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 complete dissociate the child process from the parent. The easiest
323 use POSIX qw(setsid);
324 setsid() or die "Can't start a new session: $!";
326 However, you may not be on POSIX. The following process is reported
327 to work on most Unixish systems. Non-Unix users should check their
328 Your_OS::Process module for other solutions.
334 Open /dev/tty and use the TIOCNOTTY ioctl on it. See L<tty(4)>
339 Change directory to /
343 Reopen STDIN, STDOUT, and STDERR so they're not connected to the old
348 Background yourself like this:
354 Ignore hangup signals in case you're running on a shell that doesn't
355 automatically no-hup you:
357 $SIG{HUP} = 'IGNORE'; # or whatever you'd like
361 =head2 Safe Pipe Opens
363 Another interesting approach to IPC is making your single program go
364 multiprocess and communicate between (or even amongst) yourselves. The
365 open() function will accept a file argument of either C<"-|"> or C<"|-">
366 to do a very interesting thing: it forks a child connected to the
367 filehandle you've opened. The child is running the same program as the
368 parent. This is useful for safely opening a file when running under an
369 assumed UID or GID, for example. If you open a pipe I<to> minus, you can
370 write to the filehandle you opened and your kid will find it in his
371 STDIN. If you open a pipe I<from> minus, you can read from the filehandle
372 you opened whatever your kid writes to his STDOUT.
378 $pid = open(KID_TO_WRITE, "|-");
379 unless (defined $pid) {
380 warn "cannot fork: $!";
381 die "bailing out" if $sleep_count++ > 6;
384 } until defined $pid;
387 print KID_TO_WRITE @some_data;
388 close(KID_TO_WRITE) || warn "kid exited $?";
390 ($EUID, $EGID) = ($UID, $GID); # suid progs only
391 open (FILE, "> /safe/file")
392 || die "can't open /safe/file: $!";
394 print FILE; # child's STDIN is parent's KID
396 exit; # don't forget this
399 Another common use for this construct is when you need to execute
400 something without the shell's interference. With system(), it's
401 straightforward, but you can't use a pipe open or backticks safely.
402 That's because there's no way to stop the shell from getting its hands on
403 your arguments. Instead, use lower-level control to call exec() directly.
405 Here's a safe backtick or pipe open for read:
407 # add error processing as above
408 $pid = open(KID_TO_READ, "-|");
411 while (<KID_TO_READ>) {
412 # do something interesting
414 close(KID_TO_READ) || warn "kid exited $?";
417 ($EUID, $EGID) = ($UID, $GID); # suid only
418 exec($program, @options, @args)
419 || die "can't exec program: $!";
424 And here's a safe pipe open for writing:
426 # add error processing as above
427 $pid = open(KID_TO_WRITE, "|-");
428 $SIG{ALRM} = sub { die "whoops, $program pipe broke" };
434 close(KID_TO_WRITE) || warn "kid exited $?";
437 ($EUID, $EGID) = ($UID, $GID);
438 exec($program, @options, @args)
439 || die "can't exec program: $!";
443 Note that these operations are full Unix forks, which means they may not be
444 correctly implemented on alien systems. Additionally, these are not true
445 multithreading. If you'd like to learn more about threading, see the
446 F<modules> file mentioned below in the SEE ALSO section.
448 =head2 Bidirectional Communication with Another Process
450 While this works reasonably well for unidirectional communication, what
451 about bidirectional communication? The obvious thing you'd like to do
452 doesn't actually work:
454 open(PROG_FOR_READING_AND_WRITING, "| some program |")
456 and if you forget to use the B<-w> flag, then you'll miss out
457 entirely on the diagnostic message:
459 Can't do bidirectional pipe at -e line 1.
461 If you really want to, you can use the standard open2() library function
462 to catch both ends. There's also an open3() for tridirectional I/O so you
463 can also catch your child's STDERR, but doing so would then require an
464 awkward select() loop and wouldn't allow you to use normal Perl input
467 If you look at its source, you'll see that open2() uses low-level
468 primitives like Unix pipe() and exec() calls to create all the connections.
469 While it might have been slightly more efficient by using socketpair(), it
470 would have then been even less portable than it already is. The open2()
471 and open3() functions are unlikely to work anywhere except on a Unix
472 system or some other one purporting to be POSIX compliant.
