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_CREATCreate the message queue if it does not exist.
O_EXCLUsed with
O_CREATto 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_RDWROpen 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 ...