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1=head1 NAME
2
184e9718 3perlipc - Perl interprocess communication (signals, fifos, pipes, safe subprocesses, 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
a2eb9003 17control-C or control-Z, sent to you from another process, or
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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.
a2eb9003 23Notice how all we do is set a global variable and then raise an
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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
6a3992aa 50So to check whether signal 17 and SIGALRM were the same, do just this:
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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 }
a0d0e21e 91
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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
184e9718 99based on the signal(3) function from the C library, you may sometimes be so
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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:
a0d0e21e 104
4633a7c4 105 sub REAPER {
4633a7c4 106 $waitedpid = wait;
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107 # loathe sysV: it makes us not only reinstate
108 # the handler, but place it after the wait
109 $SIG{CHLD} = \&REAPER;
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110 }
111 $SIG{CHLD} = \&REAPER;
112 # now do something that forks...
113
114or even the more elaborate:
115
6a3992aa 116 use POSIX ":sys_wait_h";
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117 sub REAPER {
118 my $child;
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119 while ($child = waitpid(-1,WNOHANG)) {
120 $Kid_Status{$child} = $?;
121 }
6a3992aa 122 $SIG{CHLD} = \&REAPER; # still loathe sysV
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123 }
124 $SIG{CHLD} = \&REAPER;
125 # do something that forks...
126
127Signal handling is also used for timeouts in Unix, While safely
128protected within an C<eval{}> block, you set a signal handler to trap
129alarm signals and then schedule to have one delivered to you in some
130number of seconds. Then try your blocking operation, clearing the alarm
131when it's done but not before you've exited your C<eval{}> block. If it
132goes off, you'll use die() to jump out of the block, much as you might
133using longjmp() or throw() in other languages.
134
135Here's an example:
136
137 eval {
138 local $SIG{ALRM} = sub { die "alarm clock restart" };
139 alarm 10;
140 flock(FH, 2); # blocking write lock
141 alarm 0;
142 };
143 if ($@ and $@ !~ /alarm clock restart/) { die }
144
145For more complex signal handling, you might see the standard POSIX
146module. Lamentably, this is almost entirely undocumented, but
147the F<t/lib/posix.t> file from the Perl source distribution has some
148examples in it.
149
150=head1 Named Pipes
151
152A named pipe (often referred to as a FIFO) is an old Unix IPC
153mechanism for processes communicating on the same machine. It works
154just like a regular, connected anonymous pipes, except that the
155processes rendezvous using a filename and don't have to be related.
156
157To create a named pipe, use the Unix command mknod(1) or on some
158systems, mkfifo(1). These may not be in your normal path.
159
160 # system return val is backwards, so && not ||
161 #
162 $ENV{PATH} .= ":/etc:/usr/etc";
163 if ( system('mknod', $path, 'p')
164 && system('mkfifo', $path) )
165 {
166 die "mk{nod,fifo} $path failed;
167 }
168
169
170A fifo is convenient when you want to connect a process to an unrelated
171one. When you open a fifo, the program will block until there's something
172on the other end.
173
174For example, let's say you'd like to have your F<.signature> file be a
175named pipe that has a Perl program on the other end. Now every time any
6a3992aa 176program (like a mailer, news reader, finger program, etc.) tries to read
4633a7c4 177from that file, the reading program will block and your program will
6a3992aa 178supply the new signature. We'll use the pipe-checking file test B<-p>
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179to find out whether anyone (or anything) has accidentally removed our fifo.
