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{PIPE} = 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. The corresponding syntax to read from command
581 pipes (with C<"|-"> in place of C<"-|">) is also implemented.
583 Note that these operations are full Unix forks, which means they may not be
584 correctly implemented on alien systems. Additionally, these are not true
585 multithreading. If you'd like to learn more about threading, see the
586 F<modules> file mentioned below in the SEE ALSO section.
588 =head2 Bidirectional Communication with Another Process
590 While this works reasonably well for unidirectional communication, what
591 about bidirectional communication? The obvious thing you'd like to do
592 doesn't actually work:
594 open(PROG_FOR_READING_AND_WRITING, "| some program |")
596 and if you forget to use the C<use warnings> pragma or the B<-w> flag,
597 then you'll miss out entirely on the diagnostic message:
599 Can't do bidirectional pipe at -e line 1.
601 If you really want to, you can use the standard open2() library function
602 to catch both ends. There's also an open3() for tridirectional I/O so you
603 can also catch your child's STDERR, but doing so would then require an
604 awkward select() loop and wouldn't allow you to use normal Perl input
607 If you look at its source, you'll see that open2() uses low-level
608 primitives like Unix pipe() and exec() calls to create all the connections.
609 While it might have been slightly more efficient by using socketpair(), it
610 would have then been even less portable than it already is. The open2()
611 and open3() functions are unlikely to work anywhere except on a Unix
612 system or some other one purporting to be POSIX compliant.
614 Here's an example of using open2():
618 $pid = open2(*Reader, *Writer, "cat -u -n" );
619 print Writer "stuff\n";
622 The problem with this is that Unix buffering is really going to
623 ruin your day. Even though your C<Writer> filehandle is auto-flushed,
624 and the process on the other end will get your data in a timely manner,
625 you can't usually do anything to force it to give it back to you
626 in a similarly quick fashion. In this case, we could, because we
627 gave I<cat> a B<-u> flag to make it unbuffered. But very few Unix
628 commands are designed to operate over pipes, so this seldom works
629 unless you yourself wrote the program on the other end of the
632 A solution to this is the nonstandard F<Comm.pl> library. It uses
633 pseudo-ttys to make your program behave more reasonably:
636 $ph = open_proc('cat -n');
638 print $ph "a line\n";
639 print "got back ", scalar <$ph>;
642 This way you don't have to have control over the source code of the
643 program you're using. The F<Comm> library also has expect()
644 and interact() functions. Find the library (and we hope its
645 successor F<IPC::Chat>) at your nearest CPAN archive as detailed
646 in the SEE ALSO section below.
648 The newer Expect.pm module from CPAN also addresses this kind of thing.
649 This module requires two other modules from CPAN: IO::Pty and IO::Stty.
650 It sets up a pseudo-terminal to interact with programs that insist on
651 using talking to the terminal device driver. If your system is
652 amongst those supported, this may be your best bet.
654 =head2 Bidirectional Communication with Yourself
656 If you want, you may make low-level pipe() and fork()
657 to stitch this together by hand. This example only
658 talks to itself, but you could reopen the appropriate
659 handles to STDIN and STDOUT and call other processes.
662 # pipe1 - bidirectional communication using two pipe pairs
663 # designed for the socketpair-challenged
664 use IO::Handle; # thousands of lines just for autoflush :-(
665 pipe(PARENT_RDR, CHILD_WTR); # XXX: failure?
666 pipe(CHILD_RDR, PARENT_WTR); # XXX: failure?
667 CHILD_WTR->autoflush(1);
668 PARENT_WTR->autoflush(1);
671 close PARENT_RDR; close PARENT_WTR;
672 print CHILD_WTR "Parent Pid $$ is sending this\n";
673 chomp($line = <CHILD_RDR>);
674 print "Parent Pid $$ just read this: `$line'\n";
675 close CHILD_RDR; close CHILD_WTR;
678 die "cannot fork: $!" unless defined $pid;
679 close CHILD_RDR; close CHILD_WTR;
680 chomp($line = <PARENT_RDR>);
681 print "Child Pid $$ just read this: `$line'\n";
682 print PARENT_WTR "Child Pid $$ is sending this\n";
683 close PARENT_RDR; close PARENT_WTR;
687 But you don't actually have to make two pipe calls. If you
688 have the socketpair() system call, it will do this all for you.
691 # pipe2 - bidirectional communication using socketpair
692 # "the best ones always go both ways"
695 use IO::Handle; # thousands of lines just for autoflush :-(
696 # We say AF_UNIX because although *_LOCAL is the
697 # POSIX 1003.1g form of the constant, many machines
698 # still don't have it.
