my $waitedpid = 0;
    my $paddr;

    sub REAPER {
	$waitedpid = wait;
	$SIG{CHLD} = \&REAPER;  # loathe sysV
	logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
    }

    $SIG{CHLD} = \&REAPER;

    for ( $waitedpid = 0;
	  ($paddr = accept(Client,Server)) || $waitedpid;
	  $waitedpid = 0, close Client)
    {
	next if $waitedpid and not $paddr;
	my($port,$iaddr) = sockaddr_in($paddr);
	my $name = gethostbyaddr($iaddr,AF_INET);

	logmsg "connection from $name [",
		inet_ntoa($iaddr), "]
		at port $port";

	spawn sub {
	    print "Hello there, $name, it's now ", scalar localtime, $EOL;
	    exec '/usr/games/fortune'		# XXX: `wrong' line terminators
		or confess "can't exec fortune: $!";
	};

    }

    sub spawn {
	my $coderef = shift;

	unless (@_ == 0 && $coderef && ref($coderef) eq 'CODE') {
	    confess "usage: spawn CODEREF";
	}

	my $pid;
	if (!defined($pid = fork)) {
	    logmsg "cannot fork: $!";
	    return;
	} elsif ($pid) {
	    logmsg "begat $pid";
	    return; # I'm the parent
	}
	# else I'm the child -- go spawn

	open(STDIN,  "<&Client")   || die "can't dup client to stdin";
	open(STDOUT, ">&Client")   || die "can't dup client to stdout";
	## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr";
	exit &$coderef();
    }

This server takes the trouble to clone off a child version via fork() for
each incoming request.  That way it can handle many requests at once,
which you might not always want.  Even if you don't fork(), the listen()
will allow that many pending connections.  Forking servers have to be
particularly careful about cleaning up their dead children (called
"zombies" in Unix parlance), because otherwise you'll quickly fill up your
process table.

We suggest that you use the B<-T> flag to use taint checking (see L<perlsec>)
even if we aren't running setuid or setgid.  This is always a good idea
for servers and other programs run on behalf of someone else (like CGI
scripts), because it lessens the chances that people from the outside will
be able to compromise your system.

Let's look at another TCP client.  This one connects to the TCP "time"
service on a number of different machines and shows how far their clocks
differ from the system on which it's being run:

    #!/usr/bin/perl  -w
    use strict;
    use Socket;

    my $SECS_of_70_YEARS = 2208988800;
    sub ctime { scalar localtime(shift) }

    my $iaddr = gethostbyname('localhost');
    my $proto = getprotobyname('tcp');
    my $port = getservbyname('time', 'tcp');
    my $paddr = sockaddr_in(0, $iaddr);
    my($host);

    $| = 1;
    printf "%-24s %8s %s\n",  "localhost", 0, ctime(time());

    foreach $host (@ARGV) {
	printf "%-24s ", $host;
	my $hisiaddr = inet_aton($host)     || die "unknown host";
	my $hispaddr = sockaddr_in($port, $hisiaddr);
	socket(SOCKET, PF_INET, SOCK_STREAM, $proto)   || die "socket: $!";
	connect(SOCKET, $hispaddr)          || die "bind: $!";
	my $rtime = '    ';
	read(SOCKET, $rtime, 4);
	close(SOCKET);
	my $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ;
	printf "%8d %s\n", $histime - time, ctime($histime);
    }

=head2 Unix-Domain TCP Clients and Servers

That's fine for Internet-domain clients and servers, but what about local
communications?  While you can use the same setup, sometimes you don't
want to.  Unix-domain sockets are local to the current host, and are often
used internally to implement pipes.  Unlike Internet domain sockets, Unix
domain sockets can show up in the file system with an ls(1) listing.

    % ls -l /dev/log
    srw-rw-rw-  1 root            0 Oct 31 07:23 /dev/log

You can test for these with Perl's B<-S> file test:

    unless ( -S '/dev/log' ) {
	die "something's wicked with the print system";
    }

Here's a sample Unix-domain client:

    #!/usr/bin/perl -w
    use Socket;
    use strict;
    my ($rendezvous, $line);

    $rendezvous = shift || '/tmp/catsock';
    socket(SOCK, PF_UNIX, SOCK_STREAM, 0)	|| die "socket: $!";
    connect(SOCK, sockaddr_un($rendezvous))	|| die "connect: $!";
    while (defined($line = <SOCK>)) {
	print $line;
    }
    exit;

And here's a corresponding server.  You don't have to worry about silly
network terminators here because Unix domain sockets are guaranteed
to be on the localhost, and thus everything works right.

