Perl and open3. What am I missing?

Question

I'm trying to make a script that communicates with mplayer using open3, but the mplayer process is showing up as defunct and I am unable to send standard input into mplayer.

Here's the code:

#!/usr/bin/env perl

{
    package mplayer::test;
    use IPC::Open3;

    sub new {
        my $class = shift;
        my $self = bless { @_ }, $class;
        $self->start_mplayer();
        $self;
    }

    sub start_mplayer{
        my $self = shift;
        local *DEVNULL;
        open DEVNULL, ">/dev/null" or die "/dev/null: $!";
        open OUTPUT, ">out.log" or die "out.log: $!";
        $self->{r} = local *MPLAYER_READ;
        $self->{w} = local *MPLAYER_WRITE;
        $self->{pid} = open3($self->{w},$self->{r},">&DEVNULL",'mplayer -slave -idle -v');
        die "Error opening mplayer!\n" unless $self->{pid};
    }
    sub do{
        my ($self, $command) = @_;
        print {$self->{w}} $command, "\n";
    }
}

mplayer::test->new;

mplayer::test->do(qq~loadfile test.mp3~);
sleep(5);

I must be missing something obvious, I'm learning open3 from examples from other modules.

Get rid of `$self->{r} = local *MPLAYER_READ; $self->{w} = local *MPLAYER_WRITE;` If it doesn't help, it'll at least get rid of two useless lines. — ikegami, Oct 11 '12 at 01:36

Schwern · Answer 1 · 2012-10-11T05:53:45.160

First off, switch to lexical filehandles. Typeglobs are package global and difficult to work with.

One problem is with local *DEVNULL. You've made *DEVNULL local to start_mplayer (and whatever it calls, including open3), but then used the associated filehandles outside start_mplayer. By that time, *DEVNULL has reverted back to its global state (ie. empty) and open3 tries to write to an empty filehandle. You should have gotten a print() on unopened filehandle DEVNULL warning, but you don't have warnings on...

Solution: don't localize it. Unfortunately this means you can't have multiple mplayer instances running at once. Normally you'd solve this by using a lexical filehandle, but unfortunately the special >& syntax only works with glob handles. The solution is to only open DEVNULL once.

Alternatively you can let open3 write to an error filehandle and just ignore them. Wastes a miniscule amount of memory.

Other changes...

Turn on strict and warnings
OUTPUT is never used.
Breaking up the command into multiple args avoids possible shell interference.
Putting localized filehandles into the object beforehand is unnecessary.
autodie is easier than typing "or die ..." all the time.

Here's your reworked start_mplayer routine. I don't have a copy of mplayer to try it with, but it works fine with cat.

use strict;
use warnings;
use autodie;

sub start_mplayer{
    my $self = shift;

    # Only open DEVNULL once, since its going to be shared.
    open DEVNULL, ">", /dev/null" unless fileno DEVNULL;

    $self->{pid} = open3($self->{r}, $self->{w}, ">&DEVNULL", 'mplayer', '-slave', '-idle', '-v');

    die "Error opening mplayer!\n" unless $self->{pid};
}

To determine if its your program or something weird about mplayer, try a different command, like 'cat'. Often you have to close the input, or make sure it sees a newline, before a program will produce output.

For a more robust way to interact with programs, see IPC::Run.

Perl and open3. What am I missing?

1 Answers1