699 socketpair(CHILD, PARENT, AF_UNIX, SOCK_STREAM, PF_UNSPEC)
700 or die "socketpair: $!";
703 PARENT->autoflush(1);
707 print CHILD "Parent Pid $$ is sending this\n";
708 chomp($line = <CHILD>);
709 print "Parent Pid $$ just read this: `$line'\n";
713 die "cannot fork: $!" unless defined $pid;
715 chomp($line = <PARENT>);
716 print "Child Pid $$ just read this: `$line'\n";
717 print PARENT "Child Pid $$ is sending this\n";
722 =head1 Sockets: Client/Server Communication
724 While not limited to Unix-derived operating systems (e.g., WinSock on PCs
725 provides socket support, as do some VMS libraries), you may not have
726 sockets on your system, in which case this section probably isn't going to do
727 you much good. With sockets, you can do both virtual circuits (i.e., TCP
728 streams) and datagrams (i.e., UDP packets). You may be able to do even more
729 depending on your system.
731 The Perl function calls for dealing with sockets have the same names as
732 the corresponding system calls in C, but their arguments tend to differ
733 for two reasons: first, Perl filehandles work differently than C file
734 descriptors. Second, Perl already knows the length of its strings, so you
735 don't need to pass that information.
737 One of the major problems with old socket code in Perl was that it used
738 hard-coded values for some of the constants, which severely hurt
739 portability. If you ever see code that does anything like explicitly
740 setting C<$AF_INET = 2>, you know you're in for big trouble: An
741 immeasurably superior approach is to use the C<Socket> module, which more
742 reliably grants access to various constants and functions you'll need.
744 If you're not writing a server/client for an existing protocol like
745 NNTP or SMTP, you should give some thought to how your server will
746 know when the client has finished talking, and vice-versa. Most
747 protocols are based on one-line messages and responses (so one party
748 knows the other has finished when a "\n" is received) or multi-line
749 messages and responses that end with a period on an empty line
750 ("\n.\n" terminates a message/response).
752 =head2 Internet Line Terminators
754 The Internet line terminator is "\015\012". Under ASCII variants of
755 Unix, that could usually be written as "\r\n", but under other systems,
756 "\r\n" might at times be "\015\015\012", "\012\012\015", or something
757 completely different. The standards specify writing "\015\012" to be
758 conformant (be strict in what you provide), but they also recommend
759 accepting a lone "\012" on input (but be lenient in what you require).
760 We haven't always been very good about that in the code in this manpage,
761 but unless you're on a Mac, you'll probably be ok.
763 =head2 Internet TCP Clients and Servers
765 Use Internet-domain sockets when you want to do client-server
766 communication that might extend to machines outside of your own system.
768 Here's a sample TCP client using Internet-domain sockets:
773 my ($remote,$port, $iaddr, $paddr, $proto, $line);
775 $remote = shift || 'localhost';
776 $port = shift || 2345; # random port
777 if ($port =~ /\D/) { $port = getservbyname($port, 'tcp') }
778 die "No port" unless $port;
779 $iaddr = inet_aton($remote) || die "no host: $remote";
780 $paddr = sockaddr_in($port, $iaddr);
782 $proto = getprotobyname('tcp');
783 socket(SOCK, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
784 connect(SOCK, $paddr) || die "connect: $!";
785 while (defined($line = <SOCK>)) {
789 close (SOCK) || die "close: $!";
792 And here's a corresponding server to go along with it. We'll
793 leave the address as INADDR_ANY so that the kernel can choose
794 the appropriate interface on multihomed hosts. If you want sit
795 on a particular interface (like the external side of a gateway
796 or firewall machine), you should fill this in with your real address
801 BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
804 my $EOL = "\015\012";
806 sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
808 my $port = shift || 2345;
809 my $proto = getprotobyname('tcp');
811 ($port) = $port =~ /^(\d+)$/ or die "invalid port";
813 socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
814 setsockopt(Server, SOL_SOCKET, SO_REUSEADDR,
815 pack("l", 1)) || die "setsockopt: $!";
816 bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!";
817 listen(Server,SOMAXCONN) || die "listen: $!";
819 logmsg "server started on port $port";
823 $SIG{CHLD} = \&REAPER;
825 for ( ; $paddr = accept(Client,Server); close Client) {
826 my($port,$iaddr) = sockaddr_in($paddr);
827 my $name = gethostbyaddr($iaddr,AF_INET);
829 logmsg "connection from $name [",
830 inet_ntoa($iaddr), "]
833 print Client "Hello there, $name, it's now ",
834 scalar localtime, $EOL;
837 And here's a multithreaded version. It's multithreaded in that
838 like most typical servers, it spawns (forks) a slave server to
839 handle the client request so that the master server can quickly
840 go back to service a new client.
