On Saturday, 12 January 2019 01:49:09 CET Jonathan Gray wrote:
> On Fri, Jan 11, 2019 at 03:21:11PM -0800, William Ahern wrote:
> > On Fri, Jan 11, 2019 at 10:43:25AM +0100, Stefan Fritsch wrote:
> > <snip>
> >
> > > /* only used for sizeof, not actually allocated */
> > > extern struct virtio_pci_common_cfg ccfg;
> >
> > <snip>
> >
> > > #define CREAD(sc, memb) _cread(sc, \
> > >
> > > offsetof(struct virtio_pci_common_cfg, memb), sizeof(ccfg.memb))
> > >
> > > The compiler should optimize this to the same code as the complicated
> > > macro above. You think this variant is acceptable?
> >
> > Maybe I'm missing something, but these are the more idiomatic constructs
> >
> > sizeof ((struct virtio_pci_common_cfg *)0)->memb
> > sizeof ((struct virtio_pci_common_cfg){ 0 }).memb
>
> No, expanding the offsetof macro and avoiding __builtin_offsetof misses
> the point of it.
It allows to get rid of the "extern struct virtio_pci_common_cfg ccfg;"
though. That's an improvement.
Stefan
