Re: [Caml-list] Portable timeout function

[Samuel Mimram <smimram@gmail.com>]

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Thanks for all your answers! I really appreciated the Gc.create_alarm hack,
which does the job alright for now, but is quite imprecise. I have
submitted a feature request for addition to the standard library [1], we'll
see...

Cheers,

Samuel.

[1] http://caml.inria.fr/mantis/view.php?id=5908


On Wed, Jan 23, 2013 at 4:03 PM, Gerd Stolpmann <info@gerd-stolpmann.de>wrote:

> 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.
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