Wikipedia

C23 (C standard revision)

C23 is the informal name for 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 was 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

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

Standard Library

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New functions

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  • 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 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]
  • 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   and exp10.[12]

Existing functions

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Preprocessor

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  • 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

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  • Add nullptr_t, a null pointer type.[22]
  • Add _BitInt(N) and unsigned _BitInt(N) types for bit-precise integers. Add BITINT_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

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  • Add nullptr constant for nullptr_t type.[22]
  • Add wb and uwb integer literal suffixes for _BitInt(N) and unsigned _BitInt(N) types,[32] 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,[33] such as 0b10101010 (equating to 0xAA).
  • Add ' digit separator to literal constants,[34] 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.[35]
  • Allow enums with no fixed underlying type to store values that are not representable by int.[36]

Keywords

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  • Add true and false 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 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

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  • 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:
    • [[unsequenced]][44] allows compiler optimizations for functions producing repeatable outputs only based on their parameters
    • [[reproducible]],[44] similar to [[unsequenced]], but for functions whose call order also matters
  • 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

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  • 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

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  • 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 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.[59]
  • 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.[60]
  • Allow storage class specifiers to appear in compound literal definition.[61]

Obsolete features

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Some 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

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The GCC 9,[67] Clang 9.0,[68] and Pelles C 11.00[69] compilers implement an experimental compiler flag to support this standard.

