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PERLIPC(1)	       Perl Programmers	Reference Guide		    PERLIPC(1)

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

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.

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 own
       process running out of stack space, or hitting a	process	file-size
       limit.

       For example, to trap an interrupt signal, set up	a handler like this:

	   our $shucks;

	   sub catch_zap {
	       my $signame = shift;
	       $shucks++;
	       die "Somebody sent me a SIG$signame";
	   }
	   $SIG{INT} = __PACKAGE__ . "::catch_zap";
	   $SIG{INT} = \&catch_zap;  # best strategy

       Prior to	Perl 5.8.0 it was necessary to do as little as you possibly
       could in	your handler; notice how all we	do is set a global variable
       and then	raise an exception.  That's because on most systems, libraries
       are not re-entrant; particularly, memory	allocation and I/O routines
       are not.	 That meant that doing nearly anything in your handler could
       in theory trigger a memory fault	and subsequent core dump - see
       "Deferred Signals (Safe Signals)" below.

       The names of the	signals	are the	ones listed out	by "kill -l" on	your
       system, or you can retrieve them	using the CPAN module IPC::Signal.

       You may also choose to assign the strings "IGNORE" or "DEFAULT" as the
       handler,	in which case Perl will	try to discard the signal or do	the
       default thing.

       On most Unix platforms, the "CHLD" (sometimes also known	as "CLD")
       signal has special behavior with	respect	to a value of "IGNORE".
       Setting $SIG{CHLD} to "IGNORE" on such a	platform has the effect	of not
       creating	zombie processes when the parent process fails to "wait()" on
       its child processes (i.e., child	processes are automatically reaped).
       Calling "wait()"	with $SIG{CHLD}	set to "IGNORE"	usually	returns	"-1"
       on such platforms.

       Some signals can	be neither trapped nor ignored,	such as	the KILL and
       STOP (but not the TSTP) signals.	Note that ignoring signals makes them
       disappear.  If you only want them blocked temporarily without them
       getting lost you'll have	to use POSIX' sigprocmask.

       Sending a signal	to a negative process ID means that you	send the
       signal to the entire Unix process group.	 This code sends a hang-up
       signal to all processes in the current process group, and also sets
       $SIG{HUP} to "IGNORE" so	it doesn't kill	itself:

	   # block scope for local
	   {
	       local $SIG{HUP} = "IGNORE";
	       kill HUP	=> -getpgrp();
	       # snazzy	writing	of: kill("HUP",	-getpgrp())
	   }

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

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

       Signal number zero may fail because you lack permission to send the
       signal when directed at a process whose real or saved UID is not
       identical to the	real or	effective UID of the sending process, even
       though the process is alive.  You may be	able to	determine the cause of
       failure using $!	or "%!".

	   unless (kill(0 => $pid) || $!{EPERM}) {
	       warn "$pid looks	dead";
	   }

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

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

       SIGCHLD handlers	require	some special care.  If a second	child dies
       while in	the signal handler caused by the first death, we won't get
       another signal. So must loop here else we will leave the	unreaped child
       as a zombie. And	the next time two children die we get another zombie.
       And so on.

	   use POSIX ":sys_wait_h";
	   $SIG{CHLD} =	sub {
	       while ((my $child = waitpid(-1, WNOHANG)) > 0) {
		   $Kid_Status{$child} = $?;
	       }
	   };
	   # do	something that forks...

       Be careful: qx(), system(), and some modules for	calling	external
       commands	do a fork(), then wait() for the result. Thus, your signal
       handler will be called. Because wait() was already called by system()
       or qx(),	the wait() in the signal handler will see no more zombies and
       will therefore block.

       The best	way to prevent this issue is to	use waitpid(), as in the
       following example:

	   use POSIX ":sys_wait_h"; # for nonblocking read

	   my %children;

	   $SIG{CHLD} =	sub {
	       # don't change $! and $?	outside	handler
	       local ($!, $?);
	       while ( (my $pid	= waitpid(-1, WNOHANG))	> 0 ) {
		   delete $children{$pid};
		   cleanup_child($pid, $?);
	       }
	   };

	   while (1) {
	       my $pid = fork();
	       die "cannot fork" unless	defined	$pid;
	       if ($pid	== 0) {
		   # ...
		   exit	0;
	       } else {
		   $children{$pid}=1;
		   # ...
		   system($command);
		   # ...
	      }
	   }

       Signal handling is also used for	timeouts in Unix.  While safely
       protected within	an "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 "eval{}" block.
       If it goes off, you'll use die()	to jump	out of the block.

       Here's an example:

	   my $ALARM_EXCEPTION = "alarm	clock restart";
	   eval	{
	       local $SIG{ALRM}	= sub {	die $ALARM_EXCEPTION };
	       alarm 10;
	       flock(FH, 2)    # blocking write	lock
			       || die "cannot flock: $!";
	       alarm 0;
	   };
	   if ($@ && $@	!~ quotemeta($ALARM_EXCEPTION))	{ die }

       If the operation	being timed out	is system() or qx(), this technique is
       liable to generate zombies.    If this matters to you, you'll need to
       do your own fork() and exec(), and kill the errant child	process.

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

   Handling the	SIGHUP Signal in Daemons
       A process that usually starts when the system boots and shuts down when
       the system is shut down is called a daemon (Disk	And Execution
       MONitor). If a daemon process has a configuration file which is
       modified	after the process has been started, there should be a way to
       tell that process to reread its configuration file without stopping the
       process.	Many daemons provide this mechanism using a "SIGHUP" signal
       handler.	When you want to tell the daemon to reread the file, simply
       send it the "SIGHUP" signal.

       The following example implements	a simple daemon, which restarts	itself
       every time the "SIGHUP" signal is received. The actual code is located
       in the subroutine "code()", which just prints some debugging info to
       show that it works; it should be	replaced with the real code.

