Date: Tue, 8 May 2007 11:28:25 -0400 From: John Baldwin <jhb@freebsd.org> To: freebsd-arch@freebsd.org Cc: Giorgos Keramidas <keramida@freebsd.org>, Hans Petter Selasky <hselasky@c2i.net> Subject: Re: Missing LIST_PREV() ? Message-ID: <200705081128.25708.jhb@freebsd.org> In-Reply-To: <20070507202517.GA88340@kobe.laptop> References: <200705051617.34162.hselasky@c2i.net> <20070507202034.GA80846@kobe.laptop> <20070507202517.GA88340@kobe.laptop>
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On Monday 07 May 2007 04:25:18 pm Giorgos Keramidas wrote:
> On 2007-05-07 23:20, Giorgos Keramidas <keramida@freebsd.org> wrote:
> >On 2007-05-05 16:17, Hans Petter Selasky <hselasky@c2i.net> wrote:
> >> Hi,
> >>
> >> Why should LISTs only be forward traversable? The following piece of
> >> code make lists backward traversable:
> >>
> >> /sys/sys/queue.h:
> >>
> >> +#define LIST_PREV(head,elm,field) \
> >> + (((elm) == LIST_FIRST(head)) ? ((__typeof(elm))0) : \
> >> + ((__typeof(elm))(((uint8_t *)((elm)->field.le_prev)) - \
> >> + ((uint8_t *)&LIST_NEXT((__typeof(elm))0,field)))))
> >>
> >> Any comments?
> >
> > 1. The use of (uint8_t *) casts is relatively ugly.
Looks like an ugly version of offsetof()
> > 2. What does LIST_PREV give us that cannot be done with TAILQ_PREV()
> > already?
>
> Even more importantly, which I missed in my original look
>
> (3) The use of the gcc-specific __typeof() extension makes this unusable
> 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.
--
John Baldwin
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