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> ...