Messages in this thread | | | Date | Fri, 28 Sep 2012 20:26:08 -0500 | From | Daniel Santos <> | Subject | Re: [PATCH 8/10] bug.h: Make BUILD_BUG_ON generate compile-time error |
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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.
Daniel
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