Event flag

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:
Additional synchronization operations are:
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 among others) includes an argument specifying the number of an event flag to set on completion.
Compare semaphore.