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 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.