Re: [PATCH 8/10] bug.h: Make BUILD_BUG_ON generate compile-time error

[Josh Triplett <josh@joshtriplett.org>]

On Sun, Sep 30, 2012 at 06:29:01PM -0500, Daniel Santos wrote:
> On 09/28/2012 09:55 PM, Josh Triplett wrote:
> > Assuming you don't call BUILD_BUG_ON_MSG more than once per line:
> >
> > /tmp$ cat test.c
> > #define BUILD_BUG_ON_MSG_INTERNAL2(cond, msg, line) \
> >     do { \
> >         extern void __build_bug_on_failed_ ## line (void) __attribute__((error(msg))); \
> >         if (cond) \
> >             __build_bug_on_failed_ ## line(); \
> >     } while (0)
> >
> > #define BUILD_BUG_ON_MSG_INTERNAL(cond, msg, line) BUILD_BUG_ON_MSG_INTERNAL2(cond, msg, line)
> > #define BUILD_BUG_ON_MSG(cond, msg) BUILD_BUG_ON_MSG_INTERNAL(cond, msg, __LINE__)
> >
> > void f(void)
> > {
> >     BUILD_BUG_ON_MSG(0, "test 1");
> >     BUILD_BUG_ON_MSG(1, "test 2");
> >     BUILD_BUG_ON_MSG(0, "test 3");
> >     BUILD_BUG_ON_MSG(1, "test 4");
> > }
> > /tmp$ gcc -c test.c
> > test.c: In function ‘f’:
> > test.c:14:119: error: call to ‘__build_bug_on_failed_14’ declared with attribute error: test 2
> > test.c:16:119: error: call to ‘__build_bug_on_failed_16’ declared with attribute error: test 4
> Thanks! This is very nice!  I've done a little more research and
> discovered that there's also a __COUNTER__ macro that is available
> in gcc 4.3+.  Before I realized that it was only available in gcc
> 4.3, I wrote this little macro:
> 
> #define _CONCAT1(a, b) a##b
> #define CONCAT(a, b) _CONCAT1(a, b)
> 
> #ifdef __COUNTER__
> # define UNIQUIFY(prefix) CONCAT(prefix, __COUNTER__)
> #else
> # define UNIQUIFY(prefix) CONCAT(prefix, __LINE__)
> #endif
> 
> However, this could lead to code might compile on gcc 4.3+, but
> not compile prior, so this is bad, right?

Yes, as long as Linux continues to support prior GCC versions (currently
anything 3.2 or newer), I'd suggest sticking with __LINE__ so that it
consistently breaks if you use more than one BUILD_BUG_ON_MSG per line.

Good to know that __COUNTER__ exists, though.

Also, the name BUILD_BUG_ON_MSG seems wrong to me somehow; I keep
parsing it as though something about the message causes the build bug.

- Josh Triplett
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