	 #!/usr/bin/perl

	 use strict;
	 use warnings;

	 use POSIX ();
	 use FindBin ();
	 use File::Basename ();
	 use File::Spec::Functions qw(catfile);

	 $| = 1;

	 # make	the daemon cross-platform, so exec always calls	the script
	 # itself with the right path, no matter how the script	was invoked.
	 my $script = File::Basename::basename($0);
	 my $SELF  = catfile($FindBin::Bin, $script);

	 # POSIX unmasks the sigprocmask properly
	 $SIG{HUP} = sub {
	     print "got	SIGHUP\n";
	     exec($SELF, @ARGV)	       || die "$0: couldn't restart: $!";
	 };

	 code();

	 sub code {
	     print "PID: $$\n";
	     print "ARGV: @ARGV\n";
	     my	$count = 0;
	     while (1) {
		 sleep 2;
		 print ++$count, "\n";
	     }
	 }

   Deferred Signals (Safe Signals)
       Before Perl 5.8.0, installing Perl code to deal with signals exposed
       you to danger from two things.  First, few system library functions are
       re-entrant.  If the signal interrupts while Perl	is executing one
       function	(like malloc(3)	or printf(3)), and your	signal handler then
       calls the same function again, you could	get unpredictable
       behavior--often,	a core dump.  Second, Perl isn't itself	re-entrant at
       the lowest levels.  If the signal interrupts Perl while Perl is
       changing	its own	internal data structures, similarly unpredictable
       behavior	may result.

       There were two things you could do, knowing this: be paranoid or	be
       pragmatic.  The paranoid	approach was to	do as little as	possible in
       your signal handler.  Set an existing integer variable that already has
       a value,	and return.  This doesn't help you if you're in	a slow system
       call, which will	just restart.  That means you have to "die" to
       longjmp(3) out of the handler.  Even this is a little cavalier for the
       true paranoiac, who avoids "die"	in a handler because the system	is out
       to get you.  The	pragmatic approach was to say "I know the risks, but
       prefer the convenience",	and to do anything you wanted in your signal
       handler,	and be prepared	to clean up core dumps now and again.

       Perl 5.8.0 and later avoid these	problems by "deferring"	signals.  That
       is, when	the signal is delivered	to the process by the system (to the C
       code that implements Perl) a flag is set, and the handler returns
       immediately.  Then at strategic "safe" points in	the Perl interpreter
       (e.g. when it is	about to execute a new opcode) the flags are checked
       and the Perl level handler from %SIG is executed. The "deferred"	scheme
       allows much more	flexibility in the coding of signal handlers as	we
       know the	Perl interpreter is in a safe state, and that we are not in a
       system library function when the	handler	is called.  However the
       implementation does differ from previous	Perls in the following ways:

       Long-running opcodes
	   As the Perl interpreter looks at signal flags only when it is about
	   to execute a	new opcode, a signal that arrives during a long-
	   running opcode (e.g.	a regular expression operation on a very large
	   string) will	not be seen until the current opcode completes.

	   If a	signal of any given type fires multiple	times during an	opcode
	   (such as from a fine-grained	timer),	the handler for	that signal
	   will	be called only once, after the opcode completes; all other
	   instances will be discarded.	 Furthermore, if your system's signal
	   queue gets flooded to the point that	there are signals that have
	   been	raised but not yet caught (and thus not	deferred) at the time
	   an opcode completes,	those signals may well be caught and deferred
	   during subsequent opcodes, with sometimes surprising	results.  For
	   example, you	may see	alarms delivered even after calling alarm(0)
	   as the latter stops the raising of alarms but does not cancel the
	   delivery of alarms raised but not yet caught.  Do not depend	on the
	   behaviors described in this paragraph as they are side effects of
	   the current implementation and may change in	future versions	of
	   Perl.

       Interrupting IO
	   When	a signal is delivered (e.g., SIGINT from a control-C) the
	   operating system breaks into	IO operations like read(2), which is
	   used	to implement Perl's readline() function, the "<>" operator. On
	   older Perls the handler was called immediately (and as "read" is
	   not "unsafe", this worked well). With the "deferred"	scheme the
	   handler is not called immediately, and if Perl is using the
	   system's "stdio" library that library may restart the "read"
	   without returning to	Perl to	give it	a chance to call the %SIG
	   handler. If this happens on your system the solution	is to use the
	   ":perlio" layer to do IO--at	least on those handles that you	want
	   to be able to break into with signals. (The ":perlio" layer checks
	   the signal flags and	calls %SIG handlers before resuming IO
	   operation.)

	   The default in Perl 5.8.0 and later is to automatically use the
	   ":perlio" layer.

	   Note	that it	is not advisable to access a file handle within	a
	   signal handler where	that signal has	interrupted an I/O operation
	   on that same	handle.	While perl will	at least try hard not to
	   crash, there	are no guarantees of data integrity; for example, some
	   data	might get dropped or written twice.

	   Some	networking library functions like gethostbyname() are known to
	   have	their own implementations of timeouts which may	conflict with
	   your	timeouts.  If you have problems	with such functions, try using
	   the POSIX sigaction() function, which bypasses Perl safe signals.
	   Be warned that this does subject you	to possible memory corruption,
	   as described	above.

	   Instead of setting $SIG{ALRM}:

	      local $SIG{ALRM} = sub { die "alarm" };

	   try something like the following:

	    use	POSIX qw(SIGALRM);
	    POSIX::sigaction(SIGALRM,
			     POSIX::SigAction->new(sub { die "alarm" }))
		     ||	die "Error setting SIGALRM handler: $!\n";

	   Another way to disable the safe signal behavior locally is to use
	   the "Perl::Unsafe::Signals" module from CPAN, which affects all
	   signals.

