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NAME    [Toc]    [Back]

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

DESCRIPTION    [Toc]    [Back]

       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    [Toc]    [Back]

       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 $signame = shift;
               die "Somebody sent me a SIG$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)"

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

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

           print "signal #17 = $signame[17]0;
           if ($signo{ALRM}) {
               print "SIGALRM is $signo{ALRM}0;

       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';
           sub more_functions {
               # interrupts still ignored, for now...

       Sending a signal to a negative process ID means that you
       send the signal to the entire Unix process-group.  This
       code sends a hang-up signal to all processes in the current
 process group (and sets $SIG{HUP} to IGNORE so it
       doesn't kill itself):
               local $SIG{HUP} = 'IGNORE';
               kill HUP => -$$;
               # snazzy writing of: kill('HUP', -$$)

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

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

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

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

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

           $SIG{INT} = sub { die "0utta here!0 };

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

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

       or better still:
           use POSIX ":sys_wait_h";
           sub REAPER {
               my $child;
               #  If a second child dies while in the signal handler caused by the
               # first death, we won't  get  another  signal.  So
must loop here else
               #  we  will  leave the unreaped child as a zombie.
And the next time
               # two children die we get another zombie.  And  so
               while (($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    [Toc]    [Back]

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

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

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

         #!/usr/bin/perl -w

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


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

         # POSIX unmasks the sigprocmask properly
         my $sigset = POSIX::SigSet->new();
         my $action = POSIX::SigAction->new('sigHUP_handler',
         POSIX::sigaction(&POSIX::SIGHUP, $action);

         sub sigHUP_handler {
             print "got SIGHUP0;
             exec($SELF, @ARGV) or die "Couldn't restart: $!0;


         sub code {
             print "PID: $$0;
             print "ARGV: @ARGV0;
             my $c = 0;
             while (++$c) {
                 sleep 2;
                 print "$c0;

Named Pipes    [Toc]    [Back]

       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 Unix command mknod(1) or
       on some systems, mkfifo(1).  These may not be in your normal

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

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

       For example, let's say you'd like to have your .signature
       file be a named pipe that has a Perl program on the other
       end.  Now every time any program (like a mailer, news
       reader, finger program, etc.) tries to read from that
       file, the reading program will block and your program will
       supply the new signature.  We'll use the pipe-checking
       file test -p to find out whether anyone (or anything) has
       accidentally removed our fifo.

           chdir; # go home
           $FIFO = '.signature';
           $ENV{PATH} .= ":/etc:/usr/games";

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

               # next line blocks until there's a reader
               open (FIFO, "> $FIFO") || die "can't write  $FIFO:
               print FIFO "John Smith (smith@host.org)0, `fortune
               close FIFO;
               sleep 2;    # to avoid dup signals
       Deferred Signals (Safe Signals)

       In Perls before Perl 5.7.3 by installing Perl code to deal
       with signals, you were exposing yourself to danger from
       two things.  First, few system library functions are
       re-entrant.  If the signal interrupts while Perl is executing
 one function (like malloc(3) or printf(3)), and
       your signal handler then calls the same function again,
       you  could get unpredictable behavior--often, a core dump.
       Second, Perl isn't itself re-entrant at the lowest levels.
       If the signal interrupts Perl while Perl is changing its
       own internal data structures, similarly unpredictable
       behaviour may result.

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

       In Perl 5.7.3 and later to avoid these problems signals
       are "deferred"-- that is when the signal is delivered to
       the process by the system (to the C code that implements
       Perl) a flag is set, and the handler returns  immediately.
       Then at strategic "safe" points in the Perl interpreter
       (e.g. when it is about to execute a new opcode) the flags
       are checked and the Perl level handler from %SIG is executed.
 The "deferred" scheme allows much more 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

       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 geth-
           ostbyname() 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} try something like the following:

               use POSIX;
               sigaction SIGALRM, new POSIX::SigAction sub {  die
"alarm0 }
                   or die "Error setting SIGALRM handler: $!0;

       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

           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, BUS are generated as a
           result of virtual memory or other "faults". These are
           normally fatal and there is little a Perl-level handler
 can do with them. (In particular the old signal
           scheme was particularly unsafe in such cases.)  However
 if a %SIG handler is set the new scheme simply
           sets a flag and returns as described above. This may
           cause the operating system to try the offending
           machine instruction again and - as nothing has changed
           - it will generate the signal again. The result of
           this is a rather odd "loop". In future Perl's signal
           mechanism may be changed to avoid this - perhaps by
           simply disallowing %SIG handlers on signals of that
           type. Until then the work-round is not to set a %SIG
           handler on those signals. (Which signals they are is
           operating system dependant.)

