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Date: Tue, 08 May 2007 12:48:20 -0700
From: Julian Elischer <julian@elischer.org>
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To: Hans Petter Selasky <hselasky@c2i.net>
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Cc: Giorgos Keramidas <keramida@freebsd.org>, freebsd-arch@freebsd.org
Subject: Re: Missing LIST_PREV() ?
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Hans Petter Selasky wrote:
> 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) \
>  (((elm) == LIST_FIRST(head)) ? ((struct type *)0) : \
>   ((struct type *)(((uint8_t *)((elm)->field.le_prev)) - \
>                    ((uint8_t *)&LIST_NEXT((struct type *)0,field)))))
> 
> What do you think?
> 
> --HPS
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I think I'd rather use offsetof() with a #ifdef offsetof around it.
and a comment saying that if you are using this you probably should 
be using a TAILQ.

The question is "should we, just because we can?"