844 BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
847 my $EOL = "\015\012";
849 sub spawn; # forward declaration
850 sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
852 my $port = shift || 2345;
853 my $proto = getprotobyname('tcp');
855 ($port) = $port =~ /^(\d+)$/ or die "invalid port";
857 socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
858 setsockopt(Server, SOL_SOCKET, SO_REUSEADDR,
859 pack("l", 1)) || die "setsockopt: $!";
860 bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!";
861 listen(Server,SOMAXCONN) || die "listen: $!";
863 logmsg "server started on port $port";
868 use POSIX ":sys_wait_h";
871 while (($waitedpid = waitpid(-1,WNOHANG)) > 0) {
872 logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
874 $SIG{CHLD} = \&REAPER; # loathe sysV
877 $SIG{CHLD} = \&REAPER;
879 for ( $waitedpid = 0;
880 ($paddr = accept(Client,Server)) || $waitedpid;
881 $waitedpid = 0, close Client)
883 next if $waitedpid and not $paddr;
884 my($port,$iaddr) = sockaddr_in($paddr);
885 my $name = gethostbyaddr($iaddr,AF_INET);
887 logmsg "connection from $name [",
888 inet_ntoa($iaddr), "]
893 print "Hello there, $name, it's now ", scalar localtime, $EOL;
894 exec '/usr/games/fortune' # XXX: `wrong' line terminators
895 or confess "can't exec fortune: $!";
903 unless (@_ == 0 && $coderef && ref($coderef) eq 'CODE') {
904 confess "usage: spawn CODEREF";
908 if (!defined($pid = fork)) {
909 logmsg "cannot fork: $!";
913 return; # I'm the parent
915 # else I'm the child -- go spawn
917 open(STDIN, "<&Client") || die "can't dup client to stdin";
918 open(STDOUT, ">&Client") || die "can't dup client to stdout";
919 ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr";
923 This server takes the trouble to clone off a child version via fork() for
924 each incoming request. That way it can handle many requests at once,
925 which you might not always want. Even if you don't fork(), the listen()
926 will allow that many pending connections. Forking servers have to be
927 particularly careful about cleaning up their dead children (called
928 "zombies" in Unix parlance), because otherwise you'll quickly fill up your
931 We suggest that you use the B<-T> flag to use taint checking (see L<perlsec>)
932 even if we aren't running setuid or setgid. This is always a good idea
933 for servers and other programs run on behalf of someone else (like CGI
934 scripts), because it lessens the chances that people from the outside will
935 be able to compromise your system.
937 Let's look at another TCP client. This one connects to the TCP "time"
938 service on a number of different machines and shows how far their clocks
939 differ from the system on which it's being run:
945 my $SECS_of_70_YEARS = 2208988800;
946 sub ctime { scalar localtime(shift) }
948 my $iaddr = gethostbyname('localhost');
949 my $proto = getprotobyname('tcp');
950 my $port = getservbyname('time', 'tcp');
951 my $paddr = sockaddr_in(0, $iaddr);
955 printf "%-24s %8s %s\n", "localhost", 0, ctime(time());
957 foreach $host (@ARGV) {
958 printf "%-24s ", $host;
959 my $hisiaddr = inet_aton($host) || die "unknown host";
960 my $hispaddr = sockaddr_in($port, $hisiaddr);
961 socket(SOCKET, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
962 connect(SOCKET, $hispaddr) || die "bind: $!";
964 read(SOCKET, $rtime, 4);
966 my $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ;
967 printf "%8d %s\n", $histime - time, ctime($histime);
970 =head2 Unix-Domain TCP Clients and Servers
972 That's fine for Internet-domain clients and servers, but what about local
973 communications? While you can use the same setup, sometimes you don't
974 want to. Unix-domain sockets are local to the current host, and are often
975 used internally to implement pipes. Unlike Internet domain sockets, Unix
976 domain sockets can show up in the file system with an ls(1) listing.
