NAME DESCRIPTION Signals Named Pipes Using open() for IPC Sockets: Client/Server Communication TCP Clients with IO::Socket TCP Servers with IO::Socket UDP: Message Passing SysV IPC NOTES BUGS AUTHOR SEE ALSO


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


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


Perl uses a simple signal handling model: the %SIG hash contains names or references of user-installed signal handlers. These handlers will be called with an argument which is the name of the signal that triggered it. A signal may be generated intentionally from a particular keyboard sequence like control-C or control-Z, sent to you from another process, or triggered automatically by the kernel when special events transpire, like a child process exiting, your process running out of stack space, or hitting file size limit.

For example, to trap an interrupt signal, set up a handler like this. Do as little as you possibly can 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 means that doing nearly anything in your handler could in theory trigger a memory fault and subsequent core dump.

sub catch_zap {

my $signame = shift; $shucks++; die

The names of the signals are the ones listed out by kill -l on your system, or you can retrieve them from the Config module. Set up an @signame list indexed by number to get the name and a %signo table indexed by name to get the number
use Config;

defined $Config{sig_name} die

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

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';

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

unless (kill 0 =

You might also want to employ anonymous functions for simple signal handlers
$SIG{INT} = sub { die
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 this
sub REAPER {

$waitedpid = wait;

  1. loathe sysV: it makes us not only reinstate
  2. the handler, but place it after the wait


or even the more elaborate

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 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.

Named Pipes

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

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

  1. system return val is backwards, so

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} .=

while (1) {

unless (-p $FIFO) { unlink $FIFO; system('mknod', $FIFO, 'p')

  1. next line blocks until there's a reader

open (FIFO,


By installing Perl code to deal with signals, you're 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 are two things you can do, knowing this: be paranoid or be pragmatic. The paranoid approach is 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 is to say ``I know the risks, but prefer the convenience'', and to do anything you want in your signal handler, prepared to clean up core dumps now and again.

To forbid signal handlers altogether would bars you from many interesting programs, including virtually everything in this manpage, since you could no longer even write SIGCHLD handlers.

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

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 { !/^(tcpudp)/ } `netstat -an 2

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



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

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

Complete Dissociation of Child from Parent

In some cases (starting server processes, for instance) you'll want to completely dissociate the child process from the parent. This is often called daemonization. A well behaved daemon will also chdir() to the root directory (so it doesn't prevent unmounting the filesystem containing the directory from which it was launched) and redirect its standard file descriptors from and to /dev/null (so that random output doesn't wind up on the user's terminal).

use POSIX 'setsid'; sub daemonize {

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

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

Safe Pipe Opens

Another interesting approach to IPC is making your single program go multiprocess and communicate between (or even amongst) yourselves. The open() function will accept a file argument of either or to do a very interesting thing: it forks a child connected to the filehandle you've opened. The child is running the same program as the parent. This is useful for safely opening a file when running under an assumed UID or GID , for example. If you open a pipe to minus, you can write to the filehandle you opened and your kid will find it in his STDIN . If you open a pipe from minus, you can read from the filehandle you opened whatever your kid writes to his STDOUT .

use English;

my $sleep_count = 0;

do {

$pid = open(KID_TO_WRITE,

if ($pid) { # parent

print KID_TO_WRITE @some_data; close(KID_TO_WRITE) warn Another common use for this construct is when you need to execute something without the shell's interference. With system(), it's straightforward, but you can't use a pipe open or backticks safely. That's because there's no way to stop the shell from getting its hands on your arguments. Instead, use lower-level control to call exec() directly.

Here's a safe backtick or pipe open for read
  1. add error processing as above

$pid = open(KID_TO_READ,

if ($pid) { # parent

while (

} else { # child

($EUID, $EGID) = ($UID, $GID); # suid only exec($program, @options, @args) die

And here's a safe pipe open for writing
  1. add error processing as above

$pid = open(KID_TO_WRITE,

if ($pid) { # parent

for (@data) { print KID_TO_WRITE; } close(KID_TO_WRITE) warn

} else { # child

($EUID, $EGID) = ($UID, $GID); exec($program, @options, @args) die Note that these operations are full Unix forks, which means they may not be correctly implemented on alien systems. Additionally, these are not true multithreading. If you'd like to learn more about threading, see the modules file mentioned below in the SEE ALSO section.