    #!/usr/bin/perl -Tw
    use strict;
    use Socket;
    use Carp;

    BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
    sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }

    my $NAME = '/tmp/catsock';
    my $uaddr = sockaddr_un($NAME);
    my $proto = getprotobyname('tcp');

    socket(Server,PF_UNIX,SOCK_STREAM,0) 	|| die "socket: $!";
    unlink($NAME);
    bind  (Server, $uaddr) 			|| die "bind: $!";
    listen(Server,SOMAXCONN)			|| die "listen: $!";

    logmsg "server started on $NAME";

    my $waitedpid;

    sub REAPER {
	$waitedpid = wait;
	$SIG{CHLD} = \&REAPER;  # loathe sysV
	logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
    }

    $SIG{CHLD} = \&REAPER;


    for ( $waitedpid = 0;
	  accept(Client,Server) || $waitedpid;
	  $waitedpid = 0, close Client)
    {
	next if $waitedpid;
	logmsg "connection on $NAME";
	spawn sub {
	    print "Hello there, it's now ", scalar localtime, "\n";
	    exec '/usr/games/fortune' or die "can't exec fortune: $!";
	};
    }

As you see, it's remarkably similar to the Internet domain TCP server, so
much so, in fact, that we've omitted several duplicate functions--spawn(),
logmsg(), ctime(), and REAPER()--which are exactly the same as in the
other server.

So why would you ever want to use a Unix domain socket instead of a
simpler named pipe?  Because a named pipe doesn't give you sessions.  You
can't tell one process's data from another's.  With socket programming,
you get a separate session for each client: that's why accept() takes two
arguments.

For example, let's say that you have a long running database server daemon
that you want folks from the World Wide Web to be able to access, but only
if they go through a CGI interface.  You'd have a small, simple CGI
program that does whatever checks and logging you feel like, and then acts
as a Unix-domain client and connects to your private server.

=head1 TCP Clients with IO::Socket

For those preferring a higher-level interface to socket programming, the
IO::Socket module provides an object-oriented approach.  IO::Socket is
included as part of the standard Perl distribution as of the 5.004
release.  If you're running an earlier version of Perl, just fetch
IO::Socket from CPAN, where you'll also find find modules providing easy
interfaces to the following systems: DNS, FTP, Ident (RFC 931), NIS and
NISPlus, NNTP, Ping, POP3, SMTP, SNMP, SSLeay, Telnet, and Time--just
to name a few.

=head2 A Simple Client

Here's a client that creates a TCP connection to the "daytime"
service at port 13 of the host name "localhost" and prints out everything
that the server there cares to provide.

    #!/usr/bin/perl -w
    use IO::Socket;
    $remote = IO::Socket::INET->new(
			Proto    => "tcp",
			PeerAddr => "localhost",
			PeerPort => "daytime(13)",
		    )
		  or die "cannot connect to daytime port at localhost";
    while ( <$remote> ) { print }

When you run this program, you should get something back that
looks like this:

    Wed May 14 08:40:46 MDT 1997

Here are what those parameters to the C<new> constructor mean:

=over

=item C<Proto>

This is which protocol to use.  In this case, the socket handle returned
will be connected to a TCP socket, because we want a stream-oriented
connection, that is, one that acts pretty much like a plain old file.
Not all sockets are this of this type.  For example, the UDP protocol
can be used to make a datagram socket, used for message-passing.

=item C<PeerAddr>

This is the name or Internet address of the remote host the server is
running on.  We could have specified a longer name like C<"www.perl.com">,
or an address like C<"204.148.40.9">.  For demonstration purposes, we've
used the special hostname C<"localhost">, which should always mean the
current machine you're running on.  The corresponding Internet address
for localhost is C<"127.1">, if you'd rather use that.

