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=head1 NAME

perlipc - Perl interprocess communication (signals, fifos, pipes, safe subprocesses, sockets, and semaphores)

=head1 DESCRIPTION

The basic IPC facilities of Perl are built out of the good old Unix
signals, named pipes, pipe opens, the Berkeley socket routines, and SysV
IPC calls.  Each is used in slightly different situations.

=head1 Signals

Perl uses a simple signal handling model: the %SIG hash contains names or
references of user-installed signal handlers.  These handlers will be called
with an argument which is the name of the signal that triggered it.  A
signal may be generated intentionally from a particular keyboard sequence like
control-C or control-Z, sent to you from another process, or
triggered automatically by the kernel when special events transpire, like
a child process exiting, your process running out of stack space, or
hitting file size limit.

For example, to trap an interrupt signal, set up a handler like this.
Notice how all we do is set a global variable and then raise an
exception.  That's because on most systems libraries are not
reentrant, so calling any print() functions (or even anything that needs to
malloc(3) more memory) could in theory trigger a memory fault
and subsequent core dump.

    sub catch_zap {
	my $signame = shift;
	$shucks++;
	die "Somebody sent me a SIG$signame";
    }
    $SIG{INT} = 'catch_zap';  # could fail in modules
    $SIG{INT} = \&catch_zap;  # best strategy

The names of the signals are the ones listed out by C<kill -l> on your
system, or you can retrieve them from the Config module.  Set up an
@signame list indexed by number to get the name and a %signo table
indexed by name to get the number:

    use Config;
    defined $Config{sig_name} || die "No sigs?";
    foreach $name (split(' ', $Config{sig_name})) {
	$signo{$name} = $i;
	$signame[$i] = $name;
	$i++;
    }

So to check whether signal 17 and SIGALRM were the same, do just this:

    print "signal #17 = $signame[17]\n";
    if ($signo{ALRM}) {
	print "SIGALRM is $signo{ALRM}\n";
    }

You may also choose to assign the strings C<'IGNORE'> or C<'DEFAULT'> as
the handler, in which case Perl will try to discard the signal or do the
default thing.  Some signals can be neither trapped nor ignored, such as
the KILL and STOP (but not the TSTP) signals.  One strategy for
temporarily ignoring signals is to use a local() statement, which will be
automatically restored once your block is exited.  (Remember that local()
values are "inherited" by functions called from within that block.)

    sub precious {
	local $SIG{INT} = 'IGNORE';
	&more_functions;
    }
    sub more_functions {
	# interrupts still ignored, for now...
    }

Sending a signal to a negative process ID means that you send the signal
to the entire Unix process-group.  This code send a hang-up signal to all
processes in the current process group I<except for> the current process
itself:

    {
	local $SIG{HUP} = 'IGNORE';
	kill HUP => -$$;
	# snazzy writing of: kill('HUP', -$$)
    }

Another interesting signal to send is signal number zero.  This doesn't
actually affect another process, but instead checks whether it's alive
or has changed its UID.

    unless (kill 0 => $kid_pid) {
	warn "something wicked happened to $kid_pid";
    }

You might also want to employ anonymous functions for simple signal
handlers:

    $SIG{INT} = sub { die "\nOutta here!\n" };

But that will be problematic for the more complicated handlers that need
to reinstall themselves.  Because Perl's signal mechanism is currently
based on the signal(3) function from the C library, you may sometimes be so
misfortunate as to run on systems where that function is "broken", that
is, it behaves in the old unreliable SysV way rather than the newer, more
reasonable BSD and POSIX fashion.  So you'll see defensive people writing
signal handlers like this:

    sub REAPER {
	$waitedpid = wait;
	# loathe sysV: it makes us not only reinstate
	# the handler, but place it after the wait
	$SIG{CHLD} = \&REAPER;
    }
    $SIG{CHLD} = \&REAPER;
    # now do something that forks...

or even the more elaborate:

    use POSIX ":sys_wait_h";
    sub REAPER {
	my $child;
        while ($child = waitpid(-1,WNOHANG)) {
	    $Kid_Status{$child} = $?;
	}
	$SIG{CHLD} = \&REAPER;  # still loathe sysV
    }
    $SIG{CHLD} = \&REAPER;
    # do something that forks...

