3 perlipc - Perl interprocess communication (signals, fifos, pipes, safe subprocceses, sockets, and semaphores)
7 The basic IPC facilities of Perl are built out of the good old Unix
8 signals, named pipes, pipe opens, the Berkeley socket routines, and SysV
9 IPC calls. Each is used in slightly different situations.
13 Perl uses a simple signal handling model: the %SIG hash contains names or
14 references of user-installed signal handlers. These handlers will be called
15 with an argument which is the name of the signal that triggered it. A
16 signal may be generated intentionally from a particular keyboard sequence like
17 control-C or control-Z, sent to you from an another process, or
18 triggered automatically by the kernel when special events transpire, like
19 a child process exiting, your process running out of stack space, or
20 hitting file size limit.
22 For example, to trap an interrupt signal, set up a handler like this.
23 Notice how all we do is set with a global variable and then raise an
24 exception. That's because on most systems libraries are not
25 re-entrant, so calling any print() functions (or even anything that needs to
26 malloc(3) more memory) could in theory trigger a memory fault
27 and subsequent core dump.
32 die "Somebody sent me a SIG$signame";
34 $SIG{INT} = 'catch_zap'; # could fail in modules
35 $SIG{INT} = \&catch_zap; # best strategy
37 The names of the signals are the ones listed out by C<kill -l> on your
38 system, or you can retrieve them from the Config module. Set up an
39 @signame list indexed by number to get the name and a %signo table
40 indexed by name to get the number:
43 defined $Config{sig_name} || die "No sigs?";
44 foreach $name (split(' ', $Config{sig_name})) {
50 So to check whether signal 17 and SIGALRM were the same, just do this:
52 print "signal #17 = $signame[17]\n";
54 print "SIGALRM is $signo{ALRM}\n";
57 You may also choose to assign the strings C<'IGNORE'> or C<'DEFAULT'> as
58 the handler, in which case Perl will try to discard the signal or do the
59 default thing. Some signals can be neither trapped nor ignored, such as
60 the KILL and STOP (but not the TSTP) signals. One strategy for
61 temporarily ignoring signals is to use a local() statement, which will be
62 automatically restored once your block is exited. (Remember that local()
63 values are "inherited" by functions called from within that block.)
66 local $SIG{INT} = 'IGNORE';
70 # interrupts still ignored, for now...
73 Sending a signal to a negative process ID means that you send the signal
74 to the entire Unix process-group. This code send a hang-up signal to all
75 processes in the current process group I<except for> the current process
79 local $SIG{HUP} = 'IGNORE';
81 # snazzy writing of: kill('HUP', -$$)
84 Another interesting signal to send is signal number zero. This doesn't
85 actually affect another process, but instead checks whether it's alive
86 or has changed its UID.
88 unless (kill 0 => $kid_pid) {
89 warn "something wicked happened to $kid_pid";
92 You might also want to employ anonymous functions for simple signal
95 $SIG{INT} = sub { die "\nOutta here!\n" };
97 But that will be problematic for the more complicated handlers that need
98 to re-install themselves. Because Perl's signal mechanism is currently
99 based on the signal(3) function from the C library, you may somtimes be so
100 misfortunate as to run on systems where that function is "broken", that
101 is, it behaves in the old unreliable SysV way rather than the newer, more
102 reasonable BSD and POSIX fashion. So you'll see defensive people writing
103 signal handlers like this:
106 $SIG{CHLD} = \&REAPER; # loathe sysV
109 $SIG{CHLD} = \&REAPER;
110 # now do something that forks...
112 or even the more elaborate:
117 $SIG{CHLD} = \&REAPER; # loathe sysV
118 while ($child = waitpid(-1,WNOHANG)) {
119 $Kid_Status{$child} = $?;
122 $SIG{CHLD} = \&REAPER;
123 # do something that forks...
125 Signal handling is also used for timeouts in Unix, While safely
126 protected within an C<eval{}> block, you set a signal handler to trap
127 alarm signals and then schedule to have one delivered to you in some
128 number of seconds. Then try your blocking operation, clearing the alarm
129 when it's done but not before you've exited your C<eval{}> block. If it
130 goes off, you'll use die() to jump out of the block, much as you might
131 using longjmp() or throw() in other languages.
