Programming Embedded Systems, 2nd Edition

Message Passing

In this section, we’ll discuss a programming technique that’s useful for certain situations where you divide up tasks—in particular, when you can specify a producer task that generates data, and a consumer task that processes the producer’s output. The use of message passing prevents tasks from stepping on each other’s data and also simplifies coding.

The message passing example is similar to the light switch-semaphore example shown earlier in this chapter, where the producer task waits for the SW0 button to be pressed, and the consumer task outputs a message. The difference this time is that a message that contains the number of times the button has been pressed is passed from the producer to the consumer. The consumer, consumerTask, waits for the message from the producer task, producerTask. Once the message is received by the consumer task, it outputs a message and toggles the green LED.

Message queues in eCos are called mailboxes. The following main function, cyg_user_start, starts by initializing the LED by calling ledInit. Next, the mailbox is initialized with a call to cyg_mbox_create. The mailbox create function is passed mailboxHdl (a handle used for subsequent calls to perform operations with that specific mailbox) and mailbox (an area of memory for the kernel’s mailbox structure). Lastly, the two tasks are created and resumed, as we saw in the prior example, and then a message is output, signifying the start of the program.

#include <cyg/kernel/kapi.h> ...