AW: [Caml-list] Portable timeout function

[Gerd Stolpmann <info@gerd-stolpmann.de>]

Am 22.01.2013 19:57:15 schrieb(en) Samuel Mimram:
> Hi,
> 
> I would like to implement a "timeout" function of type:
> 
> float -> ('a -> 'b) -> 'a -> 'b option
> 
> which takes a maximum number n of seconds to run, a function f, an  
> argument
> x, and returns Some (f x) if the computation ends before n seconds  
> and None
> otherwise. Of course, there is a simple implementation using
> Unix.setitimer, but apparently it does not work under windows because  
> of
> signals implementation (and I don't have access to a windows  
> machine...).
> Since this is a pretty standard idom I expected to find it  
> implemented in
> some library, but could not find one. Also, I'd rather not heavily  
> change
> the code (i.e. monadic threads are not really an option here, and a  
> small
> function would be appreciated).

I think this is not possible without changes in the OCaml runtime -  
what we would need here is an emulation of signals under Windows, so  
that a timer thread could be started that finally sends the signal to  
the compute thread. However, such an emulation would be limited to pure  
computations, and would not be able to interrupt system calls (no  
support from Windows).

As long as you know that your compute functions allocate memory, it  
will do garbage collections, and you could set a GC hook:


exception Timeout

let timer tmo f x =
   let t0 = Unix.gettimeofday() in
   let alarm = ref None in
   Gc.major();
   try
     let al =
       Gc.create_alarm
         (fun () ->
            let t1 = Unix.gettimeofday() in
            if t1 -. t0 > tmo then raise Timeout
         ) in
     alarm := Some al;
     let r = f x in
     Gc.delete_alarm al;
     alarm := None;
     Some r
   with Timeout ->
     ( match !alarm with
         | Some al -> Gc.delete_alarm al
         | None -> ()
     );
     None

But this does not work if the function does not allocate enough memory  
(and also note that there are several race conditions in "timer").

> Extra points if your solution also works with js_of_ocaml! :)

I don't think that there is any support in js_of_ocaml for completely  
asynchronous events (i.e. something like the regular check for signals  
the standard runtime does).

Gerd

> 
> Thanks!
> 
> Regards,
> 
> Samuel.
> 
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Gerd Stolpmann, Darmstadt, Germany    gerd@gerd-stolpmann.de
Creator of GODI and camlcity.org.
Contact details:        http://www.camlcity.org/contact.html
Company homepage:       http://www.gerd-stolpmann.de
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