474 Here's an example of using open2():
478 $pid = open2(*Reader, *Writer, "cat -u -n" );
479 Writer->autoflush(); # default here, actually
480 print Writer "stuff\n";
483 The problem with this is that Unix buffering is really going to
484 ruin your day. Even though your C<Writer> filehandle is auto-flushed,
485 and the process on the other end will get your data in a timely manner,
486 you can't usually do anything to force it to give it back to you
487 in a similarly quick fashion. In this case, we could, because we
488 gave I<cat> a B<-u> flag to make it unbuffered. But very few Unix
489 commands are designed to operate over pipes, so this seldom works
490 unless you yourself wrote the program on the other end of the
493 A solution to this is the nonstandard F<Comm.pl> library. It uses
494 pseudo-ttys to make your program behave more reasonably:
497 $ph = open_proc('cat -n');
499 print $ph "a line\n";
500 print "got back ", scalar <$ph>;
503 This way you don't have to have control over the source code of the
504 program you're using. The F<Comm> library also has expect()
505 and interact() functions. Find the library (and we hope its
506 successor F<IPC::Chat>) at your nearest CPAN archive as detailed
507 in the SEE ALSO section below.
509 The newer Expect.pm module from CPAN also addresses this kind of thing.
510 This module requires two other modules from CPAN: IO::Pty and IO::Stty.
511 It sets up a pseudo-terminal to interact with programs that insist on
512 using talking to the terminal device driver. If your system is
513 amongst those supported, this may be your best bet.
515 =head2 Bidirectional Communication with Yourself
517 If you want, you may make low-level pipe() and fork()
518 to stitch this together by hand. This example only
519 talks to itself, but you could reopen the appropriate
520 handles to STDIN and STDOUT and call other processes.
523 # pipe1 - bidirectional communication using two pipe pairs
524 # designed for the socketpair-challenged
525 use IO::Handle; # thousands of lines just for autoflush :-(
526 pipe(PARENT_RDR, CHILD_WTR); # XXX: failure?
527 pipe(CHILD_RDR, PARENT_WTR); # XXX: failure?
528 CHILD_WTR->autoflush(1);
529 PARENT_WTR->autoflush(1);
532 close PARENT_RDR; close PARENT_WTR;
533 print CHILD_WTR "Parent Pid $$ is sending this\n";
534 chomp($line = <CHILD_RDR>);
535 print "Parent Pid $$ just read this: `$line'\n";
536 close CHILD_RDR; close CHILD_WTR;
539 die "cannot fork: $!" unless defined $pid;
540 close CHILD_RDR; close CHILD_WTR;
541 chomp($line = <PARENT_RDR>);
542 print "Child Pid $$ just read this: `$line'\n";
543 print PARENT_WTR "Child Pid $$ is sending this\n";
544 close PARENT_RDR; close PARENT_WTR;
548 But you don't actually have to make two pipe calls. If you
549 have the socketpair() system call, it will do this all for you.
552 # pipe2 - bidirectional communication using socketpair
553 # "the best ones always go both ways"
556 use IO::Handle; # thousands of lines just for autoflush :-(
557 # We say AF_UNIX because although *_LOCAL is the
558 # POSIX 1003.1g form of the constant, many machines
559 # still don't have it.
560 socketpair(CHILD, PARENT, AF_UNIX, SOCK_STREAM, PF_UNSPEC)
561 or die "socketpair: $!";
564 PARENT->autoflush(1);
568 print CHILD "Parent Pid $$ is sending this\n";
569 chomp($line = <CHILD>);
570 print "Parent Pid $$ just read this: `$line'\n";
574 die "cannot fork: $!" unless defined $pid;
576 chomp($line = <PARENT>);
577 print "Child Pid $$ just read this: `$line'\n";
578 print PARENT "Child Pid $$ is sending this\n";
583 =head1 Sockets: Client/Server Communication
585 While not limited to Unix-derived operating systems (e.g., WinSock on PCs
586 provides socket support, as do some VMS libraries), you may not have
587 sockets on your system, in which case this section probably isn't going to do
588 you much good. With sockets, you can do both virtual circuits (i.e., TCP
589 streams) and datagrams (i.e., UDP packets). You may be able to do even more
590 depending on your system.
592 The Perl function calls for dealing with sockets have the same names as
593 the corresponding system calls in C, but their arguments tend to differ
594 for two reasons: first, Perl filehandles work differently than C file
595 descriptors. Second, Perl already knows the length of its strings, so you
596 don't need to pass that information.
598 One of the major problems with old socket code in Perl was that it used
599 hard-coded values for some of the constants, which severely hurt
600 portability. If you ever see code that does anything like explicitly
601 setting C<$AF_INET = 2>, you know you're in for big trouble: An
602 immeasurably superior approach is to use the C<Socket> module, which more
603 reliably grants access to various constants and functions you'll need.
