PERLIPC(1)
NNAAMMEE
perlipc - Perl interprocess communication (signals, fifos,
pipes, safe subprocesses, sockets, and semaphores)
DDEESSCCRRIIPPTTIIOONN
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.
SSiiggnnaallss
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;
$shucks++;
die "Somebody sent me a SIG$signame";
}
$SIG{INT} = 'catch_zap'; # could fail in modules
$SIG{INT} = \&catch_zap; # best strategy
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 "No sigs?";
foreach $name (split(' ', $Config{sig_name})) {
$signo{$name} = $i;
$signame[$i] = $name;
$i++;
}
So to check whether signal 17 and SIGALRM were the same,
do just this:
print "signal #17 = $signame[17]\n";
if ($signo{ALRM}) {
print "SIGALRM is $signo{ALRM}\n";
}
You may also choose to assign the strings 'IGNORE' or
'DEFAULT' as the handler, in which case Perl will try to
discard the signal or do the default thing. Some signals
can be neither trapped nor ignored, such as the KILL and
STOP (but not the TSTP) signals. One strategy for
temporarily ignoring signals is to use a local()
statement, which will be automatically restored once your
block is exited. (Remember that local() values are
"inherited" by functions called from within that block.)
sub precious {
local $SIG{INT} = 'IGNORE';
&more_functions;
}
sub more_functions {
# interrupts still ignored, for now...
}
Sending a signal to a negative process ID means that you
send the signal to the entire Unix process-group. This
code sends a hang-up signal to all processes in the
current process group (and sets $SIG{HUP} to IGNORE so it
doesn't kill itself):
{
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 another 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";
}
You might also want to employ anonymous functions for
simple signal handlers:
$SIG{INT} = sub { die "\nOutta here!\n" };
But that will be problematic for the more complicated
handlers that need to reinstall themselves. Because
Perl's signal mechanism is currently based on the
signal(3) function from the C library, you may sometimes
be so 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;
# 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 even the more elaborate:
use POSIX ":sys_wait_h";
sub REAPER {
my $child;
while ($child = waitpid(-1,WNOHANG)) {
$Kid_Status{$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 }
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.
NNaammeedd PPiippeess
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.
# 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 --pp 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
}
UUssiinngg 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 "stuff\n";
close SPOOLER || die "bad spool: $! $?";
And here's how to start up a child process you intend to
read from:
open(STATUS, "netstat -an 2>&1 |")
|| die "can't fork: $!";
while (<STATUS>) {
next if /^(tcp|udp)/;
print;
}
close STATUS || die "bad netstat: $! $?";
If one can be sure that a particular program is a Perl
script that is expecting filenames in @ARGV, the clever
programmer can write something like this:
$ program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile
and irrespective of which shell it's called from, the Perl
program will read from the file f1, the process cmd1,
standard input (tmpfile in this case), the f2 file, the
cmd2 command, and finally the f3 file. Pretty nifty, eh?
You might notice that you could use backticks for much the
same effect as opening a pipe for reading:
print grep { !/^(tcp|udp)/ } `netstat -an 2>&1`;
die "bad netstat" if $?;
While this is true on the surface, it's much more
efficient to process the file one line or record at a time
because then you don't have to read the whole thing into
memory at once. It also gives you finer control of the
whole process, letting you to kill off the child process
early if you'd like.
Be careful to check both the open() and the close() return
values. If you're writing to a pipe, you should also trap
SIGPIPE. Otherwise, think of what happens when you start
up a pipe to a command that doesn't exist: the open() will
in all likelihood succeed (it only reflects the fork()'s
success), but then your output will fail--spectacularly.
Perl can't know whether the command worked because your
command is actually running in a separate process whose
exec() might have failed. Therefore, while readers of
bogus commands return just a quick end of file, writers to
bogus command will trigger a signal they'd better be
prepared to handle. Consider:
open(FH, "|bogus");
print FH "bang\n";
close FH;
FFiilleehhaannddlleess
Both the main process and the child process share the same
STDIN, STDOUT and STDERR filehandles. If both processes
try to access them at once, strange things can happen.
