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C23 (C standard revision)
C programming language standard, current revision From Wikipedia, the free encyclopedia
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C23, formally ISO/IEC 9899:2024, is the current open standard for the C programming language, which supersedes C17 (standard ISO/IEC 9899:2018).[1] It was started in 2016 informally as C2x,[2] and was published on October 31, 2024.[3] The freely available draft most similar to the one published is document N3220[4] (see Available texts, below). The first WG14 meeting for the C2x draft was held in October 2019,[5] virtual remote meetings were held in 2020 due to the COVID-19 pandemic, then various teleconference meetings continued to occur through 2024.
In C23, the value of __STDC_VERSION__
changes from 201710L
to 202311L
. The common names "C17" and "C23" reflect these values, which are frozen prior to final adoption, rather than the years in the ISO standards identifiers (9899:2018 and 9899:2024).
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Features
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Changes integrated into the latest working draft of C23 are listed below.[6]
Standard Library
New functions
- Add
memset_explicit()
function in<string.h>
to erase sensitive data, where memory store must always be performed regardless of optimizations.[7] - Add
memccpy()
function in<string.h>
to efficiently concatenate strings – similar to POSIX and SVID C extensions.[8] - Add
strdup()
andstrndup()
functions in<string.h>
to allocate a copy of a string – similar to POSIX and SVID C extensions.[9] - Add
memalignment()
function in<stdlib.h>
to determine the byte alignment of a pointer.[10] - Add bit utility functions / macros / types in new header
<stdbit.h>
to examine many integer types. All start withstdc_
to minimize conflict with legacy code and 3rd party libraries.[11]- In the following, replace
*
withuc
,us
,ui
,ul
,ull
for five function names, or blank for a type-generic macro.[11] - Add
stdc_count_ones*()
andstdc_count_zeros*()
to count number of 1 or 0 bits in value.[11] - Add
stdc_leading_ones*()
andstdc_leading_zeros*()
to count leading 1 or 0 bits in value.[11] - Add
stdc_trailing_ones*()
andstdc_trailing_zeros*()
to count trailing 1 or 0 bits in value.[11] - Add
stdc_first_leading_one*()
andstdc_first_leading_zero*()
to find first leading bit with 1 or 0 in value.[11] - Add
stdc_first_trailing_one*()
andstdc_first_trailing_zero*()
to find first trailing bit with 1 or 0 in value.[11] - Add
stdc_has_single_bit*()
to determine if value is an exact power of 2 (return true if and only if there is a single 1 bit).[11] - Add
stdc_bit_floor*()
to determine the largest integral power of 2 that is not greater than value.[11] - Add
stdc_bit_ceil*()
to determine the smallest integral power of 2 that is not less than value.[11] - Add
stdc_bit_width*()
to determine number of bits to represent a value.[11]
- In the following, replace
- Add
timegm()
function in<time.h>
to convert time structure into calendar time value - similar to function in glibc and musl libraries.[12] - New
<math.h>
functions based on IEEE 754-2019 recommendations, such as trigonometry functions operating on units of andexp10
.[13]
Existing functions
- Add
%b
binary conversion specifier toprintf()
function family.[14] - Add
%b
binary conversion specifier toscanf()
function family.[14] - Add
0b
and0B
binary conversion support tostrtol()
andwcstol()
function families.[14] - Make the functions
bsearch()
,bsearch_s()
,memchr()
,strchr()
,strpbrk()
,strrchr()
,strstr()
, and their wide counterpartswmemchr()
,wcschr()
,wcspbrk()
,wcsrchr()
,wcsstr()
return a const qualified object if one was passed to them.[15]
Preprocessor
- Add
#elifdef
and#elifndef
directives,[16] which are essentially equivalent to#elif defined
and#elif !defined
. Both directives were added to C++23 standard and GCC 12.[17] - Add
#embed
directive for binary resource inclusion and__has_embed
allowing the availability of a resource to be checked by preprocessor directives.[18] - Add
#warning
directive for diagnostics.[19] - Add
__has_include
allowing the availability of a header to be checked by preprocessor directives.[20] - Add
__has_c_attribute
allowing the availability of an attribute to be checked by preprocessor directives.[21] (see "C++ compatibility" group for new attribute feature) - Add
__VA_OPT__
functional macro for variadic macros which expands to its argument only if a variadic argument has been passed to the containing macro.[22]
Types
- Add
nullptr_t
, a null pointer type.[23] - Add
_BitInt(N)
andunsigned _BitInt(N)
types for bit-precise integers. AddBITINT_MAXWIDTH
macro for maximum bit width.[24][25] - Add
ckd_add()
,ckd_sub()
,ckd_mul()
macros for checked integer operations.[26] - Variably-modified types (but not VLAs which are automatic variables allocated on the stack) become a mandatory feature.[27]
- Better support for using
const
with arrays.[28] - Standardization of the
typeof(...)
