Re: [PATCH] fix BUILD_BUG_ON() and a couple of bogus uses of it

Rusty Russell
2009-09-23T00:57:01Z

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.