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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.


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 -T taint checking flag to the pound-bang line for servers:

    #!/usr/bin/perl -w
    require 5.002;
    use strict;
    use sigtrap;
    use Socket;


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.


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


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

For intrepid programmers, the 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.

Source: Perl interprocess communication (signals, fifos, pipes,
Copyright: Larry Wall, et al.

Previous: UDP: Message Passing

(Corrections, notes, and links courtesy of RocketAware.com)

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