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


       Perl uses a simple signal handling model: the %SIG hash
       contains names or references of user-installed signal han­
       dlers.  These handlers will be called with an argument
       which is the name of the signal that triggered it.  A sig­
       nal may be generated intentionally from a particular key­
       board 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 hit­
       ting 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 rou­
       tines 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 Sig­
       nals)" below.

       The names of the signals are the ones listed out by "kill
       -l" on your system, or you can retrieve them from the Con­
       fig 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;

       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 strat­
       egy 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 cur­
       rent 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 "%!".

       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 $child;
               # If a second child dies while in the signal handler caused by the
               # first death, we won't get another signal. So must loop here else
               # we will leave the unreaped child as a zombie. And the next time
               # two children die we get another zombie. And so on.
               while (($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 sig­
       nal 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 lan­

       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
       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" sig­

       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 han­
       dlers 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 SIGHUP\n";
             exec($SELF, @ARGV) or die "Couldn't restart: $!\n";


         sub code {
             print "PID: $$\n";
             print "ARGV: @ARGV\n";
             my $c = 0;

       To create a named pipe, use the Unix command mknod(1) or
       on some systems, mkfifo(1).  These may not be in your nor­
       mal path.

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

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

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

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

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

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

       Deferred Signals (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 exe­
       cuting one function (like malloc(3) or printf(3)), and
       your signal handler then calls the same function again,
       in a handler because the system is out to get you.  The
       pragmatic approach was to say ``I know the risks, but pre­
       fer 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 exe­
       cuted. The "deferred" scheme allows much more flexibility
       in the coding of signal handler as we know Perl inter­
       preter 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 fol­
       lowing 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 sig­
           nal 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 ":per­
           lio" 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 implementa­
           tions of timeouts which may conflict with your time­
           outs.  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
           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 han­
           dler can do with them. (In particular the old signal
           scheme was particularly unsafe in such cases.)  How­
           ever 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 exam­
           ple 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 unidi­
       rectional 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 pro­
       cess you intend to write to:

           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 effi­
       cient 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 pre­
       pared to handle.  Consider:

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

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

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

           system("cmd &");

       The command's STDOUT and STDERR (and possibly STDIN,
       depending on your shell) will be the same as the parent's.
       You won't need to catch SIGCHLD because of the double-fork
       taking place (see below for more details).

       Complete Dissociation of Child from Parent

       In some cases (starting server processes, for instance)
       you'll want to completely dissociate the child process
       from the parent.  This is often called daemonization.  A
       well behaved daemon will also chdir() to the root direc­
       tory (so it doesn't prevent unmounting the filesystem con­
       taining 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

       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 inter­
       esting 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
           } 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 argu­
       ments.   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 $?";

       there are more than three arguments to open()), and reads
       its standard output via the "KID_PS" filehandle.  The cor­
       responding 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 sys­
       tems.  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

       While this works reasonably well for unidirectional commu­
       nication, 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 any­
       where except on a Unix system or some other one purporting
       to be POSIX compliant.

       Here's an example of using open2():

           use FileHandle;
           use IPC::Open2;
           $pid = open2(*Reader, *Writer, "cat -u -n" );
           print Writer "stuff\n";
           $got = <Reader>;

       The problem with this is that Unix buffering is really
       going to ruin your day.  Even though your "Writer" file­
       handle is auto-flushed, and the process on the other end
           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-ter­
       minal to interact with programs that insist on using talk­
       ing to the terminal device driver.  If your system is
       amongst those supported, this may be your best bet.

       Bidirectional Communication with Yourself

       If you want, you may make low-level pipe() and fork() to
       stitch this together by hand.  This example only talks to
       itself, but you could reopen the appropriate handles to
       STDIN and STDOUT and call other processes.

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

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

       But you don't actually have to make two pipe calls.  If


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

Sockets: Client/Server Communication

       While not limited to Unix-derived operating systems (e.g.,
       WinSock on PCs provides socket support, as do some VMS
       libraries), you may not have sockets on your system, in
       which case this section probably isn't going to do you
       much good.  With sockets, you can do both 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 supe­
       rior 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 pro­
       tocol like NNTP or SMTP, you should give some thought to
       how your server will know when the client has finished
       talking, and vice-versa.  Most protocols are based on one-
       line messages and responses (so one party knows the other
       has finished when a "\n" is received) or multi-line mes­
       on a Mac, you'll probably be ok.

