The assign_if() and assign_if_enable() macros are intended to be used in static initializers for function pointers, where the pointer is expected to be NULL when a compile-time condition does not hold.
These macros allow for implementing this behavior, without requiring the functions be wrapped in #ifdef conditionals, and while providing typesafety regardless of the value of the conditional. For example, the following pattern is common: #ifdef CONFIG_FOO static void foo_callback(void) { } #else #define foo_callback NULL #endif static struct foo_object foo_obj = { .callback = foo_callback, }; Usage of assign_if_enabled() allows for achieving the same effect without the preprocessor conditional, and in addition, allowing the compiler to typecheck the function regardless of CONFIG_FOO. static void foo_callback(void) { } static struct foo_object foo_obj = { .callback = assign_if_enabled(CONFIG_FOO, foo_callback), }; Cc: Andrew Morton <a...@linux-foundation.org> Signed-off-by: Josh Cartwright <jo...@codeaurora.org> --- include/linux/typecheck.h | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/include/linux/typecheck.h b/include/linux/typecheck.h index eb5b74a..04134c7 100644 --- a/include/linux/typecheck.h +++ b/include/linux/typecheck.h @@ -21,4 +21,22 @@ (void)__tmp; \ }) +/* + * Intended for use in static object initializers, + * assign_if(const_expr, function) evaluates to 'function' if 'const_expr', + * otherwise NULL. + * + * The type of the assign_if() expression is typeof(function), and therefore + * can provide typechecking regardless of 'const_expr'. + * + * gcc considers 'function' to be used and will not generate a 'defined but not + * used' warning when not 'const_expr', however, gcc is smart enough to + * eliminate 'function' if assign_if() is the only reference. + */ +#define assign_if(const_expr,function) \ + ((const_expr) ? function : NULL) + +#define assign_if_enabled(option,function) \ + assign_if(IS_ENABLED(option), function) + #endif /* TYPECHECK_H_INCLUDED */ -- Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/