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

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

	   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

       Prior to Perl 5.7.3 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 from the Config module.	 Set up an
       @signame list indexed by number to get the name and a %signo table
       indexed by name to get the number:

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

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

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

       You may also choose to assign the strings '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.	 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 sends a hang-up
       signal to all processes in the current process group (and sets
       $SIG{HUP} to IGNORE so it doesn't kill itself):

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

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

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

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

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

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

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

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

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

       or better still:

	   use POSIX ":sys_wait_h";
	   sub REAPER {
	       my $child;
	       # If a second child dies while in the signal handler caused by the
	       # first death, we won't get another signal. So must loop here else
	       # we will leave the unreaped child as a zombie. And the next time
	       # two children die we get another zombie. And so on.
	       while (($child = waitpid(-1,WNOHANG)) > 0) {
		   $Kid_Status{$child} = $?;
	       }
	       $SIG{CHLD} = \&REAPER;  # still loathe SysV
	   }
	   $SIG{CHLD} = \&REAPER;
	   # do something that forks...

       Signal handling is also used for timeouts in Unix,   While safely
       protected within an "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, 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 }

       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
       t/lib/posix.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 re-read its configuration file, without stopping
       the process. Many daemons provide this mechanism using the "SIGHUP"
       signal handler. When you want to tell the daemon to re-read the file
       you simply send it the "SIGHUP" signal.

       Not all platforms automatically reinstall their (native) signal
       handlers after a signal delivery.  This means that the handler works
       only the first time the signal is sent. The solution to this problem is
       to use "POSIX" signal handlers if available, their behaviour is well-
       defined.

       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 simply prints some debug info to show
       that it works and should be replaced with the real code.

	 #!/usr/bin/perl -w

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

	 $|=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
	 my $sigset = POSIX::SigSet->new();
	 my $action = POSIX::SigAction->new('sigHUP_handler',
					    $sigset,
					    &POSIX::SA_NODEFER);
	 POSIX::sigaction(&POSIX::SIGHUP, $action);

	 sub sigHUP_handler {
	     print "got SIGHUP\n";
	     exec($SELF, @ARGV) or die "Couldn't restart: $!\n";
	 }

	 code();

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

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 "POSIX::mkfifo()" function.

	   use POSIX qw(mkfifo);
	   mkfifo($path, 0700) or 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.

	   # 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 block and your program
       will supply the new signature.  We'll use the pipe-checking file test
       -p to find out whether anyone (or anything) has accidentally removed
       our fifo.

	   chdir; # go home
	   $FIFO = '.signature';

	   while (1) {
	       unless (-p $FIFO) {
		   unlink $FIFO;
		   require POSIX;
		   POSIX::mkfifo($FIFO, 0700)
		       or die "can't mkfifo $FIFO: $!";
	       }

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

   Deferred Signals (Safe Signals)
       In Perls before Perl 5.7.3 by installing Perl code to deal with
       signals, you were exposing yourself 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 behaviour 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.

       In Perl 5.7.3 and later to avoid these problems signals are
       "deferred"-- 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
       handler as we know 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 only looks at the signal flags 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.

	   N.B. 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 only be called once after the opcode completes, and all
	   the 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. INT control-C) the operating
	   system breaks into IO operations like "read" (used to implement
	   Perls <> 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 system's "stdio" library that library may re-start
	   the "read" without returning to Perl and giving it a chance to call
	   the %SIG handler. If this happens on your system the solution is to
	   use ":perlio" layer to do IO - at least on those handles which 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.)

	   Note that the default in Perl 5.7.3 and later is to automatically
	   use the ":perlio" layer.

	   Note that some networking library functions like gethostbyname()
	   are known to have their own implementations of timeouts which may
	   conflict with your timeouts.	 If you are having problems with such
	   functions, you can try using the POSIX sigaction() function, which
	   bypasses the Perl safe signals (note that this means subjecting
	   yourself 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" }))
		     or 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 will affect 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 5.7.3 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.

	   Note that the default ":perlio" layer will retry "read", "write"
	   and "close" as described above and that interrupted "wait" and
	   "waitpid" calls will always be retried.

       Signals as "faults"
	   Certain signals, e.g. SEGV, ILL, and BUS, are generated as a result
	   of virtual memory or other "faults". These are normally fatal and
	   there is little a Perl-level handler can do with them, so Perl now
	   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 re-issue
	   the signal as there are un-waited-for completed child processes.

       If you want the old signal behaviour back regardless of possible memory
       corruption, set the environment variable "PERL_SIGNALS" to "unsafe" (a
       new feature since Perl 5.8.1).

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

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

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

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

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

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

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

	   open(FH, "|bogus")  or die "can't fork: $!";
	   print FH "bang\n"   or die "can't write: $!";
	   close FH	       or die "can't close: $!";

       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")  or die "can't fork: $!";
	   print FH "bang\n"   or die "can't write: $!";
	   close FH	       or die "can't close: status=$?";

   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 more
       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 '/'	       or die "Can't chdir to /: $!";
	       open STDIN, '/dev/null' or die "Can't read /dev/null: $!";
	       open STDOUT, '>/dev/null'
				       or die "Can't write to /dev/null: $!";
	       defined(my $pid = fork) or die "Can't fork: $!";
	       exit if $pid;
	       die "Can't start a new session: $!" if setsid == -1;
	       open STDERR, '>&STDOUT' or die "Can't dup stdout: $!";
	   }

       The fork() has to come before the setsid() to ensure that 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
       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 '-no_match_vars';
	   my $sleep_count = 0;

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

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

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

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

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

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

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

       And here's a safe pipe open for writing:

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

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

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

       It is very easy to dead-lock a process using this form of open(), or
       indeed any use of pipe() and multiple sub-processes.  The above example
       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:

	   $pid = open WRITER, "|-";
	   defined $pid or die "fork failed; $!";
	   if ($pid) {
	       if (my $sub_pid = fork()) {
		   close WRITER;
		   # do something else...
	       }
	       else {
		   # write to WRITER...
		   exit;
	       }
	   }
	   else {
	       # do something with STDIN...
	       exit;
	   }

       In the 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 behaviour: closing it will call
       waitpid() (see "waitpid" in perlfunc), which waits for the sub-process
       to exit.	 If the child process ends up waiting for something happening
       in the section marked "do something else", then you have a deadlock.

       This can also be a problem with intermediate sub-processes 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 below:

	   pipe(READER, WRITER);
	   $pid = fork();
	   defined $pid or die "fork failed; $!";
	   if ($pid) {
	       close READER;
	       if (my $sub_pid = fork()) {
		   close WRITER;
	       }
	       else {
		   # write to WRITER...
		   exit;
	       }
	       # write to WRITER...
	   }
	   else {
	       open STDIN, "<&READER";
	       close WRITER;
	       # do something...
	       exit;
	   }

       Since Perl 5.8.0, you can also use the list form of "open" for pipes :
       the syntax

	   open KID_PS, "-|", "ps", "aux" or die $!;

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

       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 modules file mentioned below in the SEE ALSO section.

   Avoiding Pipe Deadlocks
       In general, if you have more than one sub-process, you need to be very
       careful that any process which does not need the writer half of any
       pipe you create for inter-process communication does not have it open.

       The reason for this is that any child process which is 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.

       There are some features built-in to unix to help prevent this most of
       the time.  For instance, filehandles have a 'close on exec' flag (set
       en masse with Perl using the $^F perlvar), so that any filehandles
       which you didn't explicitly route to the STDIN, STDOUT or STDERR of a
       child program will automatically be closed for you.

       So, always explicitly and immediately call close() on the writable end
       of any pipe, unless that process is actually writing to it.  If you
       don't explicitly call close() then be warned Perl will still close()
       all the filehandles during global destruction.  As warned above, if
       those filehandles were opened with Safe Pipe Open, they will also call
       waitpid() and you might again deadlock.

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

	   open(PROG_FOR_READING_AND_WRITING, "| some program |")

       and if you forget to use the "use warnings" pragma or the -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() calls 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" );
	   print Writer "stuff\n";
	   $got = <Reader>;

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

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

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

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

       The newer Expect.pm 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
       using talking to the terminal device driver.  If your system is amongst
       those supported, this may be your best bet.

   Bidirectional Communication with Yourself
       If you want, you may make low-level pipe() and fork() 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.

