in reply to Re: UDP bidirectional client
in thread UDP bidirectional client

Thank you all very much for your replies!

I have looked over ActivePerl window quirks, things that don't work, etc. I didn't find anything mentioning socket behaviour (seems forking was a concern, but now "works").

What I'm looking for is something telling me that when two processes are operating on the same socket object, one will block the other (but I can't find that statement). BUT ONLY if you are using ActivePerl (ie. unix/linux is capable of operating on the same object in a forked process).

I understand the concept of the transport layer and am looking for a script that can monitor a port for incoming datagrams and -ALSO- listen to STDIN for user input (think: UDP chat...). Implementing a canned reply in the "recv" functionality works fine, but I'm looking to have the user provide the reply to the incoming datagram..

I hope this clarifies what I'm looking to do. And I hope there is atleast one way to do this with ActivePerl. :)

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Re: Re: Re: UDP bidirectional client
by Thelonius (Priest) on Mar 06, 2003 at 21:12 UTC
    pg is on the right track when he says to use two sockets. You don't want to use the same socket to both send and receive over UDP. The code from the cookbook is for TCP sockets and won't work with UDP. If you want to use UDP, here is your code changed to use two sockets:
    #!/usr/bin/perl
# biclient - bidirectional forking client   perl_book\cookbook\ch17_11
+.htm
use IO::Socket;
use English;
use strict;

my ($lport, $host, $port, $kidpid, $sock, $line, $msg, $MAXLEN);
$MAXLEN = 1024;
$|++;

unless (@ARGV == 3) { die "usage: $0 listen_port dest_host  dest_port\
+n" }
($lport, $host, $port) = @ARGV;

# create a udp connection to the specified host and port
my $in = IO::Socket::INET->new(Proto     => "udp",
                              LocalPort => $lport )
       or die "can't connect to port $lport on localhost: $!";

my $out = IO::Socket::INET->new(Proto     => "udp",
                              PeerAddr  => $host,
                              PeerPort  => $port)
       or die "can't connect to port $port on $host: $!";


print STDOUT "[Listening on $lport, Connected to ($host:$port)]\n";
# Thelonius notes: Actually, with UDP, we 
# are NOT CONNECTED to ($host:$port).
#  However, our messages will be sent there

# split the program into two processes, identical twins
die "can't fork: $!" unless defined($kidpid = fork());

if ($kidpid) {
    print STDOUT "Parent $PID started\n";

    # parent copies the socket to standard output
    while ($in->recv($msg, $MAXLEN)) {
        print "\nReceived from ($host:$port) \"$msg\" \n";
    # Thelonius notes: You do not know where
    # the message came from unless you use
    # recvfrom.
    }
    # This loop will never exit.
    kill("TERM" => $kidpid);        # send SIGTERM to child

} else {
    print STDOUT "Child $PID started\n";
    print $out "Child $PID started";

    # child copies standard input to the socket
    while ($line = <STDIN>) {
        chomp($line);  #print STDOUT "You entered: $line\n";
        print $out $line;
    }
    # You might want to kill parent if you exit.
}
exit;

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[reply]
[d/l]
      Thanks Thelonius - but isn't there a fundamental flaw in that approach? You are sending on based on some kernel chosen port, and then listening for replies on a specified port.

      How does the peer know which port the sender is listening on?

      That's why I was trying to use one socket object. I hope that will work, or I don't know how to listen for what I sent?

[reply]
        You probably want to use TCP.
[reply]
      Thanks Thelonius - but isn't there a fundamental flaw in that approach? You are sending on based on some kernel chosen port, and then listening for replies on a specified port.

      How does the peer know which port the sender is listening on?

      That's why I was trying to use one socket object. I hope that will work, or I don't know how to listen for what I sent?

[reply]

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