C11 (C standard revision)


C11 is a past standard for the C programming language. It replaced C99 and was superseded by C17. 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.

Changes from C99

The standard includes several changes to the C99 language and library specifications, such as
  • Alignment specification
  • The _Noreturn function specifier and the <stdnoreturn.h> header
  • Type-generic expressions using the _Generic keyword. For example, the following macro cbrt translates to cbrtl, cbrt or cbrtf depending on the type of x:

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

  • Multi-threading support.
  • Improved Unicode support based on the C Unicode Technical Report ISO/IEC TR 19769:2004.
  • Removal of the gets function, which was deprecated in the previous C language standard revision, ISO/IEC 9899:1999/Cor.3:2007.
  • Bounds-checking interfaces.
  • Analyzability features.
  • More macros for querying the characteristics of floating-point types, concerning subnormal floating-point numbers and the number of decimal digits the type is able to store.
  • Anonymous structures and unions, useful when unions and structures are nested, e.g. in

struct MyStruct ;

  • Static assertions, which are evaluated during translation at a later phase than #if and #error, when types are understood by the translator.
  • An exclusive create-and-open mode for fopen. This behaves like O_CREAT|O_EXCL in POSIX, which is commonly used for lock files.
  • The quick_exit function as a third way to terminate a program, intended to do at least minimal deinitialization.
  • A new timespec_get function and corresponding structure in <time.h> with a degree of POSIX compatibility.
  • Macros for the construction of complex values.

    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.
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__

Compiler support

Some features of C11 are supported by the GCC starting with version 4.6, Clang starting with version 3.1, IBM XL C starting with version 12.1, and Microsoft Visual C++ starting with VS 2019 in September 2020.

Criticism

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. Even Microsoft, a main proponent of this interface, does not conform to the definition. In addition, Annex K does not include the more useful TR24731-2, such as and. The few open-source implementations include Open Watcom C/C++'s "Safer C" library and safeclib.