180
181 chdir; # go home
182 $FIFO = '.signature';
183 $ENV{PATH} .= ":/etc:/usr/games";
184
185 while (1) {
186 unless (-p $FIFO) {
187 unlink $FIFO;
188 system('mknod', $FIFO, 'p')
189 && die "can't mknod $FIFO: $!";
190 }
191
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`;
195 close FIFO;
6a3992aa 196 sleep 2; # to avoid dup signals
4633a7c4 197 }
a0d0e21e 198
a0d0e21e 199
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200=head1 Using open() for IPC
201
202Perl's basic open() statement can also be used for unidirectional interprocess
203communication by either appending or prepending a pipe symbol to the second
a2eb9003 204argument to open(). Here's how to start something up in a child process you
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205intend to write to:
206
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: $! $?";
212
213And here's how to start up a child process you intend to read from:
214
215 open(STATUS, "netstat -an 2>&1 |")
216 || die "can't fork: $!";
217 while (<STATUS>) {
218 next if /^(tcp|udp)/;
219 print;
220 }
a2eb9003 221 close STATUS || die "bad netstat: $! $?";
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222
223If one can be sure that a particular program is a Perl script that is
224expecting filenames in @ARGV, the clever programmer can write something
225like this:
226
227 $ program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile
228
229and irrespective of which shell it's called from, the Perl program will
230read from the file F<f1>, the process F<cmd1>, standard input (F<tmpfile>
231in this case), the F<f2> file, the F<cmd2> command, and finally the F<f3>
232file. Pretty nifty, eh?
233
6a3992aa 234You might notice that you could use back-ticks for much the
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235same effect as opening a pipe for reading:
236
237 print grep { !/^(tcp|udp)/ } `netstat -an 2>&1`;
238 die "bad netstat" if $?;
239
240While this is true on the surface, it's much more efficient to process the
241file one line or record at a time because then you don't have to read the
242whole thing into memory at once. It also gives you finer control of the
243whole process, letting you to kill off the child process early if you'd
244like.
245
246Be careful to check both the open() and the close() return values. If
247you're I<writing> to a pipe, you should also trap SIGPIPE. Otherwise,
248think of what happens when you start up a pipe to a command that doesn't
249exist: the open() will in all likelihood succeed (it only reflects the
250fork()'s success), but then your output will fail--spectacularly. Perl
251can't know whether the command worked because your command is actually
252running in a separate process whose exec() might have failed. Therefore,
6a3992aa 253while readers of bogus commands return just a quick end of file, writers
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254to bogus command will trigger a signal they'd better be prepared to
255handle. Consider:
256
257 open(FH, "|bogus");
258 print FH "bang\n";
259 close FH;
260
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261=head2 Filehandles
262
263Both the main process and the child process share the same STDIN,
264STDOUT and STDERR filehandles. If both processes try to access them
265at once, strange things can happen. You may want to close or reopen
266the filehandles for the child. You can get around this by opening
267your pipe with open(), but on some systems this means that the child
268process cannot outlive the parent.
269
270=head2 Background Processes
271
272You can run a command in the background with:
273
274 system("cmd&");
275
276The command's STDOUT and STDERR (and possibly STDIN, depending on your
277shell) will be the same as the parent's. You won't need to catch
278SIGCHLD because of the double-fork taking place (see below for more
279details).
280
281=head2 Complete Dissociation of Child from Parent
282
283In some cases (starting server processes, for instance) you'll want to
284complete dissociate the child process from the parent. The following
285process is reported to work on most Unixish systems. Non-Unix users
286should check their Your_OS::Process module for other solutions.
287
288=over 4
289
290=item *
291
292Open /dev/tty and use the the TIOCNOTTY ioctl on it. See L<tty(4)>
293for details.
294
295=item *
296
297Change directory to /
298
299=item *
300
301Reopen STDIN, STDOUT, and STDERR so they're not connected to the old
302tty.
303
304=item *
305
306Background yourself like this:
307
308 fork && exit;
309
310=back
311
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312=head2 Safe Pipe Opens
313
314Another interesting approach to IPC is making your single program go
315multiprocess and communicate between (or even amongst) yourselves. The
316open() function will accept a file argument of either C<"-|"> or C<"|-">
317to do a very interesting thing: it forks a child connected to the
318filehandle you've opened. The child is running the same program as the
319parent. This is useful for safely opening a file when running under an
320assumed UID or GID, for example. If you open a pipe I<to> minus, you can
321write to the filehandle you opened and your kid will find it in his
322STDIN. If you open a pipe I<from> minus, you can read from the filehandle
323you opened whatever your kid writes to his STDOUT.
