C23 is a draft open standard for the C programming language, which is expected to become ISO/IEC 9899:2024, replacing C17 (standard ISO/IEC 9899:2018).[1] It was started in 2016 informally as C2x,[2] and is expected to be published in 2024.[3] The most recent publicly available working draft of C23 was released on April 1, 2023.[4] 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.
Features
editChanges integrated into the latest working draft of C23 are listed below.[4]
Standard Library
editNew functions
edit- Add
memset_explicit()
function in<string.h>
to erase sensitive data, where memory store must always be performed regardless of optimizations.[6] - Add
memccpy()
function in<string.h>
to efficiently concatenate strings – similar to POSIX and SVID C extensions.[7] - Add
strdup()
andstrndup()
functions in<string.h>
to allocate a copy of a string – similar to POSIX and SVID C extensions.[8] - Add
memalignment()
function in<stdlib.h>
to determine the byte alignment of a pointer.[9] - 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.[10]- In the following, replace
*
withuc
,us
,ui
,ul
,ull
for five function names, or blank for a type-generic macro.[10] - Add
stdc_count_ones*()
andstdc_count_zeros*()
to count number of 1 or 0 bits in value.[10] - Add
stdc_leading_ones*()
andstdc_leading_zeros*()
to count leading 1 or 0 bits in value.[10] - Add
stdc_trailing_ones*()
andstdc_trailing_zeros*()
to count trailing 1 or 0 bits in value.[10] - Add
stdc_first_leading_one*()
andstdc_first_leading_zero*()
to find first leading bit with 1 or 0 in value.[10] - Add
stdc_first_trailing_one*()
andstdc_first_trailing_zero*()
to find first trailing bit with 1 or 0 in value.[10] - 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).[10] - Add
stdc_bit_floor*()
to determine the largest integral power of 2 that is not greater than value.[10] - Add
stdc_bit_ceil*()
to determine the smallest integral power of 2 that is not less than value.[10] - Add
stdc_bit_width*()
to determine number of bits to represent a value.[10]
- 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.[11] - New
<math.h>
functions based on IEEE 754-2019 recommendations, such as trigonometry functions operating on units of andexp10
.[12]
Existing functions
edit- Add
%b
binary conversion specifier toprintf()
function family.[13] - Add
%b
binary conversion specifier toscanf()
function family.[13] - Add
0b
and0B
binary conversion support tostrtol()
andwcstol()
function families.[13] - 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.[14]
Preprocessor
edit- Add
#elifdef
and#elifndef
directives,[15] which are essentially equivalent to#elif defined
and#elif !defined
. Both directives were added to C++23 standard and GCC 12.[16] - Add
#embed
directive for binary resource inclusion and__has_embed
allowing the availability of a resource to be checked by preprocessor directives.[17] - Add
#warning
directive for diagnostics.[18] - Add
__has_include
allowing the availability of a header to be checked by preprocessor directives.[19] - Add
__has_c_attribute
allowing the availability of an attribute to be checked by preprocessor directives.[20] (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.[21]
Types
edit- Add
nullptr_t
, a null pointer type.[22] - Add
_BitInt(N)
andunsigned _BitInt(N)
types for bit-precise integers. AddBITINT_MAXWIDTH
macro for maximum bit width.[23][24] - Add
ckd_add()
,ckd_sub()
,ckd_mul()
macros for checked integer operations.[25] - Variably-modified types (but not VLAs which are automatic variables allocated on the stack) become a mandatory feature.[26]
- Better support for using
const
with arrays.[27] - Standardization of the
typeof(...)