136 local $SIG{ALRM} = sub { die "alarm clock restart" };
138 flock(FH, 2); # blocking write lock
141 if ($@ and $@ !~ /alarm clock restart/) { die }
143 For more complex signal handling, you might see the standard POSIX
144 module. Lamentably, this is almost entirely undocumented, but
145 the F<t/lib/posix.t> file from the Perl source distribution has some
150 A named pipe (often referred to as a FIFO) is an old Unix IPC
151 mechanism for processes communicating on the same machine. It works
152 just like a regular, connected anonymous pipes, except that the
153 processes rendezvous using a filename and don't have to be related.
155 To create a named pipe, use the Unix command mknod(1) or on some
156 systems, mkfifo(1). These may not be in your normal path.
158 # system return val is backwards, so && not ||
160 $ENV{PATH} .= ":/etc:/usr/etc";
161 if ( system('mknod', $path, 'p')
162 && system('mkfifo', $path) )
164 die "mk{nod,fifo} $path failed;
168 A fifo is convenient when you want to connect a process to an unrelated
169 one. When you open a fifo, the program will block until there's something
172 For example, let's say you'd like to have your F<.signature> file be a
173 named pipe that has a Perl program on the other end. Now every time any
174 program (like a mailer, newsreader, finger program, etc.) tries to read
175 from that file, the reading program will block and your program will
176 supply the the new signature. We'll use the pipe-checking file test B<-p>
177 to find out whether anyone (or anything) has accidentally removed our fifo.
180 $FIFO = '.signature';
181 $ENV{PATH} .= ":/etc:/usr/games";
186 system('mknod', $FIFO, 'p')
187 && die "can't mknod $FIFO: $!";
190 # next line blocks until there's a reader
191 open (FIFO, "> $FIFO") || die "can't write $FIFO: $!";
192 print FIFO "John Smith (smith\@host.org)\n", `fortune -s`;
194 sleep 2; # to avoid dup sigs
198 =head1 Using open() for IPC
200 Perl's basic open() statement can also be used for unidirectional interprocess
201 communication by either appending or prepending a pipe symbol to the second
202 argument to open(). Here's how to start something up a child process you
205 open(SPOOLER, "| cat -v | lpr -h 2>/dev/null")
206 || die "can't fork: $!";
207 local $SIG{PIPE} = sub { die "spooler pipe broke" };
208 print SPOOLER "stuff\n";
209 close SPOOLER || die "bad spool: $! $?";
211 And here's how to start up a child process you intend to read from:
213 open(STATUS, "netstat -an 2>&1 |")
214 || die "can't fork: $!";
216 next if /^(tcp|udp)/;
219 close SPOOLER || die "bad netstat: $! $?";
221 If one can be sure that a particular program is a Perl script that is
222 expecting filenames in @ARGV, the clever programmer can write something
225 $ program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile
227 and irrespective of which shell it's called from, the Perl program will
228 read from the file F<f1>, the process F<cmd1>, standard input (F<tmpfile>
229 in this case), the F<f2> file, the F<cmd2> command, and finally the F<f3>
230 file. Pretty nifty, eh?
232 You might notice that you could use backticks for much the
233 same effect as opening a pipe for reading:
235 print grep { !/^(tcp|udp)/ } `netstat -an 2>&1`;
236 die "bad netstat" if $?;
238 While this is true on the surface, it's much more efficient to process the
239 file one line or record at a time because then you don't have to read the
240 whole thing into memory at once. It also gives you finer control of the
241 whole process, letting you to kill off the child process early if you'd
244 Be careful to check both the open() and the close() return values. If
245 you're I<writing> to a pipe, you should also trap SIGPIPE. Otherwise,
246 think of what happens when you start up a pipe to a command that doesn't
247 exist: the open() will in all likelihood succeed (it only reflects the
248 fork()'s success), but then your output will fail--spectacularly. Perl
249 can't know whether the command worked because your command is actually
250 running in a separate process whose exec() might have failed. Therefore,
251 while readers of bogus commands just return a quick end of file, writers
252 to bogus command will trigger a signal they'd better be prepared to
259 =head2 Safe Pipe Opens
261 Another interesting approach to IPC is making your single program go
262 multiprocess and communicate between (or even amongst) yourselves. The
263 open() function will accept a file argument of either C<"-|"> or C<"|-">
264 to do a very interesting thing: it forks a child connected to the
265 filehandle you've opened. The child is running the same program as the
266 parent. This is useful for safely opening a file when running under an
267 assumed UID or GID, for example. If you open a pipe I<to> minus, you can
268 write to the filehandle you opened and your kid will find it in his
269 STDIN. If you open a pipe I<from> minus, you can read from the filehandle
270 you opened whatever your kid writes to his STDOUT.
