RTEMS 4.9.1 On-Line Library

Eventflags Manager Event sets

RTEMS ITRON 3.0 API User's Guide

4.2.1: Event sets

An eventflag is used by a task (or ISR) to inform another task of the occurrence of a significant situation. One word bit-field is associated with each eventflags. 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.

A pending event is an event that has been set. An event condition is used to specify the events 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 TWF_ORW algorithm states that an event condition is satisfied when at least a single requested event is posted. The TWF_ANDW algorithm states that an event condition is satisfied when every requested event is posted.

An eventflags or condition is built by a bitwise OR of the desired events. If an event is not explicitly specified in the set or condition, then it is not present. Events are specifically designed to be mutually exclusive, therefore bitwise OR and addition operations are equivalent as long as each event appears exactly once in the event set list.

RTEMS ITRON 3.0 API User's Guide