Two tasks can synchronize their activities by using two message queues, as shown in Figure 15-4. This is called a bilateral rendez-vous and works the same as with semaphores except that both tasks may send messages to each other. A bilateral rendez-vous cannot be performed between a task and an ISR since an ISR cannot wait on a message queue.
In a bilateral rendez-vous, each message queue holds a maximum of one message. Both message queues are initially created empty. When the task on the left reaches the rendez-vous point, it sends a message to the top message queue and waits for a message to arrive on the bottom message queue. Similarly, when the task on the right reaches its rendez-vous point, it sends a message to the message queue on the bottom and waits for a message to arrive on the top message queue.
Figure 15-5 shows how to use task-message queues to perform a bilateral rendez-vous.