276 $pid = open(KID_TO_WRITE, "|-");
277 unless (defined $pid) {
278 warn "cannot fork: $!";
279 die "bailing out" if $sleep_count++ > 6;
282 } until defined $pid;
285 print KID_TO_WRITE @some_data;
286 close(KID_TO_WRITE) || warn "kid exited $?";
288 ($EUID, $EGID) = ($UID, $GID); # suid progs only
289 open (FILE, "> /safe/file")
290 || die "can't open /safe/file: $!";
292 print FILE; # child's STDIN is parent's KID
294 exit; # don't forget this
297 Another common use for this construct is when you need to execute
298 something without the shell's interference. With system(), it's
299 straigh-forward, but you can't use a pipe open or backticks safely.
300 That's because there's no way to stop the shell from getting its hands on
301 your arguments. Instead, use lower-level control to call exec() directly.
303 Here's a safe backtick or pipe open for read:
305 # add error processing as above
306 $pid = open(KID_TO_READ, "-|");
309 while (<KID_TO_READ>) {
310 # do something interesting
312 close(KID_TO_READ) || warn "kid exited $?";
315 ($EUID, $EGID) = ($UID, $GID); # suid only
316 exec($program, @options, @args)
317 || die "can't exec program: $!";
322 And here's a safe pipe open for writing:
324 # add error processing as above
325 $pid = open(KID_TO_WRITE, "|-");
326 $SIG{ALRM} = sub { die "whoops, $program pipe broke" };
332 close(KID_TO_WRITE) || warn "kid exited $?";
335 ($EUID, $EGID) = ($UID, $GID);
336 exec($program, @options, @args)
337 || die "can't exec program: $!";
341 Note that these operations are full Unix forks, which means they may not be
342 correctly implemented on alien systems. Additionally, these are not true
343 multithreading. If you'd like to learn more about threading, see the
344 F<modules> file mentioned below in the L<SEE ALSO> section.
346 =head2 Bidirectional Communication
348 While this works reasonably well for unidirectional communication, what
349 about bidirectional communication? The obvious thing you'd like to do
350 doesn't actually work:
352 open(PROG_FOR_READING_AND_WRITING, "| some program |")
354 and if you forget to use the B<-w> flag, then you'll miss out
355 entirely on the diagnostic message:
357 Can't do bidirectional pipe at -e line 1.
359 If you really want to, you can use the standard open2() library function
360 to catch both ends. There's also an open3() for tridirectional I/O so you
361 can also catch your child's STDERR, but doing so would then require an
362 awkward select() loop and wouldn't allow you to use normal Perl input
365 If you look at its source, you'll see that open2() uses low-level
366 primitives like Unix pipe() and exec() to create all the connections.
367 While it might have been slightly more efficient by using socketpair(), it
368 would have then been even less portable than it already is. The open2()
369 and open3() functions are unlikely to work anywhere except on a Unix
370 system or some other one purporting to be POSIX compliant.
