7.1 Introduction

Chapter 6 discusses activity synchronization of two or more threads of execution. Such synchronization helps tasks cooperate in order to produce an efficient real-time system. In many cases, however, task activity synchronization alone does not yield a sufficiently responsive application. Tasks must also be able to exchange messages. To facilitate inter-task data communication, kernels provide a message queue object and message queue management services.

This chapter discusses the following:

• defining message queues,

• message queue states,

• message queue content,

• typical message queue operations, and

• typical message queue use.

7.2 Defining Message Queues

A message queue is a buffer-like object through which tasks and ISRs send and receive messages to communicate and synchornize with data. A message queue is like a pipeline. It temporarily holds messages from a sender until the intended receiver is ready to read them. This temporary buffering decouples a sending and receiving task; that is, it frees the tasks from having to send and receive messages simultaneously.

As with semaphore introduced in Chapter 6, a message queue has several associated components that the kernel uses to manage the queue. When a message queue is first created, it is assigned an associated queue control block (QCB), a message queue name, a unique ID, memory buffers, a queue length, a maximum message length, and one or more task-waiting lists, as illustrated in Figure 7.1.

Figure 7.1: A message queue, its associated parameters, and supporting data structures.

It is the kernel s job to assign a unique ID to...