RT-Thread-_timer_start()函数功能虽然正常，但实现是否未达预期RT-Thread问答社区

_timer_start()函数功能虽然正常，但实现是否未达预期

发布于 2024-12-25 17:29:26 浏览：119 订阅该版

有疑问代码如下：
```c
static rt_err_t _timer_start(rt_list_t *timer_list, rt_timer_t timer)
{
    unsigned int row_lvl;
    rt_list_t *row_head[RT_TIMER_SKIP_LIST_LEVEL];
    unsigned int tst_nr;
    static unsigned int random_nr;

/* remove timer from list */
    _timer_remove(timer);
    /* change status of timer */
    timer->parent.flag &= ~RT_TIMER_FLAG_ACTIVATED;

RT_OBJECT_HOOK_CALL(rt_object_take_hook, (&(timer->parent)));

timer->timeout_tick = rt_tick_get() + timer->init_tick;

row_head[0]  = &timer_list[0];
    for (row_lvl = 0; row_lvl < RT_TIMER_SKIP_LIST_LEVEL; row_lvl++)
    {
        for (; row_head[row_lvl] != timer_list[row_lvl].prev;
             row_head[row_lvl]  = row_head[row_lvl]->next)
        {
            struct rt_timer *t;
            rt_list_t *p = row_head[row_lvl]->next;

/* fix up the entry pointer */
            t = rt_list_entry(p, struct rt_timer, row[row_lvl]);

/* If we have two timers that timeout at the same time, it's
             * preferred that the timer inserted early get called early.
             * So insert the new timer to the end the the some-timeout timer
             * list.
             */
            if ((t->timeout_tick - timer->timeout_tick) == 0)
            {
                continue;
            }
            else if ((t->timeout_tick - timer->timeout_tick) < RT_TICK_MAX / 2)
            {
                break;
            }
        }
        if (row_lvl != RT_TIMER_SKIP_LIST_LEVEL - 1)
            row_head[row_lvl + 1] = row_head[row_lvl] + 1;
    }

/* Interestingly, this super simple timer insert counter works very very
     * well on distributing the list height uniformly. By means of "very very
     * well", I mean it beats the randomness of timer->timeout_tick very easily
     * (actually, the timeout_tick is not random and easy to be attacked). */
    random_nr++;
    tst_nr = random_nr;

rt_list_insert_after(row_head[RT_TIMER_SKIP_LIST_LEVEL - 1],
                         &(timer->row[RT_TIMER_SKIP_LIST_LEVEL - 1]));
    for (row_lvl = 2; row_lvl <= RT_TIMER_SKIP_LIST_LEVEL; row_lvl++)
    {
        if (!(tst_nr & RT_TIMER_SKIP_LIST_MASK))
            rt_list_insert_after(row_head[RT_TIMER_SKIP_LIST_LEVEL - row_lvl],
                                 &(timer->row[RT_TIMER_SKIP_LIST_LEVEL - row_lvl]));
        else
            break;
        /* Shift over the bits we have tested. Works well with 1 bit and 2
         * bits. */
        tst_nr >>= (RT_TIMER_SKIP_LIST_MASK + 1) >> 1;
    }

timer->parent.flag |= RT_TIMER_FLAG_ACTIVATED;

return RT_EOK;
}
```
虽然功能都正常，但在双层循环中查找新节点插入节点时，代码中没有利用跳表的优势呀。如果将跳表设置为多层的话，里层的循环（低层的比较）未给外层循环（高层的比较）留线索呀。高层的循环每次必须从头开始比较，如果这样的话还不如直接就用单链表实现更简洁一些。