to use a static object for internal purposes without
synchronization because it could cause a data race even in programs that do not explicitly share objects between threads.
Similarly, an implementation of memcpy is not permitted to copy bytes beyond the specified length of the destination object
and then restore the original values because it could cause a data race if the program shared those bytes between threads.
This allows implementations to parallelize operations if there are no visible side effects.
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 .
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).
The message written might be of the form:
Assertion failed: expression, function abc, file xyz, line nnn.
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).
See “future library directions” (7.31.1).
The imaginary unit is a number i such that i2 = −1.
A specification for imaginary types is in informative annex G.
Values are interpreted as radians, not degrees. An implementation may set errno but is not required
to.
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.
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”.
The purpose of the pragma is to allow the implementation to use the formulas:
(x + iy) × (u + iv) = (xu − yv) + i(yu + xv) (x + iy) / (u + iv) = [(xu + yv) + i(yu − xv)]/(u2 + v 2 ) p |x + iy| = x2 + y 2
where the programmer can determine they are safe.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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).
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].
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).
For a variable z of complex type, z == creal(z)+cimag(z) I.
*
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)))
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.
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.
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.
For a variable z of complex type, z == creal(z)+cimag(z) I.
*
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).
See “future library directions” (7.31.2).
In an implementation that uses the seven-bit US ASCII character set, the printing characters are those whose values lie
from 0x20 (space) through 0x7E (tilde); the control characters are those whose values lie from 0 (NUL) through 0x1F (US),
and the character 0x7F (DEL).
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.
Synopsis
#include <ctype.h> int isalnum(int c);
Description
The isalnum function tests for any character for which isalpha or isdigit is true.
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.
The functions islower and isupper test true or false separately for each of these additional characters; all four combina-
tions are possible.
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.
Synopsis
#include <ctype.h> int iscntrl(int c);
Description
The iscntrl function tests for any control character.
Synopsis
#include <ctype.h> int isdigit(int c);
Description
The isdigit function tests for any decimal-digit character (as defined in 5.2.1).
Synopsis
#include <ctype.h> int isgraph(int c);
Description
The isgraph function tests for any printing character except space (’ ’ ).
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).
Synopsis
#include <ctype.h> int isprint(int c);
Description
The isprint function tests for any printing character including space (’ ’ ).
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.
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.
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).
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).
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.
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.
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.
The macro errno need not be the identifier of an object. It might expand to a modifiable lvalue resulting from a function
call (for example, *errno() ).
Thus, a program that uses errno for error checking should set it to zero before a library function call, then inspect it
before a subsequent library function call. Of course, a library function can save the value of errno on entry and then set it to
zero, as long as the original value is restored if errno’s value is still zero just before the return.
See “future library directions” (7.31.3).
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.
This header is designed to support the floating-point exception status flags and directed-rounding control modes required
by IEC 60559, and other similar floating-point state information. It is also designed to facilitate code portability among all
systems.
A floating-point status flag is not an object and can be set more than once within an expression.
With these conventions, a programmer can safely assume default floating-point control modes (or be unaware of them).
The responsibilities associated with accessing the floating-point environment fall on the programmer or program that does so
explicitly.
The implementation supports a floating-point exception if there are circumstances where a call to at least one of the
functions in 7.6.2, using the macro as the appropriate argument, will succeed. It is not necessary for all the functions to
succeed all the time.
See “future library directions” (7.31.4).
The macros should be distinct powers of two.
See “future library directions” (7.31.4).
Even though the rounding direction macros may expand to constants corresponding to the values of FLT_ROUNDS, they
are not required to do so.
See “future library directions” (7.31.4).
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)
The purpose of the FENV_ACCESS pragma is to allow certain optimizations that could subvert flag tests and mode changes
(e.g., global common subexpression elimination, code motion, and constant folding). In general, if the state of FENV_ACCESS
is “off”, the translator can assume that default modes are in effect and the flags are not tested.
The side effects impose a temporal ordering that requires two evaluations of x + 1 . On the other hand, without the
#pragma STDC FENV_ACCESS ON pragma, and assuming the default state is “off”, just one evaluation of x + 1 would suffice.
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.
