.. _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, see docs on :ref:`Building Linux with Clang/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 several of 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 introduce implementation-defined semantics to language entities (like variables, functions or types) without having to make significant syntactic changes to the language (e.g. adding a new keyword) [n2049]_. In some cases, attributes are optional (i.e. a compiler not supporting them should still produce proper code, even if it is slower or does not perform as many compile-time checks/diagnostics). The kernel defines pseudo-keywords (e.g. ``__pure``) instead of using directly 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