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 Do as little as you possibly can in your handler; notice how all we do is
24 set a global variable and then raise an exception. That's because on most
25 systems, libraries are not re-entrant; particularly, memory allocation and
26 I/O routines are not. That means that doing nearly I<anything> in your
27 handler could in theory trigger a memory fault 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 sends a hang-up signal to all
75 processes in the current process group (and sets $SIG{HUP} to IGNORE so
76 it doesn't kill itself):
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
201 By installing Perl code to deal with signals, you're exposing yourself
202 to danger from two things. First, few system library functions are
203 re-entrant. If the signal interrupts while Perl is executing one function
204 (like malloc(3) or printf(3)), and your signal handler then calls the
205 same function again, you could get unpredictable behavior--often, a
206 core dump. Second, Perl isn't itself re-entrant at the lowest levels.
207 If the signal interrupts Perl while Perl is changing its own internal
208 data structures, similarly unpredictable behaviour may result.
210 There are two things you can do, knowing this: be paranoid or be
211 pragmatic. The paranoid approach is to do as little as possible in your
212 signal handler. Set an existing integer variable that already has a
213 value, and return. This doesn't help you if you're in a slow system call,
214 which will just restart. That means you have to C<die> to longjump(3) out
215 of the handler. Even this is a little cavalier for the true paranoiac,
216 who avoids C<die> in a handler because the system I<is> out to get you.
217 The pragmatic approach is to say ``I know the risks, but prefer the
218 convenience'', and to do anything you want in your signal handler,
219 prepared to clean up core dumps now and again.
221 To forbid signal handlers altogether would bars you from
222 many interesting programs, including virtually everything in this manpage,
223 since you could no longer even write SIGCHLD handlers. Their dodginess
224 is expected to be addresses in the 5.005 release.
227 =head1 Using open() for IPC
229 Perl's basic open() statement can also be used for unidirectional interprocess
230 communication by either appending or prepending a pipe symbol to the second
231 argument to open(). Here's how to start something up in a child process you
234 open(SPOOLER, "| cat -v | lpr -h 2>/dev/null")
235 || die "can't fork: $!";
236 local $SIG{PIPE} = sub { die "spooler pipe broke" };
237 print SPOOLER "stuff\n";
238 close SPOOLER || die "bad spool: $! $?";
240 And here's how to start up a child process you intend to read from:
242 open(STATUS, "netstat -an 2>&1 |")
243 || die "can't fork: $!";
245 next if /^(tcp|udp)/;
248 close STATUS || die "bad netstat: $! $?";
250 If one can be sure that a particular program is a Perl script that is
251 expecting filenames in @ARGV, the clever programmer can write something
254 % program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile
256 and irrespective of which shell it's called from, the Perl program will
257 read from the file F<f1>, the process F<cmd1>, standard input (F<tmpfile>
258 in this case), the F<f2> file, the F<cmd2> command, and finally the F<f3>
259 file. Pretty nifty, eh?
261 You might notice that you could use backticks for much the
262 same effect as opening a pipe for reading:
264 print grep { !/^(tcp|udp)/ } `netstat -an 2>&1`;
265 die "bad netstat" if $?;
267 While this is true on the surface, it's much more efficient to process the
268 file one line or record at a time because then you don't have to read the
269 whole thing into memory at once. It also gives you finer control of the
270 whole process, letting you to kill off the child process early if you'd
273 Be careful to check both the open() and the close() return values. If
274 you're I<writing> to a pipe, you should also trap SIGPIPE. Otherwise,
275 think of what happens when you start up a pipe to a command that doesn't
276 exist: the open() will in all likelihood succeed (it only reflects the
277 fork()'s success), but then your output will fail--spectacularly. Perl
278 can't know whether the command worked because your command is actually
279 running in a separate process whose exec() might have failed. Therefore,
280 while readers of bogus commands return just a quick end of file, writers
281 to bogus command will trigger a signal they'd better be prepared to
284 open(FH, "|bogus") or die "can't fork: $!";
285 print FH "bang\n" or die "can't write: $!";
286 close FH or die "can't close: $!";
288 That won't blow up until the close, and it will blow up with a SIGPIPE.
289 To catch it, you could use this:
291 $SIG{PIPE} = 'IGNORE';
292 open(FH, "|bogus") or die "can't fork: $!";
293 print FH "bang\n" or die "can't write: $!";
294 close FH or die "can't close: status=$?";
298 Both the main process and any child processes it forks share the same
299 STDIN, STDOUT, and STDERR filehandles. If both processes try to access
300 them at once, strange things can happen. You'll certainly want to any
301 stdio flush output buffers before forking. You may also want to close
302 or reopen the filehandles for the child. You can get around this by
303 opening your pipe with open(), but on some systems this means that the
304 child process cannot outlive the parent.
306 =head2 Background Processes
308 You can run a command in the background with:
312 The command's STDOUT and STDERR (and possibly STDIN, depending on your
313 shell) will be the same as the parent's. You won't need to catch
314 SIGCHLD because of the double-fork taking place (see below for more
317 =head2 Complete Dissociation of Child from Parent
319 In some cases (starting server processes, for instance) you'll want to
320 completely dissociate the child process from the parent. This is
321 often called daemonization. A well behaved daemon will also chdir()
322 to the root directory (so it doesn't prevent unmounting the filesystem
323 containing the directory from which it was launched) and redirect its
324 standard file descriptors from and to F</dev/null> (so that random
325 output doesn't wind up on the user's terminal).