       Restartable system calls
	   On systems that supported it, older versions	of Perl	used the
	   SA_RESTART flag when	installing %SIG	handlers.  This	meant that
	   restartable system calls would continue rather than returning when
	   a signal arrived.  In order to deliver deferred signals promptly,
	   Perl	5.8.0 and later	do not use SA_RESTART.	Consequently,
	   restartable system calls can	fail (with $! set to "EINTR") in
	   places where	they previously	would have succeeded.

	   The default ":perlio" layer retries "read", "write" and "close" as
	   described above; interrupted	"wait" and "waitpid" calls will	always
	   be retried.

       Signals as "faults"
	   Certain signals like	SEGV, ILL, and BUS are generated by virtual
	   memory addressing errors and	similar	"faults". These	are normally
	   fatal: there	is little a Perl-level handler can do with them.  So
	   Perl	delivers them immediately rather than attempting to defer
	   them.

       Signals triggered by operating system state
	   On some operating systems certain signal handlers are supposed to
	   "do something" before returning. One	example	can be CHLD or CLD,
	   which indicates a child process has completed. On some operating
	   systems the signal handler is expected to "wait" for	the completed
	   child process. On such systems the deferred signal scheme will not
	   work	for those signals: it does not do the "wait". Again the
	   failure will	look like a loop as the	operating system will reissue
	   the signal because there are	completed child	processes that have
	   not yet been	"wait"ed for.

       If you want the old signal behavior back	despite	possible memory
       corruption, set the environment variable	"PERL_SIGNALS" to "unsafe".
       This feature first appeared in Perl 5.8.1.

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
       regular anonymous pipes,	except that the	processes rendezvous using a
       filename	and need not be	related.

       To create a named pipe, use the "POSIX::mkfifo()" function.

	   use POSIX qw(mkfifo);
	   mkfifo($path, 0700)	   ||  die "mkfifo $path failed: $!";

       You can also use	the Unix command mknod(1), or on some systems,
       mkfifo(1).  These may not be in your normal path, though.

	   # 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 .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 read the new signature
       from your program.  We'll use the pipe-checking file-test operator, -p,
       to find out whether anyone (or anything)	has accidentally removed our
       fifo.

	   chdir();    # go home
	   my $FIFO = ".signature";

	   while (1) {
	       unless (-p $FIFO) {
		   unlink $FIFO;   # discard any failure, will catch later
		   require POSIX;  # delayed loading of	heavy module
		   POSIX::mkfifo($FIFO,	0700)
				       || die "can't mkfifo $FIFO: $!";
	       }

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

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 expecting
       filenames in @ARGV, the clever programmer can write something like
       this:

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

       and no matter which sort	of shell it's called from, the Perl program
       will read from the file f1, the process cmd1, standard input (tmpfile
       in this case), the f2 file, the cmd2 command, and finally the 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 netstatus ($?)" 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 kill off the child process
       early if	you'd like.

       Be careful to check the return values from both open() and close().  If
       you're 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 EOF,	writers	to bogus commands will get hit with a
       signal, which they'd best be prepared to	handle.	 Consider:

	   open(FH, "|bogus")	   || die "can't fork: $!";
	   print FH "bang\n";	   #  neither necessary	nor sufficient
				   #  to check print retval!
	   close(FH)		   || die "can't close:	$!";

       The reason for not checking the return value from print() is because of
       pipe buffering; physical	writes are delayed.  That won't	blow up	until
       the close, and it will blow up with a SIGPIPE.  To catch	it, you	could
       use this:

	   $SIG{PIPE} =	"IGNORE";
	   open(FH, "|bogus")  || die "can't fork: $!";
	   print FH "bang\n";
	   close(FH)	       || die "can't close: status=$?";

   Filehandles
       Both the	main process and any child processes it	forks share the	same
       STDIN, STDOUT, and STDERR filehandles.  If both processes try to	access
       them at once, strange things can	happen.	 You may also 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.

   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 details.

   Complete Dissociation of Child from Parent
       In some cases (starting server processes, for instance) you'll want to
       completely dissociate the child process from the	parent.	 This is often
       called daemonization.  A	well-behaved daemon will also chdir() to the
       root directory so it doesn't prevent unmounting the filesystem
       containing the directory	from which it was launched, and	redirect its
       standard	file descriptors from and to /dev/null so that random output
       doesn't wind up on the user's terminal.

	use POSIX "setsid";

	sub daemonize {
	    chdir("/")			|| die "can't chdir to /: $!";
	    open(STDIN,	 "< /dev/null")	|| die "can't read /dev/null: $!";
	    open(STDOUT, "> /dev/null")	|| die "can't write to /dev/null: $!";
	    defined(my $pid = fork())	|| die "can't fork: $!";
	    exit if $pid;		# non-zero now means I am the parent
	    (setsid() != -1)		|| die "Can't start a new session: $!";
	    open(STDERR, ">&STDOUT")	|| die "can't dup stdout: $!";
	}

       The fork() has to come before the setsid() to ensure you	aren't a
       process group leader; the setsid() will fail if you are.	 If your
       system doesn't have the setsid()	function, open /dev/tty	and use	the
       "TIOCNOTTY" ioctl() on it instead.  See tty(4) for details.

       Non-Unix	users should check their "Your_OS::Process" module for other
       possible	solutions.