324
325 use English;
326 my $sleep_count = 0;
327
328 do {
c07a80fd 329 $pid = open(KID_TO_WRITE, "|-");
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330 unless (defined $pid) {
331 warn "cannot fork: $!";
332 die "bailing out" if $sleep_count++ > 6;
333 sleep 10;
334 }
335 } until defined $pid;
336
337 if ($pid) { # parent
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338 print KID_TO_WRITE @some_data;
339 close(KID_TO_WRITE) || warn "kid exited $?";
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340 } else { # child
341 ($EUID, $EGID) = ($UID, $GID); # suid progs only
342 open (FILE, "> /safe/file")
343 || die "can't open /safe/file: $!";
344 while (<STDIN>) {
345 print FILE; # child's STDIN is parent's KID
346 }
347 exit; # don't forget this
348 }
349
350Another common use for this construct is when you need to execute
351something without the shell's interference. With system(), it's
6a3992aa 352straightforward, but you can't use a pipe open or back-ticks safely.
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353That's because there's no way to stop the shell from getting its hands on
354your arguments. Instead, use lower-level control to call exec() directly.
355
6a3992aa 356Here's a safe back-tick or pipe open for read:
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357
358 # add error processing as above
c07a80fd 359 $pid = open(KID_TO_READ, "-|");
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360
361 if ($pid) { # parent
c07a80fd 362 while (<KID_TO_READ>) {
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363 # do something interesting
364 }
c07a80fd 365 close(KID_TO_READ) || warn "kid exited $?";
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366
367 } else { # child
368 ($EUID, $EGID) = ($UID, $GID); # suid only
369 exec($program, @options, @args)
370 || die "can't exec program: $!";
371 # NOTREACHED
372 }
373
374
375And here's a safe pipe open for writing:
376
377 # add error processing as above
c07a80fd 378 $pid = open(KID_TO_WRITE, "|-");
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379 $SIG{ALRM} = sub { die "whoops, $program pipe broke" };
380
381 if ($pid) { # parent
382 for (@data) {
c07a80fd 383 print KID_TO_WRITE;
4633a7c4 384 }
c07a80fd 385 close(KID_TO_WRITE) || warn "kid exited $?";
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386
387 } else { # child
388 ($EUID, $EGID) = ($UID, $GID);
389 exec($program, @options, @args)
390 || die "can't exec program: $!";
391 # NOTREACHED
392 }
393
394Note that these operations are full Unix forks, which means they may not be
395correctly implemented on alien systems. Additionally, these are not true
6a3992aa 396multi-threading. If you'd like to learn more about threading, see the
184e9718 397F<modules> file mentioned below in the SEE ALSO section.
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398
399=head2 Bidirectional Communication
400
401While this works reasonably well for unidirectional communication, what
402about bidirectional communication? The obvious thing you'd like to do
403doesn't actually work:
404
c07a80fd 405 open(PROG_FOR_READING_AND_WRITING, "| some program |")
4633a7c4 406
c07a80fd 407and if you forget to use the B<-w> flag, then you'll miss out
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408entirely on the diagnostic message:
409
410 Can't do bidirectional pipe at -e line 1.
411
412If you really want to, you can use the standard open2() library function
6a3992aa 413to catch both ends. There's also an open3() for tri-directional I/O so you
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414can also catch your child's STDERR, but doing so would then require an
415awkward select() loop and wouldn't allow you to use normal Perl input
416operations.
417
418If you look at its source, you'll see that open2() uses low-level
419primitives like Unix pipe() and exec() to create all the connections.
420While it might have been slightly more efficient by using socketpair(), it
421would have then been even less portable than it already is. The open2()
422and open3() functions are unlikely to work anywhere except on a Unix
423system or some other one purporting to be POSIX compliant.
424
425Here's an example of using open2():
426
427 use FileHandle;
428 use IPC::Open2;
429 $pid = open2( \*Reader, \*Writer, "cat -u -n" );
430 Writer->autoflush(); # default here, actually
431 print Writer "stuff\n";
432 $got = <Reader>;
433
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434The problem with this is that Unix buffering is really going to
435ruin your day. Even though your C<Writer> filehandle is auto-flushed,
4633a7c4 436and the process on the other end will get your data in a timely manner,
6a3992aa 437you can't usually do anything to force it to give it back to you
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438in a similarly quick fashion. In this case, we could, because we
439gave I<cat> a B<-u> flag to make it unbuffered. But very few Unix
440commands are designed to operate over pipes, so this seldom works
441unless you yourself wrote the program on the other end of the
442double-ended pipe.
443
444A solution to this is the non-standard F<Comm.pl> library. It uses
445pseudo-ttys to make your program behave more reasonably:
446
447 require 'Comm.pl';
448 $ph = open_proc('cat -n');
449 for (1..10) {
450 print $ph "a line\n";
451 print "got back ", scalar <$ph>;
452 }
a0d0e21e 453
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454This way you don't have to have control over the source code of the
455program you're using. The F<Comm> library also has expect()
6a3992aa 456and interact() functions. Find the library (and we hope its
4633a7c4 457successor F<IPC::Chat>) at your nearest CPAN archive as detailed
184e9718 458in the SEE ALSO section below.
a0d0e21e 459
4633a7c4 460=head1 Sockets: Client/Server Communication
a0d0e21e 461
6a3992aa 462While not limited to Unix-derived operating systems (e.g., WinSock on PCs
4633a7c4 463provides socket support, as do some VMS libraries), you may not have
184e9718 464sockets on your system, in which case this section probably isn't going to do
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465you much good. With sockets, you can do both virtual circuits (i.e., TCP
466streams) and datagrams (i.e., UDP packets). You may be able to do even more
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467depending on your system.
468
469The Perl function calls for dealing with sockets have the same names as
470the corresponding system calls in C, but their arguments tend to differ
471for two reasons: first, Perl filehandles work differently than C file
472descriptors. Second, Perl already knows the length of its strings, so you
473don't need to pass that information.
a0d0e21e 474
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475One of the major problems with old socket code in Perl was that it used
476hard-coded values for some of the constants, which severely hurt
477portability. If you ever see code that does anything like explicitly
478setting C<$AF_INET = 2>, you know you're in for big trouble: An
479immeasurably superior approach is to use the C<Socket> module, which more
480reliably grants access to various constants and functions you'll need.
a0d0e21e 481
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482If you're not writing a server/client for an existing protocol like
483NNTP or SMTP, you should give some thought to how your server will
484know when the client has finished talking, and vice-versa. Most
485protocols are based on one-line messages and responses (so one party
486knows the other has finished when a "\n" is received) or multiline
487messages and responses that end with a period on an empty line
488("\n.\n" terminates a message/response).
489
4633a7c4 490=head2 Internet TCP Clients and Servers
a0d0e21e 491
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492Use Internet-domain sockets when you want to do client-server
493communication that might extend to machines outside of your own system.
494
495Here's a sample TCP client using Internet-domain sockets:
496
497 #!/usr/bin/perl -w
498 require 5.002;
499 use strict;
500 use Socket;
501 my ($remote,$port, $iaddr, $paddr, $proto, $line);
502
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);
509
510 $proto = getprotobyname('tcp');
511 socket(SOCK, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
512 connect(SOCK, $paddr) || die "connect: $!";
513 while ($line = <SOCK>) {
514 print $line;
515 }
516
517 close (SOCK) || die "close: $!";
518 exit;
519
520And here's a corresponding server to go along with it. We'll
521leave the address as INADDR_ANY so that the kernel can choose
6a3992aa 522the appropriate interface on multi-homed hosts. If you want sit
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523on a particular interface (like the external side of a gateway
524or firewall machine), you should fill this in with your real address
525instead.
526
527 #!/usr/bin/perl -Tw
528 require 5.002;
529 use strict;
530 BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
531 use Socket;
532 use Carp;
533
534 sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
535
536 my $port = shift || 2345;
537 my $proto = getprotobyname('tcp');
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538 $port = $1 if $port =~ /(\d+)/; # untaint port number
539
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540 socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
541 setsockopt(Server, SOL_SOCKET, SO_REUSEADDR,
542 pack("l", 1)) || die "setsockopt: $!";
543 bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!";
544 listen(Server,SOMAXCONN) || die "listen: $!";
545
546 logmsg "server started on port $port";
547
548 my $paddr;
549
550 $SIG{CHLD} = \&REAPER;
551
552 for ( ; $paddr = accept(Client,Server); close Client) {
553 my($port,$iaddr) = sockaddr_in($paddr);
554 my $name = gethostbyaddr($iaddr,AF_INET);
555
556 logmsg "connection from $name [",
557 inet_ntoa($iaddr), "]
558 at port $port";
559
80aa6872 560 print Client "Hello there, $name, it's now ",
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561 scalar localtime, "\n";
562 }
563
6a3992aa 564And here's a multi-threaded version. It's multi-threaded in that
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565like most typical servers, it spawns (forks) a slave server to
566handle the client request so that the master server can quickly
567go back to service a new client.
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568
569 #!/usr/bin/perl -Tw
570 require 5.002;
571 use strict;
572 BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
a0d0e21e 573 use Socket;
4633a7c4 574 use Carp;
a0d0e21e 575
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576 sub spawn; # forward declaration
577 sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
a0d0e21e 578
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579 my $port = shift || 2345;
580 my $proto = getprotobyname('tcp');
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581 $port = $1 if $port =~ /(\d+)/; # untaint port number
582
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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: $!";
a0d0e21e 588
4633a7c4 589 logmsg "server started on port $port";
a0d0e21e 590
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591 my $waitedpid = 0;
592 my $paddr;
a0d0e21e 593
4633a7c4 594 sub REAPER {
4633a7c4 595 $waitedpid = wait;
6a3992aa 596 $SIG{CHLD} = \&REAPER; # loathe sysV
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597 logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
598 }
599
600 $SIG{CHLD} = \&REAPER;
601
602 for ( $waitedpid = 0;
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603 ($paddr = accept(Client,Server)) || $waitedpid;
604 $waitedpid = 0, close Client)
4633a7c4 605 {
6a3992aa 606 next if $waitedpid and not $paddr;
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607 my($port,$iaddr) = sockaddr_in($paddr);
608 my $name = gethostbyaddr($iaddr,AF_INET);
609
610 logmsg "connection from $name [",
611 inet_ntoa($iaddr), "]
612 at port $port";
a0d0e21e 613
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614 spawn sub {
615 print "Hello there, $name, it's now ", scalar localtime, "\n";
616 exec '/usr/games/fortune'
617 or confess "can't exec fortune: $!";
618 };
a0d0e21e 619
4633a7c4 620 }
a0d0e21e 621
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622 sub spawn {
623 my $coderef = shift;
a0d0e21e 624
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625 unless (@_ == 0 && $coderef && ref($coderef) eq 'CODE') {
626 confess "usage: spawn CODEREF";
a0d0e21e 627 }
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628
629 my $pid;
630 if (!defined($pid = fork)) {
631 logmsg "cannot fork: $!";
632 return;
633 } elsif ($pid) {
634 logmsg "begat $pid";
6a3992aa 635 return; # I'm the parent
4633a7c4 636 }
6a3992aa 637 # else I'm the child -- go spawn
4633a7c4 638
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639 open(STDIN, "<&Client") || die "can't dup client to stdin";
640 open(STDOUT, ">&Client") || die "can't dup client to stdout";
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641 ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr";
642 exit &$coderef();
643 }
644
645This server takes the trouble to clone off a child version via fork() for
646each incoming request. That way it can handle many requests at once,
647which you might not always want. Even if you don't fork(), the listen()
648will allow that many pending connections. Forking servers have to be
649particularly careful about cleaning up their dead children (called
650"zombies" in Unix parlance), because otherwise you'll quickly fill up your
651process table.
652
653We suggest that you use the B<-T> flag to use taint checking (see L<perlsec>)
654even if we aren't running setuid or setgid. This is always a good idea
655for servers and other programs run on behalf of someone else (like CGI
656scripts), because it lessens the chances that people from the outside will
657be able to compromise your system.
658
659Let's look at another TCP client. This one connects to the TCP "time"
660service on a number of different machines and shows how far their clocks
661differ from the system on which it's being run:
662
663 #!/usr/bin/perl -w
664 require 5.002;
665 use strict;
666 use Socket;
667
668 my $SECS_of_70_YEARS = 2208988800;
669 sub ctime { scalar localtime(shift) }
670
671 my $iaddr = gethostbyname('localhost');
672 my $proto = getprotobyname('tcp');
673 my $port = getservbyname('time', 'tcp');
674 my $paddr = sockaddr_in(0, $iaddr);
675 my($host);
676
677 $| = 1;
678 printf "%-24s %8s %s\n", "localhost", 0, ctime(time());
679
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: $!";
686 my $rtime = ' ';
687 read(SOCKET, $rtime, 4);
688 close(SOCKET);
689 my $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ;
690 printf "%8d %s\n", $histime - time, ctime($histime);
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691 }
692
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693=head2 Unix-Domain TCP Clients and Servers
694
a2eb9003 695That's fine for Internet-domain clients and servers, but what about local
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696communications? While you can use the same setup, sometimes you don't
697want to. Unix-domain sockets are local to the current host, and are often
698used internally to implement pipes. Unlike Internet domain sockets, UNIX
699domain sockets can show up in the file system with an ls(1) listing.
700
701 $ ls -l /dev/log
702 srw-rw-rw- 1 root 0 Oct 31 07:23 /dev/log
a0d0e21e 703
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704You can test for these with Perl's B<-S> file test:
705
706 unless ( -S '/dev/log' ) {
707 die "something's wicked with the print system";
708 }
709
710Here's a sample Unix-domain client:
711
712 #!/usr/bin/perl -w
713 require 5.002;
714 use Socket;
715 use strict;
716 my ($rendezvous, $line);
717
718 $rendezvous = shift || '/tmp/catsock';
719 socket(SOCK, PF_UNIX, SOCK_STREAM, 0) || die "socket: $!";
9607fc9c 720 connect(SOCK, sockaddr_un($rendezvous)) || die "connect: $!";
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721 while ($line = <SOCK>) {
722 print $line;
723 }
724 exit;
725
726And here's a corresponding server.
727
728 #!/usr/bin/perl -Tw
729 require 5.002;
730 use strict;
731 use Socket;
732 use Carp;
733
734 BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
735
736 my $NAME = '/tmp/catsock';
737 my $uaddr = sockaddr_un($NAME);
738 my $proto = getprotobyname('tcp');
739
c07a80fd 740 socket(Server,PF_UNIX,SOCK_STREAM,0) || die "socket: $!";
4633a7c4 741 unlink($NAME);
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742 bind (Server, $uaddr) || die "bind: $!";
743 listen(Server,SOMAXCONN) || die "listen: $!";
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744
745 logmsg "server started on $NAME";
746
747 $SIG{CHLD} = \&REAPER;
748
749 for ( $waitedpid = 0;
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750 accept(Client,Server) || $waitedpid;
751 $waitedpid = 0, close Client)
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752 {
753 next if $waitedpid;
754 logmsg "connection on $NAME";
755 spawn sub {
756 print "Hello there, it's now ", scalar localtime, "\n";
757 exec '/usr/games/fortune' or die "can't exec fortune: $!";
758 };
759 }
760
761As you see, it's remarkably similar to the Internet domain TCP server, so
762much so, in fact, that we've omitted several duplicate functions--spawn(),
763logmsg(), ctime(), and REAPER()--which are exactly the same as in the
764other server.
765
766So why would you ever want to use a Unix domain socket instead of a
767simpler named pipe? Because a named pipe doesn't give you sessions. You
768can't tell one process's data from another's. With socket programming,
769you get a separate session for each client: that's why accept() takes two
770arguments.
771
772For example, let's say that you have a long running database server daemon
773that you want folks from the World Wide Web to be able to access, but only
774if they go through a CGI interface. You'd have a small, simple CGI
775program that does whatever checks and logging you feel like, and then acts
776as a Unix-domain client and connects to your private server.
777
778=head2 UDP: Message Passing
779
780Another kind of client-server setup is one that uses not connections, but
781messages. UDP communications involve much lower overhead but also provide
782less reliability, as there are no promises that messages will arrive at
783all, let alone in order and unmangled. Still, UDP offers some advantages
784over TCP, including being able to "broadcast" or "multicast" to a whole
785bunch of destination hosts at once (usually on your local subnet). If you
786find yourself overly concerned about reliability and start building checks
6a3992aa 787into your message system, then you probably should use just TCP to start
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788with.
789
790Here's a UDP program similar to the sample Internet TCP client given
791above. However, instead of checking one host at a time, the UDP version
792will check many of them asynchronously by simulating a multicast and then
793using select() to do a timed-out wait for I/O. To do something similar
794with TCP, you'd have to use a different socket handle for each host.
795
796 #!/usr/bin/perl -w
797 use strict;
798 require 5.002;
799 use Socket;
800 use Sys::Hostname;
801
802 my ( $count, $hisiaddr, $hispaddr, $histime,
803 $host, $iaddr, $paddr, $port, $proto,
804 $rin, $rout, $rtime, $SECS_of_70_YEARS);
805
806 $SECS_of_70_YEARS = 2208988800;
807
808 $iaddr = gethostbyname(hostname());
809 $proto = getprotobyname('udp');
810 $port = getservbyname('time', 'udp');
811 $paddr = sockaddr_in(0, $iaddr); # 0 means let kernel pick
812
813 socket(SOCKET, PF_INET, SOCK_DGRAM, $proto) || die "socket: $!";
814 bind(SOCKET, $paddr) || die "bind: $!";
815
816 $| = 1;
817 printf "%-12s %8s %s\n", "localhost", 0, scalar localtime time;
818 $count = 0;
819 for $host (@ARGV) {
820 $count++;
821 $hisiaddr = inet_aton($host) || die "unknown host";
822 $hispaddr = sockaddr_in($port, $hisiaddr);
823 defined(send(SOCKET, 0, 0, $hispaddr)) || die "send $host: $!";
824 }
825
826 $rin = '';
827 vec($rin, fileno(SOCKET), 1) = 1;
828
829 # timeout after 10.0 seconds
830 while ($count && select($rout = $rin, undef, undef, 10.0)) {
831 $rtime = '';
832 ($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);
838 $count--;
839 }
840
841=head1 SysV IPC
842
843While System V IPC isn't so widely used as sockets, it still has some
844interesting uses. You can't, however, effectively use SysV IPC or
845Berkeley mmap() to have shared memory so as to share a variable amongst
846several processes. That's because Perl would reallocate your string when
847you weren't wanting it to.
848
849
850Here's a small example showing shared memory usage.
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851
852 $IPC_PRIVATE = 0;
853 $IPC_RMID = 0;
854 $size = 2000;
855 $key = shmget($IPC_PRIVATE, $size , 0777 );
4633a7c4 856 die unless defined $key;
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857
858 $message = "Message #1";
859 shmwrite($key, $message, 0, 60 ) || die "$!";
860 shmread($key,$buff,0,60) || die "$!";
861
862 print $buff,"\n";
863
864 print "deleting $key\n";
865 shmctl($key ,$IPC_RMID, 0) || die "$!";
866
867Here's an example of a semaphore:
868
869 $IPC_KEY = 1234;
870 $IPC_RMID = 0;
871 $IPC_CREATE = 0001000;
872 $key = semget($IPC_KEY, $nsems , 0666 | $IPC_CREATE );
873 die if !defined($key);
874 print "$key\n";
875
a2eb9003 876Put this code in a separate file to be run in more than one process.