You may 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.
BBaacckkggrroouunndd PPrroocceesssseess
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).
CCoommpplleettee DDiissssoocciiaattiioonn ooff CChhiilldd ffrroomm PPaarreenntt
In some cases (starting server processes, for instance)
you'll want to complete dissociate the child process from
the parent. The following process is reported to work on
most Unixish systems. Non-Unix users should check their
Your_OS::Process module for other solutions.
+o Open /dev/tty and use the TIOCNOTTY ioctl on it. See
the tty(4) manpage for details.
+o Change directory to /
+o Reopen STDIN, STDOUT, and STDERR so they're not
connected to the old tty.
+o Background yourself like this:
fork && exit;
SSaaffee PPiippee OOppeennss
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, "|-");
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() directly.
Here's a safe backtick or pipe open for read:
# add error processing as above
$pid = open(KID_TO_READ, "-|");
if ($pid) { # parent
while (<KID_TO_READ>) {
# do something interesting
}
close(KID_TO_READ) || warn "kid exited $?";
} else { # child
($EUID, $EGID) = ($UID, $GID); # suid only
exec($program, @options, @args)
|| die "can't exec program: $!";
# NOTREACHED
}
And here's a safe pipe open for writing:
# add error processing as above
$pid = open(KID_TO_WRITE, "|-");
$SIG{ALRM} = 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
}
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.
BBiiddiirreeccttiioonnaall CCoommmmuunniiccaattiioonn wwiitthh AAnnootthheerr PPrroocceessss
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 --ww 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() 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" );
Writer->autoflush(); # default here, actually
print Writer "stuff\n";
$got = <Reader>;
The problem with this is that Unix buffering is really
going to ruin your day. Even though your Writer
filehandle is auto-flushed, and the process on the other
end will get your data in a timely manner, you can't
usually do anything to force it to give it back to you in
a similarly quick fashion. In this case, we could,
because we gave cat a --uu flag to make it unbuffered. But
very few Unix commands are designed to operate over pipes,
so this seldom works unless you yourself wrote the program
on the other end of the double-ended pipe.
A solution to this is the nonstandard Comm.pl library. It
uses pseudo-ttys to make your program behave more
reasonably:
require 'Comm.pl';
$ph = open_proc('cat -n');
for (1..10) {
print $ph "a line\n";
print "got back ", scalar <$ph>;
}
This way you don't have to have control over the source
code of the program you're using. The Comm library also
has expect() and interact() functions. Find the library
(and we hope its successor IPC::Chat) at your nearest CPAN
archive as detailed in the SEE ALSO section below.
SSoocckkeettss:: CClliieenntt//SSeerrvveerr CCoommmmuunniiccaattiioonn
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 $AF_INET = 2,
you know you're in for big trouble: An immeasurably
superior approach is to use the Socket module, which more
reliably grants access to various constants and functions
you'll need.
If you're not writing a server/client for an existing
protocol like NNTP or SMTP, you should give some thought
to how your server will know when the client has finished
talking, and vice-versa. Most protocols are based on one-
line messages and responses (so one party knows the other
has finished when a "\n" is received) or multi-line
messages and responses that end with a period on an empty
line ("\n.\n" terminates a message/response).
IInntteerrnneett TTCCPP CClliieennttss aanndd SSeerrvveerrss
Use Internet-domain sockets when you want to do client-
server communication that might extend to machines outside
of your own system.
