Re: [PATCH v2 09/10] bug.h: Add BUILD_BUG_ON_MSG & BUILD_BUG_INTERNAL{,2}

Steven Rostedt
2012-10-05T21:04:14Z

On Fri, 2012-10-05 at 22:58 +0200, Borislav Petkov wrote:
> On Fri, Oct 05, 2012 at 02:42:48PM -0500, danielfsantos@att.net wrote:
> > Add BUILD_BUG_ON_MSG which behaves like BUILD_BUG_ON (with optimizations
> > turned enabled), except that it allows you to specify the error message
> > you want emitted as the third parameter. Under the hood, this relies on
> > BUILD_BUG_INTERNAL{,2}, which does the actual work and is pretty-much
> > identical to BUILD_BUG_ON.
> >
> > Signed-off-by: Daniel Santos <daniel.santos@pobox.com>
> > ---
> > include/linux/bug.h | 26 ++++++++++++++++++++++++++
> > 1 files changed, 26 insertions(+), 0 deletions(-)
> >
> > diff --git a/include/linux/bug.h b/include/linux/bug.h
> > index 1b43ea2..91bd9d5 100644
> > --- a/include/linux/bug.h
> > +++ b/include/linux/bug.h
> > @@ -16,6 +16,7 @@ struct pt_regs;
> > #define BUILD_BUG_ON_NOT_POWER_OF_2(n)
> > #define BUILD_BUG_ON_ZERO(e) (0)
> > #define BUILD_BUG_ON_NULL(e) ((void*)0)
> > +#define BUILD_BUG_ON_MSG(cond, msg) (0)
> > #define BUILD_BUG_ON(condition) (0)
> > #define BUILD_BUG() (0)
> > #else /* __CHECKER__ */
> > @@ -38,6 +39,31 @@ struct pt_regs;
> > */
> > #define BUILD_BUG_ON_INVALID(e) ((void)(sizeof((__force long)(e))))
> >
> > +#define _CONCAT1(a, b) a##b
> > +#define CONCAT(a, b) _CONCAT1(a, b)
>
> Let's call the indirection _CONCAT without the "1".

You're stricter than I ;-)

>
> > +#define UNIQUIFY(prefix) CONCAT(prefix, __LINE__)
> > +
> > +#define BUILD_BUG_INTERNAL2(condition, msg, fn) \
> > + do { \
> > + extern void fn (void) __compiletime_error(msg); \
> > + __compiletime_error_fallback(condition); \
> > + if (condition) \
> > + fn(); \
> > + } while (0)
> > +
> > +#define BUILD_BUG_INTERNAL(condition, msg, fn) \
> > + BUILD_BUG_INTERNAL2(condition, msg, fn)
>
> Ditto. BUILD_BUG_INTERNAL2 should be __BUILD_BUG_INTERNAL and the one
> calling it _BUILD_BUG_INTERNAL (with one underscore).

I thought about commenting about the '2' too, but figured it's only used
internally by BUILD_BUG_INTERNAL and was really not too concerned about
such a trivial thing. It could have been 42 for all I care ;-)

>
> > +
> > +/**
> > + * BUILD_BUG_ON_MSG - break compile if a condition is true & emit supplied
> > + * error message.
> > + * @condition: the condition which the compiler should know is false.
> > + *
> > + * See BUILD_BUG_ON for description.
> > + */
> > +#define BUILD_BUG_ON_MSG(cond, msg) \
> > + BUILD_BUG_INTERNAL(cond, msg, UNIQUIFY(__build_bug_on_failed_))
>
> Btw, why are we adding the line at all? It is issued by gcc anyway:
>
> cc -Wall macros.c -o macros
> macros.c: In function ‘main’:
> macros.c:22:1: error: ‘__build_bug_on_failed_22’ undeclared (first use in this function)
> ^^^^
>
> It is in front of the filename here.
>
> macros.c:22:1: note: each undeclared identifier is reported only once for each function it appears in
> make: *** [macros] Error 1

I was thinking that the number was added as a safety measure that the
line number would be shown for all versions of the compiler. I'm not
sure it is.

-- Steve