MirOS Manual: perlipc(1)


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

NAME

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

DESCRIPTION

     The basic IPC facilities of Perl are built out of the good
     old Unix signals, named pipes, pipe opens, the Berkeley
     socket routines, and SysV IPC calls.  Each is used in
     slightly different situations.

Signals

     Perl uses a simple signal handling model: the %SIG hash con-
     tains 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 gen-
     erated intentionally from a particular keyboard sequence
     like control-C or control-Z, sent to you from another pro-
     cess, 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; par-
     ticularly, 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 plat-
     form 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 pro-
     cess 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) func-
     tion from the C library, you may sometimes be so misfor-
     tunate 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 undocu-
     mented, 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 con-
     figuration 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.

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     Many daemons provide this mechanism using the "SIGHUP" sig-
     nal 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 res-
     tarts 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();

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

Named Pipes

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

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

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

     You can also use the Unix command mknod(1) or on some sys-
     tems, 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';

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

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

     Deferred Signals (Safe Signals)

     In Perls before Perl 5.7.3 by installing Perl code to deal
     with signals, you were exposing yourself to danger from two
     things.  First, few system library functions are re-entrant.
     If the signal interrupts while Perl is executing one func-
     tion (like malloc(3) or printf(3)), and your signal handler
     then calls the same function again, you could get unpredict-
     able 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 exist-
     ing 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 con-
     venience", 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

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     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 completes.

     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 implementa-
         tions 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";

     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

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         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 inter-
         rupted "wait" and "waitpid" calls will always be
         retried.

     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 instruc-
         tion again and - as nothing has changed - it will gen-
         erate 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 sig-
         nals. (Which signals they are is operating system depen-
         dent.)

     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 com-
         pleted child processes.

     If you want the old signal behaviour back regardless of pos-
     sible 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 unidirec-
     tional 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 pro-
     grammer 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, stan-
     dard 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 suc-
     cess), 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.

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     Consider:

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

     That won't blow up until the close, and it will blow up with
     a SIGPIPE. To catch it, you could use this:

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

     Filehandles

     Both the main process and any child processes it forks share
     the same STDIN, STDOUT, and STDERR filehandles.  If both
     processes try to access them at once, strange things can
     happen.  You may also want to close or reopen the filehan-
     dles 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, depend-
     ing on your shell) will be the same as the parent's.  You
     won't need to catch SIGCHLD because of the double-fork tak-
     ing 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 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';

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         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() func-
     tion, 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 run-
     ning 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;

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         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 $?";

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         } 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 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.

     Bidirectional Communication with Another Process

     While this works reasonably well for unidirectional communi-
     cation, what about bidirectional communication?  The obvious
     thing you'd like to do doesn't actually work:

         open(PROG_FOR_READING_AND_WRITING, "| some program |")

     and if you forget to use the "use warnings" pragma or the -w
     flag, then you'll miss out entirely on the diagnostic mes-
     sage:

         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

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     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 reason-
     ably:

         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.

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

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

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

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

         use Socket;
         use IO::Handle;     # thousands of lines just for autoflush :-(
         # We say AF_UNIX because although *_LOCAL is the
         # POSIX 1003.1g form of the constant, many machines
         # still don't have it.
         socketpair(CHILD, PARENT, AF_UNIX, SOCK_STREAM, PF_UNSPEC)
                                     or  die "socketpair: $!";

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

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         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 proto-
     col 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 mes-
     sages and responses (so one party knows the other has fin-
     ished 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).

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     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 dif-
     ferent.  The standards specify writing "\015\012" to be con-
     formant (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" }

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         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 {
                 $|=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: $!";
             };

         }

         sub spawn {
             my $coderef = shift;

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

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

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

     This server takes the trouble to clone off a child version
     via fork() for each incoming request.  That way it can han-
     dle 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 particu-
     larly 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) }

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

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

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

     Unix-Domain TCP Clients and Servers

     That's fine for Internet-domain clients and servers, but
     what about local communications?  While you can use the same
     setup, sometimes you don't want to.  Unix-domain sockets are
     local to the current host, and are often used internally to
     implement pipes.  Unlike Internet domain sockets, Unix
     domain sockets can show up in the file system with an ls(1)
     listing.

