[gambit-list] call/cc across threads

Guillaume Germain guillaume.germain at gmail.com
Tue Feb 20 10:48:00 EST 2007

On 2/20/07, Lang Martin <lang-gb at coptix.com> wrote:
> I have a (possibly irrational) bias against mutexes, and attempted to
> solve a threading problem with just message passing. The code works,
> but only part of the time.

I haven't run your code myself, but I think I see a few things wrong with

This code:
> (let ((th (thread-start!
>             (make-thread
>              (lambda ()
>                (let lp ()
>                  (let ((p (thread-receive 0 #f)))
>                    (if p
>                        (p 40)
>                        (begin
>                          (thread-yield!)
>                          (lp))))))))))
>         (+ 2 (call/cc
>          (lambda (ret)
>            (thread-send th ret)
>            (thread-sleep! 4)))))
> Occasionally fails by attempting to add 2 to #!void.

Let's call the original thread "thread #1" and the newly created one "thread

Thread #1 will start thread #2, capture its own continuation and send it to
thread #2.  Then it adds the result of (thread-sleep! 4) [which is #!void]
to 2, which cause your error after 4 seconds.

In thread #2, calling (p 40) will make that thread invoke thread #1's
continuation, resulting in a final result of 42.  Note that thread #2 will
not loop if it gets the continuation as a message, because it discards its
own continuation when invoking (p 40).  It basically becomes thread #1 at
that point.

Other notes: you should just wait in thread #2 for a message instead of
waiting in a spin lock (ie using 'thread-receive' with a timeout of 0).
Also, calling 'thread-yield!' isn't necessary since Gambit's thread system
is preemptive.

Finally, for debugging it might help to use Gambit's command-line options so
that threads other than the primordial thread will also start a repl on
crashes, using for example:
% gsi -:dar
(for more details see Gambit's documentation).

Hope this helps and that I didn't misinterpret the code.  Personally, I
don't think you will have to use mutexes if you design your program

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