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877Call the file F<take>:
878
879 # create a semaphore
880
881 $IPC_KEY = 1234;
882 $key = semget($IPC_KEY, 0 , 0 );
883 die if !defined($key);
884
885 $semnum = 0;
886 $semflag = 0;
887
888 # 'take' semaphore
889 # wait for semaphore to be zero
890 $semop = 0;
891 $opstring1 = pack("sss", $semnum, $semop, $semflag);
892
893 # Increment the semaphore count
894 $semop = 1;
895 $opstring2 = pack("sss", $semnum, $semop, $semflag);
896 $opstring = $opstring1 . $opstring2;
897
898 semop($key,$opstring) || die "$!";
899
a2eb9003 900Put this code in a separate file to be run in more than one process.
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901Call this file F<give>:
902
4633a7c4 903 # 'give' the semaphore
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904 # run this in the original process and you will see
905 # that the second process continues
906
907 $IPC_KEY = 1234;
908 $key = semget($IPC_KEY, 0, 0);
909 die if !defined($key);
910
911 $semnum = 0;
912 $semflag = 0;
913
914 # Decrement the semaphore count
915 $semop = -1;
916 $opstring = pack("sss", $semnum, $semop, $semflag);
917
918 semop($key,$opstring) || die "$!";
919
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920=head1 WARNING
921
922The SysV IPC code above was written long ago, and it's definitely clunky
923looking. It should at the very least be made to C<use strict> and
924C<require "sys/ipc.ph">. Better yet, perhaps someone should create an
925C<IPC::SysV> module the way we have the C<Socket> module for normal
926client-server communications.
927
928(... time passes)
929
930Voila! Check out the IPC::SysV modules written by Jack Shirazi. You can
931find them at a CPAN store near you.
932
933=head1 NOTES
934
935If you are running under version 5.000 (dubious) or 5.001, you can still
936use most of the examples in this document. You may have to remove the
937C<use strict> and some of the my() statements for 5.000, and for both
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938you'll have to load in version 1.2 or older of the F<Socket.pm> module, which
939is included in I<perl5.002>.
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940
941Most of these routines quietly but politely return C<undef> when they fail
942instead of causing your program to die right then and there due to an
943uncaught exception. (Actually, some of the new I<Socket> conversion
944functions croak() on bad arguments.) It is therefore essential
a2eb9003 945that you should check the return values of these functions. Always begin
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946your socket programs this way for optimal success, and don't forget to add
947B<-T> taint checking flag to the pound-bang line for servers:
948
949 #!/usr/bin/perl -w
950 require 5.002;
951 use strict;
952 use sigtrap;
953 use Socket;
954
955=head1 BUGS
956
957All these routines create system-specific portability problems. As noted
958elsewhere, Perl is at the mercy of your C libraries for much of its system
959behaviour. It's probably safest to assume broken SysV semantics for
6a3992aa 960signals and to stick with simple TCP and UDP socket operations; e.g., don't
a2eb9003 961try to pass open file descriptors over a local UDP datagram socket if you
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962want your code to stand a chance of being portable.
963
964Because few vendors provide C libraries that are safely
965re-entrant, the prudent programmer will do little else within
966a handler beyond die() to raise an exception and longjmp(3) out.
967
968=head1 AUTHOR
969
970Tom Christiansen, with occasional vestiges of Larry Wall's original
971version.
972
973=head1 SEE ALSO
974
975Besides the obvious functions in L<perlfunc>, you should also check out
976the F<modules> file at your nearest CPAN site. (See L<perlmod> or best
977yet, the F<Perl FAQ> for a description of what CPAN is and where to get it.)
978Section 5 of the F<modules> file is devoted to "Networking, Device Control
6a3992aa 979(modems), and Interprocess Communication", and contains numerous unbundled
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980modules numerous networking modules, Chat and Expect operations, CGI
981programming, DCE, FTP, IPC, NNTP, Proxy, Ptty, RPC, SNMP, SMTP, Telnet,
982Threads, and ToolTalk--just to name a few.