Here's a sample TCP client using Internet-domain sockets:
#!/usr/bin/perl -w
require 5.002;
use strict;
use Socket;
my ($remote,$port, $iaddr, $paddr, $proto, $line);
$remote = shift || 'localhost';
$port = shift || 2345; # random port
if ($port =~ /\D/) { $port = getservbyname($port, 'tcp') }
die "No port" unless $port;
$iaddr = inet_aton($remote) || die "no host: $remote";
$paddr = sockaddr_in($port, $iaddr);
$proto = getprotobyname('tcp');
socket(SOCK, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
connect(SOCK, $paddr) || die "connect: $!";
while (defined($line = <SOCK>)) {
print $line;
}
close (SOCK) || die "close: $!";
exit;
And here's a corresponding server to go along with it.
We'll leave the address as INADDR_ANY so that the kernel
can choose the appropriate interface on multihomed hosts.
If you want sit on a particular interface (like the
external side of a gateway or firewall machine), you
should fill this in with your real address instead.
#!/usr/bin/perl -Tw
require 5.002;
use strict;
BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
use Socket;
use Carp;
sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
my $port = shift || 2345;
my $proto = getprotobyname('tcp');
$port = $1 if $port =~ /(\d+)/; # untaint port number
socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
setsockopt(Server, SOL_SOCKET, SO_REUSEADDR,
pack("l", 1)) || die "setsockopt: $!";
bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!";
listen(Server,SOMAXCONN) || die "listen: $!";
logmsg "server started on port $port";
my $paddr;
$SIG{CHLD} = \&REAPER;
for ( ; $paddr = accept(Client,Server); close Client) {
my($port,$iaddr) = sockaddr_in($paddr);
my $name = gethostbyaddr($iaddr,AF_INET);
logmsg "connection from $name [",
inet_ntoa($iaddr), "]
at port $port";
print Client "Hello there, $name, it's now ",
scalar localtime, "\n";
}
And here's a multithreaded version. It's multithreaded in
that like most typical servers, it spawns (forks) a slave
server to handle the client request so that the master
server can quickly go back to service a new client.
#!/usr/bin/perl -Tw
require 5.002;
use strict;
BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
use Socket;
use Carp;
sub spawn; # forward declaration
sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
my $port = shift || 2345;
my $proto = getprotobyname('tcp');
$port = $1 if $port =~ /(\d+)/; # untaint port number
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;
sub REAPER {
$waitedpid = wait;
$SIG{CHLD} = \&REAPER; # loathe sysV
logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
}
$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, "\n";
exec '/usr/games/fortune'
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: $!";
return;
} elsif ($pid) {
logmsg "begat $pid";
return; # I'm the parent
}
# else I'm the child -- go spawn
open(STDIN, "<&Client") || die "can't dup client to stdin";
open(STDOUT, ">&Client") || die "can't dup client to stdout";
## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr";
exit &$coderef();
}
This server takes the trouble to clone off a child version
via fork() for each incoming request. That way it can
handle many requests at once, which you might not always
want. Even if you don't fork(), the listen() will allow
that many pending connections. Forking servers have to be
particularly careful about cleaning up their dead children
(called "zombies" in Unix parlance), because otherwise
you'll quickly fill up your process table.
We suggest that you use the --TT flag to use taint checking
(see the perlsec manpage) 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
require 5.002;
use strict;
use Socket;
my $SECS_of_70_YEARS = 2208988800;
sub ctime { scalar localtime(shift) }
my $iaddr = gethostbyname('localhost');
my $proto = getprotobyname('tcp');
my $port = getservbyname('time', 'tcp');
my $paddr = sockaddr_in(0, $iaddr);
my($host);
$| = 1;
printf "%-24s %8s %s\n", "localhost", 0, ctime(time());
foreach $host (@ARGV) {
printf "%-24s ", $host;
my $hisiaddr = inet_aton($host) || die "unknown host";
my $hispaddr = sockaddr_in($port, $hisiaddr);
socket(SOCKET, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
connect(SOCKET, $hispaddr) || die "bind: $!";
my $rtime = ' ';
read(SOCKET, $rtime, 4);
close(SOCKET);
my $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ;
printf "%8d %s\n", $histime - time, ctime($histime);
}
UUnniixx--DDoommaaiinn TTCCPP CClliieennttss aanndd SSeerrvveerrss
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 --SS file test:
unless ( -S '/dev/log' ) {
die "something's wicked with the print system";
}
Here's a sample Unix-domain client:
#!/usr/bin/perl -w
require 5.002;
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>)) {
print $line;
}
exit;
And here's a corresponding server.