Signal handling is also used for timeouts in Unix,   While safely
protected within an C<eval{}> block, you set a signal handler to trap
alarm signals and then schedule to have one delivered to you in some
number of seconds.  Then try your blocking operation, clearing the alarm
when it's done but not before you've exited your C<eval{}> block.  If it
goes off, you'll use die() to jump out of the block, much as you might
using longjmp() or throw() in other languages.

Here's an example:

    eval {
        local $SIG{ALRM} = sub { die "alarm clock restart" };
        alarm 10;
        flock(FH, 2);   # blocking write lock
        alarm 0;
    };
    if ($@ and $@ !~ /alarm clock restart/) { die }

For more complex signal handling, you might see the standard POSIX
module.  Lamentably, this is almost entirely undocumented, but
the F<t/lib/posix.t> file from the Perl source distribution has some
examples in it.

=head1 Named Pipes

A named pipe (often referred to as a FIFO) is an old Unix IPC
mechanism for processes communicating on the same machine.  It works
just like a regular, connected anonymous pipes, except that the
processes rendezvous using a filename and don't have to be related.

To create a named pipe, use the Unix command mknod(1) or on some
systems, mkfifo(1).  These may not be in your normal path.

    # system return val is backwards, so && not ||
    #
    $ENV{PATH} .= ":/etc:/usr/etc";
    if  (      system('mknod',  $path, 'p')
	    && system('mkfifo', $path) )
    {
	die "mk{nod,fifo} $path failed;
    }


A fifo is convenient when you want to connect a process to an unrelated
one.  When you open a fifo, the program will block until there's something
on the other end.

For example, let's say you'd like to have your F<.signature> file be a
named pipe that has a Perl program on the other end.  Now every time any
program (like a mailer, news reader, finger program, etc.) tries to read
from that file, the reading program will block and your program will
supply the new signature.  We'll use the pipe-checking file test B<-p>
to find out whether anyone (or anything) has accidentally removed our fifo.

    chdir; # go home
    $FIFO = '.signature';
    $ENV{PATH} .= ":/etc:/usr/games";

    while (1) {
	unless (-p $FIFO) {
	    unlink $FIFO;
	    system('mknod', $FIFO, 'p')
		&& die "can't mknod $FIFO: $!";
	}

	# next line blocks until there's a reader
	open (FIFO, "> $FIFO") || die "can't write $FIFO: $!";
	print FIFO "John Smith (smith\@host.org)\n", `fortune -s`;
	close FIFO;
	sleep 2;    # to avoid dup signals
    }


=head1 Using open() for IPC

Perl's basic open() statement can also be used for unidirectional interprocess
communication by either appending or prepending a pipe symbol to the second
argument to open().  Here's how to start something up in a child process you
intend to write to:

    open(SPOOLER, "| cat -v | lpr -h 2>/dev/null")
		    || die "can't fork: $!";
    local $SIG{PIPE} = sub { die "spooler pipe broke" };
    print SPOOLER "stuff\n";
    close SPOOLER || die "bad spool: $! $?";

And here's how to start up a child process you intend to read from:

    open(STATUS, "netstat -an 2>&1 |")
		    || die "can't fork: $!";
    while (<STATUS>) {
	next if /^(tcp|udp)/;
	print;
    }
    close STATUS || die "bad netstat: $! $?";

If one can be sure that a particular program is a Perl script that is
expecting filenames in @ARGV, the clever programmer can write something
like this:

    $ program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile

and irrespective of which shell it's called from, the Perl program will
read from the file F<f1>, the process F<cmd1>, standard input (F<tmpfile>
in this case), the F<f2> file, the F<cmd2> command, and finally the F<f3>
file.  Pretty nifty, eh?