372 Here's an example of using open2():
376 $pid = open2( \*Reader, \*Writer, "cat -u -n" );
377 Writer->autoflush(); # default here, actually
378 print Writer "stuff\n";
381 The problem with this is that Unix buffering is going to really
382 ruin your day. Even though your C<Writer> filehandle is autoflushed,
383 and the process on the other end will get your data in a timely manner,
384 you can't usually do anything to force it to actually give it back to you
385 in a similarly quick fashion. In this case, we could, because we
386 gave I<cat> a B<-u> flag to make it unbuffered. But very few Unix
387 commands are designed to operate over pipes, so this seldom works
388 unless you yourself wrote the program on the other end of the
391 A solution to this is the non-standard F<Comm.pl> library. It uses
392 pseudo-ttys to make your program behave more reasonably:
395 $ph = open_proc('cat -n');
397 print $ph "a line\n";
398 print "got back ", scalar <$ph>;
401 This way you don't have to have control over the source code of the
402 program you're using. The F<Comm> library also has expect()
403 and interact() functions. Find the library (and hopefully its
404 successor F<IPC::Chat>) at your nearest CPAN archive as detailed
405 in the L<SEE ALSO> section below.
407 =head1 Sockets: Client/Server Communication
409 While not limited to Unix-derived operating systems (e.g. WinSock on PCs
410 provides socket support, as do some VMS libraries), you may not have
411 sockets on your system, in which this section probably isn't going to do
412 you much good. With sockets, you can do both virtual circuits (i.e. TCP
413 streams) and datagrams (i.e. UDP packets). You may be able to do even more
414 depending on your system.
416 The Perl function calls for dealing with sockets have the same names as
417 the corresponding system calls in C, but their arguments tend to differ
418 for two reasons: first, Perl filehandles work differently than C file
419 descriptors. Second, Perl already knows the length of its strings, so you
420 don't need to pass that information.
422 One of the major problems with old socket code in Perl was that it used
423 hard-coded values for some of the constants, which severely hurt
424 portability. If you ever see code that does anything like explicitly
425 setting C<$AF_INET = 2>, you know you're in for big trouble: An
426 immeasurably superior approach is to use the C<Socket> module, which more
427 reliably grants access to various constants and functions you'll need.
429 =head2 Internet TCP Clients and Servers
431 Use Internet-domain sockets when you want to do client-server
432 communication that might extend to machines outside of your own system.
434 Here's a sample TCP client using Internet-domain sockets:
440 my ($remote,$port, $iaddr, $paddr, $proto, $line);
442 $remote = shift || 'localhost';
443 $port = shift || 2345; # random port
444 if ($port =~ /\D/) { $port = getservbyname($port, 'tcp') }
445 die "No port" unless $port;
446 $iaddr = inet_aton($remote) || die "no host: $remote";
447 $paddr = sockaddr_in($port, $iaddr);
449 $proto = getprotobyname('tcp');
450 socket(SOCK, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
451 connect(SOCK, $paddr) || die "connect: $!";
452 while ($line = <SOCK>) {
456 close (SOCK) || die "close: $!";
459 And here's a corresponding server to go along with it. We'll
460 leave the address as INADDR_ANY so that the kernel can choose
461 the appropriate interface on multihomed hosts. If you want sit
462 on a particular interface (like the external side of a gateway
463 or firewall machine), you should fill this in with your real address
469 BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
473 sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
475 my $port = shift || 2345;
476 my $proto = getprotobyname('tcp');
477 socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
478 setsockopt(Server, SOL_SOCKET, SO_REUSEADDR,
479 pack("l", 1)) || die "setsockopt: $!";
480 bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!";
481 listen(Server,SOMAXCONN) || die "listen: $!";
483 logmsg "server started on port $port";
487 $SIG{CHLD} = \&REAPER;
489 for ( ; $paddr = accept(Client,Server); close Client) {
490 my($port,$iaddr) = sockaddr_in($paddr);
491 my $name = gethostbyaddr($iaddr,AF_INET);
493 logmsg "connection from $name [",
494 inet_ntoa($iaddr), "]
497 print CLIENT "Hello there, $name, it's now ",
498 scalar localtime, "\n";
501 And here's a multithreaded version. It's multithreaded in that
502 like most typical servers, it spawns (forks) a slave server to
503 handle the client request so that the master server can quickly
504 go back to service a new client.
