PERLIPC(1) PERLIPC(1)
NAME
perlipc - Perl interprocess communication (signals, fifos, pipes, safe
subprocesses, sockets, and semaphores)
DESCRIPTION
The basic IPC facilities of Perl are built out of the good old Unix
signals, named pipes, pipe opens, the Berkeley socket routines, and SysV
IPC calls. Each is used in slightly different situations.
Signals
Perl uses a simple signal handling model: the %SIG hash 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:
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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:
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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.
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.
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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 -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
}
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:
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open(SPOOLER, "| cat -v | lpr -h 2>/dev/null")
|| die "can't fork: $!";
local $SIG{PIPE} = sub { die "spooler pipe broke" };
print SPOOLER "stuff\n";
close SPOOLER || die "bad spool: $! $?";
And here's how to start up a child process you intend to read from:
open(STATUS, "netstat -an 2>&1 |")
|| die "can't fork: $!";
while (<STATUS>) {
next if /^(tcp|udp)/;
print;
}
close STATUS || die "bad netstat: $! $?";
If one can be sure that a particular program is a Perl script that is
expecting filenames in @ARGV, the clever 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:
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open(FH, "|bogus");
print FH "bang\n";
close FH;
Filehandles
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.
Background Processes
You can run a command in the background with:
system("cmd &");
The command's STDOUT and STDERR (and possibly STDIN, depending on your
shell) will be the same as the parent's. You won't need to catch SIGCHLD
because of the double-fork taking place (see below for more details).
Complete Dissociation of Child from Parent
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.
⊕ Open /dev/tty and use the TIOCNOTTY ioctl on it. See the tty(4)
manpage for details.
⊕ Change directory to /
⊕ Reopen STDIN, STDOUT, and STDERR so they're not connected to the old
tty.
⊕ Background yourself like this:
fork && exit;
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
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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 $?";
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} else { # child
($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.
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 work:
open(PROG_FOR_READING_AND_WRITING, "| some program |")
and if you forget to use 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.
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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 -u flag to make
it unbuffered. But very few Unix commands are designed to operate over
pipes, so this seldom works unless you yourself wrote the program on the
other end of the double-ended pipe.
A solution to this is the nonstandard Comm.pl library. It uses pseudo-
ttys to make your program behave more reasonably:
require 'Comm.pl';
$ph = open_proc('cat -n');
for (1..10) {
print $ph "a line\n";
print "got back ", scalar <$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.
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
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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 $AF_INET = 2, you know you're in for big trouble: An
immeasurably superior approach is to use the Socket module, which more
reliably grants access to various constants and functions you'll need.
If you're not writing a server/client for an existing protocol like NNTP
or SMTP, you should give some thought to how your server will know when
the client has finished talking, and vice-versa. Most protocols are
based on one-line messages and responses (so one party knows the other
has finished when a "\n" is received) or multi-line messages and
responses that end with a period on an empty line ("\n.\n" terminates a
message/response).
Internet TCP Clients and Servers
Use Internet-domain sockets when you want to do client-server
communication that might extend to machines outside of your own system.
Here's a sample TCP client using Internet-domain sockets:
#!/usr/bin/perl -w
require 5.002;
use strict;
use Socket;
my ($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.
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#!/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" }
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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";
}
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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 -T 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());
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foreach $host (@ARGV) {
printf "%-24s ", $host;
my $hisiaddr = inet_aton($host) || die "unknown host";
my $hispaddr = sockaddr_in($port, $hisiaddr);
socket(SOCKET, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
connect(SOCKET, $hispaddr) || die "bind: $!";
my $rtime = ' ';
read(SOCKET, $rtime, 4);
close(SOCKET);
my $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ;
printf "%8d %s\n", $histime - time, ctime($histime);
}
Unix-Domain TCP Clients and Servers
That's fine for Internet-domain clients and servers, but what about local
communications? While you can use the same setup, sometimes you don't
want to. Unix-domain sockets are local to the current host, and are
often used internally to implement pipes. Unlike Internet domain
sockets, Unix domain sockets can show up in the file system with an ls(1)
listing.
