1PERLIPC(1)             Perl Programmers Reference Guide             PERLIPC(1)
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NAME

6       perlipc - Perl interprocess communication (signals, fifos, pipes, safe
7       subprocesses, sockets, and semaphores)
8

DESCRIPTION

10       The basic IPC facilities of Perl are built out of the good old Unix
11       signals, named pipes, pipe opens, the Berkeley socket routines, and
12       SysV IPC calls.  Each is used in slightly different situations.
13

Signals

15       Perl uses a simple signal handling model: the %SIG hash contains names
16       or references of user-installed signal handlers.  These handlers will
17       be called with an argument which is the name of the signal that
18       triggered it.  A signal may be generated intentionally from a
19       particular keyboard sequence like control-C or control-Z, sent to you
20       from another process, or triggered automatically by the kernel when
21       special events transpire, like a child process exiting, your own
22       process running out of stack space, or hitting a process file-size
23       limit.
24
25       For example, to trap an interrupt signal, set up a handler like this:
26
27           our $shucks;
28
29           sub catch_zap {
30               my $signame = shift;
31               $shucks++;
32               die "Somebody sent me a SIG$signame";
33           }
34           $SIG{INT} = __PACKAGE__ . "::catch_zap";
35           $SIG{INT} = \&catch_zap;  # best strategy
36
37       Prior to Perl 5.8.0 it was necessary to do as little as you possibly
38       could in your handler; notice how all we do is set a global variable
39       and then raise an exception.  That's because on most systems, libraries
40       are not re-entrant; particularly, memory allocation and I/O routines
41       are not.  That meant that doing nearly anything in your handler could
42       in theory trigger a memory fault and subsequent core dump - see
43       "Deferred Signals (Safe Signals)" below.
44
45       The names of the signals are the ones listed out by "kill -l" on your
46       system, or you can retrieve them using the CPAN module IPC::Signal.
47
48       You may also choose to assign the strings "IGNORE" or "DEFAULT" as the
49       handler, in which case Perl will try to discard the signal or do the
50       default thing.
51
52       On most Unix platforms, the "CHLD" (sometimes also known as "CLD")
53       signal has special behavior with respect to a value of "IGNORE".
54       Setting $SIG{CHLD} to "IGNORE" on such a platform has the effect of not
55       creating zombie processes when the parent process fails to "wait()" on
56       its child processes (i.e., child processes are automatically reaped).
57       Calling "wait()" with $SIG{CHLD} set to "IGNORE" usually returns "-1"
58       on such platforms.
59
60       Some signals can be neither trapped nor ignored, such as the KILL and
61       STOP (but not the TSTP) signals. Note that ignoring signals makes them
62       disappear.  If you only want them blocked temporarily without them
63       getting lost you'll have to use POSIX' sigprocmask.
64
65       Sending a signal to a negative process ID means that you send the
66       signal to the entire Unix process group.  This code sends a hang-up
67       signal to all processes in the current process group, and also sets
68       $SIG{HUP} to "IGNORE" so it doesn't kill itself:
69
70           # block scope for local
71           {
72               local $SIG{HUP} = "IGNORE";
73               kill HUP => -$$;
74               # snazzy writing of: kill("HUP", -$$)
75           }
76
77       Another interesting signal to send is signal number zero.  This doesn't
78       actually affect a child process, but instead checks whether it's alive
79       or has changed its UIDs.
80
81           unless (kill 0 => $kid_pid) {
82               warn "something wicked happened to $kid_pid";
83           }
84
85       Signal number zero may fail because you lack permission to send the
86       signal when directed at a process whose real or saved UID is not
87       identical to the real or effective UID of the sending process, even
88       though the process is alive.  You may be able to determine the cause of
89       failure using $! or "%!".
90
91           unless (kill(0 => $pid) || $!{EPERM}) {
92               warn "$pid looks dead";
93           }
94
95       You might also want to employ anonymous functions for simple signal
96       handlers:
97
98           $SIG{INT} = sub { die "\nOutta here!\n" };
99
100       SIGCHLD handlers require some special care.  If a second child dies
101       while in the signal handler caused by the first death, we won't get
102       another signal. So must loop here else we will leave the unreaped child
103       as a zombie. And the next time two children die we get another zombie.
104       And so on.
105
106           use POSIX ":sys_wait_h";
107           $SIG{CHLD} = sub {
108               while ((my $child = waitpid(-1, WNOHANG)) > 0) {
109                   $Kid_Status{$child} = $?;
110               }
111           };
112           # do something that forks...
113
114       Be careful: qx(), system(), and some modules for calling external
115       commands do a fork(), then wait() for the result. Thus, your signal
116       handler will be called. Because wait() was already called by system()
117       or qx(), the wait() in the signal handler will see no more zombies and
118       will therefore block.
119
120       The best way to prevent this issue is to use waitpid(), as in the
121       following example:
122
123           use POSIX ":sys_wait_h"; # for nonblocking read
124
125           my %children;
126
127           $SIG{CHLD} = sub {
128               # don't change $! and $? outside handler
129               local ($!, $?);
130               while ( (my $pid = waitpid(-1, WNOHANG)) > 0 ) {
131                   delete $children{$pid};
132                   cleanup_child($pid, $?);
133               }
134           };
135
136           while (1) {
137               my $pid = fork();
138               die "cannot fork" unless defined $pid;
139               if ($pid == 0) {
140                   # ...
141                   exit 0;
142               } else {
143                   $children{$pid}=1;
144                   # ...
145                   system($command);
146                   # ...
147              }
148           }
149
150       Signal handling is also used for timeouts in Unix.  While safely
151       protected within an "eval{}" block, you set a signal handler to trap
152       alarm signals and then schedule to have one delivered to you in some
153       number of seconds.  Then try your blocking operation, clearing the
154       alarm when it's done but not before you've exited your "eval{}" block.
155       If it goes off, you'll use die() to jump out of the block.
156
157       Here's an example:
158
159           my $ALARM_EXCEPTION = "alarm clock restart";
160           eval {
161               local $SIG{ALRM} = sub { die $ALARM_EXCEPTION };
162               alarm 10;
163               flock(FH, 2)    # blocking write lock
164                               || die "cannot flock: $!";
165               alarm 0;
166           };
167           if ($@ && $@ !~ quotemeta($ALARM_EXCEPTION)) { die }
168
169       If the operation being timed out is system() or qx(), this technique is
170       liable to generate zombies.    If this matters to you, you'll need to
171       do your own fork() and exec(), and kill the errant child process.
172
173       For more complex signal handling, you might see the standard POSIX
174       module.  Lamentably, this is almost entirely undocumented, but the
175       ext/POSIX/t/sigaction.t file from the Perl source distribution has some
176       examples in it.
177
178   Handling the SIGHUP Signal in Daemons
179       A process that usually starts when the system boots and shuts down when
180       the system is shut down is called a daemon (Disk And Execution
181       MONitor). If a daemon process has a configuration file which is
182       modified after the process has been started, there should be a way to
183       tell that process to reread its configuration file without stopping the
184       process. Many daemons provide this mechanism using a "SIGHUP" signal
185       handler. When you want to tell the daemon to reread the file, simply
186       send it the "SIGHUP" signal.
187
188       The following example implements a simple daemon, which restarts itself
189       every time the "SIGHUP" signal is received. The actual code is located
190       in the subroutine "code()", which just prints some debugging info to
191       show that it works; it should be replaced with the real code.
192
193         #!/usr/bin/perl
194
195         use strict;
196         use warnings;
197
198         use POSIX ();
199         use FindBin ();
200         use File::Basename ();
201         use File::Spec::Functions qw(catfile);
202
203         $| = 1;
204
205         # make the daemon cross-platform, so exec always calls the script
206         # itself with the right path, no matter how the script was invoked.
207         my $script = File::Basename::basename($0);
208         my $SELF  = catfile($FindBin::Bin, $script);
209
210         # POSIX unmasks the sigprocmask properly
211         $SIG{HUP} = sub {
212             print "got SIGHUP\n";
213             exec($SELF, @ARGV)        || die "$0: couldn't restart: $!";
214         };
215
216         code();
217
218         sub code {
219             print "PID: $$\n";
220             print "ARGV: @ARGV\n";
221             my $count = 0;
222             while (1) {
223                 sleep 2;
224                 print ++$count, "\n";
225             }
226         }
227
228   Deferred Signals (Safe Signals)
229       Before Perl 5.8.0, installing Perl code to deal with signals exposed
230       you to danger from two things.  First, few system library functions are
231       re-entrant.  If the signal interrupts while Perl is executing one
232       function (like malloc(3) or printf(3)), and your signal handler then
233       calls the same function again, you could get unpredictable
234       behavior--often, a core dump.  Second, Perl isn't itself re-entrant at
235       the lowest levels.  If the signal interrupts Perl while Perl is
236       changing its own internal data structures, similarly unpredictable
237       behavior may result.
238
239       There were two things you could do, knowing this: be paranoid or be
240       pragmatic.  The paranoid approach was to do as little as possible in
241       your signal handler.  Set an existing integer variable that already has
242       a value, and return.  This doesn't help you if you're in a slow system
243       call, which will just restart.  That means you have to "die" to
244       longjmp(3) out of the handler.  Even this is a little cavalier for the
245       true paranoiac, who avoids "die" in a handler because the system is out
246       to get you.  The pragmatic approach was to say "I know the risks, but
247       prefer the convenience", and to do anything you wanted in your signal
248       handler, and be prepared to clean up core dumps now and again.
249
250       Perl 5.8.0 and later avoid these problems by "deferring" signals.  That
251       is, when the signal is delivered to the process by the system (to the C
252       code that implements Perl) a flag is set, and the handler returns
253       immediately.  Then at strategic "safe" points in the Perl interpreter
254       (e.g. when it is about to execute a new opcode) the flags are checked
255       and the Perl level handler from %SIG is executed. The "deferred" scheme
256       allows much more flexibility in the coding of signal handlers as we
257       know the Perl interpreter is in a safe state, and that we are not in a
258       system library function when the handler is called.  However the
259       implementation does differ from previous Perls in the following ways:
260
261       Long-running opcodes
262           As the Perl interpreter looks at signal flags only when it is about
263           to execute a new opcode, a signal that arrives during a long-
264           running opcode (e.g. a regular expression operation on a very large
265           string) will not be seen until the current opcode completes.
266
267           If a signal of any given type fires multiple times during an opcode
268           (such as from a fine-grained timer), the handler for that signal
269           will be called only once, after the opcode completes; all other
270           instances will be discarded.  Furthermore, if your system's signal
271           queue gets flooded to the point that there are signals that have
272           been raised but not yet caught (and thus not deferred) at the time
273           an opcode completes, those signals may well be caught and deferred
274           during subsequent opcodes, with sometimes surprising results.  For
275           example, you may see alarms delivered even after calling alarm(0)
276           as the latter stops the raising of alarms but does not cancel the
277           delivery of alarms raised but not yet caught.  Do not depend on the
278           behaviors described in this paragraph as they are side effects of
279           the current implementation and may change in future versions of
280           Perl.
281
282       Interrupting IO
283           When a signal is delivered (e.g., SIGINT from a control-C) the
284           operating system breaks into IO operations like read(2), which is
285           used to implement Perl's readline() function, the "<>" operator. On
286           older Perls the handler was called immediately (and as "read" is
287           not "unsafe", this worked well). With the "deferred" scheme the
288           handler is not called immediately, and if Perl is using the
289           system's "stdio" library that library may restart the "read"
290           without returning to Perl to give it a chance to call the %SIG
291           handler. If this happens on your system the solution is to use the
292           ":perlio" layer to do IO--at least on those handles that you want
293           to be able to break into with signals. (The ":perlio" layer checks
294           the signal flags and calls %SIG handlers before resuming IO
295           operation.)
296
297           The default in Perl 5.8.0 and later is to automatically use the
298           ":perlio" layer.
299
300           Note that it is not advisable to access a file handle within a
301           signal handler where that signal has interrupted an I/O operation
302           on that same handle. While perl will at least try hard not to
303           crash, there are no guarantees of data integrity; for example, some
304           data might get dropped or written twice.
305
306           Some networking library functions like gethostbyname() are known to
307           have their own implementations of timeouts which may conflict with
308           your timeouts.  If you have problems with such functions, try using
309           the POSIX sigaction() function, which bypasses Perl safe signals.
310           Be warned that this does subject you to possible memory corruption,
311           as described above.
312
313           Instead of setting $SIG{ALRM}:
314
315              local $SIG{ALRM} = sub { die "alarm" };
316
317           try something like the following:
318
319            use POSIX qw(SIGALRM);
320            POSIX::sigaction(SIGALRM,
321                             POSIX::SigAction->new(sub { die "alarm" }))
322                     || die "Error setting SIGALRM handler: $!\n";
323
324           Another way to disable the safe signal behavior locally is to use
325           the "Perl::Unsafe::Signals" module from CPAN, which affects all
326           signals.
327
328       Restartable system calls
329           On systems that supported it, older versions of Perl used the
330           SA_RESTART flag when installing %SIG handlers.  This meant that
331           restartable system calls would continue rather than returning when
332           a signal arrived.  In order to deliver deferred signals promptly,
333           Perl 5.8.0 and later do not use SA_RESTART.  Consequently,
334           restartable system calls can fail (with $! set to "EINTR") in
335           places where they previously would have succeeded.
336
337           The default ":perlio" layer retries "read", "write" and "close" as
338           described above; interrupted "wait" and "waitpid" calls will always
339           be retried.
340
341       Signals as "faults"
342           Certain signals like SEGV, ILL, and BUS are generated by virtual
343           memory addressing errors and similar "faults". These are normally
344           fatal: there is little a Perl-level handler can do with them.  So
345           Perl delivers them immediately rather than attempting to defer
346           them.
347
348       Signals triggered by operating system state
349           On some operating systems certain signal handlers are supposed to
350           "do something" before returning. One example can be CHLD or CLD,
351           which indicates a child process has completed. On some operating
352           systems the signal handler is expected to "wait" for the completed
353           child process. On such systems the deferred signal scheme will not
354           work for those signals: it does not do the "wait". Again the
355           failure will look like a loop as the operating system will reissue
356           the signal because there are completed child processes that have
357           not yet been "wait"ed for.
358
359       If you want the old signal behavior back despite possible memory
360       corruption, set the environment variable "PERL_SIGNALS" to "unsafe".
361       This feature first appeared in Perl 5.8.1.
362