#!/usr/bin/perl -Tw
require 5.002;
use strict;
use Socket;
use Carp;
BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
my $NAME = '/tmp/catsock';
my $uaddr = sockaddr_un($NAME);
my $proto = getprotobyname('tcp');
socket(Server,PF_UNIX,SOCK_STREAM,0) || die "socket: $!";
unlink($NAME);
bind (Server, $uaddr) || die "bind: $!";
listen(Server,SOMAXCONN) || die "listen: $!";
logmsg "server started on $NAME";
$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: $!";
};
}
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.
TTCCPP CClliieennttss wwiitthh IIOO::::SSoocckkeett
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 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.
AA SSiimmppllee CClliieenntt
Here's a client that creates a TCP connection to the
"daytime" service at port 13 of the host name "localhost"
and prints out everything that the server there cares to
provide.
#!/usr/bin/perl -w
use IO::Socket;
$remote = IO::Socket::INET->new(
Proto => "tcp",
PeerAddr => "localhost",
PeerPort => "daytime(13)",
)
or die "cannot connect to daytime port at localhost";
while ( <$remote> ) { print }
When you run this program, you should get something back
that looks like this:
Wed May 14 08:40:46 MDT 1997
Here are what those parameters to the new constructor
mean:
Proto
This is which protocol to use. In this case, the
socket handle returned will be connected to a TCP
socket, because we want a stream-oriented connection,
that is, one that acts pretty much like a plain old
file. Not all sockets are this of this type. For
example, the UDP protocol can be used to make a
datagram socket, used for message-passing.
PeerAddr
This is the name or Internet address of the remote
host the server is running on. We could have
specified a longer name like "www.perl.com", or an
address like "204.148.40.9". For demonstration
purposes, we've used the special hostname
"localhost", which should always mean the current
machine you're running on. The corresponding
Internet address for localhost is "127.1", if you'd
rather use that.
PeerPort
This is the service name or port number we'd like to
connect to. We could have gotten away with using
just "daytime" on systems with a well-configured
system services file,[FOOTNOTE: The system services
file is in /etc/services under Unix] but just in
case, we've specified the port number (13) in
parentheses. Using just the number would also have
worked, but constant numbers make careful programmers
nervous.
Notice how the return value from the new constructor is
used as a filehandle in the while loop? That's what's
called an indirect filehandle, a scalar variable
containing a filehandle. You can use it the same way you
would a normal filehandle. For example, you can read one
line from it this way:
$line = <$handle>;
all remaining lines from is this way:
@lines = <$handle>;
and send a line of data to it this way:
print $handle "some data\n";
AA WWeebbggeett CClliieenntt
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);
foreach $document ( @ARGV ) {
$remote = IO::Socket::INET->new( Proto => "tcp",
PeerAddr => $host,
PeerPort => "http(80)",
);
unless ($remote) { die "cannot connect to http daemon on $host" }
$remote->autoflush(1);
print $remote "GET $document HTTP/1.0\n\n";
while ( <$remote> ) { print }
close $remote;
}
The web server handing the "http" service, which is
assumed to be at its standard port, number 80. If your
the web server you're trying to connect to is at a
different port (like 1080 or 8080), you should specify as
the named-parameter pair, PeerPort => 8080. The autoflush
method is used on the socket because otherwise the system
would buffer up the output we sent it. (If you're on a
Mac, you'll also need to change every "\n" in your code
that sends data over the network to be a "\015\012"
instead.)