   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 "-|" or "|-" 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 to minus, you can
       write to	the filehandle you opened and your kid will find it in his
       STDIN.  If you open a pipe from minus, you can read from	the filehandle
       you opened whatever your	kid writes to his STDOUT.

	   use English;
	   my $PRECIOUS	= "/path/to/some/safe/file";
	   my $sleep_count;
	   my $pid;

	   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) {		       # I am the parent
	       print KID_TO_WRITE @some_data;
	       close(KID_TO_WRITE)     || warn "kid exited $?";
	   } else {		       # I am the child
	       # drop permissions in setuid and/or setgid programs:
	       ($EUID, $EGID) =	($UID, $GID);
	       open (OUTFILE, "> $PRECIOUS")
				       || die "can't open $PRECIOUS: $!";
	       while (<STDIN>) {
		   print OUTFILE;      # child's STDIN is parent's KID_TO_WRITE
	       }
	       close(OUTFILE)	       || die "can't close $PRECIOUS: $!";
	       exit(0);		       # 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:

	   my $pid = open(KID_TO_READ, "-|");
	   defined($pid)	   || die "can't fork: $!";

	   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:

	   my $pid = open(KID_TO_WRITE,	"|-");
	   defined($pid)	   || die "can't fork: $!";

	   $SIG{PIPE} =	sub { die "whoops, $program pipe broke"	};

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

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

       It is very easy to dead-lock a process using this form of open(), or
       indeed with any use of pipe() with multiple subprocesses.  The example
       above is	"safe" because it is simple and	calls exec().  See "Avoiding
       Pipe Deadlocks" for general safety principles, but there	are extra
       gotchas with Safe Pipe Opens.

       In particular, if you opened the	pipe using "open FH, "|-"", then you
       cannot simply use close() in the	parent process to close	an unwanted
       writer.	Consider this code:

	   my $pid = open(WRITER, "|-");	# fork open a kid
	   defined($pid)	       || die "first fork failed: $!";
	   if ($pid) {
	       if (my $sub_pid = fork()) {
		   defined($sub_pid)   || die "second fork failed: $!";
		   close(WRITER)       || die "couldn't	close WRITER: $!";
		   # now do something else...
	       }
	       else {
		   # first write to WRITER
		   # ...
		   # then when finished
		   close(WRITER)       || die "couldn't	close WRITER: $!";
		   exit(0);
	       }
	   }
	   else	{
	       # first do something with STDIN,	then
	       exit(0);
	   }

       In the example above, the true parent does not want to write to the
       WRITER filehandle, so it	closes it.  However, because WRITER was	opened
       using "open FH, "|-"", it has a special behavior: closing it calls
       waitpid() (see "waitpid"	in perlfunc), which waits for the subprocess
       to exit.	 If the	child process ends up waiting for something happening
       in the section marked "do something else", you have deadlock.

       This can	also be	a problem with intermediate subprocesses in more
       complicated code, which will call waitpid() on all open filehandles
       during global destruction--in no	predictable order.

       To solve	this, you must manually	use pipe(), fork(), and	the form of
       open() which sets one file descriptor to	another, as shown below:

	   pipe(READER,	WRITER)	       || die "pipe failed: $!";
	   $pid	= fork();
	   defined($pid)	       || die "first fork failed: $!";
	   if ($pid) {
	       close READER;
	       if (my $sub_pid = fork()) {
		   defined($sub_pid)   || die "first fork failed: $!";
		   close(WRITER)       || die "can't close WRITER: $!";
	       }
	       else {
		   # write to WRITER...
		   # ...
		   # then  when	finished
		   close(WRITER)       || die "can't close WRITER: $!";
		   exit(0);
	       }
	       # write to WRITER...
	   }
	   else	{
	       open(STDIN, "<&READER") || die "can't reopen STDIN: $!";
	       close(WRITER)	       || die "can't close WRITER: $!";
	       # do something...
	       exit(0);
	   }

       Since Perl 5.8.0, you can also use the list form	of "open" for pipes.
       This is preferred when you wish to avoid	having the shell interpret
       metacharacters that may be in your command string.

       So for example, instead of using:

	   open(PS_PIPE, "ps aux|")    || die "can't open ps pipe: $!";

       One would use either of these:

	   open(PS_PIPE, "-|", "ps", "aux")
				       || die "can't open ps pipe: $!";

	   @ps_args = qw[ ps aux ];
	   open(PS_PIPE, "-|", @ps_args)
				       || die "can't open @ps_args|: $!";

       Because there are more than three arguments to open(), forks the	ps(1)
       command without spawning	a shell, and reads its standard	output via the
       "PS_PIPE" filehandle.  The corresponding	syntax to write	to command
       pipes is	to use "|-" in place of	"-|".

       This was	admittedly a rather silly example, because you're using	string
       literals	whose content is perfectly safe.  There	is therefore no	cause
       to resort to the	harder-to-read,	multi-argument form of pipe open().
       However,	whenever you cannot be assured that the	program	arguments are
       free of shell metacharacters, the fancier form of open()	should be
       used.  For example:

	   @grep_args =	("egrep", "-i",	$some_pattern, @many_files);
	   open(GREP_PIPE, "-|", @grep_args)
			       || die "can't open @grep_args|: $!";

       Here the	multi-argument form of pipe open() is preferred	because	the
       pattern and indeed even the filenames themselves	might hold
       metacharacters.

       Be aware	that these operations are full Unix forks, which means they
       may not be correctly implemented	on all alien systems.

   Avoiding Pipe Deadlocks
       Whenever	you have more than one subprocess, you must be careful that
       each closes whichever half of any pipes created for interprocess
       communication it	is not using.  This is because any child process
       reading from the	pipe and expecting an EOF will never receive it, and
       therefore never exit. A single process closing a	pipe is	not enough to
       close it; the last process with the pipe	open must close	it for it to
       read EOF.

       Certain built-in	Unix features help prevent this	most of	the time.  For
       instance, filehandles have a "close on exec" flag, which	is set en
       masse under control of the $^F variable.	 This is so any	filehandles
       you didn't explicitly route to the STDIN, STDOUT	or STDERR of a child
       program will be automatically closed.