509 BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
513 sub spawn; # forward declaration
514 sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
516 my $port = shift || 2345;
517 my $proto = getprotobyname('tcp');
518 socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
519 setsockopt(Server, SOL_SOCKET, SO_REUSEADDR,
520 pack("l", 1)) || die "setsockopt: $!";
521 bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!";
522 listen(Server,SOMAXCONN) || die "listen: $!";
524 logmsg "server started on port $port";
530 $SIG{CHLD} = \&REAPER; # loathe sysV
532 logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
535 $SIG{CHLD} = \&REAPER;
537 for ( $waitedpid = 0;
538 ($paddr = accept(Client,Server)) || $waitedpid;
539 $waitedpid = 0, close Client)
542 my($port,$iaddr) = sockaddr_in($paddr);
543 my $name = gethostbyaddr($iaddr,AF_INET);
545 logmsg "connection from $name [",
546 inet_ntoa($iaddr), "]
550 print "Hello there, $name, it's now ", scalar localtime, "\n";
551 exec '/usr/games/fortune'
552 or confess "can't exec fortune: $!";
560 unless (@_ == 0 && $coderef && ref($coderef) eq 'CODE') {
561 confess "usage: spawn CODEREF";
565 if (!defined($pid = fork)) {
566 logmsg "cannot fork: $!";
570 return; # i'm the parent
572 # else i'm the child -- go spawn
574 open(STDIN, "<&Client") || die "can't dup client to stdin";
575 open(STDOUT, ">&Client") || die "can't dup client to stdout";
576 ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr";
580 This server takes the trouble to clone off a child version via fork() for
581 each incoming request. That way it can handle many requests at once,
582 which you might not always want. Even if you don't fork(), the listen()
583 will allow that many pending connections. Forking servers have to be
584 particularly careful about cleaning up their dead children (called
585 "zombies" in Unix parlance), because otherwise you'll quickly fill up your
588 We suggest that you use the B<-T> flag to use taint checking (see L<perlsec>)
589 even if we aren't running setuid or setgid. This is always a good idea
590 for servers and other programs run on behalf of someone else (like CGI
591 scripts), because it lessens the chances that people from the outside will
592 be able to compromise your system.
594 Let's look at another TCP client. This one connects to the TCP "time"
595 service on a number of different machines and shows how far their clocks
596 differ from the system on which it's being run:
603 my $SECS_of_70_YEARS = 2208988800;
604 sub ctime { scalar localtime(shift) }
606 my $iaddr = gethostbyname('localhost');
607 my $proto = getprotobyname('tcp');
608 my $port = getservbyname('time', 'tcp');
609 my $paddr = sockaddr_in(0, $iaddr);
613 printf "%-24s %8s %s\n", "localhost", 0, ctime(time());
615 foreach $host (@ARGV) {
616 printf "%-24s ", $host;
617 my $hisiaddr = inet_aton($host) || die "unknown host";
618 my $hispaddr = sockaddr_in($port, $hisiaddr);
619 socket(SOCKET, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
620 connect(SOCKET, $hispaddr) || die "bind: $!";
622 read(SOCKET, $rtime, 4);
624 my $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ;
625 printf "%8d %s\n", $histime - time, ctime($histime);
628 =head2 Unix-Domain TCP Clients and Servers
630 That's fine for Internet-domain clients and servers, but what local
631 communications? While you can use the same setup, sometimes you don't
632 want to. Unix-domain sockets are local to the current host, and are often
633 used internally to implement pipes. Unlike Internet domain sockets, UNIX
634 domain sockets can show up in the file system with an ls(1) listing.