330 chdir '/' or die "Can't chdir to /: $!";
331 open STDIN, '/dev/null' or die "Can't read /dev/null: $!";
332 open STDOUT, '>/dev/null'
333 or die "Can't write to /dev/null: $!";
334 defined(my $pid = fork) or die "Can't fork: $!";
336 setsid or die "Can't start a new session: $!";
337 open STDERR, '>&STDOUT' or die "Can't dup stdout: $!";
340 The fork() has to come before the setsid() to ensure that you aren't a
341 process group leader (the setsid() will fail if you are). If your
342 system doesn't have the setsid() function, open F</dev/tty> and use the
343 C<TIOCNOTTY> ioctl() on it instead. See L<tty(4)> for details.
345 Non-Unix users should check their Your_OS::Process module for other
348 =head2 Safe Pipe Opens
350 Another interesting approach to IPC is making your single program go
351 multiprocess and communicate between (or even amongst) yourselves. The
352 open() function will accept a file argument of either C<"-|"> or C<"|-">
353 to do a very interesting thing: it forks a child connected to the
354 filehandle you've opened. The child is running the same program as the
355 parent. This is useful for safely opening a file when running under an
356 assumed UID or GID, for example. If you open a pipe I<to> minus, you can
357 write to the filehandle you opened and your kid will find it in his
358 STDIN. If you open a pipe I<from> minus, you can read from the filehandle
359 you opened whatever your kid writes to his STDOUT.
365 $pid = open(KID_TO_WRITE, "|-");
366 unless (defined $pid) {
367 warn "cannot fork: $!";
368 die "bailing out" if $sleep_count++ > 6;
371 } until defined $pid;
374 print KID_TO_WRITE @some_data;
375 close(KID_TO_WRITE) || warn "kid exited $?";
377 ($EUID, $EGID) = ($UID, $GID); # suid progs only
378 open (FILE, "> /safe/file")
379 || die "can't open /safe/file: $!";
381 print FILE; # child's STDIN is parent's KID
383 exit; # don't forget this
386 Another common use for this construct is when you need to execute
387 something without the shell's interference. With system(), it's
388 straightforward, but you can't use a pipe open or backticks safely.
389 That's because there's no way to stop the shell from getting its hands on
390 your arguments. Instead, use lower-level control to call exec() directly.
392 Here's a safe backtick or pipe open for read:
394 # add error processing as above
395 $pid = open(KID_TO_READ, "-|");
398 while (<KID_TO_READ>) {
399 # do something interesting
401 close(KID_TO_READ) || warn "kid exited $?";
404 ($EUID, $EGID) = ($UID, $GID); # suid only
405 exec($program, @options, @args)
406 || die "can't exec program: $!";
411 And here's a safe pipe open for writing:
413 # add error processing as above
414 $pid = open(KID_TO_WRITE, "|-");
415 $SIG{ALRM} = sub { die "whoops, $program pipe broke" };
421 close(KID_TO_WRITE) || warn "kid exited $?";
424 ($EUID, $EGID) = ($UID, $GID);
425 exec($program, @options, @args)
426 || die "can't exec program: $!";
430 Note that these operations are full Unix forks, which means they may not be
431 correctly implemented on alien systems. Additionally, these are not true
432 multithreading. If you'd like to learn more about threading, see the
433 F<modules> file mentioned below in the SEE ALSO section.
435 =head2 Bidirectional Communication with Another Process
437 While this works reasonably well for unidirectional communication, what
438 about bidirectional communication? The obvious thing you'd like to do
439 doesn't actually work:
441 open(PROG_FOR_READING_AND_WRITING, "| some program |")
443 and if you forget to use the B<-w> flag, then you'll miss out
444 entirely on the diagnostic message:
446 Can't do bidirectional pipe at -e line 1.
448 If you really want to, you can use the standard open2() library function
449 to catch both ends. There's also an open3() for tridirectional I/O so you
450 can also catch your child's STDERR, but doing so would then require an
451 awkward select() loop and wouldn't allow you to use normal Perl input
454 If you look at its source, you'll see that open2() uses low-level
455 primitives like Unix pipe() and exec() calls to create all the connections.
456 While it might have been slightly more efficient by using socketpair(), it
457 would have then been even less portable than it already is. The open2()
458 and open3() functions are unlikely to work anywhere except on a Unix
459 system or some other one purporting to be POSIX compliant.
461 Here's an example of using open2():
465 $pid = open2(*Reader, *Writer, "cat -u -n" );
466 Writer->autoflush(); # default here, actually
467 print Writer "stuff\n";
470 The problem with this is that Unix buffering is really going to
471 ruin your day. Even though your C<Writer> filehandle is auto-flushed,
472 and the process on the other end will get your data in a timely manner,
473 you can't usually do anything to force it to give it back to you
474 in a similarly quick fashion. In this case, we could, because we
475 gave I<cat> a B<-u> flag to make it unbuffered. But very few Unix
476 commands are designed to operate over pipes, so this seldom works
477 unless you yourself wrote the program on the other end of the
480 A solution to this is the nonstandard F<Comm.pl> library. It uses
481 pseudo-ttys to make your program behave more reasonably:
484 $ph = open_proc('cat -n');
486 print $ph "a line\n";
487 print "got back ", scalar <$ph>;
490 This way you don't have to have control over the source code of the
491 program you're using. The F<Comm> library also has expect()
492 and interact() functions. Find the library (and we hope its
493 successor F<IPC::Chat>) at your nearest CPAN archive as detailed
494 in the SEE ALSO section below.
