C2x is an informal name for the next (after C17) major C language standard revision.^[1][2] It is expected to be voted on in 2023 and would therefore become C23.^[3] The most recent publicly available draft of C23 was released on January 24, 2023.^[4]

The first WG14 meeting for the C2x draft was held in October 2019, then virtual remote meetings were held due to COVID-19 pandemic in March/April 2020, October/November 2020, March/April 2021, August/September 2021, November/December 2021, January/February 2022, May 2022, July 2022, January 2023.^[3][4]

Features

Changes integrated into the latest C23 draft are:^[4]

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.^[5]
• add memccpy() function in <string.h> to efficiently concatenate strings - similar to POSIX and SVID C extensions.^[6]
• add strdup() and strndup() functions in <string.h> to allocate a copy of a string – similar to POSIX and SVID C extensions.^[7]
• add memalignment() function in <stdlib.h> to determine the byte alignment of a pointer.^[8]
• 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.^[9]
  • In the following, replace * with uc, us, ui, ul, ull for five function names, or blank for a type-generic macro.^[9]
  • add stdc_count_ones*() and stdc_count_zeros*() to count number of 1 or 0 bits in value.^[9]
  • add stdc_leading_ones*() and stdc_leading_zeros*() to count leading 1 or 0 bits in value.^[9]
  • add stdc_trailing_ones*() and stdc_trailing_zeros*() to count trailing 1 or 0 bits in value.^[9]
  • add stdc_first_leading_one*() and stdc_first_leading_zero*() to find first leading bit with 1 or 0 in value.^[9]
  • add stdc_first_trailing_one*() and stdc_first_trailing_zero*() to find first trailing bit with 1 or 0 in value.^[9]
  • 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).^[9]
  • add stdc_bit_floor*() to determine the largest integral power of 2 that is not greater than value.^[9]
  • add stdc_bit_ceil*() to determine the smallest integral power of 2 that is not less than value.^[9]
  • add stdc_bit_width*() to determine number of bits to represent a value.^[9]

Standard Library (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.^[10]
• add %b binary conversion specifier to scanf() function family.^[10]
• add 0b and 0B binary conversion support to strtol() and wcstol() function families.^[10]
• 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.^[11]

Preprocessor

• add #elifdef and #elifndef directives^[12] are essentially equivalent to #elif defined and #elif !defined. Both directives were added to C++23 and GCC 12 too.^[13]
• add #embed directive for binary resource inclusion.^[14]
• add #warning directive for diagnostics.^[15]
• add __has_include allowing the availability of a header to be checked by preprocessor directives.^[16]
• add __has_c_attribute allowing the availability of an attribute to be checked by preprocessor directives.^[17] (see C++ compatibility group below 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.^[18]

Types

• add nullptr_t type.^[19]
• add _BitInt(N) and unsigned _BitInt(N) types for bit-precise integers. Add BITINT_MAXWIDTH macro for maximum bit width.^[20][21] Add ckd_add(), ckd_sub(), ckd_mul() macros for checked integer operations.^[22]
• Variably-modified types (but not VLAs which are automatic variables allocated on the stack) become a mandatory feature.^[23]
• Standardization of the typeof(...) operator.^[24]
• 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.^[25]

Constants

• add nullptr constant for nullptr_t type.^[19]
• add wb and uwb integer literal suffixes for _BitInt(N) and unsigned _BitInt(N) types,^[26] 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,^[27] such as 0b10101010 (equating to 0xAA).
• add ' digit separator to literal constants,^[28] 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.^[29]

C++ compatibility

• add C++11 style attribute syntax^[30] using double square brackets . Add attributes ^[31] deprecated,^[32] fallthrough,^[33] maybe_unused,^[34] nodiscard,^[35] and noreturn attribute for compatibility with C++11, then deprecate _Noreturn, noreturn, header <stdnoreturn.h> features introduced in C11.^[36] Duplicate attributes are allowed for compatility with C++23.^[37]
• add u8 prefix for character literals to represent UTF-8 encoding for compatibility with C++17.^[38][39]
• add #elifdef and #elifndef preprocessing directives for compatibility with C++23.^[12] (see preprocessor group above)

Other

• 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.^[40][41]
• Single-argument _Static_assert.^[42]
• Labels can appear before declarations and at the end of compound statements.^[43]
• Unnamed parameters in function definitions.^[44]
• Better support for using const with arrays.^[45]
• Zero initialization with {} (including initialization of VLAs).^[46]
• alignas, alignof, bool, true, false, static_assert, thread_local become keywords.^[47][48]
• The constexpr specifier for objects but not functions, unlike C++'s equivalent.^[49]
• 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.^[50]
• add the 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.^[51]
• allow storage class specifiers to appear in compound literal definition.^[52]

Obsolete featuresedit

Some old obsolete features are either removed or deprecated:

• Remove Trigraphs.^[53]
• Remove K&R function definitions/declarations.^[54][55]
• Remove representations for signed integers other than two's complement. Two's complement signed integer representation will be required.^[56]
• The *_HAS_SUBNORM macros in <float.h> are obsolescent features.^[57]

Compiler supportedit

The GCC 9,^[58] Clang 9.0,^[59] and Pelles C 11.00^[60] compilers implement an experimental compiler flag to support this standard.

See alsoedit

• C++23, C++20, C++17, C++14, C++11, C++03, C++98, versions of the C++ programming language standard
• Compatibility of C and C++

Referencesedit

External linksedit

• C Working Group WG14 Documents
• N3088, draft of C23 standard (January 2023)