You might notice that you could use backticks for much the
same effect as opening a pipe for reading:

    print grep { !/^(tcp|udp)/ } `netstat -an 2>&1`;
    die "bad netstat" if $?;

While this is true on the surface, it's much more efficient to process the
file one line or record at a time because then you don't have to read the
whole thing into memory at once. It also gives you finer control of the
whole process, letting you to kill off the child process early if you'd
like.

Be careful to check both the open() and the close() return values.  If
you're I<writing> to a pipe, you should also trap SIGPIPE.  Otherwise,
think of what happens when you start up a pipe to a command that doesn't
exist: the open() will in all likelihood succeed (it only reflects the
fork()'s success), but then your output will fail--spectacularly.  Perl
can't know whether the command worked because your command is actually
running in a separate process whose exec() might have failed.  Therefore,
while readers of bogus commands return just a quick end of file, writers
to bogus command will trigger a signal they'd better be prepared to
handle.  Consider:

    open(FH, "|bogus");
    print FH "bang\n";
    close FH;

=head2 Filehandles

Both the main process and the child process share the same STDIN,
STDOUT and STDERR filehandles.  If both processes try to access them
at once, strange things can happen.  You may want to close or reopen
the filehandles for the child.  You can get around this by opening
your pipe with open(), but on some systems this means that the child
process cannot outlive the parent.

=head2 Background Processes

You can run a command in the background with:

    system("cmd&");

The command's STDOUT and STDERR (and possibly STDIN, depending on your
shell) will be the same as the parent's.  You won't need to catch
SIGCHLD because of the double-fork taking place (see below for more
details).

=head2 Complete Dissociation of Child from Parent

In some cases (starting server processes, for instance) you'll want to
complete dissociate the child process from the parent.  The following
process is reported to work on most Unixish systems.  Non-Unix users
should check their Your_OS::Process module for other solutions.

=over 4

=item *

Open /dev/tty and use the TIOCNOTTY ioctl on it.  See L<tty(4)>
for details.

=item *

Change directory to /

=item *

Reopen STDIN, STDOUT, and STDERR so they're not connected to the old
tty.

=item *

Background yourself like this:

    fork && exit;

=back

=head2 Safe Pipe Opens

Another interesting approach to IPC is making your single program go
multiprocess and communicate between (or even amongst) yourselves.  The
open() function will accept a file argument of either C<"-|"> or C<"|-">
to do a very interesting thing: it forks a child connected to the
filehandle you've opened.  The child is running the same program as the
parent.  This is useful for safely opening a file when running under an
assumed UID or GID, for example.  If you open a pipe I<to> minus, you can
write to the filehandle you opened and your kid will find it in his
STDIN.  If you open a pipe I<from> minus, you can read from the filehandle
you opened whatever your kid writes to his STDOUT.

    use English;
    my $sleep_count = 0;

    do {
	$pid = open(KID_TO_WRITE, "|-");
	unless (defined $pid) {
	    warn "cannot fork: $!";
	    die "bailing out" if $sleep_count++ > 6;
	    sleep 10;
	}
    } until defined $pid;

    if ($pid) {  # parent
	print KID_TO_WRITE @some_data;
	close(KID_TO_WRITE) || warn "kid exited $?";
    } else {     # child
	($EUID, $EGID) = ($UID, $GID); # suid progs only
	open (FILE, "> /safe/file")
	    || die "can't open /safe/file: $!";
	while (<STDIN>) {
	    print FILE; # child's STDIN is parent's KID
	}
	exit;  # don't forget this
    }

Another common use for this construct is when you need to execute
something without the shell's interference.  With system(), it's
straightforward, but you can't use a pipe open or backticks safely.
That's because there's no way to stop the shell from getting its hands on
your arguments.   Instead, use lower-level control to call exec() directly.

