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PERLIPC

Section: Perl Programmers Reference Guide (1)
Updated: perl 5.004, patch 04
Index

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 Unixsignals, named pipes, pipe opens, the Berkeley socket routines, and SysVIPC calls. Each is used in slightly different situations.

Signals

Perl uses a simple signal handling model: the %SIG hash contains names orreferences of user-installed signal handlers. These handlers will be calledwith an argument which is the name of the signal that triggered it. Asignal may be generated intentionally from a particular keyboard sequence likecontrol-C or control-Z, sent to you from another process, ortriggered automatically by the kernel when special events transpire, likea child process exiting, your process running out of stack space, orhitting 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 isset a global variable and then raise an exception. That's because on mostsystems, libraries are not re-entrant; particularly, memory allocation andI/O routines are not. That means that doing nearly anything in yourhandler 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 yoursystem, or you can retrieve them from the Config module. Set up an@signame list indexed by number to get the name and a %signo tableindexed 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' asthe handler, in which case Perl will try to discard the signal or do thedefault thing. Some signals can be neither trapped nor ignored, such asthe KILL and STOP (but not the TSTP) signals. One strategy fortemporarily ignoring signals is to use a local() statement, which will beautomatically 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 signalto the entire Unix process-group. This code sends a hang-up signal to allprocesses in the current process group (and sets $SIG{HUP} to IGNORE soit 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'tactually affect another process, but instead checks whether it's aliveor 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 signalhandlers:

    $SIG{INT} = sub { die "\nOutta here!\n" };

But that will be problematic for the more complicated handlers that needto reinstall themselves. Because Perl's signal mechanism is currentlybased on the signal(3) function from the C library, you may sometimes be somisfortunate as to run on systems where that function is ``broken'', thatis, it behaves in the old unreliable SysV way rather than the newer, morereasonable BSD and POSIX fashion. So you'll see defensive people writingsignal 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 safelyprotected within an eval{} block, you set a signal handler to trapalarm signals and then schedule to have one delivered to you in somenumber of seconds. Then try your blocking operation, clearing the alarmwhen it's done but not before you've exited your eval{} block. If itgoes off, you'll use die() to jump out of the block, much as you mightusing 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 POSIXmodule. Lamentably, this is almost entirely undocumented, butthe t/lib/posix.t file from the Perl source distribution has someexamples in it.

Named Pipes

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

To create a named pipe, use the Unix command mknod(1) or on somesystems, 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 unrelatedone. When you open a fifo, the program will block until there's somethingon the other end.

For example, let's say you'd like to have your .signature file be anamed pipe that has a Perl program on the other end. Now every time anyprogram (like a mailer, news reader, finger program, etc.) tries to readfrom that file, the reading program will block and your program willsupply the new signature. We'll use the pipe-checking file test -pto 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 interprocesscommunication by either appending or prepending a pipe symbol to the secondargument to open(). Here's how to start something up in a child process youintend 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 isexpecting filenames in @ARGV, the clever programmer can write somethinglike this:

    $ program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile

and irrespective of which shell it's called from, the Perl program willread from the file f1, the process cmd1, standard input (tmpfilein this case), the f2 file, the cmd2 command, and finally the f3file. Pretty nifty, eh?

You might notice that you could use backticks for much thesame 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 thefile one line or record at a time because then you don't have to read thewhole thing into memory at once. It also gives you finer control of thewhole process, letting you to kill off the child process early if you'dlike.

Be careful to check both the open() and the close() return values. Ifyou'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'texist: the open() will in all likelihood succeed (it only reflects thefork()'s success), but then your output will fail---spectacularly. Perlcan't know whether the command worked because your command is actuallyrunning in a separate process whose exec() might have failed. Therefore,while readers of bogus commands return just a quick end of file, writersto bogus command will trigger a signal they'd better be prepared tohandle. Consider:

    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 themat once, strange things can happen. You may want to close or reopenthe filehandles for the child. You can get around this by openingyour pipe with open(), but on some systems this means that the childprocess 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 yourshell) will be the same as the parent's. You won't need to catchSIGCHLD because of the double-fork taking place (see below for moredetails).

