C11 (formerly C1X) is an informal name for ISO/IEC 9899:2011, the current standard for the C programming language. It replaces the previous C standard, informally known as C99. This new version mainly standardizes features that have already been 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.
Contents
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. Some features of C11 are supported by the GCC starting with version 4.6, Clang starting with version 3.1, and IBM XL C starting with version 12.1.
Changes from C99
The standard includes several changes to the C99 language and library specifications, such as:
_Alignas specifier, _Alignof operator, aligned_alloc function, <stdalign.h> header file)_Noreturn function specifier and the <stdnoreturn.h> header file_Generic keyword. For example, the following macro cbrt(x) translates to cbrtl(x), cbrt(x) or cbrtf(x) depending on the type of 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 the _Atomic type qualifier and <stdatomic.h> for uninterruptible object access).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 function, deprecated in the previous C language standard revision, ISO/IEC 9899:1999/Cor.3:2007(E), in favor of a new safe alternative, gets_s.struct T { int tag; union { float x; int n; }; };.#if and #error, when types are understood by the translator."…x" suffix) for fopen. This behaves like O_CREAT|O_EXCL in POSIX, which is commonly used for lock files.quick_exit function as a third way to terminate a program, intended to do at least minimal deinitialization if termination with exit fails.real + imaginary*I might not yield the expected value if imaginary is infinite or NaN).Optional features
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.
Criticism
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. (The open-source Open Watcom C/C++ contains a "Safer C" library that is considered a nearly conforming implementation.)