Bidirectional Communication with Another Process

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

and if you forget to use the use warnings pragma or the -w flag, then you'll miss out entirely on the diagnostic message
Can't do bidirectional pipe at -e line 1.

If you really want to, you can use the standard open2() library function to catch both ends. There's also an open3() for tridirectional I/O so you can also catch your child's STDERR , but doing so would then require an awkward select() loop and wouldn't allow you to use normal Perl input operations.

If you look at its source, you'll see that open2() uses low-level primitives like Unix pipe() and exec() calls to create all the connections. While it might have been slightly more efficient by using socketpair(), it would have then been even less portable than it already is. The open2() and open3() functions are unlikely to work anywhere except on a Unix system or some other one purporting to be POSIX compliant.

Here's an example of using open2()
use !FileHandle?;

use IPC::Open2; $pid = open2(*Reader, *Writer, 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 library. It uses pseudo-ttys to make your program behave

more reasonably
require '';

$ph = open_proc('cat -n'); for (1..10) { print $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 module from CPAN also addresses this kind of thing. This module requires two other modules from CPAN: IO::Pty and IO::Stty. It sets up a pseudo-terminal to interact with programs that insist on using talking to the terminal device driver. If your system is amongst those supported, this may be your best bet.

Bidirectional Communication with Yourself

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

  1. /usr/bin/perl -w

  2. pipe1 - bidirectional communication using two pipe pairs
  3. designed for the socketpair-challenged

use IO::Handle; # thousands of lines just for autoflush :-( pipe(PARENT_RDR, CHILD_WTR); # XXX: failure? pipe(CHILD_RDR, PARENT_WTR); # XXX: failure? CHILD_WTR-

if ($pid = fork) {

close PARENT_RDR; close PARENT_WTR; print CHILD_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.

  1. /usr/bin/perl -w

  2. pipe2 - bidirectional communication using socketpair


use Socket;

use IO::Handle; # thousands of lines just for autoflush :-(

  1. We say AF_UNIX because although *_LOCAL is the
  2. POSIX 1003.1g form of the constant, many machines
  3. still don't have it.


CHILD- if ($pid = fork) {

close PARENT; print CHILD

Sockets: Client/Server Communication

While not limited to Unix-derived operating systems (e.g.,

WinSock? on PCs provides socket support, as do some

VMS libraries), you may not have sockets on your system, in which case this section probably isn't going to do you much good. With sockets, you can do both virtual circuits (i.e., TCP streams) and datagrams (i.e., UDP packets). You may be able to do even more depending on your system.

The Perl function calls for dealing with sockets have the same names as the corresponding system calls in C, but their arguments tend to differ for two reasons: first, Perl filehandles work differently than C file descriptors. Second, Perl already knows the length of its strings, so you don't need to pass that information.

One of the major problems with old socket code in Perl was that it used hard-coded values for some of the constants, which severely hurt portability. If you ever see code that does anything like explicitly setting $AF_INET = 2, you know you're in for big trouble: An immeasurably superior approach is to use the Socket module, which more reliably grants access to various constants and functions you'll need.

If you're not writing a server/client for an existing protocol like NNTP or SMTP , you should give some thought to how your server will know when the client has finished talking, and vice-versa. Most protocols are based on one-line messages and responses (so one party knows the other has finished when a ``n is received) or multi-line messages and responses that end with a period on an empty line (``n.n terminates a message/response).

Internet Line Terminators

The Internet line terminator is ``015012. Under ASCII variants of Unix, that could usually be written as ``rn, but under other systems, ``rn might at times be ``015015012, ``012012015, or something completely different. The standards specify writing ``015012 to be conformant (be strict in what you provide), but they also recommend accepting a lone ``012'' on input (but be lenient in what you require). We haven't always been very good about that in the code in this manpage, but unless you're on a Mac, you'll probably be ok.