Here's a safe backtick or pipe open for read:

    # add error processing as above
    $pid = open(KID_TO_READ, "-|");

    if ($pid) {   # parent
	while (<KID_TO_READ>) {
	    # do something interesting
	}
	close(KID_TO_READ) || warn "kid exited $?";

    } else {      # child
	($EUID, $EGID) = ($UID, $GID); # suid only
	exec($program, @options, @args)
	    || die "can't exec program: $!";
	# NOTREACHED
    }


And here's a safe pipe open for writing:

    # add error processing as above
    $pid = open(KID_TO_WRITE, "|-");
    $SIG{ALRM} = sub { die "whoops, $program pipe broke" };

    if ($pid) {  # parent
	for (@data) {
	    print KID_TO_WRITE;
	}
	close(KID_TO_WRITE) || warn "kid exited $?";

    } else {     # child
	($EUID, $EGID) = ($UID, $GID);
	exec($program, @options, @args)
	    || die "can't exec program: $!";
	# NOTREACHED
    }

Note that these operations are full Unix forks, which means they may not be
correctly implemented on alien systems.  Additionally, these are not true
multithreading.  If you'd like to learn more about threading, see the
F<modules> file mentioned below in the SEE ALSO section.

=head2 Bidirectional Communication

While this works reasonably well for unidirectional communication, what
about bidirectional communication?  The obvious thing you'd like to do
doesn't actually work:

    open(PROG_FOR_READING_AND_WRITING, "| some program |")

and if you forget to use the B<-w> flag, then you'll miss out
entirely on the diagnostic message:

    Can't do bidirectional pipe at -e line 1.

If you really want to, you can use the standard open2() library function
to catch both ends.  There's also an open3() for tri-directional I/O so you
can also catch your child's STDERR, but doing so would then require an
awkward select() loop and wouldn't allow you to use normal Perl input
operations.

If you look at its source, you'll see that open2() uses low-level
primitives like Unix pipe() and exec() to create all the connections.
While it might have been slightly more efficient by using socketpair(), it
would have then been even less portable than it already is.  The open2()
and open3() functions are  unlikely to work anywhere except on a Unix
system or some other one purporting to be POSIX compliant.

Here's an example of using open2():

    use FileHandle;
    use IPC::Open2;
    $pid = open2( \*Reader, \*Writer, "cat -u -n" );
    Writer->autoflush(); # default here, actually
    print Writer "stuff\n";
    $got = <Reader>;

The problem with this is that Unix buffering is really going to
ruin your day.  Even though your C<Writer> filehandle is auto-flushed,
and the process on the other end will get your data in a timely manner,
you can't usually do anything to force it to give it back to you
in a similarly quick fashion.  In this case, we could, because we
gave I<cat> a B<-u> flag to make it unbuffered.  But very few Unix
commands are designed to operate over pipes, so this seldom works
unless you yourself wrote the program on the other end of the
double-ended pipe.

A solution to this is the nonstandard F<Comm.pl> library.  It uses
pseudo-ttys to make your program behave more reasonably:

    require 'Comm.pl';
    $ph = open_proc('cat -n');
    for (1..10) {
	print $ph "a line\n";
	print "got back ", scalar <$ph>;
    }

This way you don't have to have control over the source code of the
program you're using.  The F<Comm> library also has expect()
and interact() functions.  Find the library (and we hope its
successor F<IPC::Chat>) at your nearest CPAN archive as detailed
in the SEE ALSO section below.

=head1 Sockets: Client/Server Communication

While not limited to Unix-derived operating systems (e.g., WinSock on PCs
provides socket support, as do some VMS libraries), you may not have
sockets on your system, in which case this section probably isn't going to do
you much good.  With sockets, you can do both virtual circuits (i.e., TCP
streams) and datagrams (i.e., UDP packets).  You may be able to do even more
depending on your system.