979 srw-rw-rw- 1 root 0 Oct 31 07:23 /dev/log
981 You can test for these with Perl's B<-S> file test:
983 unless ( -S '/dev/log' ) {
984 die "something's wicked with the log system";
987 Here's a sample Unix-domain client:
992 my ($rendezvous, $line);
994 $rendezvous = shift || '/tmp/catsock';
995 socket(SOCK, PF_UNIX, SOCK_STREAM, 0) || die "socket: $!";
996 connect(SOCK, sockaddr_un($rendezvous)) || die "connect: $!";
997 while (defined($line = <SOCK>)) {
1002 And here's a corresponding server. You don't have to worry about silly
1003 network terminators here because Unix domain sockets are guaranteed
1004 to be on the localhost, and thus everything works right.
1011 BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
1012 sub spawn; # forward declaration
1013 sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
1015 my $NAME = '/tmp/catsock';
1016 my $uaddr = sockaddr_un($NAME);
1017 my $proto = getprotobyname('tcp');
1019 socket(Server,PF_UNIX,SOCK_STREAM,0) || die "socket: $!";
1021 bind (Server, $uaddr) || die "bind: $!";
1022 listen(Server,SOMAXCONN) || die "listen: $!";
1024 logmsg "server started on $NAME";
1028 use POSIX ":sys_wait_h";
1031 while (($waitedpid = waitpid(-1,WNOHANG)) > 0) {
1032 logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
1034 $SIG{CHLD} = \&REAPER; # loathe sysV
1037 $SIG{CHLD} = \&REAPER;
1040 for ( $waitedpid = 0;
1041 accept(Client,Server) || $waitedpid;
1042 $waitedpid = 0, close Client)
1045 logmsg "connection on $NAME";
1047 print "Hello there, it's now ", scalar localtime, "\n";
1048 exec '/usr/games/fortune' or die "can't exec fortune: $!";
1053 my $coderef = shift;
1055 unless (@_ == 0 && $coderef && ref($coderef) eq 'CODE') {
1056 confess "usage: spawn CODEREF";
1060 if (!defined($pid = fork)) {
1061 logmsg "cannot fork: $!";
1064 logmsg "begat $pid";
1065 return; # I'm the parent
1067 # else I'm the child -- go spawn
1069 open(STDIN, "<&Client") || die "can't dup client to stdin";
1070 open(STDOUT, ">&Client") || die "can't dup client to stdout";
1071 ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr";
1075 As you see, it's remarkably similar to the Internet domain TCP server, so
1076 much so, in fact, that we've omitted several duplicate functions--spawn(),
1077 logmsg(), ctime(), and REAPER()--which are exactly the same as in the
1080 So why would you ever want to use a Unix domain socket instead of a
1081 simpler named pipe? Because a named pipe doesn't give you sessions. You
1082 can't tell one process's data from another's. With socket programming,
1083 you get a separate session for each client: that's why accept() takes two
1086 For example, let's say that you have a long running database server daemon
1087 that you want folks from the World Wide Web to be able to access, but only
1088 if they go through a CGI interface. You'd have a small, simple CGI
1089 program that does whatever checks and logging you feel like, and then acts
1090 as a Unix-domain client and connects to your private server.
1092 =head1 TCP Clients with IO::Socket
1094 For those preferring a higher-level interface to socket programming, the
1095 IO::Socket module provides an object-oriented approach. IO::Socket is
1096 included as part of the standard Perl distribution as of the 5.004
1097 release. If you're running an earlier version of Perl, just fetch
1098 IO::Socket from CPAN, where you'll also find modules providing easy
1099 interfaces to the following systems: DNS, FTP, Ident (RFC 931), NIS and
1100 NISPlus, NNTP, Ping, POP3, SMTP, SNMP, SSLeay, Telnet, and Time--just
1103 =head2 A Simple Client
1105 Here's a client that creates a TCP connection to the "daytime"
1106 service at port 13 of the host name "localhost" and prints out everything
1107 that the server there cares to provide.
1111 $remote = IO::Socket::INET->new(
1113 PeerAddr => "localhost",
1114 PeerPort => "daytime(13)",
1116 or die "cannot connect to daytime port at localhost";
1117 while ( <$remote> ) { print }
1119 When you run this program, you should get something back that
1122 Wed May 14 08:40:46 MDT 1997
1124 Here are what those parameters to the C<new> constructor mean:
1130 This is which protocol to use. In this case, the socket handle returned
1131 will be connected to a TCP socket, because we want a stream-oriented
1132 connection, that is, one that acts pretty much like a plain old file.
1133 Not all sockets are this of this type. For example, the UDP protocol
1134 can be used to make a datagram socket, used for message-passing.
