mailbox接收函数对于超时时间的处理疑问

发布于 2020-08-24 15:01:31 浏览：1178 订阅该版

在`rt_mb_recv`函数中为什么要通过一个while循环来判断是否有邮件到达，按说`rt_schedule`返回之后，如果是因为超时或者邮箱被删除或脱离，就会直接返回错误了；否则就是收到了邮件，`rt_schedule`才会返回。如果是这样，为什么最后还要判断timeout是否大于0呢？为什么不是与semaphore或者mutex等相同的处理方式？  
```c
/* src/ipc.c */

rt_err_t rt_mb_recv(rt_mailbox_t mb, rt_ubase_t *value, rt_int32_t timeout)
{
    ...
    /* mailbox is empty */
    while (mb->entry == 0)
    {
        /* reset error number in thread */
        thread->error = RT_EOK;

/* no waiting, return timeout */
        if (timeout == 0)
        {
            /* enable interrupt */
            rt_hw_interrupt_enable(temp);

thread->error = -RT_ETIMEOUT;

return -RT_ETIMEOUT;
        }

RT_DEBUG_IN_THREAD_CONTEXT;
        /* suspend current thread */
        rt_ipc_list_suspend(&(mb->parent.suspend_thread),
                            thread,
                            mb->parent.parent.flag);

/* has waiting time, start thread timer */
        if (timeout > 0)
        {
            /* get the start tick of timer */
            tick_delta = rt_tick_get();

RT_DEBUG_LOG(RT_DEBUG_IPC, ("mb_recv: start timer of thread:%s\n",
                                        thread->name));

/* reset the timeout of thread timer and start it */
            rt_timer_control(&(thread->thread_timer),
                             RT_TIMER_CTRL_SET_TIME,
                             &timeout);
            rt_timer_start(&(thread->thread_timer));
        }

/* enable interrupt */
        rt_hw_interrupt_enable(temp);

/* re-schedule */
        rt_schedule();

/* resume from suspend state */
        if (thread->error != RT_EOK)
        {
            /* return error */
            return thread->error;
        }

/* disable interrupt */
        temp = rt_hw_interrupt_disable();

/* if it's not waiting forever and then re-calculate timeout tick */
        if (timeout > 0)
        {
            tick_delta = rt_tick_get() - tick_delta;
            timeout -= tick_delta;
            if (timeout < 0)
                timeout = 0;
        }
    }
    ...
}
```