.. _programming_language: Programming Language==================== The kernel is written in the C programming language [c-language]_.More precisely, the kernel is typically compiled with ``gcc`` [gcc]_under ``-std=gnu89`` [gcc-c-dialect-options]_: the GNU dialect of ISO C90(including some C99 features). ``clang`` [clang]_ is also supported, seedocs on :ref:`Building Linux with Clang/LLVM <kbuild_llvm>`. This dialect contains many extensions to the language [gnu-extensions]_,and many of them are used within the kernel as a matter of course. There is some support for compiling the kernel with ``icc`` [icc]_ for severalof the architectures, although at the time of writing it is not completed,requiring third-party patches. Attributes---------- One of the common extensions used throughout the kernel are attributes[gcc-attribute-syntax]_. Attributes allow to introduceimplementation-defined semantics to language entities (like variables,functions or types) without having to make significant syntactic changesto the language (e.g. adding a new keyword) [n2049]_. In some cases, attributes are optional (i.e. a compiler not supporting themshould still produce proper code, even if it is slower or does not performas many compile-time checks/diagnostics). The kernel defines pseudo-keywords (e.g. ``__pure``) instead of usingdirectly the GNU attribute syntax (e.g. ``__attribute__((__pure__))``)in order to feature detect which ones can be used and/or to shorten the code. Please refer to ``include/linux/compiler_attributes.h`` for more information. .. [c-language] http://www.open-std.org/jtc1/sc22/wg14/www/standards.. [gcc] https://gcc.gnu.org.. [clang] https://clang.llvm.org.. [icc] https://software.intel.com/en-us/c-compilers.. [gcc-c-dialect-options] https://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html.. [gnu-extensions] https://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html.. [gcc-attribute-syntax] https://gcc.gnu.org/onlinedocs/gcc/Attribute-Syntax.html.. [n2049] http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2049.pdf