1.. _programming_language:
2
3Programming Language
4====================
5
6The kernel is written in the C programming language [c-language]_.
7More precisely, the kernel is typically compiled with ``gcc`` [gcc]_
8under ``-std=gnu89`` [gcc-c-dialect-options]_: the GNU dialect of ISO C90
9(including some C99 features). ``clang`` [clang]_ is also supported, see
10docs on :ref:`Building Linux with Clang/LLVM <kbuild_llvm>`.
11
12This dialect contains many extensions to the language [gnu-extensions]_,
13and many of them are used within the kernel as a matter of course.
14
15There is some support for compiling the kernel with ``icc`` [icc]_ for several
16of the architectures, although at the time of writing it is not completed,
17requiring third-party patches.
18
19Attributes
20----------
21
22One of the common extensions used throughout the kernel are attributes
23[gcc-attribute-syntax]_. Attributes allow to introduce
24implementation-defined semantics to language entities (like variables,
25functions or types) without having to make significant syntactic changes
26to the language (e.g. adding a new keyword) [n2049]_.
27
28In some cases, attributes are optional (i.e. a compiler not supporting them
29should still produce proper code, even if it is slower or does not perform
30as many compile-time checks/diagnostics).
31
32The kernel defines pseudo-keywords (e.g. ``__pure``) instead of using
33directly the GNU attribute syntax (e.g. ``__attribute__((__pure__))``)
34in order to feature detect which ones can be used and/or to shorten the code.
35
36Please refer to ``include/linux/compiler_attributes.h`` for more information.
37
38.. [c-language] http://www.open-std.org/jtc1/sc22/wg14/www/standards
39.. [gcc] https://gcc.gnu.org
40.. [clang] https://clang.llvm.org
41.. [icc] https://software.intel.com/en-us/c-compilers
42.. [gcc-c-dialect-options] https://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html
43.. [gnu-extensions] https://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html
44.. [gcc-attribute-syntax] https://gcc.gnu.org/onlinedocs/gcc/Attribute-Syntax.html
45.. [n2049] http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2049.pdf
46
47