Named Pipes

364       A named pipe (often referred to as a FIFO) is an old Unix IPC mechanism
365       for processes communicating on the same machine.  It works just like
366       regular anonymous pipes, except that the processes rendezvous using a
367       filename and need not be related.
368
369       To create a named pipe, use the "POSIX::mkfifo()" function.
370
371           use POSIX qw(mkfifo);
372           mkfifo($path, 0700)     ||  die "mkfifo $path failed: $!";
373
374       You can also use the Unix command mknod(1), or on some systems,
375       mkfifo(1).  These may not be in your normal path, though.
376
377           # system return val is backwards, so && not ||
378           #
379           $ENV{PATH} .= ":/etc:/usr/etc";
380           if  (      system("mknod",  $path, "p")
381                   && system("mkfifo", $path) )
382           {
383               die "mk{nod,fifo} $path failed";
384           }
385
386       A fifo is convenient when you want to connect a process to an unrelated
387       one.  When you open a fifo, the program will block until there's
388       something on the other end.
389
390       For example, let's say you'd like to have your .signature file be a
391       named pipe that has a Perl program on the other end.  Now every time
392       any program (like a mailer, news reader, finger program, etc.) tries to
393       read from that file, the reading program will read the new signature
394       from your program.  We'll use the pipe-checking file-test operator, -p,
395       to find out whether anyone (or anything) has accidentally removed our
396       fifo.
397
398           chdir();    # go home
399           my $FIFO = ".signature";
400
401           while (1) {
402               unless (-p $FIFO) {
403                   unlink $FIFO;   # discard any failure, will catch later
404                   require POSIX;  # delayed loading of heavy module
405                   POSIX::mkfifo($FIFO, 0700)
406                                       || die "can't mkfifo $FIFO: $!";
407               }
408
409               # next line blocks till there's a reader
410               open (FIFO, "> $FIFO")  || die "can't open $FIFO: $!";
411               print FIFO "John Smith (smith\@host.org)\n", `fortune -s`;
412               close(FIFO)             || die "can't close $FIFO: $!";
413               sleep 2;                # to avoid dup signals
414           }
415

