Re: [Qemu-devel] [Qemu-ppc] [QEMU PATCH v6 2/2] migration: migrate QTAILQ

[Paolo Bonzini <pbonzini@redhat.com>]

> > Another possibility is a macro like
> > 
> > #define field_at_offset(base, offset, type) \
> >    ((type) (((char *) (base)) + (offset)))
> > 
> > so that you can do
> > 
> >    *field_at_offset(void **, elm, (entry) + QTAILQ_NEXT_OFFSET) = NULL;
> >    *field_at_offset(void ***, elm, (entry) + QTAILQ_PREV_OFFSET) =
> >        *field_at_offset(void ***, head, QTAILQ_LAST_OFFSET);
> >    **field_at_offset(void ***, head, QTAILQ_LAST_OFFSET) = (elm);
> >    *field_at_offset(void ***, head, QTAILQ_LAST_OFFSET) =
> >         field_at_offset(void **, elm, (entry) + QTAILQ_NEXT_OFFSET);
> 
> The thing is that we don't know type at all. So only the offset values
> is used. This is intended for QTAILQ of any type.

Sure, my code is identical to yours---just with more macros for readability,
and a little more type-safe.  Another possibility:

#define QTAILQ_RAW_NEXT(elm, entry) \
   (*field_at_offset(void **, elm, (entry) + QTAILQ_NEXT_OFFSET))
#define QTAILQ_RAW_PREV(elm, entry) \
   (*field_at_offset(void ***, elm, (entry) + QTAILQ_PREV_OFFSET))
#define QTAILQ_RAW_LAST(head) \
   (*field_at_offset(void ***, head, QTAILQ_LAST_OFFSET))

    QTAILQ_RAW_NEXT(elm, entry) = NULL;
    QTAILQ_RAW_PREV(elm, entry) = QTAILQ_RAW_LAST(head);
    *QTAILQ_RAW_LAST(head) = (elm);
    QTAILQ_RAW_LAST(head) = &QTAILQ_RAW_NEXT(elm, entry);

Thanks,

Paolo

> Thanks,
> Jianjun
> 
> > 
> > or something like that (note that I've always used the same type for
> > next and last, by the way).
> > 
> > Paolo
> > 
> 
>