         % ls -l /dev/log
         srw-rw-rw-  1 root            0 Oct 31 07:23 /dev/log

     You can test for these with Perl's -S file test:

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

     Here's a sample Unix-domain client:

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

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

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

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

         BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
         sub spawn;  # forward declaration
         sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }

         my $NAME = 'catsock';
         my $uaddr = sockaddr_un($NAME);
         my $proto = getprotobyname('tcp');

         socket(Server,PF_UNIX,SOCK_STREAM,0)        || die "socket: $!";
         unlink($NAME);
         bind  (Server, $uaddr)                      || die "bind: $!";
         listen(Server,SOMAXCONN)                    || die "listen: $!";

         logmsg "server started on $NAME";

         my $waitedpid;

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

         $SIG{CHLD} = \&REAPER;

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         for ( $waitedpid = 0;
               accept(Client,Server) || $waitedpid;
               $waitedpid = 0, close Client)
         {
             next if $waitedpid;
             logmsg "connection on $NAME";
             spawn sub {
                 print "Hello there, it's now ", scalar localtime, "\n";
                 exec '/usr/games/fortune' or die "can't exec fortune: $!";
             };
         }

         sub spawn {
             my $coderef = shift;

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

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

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

     As you see, it's remarkably similar to the Internet domain
     TCP server, so much so, in fact, that we've omitted several
     duplicate functions--spawn(), logmsg(), ctime(), and
     REAPER()--which are exactly the same as in the other server.

     So why would you ever want to use a Unix domain socket
     instead of a simpler named pipe?  Because a named pipe
     doesn't give you sessions.  You can't tell one process's
     data from another's.  With socket programming, you get a
     separate session for each client: that's why accept() takes
     two arguments.

     For example, let's say that you have a long running database
     server daemon that you want folks from the World Wide Web to
     be able to access, but only if they go through a CGI inter-
     face.  You'd have a small, simple CGI program that does
     whatever checks and logging you feel like, and then acts as

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     a Unix-domain client and connects to your private server.

TCP Clients with IO::Socket
     For those preferring a higher-level interface to socket pro-
     gramming, 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 run-
     ning an earlier version of Perl, just fetch IO::Socket from
     CPAN, where you'll also find modules providing easy inter-
     faces to the following systems: DNS, FTP, Ident (RFC 931),
     NIS and NISPlus, NNTP, Ping, POP3, SMTP, SNMP, SSLeay, Tel-
     net, and Time--just to name a few.

     A Simple Client

     Here's a client that creates a TCP connection to the "day-
     time" service at port 13 of the host name "localhost" and
     prints out everything that the server there cares to pro-
     vide.

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

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

         Wed May 14 08:40:46 MDT 1997

     Here are what those parameters to the "new" constructor
     mean:

     "Proto"
         This is which protocol to use.  In this case, the socket
         handle returned will be connected to a TCP socket,
         because we want a stream-oriented connection, that is,
         one that acts pretty much like a plain old file. Not all
         sockets are this of this type.  For example, the UDP
         protocol can be used to make a datagram socket, used for
         message-passing.

     "PeerAddr"
         This is the name or Internet address of the remote host
         the server is running on.  We could have specified a
         longer name like "www.perl.com", or an address like
         "204.148.40.9".  For demonstration purposes, we've used

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         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 ser-
         vices file,[FOOTNOTE: The system services file is in
         /etc/services under Unix] but just in case, we've speci-
         fied 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 nor-
     mal 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.

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

     The web server handing the "http" service, which is assumed
     to be at its standard port, number 80.  If the web server
     you're trying to connect to is at a different port (like
     1080 or 8080), you should specify as the named-parameter
     pair, "PeerPort => 8080".  The "autoflush" method is used on
     the socket because otherwise the system would buffer up the
     output we sent it.  (If you're on a Mac, you'll also need to
     change every "\n" in your code that sends data over the net-
     work to be a "\015\012" instead.)

     Connecting to the server is only the first part of the pro-
     cess: 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

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         <HEAD><TITLE>404 File Not Found</TITLE></HEAD>
         <BODY><H1>File Not Found</H1>
         The requested URL /guanaco.html was not found on this server.<P>
         </BODY>

     Ok, so that's not very interesting, because it didn't find
     that particular document.  But a long response wouldn't have
     fit on this page.

     For a more fully-featured version of this program, you
     should look to the lwp-request program included with the LWP
     modules from CPAN.

     Interactive Client with IO::Socket

     Well, that's all fine if you want to send one command and
     get one answer, but what about setting up something fully
     interactive, somewhat like the way telnet works?  That way
     you can type a line, get the answer, type a line, get the
     answer, etc.

