C23 (C standard revision)

C23 is the informal name for what will likely become ISO/IEC 9899:2024, the next standard for the C programming language, which will replace C17 (standard ISO/IEC 9899:2018).[1] It was started in 2016 informally as C2x,[2] and 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 2023.

C language revisions

Features

Changes integrated into the latest working draft of C23 are listed below.[4]

New functions
  • 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() and strndup() 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 with stdc_ to minimize conflict with legacy code and 3rd party libraries.[10]
    • In the following, replace * with uc, us, ui, ul, ull for five function names, or blank for a type-generic macro.[10]
    • add stdc_count_ones*() and stdc_count_zeros*() to count number of 1 or 0 bits in value.[10]
    • add stdc_leading_ones*() and stdc_leading_zeros*() to count leading 1 or 0 bits in value.[10]
    • add stdc_trailing_ones*() and stdc_trailing_zeros*() to count trailing 1 or 0 bits in value.[10]
    • add stdc_first_leading_one*() and stdc_first_leading_zero*() to find first leading bit with 1 or 0 in value.[10]
    • add stdc_first_trailing_one*() and stdc_first_trailing_zero*() to find first trailing bit with 1 or 0 in value.[10]
    • add stdc_has_single_bit*() to determine if value 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]
  • add timegm() function in <time.h> to convert time structure into calendar time value - similar to function in glibc and musl libraries.[11]

Existing functions
  • add %b binary conversion specifier to printf() function family, prepending non-zero values with 0b, similar to how %x works. Implementations that previously didn't use %B as their own extension are encouraged to implement and prepend non-zero values with 0B, similar to how %X works.[12]
  • add %b binary conversion specifier to scanf() function family.[12]
  • add 0b and 0B binary conversion support to strtol() and wcstol() function families.[12]
  • make the functions bsearch(), bsearch_s(), memchr(), strchr(), strpbrk(), strrchr(), strstr(), and their wide counterparts wmemchr(), wcschr(), wcspbrk(), wcsrchr(), wcsstr() return a const qualified object if one was passed to them.[13]

Preprocessor
  • add #elifdef and #elifndef directives[14] are essentially equivalent to #elif defined and #elif !defined. Both directives were added to C++23 standard and GCC 12.[15]
  • add #embed directive for binary resource inclusion and __has_embed allowing the availability of a resource to be checked by preprocessor directives.[16]
  • add #warning directive for diagnostics.[17]
  • add __has_include allowing the availability of a header to be checked by preprocessor directives.[18]
  • add __has_c_attribute allowing the availability of an attribute to be checked by preprocessor directives.[19] (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.[20]

Types
  • add nullptr_t, a null pointer type.[21]
  • add _BitInt(N) and unsigned _BitInt(N) types for bit-precise integers. Add BITINT_MAXWIDTH macro for maximum bit width.[22][23] Add ckd_add(), ckd_sub(), ckd_mul() macros for checked integer operations.[24]
  • Variably-modified types (but not VLAs which are automatic variables allocated on the stack) become a mandatory feature.[25]
  • Better support for using const with arrays.[26]
  • Standardization of the typeof(...) operator.[27]
  • 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).[28]
  • compatibility rules for structure, union, and enumerated types were changed to allow a redeclaration of a compatible type with the same tag.[29]

Constants
  • add nullptr constant for nullptr_t type.[21]
  • add wb and uwb integer literal suffixes for _BitInt(N) and unsigned _BitInt(N) types,[30] such as 6uwb yields an unsigned _BitInt(3), and -6wb yields a signed _BitInt(4) which has three value bits and one sign bit.
  • add 0b and 0B binary literal constant prefixes,[31] such as 0b10101010 (equating to 0xAA).
  • add ' digit separator to literal constants,[32] such as 0xFE'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.[33]
  • allow enums with no fixed underlying type to store values that aren't representable by int.[34]

Keywords
  • add true and false keywords.[35]
  • add alignas, alignof, bool, static_assert, thread_local keywords. Previously defined keywords become alternative spellings: _Alignas, _Alignof, _Bool, _Static_assert, _Thread_local.[36]
  • add _BitInt keyword (see "types" group)
  • add typeof and typeof_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.[37]
  • Unnamed parameters in function definitions.[38]
  • Zero initialization with {} (including initialization of VLAs).[39]
  • 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.[40]
  • add C++11 style attribute syntax[41] using double square brackets [[]].
  • add single-argument _Static_assert for compatibility with C++17.[42]
  • 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
  • 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.[43]
  • add C++-style attributes (see Syntax). Add attributes [44] [[deprecated]],[45] [[fallthrough]],[46] [[maybe_unused]],[47] [[nodiscard]],[48] and [[noreturn]] attribute for compatibility with C++11, then deprecate _Noreturn, noreturn, header <stdnoreturn.h> features introduced in C11.[49] Duplicate attributes are allowed for compatibility with C++23.[50] 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.[51][52]
  • add #elifdef and #elifndef preprocessing directives for compatibility with C++23.[14] (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.[53][54]
  • The constexpr specifier for objects but not functions, unlike C++'s equivalent.[55]
  • add char8_t type for storing UTF-8 encoded data and change the type of u8 character constants and string literals to char8_t. Also, the functions mbrtoc8() and c8rtomb() 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.[56]
  • clarify that all char16_t strings and literals shall be UTF-16 encoded, and all char32_t strings and literals shall be UTF-32 encoded, unless otherwise explicitly specified.[57]
  • allow storage class specifiers to appear in compound literal definition.[58]

Obsolete features

Some old obsolete features are either removed or deprecated from the working draft of C23:

  • Remove Trigraphs.[59]
  • Remove K&R function definitions/declarations (with no information about the function arguments).[60][61]
  • Remove representations for signed integers other than two's complement. Two's complement signed integer representation will be required.[62]
  • The *_HAS_SUBNORM macros in <float.h> are obsolescent features.[63]

Compiler support

The GCC 9,[64] Clang 9.0,[65] and Pelles C 11.00[66] compilers implement an experimental compiler flag to support this standard.