       Always explicitly and immediately call close() on the writable end of
       any pipe, unless	that process is	actually writing to it.	 Even if you
       don't explicitly	call close(), Perl will	still close() all filehandles
       during global destruction.  As previously discussed, if those
       filehandles have	been opened with Safe Pipe Open, this will result in
       calling waitpid(), which	may again deadlock.

   Bidirectional Communication with Another Process
       While this works	reasonably well	for unidirectional communication, what
       about bidirectional communication?  The most obvious approach doesn't
       work:

	   # THIS DOES NOT WORK!!
	   open(PROG_FOR_READING_AND_WRITING, "| some program |")

       If you forget to	"use warnings",	you'll miss out	entirely on the
       helpful diagnostic message:

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

       If you really want to, you can use the standard open2() from the
       "IPC::Open2" module to catch both ends.	There's	also an	open3()	in
       "IPC::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 the pipe() and exec() syscalls to create	all the
       connections.  Although it might have been more efficient	by using
       socketpair(), this would	have been even less portable than it already
       is. The open2() and open3() functions are unlikely to work anywhere
       except on a Unix	system,	or at least one	purporting POSIX compliance.

       Here's an example of using open2():

	   use FileHandle;
	   use IPC::Open2;
	   $pid	= open2(*Reader, *Writer, "cat -un");
	   print Writer	"stuff\n";
	   $got	= <Reader>;

       The problem with	this is	that buffering is really going to ruin your
       day.  Even though your "Writer" filehandle is auto-flushed so the
       process on the other end	gets your data in a timely manner, you can't
       usually do anything to force that process to give its data to you in a
       similarly quick fashion.	 In this special case, we could	actually so,
       because we gave cat a -u	flag to	make it	unbuffered.  But very few
       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 to	use a library which uses pseudottys to make
       your program behave more	reasonably.  This way you don't	have to	have
       control over the	source code of the program you're using.  The "Expect"
       module from CPAN	also addresses this kind of thing.  This module
       requires	two other modules from CPAN, "IO::Pty" and "IO::Stty".	It
       sets up a pseudo	terminal to interact with programs that	insist on
       talking to the terminal device driver.  If your system is supported,
       this may	be your	best bet.

   Bidirectional Communication with Yourself
       If you want, you	may make low-level pipe() and fork() syscalls to
       stitch this together by hand.  This example only	talks to itself, but
       you could reopen	the appropriate	handles	to STDIN and STDOUT and	call
       other processes.	 (The following	example	lacks proper error checking.)

	#!/usr/bin/perl	-w
	# pipe1	- bidirectional	communication using two	pipe pairs
	#	  designed for the socketpair-challenged
	use IO::Handle;		    # thousands	of lines just for autoflush :-(
	pipe(PARENT_RDR, CHILD_WTR);  #	XXX: check failure?
	pipe(CHILD_RDR,	 PARENT_WTR); #	XXX: check failure?
	CHILD_WTR->autoflush(1);
	PARENT_WTR->autoflush(1);

	if ($pid = fork()) {
	    close PARENT_RDR;
	    close PARENT_WTR;
	    print CHILD_WTR "Parent Pid	$$ is sending this\n";
	    chomp($line	= <CHILD_RDR>);
	    print "Parent Pid $$ just read this: '$line'\n";
	    close CHILD_RDR; close CHILD_WTR;
	    waitpid($pid, 0);
	} else {
	    die	"cannot	fork: $!" unless defined $pid;
	    close CHILD_RDR;
	    close CHILD_WTR;
	    chomp($line	= <PARENT_RDR>);
	    print "Child Pid $$	just read this:	'$line'\n";
	    print PARENT_WTR "Child Pid	$$ is sending this\n";
	    close PARENT_RDR;
	    close PARENT_WTR;
	    exit(0);
	}

       But you don't actually have to make two pipe calls.  If you have	the
       socketpair() system call, it will do this all for you.

	#!/usr/bin/perl	-w
	# pipe2	- bidirectional	communication using socketpair
	#   "the best ones always go both ways"

	use Socket;
	use IO::Handle;	 # thousands of	lines just for autoflush :-(

	# We say AF_UNIX because although *_LOCAL is the
	# POSIX	1003.1g	form of	the constant, many machines
	# still	don't have it.
	socketpair(CHILD, PARENT, AF_UNIX, SOCK_STREAM,	PF_UNSPEC)
				    ||	die "socketpair: $!";

	CHILD->autoflush(1);
	PARENT->autoflush(1);

	if ($pid = fork()) {
	    close PARENT;
	    print CHILD	"Parent	Pid $$ is sending this\n";
	    chomp($line	= <CHILD>);
	    print "Parent Pid $$ just read this: '$line'\n";
	    close CHILD;
	    waitpid($pid, 0);
	} else {
	    die	"cannot	fork: $!" unless defined $pid;
	    close CHILD;
	    chomp($line	= <PARENT>);
	    print "Child Pid $$	just read this:	'$line'\n";
	    print PARENT "Child	Pid $$ is sending this\n";
	    close PARENT;
	    exit(0);
	}

Sockets: Client/Server Communication
       While not entirely limited to Unix-derived operating systems (e.g.,
       WinSock on PCs provides socket support, as do some VMS libraries), you
       might 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 like TCP streams and datagrams like UDP packets.
       You may be able to do even more depending on your system.

       The Perl	functions 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 ancient, antemillennial 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	"$AF_INET = 2",	you know you're	in for
       big trouble.  An	immeasurably superior approach is to use the "Socket"
       module, which more reliably grants access to the	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).

   Internet Line Terminators
       The Internet line terminator is "\015\012".  Under ASCII	variants of
       Unix, that could	usually	be written as "\r\n", but under	other systems,
       "\r\n" might at times be	"\015\015\012",	"\012\012\015",	or something
       completely different.  The standards specify writing "\015\012" to be
       conformant (be strict in	what you provide), but they also recommend
       accepting a lone	"\012" on input	(be lenient in what you	require).  We
       haven't always been very	good about that	in the code in this manpage,
       but unless you're on a Mac from way back	in its pre-Unix	dark ages,
       you'll probably be ok.

