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
14 or references of user-installed signal handlers. These handlers will
15 be called with an argument which is the name of the signal that
16 triggered it. A signal may be generated intentionally from a
17 particular keyboard sequence like control-C or control-Z, sent to you
18 from another process, or triggered automatically by the kernel when
19 special events transpire, like a child process exiting, your process
20 running out of stack space, or hitting file size limit.
22 For example, to trap an interrupt signal, set up a handler like this:
27 die "Somebody sent me a SIG$signame";
29 $SIG{INT} = 'catch_zap'; # could fail in modules
30 $SIG{INT} = \&catch_zap; # best strategy
32 Prior to Perl 5.7.3 it was necessary to do as little as you possibly
33 could in your handler; notice how all we do is set a global variable
34 and then raise an exception. That's because on most systems,
35 libraries are not re-entrant; particularly, memory allocation and I/O
36 routines are not. That meant that doing nearly I<anything> in your
37 handler could in theory trigger a memory fault and subsequent core
38 dump - see L<Deferred Signals> below.
40 The names of the signals are the ones listed out by C<kill -l> on your
41 system, or you can retrieve them from the Config module. Set up an
42 @signame list indexed by number to get the name and a %signo table
43 indexed by name to get the number:
46 defined $Config{sig_name} || die "No sigs?";
47 foreach $name (split(' ', $Config{sig_name})) {
53 So to check whether signal 17 and SIGALRM were the same, do just this:
55 print "signal #17 = $signame[17]\n";
57 print "SIGALRM is $signo{ALRM}\n";
60 You may also choose to assign the strings C<'IGNORE'> or C<'DEFAULT'> as
61 the handler, in which case Perl will try to discard the signal or do the
64 On most Unix platforms, the C<CHLD> (sometimes also known as C<CLD>) signal
65 has special behavior with respect to a value of C<'IGNORE'>.
66 Setting C<$SIG{CHLD}> to C<'IGNORE'> on such a platform has the effect of
67 not creating zombie processes when the parent process fails to C<wait()>
68 on its child processes (i.e. child processes are automatically reaped).
69 Calling C<wait()> with C<$SIG{CHLD}> set to C<'IGNORE'> usually returns
70 C<-1> on such platforms.
72 Some signals can be neither trapped nor ignored, such as
73 the KILL and STOP (but not the TSTP) signals. One strategy for
74 temporarily ignoring signals is to use a local() statement, which will be
75 automatically restored once your block is exited. (Remember that local()
76 values are "inherited" by functions called from within that block.)
79 local $SIG{INT} = 'IGNORE';
83 # interrupts still ignored, for now...
86 Sending a signal to a negative process ID means that you send the signal
87 to the entire Unix process-group. This code sends a hang-up signal to all
88 processes in the current process group (and sets $SIG{HUP} to IGNORE so
89 it doesn't kill itself):
92 local $SIG{HUP} = 'IGNORE';
94 # snazzy writing of: kill('HUP', -$$)
97 Another interesting signal to send is signal number zero. This doesn't
98 actually affect another process, but instead checks whether it's alive
99 or has changed its UID.
101 unless (kill 0 => $kid_pid) {
102 warn "something wicked happened to $kid_pid";
105 You might also want to employ anonymous functions for simple signal
108 $SIG{INT} = sub { die "\nOutta here!\n" };
110 But that will be problematic for the more complicated handlers that need
111 to reinstall themselves. Because Perl's signal mechanism is currently
112 based on the signal(3) function from the C library, you may sometimes be so
113 misfortunate as to run on systems where that function is "broken", that
114 is, it behaves in the old unreliable SysV way rather than the newer, more
115 reasonable BSD and POSIX fashion. So you'll see defensive people writing
116 signal handlers like this:
120 # loathe sysV: it makes us not only reinstate
121 # the handler, but place it after the wait
122 $SIG{CHLD} = \&REAPER;
124 $SIG{CHLD} = \&REAPER;
125 # now do something that forks...
129 use POSIX ":sys_wait_h";
132 # If a second child dies while in the signal handler caused by the
133 # first death, we won't get another signal. So must loop here else
134 # we will leave the unreaped child as a zombie. And the next time
135 # two children die we get another zombie. And so on.
136 while (($child = waitpid(-1,WNOHANG)) > 0) {
137 $Kid_Status{$child} = $?;
139 $SIG{CHLD} = \&REAPER; # still loathe sysV
141 $SIG{CHLD} = \&REAPER;
142 # do something that forks...
144 Signal handling is also used for timeouts in Unix, While safely
145 protected within an C<eval{}> block, you set a signal handler to trap
146 alarm signals and then schedule to have one delivered to you in some
147 number of seconds. Then try your blocking operation, clearing the alarm
148 when it's done but not before you've exited your C<eval{}> block. If it
149 goes off, you'll use die() to jump out of the block, much as you might
150 using longjmp() or throw() in other languages.
155 local $SIG{ALRM} = sub { die "alarm clock restart" };
157 flock(FH, 2); # blocking write lock
160 if ($@ and $@ !~ /alarm clock restart/) { die }
162 If the operation being timed out is system() or qx(), this technique
163 is liable to generate zombies. If this matters to you, you'll
164 need to do your own fork() and exec(), and kill the errant child process.
166 For more complex signal handling, you might see the standard POSIX
167 module. Lamentably, this is almost entirely undocumented, but
168 the F<t/lib/posix.t> file from the Perl source distribution has some
171 =head2 Handling the SIGHUP Signal in Daemons
173 A process that usually starts when the system boots and shuts down
174 when the system is shut down is called a daemon (Disk And Execution
175 MONitor). If a daemon process has a configuration file which is
176 modified after the process has been started, there should be a way to
177 tell that process to re-read its configuration file, without stopping
178 the process. Many daemons provide this mechanism using the C<SIGHUP>
179 signal handler. When you want to tell the daemon to re-read the file
180 you simply send it the C<SIGHUP> signal.
182 Not all platforms automatically reinstall their (native) signal
183 handlers after a signal delivery. This means that the handler works
184 only the first time the signal is sent. The solution to this problem
185 is to use C<POSIX> signal handlers if available, their behaviour
188 The following example implements a simple daemon, which restarts
189 itself every time the C<SIGHUP> signal is received. The actual code is
190 located in the subroutine C<code()>, which simply prints some debug
191 info to show that it works and should be replaced with the real code.
197 use File::Basename ();
198 use File::Spec::Functions;
202 # make the daemon cross-platform, so exec always calls the script
203 # itself with the right path, no matter how the script was invoked.
204 my $script = File::Basename::basename($0);
205 my $SELF = catfile $FindBin::Bin, $script;
207 # POSIX unmasks the sigprocmask properly
208 my $sigset = POSIX::SigSet->new();
209 my $action = POSIX::SigAction->new('sigHUP_handler',
212 POSIX::sigaction(&POSIX::SIGHUP, $action);
215 print "got SIGHUP\n";
216 exec($SELF, @ARGV) or die "Couldn't restart: $!\n";
223 print "ARGV: @ARGV\n";
235 A named pipe (often referred to as a FIFO) is an old Unix IPC
236 mechanism for processes communicating on the same machine. It works
237 just like a regular, connected anonymous pipes, except that the
238 processes rendezvous using a filename and don't have to be related.