operator.[29] - The meaning of the
auto
keyword was changed to cause type inference while also retaining its old meaning of a storage class specifier if used alongside a type. Unlike C++, C23 allows type inference only for object definitions (no inferring function return type or function parameter type).[30] - Compatibility rules for structure, union, and enumerated types were changed to allow a redeclaration of a compatible type with the same tag.[31]
- Exact-width integer may now exceed
intmax_t
(N2888).[32]
Constants
- Add
nullptr
constant fornullptr_t
type.[23] - Add
wb
anduwb
integer literal suffixes for_BitInt(N)
andunsigned _BitInt(N)
types,[33] such as6uwb
yields anunsigned _BitInt(3)
, and-6wb
yields asigned _BitInt(4)
which has three value bits and one sign bit. - Add
0b
and0B
binary literal constant prefixes,[34] such as0b10101010
(equating to 0xAA). - Add
'
digit separator to literal constants,[35] such as0xFE'DC'BA'98
(equating to 0xFEDCBA98),299'792'458
(equating to 299792458),1.414'213'562
(equating to 1.414213562). - Add the ability to specify the underlying type of an
enum
.[36] - Allow
enum
s with no fixed underlying type to store values that are not representable byint
.[37]
Keywords
- Add
true
andfalse
keywords.[38] - Add
alignas
,alignof
,bool
,static_assert
,thread_local
keywords. Previously defined keywords become alternative spellings:_Alignas
,_Alignof
,_Bool
,_Static_assert
,_Thread_local
.[39] - Add
_BitInt
keyword (see "types" group) - Add
typeof
andtypeof_unqual
keywords (see "types" group) - Add
nullptr
keyword (see "constants" group) - Add
constexpr
keyword (see "other" group) - Add
_Decimal32
,_Decimal64
,_Decimal128
keywords for (optional) decimal floating-point arithmetic (see "other" group)
Syntax
- Labels can appear before declarations and at the end of compound statements.[40]
- Unnamed parameters in function definitions.[41]
- Zero initialization with
{}
(including initialization of VLAs).[42] - Variadic functions no longer need a named argument before the ellipsis and the
va_start
macro no longer needs a second argument nor does it evaluate any argument after the first one if present.[43] - Add C++11 style attribute syntax[44] using double square brackets
[[]]
. In addition to C++11 attributes listed below, add new attributes: - Add single-argument
_Static_assert
for compatibility with C++17.[46] - Functions with no arguments listed in the prototype (e.g.
void foo()
) are understood as taking no arguments (see removal of K&R function declarations)
C++ compatibility
- Various syntax changes improve compatibility with C++, such as labels before declarations, unnamed function arguments, zero initialization with
{}
, variadic functions without named argument, C++11 style attributes,_Static_assert
(see Syntax). For labels at the end of compound statements a corresponding change was made to C++23.[47] - Add C++-style attributes (see Syntax). Add attributes[48]
[[deprecated]]
,[49][[fallthrough]]
,[50][[maybe_unused]]
,[51][[nodiscard]]
,[52] and[[noreturn]]
attribute for compatibility with C++11, then deprecate_Noreturn
,noreturn
, header<stdnoreturn.h>
features introduced in C11.[53] Duplicate attributes are allowed for compatibility with C++23.[54] All standard attributes can also be surrounded by double underscores (e.g.[[__deprecated__]]
is equivalent to[[deprecated]]
). - Add
u8
prefix for character literals to represent UTF-8 encoding for compatibility with C++17.[55][56] - Add
#elifdef
and#elifndef
preprocessing directives for compatibility with C++23.[16] (see "preprocessor" group)
Other features
- Support for the ISO/IEC 60559:2020, the current version of the IEEE 754 standard for floating-point arithmetic, with extended binary floating-point arithmetic and (optional) decimal floating-point arithmetic.[57][58]
- The
constexpr
specifier for objects but not functions, unlike C++'s equivalent.[59] - Add
char8_t
type for storing UTF-8 encoded data and change the type of u8 character constants and string literals tochar8_t
. Also, the functionsmbrtoc8()
andc8rtomb()
to convert a narrow multibyte character to UTF-8 encoding and a single code point from UTF-8 to a narrow multibyte character representation respectively.[60] - Clarify that all
char16_t
strings and literals shall be UTF-16 encoded, and allchar32_t
strings and literals shall be UTF-32 encoded, unless otherwise explicitly specified.[61] - Allow storage class specifiers to appear in compound literal definition.[62]
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Obsolete features
Some old obsolete features are either removed or deprecated from the working draft of C23:
- Remove trigraphs.[63]
- Remove K&R function definitions/declarations (with no information about the function arguments).[64][65]
- Remove representations for signed integers other than two's complement. Two's complement signed integer representation will be required.[66]
- The
*_HAS_SUBNORM
macros in<float.h>
are obsolescent features.[67]
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Compiler support
The following compilers implement an experimental compiler flag to support this standard:
Additionally, versions of GCC beyond GCC 15 now use C23 by default.[71]
Available texts
Like other editions of the C standard, the official ISO text of the standard is not freely available.
The latest working draft pre-C23 that was made public was N3096,[6] dated 2023-04-01. In the months that followed this draft, hundreds of changes[72] were made before producing the working draft N3149 dated 2023-07-09 and the official draft standard N3219 dated 2024-02-22.[72][73] Neither of these later drafts are public.[72][73]
On the same date that the draft standard N3219 was announced, a new working draft N3220[4] was made public. While this document is officially described[73] as a draft of the future version "C2Y" of the standard, the accompanying "Editor's Report" specifies that N3220 differs from the draft C23 standard N3219 only by a fix to one footnote in Annex K.[73]
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See also
References
Further reading
External links
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