637 srw-rw-rw- 1 root 0 Oct 31 07:23 /dev/log
639 You can test for these with Perl's B<-S> file test:
641 unless ( -S '/dev/log' ) {
642 die "something's wicked with the print system";
645 Here's a sample Unix-domain client:
651 my ($rendezvous, $line);
653 $rendezvous = shift || '/tmp/catsock';
654 socket(SOCK, PF_UNIX, SOCK_STREAM, 0) || die "socket: $!";
655 connect(SOCK, sockaddr_un($remote)) || die "connect: $!";
656 while ($line = <SOCK>) {
661 And here's a corresponding server.
669 BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
671 my $NAME = '/tmp/catsock';
672 my $uaddr = sockaddr_un($NAME);
673 my $proto = getprotobyname('tcp');
675 socket(Server,PF_UNIX,SOCK_STREAM,0) || die "socket: $!";
677 bind (Server, $uaddr) || die "bind: $!";
678 listen(Server,SOMAXCONN) || die "listen: $!";
680 logmsg "server started on $NAME";
682 $SIG{CHLD} = \&REAPER;
684 for ( $waitedpid = 0;
685 accept(Client,Server) || $waitedpid;
686 $waitedpid = 0, close Client)
689 logmsg "connection on $NAME";
691 print "Hello there, it's now ", scalar localtime, "\n";
692 exec '/usr/games/fortune' or die "can't exec fortune: $!";
696 As you see, it's remarkably similar to the Internet domain TCP server, so
697 much so, in fact, that we've omitted several duplicate functions--spawn(),
698 logmsg(), ctime(), and REAPER()--which are exactly the same as in the
701 So why would you ever want to use a Unix domain socket instead of a
702 simpler named pipe? Because a named pipe doesn't give you sessions. You
703 can't tell one process's data from another's. With socket programming,
704 you get a separate session for each client: that's why accept() takes two
707 For example, let's say that you have a long running database server daemon
708 that you want folks from the World Wide Web to be able to access, but only
709 if they go through a CGI interface. You'd have a small, simple CGI
710 program that does whatever checks and logging you feel like, and then acts
711 as a Unix-domain client and connects to your private server.
713 =head2 UDP: Message Passing
715 Another kind of client-server setup is one that uses not connections, but
716 messages. UDP communications involve much lower overhead but also provide
717 less reliability, as there are no promises that messages will arrive at
718 all, let alone in order and unmangled. Still, UDP offers some advantages
719 over TCP, including being able to "broadcast" or "multicast" to a whole
720 bunch of destination hosts at once (usually on your local subnet). If you
721 find yourself overly concerned about reliability and start building checks
722 into your message system, then you probably should just use TCP to start
725 Here's a UDP program similar to the sample Internet TCP client given
726 above. However, instead of checking one host at a time, the UDP version
727 will check many of them asynchronously by simulating a multicast and then
728 using select() to do a timed-out wait for I/O. To do something similar
729 with TCP, you'd have to use a different socket handle for each host.
737 my ( $count, $hisiaddr, $hispaddr, $histime,
738 $host, $iaddr, $paddr, $port, $proto,
739 $rin, $rout, $rtime, $SECS_of_70_YEARS);
741 $SECS_of_70_YEARS = 2208988800;
743 $iaddr = gethostbyname(hostname());
744 $proto = getprotobyname('udp');
745 $port = getservbyname('time', 'udp');
746 $paddr = sockaddr_in(0, $iaddr); # 0 means let kernel pick
748 socket(SOCKET, PF_INET, SOCK_DGRAM, $proto) || die "socket: $!";
749 bind(SOCKET, $paddr) || die "bind: $!";
752 printf "%-12s %8s %s\n", "localhost", 0, scalar localtime time;
756 $hisiaddr = inet_aton($host) || die "unknown host";
757 $hispaddr = sockaddr_in($port, $hisiaddr);
758 defined(send(SOCKET, 0, 0, $hispaddr)) || die "send $host: $!";
762 vec($rin, fileno(SOCKET), 1) = 1;
764 # timeout after 10.0 seconds
765 while ($count && select($rout = $rin, undef, undef, 10.0)) {
767 ($hispaddr = recv(SOCKET, $rtime, 4, 0)) || die "recv: $!";
768 ($port, $hisiaddr) = sockaddr_in($hispaddr);
769 $host = gethostbyaddr($hisiaddr, AF_INET);
770 $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ;
771 printf "%-12s ", $host;
772 printf "%8d %s\n", $histime - time, scalar localtime($histime);
778 While System V IPC isn't so widely used as sockets, it still has some
779 interesting uses. You can't, however, effectively use SysV IPC or
780 Berkeley mmap() to have shared memory so as to share a variable amongst
781 several processes. That's because Perl would reallocate your string when
782 you weren't wanting it to.