240 To create a named pipe, use the Unix command mknod(1) or on some
241 systems, mkfifo(1). These may not be in your normal path.
243 # system return val is backwards, so && not ||
245 $ENV{PATH} .= ":/etc:/usr/etc";
246 if ( system('mknod', $path, 'p')
247 && system('mkfifo', $path) )
249 die "mk{nod,fifo} $path failed";
253 A fifo is convenient when you want to connect a process to an unrelated
254 one. When you open a fifo, the program will block until there's something
257 For example, let's say you'd like to have your F<.signature> file be a
258 named pipe that has a Perl program on the other end. Now every time any
259 program (like a mailer, news reader, finger program, etc.) tries to read
260 from that file, the reading program will block and your program will
261 supply the new signature. We'll use the pipe-checking file test B<-p>
262 to find out whether anyone (or anything) has accidentally removed our fifo.
265 $FIFO = '.signature';
266 $ENV{PATH} .= ":/etc:/usr/games";
271 system('mknod', $FIFO, 'p')
272 && die "can't mknod $FIFO: $!";
275 # next line blocks until there's a reader
276 open (FIFO, "> $FIFO") || die "can't write $FIFO: $!";
277 print FIFO "John Smith (smith\@host.org)\n", `fortune -s`;
279 sleep 2; # to avoid dup signals
282 =head2 Deferred Signals
284 In Perls before Perl 5.7.3 by installing Perl code to deal with
285 signals, you were exposing yourself to danger from two things. First,
286 few system library functions are re-entrant. If the signal interrupts
287 while Perl is executing one function (like malloc(3) or printf(3)),
288 and your signal handler then calls the same function again, you could
289 get unpredictable behavior--often, a core dump. Second, Perl isn't
290 itself re-entrant at the lowest levels. If the signal interrupts Perl
291 while Perl is changing its own internal data structures, similarly
292 unpredictable behaviour may result.
294 There were two things you could do, knowing this: be paranoid or be
295 pragmatic. The paranoid approach was to do as little as possible in your
296 signal handler. Set an existing integer variable that already has a
297 value, and return. This doesn't help you if you're in a slow system call,
298 which will just restart. That means you have to C<die> to longjump(3) out
299 of the handler. Even this is a little cavalier for the true paranoiac,
300 who avoids C<die> in a handler because the system I<is> out to get you.
301 The pragmatic approach was to say ``I know the risks, but prefer the
302 convenience'', and to do anything you wanted in your signal handler,
303 and be prepared to clean up core dumps now and again.
305 In Perl 5.7.3 and later to avoid these problems signals are
306 "deferred"-- that is when the signal is delivered to the process by
307 the system (to the C code that implements Perl) a flag is set, and the
308 handler returns immediately. Then at strategic "safe" points in the
309 Perl interpreter (e.g. when it is about to execute a new opcode) the
310 flags are checked and the Perl level handler from %SIG is
311 executed. The "deferred" scheme allows much more flexibility in the
312 coding of signal handler as we know Perl interpreter is in a safe
313 state, and that we are not in a system library function when the
314 handler is called. However the implementation does differ from
315 previous Perls in the following ways:
319 =item Long running opcodes
321 As Perl interpreter only looks at the signal flags when it about to
322 execute a new opcode if a signal arrives during a long running opcode
323 (e.g. a regular expression operation on a very large string) then
324 signal will not be seen until operation completes.
326 =item Interrupting IO
328 When a signal is delivered (e.g. INT control-C) the operating system
329 breaks into IO operations like C<read> (used to implement Perls
330 E<lt>E<gt> operator). On older Perls the handler was called
331 immediately (and as C<read> is not "unsafe" this worked well). With
332 the "deferred" scheme the handler is not called immediately, and if
333 Perl is using system's C<stdio> library that library may re-start the
334 C<read> without returning to Perl and giving it a chance to call the
335 %SIG handler. If this happens on your system the solution is to use
336 C<:perlio> layer to do IO - at least on those handles which you want
337 to be able to break into with signals. (The C<:perlio> layer checks
338 the signal flags and calls %SIG handlers before resuming IO operation.)
340 Note that the default in Perl 5.7.3 and later is to automatically use
341 the C<:perlio> layer.
343 =item Signals as "faults"
345 Certain signals e.g. SEGV, ILL, BUS are generated as a result of
346 virtual memory or other "faults". These are normally fatal and there
347 is little a Perl-level handler can do with them. (In particular the
348 old signal scheme was particularly unsafe in such cases.) However if
349 a %SIG handler is set the new scheme simply sets a flag and returns as
350 described above. This may cause the operating system to try the
351 offending machine instruction again and - as nothing has changed - it
352 will generate the signal again. The result of this is a rather odd
353 "loop". In future Perl's signal mechanism may be changed to avoid this
354 - perhaps by simply disallowing %SIG handlers on signals of that
355 type. Until then the work-round is not to set a %SIG handler on those
356 signals. (Which signals they are is operating system dependant.)
358 =item Signals triggered by operating system state
360 On some operating systems certain signal handlers are supposed to "do
361 something" before returning. One example can be CHLD or CLD which
362 indicates a child process has completed. On some operating systems the
363 signal handler is expected to C<wait> for the completed child
364 process. On such systems the deferred signal scheme will not work for
365 those signals (it does not do the C<wait>). Again the failure will
366 look like a loop as the operating system will re-issue the signal as
367 there are un-waited-for completed child processes.
371 =head1 Using open() for IPC
373 Perl's basic open() statement can also be used for unidirectional
374 interprocess communication by either appending or prepending a pipe
375 symbol to the second argument to open(). Here's how to start
376 something up in a child process you intend to write to:
378 open(SPOOLER, "| cat -v | lpr -h 2>/dev/null")
379 || die "can't fork: $!";
380 local $SIG{PIPE} = sub { die "spooler pipe broke" };
381 print SPOOLER "stuff\n";
382 close SPOOLER || die "bad spool: $! $?";
384 And here's how to start up a child process you intend to read from:
386 open(STATUS, "netstat -an 2>&1 |")
387 || die "can't fork: $!";
389 next if /^(tcp|udp)/;
392 close STATUS || die "bad netstat: $! $?";
394 If one can be sure that a particular program is a Perl script that is
395 expecting filenames in @ARGV, the clever programmer can write something
398 % program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile
400 and irrespective of which shell it's called from, the Perl program will
401 read from the file F<f1>, the process F<cmd1>, standard input (F<tmpfile>
402 in this case), the F<f2> file, the F<cmd2> command, and finally the F<f3>
403 file. Pretty nifty, eh?
