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. 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...
>
> Cheers,
> Rusty.
>
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