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Date: Tue, 08 May 2007 14:51:42 -0700
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To: Kostik Belousov <kostikbel@gmail.com>
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Cc: freebsd-arch@freebsd.org, Giorgos Keramidas <keramida@freebsd.org>,
	Hans Petter Selasky <hselasky@c2i.net>
Subject: Re: Missing LIST_PREV() ?
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Kostik Belousov wrote:
> On Tue, May 08, 2007 at 11:28:25AM -0400, John Baldwin wrote:
>> 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()
>>
> ...
>> I'm not sure how portable offsetof() would be though.  In general if you want 
> 
> offsetof() itself is defined by C standard to be present both in
> freestanding and hosted environment (and be available by stddef.h).


You learn something every day! interesting (but still scary)..