Complete Dissociation of Child from Parent

In some cases (starting server processes, for instance) you'll want tocomplete dissociate the child process from the parent. The followingprocess is reported to work on most Unixish systems. Non-Unix usersshould check their Your_OS::Process module for other solutions.

•

Open /dev/tty and use the TIOCNOTTY ioctl on it. See the tty(4) manpagefor details.

•

Change directory to /

•

Reopen STDIN, STDOUT, and STDERR so they're not connected to the oldtty.

•

Background yourself like this:

    fork && exit;

Safe Pipe Opens

Another interesting approach to IPC is making your single program gomultiprocess and communicate between (or even amongst) yourselves. Theopen() function will accept a file argument of either "-|" or "|-"to do a very interesting thing: it forks a child connected to thefilehandle you've opened. The child is running the same program as theparent. This is useful for safely opening a file when running under anassumed UID or GID, for example. If you open a pipe to minus, you canwrite to the filehandle you opened and your kid will find it in hisSTDIN. If you open a pipe from minus, you can read from the filehandleyou 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 executesomething without the shell's interference. With system(), it'sstraightforward, 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 onyour 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 becorrectly implemented on alien systems. Additionally, these are not truemultithreading. If you'd like to learn more about threading, see themodules file mentioned below in the SEE ALSO section.

Bidirectional Communication with Another Process

While this works reasonably well for unidirectional communication, whatabout bidirectional communication? The obvious thing you'd like to dodoesn't actually work:

    open(PROG_FOR_READING_AND_WRITING, "| some program |")

and if you forget to use the -w flag, then you'll miss outentirely 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 functionto catch both ends. There's also an open3() for tridirectional I/O so youcan also catch your child's STDERR, but doing so would then require anawkward select() loop and wouldn't allow you to use normal Perl inputoperations.

If you look at its source, you'll see that open2() uses low-levelprimitives like Unix pipe() and exec() to create all the connections.While it might have been slightly more efficient by using socketpair(), itwould have then been even less portable than it already is. The open2()and open3() functions are unlikely to work anywhere except on a Unixsystem 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 toruin 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 youin a similarly quick fashion. In this case, we could, because wegave cat a -u flag to make it unbuffered. But very few Unixcommands are designed to operate over pipes, so this seldom worksunless you yourself wrote the program on the other end of thedouble-ended pipe.

A solution to this is the nonstandard Comm.pl library. It usespseudo-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 theprogram you're using. The Comm library also has expect()and interact() functions. Find the library (and we hope itssuccessor IPC::Chat) at your nearest CPAN archive as detailedin the SEE ALSO section below.

Sockets: Client/Server Communication

While not limited to Unix-derived operating systems (e.g., WinSock on PCsprovides socket support, as do some VMS libraries), you may not havesockets on your system, in which case this section probably isn't going to doyou much good. With sockets, you can do both virtual circuits (i.e., TCPstreams) and datagrams (i.e., UDP packets). You may be able to do even moredepending on your system.

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

One of the major problems with old socket code in Perl was that it usedhard-coded values for some of the constants, which severely hurtportability. If you ever see code that does anything like explicitlysetting $AF_INET = 2, you know you're in for big trouble: Animmeasurably superior approach is to use the Socket module, which morereliably grants access to various constants and functions you'll need.

If you're not writing a server/client for an existing protocol likeNNTP or SMTP, you should give some thought to how your server willknow when the client has finished talking, and vice-versa. Mostprotocols are based on one-line messages and responses (so one partyknows the other has finished when a ``\n'' is received) or multi-linemessages 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-servercommunication 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'llleave the address as INADDR_ANY so that the kernel can choosethe appropriate interface on multihomed hosts. If you want siton a particular interface (like the external side of a gatewayor firewall machine), you should fill this in with your real addressinstead.

