C11 (C standard revision)

C11 is an informal name for ISO/IEC 9899:2011, a past standard for the C programming language. It replaced C99 and has been superseded by C18. 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.
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:

  1. define cbrt _Generic, long double: cbrtl, \
default: cbrt, \
float: cbrtf)

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.
FeatureFeature test macroAvailability in C99
Analyzability __STDC_ANALYZABLE__
Bounds-checking interfaces __STDC_LIB_EXT1__
Multithreading __STDC_NO_THREADS__
Atomic primitives and types __STDC_NO_ATOMICS__
IEC 60559 floating-point arithmetic __STDC_IEC_559__
IEC 60559 compatible complex arithmetic __STDC_IEC_559_COMPLEX__
Complex types __STDC_NO_COMPLEX__
Variable length arrays__STDC_NO_VLA__


The optional bounds-checking interfaces remain controversial and have not been widely implemented, and their deprecation or removal from the next standard revision has been proposed.