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 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

Changes integrated into the latest working draft of C23 are listed below.^[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.^[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 ${\displaystyle \pi x}$ and exp10.^[12]

Existing functions

• Add %b binary conversion specifier to printf() function family.^[13]
• Add %b binary conversion specifier to scanf() function family.^[13]
• Add 0b and 0B binary conversion support to strtol() and wcstol() function families.^[13]
• 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.^[14]

Preprocessor

• 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

• 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

• 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

• 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

• 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 [[]].
• Add single-argument _Static_assert for compatibility with C++17.^[44]
• 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.^[45]
• Add C++-style attributes (see Syntax). Add attributes ^[46] [[deprecated]],^[47] [[fallthrough]],^[48] [[maybe_unused]],^[49] [[nodiscard]],^[50] and [[noreturn]] attribute for compatibility with C++11, then deprecate _Noreturn, noreturn, header <stdnoreturn.h> features introduced in C11.^[51] Duplicate attributes are allowed for compatibility with C++23.^[52] 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.^[53][54]
• Add #elifdef and #elifndef preprocessing directives for compatibility with C++23.^[15] (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.^[55][56]

• The constexpr specifier for objects but not functions, unlike C++'s equivalent.^[57]
• 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.^[58]
• 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.^[59]
• Allow storage class specifiers to appear in compound literal definition.^[60]

Obsolete features

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

• Remove trigraphs.^[61]
• Remove K&R function definitions/declarations (with no information about the function arguments).^[62][63]
• Remove representations for signed integers other than two's complement. Two's complement signed integer representation will be required.^[64]
• The *_HAS_SUBNORM macros in <float.h> are obsolescent features.^[65]

Compiler support

The GCC 9,^[66] Clang 9.0,^[67] and Pelles C 11.00^[68] compilers implement an experimental compiler flag to support this standard.