605 If you're not writing a server/client for an existing protocol like
606 NNTP or SMTP, you should give some thought to how your server will
607 know when the client has finished talking, and vice-versa. Most
608 protocols are based on one-line messages and responses (so one party
609 knows the other has finished when a "\n" is received) or multi-line
610 messages and responses that end with a period on an empty line
611 ("\n.\n" terminates a message/response).
613 =head2 Internet Line Terminators
615 The Internet line terminator is "\015\012". Under ASCII variants of
616 Unix, that could usually be written as "\r\n", but under other systems,
617 "\r\n" might at times be "\015\015\012", "\012\012\015", or something
618 completely different. The standards specify writing "\015\012" to be
619 conformant (be strict in what you provide), but they also recommend
620 accepting a lone "\012" on input (but be lenient in what you require).
621 We haven't always been very good about that in the code in this manpage,
622 but unless you're on a Mac, you'll probably be ok.
624 =head2 Internet TCP Clients and Servers
626 Use Internet-domain sockets when you want to do client-server
627 communication that might extend to machines outside of your own system.
629 Here's a sample TCP client using Internet-domain sockets:
634 my ($remote,$port, $iaddr, $paddr, $proto, $line);
636 $remote = shift || 'localhost';
637 $port = shift || 2345; # random port
638 if ($port =~ /\D/) { $port = getservbyname($port, 'tcp') }
639 die "No port" unless $port;
640 $iaddr = inet_aton($remote) || die "no host: $remote";
641 $paddr = sockaddr_in($port, $iaddr);
643 $proto = getprotobyname('tcp');
644 socket(SOCK, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
645 connect(SOCK, $paddr) || die "connect: $!";
646 while (defined($line = <SOCK>)) {
650 close (SOCK) || die "close: $!";
653 And here's a corresponding server to go along with it. We'll
654 leave the address as INADDR_ANY so that the kernel can choose
655 the appropriate interface on multihomed hosts. If you want sit
656 on a particular interface (like the external side of a gateway
657 or firewall machine), you should fill this in with your real address
662 BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
667 sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
669 my $port = shift || 2345;
670 my $proto = getprotobyname('tcp');
671 $port = $1 if $port =~ /(\d+)/; # untaint port number
673 socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
674 setsockopt(Server, SOL_SOCKET, SO_REUSEADDR,
675 pack("l", 1)) || die "setsockopt: $!";
676 bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!";
677 listen(Server,SOMAXCONN) || die "listen: $!";
679 logmsg "server started on port $port";
683 $SIG{CHLD} = \&REAPER;
685 for ( ; $paddr = accept(Client,Server); close Client) {
686 my($port,$iaddr) = sockaddr_in($paddr);
687 my $name = gethostbyaddr($iaddr,AF_INET);
689 logmsg "connection from $name [",
690 inet_ntoa($iaddr), "]
693 print Client "Hello there, $name, it's now ",
694 scalar localtime, $EOL;
697 And here's a multithreaded version. It's multithreaded in that
698 like most typical servers, it spawns (forks) a slave server to
699 handle the client request so that the master server can quickly
700 go back to service a new client.
704 BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
709 sub spawn; # forward declaration
710 sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
712 my $port = shift || 2345;
713 my $proto = getprotobyname('tcp');
714 $port = $1 if $port =~ /(\d+)/; # untaint port number
716 socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
717 setsockopt(Server, SOL_SOCKET, SO_REUSEADDR,
718 pack("l", 1)) || die "setsockopt: $!";
719 bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!";
720 listen(Server,SOMAXCONN) || die "listen: $!";
722 logmsg "server started on port $port";
729 $SIG{CHLD} = \&REAPER; # loathe sysV
730 logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
733 $SIG{CHLD} = \&REAPER;
735 for ( $waitedpid = 0;
736 ($paddr = accept(Client,Server)) || $waitedpid;
737 $waitedpid = 0, close Client)
739 next if $waitedpid and not $paddr;
740 my($port,$iaddr) = sockaddr_in($paddr);
741 my $name = gethostbyaddr($iaddr,AF_INET);
743 logmsg "connection from $name [",
744 inet_ntoa($iaddr), "]
748 print "Hello there, $name, it's now ", scalar localtime, $EOL;
749 exec '/usr/games/fortune' # XXX: `wrong' line terminators
750 or confess "can't exec fortune: $!";
758 unless (@_ == 0 && $coderef && ref($coderef) eq 'CODE') {
759 confess "usage: spawn CODEREF";
763 if (!defined($pid = fork)) {
764 logmsg "cannot fork: $!";
768 return; # I'm the parent
770 # else I'm the child -- go spawn
772 open(STDIN, "<&Client") || die "can't dup client to stdin";
773 open(STDOUT, ">&Client") || die "can't dup client to stdout";
774 ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr";
778 This server takes the trouble to clone off a child version via fork() for
779 each incoming request. That way it can handle many requests at once,
780 which you might not always want. Even if you don't fork(), the listen()
781 will allow that many pending connections. Forking servers have to be
782 particularly careful about cleaning up their dead children (called
783 "zombies" in Unix parlance), because otherwise you'll quickly fill up your
786 We suggest that you use the B<-T> flag to use taint checking (see L<perlsec>)
787 even if we aren't running setuid or setgid. This is always a good idea
788 for servers and other programs run on behalf of someone else (like CGI
789 scripts), because it lessens the chances that people from the outside will
790 be able to compromise your system.