       Internet TCP Clients and Servers

       Use Internet-domain sockets when you want to do client-
       server communication that might extend to machines outside
       of your own system.

       Here's a sample TCP client using Internet-domain sockets:

           #!/usr/bin/perl -w
           use strict;
           use Socket;
           my ($remote,$port, $iaddr, $paddr, $proto, $line);

           $remote  = shift || 'localhost';
           $port    = shift || 2345;  # random port
           if ($port =~ /\D/) { $port = getservbyname($port, 'tcp') }
           die "No port" unless $port;
           $iaddr   = inet_aton($remote)               || die "no host: $remote";
           $paddr   = sockaddr_in($port, $iaddr);

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

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

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

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

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

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

           ($port) = $port =~ /^(\d+)$/                        or die "invalid port";
               my($port,$iaddr) = sockaddr_in($paddr);
               my $name = gethostbyaddr($iaddr,AF_INET);

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

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

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

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

           sub spawn;  # forward declaration
           sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }

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

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

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

           logmsg "server started on port $port";

           my $waitedpid = 0;
           my $paddr;

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

           $SIG{CHLD} = \&REAPER;

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

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

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

       Unix-Domain TCP Clients and Servers

       That's fine for Internet-domain clients and servers, but
       what about local communications?  While you can use the
       same setup, sometimes you don't want to.  Unix-domain
       sockets are local to the current host, and are often used
       internally to implement pipes.  Unlike Internet domain
       sockets, Unix domain sockets can show up in the file sys­
       tem 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 || '/tmp/catsock';
           socket(SOCK, PF_UNIX, SOCK_STREAM, 0)       || die "socket: $!";
           connect(SOCK, sockaddr_un($rendezvous))     || die "connect: $!";
           while (defined($line = <SOCK>)) {
           BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
           sub spawn;  # forward declaration
           sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }

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

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

           logmsg "server started on $NAME";

           my $waitedpid;

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

           $SIG{CHLD} = \&REAPER;

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

           sub spawn {
               my $coderef = shift;

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

               my $pid;
               if (!defined($pid = fork)) {
                   logmsg "cannot fork: $!";
               } elsif ($pid) {

       TCP server, so much so, in fact, that we've omitted sev­
       eral 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-ori­
       ented 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 pro­
       viding easy interfaces to the following systems: DNS, FTP,
       Ident (RFC 931), NIS and NISPlus, NNTP, Ping, POP3, SMTP,
       SNMP, SSLeay, Telnet, and Time--just to name a few.

       A Simple Client

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

           #!/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
           This is the name or Internet address of the remote
           host the server is running on.  We could have speci­
           fied 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 contain­
       ing a filehandle.  You can use it the same way you would a
       normal filehandle.  For example, you can read one line
       from it this way:

           $line = <$handle>;

       all remaining lines from is this way:

           @lines = <$handle>;

       and send a line of data to it this way:

           print $handle "some data\n";

       A Webget Client

       Here's a simple client that takes a remote host to fetch a
       document from, and then a list of documents to get from
       that host.  This is a more interesting client than the
       previous one because it first sends something to the
       server before fetching the server's response.

               $remote = IO::Socket::INET->new( Proto     => "tcp",
                                                PeerAddr  => $host,
                                                PeerPort  => "http(80)",
               unless ($remote) { die "cannot connect to http daemon on $host" }
               print $remote "GET $document HTTP/1.0" . $BLANK;
               while ( <$remote> ) { print }
               close $remote;

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

       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 con­
       nection for each document, even though it's the same host.
       That's the way you always used to have to speak HTTP.
       Recent versions of web browsers may request that the
       remote server leave the connection open a little while,
       but the server doesn't have to honor such a request.

       Here's an example of running that program, which we'll
       call webget:

           % webget www.perl.com /guanaco.html
           HTTP/1.1 404 File Not Found
           Date: Thu, 08 May 1997 18:02:32 GMT
           Server: Apache/1.2b6
           Connection: close
           Content-type: text/html

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

       Ok, so that's not very interesting, because it didn't find
       that particular document.  But a long response wouldn't
       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 out­
       put, while the child simultaneously copies everything from
       standard input to the socket.  To accomplish the same
       thing using just one process would be much harder, because
       it's easier to code two processes to do one thing than it
       is to code one process to do two things.  (This keep-it-
       simple principle a cornerstones of the Unix philosophy,
       and good software engineering as well, which is probably
       why it's spread to other systems.)

