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

Josh Triplett
2012-09-29T02:55:44Z

On Fri, Sep 28, 2012 at 08:26:08PM -0500, Daniel Santos wrote:
> On 09/28/2012 07:32 PM, Josh Triplett wrote:
> > On Fri, Sep 28, 2012 at 06:20:09PM -0500, Daniel Santos wrote:
> >> Negative sized arrays wont create a compile-time error in some cases
> >> starting with gcc 4.4 (e.g., inlined functions), but gcc 4.3 introduced
> >> the error function attribute that will. This patch modifies
> >> BUILD_BUG_ON to behave like BUILD_BUG already does, using the error
> >> function attribute so that you don't have to build the entire kernel to
> >> discover that you have a problem, and then enjoy trying to track it down
> >> from a link-time error.
> >
> > Rather than doing both, and potentially producing two errors for the
> > same issue, how about using __compiletime_error only, and only using the
> > negative-sized array when __compiletime_error has no useful definition?
> >
> > For instance, in compiler.h, when defining __compiletime_error as an
> > empty macro in the fallback case, you could define a
> > __compiletime_error_fallback() macro that declares a negative-sized
> > array; you could then define __compiletime_error_fallback() as an empty
> > macro when it doesn't exist.
> It may also make sense to define
> #define BUILD_BUG() BUILD_BUG_ON(1)
>
> I haven't examined this really closely yet and my eyes are getting a
> little bleary :)
>
>
> Really, I would have liked to be able to set the error message as
> well, but we would have to have the macro generate a unique function
> name for each time its expanded to make that work. Example:
>
> #define BUILD_BUG_ON(cond) BUILD_BUG_ON_MSG(cond, #cond)
> #define BUILD_BUG_ON_MSG(cond, msg) \
> do { \
> extern void __build_bug_on_failed ## something_unique(void)\
> __compiletime_error("BUILD_BUG_ON failed: " msg); \
> __compiletime_error_fallback(cond); \
> if (cond) \
> __build_bug_on_failed(); \
> } while(0)
>
> I just don't know any tricks to generate unique pre-processor
> value automatically.

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

- Josh Triplett