Using open() for IPC

417       Perl's basic open() statement can also be used for unidirectional
418       interprocess communication by either appending or prepending a pipe
419       symbol to the second argument to open().  Here's how to start something
420       up in a child process you intend to write to:
421
422           open(SPOOLER, "| cat -v | lpr -h 2>/dev/null")
423                               || die "can't fork: $!";
424           local $SIG{PIPE} = sub { die "spooler pipe broke" };
425           print SPOOLER "stuff\n";
426           close SPOOLER       || die "bad spool: $! $?";
427
428       And here's how to start up a child process you intend to read from:
429
430           open(STATUS, "netstat -an 2>&1 |")
431                               || die "can't fork: $!";
432           while (<STATUS>) {
433               next if /^(tcp|udp)/;
434               print;
435           }
436           close STATUS        || die "bad netstat: $! $?";
437
438       If one can be sure that a particular program is a Perl script expecting
439       filenames in @ARGV, the clever programmer can write something like
440       this:
441
442           % program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile
443
444       and no matter which sort of shell it's called from, the Perl program
445       will read from the file f1, the process cmd1, standard input (tmpfile
446       in this case), the f2 file, the cmd2 command, and finally the f3 file.
447       Pretty nifty, eh?
448
449       You might notice that you could use backticks for much the same effect
450       as opening a pipe for reading:
451
452           print grep { !/^(tcp|udp)/ } `netstat -an 2>&1`;
453           die "bad netstatus ($?)" if $?;
454
455       While this is true on the surface, it's much more efficient to process
456       the file one line or record at a time because then you don't have to
457       read the whole thing into memory at once.  It also gives you finer
458       control of the whole process, letting you kill off the child process
459       early if you'd like.
460
461       Be careful to check the return values from both open() and close().  If
462       you're writing to a pipe, you should also trap SIGPIPE.  Otherwise,
463       think of what happens when you start up a pipe to a command that
464       doesn't exist: the open() will in all likelihood succeed (it only
465       reflects the fork()'s success), but then your output will
466       fail--spectacularly.  Perl can't know whether the command worked,
467       because your command is actually running in a separate process whose
468       exec() might have failed.  Therefore, while readers of bogus commands
469       return just a quick EOF, writers to bogus commands will get hit with a
470       signal, which they'd best be prepared to handle.  Consider:
471
472           open(FH, "|bogus")      || die "can't fork: $!";
473           print FH "bang\n";      #  neither necessary nor sufficient
474                                   #  to check print retval!
475           close(FH)               || die "can't close: $!";
476
477       The reason for not checking the return value from print() is because of
478       pipe buffering; physical writes are delayed.  That won't blow up until
479       the close, and it will blow up with a SIGPIPE.  To catch it, you could
480       use this:
481
482           $SIG{PIPE} = "IGNORE";
483           open(FH, "|bogus")  || die "can't fork: $!";
484           print FH "bang\n";
485           close(FH)           || die "can't close: status=$?";
486
487   Filehandles
488       Both the main process and any child processes it forks share the same
489       STDIN, STDOUT, and STDERR filehandles.  If both processes try to access
490       them at once, strange things can happen.  You may also want to close or
491       reopen the filehandles for the child.  You can get around this by
492       opening your pipe with open(), but on some systems this means that the
493       child process cannot outlive the parent.
494
495   Background Processes
496       You can run a command in the background with:
497
498           system("cmd &");
499
500       The command's STDOUT and STDERR (and possibly STDIN, depending on your
501       shell) will be the same as the parent's.  You won't need to catch
502       SIGCHLD because of the double-fork taking place; see below for details.
503
504   Complete Dissociation of Child from Parent
505       In some cases (starting server processes, for instance) you'll want to
506       completely dissociate the child process from the parent.  This is often
507       called daemonization.  A well-behaved daemon will also chdir() to the
508       root directory so it doesn't prevent unmounting the filesystem
509       containing the directory from which it was launched, and redirect its
510       standard file descriptors from and to /dev/null so that random output
511       doesn't wind up on the user's terminal.
512
513        use POSIX "setsid";
514
515        sub daemonize {
516            chdir("/")                  || die "can't chdir to /: $!";
517            open(STDIN,  "< /dev/null") || die "can't read /dev/null: $!";
518            open(STDOUT, "> /dev/null") || die "can't write to /dev/null: $!";
519            defined(my $pid = fork())   || die "can't fork: $!";
520            exit if $pid;               # non-zero now means I am the parent
521            (setsid() != -1)            || die "Can't start a new session: $!";
522            open(STDERR, ">&STDOUT")    || die "can't dup stdout: $!";
523        }
524
525       The fork() has to come before the setsid() to ensure you aren't a
526       process group leader; the setsid() will fail if you are.  If your
527       system doesn't have the setsid() function, open /dev/tty and use the
528       "TIOCNOTTY" ioctl() on it instead.  See tty(4) for details.
529
530       Non-Unix users should check their "Your_OS::Process" module for other
531       possible solutions.
532
533   Safe Pipe Opens
534       Another interesting approach to IPC is making your single program go
535       multiprocess and communicate between--or even amongst--yourselves.  The
536       open() function will accept a file argument of either "-|" or "|-" to
537       do a very interesting thing: it forks a child connected to the
538       filehandle you've opened.  The child is running the same program as the
539       parent.  This is useful for safely opening a file when running under an
540       assumed UID or GID, for example.  If you open a pipe to minus, you can
541       write to the filehandle you opened and your kid will find it in his
542       STDIN.  If you open a pipe from minus, you can read from the filehandle
543       you opened whatever your kid writes to his STDOUT.
544
545           use English;
546           my $PRECIOUS = "/path/to/some/safe/file";
547           my $sleep_count;
548           my $pid;
549
550           do {
551               $pid = open(KID_TO_WRITE, "|-");
552               unless (defined $pid) {
553                   warn "cannot fork: $!";
554                   die "bailing out" if $sleep_count++ > 6;
555                   sleep 10;
556               }
557           } until defined $pid;
558
559           if ($pid) {                 # I am the parent
560               print KID_TO_WRITE @some_data;
561               close(KID_TO_WRITE)     || warn "kid exited $?";
562           } else {                    # I am the child
563               # drop permissions in setuid and/or setgid programs:
564               ($EUID, $EGID) = ($UID, $GID);
565               open (OUTFILE, "> $PRECIOUS")
566                                       || die "can't open $PRECIOUS: $!";
567               while (<STDIN>) {
568                   print OUTFILE;      # child's STDIN is parent's KID_TO_WRITE
569               }
570               close(OUTFILE)          || die "can't close $PRECIOUS: $!";
571               exit(0);                # don't forget this!!
572           }
573
574       Another common use for this construct is when you need to execute
575       something without the shell's interference.  With system(), it's
576       straightforward, but you can't use a pipe open or backticks safely.
577       That's because there's no way to stop the shell from getting its hands
578       on your arguments.   Instead, use lower-level control to call exec()
579       directly.
580
581       Here's a safe backtick or pipe open for read:
582
583           my $pid = open(KID_TO_READ, "-|");
584           defined($pid)           || die "can't fork: $!";
585
586           if ($pid) {             # parent
587               while (<KID_TO_READ>) {
588                                   # do something interesting
589               }
590               close(KID_TO_READ)  || warn "kid exited $?";
591
592           } else {                # child
593               ($EUID, $EGID) = ($UID, $GID); # suid only
594               exec($program, @options, @args)
595                                   || die "can't exec program: $!";
596               # NOTREACHED
597           }
598
599       And here's a safe pipe open for writing:
600
601           my $pid = open(KID_TO_WRITE, "|-");
602           defined($pid)           || die "can't fork: $!";
603
604           $SIG{PIPE} = sub { die "whoops, $program pipe broke" };
605
606           if ($pid) {             # parent
607               print KID_TO_WRITE @data;
608               close(KID_TO_WRITE) || warn "kid exited $?";
609
610           } else {                # child
611               ($EUID, $EGID) = ($UID, $GID);
612               exec($program, @options, @args)
613                                   || die "can't exec program: $!";
614               # NOTREACHED
615           }
616
617       It is very easy to dead-lock a process using this form of open(), or
618       indeed with any use of pipe() with multiple subprocesses.  The example
619       above is "safe" because it is simple and calls exec().  See "Avoiding
620       Pipe Deadlocks" for general safety principles, but there are extra
621       gotchas with Safe Pipe Opens.
622
623       In particular, if you opened the pipe using "open FH, "|-"", then you
624       cannot simply use close() in the parent process to close an unwanted
625       writer.  Consider this code:
626
627           my $pid = open(WRITER, "|-");        # fork open a kid
628           defined($pid)               || die "first fork failed: $!";
629           if ($pid) {
630               if (my $sub_pid = fork()) {
631                   defined($sub_pid)   || die "second fork failed: $!";
632                   close(WRITER)       || die "couldn't close WRITER: $!";
633                   # now do something else...
634               }
635               else {
636                   # first write to WRITER
637                   # ...
638                   # then when finished
639                   close(WRITER)       || die "couldn't close WRITER: $!";
640                   exit(0);
641               }
642           }
643           else {
644               # first do something with STDIN, then
645               exit(0);
646           }
647
648       In the example above, the true parent does not want to write to the
649       WRITER filehandle, so it closes it.  However, because WRITER was opened
650       using "open FH, "|-"", it has a special behavior: closing it calls
651       waitpid() (see "waitpid" in perlfunc), which waits for the subprocess
652       to exit.  If the child process ends up waiting for something happening
653       in the section marked "do something else", you have deadlock.
654
655       This can also be a problem with intermediate subprocesses in more
656       complicated code, which will call waitpid() on all open filehandles
657       during global destruction--in no predictable order.
658
659       To solve this, you must manually use pipe(), fork(), and the form of
660       open() which sets one file descriptor to another, as shown below:
661
662           pipe(READER, WRITER)        || die "pipe failed: $!";
663           $pid = fork();
664           defined($pid)               || die "first fork failed: $!";
665           if ($pid) {
666               close READER;
667               if (my $sub_pid = fork()) {
668                   defined($sub_pid)   || die "first fork failed: $!";
669                   close(WRITER)       || die "can't close WRITER: $!";
670               }
671               else {
672                   # write to WRITER...
673                   # ...
674                   # then  when finished
675                   close(WRITER)       || die "can't close WRITER: $!";
676                   exit(0);
677               }
678               # write to WRITER...
679           }
680           else {
681               open(STDIN, "<&READER") || die "can't reopen STDIN: $!";
682               close(WRITER)           || die "can't close WRITER: $!";
683               # do something...
684               exit(0);
685           }
686
687       Since Perl 5.8.0, you can also use the list form of "open" for pipes.
688       This is preferred when you wish to avoid having the shell interpret
689       metacharacters that may be in your command string.
690
691       So for example, instead of using:
692
693           open(PS_PIPE, "ps aux|")    || die "can't open ps pipe: $!";
694
695       One would use either of these:
696
697           open(PS_PIPE, "-|", "ps", "aux")
698                                       || die "can't open ps pipe: $!";
699
700           @ps_args = qw[ ps aux ];
701           open(PS_PIPE, "-|", @ps_args)
702                                       || die "can't open @ps_args|: $!";
703
704       Because there are more than three arguments to open(), forks the ps(1)
705       command without spawning a shell, and reads its standard output via the
706       "PS_PIPE" filehandle.  The corresponding syntax to write to command
707       pipes is to use "|-" in place of "-|".
708
709       This was admittedly a rather silly example, because you're using string
710       literals whose content is perfectly safe.  There is therefore no cause
711       to resort to the harder-to-read, multi-argument form of pipe open().
712       However, whenever you cannot be assured that the program arguments are
713       free of shell metacharacters, the fancier form of open() should be
714       used.  For example:
715
716           @grep_args = ("egrep", "-i", $some_pattern, @many_files);
717           open(GREP_PIPE, "-|", @grep_args)
718                               || die "can't open @grep_args|: $!";
719
720       Here the multi-argument form of pipe open() is preferred because the
721       pattern and indeed even the filenames themselves might hold
722       metacharacters.
723
724       Be aware that these operations are full Unix forks, which means they
725       may not be correctly implemented on all alien systems.
726
727   Avoiding Pipe Deadlocks
728       Whenever you have more than one subprocess, you must be careful that
729       each closes whichever half of any pipes created for interprocess
730       communication it is not using.  This is because any child process
731       reading from the pipe and expecting an EOF will never receive it, and
732       therefore never exit. A single process closing a pipe is not enough to
733       close it; the last process with the pipe open must close it for it to
734       read EOF.
735
736       Certain built-in Unix features help prevent this most of the time.  For
737       instance, filehandles have a "close on exec" flag, which is set en
738       masse under control of the $^F variable.  This is so any filehandles
739       you didn't explicitly route to the STDIN, STDOUT or STDERR of a child
740       program will be automatically closed.
741
742       Always explicitly and immediately call close() on the writable end of
743       any pipe, unless that process is actually writing to it.  Even if you
744       don't explicitly call close(), Perl will still close() all filehandles
745       during global destruction.  As previously discussed, if those
746       filehandles have been opened with Safe Pipe Open, this will result in
747       calling waitpid(), which may again deadlock.
748
749   Bidirectional Communication with Another Process
750       While this works reasonably well for unidirectional communication, what
751       about bidirectional communication?  The most obvious approach doesn't
752       work:
753
754           # THIS DOES NOT WORK!!
755           open(PROG_FOR_READING_AND_WRITING, "| some program |")
756
757       If you forget to "use warnings", you'll miss out entirely on the
758       helpful diagnostic message:
759
760           Can't do bidirectional pipe at -e line 1.
761
762       If you really want to, you can use the standard open2() from the
763       "IPC::Open2" module to catch both ends.  There's also an open3() in
764       "IPC::Open3" for tridirectional I/O so you can also catch your child's
765       STDERR, but doing so would then require an awkward select() loop and
766       wouldn't allow you to use normal Perl input operations.
767
768       If you look at its source, you'll see that open2() uses low-level
769       primitives like the pipe() and exec() syscalls to create all the
770       connections.  Although it might have been more efficient by using
771       socketpair(), this would have been even less portable than it already
772       is. The open2() and open3() functions are unlikely to work anywhere
773       except on a Unix system, or at least one purporting POSIX compliance.
774
775       Here's an example of using open2():
776
777           use FileHandle;
778           use IPC::Open2;
779           $pid = open2(*Reader, *Writer, "cat -un");
780           print Writer "stuff\n";
781           $got = <Reader>;
782
783       The problem with this is that buffering is really going to ruin your
784       day.  Even though your "Writer" filehandle is auto-flushed so the
785       process on the other end gets your data in a timely manner, you can't
786       usually do anything to force that process to give its data to you in a
787       similarly quick fashion.  In this special case, we could actually so,
788       because we gave cat a -u flag to make it unbuffered.  But very few
789       commands are designed to operate over pipes, so this seldom works
790       unless you yourself wrote the program on the other end of the double-
791       ended pipe.
792
793       A solution to this is to use a library which uses pseudottys to make
794       your program behave more reasonably.  This way you don't have to have
795       control over the source code of the program you're using.  The "Expect"
796       module from CPAN also addresses this kind of thing.  This module
797       requires two other modules from CPAN, "IO::Pty" and "IO::Stty".  It
798       sets up a pseudo terminal to interact with programs that insist on
799       talking to the terminal device driver.  If your system is supported,
800       this may be your best bet.
801
802   Bidirectional Communication with Yourself
803       If you want, you may make low-level pipe() and fork() syscalls to
804       stitch this together by hand.  This example only talks to itself, but
805       you could reopen the appropriate handles to STDIN and STDOUT and call
806       other processes.  (The following example lacks proper error checking.)
807
808        #!/usr/bin/perl -w
809        # pipe1 - bidirectional communication using two pipe pairs
810        #         designed for the socketpair-challenged
811        use IO::Handle;             # thousands of lines just for autoflush :-(
812        pipe(PARENT_RDR, CHILD_WTR);  # XXX: check failure?
813        pipe(CHILD_RDR,  PARENT_WTR); # XXX: check failure?
814        CHILD_WTR->autoflush(1);
815        PARENT_WTR->autoflush(1);
816
817        if ($pid = fork()) {
818            close PARENT_RDR;
819            close PARENT_WTR;
820            print CHILD_WTR "Parent Pid $$ is sending this\n";
821            chomp($line = <CHILD_RDR>);
822            print "Parent Pid $$ just read this: '$line'\n";
823            close CHILD_RDR; close CHILD_WTR;
824            waitpid($pid, 0);
825        } else {
826            die "cannot fork: $!" unless defined $pid;
827            close CHILD_RDR;
828            close CHILD_WTR;
829            chomp($line = <PARENT_RDR>);
830            print "Child Pid $$ just read this: '$line'\n";
831            print PARENT_WTR "Child Pid $$ is sending this\n";
832            close PARENT_RDR;
833            close PARENT_WTR;
834            exit(0);
835        }
836
837       But you don't actually have to make two pipe calls.  If you have the
838       socketpair() system call, it will do this all for you.
839
840        #!/usr/bin/perl -w
841        # pipe2 - bidirectional communication using socketpair
842        #   "the best ones always go both ways"
843
844        use Socket;
845        use IO::Handle;  # thousands of lines just for autoflush :-(
846
847        # We say AF_UNIX because although *_LOCAL is the
848        # POSIX 1003.1g form of the constant, many machines
849        # still don't have it.
850        socketpair(CHILD, PARENT, AF_UNIX, SOCK_STREAM, PF_UNSPEC)
851                                    ||  die "socketpair: $!";
852
853        CHILD->autoflush(1);
854        PARENT->autoflush(1);
855
856        if ($pid = fork()) {
857            close PARENT;
858            print CHILD "Parent Pid $$ is sending this\n";
859            chomp($line = <CHILD>);
860            print "Parent Pid $$ just read this: '$line'\n";
861            close CHILD;
862            waitpid($pid, 0);
863        } else {
864            die "cannot fork: $!" unless defined $pid;
865            close CHILD;
866            chomp($line = <PARENT>);
867            print "Child Pid $$ just read this: '$line'\n";
868            print PARENT "Child Pid $$ is sending this\n";
869            close PARENT;
870            exit(0);
871        }
872

Sockets: Client/Server Communication

874       While not entirely limited to Unix-derived operating systems (e.g.,
875       WinSock on PCs provides socket support, as do some VMS libraries), you
876       might not have sockets on your system, in which case this section
877       probably isn't going to do you much good.  With sockets, you can do
878       both virtual circuits like TCP streams and datagrams like UDP packets.
879       You may be able to do even more depending on your system.
880
881       The Perl functions for dealing with sockets have the same names as the
882       corresponding system calls in C, but their arguments tend to differ for
883       two reasons.  First, Perl filehandles work differently than C file
884       descriptors.  Second, Perl already knows the length of its strings, so
885       you don't need to pass that information.
886
887       One of the major problems with ancient, antemillennial socket code in
888       Perl was that it used hard-coded values for some of the constants,
889       which severely hurt portability.  If you ever see code that does
890       anything like explicitly setting "$AF_INET = 2", you know you're in for
891       big trouble.  An immeasurably superior approach is to use the "Socket"
892       module, which more reliably grants access to the various constants and
893       functions you'll need.
894
895       If you're not writing a server/client for an existing protocol like
896       NNTP or SMTP, you should give some thought to how your server will know
897       when the client has finished talking, and vice-versa.  Most protocols
898       are based on one-line messages and responses (so one party knows the
899       other has finished when a "\n" is received) or multi-line messages and
900       responses that end with a period on an empty line ("\n.\n" terminates a
901       message/response).
902
903   Internet Line Terminators
904       The Internet line terminator is "\015\012".  Under ASCII variants of
905       Unix, that could usually be written as "\r\n", but under other systems,
906       "\r\n" might at times be "\015\015\012", "\012\012\015", or something
907       completely different.  The standards specify writing "\015\012" to be
908       conformant (be strict in what you provide), but they also recommend
909       accepting a lone "\012" on input (be lenient in what you require).  We
910       haven't always been very good about that in the code in this manpage,
911       but unless you're on a Mac from way back in its pre-Unix dark ages,
912       you'll probably be ok.
913
914   Internet TCP Clients and Servers
915       Use Internet-domain sockets when you want to do client-server
916       communication that might extend to machines outside of your own system.
917
918       Here's a sample TCP client using Internet-domain sockets:
919
920           #!/usr/bin/perl -w
921           use strict;
922           use Socket;
923           my ($remote, $port, $iaddr, $paddr, $proto, $line);
924
925           $remote  = shift || "localhost";
926           $port    = shift || 2345;  # random port
927           if ($port =~ /\D/) { $port = getservbyname($port, "tcp") }
928           die "No port" unless $port;
929           $iaddr   = inet_aton($remote)       || die "no host: $remote";
930           $paddr   = sockaddr_in($port, $iaddr);
931
932           $proto   = getprotobyname("tcp");
933           socket(SOCK, PF_INET, SOCK_STREAM, $proto)  || die "socket: $!";
934           connect(SOCK, $paddr)               || die "connect: $!";
935           while ($line = <SOCK>) {
936               print $line;
937           }
938
939           close (SOCK)                        || die "close: $!";
940           exit(0);
941
942       And here's a corresponding server to go along with it.  We'll leave the
943       address as "INADDR_ANY" so that the kernel can choose the appropriate
944       interface on multihomed hosts.  If you want sit on a particular
945       interface (like the external side of a gateway or firewall machine),
946       fill this in with your real address instead.
947
948        #!/usr/bin/perl -Tw
949        use strict;
950        BEGIN { $ENV{PATH} = "/usr/bin:/bin" }
951        use Socket;
952        use Carp;
953        my $EOL = "\015\012";
954
955        sub logmsg { print "$0 $$: @_ at ", scalar localtime(), "\n" }
956
957        my $port  = shift || 2345;
958        die "invalid port" unless $port =~ /^ \d+ $/x;
959
960        my $proto = getprotobyname("tcp");
961
962        socket(Server, PF_INET, SOCK_STREAM, $proto)   || die "socket: $!";
963        setsockopt(Server, SOL_SOCKET, SO_REUSEADDR, pack("l", 1))
964                                                       || die "setsockopt: $!";
965        bind(Server, sockaddr_in($port, INADDR_ANY))   || die "bind: $!";
966        listen(Server, SOMAXCONN)                      || die "listen: $!";
967
968        logmsg "server started on port $port";
969
970        my $paddr;
971
972        for ( ; $paddr = accept(Client, Server); close Client) {
973            my($port, $iaddr) = sockaddr_in($paddr);
974            my $name = gethostbyaddr($iaddr, AF_INET);
975
976            logmsg "connection from $name [",
977                    inet_ntoa($iaddr), "]
978                    at port $port";
979
980            print Client "Hello there, $name, it's now ",
981                            scalar localtime(), $EOL;
982        }
983
984       And here's a multitasking version.  It's multitasked in that like most
985       typical servers, it spawns (fork()s) a slave server to handle the
986       client request so that the master server can quickly go back to service
987       a new client.
988
989        #!/usr/bin/perl -Tw
990        use strict;
991        BEGIN { $ENV{PATH} = "/usr/bin:/bin" }
992        use Socket;
993        use Carp;
994        my $EOL = "\015\012";
995
996        sub spawn;  # forward declaration
997        sub logmsg { print "$0 $$: @_ at ", scalar localtime(), "\n" }
998
999        my $port  = shift || 2345;
1000        die "invalid port" unless $port =~ /^ \d+ $/x;
1001
1002        my $proto = getprotobyname("tcp");
1003
1004        socket(Server, PF_INET, SOCK_STREAM, $proto)   || die "socket: $!";
1005        setsockopt(Server, SOL_SOCKET, SO_REUSEADDR, pack("l", 1))
1006                                                       || die "setsockopt: $!";
1007        bind(Server, sockaddr_in($port, INADDR_ANY))   || die "bind: $!";
1008        listen(Server, SOMAXCONN)                      || die "listen: $!";
1009
1010        logmsg "server started on port $port";
1011
1012        my $waitedpid = 0;
1013        my $paddr;
1014
1015        use POSIX ":sys_wait_h";
1016        use Errno;
1017
1018        sub REAPER {
1019            local $!;   # don't let waitpid() overwrite current error
1020            while ((my $pid = waitpid(-1, WNOHANG)) > 0 && WIFEXITED($?)) {
1021                logmsg "reaped $waitedpid" . ($? ? " with exit $?" : "");
1022            }
1023            $SIG{CHLD} = \&REAPER;  # loathe SysV
1024        }
1025
1026        $SIG{CHLD} = \&REAPER;
1027
1028        while (1) {
1029            $paddr = accept(Client, Server) || do {
1030                # try again if accept() returned because got a signal
1031                next if $!{EINTR};
1032                die "accept: $!";
1033            };
1034            my ($port, $iaddr) = sockaddr_in($paddr);
1035            my $name = gethostbyaddr($iaddr, AF_INET);
1036
1037            logmsg "connection from $name [",
1038                   inet_ntoa($iaddr),
1039                   "] at port $port";
1040
1041            spawn sub {
1042                $| = 1;
1043                print "Hello there, $name, it's now ",
1044                      scalar localtime(),
1045                      $EOL;
1046                exec "/usr/games/fortune"       # XXX: "wrong" line terminators
1047                    or confess "can't exec fortune: $!";
1048            };
1049            close Client;
1050        }
1051
1052        sub spawn {
1053            my $coderef = shift;
1054
1055            unless (@_ == 0 && $coderef && ref($coderef) eq "CODE") {
1056                confess "usage: spawn CODEREF";
1057            }
1058
1059            my $pid;
1060            unless (defined($pid = fork())) {
1061                logmsg "cannot fork: $!";
1062                return;
1063            }
1064            elsif ($pid) {
1065                logmsg "begat $pid";
1066                return; # I'm the parent
1067            }
1068            # else I'm the child -- go spawn
1069
1070            open(STDIN,  "<&Client")    || die "can't dup client to stdin";
1071            open(STDOUT, ">&Client")    || die "can't dup client to stdout";
1072            ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr";
1073            exit($coderef->());
1074        }
1075
