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RedHat 9 (Linux i386) - man page for perlipc (redhat section 1)

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

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

       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.

       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

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

	   sub catch_zap {
	       my $signame = shift;
	       die "Somebody sent me a SIG$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 han-
       dler; 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 the-
       ory trigger a memory fault and subsequent core dump - see "Deferred 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;

       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() state-
       ment, 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';
	   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
       another process, but instead checks whether it's alive or has changed its UID.

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

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

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

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

	 #!/usr/bin/perl -w

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


	 # 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',
	 POSIX::sigaction(&POSIX::SIGHUP, $action);

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


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

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

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

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

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

       For example, let's say you'd like to have your .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';
	   $ENV{PATH} .= ":/etc:/usr/games";

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

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

       Deferred 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 para-
       noid 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 longjump(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 flex-
       ibility 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 Perl interpreter only looks at the signal flags when it about to execute a new
	   opcode if a signal arrives during a long running opcode (e.g. a regular expression
	   operation on a very large string) then signal will not be seen until operation com-

       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 han-
	   dler 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 solu-
	   tion 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"

       Signals as "faults"
	   Certain signals e.g. SEGV, ILL, 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. (In particular the old signal scheme was particularly unsafe in such
	   cases.)  However if a %SIG handler is set the new scheme simply sets a flag and
	   returns as described above. This may cause the operating system to try the offending
	   machine instruction again and - as nothing has changed - it will generate the signal
	   again. The result of this is a rather odd "loop". In future Perl's signal mechanism
	   may be changed to avoid this - perhaps by simply disallowing %SIG handlers on signals
	   of that type. Until then the work-round is not to set a %SIG handler on those signals.
	   (Which signals they are is operating system dependant.)

       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.

Using open() for IPC
       Perl's basic open() statement can also be used for unidirectional interprocess communica-
       tion 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)/;
	   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 com-
       mand, 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: $!";
	   close FH	       or die "can't close: status=$?";


       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 hap-
       pen.  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 dissoci-
       ate 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 stan-
       dard 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;
	       setsid		       or die "Can't start a new session: $!";
	       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

       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 com-
       municate 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 file-
       handle 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
	       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

       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 read from 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

       Bidirectional Communication with Another Process

       While this works reasonably well for unidirectional communication, what about bidirec-
       tional 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?

	   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;
	   } 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;

       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.
				       or  die "socketpair: $!";


	   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;
	   } 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;

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 vir-
       tual 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 file-
       handles work differently than C file descriptors.  Second, Perl already knows the length
       of its strings, so you don't need to pass that information.

       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 talk-
       ing, 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 usu-
       ally 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 recom-
       mend 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: $!";

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

	   $SIG{CHLD} = \&REAPER;

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

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

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


	   sub spawn {
	       my $coderef = shift;

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

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

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

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

       We suggest that you use the -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);

	   $| = 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);
	       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)

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

       And here's a corresponding server.  You don't have to worry about silly network termina-
       tors 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 = '/tmp/catsock';
	   my $uaddr = sockaddr_un($NAME);
	   my $proto = getprotobyname('tcp');

	   socket(Server,PF_UNIX,SOCK_STREAM,0)        || die "socket: $!";
	   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: $!";
	       } elsif ($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:

	   This is which protocol to use.  In this case, the socket handle returned will be con-
	   nected 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 mes-

	   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
	   "".  For demonstration purposes, we've used the special hostname "local-
	   host", which should always mean the current machine you're running on.  The corre-
	   sponding Internet address for localhost is "127.1", if you'd rather use that.

	   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" }
	       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 connec-
       tion, you have to use the server's language.  Each server on the network has its own lit-
       tle 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>

       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 pro-
       gram 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 prob-
       ably 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.

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

	   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.

	   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 maxi-
	   mum, which is SOMAXCONN.

	   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" vari-
       ant 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()) {
	  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) espe-
       cially 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 asyn-
       chronously 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) {
	       $hisiaddr = inet_aton($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);

       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.

       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.


	   $size = 2000;
	   $id = shmget(IPC_PRIVATE, $size, S_IRWXU) || die "$!";
	   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

       A small example demonstrating SysV message queues:


	   my $id = msgget(IPC_PRIVATE, IPC_CREAT | S_IRWXU);

	   my $sent = "message";
	   my $type = 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";

       Most of these routines quietly but politely return "undef" when they fail instead of caus-
       ing 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;

       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.

       As mentioned in the signals section, because few vendors provide C libraries that are
       safely re-entrant, the prudent programmer will do little else within a handler beyond set-
       ting a numeric variable that already exists; or, if locked into a slow (restarting) system
       call, using die() to raise an exception and longjmp(3) out.  In fact, even these may in
       some cases cause a core dump.  It's probably best to avoid signals except where they are
       absolutely inevitable.  This will be addressed in a future release of Perl.

       Tom Christiansen, with occasional vestiges of Larry Wall's original version and sugges-
       tions from the Perl Porters.

       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 Edi-
       tion, 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 v5.8.0				    2003-02-18				       PERLIPC(1)

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