1138 This is the name or Internet address of the remote host the server is
1139 running on. We could have specified a longer name like C<"www.perl.com">,
1140 or an address like C<"204.148.40.9">. For demonstration purposes, we've
1141 used the special hostname C<"localhost">, which should always mean the
1142 current machine you're running on. The corresponding Internet address
1143 for localhost is C<"127.1">, if you'd rather use that.
1147 This is the service name or port number we'd like to connect to.
1148 We could have gotten away with using just C<"daytime"> on systems with a
1149 well-configured system services file,[FOOTNOTE: The system services file
1150 is in I</etc/services> under Unix] but just in case, we've specified the
1151 port number (13) in parentheses. Using just the number would also have
1152 worked, but constant numbers make careful programmers nervous.
1156 Notice how the return value from the C<new> constructor is used as
1157 a filehandle in the C<while> loop? That's what's called an indirect
1158 filehandle, a scalar variable containing a filehandle. You can use
1159 it the same way you would a normal filehandle. For example, you
1160 can read one line from it this way:
1164 all remaining lines from is this way:
1168 and send a line of data to it this way:
1170 print $handle "some data\n";
1172 =head2 A Webget Client
1174 Here's a simple client that takes a remote host to fetch a document
1175 from, and then a list of documents to get from that host. This is a
1176 more interesting client than the previous one because it first sends
1177 something to the server before fetching the server's response.
1181 unless (@ARGV > 1) { die "usage: $0 host document ..." }
1182 $host = shift(@ARGV);
1185 foreach $document ( @ARGV ) {
1186 $remote = IO::Socket::INET->new( Proto => "tcp",
1188 PeerPort => "http(80)",
1190 unless ($remote) { die "cannot connect to http daemon on $host" }
1191 $remote->autoflush(1);
1192 print $remote "GET $document HTTP/1.0" . $BLANK;
1193 while ( <$remote> ) { print }
1197 The web server handing the "http" service, which is assumed to be at
1198 its standard port, number 80. If the web server you're trying to
1199 connect to is at a different port (like 1080 or 8080), you should specify
1200 as the named-parameter pair, C<< PeerPort => 8080 >>. The C<autoflush>
1201 method is used on the socket because otherwise the system would buffer
1202 up the output we sent it. (If you're on a Mac, you'll also need to
1203 change every C<"\n"> in your code that sends data over the network to
1204 be a C<"\015\012"> instead.)
1206 Connecting to the server is only the first part of the process: once you
1207 have the connection, you have to use the server's language. Each server
1208 on the network has its own little command language that it expects as
1209 input. The string that we send to the server starting with "GET" is in
1210 HTTP syntax. In this case, we simply request each specified document.
1211 Yes, we really are making a new connection for each document, even though
1212 it's the same host. That's the way you always used to have to speak HTTP.
1213 Recent versions of web browsers may request that the remote server leave
1214 the connection open a little while, but the server doesn't have to honor
1217 Here's an example of running that program, which we'll call I<webget>:
1219 % webget www.perl.com /guanaco.html
1220 HTTP/1.1 404 File Not Found
1221 Date: Thu, 08 May 1997 18:02:32 GMT
1222 Server: Apache/1.2b6
1224 Content-type: text/html
1226 <HEAD><TITLE>404 File Not Found</TITLE></HEAD>
1227 <BODY><H1>File Not Found</H1>
1228 The requested URL /guanaco.html was not found on this server.<P>
1231 Ok, so that's not very interesting, because it didn't find that
1232 particular document. But a long response wouldn't have fit on this page.
1234 For a more fully-featured version of this program, you should look to
1235 the I<lwp-request> program included with the LWP modules from CPAN.
1237 =head2 Interactive Client with IO::Socket
1239 Well, that's all fine if you want to send one command and get one answer,
1240 but what about setting up something fully interactive, somewhat like
1241 the way I<telnet> works? That way you can type a line, get the answer,
1242 type a line, get the answer, etc.
1244 This client is more complicated than the two we've done so far, but if
1245 you're on a system that supports the powerful C<fork> call, the solution
1246 isn't that rough. Once you've made the connection to whatever service
1247 you'd like to chat with, call C<fork> to clone your process. Each of
1248 these two identical process has a very simple job to do: the parent
1249 copies everything from the socket to standard output, while the child
1250 simultaneously copies everything from standard input to the socket.
1251 To accomplish the same thing using just one process would be I<much>
1252 harder, because it's easier to code two processes to do one thing than it
1253 is to code one process to do two things. (This keep-it-simple principle
1254 a cornerstones of the Unix philosophy, and good software engineering as
1255 well, which is probably why it's spread to other systems.)
