Event Flags (EVENT)

The EVENT feature manages sets of event flags. An event flag set is a set of bits in memory associated with a thread-wait queue. Each bit is associated with one event. In this feature, the set is implemented as an unsigned integer, therefore the maximum number of flags in a set is 8*sizeof(int). Inside a set, each event flag is designated by an integer 0 and 8*sizeof(int).

When a flag is set, it is said to be posted, and the associated event is considered to have occurred. Otherwise, the associated event has not yet occurred. Both threads and interrupt handlers can use event flag sets for signaling purposes.

A thread can wait for a conjunctive (and) or disjunctive (or) subset of the events in one event flag set. Several threads can wait on the same event, in which case each of the threads will be made eligible to run when the event occurs.

Three functions are available on event flag sets: eventWait(), eventPost(), and eventClear().

Event flag sets are data structures allocated in the client actors' address spaces. No microkernel data structure is allocated for these objects. They are simply designated by the address of the structures. The number of these types of objects that threads can use is thus unlimited.

For details, see the EVENT(5FEA) man page.

EVENT API

The EVENT API is summarized in the following table: