3 perlipc - Perl interprocess communication (signals, fifos, pipes, safe subprocesses, 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 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 a global variable and then raise an
24 exception. That's because on most systems libraries are not
25 reentrant, 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, do just 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 reinstall themselves. Because Perl's signal mechanism is currently
99 based on the signal(3) function from the C library, you may sometimes 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:
107 # loathe sysV: it makes us not only reinstate
108 # the handler, but place it after the wait
109 $SIG{CHLD} = \&REAPER;
111 $SIG{CHLD} = \&REAPER;
112 # now do something that forks...
114 or even the more elaborate:
116 use POSIX ":sys_wait_h";
119 while ($child = waitpid(-1,WNOHANG)) {
120 $Kid_Status{$child} = $?;
122 $SIG{CHLD} = \&REAPER; # still loathe sysV
124 $SIG{CHLD} = \&REAPER;
125 # do something that forks...
127 Signal handling is also used for timeouts in Unix, While safely
128 protected within an C<eval{}> block, you set a signal handler to trap
129 alarm signals and then schedule to have one delivered to you in some
130 number of seconds. Then try your blocking operation, clearing the alarm
131 when it's done but not before you've exited your C<eval{}> block. If it
132 goes off, you'll use die() to jump out of the block, much as you might
133 using longjmp() or throw() in other languages.
138 local $SIG{ALRM} = sub { die "alarm clock restart" };
140 flock(FH, 2); # blocking write lock
143 if ($@ and $@ !~ /alarm clock restart/) { die }
145 For more complex signal handling, you might see the standard POSIX
146 module. Lamentably, this is almost entirely undocumented, but
147 the F<t/lib/posix.t> file from the Perl source distribution has some
152 A named pipe (often referred to as a FIFO) is an old Unix IPC
153 mechanism for processes communicating on the same machine. It works
154 just like a regular, connected anonymous pipes, except that the
155 processes rendezvous using a filename and don't have to be related.
157 To create a named pipe, use the Unix command mknod(1) or on some
158 systems, mkfifo(1). These may not be in your normal path.
160 # system return val is backwards, so && not ||
162 $ENV{PATH} .= ":/etc:/usr/etc";
163 if ( system('mknod', $path, 'p')
164 && system('mkfifo', $path) )
166 die "mk{nod,fifo} $path failed;
170 A fifo is convenient when you want to connect a process to an unrelated
171 one. When you open a fifo, the program will block until there's something
174 For example, let's say you'd like to have your F<.signature> file be a
175 named pipe that has a Perl program on the other end. Now every time any
176 program (like a mailer, news reader, finger program, etc.) tries to read
177 from that file, the reading program will block and your program will
178 supply the new signature. We'll use the pipe-checking file test B<-p>
179 to find out whether anyone (or anything) has accidentally removed our fifo.
182 $FIFO = '.signature';
183 $ENV{PATH} .= ":/etc:/usr/games";
188 system('mknod', $FIFO, 'p')
189 && die "can't mknod $FIFO: $!";
192 # next line blocks until there's a reader
193 open (FIFO, "> $FIFO") || die "can't write $FIFO: $!";
194 print FIFO "John Smith (smith\@host.org)\n", `fortune -s`;
196 sleep 2; # to avoid dup signals
200 =head1 Using open() for IPC
202 Perl's basic open() statement can also be used for unidirectional interprocess
203 communication by either appending or prepending a pipe symbol to the second
204 argument to open(). Here's how to start something up in a child process you
207 open(SPOOLER, "| cat -v | lpr -h 2>/dev/null")
208 || die "can't fork: $!";
209 local $SIG{PIPE} = sub { die "spooler pipe broke" };
210 print SPOOLER "stuff\n";
211 close SPOOLER || die "bad spool: $! $?";
213 And here's how to start up a child process you intend to read from:
215 open(STATUS, "netstat -an 2>&1 |")
216 || die "can't fork: $!";
218 next if /^(tcp|udp)/;
221 close STATUS || die "bad netstat: $! $?";
223 If one can be sure that a particular program is a Perl script that is
224 expecting filenames in @ARGV, the clever programmer can write something
227 $ program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile
229 and irrespective of which shell it's called from, the Perl program will
230 read from the file F<f1>, the process F<cmd1>, standard input (F<tmpfile>
231 in this case), the F<f2> file, the F<cmd2> command, and finally the F<f3>
232 file. Pretty nifty, eh?
234 You might notice that you could use backticks for much the
235 same effect as opening a pipe for reading:
237 print grep { !/^(tcp|udp)/ } `netstat -an 2>&1`;
238 die "bad netstat" if $?;
240 While this is true on the surface, it's much more efficient to process the
241 file one line or record at a time because then you don't have to read the
242 whole thing into memory at once. It also gives you finer control of the
243 whole process, letting you to kill off the child process early if you'd
246 Be careful to check both the open() and the close() return values. If
247 you're I<writing> to a pipe, you should also trap SIGPIPE. Otherwise,
248 think of what happens when you start up a pipe to a command that doesn't
249 exist: the open() will in all likelihood succeed (it only reflects the
250 fork()'s success), but then your output will fail--spectacularly. Perl
251 can't know whether the command worked because your command is actually
252 running in a separate process whose exec() might have failed. Therefore,
253 while readers of bogus commands return just a quick end of file, writers
254 to bogus command will trigger a signal they'd better be prepared to
263 Both the main process and the child process share the same STDIN,
264 STDOUT and STDERR filehandles. If both processes try to access them
265 at once, strange things can happen. You may want to close or reopen
266 the filehandles for the child. You can get around this by opening
267 your pipe with open(), but on some systems this means that the child
268 process cannot outlive the parent.
270 =head2 Background Processes
272 You can run a command in the background with:
276 The command's STDOUT and STDERR (and possibly STDIN, depending on your
277 shell) will be the same as the parent's. You won't need to catch
278 SIGCHLD because of the double-fork taking place (see below for more
281 =head2 Complete Dissociation of Child from Parent
283 In some cases (starting server processes, for instance) you'll want to
284 complete dissociate the child process from the parent. The following
285 process is reported to work on most Unixish systems. Non-Unix users
286 should check their Your_OS::Process module for other solutions.
292 Open /dev/tty and use the TIOCNOTTY ioctl on it. See L<tty(4)>
297 Change directory to /
301 Reopen STDIN, STDOUT, and STDERR so they're not connected to the old
306 Background yourself like this:
312 =head2 Safe Pipe Opens
314 Another interesting approach to IPC is making your single program go
315 multiprocess and communicate between (or even amongst) yourselves. The
316 open() function will accept a file argument of either C<"-|"> or C<"|-">
317 to do a very interesting thing: it forks a child connected to the
318 filehandle you've opened. The child is running the same program as the
319 parent. This is useful for safely opening a file when running under an
320 assumed UID or GID, for example. If you open a pipe I<to> minus, you can
321 write to the filehandle you opened and your kid will find it in his
322 STDIN. If you open a pipe I<from> minus, you can read from the filehandle
323 you opened whatever your kid writes to his STDOUT.
329 $pid = open(KID_TO_WRITE, "|-");
330 unless (defined $pid) {
331 warn "cannot fork: $!";
332 die "bailing out" if $sleep_count++ > 6;
335 } until defined $pid;
338 print KID_TO_WRITE @some_data;
339 close(KID_TO_WRITE) || warn "kid exited $?";
341 ($EUID, $EGID) = ($UID, $GID); # suid progs only
342 open (FILE, "> /safe/file")
343 || die "can't open /safe/file: $!";
345 print FILE; # child's STDIN is parent's KID
347 exit; # don't forget this
350 Another common use for this construct is when you need to execute
351 something without the shell's interference. With system(), it's
352 straightforward, but you can't use a pipe open or backticks safely.
353 That's because there's no way to stop the shell from getting its hands on
354 your arguments. Instead, use lower-level control to call exec() directly.
356 Here's a safe backtick or pipe open for read:
358 # add error processing as above
359 $pid = open(KID_TO_READ, "-|");
362 while (<KID_TO_READ>) {
363 # do something interesting
365 close(KID_TO_READ) || warn "kid exited $?";
368 ($EUID, $EGID) = ($UID, $GID); # suid only
369 exec($program, @options, @args)
370 || die "can't exec program: $!";
375 And here's a safe pipe open for writing:
377 # add error processing as above
378 $pid = open(KID_TO_WRITE, "|-");
379 $SIG{ALRM} = sub { die "whoops, $program pipe broke" };
385 close(KID_TO_WRITE) || warn "kid exited $?";
388 ($EUID, $EGID) = ($UID, $GID);
389 exec($program, @options, @args)
390 || die "can't exec program: $!";
394 Note that these operations are full Unix forks, which means they may not be
395 correctly implemented on alien systems. Additionally, these are not true
396 multithreading. If you'd like to learn more about threading, see the
397 F<modules> file mentioned below in the SEE ALSO section.
399 =head2 Bidirectional Communication
401 While this works reasonably well for unidirectional communication, what
402 about bidirectional communication? The obvious thing you'd like to do
403 doesn't actually work:
405 open(PROG_FOR_READING_AND_WRITING, "| some program |")
407 and if you forget to use the B<-w> flag, then you'll miss out
408 entirely on the diagnostic message:
410 Can't do bidirectional pipe at -e line 1.
412 If you really want to, you can use the standard open2() library function
413 to catch both ends. There's also an open3() for tri-directional I/O so you
414 can also catch your child's STDERR, but doing so would then require an
415 awkward select() loop and wouldn't allow you to use normal Perl input
418 If you look at its source, you'll see that open2() uses low-level
419 primitives like Unix pipe() and exec() to create all the connections.
420 While it might have been slightly more efficient by using socketpair(), it
421 would have then been even less portable than it already is. The open2()
422 and open3() functions are unlikely to work anywhere except on a Unix
423 system or some other one purporting to be POSIX compliant.
425 Here's an example of using open2():
429 $pid = open2( \*Reader, \*Writer, "cat -u -n" );
430 Writer->autoflush(); # default here, actually
431 print Writer "stuff\n";
434 The problem with this is that Unix buffering is really going to
435 ruin your day. Even though your C<Writer> filehandle is auto-flushed,
436 and the process on the other end will get your data in a timely manner,
437 you can't usually do anything to force it to give it back to you
438 in a similarly quick fashion. In this case, we could, because we
439 gave I<cat> a B<-u> flag to make it unbuffered. But very few Unix
440 commands are designed to operate over pipes, so this seldom works
441 unless you yourself wrote the program on the other end of the
444 A solution to this is the nonstandard F<Comm.pl> library. It uses
445 pseudo-ttys to make your program behave more reasonably:
448 $ph = open_proc('cat -n');
450 print $ph "a line\n";
451 print "got back ", scalar <$ph>;
454 This way you don't have to have control over the source code of the
455 program you're using. The F<Comm> library also has expect()
456 and interact() functions. Find the library (and we hope its
457 successor F<IPC::Chat>) at your nearest CPAN archive as detailed
458 in the SEE ALSO section below.
460 =head1 Sockets: Client/Server Communication
462 While not limited to Unix-derived operating systems (e.g., WinSock on PCs
463 provides socket support, as do some VMS libraries), you may not have
464 sockets on your system, in which case this section probably isn't going to do
465 you much good. With sockets, you can do both virtual circuits (i.e., TCP
466 streams) and datagrams (i.e., UDP packets). You may be able to do even more
467 depending on your system.
469 The Perl function calls for dealing with sockets have the same names as
470 the corresponding system calls in C, but their arguments tend to differ
471 for two reasons: first, Perl filehandles work differently than C file
472 descriptors. Second, Perl already knows the length of its strings, so you
473 don't need to pass that information.
475 One of the major problems with old socket code in Perl was that it used
476 hard-coded values for some of the constants, which severely hurt
477 portability. If you ever see code that does anything like explicitly
478 setting C<$AF_INET = 2>, you know you're in for big trouble: An
479 immeasurably superior approach is to use the C<Socket> module, which more
480 reliably grants access to various constants and functions you'll need.
482 If you're not writing a server/client for an existing protocol like
483 NNTP or SMTP, you should give some thought to how your server will
484 know when the client has finished talking, and vice-versa. Most
485 protocols are based on one-line messages and responses (so one party
486 knows the other has finished when a "\n" is received) or multi-line
487 messages and responses that end with a period on an empty line
488 ("\n.\n" terminates a message/response).
490 =head2 Internet TCP Clients and Servers
492 Use Internet-domain sockets when you want to do client-server
493 communication that might extend to machines outside of your own system.
495 Here's a sample TCP client using Internet-domain sockets:
501 my ($remote,$port, $iaddr, $paddr, $proto, $line);
503 $remote = shift || 'localhost';
504 $port = shift || 2345; # random port
505 if ($port =~ /\D/) { $port = getservbyname($port, 'tcp') }
506 die "No port" unless $port;
507 $iaddr = inet_aton($remote) || die "no host: $remote";
508 $paddr = sockaddr_in($port, $iaddr);
510 $proto = getprotobyname('tcp');
511 socket(SOCK, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
512 connect(SOCK, $paddr) || die "connect: $!";
513 while (defined($line = <SOCK>)) {
517 close (SOCK) || die "close: $!";
520 And here's a corresponding server to go along with it. We'll
521 leave the address as INADDR_ANY so that the kernel can choose
522 the appropriate interface on multihomed hosts. If you want sit
523 on a particular interface (like the external side of a gateway
524 or firewall machine), you should fill this in with your real address
530 BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
534 sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
536 my $port = shift || 2345;
537 my $proto = getprotobyname('tcp');
538 $port = $1 if $port =~ /(\d+)/; # untaint port number
540 socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
541 setsockopt(Server, SOL_SOCKET, SO_REUSEADDR,
542 pack("l", 1)) || die "setsockopt: $!";
543 bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!";
544 listen(Server,SOMAXCONN) || die "listen: $!";
546 logmsg "server started on port $port";
550 $SIG{CHLD} = \&REAPER;
552 for ( ; $paddr = accept(Client,Server); close Client) {
553 my($port,$iaddr) = sockaddr_in($paddr);
554 my $name = gethostbyaddr($iaddr,AF_INET);
556 logmsg "connection from $name [",
557 inet_ntoa($iaddr), "]
560 print Client "Hello there, $name, it's now ",
561 scalar localtime, "\n";
564 And here's a multithreaded version. It's multithreaded in that
565 like most typical servers, it spawns (forks) a slave server to
566 handle the client request so that the master server can quickly
567 go back to service a new client.
572 BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
576 sub spawn; # forward declaration
577 sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
579 my $port = shift || 2345;
580 my $proto = getprotobyname('tcp');
581 $port = $1 if $port =~ /(\d+)/; # untaint port number
583 socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
584 setsockopt(Server, SOL_SOCKET, SO_REUSEADDR,
585 pack("l", 1)) || die "setsockopt: $!";
586 bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!";
587 listen(Server,SOMAXCONN) || die "listen: $!";
589 logmsg "server started on port $port";
596 $SIG{CHLD} = \&REAPER; # loathe sysV
597 logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
600 $SIG{CHLD} = \&REAPER;
602 for ( $waitedpid = 0;
603 ($paddr = accept(Client,Server)) || $waitedpid;
604 $waitedpid = 0, close Client)
606 next if $waitedpid and not $paddr;
607 my($port,$iaddr) = sockaddr_in($paddr);
608 my $name = gethostbyaddr($iaddr,AF_INET);
610 logmsg "connection from $name [",
611 inet_ntoa($iaddr), "]
615 print "Hello there, $name, it's now ", scalar localtime, "\n";
616 exec '/usr/games/fortune'
617 or confess "can't exec fortune: $!";
625 unless (@_ == 0 && $coderef && ref($coderef) eq 'CODE') {
626 confess "usage: spawn CODEREF";
630 if (!defined($pid = fork)) {
631 logmsg "cannot fork: $!";
635 return; # I'm the parent
637 # else I'm the child -- go spawn
639 open(STDIN, "<&Client") || die "can't dup client to stdin";
640 open(STDOUT, ">&Client") || die "can't dup client to stdout";
641 ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr";
645 This server takes the trouble to clone off a child version via fork() for
646 each incoming request. That way it can handle many requests at once,
647 which you might not always want. Even if you don't fork(), the listen()
648 will allow that many pending connections. Forking servers have to be
649 particularly careful about cleaning up their dead children (called
650 "zombies" in Unix parlance), because otherwise you'll quickly fill up your
653 We suggest that you use the B<-T> flag to use taint checking (see L<perlsec>)
654 even if we aren't running setuid or setgid. This is always a good idea
655 for servers and other programs run on behalf of someone else (like CGI
656 scripts), because it lessens the chances that people from the outside will
657 be able to compromise your system.
659 Let's look at another TCP client. This one connects to the TCP "time"
660 service on a number of different machines and shows how far their clocks
661 differ from the system on which it's being run:
668 my $SECS_of_70_YEARS = 2208988800;
669 sub ctime { scalar localtime(shift) }
671 my $iaddr = gethostbyname('localhost');
672 my $proto = getprotobyname('tcp');
673 my $port = getservbyname('time', 'tcp');
674 my $paddr = sockaddr_in(0, $iaddr);
678 printf "%-24s %8s %s\n", "localhost", 0, ctime(time());
680 foreach $host (@ARGV) {
681 printf "%-24s ", $host;
682 my $hisiaddr = inet_aton($host) || die "unknown host";
683 my $hispaddr = sockaddr_in($port, $hisiaddr);
684 socket(SOCKET, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
685 connect(SOCKET, $hispaddr) || die "bind: $!";
687 read(SOCKET, $rtime, 4);
689 my $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ;
690 printf "%8d %s\n", $histime - time, ctime($histime);
693 =head2 Unix-Domain TCP Clients and Servers
695 That's fine for Internet-domain clients and servers, but what about local
696 communications? While you can use the same setup, sometimes you don't
697 want to. Unix-domain sockets are local to the current host, and are often
698 used internally to implement pipes. Unlike Internet domain sockets, Unix
699 domain sockets can show up in the file system with an ls(1) listing.
702 srw-rw-rw- 1 root 0 Oct 31 07:23 /dev/log
704 You can test for these with Perl's B<-S> file test:
706 unless ( -S '/dev/log' ) {
707 die "something's wicked with the print system";
710 Here's a sample Unix-domain client:
716 my ($rendezvous, $line);
718 $rendezvous = shift || '/tmp/catsock';
719 socket(SOCK, PF_UNIX, SOCK_STREAM, 0) || die "socket: $!";
720 connect(SOCK, sockaddr_un($rendezvous)) || die "connect: $!";
721 while (defined($line = <SOCK>)) {
726 And here's a corresponding server.
734 BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
736 my $NAME = '/tmp/catsock';
737 my $uaddr = sockaddr_un($NAME);
738 my $proto = getprotobyname('tcp');
740 socket(Server,PF_UNIX,SOCK_STREAM,0) || die "socket: $!";
742 bind (Server, $uaddr) || die "bind: $!";
743 listen(Server,SOMAXCONN) || die "listen: $!";
745 logmsg "server started on $NAME";
747 $SIG{CHLD} = \&REAPER;
749 for ( $waitedpid = 0;
750 accept(Client,Server) || $waitedpid;
751 $waitedpid = 0, close Client)
754 logmsg "connection on $NAME";
756 print "Hello there, it's now ", scalar localtime, "\n";
757 exec '/usr/games/fortune' or die "can't exec fortune: $!";
761 As you see, it's remarkably similar to the Internet domain TCP server, so
762 much so, in fact, that we've omitted several duplicate functions--spawn(),
763 logmsg(), ctime(), and REAPER()--which are exactly the same as in the
766 So why would you ever want to use a Unix domain socket instead of a
767 simpler named pipe? Because a named pipe doesn't give you sessions. You
768 can't tell one process's data from another's. With socket programming,
769 you get a separate session for each client: that's why accept() takes two
772 For example, let's say that you have a long running database server daemon
773 that you want folks from the World Wide Web to be able to access, but only
774 if they go through a CGI interface. You'd have a small, simple CGI
775 program that does whatever checks and logging you feel like, and then acts
776 as a Unix-domain client and connects to your private server.
778 =head2 UDP: Message Passing
780 Another kind of client-server setup is one that uses not connections, but
781 messages. UDP communications involve much lower overhead but also provide
782 less reliability, as there are no promises that messages will arrive at
783 all, let alone in order and unmangled. Still, UDP offers some advantages
784 over TCP, including being able to "broadcast" or "multicast" to a whole
785 bunch of destination hosts at once (usually on your local subnet). If you
786 find yourself overly concerned about reliability and start building checks
787 into your message system, then you probably should use just TCP to start
790 Here's a UDP program similar to the sample Internet TCP client given
791 above. However, instead of checking one host at a time, the UDP version
792 will check many of them asynchronously by simulating a multicast and then
793 using select() to do a timed-out wait for I/O. To do something similar
794 with TCP, you'd have to use a different socket handle for each host.
802 my ( $count, $hisiaddr, $hispaddr, $histime,
803 $host, $iaddr, $paddr, $port, $proto,
804 $rin, $rout, $rtime, $SECS_of_70_YEARS);
806 $SECS_of_70_YEARS = 2208988800;
808 $iaddr = gethostbyname(hostname());
809 $proto = getprotobyname('udp');
810 $port = getservbyname('time', 'udp');
811 $paddr = sockaddr_in(0, $iaddr); # 0 means let kernel pick
813 socket(SOCKET, PF_INET, SOCK_DGRAM, $proto) || die "socket: $!";
814 bind(SOCKET, $paddr) || die "bind: $!";
817 printf "%-12s %8s %s\n", "localhost", 0, scalar localtime time;
821 $hisiaddr = inet_aton($host) || die "unknown host";
822 $hispaddr = sockaddr_in($port, $hisiaddr);
823 defined(send(SOCKET, 0, 0, $hispaddr)) || die "send $host: $!";
827 vec($rin, fileno(SOCKET), 1) = 1;
829 # timeout after 10.0 seconds
830 while ($count && select($rout = $rin, undef, undef, 10.0)) {
832 ($hispaddr = recv(SOCKET, $rtime, 4, 0)) || die "recv: $!";
833 ($port, $hisiaddr) = sockaddr_in($hispaddr);
834 $host = gethostbyaddr($hisiaddr, AF_INET);
835 $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ;
836 printf "%-12s ", $host;
837 printf "%8d %s\n", $histime - time, scalar localtime($histime);
843 While System V IPC isn't so widely used as sockets, it still has some
844 interesting uses. You can't, however, effectively use SysV IPC or
845 Berkeley mmap() to have shared memory so as to share a variable amongst
846 several processes. That's because Perl would reallocate your string when
847 you weren't wanting it to.
850 Here's a small example showing shared memory usage.
855 $key = shmget($IPC_PRIVATE, $size , 0777 );
856 die unless defined $key;
858 $message = "Message #1";
859 shmwrite($key, $message, 0, 60 ) || die "$!";
860 shmread($key,$buff,0,60) || die "$!";
864 print "deleting $key\n";
865 shmctl($key ,$IPC_RMID, 0) || die "$!";
867 Here's an example of a semaphore:
871 $IPC_CREATE = 0001000;
872 $key = semget($IPC_KEY, $nsems , 0666 | $IPC_CREATE );
873 die if !defined($key);
876 Put this code in a separate file to be run in more than one process.
877 Call the file F<take>:
882 $key = semget($IPC_KEY, 0 , 0 );
883 die if !defined($key);
889 # wait for semaphore to be zero
891 $opstring1 = pack("sss", $semnum, $semop, $semflag);
893 # Increment the semaphore count
895 $opstring2 = pack("sss", $semnum, $semop, $semflag);
896 $opstring = $opstring1 . $opstring2;
898 semop($key,$opstring) || die "$!";
900 Put this code in a separate file to be run in more than one process.
901 Call this file F<give>:
903 # 'give' the semaphore
904 # run this in the original process and you will see
905 # that the second process continues
908 $key = semget($IPC_KEY, 0, 0);
909 die if !defined($key);
914 # Decrement the semaphore count
916 $opstring = pack("sss", $semnum, $semop, $semflag);
918 semop($key,$opstring) || die "$!";
922 The SysV IPC code above was written long ago, and it's definitely clunky
923 looking. It should at the very least be made to C<use strict> and
924 C<require "sys/ipc.ph">. Better yet, perhaps someone should create an
925 C<IPC::SysV> module the way we have the C<Socket> module for normal
926 client-server communications.
930 Voila! Check out the IPC::SysV modules written by Jack Shirazi. You can
931 find them at a CPAN store near you.
935 If you are running under version 5.000 (dubious) or 5.001, you can still
936 use most of the examples in this document. You may have to remove the
937 C<use strict> and some of the my() statements for 5.000, and for both
938 you'll have to load in version 1.2 or older of the F<Socket.pm> module, which
939 is included in I<perl5.002>.
941 Most of these routines quietly but politely return C<undef> when they fail
942 instead of causing your program to die right then and there due to an
943 uncaught exception. (Actually, some of the new I<Socket> conversion
944 functions croak() on bad arguments.) It is therefore essential
945 that you should check the return values of these functions. Always begin
946 your socket programs this way for optimal success, and don't forget to add
947 B<-T> taint checking flag to the pound-bang line for servers:
957 All these routines create system-specific portability problems. As noted
958 elsewhere, Perl is at the mercy of your C libraries for much of its system
959 behaviour. It's probably safest to assume broken SysV semantics for
960 signals and to stick with simple TCP and UDP socket operations; e.g., don't
961 try to pass open file descriptors over a local UDP datagram socket if you
962 want your code to stand a chance of being portable.
964 Because few vendors provide C libraries that are safely
965 reentrant, the prudent programmer will do little else within
966 a handler beyond die() to raise an exception and longjmp(3) out.
970 Tom Christiansen, with occasional vestiges of Larry Wall's original
975 Besides the obvious functions in L<perlfunc>, you should also check out
976 the F<modules> file at your nearest CPAN site. (See L<perlmod> or best
977 yet, the F<Perl FAQ> for a description of what CPAN is and where to get it.)
978 Section 5 of the F<modules> file is devoted to "Networking, Device Control
979 (modems), and Interprocess Communication", and contains numerous unbundled
980 modules numerous networking modules, Chat and Expect operations, CGI
981 programming, DCE, FTP, IPC, NNTP, Proxy, Ptty, RPC, SNMP, SMTP, Telnet,
982 Threads, and ToolTalk--just to name a few.