The Perl function calls for dealing with sockets have the same names as
the corresponding system calls in C, but their arguments tend to differ
for two reasons: first, Perl filehandles work differently than C file
descriptors.  Second, Perl already knows the length of its strings, so you
don't need to pass that information.

One of the major problems with old socket code in Perl was that it used
hard-coded values for some of the constants, which severely hurt
portability.  If you ever see code that does anything like explicitly
setting C<$AF_INET = 2>, you know you're in for big trouble:  An
immeasurably superior approach is to use the C<Socket> module, which more
reliably grants access to various constants and functions you'll need.

If you're not writing a server/client for an existing protocol like
NNTP or SMTP, you should give some thought to how your server will
know when the client has finished talking, and vice-versa.  Most
protocols are based on one-line messages and responses (so one party
knows the other has finished when a "\n" is received) or multi-line
messages and responses that end with a period on an empty line
("\n.\n" terminates a message/response).

=head2 Internet TCP Clients and Servers

Use Internet-domain sockets when you want to do client-server
communication that might extend to machines outside of your own system.

Here's a sample TCP client using Internet-domain sockets:

    #!/usr/bin/perl -w
    require 5.002;
    use strict;
    use Socket;
    my ($remote,$port, $iaddr, $paddr, $proto, $line);

    $remote  = shift || 'localhost';
    $port    = shift || 2345;  # random port
    if ($port =~ /\D/) { $port = getservbyname($port, 'tcp') }
    die "No port" unless $port;
    $iaddr   = inet_aton($remote) 		|| die "no host: $remote";
    $paddr   = sockaddr_in($port, $iaddr);

    $proto   = getprotobyname('tcp');
    socket(SOCK, PF_INET, SOCK_STREAM, $proto)	|| die "socket: $!";
    connect(SOCK, $paddr)    || die "connect: $!";
    while (defined($line = <SOCK>)) {
	print $line;
    }

    close (SOCK)	    || die "close: $!";
    exit;

And here's a corresponding server to go along with it.  We'll
leave the address as INADDR_ANY so that the kernel can choose
the appropriate interface on multihomed hosts.  If you want sit
on a particular interface (like the external side of a gateway
or firewall machine), you should fill this in with your real address
instead.

    #!/usr/bin/perl -Tw
    require 5.002;
    use strict;
    BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
    use Socket;
    use Carp;

    sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }

    my $port = shift || 2345;
    my $proto = getprotobyname('tcp');
    $port = $1 if $port =~ /(\d+)/; # untaint port number

    socket(Server, PF_INET, SOCK_STREAM, $proto)	|| die "socket: $!";
    setsockopt(Server, SOL_SOCKET, SO_REUSEADDR,
					pack("l", 1)) 	|| die "setsockopt: $!";
    bind(Server, sockaddr_in($port, INADDR_ANY))	|| die "bind: $!";
    listen(Server,SOMAXCONN) 				|| die "listen: $!";

    logmsg "server started on port $port";

    my $paddr;

    $SIG{CHLD} = \&REAPER;

    for ( ; $paddr = accept(Client,Server); close Client) {
	my($port,$iaddr) = sockaddr_in($paddr);
	my $name = gethostbyaddr($iaddr,AF_INET);

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

	print Client "Hello there, $name, it's now ",
			scalar localtime, "\n";
    }

And here's a multithreaded version.  It's multithreaded in that
like most typical servers, it spawns (forks) a slave server to
handle the client request so that the master server can quickly
go back to service a new client.

    #!/usr/bin/perl -Tw
    require 5.002;
    use strict;
    BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
    use Socket;
    use Carp;

    sub spawn;  # forward declaration
    sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }

    my $port = shift || 2345;
    my $proto = getprotobyname('tcp');
    $port = $1 if $port =~ /(\d+)/; # untaint port number

    socket(Server, PF_INET, SOCK_STREAM, $proto)	|| die "socket: $!";
    setsockopt(Server, SOL_SOCKET, SO_REUSEADDR,
					pack("l", 1)) 	|| die "setsockopt: $!";
    bind(Server, sockaddr_in($port, INADDR_ANY))	|| die "bind: $!";
    listen(Server,SOMAXCONN) 				|| die "listen: $!";

    logmsg "server started on port $port";

    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, "\n";
	    exec '/usr/games/fortune'
		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
    require 5.002;
    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
    require 5.002;
    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.