496 The newer Expect.pm module from CPAN also addresses this kind of thing.
497 This module requires two other modules from CPAN: IO::Pty and IO::Stty.
498 It sets up a pseudo-terminal to interact with programs that insist on
499 using talking to the terminal device driver. If your system is
500 amongst those supported, this may be your best bet.
502 =head2 Bidirectional Communication with Yourself
504 If you want, you may make low-level pipe() and fork()
505 to stitch this together by hand. This example only
506 talks to itself, but you could reopen the appropriate
507 handles to STDIN and STDOUT and call other processes.
510 # pipe1 - bidirectional communication using two pipe pairs
511 # designed for the socketpair-challenged
512 use IO::Handle; # thousands of lines just for autoflush :-(
513 pipe(PARENT_RDR, CHILD_WTR); # XXX: failure?
514 pipe(CHILD_RDR, PARENT_WTR); # XXX: failure?
515 CHILD_WTR->autoflush(1);
516 PARENT_WTR->autoflush(1);
519 close PARENT_RDR; close PARENT_WTR;
520 print CHILD_WTR "Parent Pid $$ is sending this\n";
521 chomp($line = <CHILD_RDR>);
522 print "Parent Pid $$ just read this: `$line'\n";
523 close CHILD_RDR; close CHILD_WTR;
526 die "cannot fork: $!" unless defined $pid;
527 close CHILD_RDR; close CHILD_WTR;
528 chomp($line = <PARENT_RDR>);
529 print "Child Pid $$ just read this: `$line'\n";
530 print PARENT_WTR "Child Pid $$ is sending this\n";
531 close PARENT_RDR; close PARENT_WTR;
535 But you don't actually have to make two pipe calls. If you
536 have the socketpair() system call, it will do this all for you.
539 # pipe2 - bidirectional communication using socketpair
540 # "the best ones always go both ways"
543 use IO::Handle; # thousands of lines just for autoflush :-(
544 # We say AF_UNIX because although *_LOCAL is the
545 # POSIX 1003.1g form of the constant, many machines
546 # still don't have it.
547 socketpair(CHILD, PARENT, AF_UNIX, SOCK_STREAM, PF_UNSPEC)
548 or die "socketpair: $!";
551 PARENT->autoflush(1);
555 print CHILD "Parent Pid $$ is sending this\n";
556 chomp($line = <CHILD>);
557 print "Parent Pid $$ just read this: `$line'\n";
561 die "cannot fork: $!" unless defined $pid;
563 chomp($line = <PARENT>);
564 print "Child Pid $$ just read this: `$line'\n";
565 print PARENT "Child Pid $$ is sending this\n";
570 =head1 Sockets: Client/Server Communication
572 While not limited to Unix-derived operating systems (e.g., WinSock on PCs
573 provides socket support, as do some VMS libraries), you may not have
574 sockets on your system, in which case this section probably isn't going to do
575 you much good. With sockets, you can do both virtual circuits (i.e., TCP
576 streams) and datagrams (i.e., UDP packets). You may be able to do even more
577 depending on your system.
579 The Perl function calls for dealing with sockets have the same names as
580 the corresponding system calls in C, but their arguments tend to differ
581 for two reasons: first, Perl filehandles work differently than C file
582 descriptors. Second, Perl already knows the length of its strings, so you
583 don't need to pass that information.
585 One of the major problems with old socket code in Perl was that it used
586 hard-coded values for some of the constants, which severely hurt
587 portability. If you ever see code that does anything like explicitly
588 setting C<$AF_INET = 2>, you know you're in for big trouble: An
589 immeasurably superior approach is to use the C<Socket> module, which more
590 reliably grants access to various constants and functions you'll need.
592 If you're not writing a server/client for an existing protocol like
593 NNTP or SMTP, you should give some thought to how your server will
594 know when the client has finished talking, and vice-versa. Most
595 protocols are based on one-line messages and responses (so one party
596 knows the other has finished when a "\n" is received) or multi-line
597 messages and responses that end with a period on an empty line
598 ("\n.\n" terminates a message/response).
600 =head2 Internet Line Terminators
602 The Internet line terminator is "\015\012". Under ASCII variants of
603 Unix, that could usually be written as "\r\n", but under other systems,
604 "\r\n" might at times be "\015\015\012", "\012\012\015", or something
605 completely different. The standards specify writing "\015\012" to be
606 conformant (be strict in what you provide), but they also recommend
607 accepting a lone "\012" on input (but be lenient in what you require).
608 We haven't always been very good about that in the code in this manpage,
609 but unless you're on a Mac, you'll probably be ok.
611 =head2 Internet TCP Clients and Servers
613 Use Internet-domain sockets when you want to do client-server
614 communication that might extend to machines outside of your own system.
