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 among others) includes an argument specifying the number of an event flag to set on completion.
Compare semaphore.
Famous quotes containing the words event and/or flag:
“There is not any present moment that is unconnected with some future one. The life of every man is a continued chain of incidents, each link of which hangs upon the former. The transition from cause to effect, from event to event, is often carried on by secret steps, which our foresight cannot divine, and our sagacity is unable to trace. Evil may at some future period bring forth good; and good may bring forth evil, both equally unexpected.”
—Joseph Addison (1672–1719)
“My dream is that as the years go by and the world knows more and more of America, it ... will turn to America for those moral inspirations that lie at the basis of all freedom ... that America will come into the full light of the day when all shall know that she puts human rights above all other rights, and that her flag is the flag not only of America but of humanity.”
—Woodrow Wilson (1856–1924)