Messages in this thread | | | From | Rusty Russell <> | Subject | Re: [PATCH] fix BUILD_BUG_ON() and a couple of bogus uses of it | Date | Wed, 23 Sep 2009 10:27:00 +0930 |
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On Wed, 19 Aug 2009 01:29:25 am Jan Beulich wrote: > gcc permitting variable length arrays makes the current construct > used for BUILD_BUG_ON() useless, as that doesn't produce any diagnostic > if the controlling expression isn't really constant. Instead, this > patch makes it so that a bit field gets used here. Consequently, those > uses where the condition isn't really constant now also need fixing. > > Note that in the gfp.h, kmemcheck.h, and virtio_config.h cases > MAYBE_BUILD_BUG_ON() really just serves documentation purposes - even > if the expression is compile time constant (__builtin_constant_p() > yields true), the array is still deemed of variable length by gcc, and > hence the whole expression doesn't have the intended effect. > > Signed-off-by: Jan Beulich <jbeulich@novell.com>
We used to use an undefined symbol here; diagnostics are worse but it catches more stuff.
Perhaps a hybrid is the way to go?
#ifndef __OPTIMIZE__ #define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)])) #else /* If it's a constant, catch it at compile time, otherwise at link time. */ 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; \ } while(0) #endif
Thanks, Rusty.
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