792 Let's look at another TCP client. This one connects to the TCP "time"
793 service on a number of different machines and shows how far their clocks
794 differ from the system on which it's being run:
800 my $SECS_of_70_YEARS = 2208988800;
801 sub ctime { scalar localtime(shift) }
803 my $iaddr = gethostbyname('localhost');
804 my $proto = getprotobyname('tcp');
805 my $port = getservbyname('time', 'tcp');
806 my $paddr = sockaddr_in(0, $iaddr);
810 printf "%-24s %8s %s\n", "localhost", 0, ctime(time());
812 foreach $host (@ARGV) {
813 printf "%-24s ", $host;
814 my $hisiaddr = inet_aton($host) || die "unknown host";
815 my $hispaddr = sockaddr_in($port, $hisiaddr);
816 socket(SOCKET, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
817 connect(SOCKET, $hispaddr) || die "bind: $!";
819 read(SOCKET, $rtime, 4);
821 my $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ;
822 printf "%8d %s\n", $histime - time, ctime($histime);
825 =head2 Unix-Domain TCP Clients and Servers
827 That's fine for Internet-domain clients and servers, but what about local
828 communications? While you can use the same setup, sometimes you don't
829 want to. Unix-domain sockets are local to the current host, and are often
830 used internally to implement pipes. Unlike Internet domain sockets, Unix
831 domain sockets can show up in the file system with an ls(1) listing.
834 srw-rw-rw- 1 root 0 Oct 31 07:23 /dev/log
836 You can test for these with Perl's B<-S> file test:
838 unless ( -S '/dev/log' ) {
839 die "something's wicked with the print system";
842 Here's a sample Unix-domain client:
847 my ($rendezvous, $line);
849 $rendezvous = shift || '/tmp/catsock';
850 socket(SOCK, PF_UNIX, SOCK_STREAM, 0) || die "socket: $!";
851 connect(SOCK, sockaddr_un($rendezvous)) || die "connect: $!";
852 while (defined($line = <SOCK>)) {
857 And here's a corresponding server. You don't have to worry about silly
858 network terminators here because Unix domain sockets are guaranteed
859 to be on the localhost, and thus everything works right.
866 BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
867 sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
869 my $NAME = '/tmp/catsock';
870 my $uaddr = sockaddr_un($NAME);
871 my $proto = getprotobyname('tcp');
873 socket(Server,PF_UNIX,SOCK_STREAM,0) || die "socket: $!";
875 bind (Server, $uaddr) || die "bind: $!";
876 listen(Server,SOMAXCONN) || die "listen: $!";
878 logmsg "server started on $NAME";
884 $SIG{CHLD} = \&REAPER; # loathe sysV
885 logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
888 $SIG{CHLD} = \&REAPER;
891 for ( $waitedpid = 0;
892 accept(Client,Server) || $waitedpid;
893 $waitedpid = 0, close Client)
896 logmsg "connection on $NAME";
898 print "Hello there, it's now ", scalar localtime, "\n";
899 exec '/usr/games/fortune' or die "can't exec fortune: $!";
903 As you see, it's remarkably similar to the Internet domain TCP server, so
904 much so, in fact, that we've omitted several duplicate functions--spawn(),
905 logmsg(), ctime(), and REAPER()--which are exactly the same as in the
908 So why would you ever want to use a Unix domain socket instead of a
909 simpler named pipe? Because a named pipe doesn't give you sessions. You
910 can't tell one process's data from another's. With socket programming,
911 you get a separate session for each client: that's why accept() takes two
914 For example, let's say that you have a long running database server daemon
915 that you want folks from the World Wide Web to be able to access, but only
916 if they go through a CGI interface. You'd have a small, simple CGI
917 program that does whatever checks and logging you feel like, and then acts
918 as a Unix-domain client and connects to your private server.
