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From: Joerg Czeranski <jc@joerch.org>
To: Xavier Leroy <Xavier.Leroy@trusted-logic.fr>
Cc: caml-list@inria.fr
In-Reply-To: <19990518181020.A24993@hoedic.trusted-logic.fr>
Date: Wed, 19 May 1999 20:32:31 +0200 (MET DST)
Message-ID: <5vbM3S6mM$17V$1@joerch.org>
Subject: Re: more patches (for Unix signal mask)
Sender: weis

Xavier Leroy answered to my mail:
> > I replaced all sigsetjmp() calls with _setjmp() calls (setjmp() is
> > allowed to modify the signal mask, too, as per Single Unix Spec v2)
> > and handled jumps out of signal handlers separately.
> 
> That's an interesting idea; I'll have to think about it.  By the way,
> you can just do sigsetjmp(..., 0) if you don't want the signal mask to
> be saved and restored; this is more portable than _setjmp.  

Ah, I missed that in the man page, I'll have a look if it has no
unexpected side effects ;-) to use sigsetjmp(..., 0).

> > Exceptions on the other hand go straight up the stack until they find
> > a handler, and then *immediately* invalidate the handler.
> > In a non-pure programming language like O'Caml this creates unavoidable
> > race conditions:
> > 	let resource = acquire () in
> > 	try
> > 		use resource;
> > 		release resource
> > 	with e ->
> > 		release resource;
> > 		raise e
> > "release" is never called if two exceptions arrive at virtually
> > the same time, and neither if an exception arrives after the call
> > to "acquire", but before the "try".
> 
> Yes, asynchronous exceptions (such as those generated from a signal
> handler) are very hard to use because of this.  The programming idiom
> you showed above is safe for synchronous exceptions (exceptions that
> can only be raised by "use resource"), however.

What about resource problems like stack overflows and running out
of heap memory?  I think they might happen at the call to "release".
But it probably depends on the interna of the release function.

> My take on this is that exceptions as they are implemented now are
> just fine as a non-local control structure inside a sequential
> program, but that something else is needed for multithreaded and
> signal-based processing.  The thread cancellation model of Posix threads
> is an interesting example of how inter-thread asynchronous
> notifications can be made safe.

Maybe signals should be handled in a completely different way,
not via handlers, but as a special kind of input stream.
A function similar to Unix.select (even a wrapper around it) could provide
signal notification service.  Such a model might be useful for C programs
too.
I never got around to trying this idea though.  Maybe I'll try to
write such a function for O'Caml as an alternative to the Sys.signal
facility.

joerch