Connecting to the server is only the first part of the
process: once you have the connection, you have to use the
server's language. Each server on the network has its own
little command language that it expects as input. The
string that we send to the server starting with "GET" is
in HTTP syntax. In this case, we simply request each
specified document. Yes, we really are making a new
connection for each document, even though it's the same
host. That's the way you always used to have to speak
HTTP. Recent versions of web browsers may request that
the remote server leave the connection open a little
while, but the server doesn't have to honor such a
request.
Here's an example of running that program, which we'll
call webget:
shell_prompt$ webget www.perl.com /guanaco.html
HTTP/1.1 404 File Not Found
Date: Thu, 08 May 1997 18:02:32 GMT
Server: Apache/1.2b6
Connection: close
Content-type: text/html
<HEAD><TITLE>404 File Not Found</TITLE></HEAD>
<BODY><H1>File Not Found</H1>
The requested URL /guanaco.html was not found on this server.<P>
</BODY>
Ok, so that's not very interesting, because it didn't find
that particular document. But a long response wouldn't
have fit on this page.
For a more fully-featured version of this program, you
should look to the lwp-request program included with the
LWP modules from CPAN.
IInntteerraaccttiivvee CClliieenntt wwiitthh IIOO::::SSoocckkeett
Well, that's all fine if you want to send one command and
get one answer, but what about setting up something fully
interactive, somewhat like the way telnet works? That way
you can type a line, get the answer, type a line, get the
answer, etc.
This client is more complicated than the two we've done so
far, but if you're on a system that supports the powerful
fork call, the solution isn't that rough. Once you've
made the connection to whatever service you'd like to chat
with, call fork to clone your process. Each of these two
identical process has a very simple job to do: the parent
copies everything from the socket to standard output,
while the child simultaneously copies everything from
standard input to the socket. To accomplish the same
thing using just one process would be much harder, because
it's easier to code two processes to do one thing than it
is to code one process to do two things. (This keep-it-
simple principle is one of the cornerstones of the Unix
philosophy, and good software engineering as well, which
is probably why it's spread to other systems as well.)
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>)) {
print $handle $line;
}
}
The kill function in the parent's if block is there to
send a signal to our child process (current running in the
else block) as soon as the remote server has closed its
end of the connection.
The kill at the end of the parent's block is there to
eliminate the child process as soon as the server we
connect to closes its end.
If the remote server sends data a byte at time, and you
need that data immediately without waiting for a newline
(which might not happen), you may wish to replace the
while loop in the parent with the following:
my $byte;
while (sysread($handle, $byte, 1) == 1) {
print STDOUT $byte;
}
Making a system call for each byte you want to read is not
very efficient (to put it mildly) but is the simplest to
explain and works reasonably well.
TTCCPP SSeerrvveerrss wwiitthh IIOO::::SSoocckkeett
Setting up server is little bit more involved than running
a client. The model is that the server creates a special
kind of socket that does nothing but listen on a
particular port for incoming connections. It does this by
calling the IO::Socket::INET->new() method with slightly
different arguments than the client did.
Proto
This is which protocol to use. Like our clients,
we'll still specify "tcp" here.
LocalPort
We specify a local port in the LocalPort argument,
which we didn't do for the client. This is service
name or port number for which you want to be the
server. (Under Unix, ports under 1024 are restricted
to the superuser.) In our sample, we'll use port
9000, but you can use any port that's not currently
in use on your system. If you try to use one already
in used, you'll get an "Address already in use"
message. Under Unix, the netstat -a command will show
which services current have servers.
Listen
The Listen parameter is set to the maximum number of
pending connections we can accept until we turn away
incoming clients. Think of it as a call-waiting
queue for your telephone. The low-level Socket
module has a special symbol for the system maximum,
which is SOMAXCONN.
Reuse
The Reuse parameter is needed so that we restart our
server manually without waiting a few minutes to
allow system buffers to clear out.
Once the generic server socket has been created using the
parameters listed above, the server then waits for a new
client to connect to it. The server blocks in the accept
method, which eventually an bidirectional connection to
the remote client. (Make sure to autoflush this handle to
circumvent buffering.)
