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Date: Mon, 7 May 2007 23:25:18 +0300
From: Giorgos Keramidas <keramida@freebsd.org>
To: Hans Petter Selasky <hselasky@c2i.net>
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Subject: Re: Missing LIST_PREV() ?
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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.
>
> 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.

The entire <sys/queue.h> header is otherwise very portable and I already
use it successfully on other systems too (i.e. Solaris with the Sun
Studio 11 compilers).  Introducing unportable constructs like __typeof()
shouldn't be allowed, IMHO.

- Giorgos