       Here's the code:

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

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

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

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

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

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

           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 cre­
       ates a special kind of socket that does nothing but listen
       on a particular port for incoming connections.  It does
       this by calling the "IO::Socket::INET->new()" method with
       slightly different arguments than the client did.

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

           We specify a local port in the "LocalPort" argument,
           which we didn't do for the client.  This is service
           name or port number for which you want to be the
           server. (Under Unix, ports under 1024 are restricted
           to the superuser.)  In our sample, we'll use port
           9000, but you can use any port that's not currently in
           use on your system.  If you try to use one already in
           used, you'll get an "Address already in use" message.
           Under Unix, the "netstat -a" command will show which
           services current have servers.

           The "Listen" parameter is set to the maximum number of
           pending connections we can accept until we turn away
           incoming clients.  Think of it as a call-waiting queue
           for your telephone.  The low-level Socket module has a
           special symbol for the system maximum, which is SOMAX­

           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 aut­
       oflush this handle to circumvent buffering.)
        #!/usr/bin/perl -w
        use IO::Socket;
        use Net::hostent;              # for OO version of gethostbyaddr

        $PORT = 9000;                  # pick something not in use

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

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

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

UDP: Message Passing

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

       Note that UDP datagrams are not a bytestream and should
       not be treated as such. This makes using I/O mechanisms
       with internal buffering like stdio (i.e. print() and
           use Sys::Hostname;

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

           $SECS_of_70_YEARS      = 2208988800;

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

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

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

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

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

       Note that this example does not include any retries and
       may consequently fail to contact a reachable host. The
       most prominent reason for this is congestion of the queues
       on the sending host if the number of list of hosts to con­
       tact is sufficiently large.


       While System V IPC isn't so widely used as sockets, it
       still has some interesting uses.  You can't, however,
       effectively use SysV IPC or Berkeley mmap() to have shared
       memory so as to share a variable amongst several pro­
           shmread($id, $buff, 0, 60) || die "$!";
           print "read : '$buff'\n";

           # the buffer of shmread is zero-character end-padded.
           substr($buff, index($buff, "\0")) = '';
           print "un" unless $buff eq $message;
           print "swell\n";

           print "deleting shm $id\n";
           shmctl($id, IPC_RMID, 0) || die "$!";

       Here's an example of a semaphore:

           use IPC::SysV qw(IPC_CREAT);

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

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

           # create a semaphore

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

           $semnum = 0;
           $semflag = 0;

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

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

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

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

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

           $IPC_KEY = 1234;
           $id = semget($IPC_KEY, 0, 0);

       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:


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

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

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


       Most of these routines quietly but politely return "undef"
       when they fail instead of causing your program to die
       right then and there due to an uncaught exception.  (Actu­
       ally, some of the new Socket conversion functions  croak()
       on bad arguments.)  It is therefore essential to check
       return values from these functions.  Always begin your
       socket programs this way for optimal success, and don't
       forget to add -T taint checking flag to the #! line for

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


       die() to raise an exception and longjmp(3) out.  In fact,
       even these may in some cases cause a core dump.  It's
       probably best to avoid signals except where they are abso­
       lutely inevitable.  This will be addressed in a future
       release of Perl.


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


       There's a lot more to networking than this, but this
       should get you started.

       For intrepid programmers, the indispensable textbook is
       Unix Network Programming, 2nd Edition, Volume 1 by W.
       Richard Stevens (published by Prentice-Hall).  Note that
       most books on networking address the subject from the per­
       spective 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 inter­
       face to sockets.  Besides the obvious functions in perl­
       func, 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

       Section 5 of the modules file is devoted to "Networking,
       Device Control (modems), and Interprocess Communication",
       and contains numerous unbundled modules numerous network­
       ing modules, Chat and Expect operations, CGI programming,
       DCE, FTP, IPC, NNTP, Proxy, Ptty, RPC, SNMP, SMTP, Telnet,
       Threads, and ToolTalk--just to name a few.

perl v5.8.1                 2003-09-02                 PERLIPC(1)
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