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author | Josh Triplett <josh@joshtriplett.org> | 2014-10-29 11:15:17 -0700 |
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committer | Jonathan Corbet <corbet@lwn.net> | 2014-11-03 11:46:02 -0500 |
commit | 21228a1868692c34ade648dbb0bc3db0069ab551 (patch) | |
tree | a545010214c9e42931bef25b2fc06880be885bec | |
parent | 0df1f2487d2f0d04703f142813d53615d62a1da4 (diff) | |
download | linux-21228a1868692c34ade648dbb0bc3db0069ab551.tar.bz2 |
CodingStyle: Add a chapter on conditional compilation
Document several common practices and conventions regarding conditional
compilation, most notably the preference for ifdefs in headers rather
than .c files.
Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
-rw-r--r-- | Documentation/CodingStyle | 43 |
1 files changed, 43 insertions, 0 deletions
diff --git a/Documentation/CodingStyle b/Documentation/CodingStyle index 3171822c22a5..9f28b140dc89 100644 --- a/Documentation/CodingStyle +++ b/Documentation/CodingStyle @@ -845,6 +845,49 @@ next instruction in the assembly output: : /* outputs */ : /* inputs */ : /* clobbers */); + Chapter 20: Conditional Compilation + +Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c +files; doing so makes code harder to read and logic harder to follow. Instead, +use such conditionals in a header file defining functions for use in those .c +files, providing no-op stub versions in the #else case, and then call those +functions unconditionally from .c files. The compiler will avoid generating +any code for the stub calls, producing identical results, but the logic will +remain easy to follow. + +Prefer to compile out entire functions, rather than portions of functions or +portions of expressions. Rather than putting an ifdef in an expression, factor +out part or all of the expression into a separate helper function and apply the +conditional to that function. + +If you have a function or variable which may potentially go unused in a +particular configuration, and the compiler would warn about its definition +going unused, mark the definition as __maybe_unused rather than wrapping it in +a preprocessor conditional. (However, if a function or variable *always* goes +unused, delete it.) + +Within code, where possible, use the IS_ENABLED macro to convert a Kconfig +symbol into a C boolean expression, and use it in a normal C conditional: + + if (IS_ENABLED(CONFIG_SOMETHING)) { + ... + } + +The compiler will constant-fold the conditional away, and include or exclude +the block of code just as with an #ifdef, so this will not add any runtime +overhead. However, this approach still allows the C compiler to see the code +inside the block, and check it for correctness (syntax, types, symbol +references, etc). Thus, you still have to use an #ifdef if the code inside the +block references symbols that will not exist if the condition is not met. + +At the end of any non-trivial #if or #ifdef block (more than a few lines), +place a comment after the #endif on the same line, noting the conditional +expression used. For instance: + +#ifdef CONFIG_SOMETHING +... +#endif /* CONFIG_SOMETHING */ + Appendix I: References |