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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:









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           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):







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           {
               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:








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           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



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       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__




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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



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           }

   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



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           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



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           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:








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           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:




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           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: $!";
           }



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       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



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       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:







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           $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;



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           }

       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



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       -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



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       (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



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           # 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



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       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.






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           #!/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;



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           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') {



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                   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.




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       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:



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           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
           }



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           $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



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       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



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           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;



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               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



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       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



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               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



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           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`;      }



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            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: $!";



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           $| = 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";




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           # 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;




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           # 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:





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           #!/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.













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