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1=head1 NAME
2
cb1a09d0 3perlipc - Perl interprocess communication (signals, fifos, pipes, safe subprocceses, sockets, and semaphores)
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4
5=head1 DESCRIPTION
6
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7The basic IPC facilities of Perl are built out of the good old Unix
8signals, named pipes, pipe opens, the Berkeley socket routines, and SysV
9IPC calls. Each is used in slightly different situations.
10
11=head1 Signals
12
13Perl uses a simple signal handling model: the %SIG hash contains names or
14references of user-installed signal handlers. These handlers will be called
15with an argument which is the name of the signal that triggered it. A
16signal may be generated intentionally from a particular keyboard sequence like
17control-C or control-Z, sent to you from an another process, or
18triggered automatically by the kernel when special events transpire, like
19a child process exiting, your process running out of stack space, or
20hitting file size limit.
21
22For example, to trap an interrupt signal, set up a handler like this.
23Notice how all we do is set with a global variable and then raise an
24exception. That's because on most systems libraries are not
25re-entrant, so calling any print() functions (or even anything that needs to
26malloc(3) more memory) could in theory trigger a memory fault
27and subsequent core dump.
28
29 sub catch_zap {
30 my $signame = shift;
31 $shucks++;
32 die "Somebody sent me a SIG$signame";
33 }
34 $SIG{INT} = 'catch_zap'; # could fail in modules
35 $SIG{INT} = \&catch_zap; # best strategy
36
37The names of the signals are the ones listed out by C<kill -l> on your
38system, 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
40indexed by name to get the number:
41
42 use Config;
43 defined $Config{sig_name} || die "No sigs?";
44 foreach $name (split(' ', $Config{sig_name})) {
45 $signo{$name} = $i;
46 $signame[$i] = $name;
47 $i++;
48 }
49
50So to check whether signal 17 and SIGALRM were the same, just do this:
51
52 print "signal #17 = $signame[17]\n";
53 if ($signo{ALRM}) {
54 print "SIGALRM is $signo{ALRM}\n";
55 }
56
57You may also choose to assign the strings C<'IGNORE'> or C<'DEFAULT'> as
58the handler, in which case Perl will try to discard the signal or do the
59default thing. Some signals can be neither trapped nor ignored, such as
60the KILL and STOP (but not the TSTP) signals. One strategy for
61temporarily ignoring signals is to use a local() statement, which will be
62automatically restored once your block is exited. (Remember that local()
63values are "inherited" by functions called from within that block.)
64
65 sub precious {
66 local $SIG{INT} = 'IGNORE';
67 &more_functions;
68 }
69 sub more_functions {
70 # interrupts still ignored, for now...
71 }
72
73Sending a signal to a negative process ID means that you send the signal
74to the entire Unix process-group. This code send a hang-up signal to all
75processes in the current process group I<except for> the current process
76itself:
77
78 {
79 local $SIG{HUP} = 'IGNORE';
80 kill HUP => -$$;
81 # snazzy writing of: kill('HUP', -$$)
82 }
a0d0e21e 83
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84Another interesting signal to send is signal number zero. This doesn't
85actually affect another process, but instead checks whether it's alive
86or has changed its UID.
a0d0e21e 87
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88 unless (kill 0 => $kid_pid) {
89 warn "something wicked happened to $kid_pid";
90 }
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92You might also want to employ anonymous functions for simple signal
93handlers:
a0d0e21e 94
4633a7c4 95 $SIG{INT} = sub { die "\nOutta here!\n" };
a0d0e21e 96
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97But that will be problematic for the more complicated handlers that need
98to re-install themselves. Because Perl's signal mechanism is currently
99based on the signal(3) function from the C library, you may somtimes be so
100misfortunate as to run on systems where that function is "broken", that
101is, it behaves in the old unreliable SysV way rather than the newer, more
102reasonable BSD and POSIX fashion. So you'll see defensive people writing
103signal handlers like this:
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105 sub REAPER {
106 $SIG{CHLD} = \&REAPER; # loathe sysV
107 $waitedpid = wait;
108 }
109 $SIG{CHLD} = \&REAPER;
110 # now do something that forks...
111
112or even the more elaborate:
113
114 use POSIX "wait_h";
115 sub REAPER {
116 my $child;
117 $SIG{CHLD} = \&REAPER; # loathe sysV
118 while ($child = waitpid(-1,WNOHANG)) {
119 $Kid_Status{$child} = $?;
120 }
121 }
122 $SIG{CHLD} = \&REAPER;
123 # do something that forks...
124
125Signal handling is also used for timeouts in Unix, While safely
126protected within an C<eval{}> block, you set a signal handler to trap
127alarm signals and then schedule to have one delivered to you in some
128number of seconds. Then try your blocking operation, clearing the alarm
129when it's done but not before you've exited your C<eval{}> block. If it
130goes off, you'll use die() to jump out of the block, much as you might
131using longjmp() or throw() in other languages.
132
133Here's an example:
134
135 eval {
136 local $SIG{ALRM} = sub { die "alarm clock restart" };
137 alarm 10;
138 flock(FH, 2); # blocking write lock
139 alarm 0;
140 };
141 if ($@ and $@ !~ /alarm clock restart/) { die }
142
143For more complex signal handling, you might see the standard POSIX
144module. Lamentably, this is almost entirely undocumented, but
145the F<t/lib/posix.t> file from the Perl source distribution has some
146examples in it.
147
148=head1 Named Pipes
149
150A named pipe (often referred to as a FIFO) is an old Unix IPC
151mechanism for processes communicating on the same machine. It works
152just like a regular, connected anonymous pipes, except that the
153processes rendezvous using a filename and don't have to be related.
154
155To create a named pipe, use the Unix command mknod(1) or on some
156systems, mkfifo(1). These may not be in your normal path.
157
158 # system return val is backwards, so && not ||
159 #
160 $ENV{PATH} .= ":/etc:/usr/etc";
161 if ( system('mknod', $path, 'p')
162 && system('mkfifo', $path) )
163 {
164 die "mk{nod,fifo} $path failed;
165 }
166
167
168A fifo is convenient when you want to connect a process to an unrelated
169one. When you open a fifo, the program will block until there's something
170on the other end.
171
172For example, let's say you'd like to have your F<.signature> file be a
173named pipe that has a Perl program on the other end. Now every time any
174program (like a mailer, newsreader, finger program, etc.) tries to read
175from that file, the reading program will block and your program will
176supply the the new signature. We'll use the pipe-checking file test B<-p>
177to find out whether anyone (or anything) has accidentally removed our fifo.
178
179 chdir; # go home
180 $FIFO = '.signature';
181 $ENV{PATH} .= ":/etc:/usr/games";
182
183 while (1) {
184 unless (-p $FIFO) {
185 unlink $FIFO;
186 system('mknod', $FIFO, 'p')
187 && die "can't mknod $FIFO: $!";
188 }
189
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`;
193 close FIFO;
194 sleep 2; # to avoid dup sigs
195 }
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198=head1 Using open() for IPC
199
200Perl's basic open() statement can also be used for unidirectional interprocess
201communication by either appending or prepending a pipe symbol to the second
202argument to open(). Here's how to start something up a child process you
203intend to write to:
204
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: $! $?";
210
211And here's how to start up a child process you intend to read from:
212
213 open(STATUS, "netstat -an 2>&1 |")
214 || die "can't fork: $!";
215 while (<STATUS>) {
216 next if /^(tcp|udp)/;
217 print;
218 }
219 close SPOOLER || die "bad netstat: $! $?";
220
221If one can be sure that a particular program is a Perl script that is
222expecting filenames in @ARGV, the clever programmer can write something
223like this:
224
225 $ program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile
226
227and irrespective of which shell it's called from, the Perl program will
228read from the file F<f1>, the process F<cmd1>, standard input (F<tmpfile>
229in this case), the F<f2> file, the F<cmd2> command, and finally the F<f3>
230file. Pretty nifty, eh?
231
232You might notice that you could use backticks for much the
233same effect as opening a pipe for reading:
234
235 print grep { !/^(tcp|udp)/ } `netstat -an 2>&1`;
236 die "bad netstat" if $?;
237
238While this is true on the surface, it's much more efficient to process the
239file one line or record at a time because then you don't have to read the
240whole thing into memory at once. It also gives you finer control of the
241whole process, letting you to kill off the child process early if you'd
242like.
243
244Be careful to check both the open() and the close() return values. If
245you're I<writing> to a pipe, you should also trap SIGPIPE. Otherwise,
246think of what happens when you start up a pipe to a command that doesn't
247exist: the open() will in all likelihood succeed (it only reflects the
248fork()'s success), but then your output will fail--spectacularly. Perl
249can't know whether the command worked because your command is actually
250running in a separate process whose exec() might have failed. Therefore,
251while readers of bogus commands just return a quick end of file, writers
252to bogus command will trigger a signal they'd better be prepared to
253handle. Consider:
254
255 open(FH, "|bogus");
256 print FH "bang\n";
257 close FH;
258
259=head2 Safe Pipe Opens
260
261Another interesting approach to IPC is making your single program go
262multiprocess and communicate between (or even amongst) yourselves. The
263open() function will accept a file argument of either C<"-|"> or C<"|-">
264to do a very interesting thing: it forks a child connected to the
265filehandle you've opened. The child is running the same program as the
266parent. This is useful for safely opening a file when running under an
267assumed UID or GID, for example. If you open a pipe I<to> minus, you can
268write to the filehandle you opened and your kid will find it in his
269STDIN. If you open a pipe I<from> minus, you can read from the filehandle
270you opened whatever your kid writes to his STDOUT.
271
272 use English;
273 my $sleep_count = 0;
274
275 do {
276 $pid = open(KID, "-|");
277 unless (defined $pid) {
278 warn "cannot fork: $!";
279 die "bailing out" if $sleep_count++ > 6;
280 sleep 10;
281 }
282 } until defined $pid;
283
284 if ($pid) { # parent
285 print KID @some_data;
286 close(KID) || warn "kid exited $?";
287 } else { # child
288 ($EUID, $EGID) = ($UID, $GID); # suid progs only
289 open (FILE, "> /safe/file")
290 || die "can't open /safe/file: $!";
291 while (<STDIN>) {
292 print FILE; # child's STDIN is parent's KID
293 }
294 exit; # don't forget this
295 }
296
297Another common use for this construct is when you need to execute
298something without the shell's interference. With system(), it's
299straigh-forward, but you can't use a pipe open or backticks safely.
300That's because there's no way to stop the shell from getting its hands on
301your arguments. Instead, use lower-level control to call exec() directly.
302
303Here's a safe backtick or pipe open for read:
304
305 # add error processing as above
306 $pid = open(KID, "-|");
307
308 if ($pid) { # parent
309 while (<KID>) {
310 # do something interesting
311 }
312 close(KID) || warn "kid exited $?";
313
314 } else { # child
315 ($EUID, $EGID) = ($UID, $GID); # suid only
316 exec($program, @options, @args)
317 || die "can't exec program: $!";
318 # NOTREACHED
319 }
320
321
322And here's a safe pipe open for writing:
323
324 # add error processing as above
325 $pid = open(KID, "|-");
326 $SIG{ALRM} = sub { die "whoops, $program pipe broke" };
327
328 if ($pid) { # parent
329 for (@data) {
330 print KID;
331 }
332 close(KID) || warn "kid exited $?";
333
334 } else { # child
335 ($EUID, $EGID) = ($UID, $GID);
336 exec($program, @options, @args)
337 || die "can't exec program: $!";
338 # NOTREACHED
339 }
340
341Note that these operations are full Unix forks, which means they may not be
342correctly implemented on alien systems. Additionally, these are not true
343multithreading. If you'd like to learn more about threading, see the
344F<modules> file mentioned below in the L<SEE ALSO> section.
345
346=head2 Bidirectional Communication
347
348While this works reasonably well for unidirectional communication, what
349about bidirectional communication? The obvious thing you'd like to do
350doesn't actually work:
351
352 open(KID, "| some program |")
353
354and if you forgot to use the B<-w> flag, then you'll miss out
355entirely on the diagnostic message:
356
357 Can't do bidirectional pipe at -e line 1.
358
359If you really want to, you can use the standard open2() library function
360to catch both ends. There's also an open3() for tridirectional I/O so you
361can also catch your child's STDERR, but doing so would then require an
362awkward select() loop and wouldn't allow you to use normal Perl input
363operations.
364
365If you look at its source, you'll see that open2() uses low-level
366primitives like Unix pipe() and exec() to create all the connections.
367While it might have been slightly more efficient by using socketpair(), it
368would have then been even less portable than it already is. The open2()
369and open3() functions are unlikely to work anywhere except on a Unix
370system or some other one purporting to be POSIX compliant.
371
372Here's an example of using open2():
373
374 use FileHandle;
375 use IPC::Open2;
376 $pid = open2( \*Reader, \*Writer, "cat -u -n" );
377 Writer->autoflush(); # default here, actually
378 print Writer "stuff\n";
379 $got = <Reader>;
380
381The problem with this is that Unix buffering is going to really
382ruin your day. Even though your C<Writer> filehandle is autoflushed,
383and the process on the other end will get your data in a timely manner,
384you can't usually do anything to force it to actually give it back to you
385in a similarly quick fashion. In this case, we could, because we
386gave I<cat> a B<-u> flag to make it unbuffered. But very few Unix
387commands are designed to operate over pipes, so this seldom works
388unless you yourself wrote the program on the other end of the
389double-ended pipe.
390
391A solution to this is the non-standard F<Comm.pl> library. It uses
392pseudo-ttys to make your program behave more reasonably:
393
394 require 'Comm.pl';
395 $ph = open_proc('cat -n');
396 for (1..10) {
397 print $ph "a line\n";
398 print "got back ", scalar <$ph>;
399 }
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401This way you don't have to have control over the source code of the
402program you're using. The F<Comm> library also has expect()
403and interact() functions. Find the library (and hopefully its
404successor F<IPC::Chat>) at your nearest CPAN archive as detailed
405in the L<SEE ALSO> section below.
a0d0e21e 406
4633a7c4 407=head1 Sockets: Client/Server Communication
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409While not limited to Unix-derived operating systems (e.g. WinSock on PCs
410provides socket support, as do some VMS libraries), you may not have
411sockets on your system, in which this section probably isn't going to do
412you much good. With sockets, you can do both virtual circuits (i.e. TCP
413streams) and datagrams (i.e. UDP packets). You may be able to do even more
414depending on your system.
415
416The Perl function calls for dealing with sockets have the same names as
417the corresponding system calls in C, but their arguments tend to differ
418for two reasons: first, Perl filehandles work differently than C file
419descriptors. Second, Perl already knows the length of its strings, so you
420don't need to pass that information.
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422One of the major problems with old socket code in Perl was that it used
423hard-coded values for some of the constants, which severely hurt
424portability. If you ever see code that does anything like explicitly
425setting C<$AF_INET = 2>, you know you're in for big trouble: An
426immeasurably superior approach is to use the C<Socket> module, which more
427reliably grants access to various constants and functions you'll need.
a0d0e21e 428
4633a7c4 429=head2 Internet TCP Clients and Servers
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431Use Internet-domain sockets when you want to do client-server
432communication that might extend to machines outside of your own system.
433
434Here's a sample TCP client using Internet-domain sockets:
435
436 #!/usr/bin/perl -w
437 require 5.002;
438 use strict;
439 use Socket;
440 my ($remote,$port, $iaddr, $paddr, $proto, $line);
441
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);
448
449 $proto = getprotobyname('tcp');
450 socket(SOCK, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
451 connect(SOCK, $paddr) || die "connect: $!";
452 while ($line = <SOCK>) {
453 print $line;
454 }
455
456 close (SOCK) || die "close: $!";
457 exit;
458
459And here's a corresponding server to go along with it. We'll
460leave the address as INADDR_ANY so that the kernel can choose
461the appropriate interface on multihomed hosts:
462
463 #!/usr/bin/perl -Tw
464 require 5.002;
465 use strict;
466 BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
a0d0e21e 467 use Socket;
4633a7c4 468 use Carp;
a0d0e21e 469
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470 sub spawn; # forward declaration
471 sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
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473 my $port = shift || 2345;
474 my $proto = getprotobyname('tcp');
475 socket(SERVER, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
476 setsockopt(SERVER, SOL_SOCKET, SO_REUSEADDR, 1) || die "setsockopt: $!";
477 bind(SERVER, sockaddr_in($port, INADDR_ANY)) || die "bind: $!";
478 listen(SERVER,5) || die "listen: $!";
a0d0e21e 479
4633a7c4 480 logmsg "server started on port $port";
a0d0e21e 481
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482 my $waitedpid = 0;
483 my $paddr;
a0d0e21e 484
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485 sub REAPER {
486 $SIG{CHLD} = \&REAPER; # loathe sysV
487 $waitedpid = wait;
488 logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
489 }
490
491 $SIG{CHLD} = \&REAPER;
492
493 for ( $waitedpid = 0;
494 ($paddr = accept(CLIENT,SERVER)) || $waitedpid;
495 $waitedpid = 0, close CLIENT)
496 {
497 next if $waitedpid;
498 my($port,$iaddr) = sockaddr_in($paddr);
499 my $name = gethostbyaddr($iaddr,AF_INET);
500
501 logmsg "connection from $name [",
502 inet_ntoa($iaddr), "]
503 at port $port";
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505 spawn sub {
506 print "Hello there, $name, it's now ", scalar localtime, "\n";
507 exec '/usr/games/fortune'
508 or confess "can't exec fortune: $!";
509 };
a0d0e21e 510
4633a7c4 511 }
a0d0e21e 512
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513 sub spawn {
514 my $coderef = shift;
a0d0e21e 515
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516 unless (@_ == 0 && $coderef && ref($coderef) eq 'CODE') {
517 confess "usage: spawn CODEREF";
a0d0e21e 518 }
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519
520 my $pid;
521 if (!defined($pid = fork)) {
522 logmsg "cannot fork: $!";
523 return;
524 } elsif ($pid) {
525 logmsg "begat $pid";
526 return; # i'm the parent
527 }
528 # else i'm the child -- go spawn
529
530 open(STDIN, "<&CLIENT") || die "can't dup client to stdin";
531 open(STDOUT, ">&CLIENT") || die "can't dup client to stdout";
532 ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr";
533 exit &$coderef();
534 }
535
536This server takes the trouble to clone off a child version via fork() for
537each incoming request. That way it can handle many requests at once,
538which you might not always want. Even if you don't fork(), the listen()
539will allow that many pending connections. Forking servers have to be
540particularly careful about cleaning up their dead children (called
541"zombies" in Unix parlance), because otherwise you'll quickly fill up your
542process table.
543
544We suggest that you use the B<-T> flag to use taint checking (see L<perlsec>)
545even if we aren't running setuid or setgid. This is always a good idea
546for servers and other programs run on behalf of someone else (like CGI
547scripts), because it lessens the chances that people from the outside will
548be able to compromise your system.
549
550Let's look at another TCP client. This one connects to the TCP "time"
551service on a number of different machines and shows how far their clocks
552differ from the system on which it's being run:
553
554 #!/usr/bin/perl -w
555 require 5.002;
556 use strict;
557 use Socket;
558
559 my $SECS_of_70_YEARS = 2208988800;
560 sub ctime { scalar localtime(shift) }
561
562 my $iaddr = gethostbyname('localhost');
563 my $proto = getprotobyname('tcp');
564 my $port = getservbyname('time', 'tcp');
565 my $paddr = sockaddr_in(0, $iaddr);
566 my($host);
567
568 $| = 1;
569 printf "%-24s %8s %s\n", "localhost", 0, ctime(time());
570
571 foreach $host (@ARGV) {
572 printf "%-24s ", $host;
573 my $hisiaddr = inet_aton($host) || die "unknown host";
574 my $hispaddr = sockaddr_in($port, $hisiaddr);
575 socket(SOCKET, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
576 connect(SOCKET, $hispaddr) || die "bind: $!";
577 my $rtime = ' ';
578 read(SOCKET, $rtime, 4);
579 close(SOCKET);
580 my $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ;
581 printf "%8d %s\n", $histime - time, ctime($histime);
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582 }
583
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584=head2 Unix-Domain TCP Clients and Servers
585
586That's fine for Internet-domain clients and servers, but what local
587communications? While you can use the same setup, sometimes you don't
588want to. Unix-domain sockets are local to the current host, and are often
589used internally to implement pipes. Unlike Internet domain sockets, UNIX
590domain sockets can show up in the file system with an ls(1) listing.
591
592 $ ls -l /dev/log
593 srw-rw-rw- 1 root 0 Oct 31 07:23 /dev/log
a0d0e21e 594
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595You can test for these with Perl's B<-S> file test:
596
597 unless ( -S '/dev/log' ) {
598 die "something's wicked with the print system";
599 }
600
601Here's a sample Unix-domain client:
602
603 #!/usr/bin/perl -w
604 require 5.002;
605 use Socket;
606 use strict;
607 my ($rendezvous, $line);
608
609 $rendezvous = shift || '/tmp/catsock';
610 socket(SOCK, PF_UNIX, SOCK_STREAM, 0) || die "socket: $!";
611 connect(SOCK, sockaddr_un($remote)) || die "connect: $!";
612 while ($line = <SOCK>) {
613 print $line;
614 }
615 exit;
616
617And here's a corresponding server.
618
619 #!/usr/bin/perl -Tw
620 require 5.002;
621 use strict;
622 use Socket;
623 use Carp;
624
625 BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
626
627 my $NAME = '/tmp/catsock';
628 my $uaddr = sockaddr_un($NAME);
629 my $proto = getprotobyname('tcp');
630
631 socket(SERVER,PF_UNIX,SOCK_STREAM,0) || die "socket: $!";
632 unlink($NAME);
633 bind (SERVER, $uaddr) || die "bind: $!";
634 listen(SERVER,5) || die "listen: $!";
635
636 logmsg "server started on $NAME";
637
638 $SIG{CHLD} = \&REAPER;
639
640 for ( $waitedpid = 0;
641 accept(CLIENT,SERVER) || $waitedpid;
642 $waitedpid = 0, close CLIENT)
643 {
644 next if $waitedpid;
645 logmsg "connection on $NAME";
646 spawn sub {
647 print "Hello there, it's now ", scalar localtime, "\n";
648 exec '/usr/games/fortune' or die "can't exec fortune: $!";
649 };
650 }
651
652As you see, it's remarkably similar to the Internet domain TCP server, so
653much so, in fact, that we've omitted several duplicate functions--spawn(),
654logmsg(), ctime(), and REAPER()--which are exactly the same as in the
655other server.
656
657So why would you ever want to use a Unix domain socket instead of a
658simpler named pipe? Because a named pipe doesn't give you sessions. You
659can't tell one process's data from another's. With socket programming,
660you get a separate session for each client: that's why accept() takes two
661arguments.
662
663For example, let's say that you have a long running database server daemon
664that you want folks from the World Wide Web to be able to access, but only
665if they go through a CGI interface. You'd have a small, simple CGI
666program that does whatever checks and logging you feel like, and then acts
667as a Unix-domain client and connects to your private server.
668
669=head2 UDP: Message Passing
670
671Another kind of client-server setup is one that uses not connections, but
672messages. UDP communications involve much lower overhead but also provide
673less reliability, as there are no promises that messages will arrive at
674all, let alone in order and unmangled. Still, UDP offers some advantages
675over TCP, including being able to "broadcast" or "multicast" to a whole
676bunch of destination hosts at once (usually on your local subnet). If you
677find yourself overly concerned about reliability and start building checks
678into your message system, then you probably should just use TCP to start
679with.
680
681Here's a UDP program similar to the sample Internet TCP client given
682above. However, instead of checking one host at a time, the UDP version
683will check many of them asynchronously by simulating a multicast and then
684using select() to do a timed-out wait for I/O. To do something similar
685with TCP, you'd have to use a different socket handle for each host.
686
687 #!/usr/bin/perl -w
688 use strict;
689 require 5.002;
690 use Socket;
691 use Sys::Hostname;
692
693 my ( $count, $hisiaddr, $hispaddr, $histime,
694 $host, $iaddr, $paddr, $port, $proto,
695 $rin, $rout, $rtime, $SECS_of_70_YEARS);
696
697 $SECS_of_70_YEARS = 2208988800;
698
699 $iaddr = gethostbyname(hostname());
700 $proto = getprotobyname('udp');
701 $port = getservbyname('time', 'udp');
702 $paddr = sockaddr_in(0, $iaddr); # 0 means let kernel pick
703
704 socket(SOCKET, PF_INET, SOCK_DGRAM, $proto) || die "socket: $!";
705 bind(SOCKET, $paddr) || die "bind: $!";
706
707 $| = 1;
708 printf "%-12s %8s %s\n", "localhost", 0, scalar localtime time;
709 $count = 0;
710 for $host (@ARGV) {
711 $count++;
712 $hisiaddr = inet_aton($host) || die "unknown host";
713 $hispaddr = sockaddr_in($port, $hisiaddr);
714 defined(send(SOCKET, 0, 0, $hispaddr)) || die "send $host: $!";
715 }
716
717 $rin = '';
718 vec($rin, fileno(SOCKET), 1) = 1;
719
720 # timeout after 10.0 seconds
721 while ($count && select($rout = $rin, undef, undef, 10.0)) {
722 $rtime = '';
723 ($hispaddr = recv(SOCKET, $rtime, 4, 0)) || die "recv: $!";
724 ($port, $hisiaddr) = sockaddr_in($hispaddr);
725 $host = gethostbyaddr($hisiaddr, AF_INET);
726 $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ;
727 printf "%-12s ", $host;
728 printf "%8d %s\n", $histime - time, scalar localtime($histime);
729 $count--;
730 }
731
732=head1 SysV IPC
733
734While System V IPC isn't so widely used as sockets, it still has some
735interesting uses. You can't, however, effectively use SysV IPC or
736Berkeley mmap() to have shared memory so as to share a variable amongst
737several processes. That's because Perl would reallocate your string when
738you weren't wanting it to.
739
740
741Here's a small example showing shared memory usage.
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742
743 $IPC_PRIVATE = 0;
744 $IPC_RMID = 0;
745 $size = 2000;
746 $key = shmget($IPC_PRIVATE, $size , 0777 );
4633a7c4 747 die unless defined $key;
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748
749 $message = "Message #1";
750 shmwrite($key, $message, 0, 60 ) || die "$!";
751 shmread($key,$buff,0,60) || die "$!";
752
753 print $buff,"\n";
754
755 print "deleting $key\n";
756 shmctl($key ,$IPC_RMID, 0) || die "$!";
757
758Here's an example of a semaphore:
759
760 $IPC_KEY = 1234;
761 $IPC_RMID = 0;
762 $IPC_CREATE = 0001000;
763 $key = semget($IPC_KEY, $nsems , 0666 | $IPC_CREATE );
764 die if !defined($key);
765 print "$key\n";
766
767Put this code in a separate file to be run in more that one process
768Call the file F<take>:
769
770 # create a semaphore
771
772 $IPC_KEY = 1234;
773 $key = semget($IPC_KEY, 0 , 0 );
774 die if !defined($key);
775
776 $semnum = 0;
777 $semflag = 0;
778
779 # 'take' semaphore
780 # wait for semaphore to be zero
781 $semop = 0;
782 $opstring1 = pack("sss", $semnum, $semop, $semflag);
783
784 # Increment the semaphore count
785 $semop = 1;
786 $opstring2 = pack("sss", $semnum, $semop, $semflag);
787 $opstring = $opstring1 . $opstring2;
788
789 semop($key,$opstring) || die "$!";
790
791Put this code in a separate file to be run in more that one process
792Call this file F<give>:
793
4633a7c4 794 # 'give' the semaphore
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795 # run this in the original process and you will see
796 # that the second process continues
797
798 $IPC_KEY = 1234;
799 $key = semget($IPC_KEY, 0, 0);
800 die if !defined($key);
801
802 $semnum = 0;
803 $semflag = 0;
804
805 # Decrement the semaphore count
806 $semop = -1;
807 $opstring = pack("sss", $semnum, $semop, $semflag);
808
809 semop($key,$opstring) || die "$!";
810
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811=head1 WARNING
812
813The SysV IPC code above was written long ago, and it's definitely clunky
814looking. It should at the very least be made to C<use strict> and
815C<require "sys/ipc.ph">. Better yet, perhaps someone should create an
816C<IPC::SysV> module the way we have the C<Socket> module for normal
817client-server communications.
818
819(... time passes)
820
821Voila! Check out the IPC::SysV modules written by Jack Shirazi. You can
822find them at a CPAN store near you.
823
824=head1 NOTES
825
826If you are running under version 5.000 (dubious) or 5.001, you can still
827use most of the examples in this document. You may have to remove the
828C<use strict> and some of the my() statements for 5.000, and for both
829you'll have to load in version 1.2 of the F<Socket.pm> module, which
830was/is/shall-be included in I<perl5.001o>.
831
832Most of these routines quietly but politely return C<undef> when they fail
833instead of causing your program to die right then and there due to an
834uncaught exception. (Actually, some of the new I<Socket> conversion
835functions croak() on bad arguments.) It is therefore essential
836that you should check the return values fo these functions. Always begin
837your socket programs this way for optimal success, and don't forget to add
838B<-T> taint checking flag to the pound-bang line for servers:
839
840 #!/usr/bin/perl -w
841 require 5.002;
842 use strict;
843 use sigtrap;
844 use Socket;
845
846=head1 BUGS
847
848All these routines create system-specific portability problems. As noted
849elsewhere, Perl is at the mercy of your C libraries for much of its system
850behaviour. It's probably safest to assume broken SysV semantics for
851signals and to stick with simple TCP and UDP socket operations; e.g. don't
852try to pass open filedescriptors over a local UDP datagram socket if you
853want your code to stand a chance of being portable.
854
855Because few vendors provide C libraries that are safely
856re-entrant, the prudent programmer will do little else within
857a handler beyond die() to raise an exception and longjmp(3) out.
858
859=head1 AUTHOR
860
861Tom Christiansen, with occasional vestiges of Larry Wall's original
862version.
863
864=head1 SEE ALSO
865
866Besides the obvious functions in L<perlfunc>, you should also check out
867the F<modules> file at your nearest CPAN site. (See L<perlmod> or best
868yet, the F<Perl FAQ> for a description of what CPAN is and where to get it.)
869Section 5 of the F<modules> file is devoted to "Networking, Device Control
870(modems) and Interprocess Communication", and contains numerous unbundled
871modules numerous networking modules, Chat and Expect operations, CGI
872programming, DCE, FTP, IPC, NNTP, Proxy, Ptty, RPC, SNMP, SMTP, Telnet,
873Threads, and ToolTalk--just to name a few.