	   #!/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: failure?
	   pipe(CHILD_RDR,  PARENT_WTR);	       # XXX: 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;
	   }

       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)
				       or  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;
	   }

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

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

       One of the major problems with old socket code in Perl was that it used
       hard-coded values for some of the constants, which severely hurt
       portability.  If you ever see code that does anything like explicitly
       setting "$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 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 (but 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, 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: $!";
	   while (defined($line = <SOCK>)) {
	       print $line;
	   }

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

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

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

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

	   my $port = shift || 2345;
	   my $proto = getprotobyname('tcp');

	   ($port) = $port =~ /^(\d+)$/			       or die "invalid port";

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

	   logmsg "server started on port $port";

	   my $paddr;

	   $SIG{CHLD} = \&REAPER;

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

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

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

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

	   #!/usr/bin/perl -Tw
	   use strict;
	   BEGIN { $ENV{PATH} = '/usr/ucb:/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;
	   my $proto = getprotobyname('tcp');

	   ($port) = $port =~ /^(\d+)$/			       or die "invalid port";

	   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 a signal was received
		   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 $pid;
	       if (! defined($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.  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 that needs to
       be reported.  However the introduction of safe signals (see "Deferred
       Signals (Safe Signals)" above) in Perl 5.7.3 means that accept() may
       also be interrupted when the process receives a signal.	This typically
       happens when one of the forked sub-processes exits and notifies the
       parent process with a CHLD signal.

       If accept() is interrupted by a signal then $! will be set to EINTR.
       If this happens then we can safely continue to the next iteration of
       the loop and another call to accept().  It is important that your
       signal handling code doesn't modify the value of $! or this test will
       most 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 will
       update the local copy leaving the original unchanged.

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

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

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

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

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

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

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

   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;

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

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

	   BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
	   sub 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' or die "can't exec fortune: $!";
	       };
	   }

	   sub spawn {
	       my $coderef = shift;

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

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

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

       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.

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

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

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

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

	   Wed May 14 08:40:46 MDT 1997

       Here are what those parameters to the "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 "204.148.40.9".  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
	   "127.1", 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 in /etc/services under Unix] but just in case,
	   we've specified the port number (13) in parentheses.	 Using just
	   the number would also have worked, but constant numbers make
	   careful programmers nervous.

       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 documents to get from that host.  This is a
       more interesting client than the previous one because it first sends
       something to the server before fetching the server's response.

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

       The web server handing the "http" service, which is assumed to be at
       its standard port, number 80.  If the web server you're trying to
       connect to is at a different port (like 1080 or 8080), you should
       specify as the named-parameter pair, "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 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 fully-featured 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)
		  or 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;
	       }
	   }

       The "kill" function in the parent's "if" block is there to send a
       signal to our child process (current 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.  Note that unlike most network servers, this one only
       handles one incoming client at a time.  Multithreaded servers are
       covered in Chapter 6 of the Camel.

       Here's the code.	 We'll

	#!/usr/bin/perl -w
	use IO::Socket;
	use Net::hostent;	       # for OO 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.

       Note that 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	  = 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--;
	   }

       Note that 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 list 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.  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.

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

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

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

       Here's an example of a semaphore:

	   use IPC::SysV qw(IPC_CREAT);

	   $IPC_KEY = 1234;
	   $id = semget($IPC_KEY, 10, 0666 | IPC_CREAT ) || die "$!";
	   print "shm key $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 );
	   die if !defined($id);

	   $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 "$!";

       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 if !defined($id);

	   $semnum = 0;
	   $semflag = 0;

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

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

       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 which
       is included with Perl starting from Perl 5.005.

       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);

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

	   if (defined $id) {
	       if (msgsnd($id, pack("l! a*", $type_sent, $sent), 0)) {
		   if (msgrcv($id, $rcvd, 60, 0, 0)) {
		       ($type_rcvd, $rcvd) = unpack("l! a*", $rcvd);
		       if ($rcvd eq $sent) {
			   print "okay\n";
		       } else {
			   print "not okay\n";
		       }
		   } else {
		       die "# msgrcv failed\n";
		   }
	       } else {
		   die "# msgsnd failed\n";
	       }
	       msgctl($id, IPC_RMID, 0) || die "# msgctl failed: $!\n";
	   } else {
	       die "# msgget 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  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 -T taint
       checking flag to the #! line for servers:

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

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

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).	Note that 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.  (See perlmodlib or best
       yet, the Perl FAQ for a description of what CPAN is and where to get
       it.)

       Section 5 of the 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.

perl v5.10.1			  2009-08-10			    PERLIPC(1)
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