616 Here's a sample TCP client using Internet-domain sockets:
621 my ($remote,$port, $iaddr, $paddr, $proto, $line);
623 $remote = shift || 'localhost';
624 $port = shift || 2345; # random port
625 if ($port =~ /\D/) { $port = getservbyname($port, 'tcp') }
626 die "No port" unless $port;
627 $iaddr = inet_aton($remote) || die "no host: $remote";
628 $paddr = sockaddr_in($port, $iaddr);
630 $proto = getprotobyname('tcp');
631 socket(SOCK, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
632 connect(SOCK, $paddr) || die "connect: $!";
633 while (defined($line = <SOCK>)) {
637 close (SOCK) || die "close: $!";
640 And here's a corresponding server to go along with it. We'll
641 leave the address as INADDR_ANY so that the kernel can choose
642 the appropriate interface on multihomed hosts. If you want sit
643 on a particular interface (like the external side of a gateway
644 or firewall machine), you should fill this in with your real address
649 BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
654 sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
656 my $port = shift || 2345;
657 my $proto = getprotobyname('tcp');
658 $port = $1 if $port =~ /(\d+)/; # untaint port number
660 socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
661 setsockopt(Server, SOL_SOCKET, SO_REUSEADDR,
662 pack("l", 1)) || die "setsockopt: $!";
663 bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!";
664 listen(Server,SOMAXCONN) || die "listen: $!";
666 logmsg "server started on port $port";
670 $SIG{CHLD} = \&REAPER;
672 for ( ; $paddr = accept(Client,Server); close Client) {
673 my($port,$iaddr) = sockaddr_in($paddr);
674 my $name = gethostbyaddr($iaddr,AF_INET);
676 logmsg "connection from $name [",
677 inet_ntoa($iaddr), "]
680 print Client "Hello there, $name, it's now ",
681 scalar localtime, $EOL;
684 And here's a multithreaded version. It's multithreaded in that
685 like most typical servers, it spawns (forks) a slave server to
686 handle the client request so that the master server can quickly
687 go back to service a new client.
691 BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
696 sub spawn; # forward declaration
697 sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
699 my $port = shift || 2345;
700 my $proto = getprotobyname('tcp');
701 $port = $1 if $port =~ /(\d+)/; # untaint port number
703 socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
704 setsockopt(Server, SOL_SOCKET, SO_REUSEADDR,
705 pack("l", 1)) || die "setsockopt: $!";
706 bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!";
707 listen(Server,SOMAXCONN) || die "listen: $!";
709 logmsg "server started on port $port";
716 $SIG{CHLD} = \&REAPER; # loathe sysV
717 logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
720 $SIG{CHLD} = \&REAPER;
722 for ( $waitedpid = 0;
723 ($paddr = accept(Client,Server)) || $waitedpid;
724 $waitedpid = 0, close Client)
726 next if $waitedpid and not $paddr;
727 my($port,$iaddr) = sockaddr_in($paddr);
728 my $name = gethostbyaddr($iaddr,AF_INET);
730 logmsg "connection from $name [",
731 inet_ntoa($iaddr), "]
735 print "Hello there, $name, it's now ", scalar localtime, $EOL;
736 exec '/usr/games/fortune' # XXX: `wrong' line terminators
737 or confess "can't exec fortune: $!";
745 unless (@_ == 0 && $coderef && ref($coderef) eq 'CODE') {
746 confess "usage: spawn CODEREF";
750 if (!defined($pid = fork)) {
751 logmsg "cannot fork: $!";
755 return; # I'm the parent
757 # else I'm the child -- go spawn
759 open(STDIN, "<&Client") || die "can't dup client to stdin";
760 open(STDOUT, ">&Client") || die "can't dup client to stdout";
761 ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr";
765 This server takes the trouble to clone off a child version via fork() for
766 each incoming request. That way it can handle many requests at once,
767 which you might not always want. Even if you don't fork(), the listen()
768 will allow that many pending connections. Forking servers have to be
769 particularly careful about cleaning up their dead children (called
770 "zombies" in Unix parlance), because otherwise you'll quickly fill up your
773 We suggest that you use the B<-T> flag to use taint checking (see L<perlsec>)
774 even if we aren't running setuid or setgid. This is always a good idea
775 for servers and other programs run on behalf of someone else (like CGI
776 scripts), because it lessens the chances that people from the outside will
777 be able to compromise your system.
779 Let's look at another TCP client. This one connects to the TCP "time"
780 service on a number of different machines and shows how far their clocks
781 differ from the system on which it's being run:
787 my $SECS_of_70_YEARS = 2208988800;
788 sub ctime { scalar localtime(shift) }
790 my $iaddr = gethostbyname('localhost');
791 my $proto = getprotobyname('tcp');
792 my $port = getservbyname('time', 'tcp');
793 my $paddr = sockaddr_in(0, $iaddr);
797 printf "%-24s %8s %s\n", "localhost", 0, ctime(time());
799 foreach $host (@ARGV) {
800 printf "%-24s ", $host;
801 my $hisiaddr = inet_aton($host) || die "unknown host";
802 my $hispaddr = sockaddr_in($port, $hisiaddr);
803 socket(SOCKET, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
804 connect(SOCKET, $hispaddr) || die "bind: $!";
806 read(SOCKET, $rtime, 4);
808 my $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ;
809 printf "%8d %s\n", $histime - time, ctime($histime);
812 =head2 Unix-Domain TCP Clients and Servers
814 That's fine for Internet-domain clients and servers, but what about local
815 communications? While you can use the same setup, sometimes you don't
816 want to. Unix-domain sockets are local to the current host, and are often
817 used internally to implement pipes. Unlike Internet domain sockets, Unix
818 domain sockets can show up in the file system with an ls(1) listing.
