Re: Exception Propagation

[dennison@telepath.com]

In article <0nc73.5376$y6.3195132@WReNphoon3>,
  cdecker@snet.net (Decker, Christian R) wrote:
> OK, everyone is mis-understanding my question, and i apologize.....
> I understand that when an exception is raised in a task that it is not
> deterministic as to where the exception gets
> propagated as far as being able to trap and handle it.

There certianly is a misunderstanding here. I hope I didn't cause it.

What I was trying to say is that an exception is *not* propagated into
any task other than the one it occurred in (except in a rendezvous). If
I were to create language TERD, which did have that behavior, then in
TERD it would be completly non-deterministic in the parent task where an
exception raised from the child task popped up. Therefore there would be
no good way to handle and recover from exceptions in child tasks. That's
probably one reason why the designers of Ada did not do that! In Ada
tasks can only get exceptions from themselves or from tasks they
rendezvous with during the rendezvous. That makes it quite
deterministic.

As for warning you that a task died, in my experinece most compilers do
*not* do that. Whether that is from concerns about synchronizing writes
to standard output, or from sheer lazieness I can't say. But I know that
both Gnat and GreenHills behave that way, and I suspect ObjectAda does
as well.

If you *do* write such code in last-ditch handlers on your tasks, make
sure to handle the IO synchronization problem yourself, or you are
liable to get interspersed output, or different exceptions than the ones
you are looking for.

--
T.E.D.


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