linux-kernel - Re: [PATCH] debug: fix BUILD_BUG_ON() for non-constant expressions

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Message-Id: <200808181109.43203.rusty@rustcorp.com.au>
Date:	Mon, 18 Aug 2008 11:09:42 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Sam Ravnborg <sam@...nborg.org>
Subject: Re: [PATCH] debug: fix BUILD_BUG_ON() for non-constant expressions

On Monday 18 August 2008 03:33:19 Ingo Molnar wrote:
> * Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> > Gag me now.
> >
> > Why not just do
> >
> >  #define __BBO(c)                sizeof(const char[1 - 2*!!(c)])
> >  #define __BBONC(c)              __BBO(!__builtin_constant_p(c))
> >  #define BUILD_BUG_ON_ZERO(c)    (__BBO(c) - __BBONC(c))
> >  #define BUILD_BUG_ON(c)         (void)BUILD_BUG_ON_ZERO(c)
> >
> > and be done with it?
>
> yeah, i first tried a few variants of that (compile-time warnings are
> much better than link time warnings), but none worked when i tested
> various failure modes.

Hey, I thought I was the "undisputed ruler of Ugly-land".

How about this instead:

#define BUILD_BUG_ON(condition)						\
do {									\
	static struct { char arr[1 - 2*!!(condition)]; } x __maybe_unused;	\
} while(0)

Cheers,
Rusty.
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