Path: utzoo!utgpu!water!watmath!clyde!att!chinet!jkingdon
From: jkingdon@chinet.chi.il.us (James Kingdon)
Newsgroups: comp.os.vms
Subject: Re: Event flags and AST
Summary: Use locks instead
Message-ID: <6096@chinet.chi.il.us>
Date: 26 Jul 88 01:29:47 GMT
References: <8807251248.AA15765@ucbvax.berkeley.edu>
Reply-To: jkingdon@chinet.chi.il.us (James Kingdon)
Organization: Chinet - Public Access Unix
Lines: 15

When I had to do this, I just abandoned event flags and used locks as follows:
1.  The program that wants to be interrupted later lakes out an exclusive
mode lock, with a blocking AST.  Then it goes about it's business.
2.  When the other program wants to interrupt it, it asks for the lock (also
exclusive mode).  The first program executes the blocking AST, which does
whatever you want to do, and then converts the lock to null, and then converts
the lock back to exclusive (this allows the second program to wait for the
first one to do something). 

One thing to be careful of in using locks for this kind of thing is that
new lock requests and lock conversions are placed in separate queues.
I don't remember whether this might cause a problem in this particular
example, but I remember I did think of some scenerio where things could
get granted in the wrong order.  The fix is simple:  When requesting a
new lock, request a null mode lock, then convert it to exclusive.