821 srw-rw-rw- 1 root 0 Oct 31 07:23 /dev/log
823 You can test for these with Perl's B<-S> file test:
825 unless ( -S '/dev/log' ) {
826 die "something's wicked with the print system";
829 Here's a sample Unix-domain client:
834 my ($rendezvous, $line);
836 $rendezvous = shift || '/tmp/catsock';
837 socket(SOCK, PF_UNIX, SOCK_STREAM, 0) || die "socket: $!";
838 connect(SOCK, sockaddr_un($rendezvous)) || die "connect: $!";
839 while (defined($line = <SOCK>)) {
844 And here's a corresponding server. You don't have to worry about silly
845 network terminators here because Unix domain sockets are guaranteed
846 to be on the localhost, and thus everything works right.
853 BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
854 sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
856 my $NAME = '/tmp/catsock';
857 my $uaddr = sockaddr_un($NAME);
858 my $proto = getprotobyname('tcp');
860 socket(Server,PF_UNIX,SOCK_STREAM,0) || die "socket: $!";
862 bind (Server, $uaddr) || die "bind: $!";
863 listen(Server,SOMAXCONN) || die "listen: $!";
865 logmsg "server started on $NAME";
871 $SIG{CHLD} = \&REAPER; # loathe sysV
872 logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
875 $SIG{CHLD} = \&REAPER;
878 for ( $waitedpid = 0;
879 accept(Client,Server) || $waitedpid;
880 $waitedpid = 0, close Client)
883 logmsg "connection on $NAME";
885 print "Hello there, it's now ", scalar localtime, "\n";
886 exec '/usr/games/fortune' or die "can't exec fortune: $!";
890 As you see, it's remarkably similar to the Internet domain TCP server, so
891 much so, in fact, that we've omitted several duplicate functions--spawn(),
892 logmsg(), ctime(), and REAPER()--which are exactly the same as in the
895 So why would you ever want to use a Unix domain socket instead of a
896 simpler named pipe? Because a named pipe doesn't give you sessions. You
897 can't tell one process's data from another's. With socket programming,
898 you get a separate session for each client: that's why accept() takes two
901 For example, let's say that you have a long running database server daemon
902 that you want folks from the World Wide Web to be able to access, but only
903 if they go through a CGI interface. You'd have a small, simple CGI
904 program that does whatever checks and logging you feel like, and then acts
905 as a Unix-domain client and connects to your private server.
907 =head1 TCP Clients with IO::Socket
909 For those preferring a higher-level interface to socket programming, the
910 IO::Socket module provides an object-oriented approach. IO::Socket is
911 included as part of the standard Perl distribution as of the 5.004
912 release. If you're running an earlier version of Perl, just fetch
913 IO::Socket from CPAN, where you'll also find find modules providing easy
914 interfaces to the following systems: DNS, FTP, Ident (RFC 931), NIS and
915 NISPlus, NNTP, Ping, POP3, SMTP, SNMP, SSLeay, Telnet, and Time--just
918 =head2 A Simple Client
920 Here's a client that creates a TCP connection to the "daytime"
921 service at port 13 of the host name "localhost" and prints out everything
922 that the server there cares to provide.
926 $remote = IO::Socket::INET->new(
928 PeerAddr => "localhost",
929 PeerPort => "daytime(13)",
931 or die "cannot connect to daytime port at localhost";
932 while ( <$remote> ) { print }
934 When you run this program, you should get something back that
937 Wed May 14 08:40:46 MDT 1997
939 Here are what those parameters to the C<new> constructor mean:
945 This is which protocol to use. In this case, the socket handle returned
946 will be connected to a TCP socket, because we want a stream-oriented
947 connection, that is, one that acts pretty much like a plain old file.
948 Not all sockets are this of this type. For example, the UDP protocol
949 can be used to make a datagram socket, used for message-passing.
953 This is the name or Internet address of the remote host the server is
954 running on. We could have specified a longer name like C<"www.perl.com">,
955 or an address like C<"204.148.40.9">. For demonstration purposes, we've
956 used the special hostname C<"localhost">, which should always mean the
957 current machine you're running on. The corresponding Internet address
958 for localhost is C<"127.1">, if you'd rather use that.
962 This is the service name or port number we'd like to connect to.
963 We could have gotten away with using just C<"daytime"> on systems with a
964 well-configured system services file,[FOOTNOTE: The system services file
965 is in I</etc/services> under Unix] but just in case, we've specified the
966 port number (13) in parentheses. Using just the number would also have
967 worked, but constant numbers make careful programmers nervous.