920 =head1 TCP Clients with IO::Socket
922 For those preferring a higher-level interface to socket programming, the
923 IO::Socket module provides an object-oriented approach. IO::Socket is
924 included as part of the standard Perl distribution as of the 5.004
925 release. If you're running an earlier version of Perl, just fetch
926 IO::Socket from CPAN, where you'll also find find modules providing easy
927 interfaces to the following systems: DNS, FTP, Ident (RFC 931), NIS and
928 NISPlus, NNTP, Ping, POP3, SMTP, SNMP, SSLeay, Telnet, and Time--just
931 =head2 A Simple Client
933 Here's a client that creates a TCP connection to the "daytime"
934 service at port 13 of the host name "localhost" and prints out everything
935 that the server there cares to provide.
939 $remote = IO::Socket::INET->new(
941 PeerAddr => "localhost",
942 PeerPort => "daytime(13)",
944 or die "cannot connect to daytime port at localhost";
945 while ( <$remote> ) { print }
947 When you run this program, you should get something back that
950 Wed May 14 08:40:46 MDT 1997
952 Here are what those parameters to the C<new> constructor mean:
958 This is which protocol to use. In this case, the socket handle returned
959 will be connected to a TCP socket, because we want a stream-oriented
960 connection, that is, one that acts pretty much like a plain old file.
961 Not all sockets are this of this type. For example, the UDP protocol
962 can be used to make a datagram socket, used for message-passing.
966 This is the name or Internet address of the remote host the server is
967 running on. We could have specified a longer name like C<"www.perl.com">,
968 or an address like C<"204.148.40.9">. For demonstration purposes, we've
969 used the special hostname C<"localhost">, which should always mean the
970 current machine you're running on. The corresponding Internet address
971 for localhost is C<"127.1">, if you'd rather use that.
975 This is the service name or port number we'd like to connect to.
976 We could have gotten away with using just C<"daytime"> on systems with a
977 well-configured system services file,[FOOTNOTE: The system services file
978 is in I</etc/services> under Unix] but just in case, we've specified the
979 port number (13) in parentheses. Using just the number would also have
980 worked, but constant numbers make careful programmers nervous.
984 Notice how the return value from the C<new> constructor is used as
985 a filehandle in the C<while> loop? That's what's called an indirect
986 filehandle, a scalar variable containing a filehandle. You can use
987 it the same way you would a normal filehandle. For example, you
988 can read one line from it this way:
992 all remaining lines from is this way:
996 and send a line of data to it this way:
998 print $handle "some data\n";
1000 =head2 A Webget Client
1002 Here's a simple client that takes a remote host to fetch a document
1003 from, and then a list of documents to get from that host. This is a
1004 more interesting client than the previous one because it first sends
1005 something to the server before fetching the server's response.
1009 unless (@ARGV > 1) { die "usage: $0 host document ..." }
1010 $host = shift(@ARGV);
1013 foreach $document ( @ARGV ) {
1014 $remote = IO::Socket::INET->new( Proto => "tcp",
1016 PeerPort => "http(80)",
1018 unless ($remote) { die "cannot connect to http daemon on $host" }
1019 $remote->autoflush(1);
1020 print $remote "GET $document HTTP/1.0" . $BLANK;
1021 while ( <$remote> ) { print }
1025 The web server handing the "http" service, which is assumed to be at
1026 its standard port, number 80. If your the web server you're trying to
1027 connect to is at a different port (like 1080 or 8080), you should specify
1028 as the named-parameter pair, C<PeerPort =E<gt> 8080>. The C<autoflush>
1029 method is used on the socket because otherwise the system would buffer
1030 up the output we sent it. (If you're on a Mac, you'll also need to
1031 change every C<"\n"> in your code that sends data over the network to
1032 be a C<"\015\012"> instead.)
1034 Connecting to the server is only the first part of the process: once you
1035 have the connection, you have to use the server's language. Each server
1036 on the network has its own little command language that it expects as
1037 input. The string that we send to the server starting with "GET" is in
1038 HTTP syntax. In this case, we simply request each specified document.
1039 Yes, we really are making a new connection for each document, even though
1040 it's the same host. That's the way you always used to have to speak HTTP.
