>>> Hollis Blanchard <hollisb@xxxxxxxxxx> 02.10.09 17:48 >>>
>On Wed, 2009-09-30 at 07:35 +0100, Jan Beulich wrote:
>> The one Rusty suggested the other day may help here. I don't like it
>> as a drop-in replacement for BUILD_BUG_ON() though (due to it
>> deferring the error generated to the linking stage), I'd rather view
>> this as an improvement to MAYBE_BUILD_BUG_ON() (which should
>> then be used here).
>
>Can you be more specific?
>
>I have no idea what Rusty suggested where. I can't even guess what
I'm attaching Rusty's response I was referring to.
>MAYBE_BUILD_BUG_ON() is supposed to do (sounds like a terrible name).
Agreed - but presumably better than just deleting the bogus instances
altogether...
Jan
--- Begin Message ---
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@xxxxxxxxxx>
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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