   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
	   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(0);

       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),
       fill this in with your real address instead.

	#!/usr/bin/perl	-Tw
	use strict;
	BEGIN {	$ENV{PATH} = "/usr/bin:/bin" }
	use Socket;
	use Carp;
	my $EOL	= "\015\012";

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

	my $port  = shift || 2345;
	die "invalid port" unless $port	=~ /^ \d+ $/x;

	my $proto = getprotobyname("tcp");

	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;

	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(),	$EOL;
	}

       And here's a multitasking version.  It's	multitasked in that like most
       typical servers,	it spawns (fork()s) 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
	use strict;
	BEGIN {	$ENV{PATH} = "/usr/bin:/bin" }
	use Socket;
	use Carp;
	my $EOL	= "\015\012";

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

	my $port  = shift || 2345;
	die "invalid port" unless $port	=~ /^ \d+ $/x;

	my $proto = getprotobyname("tcp");

	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;

	use POSIX ":sys_wait_h";
	use Errno;

	sub REAPER {
	    local $!;	# don't	let waitpid() overwrite	current	error
	    while ((my $pid = waitpid(-1, WNOHANG)) > 0	&& WIFEXITED($?)) {
		logmsg "reaped $waitedpid" . ($? ? " with exit $?" : "");
	    }
	    $SIG{CHLD} = \&REAPER;  # loathe SysV
	}

	$SIG{CHLD} = \&REAPER;

	while (1) {
	    $paddr = accept(Client, Server) || do {
		# try again if accept()	returned because got a signal
		next if	$!{EINTR};
		die "accept: $!";
	    };
	    my ($port, $iaddr) = sockaddr_in($paddr);
	    my $name = gethostbyaddr($iaddr, AF_INET);

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

	    spawn sub {
		$| = 1;
		print "Hello there, $name, it's	now ",
		      scalar localtime(),
		      $EOL;
		exec "/usr/games/fortune"	# XXX: "wrong" line terminators
		    or confess "can't exec fortune: $!";
	    };
	    close Client;
	}

	sub spawn {
	    my $coderef	= shift;

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

	    my $pid;
	    unless (defined($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.  The	REAPER subroutine is used here to call
       waitpid() for any child processes that have finished, thereby ensuring
       that they terminate cleanly and don't join the ranks of the living
       dead.

       Within the while	loop we	call accept() and check	to see if it returns a
       false value.  This would	normally indicate a system error needs to be
       reported.  However, the introduction of safe signals (see "Deferred
       Signals (Safe Signals)" above) in Perl 5.8.0 means that accept()	might
       also be interrupted when	the process receives a signal.	This typically
       happens when one	of the forked subprocesses exits and notifies the
       parent process with a CHLD signal.

       If accept() is interrupted by a signal, $! will be set to EINTR.	 If
       this happens, we	can safely continue to the next	iteration of the loop
       and another call	to accept().  It is important that your	signal
       handling	code not modify	the value of $!, or else this test will	likely
       fail.  In the REAPER subroutine we create a local version of $! before
       calling waitpid().  When	waitpid() sets $! to ECHILD as it inevitably
       does when it has	no more	children waiting, it updates the local copy
       and leaves the original unchanged.

       You should use the -T flag to enable taint checking (see	perlsec) even
       if we aren't running setuid or setgid.  This is always a	good idea for
       servers or any program 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()	|| time()) }

	   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();

	   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 "connect: $!";
	       my $rtime = pack("C4", ());
	       read(SOCKET, $rtime, 4);
	       close(SOCKET);
	       my $histime = unpack("N", $rtime) - $SECS_OF_70_YEARS;
	       printf "%8d %s\n", $histime - time(), ctime($histime);
	   }

   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 -S file test:

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

       Here's a	sample Unix-domain client:

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

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

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

	   my $NAME = "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;

	   use POSIX ":sys_wait_h";
	   sub REAPER {
	       my $child;
	       while (($waitedpid = waitpid(-1,	WNOHANG)) > 0) {
		   logmsg "reaped $waitedpid" .	($? ? "	with exit $?" :	"");
	       }
	       $SIG{CHLD} = \&REAPER;  # loathe	SysV
	   }

	   $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")  || die "can't exec fortune: $!";
	       };
	   }

	   sub spawn {
	       my $coderef = shift();

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

	       my $pid;
	       unless (defined($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->());
	   }

       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 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 to be	able to	access from the	Web, 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.

TCP Clients with IO::Socket
       For those preferring a higher-level interface to	socket programming,
       the IO::Socket module provides an object-oriented approach.  If for
       some reason you lack this module, you can just fetch IO::Socket from
       CPAN, where you'll also 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--to name just a	few.

   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;
	   $remote = IO::Socket::INET->new(
			       Proto	=> "tcp",
			       PeerAddr	=> "localhost",
			       PeerPort	=> "daytime(13)",
			   )
			|| die "can't connect to daytime service on localhost";
	   while (<$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 new() constructor mean:

       "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.

       "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
	   "www.perl.com", or an address like "207.171.7.72".  For
	   demonstration purposes, we've used the special hostname
	   "localhost",	which should always mean the current machine you're
	   running on.	The corresponding Internet address for localhost is
	   "127.0.0.1",	if you'd rather	use that.