1041 Recent versions of web browsers may request that the remote server leave
1042 the connection open a little while, but the server doesn't have to honor
1045 Here's an example of running that program, which we'll call I<webget>:
1047 % webget www.perl.com /guanaco.html
1048 HTTP/1.1 404 File Not Found
1049 Date: Thu, 08 May 1997 18:02:32 GMT
1050 Server: Apache/1.2b6
1052 Content-type: text/html
1054 <HEAD><TITLE>404 File Not Found</TITLE></HEAD>
1055 <BODY><H1>File Not Found</H1>
1056 The requested URL /guanaco.html was not found on this server.<P>
1059 Ok, so that's not very interesting, because it didn't find that
1060 particular document. But a long response wouldn't have fit on this page.
1062 For a more fully-featured version of this program, you should look to
1063 the I<lwp-request> program included with the LWP modules from CPAN.
1065 =head2 Interactive Client with IO::Socket
1067 Well, that's all fine if you want to send one command and get one answer,
1068 but what about setting up something fully interactive, somewhat like
1069 the way I<telnet> works? That way you can type a line, get the answer,
1070 type a line, get the answer, etc.
1072 This client is more complicated than the two we've done so far, but if
1073 you're on a system that supports the powerful C<fork> call, the solution
1074 isn't that rough. Once you've made the connection to whatever service
1075 you'd like to chat with, call C<fork> to clone your process. Each of
1076 these two identical process has a very simple job to do: the parent
1077 copies everything from the socket to standard output, while the child
1078 simultaneously copies everything from standard input to the socket.
1079 To accomplish the same thing using just one process would be I<much>
1080 harder, because it's easier to code two processes to do one thing than it
1081 is to code one process to do two things. (This keep-it-simple principle
1082 a cornerstones of the Unix philosophy, and good software engineering as
1083 well, which is probably why it's spread to other systems.)
1090 my ($host, $port, $kidpid, $handle, $line);
1092 unless (@ARGV == 2) { die "usage: $0 host port" }
1093 ($host, $port) = @ARGV;
1095 # create a tcp connection to the specified host and port
1096 $handle = IO::Socket::INET->new(Proto => "tcp",
1099 or die "can't connect to port $port on $host: $!";
1101 $handle->autoflush(1); # so output gets there right away
1102 print STDERR "[Connected to $host:$port]\n";
1104 # split the program into two processes, identical twins
1105 die "can't fork: $!" unless defined($kidpid = fork());
1107 # the if{} block runs only in the parent process
1109 # copy the socket to standard output
1110 while (defined ($line = <$handle>)) {
1113 kill("TERM", $kidpid); # send SIGTERM to child
1115 # the else{} block runs only in the child process
1117 # copy standard input to the socket
1118 while (defined ($line = <STDIN>)) {
1119 print $handle $line;
1123 The C<kill> function in the parent's C<if> block is there to send a
1124 signal to our child process (current running in the C<else> block)
1125 as soon as the remote server has closed its end of the connection.
1127 If the remote server sends data a byte at time, and you need that
1128 data immediately without waiting for a newline (which might not happen),
1129 you may wish to replace the C<while> loop in the parent with the
1133 while (sysread($handle, $byte, 1) == 1) {
1137 Making a system call for each byte you want to read is not very efficient
1138 (to put it mildly) but is the simplest to explain and works reasonably
1141 =head1 TCP Servers with IO::Socket
1143 As always, setting up a server is little bit more involved than running a client.
1144 The model is that the server creates a special kind of socket that
1145 does nothing but listen on a particular port for incoming connections.
1146 It does this by calling the C<IO::Socket::INET-E<gt>new()> method with
1147 slightly different arguments than the client did.
1153 This is which protocol to use. Like our clients, we'll
1154 still specify C<"tcp"> here.
1159 port in the C<LocalPort> argument, which we didn't do for the client.
1160 This is service name or port number for which you want to be the
1161 server. (Under Unix, ports under 1024 are restricted to the
1162 superuser.) In our sample, we'll use port 9000, but you can use
1163 any port that's not currently in use on your system. If you try
1164 to use one already in used, you'll get an "Address already in use"
1165 message. Under Unix, the C<netstat -a> command will show
1166 which services current have servers.
1170 The C<Listen> parameter is set to the maximum number of
1171 pending connections we can accept until we turn away incoming clients.
1172 Think of it as a call-waiting queue for your telephone.
1173 The low-level Socket module has a special symbol for the system maximum, which
1178 The C<Reuse> parameter is needed so that we restart our server
1179 manually without waiting a few minutes to allow system buffers to
1184 Once the generic server socket has been created using the parameters
1185 listed above, the server then waits for a new client to connect
1186 to it. The server blocks in the C<accept> method, which eventually an
1187 bidirectional connection to the remote client. (Make sure to autoflush
1188 this handle to circumvent buffering.)