$ ls -l /dev/log
srw-rw-rw- 1 root 0 Oct 31 07:23 /dev/log
You can test for these with Perl's -S file test:
unless ( -S '/dev/log' ) {
die "something's wicked with the 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.
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#!/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.
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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 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.
#!/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.
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PERLIPC(1) PERLIPC(1)
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";
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.
#!/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 $remote;
}
The web server handing the "http" service, which is assumed to be at its
standard port, number 80. If your the web server you're trying to
connect to is at a different port (like 1080 or 8080), you should specify
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PERLIPC(1) PERLIPC(1)
as the named-parameter pair, PeerPort => 8080. The autoflush method is
used on the socket because otherwise the system would buffer up the
output we sent it. (If you're on a Mac, you'll also need to change every
"\n" in your code that sends data over the network to be a "\015\012"
instead.)
Connecting to the server is only the first part of the process: once you
have the connection, you have to use the server's language. Each server
on the network has its own little command language that it expects as
input. The string that we send to the server starting with "GET" is in
HTTP syntax. In this case, we simply request each specified document.
Yes, we really are making a new connection for each document, even though
it's the same host. That's the way you always used to have to speak
HTTP. Recent versions of web browsers may request that the remote server
leave the connection open a little while, but the server doesn't have to
honor such a request.
Here's an example of running that program, which we'll call webget:
shell_prompt$ 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.
Interactive Client with IO::Socket
Well, that's all fine if you want to send one command and get one answer,
but what about setting up something fully interactive, somewhat like the
way telnet works? That way you can type a line, get the answer, type a
line, get the answer, etc.
This client is more complicated than the two we've done so far, but if
you're on a system that supports the powerful fork call, the solution
isn't that rough. Once you've made the connection to whatever service
you'd like to chat with, call fork to clone your process. Each of these
two identical process has a very simple job to do: the parent copies
everything from the socket to standard output, while the child
simultaneously copies everything from standard input to the socket. To
accomplish the same thing using just one process would be much harder,
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PERLIPC(1) PERLIPC(1)
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.
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PERLIPC(1) PERLIPC(1)
If the remote server sends data a byte at time, and you need that data
immediately without waiting for a newline (which might not happen), you
may wish to replace the while loop in the parent with the following:
my $byte;
while (sysread($handle, $byte, 1) == 1) {
print STDOUT $byte;
}
Making a system call for each byte you want to read is not very efficient
(to put it mildly) but is the simplest to explain and works reasonably
well.
TCP Servers with IO::Socket
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.)
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PERLIPC(1) PERLIPC(1)
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;
}
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
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PERLIPC(1) PERLIPC(1)
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;
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PERLIPC(1) PERLIPC(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--;
}
SysV IPC
While System V IPC isn't so widely used as sockets, it still has some
interesting uses. You can't, however, effectively use SysV IPC or
Berkeley mmap() to have shared memory so as to share a variable amongst
several processes. That's because Perl would reallocate your string when
you weren't wanting it to.
Here's a small example showing shared memory usage.
$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 "$key\n";
Put this code in a separate file to be run in more than one process.
Call the file take:
# create a semaphore
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$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.
NOTES
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.
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PERLIPC(1) PERLIPC(1)
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 -T
taint checking flag to the pound-bang line for servers:
#!/usr/bin/perl -w
require 5.002;
use strict;
use sigtrap;
use Socket;
BUGS
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.
AUTHOR
Tom Christiansen, with occasional vestiges of Larry Wall's original
version and suggestions from the Perl Porters.
SEE ALSO
There's a lot more to networking than this, but this should get you
started.
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.)
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Section 5 of the modules file is devoted to "Networking, Device Control
(modems), and Interprocess Communication", and contains numerous
unbundled modules numerous networking modules, Chat and Expect
operations, CGI programming, DCE, FTP, IPC, NNTP, Proxy, Ptty, RPC, SNMP,
SMTP, Telnet, Threads, and ToolTalk--just to name a few.
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