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>
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@ -845,6 +845,49 @@ next instruction in the assembly output:
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: /* outputs */ : /* inputs */ : /* clobbers */);
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Chapter 20: Conditional Compilation
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Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c
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files; doing so makes code harder to read and logic harder to follow. Instead,
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use such conditionals in a header file defining functions for use in those .c
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files, providing no-op stub versions in the #else case, and then call those
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functions unconditionally from .c files. The compiler will avoid generating
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any code for the stub calls, producing identical results, but the logic will
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remain easy to follow.
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Prefer to compile out entire functions, rather than portions of functions or
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portions of expressions. Rather than putting an ifdef in an expression, factor
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out part or all of the expression into a separate helper function and apply the
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conditional to that function.
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If you have a function or variable which may potentially go unused in a
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particular configuration, and the compiler would warn about its definition
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going unused, mark the definition as __maybe_unused rather than wrapping it in
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a preprocessor conditional. (However, if a function or variable *always* goes
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unused, delete it.)
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Within code, where possible, use the IS_ENABLED macro to convert a Kconfig
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symbol into a C boolean expression, and use it in a normal C conditional:
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if (IS_ENABLED(CONFIG_SOMETHING)) {
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...
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}
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The compiler will constant-fold the conditional away, and include or exclude
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the block of code just as with an #ifdef, so this will not add any runtime
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overhead. However, this approach still allows the C compiler to see the code
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inside the block, and check it for correctness (syntax, types, symbol
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references, etc). Thus, you still have to use an #ifdef if the code inside the
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block references symbols that will not exist if the condition is not met.
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At the end of any non-trivial #if or #ifdef block (more than a few lines),
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place a comment after the #endif on the same line, noting the conditional
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expression used. For instance:
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#ifdef CONFIG_SOMETHING
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...
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#endif /* CONFIG_SOMETHING */
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Appendix I: References
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