1076       This server takes the trouble to clone off a child version via fork()
1077       for each incoming request.  That way it can handle many requests at
1078       once, which you might not always want.  Even if you don't fork(), the
1079       listen() will allow that many pending connections.  Forking servers
1080       have to be particularly careful about cleaning up their dead children
1081       (called "zombies" in Unix parlance), because otherwise you'll quickly
1082       fill up your process table.  The REAPER subroutine is used here to call
1083       waitpid() for any child processes that have finished, thereby ensuring
1084       that they terminate cleanly and don't join the ranks of the living
1085       dead.
1086
1087       Within the while loop we call accept() and check to see if it returns a
1088       false value.  This would normally indicate a system error needs to be
1089       reported.  However, the introduction of safe signals (see "Deferred
1090       Signals (Safe Signals)" above) in Perl 5.8.0 means that accept() might
1091       also be interrupted when the process receives a signal.  This typically
1092       happens when one of the forked subprocesses exits and notifies the
1093       parent process with a CHLD signal.
1094
1095       If accept() is interrupted by a signal, $! will be set to EINTR.  If
1096       this happens, we can safely continue to the next iteration of the loop
1097       and another call to accept().  It is important that your signal
1098       handling code not modify the value of $!, or else this test will likely
1099       fail.  In the REAPER subroutine we create a local version of $! before
1100       calling waitpid().  When waitpid() sets $! to ECHILD as it inevitably
1101       does when it has no more children waiting, it updates the local copy
1102       and leaves the original unchanged.
1103
1104       You should use the -T flag to enable taint checking (see perlsec) even
1105       if we aren't running setuid or setgid.  This is always a good idea for
1106       servers or any program run on behalf of someone else (like CGI
1107       scripts), because it lessens the chances that people from the outside
1108       will be able to compromise your system.
1109
1110       Let's look at another TCP client.  This one connects to the TCP "time"
1111       service on a number of different machines and shows how far their
1112       clocks differ from the system on which it's being run:
1113
1114           #!/usr/bin/perl  -w
1115           use strict;
1116           use Socket;
1117
1118           my $SECS_OF_70_YEARS = 2208988800;
1119           sub ctime { scalar localtime(shift() || time()) }
1120
1121           my $iaddr = gethostbyname("localhost");
1122           my $proto = getprotobyname("tcp");
1123           my $port = getservbyname("time", "tcp");
1124           my $paddr = sockaddr_in(0, $iaddr);
1125           my($host);
1126
1127           $| = 1;
1128           printf "%-24s %8s %s\n", "localhost", 0, ctime();
1129
1130           foreach $host (@ARGV) {
1131               printf "%-24s ", $host;
1132               my $hisiaddr = inet_aton($host)     || die "unknown host";
1133               my $hispaddr = sockaddr_in($port, $hisiaddr);
1134               socket(SOCKET, PF_INET, SOCK_STREAM, $proto)
1135                                                   || die "socket: $!";
1136               connect(SOCKET, $hispaddr)          || die "connect: $!";
1137               my $rtime = pack("C4", ());
1138               read(SOCKET, $rtime, 4);
1139               close(SOCKET);
1140               my $histime = unpack("N", $rtime) - $SECS_OF_70_YEARS;
1141               printf "%8d %s\n", $histime - time(), ctime($histime);
1142           }
1143
1144   Unix-Domain TCP Clients and Servers
1145       That's fine for Internet-domain clients and servers, but what about
1146       local communications?  While you can use the same setup, sometimes you
1147       don't want to.  Unix-domain sockets are local to the current host, and
1148       are often used internally to implement pipes.  Unlike Internet domain
1149       sockets, Unix domain sockets can show up in the file system with an
1150       ls(1) listing.
1151
1152           % ls -l /dev/log
1153           srw-rw-rw-  1 root            0 Oct 31 07:23 /dev/log
1154
1155       You can test for these with Perl's -S file test:
1156
1157           unless (-S "/dev/log") {
1158               die "something's wicked with the log system";
1159           }
1160
1161       Here's a sample Unix-domain client:
1162
1163           #!/usr/bin/perl -w
1164           use Socket;
1165           use strict;
1166           my ($rendezvous, $line);
1167
1168           $rendezvous = shift || "catsock";
1169           socket(SOCK, PF_UNIX, SOCK_STREAM, 0)     || die "socket: $!";
1170           connect(SOCK, sockaddr_un($rendezvous))   || die "connect: $!";
1171           while (defined($line = <SOCK>)) {
1172               print $line;
1173           }
1174           exit(0);
1175
1176       And here's a corresponding server.  You don't have to worry about silly
1177       network terminators here because Unix domain sockets are guaranteed to
1178       be on the localhost, and thus everything works right.
1179
1180           #!/usr/bin/perl -Tw
1181           use strict;
1182           use Socket;
1183           use Carp;
1184
1185           BEGIN { $ENV{PATH} = "/usr/bin:/bin" }
1186           sub spawn;  # forward declaration
1187           sub logmsg { print "$0 $$: @_ at ", scalar localtime(), "\n" }
1188
1189           my $NAME = "catsock";
1190           my $uaddr = sockaddr_un($NAME);
1191           my $proto = getprotobyname("tcp");
1192
1193           socket(Server, PF_UNIX, SOCK_STREAM, 0) || die "socket: $!";
1194           unlink($NAME);
1195           bind  (Server, $uaddr)                  || die "bind: $!";
1196           listen(Server, SOMAXCONN)               || die "listen: $!";
1197
1198           logmsg "server started on $NAME";
1199
1200           my $waitedpid;
1201
1202           use POSIX ":sys_wait_h";
1203           sub REAPER {
1204               my $child;
1205               while (($waitedpid = waitpid(-1, WNOHANG)) > 0) {
1206                   logmsg "reaped $waitedpid" . ($? ? " with exit $?" : "");
1207               }
1208               $SIG{CHLD} = \&REAPER;  # loathe SysV
1209           }
1210
1211           $SIG{CHLD} = \&REAPER;
1212
1213
1214           for ( $waitedpid = 0;
1215                 accept(Client, Server) || $waitedpid;
1216                 $waitedpid = 0, close Client)
1217           {
1218               next if $waitedpid;
1219               logmsg "connection on $NAME";
1220               spawn sub {
1221                   print "Hello there, it's now ", scalar localtime(), "\n";
1222                   exec("/usr/games/fortune")  || die "can't exec fortune: $!";
1223               };
1224           }
1225
1226           sub spawn {
1227               my $coderef = shift();
1228
1229               unless (@_ == 0 && $coderef && ref($coderef) eq "CODE") {
1230                   confess "usage: spawn CODEREF";
1231               }
1232
1233               my $pid;
1234               unless (defined($pid = fork())) {
1235                   logmsg "cannot fork: $!";
1236                   return;
1237               }
1238               elsif ($pid) {
1239                   logmsg "begat $pid";
1240                   return; # I'm the parent
1241               }
1242               else {
1243                   # I'm the child -- go spawn
1244               }
1245
1246               open(STDIN,  "<&Client")    || die "can't dup client to stdin";
1247               open(STDOUT, ">&Client")    || die "can't dup client to stdout";
1248               ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr";
1249               exit($coderef->());
1250           }
1251
1252       As you see, it's remarkably similar to the Internet domain TCP server,
1253       so much so, in fact, that we've omitted several duplicate
1254       functions--spawn(), logmsg(), ctime(), and REAPER()--which are the same
1255       as in the other server.
1256
1257       So why would you ever want to use a Unix domain socket instead of a
1258       simpler named pipe?  Because a named pipe doesn't give you sessions.
1259       You can't tell one process's data from another's.  With socket
1260       programming, you get a separate session for each client; that's why
1261       accept() takes two arguments.
1262
1263       For example, let's say that you have a long-running database server
1264       daemon that you want folks to be able to access from the Web, but only
1265       if they go through a CGI interface.  You'd have a small, simple CGI
1266       program that does whatever checks and logging you feel like, and then
1267       acts as a Unix-domain client and connects to your private server.
1268

