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From: Hans Petter Selasky <hselasky@c2i.net>
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Date: Tue, 8 May 2007 21:00:51 +0200
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Cc: Julian Elischer <julian@elischer.org>,
	Giorgos Keramidas <keramida@freebsd.org>
Subject: Re: Missing LIST_PREV() ?
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On Tuesday 08 May 2007 20:45, Julian Elischer wrote:
> John Baldwin wrote:
> > On Monday 07 May 2007 04:25:18 pm Giorgos Keramidas wrote:
> >> with other compilers.
> >
> > This can be fixed by passing the type as an argument which is what
> > TAILQ_PREV() does:
> >
> > #define TAILQ_PREV(elm, headname, field)                               =
 \
> >         (*(((struct headname *)((elm)->field.tqe_prev))->tqh_last))
> >
> > I'm not sure how portable offsetof() would be though.  In general if you
> > want this feature, you should just use a TAILQ though.  TAILQ_ENTRY() is
> > the same size as a LIST_ENTRY(), it just adds one more pointer to the
> > HEAD structure. It is also specifically designed to make TAILQ_PREV()
> > work w/o needing the offsetof() hack.
>
> I agree with this.. that's why we have the different types.
> The suggested change in ingenious but I don't know how portable it is..

I suggested the following at hacker's:

#define LIST_PREV(head,elm,field,type) \
=A0(((elm) =3D=3D LIST_FIRST(head)) ? ((struct type *)0) : \
=A0 ((struct type *)(((uint8_t *)((elm)->field.le_prev)) - \
=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0((uint8_t *)&LIST_NEXT((struct type =
*)0,field)))))

What do you think?

=2D-HPS