operator.[28] - 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).[29] - Compatibility rules for structure, union, and enumerated types were changed to allow a redeclaration of a compatible type with the same tag.[30]
- Exact-width integer may now exceed
intmax_t
(N2888).[31]
Constants
edit- Add
nullptr
constant fornullptr_t
type.[22] - Add
wb
anduwb
integer literal suffixes for_BitInt(N)
andunsigned _BitInt(N)
types,[32] 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,[33] such as0b10101010
(equating to 0xAA). - Add
'
digit separator to literal constants,[34] 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
.[35] - Allow
enum
s with no fixed underlying type to store values that are not representable byint
.[36]
Keywords
edit- Add
true
andfalse
keywords.[37] - Add
alignas
,alignof
,bool
,static_assert
,thread_local
keywords. Previously defined keywords become alternative spellings:_Alignas
,_Alignof
,_Bool
,_Static_assert
,_Thread_local
.[38] - 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
edit- Labels can appear before declarations and at the end of compound statements.[39]
- Unnamed parameters in function definitions.[40]
- Zero initialization with
{}
(including initialization of VLAs).[41] - 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.[42] - Add C++11 style attribute syntax[43] 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.[45] - Functions with no arguments listed in the prototype
void foo()
are understood as taking no arguments (see removal of K&R function declarations)
C++ compatibility
edit- 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.[46] - Add C++-style attributes (see Syntax). Add attributes[47]
[[deprecated]]
,[48][[fallthrough]]
,[49][[maybe_unused]]
,[50][[nodiscard]]
,[51] and[[noreturn]]
attribute for compatibility with C++11, then deprecate_Noreturn
,noreturn
, header<stdnoreturn.h>
features introduced in C11.[52] Duplicate attributes are allowed for compatibility with C++23.[53] 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.[54][55] - Add
#elifdef
and#elifndef
preprocessing directives for compatibility with C++23.[15] (see "preprocessor" group)
Other features
edit- 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.[56][57]
- The
constexpr
specifier for objects but not functions, unlike C++'s equivalent.[58] - 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.[59] - 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.[60] - Allow storage class specifiers to appear in compound literal definition.[61]
Obsolete features
editSome old obsolete features are either removed or deprecated from the working draft of C23:
- Remove trigraphs.[62]
- Remove K&R function definitions/declarations (with no information about the function arguments).[63][64]
- Remove representations for signed integers other than two's complement. Two's complement signed integer representation will be required.[65]
- The
*_HAS_SUBNORM
macros in<float.h>
are obsolescent features.[66]
Compiler support
editThe following compilers implement an experimental compiler flag to support this standard:
See also
editReferences
edit- ^ "History of C". cppreference.com. 2022-06-27. Archived from the original on October 19, 2022.
- ^ "WG14-N2086: C2x Charter". open-std.org. 2016-09-20. Archived from the original on December 22, 2022.
- ^ "WG14-N3156: Updated C23 Schedule" (PDF). open-std.org. 2023-07-19. Archived (PDF) from the original on January 29, 2024.
- ^ a b "WG14-N3096: Draft for ISO/IEC 9899:2024" (PDF). open-std.org. April 1, 2023. Archived (PDF) from the original on April 2, 2023.
- ^ "WG14-N2437: Agenda for October 2019". open-std.org. 2019-10-21. Archived from the original on March 5, 2021.
- ^ "WG14-N2897: memset_explicit()". open-std.org. 2021-12-27. Archived from the original on October 25, 2022.
- ^ "WG14-N2349: Toward more efficient string copying and concatenation". open-std.org. 2019-03-18. Archived from the original on September 30, 2022.
- ^ "WG14-N2353: strdup() and strndup()". open-std.org. 2019-03-18. Archived from the original on December 24, 2022.
- ^ "WG14-N2974: Queryable pointer alignment" (PDF). open-std.org. 2022-04-15. Archived (PDF) from the original on October 13, 2022.
- ^ a b c d e f g h i j k "WG14-N3022: Modern Bit Utilities". open-std.org. 2022-07-06. Archived from the original on December 24, 2022.
- ^ "WG14-N2833: Add timegm() as non-optional part of time.h". open-std.org. 2021-10-07. Archived from the original on December 1, 2021.
- ^ See N3096 § B.11 for a useful overview. The functions were added in separate documents: N2488, its updated versions, and its refs.
- ^ a b c "WG14-N2630: formatted input/output of binary integer numbers" (PDF). open-std.org. 2021-01-01. Archived (PDF) from the original on December 14, 2022.
- ^ "WG14-N3020: Qualifier-preserving standard library functions" (PDF). open-std.org. 2022-06-13. Archived (PDF) from the original on October 13, 2022.
- ^ a b "WG14-N2645: Add support for preprocessing directives #elifdef and #elifndef" (PDF). open-std.org. 2020-01-25. Archived (PDF) from the original on November 28, 2022.
- ^ "GCC 12 Adds Support For New #elifdef #elifndef Directives". phoronix. May 12, 2021. Archived from the original on December 27, 2022.
