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

[Wenchao Xia]

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.

> > 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