RTEMS 4.5.1-pre3 On-Line Library

Event Manager Event Sets

RTEMS Ada User's Guide

11.2.1: Event Sets

An event flag is used by a task (or ISR) to inform another task of the occurrence of a significant situation. Thirty-two event flags are associated with each task. A collection of one or more event flags is referred to as an event set. The data type rtems.event_set is used to manage event sets.

The application developer should remember the following key characteristics of event operations when utilizing the event manager:

Events provide a simple synchronization facility.
Events are aimed at tasks.
Tasks can wait on more than one event simultaneously.
Events are independent of one another.
Events do not hold or transport data.
Events are not queued. In other words, if an event is sent more than once to a task before being received, the second and subsequent send operations to that same task have no effect.

An event set is posted when it is directed (or sent) to a task. A pending event is an event that has been posted but not received. An event condition is used to specify the event set which the task desires to receive and the algorithm which will be used to determine when the request is satisfied. An event condition is satisfied based upon one of two algorithms which are selected by the user. The RTEMS.EVENT_ANY algorithm states that an event condition is satisfied when at least a single requested event is posted. The RTEMS.EVENT_ALL algorithm states that an event condition is satisfied when every requested event is posted.

RTEMS Ada User's Guide