405 You might notice that you could use backticks for much the
406 same effect as opening a pipe for reading:
408 print grep { !/^(tcp|udp)/ } `netstat -an 2>&1`;
409 die "bad netstat" if $?;
411 While this is true on the surface, it's much more efficient to process the
412 file one line or record at a time because then you don't have to read the
413 whole thing into memory at once. It also gives you finer control of the
414 whole process, letting you to kill off the child process early if you'd
417 Be careful to check both the open() and the close() return values. If
418 you're I<writing> to a pipe, you should also trap SIGPIPE. Otherwise,
419 think of what happens when you start up a pipe to a command that doesn't
420 exist: the open() will in all likelihood succeed (it only reflects the
421 fork()'s success), but then your output will fail--spectacularly. Perl
422 can't know whether the command worked because your command is actually
423 running in a separate process whose exec() might have failed. Therefore,
424 while readers of bogus commands return just a quick end of file, writers
425 to bogus command will trigger a signal they'd better be prepared to
428 open(FH, "|bogus") or die "can't fork: $!";
429 print FH "bang\n" or die "can't write: $!";
430 close FH or die "can't close: $!";
432 That won't blow up until the close, and it will blow up with a SIGPIPE.
433 To catch it, you could use this:
435 $SIG{PIPE} = 'IGNORE';
436 open(FH, "|bogus") or die "can't fork: $!";
437 print FH "bang\n" or die "can't write: $!";
438 close FH or die "can't close: status=$?";
442 Both the main process and any child processes it forks share the same
443 STDIN, STDOUT, and STDERR filehandles. If both processes try to access
444 them at once, strange things can happen. You may also want to close
445 or reopen the filehandles for the child. You can get around this by
446 opening your pipe with open(), but on some systems this means that the
447 child process cannot outlive the parent.
449 =head2 Background Processes
451 You can run a command in the background with:
455 The command's STDOUT and STDERR (and possibly STDIN, depending on your
456 shell) will be the same as the parent's. You won't need to catch
457 SIGCHLD because of the double-fork taking place (see below for more
460 =head2 Complete Dissociation of Child from Parent
462 In some cases (starting server processes, for instance) you'll want to
463 completely dissociate the child process from the parent. This is
464 often called daemonization. A well behaved daemon will also chdir()
465 to the root directory (so it doesn't prevent unmounting the filesystem
466 containing the directory from which it was launched) and redirect its
467 standard file descriptors from and to F</dev/null> (so that random
468 output doesn't wind up on the user's terminal).
473 chdir '/' or die "Can't chdir to /: $!";
474 open STDIN, '/dev/null' or die "Can't read /dev/null: $!";
475 open STDOUT, '>/dev/null'
476 or die "Can't write to /dev/null: $!";
477 defined(my $pid = fork) or die "Can't fork: $!";
479 setsid or die "Can't start a new session: $!";
480 open STDERR, '>&STDOUT' or die "Can't dup stdout: $!";
483 The fork() has to come before the setsid() to ensure that you aren't a
484 process group leader (the setsid() will fail if you are). If your
485 system doesn't have the setsid() function, open F</dev/tty> and use the
486 C<TIOCNOTTY> ioctl() on it instead. See L<tty(4)> for details.
488 Non-Unix users should check their Your_OS::Process module for other
491 =head2 Safe Pipe Opens
493 Another interesting approach to IPC is making your single program go
494 multiprocess and communicate between (or even amongst) yourselves. The
495 open() function will accept a file argument of either C<"-|"> or C<"|-">
496 to do a very interesting thing: it forks a child connected to the
497 filehandle you've opened. The child is running the same program as the
498 parent. This is useful for safely opening a file when running under an
499 assumed UID or GID, for example. If you open a pipe I<to> minus, you can
500 write to the filehandle you opened and your kid will find it in his
501 STDIN. If you open a pipe I<from> minus, you can read from the filehandle
502 you opened whatever your kid writes to his STDOUT.
504 use English '-no_match_vars';
508 $pid = open(KID_TO_WRITE, "|-");
509 unless (defined $pid) {
510 warn "cannot fork: $!";
511 die "bailing out" if $sleep_count++ > 6;
514 } until defined $pid;
517 print KID_TO_WRITE @some_data;
518 close(KID_TO_WRITE) || warn "kid exited $?";
520 ($EUID, $EGID) = ($UID, $GID); # suid progs only
521 open (FILE, "> /safe/file")
522 || die "can't open /safe/file: $!";
524 print FILE; # child's STDIN is parent's KID
526 exit; # don't forget this
529 Another common use for this construct is when you need to execute
530 something without the shell's interference. With system(), it's
531 straightforward, but you can't use a pipe open or backticks safely.
532 That's because there's no way to stop the shell from getting its hands on
533 your arguments. Instead, use lower-level control to call exec() directly.
535 Here's a safe backtick or pipe open for read:
537 # add error processing as above
538 $pid = open(KID_TO_READ, "-|");
541 while (<KID_TO_READ>) {
542 # do something interesting
544 close(KID_TO_READ) || warn "kid exited $?";
547 ($EUID, $EGID) = ($UID, $GID); # suid only
548 exec($program, @options, @args)
549 || die "can't exec program: $!";
554 And here's a safe pipe open for writing:
556 # add error processing as above
557 $pid = open(KID_TO_WRITE, "|-");
558 $SIG{ALRM} = sub { die "whoops, $program pipe broke" };
564 close(KID_TO_WRITE) || warn "kid exited $?";
567 ($EUID, $EGID) = ($UID, $GID);
568 exec($program, @options, @args)
569 || die "can't exec program: $!";
573 Since Perl 5.8.0, you can also use the list form of C<open> for pipes :
576 open KID_PS, "-|", "ps", "aux" or die $!;
578 forks the ps(1) command (without spawning a shell, as there are more than
579 three arguments to open()), and reads its standard output via the
580 C<KID_PS> filehandle.
582 Note that these operations are full Unix forks, which means they may not be
583 correctly implemented on alien systems. Additionally, these are not true
584 multithreading. If you'd like to learn more about threading, see the
585 F<modules> file mentioned below in the SEE ALSO section.
587 =head2 Bidirectional Communication with Another Process
589 While this works reasonably well for unidirectional communication, what
590 about bidirectional communication? The obvious thing you'd like to do
591 doesn't actually work:
593 open(PROG_FOR_READING_AND_WRITING, "| some program |")
595 and if you forget to use the C<use warnings> pragma or the B<-w> flag,
596 then you'll miss out entirely on the diagnostic message:
598 Can't do bidirectional pipe at -e line 1.
600 If you really want to, you can use the standard open2() library function
601 to catch both ends. There's also an open3() for tridirectional I/O so you
602 can also catch your child's STDERR, but doing so would then require an
603 awkward select() loop and wouldn't allow you to use normal Perl input
606 If you look at its source, you'll see that open2() uses low-level
607 primitives like Unix pipe() and exec() calls to create all the connections.