The functions fetestexcept, feraiseexcept, and feclearexcept support the basic abstraction of flags that are either
set or clear. An implementation may endow floating-point status flags with more information — for example, the address of
the code which first raised the floating-point exception; the functions fegetexceptflag and fesetexceptflag deal with
the full content of flags.
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.
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.
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.
The effect is intended to be similar to that of floating-point exceptions raised by arithmetic operations. Hence, enabled
traps for floating-point exceptions raised by this function are taken. The specification in F.8.6 is in the same spirit.
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.
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(); /* ... */ }
This mechanism allows testing several floating-point exceptions with just one function call.
The fegetround and fesetround functions provide control of rounding direction modes.
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.
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); /* ... */ }
The functions in this section manage the floating-point environment — status flags and control
modes — as one entity.
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.
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.
IEC 60559 systems have a default non-stop mode, and typically at least one other mode for trap handling or aborting; if
the system provides only the non-stop mode then installing it is trivial. For such systems, the feholdexcept function can be
used in conjunction with the feupdateenv function to write routines that hide spurious floating-point exceptions from their
callers.
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.
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; }
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.
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).
See “future library directions” (7.31.5).
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; }
Separate macros are given for use with fprintf and fscanf functions because, in the general case, different format
specifiers may be required for fprintf and fscanf, even when the type is the same.
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.
The absolute value of the most negative number cannot be represented in two’s complement.
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.
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).
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).
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 ^=
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.
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.
ISO/IEC 9945–2 specifies locale and charmap formats that may be used to specify locales for C.
See “future library directions” (7.31.6).
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).
The only functions in 7.4 whose behavior is not affected by the current locale are isdigit and isxdigit.
The implementation shall arrange to encode in a string the various categories due to a heterogeneous locale when
category has the value LC_ALL.
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:
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:
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:
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
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>.
Particularly on systems with wide expression evaluation, a <math.h> function might pass arguments and return values
in wider format than the synopsis prototype indicates.
The types float_t and double_t are intended to be the implementation’s most efficient types at least as wide as
float and double, respectively. For FLT_EVAL_METHOD equal 0, 1, or 2, the type float_t is the narrowest type used by the
implementation to evaluate floating expressions.
HUGE_VAL, HUGE_VALF, and HUGE_VALL can be positive infinities in an implementation that supports infinities.
In this case, using INFINITY will violate the constraint in 6.4.4 and thus require a diagnostic.
Typically, the FP_FAST_FMA macro is defined if and only if the fma function is implemented directly with a hardware
multiply-add instruction. Software implementations are expected to be substantially slower.
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.
In an implementation that supports infinities, this allows an infinity as an argument to be a domain error if the
mathematical domain of the function does not include the infinity.
The term underflow here is intended to encompass both “gradual underflow” as in IEC 60559 and also “flush-to-zero”
underflow.
Math errors are being indicated by the floating-point exception flags rather than by errno.
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.
In the synopses in this subclause, real-floating indicates that the argument shall be an expression of
real floating type.
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.
Since an expression can be evaluated with more range and precision than its type has, it is important to know the type
that classification is based on. For example, a normal long double value might become subnormal when converted to
double, and zero when converted to float.
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.
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.
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.
For the isnan macro, the type for determination does not matter unless the implementation supports NaNs in the
evaluation type but not in the semantic type.
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.
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.
The signbit macro reports the sign of all values, including infinities, zeros, and NaNs. If zero is unsigned, it is treated
as positive.
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.
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.
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.
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.
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.
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.
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.
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, +∞].
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.
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.
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.
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.
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.
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 .
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 .
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.
For small magnitude x , expm1(x) is expected to be more accurate than exp(x)-1.
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.
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).
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 .
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.
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.
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).
For small magnitude x , log1p(x) is expected to be more accurate than log(1 + x).
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.
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.
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.
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 .
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 .
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|.
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 .
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 .
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.
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 −t2
The erf functions return erf x = √2π e dt.
0
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∞ −t2
The erfc functions return erfc x = 1 − erf x = √2π e dt.
x
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)|.
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).
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
“When y 6= 0, the remainder r = x REM y is defined regardless of the rounding mode by the mathematical relation
r = x − ny, where n is the integer nearest the exact value of x/y; whenever |n − x/y| = 1/2, then n is even. If r = 0, its sign
shall be that of x.” This definition is applicable for all implementations.
