On Wed, May 29, 2013 at 09:27:02AM +0200, Paolo Bonzini wrote:
> Il 29/05/2013 06:33, Rusty Russell ha scritto:
> > Paolo Bonzini <pbonzini@xxxxxxxxxx> writes:
> >> Il 28/05/2013 19:32, Michael S. Tsirkin ha scritto:
> >>>>>>> +
> >>>>>>> + switch (addr) {
> >>>>>>> + case offsetof(struct virtio_pci_common_cfg, device_feature_select):
> >>>>>>> + return proxy->device_feature_select;
> >>>>>
> >>>>> Oh dear no... Please use defines like the rest of QEMU.
> >>> Any good reason not to use offsetof?
> >>
> >> I'm not sure it's portable to use it in case labels. IIRC, the
> >> definition ((int)&(((T *)0)->field)) is not a valid C integer constant
> >> expression. Laszlo?
> >
> > It's defined to yield an integer constant expression in the ISO standard
> > (and I think ANSI too, though that's not at hand):
>
> It's not in C89. The oldest compiler QEMU cares about is GCC 4.2. I
> don't know if it has a builtin offsetof, probably it does.
Yes, I think __builtin_offsetof was added in 4.0:
http://gcc.gnu.org/onlinedocs/gcc-4.0.0/gcc/Offsetof.html
> But I'm not sure about other users of virtio headers.
>
> Paolo
> >
> > 7.19, para 3:
> >
> > ...offsetof(type, member-designator)
> > which expands to an integer constant expression that has type
> > size_t, ...
> >
> > The real question is whether compilers qemu cares about meet the
> > standard (there's some evidence that older compilers fail this). If
> > not, we'll have to define them as raw offsets...
So I think the question is whether we care about GCC 3. We used to when
mingw in debian stable was GCC 3, but not anymore ...
> >
> > Cheers,
> > Rusty.
> >
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