Programming Embedded Systems, 2nd Edition

Message Passing

The Linux message passing example is a light switch, as is the eCos example, with a producer and a consumer task. The producer task monitors button SW0. Once the button is pressed, the producer sends the button press count to the consumer task. The consumer task waits for this message, outputs the button count, and then toggles the green LED. This example uses a POSIX message queue for passing the message from the producer to the consumer task.

Tip

The source code for the message queue example is not included on the book’s web site because it does not work on the Arcom board as shipped. In order to get the message queue code running on the Arcom board, the Linux kernel needs to be rebuilt with message queue support included.

The main routine demonstrates how to create the message queue. First, the LED is initialized by calling ledInit. Then the message queue is created by calling mq_open. The first parameter specifies the name of the queue as message queue. The second parameter, which is the OR of the file status flags and access modes, specifies the following:

O_CREAT

Create the message queue if it does not exist.

O_EXCL

Used with O_CREAT to create and open a message queue if a queue of the same name does not already exist. If a queue does exist with the same name, the message queue is not opened.

O_RDWR

Open for read and write access.

After the message queue is created successfully, the tasks are created as shown previously. The techniques we’ve just discussed are shown ...