| .. _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 <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 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 | |