[Caml-list] Timeouts and event

[Caml-list] Timeouts and event

[Nicolas George]

Is it possible to do something like Event.select, but that would only block
for a limited amount of time? It is possible to loop on Event.poll and
Thread.delay, but that keeps the thread a bit busy.
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Re: [Caml-list] Timeouts and event

[Eric C. Cooper]

Nicolas George wrote:
> 
> Is it possible to do something like Event.select, but that would only block
> for a limited amount of time? It is possible to loop on Event.poll and
> Thread.delay, but that keeps the thread a bit busy.

You can do this nicely using a "watchdog event" that produces a value
after a timeout.

let watchdog seconds v =
  let channel = Event.new_channel () in
  let watchdog_thread () =
    Thread.delay seconds;
    Event.sync (Event.send channel v)
  in
  ignore (Thread.create watchdog_thread ());
  Event.receive channel

Using the watchdog event, you can make a version of sync that returns a
known value in the case of a timeout:

let timed_sync timeout_interval timeout_val event =
  Event.sync (Event.choose [event;
			    watchdog timeout_interval timeout_val])

Or you can make a version that returns an option (None for a timeout,
Some x for a "real" sync):

let optional_sync timeout event =
  Event.sync (Event.choose [Event.wrap event (fun x -> Some x);
			    watchdog timeout None])


----
Eric Cooper
ecc@cmu.edu
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Re: [Caml-list] Timeouts and event

[Nicolas George]

Le jeudi 13 septembre 2001 à 15:16, Eric C. Cooper a écrit :
> You can do this nicely using a "watchdog event" that produces a value
> after a timeout.

Ok, you create a new thread, who will only send the event after the
timeout. I am always a bit reluctant in creating new threads, especially if
I use system threads. If there were some builtin timeout event, it would be
great. And since we are in feature wish, event corresponding to wait_read
and wait_write.

> let watchdog seconds v =
>   let channel = Event.new_channel () in
>   let watchdog_thread () =
>     Thread.delay seconds;
>     Event.sync (Event.send channel v)
>   in
>   ignore (Thread.create watchdog_thread ());
>   Event.receive channel

I would write the two last lines like that:

let child = Thread.create watchdog_thread () in
let event = Event.receive channel in
Event.wrap_abort event ~f:(fun () -> Thread.kill child)

Aargh, Thread.kill does not exist with system threads!
(I see one solution, which is to start a new thread that will only sync the
event, but I think it is a waste.)
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