1262 my ($host, $port, $kidpid, $handle, $line);
1264 unless (@ARGV == 2) { die "usage: $0 host port" }
1265 ($host, $port) = @ARGV;
1267 # create a tcp connection to the specified host and port
1268 $handle = IO::Socket::INET->new(Proto => "tcp",
1271 or die "can't connect to port $port on $host: $!";
1273 $handle->autoflush(1); # so output gets there right away
1274 print STDERR "[Connected to $host:$port]\n";
1276 # split the program into two processes, identical twins
1277 die "can't fork: $!" unless defined($kidpid = fork());
1279 # the if{} block runs only in the parent process
1281 # copy the socket to standard output
1282 while (defined ($line = <$handle>)) {
1285 kill("TERM", $kidpid); # send SIGTERM to child
1287 # the else{} block runs only in the child process
1289 # copy standard input to the socket
1290 while (defined ($line = <STDIN>)) {
1291 print $handle $line;
1295 The C<kill> function in the parent's C<if> block is there to send a
1296 signal to our child process (current running in the C<else> block)
1297 as soon as the remote server has closed its end of the connection.
1299 If the remote server sends data a byte at time, and you need that
1300 data immediately without waiting for a newline (which might not happen),
1301 you may wish to replace the C<while> loop in the parent with the
1305 while (sysread($handle, $byte, 1) == 1) {
1309 Making a system call for each byte you want to read is not very efficient
1310 (to put it mildly) but is the simplest to explain and works reasonably
1313 =head1 TCP Servers with IO::Socket
1315 As always, setting up a server is little bit more involved than running a client.
1316 The model is that the server creates a special kind of socket that
1317 does nothing but listen on a particular port for incoming connections.
1318 It does this by calling the C<< IO::Socket::INET->new() >> method with
1319 slightly different arguments than the client did.
1325 This is which protocol to use. Like our clients, we'll
1326 still specify C<"tcp"> here.
1331 port in the C<LocalPort> argument, which we didn't do for the client.
1332 This is service name or port number for which you want to be the
1333 server. (Under Unix, ports under 1024 are restricted to the
1334 superuser.) In our sample, we'll use port 9000, but you can use
1335 any port that's not currently in use on your system. If you try
1336 to use one already in used, you'll get an "Address already in use"
1337 message. Under Unix, the C<netstat -a> command will show
1338 which services current have servers.
1342 The C<Listen> parameter is set to the maximum number of
1343 pending connections we can accept until we turn away incoming clients.
1344 Think of it as a call-waiting queue for your telephone.
1345 The low-level Socket module has a special symbol for the system maximum, which
1350 The C<Reuse> parameter is needed so that we restart our server
1351 manually without waiting a few minutes to allow system buffers to
1356 Once the generic server socket has been created using the parameters
1357 listed above, the server then waits for a new client to connect
1358 to it. The server blocks in the C<accept> method, which eventually accepts a
1359 bidirectional connection from the remote client. (Make sure to autoflush
1360 this handle to circumvent buffering.)
1362 To add to user-friendliness, our server prompts the user for commands.
1363 Most servers don't do this. Because of the prompt without a newline,
1364 you'll have to use the C<sysread> variant of the interactive client above.
1366 This server accepts one of five different commands, sending output
1367 back to the client. Note that unlike most network servers, this one
1368 only handles one incoming client at a time. Multithreaded servers are
1369 covered in Chapter 6 of the Camel.
1371 Here's the code. We'll
1375 use Net::hostent; # for OO version of gethostbyaddr
1377 $PORT = 9000; # pick something not in use
1379 $server = IO::Socket::INET->new( Proto => 'tcp',
1381 Listen => SOMAXCONN,
1384 die "can't setup server" unless $server;
1385 print "[Server $0 accepting clients]\n";
1387 while ($client = $server->accept()) {
1388 $client->autoflush(1);
1389 print $client "Welcome to $0; type help for command list.\n";
1390 $hostinfo = gethostbyaddr($client->peeraddr);
1391 printf "[Connect from %s]\n", $hostinfo ? $hostinfo->name : $client->peerhost;
1392 print $client "Command? ";
1393 while ( <$client>) {
1394 next unless /\S/; # blank line
1395 if (/quit|exit/i) { last; }
1396 elsif (/date|time/i) { printf $client "%s\n", scalar localtime; }
1397 elsif (/who/i ) { print $client `who 2>&1`; }
1398 elsif (/cookie/i ) { print $client `/usr/games/fortune 2>&1`; }
1399 elsif (/motd/i ) { print $client `cat /etc/motd 2>&1`; }
1401 print $client "Commands: quit date who cookie motd\n";
1404 print $client "Command? ";
1409 =head1 UDP: Message Passing
1411 Another kind of client-server setup is one that uses not connections, but
1412 messages. UDP communications involve much lower overhead but also provide
1413 less reliability, as there are no promises that messages will arrive at
1414 all, let alone in order and unmangled. Still, UDP offers some advantages
1415 over TCP, including being able to "broadcast" or "multicast" to a whole
1416 bunch of destination hosts at once (usually on your local subnet). If you
1417 find yourself overly concerned about reliability and start building checks
1418 into your message system, then you probably should use just TCP to start
1421 Note that UDP datagrams are I<not> a bytestream and should not be treated
1422 as such. This makes using I/O mechanisms with internal buffering
1423 like stdio (i.e. print() and friends) especially cumbersome. Use syswrite(),
1424 or better send(), like in the example below.