971 Notice how the return value from the C<new> constructor is used as
972 a filehandle in the C<while> loop? That's what's called an indirect
973 filehandle, a scalar variable containing a filehandle. You can use
974 it the same way you would a normal filehandle. For example, you
975 can read one line from it this way:
979 all remaining lines from is this way:
983 and send a line of data to it this way:
985 print $handle "some data\n";
987 =head2 A Webget Client
989 Here's a simple client that takes a remote host to fetch a document
990 from, and then a list of documents to get from that host. This is a
991 more interesting client than the previous one because it first sends
992 something to the server before fetching the server's response.
996 unless (@ARGV > 1) { die "usage: $0 host document ..." }
997 $host = shift(@ARGV);
1000 foreach $document ( @ARGV ) {
1001 $remote = IO::Socket::INET->new( Proto => "tcp",
1003 PeerPort => "http(80)",
1005 unless ($remote) { die "cannot connect to http daemon on $host" }
1006 $remote->autoflush(1);
1007 print $remote "GET $document HTTP/1.0" . $BLANK;
1008 while ( <$remote> ) { print }
1012 The web server handing the "http" service, which is assumed to be at
1013 its standard port, number 80. If your the web server you're trying to
1014 connect to is at a different port (like 1080 or 8080), you should specify
1015 as the named-parameter pair, C<PeerPort =E<gt> 8080>. The C<autoflush>
1016 method is used on the socket because otherwise the system would buffer
1017 up the output we sent it. (If you're on a Mac, you'll also need to
1018 change every C<"\n"> in your code that sends data over the network to
1019 be a C<"\015\012"> instead.)
1021 Connecting to the server is only the first part of the process: once you
1022 have the connection, you have to use the server's language. Each server
1023 on the network has its own little command language that it expects as
1024 input. The string that we send to the server starting with "GET" is in
1025 HTTP syntax. In this case, we simply request each specified document.
1026 Yes, we really are making a new connection for each document, even though
1027 it's the same host. That's the way you always used to have to speak HTTP.
1028 Recent versions of web browsers may request that the remote server leave
1029 the connection open a little while, but the server doesn't have to honor
1032 Here's an example of running that program, which we'll call I<webget>:
1034 % webget www.perl.com /guanaco.html
1035 HTTP/1.1 404 File Not Found
1036 Date: Thu, 08 May 1997 18:02:32 GMT
1037 Server: Apache/1.2b6
1039 Content-type: text/html
1041 <HEAD><TITLE>404 File Not Found</TITLE></HEAD>
1042 <BODY><H1>File Not Found</H1>
1043 The requested URL /guanaco.html was not found on this server.<P>
1046 Ok, so that's not very interesting, because it didn't find that
1047 particular document. But a long response wouldn't have fit on this page.
1049 For a more fully-featured version of this program, you should look to
1050 the I<lwp-request> program included with the LWP modules from CPAN.
1052 =head2 Interactive Client with IO::Socket
1054 Well, that's all fine if you want to send one command and get one answer,
1055 but what about setting up something fully interactive, somewhat like
1056 the way I<telnet> works? That way you can type a line, get the answer,
1057 type a line, get the answer, etc.
1059 This client is more complicated than the two we've done so far, but if
1060 you're on a system that supports the powerful C<fork> call, the solution
1061 isn't that rough. Once you've made the connection to whatever service
1062 you'd like to chat with, call C<fork> to clone your process. Each of
1063 these two identical process has a very simple job to do: the parent
1064 copies everything from the socket to standard output, while the child
1065 simultaneously copies everything from standard input to the socket.
1066 To accomplish the same thing using just one process would be I<much>
1067 harder, because it's easier to code two processes to do one thing than it
1068 is to code one process to do two things. (This keep-it-simple principle
1069 a cornerstones of the Unix philosophy, and good software engineering as
1070 well, which is probably why it's spread to other systems.)
1077 my ($host, $port, $kidpid, $handle, $line);
1079 unless (@ARGV == 2) { die "usage: $0 host port" }
1080 ($host, $port) = @ARGV;
1082 # create a tcp connection to the specified host and port
1083 $handle = IO::Socket::INET->new(Proto => "tcp",
1086 or die "can't connect to port $port on $host: $!";
1088 $handle->autoflush(1); # so output gets there right away
1089 print STDERR "[Connected to $host:$port]\n";
1091 # split the program into two processes, identical twins
1092 die "can't fork: $!" unless defined($kidpid = fork());
1094 # the if{} block runs only in the parent process
1096 # copy the socket to standard output
1097 while (defined ($line = <$handle>)) {
1100 kill("TERM", $kidpid); # send SIGTERM to child
1102 # the else{} block runs only in the child process
1104 # copy standard input to the socket
1105 while (defined ($line = <STDIN>)) {
1106 print $handle $line;
1110 The C<kill> function in the parent's C<if> block is there to send a
1111 signal to our child process (current running in the C<else> block)
1112 as soon as the remote server has closed its end of the connection.
1114 If the remote server sends data a byte at time, and you need that
1115 data immediately without waiting for a newline (which might not happen),
1116 you may wish to replace the C<while> loop in the parent with the
1120 while (sysread($handle, $byte, 1) == 1) {
1124 Making a system call for each byte you want to read is not very efficient
1125 (to put it mildly) but is the simplest to explain and works reasonably
1128 =head1 TCP Servers with IO::Socket
1130 As always, setting up a server is little bit more involved than running a client.
1131 The model is that the server creates a special kind of socket that
1132 does nothing but listen on a particular port for incoming connections.