    #!/usr/bin/perl -Tw
    require 5.002;
    use strict;
    use Socket;
    use Carp;

    BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }

    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";

    $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.

=head2 UDP: Message Passing

Another kind of client-server setup is one that uses not connections, but
messages.  UDP communications involve much lower overhead but also provide
less reliability, as there are no promises that messages will arrive at
all, let alone in order and unmangled.  Still, UDP offers some advantages
over TCP, including being able to "broadcast" or "multicast" to a whole
bunch of destination hosts at once (usually on your local subnet).  If you
find yourself overly concerned about reliability and start building checks
into your message system, then you probably should use just TCP to start
with.

Here's a UDP program similar to the sample Internet TCP client given
above.  However, instead of checking one host at a time, the UDP version
will check many of them asynchronously by simulating a multicast and then
using select() to do a timed-out wait for I/O.  To do something similar
with TCP, you'd have to use a different socket handle for each host.

    #!/usr/bin/perl -w
    use strict;
    require 5.002;
    use Socket;
    use Sys::Hostname;

    my ( $count, $hisiaddr, $hispaddr, $histime,
	 $host, $iaddr, $paddr, $port, $proto,
	 $rin, $rout, $rtime, $SECS_of_70_YEARS);

    $SECS_of_70_YEARS      = 2208988800;

    $iaddr = gethostbyname(hostname());
    $proto = getprotobyname('udp');
    $port = getservbyname('time', 'udp');
    $paddr = sockaddr_in(0, $iaddr); # 0 means let kernel pick

    socket(SOCKET, PF_INET, SOCK_DGRAM, $proto)   || die "socket: $!";
    bind(SOCKET, $paddr)                          || die "bind: $!";

    $| = 1;
    printf "%-12s %8s %s\n",  "localhost", 0, scalar localtime time;
    $count = 0;
    for $host (@ARGV) {
	$count++;
	$hisiaddr = inet_aton($host) 	|| die "unknown host";
	$hispaddr = sockaddr_in($port, $hisiaddr);
	defined(send(SOCKET, 0, 0, $hispaddr))    || die "send $host: $!";
    }

    $rin = '';
    vec($rin, fileno(SOCKET), 1) = 1;

    # timeout after 10.0 seconds
    while ($count && select($rout = $rin, undef, undef, 10.0)) {
	$rtime = '';
	($hispaddr = recv(SOCKET, $rtime, 4, 0)) 	|| die "recv: $!";
	($port, $hisiaddr) = sockaddr_in($hispaddr);
	$host = gethostbyaddr($hisiaddr, AF_INET);
	$histime = unpack("N", $rtime) - $SECS_of_70_YEARS ;
	printf "%-12s ", $host;
	printf "%8d %s\n", $histime - time, scalar localtime($histime);
	$count--;
    }

=head1 SysV IPC

While System V IPC isn't so widely used as sockets, it still has some
interesting uses.  You can't, however, effectively use SysV IPC or
Berkeley mmap() to have shared memory so as to share a variable amongst
several processes.  That's because Perl would reallocate your string when
you weren't wanting it to.


Here's a small example showing shared memory usage.

