Please look at the code below:

#!/usr/bin/env perl -w

use strict;
use IO::Pty;

my $master = new IO::Pty;
my $slave  = $master->slave();

my $pid = fork();
die "Couldn't fork: $!" unless defined $pid;

if ($pid) {
    $master->close_slave();

    while ( !$master->eof ) {
        my $line = $master->getline;
        chomp $line;
        print "TIME: " . localtime() . " OUTPUT: " . $line . "\n";

        if ( $line == 3 ) {
            $master->print("2\n");
            $master->print("4\n");
            $master->print("6\n");
        }
    }
    wait();
}
else {
    $master->close();
    $master->make_slave_controlling_terminal();

    $slave->print("1\n");
    $slave->print("3\n");
    $slave->print("5\n");
}

[download]

and the result is:

$ perl  realtime5d.pl
TIME: Thu Jul  3 21:05:10 2014 OUTPUT: 1
TIME: Thu Jul  3 21:05:10 2014 OUTPUT: 3
TIME: Thu Jul  3 21:05:10 2014 OUTPUT: 2
TIME: Thu Jul  3 21:05:10 2014 OUTPUT: 4
TIME: Thu Jul  3 21:05:10 2014 OUTPUT: 6

Question 1: I expect 1, 3, 2, 4, 6, 5, why 5 written to $slave is lost? If you look at the picture from here: http://en.wikipedia.org/wiki/Pseudo_terminal, the traffic is bidirectional from master <=> slave, no?

Question 2: How do I turn off the echo? i.e. I do not want to read what I wrote myself, 2, 4, 6 in this case.

Question 3: Above 5 line output was from Perl 5.16 on Mac OS X 10.9.3. I did not think it is OS dependent for this simple script, but when I run with Perl 5.8 on Linux kernel 2.6, I got the following:

$ perl realtime5d.pl 
TIME: Fri Jul  4 10:08:50 2014 OUTPUT: 1
TIME: Fri Jul  4 10:08:50 2014 OUTPUT: 3

It looks that the echo is turned off, but once I write to the master, the is no more reading from slave. Why?

Question 4: The other very similar script http://www.perlmonks.org/?node_id=1092227 produces the same behavior on Perl 5.16 on Mac OS X 10.9.3 and on Perl 5.8 on Linux kernel 2.6, what is the real difference that these two scripts that causes the different behavior?

Thanks.

In reply to [IO::Pty] How to read from and write to slave? by puravida

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