要传输较大数据时，使用队列。
传输较小数值时，使用邮箱。

队列、邮箱用来传递数据。
如果只是用来传递资源的个数，可以使用信号量。

A车与B车只需要传递信号量（代表资源）。

信号量

获取信号量

1. 如果value>0，value–，return OK;如果value0，timeout0，return -ETIMEOUT;如果value==0，timeout>0，从就绪链表移除（挂起），把自己记录在信号量的等待链表。
2. 被唤醒：如果thread->err!=0，return thread->err；return EOK;

释放信号量

1. value++;如果有等待信号量线程，唤醒线程。

创建信号量

/**
 * This function will create a semaphore from system resource
 *
 * @param name the name of semaphore
 * @param value the initial value of semaphore
 * @param flag the flag of semaphore
 *
 * @return the created semaphore, RT_NULL on error happen
 *
 * @see rt_sem_init
 */
rt_sem_t rt_sem_create(const char *name, rt_uint32_t value, rt_uint8_t flag)

信号量结构体

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

    rt_uint16_t          value;                         /**< value of semaphore. */
    rt_uint16_t          reserved;                      /**< reserved field */
};

struct rt_ipc_object
{
    struct rt_object parent;                            /**< inherit from rt_object */

    rt_list_t        suspend_thread;                    /**< threads pended on this resource */
};

获取信号量

rt_err_t rt_sem_take(rt_sem_t sem, rt_int32_t time)
{
    register rt_base_t temp;
    struct rt_thread *thread;

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

    RT_OBJECT_HOOK_CALL(rt_object_trytake_hook, (&(sem->parent.parent)));

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

    RT_DEBUG_LOG(RT_DEBUG_IPC, ("thread %s take sem:%s, which value is: %d\n",
                                rt_thread_self()->name,
                                ((struct rt_object *)sem)->name,
                                sem->value));

    if (sem->value > 0) //如果有资源
    {
        /* semaphore is available */
        sem->value --;

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

            return -RT_ETIMEOUT;
        }
        else
        {
            /* current context checking */
            RT_DEBUG_IN_THREAD_CONTEXT;

            /* semaphore is unavailable, push to suspend list */
            /* get current thread */
            thread = rt_thread_self();

            /* reset thread error number */
            thread->error = RT_EOK;

            RT_DEBUG_LOG(RT_DEBUG_IPC, ("sem take: suspend thread - %s\n",
                                        thread->name));

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

            /* has waiting time, start thread timer */
            if (time > 0)
            {
                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,
                                 &time);
                rt_timer_start(&(thread->thread_timer));
            }

            /* enable interrupt */
            rt_hw_interrupt_enable(temp);

            /* do schedule */
            rt_schedule();

            if (thread->error != RT_EOK)
            {
                return thread->error;
            }
        }
    }

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

    return RT_EOK; //获取到了资源成功返回
}
RTM_EXPORT(rt_sem_take);

释放信号量

rt_err_t rt_sem_release(rt_sem_t sem)
{
    register rt_base_t temp;
    register rt_bool_t need_schedule;

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

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

    need_schedule = RT_FALSE;

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

    RT_DEBUG_LOG(RT_DEBUG_IPC, ("thread %s releases sem:%s, which value is: %d\n",
                                rt_thread_self()->name,
                                ((struct rt_object *)sem)->name,
                                sem->value));

    if (!rt_list_isempty(&sem->parent.suspend_thread)) //是否有等待线程
    {
        /* resume the suspended thread */
        rt_ipc_list_resume(&(sem->parent.suspend_thread));//唤醒++，接收--，所以没有对sem->value做任何操作。
        need_schedule = RT_TRUE;
    }
    else
        sem->value ++; /* increase value */

    /* enable interrupt */
    rt_hw_interrupt_enable(temp);

    /* resume a thread, re-schedule */
    if (need_schedule == RT_TRUE)
        rt_schedule();

    return RT_EOK;
}
RTM_EXPORT(rt_sem_release);