     This client is more complicated than the two we've done so
     far, but if you're on a system that supports the powerful
     "fork" call, the solution isn't that rough.  Once you've
     made the connection to whatever service you'd like to chat
     with, call "fork" to clone your process.  Each of these two
     identical process has a very simple job to do: the parent
     copies everything from the socket to standard output, while
     the child simultaneously copies everything from standard
     input to the socket. To accomplish the same thing using just
     one process would be much harder, because it's easier to
     code two processes to do one thing than it is to code one
     process to do two things.  (This keep-it-simple principle a
     cornerstones of the Unix philosophy, and good software
     engineering as well, which is probably why it's spread to
     other systems.)

     Here's the code:

         #!/usr/bin/perl -w
         use strict;
         use IO::Socket;
         my ($host, $port, $kidpid, $handle, $line);

         unless (@ARGV == 2) { die "usage: $0 host port" }
         ($host, $port) = @ARGV;

         # create a tcp connection to the specified host and port
         $handle = IO::Socket::INET->new(Proto     => "tcp",
                                         PeerAddr  => $host,
                                         PeerPort  => $port)
                or die "can't connect to port $port on $host: $!";

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

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

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

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

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

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

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

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

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

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     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 SOMAX-
         CONN.

     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

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

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

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

UDP: Message Passing
     Another kind of client-server setup is one that uses not
     connections, but messages.  UDP communications involve much
     lower overhead but also provide less reliability, as there
     are no promises that messages will arrive at all, let alone
     in order and unmangled.  Still, UDP offers some advantages
     over TCP, including being able to "broadcast" or "multicast"
     to a whole bunch of destination hosts at once (usually on
     your local subnet).  If you find yourself overly concerned
     about reliability and start building checks into your mes-
     sage 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

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     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 han-
     dle for each host.

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

         my ( $count, $hisiaddr, $hispaddr, $histime,
              $host, $iaddr, $paddr, $port, $proto,
              $rin, $rout, $rtime, $SECS_of_70_YEARS);

         $SECS_of_70_YEARS      = 2208988800;

         $iaddr = gethostbyname(hostname());
         $proto = getprotobyname('udp');
         $port = getservbyname('time', 'udp');
         $paddr = sockaddr_in(0, $iaddr); # 0 means let kernel pick

         socket(SOCKET, PF_INET, SOCK_DGRAM, $proto)   || die "socket: $!";
         bind(SOCKET, $paddr)                          || die "bind: $!";

         $| = 1;
         printf "%-12s %8s %s\n",  "localhost", 0, scalar localtime time;
         $count = 0;
         for $host (@ARGV) {
             $count++;
             $hisiaddr = inet_aton($host)    || die "unknown host";
             $hispaddr = sockaddr_in($port, $hisiaddr);
             defined(send(SOCKET, 0, 0, $hispaddr))    || die "send $host: $!";
         }

         $rin = '';
         vec($rin, fileno(SOCKET), 1) = 1;

         # timeout after 10.0 seconds
         while ($count && select($rout = $rin, undef, undef, 10.0)) {
             $rtime = '';
             ($hispaddr = recv(SOCKET, $rtime, 4, 0))        || die "recv: $!";
             ($port, $hisiaddr) = sockaddr_in($hispaddr);
             $host = gethostbyaddr($hisiaddr, AF_INET);
             $histime = unpack("N", $rtime) - $SECS_of_70_YEARS;
             printf "%-12s ", $host;
             printf "%8d %s\n", $histime - time, scalar localtime($histime);
             $count--;
         }

     Note that this example does not include any retries and may
     consequently fail to contact a reachable host. The most
     prominent reason for this is congestion of the queues on the

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     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_IRWXU);

         $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;

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

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

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

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

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

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

         $semnum = 0;
         $semflag = 0;

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

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

     The SysV IPC code above was written long ago, and it's
     definitely clunky looking.  For a more modern look, see the
     IPC::SysV module which is included with Perl starting from
     Perl 5.005.

     A small example demonstrating SysV message queues:

         use IPC::SysV qw(IPC_PRIVATE IPC_RMID IPC_CREAT S_IRWXU);

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

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

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         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 pro-
     grams this way for optimal success, and don't forget to add
     -T taint checking flag to the #! line for servers:

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

BUGS

     All these routines create system-specific portability prob-
     lems.  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.

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     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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