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1 | .. _programming_language: |
2 | ||
3 | Programming Language | |
4 | ==================== | |
5 | ||
6 | The kernel is written in the C programming language [c-language]_. | |
7 | More precisely, the kernel is typically compiled with ``gcc`` [gcc]_ | |
8 | under ``-std=gnu89`` [gcc-c-dialect-options]_: the GNU dialect of ISO C90 | |
9 | (including some C99 features). | |
10 | ||
11 | This dialect contains many extensions to the language [gnu-extensions]_, | |
12 | and many of them are used within the kernel as a matter of course. | |
13 | ||
14 | There is some support for compiling the kernel with ``clang`` [clang]_ | |
15 | and ``icc`` [icc]_ for several of the architectures, although at the time | |
16 | of writing it is not completed, requiring third-party patches. | |
17 | ||
18 | Attributes | |
19 | ---------- | |
20 | ||
21 | One of the common extensions used throughout the kernel are attributes | |
22 | [gcc-attribute-syntax]_. Attributes allow to introduce | |
23 | implementation-defined semantics to language entities (like variables, | |
24 | functions or types) without having to make significant syntactic changes | |
25 | to the language (e.g. adding a new keyword) [n2049]_. | |
26 | ||
27 | In some cases, attributes are optional (i.e. a compiler not supporting them | |
28 | should still produce proper code, even if it is slower or does not perform | |
29 | as many compile-time checks/diagnostics). | |
30 | ||
31 | The kernel defines pseudo-keywords (e.g. ``__pure``) instead of using | |
32 | directly the GNU attribute syntax (e.g. ``__attribute__((__pure__))``) | |
33 | in order to feature detect which ones can be used and/or to shorten the code. | |
34 | ||
35 | Please refer to ``include/linux/compiler_attributes.h`` for more information. | |
36 | ||
37 | .. [c-language] http://www.open-std.org/jtc1/sc22/wg14/www/standards | |
38 | .. [gcc] https://gcc.gnu.org | |
39 | .. [clang] https://clang.llvm.org | |
40 | .. [icc] https://software.intel.com/en-us/c-compilers | |
41 | .. [gcc-c-dialect-options] https://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html | |
42 | .. [gnu-extensions] https://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html | |
43 | .. [gcc-attribute-syntax] https://gcc.gnu.org/onlinedocs/gcc/Attribute-Syntax.html | |
44 | .. [n2049] http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2049.pdf | |
45 |