To add to user-friendliness, our server prompts the user
for commands. Most servers don't do this. Because of the
prompt without a newline, you'll have to use the sysread
variant of the interactive client above.
This server accepts one of five different commands,
sending output back to the client. Note that unlike most
network servers, this one only handles one incoming client
at a time. Multithreaded servers are covered in Chapter 6
of the Camel or in the perlipc(1) manpage.
Here's the code. We'll
#!/usr/bin/perl -w
use IO::Socket;
use Net::hostent; # for OO version of gethostbyaddr
$PORT = 9000; # pick something not in use
$server = IO::Socket::INET->new( Proto => 'tcp',
LocalPort => $PORT,
Listen => SOMAXCONN,
Reuse => 1);
die "can't setup server" unless $server;
print "[Server $0 accepting clients]\n";
while ($client = $server->accept()) {
$client->autoflush(1);
print $client "Welcome to $0; type help for command list.\n";
$hostinfo = gethostbyaddr($client->peeraddr);
printf "[Connect from %s]\n", $hostinfo->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;
}
UUDDPP:: MMeessssaaggee PPaassssiinngg
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.
Here's a UDP program similar to the sample Internet TCP
client given earlier. However, instead of checking one
host at a time, the UDP version will check many of them
asynchronously by simulating a multicast and then using
select() to do a timed-out wait for I/O. To do something
similar with TCP, you'd have to use a different socket
handle for each host.
#!/usr/bin/perl -w
use strict;
require 5.002;
use Socket;
use Sys::Hostname;
my ( $count, $hisiaddr, $hispaddr, $histime,
$host, $iaddr, $paddr, $port, $proto,
$rin, $rout, $rtime, $SECS_of_70_YEARS);
$SECS_of_70_YEARS = 2208988800;
$iaddr = gethostbyname(hostname());
$proto = getprotobyname('udp');
$port = getservbyname('time', 'udp');
$paddr = sockaddr_in(0, $iaddr); # 0 means let kernel pick
socket(SOCKET, PF_INET, SOCK_DGRAM, $proto) || die "socket: $!";
bind(SOCKET, $paddr) || die "bind: $!";
$| = 1;
printf "%-12s %8s %s\n", "localhost", 0, scalar localtime time;
$count = 0;
for $host (@ARGV) {
$count++;
$hisiaddr = inet_aton($host) || die "unknown host";
$hispaddr = sockaddr_in($port, $hisiaddr);
defined(send(SOCKET, 0, 0, $hispaddr)) || die "send $host: $!";
}
$rin = '';
vec($rin, fileno(SOCKET), 1) = 1;
# timeout after 10.0 seconds
while ($count && select($rout = $rin, undef, undef, 10.0)) {
$rtime = '';
($hispaddr = recv(SOCKET, $rtime, 4, 0)) || die "recv: $!";
($port, $hisiaddr) = sockaddr_in($hispaddr);
$host = gethostbyaddr($hisiaddr, AF_INET);
$histime = unpack("N", $rtime) - $SECS_of_70_YEARS ;
printf "%-12s ", $host;
printf "%8d %s\n", $histime - time, scalar localtime($histime);
$count--;
}
SSyyssVV IIPPCC
While System V IPC isn't so widely used as sockets, it
still has some interesting uses. You can't, however,
effectively use SysV IPC or Berkeley mmap() to have shared
memory so as to share a variable amongst several
processes. That's because Perl would reallocate your
string when you weren't wanting it to.
Here's a small example showing shared memory usage.