- ^ "WG14-N3017: #embed - a scannable, tooling-friendly binary resource inclusion mechanism". open-std.org. 2022-06-27. Archived from the original on December 24, 2022.
- ^ "WG14-N2686: #warning" (PDF). open-std.org. 2022-07-22. Archived (PDF) from the original on November 28, 2022.
- ^ "WG14-N2799: __has_include for C" (PDF). open-std.org. 2021-08-30. Archived (PDF) from the original on December 24, 2022.
- ^ "WG14-N2553: Querying attribute support" (PDF). open-std.org. 2020-08-04. Archived (PDF) from the original on October 14, 2022.
- ^ "WG14-N3033: Comma omission and comma deletion". open-std.org. 2022-07-20. Archived from the original on December 27, 2022.
- ^ a b "WR14-N3042: Introduce the nullptr constant". open-std.org. 2022-07-22. Archived from the original on December 24, 2022.
- ^ "WG14-N2763: Adding a Fundamental Type for N-bit integers" (PDF). open-std.org. 2021-06-21. Archived (PDF) from the original on December 27, 2022.
- ^ "WG14-N3035: _BitInt Fixes" (PDF). open-std.org. 2022-07-21. Archived (PDF) from the original on October 13, 2022.
- ^ "WG14-N2867: Checked N-Bit Integers" (PDF). open-std.org. 2021-11-28. Archived (PDF) from the original on December 14, 2022.
- ^ "WG14-N2778: Variably-Modified Types" (PDF). open-std.org. 2021-07-11. Archived (PDF) from the original on December 22, 2022.
- ^ "WG14-N2607: Compatibility of Pointers to Arrays with Qualifiers" (PDF). open-std.org. 2020-10-31. Archived (PDF) from the original on October 13, 2022.
- ^ "WG14-N2899: Not-so-magic - typeof for C". open-std.org. 2022-01-21. Archived from the original on December 24, 2022.
- ^ "WG14-N3007: Type inference for object definitions". open-std.org. 2022-06-10. Archived from the original on December 24, 2022.
- ^ "WG14-N3037: Improved Rules for Tag Compatibility (updates N3032)" (PDF).
- ^ "C23 is Finished: Here is What is on the Menu". The Pasture. 31 July 2022.
- ^ "WG14-N2775: Literal suffixes for bit-precise integers" (PDF). open-std.org. 2021-07-13. Archived (PDF) from the original on December 27, 2022.
- ^ "WG14-N2549: Allow for binary integer constants" (PDF). open-std.org. 2020-07-30. Archived (PDF) from the original on December 22, 2022.
- ^ "WG14-N2626: Digit separators" (PDF). open-std.org. 2020-12-15. Archived (PDF) from the original on December 19, 2022.
- ^ "WG14-N3030: Enhancements to Enumerations". open-std.org. 2022-07-19. Archived from the original on November 26, 2022.
- ^ "WG14-N3029: Improved Normal Enumerations". open-std.org. 2022-07-19. Archived from the original on January 29, 2023.
- ^ "WG14-N2935: Make false and true first-class language features" (PDF). open-std.org. 2022-02-15. Archived (PDF) from the original on November 21, 2022.
- ^ "WG14-N2934: Revise spelling of keywords" (PDF). open-std.org. 2022-02-15. Archived (PDF) from the original on December 24, 2022.
- ^ "WG14-N2508: Free Positioning of Labels Inside Compound Statements" (PDF). open-std.org. 2020-03-28. Archived (PDF) from the original on December 27, 2022.
- ^ "WG14-N2510: Allowing unnamed parameters in a function definition" (PDF). open-std.org. 2020-04-09. Archived (PDF) from the original on December 24, 2022.
- ^ "WG14-N2900: Consistent, Warningless, and Intuitive Initialization with {}". open-std.org. 2022-01-01. Archived from the original on December 27, 2022.
- ^ "WG14-N2975: Relax requirements for variadic parameter lists" (PDF). open-std.org. 2022-04-15. Archived (PDF) from the original on November 28, 2022.
- ^ "WG14-N2335: Attributes in C" (PDF). open-std.org. 2019-03-09. Archived (PDF) from the original on October 26, 2022.
- ^ a b "Unsequenced functions". open-std.org. Retrieved 2024-07-18.
- ^ "WG14-N2265: Harmonizing static_assert with C++" (PDF). open-std.org. 2018-07-06. Archived (PDF) from the original on March 28, 2023.