1190 To add to user-friendliness, our server prompts the user for commands.
1191 Most servers don't do this. Because of the prompt without a newline,
1192 you'll have to use the C<sysread> variant of the interactive client above.
1194 This server accepts one of five different commands, sending output
1195 back to the client. Note that unlike most network servers, this one
1196 only handles one incoming client at a time. Multithreaded servers are
1197 covered in Chapter 6 of the Camel as well as later in this manpage.
1199 Here's the code. We'll
1203 use Net::hostent; # for OO version of gethostbyaddr
1205 $PORT = 9000; # pick something not in use
1207 $server = IO::Socket::INET->new( Proto => 'tcp',
1209 Listen => SOMAXCONN,
1212 die "can't setup server" unless $server;
1213 print "[Server $0 accepting clients]\n";
1215 while ($client = $server->accept()) {
1216 $client->autoflush(1);
1217 print $client "Welcome to $0; type help for command list.\n";
1218 $hostinfo = gethostbyaddr($client->peeraddr);
1219 printf "[Connect from %s]\n", $hostinfo->name || $client->peerhost;
1220 print $client "Command? ";
1221 while ( <$client>) {
1222 next unless /\S/; # blank line
1223 if (/quit|exit/i) { last; }
1224 elsif (/date|time/i) { printf $client "%s\n", scalar localtime; }
1225 elsif (/who/i ) { print $client `who 2>&1`; }
1226 elsif (/cookie/i ) { print $client `/usr/games/fortune 2>&1`; }
1227 elsif (/motd/i ) { print $client `cat /etc/motd 2>&1`; }
1229 print $client "Commands: quit date who cookie motd\n";
1232 print $client "Command? ";
1237 =head1 UDP: Message Passing
1239 Another kind of client-server setup is one that uses not connections, but
1240 messages. UDP communications involve much lower overhead but also provide
1241 less reliability, as there are no promises that messages will arrive at
1242 all, let alone in order and unmangled. Still, UDP offers some advantages
1243 over TCP, including being able to "broadcast" or "multicast" to a whole
1244 bunch of destination hosts at once (usually on your local subnet). If you
1245 find yourself overly concerned about reliability and start building checks
1246 into your message system, then you probably should use just TCP to start
1249 Here's a UDP program similar to the sample Internet TCP client given
1250 earlier. However, instead of checking one host at a time, the UDP version
1251 will check many of them asynchronously by simulating a multicast and then
1252 using select() to do a timed-out wait for I/O. To do something similar
1253 with TCP, you'd have to use a different socket handle for each host.
1260 my ( $count, $hisiaddr, $hispaddr, $histime,
1261 $host, $iaddr, $paddr, $port, $proto,
1262 $rin, $rout, $rtime, $SECS_of_70_YEARS);
1264 $SECS_of_70_YEARS = 2208988800;
1266 $iaddr = gethostbyname(hostname());
1267 $proto = getprotobyname('udp');
1268 $port = getservbyname('time', 'udp');
1269 $paddr = sockaddr_in(0, $iaddr); # 0 means let kernel pick
1271 socket(SOCKET, PF_INET, SOCK_DGRAM, $proto) || die "socket: $!";
1272 bind(SOCKET, $paddr) || die "bind: $!";
1275 printf "%-12s %8s %s\n", "localhost", 0, scalar localtime time;
1279 $hisiaddr = inet_aton($host) || die "unknown host";
1280 $hispaddr = sockaddr_in($port, $hisiaddr);
1281 defined(send(SOCKET, 0, 0, $hispaddr)) || die "send $host: $!";
1285 vec($rin, fileno(SOCKET), 1) = 1;
1287 # timeout after 10.0 seconds
1288 while ($count && select($rout = $rin, undef, undef, 10.0)) {
1290 ($hispaddr = recv(SOCKET, $rtime, 4, 0)) || die "recv: $!";
1291 ($port, $hisiaddr) = sockaddr_in($hispaddr);
1292 $host = gethostbyaddr($hisiaddr, AF_INET);
1293 $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ;
1294 printf "%-12s ", $host;
1295 printf "%8d %s\n", $histime - time, scalar localtime($histime);
1301 While System V IPC isn't so widely used as sockets, it still has some
1302 interesting uses. You can't, however, effectively use SysV IPC or
1303 Berkeley mmap() to have shared memory so as to share a variable amongst
1304 several processes. That's because Perl would reallocate your string when
1305 you weren't wanting it to.