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.
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.
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).
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.
The argument values are converted to the type of the function, even by a macro implementation of the function.
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)
The result of the nexttoward functions is determined in the type of the function, without loss of range or precision in a
floating second argument.
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.
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.
NaN arguments are treated as missing data: if one argument is a NaN and the other numeric, then the fmax functions
choose the numeric value. See F.10.9.2.
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.
The fmin functions are analogous to the fmax functions in their treatment of NaNs.
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.
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)
IEC 60559 requires that the built-in relational operators raise the “invalid” floating-point exception if the operands
compare unordered, as an error indicator for programs written without consideration of NaNs; the result in these cases is
false.
If any argument is of integer type, or any other type that is not a real floating type, the behavior is undefined.
Whether an argument represented in a format wider than its semantic type is converted to the semantic type is unspecified.
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) .
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) .
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).
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) .
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) .
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.
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.
These functions are useful for dealing with unusual conditions encountered in a low-level function of a program.
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.
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 }
For example, by executing a return statement or because another longjmp call has caused a transfer to a setjmp
invocation in a function earlier in the set of nested calls.
This includes, but is not limited to, the floating-point status flags and the state of open files.
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
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.
See “future library directions” (7.31.7). The names of the signal numbers reflect the following terms (respectively): abort,
floating-point exception, illegal instruction, interrupt, segmentation violation, and termination.
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).
This includes functions called indirectly via standard library functions (e.g., a SIGABRT handler called via the abort
function).
If any signal is generated by an asynchronous signal handler, the behavior is undefined.
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.
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.
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)
It is permitted to create a pointer to a va_list and pass that pointer to another function, in which case the original
function may make further use of the original list after the other function returns.
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.
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.
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.
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.
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); }
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.
See “future library directions” (7.31.8).
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);
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);
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.
See “future library directions” (7.31.8).
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.
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.
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.
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.
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.
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)
obj may be a null pointer.
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.
See “future library directions” (7.31.8).
The same representation and alignment requirements are meant to imply interchangeability as arguments to functions,
return values from functions, and members of unions.
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.
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.
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.
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.
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.
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:
or | bitwise inclusive or
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.
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;
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.
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.
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)
See “future library directions” (7.31.9).
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.
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”).
See “future library directions” (7.31.10).
Some of these types may denote implementation-defined extended 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).
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.
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.
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.
The designated type is not guaranteed to be fastest for all purposes; if the implementation has no clear grounds for
choosing one type over another, it will simply pick some integer type satisfying the signedness and width requirements.
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.
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.
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.
— minimum values of exact-width signed integer types
INTN_MIN exactly −(2N −1 )
INTN_MAX exactly 2N −1 − 1
UINTN_MAX exactly 2N − 1
— minimum values of minimum-width signed integer types
INT_LEASTN_MIN −(2N −1 − 1)
INT_LEASTN_MAX 2N −1 − 1
UINT_LEASTN_MAX 2N − 1
— minimum values of fastest minimum-width signed integer types
INT_FASTN_MIN −(2N −1 − 1)
INT_FASTN_MAX 2N −1 − 1
UINT_FASTN_MAX 2N − 1
— minimum value of pointer-holding signed integer type
INTPTR_MIN −(215 − 1)
INTPTR_MAX 215 − 1
UINTPTR_MAX 216 − 1
— minimum value of greatest-width signed integer type
INTMAX_MIN −(263 − 1)
INTMAX_MAX 263 − 1
UINTMAX_MAX 264 − 1
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)
PTRDIFF_MIN -65535 PTRDIFF_MAX +65535
SIG_ATOMIC_MIN see below SIG_ATOMIC_MAX see below
SIZE_MAX 65535
WCHAR_MIN see below WCHAR_MAX see below
WINT_MIN see below WINT_MAX see below
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.
A freestanding implementation need not provide all of these types.
The values WCHAR_MIN and WCHAR_MAX do not necessarily correspond to members of the extended character set.
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.
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.
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)
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).
If the implementation imposes no practical limit on the length of file name strings, the value of FILENAME_MAX should
instead be the recommended size of an array intended to hold a file name string. Of course, file name string contents are
subject to other system-specific constraints; therefore all possible strings of length FILENAME_MAX cannot be expected to be
opened successfully.
