Greetings Brothers, I've been banging my head against the wall trying to nail down good IPC technique for a couple months now, and just when I think it all makes sense I can't quite test my understandings successfully. In particular, my research indicates that it's essential basically to use IO::Select to find out if read and write handles are ready for reading and writing before using them. In particular I've studied some nice code here:

http://www.perlmonks.org/?node_id=151886

But what's happening when I try my own implementation (including *writing* to the child process) is IO::Select seems to let the write file handle sneak through as ready for reading! Here is my "server" code: 

#!/usr/bin/perl

print "What is your name?\n";
my $name = prompt_for_input();

print "What is your mission?\n";
my $mission = prompt_for_input();

print "What is your favorite color?\n";
my $color = prompt_for_input();

print "Your name is $name, your mission is $mission, and your favorite
+ color is $color. Have a nice day!\n";

sub prompt_for_input {
    local ($|) = (1);
    my $resp = <STDIN>;
    chomp $resp;
    return $resp;
}

[download]
Simple enough, right? Here is my "client": 
#!/usr/bin/perl
use strict;
use warnings;
use diagnostics;


use IPC::Open3;
use IO::Select;

my @answers = qw/galahad seekthegrail bluenored!/;

my( $wtr, $rdr, $err );
use Symbol 'gensym';
$err = gensym;
my $pid = open3( $wtr, $rdr, $err, 'server.pl' );

print "IN: $wtr     OUT: $rdr     ERR: $err\n";
print "PID was $pid\n";

my $sel = new IO::Select;
$sel->add( $wtr, $rdr );

my $line;
foreach my $readhandle ( $sel->can_read(1) ) {
    print "readhandle is $readhandle \n";
    next if $readhandle != $rdr;
    print "past next\n";
    my $len = sysread $readhandle, $line, 4096;
    if( not defined $len ) {
        die "Error from child: $!\n";
    }

    print $line;
    write_answers();


}

waitpid( $pid, 0 );


sub write_answers {

    foreach my $writehandle ( $sel->can_write ) {
        print "write handle is $writehandle\n";
        return if $writehandle != $wtr;
        my $answer = shift @answers;
        print "writing $answer\n";
        print $writehandle $answer . "\n";
    }
}

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So what happens is very strange: 
rasto@frodo:~/work$ ./client.pl 
IN: GLOB(0x880fe78)     OUT: GLOB(0x880fe88)     ERR: GLOB(0x880fe28)
PID was 6834
readhandle is GLOB(0x880fe78) 
readhandle is GLOB(0x880fe88) 
past next
write handle is GLOB(0x880fe78)
writing galahad
rasto@frodo:~/work$

[download]
So there are two things happening here that I don't understand. 1. Why did IO::Select brings GLOB(0x880fe78) up as ready for reading at all, when it was opened as the write (input) filehandle? To paraphrase Basil Fawlty, "What's the point of the bl**dy thing if it doesn't work when you really need it?" lol.

And 2. I don't understand why the program terminates after writing the answer to the first question. I suspect the two things are interrelated, and I feel like I'm *so close* to really understanding how this process is supposed to work--any input on understanding this would be greatly appreciated!

In reply to Weirdness with IO::Select and IPC::Open3 by rastoboy

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