1426 Here's a UDP program similar to the sample Internet TCP client given
1427 earlier. However, instead of checking one host at a time, the UDP version
1428 will check many of them asynchronously by simulating a multicast and then
1429 using select() to do a timed-out wait for I/O. To do something similar
1430 with TCP, you'd have to use a different socket handle for each host.
1437 my ( $count, $hisiaddr, $hispaddr, $histime,
1438 $host, $iaddr, $paddr, $port, $proto,
1439 $rin, $rout, $rtime, $SECS_of_70_YEARS);
1441 $SECS_of_70_YEARS = 2208988800;
1443 $iaddr = gethostbyname(hostname());
1444 $proto = getprotobyname('udp');
1445 $port = getservbyname('time', 'udp');
1446 $paddr = sockaddr_in(0, $iaddr); # 0 means let kernel pick
1448 socket(SOCKET, PF_INET, SOCK_DGRAM, $proto) || die "socket: $!";
1449 bind(SOCKET, $paddr) || die "bind: $!";
1452 printf "%-12s %8s %s\n", "localhost", 0, scalar localtime time;
1456 $hisiaddr = inet_aton($host) || die "unknown host";
1457 $hispaddr = sockaddr_in($port, $hisiaddr);
1458 defined(send(SOCKET, 0, 0, $hispaddr)) || die "send $host: $!";
1462 vec($rin, fileno(SOCKET), 1) = 1;
1464 # timeout after 10.0 seconds
1465 while ($count && select($rout = $rin, undef, undef, 10.0)) {
1467 ($hispaddr = recv(SOCKET, $rtime, 4, 0)) || die "recv: $!";
1468 ($port, $hisiaddr) = sockaddr_in($hispaddr);
1469 $host = gethostbyaddr($hisiaddr, AF_INET);
1470 $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ;
1471 printf "%-12s ", $host;
1472 printf "%8d %s\n", $histime - time, scalar localtime($histime);
1476 Note that this example does not include any retries and may consequently
1477 fail to contact a reachable host. The most prominent reason for this
1478 is congestion of the queues on the sending host if the number of
1479 list of hosts to contact is sufficiently large.
1483 While System V IPC isn't so widely used as sockets, it still has some
1484 interesting uses. You can't, however, effectively use SysV IPC or
1485 Berkeley mmap() to have shared memory so as to share a variable amongst
1486 several processes. That's because Perl would reallocate your string when
1487 you weren't wanting it to.
1489 Here's a small example showing shared memory usage.
1491 use IPC::SysV qw(IPC_PRIVATE IPC_RMID S_IRWXU);
1494 $id = shmget(IPC_PRIVATE, $size, S_IRWXU) || die "$!";
1495 print "shm key $id\n";
1497 $message = "Message #1";
1498 shmwrite($id, $message, 0, 60) || die "$!";
1499 print "wrote: '$message'\n";
1500 shmread($id, $buff, 0, 60) || die "$!";
1501 print "read : '$buff'\n";
1503 # the buffer of shmread is zero-character end-padded.
