[erlang-questions] Subordinate processes and stdout

[Vlad Dumitrescu]

Hi Craig,

Without the io:formats, the main process kills the chidren before they
manage to get executed. Try adding "receive after 3000 -> ok end" before
killing the children.

regards,
Vlad


On Sun, Oct 5, 2014 at 5:00 PM, zxq9 <zxq9@REDACTED> wrote:

> I wrote a toy broadcast to pid example today with R17, and in it I had the
> spawned processes output with io:format/2 instead of returning a message to
> the caller. It turns out, though, that unless the caller prints to stdout
> after the subordinates should have executed their own io call nothing is
> displayed if this is run in the shell (I haven't tried it as a script).
>
> Here is a sample:
>
> % Elementary broadcasting.
> -module(bcast).
> -export([start/0]).
>
> start() ->
>     Pids = [spawn(fun() -> listener() end) || _ <- lists:seq(1,3)],
>     Message = "This is my message.",
>     ok = bcast(Pids, Message),
>     io:format("~p ~p: Weirdly initiates the first batch of messages.~n",
>               [self(), now()]),
>     io:format("~p ~p: Initiates the second batch.~n",
>               [self(), now()]),
>     [exit(P, kill) || P <- Pids],
>     ok.
>
> listener() ->
>     receive
>         {bcast, Message} ->
>             Now = now(),
>             io:format(user, "~p: ~p~n", [self(), Now]),
>             io:format(user, "~p ~p: Received: ~p~n", [self(), now(),
> Message]),
>             listener()
>     end.
>
> bcast(Pids, Message) ->
>     [P ! {bcast, Message} || P <- Pids],
>     ok.
>
> And in the shell:
>
> 24> c(bcast).
> {ok,bcast}
> 25> bcast:start().
> <0.138.0>: {1412,520755,828598}
> <0.33.0> {1412,520755,828584}: Weirdly initiates the first batch of
> messages.
> <0.139.0>: {1412,520755,828682}
> <0.140.0>: {1412,520755,828690}
> <0.138.0> {1412,520755,828803}: Received: "This is my message."
> <0.33.0> {1412,520755,828812}: Initiates the second batch.
> <0.139.0> {1412,520755,828815}: Received: "This is my message."
> <0.140.0> {1412,520755,828845}: Received: "This is my message."
> ok
> 26> bcast:start().
> <0.33.0> {1412,520757,650460}: Weirdly initiates the first batch of
> messages.
> <0.142.0>: {1412,520757,650474}
> <0.143.0>: {1412,520757,650482}
> <0.144.0>: {1412,520757,650486}
> <0.33.0> {1412,520757,650656}: Initiates the second batch.
> <0.142.0> {1412,520757,650665}: Received: "This is my message."
> <0.143.0> {1412,520757,650671}: Received: "This is my message."
> <0.144.0> {1412,520757,650676}: Received: "This is my message."
> ok
>
> The two io:format/2 calls in start/0 appear to be necessary to permit the
> subordinates to access stdout. If I comment out one, then only the first
> subordinate process' io:format/2 is displayed, if I comment out both
> bcast:start/0 quietly returns 'ok'. Even more weird, though, is that the
> timestamps demonstrate that the calls to now/0 are happening after
> start/0's
> calls to io -- which seems very odd.
>
> What is happening here? I understand the group leader thing with stdout (at
> least I think I do -- this is all on a single, local node, anyway), but am
> puzzled at this behavior.
>
> Regards,
> -Craig
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