30 下列放出的消息队列接收消息函数中，后面有备注步骤几的链表是怎么连接的？

发布于 2024-12-17 16:35:23 浏览：55 订阅该版

```c
rt_err_t rt_mq_recv(rt_mq_t    mq,
                    void      *buffer,
                    rt_size_t  size,
                    rt_int32_t timeout)
{
    struct rt_thread *thread;
    register rt_ubase_t temp;
    struct rt_mq_message *msg;
    rt_uint32_t tick_delta;

RT_ASSERT(mq != RT_NULL);
    RT_ASSERT(buffer != RT_NULL);
    RT_ASSERT(size != 0);

/* initialize delta tick */
    tick_delta = 0;
    /* get current thread */
    thread = rt_thread_self();
    RT_OBJECT_HOOK_CALL(rt_object_trytake_hook, (&(mq->parent.parent)));

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

/* for non-blocking call */
    if (mq->entry == 0 && timeout == 0)
    {
        rt_hw_interrupt_enable(temp);

return -RT_ETIMEOUT;
    }

/* message queue is empty */
    while (mq->entry == 0)
    {
        RT_DEBUG_IN_THREAD_CONTEXT;

/* 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;
        }

/* suspend current thread */
        rt_ipc_list_suspend(&(mq->parent.suspend_thread),
                            thread,
                            mq->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, ("set thread:%s to timer list\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();

/* recv message */
        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;
        }
    }

/* get message from queue */
    msg = (struct rt_mq_message *)mq->msg_queue_head;   // 步骤 1

/* move message queue head */
    mq->msg_queue_head = msg->next;   // 步骤 2
    /* reach queue tail, set to NULL */
    if (mq->msg_queue_tail == msg)
        mq->msg_queue_tail = RT_NULL;

/* decrease message entry */
    mq->entry --;

/* enable interrupt */
    rt_hw_interrupt_enable(temp);

/* copy message */
    rt_memcpy(buffer, msg + 1, size > mq->msg_size ? mq->msg_size : size);

/* disable interrupt */
    temp = rt_hw_interrupt_disable();
    /* put message to free list */
    msg->next = (struct rt_mq_message *)mq->msg_queue_free;  // 步骤 3
    mq->msg_queue_free = msg;   // 步骤 4
    /* enable interrupt */
    rt_hw_interrupt_enable(temp);

RT_OBJECT_HOOK_CALL(rt_object_take_hook, (&(mq->parent.parent)));

return RT_EOK;
}
```

- 我的理解是： 消息队列中有两个链表，一个一个空闲链表，一个消息链表
- mq->msg_queue_head 代表消息链表的表头
- mq->msg_queue_tail 代表消息链表的表尾

- 步骤1的时候：msg得到消息链表的表头
- 步骤2的时候：刷新消息链表的表头，因为原本的表头中的消息被取走了。
- 步骤3的时候：`msg->next`指向当前空闲链表的头部。   这步是为什么？ `msg->next`不是应该还存在消息吗，不是应该这样吗   `msg=(struct rt_mq_message *)mq->msg_queue_free;`  因为msg的消息给搬走了
- 步骤4的时候：  请问这步 做了什么？