Re: [Qemu-devel] [PATCH V2 2/6] libqblock type and structure defines

[Wenchao Xia <xiawenc@linux.vnet.ibm.com>]

于 2012-9-15 2:11, Blue Swirl 写道:
> On Thu, Sep 13, 2012 at 3:49 AM, Eric Blake <eblake@redhat.com> wrote:
>> On 09/12/2012 09:33 PM, Eric Blake wrote:
>>>>   OK ,then I think
>>>> #if __GNUC__ >= 4
>>>> ....
>>>> #else
>>>>    [warn name space pollution may happen]
>>>> #endif
>>>> would be better.
>>>
>>> It may be shorter, but it is definitely not better, at least not in the
>>> current context of qemu.  Using the short form will fail a -Werror
>>> build, unless you also write a patch to qemu's configure to quit
>>> supplying -Wundef during builds.  But as touching configure has a bigger
>>> impact to the overall qemu project, you're going to need a lot more
>>> buy-in from other developers that -Wundef is not helping qemu gain any
>>> portability, and that it is safe to ditch it (or get enough
>>> counter-arguments from other developers why qemu insists on the
>>> anachronistic style enforced by -Wundef, at which point you must comply
>>> and use the longer form).
>>
>> On second thought, this _particular_ usage will never fail a -Wundef
>> -Werror build, precisely because -Wundef is a gcc warning, which impies
>> the warning is only ever useful in the same scenarios that the __GNUC__
>> macro is always defined (that is, __GNUC__ is undefined only on a
>> non-gcc compiler, but what non-gcc compiler supports -Wundef -Werror?).
>
> The library could be used by a project that does not use GCC or pick
> CFLAGS from QEMU configuration. Supporting for example MSVC or C++
> users for the library could be interesting one day, even if we didn't
> support MSVC or C++ at all for building the rest of QEMU.
>
   Each compiler would have its own predefined macro, so I think now
I can just support gcc and give a warning when gcc not found. If
more compiler is needed, extend the macro in the future.

>>
>> But why should this line be the one exemption to the rules?  Either qemu
>> insists on the -Wundef style of coding (and you should use the long form
>> to conform to that style, on the off-chance that someone ever wants to
>> port to a non-gcc compiler, even in this one place where gcc can't warn
>> you about the violation of that style), or we should change the qemu
>> style (at which point, the short form is nicer here, but it also implies
>> the potential for cleaning up lots of other places to also use short
>> forms and rely on preprocessor 0 computation).
>>
>> --
>> Eric Blake   eblake@redhat.com    +1-919-301-3266
>> Libvirt virtualization library http://libvirt.org
>>
>


-- 
Best Regards

Wenchao Xia