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From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [Qemu-ppc] [QEMU PATCH v6 2/2] migration: migrate
 QTAILQ
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To: Jianjun Duan <duanj@linux.vnet.ibm.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>, qemu-devel@nongnu.org, qemu-ppc@nongnu.org, dmitry@daynix.com, peter maydell <peter.maydell@linaro.org>, kraxel@redhat.com, mst@redhat.com, david@gibson.dropbear.id.au, veroniabahaa@gmail.com, quintela@redhat.com, amit shah <amit.shah@redhat.com>, mreitz@redhat.com, kwolf@redhat.com, rth@twiddle.net, aurelien@aurel32.net, leon alrae <leon.alrae@imgtec.com>, blauwirbel@gmail.com, mark cave-ayland <mark.cave-ayland@ilande.co.uk>, mdroth@linux.vnet.ibm.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
> > 
> 
>