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authorJonathan Corbet <corbet@lwn.net>2014-11-24 10:50:40 -0700
committerJonathan Corbet <corbet@lwn.net>2014-11-24 10:50:40 -0700
commit86d3e023e05d90b2b5f88dcbf2e334b5835131f8 (patch)
tree7695c67ca0ae65bbe04f7b6d7f244ab395f8dabc /Documentation/CodingStyle
parent690b0543a813b0ecfc51b0374c0ce6c8275435f0 (diff)
parent3c415707b37f1e4483c418c77f57692b89bcfd5e (diff)
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@@ -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