    $IPC_PRIVATE = 0;
    $IPC_RMID = 0;
    $size = 2000;
    $key = shmget($IPC_PRIVATE, $size , 0777 );
    die unless defined $key;

    $message = "Message #1";
    shmwrite($key, $message, 0, 60 ) || die "$!";
    shmread($key,$buff,0,60) || die "$!";

    print $buff,"\n";

    print "deleting $key\n";
    shmctl($key ,$IPC_RMID, 0) || die "$!";

Here's an example of a semaphore:

    $IPC_KEY = 1234;
    $IPC_RMID = 0;
    $IPC_CREATE = 0001000;
    $key = semget($IPC_KEY, $nsems , 0666 | $IPC_CREATE );
    die if !defined($key);
    print "$key\n";

Put this code in a separate file to be run in more than one process.
Call the file F<take>:

    # create a semaphore

    $IPC_KEY = 1234;
    $key = semget($IPC_KEY,  0 , 0 );
    die if !defined($key);

    $semnum = 0;
    $semflag = 0;

    # 'take' semaphore
    # wait for semaphore to be zero
    $semop = 0;
    $opstring1 = pack("sss", $semnum, $semop, $semflag);

    # Increment the semaphore count
    $semop = 1;
    $opstring2 = pack("sss", $semnum, $semop,  $semflag);
    $opstring = $opstring1 . $opstring2;

    semop($key,$opstring) || die "$!";

Put this code in a separate file to be run in more than one process.
Call this file F<give>:

    # 'give' the semaphore
    # run this in the original process and you will see
    # that the second process continues

    $IPC_KEY = 1234;
    $key = semget($IPC_KEY, 0, 0);
    die if !defined($key);

    $semnum = 0;
    $semflag = 0;

    # Decrement the semaphore count
    $semop = -1;
    $opstring = pack("sss", $semnum, $semop, $semflag);

    semop($key,$opstring) || die "$!";

=head1 WARNING

The SysV IPC code above was written long ago, and it's definitely clunky
looking.  It should at the very least be made to C<use strict> and
C<require "sys/ipc.ph">.  Better yet, perhaps someone should create an
C<IPC::SysV> module the way we have the C<Socket> module for normal
client-server communications.

(... time passes)

Voila!  Check out the IPC::SysV modules written by Jack Shirazi.  You can
find them at a CPAN store near you.

=head1 NOTES

If you are running under version 5.000 (dubious) or 5.001, you can still
use most of the examples in this document.  You may have to remove the
C<use strict> and some of the my() statements for 5.000, and for both
you'll have to load in version 1.2 or older of the F<Socket.pm> module, which
is included in I<perl5.002>.

Most of these routines quietly but politely return C<undef> when they fail
instead of causing your program to die right then and there due to an
uncaught exception.  (Actually, some of the new I<Socket> conversion
functions  croak() on bad arguments.)  It is therefore essential
that you should check the return values of these functions.  Always begin
your socket programs this way for optimal success, and don't forget to add
B<-T> taint checking flag to the pound-bang line for servers:

    #!/usr/bin/perl -w
    require 5.002;
    use strict;
    use sigtrap;
    use Socket;

=head1 BUGS

All these routines create system-specific portability problems.  As noted
elsewhere, Perl is at the mercy of your C libraries for much of its system
behaviour.  It's probably safest to assume broken SysV semantics for
signals and to stick with simple TCP and UDP socket operations; e.g., don't
try to pass open file descriptors over a local UDP datagram socket if you
want your code to stand a chance of being portable.

Because few vendors provide C libraries that are safely
reentrant, the prudent programmer will do little else within
a handler beyond die() to raise an exception and longjmp(3) out.

=head1 AUTHOR

Tom Christiansen, with occasional vestiges of Larry Wall's original
version.

=head1 SEE ALSO

Besides the obvious functions in L<perlfunc>, you should also check out
the F<modules> file at your nearest CPAN site.  (See L<perlmodlib> or best
yet, the F<Perl FAQ> for a description of what CPAN is and where to get it.)
Section 5 of the F<modules> file is devoted to "Networking, Device Control
(modems), and Interprocess Communication", and contains numerous unbundled
modules numerous networking modules, Chat and Expect operations, CGI
programming, DCE, FTP, IPC, NNTP, Proxy, Ptty, RPC, SNMP, SMTP, Telnet,
Threads, and ToolTalk--just to name a few.