TCP Clients with IO::Socket

1270       For those preferring a higher-level interface to socket programming,
1271       the IO::Socket module provides an object-oriented approach.  If for
1272       some reason you lack this module, you can just fetch IO::Socket from
1273       CPAN, where you'll also find modules providing easy interfaces to the
1274       following systems: DNS, FTP, Ident (RFC 931), NIS and NISPlus, NNTP,
1275       Ping, POP3, SMTP, SNMP, SSLeay, Telnet, and Time--to name just a few.
1276
1277   A Simple Client
1278       Here's a client that creates a TCP connection to the "daytime" service
1279       at port 13 of the host name "localhost" and prints out everything that
1280       the server there cares to provide.
1281
1282           #!/usr/bin/perl -w
1283           use IO::Socket;
1284           $remote = IO::Socket::INET->new(
1285                               Proto    => "tcp",
1286                               PeerAddr => "localhost",
1287                               PeerPort => "daytime(13)",
1288                           )
1289                        || die "can't connect to daytime service on localhost";
1290           while (<$remote>) { print }
1291
1292       When you run this program, you should get something back that looks
1293       like this:
1294
1295           Wed May 14 08:40:46 MDT 1997
1296
1297       Here are what those parameters to the new() constructor mean:
1298
1299       "Proto"
1300           This is which protocol to use.  In this case, the socket handle
1301           returned will be connected to a TCP socket, because we want a
1302           stream-oriented connection, that is, one that acts pretty much like
1303           a plain old file.  Not all sockets are this of this type.  For
1304           example, the UDP protocol can be used to make a datagram socket,
1305           used for message-passing.
1306
1307       "PeerAddr"
1308           This is the name or Internet address of the remote host the server
1309           is running on.  We could have specified a longer name like
1310           "www.perl.com", or an address like "207.171.7.72".  For
1311           demonstration purposes, we've used the special hostname
1312           "localhost", which should always mean the current machine you're
1313           running on.  The corresponding Internet address for localhost is
1314           "127.0.0.1", if you'd rather use that.
1315
1316       "PeerPort"
1317           This is the service name or port number we'd like to connect to.
1318           We could have gotten away with using just "daytime" on systems with
1319           a well-configured system services file,[FOOTNOTE: The system
1320           services file is found in /etc/services under Unixy systems.] but
1321           here we've specified the port number (13) in parentheses.  Using
1322           just the number would have also worked, but numeric literals make
1323           careful programmers nervous.
1324
1325       Notice how the return value from the "new" constructor is used as a
1326       filehandle in the "while" loop?  That's what's called an indirect
1327       filehandle, a scalar variable containing a filehandle.  You can use it
1328       the same way you would a normal filehandle.  For example, you can read
1329       one line from it this way:
1330
1331           $line = <$handle>;
1332
1333       all remaining lines from is this way:
1334
1335           @lines = <$handle>;
1336
1337       and send a line of data to it this way:
1338
1339           print $handle "some data\n";
1340
1341   A Webget Client
1342       Here's a simple client that takes a remote host to fetch a document
1343       from, and then a list of files to get from that host.  This is a more
1344       interesting client than the previous one because it first sends
1345       something to the server before fetching the server's response.
1346
1347           #!/usr/bin/perl -w
1348           use IO::Socket;
1349           unless (@ARGV > 1) { die "usage: $0 host url ..." }
1350           $host = shift(@ARGV);
1351           $EOL = "\015\012";
1352           $BLANK = $EOL x 2;
1353           for my $document (@ARGV) {
1354               $remote = IO::Socket::INET->new( Proto     => "tcp",
1355                                                PeerAddr  => $host,
1356                                                PeerPort  => "http(80)",
1357                         )     || die "cannot connect to httpd on $host";
1358               $remote->autoflush(1);
1359               print $remote "GET $document HTTP/1.0" . $BLANK;
1360               while ( <$remote> ) { print }
1361               close $remote;
1362           }
1363
1364       The web server handling the HTTP service is assumed to be at its
1365       standard port, number 80.  If the server you're trying to connect to is
1366       at a different port, like 1080 or 8080, you should specify it as the
1367       named-parameter pair, "PeerPort => 8080".  The "autoflush" method is
1368       used on the socket because otherwise the system would buffer up the
1369       output we sent it.  (If you're on a prehistoric Mac, you'll also need
1370       to change every "\n" in your code that sends data over the network to
1371       be a "\015\012" instead.)
1372
1373       Connecting to the server is only the first part of the process: once
1374       you have the connection, you have to use the server's language.  Each
1375       server on the network has its own little command language that it
1376       expects as input.  The string that we send to the server starting with
1377       "GET" is in HTTP syntax.  In this case, we simply request each
1378       specified document.  Yes, we really are making a new connection for
1379       each document, even though it's the same host.  That's the way you
1380       always used to have to speak HTTP.  Recent versions of web browsers may
1381       request that the remote server leave the connection open a little
1382       while, but the server doesn't have to honor such a request.
1383
1384       Here's an example of running that program, which we'll call webget:
1385
1386           % webget www.perl.com /guanaco.html
1387           HTTP/1.1 404 File Not Found
1388           Date: Thu, 08 May 1997 18:02:32 GMT
1389           Server: Apache/1.2b6
1390           Connection: close
1391           Content-type: text/html
1392
1393           <HEAD><TITLE>404 File Not Found</TITLE></HEAD>
1394           <BODY><H1>File Not Found</H1>
1395           The requested URL /guanaco.html was not found on this server.<P>
1396           </BODY>
1397
1398       Ok, so that's not very interesting, because it didn't find that
1399       particular document.  But a long response wouldn't have fit on this
1400       page.
1401
1402       For a more featureful version of this program, you should look to the
1403       lwp-request program included with the LWP modules from CPAN.
1404
1405   Interactive Client with IO::Socket
1406       Well, that's all fine if you want to send one command and get one
1407       answer, but what about setting up something fully interactive, somewhat
1408       like the way telnet works?  That way you can type a line, get the
1409       answer, type a line, get the answer, etc.
1410
1411       This client is more complicated than the two we've done so far, but if
1412       you're on a system that supports the powerful "fork" call, the solution
1413       isn't that rough.  Once you've made the connection to whatever service
1414       you'd like to chat with, call "fork" to clone your process.  Each of
1415       these two identical process has a very simple job to do: the parent
1416       copies everything from the socket to standard output, while the child
1417       simultaneously copies everything from standard input to the socket.  To
1418       accomplish the same thing using just one process would be much harder,
1419       because it's easier to code two processes to do one thing than it is to
1420       code one process to do two things.  (This keep-it-simple principle a
1421       cornerstones of the Unix philosophy, and good software engineering as
1422       well, which is probably why it's spread to other systems.)
1423
1424       Here's the code:
1425
1426           #!/usr/bin/perl -w
1427           use strict;
1428           use IO::Socket;
1429           my ($host, $port, $kidpid, $handle, $line);
1430
1431           unless (@ARGV == 2) { die "usage: $0 host port" }
1432           ($host, $port) = @ARGV;
1433
1434           # create a tcp connection to the specified host and port
1435           $handle = IO::Socket::INET->new(Proto     => "tcp",
1436                                           PeerAddr  => $host,
1437                                           PeerPort  => $port)
1438                      || die "can't connect to port $port on $host: $!";
1439
1440           $handle->autoflush(1);       # so output gets there right away
1441           print STDERR "[Connected to $host:$port]\n";
1442
1443           # split the program into two processes, identical twins
1444           die "can't fork: $!" unless defined($kidpid = fork());
1445
1446           # the if{} block runs only in the parent process
1447           if ($kidpid) {
1448               # copy the socket to standard output
1449               while (defined ($line = <$handle>)) {
1450                   print STDOUT $line;
1451               }
1452               kill("TERM", $kidpid);   # send SIGTERM to child
1453           }
1454           # the else{} block runs only in the child process
1455           else {
1456               # copy standard input to the socket
1457               while (defined ($line = <STDIN>)) {
1458                   print $handle $line;
1459               }
1460               exit(0);                # just in case
1461           }
1462
1463       The "kill" function in the parent's "if" block is there to send a
1464       signal to our child process, currently running in the "else" block, as
1465       soon as the remote server has closed its end of the connection.
1466
1467       If the remote server sends data a byte at time, and you need that data
1468       immediately without waiting for a newline (which might not happen), you
1469       may wish to replace the "while" loop in the parent with the following:
1470
1471           my $byte;
1472           while (sysread($handle, $byte, 1) == 1) {
1473               print STDOUT $byte;
1474           }
1475
1476       Making a system call for each byte you want to read is not very
1477       efficient (to put it mildly) but is the simplest to explain and works
1478       reasonably well.
1479

TCP Servers with IO::Socket

1481       As always, setting up a server is little bit more involved than running
1482       a client.  The model is that the server creates a special kind of
1483       socket that does nothing but listen on a particular port for incoming
1484       connections.  It does this by calling the "IO::Socket::INET->new()"
1485       method with slightly different arguments than the client did.
1486
1487       Proto
1488           This is which protocol to use.  Like our clients, we'll still
1489           specify "tcp" here.
1490
1491       LocalPort
1492           We specify a local port in the "LocalPort" argument, which we
1493           didn't do for the client.  This is service name or port number for
1494           which you want to be the server. (Under Unix, ports under 1024 are
1495           restricted to the superuser.)  In our sample, we'll use port 9000,
1496           but you can use any port that's not currently in use on your
1497           system.  If you try to use one already in used, you'll get an
1498           "Address already in use" message.  Under Unix, the "netstat -a"
1499           command will show which services current have servers.
1500
1501       Listen
1502           The "Listen" parameter is set to the maximum number of pending
1503           connections we can accept until we turn away incoming clients.
1504           Think of it as a call-waiting queue for your telephone.  The low-
1505           level Socket module has a special symbol for the system maximum,
1506           which is SOMAXCONN.
1507
1508       Reuse
1509           The "Reuse" parameter is needed so that we restart our server
1510           manually without waiting a few minutes to allow system buffers to
1511           clear out.
1512
1513       Once the generic server socket has been created using the parameters
1514       listed above, the server then waits for a new client to connect to it.
1515       The server blocks in the "accept" method, which eventually accepts a
1516       bidirectional connection from the remote client.  (Make sure to
1517       autoflush this handle to circumvent buffering.)
1518
1519       To add to user-friendliness, our server prompts the user for commands.
1520       Most servers don't do this.  Because of the prompt without a newline,
1521       you'll have to use the "sysread" variant of the interactive client
1522       above.
1523
1524       This server accepts one of five different commands, sending output back
1525       to the client.  Unlike most network servers, this one handles only one
1526       incoming client at a time.  Multitasking servers are covered in Chapter
1527       16 of the Camel.
1528
1529       Here's the code.  We'll
1530
1531        #!/usr/bin/perl -w
1532        use IO::Socket;
1533        use Net::hostent;      # for OOish version of gethostbyaddr
1534
1535        $PORT = 9000;          # pick something not in use
1536
1537        $server = IO::Socket::INET->new( Proto     => "tcp",
1538                                         LocalPort => $PORT,
1539                                         Listen    => SOMAXCONN,
1540                                         Reuse     => 1);
1541
1542        die "can't setup server" unless $server;
1543        print "[Server $0 accepting clients]\n";
1544
1545        while ($client = $server->accept()) {
1546          $client->autoflush(1);
1547          print $client "Welcome to $0; type help for command list.\n";
1548          $hostinfo = gethostbyaddr($client->peeraddr);
1549          printf "[Connect from %s]\n",
1550                 $hostinfo ? $hostinfo->name : $client->peerhost;
1551          print $client "Command? ";
1552          while ( <$client>) {
1553            next unless /\S/;     # blank line
1554            if    (/quit|exit/i)  { last                                      }
1555            elsif (/date|time/i)  { printf $client "%s\n", scalar localtime() }
1556            elsif (/who/i )       { print  $client `who 2>&1`                 }
1557            elsif (/cookie/i )    { print  $client `/usr/games/fortune 2>&1`  }
1558            elsif (/motd/i )      { print  $client `cat /etc/motd 2>&1`       }
1559            else {
1560              print $client "Commands: quit date who cookie motd\n";
1561            }
1562          } continue {
1563             print $client "Command? ";
1564          }
1565          close $client;
1566        }
1567