       Signals triggered by operating system state
           On some operating systems certain signal handlers are
           supposed to "do something" before returning. One example
 can be CHLD or CLD which indicates a child process
           has completed. On some operating systems the signal
           handler is expected to "wait" for the completed child
           process. On such systems the deferred signal scheme
           will not work for those signals (it does not do the
           "wait"). Again the failure will look like a loop as
           the operating system will re-issue the signal as there
           are un-waited-for completed child processes.

       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

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

           open(SPOOLER, "| cat -v | lpr -h 2>/dev/null")
                           || die "can't fork: $!";
           local $SIG{PIPE} = sub { die "spooler pipe broke" };
           print SPOOLER "stuff0;
           close SPOOLER || die "bad spool: $! $?";

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

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

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

           % program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile
       and irrespective of which shell it's called from, the Perl
       program will read from the file f1, the process cmd1,
       standard input (tmpfile in this case), the f2 file, the
       cmd2  command, and finally the f3 file.  Pretty nifty, eh?

       You might notice that you could use backticks for much the
       same effect as opening a pipe for reading:

           print grep { !/^(tcp|udp)/ } `netstat -an 2>&1`;
           die "bad netstat" if $?;

       While this is true on the surface, it's much more efficient
 to process the file one line or record at a time
       because then you don't have to read the whole thing into
       memory at once.  It also gives you finer control of the
       whole process, letting you to kill off the child process
       early if you'd like.

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

           open(FH, "|bogus")  or die "can't fork: $!";
           print FH "bang0   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 "bang0   or die "can't write: $!";
           close FH            or die "can't close: status=$?";

       Filehandles    [Toc]    [Back]

       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    [Toc]    [Back]

       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;
               setsid                   or die "Can't start a new
session: $!";
               open STDERR, '>&STDOUT' or die "Can't dup  stdout:

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

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

       Safe Pipe Opens    [Toc]    [Back]

       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
               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()

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

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

           if ($pid) {   # parent
               while (<KID_TO_READ>) {
                   # do something interesting
               close(KID_TO_READ) || warn "kid exited $?";
           } else {      # child
               ($EUID, $EGID) = ($UID, $GID); # suid only
               exec($program, @options, @args)
                   || die "can't exec program: $!";
               # NOTREACHED

       And here's a safe pipe open for writing:

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

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

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

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

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

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

       Note that these operations are full Unix forks, which
       means they may not be correctly implemented on alien systems.
  Additionally, these are not true multithreading.
       If you'd like to learn more about threading, see the mod-
       ules file mentioned below in the SEE ALSO section.

       Bidirectional Communication with Another Process    [Toc]    [Back]

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

           open(PROG_FOR_READING_AND_WRITING, "| some program |")

       and if you forget to use the "use warnings" pragma or the
       -w flag, then you'll miss out entirely on the diagnostic
           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 "stuff0;
           $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 $ph "a line0;
               print "got back ", scalar <$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    [Toc]    [Back]

       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
           #         designed for the socketpair-challenged
           use IO::Handle;     # thousands of lines just for autoflush :-(
           pipe(PARENT_RDR,   CHILD_WTR);                 #  XXX:
           pipe(CHILD_RDR,   PARENT_WTR);                #   XXX:

           if ($pid = fork) {
               close PARENT_RDR; close PARENT_WTR;
               print CHILD_WTR "Parent Pid $$ is sending this0;
               chomp($line = <CHILD_RDR>);
               print "Parent Pid $$ just read this: `$line'0;
               close CHILD_RDR; close CHILD_WTR;
           } else {
               die "cannot fork: $!" unless defined $pid;
               close CHILD_RDR; close CHILD_WTR;
               chomp($line = <PARENT_RDR>);
               print "Child Pid $$ just read this: `$line'0;
               print PARENT_WTR "Child Pid $$ is sending this0;
               close PARENT_RDR; close PARENT_WTR;

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

           #!/usr/bin/perl -w
           # pipe2 - bidirectional communication using socketpair
           #   "the best ones always go both ways"
           use Socket;
           use IO::Handle;     # thousands of lines just for autoflush :-(
           # We say AF_UNIX because although *_LOCAL is the
           # POSIX 1003.1g form of the constant, many machines
           # still don't have it.
                                       or  die "socketpair: $!";


           if ($pid = fork) {
               close PARENT;
               print CHILD "Parent Pid $$ is sending this0;
               chomp($line = <CHILD>);
               print "Parent Pid $$ just read this: `$line'0;
               close CHILD;
           } else {
               die "cannot fork: $!" unless defined $pid;
               close CHILD;
               chomp($line = <PARENT>);
               print "Child Pid $$ just read this: `$line'0;
               print PARENT "Child Pid $$ is sending this0;
               close PARENT;

Sockets: Client/Server Communication
       While not limited to Unix-derived operating systems (e.g.,
       WinSock on PCs provides socket support, as do some VMS
       libraries), you may not have sockets on your system, in
       which case this section probably isn't going to do you
       much good.  With sockets, you can do both 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 oneline
 messages and responses (so one party knows the other
       has finished when a "0 is received) or multi-line messages
 and responses that end with a period on an empty
       line ("00 terminates a message/response).