Internet TCP Clients and Servers

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

Here's a sample TCP client using

Internet-domain sockets
  1. /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

$proto = getprotobyname('tcp');

socket(SOCK, PF_INET, SOCK_STREAM, $proto) die

close (SOCK) die

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.

  1. /usr/bin/perl -Tw

use strict; BEGIN { $ENV{PATH} = '/usr/ucb:/bin' } use Socket; use Carp; my $EOL =

sub logmsg { print my $port = shift 2345;

my $proto = getprotobyname('tcp');

($port) = $port = /^(d+)$/ or die socket(Server, PF_INET, SOCK_STREAM, $proto) die logmsg my $paddr; $SIG{CHLD} = for ( ; $paddr = accept(Client,Server); close Client) {

my($port,$iaddr) = sockaddr_in($paddr); my $name = gethostbyaddr($iaddr,AF_INET);

logmsg print Client

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.

  1. /usr/bin/perl -Tw

use strict; BEGIN { $ENV{PATH} = '/usr/ucb:/bin' } use Socket; use Carp; my $EOL =

sub spawn; # forward declaration

sub logmsg { print

my $port = shift 2345;

my $proto = getprotobyname('tcp');

($port) = $port = /^(d+)$/ or die socket(Server, PF_INET, SOCK_STREAM, $proto) die logmsg my $waitedpid = 0;

my $paddr;

sub REAPER {

$waitedpid = wait; $SIG{CHLD} =

$SIG{CHLD} = 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 spawn sub {

$=1; print

} sub spawn {

my $coderef = shift;

unless (@_ == 0 my $pid;

if (!defined($pid = fork)) { logmsg


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
  1. /usr/bin/perl -w

use strict; use Socket;

my $SECS_of_70_YEARS = 2208988800;

sub ctime { scalar localtime(shift) }

my $iaddr = gethostbyname('localhost');

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

$ = 1;


foreach $host (@ARGV) {


Unix-Domain TCP Clients and Servers

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

% ls -l /dev/log

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

You can test for these with Perl's -S file test
unless ( -S '/dev/log' ) {


Here's a sample Unix-domain client
  1. /usr/bin/perl -w

use Socket; use strict; my ($rendezvous, $line);

$rendezvous = shift '/tmp/catsock';

socket(SOCK, PF_UNIX, SOCK_STREAM, 0) die 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.

  1. /usr/bin/perl -Tw

use strict; use Socket; use Carp;

BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }

sub spawn; # forward declaration sub logmsg { print

my $NAME = '/tmp/catsock';

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

socket(Server,PF_UNIX,SOCK_STREAM,0) die logmsg my $waitedpid; sub REAPER {

$waitedpid = wait; $SIG{CHLD} =

$SIG{CHLD} = for ( $waitedpid = 0;

accept(Client,Server) $waitedpid; $waitedpid = 0, close Client) { next if $waitedpid; logmsg

sub spawn {

my $coderef = shift;

unless (@_ == 0 my $pid;

if (!defined($pid = fork)) { logmsg


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

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

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

TCP Clients with IO::Socket

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

A Simple Client

Here's a client that creates a TCP connection to the ``daytime service at port 13 of the host name ``localhost and prints out everything that the server there cares to provide.

  1. /usr/bin/perl -w

use IO::Socket; $remote = IO::Socket::INET-

When you run this program, you should get something back that looks like this
Wed May 14 08:40:46 MDT 1997

Here are what those parameters to the new constructor mean:


This is which protocol to use. In this case, the socket handle returned will be 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 , or an address like . For demonstration purposes, we've used the special hostname , which should always mean the current machine you're running on. The corresponding Internet address for localhost is , 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

on systems with a

well-configured system services file,

Fatal Error:

lib/CachedMarkup.php (In template 'browse' < 'body' < 'html'):257: Error: Pure virtual

Fatal PhpWiki Error

lib/CachedMarkup.php (In template 'browse' < 'body' < 'html'):257: Error: Pure virtual