Commit 21228a18 authored by Josh Triplett's avatar Josh Triplett Committed by Jonathan Corbet

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's avatarJosh Triplett <josh@joshtriplett.org>
Acked-by: default avatarGeert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: default avatarJonathan Corbet <corbet@lwn.net>
parent 0df1f248
......@@ -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
......
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