785 Here's a small example showing shared memory usage.
790 $key = shmget($IPC_PRIVATE, $size , 0777 );
791 die unless defined $key;
793 $message = "Message #1";
794 shmwrite($key, $message, 0, 60 ) || die "$!";
795 shmread($key,$buff,0,60) || die "$!";
799 print "deleting $key\n";
800 shmctl($key ,$IPC_RMID, 0) || die "$!";
802 Here's an example of a semaphore:
806 $IPC_CREATE = 0001000;
807 $key = semget($IPC_KEY, $nsems , 0666 | $IPC_CREATE );
808 die if !defined($key);
811 Put this code in a separate file to be run in more that one process
812 Call the file F<take>:
817 $key = semget($IPC_KEY, 0 , 0 );
818 die if !defined($key);
824 # wait for semaphore to be zero
826 $opstring1 = pack("sss", $semnum, $semop, $semflag);
828 # Increment the semaphore count
830 $opstring2 = pack("sss", $semnum, $semop, $semflag);
831 $opstring = $opstring1 . $opstring2;
833 semop($key,$opstring) || die "$!";
835 Put this code in a separate file to be run in more that one process
836 Call this file F<give>:
838 # 'give' the semaphore
839 # run this in the original process and you will see
840 # that the second process continues
843 $key = semget($IPC_KEY, 0, 0);
844 die if !defined($key);
849 # Decrement the semaphore count
851 $opstring = pack("sss", $semnum, $semop, $semflag);
853 semop($key,$opstring) || die "$!";
857 The SysV IPC code above was written long ago, and it's definitely clunky
858 looking. It should at the very least be made to C<use strict> and
859 C<require "sys/ipc.ph">. Better yet, perhaps someone should create an
860 C<IPC::SysV> module the way we have the C<Socket> module for normal
861 client-server communications.
865 Voila! Check out the IPC::SysV modules written by Jack Shirazi. You can
866 find them at a CPAN store near you.
870 If you are running under version 5.000 (dubious) or 5.001, you can still
871 use most of the examples in this document. You may have to remove the
872 C<use strict> and some of the my() statements for 5.000, and for both
873 you'll have to load in version 1.2 of the F<Socket.pm> module, which
874 was/is/shall-be included in I<perl5.001o>.
876 Most of these routines quietly but politely return C<undef> when they fail
877 instead of causing your program to die right then and there due to an
878 uncaught exception. (Actually, some of the new I<Socket> conversion
879 functions croak() on bad arguments.) It is therefore essential
880 that you should check the return values fo these functions. Always begin
881 your socket programs this way for optimal success, and don't forget to add
882 B<-T> taint checking flag to the pound-bang line for servers:
892 All these routines create system-specific portability problems. As noted
893 elsewhere, Perl is at the mercy of your C libraries for much of its system
894 behaviour. It's probably safest to assume broken SysV semantics for
895 signals and to stick with simple TCP and UDP socket operations; e.g. don't
896 try to pass open filedescriptors over a local UDP datagram socket if you
897 want your code to stand a chance of being portable.
899 Because few vendors provide C libraries that are safely
900 re-entrant, the prudent programmer will do little else within
901 a handler beyond die() to raise an exception and longjmp(3) out.
905 Tom Christiansen, with occasional vestiges of Larry Wall's original
910 Besides the obvious functions in L<perlfunc>, you should also check out
911 the F<modules> file at your nearest CPAN site. (See L<perlmod> or best
912 yet, the F<Perl FAQ> for a description of what CPAN is and where to get it.)
913 Section 5 of the F<modules> file is devoted to "Networking, Device Control
914 (modems) and Interprocess Communication", and contains numerous unbundled
915 modules numerous networking modules, Chat and Expect operations, CGI
916 programming, DCE, FTP, IPC, NNTP, Proxy, Ptty, RPC, SNMP, SMTP, Telnet,
917 Threads, and ToolTalk--just to name a few.