Date: Tue, 06 Feb 2001 10:27:52 -0800 From: Peter Wemm <peter@netplex.com.au> To: Poul-Henning Kamp <phk@critter.freebsd.dk> Cc: Garrett Wollman <wollman@khavrinen.lcs.mit.edu>, cvs-committers@FreeBSD.org, cvs-all@FreeBSD.org Subject: Re: cvs commit: src/sys/dev/awi awi.c src/sys/dev/ed if_ed.c src/sys/dev/fe if_fe.c src/sys/dev/ie if_ie.c src/sys/dev/lnc if_lnc.c src/sys/dev/pdq pdq_ifsubr.c src/sys/dev/ray if_ray.c src/sys/dev/sn if_sn.c src/sys/dev/snc dp83932.c src/sys/dev/usb ... Message-ID: <200102061827.f16IRqt49819@mobile.wemm.org> In-Reply-To: <35662.981479283@critter>
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Poul-Henning Kamp wrote:
> In message <200102061704.f16H4ct48889@mobile.wemm.org>, Peter Wemm writes:
> >Garrett Wollman wrote:
> >> <<On Tue, 06 Feb 2001 08:41:00 -0800, Peter Wemm <peter@netplex.com.au> sa
id:
> >>
> >> > I thought LIST was doubly linked, which means we should be able to have
a
> >> > LIST_PREV(), right? SLIST is singly linked as is STAILQ.
> >>
> >> There is a LIST_PREV macro, but in order to traverse the entire list
> >> backwards, one must first get to the very end, which requires a loop
> >> in LIST and a single dereference in TAILQ.
> >
> >Do you know where we can get one? For completeness we should have it and it
> >is missing from our queue.h. It could probably be a simple enough variant
> >of the recently added TAILQ_PREV() macro - which still gives me a headache
> >trying to understand. :-)
>
> If you want to implement it, you have to put a second pointer in the
> LIST_HEAD, and then you might as well make that pointer point to the last
> element at which point a global s/LIST/TAILQ/g makes more sense.
No, you dont. I've figured it out.
> The TAILQ_PREV is nasty, but as far as I can tell safe.
TAILQ_PREV is only nasty because of a HORRIBLE hack. There is no structure
for the tailq entry nodes, so it abuses the head structure definition.
TAILQ_PREV can be quite easily be fixed to not need to do this abuse if
we put a differient hack in another place. LIST_PREV can do the same thing
without needing a head structure.. But it will need to know what the head
of the list is so that it can detect when it hits it.
Cheers,
-Peter
--
Peter Wemm - peter@FreeBSD.org; peter@yahoo-inc.com; peter@netplex.com.au
"All of this is for nothing if we don't go to the stars" - JMS/B5
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