See also

References
  1. "History of C". cppreference.com. 2022-06-27. Archived from the original on October 19, 2022.
  2. "WG14-N2086 : C2x Charter". open-std.org. 2016-09-20. Archived from the original on December 22, 2022.
  3. "WG14-N3132 : Revised C23 Schedule" (PDF). open-std.org. 2023-06-04. Archived (PDF) from the original on June 9, 2023.
  4. "WG14-N3096 : Draft for ISO/IEC 9899:2023" (PDF). open-std.org. April 1, 2023. Archived (PDF) from the original on April 2, 2023.
  5. "WG14-N2437 : Agenda for October 2019". open-std.org. 2019-10-21. Archived from the original on March 5, 2021.
  6. "WG14-N2897 : memset_explicit()". open-std.org. 2021-12-27. Archived from the original on October 25, 2022.
  7. "WG14-N2349 : Toward more efficient string copying and concatenation". open-std.org. 2019-03-18. Archived from the original on September 30, 2022.
  8. "WG14-N2353 : strdup() and strndup()". open-std.org. 2019-03-18. Archived from the original on December 24, 2022.
  9. "WG14-N2974 : Queryable pointer alignment" (PDF). open-std.org. 2022-04-15. Archived (PDF) from the original on October 13, 2022.
  10. "WG14-N3022 : Modern Bit Utilities". open-std.org. 2022-07-06. Archived from the original on December 24, 2022.
  11. "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.
  12. "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.
  13. "WG14-N3020 : Qualifier-preserving standard library functions" (PDF). open-std.org. 2022-06-13. Archived (PDF) from the original on October 13, 2022.
  14. "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.
  15. "GCC 12 Adds Support For New #elifdef #elifndef Directives". phoronix. May 12, 2021. Archived from the original on December 27, 2022.
  16. "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.
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  21. "WR14-N3042 : Introduce the nullptr constant". open-std.org. 2022-07-22. Archived from the original on December 24, 2022.
  22. "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.
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  24. "WG14-N2867 : Checked N-Bit Integers" (PDF). open-std.org. 2021-11-28. Archived (PDF) from the original on December 14, 2022.
  25. "WG14-N2778 : Variably-Modified Types" (PDF). open-std.org. 2021-07-11. Archived (PDF) from the original on December 22, 2022.
  26. "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.
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  28. "WG14-N3007 : Type inference for object definitions". open-std.org. 2022-06-10. Archived from the original on December 24, 2022.
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  30. "WG14-N2775 : Literal suffixes for bit-precise integers" (PDF). open-std.org. 2021-07-13. Archived (PDF) from the original on December 27, 2022.
  31. "WG14-N2549 : Allow for binary integer constants" (PDF). open-std.org. 2020-07-30. Archived (PDF) from the original on December 22, 2022.
  32. "WG14-N2626 : Digit separators" (PDF). open-std.org. 2020-12-15. Archived (PDF) from the original on December 19, 2022.
  33. "WG14-N3030 : Enhancements to Enumerations". open-std.org. 2022-07-19. Archived from the original on November 26, 2022.
  34. "WG14-N3029 : Improved Normal Enumerations". open-std.org. 2022-07-19. Archived from the original on January 29, 2023.
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  36. "WG14-N2934 : Revise spelling of keywords" (PDF). open-std.org. 2022-02-15. Archived (PDF) from the original on December 24, 2022.
  37. "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.
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  39. "WG14-N2900 : Consistent, Warningless, and Intuitive Initialization with {}". open-std.org. 2022-01-01. Archived from the original on December 27, 2022.
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  47. "WG14-N2270 : The maybe_unused attribute" (PDF). open-std.org. 2018-07-06. Archived (PDF) from the original on December 25, 2022.
  48. "WG14-N2267 : The nodiscard attribute" (PDF). open-std.org. 2018-07-06. Archived (PDF) from the original on October 19, 2022.
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  52. What is the point of the UTF-8 character literals proposed for C++17?; Stack Overflow.
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  58. "WG14-N3038 : Introduce storage-class specifiers for compound literals". open-std.org. 2022-07-21. Archived from the original on November 26, 2022.
  59. "WG14-N2940 : Removing trigraphs??!" (PDF). open-std.org. 2022-03-02. Archived (PDF) from the original on October 26, 2022.
  60. "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.
  61. "WG14-N2841 : No function declarators without prototypes". open-std.org. 2021-10-10. Archived from the original on November 12, 2022.
  62. "WG14-N2412 : Two's complement sign representation" (PDF). open-std.org. August 11, 2019. Archived (PDF) from the original on December 27, 2022.
  63. "WG14-N2993 : Make *_HAS_SUBNORM be obsolescent". open-std.org. 2022-06-06. Archived from the original on December 5, 2022.
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