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).
An implementation need not distinguish between text streams and binary streams. In such an implementation, there
need be no new-line characters in a text stream nor any limit to the length of a line.
The three predefined streams stdin, stdout, and stderr are unoriented at program startup.
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).
Setting the file position indicator to end-of-file, as with fseek(file, 0, SEEK_END), has undefined behavior for a
binary stream (because of possible trailing null characters) or for any stream with state-dependent encoding that does not
assuredly end in the initial shift state.
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.
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.
Among the reasons the implementation may cause the rename function to fail are that the file is open or that it is necessary
to copy its contents to effectuate its renaming.
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).
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.
Files created using strings generated by the tmpnam function are temporary only in the sense that their names should not
collide with those generated by conventional naming rules for the implementation. It is still necessary to use the remove
function to remove such files when their use is ended, and before program termination.
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.
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).
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).
If the string begins with one of the above sequences, the implementation might choose to ignore the remaining characters,
or it might use them to select different kinds of a file (some of which might not conform to the properties in 7.21.2).
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.
The primary use of the freopen function is to change the file associated with a standard text stream (stderr, stdin,
or stdout), as those identifiers need not be modifiable lvalues to which the value returned by the fopen function may be
assigned.
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).
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.
The buffer has to have a lifetime at least as great as the open stream, so the stream should be closed before a buffer that
has automatic storage duration is deallocated upon block exit.
The formatted input/output functions shall behave as if there is a sequence point after the actions
associated with each specifier.278)
The fprintf functions perform writes to memory for the %n specifier.
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:
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 length modifiers and their meanings are:
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.
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:
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.
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).
Note that 0 is taken as a flag, not as the beginning of a field width.
The results of all floating conversions of a negative zero, and of negative values that round to zero, include a minus sign.
When applied to infinite and NaN values, the -, +, and space flag characters have their usual meaning; the # and 0 flag
characters have no effect.
Binary implementations can choose the hexadecimal digit to the left of the decimal-point character so that subsequent
digits align to nibble (4-bit) boundaries.
The precision p is sufficient to distinguish values of the source type if 16p−1 > bn where b is FLT_RADIX and n is the
number of base-b digits in the significand of the source type. A smaller p might suffice depending on the implementation’s
scheme for determining the digit to the left of the decimal-point character.
No special provisions are made for multibyte characters.
Redundant shift sequences may result if multibyte characters have a state-dependent encoding.
See “future library directions” (7.31.11).
For binary-to-decimal conversion, the result format’s values are the numbers representable with the given format specifier.
The number of significant digits is determined by the format specifier, and in the case of fixed-point conversion by the source
value as well.
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.
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).
These white-space characters are not counted against a specified field width.
fscanf pushes back at most one input character onto the input stream. Therefore, some sequences that are acceptable to
strtod, strtol, etc., are unacceptable to fscanf.
No special provisions are made for multibyte characters in the matching rules used by the c, s, and [ conversion specifiers
— the extent of the input field is determined on a byte-by-byte basis. The resulting field is nevertheless a sequence of multibyte
characters that begins in the initial shift state.
See “future library directions” (7.31.11).
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.
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.
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.
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.
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.
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); }
As the functions vfprintf , vfscanf , vprintf , vscanf , vsnprintf , vsprintf , and vsscanf invoke the va_arg macro,
the value of arg after the return is indeterminate.
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.
As the functions vfprintf , vfscanf , vprintf , vscanf , vsnprintf , vsprintf , and vsscanf invoke the va_arg macro,
the value of arg after the return is indeterminate.
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.
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.
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.
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.
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.
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)
An end-of-file and a read error can be distinguished by use of the feof and ferror functions.
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.
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.
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.
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.
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.
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.
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.
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.
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).
Note that a file positioning function may further modify the file position indicator after discarding any pushed-back
characters.
See “future library directions” (7.31.11).
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.
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.
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).
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).
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.
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.
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.
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.
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.
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.
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).
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.
See “future library directions” (7.31.12).
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.
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).
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).
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:
n-char-sequence: digit nondigit n-char-sequence digit n-char-sequence nondigit
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