Re: [PATCH v2] Suppress -Wcast-qual warnings in bsearch

[Joseph Myers <joseph@codesourcery.com>]

On Thu, 30 Sep 2021, Adhemerval Zanella via Libc-alpha wrote:

> On 30/09/2021 12:07, Joseph Myers wrote:
> > On Thu, 30 Sep 2021, Adhemerval Zanella via Libc-alpha wrote:
> > 
> >>>>        else
> >>>> +#if __GNUC_PREREQ(4, 6)
> >>>> +# pragma GCC diagnostic push
> >>>> +# pragma GCC diagnostic ignored "-Wcast-qual"
> >>>> +#endif
> >>>>  	return (void *) __p;
> >>>> +#if __GNUC_PREREQ(4, 6)
> >>>> +# pragma GCC diagnostic pop
> >>>> +#endif
> >>>>      }
> > 
> > I think braces may need adding around those pragmas to avoid a pragma 
> > being considered the body of the else in some cases.
> 
> But how exactly the pragma is changing the code semantic in this case?

The effect, when the #pragma is treated as a statement by the C parser, is 
that the prior cases of the "if" fall through to the "return", which 
becomes unconditional, when otherwise they would not have returned.

> Would it be safe for all supported gcc (since it is an installed header)?

The #if conditionals are still needed.  It's just that braces should be 
added before the first #if and after the last #endif, to avoid a single 
#pragma being considered as the else body (see GCC bug 41517 and other 
bugs related to #pragma parsing).

-- 
Joseph S. Myers
joseph@codesourcery.com