5.002 beta 1

If you're adventurous, have a look at

    ftp://ftp.sems.com/pub/outgoing/perl5.0/perl5.002beta1.tar.gz

Many thanks to Andy for doing the integration.

Obviously, if you consult the bugs database, you'll note there are
still plenty of buglets that need fixing, and several enhancements that
I've intended to put in still haven't made it in (Hi, Tim and Ilya).
But I think it'll be pretty stable.  And you can start to fiddle around
with prototypes (which are, of course, still totally undocumented).

Packrats, don't worry too much about readvertising this widely.
Nowadays we're on a T1 here, so our bandwidth is okay.

Have the appropriate amount of jollity.

Larry

1 files changed, 774 insertions, 68 deletions

diff --git a/pod/perlipc.pod b/pod/perlipc.pod
index 5a43660fb2..3166f1a75e 100644
--- a/pod/perlipc.pod
+++ b/pod/perlipc.pod
@@ -1,108 +1,751 @@
 =head1 NAME
 
-perlipc - Perl interprocess communication
+perlipc - Perl interprocess communication (signals, fifos, pipes, safe
+subprocceses, sockets, and semaphores)
 
 =head1 DESCRIPTION
 
-The IPC facilities of Perl are built on the Berkeley socket mechanism.
-If you don't have sockets, you can ignore this section.  The calls have
-the same names as the corresponding system calls, but the arguments
-tend to differ, for two reasons.  First, Perl file handles work
-differently than C file descriptors.  Second, Perl already knows the
-length of its strings, so you don't need to pass that information.
+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 an 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 with a global variable and then raise an
+exception.  That's because on most systems libraries are not
+re-entrant, 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, just do 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 => -$$;
+	# snazzy writing of: kill('HUP', -$$)
+    }
 
-=head2 Client/Server Communication
+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.  
 
-Here's a sample TCP client.
+    unless (kill 0 => $kid_pid) {
+	warn "something wicked happened to $kid_pid";
+    } 
 
-    ($them,$port) = @ARGV;
-    $port = 2345 unless $port;
-    $them = 'localhost' unless $them;
+You might also want to employ anonymous functions for simple signal
+handlers:
 
-    $SIG{'INT'} = 'dokill';
-    sub dokill { kill 9,$child if $child; }
+    $SIG{INT} = sub { die "\nOutta here!\n" };
 
-    use Socket;
+But that will be problematic for the more complicated handlers that need
+to re-install themselves.  Because Perl's signal mechanism is currently
+based on the signal(3) function from the C library, you may somtimes 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:
 
-    $sockaddr = 'S n a4 x8';
-    chop($hostname = `hostname`);
+    sub REAPER { 
+	$SIG{CHLD} = \&REAPER;  # loathe sysV
+	$waitedpid = wait;
+    }
+    $SIG{CHLD} = \&REAPER;
+    # now do something that forks...
+
+or even the more elaborate:
+
+    use POSIX "wait_h";
+    sub REAPER { 
+	my $child;
+	$SIG{CHLD} = \&REAPER;  # loathe sysV
+        while ($child = waitpid(-1,WNOHANG)) {
+	    $Kid_Status{$child} = $?;
+	} 
+    }
+    $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, newsreader, finger program, etc.) tries to read
+from that file, the reading program will block and your program will
+supply the 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 sigs
+    }
 
-    ($name, $aliases, $proto) = getprotobyname('tcp');
-    ($name, $aliases, $port) = getservbyname($port, 'tcp')
-	unless $port =~ /^\d+$/;
-    ($name, $aliases, $type, $len, $thisaddr) =
-		    gethostbyname($hostname);
-    ($name, $aliases, $type, $len, $thataddr) = gethostbyname($them);
 
-    $this = pack($sockaddr, AF_INET, 0, $thisaddr);
-    $that = pack($sockaddr, AF_INET, $port, $thataddr);
+=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 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 SPOOLER || 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 just return 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 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, "-|");
+	unless (defined $pid) {
+	    warn "cannot fork: $!";
+	    die "bailing out" if $sleep_count++ > 6;
+	    sleep 10;
+	} 
+    } until defined $pid;
+
+    if ($pid) {  # parent
+	print KID @some_data;
+	close(KID) || 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
+straigh-forward, 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
+    $pid = open(KID, "-|");
+
+    if ($pid) {   # parent
+	while (<KID>) {
+	    # do something interesting
+	} 	  
+	close(KID) || 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, "|-");
+    $SIG{ALRM} = sub { die "whoops, $program pipe broke" };
+
+    if ($pid) {  # parent
+	for (@data) {
+	    print KID;
+	} 
+	close(KID) || 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 L<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(KID, "| some program |")
+
+and if you forgot 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 tridirectional 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 going to really
+ruin your day.  Even though your C<Writer> filehandle is autoflushed,
+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 actually 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 non-standard 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>;
+    }
 
-    socket(S, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
-    bind(S, $this) || die "bind: $!";
-    connect(S, $that) || die "connect: $!";
+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 hopefully its 
+successor F<IPC::Chat>) at your nearest CPAN archive as detailed
+in the L<SEE ALSO> section below.
 
