I have a situation in which I want to run a REXX script periodically, but
with the ability to time out if something goes wrong.  For example, I have
a program (BinkleyTerm) which is started from a batch file and which is
usually running, but which exits with certain errorlevels at scheduled
times.  One of these errorlevels causes the parent batch file to run a
particular REXX script.  Because there are certain real-time constraints, I
need to be able to limit the amount of time that the REXX script runs,
killing it ungracefully if necessary as a last ditch effort to keep the
system running.

If the program in question was an EXE rather than a REXX script, then I
could use something like DosKillProcess().  But this does not work because
simply killing the REXX script process has the effect of killing the parent
batch, too, since CMD itself is the process.

The ideal approach to doing a time out would be to build it directly into
the program, which would naturally be accomplished by multithreading. 
However, there is no way to multithread a REXX program, as far as I know.

It seems to me that there are two ways out of this.  The first way is to
interpose some sort of parent process for the REXX script which is itself
not written in REXX and which therefore can be multithreaded, and have the
main thread hold onto the child process ID when it spawns CMD to run the
REXX script.  The time-out could then be accomplished by running a separate
thread that would request a mutex semaphore that would be owned by the main
thread while the child process is running, and the return code from
DosRequestMutexSem() could be inspected to determine whether the call had
returned successfully, meaning the child had terminated normally, or had
timed out, meaning the child needed to be killed ungracefully.

The second approach is to devise some sort of REXX DLL which the REXX
script could use to set a time out on itself.  The problem here is again
that the DLL would only be able to kill the REXX process, which is really
that of the parent batch.  I would think that there is some way to do this,
since hitting Ctrl-Break during a REXX script will terminate it without
terminating the parent batch, but I am not sure exactly how to make that
distinction.
 
-- Mike


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