- ^ "Labels at the end of compound statements (C compatibility)" (PDF). 2022-01-13.
- ^ "WG14-N2554: Minor attribute wording cleanups" (PDF). open-std.org. 2020-08-04. Archived (PDF) from the original on November 28, 2022.
- ^ "WG14-N2334: The deprecated attribute" (PDF). open-std.org. 2019-01-22. Archived (PDF) from the original on October 19, 2022.
- ^ "WG14-N2408: The fallthrough attribute" (PDF). open-std.org. 2019-08-11. Archived (PDF) from the original on December 25, 2022.
- ^ "WG14-N2270: The maybe_unused attribute" (PDF). open-std.org. 2018-07-06. Archived (PDF) from the original on December 25, 2022.
- ^ "WG14-N2267: The nodiscard attribute" (PDF). open-std.org. 2018-07-06. Archived (PDF) from the original on October 19, 2022.
- ^ "WG14-N2764: The noreturn attribute" (PDF). open-std.org. 2021-06-21. Archived (PDF) from the original on December 25, 2022.
- ^ "WG14-N2557: Allow Duplicate Attributes" (PDF). open-std.org. 2020-09-01. Archived (PDF) from the original on November 28, 2022.
- ^ "WG14-N2418: Adding the u8 character prefix" (PDF). open-std.org. 2019-09-02. Archived (PDF) from the original on January 13, 2023.
- ^ What is the point of the UTF-8 character literals proposed for C++17?; Stack Overflow.
- ^ "WG14-N2341: ISO/IEC TS 18661-2 - Floating-point extensions for C - Part 2: Decimal floating-point arithmetic" (PDF). open-std.org. February 26, 2019. Archived (PDF) from the original on November 21, 2022.
- ^ "WG14-N2601: Annex X - IEC 60559 interchange and extended types" (PDF). open-std.org. October 15, 2020. Archived (PDF) from the original on October 14, 2022.
- ^ "WG14-N3018: The constexpr specifier for object definitions". open-std.org. 2022-07-06. Archived from the original on December 24, 2022.
- ^ "WG14-N2653: char8_t: A type for UTF-8 characters and strings (Revision 1)". open-std.org. 2021-06-04. Archived from the original on May 27, 2023.
- ^ "WG14-N2728: char16_t & char32_t string literals shall be UTF-16 & UTF-32". open-std.org. 2021-05-15. Archived from the original on May 27, 2023.
- ^ "WG14-N3038: Introduce storage-class specifiers for compound literals". open-std.org. 2022-07-21. Archived from the original on November 26, 2022.
- ^ "WG14-N2940: Removing trigraphs??!" (PDF). open-std.org. 2022-03-02. Archived (PDF) from the original on October 26, 2022.
- ^ "WG14-N2432: Remove support for function definitions with identifier lists proposal" (PDF). open-std.org. September 25, 2019. Archived (PDF) from the original on December 27, 2022.
- ^ "WG14-N2841: No function declarators without prototypes". open-std.org. 2021-10-10. Archived from the original on November 12, 2022.
- ^ "WG14-N2412: Two's complement sign representation" (PDF). open-std.org. August 11, 2019. Archived (PDF) from the original on December 27, 2022.
- ^ "WG14-N2993: Make *_HAS_SUBNORM be obsolescent". open-std.org. 2022-06-06. Archived from the original on December 5, 2022.
- ^ "GCC 9 Release Notes". GNU Project. Archived from the original on December 27, 2022.
- ^ "Clang 9.0 - add new language mode for C2x". LLVM Project Repository. May 14, 2019. Archived from the original on December 27, 2022.
- ^ "Pelles C - major changes between 10.00 and 11.00". smorgasbordet.com. Archived from the original on December 27, 2022.
Further reading
edit- N3096 (working draft of C23 standard); WG14; April 2023. (free download)
- N3149 (working draft of C23 standard); WG14; July 2023. (not available to public)
- N3219 (ISO/IEC 9899:2023 DIS Draft); WG14; February 2024. (ISO draft available but not free)
- ISO/IEC 9899:2024 (official C23 standard); ISO; 2024. (planning for release in 2024)
External links
edit- WG14 Document Repository
- WG14 Meetings - agenda and minutes
- WG14 Charters: C2x Charter, C23 Charter, Interpreting the C23 Charter, C Standard Charter