Re: [PATCH RFC] virtio-pci: new config layout: using memory BAR

[Rusty Russell <rusty@rustcorp.com.au>]

Paolo Bonzini <pbonzini@redhat.com> 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):

    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...

Cheers,
Rusty.