    #!/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 thatlike most typical servers, it spawns (forks) a slave server tohandle the client request so that the master server can quicklygo 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() foreach 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 beparticularly careful about cleaning up their dead children (called``zombies'' in Unix parlance), because otherwise you'll quickly fill up yourprocess 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 ideafor servers and other programs run on behalf of someone else (like CGIscripts), because it lessens the chances that people from the outside willbe 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 clocksdiffer 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);    }

Unix-Domain TCP Clients and Servers

That's fine for Internet-domain clients and servers, but what about localcommunications? While you can use the same setup, sometimes you don'twant to. Unix-domain sockets are local to the current host, and are oftenused internally to implement pipes. Unlike Internet domain sockets, Unixdomain 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.

    #!/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, somuch so, in fact, that we've omitted several duplicate functions--spawn(),logmsg(), ctime(), and REAPER()--which are exactly the same as in theother server.

So why would you ever want to use a Unix domain socket instead of asimpler named pipe? Because a named pipe doesn't give you sessions. Youcan'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 twoarguments.

For example, let's say that you have a long running database server daemonthat you want folks from the World Wide Web to be able to access, but onlyif they go through a CGI interface. You'd have a small, simple CGIprogram that does whatever checks and logging you feel like, and then actsas 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, theIO::Socket module provides an object-oriented approach. IO::Socket isincluded as part of the standard Perl distribution as of the 5.004release. If you're running an earlier version of Perl, just fetchIO::Socket from CPAN, where you'll also find find modules providing easyinterfaces to the following systems: DNS, FTP, Ident (RFC 931), NIS andNISPlus, NNTP, Ping, POP3, SMTP, SNMP, SSLeay, Telnet, and Time---justto 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 everythingthat 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 thatlooks 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 returnedwill be connected to a TCP socket, because we want a stream-orientedconnection, that is, one that acts pretty much like a plain old file.Not all sockets are this of this type. For example, the UDP protocolcan 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 isrunning on. We could have specified a longer name like "www.perl.com",or an address like "204.148.40.9". For demonstration purposes, we'veused the special hostname "localhost", which should always mean thecurrent machine you're running on. The corresponding Internet addressfor 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 awell-configured system services file,[FOOTNOTE: The system services fileis in /etc/services under Unix] but just in case, we've specified theport number (13) in parentheses. Using just the number would also haveworked, but constant numbers make careful programmers nervous.

Notice how the return value from the new constructor is used asa filehandle in the while loop? That's what's called an indirectfilehandle, a scalar variable containing a filehandle. You can useit the same way you would a normal filehandle. For example, youcan 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 documentfrom, and then a list of documents to get from that host. This is amore interesting client than the previous one because it first sendssomething 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 atits standard port, number 80. If your the web server you're trying toconnect to is at a different port (like 1080 or 8080), you should specifyas the named-parameter pair, PeerPort => 8080. The autoflushmethod is used on the socket because otherwise the system would bufferup the output we sent it. (If you're on a Mac, you'll also need tochange every "\n" in your code that sends data over the network tobe a "\015\012" instead.)

Connecting to the server is only the first part of the process: once youhave the connection, you have to use the server's language. Each serveron the network has its own little command language that it expects asinput. The string that we send to the server starting with ``GET'' is inHTTP syntax. In this case, we simply request each specified document.Yes, we really are making a new connection for each document, even thoughit'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 leavethe connection open a little while, but the server doesn't have to honorsuch 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 thatparticular document. But a long response wouldn't have fit on this page.