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

       Notice how the return value from	the "new" constructor is used as a
       filehandle in the "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";

   A Webget Client
       Here's a	simple client that takes a remote host to fetch	a document
       from, and then a	list of	files 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 url	..." }
	   $host = shift(@ARGV);
	   $EOL	= "\015\012";
	   $BLANK = $EOL x 2;
	   for my $document (@ARGV) {
	       $remote = IO::Socket::INET->new(	Proto	  => "tcp",
						PeerAddr  => $host,
						PeerPort  => "http(80)",
			 )     || die "cannot connect to httpd on $host";
	       $remote->autoflush(1);
	       print $remote "GET $document HTTP/1.0" .	$BLANK;
	       while ( <$remote> ) { print }
	       close $remote;
	   }

       The web server handling the HTTP	service	is assumed to be at its
       standard	port, number 80.  If the server	you're trying to connect to is
       at a different port, like 1080 or 8080, you should specify it as	the
       named-parameter pair, "PeerPort => 8080".  The "autoflush" method is
       used on the socket because otherwise the	system would buffer up the
       output we sent it.  (If you're on a prehistoric Mac, you'll also	need
       to change every "\n" in your code that sends data over the network to
       be a "\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 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 featureful version of	this program, you should look to the
       lwp-request program included with the LWP modules from CPAN.

   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 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 "fork" call, the solution
       isn't that rough.  Once you've made the connection to whatever service
       you'd like to chat with,	call "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 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.)

       Here's the code:

	   #!/usr/bin/perl -w
	   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)
		      || die "can't connect to port $port on $host: $!";

	   $handle->autoflush(1);	# so output gets there right away
	   print STDERR	"[Connected to $host:$port]\n";

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

	   # the if{} block runs only in the parent process
	   if ($kidpid)	{
	       # copy the socket to standard output
	       while (defined ($line = <$handle>)) {
		   print STDOUT	$line;
	       }
	       kill("TERM", $kidpid);	# send SIGTERM to child
	   }
	   # the else{}	block runs only	in the child process
	   else	{
	       # copy standard input to	the socket
	       while (defined ($line = <STDIN>)) {
		   print $handle $line;
	       }
	       exit(0);		       # just in case
	   }

       The "kill" function in the parent's "if"	block is there to send a
       signal to our child process, currently running in the "else" block, as
       soon as the remote server has closed its	end of the connection.

       If the remote server sends data a byte at time, and you need that data
       immediately without waiting for a newline (which	might not happen), you
       may wish	to replace the "while" loop in the parent with the following:

	   my $byte;
	   while (sysread($handle, $byte, 1) ==	1) {
	       print STDOUT $byte;
	   }

       Making a	system call for	each byte you want to read is not very
       efficient (to put it mildly) but	is the simplest	to explain and works
       reasonably well.

TCP Servers with IO::Socket
       As always, setting up a server is little	bit more involved than running
       a client.  The model is that the	server creates a special kind of
       socket that does	nothing	but listen on a	particular port	for incoming
       connections.  It	does this by calling the "IO::Socket::INET->new()"
       method with slightly different arguments	than the client	did.

       Proto
	   This	is which protocol to use.  Like	our clients, we'll still
	   specify "tcp" here.

       LocalPort
	   We specify a	local port in the "LocalPort" argument,	which we
	   didn't do for the client.  This is service name or port number for
	   which you want to be	the server. (Under Unix, ports under 1024 are
	   restricted to the superuser.)  In our sample, we'll use port	9000,
	   but you can use any port that's not currently in use	on your
	   system.  If you try to use one already in used, you'll get an
	   "Address already in use" message.  Under Unix, the "netstat -a"
	   command will	show which services current have servers.

       Listen
	   The "Listen"	parameter is set to the	maximum	number of pending
	   connections we can accept until we turn away	incoming clients.
	   Think of it as a call-waiting queue for your	telephone.  The	low-
	   level Socket	module has a special symbol for	the system maximum,
	   which is SOMAXCONN.

       Reuse
	   The "Reuse" parameter is needed so that we restart our server
	   manually without waiting a few minutes to allow system buffers to
	   clear out.

       Once the	generic	server socket has been created using the parameters
       listed above, the server	then waits for a new client to connect to it.
       The server blocks in the	"accept" method, which eventually accepts a
       bidirectional connection	from the remote	client.	 (Make sure to
       autoflush this handle to	circumvent buffering.)

       To add to user-friendliness, our	server prompts the user	for commands.
       Most servers don't do this.  Because of the prompt without a newline,
       you'll have to use the "sysread"	variant	of the interactive client
       above.

       This server accepts one of five different commands, sending output back
       to the client.  Unlike most network servers, this one handles only one
       incoming	client at a time.  Multitasking	servers	are covered in Chapter
       16 of the Camel.

       Here's the code.	 We'll

	#!/usr/bin/perl	-w
	use IO::Socket;
	use Net::hostent;      # for OOish version of gethostbyaddr

	$PORT =	9000;	       # pick something	not in use

	$server	= IO::Socket::INET->new( Proto	   => "tcp",
					 LocalPort => $PORT,
					 Listen	   => SOMAXCONN,
					 Reuse	   => 1);

	die "can't setup server" unless	$server;
	print "[Server $0 accepting clients]\n";

	while ($client = $server->accept()) {
	  $client->autoflush(1);
	  print	$client	"Welcome to $0;	type help for command list.\n";
	  $hostinfo = gethostbyaddr($client->peeraddr);
	  printf "[Connect from	%s]\n",
		 $hostinfo ? $hostinfo->name : $client->peerhost;
	  print	$client	"Command? ";
	  while	( <$client>) {
	    next unless	/\S/;	  # blank line
	    if	  (/quit|exit/i)  { last				      }
	    elsif (/date|time/i)  { printf $client "%s\n", scalar localtime() }
	    elsif (/who/i )	  { print  $client `who	2>&1`		      }
	    elsif (/cookie/i )	  { print  $client `/usr/games/fortune 2>&1`  }
	    elsif (/motd/i )	  { print  $client `cat	/etc/motd 2>&1`	      }
	    else {
	      print $client "Commands: quit date who cookie motd\n";
	    }
	  } continue {
	     print $client "Command? ";
	  }
	  close	$client;
	}

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.

