The kernel is written in the C programming language [c-language].
More precisely, the kernel is typically compiled with
-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.
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.
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.