Perl Cookbook

Writing Bidirectional Clients

Problem

You want set up a fully interactive client so you can type a line, get the answer, type a line, get the answer, etc., somewhat like telnet.

Solution

Once you’ve connected, fork off a duplicate process. One twin only reads your input and passes it on to the server, and the other only reads the server’s output and sends it to your own output.

Discussion

In a client-server relationship, it is difficult to know whose turn it is to talk. Single-threaded solutions involving the four-argument version of select are hard to write and maintain. But there’s no reason to ignore multitasking solutions. The fork function dramatically simplifies this problem.

Once you’ve connected to the service you’d like to chat with, call fork to clone a twin. Each of these two (nearly) identical processes has a simple job. The parent copies everything from the socket to standard output, and the child simultaneously copies everything from standard input to the socket.

The code is in Example 17.4.

Example 17-4. biclient

#!/usr/bin/perl -w
# biclient - bidirectional forking client use strict; use IO::Socket; my ($host, $port, $kidpid, $handle, $line); unless (@ARGV == 2) { die "usage: $0 host port" } ($host, $port) = @ARGV; # create a tcp connection to the specified host and port $handle = IO::Socket::INET->new(Proto => "tcp", PeerAddr => $host, PeerPort => $port) or die "can't connect to port $port on $host: $!"; $handle->autoflush(1); # so output gets there right away print ...