=item C<PeerPort>

This is the service name or port number we'd like to connect to.
We could have gotten away with using just C<"daytime"> on systems with a
well-configured system services file,[FOOTNOTE: The system services file
is in I</etc/services> under Unix] but just in case, we've specified the
port number (13) in parentheses.  Using just the number would also have
worked, but constant numbers make careful programmers nervous.

=back

Notice how the return value from the C<new> constructor is used as
a filehandle in the C<while> loop?  That's what's called an indirect
filehandle, a scalar variable containing a filehandle.  You can use
it the same way you would a normal filehandle.  For example, you
can read one line from it this way:

    $line = <$handle>;

all remaining lines from is this way:

    @lines = <$handle>;

and send a line of data to it this way:

    print $handle "some data\n";

=head2 A Webget Client

Here's a simple client that takes a remote host to fetch a document
from, and then a list of documents to get from that host.  This is a
more interesting client than the previous one because it first sends
something to the server before fetching the server's response.

    #!/usr/bin/perl -w
    use IO::Socket;
    unless (@ARGV > 1) { die "usage: $0 host document ..." }
    $host = shift(@ARGV);
    $EOL = "\015\012";
    $BLANK = $EOL x 2;
    foreach $document ( @ARGV ) {
	$remote = IO::Socket::INET->new( Proto     => "tcp",
					 PeerAddr  => $host,
					 PeerPort  => "http(80)",
				        );
	unless ($remote) { die "cannot connect to http daemon on $host" }
	$remote->autoflush(1);
	print $remote "GET $document HTTP/1.0" . $BLANK;
	while ( <$remote> ) { print }
	close $remote;
    }

The web server handing the "http" service, which is assumed to be at
its standard port, number 80.  If your the web server you're trying to
connect to is at a different port (like 1080 or 8080), you should specify
as the named-parameter pair, C<PeerPort =E<gt> 8080>.  The C<autoflush>
method is used on the socket because otherwise the system would buffer
up the output we sent it.  (If you're on a Mac, you'll also need to
change every C<"\n"> in your code that sends data over the network to
be a C<"\015\012"> instead.)

Connecting to the server is only the first part of the process: once you
have the connection, you have to use the server's language.  Each server
on the network has its own little command language that it expects as
input.  The string that we send to the server starting with "GET" is in
HTTP syntax.  In this case, we simply request each specified document.
Yes, we really are making a new connection for each document, even though
it's the same host.  That's the way you always used to have to speak HTTP.
Recent versions of web browsers may request that the remote server leave
the connection open a little while, but the server doesn't have to honor
such a request.

Here's an example of running that program, which we'll call I<webget>:

    % webget www.perl.com /guanaco.html
    HTTP/1.1 404 File Not Found
    Date: Thu, 08 May 1997 18:02:32 GMT
    Server: Apache/1.2b6
    Connection: close
    Content-type: text/html

    <HEAD><TITLE>404 File Not Found</TITLE></HEAD>
    <BODY><H1>File Not Found</H1>
    The requested URL /guanaco.html was not found on this server.<P>
    </BODY>

Ok, so that's not very interesting, because it didn't find that
particular document.  But a long response wouldn't have fit on this page.

For a more fully-featured version of this program, you should look to
the I<lwp-request> program included with the LWP modules from CPAN.

=head2 Interactive Client with IO::Socket

Well, that's all fine if you want to send one command and get one answer,
but what about setting up something fully interactive, somewhat like
the way I<telnet> works?  That way you can type a line, get the answer,
type a line, get the answer, etc.

This client is more complicated than the two we've done so far, but if
you're on a system that supports the powerful C<fork> call, the solution
isn't that rough.  Once you've made the connection to whatever service
you'd like to chat with, call C<fork> to clone your process.  Each of
these two identical process has a very simple job to do: the parent
copies everything from the socket to standard output, while the child
simultaneously copies everything from standard input to the socket.
To accomplish the same thing using just one process would be I<much>
harder, because it's easier to code two processes to do one thing than it
is to code one process to do two things.  (This keep-it-simple principle
a cornerstones of the Unix philosophy, and good software engineering as
well, which is probably why it's spread to other systems.)