{{notability, date=March 2019
An event flag is a process
synchronization primitive in the
OpenVMS operating system. It has two possible states, ''set'' or ''cleared''. The following basic primitive operations are provided:
* Set event flag ($SETEF)
* Clear event flag ($CLREF)
* Wait for event flag ($WAITFR)--if the flag was clear, this would make the process wait until it was set. If the flag was already set, this would immediately return, leaving the flag set.
Additional synchronization operations are:
* $WFLOR--wait for any of the specified event flags to be set.
* $WFLAND--wait for ''all'' the specified event flags to be set.
Event flags can be either ''local'' (per-process) or ''common'' (accessible by more than one process). Each process has its own set of 64 local flags, numbered 0-63. It is also possible to ''associate'' the process with up to 2 sets of common event flags. These come in sets of 32 each, and the process can assign them numbers 64-95 or 96-127. They come into existence when the first process associates with them, and disappear when the last process removes its association.
Every potentially time-consuming asynchronous system call (
QIO
QIO (Queue I/O) is a term used in several computer operating systems designed by the former Digital Equipment Corporation ( DEC) of Maynard, Massachusetts.
I/O operations on these systems are initiated by issuing a QIO call to the kernel. There ...
among others) includes an argument specifying the number of an event flag to set on completion.
Compare
semaphore.
Operating system technology
OpenVMS