608 While it might have been slightly more efficient by using socketpair(), it
609 would have then been even less portable than it already is. The open2()
610 and open3() functions are unlikely to work anywhere except on a Unix
611 system or some other one purporting to be POSIX compliant.
613 Here's an example of using open2():
617 $pid = open2(*Reader, *Writer, "cat -u -n" );
618 print Writer "stuff\n";
621 The problem with this is that Unix buffering is really going to
622 ruin your day. Even though your C<Writer> filehandle is auto-flushed,
623 and the process on the other end will get your data in a timely manner,
624 you can't usually do anything to force it to give it back to you
625 in a similarly quick fashion. In this case, we could, because we
626 gave I<cat> a B<-u> flag to make it unbuffered. But very few Unix
627 commands are designed to operate over pipes, so this seldom works
628 unless you yourself wrote the program on the other end of the
631 A solution to this is the nonstandard F<Comm.pl> library. It uses
632 pseudo-ttys to make your program behave more reasonably:
635 $ph = open_proc('cat -n');
637 print $ph "a line\n";
638 print "got back ", scalar <$ph>;
641 This way you don't have to have control over the source code of the
642 program you're using. The F<Comm> library also has expect()
643 and interact() functions. Find the library (and we hope its
644 successor F<IPC::Chat>) at your nearest CPAN archive as detailed
645 in the SEE ALSO section below.
647 The newer Expect.pm module from CPAN also addresses this kind of thing.
648 This module requires two other modules from CPAN: IO::Pty and IO::Stty.
649 It sets up a pseudo-terminal to interact with programs that insist on
650 using talking to the terminal device driver. If your system is
651 amongst those supported, this may be your best bet.
653 =head2 Bidirectional Communication with Yourself
655 If you want, you may make low-level pipe() and fork()
656 to stitch this together by hand. This example only
657 talks to itself, but you could reopen the appropriate
658 handles to STDIN and STDOUT and call other processes.
661 # pipe1 - bidirectional communication using two pipe pairs
662 # designed for the socketpair-challenged
663 use IO::Handle; # thousands of lines just for autoflush :-(
664 pipe(PARENT_RDR, CHILD_WTR); # XXX: failure?
665 pipe(CHILD_RDR, PARENT_WTR); # XXX: failure?
666 CHILD_WTR->autoflush(1);
667 PARENT_WTR->autoflush(1);
670 close PARENT_RDR; close PARENT_WTR;
671 print CHILD_WTR "Parent Pid $$ is sending this\n";
672 chomp($line = <CHILD_RDR>);
673 print "Parent Pid $$ just read this: `$line'\n";
674 close CHILD_RDR; close CHILD_WTR;
677 die "cannot fork: $!" unless defined $pid;
678 close CHILD_RDR; close CHILD_WTR;
679 chomp($line = <PARENT_RDR>);
680 print "Child Pid $$ just read this: `$line'\n";
681 print PARENT_WTR "Child Pid $$ is sending this\n";
682 close PARENT_RDR; close PARENT_WTR;
686 But you don't actually have to make two pipe calls. If you
687 have the socketpair() system call, it will do this all for you.
690 # pipe2 - bidirectional communication using socketpair
691 # "the best ones always go both ways"
694 use IO::Handle; # thousands of lines just for autoflush :-(
695 # We say AF_UNIX because although *_LOCAL is the
696 # POSIX 1003.1g form of the constant, many machines
697 # still don't have it.
698 socketpair(CHILD, PARENT, AF_UNIX, SOCK_STREAM, PF_UNSPEC)
699 or die "socketpair: $!";
702 PARENT->autoflush(1);
706 print CHILD "Parent Pid $$ is sending this\n";
707 chomp($line = <CHILD>);
708 print "Parent Pid $$ just read this: `$line'\n";
712 die "cannot fork: $!" unless defined $pid;
714 chomp($line = <PARENT>);
715 print "Child Pid $$ just read this: `$line'\n";
716 print PARENT "Child Pid $$ is sending this\n";
721 =head1 Sockets: Client/Server Communication
723 While not limited to Unix-derived operating systems (e.g., WinSock on PCs
724 provides socket support, as do some VMS libraries), you may not have
725 sockets on your system, in which case this section probably isn't going to do
726 you much good. With sockets, you can do both virtual circuits (i.e., TCP
727 streams) and datagrams (i.e., UDP packets). You may be able to do even more
728 depending on your system.
730 The Perl function calls for dealing with sockets have the same names as
731 the corresponding system calls in C, but their arguments tend to differ
732 for two reasons: first, Perl filehandles work differently than C file
733 descriptors. Second, Perl already knows the length of its strings, so you
734 don't need to pass that information.
736 One of the major problems with old socket code in Perl was that it used
737 hard-coded values for some of the constants, which severely hurt
738 portability. If you ever see code that does anything like explicitly
739 setting C<$AF_INET = 2>, you know you're in for big trouble: An
740 immeasurably superior approach is to use the C<Socket> module, which more
741 reliably grants access to various constants and functions you'll need.
743 If you're not writing a server/client for an existing protocol like
744 NNTP or SMTP, you should give some thought to how your server will
745 know when the client has finished talking, and vice-versa. Most
746 protocols are based on one-line messages and responses (so one party
747 knows the other has finished when a "\n" is received) or multi-line
748 messages and responses that end with a period on an empty line
749 ("\n.\n" terminates a message/response).
751 =head2 Internet Line Terminators
753 The Internet line terminator is "\015\012". Under ASCII variants of
754 Unix, that could usually be written as "\r\n", but under other systems,
755 "\r\n" might at times be "\015\015\012", "\012\012\015", or something
756 completely different. The standards specify writing "\015\012" to be
757 conformant (be strict in what you provide), but they also recommend
758 accepting a lone "\012" on input (but be lenient in what you require).
759 We haven't always been very good about that in the code in this manpage,
760 but unless you're on a Mac, you'll probably be ok.
762 =head2 Internet TCP Clients and Servers
764 Use Internet-domain sockets when you want to do client-server
765 communication that might extend to machines outside of your own system.
767 Here's a sample TCP client using Internet-domain sockets:
772 my ($remote,$port, $iaddr, $paddr, $proto, $line);
774 $remote = shift || 'localhost';
775 $port = shift || 2345; # random port
776 if ($port =~ /\D/) { $port = getservbyname($port, 'tcp') }
777 die "No port" unless $port;
778 $iaddr = inet_aton($remote) || die "no host: $remote";
779 $paddr = sockaddr_in($port, $iaddr);
781 $proto = getprotobyname('tcp');
782 socket(SOCK, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
783 connect(SOCK, $paddr) || die "connect: $!";
784 while (defined($line = <SOCK>)) {
788 close (SOCK) || die "close: $!";
791 And here's a corresponding server to go along with it. We'll
792 leave the address as INADDR_ANY so that the kernel can choose
793 the appropriate interface on multihomed hosts. If you want sit
794 on a particular interface (like the external side of a gateway
795 or firewall machine), you should fill this in with your real address
800 BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
803 my $EOL = "\015\012";
805 sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
807 my $port = shift || 2345;
808 my $proto = getprotobyname('tcp');
810 ($port) = $port =~ /^(\d+)$/ or die "invalid port";
812 socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
813 setsockopt(Server, SOL_SOCKET, SO_REUSEADDR,
814 pack("l", 1)) || die "setsockopt: $!";
815 bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!";
816 listen(Server,SOMAXCONN) || die "listen: $!";
818 logmsg "server started on port $port";
822 $SIG{CHLD} = \&REAPER;
824 for ( ; $paddr = accept(Client,Server); close Client) {
825 my($port,$iaddr) = sockaddr_in($paddr);
826 my $name = gethostbyaddr($iaddr,AF_INET);
828 logmsg "connection from $name [",
829 inet_ntoa($iaddr), "]
832 print Client "Hello there, $name, it's now ",
833 scalar localtime, $EOL;
836 And here's a multithreaded version. It's multithreaded in that
837 like most typical servers, it spawns (forks) a slave server to
838 handle the client request so that the master server can quickly
839 go back to service a new client.
843 BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
846 my $EOL = "\015\012";
848 sub spawn; # forward declaration
849 sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
851 my $port = shift || 2345;
852 my $proto = getprotobyname('tcp');
854 ($port) = $port =~ /^(\d+)$/ or die "invalid port";
856 socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
857 setsockopt(Server, SOL_SOCKET, SO_REUSEADDR,
858 pack("l", 1)) || die "setsockopt: $!";
859 bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!";
860 listen(Server,SOMAXCONN) || die "listen: $!";
862 logmsg "server started on port $port";
867 use POSIX ":sys_wait_h";
870 while (($waitedpid = waitpid(-1,WNOHANG)) > 0) {
871 logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
873 $SIG{CHLD} = \&REAPER; # loathe sysV
876 $SIG{CHLD} = \&REAPER;
878 for ( $waitedpid = 0;
879 ($paddr = accept(Client,Server)) || $waitedpid;
880 $waitedpid = 0, close Client)
882 next if $waitedpid and not $paddr;
883 my($port,$iaddr) = sockaddr_in($paddr);
884 my $name = gethostbyaddr($iaddr,AF_INET);
886 logmsg "connection from $name [",
887 inet_ntoa($iaddr), "]
892 print "Hello there, $name, it's now ", scalar localtime, $EOL;
893 exec '/usr/games/fortune' # XXX: `wrong' line terminators
894 or confess "can't exec fortune: $!";
902 unless (@_ == 0 && $coderef && ref($coderef) eq 'CODE') {
903 confess "usage: spawn CODEREF";
907 if (!defined($pid = fork)) {
908 logmsg "cannot fork: $!";
912 return; # I'm the parent
914 # else I'm the child -- go spawn
916 open(STDIN, "<&Client") || die "can't dup client to stdin";
917 open(STDOUT, ">&Client") || die "can't dup client to stdout";
918 ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr";
922 This server takes the trouble to clone off a child version via fork() for
923 each incoming request. That way it can handle many requests at once,
924 which you might not always want. Even if you don't fork(), the listen()
925 will allow that many pending connections. Forking servers have to be
926 particularly careful about cleaning up their dead children (called
927 "zombies" in Unix parlance), because otherwise you'll quickly fill up your
930 We suggest that you use the B<-T> flag to use taint checking (see L<perlsec>)
931 even if we aren't running setuid or setgid. This is always a good idea
932 for servers and other programs run on behalf of someone else (like CGI
933 scripts), because it lessens the chances that people from the outside will
934 be able to compromise your system.
936 Let's look at another TCP client. This one connects to the TCP "time"
937 service on a number of different machines and shows how far their clocks
938 differ from the system on which it's being run:
944 my $SECS_of_70_YEARS = 2208988800;
945 sub ctime { scalar localtime(shift) }
947 my $iaddr = gethostbyname('localhost');
948 my $proto = getprotobyname('tcp');
949 my $port = getservbyname('time', 'tcp');
950 my $paddr = sockaddr_in(0, $iaddr);
954 printf "%-24s %8s %s\n", "localhost", 0, ctime(time());
956 foreach $host (@ARGV) {
957 printf "%-24s ", $host;
958 my $hisiaddr = inet_aton($host) || die "unknown host";
959 my $hispaddr = sockaddr_in($port, $hisiaddr);
960 socket(SOCKET, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
961 connect(SOCKET, $hispaddr) || die "bind: $!";
963 read(SOCKET, $rtime, 4);
965 my $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ;
966 printf "%8d %s\n", $histime - time, ctime($histime);
969 =head2 Unix-Domain TCP Clients and Servers
971 That's fine for Internet-domain clients and servers, but what about local
972 communications? While you can use the same setup, sometimes you don't
973 want to. Unix-domain sockets are local to the current host, and are often
974 used internally to implement pipes. Unlike Internet domain sockets, Unix
975 domain sockets can show up in the file system with an ls(1) listing.
978 srw-rw-rw- 1 root 0 Oct 31 07:23 /dev/log
980 You can test for these with Perl's B<-S> file test:
982 unless ( -S '/dev/log' ) {
983 die "something's wicked with the log system";
986 Here's a sample Unix-domain client:
991 my ($rendezvous, $line);
993 $rendezvous = shift || '/tmp/catsock';
994 socket(SOCK, PF_UNIX, SOCK_STREAM, 0) || die "socket: $!";
995 connect(SOCK, sockaddr_un($rendezvous)) || die "connect: $!";
996 while (defined($line = <SOCK>)) {
1001 And here's a corresponding server. You don't have to worry about silly
1002 network terminators here because Unix domain sockets are guaranteed
1003 to be on the localhost, and thus everything works right.
