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

[Blue Swirl]

On Mon, Sep 17, 2012 at 2:23 AM, Wenchao Xia <address@hidden> wrote:
> 于 2012-9-15 2:11, Blue Swirl 写道:
>
>> On Thu, Sep 13, 2012 at 3:49 AM, Eric Blake <address@hidden> 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.

Yes, that's OK for now. It's also possible that any interest for
non-GCC compilation or C++ will never materialize. But please support
-Wundef.

>
>
>>>
>>> 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   address@hidden    +1-919-301-3266
>>> Libvirt virtualization library http://libvirt.org
>>>
>>
>
>
> --
> Best Regards
>
> Wenchao Xia
>