On 2011-09-28 05:38, Wayne Xia wrote:
> Hi, during my coding, I found macro a bit different from other
> QTAIL macros.
>
> QTAILQ_INSERT_AFTER was defined as:
> -----------------------------------------------------------
> #define QTAILQ_INSERT_AFTER(head, listelm, elm, field) do { \
> if (((elm)->field.tqe_next = (listelm)->field.tqe_next) != NULL)\
> (elm)->field.tqe_next->field.tqe_prev = \
> &(elm)->field.tqe_next; \
> else \
> (head)->tqh_last = &(elm)->field.tqe_next; \
> (listelm)->field.tqe_next = (elm); \
> (elm)->field.tqe_prev = &(listelm)->field.tqe_next; \
> } while (/*CONSTCOND*/0)
> ---------------------------------------------------------
> QTAILQ_INSERT_BEFORE is defined as following:
>
> #define QTAILQ_INSERT_BEFORE(listelm, elm, field) do {
> (elm)->field.tqe_prev = (listelm)->field.tqe_prev; \
> (elm)->field.tqe_next = (listelm); \
> *(listelm)->field.tqe_prev = (elm); \
> (listelm)->field.tqe_prev = &(elm)->field.tqe_next; \
> } while (/*CONSTCOND*/0)
> ----------------------------------------------------------
>
> It did not take care of "head" as QTAILQ_INSERT_AFTER did, so I am
> wondering what would happen if I use QTAILQ_INSERT_BEFORE to insert one
> element to a queue that have only one element in it, would it happen
> that the queue head pointer is not updated and the real first element
> is lost? Currently some codes in qemu have used this macro.
The code is fine: a QTAILQ head consists of a dummy entry element that
looks for the first element as if a normal element would precede it. In
contrast, there is no dummy "end element", the last one just points to
NULL. Therefore we need to handle this separately.
Jan
--
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Corporate Competence Center Embedded Linux
