Re: [Qemu-devel] [PATCH 1/9] build: disable Wstrict-prototypes

[Laszlo Ersek]

On 02/20/13 16:44, Kevin Wolf wrote:
> On Wed, Feb 20, 2013 at 04:33:50PM +0100, Laszlo Ersek wrote:

>> (a) #pragma PP_TOKEN1 PP_TOKEN2 ..., where PP_TOKEN1 is not STDC, has
>> implementation-defined effect. (Ie. the platform can do whatever it
>> wants, but has to document it). Non-STDC #pragmas that are not
>> recognized by the implementation must be ignored (C99 6.10.6p1
>> requirement). So wrapping "#pragma GCC diagnostic" is not really useful:
>> non-gcc compilers will surely not recognize "#pragma GCC whatever", and
>> an older gcc (not recognizing "#pragma GCC diagnostic") is required to
>> ignore it.
> 
> gcc produces a warning for unknown pragmas, and -Werror turns it into an
> error, so it's not quite useless.

(To be quashed with -Wno-unknown-pragmas ;))

>> (b) My main point was, if you build on a gcc which understands
>> -Wstrict-prototypes and chokes on gtkitemfactory.h, but doesn't
>> understand the #pragma, then compilation will fail.
>>
>> I think that on such a platform "qemu-pixman.h" / <pixman.h> from
>> pixman-0.16.0 would fail to parse as well.
> 
> I think you're right here. When I originally introduced the first one of
> these, I had to deal with a new warning in 4.6, so this was obviously
> not a problem. But qemu-pixman.h as well as this one can't rely on it.
> 
> Maybe we need to remove -Werror if configure detects that the #pragma
> doesn't work.

Sounds good to me. I assume developers (who should care the most about
warnings) work with recent toolstacks (in RHEL-6.3 for example,
devtoolset-1.1 ships gcc-4.7.2 (+patches)), hence the #pragma would work
for them. For users building qemu on an older toolstack (where the
#pragma would not be recognized and suppress the warning) the warning
should not be fatal.

Clang was also mentioned, I'm not sure about it. I suppose clang wishes
to be UI-compatible with gcc (that's why it supports the -W* options),
so maybe they'll pick up the #pragma too in time?...

Thanks
Laszlo