Programming Embedded Systems, 2nd Edition

Message Passing

While semaphores can be used to signal from one task to another, there may be times when data needs to be passed in addition to the signal. For this purpose, operating systems frequently provide another mechanism called a message queue (or mailbox).

The operating system handles the buffering and buffer management for message passing as well as the safe communication of data between tasks. Message passing is therefore an alternative to the simple expedient of storing data in a global variable, and offers a much cleaner and more bug-free method of data exchange. Real-time operating systems typically use pointers for accessing the message data for reasons of speed and memory conservation.

Many applications that use message queues consist of a producer task that sends the data and a consumer task that receives it. There can be multiple producer and/or consumer tasks. The message content is typically understood between the sender and receiver ahead of time.

If the message queue is full, the operating system can block the sending task until space is available for the message. Similarly, if no message is present when the receiver attempts to read the message, the operating system blocks the receiving task. The size of the queue may vary depending on the message traffic.