exit in fun running via timer:apply_interval/4

[Mark Scandariato]

On Jun 7, 2005, at 7:11 AM, Bengt Kleberg wrote:

> On 2005-06-03 20:53, Reto Kramer wrote:
>> Folks,
>> I'm looking for the best (elegant) way to make sure that if there's 
>> an exit in the Mod:Fun that I run via timer:apply_interval/4, the 
>> "parent" process (the one that scheduled the timer) will receive the 
>> exit message, so I can programmatically react to it (and e.g. 
>> shutdown my system).
>> I'm sure someone out there has an elegant solution.
>
> i do not have a solution. instead i have a follow-up question:
>
> what does the following from the man page of the timer module mean?
>
>         An interval timer, i.e. a timer created by evaluating any of 
> the  func-
>        tions apply_interval/4, send_interval/3, and send_interval/2, 
> is linked
>        to the process towards which the timer performs its task.
>
> i interpreted it to mean that the process that called 
> timer:apply_interval will be linked to the (temporary) process doing 
> the  module:function() supplied as argument to apply_interval/4.
> this is not the case, so what does it really mean?
> who is linked, and to whom?
>

The caller of timer:apply_interval is linked to the timer_server 
process - when the caller exits, the timer is deleted.

If you pass the pid of the caller to the function, it could be 
notified. Something like this:


-module(foo).
-export([main/0, tick/3]).

action(Arg) -> whatever.

tick(Parent, Ref, Arg) ->
	try action(Arg)
	catch
		exit:Reason -> Parent ! {Ref, Reason}
	end.

main() ->
	Ref = make_ref(),
	timer:apply_interval(100, foo, tick, [self(), Ref, Arg]),
	receive
		{Ref, Reason} -> whatever
	end.


On the other hand, if you're that concerned about the execution of the 
function, you might want to rethink  the use of apply_interval.

Mark.

>
> bengt, who has included a little test module to play with when 
> investigating this problem.
>
>
> -module(interval).
> -export([main/1, crashing/0]).
>
>
> main(_) ->
> 	timer:apply_interval(1000,  interval,  crashing,  []),
> 	loop(0),
> 	init:stop().
>
> crashing() ->
> 	io:fwrite("~w~n", [erlang:self()]),
> 	erlang:exit(crashing).
>
> loop(N) ->
> 	receive
> 	Something ->
> 		io:fwrite("~w~n", [Something])
> 	after 500 ->
> 		io:fwrite("~w~n", [N])
> 	end,
> 	loop(N+1).
>