For a more fully-featured version of this program, you should look tothe 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 likethe 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 ifyou're on a system that supports the powerful fork call, the solutionisn't that rough. Once you've made the connection to whatever serviceyou'd like to chat with, call fork to clone your process. Each ofthese two identical process has a very simple job to do: the parentcopies everything from the socket to standard output, while the childsimultaneously copies everything from standard input to the socket.To accomplish the same thing using just one process would be muchharder, because it's easier to code two processes to do one thing than itis to code one process to do two things. (This keep-it-simple principleis one of the cornerstones of the Unix philosophy, and good softwareengineering as well, which is probably why it's spread to other systemsas 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 asignal 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 thechild 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 thatdata immediately without waiting for a newline (which might not happen),you may wish to replace the while loop in the parent with thefollowing:

    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 reasonablywell.

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 thatdoes nothing but listen on a particular port for incoming connections.It does this by calling the IO::Socket::INET->new() method withslightly different arguments than the client did.

Proto: This is which protocol to use. Like our clients, we'llstill specify "tcp" here.
LocalPort: We specify a localport 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 theserver. (Under Unix, ports under 1024 are restricted to thesuperuser.) In our sample, we'll use port 9000, but you can useany port that's not currently in use on your system. If you tryto use one already in used, you'll get an ``Address already in use''message. Under Unix, the netstat -a command will showwhich services current have servers.
Listen: The Listen parameter is set to the maximum number ofpending 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, whichis SOMAXCONN.
Reuse: The Reuse parameter is needed so that we restart our servermanually without waiting a few minutes to allow system buffers toclear out.

Once the generic server socket has been created using the parameterslisted above, the server then waits for a new client to connectto it. The server blocks in the accept method, which eventually anbidirectional connection to the remote client. (Make sure to autoflushthis 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 outputback to the client. Note that unlike most network servers, this oneonly handles one incoming client at a time. Multithreaded servers arecovered 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, butmessages. UDP communications involve much lower overhead but also provideless reliability, as there are no promises that messages will arrive atall, let alone in order and unmangled. Still, UDP offers some advantagesover TCP, including being able to ``broadcast'' or ``multicast'' to a wholebunch of destination hosts at once (usually on your local subnet). If youfind yourself overly concerned about reliability and start building checksinto your message system, then you probably should use just TCP to startwith.

Here's a UDP program similar to the sample Internet TCP client givenearlier. However, instead of checking one host at a time, the UDP versionwill check many of them asynchronously by simulating a multicast and thenusing select() to do a timed-out wait for I/O. To do something similarwith 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--;    }

SysV IPC

While System V IPC isn't so widely used as sockets, it still has someinteresting uses. You can't, however, effectively use SysV IPC orBerkeley mmap() to have shared memory so as to share a variable amongstseveral processes. That's because Perl would reallocate your string whenyou 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 definitelyclunky looking. It should at the very least be made to use strictand require "sys/ipc.ph". Better yet, check out the IPC::SysV moduleson CPAN.

NOTES

If you are running under version 5.000 (dubious) or 5.001, you can stilluse most of the examples in this document. You may have to remove theuse strict and some of the my() statements for 5.000, and for bothyou'll have to load in version 1.2 or older of the Socket.pm module, whichis included in perl5.002.

Most of these routines quietly but politely return undef when they failinstead of causing your program to die right then and there due to anuncaught exception. (Actually, some of the new Socket conversionfunctions croak() on bad arguments.) It is therefore essentialthat you should check the return values of these functions. Always beginyour 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 notedelsewhere, Perl is at the mercy of your C libraries for much of its systembehaviour. It's probably safest to assume broken SysV semantics forsignals and to stick with simple TCP and UDP socket operations; e.g., don'ttry to pass open file descriptors over a local UDP datagram socket if youwant 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 beyondsetting a numeric variable that already exists; or, if locked intoa slow (restarting) system call, using die() to raise an exceptionand longjmp(3) out. In fact, even these may in some cases cause acore dump. It's probably best to avoid signals except where they areabsolutely inevitable. This perilous problems will be addressed in afuture release of Perl.

AUTHOR

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

This document was created byman2html,using the manual pages.