       Internet Line Terminators    [Toc]    [Back]

       The Internet line terminator is " 15 12".  Under ASC0,
       variants of Unix, that cou0dmightlat timesibeen as "
       but under other systems, "
       " 15 15 12", " 12 12 15", or something completely
       different.  The standards specify writing " 15 12" to be
       conformant (be strict in what you provide), but they also
       recommend accepting a lone " 12" 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    [Toc]    [Back]

       Use Internet-domain sockets when you want to do clientserver
 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 =~ /           die "No port" unless $port;
           $iaddr   = inet_aton($remote)               || die "no
host: $remote";
           $paddr   = sockaddr_in($port, $iaddr);

           $proto   = getprotobyname('tcp');
           socket(SOCK,  PF_INET,  SOCK_STREAM,  $proto)   || die
"socket: $!";
           connect(SOCK, $paddr)    || die "connect: $!";
           while (defined($line = <SOCK>)) {
               print $line;

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

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

           #!/usr/bin/perl -Tw
           use strict;
           BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
           use Socket;
           use Carp;
           my $EOL = " 15 12";

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

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

           ($port)  =  $port =~ /^(+)$/                        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 = " 15 12";
           sub spawn;  # forward declaration
           sub logmsg { print "$0 $$: @_ at ", scalar  localtime,
"0 }

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

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

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

           logmsg "server started on port $port";

           my $waitedpid = 0;
           my $paddr;

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

           $SIG{CHLD} = REAPER;

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

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

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


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

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

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

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

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

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

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

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

           my $iaddr = gethostbyname('localhost');
           my $proto = getprotobyname('tcp');
           my $port = getservbyname('time', 'tcp');
           my $paddr = sockaddr_in(0, $iaddr);
           $| = 1;
           printf "%-24s %8s %s0,  "localhost", 0, ctime(time());

           foreach $host (@ARGV) {
               printf "%-24s ", $host;
               my $hisiaddr = inet_aton($host)      ||  die  "unknown host";
               my $hispaddr = sockaddr_in($port, $hisiaddr);
               socket(SOCKET,  PF_INET, SOCK_STREAM, $proto)   ||
die "socket: $!";
               connect(SOCKET, $hispaddr)          || die  "bind:
               my $rtime = '    ';
               read(SOCKET, $rtime, 4);
               my    $histime    =    unpack("N",    $rtime)    -
$SECS_of_70_YEARS ;
               printf "%8d %s0, $histime - time, ctime($histime);

       Unix-Domain TCP Clients and Servers    [Toc]    [Back]

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

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

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

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

       Here's a sample Unix-domain client:

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

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

       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,
"0 }

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

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

           logmsg "server started on $NAME";

           my $waitedpid;

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

           $SIG{CHLD} = REAPER;

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

           sub spawn {
               my $coderef = shift;

               unless (@_ == 0 && $coderef  &&  ref($coderef)  eq
'CODE') {
                   confess "usage: spawn CODEREF";
               my $pid;
               if (!defined($pid = fork)) {
                   logmsg "cannot fork: $!";
               } elsif ($pid) {
                   logmsg "begat $pid";
                   return; # I'm the parent
               # else I'm the child -- go spawn

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

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

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

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

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

       A Simple Client    [Toc]    [Back]

       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

           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.

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

           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 data0;

       A Webget Client    [Toc]    [Back]

       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 = " 15 12";
           $BLANK = $EOL x 2;
           foreach $document ( @ARGV ) {
               $remote  =  IO::Socket::INET->new(  Proto       =>
                                                PeerAddr       =>
                                                PeerPort       =>
               unless  ($remote)  {  die  "cannot connect to http
daemon on $host" }
               print $remote "GET $document HTTP/1.0" . $BLANK;
               while ( <$remote> ) { print }
               close $remote;

       The web server handing the "http" service, which is
       assumed to be at its standard port, number 80.  If the web
       server you're trying to connect to is at a different port
       (like 1080 or 8080), you should specify as the namedparameter
 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 "0 in your code
       that sends data over the network to be a " 15 12"

       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

       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

       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, b

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