See also

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References

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  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-N3156: Updated C23 Schedule" (PDF). open-std.org. 2023-07-19. Archived (PDF) from the original on January 29, 2024.
  4. ^ 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.
  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. ^ 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.
  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. ^ See N3096 § B.11 for a useful overview. The functions were added in separate documents: N2488, its updated versions, and its refs.
  13. ^ 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.
  14. ^ "WG14-N3020: Qualifier-preserving standard library functions" (PDF). open-std.org. 2022-06-13. Archived (PDF) from the original on October 13, 2022.
  15. ^ 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.
  16. ^ "GCC 12 Adds Support For New #elifdef #elifndef Directives". phoronix. May 12, 2021. Archived from the original on December 27, 2022.
  17. ^ "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.
  18. ^ "WG14-N2686: #warning" (PDF). open-std.org. 2022-07-22. Archived (PDF) from the original on November 28, 2022.
  19. ^ "WG14-N2799: __has_include for C" (PDF). open-std.org. 2021-08-30. Archived (PDF) from the original on December 24, 2022.
  20. ^ "WG14-N2553: Querying attribute support" (PDF). open-std.org. 2020-08-04. Archived (PDF) from the original on October 14, 2022.
  21. ^ "WG14-N3033: Comma omission and comma deletion". open-std.org. 2022-07-20. Archived from the original on December 27, 2022.
  22. ^ a b "WR14-N3042: Introduce the nullptr constant". open-std.org. 2022-07-22. Archived from the original on December 24, 2022.
  23. ^ "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.
  24. ^ "WG14-N3035: _BitInt Fixes" (PDF). open-std.org. 2022-07-21. Archived (PDF) from the original on October 13, 2022.
  25. ^ "WG14-N2867: Checked N-Bit Integers" (PDF). open-std.org. 2021-11-28. Archived (PDF) from the original on December 14, 2022.
  26. ^ "WG14-N2778: Variably-Modified Types" (PDF). open-std.org. 2021-07-11. Archived (PDF) from the original on December 22, 2022.
  27. ^ "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.
  28. ^ "WG14-N2899: Not-so-magic - typeof for C". open-std.org. 2022-01-21. Archived from the original on December 24, 2022.
  29. ^ "WG14-N3007: Type inference for object definitions". open-std.org. 2022-06-10. Archived from the original on December 24, 2022.
  30. ^ "WG14-N3037: Improved Rules for Tag Compatibility (updates N3032)" (PDF).
  31. ^ "C23 is Finished: Here is What is on the Menu". The Pasture. 31 July 2022.
  32. ^ "WG14-N2775: Literal suffixes for bit-precise integers" (PDF). open-std.org. 2021-07-13. Archived (PDF) from the original on December 27, 2022.
  33. ^ "WG14-N2549: Allow for binary integer constants" (PDF). open-std.org. 2020-07-30. Archived (PDF) from the original on December 22, 2022.
  34. ^ "WG14-N2626: Digit separators" (PDF). open-std.org. 2020-12-15. Archived (PDF) from the original on December 19, 2022.
  35. ^ "WG14-N3030: Enhancements to Enumerations". open-std.org. 2022-07-19. Archived from the original on November 26, 2022.
  36. ^ "WG14-N3029: Improved Normal Enumerations". open-std.org. 2022-07-19. Archived from the original on January 29, 2023.
  37. ^ "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.
  38. ^ "WG14-N2934: Revise spelling of keywords" (PDF). open-std.org. 2022-02-15. Archived (PDF) from the original on December 24, 2022.
  39. ^ "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.
  40. ^ "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.
  41. ^ "WG14-N2900: Consistent, Warningless, and Intuitive Initialization with {}". open-std.org. 2022-01-01. Archived from the original on December 27, 2022.
  42. ^ "WG14-N2975: Relax requirements for variadic parameter lists" (PDF). open-std.org. 2022-04-15. Archived (PDF) from the original on November 28, 2022.
  43. ^ "WG14-N2335: Attributes in C" (PDF). open-std.org. 2019-03-09. Archived (PDF) from the original on October 26, 2022.
  44. ^ a b "Unsequenced functions". open-std.org. Retrieved 2024-07-18.
  45. ^ "WG14-N2265: Harmonizing static_assert with C++" (PDF). open-std.org. 2018-07-06. Archived (PDF) from the original on March 28, 2023.
  46. ^ "Labels at the end of compound statements (C compatibility)" (PDF). 2022-01-13.
  47. ^ "WG14-N2554: Minor attribute wording cleanups" (PDF). open-std.org. 2020-08-04. Archived (PDF) from the original on November 28, 2022.
  48. ^ "WG14-N2334: The deprecated attribute" (PDF). open-std.org. 2019-01-22. Archived (PDF) from the original on October 19, 2022.
  49. ^ "WG14-N2408: The fallthrough attribute" (PDF). open-std.org. 2019-08-11. Archived (PDF) from the original on December 25, 2022.
  50. ^ "WG14-N2270: The maybe_unused attribute" (PDF). open-std.org. 2018-07-06. Archived (PDF) from the original on December 25, 2022.
  51. ^ "WG14-N2267: The nodiscard attribute" (PDF). open-std.org. 2018-07-06. Archived (PDF) from the original on October 19, 2022.
  52. ^ "WG14-N2764: The noreturn attribute" (PDF). open-std.org. 2021-06-21. Archived (PDF) from the original on December 25, 2022.
  53. ^ "WG14-N2557: Allow Duplicate Attributes" (PDF). open-std.org. 2020-09-01. Archived (PDF) from the original on November 28, 2022.
  54. ^ "WG14-N2418: Adding the u8 character prefix" (PDF). open-std.org. 2019-09-02. Archived (PDF) from the original on January 13, 2023.
  55. ^ What is the point of the UTF-8 character literals proposed for C++17?; Stack Overflow.
  56. ^ "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.
  57. ^ "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.
  58. ^ "WG14-N3018: The constexpr specifier for object definitions". open-std.org. 2022-07-06. Archived from the original on December 24, 2022.
  59. ^ "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.
  60. ^ "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.
  61. ^ "WG14-N3038: Introduce storage-class specifiers for compound literals". open-std.org. 2022-07-21. Archived from the original on November 26, 2022.
  62. ^ "WG14-N2940: Removing trigraphs??!" (PDF). open-std.org. 2022-03-02. Archived (PDF) from the original on October 26, 2022.
  63. ^ "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.
  64. ^ "WG14-N2841: No function declarators without prototypes". open-std.org. 2021-10-10. Archived from the original on November 12, 2022.
  65. ^ "WG14-N2412: Two's complement sign representation" (PDF). open-std.org. August 11, 2019. Archived (PDF) from the original on December 27, 2022.
  66. ^ "WG14-N2993: Make *_HAS_SUBNORM be obsolescent". open-std.org. 2022-06-06. Archived from the original on December 5, 2022.
  67. ^ "GCC 9 Release Notes". GNU Project. Archived from the original on December 27, 2022.
  68. ^ "Clang 9.0 - add new language mode for C2x". LLVM Project Repository. May 14, 2019. Archived from the original on December 27, 2022.
  69. ^ "Pelles C - major changes between 10.00 and 11.00". smorgasbordet.com. Archived from the original on December 27, 2022.

Further reading

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  • 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)
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Preceded by C language standards Latest
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