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).

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 clang [clang] and 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