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

Daniel Santos
2012-10-02T14:57:56Z

On 10/01/2012 07:48 PM, Michel Lespinasse wrote:
> On Fri, Sep 28, 2012 at 4:20 PM, Daniel Santos <daniel.santos@pobox.com> 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.
> Few other alternatives I've seen used in other projects (from memory,
> so I may have gotten the details wrong):
>
> 1) if (condition) { __asm__(".error \"Some error message\""); }
> 2) switch (0) {
> case 0: break;
> case !condition: break;
> }
> (fails to compile if !condition evaluates to 0)
>
> If you can get the first suggestion to work it'd be nice, as you could
> get some descriptive error message (you can add the line number there
> too).
Thanks for this info. I'm still pretty noob-ish in modern assembly, but
is the .error directive available on all supported assemblers? It
appears to have been added to GNU binutils in version 2.16 from what I
can tell (http://sourceware.org/binutils/docs-2.16/as/Error.html). If
it is supported, I would definitely prefer that one as a fallback option.

I did a search on sources for the regex \.error\s+\\?" and didn't turn
up anything outside of arch/, so I'm guessing this may not be completely
portable. Note also that these BUILD_BUG* macros do emit an error
message on gcc 4.3+ using __attribute__((error("msg"))).

Daniel