1307 Here's a small example showing shared memory usage.
1309 use IPC::SysV qw(IPC_PRIVATE IPC_RMID S_IRWXU S_IRWXG S_IRWXO);
1312 $key = shmget(IPC_PRIVATE, $size, S_IRWXU|S_IRWXG|S_IRWXO) || die "$!";
1313 print "shm key $key\n";
1315 $message = "Message #1";
1316 shmwrite($key, $message, 0, 60) || die "$!";
1317 print "wrote: '$message'\n";
1318 shmread($key, $buff, 0, 60) || die "$!";
1319 print "read : '$buff'\n";
1321 # the buffer of shmread is zero-character end-padded.
1322 substr($buff, index($buff, "\0")) = '';
1323 print "un" unless $buff eq $message;
1326 print "deleting shm $key\n";
1327 shmctl($key, IPC_RMID, 0) || die "$!";
1329 Here's an example of a semaphore:
1331 use IPC::SysV qw(IPC_CREAT);
1334 $key = semget($IPC_KEY, 10, 0666 | IPC_CREAT ) || die "$!";
1335 print "shm key $key\n";
1337 Put this code in a separate file to be run in more than one process.
1338 Call the file F<take>:
1340 # create a semaphore
1343 $key = semget($IPC_KEY, 0 , 0 );
1344 die if !defined($key);
1350 # wait for semaphore to be zero
1352 $opstring1 = pack("sss", $semnum, $semop, $semflag);
1354 # Increment the semaphore count
1356 $opstring2 = pack("sss", $semnum, $semop, $semflag);
1357 $opstring = $opstring1 . $opstring2;
1359 semop($key,$opstring) || die "$!";
1361 Put this code in a separate file to be run in more than one process.
1362 Call this file F<give>:
1364 # 'give' the semaphore
1365 # run this in the original process and you will see
1366 # that the second process continues
1369 $key = semget($IPC_KEY, 0, 0);
1370 die if !defined($key);
1375 # Decrement the semaphore count
1377 $opstring = pack("sss", $semnum, $semop, $semflag);
1379 semop($key,$opstring) || die "$!";
1381 The SysV IPC code above was written long ago, and it's definitely
1382 clunky looking. For a more modern look, see the IPC::SysV module
1383 which is included with Perl starting from Perl 5.005.
1387 Most of these routines quietly but politely return C<undef> when they
1388 fail instead of causing your program to die right then and there due to
1389 an uncaught exception. (Actually, some of the new I<Socket> conversion
1390 functions croak() on bad arguments.) It is therefore essential to
1391 check return values from these functions. Always begin your socket
1392 programs this way for optimal success, and don't forget to add B<-T>
1393 taint checking flag to the #! line for servers:
1402 All these routines create system-specific portability problems. As noted
1403 elsewhere, Perl is at the mercy of your C libraries for much of its system
1404 behaviour. It's probably safest to assume broken SysV semantics for
1405 signals and to stick with simple TCP and UDP socket operations; e.g., don't
1406 try to pass open file descriptors over a local UDP datagram socket if you
1407 want your code to stand a chance of being portable.
1409 As mentioned in the signals section, because few vendors provide C
1410 libraries that are safely re-entrant, the prudent programmer will do
1411 little else within a handler beyond setting a numeric variable that
1412 already exists; or, if locked into a slow (restarting) system call,
1413 using die() to raise an exception and longjmp(3) out. In fact, even
1414 these may in some cases cause a core dump. It's probably best to avoid
1415 signals except where they are absolutely inevitable. This
1416 will be addressed in a future release of Perl.
1420 Tom Christiansen, with occasional vestiges of Larry Wall's original
1421 version and suggestions from the Perl Porters.
1425 There's a lot more to networking than this, but this should get you
1428 For intrepid programmers, the indispensable textbook is I<Unix Network
1429 Programming> by W. Richard Stevens (published by Addison-Wesley). Note
1430 that most books on networking address networking from the perspective of
1431 a C programmer; translation to Perl is left as an exercise for the reader.
1433 The IO::Socket(3) manpage describes the object library, and the Socket(3)
1434 manpage describes the low-level interface to sockets. Besides the obvious
1435 functions in L<perlfunc>, you should also check out the F<modules> file
1436 at your nearest CPAN site. (See L<perlmodlib> or best yet, the F<Perl
1437 FAQ> for a description of what CPAN is and where to get it.)
1439 Section 5 of the F<modules> file is devoted to "Networking, Device Control
1440 (modems), and Interprocess Communication", and contains numerous unbundled
1441 modules numerous networking modules, Chat and Expect operations, CGI
1442 programming, DCE, FTP, IPC, NNTP, Proxy, Ptty, RPC, SNMP, SMTP, Telnet,
1443 Threads, and ToolTalk--just to name a few.