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

[Wenchao Xia]

于 2012-9-12 6:52, Eric Blake 写道:
> On 09/11/2012 02:31 PM, Blue Swirl wrote:
> > On Mon, Sep 10, 2012 at 8:26 AM, Wenchao Xia <address@hidden> wrote:
> > >    This patch contains type and defines used in APIs, one file for public usage
> > > by user, one for libqblock internal usage.
>
> > > +
> > > +#if __GNUC__ >= 4
> >
> > #if defined(__GNUC__) && __GNUC__ ...
>
> Seriously?  We require a C99-compliant compiler, which is required to
> treat the more compact version identically (all undefined names evaluate
> to 0 in the preprocessor), and HACKING doesn't mandate that we spell out
> a defined-ness check first.  Okay, so configure adds -Wundef to the set
> of CFLAGS, but I fail to see why we want -Wundef (that's an
> anachronistic warning, only there to help you if you are writing code
> portable to K&R).
  So if the preprocessor replaced __GNUC__ to 0, is there difference
between these two kinds of macoros?
#if __GNUC__ >= 4
#if defined(__GNUC__) && __GNUC__ >=4

  I guess they are the same.

--
Best Regards

Wenchao Xia