$IPC_PRIVATE = 0;
$IPC_RMID = 0;
$size = 2000;
$key = shmget($IPC_PRIVATE, $size , 0777 );
die unless defined $key;
$message = "Message #1";
shmwrite($key, $message, 0, 60 ) || die "$!";
shmread($key,$buff,0,60) || die "$!";
print $buff,"\n";
print "deleting $key\n";
shmctl($key ,$IPC_RMID, 0) || die "$!";
Here's an example of a semaphore:
$IPC_KEY = 1234;
$IPC_RMID = 0;
$IPC_CREATE = 0001000;
$key = semget($IPC_KEY, $nsems , 0666 | $IPC_CREATE );
die if !defined($key);
print "$key\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;
$key = semget($IPC_KEY, 0 , 0 );
die if !defined($key);
$semnum = 0;
$semflag = 0;
# 'take' semaphore
# wait for semaphore to be zero
$semop = 0;
$opstring1 = pack("sss", $semnum, $semop, $semflag);
# Increment the semaphore count
$semop = 1;
$opstring2 = pack("sss", $semnum, $semop, $semflag);
$opstring = $opstring1 . $opstring2;
semop($key,$opstring) || die "$!";
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;
$key = semget($IPC_KEY, 0, 0);
die if !defined($key);
$semnum = 0;
$semflag = 0;
# Decrement the semaphore count
$semop = -1;
$opstring = pack("sss", $semnum, $semop, $semflag);
semop($key,$opstring) || die "$!";
The SysV IPC code above was written long ago, and it's
definitely clunky looking. It should at the very least be
made to use strict and require "sys/ipc.ph". Better yet,
check out the IPC::SysV modules on CPAN.
NNOOTTEESS
If you are running under version 5.000 (dubious) or 5.001,
you can still use most of the examples in this document.
You may have to remove the use strict and some of the my()
statements for 5.000, and for both you'll have to load in
version 1.2 or older of the Socket.pm module, which is
included in perl5.002.
Most of these routines quietly but politely return undef
when they fail instead of causing your program to die
right then and there due to an uncaught exception.
(Actually, some of the new Socket conversion functions
croak() on bad arguments.) It is therefore essential that
you should check the return values of these functions.
Always begin your socket programs this way for optimal
success, and don't forget to add --TT taint checking flag to
the pound-bang line for servers:
#!/usr/bin/perl -w
require 5.002;
use strict;
use sigtrap;
use Socket;
BBUUGGSS
All these routines create system-specific portability
problems. As noted elsewhere, Perl is at the mercy of
your C libraries for much of its system behaviour. It's
probably safest to assume broken SysV semantics for
signals and to stick with simple TCP and UDP socket
operations; e.g., don't try to pass open file descriptors
over a local UDP datagram socket if you want your code to
stand a chance of being portable.
Because few vendors provide C libraries that are safely
re-entrant, the prudent programmer will do little else
within a handler beyond setting a numeric variable that
already exists; or, if locked into a slow (restarting)
system call, using 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 absolutely inevitable. This
perilous problems will be addressed in a future release of
Perl.
AAUUTTHHOORR
Tom Christiansen, with occasional vestiges of Larry Wall's
original version and suggestions from the Perl Porters.
SSEEEE AALLSSOO
There's a lot more to networking than this, but this
should get you started.
For intrepid programmers, the classic textbook Unix
Network Programming by Richard Stevens (published by
Addison-Wesley). Note that most books on networking
address networking from the perspective of a C programmer;
translation to Perl is left as an exercise for the reader.
The IO::Socket(3) manpage describes the object library,
and the Socket(3) manpage describes the low-level
interface to sockets. Besides the obvious functions in
the perlfunc manpage, you should also check out the
modules file at your nearest CPAN site. (See the
perlmodlib manpage or best yet, the Perl FAQ for a
description of what CPAN is and where to get it.)
Section 5 of the modules file is devoted to "Networking,
Device Control (modems), and Interprocess Communication",
and contains numerous unbundled modules numerous
networking modules, Chat and Expect operations, CGI
programming, DCE, FTP, IPC, NNTP, Proxy, Ptty, RPC, SNMP,
SMTP, Telnet, Threads, and ToolTalk--just to name a few.