#!/usr/bin/perl
use warnings;
use strict;

# by Phil Letschert on gtk2-perl maillist
#It is common behaviour that the C library uses isatty(3) to decide if stdout
#is line-buffered or block-buffered. When reading from a pipe to an external
#command the output is block-buffered, which explains the reported behaviour.
#Solution is using a pseudo-terminal and connect the watcher to it, following is
#a complete example:

use Glib;
use IO::Pty;

my $pty = IO::Pty->new;
my $cmd = "top -d .5";
my $pid;

unless ($pid = fork) {  # child process
        die "problem spawning program: $!\n" unless defined $pid;

        use POSIX ();
        POSIX::setsid
            or die "setsid failed: $!";

        my $tty = $pty->slave;
        my $tty_fd = $tty->fileno;
 
        open STDIN, "<&$tty_fd" or die $!;
        open STDOUT, ">&$tty_fd" or die $!;
        open STDERR, ">&STDOUT" or die $!;
        close $pty;
        close $tty;

        exec $cmd;
}

my $main_loop = Glib::MainLoop->new;

my $watcher;
$watcher = Glib::IO->add_watch( fileno( $pty ), ['in', 'hup'], \&callback);

$main_loop->run;

sub callback {
	if ( $pty->eof ) {
	        Gtk2::Helper->remove_watch( $watcher );
		close( $pty );
	}
	else {
		my $line = $pty->getline;
		print $line;
	}
}