Re: Missing LIST_PREV() ?



On 2007-05-07 23:20, Giorgos Keramidas <keramida@xxxxxxxxxxx> wrote:
On 2007-05-05 16:17, Hans Petter Selasky <hselasky@xxxxxxx> 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

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