Mastering Algorithms with C

Queue Example: Event Handling

One popular application of queues is handling events in event-driven applications. Event-driven applications execute largely under the direction of real-time occurrences called events. In a graphical user interface developed in Java, X, or Windows, for example, the behavior of an application depends a great deal on key presses, mouse movements, and other events triggered by the user. Other examples of event-driven applications occur frequently in control systems such as those found in aircraft or factory equipment.

In nearly all event-driven applications, events can occur at any moment, so queues play an important role in storing events until an application is ready to deal with them. A queue works well for this because applications handle events more or less in the same order as they occur.

Example 6.5 presents two functions for handling events: receive_event and process_event . Both functions operate on a queue containing events of type Event. Event is defined in event.h, which is not shown. An application calls receive_event to enqueue an event it has been notified about. Exactly how an application is notified of an event varies, but notification often begins with a hardware interrupt. When the application decides it is time to process an event, it calls process_event. Inside of process_event, an event is dequeued from the event queue and is passed to an application-specific dispatch function. The dispatch function is passed to process_event as ...