1010 BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
1011 sub spawn; # forward declaration
1012 sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
1014 my $NAME = '/tmp/catsock';
1015 my $uaddr = sockaddr_un($NAME);
1016 my $proto = getprotobyname('tcp');
1018 socket(Server,PF_UNIX,SOCK_STREAM,0) || die "socket: $!";
1020 bind (Server, $uaddr) || die "bind: $!";
1021 listen(Server,SOMAXCONN) || die "listen: $!";
1023 logmsg "server started on $NAME";
1027 use POSIX ":sys_wait_h";
1030 while (($waitedpid = waitpid(-1,WNOHANG)) > 0) {
1031 logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
1033 $SIG{CHLD} = \&REAPER; # loathe sysV
1036 $SIG{CHLD} = \&REAPER;
1039 for ( $waitedpid = 0;
1040 accept(Client,Server) || $waitedpid;
1041 $waitedpid = 0, close Client)
1044 logmsg "connection on $NAME";
1046 print "Hello there, it's now ", scalar localtime, "\n";
1047 exec '/usr/games/fortune' or die "can't exec fortune: $!";
1052 my $coderef = shift;
1054 unless (@_ == 0 && $coderef && ref($coderef) eq 'CODE') {
1055 confess "usage: spawn CODEREF";
1059 if (!defined($pid = fork)) {
1060 logmsg "cannot fork: $!";
1063 logmsg "begat $pid";
1064 return; # I'm the parent
1066 # else I'm the child -- go spawn
1068 open(STDIN, "<&Client") || die "can't dup client to stdin";
1069 open(STDOUT, ">&Client") || die "can't dup client to stdout";
1070 ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr";
1074 As you see, it's remarkably similar to the Internet domain TCP server, so
1075 much so, in fact, that we've omitted several duplicate functions--spawn(),
1076 logmsg(), ctime(), and REAPER()--which are exactly the same as in the
1079 So why would you ever want to use a Unix domain socket instead of a
1080 simpler named pipe? Because a named pipe doesn't give you sessions. You
1081 can't tell one process's data from another's. With socket programming,
1082 you get a separate session for each client: that's why accept() takes two
1085 For example, let's say that you have a long running database server daemon
1086 that you want folks from the World Wide Web to be able to access, but only
1087 if they go through a CGI interface. You'd have a small, simple CGI
1088 program that does whatever checks and logging you feel like, and then acts
1089 as a Unix-domain client and connects to your private server.
1091 =head1 TCP Clients with IO::Socket
1093 For those preferring a higher-level interface to socket programming, the
1094 IO::Socket module provides an object-oriented approach. IO::Socket is
1095 included as part of the standard Perl distribution as of the 5.004
1096 release. If you're running an earlier version of Perl, just fetch
1097 IO::Socket from CPAN, where you'll also find modules providing easy
1098 interfaces to the following systems: DNS, FTP, Ident (RFC 931), NIS and
1099 NISPlus, NNTP, Ping, POP3, SMTP, SNMP, SSLeay, Telnet, and Time--just
1102 =head2 A Simple Client
1104 Here's a client that creates a TCP connection to the "daytime"
1105 service at port 13 of the host name "localhost" and prints out everything
1106 that the server there cares to provide.
1110 $remote = IO::Socket::INET->new(
1112 PeerAddr => "localhost",
1113 PeerPort => "daytime(13)",
1115 or die "cannot connect to daytime port at localhost";
1116 while ( <$remote> ) { print }
1118 When you run this program, you should get something back that
1121 Wed May 14 08:40:46 MDT 1997
1123 Here are what those parameters to the C<new> constructor mean:
1129 This is which protocol to use. In this case, the socket handle returned
1130 will be connected to a TCP socket, because we want a stream-oriented
1131 connection, that is, one that acts pretty much like a plain old file.
1132 Not all sockets are this of this type. For example, the UDP protocol
1133 can be used to make a datagram socket, used for message-passing.
1137 This is the name or Internet address of the remote host the server is
1138 running on. We could have specified a longer name like C<"www.perl.com">,
1139 or an address like C<"204.148.40.9">. For demonstration purposes, we've
1140 used the special hostname C<"localhost">, which should always mean the
1141 current machine you're running on. The corresponding Internet address
1142 for localhost is C<"127.1">, if you'd rather use that.
1146 This is the service name or port number we'd like to connect to.
1147 We could have gotten away with using just C<"daytime"> on systems with a
1148 well-configured system services file,[FOOTNOTE: The system services file
1149 is in I</etc/services> under Unix] but just in case, we've specified the
1150 port number (13) in parentheses. Using just the number would also have
1151 worked, but constant numbers make careful programmers nervous.
1155 Notice how the return value from the C<new> constructor is used as
1156 a filehandle in the C<while> loop? That's what's called an indirect
1157 filehandle, a scalar variable containing a filehandle. You can use
1158 it the same way you would a normal filehandle. For example, you
1159 can read one line from it this way:
1163 all remaining lines from is this way:
1167 and send a line of data to it this way:
1169 print $handle "some data\n";
1171 =head2 A Webget Client
1173 Here's a simple client that takes a remote host to fetch a document
1174 from, and then a list of documents to get from that host. This is a
1175 more interesting client than the previous one because it first sends
1176 something to the server before fetching the server's response.
1180 unless (@ARGV > 1) { die "usage: $0 host document ..." }
1181 $host = shift(@ARGV);
1184 foreach $document ( @ARGV ) {
1185 $remote = IO::Socket::INET->new( Proto => "tcp",
1187 PeerPort => "http(80)",
1189 unless ($remote) { die "cannot connect to http daemon on $host" }
1190 $remote->autoflush(1);
1191 print $remote "GET $document HTTP/1.0" . $BLANK;
1192 while ( <$remote> ) { print }
1196 The web server handing the "http" service, which is assumed to be at
1197 its standard port, number 80. If the web server you're trying to
1198 connect to is at a different port (like 1080 or 8080), you should specify
1199 as the named-parameter pair, C<< PeerPort => 8080 >>. The C<autoflush>
1200 method is used on the socket because otherwise the system would buffer
1201 up the output we sent it. (If you're on a Mac, you'll also need to
1202 change every C<"\n"> in your code that sends data over the network to
1203 be a C<"\015\012"> instead.)
1205 Connecting to the server is only the first part of the process: once you
1206 have the connection, you have to use the server's language. Each server
1207 on the network has its own little command language that it expects as
1208 input. The string that we send to the server starting with "GET" is in
1209 HTTP syntax. In this case, we simply request each specified document.
1210 Yes, we really are making a new connection for each document, even though
1211 it's the same host. That's the way you always used to have to speak HTTP.
1212 Recent versions of web browsers may request that the remote server leave
1213 the connection open a little while, but the server doesn't have to honor
1216 Here's an example of running that program, which we'll call I<webget>:
1218 % webget www.perl.com /guanaco.html
1219 HTTP/1.1 404 File Not Found
1220 Date: Thu, 08 May 1997 18:02:32 GMT
1221 Server: Apache/1.2b6
1223 Content-type: text/html
1225 <HEAD><TITLE>404 File Not Found</TITLE></HEAD>
1226 <BODY><H1>File Not Found</H1>
1227 The requested URL /guanaco.html was not found on this server.<P>
1230 Ok, so that's not very interesting, because it didn't find that
1231 particular document. But a long response wouldn't have fit on this page.
1233 For a more fully-featured version of this program, you should look to
1234 the I<lwp-request> program included with the LWP modules from CPAN.
1236 =head2 Interactive Client with IO::Socket
1238 Well, that's all fine if you want to send one command and get one answer,
1239 but what about setting up something fully interactive, somewhat like
1240 the way I<telnet> works? That way you can type a line, get the answer,
1241 type a line, get the answer, etc.
