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:

  • 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.

Other articles related to "flag, events, event flag, event":

2002 Winter Olympics - Highlights
... They included the flag that flew at Ground Zero, NYPD officer Daniel Rodriguez singing "God Bless America", and honor guards of NYPD and FDNY members ... Along with the flag that flew at the World Trade Center site, the Challenger flag was also carried into the stadium ... biathlete Ole Einar Bjørndalen of Norway, winning gold in all four men's events (10 k, 12.5 k, 20 k, 4 x 7.5 relay), Nordic combined athlete Samppa Lajunen of Finland ...
PCLSRing - Asynchronous Approaches
... set of available notifications (such as set an event flag, or deliver an asynchronous system trap), as well as a set of system calls for explicitly suspending the process while waiting for these, which ... asynchronous operation" followed by "wait until the operation sets the event flag" ... context during this time will see it about to (re)enter the wait-for-event-flag call ...
QIO Arguments in VMS
... Under VMS, the arguments to the QIO call are The event flag to set when the operation completes ... It isn't possible to not specify an event flag flag 0 is valid ... to have multiple simultaneous operations which set the same event flag on completion it is then up to the application to sort out any confusion this might cause, or just ignore that event flag ...

Famous quotes containing the words flag and/or event:

    “Justice” was done, and the President of the Immortals, in Æschylean phrase, had ended his sport with Tess. And the d’Urberville knights and dames slept on in their tombs unknowing. The two speechless gazers bent themselves down to the earth, as if in prayer, and remained thus a long time, absolutely motionless: the flag continued to wave silently. As soon as they had strength they arose, joined hands again, and went on.
    The End
    Thomas Hardy (1840–1928)

    The awakening of the people of China to the possibilities under free government is the most significant, if not the most momentous, event of our generation.
    Woodrow Wilson (1856–1924)