1133 It does this by calling the C<IO::Socket::INET-E<gt>new()> method with
1134 slightly different arguments than the client did.
1140 This is which protocol to use. Like our clients, we'll
1141 still specify C<"tcp"> here.
1146 port in the C<LocalPort> argument, which we didn't do for the client.
1147 This is service name or port number for which you want to be the
1148 server. (Under Unix, ports under 1024 are restricted to the
1149 superuser.) In our sample, we'll use port 9000, but you can use
1150 any port that's not currently in use on your system. If you try
1151 to use one already in used, you'll get an "Address already in use"
1152 message. Under Unix, the C<netstat -a> command will show
1153 which services current have servers.
1157 The C<Listen> parameter is set to the maximum number of
1158 pending connections we can accept until we turn away incoming clients.
1159 Think of it as a call-waiting queue for your telephone.
1160 The low-level Socket module has a special symbol for the system maximum, which
1165 The C<Reuse> parameter is needed so that we restart our server
1166 manually without waiting a few minutes to allow system buffers to
1171 Once the generic server socket has been created using the parameters
1172 listed above, the server then waits for a new client to connect
1173 to it. The server blocks in the C<accept> method, which eventually an
1174 bidirectional connection to the remote client. (Make sure to autoflush
1175 this handle to circumvent buffering.)
1177 To add to user-friendliness, our server prompts the user for commands.
1178 Most servers don't do this. Because of the prompt without a newline,
1179 you'll have to use the C<sysread> variant of the interactive client above.
1181 This server accepts one of five different commands, sending output
1182 back to the client. Note that unlike most network servers, this one
1183 only handles one incoming client at a time. Multithreaded servers are
1184 covered in Chapter 6 of the Camel as well as later in this manpage.
1186 Here's the code. We'll
1190 use Net::hostent; # for OO version of gethostbyaddr
1192 $PORT = 9000; # pick something not in use
1194 $server = IO::Socket::INET->new( Proto => 'tcp',
1196 Listen => SOMAXCONN,
1199 die "can't setup server" unless $server;
1200 print "[Server $0 accepting clients]\n";
1202 while ($client = $server->accept()) {
1203 $client->autoflush(1);
1204 print $client "Welcome to $0; type help for command list.\n";
1205 $hostinfo = gethostbyaddr($client->peeraddr);
1206 printf "[Connect from %s]\n", $hostinfo->name || $client->peerhost;
1207 print $client "Command? ";
1208 while ( <$client>) {
1209 next unless /\S/; # blank line
1210 if (/quit|exit/i) { last; }
1211 elsif (/date|time/i) { printf $client "%s\n", scalar localtime; }
1212 elsif (/who/i ) { print $client `who 2>&1`; }
1213 elsif (/cookie/i ) { print $client `/usr/games/fortune 2>&1`; }
1214 elsif (/motd/i ) { print $client `cat /etc/motd 2>&1`; }
1216 print $client "Commands: quit date who cookie motd\n";
1219 print $client "Command? ";
1224 =head1 UDP: Message Passing
1226 Another kind of client-server setup is one that uses not connections, but
1227 messages. UDP communications involve much lower overhead but also provide
1228 less reliability, as there are no promises that messages will arrive at
1229 all, let alone in order and unmangled. Still, UDP offers some advantages
1230 over TCP, including being able to "broadcast" or "multicast" to a whole
1231 bunch of destination hosts at once (usually on your local subnet). If you
1232 find yourself overly concerned about reliability and start building checks
1233 into your message system, then you probably should use just TCP to start
1236 Here's a UDP program similar to the sample Internet TCP client given
1237 earlier. However, instead of checking one host at a time, the UDP version
1238 will check many of them asynchronously by simulating a multicast and then
1239 using select() to do a timed-out wait for I/O. To do something similar
1240 with TCP, you'd have to use a different socket handle for each host.
1247 my ( $count, $hisiaddr, $hispaddr, $histime,
1248 $host, $iaddr, $paddr, $port, $proto,
1249 $rin, $rout, $rtime, $SECS_of_70_YEARS);
1251 $SECS_of_70_YEARS = 2208988800;
1253 $iaddr = gethostbyname(hostname());
1254 $proto = getprotobyname('udp');
1255 $port = getservbyname('time', 'udp');
1256 $paddr = sockaddr_in(0, $iaddr); # 0 means let kernel pick
1258 socket(SOCKET, PF_INET, SOCK_DGRAM, $proto) || die "socket: $!";
1259 bind(SOCKET, $paddr) || die "bind: $!";
1262 printf "%-12s %8s %s\n", "localhost", 0, scalar localtime time;
1266 $hisiaddr = inet_aton($host) || die "unknown host";
1267 $hispaddr = sockaddr_in($port, $hisiaddr);
1268 defined(send(SOCKET, 0, 0, $hispaddr)) || die "send $host: $!";
1272 vec($rin, fileno(SOCKET), 1) = 1;
1274 # timeout after 10.0 seconds
1275 while ($count && select($rout = $rin, undef, undef, 10.0)) {
1277 ($hispaddr = recv(SOCKET, $rtime, 4, 0)) || die "recv: $!";
1278 ($port, $hisiaddr) = sockaddr_in($hispaddr);
1279 $host = gethostbyaddr($hisiaddr, AF_INET);
1280 $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ;
1281 printf "%-12s ", $host;
1282 printf "%8d %s\n", $histime - time, scalar localtime($histime);
1288 While System V IPC isn't so widely used as sockets, it still has some
1289 interesting uses. You can't, however, effectively use SysV IPC or
1290 Berkeley mmap() to have shared memory so as to share a variable amongst
1291 several processes. That's because Perl would reallocate your string when
1292 you weren't wanting it to.