1243 This client is more complicated than the two we've done so far, but if
1244 you're on a system that supports the powerful C<fork> call, the solution
1245 isn't that rough. Once you've made the connection to whatever service
1246 you'd like to chat with, call C<fork> to clone your process. Each of
1247 these two identical process has a very simple job to do: the parent
1248 copies everything from the socket to standard output, while the child
1249 simultaneously copies everything from standard input to the socket.
1250 To accomplish the same thing using just one process would be I<much>
1251 harder, because it's easier to code two processes to do one thing than it
1252 is to code one process to do two things. (This keep-it-simple principle
1253 a cornerstones of the Unix philosophy, and good software engineering as
1254 well, which is probably why it's spread to other systems.)
1261 my ($host, $port, $kidpid, $handle, $line);
1263 unless (@ARGV == 2) { die "usage: $0 host port" }
1264 ($host, $port) = @ARGV;
1266 # create a tcp connection to the specified host and port
1267 $handle = IO::Socket::INET->new(Proto => "tcp",
1270 or die "can't connect to port $port on $host: $!";
1272 $handle->autoflush(1); # so output gets there right away
1273 print STDERR "[Connected to $host:$port]\n";
1275 # split the program into two processes, identical twins
1276 die "can't fork: $!" unless defined($kidpid = fork());
1278 # the if{} block runs only in the parent process
1280 # copy the socket to standard output
1281 while (defined ($line = <$handle>)) {
1284 kill("TERM", $kidpid); # send SIGTERM to child
1286 # the else{} block runs only in the child process
1288 # copy standard input to the socket
1289 while (defined ($line = <STDIN>)) {
1290 print $handle $line;
1294 The C<kill> function in the parent's C<if> block is there to send a
1295 signal to our child process (current running in the C<else> block)
1296 as soon as the remote server has closed its end of the connection.
1298 If the remote server sends data a byte at time, and you need that
1299 data immediately without waiting for a newline (which might not happen),
1300 you may wish to replace the C<while> loop in the parent with the
1304 while (sysread($handle, $byte, 1) == 1) {
1308 Making a system call for each byte you want to read is not very efficient
1309 (to put it mildly) but is the simplest to explain and works reasonably
1312 =head1 TCP Servers with IO::Socket
1314 As always, setting up a server is little bit more involved than running a client.
1315 The model is that the server creates a special kind of socket that
1316 does nothing but listen on a particular port for incoming connections.
1317 It does this by calling the C<< IO::Socket::INET->new() >> method with
1318 slightly different arguments than the client did.
1324 This is which protocol to use. Like our clients, we'll
1325 still specify C<"tcp"> here.
1330 port in the C<LocalPort> argument, which we didn't do for the client.
1331 This is service name or port number for which you want to be the
1332 server. (Under Unix, ports under 1024 are restricted to the
1333 superuser.) In our sample, we'll use port 9000, but you can use
1334 any port that's not currently in use on your system. If you try
1335 to use one already in used, you'll get an "Address already in use"
1336 message. Under Unix, the C<netstat -a> command will show
1337 which services current have servers.
1341 The C<Listen> parameter is set to the maximum number of
1342 pending connections we can accept until we turn away incoming clients.
1343 Think of it as a call-waiting queue for your telephone.
1344 The low-level Socket module has a special symbol for the system maximum, which
1349 The C<Reuse> parameter is needed so that we restart our server
1350 manually without waiting a few minutes to allow system buffers to
1355 Once the generic server socket has been created using the parameters
1356 listed above, the server then waits for a new client to connect
1357 to it. The server blocks in the C<accept> method, which eventually accepts a
1358 bidirectional connection from the remote client. (Make sure to autoflush
1359 this handle to circumvent buffering.)
1361 To add to user-friendliness, our server prompts the user for commands.
1362 Most servers don't do this. Because of the prompt without a newline,
1363 you'll have to use the C<sysread> variant of the interactive client above.
1365 This server accepts one of five different commands, sending output
1366 back to the client. Note that unlike most network servers, this one
1367 only handles one incoming client at a time. Multithreaded servers are
1368 covered in Chapter 6 of the Camel.
1370 Here's the code. We'll
1374 use Net::hostent; # for OO version of gethostbyaddr
1376 $PORT = 9000; # pick something not in use
1378 $server = IO::Socket::INET->new( Proto => 'tcp',
1380 Listen => SOMAXCONN,
1383 die "can't setup server" unless $server;
1384 print "[Server $0 accepting clients]\n";
1386 while ($client = $server->accept()) {
1387 $client->autoflush(1);
1388 print $client "Welcome to $0; type help for command list.\n";
1389 $hostinfo = gethostbyaddr($client->peeraddr);
1390 printf "[Connect from %s]\n", $hostinfo->name || $client->peerhost;
1391 print $client "Command? ";
1392 while ( <$client>) {
1393 next unless /\S/; # blank line
1394 if (/quit|exit/i) { last; }
1395 elsif (/date|time/i) { printf $client "%s\n", scalar localtime; }
1396 elsif (/who/i ) { print $client `who 2>&1`; }
1397 elsif (/cookie/i ) { print $client `/usr/games/fortune 2>&1`; }
1398 elsif (/motd/i ) { print $client `cat /etc/motd 2>&1`; }
1400 print $client "Commands: quit date who cookie motd\n";
1403 print $client "Command? ";
1408 =head1 UDP: Message Passing
1410 Another kind of client-server setup is one that uses not connections, but
1411 messages. UDP communications involve much lower overhead but also provide
1412 less reliability, as there are no promises that messages will arrive at
1413 all, let alone in order and unmangled. Still, UDP offers some advantages
1414 over TCP, including being able to "broadcast" or "multicast" to a whole
1415 bunch of destination hosts at once (usually on your local subnet). If you
1416 find yourself overly concerned about reliability and start building checks
1417 into your message system, then you probably should use just TCP to start
1420 Note that UDP datagrams are I<not> a bytestream and should not be treated
1421 as such. This makes using I/O mechanisms with internal buffering
1422 like stdio (i.e. print() and friends) especially cumbersome. Use syswrite(),
1423 or better send(), like in the example below.
1425 Here's a UDP program similar to the sample Internet TCP client given
1426 earlier. However, instead of checking one host at a time, the UDP version
1427 will check many of them asynchronously by simulating a multicast and then
1428 using select() to do a timed-out wait for I/O. To do something similar
1429 with TCP, you'd have to use a different socket handle for each host.
