bug.h: make BUILD_BUG_ON generate compile-time error
RFC PATCH v3 4/6 bug.h: make BUILD_BUG_ON generate compile-time error
Negative sized arrays wont create a compile-time error in some cases starting with gcc 4.4 (e.g., inlined functions), but gcc 4.3 introduced the error function attribute that will. This patch modifies BUILD_BUG_ON to behave like BUILD_BUG already does, using the error function attribute so that you don't have to build the entire kernel to discover that you have a problem, and then enjoy trying to track it down from a link-time error.
This is not only nicer, but it makes it behave consistiently with BUILD_BUG.
Signed-off-by Daniel Santos <[email protected]>
--- include/linux/bug.h | 23 +++++++++++++---------- 1 files changed, 13 insertions(+), 10 deletions(-)
include/linux/bug.h +13 -10
@@ -42,24 +42,27 @@ struct pt_regs;* @condition: the condition which the compiler should know is false. * * If you have some code which relies on certain constants being equal, or -* other compile-time-evaluated condition, you should use BUILD_BUG_ON to +* some other compile-time-evaluated condition, you should use BUILD_BUG_ON to * detect if someone changes it. * * The implementation uses gcc's reluctance to create a negative array, but * gcc (as of 4.4) only emits that error for obvious cases (eg. not arguments -* to inline functions). So as a fallback we use the optimizer; if it can't -* prove the condition is false, it will cause a link error on the undefined -* "__build_bug_on_failed". This error message can be harder to track down -* though, hence the two different methods. +* to inline functions). Luckily, in 4.3 they added the "error" function +* attribute just for this type of case. Thus, we use a negative sized array +* (should always create an error pre-gcc-4.4) and then call an undefined +* function with the error attribute (should always creates an error 4.3+). If +* for some reason, neither creates a compile-time error, we'll still have a +* link-time error, which is harder to track down. */ #ifndef __OPTIMIZE__#define BUILD_BUG_ON (condition) ((void)sizeof(char[1 - 2*!!(condition)]))#else -extern int __build_bug_on_failed; -#define BUILD_BUG_ON (condition) \ - do { \ - ((void)sizeof(char[1 - 2*!!(condition)])); \ - if (condition) __build_bug_on_failed = 1; \ +#define BUILD_BUG_ON (condition) \ + do { \ + extern void __build_bug_on_failed(void) \ + __compiletime_error("BUILD_BUG_ON failed"); \ + ((void)sizeof(char[1 - 2*!!(condition)])); \ + if (condition) __build_bug_on_failed(); \ } while(0)#endif -- 1.7.3.4