RT-Thread内核源码分析-消息队列实现原理

目录

概念

消息队列

消息队列初始化

消息队列-发送消息

消息队列-读取消息

总结

---

概念

消息队列本质上是存放消息的链接表 ,存放在内核中，内核通过维护这个链表来维护消息队列。 消息队列的初始化就相当于创建一个空闲的链表， 能够存放一定数量的消息；向消息队列发送消息，就是向这个链表中插入一个新节点；从消息队列中都数据，实际就是从链表中删除一个节点。

消息队列

消息队列结构体如下：

struct rt_messagequeue
{
    struct rt_ipc_object parent;                        /**< inherit from ipc_object */

    void                *msg_pool;                      /**< start address of message queue */

    rt_uint16_t          msg_size;                      /**< message size of each message */
    rt_uint16_t          max_msgs;                      /**< max number of messages */

    rt_uint16_t          entry;                         /**< index of messages in the queue */

    void                *msg_queue_head;                /**< list head */
    void                *msg_queue_tail;                /**< list tail */
    void                *msg_queue_free;                /**< pointer indicated the free node of queue */
};
typedef struct rt_messagequeue *rt_mq_t;

可以看出，RT-Thread的消息队列是采用静态分配的， 即在初始化时就分配一大块空间，然后按照消息的大小划分为一个个的小块，将这些小块连接成链表以方便管理。

另外，消息队列也没有所有者的概念，意味着任何线程都可以随意的向消息队列中发送或读取消息(mutex的实现就存在所有者的概念)， 消息队列可以传递较长的数据，相比于消息邮箱，消息邮箱只能传递一个四字节的数据。

消息队列初始化

rt_err_t rt_mq_init(rt_mq_t     mq,
                    const char *name,
                    void       *msgpool,
                    rt_size_t   msg_size,
                    rt_size_t   pool_size,
                    rt_uint8_t  flag)
{
    struct rt_mq_message *head;
    register rt_base_t temp;

    /* parameter check */
    RT_ASSERT(mq != RT_NULL);

    /* init object */
    rt_object_init(&(mq->parent.parent), RT_Object_Class_MessageQueue, name);

    /* set parent flag */
    mq->parent.parent.flag = flag;

    /* init ipc object */
    rt_ipc_object_init(&(mq->parent));

    /* set messasge pool */
    mq->msg_pool = msgpool;

    /* get correct message size */
    mq->msg_size = RT_ALIGN(msg_size, RT_ALIGN_SIZE);
    mq->max_msgs = pool_size / (mq->msg_size + sizeof(struct rt_mq_message));

    /* init message list */
    mq->msg_queue_head = RT_NULL;
    mq->msg_queue_tail = RT_NULL;

    /* init message empty list */
    mq->msg_queue_free = RT_NULL;
    for (temp = 0; temp < mq->max_msgs; temp ++)
    {
        head = (struct rt_mq_message *)((rt_uint8_t *)mq->msg_pool +
                                        temp * (mq->msg_size + sizeof(struct rt_mq_message)));
        head->next = mq->msg_queue_free;
        mq->msg_queue_free = head;
    }//for循环将整块内存空间msgpool划分为一个个小块，作为链表的元素给连接起来

    /* the initial entry is zero */
    mq->entry = 0;

    return RT_EOK;
}

消息队列-发送消息

rt_err_t rt_mq_send(rt_mq_t mq, void *buffer, rt_size_t size)
{
    register rt_ubase_t temp;
    struct rt_mq_message *msg;

    /* parameter check */
    RT_ASSERT(mq != RT_NULL);
    RT_ASSERT(rt_object_get_type(&mq->parent.parent) == RT_Object_Class_MessageQueue);
    RT_ASSERT(buffer != RT_NULL);
    RT_ASSERT(size != 0);

    /* greater than one message size */
    if (size > mq->msg_size)
        return -RT_ERROR;

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

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

    /* get a free list, there must be an empty item */
    msg = (struct rt_mq_message *)mq->msg_queue_free;//获取第一个空闲的消息空间
    /* message queue is full */
    if (msg == RT_NULL)
    {
        /* enable interrupt */
        rt_hw_interrupt_enable(temp);

        return -RT_EFULL;
    }
    /* move free list pointer */
    mq->msg_queue_free = msg->next;

    /* enable interrupt */
    rt_hw_interrupt_enable(temp);

    /* the msg is the new tailer of list, the next shall be NULL */
    msg->next = RT_NULL;
    /* copy buffer */
    rt_memcpy(msg + 1, buffer, size);

    /* disable interrupt */
    temp = rt_hw_interrupt_disable();
    /* link msg to message queue */
    if (mq->msg_queue_tail != RT_NULL)
    {
        /* if the tail exists, */
        ((struct rt_mq_message *)mq->msg_queue_tail)->next = msg;//将新消息连接到消息列表的末尾
    }

    /* set new tail */
    mq->msg_queue_tail = msg;
    /* if the head is empty, set head */
    if (mq->msg_queue_head == RT_NULL)
        mq->msg_queue_head = msg;

    /* increase message entry */
    mq->entry ++;//有效消息数量增加

    /* resume suspended thread */
    if (!rt_list_isempty(&mq->parent.suspend_thread))
    {//有挂起到该消息队列的线程，则需要换新被挂起的第一个线程，然后执行调度操作
        rt_ipc_list_resume(&(mq->parent.suspend_thread));

        /* enable interrupt */
        rt_hw_interrupt_enable(temp);

        rt_schedule();

        return RT_EOK;
    }

    /* enable interrupt */
    rt_hw_interrupt_enable(temp);

    return RT_EOK;
}

消息队列-读取消息

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;

    /* parameter check */
    RT_ASSERT(mq != RT_NULL);
    RT_ASSERT(rt_object_get_type(&mq->parent.parent) == RT_Object_Class_MessageQueue);
    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;

    /* move message queue head */
    mq->msg_queue_head = msg->next;
    /* 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;
    mq->msg_queue_free = msg;
    /* enable interrupt */
    rt_hw_interrupt_enable(temp);

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

    return RT_EOK;
}

读取消息本质上就是从链表中取走一个有效的数据，代码之所以这么复杂，主要是因为读取操作涉及到延时时间和线程挂起。对于被挂起的线程， 会在其他线程调用rt_mq_send发送消息时被唤醒。

 

总结

消息队列用于线程间通信，相比sem,mutex而言可以传输更多的数据， 但是消息的大小和允许消息的数量都受限于内核的配置，本质上和Linux平台的实现原理类似。 一个消息只会被一个线程读取，消息队列没有所有者的概念，意味着任何线程均可以对消息队列进行发送消息或读取消息操作。