1436 my ( $count, $hisiaddr, $hispaddr, $histime,
1437 $host, $iaddr, $paddr, $port, $proto,
1438 $rin, $rout, $rtime, $SECS_of_70_YEARS);
1440 $SECS_of_70_YEARS = 2208988800;
1442 $iaddr = gethostbyname(hostname());
1443 $proto = getprotobyname('udp');
1444 $port = getservbyname('time', 'udp');
1445 $paddr = sockaddr_in(0, $iaddr); # 0 means let kernel pick
1447 socket(SOCKET, PF_INET, SOCK_DGRAM, $proto) || die "socket: $!";
1448 bind(SOCKET, $paddr) || die "bind: $!";
1451 printf "%-12s %8s %s\n", "localhost", 0, scalar localtime time;
1455 $hisiaddr = inet_aton($host) || die "unknown host";
1456 $hispaddr = sockaddr_in($port, $hisiaddr);
1457 defined(send(SOCKET, 0, 0, $hispaddr)) || die "send $host: $!";
1461 vec($rin, fileno(SOCKET), 1) = 1;
1463 # timeout after 10.0 seconds
1464 while ($count && select($rout = $rin, undef, undef, 10.0)) {
1466 ($hispaddr = recv(SOCKET, $rtime, 4, 0)) || die "recv: $!";
1467 ($port, $hisiaddr) = sockaddr_in($hispaddr);
1468 $host = gethostbyaddr($hisiaddr, AF_INET);
1469 $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ;
1470 printf "%-12s ", $host;
1471 printf "%8d %s\n", $histime - time, scalar localtime($histime);
1475 Note that this example does not include any retries and may consequently
1476 fail to contact a reachable host. The most prominent reason for this
1477 is congestion of the queues on the sending host if the number of
1478 list of hosts to contact is sufficiently large.
1482 While System V IPC isn't so widely used as sockets, it still has some
1483 interesting uses. You can't, however, effectively use SysV IPC or
1484 Berkeley mmap() to have shared memory so as to share a variable amongst
1485 several processes. That's because Perl would reallocate your string when
1486 you weren't wanting it to.
1488 Here's a small example showing shared memory usage.
1490 use IPC::SysV qw(IPC_PRIVATE IPC_RMID S_IRWXU);
1493 $id = shmget(IPC_PRIVATE, $size, S_IRWXU) || die "$!";
1494 print "shm key $id\n";
1496 $message = "Message #1";
1497 shmwrite($id, $message, 0, 60) || die "$!";
1498 print "wrote: '$message'\n";
1499 shmread($id, $buff, 0, 60) || die "$!";
1500 print "read : '$buff'\n";
1502 # the buffer of shmread is zero-character end-padded.
1503 substr($buff, index($buff, "\0")) = '';
1504 print "un" unless $buff eq $message;
1507 print "deleting shm $id\n";
1508 shmctl($id, IPC_RMID, 0) || die "$!";
1510 Here's an example of a semaphore:
1512 use IPC::SysV qw(IPC_CREAT);
1515 $id = semget($IPC_KEY, 10, 0666 | IPC_CREAT ) || die "$!";
1516 print "shm key $id\n";
1518 Put this code in a separate file to be run in more than one process.
1519 Call the file F<take>:
1521 # create a semaphore
1524 $id = semget($IPC_KEY, 0 , 0 );
1525 die if !defined($id);
1531 # wait for semaphore to be zero
1533 $opstring1 = pack("s!s!s!", $semnum, $semop, $semflag);
1535 # Increment the semaphore count
1537 $opstring2 = pack("s!s!s!", $semnum, $semop, $semflag);
1538 $opstring = $opstring1 . $opstring2;
1540 semop($id,$opstring) || die "$!";
1542 Put this code in a separate file to be run in more than one process.
1543 Call this file F<give>:
1545 # 'give' the semaphore
1546 # run this in the original process and you will see
1547 # that the second process continues
1550 $id = semget($IPC_KEY, 0, 0);
1551 die if !defined($id);
1556 # Decrement the semaphore count
1558 $opstring = pack("s!s!s!", $semnum, $semop, $semflag);
1560 semop($id,$opstring) || die "$!";
1562 The SysV IPC code above was written long ago, and it's definitely
1563 clunky looking. For a more modern look, see the IPC::SysV module
1564 which is included with Perl starting from Perl 5.005.
1566 A small example demonstrating SysV message queues:
1568 use IPC::SysV qw(IPC_PRIVATE IPC_RMID IPC_CREAT S_IRWXU);
1570 my $id = msgget(IPC_PRIVATE, IPC_CREAT | S_IRWXU);
1572 my $sent = "message";
1578 if (msgsnd($id, pack("l! a*", $type_sent, $sent), 0)) {
1579 if (msgrcv($id, $rcvd, 60, 0, 0)) {
1580 ($type_rcvd, $rcvd) = unpack("l! a*", $rcvd);
1581 if ($rcvd eq $sent) {
1587 die "# msgrcv failed\n";
1590 die "# msgsnd failed\n";
1592 msgctl($id, IPC_RMID, 0) || die "# msgctl failed: $!\n";
1594 die "# msgget failed\n";
1599 Most of these routines quietly but politely return C<undef> when they
1600 fail instead of causing your program to die right then and there due to
1601 an uncaught exception. (Actually, some of the new I<Socket> conversion
1602 functions croak() on bad arguments.) It is therefore essential to
1603 check return values from these functions. Always begin your socket
1604 programs this way for optimal success, and don't forget to add B<-T>
1605 taint checking flag to the #! line for servers:
1614 All these routines create system-specific portability problems. As noted
1615 elsewhere, Perl is at the mercy of your C libraries for much of its system
1616 behaviour. It's probably safest to assume broken SysV semantics for
1617 signals and to stick with simple TCP and UDP socket operations; e.g., don't
1618 try to pass open file descriptors over a local UDP datagram socket if you
1619 want your code to stand a chance of being portable.
1621 As mentioned in the signals section, because few vendors provide C
1622 libraries that are safely re-entrant, the prudent programmer will do
1623 little else within a handler beyond setting a numeric variable that
1624 already exists; or, if locked into a slow (restarting) system call,
1625 using die() to raise an exception and longjmp(3) out. In fact, even
1626 these may in some cases cause a core dump. It's probably best to avoid
1627 signals except where they are absolutely inevitable. This
1628 will be addressed in a future release of Perl.
1632 Tom Christiansen, with occasional vestiges of Larry Wall's original
1633 version and suggestions from the Perl Porters.
1637 There's a lot more to networking than this, but this should get you
1640 For intrepid programmers, the indispensable textbook is I<Unix Network
1641 Programming> by W. Richard Stevens (published by Addison-Wesley). Note
1642 that most books on networking address networking from the perspective of
1643 a C programmer; translation to Perl is left as an exercise for the reader.
1645 The IO::Socket(3) manpage describes the object library, and the Socket(3)
1646 manpage describes the low-level interface to sockets. Besides the obvious
1647 functions in L<perlfunc>, you should also check out the F<modules> file
1648 at your nearest CPAN site. (See L<perlmodlib> or best yet, the F<Perl
1649 FAQ> for a description of what CPAN is and where to get it.)
1651 Section 5 of the F<modules> file is devoted to "Networking, Device Control
1652 (modems), and Interprocess Communication", and contains numerous unbundled
1653 modules numerous networking modules, Chat and Expect operations, CGI
1654 programming, DCE, FTP, IPC, NNTP, Proxy, Ptty, RPC, SNMP, SMTP, Telnet,
1655 Threads, and ToolTalk--just to name a few.