[erlang-questions] Ports and piping output between them

Nathan Fiedler <>
Mon Jul 18 02:02:02 CEST 2016


That's awesome, I love it. I've never heard of this, but it's very handy to
know. Thanks for pointing it out. I'll use that in the shell script that I
generate so I can return that exit code specifically.

n


On Sun, Jul 17, 2016 at 3:15 AM, Pierre Fenoll <>
wrote:

> If your issue has more to do with getting the output error code then why
> not echo PIPESTATUS?
>
> http://stackoverflow.com/a/9168523
>
> On 16 Jul 2016, at 19:13, Nathan Fiedler <> wrote:
>
> My ultimate goal is to run two external processes, with the output of one
> feeding into the input of the other, a la shell piping. So something like
> `zfs send | zfs recv` or `tar | split`, but from within my Erlang
> application. Below is my simple example that fairly quickly balloons memory
> out of control and eventually explodes (i.e. the OS kills it).
>
> #!/usr/bin/env escript
> %%!
>
> main(_Args) ->
>     SendCmd = "cat /dev/random",
>     %
>     % not using 'binary' with open_port makes memory size grow really fast,
>     % but that's fine, I want that option anyway, just pointing it out
>     %
>     SendPort = erlang:open_port({spawn, SendCmd}, [exit_status, binary]),
>     RecvCmd = "strings",
>     RecvPort = erlang:open_port({spawn, RecvCmd}, [exit_status, binary]),
>     {ok, 0} = pipe_until_exit(SendPort, RecvPort, 0),
>     ensure_port_closed(SendPort),
>     ensure_port_closed(RecvPort),
>     ok.
>
> pipe_until_exit(SendPort, RecvPort, N) ->
>     %
>     % invoking garbage_collect/0 helps a little bit, but memory still grows
>     %
>     io:format("iteration ~w~n", [N]),
>     receive
>         {SendPort, {exit_status, Status}} ->
>             {ok, Status};
>         {RecvPort, {exit_status, Status}} ->
>             io:format("error: receive port exited ~w~n", [Status]);
>         {SendPort, {data, Data}} ->
>             true = erlang:port_command(RecvPort, Data),
>             pipe_until_exit(SendPort, RecvPort, N + 1);
>         {RecvPort, {data, _Data}} ->
>             pipe_until_exit(SendPort, RecvPort, N + 1);
>         Msg ->
>             io:format("some other message: ~w", [Msg])
>     end.
>
> ensure_port_closed(Port) ->
>     case erlang:port_info(Port) of
>         undefined -> ok;
>         _         -> erlang:port_close(Port)
>     end.
>
> My assumption is that I am doing something wrong, but I can't see it. I'll
> explore other solutions, such as simply generating a shell script and
> invoking it. In the mean time, if you can see something obvious, please let
> me know.
>
> Yes, I am aware of os:cmd/1, but that does not return the exit code, which
> I really want to have so I know that something at least appeared to work
> (zfs send|zfs recv and tar|split do not generate any success indication
> other than exit code).
>
> Thanks
>
> n
>
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> 
> http://erlang.org/mailman/listinfo/erlang-questions
>
>
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