       UDP datagrams are not a bytestream and should not be treated as such.
       This makes using	I/O mechanisms with internal buffering like stdio
       (i.e.  print() and friends) especially cumbersome. Use syswrite(), or
       better send(), like in the example below.

       Here's a	UDP program similar to the sample Internet TCP client given
       earlier.	 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;
	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 = 2_208_988_800;

	$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();
	$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--;
	}

       This example does not include any retries and may consequently fail to
       contact a reachable host. The most prominent reason for this is
       congestion of the queues	on the sending host if the number of hosts to
       contact is sufficiently large.

SysV IPC
       While System V IPC isn't	so widely used as sockets, it still has	some
       interesting uses.  However, you cannot use SysV IPC or Berkeley mmap()
       to have a variable shared amongst several processes.  That's because
       Perl would reallocate your string when you weren't wanting it to.  You
       might look into the "IPC::Shareable" or "threads::shared" modules for
       that.

       Here's a	small example showing shared memory usage.

	   use IPC::SysV qw(IPC_PRIVATE	IPC_RMID S_IRUSR S_IWUSR);

	   $size = 2000;
	   $id = shmget(IPC_PRIVATE, $size, S_IRUSR | S_IWUSR);
	   defined($id)			   || die "shmget: $!";
	   print "shm key $id\n";

	   $message = "Message #1";
	   shmwrite($id, $message, 0, 60)  || die "shmwrite: $!";
	   print "wrote: '$message'\n";
	   shmread($id,	$buff, 0, 60)	   || die "shmread: $!";
	   print "read : '$buff'\n";

	   # the buffer	of shmread is zero-character end-padded.
	   substr($buff, index($buff, "\0")) = "";
	   print "un" unless $buff eq $message;
	   print "swell\n";

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

       Here's an example of a semaphore:

	   use IPC::SysV qw(IPC_CREAT);

	   $IPC_KEY = 1234;
	   $id = semget($IPC_KEY, 10, 0666 | IPC_CREAT);
	   defined($id)			   || die "semget: $!";
	   print "sem id $id\n";

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

	   # create a semaphore

	   $IPC_KEY = 1234;
	   $id = semget($IPC_KEY, 0, 0);
	   defined($id)			   || die "semget: $!";

	   $semnum  = 0;
	   $semflag = 0;

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

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

	   semop($id, $opstring)   || die "semop: $!";

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

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

	   $IPC_KEY = 1234;
	   $id = semget($IPC_KEY, 0, 0);
	   die unless defined($id);

	   $semnum  = 0;
	   $semflag = 0;

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

	   semop($id, $opstring)   || die "semop: $!";

       The SysV	IPC code above was written long	ago, and it's definitely
       clunky looking.	For a more modern look,	see the	IPC::SysV module.

       A small example demonstrating SysV message queues:

	   use IPC::SysV qw(IPC_PRIVATE	IPC_RMID IPC_CREAT S_IRUSR S_IWUSR);

	   my $id = msgget(IPC_PRIVATE,	IPC_CREAT | S_IRUSR | S_IWUSR);
	   defined($id)		       || die "msgget failed: $!";

	   my $sent	 = "message";
	   my $type_sent = 1234;

	   msgsnd($id, pack("l!	a*", $type_sent, $sent), 0)
				       || die "msgsnd failed: $!";

	   msgrcv($id, my $rcvd_buf, 60, 0, 0)
				       || die "msgrcv failed: $!";

	   my($type_rcvd, $rcvd) = unpack("l! a*", $rcvd_buf);

	   if ($rcvd eq	$sent) {
	       print "okay\n";
	   } else {
	       print "not okay\n";
	   }

	   msgctl($id, IPC_RMID, 0)    || die "msgctl failed: $!\n";

NOTES
       Most of these routines quietly but politely return "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 Socket conversion
       functions do croak() on bad arguments.)	It is therefore	essential to
       check return values from	these functions.  Always begin your socket
       programs	this way for optimal success, and don't	forget to add the -T
       taint-checking flag to the "#!" line for	servers:

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

BUGS
       These routines all create system-specific portability problems.	As
       noted elsewhere,	Perl is	at the mercy of	your C libraries for much of
       its system behavior.  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.

AUTHOR
       Tom Christiansen, with occasional vestiges of Larry Wall's original
       version and suggestions from the	Perl Porters.

SEE ALSO
       There's a lot more to networking	than this, but this should get you
       started.

       For intrepid programmers, the indispensable textbook is Unix Network
       Programming, 2nd	Edition, Volume	1 by W.	Richard	Stevens	(published by
       Prentice-Hall).	Most books on networking address the subject from the
       perspective of a	C programmer; translation to Perl is left as an
       exercise	for the	reader.

       The IO::Socket(3) manpage describes the object library, and the
       Socket(3) manpage describes the low-level interface to sockets.
       Besides the obvious functions in	perlfunc, you should also check	out
       the modules file	at your	nearest	CPAN site, especially
       <http://www.cpan.org/modules/00modlist.long.html#ID5_Networking_>.  See
       perlmodlib or best yet, the Perl	FAQ for	a description of what CPAN is
       and where to get	it if the previous link	doesn't	work for you.

       Section 5 of CPAN's 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--to name just a few.

perl v5.28.3			  2020-05-14			    PERLIPC(1)

NAME | DESCRIPTION | Signals | Named Pipes | Using open() for IPC | Sockets: Client/Server Communication | TCP Clients with IO::Socket | TCP Servers with IO::Socket | UDP: Message Passing | SysV IPC | NOTES | BUGS | AUTHOR | SEE ALSO

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