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Ask the Wizard Questions

Language: Enqueue lock from DCL

The Question is:

I want to write a program to be called from DCL to enqueue a lock and
have the lock stay enqueued after the program exits.  At a later point
in the DCL I want to call another program to dequeue the lock.  How do
I get the lock to remain after the first program exits?

The Answer is:

      If your process or the image has CMEXEC privilege, you can enqueue a
    lock which will survive image rundown. To do this you need to execute
    the call to SYS$ENQW in EXECUTIVE mode. If you wish the lock to be
    visible beyond your immediate UIC group, you will also need SYSLCK
    privilege and include the flag LCK$M_SYSTEM in the $ENQ.

      To $DEQ the lock in a later image activation, you first need to
    find the LKID using SYS$GETLKIW. This will also need to be done in
    EXECUTIVE mode. The method I would suggest is to use a wildcard
    search for a lock held by your PID and with the same resource name.

      Beware - exceptions in EXECUTIVE mode will delete your process, and,
    in some instances can crash your system. You also cannot reliably use
    I/O from executive mode, nor can you use DEBUG. A rough outline of
    the code required is:

    To take out the lock:

    END EnqRoutine

    To release the lock

    Use $GETJPI to determine your own PID => Self

    init GETLKI item list to return LKI$_RESNAM, LKI$_PID and LKI$_LKID
    LKSB=0 (wildcard)
    UNTIL bad status OR ((PID=Self) AND (RESNAM='your-resource-name')
    IF good status THEN $DEQ(lkid=LKID)
    END DeqRoutine

    Be careful with this stuff, as well as being privileged and difficult
    to debug, it's very easy to deadlock yourself.