1294 Here's a small example showing shared memory usage.
1296 use IPC::SysV qw(IPC_PRIVATE IPC_RMID S_IRWXU S_IRWXG S_IRWXO);
1299 $key = shmget(IPC_PRIVATE, $size, S_IRWXU|S_IRWXG|S_IRWXO) || die "$!";
1300 print "shm key $key\n";
1302 $message = "Message #1";
1303 shmwrite($key, $message, 0, 60) || die "$!";
1304 print "wrote: '$message'\n";
1305 shmread($key, $buff, 0, 60) || die "$!";
1306 print "read : '$buff'\n";
1308 # the buffer of shmread is zero-character end-padded.
1309 substr($buff, index($buff, "\0")) = '';
1310 print "un" unless $buff eq $message;
1313 print "deleting shm $key\n";
1314 shmctl($key, IPC_RMID, 0) || die "$!";
1316 Here's an example of a semaphore:
1318 use IPC::SysV qw(IPC_CREAT);
1321 $key = semget($IPC_KEY, 10, 0666 | IPC_CREAT ) || die "$!";
1322 print "shm key $key\n";
1324 Put this code in a separate file to be run in more than one process.
1325 Call the file F<take>:
1327 # create a semaphore
1330 $key = semget($IPC_KEY, 0 , 0 );
1331 die if !defined($key);
1337 # wait for semaphore to be zero
1339 $opstring1 = pack("sss", $semnum, $semop, $semflag);
1341 # Increment the semaphore count
1343 $opstring2 = pack("sss", $semnum, $semop, $semflag);
1344 $opstring = $opstring1 . $opstring2;
1346 semop($key,$opstring) || die "$!";
1348 Put this code in a separate file to be run in more than one process.
1349 Call this file F<give>:
1351 # 'give' the semaphore
1352 # run this in the original process and you will see
1353 # that the second process continues
1356 $key = semget($IPC_KEY, 0, 0);
1357 die if !defined($key);
1362 # Decrement the semaphore count
1364 $opstring = pack("sss", $semnum, $semop, $semflag);
1366 semop($key,$opstring) || die "$!";
1368 The SysV IPC code above was written long ago, and it's definitely
1369 clunky looking. For a more modern look, see the IPC::SysV module
1370 which is included with Perl starting from Perl 5.005.
1374 Most of these routines quietly but politely return C<undef> when they
1375 fail instead of causing your program to die right then and there due to
1376 an uncaught exception. (Actually, some of the new I<Socket> conversion
1377 functions croak() on bad arguments.) It is therefore essential to
1378 check return values from these functions. Always begin your socket
1379 programs this way for optimal success, and don't forget to add B<-T>
1380 taint checking flag to the #! line for servers:
1389 All these routines create system-specific portability problems. As noted
1390 elsewhere, Perl is at the mercy of your C libraries for much of its system
1391 behaviour. It's probably safest to assume broken SysV semantics for
1392 signals and to stick with simple TCP and UDP socket operations; e.g., don't
1393 try to pass open file descriptors over a local UDP datagram socket if you
1394 want your code to stand a chance of being portable.
1396 As mentioned in the signals section, because few vendors provide C
1397 libraries that are safely re-entrant, the prudent programmer will do
1398 little else within a handler beyond setting a numeric variable that
1399 already exists; or, if locked into a slow (restarting) system call,
1400 using die() to raise an exception and longjmp(3) out. In fact, even
1401 these may in some cases cause a core dump. It's probably best to avoid
1402 signals except where they are absolutely inevitable. This
1403 will be addressed in a future release of Perl.
1407 Tom Christiansen, with occasional vestiges of Larry Wall's original
1408 version and suggestions from the Perl Porters.
1412 There's a lot more to networking than this, but this should get you
1415 For intrepid programmers, the indispensable textbook is I<Unix Network
1416 Programming> by W. Richard Stevens (published by Addison-Wesley). Note
1417 that most books on networking address networking from the perspective of
1418 a C programmer; translation to Perl is left as an exercise for the reader.
1420 The IO::Socket(3) manpage describes the object library, and the Socket(3)
1421 manpage describes the low-level interface to sockets. Besides the obvious
1422 functions in L<perlfunc>, you should also check out the F<modules> file
1423 at your nearest CPAN site. (See L<perlmodlib> or best yet, the F<Perl
1424 FAQ> for a description of what CPAN is and where to get it.)
1426 Section 5 of the F<modules> file is devoted to "Networking, Device Control
1427 (modems), and Interprocess Communication", and contains numerous unbundled
1428 modules numerous networking modules, Chat and Expect operations, CGI
1429 programming, DCE, FTP, IPC, NNTP, Proxy, Ptty, RPC, SNMP, SMTP, Telnet,
1430 Threads, and ToolTalk--just to name a few.