-    select(S); $| = 1; select(stdout);
+=head1 Sockets: Client/Server Communication
 
-    if ($child = fork) {
-	while (<>) {
-	    print S;
-	}
-	sleep 3;
-	do dokill();
-    }
-    else {
-	while (<S>) {
-	    print;
-	}
-    }
+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 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.
 
-And here's a server:
+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.
 
-    ($port) = @ARGV;
-    $port = 2345 unless $port;
+=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: $!";
+    while ($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:
+
+    #!/usr/bin/perl -Tw
+    require 5.002;
+    use strict;
+    BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
     use Socket;
+    use Carp;
 
-    $sockaddr = 'S n a4 x8';
+    sub spawn;  # forward declaration
+    sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" } 
 
-    ($name, $aliases, $proto) = getprotobyname('tcp');
-    ($name, $aliases, $port) = getservbyname($port, 'tcp')
-	unless $port =~ /^\d+$/;
+    my $port = shift || 2345;
+    my $proto = getprotobyname('tcp');
+    socket(SERVER, PF_INET, SOCK_STREAM, $proto)	|| die "socket: $!";
+    setsockopt(SERVER, SOL_SOCKET, SO_REUSEADDR, 1) 	|| die "setsockopt: $!";
+    bind(SERVER, sockaddr_in($port, INADDR_ANY))	|| die "bind: $!";
+    listen(SERVER,5) 					|| die "listen: $!";
 
-    $this = pack($sockaddr, AF_INET, $port, "\0\0\0\0");
+    logmsg "server started on port $port";
 
-    select(NS); $| = 1; select(stdout);
+    my $waitedpid = 0;
+    my $paddr;
 
-    socket(S, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
-    bind(S, $this) || die "bind: $!";
-    listen(S, 5) || die "connect: $!";
+    sub REAPER { 
+	$SIG{CHLD} = \&REAPER;  # loathe sysV
+	$waitedpid = wait;
+	logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
+    }
+
+    $SIG{CHLD} = \&REAPER;
+
+    for ( $waitedpid = 0; 
+	  ($paddr = accept(CLIENT,SERVER)) || $waitedpid; 
+	  $waitedpid = 0, close CLIENT) 
+    {
+	next if $waitedpid;
+	my($port,$iaddr) = sockaddr_in($paddr);
+	my $name = gethostbyaddr($iaddr,AF_INET);
+
+	logmsg "connection from $name [", 
+		inet_ntoa($iaddr), "] 
+		at port $port";
 
-    select(S); $| = 1; select(stdout);
+	spawn sub { 
+	    print "Hello there, $name, it's now ", scalar localtime, "\n";
+	    exec '/usr/games/fortune' 
+		or confess "can't exec fortune: $!";
+	};
 
-    for (;;) {
-	print "Listening again\n";
-	($addr = accept(NS,S)) || die $!;
-	print "accept ok\n";
+    } 
 
-	($af,$port,$inetaddr) = unpack($sockaddr,$addr);
-	@inetaddr = unpack('C4',$inetaddr);
-	print "$af $port @inetaddr\n";
+    sub spawn {
+	my $coderef = shift;
 
-	while (<NS>) {
-	    print;
-	    print NS;
+	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 SysV IPC
+=head2 Unix-Domain TCP Clients and Servers
+
+That's fine for Internet-domain clients and servers, but what 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
 
-Here's a small example showing shared memory usage:
+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: $!";
+    connect(SOCK, sockaddr_un($remote))		|| die "connect: $!";
+    while ($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,5)			|| 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 just use 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, $size , 0777 );
-    die if !defined($key);
+    die unless defined $key;
 
     $message = "Message #1";
     shmwrite($key, $message, 0, 60 ) || die "$!";
@@ -149,7 +792,7 @@ Call the file F<take>:
 Put this code in a separate file to be run in more that one process
 Call this file F<give>:
 
-    #'give' the semaphore
+    # 'give' the semaphore
     # run this in the original process and you will see
     # that the second process continues
 
@@ -166,3 +809,66 @@ Call this file F<give>:
 
     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 of the F<Socket.pm> module, which
+was/is/shall-be included in I<perl5.001o>.
+
+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 fo 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 filedescriptors 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 
+re-entrant, 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<perlmod> 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.