C11 (formerly C1X) is an informal name for ISO/IEC 9899:2011,[1] a past standard for the C programming language. It replaced C99 (standard ISO/IEC 9899:1999) and has been superseded by C17 (standard ISO/IEC 9899:2018). C11 mainly standardizes features already supported by common contemporary compilers, and includes a detailed memory model to better support multiple threads of execution. Due to delayed availability of conforming C99 implementations, C11 makes certain features optional, to make it easier to comply with the core language standard.[2]
The final draft, N1570, was published in April 2011. The new standard passed its final draft review on October 10, 2011 and was officially ratified by ISO and published as ISO/IEC 9899:2011 on December 8, 2011, with no comments requiring resolution by participating national bodies.
A standard macro __STDC_VERSION__ is defined with value 201112L to indicate that C11 support is available.[3]
The standard includes several changes to the C99 language and library specifications, such as
_Alignas specifier, _Alignof operator, aligned_alloc function, <stdalign.h> header)_Noreturn function specifier and the <stdnoreturn.h> header_Generic keyword. For example, the following macro [[Cube root|cbrt(x)]] translates to cbrtl(x), cbrt(x) or cbrtf(x) depending on the type of x:default: cbrt, \ float: cbrtf)(x)
_Thread_local storage-class specifier, <threads.h> header including thread creation/management functions, mutex, condition variable and thread-specific storage functionality, as well as <stdatomic.h>[4] for atomic operations supporting the C11 memory model).char16_t and char32_t types for storing UTF-16/UTF-32 encoded data, including conversion functions in <uchar.h> and the corresponding u and U string literal prefixes, as well as the u8 prefix for UTF-8 encoded literals).[[gets|gets]] function (in favor of safer fgets), which was deprecated in the previous C language standard revision, ISO/IEC 9899:1999/Cor.3:2007(E).#if and #error, when types are understood by the translator."…x" suffix) for [[C file input/output#Opening a file using fopen|fopen]]. This behaves like O_CREAT|O_EXCL in POSIX, which is commonly used for lock files.[[C process control|quick_exit]] function as a third way to terminate a program, intended to do at least minimal deinitialization.[6]timespec_get function and corresponding structure in <time.h> with a degree of POSIX compatibility.real + imaginary*I might not yield the expected value if imaginary is infinite or NaN).The new revision allows implementations to not support certain parts of the standard — including some that had been mandatory to support in the 1999 revision. Programs can use predefined macros to determine whether an implementation supports a certain feature or not.
| Analyzability (Annex L) | __STDC_ANALYZABLE__ | ||
| Bounds-checking interfaces (Annex K) | __STDC_LIB_EXT1__ | ||
Multithreading (<threads.h>) | __STDC_NO_THREADS__ | ||
Atomic primitives and types (<stdatomic.h> and the _Atomic type qualifier)[7] | __STDC_NO_ATOMICS__ | ||
| IEC 60559 floating-point arithmetic (Annex F) | __STDC_IEC_559__ | ||
| IEC 60559 compatible complex arithmetic (Annex G) | __STDC_IEC_559_COMPLEX__ | ||
Complex types (<complex.h>) | __STDC_NO_COMPLEX__ | ||
| Variable-length arrays[8] | __STDC_NO_VLA__ |
Some features of C11 are supported by the GCC starting with version 4.6,[9] Clang starting with version 3.1,[10] IBM XL C starting with version 12.1,[11] and Microsoft Visual C++ starting with VS 2019 (16.8)[12] in September 2020.
The optional bounds-checking interfaces (Annex K) remain controversial and have not been widely implemented, and their deprecation or removal from the next standard revision has been proposed. Even Microsoft, a main proponent of this interface, does not conform to the definition.[13] In addition, Annex K does not include the more useful TR24731-2 (dynamic allocation functions), such as and .[14] The few open-source implementations include Open Watcom C/C++'s "Safer C" library[15] and safeclib.[16]