1504 substr($buff, index($buff, "\0")) = '';
1505 print "un" unless $buff eq $message;
1508 print "deleting shm $id\n";
1509 shmctl($id, IPC_RMID, 0) || die "$!";
1511 Here's an example of a semaphore:
1513 use IPC::SysV qw(IPC_CREAT);
1516 $id = semget($IPC_KEY, 10, 0666 | IPC_CREAT ) || die "$!";
1517 print "shm key $id\n";
1519 Put this code in a separate file to be run in more than one process.
1520 Call the file F<take>:
1522 # create a semaphore
1525 $id = semget($IPC_KEY, 0 , 0 );
1526 die if !defined($id);
1532 # wait for semaphore to be zero
1534 $opstring1 = pack("s!s!s!", $semnum, $semop, $semflag);
1536 # Increment the semaphore count
1538 $opstring2 = pack("s!s!s!", $semnum, $semop, $semflag);
1539 $opstring = $opstring1 . $opstring2;
1541 semop($id,$opstring) || die "$!";
1543 Put this code in a separate file to be run in more than one process.
1544 Call this file F<give>:
1546 # 'give' the semaphore
1547 # run this in the original process and you will see
1548 # that the second process continues
1551 $id = semget($IPC_KEY, 0, 0);
1552 die if !defined($id);
1557 # Decrement the semaphore count
1559 $opstring = pack("s!s!s!", $semnum, $semop, $semflag);
1561 semop($id,$opstring) || die "$!";
1563 The SysV IPC code above was written long ago, and it's definitely
1564 clunky looking. For a more modern look, see the IPC::SysV module
1565 which is included with Perl starting from Perl 5.005.
1567 A small example demonstrating SysV message queues:
1569 use IPC::SysV qw(IPC_PRIVATE IPC_RMID IPC_CREAT S_IRWXU);
1571 my $id = msgget(IPC_PRIVATE, IPC_CREAT | S_IRWXU);
1573 my $sent = "message";
1579 if (msgsnd($id, pack("l! a*", $type_sent, $sent), 0)) {
1580 if (msgrcv($id, $rcvd, 60, 0, 0)) {
1581 ($type_rcvd, $rcvd) = unpack("l! a*", $rcvd);
1582 if ($rcvd eq $sent) {
1588 die "# msgrcv failed\n";
1591 die "# msgsnd failed\n";
1593 msgctl($id, IPC_RMID, 0) || die "# msgctl failed: $!\n";
1595 die "# msgget failed\n";
1600 Most of these routines quietly but politely return C<undef> when they
1601 fail instead of causing your program to die right then and there due to
1602 an uncaught exception. (Actually, some of the new I<Socket> conversion
1603 functions croak() on bad arguments.) It is therefore essential to
1604 check return values from these functions. Always begin your socket
1605 programs this way for optimal success, and don't forget to add B<-T>
1606 taint checking flag to the #! line for servers:
1615 All these routines create system-specific portability problems. As noted
1616 elsewhere, Perl is at the mercy of your C libraries for much of its system
1617 behaviour. It's probably safest to assume broken SysV semantics for
1618 signals and to stick with simple TCP and UDP socket operations; e.g., don't
1619 try to pass open file descriptors over a local UDP datagram socket if you
1620 want your code to stand a chance of being portable.
1622 As mentioned in the signals section, because few vendors provide C
1623 libraries that are safely re-entrant, the prudent programmer will do
1624 little else within a handler beyond setting a numeric variable that
1625 already exists; or, if locked into a slow (restarting) system call,
1626 using die() to raise an exception and longjmp(3) out. In fact, even
1627 these may in some cases cause a core dump. It's probably best to avoid
1628 signals except where they are absolutely inevitable. This
1629 will be addressed in a future release of Perl.
1633 Tom Christiansen, with occasional vestiges of Larry Wall's original
1634 version and suggestions from the Perl Porters.
1638 There's a lot more to networking than this, but this should get you
1641 For intrepid programmers, the indispensable textbook is I<Unix
1642 Network Programming, 2nd Edition, Volume 1> by W. Richard Stevens
1643 (published by Prentice-Hall). Note that most books on networking
1644 address the subject from the perspective of a C programmer; translation
1645 to Perl is left as an exercise for the reader.
1647 The IO::Socket(3) manpage describes the object library, and the Socket(3)
1648 manpage describes the low-level interface to sockets. Besides the obvious
1649 functions in L<perlfunc>, you should also check out the F<modules> file
1650 at your nearest CPAN site. (See L<perlmodlib> or best yet, the F<Perl
1651 FAQ> for a description of what CPAN is and where to get it.)
1653 Section 5 of the F<modules> file is devoted to "Networking, Device Control
1654 (modems), and Interprocess Communication", and contains numerous unbundled
1655 modules numerous networking modules, Chat and Expect operations, CGI
1656 programming, DCE, FTP, IPC, NNTP, Proxy, Ptty, RPC, SNMP, SMTP, Telnet,
1657 Threads, and ToolTalk--just to name a few.