UDP: Message Passing

1569       Another kind of client-server setup is one that uses not connections,
1570       but messages.  UDP communications involve much lower overhead but also
1571       provide less reliability, as there are no promises that messages will
1572       arrive at all, let alone in order and unmangled.  Still, UDP offers
1573       some advantages over TCP, including being able to "broadcast" or
1574       "multicast" to a whole bunch of destination hosts at once (usually on
1575       your local subnet).  If you find yourself overly concerned about
1576       reliability and start building checks into your message system, then
1577       you probably should use just TCP to start with.
1578
1579       UDP datagrams are not a bytestream and should not be treated as such.
1580       This makes using I/O mechanisms with internal buffering like stdio
1581       (i.e.  print() and friends) especially cumbersome. Use syswrite(), or
1582       better send(), like in the example below.
1583
1584       Here's a UDP program similar to the sample Internet TCP client given
1585       earlier.  However, instead of checking one host at a time, the UDP
1586       version will check many of them asynchronously by simulating a
1587       multicast and then using select() to do a timed-out wait for I/O.  To
1588       do something similar with TCP, you'd have to use a different socket
1589       handle for each host.
1590
1591        #!/usr/bin/perl -w
1592        use strict;
1593        use Socket;
1594        use Sys::Hostname;
1595
1596        my ( $count, $hisiaddr, $hispaddr, $histime,
1597             $host, $iaddr, $paddr, $port, $proto,
1598             $rin, $rout, $rtime, $SECS_OF_70_YEARS);
1599
1600        $SECS_OF_70_YEARS = 2_208_988_800;
1601
1602        $iaddr = gethostbyname(hostname());
1603        $proto = getprotobyname("udp");
1604        $port = getservbyname("time", "udp");
1605        $paddr = sockaddr_in(0, $iaddr); # 0 means let kernel pick
1606
1607        socket(SOCKET, PF_INET, SOCK_DGRAM, $proto)   || die "socket: $!";
1608        bind(SOCKET, $paddr)                          || die "bind: $!";
1609
1610        $| = 1;
1611        printf "%-12s %8s %s\n",  "localhost", 0, scalar localtime();
1612        $count = 0;
1613        for $host (@ARGV) {
1614            $count++;
1615            $hisiaddr = inet_aton($host)              || die "unknown host";
1616            $hispaddr = sockaddr_in($port, $hisiaddr);
1617            defined(send(SOCKET, 0, 0, $hispaddr))    || die "send $host: $!";
1618        }
1619
1620        $rin = "";
1621        vec($rin, fileno(SOCKET), 1) = 1;
1622
1623        # timeout after 10.0 seconds
1624        while ($count && select($rout = $rin, undef, undef, 10.0)) {
1625            $rtime = "";
1626            $hispaddr = recv(SOCKET, $rtime, 4, 0)    || die "recv: $!";
1627            ($port, $hisiaddr) = sockaddr_in($hispaddr);
1628            $host = gethostbyaddr($hisiaddr, AF_INET);
1629            $histime = unpack("N", $rtime) - $SECS_OF_70_YEARS;
1630            printf "%-12s ", $host;
1631            printf "%8d %s\n", $histime - time(), scalar localtime($histime);
1632            $count--;
1633        }
1634
1635       This example does not include any retries and may consequently fail to
1636       contact a reachable host. The most prominent reason for this is
1637       congestion of the queues on the sending host if the number of hosts to
1638       contact is sufficiently large.
1639

SysV IPC

1641       While System V IPC isn't so widely used as sockets, it still has some
1642       interesting uses.  However, you cannot use SysV IPC or Berkeley mmap()
1643       to have a variable shared amongst several processes.  That's because
1644       Perl would reallocate your string when you weren't wanting it to.  You
1645       might look into the "IPC::Shareable" or "threads::shared" modules for
1646       that.
1647
1648       Here's a small example showing shared memory usage.
1649
1650           use IPC::SysV qw(IPC_PRIVATE IPC_RMID S_IRUSR S_IWUSR);
1651
1652           $size = 2000;
1653           $id = shmget(IPC_PRIVATE, $size, S_IRUSR | S_IWUSR);
1654           defined($id)                    || die "shmget: $!";
1655           print "shm key $id\n";
1656
1657           $message = "Message #1";
1658           shmwrite($id, $message, 0, 60)  || die "shmwrite: $!";
1659           print "wrote: '$message'\n";
1660           shmread($id, $buff, 0, 60)      || die "shmread: $!";
1661           print "read : '$buff'\n";
1662
1663           # the buffer of shmread is zero-character end-padded.
1664           substr($buff, index($buff, "\0")) = "";
1665           print "un" unless $buff eq $message;
1666           print "swell\n";
1667
1668           print "deleting shm $id\n";
1669           shmctl($id, IPC_RMID, 0)        || die "shmctl: $!";
1670
1671       Here's an example of a semaphore:
1672
1673           use IPC::SysV qw(IPC_CREAT);
1674
1675           $IPC_KEY = 1234;
1676           $id = semget($IPC_KEY, 10, 0666 | IPC_CREAT);
1677           defined($id)                    || die "semget: $!";
1678           print "sem id $id\n";
1679
1680       Put this code in a separate file to be run in more than one process.
1681       Call the file take:
1682
1683           # create a semaphore
1684
1685           $IPC_KEY = 1234;
1686           $id = semget($IPC_KEY, 0, 0);
1687           defined($id)                    || die "semget: $!";
1688
1689           $semnum  = 0;
1690           $semflag = 0;
1691
1692           # "take" semaphore
1693           # wait for semaphore to be zero
1694           $semop = 0;
1695           $opstring1 = pack("s!s!s!", $semnum, $semop, $semflag);
1696
1697           # Increment the semaphore count
1698           $semop = 1;
1699           $opstring2 = pack("s!s!s!", $semnum, $semop,  $semflag);
1700           $opstring  = $opstring1 . $opstring2;
1701
1702           semop($id, $opstring)   || die "semop: $!";
1703
1704       Put this code in a separate file to be run in more than one process.
1705       Call this file give:
1706
1707           # "give" the semaphore
1708           # run this in the original process and you will see
1709           # that the second process continues
1710
1711           $IPC_KEY = 1234;
1712           $id = semget($IPC_KEY, 0, 0);
1713           die unless defined($id);
1714
1715           $semnum  = 0;
1716           $semflag = 0;
1717
1718           # Decrement the semaphore count
1719           $semop = -1;
1720           $opstring = pack("s!s!s!", $semnum, $semop, $semflag);
1721
1722           semop($id, $opstring)   || die "semop: $!";
1723
1724       The SysV IPC code above was written long ago, and it's definitely
1725       clunky looking.  For a more modern look, see the IPC::SysV module.
1726
1727       A small example demonstrating SysV message queues:
1728
1729           use IPC::SysV qw(IPC_PRIVATE IPC_RMID IPC_CREAT S_IRUSR S_IWUSR);
1730
1731           my $id = msgget(IPC_PRIVATE, IPC_CREAT | S_IRUSR | S_IWUSR);
1732           defined($id)                || die "msgget failed: $!";
1733
1734           my $sent      = "message";
1735           my $type_sent = 1234;
1736
1737           msgsnd($id, pack("l! a*", $type_sent, $sent), 0)
1738                                       || die "msgsnd failed: $!";
1739
1740           msgrcv($id, my $rcvd_buf, 60, 0, 0)
1741                                       || die "msgrcv failed: $!";
1742
1743           my($type_rcvd, $rcvd) = unpack("l! a*", $rcvd_buf);
1744
1745           if ($rcvd eq $sent) {
1746               print "okay\n";
1747           } else {
1748               print "not okay\n";
1749           }
1750
1751           msgctl($id, IPC_RMID, 0)    || die "msgctl failed: $!\n";
1752

NOTES

1754       Most of these routines quietly but politely return "undef" when they
1755       fail instead of causing your program to die right then and there due to
1756       an uncaught exception.  (Actually, some of the new Socket conversion
1757       functions do croak() on bad arguments.)  It is therefore essential to
1758       check return values from these functions.  Always begin your socket
1759       programs this way for optimal success, and don't forget to add the -T
1760       taint-checking flag to the "#!" line for servers:
1761
1762           #!/usr/bin/perl -Tw
1763           use strict;
1764           use sigtrap;
1765           use Socket;
1766

BUGS

1768       These routines all create system-specific portability problems.  As
1769       noted elsewhere, Perl is at the mercy of your C libraries for much of
1770       its system behavior.  It's probably safest to assume broken SysV
1771       semantics for signals and to stick with simple TCP and UDP socket
1772       operations; e.g., don't try to pass open file descriptors over a local
1773       UDP datagram socket if you want your code to stand a chance of being
1774       portable.
1775

AUTHOR

1777       Tom Christiansen, with occasional vestiges of Larry Wall's original
1778       version and suggestions from the Perl Porters.
1779

SEE ALSO

1781       There's a lot more to networking than this, but this should get you
1782       started.
1783
1784       For intrepid programmers, the indispensable textbook is Unix Network
1785       Programming, 2nd Edition, Volume 1 by W. Richard Stevens (published by
1786       Prentice-Hall).  Most books on networking address the subject from the
1787       perspective of a C programmer; translation to Perl is left as an
1788       exercise for the reader.
1789
1790       The IO::Socket(3) manpage describes the object library, and the
1791       Socket(3) manpage describes the low-level interface to sockets.
1792       Besides the obvious functions in perlfunc, you should also check out
1793       the modules file at your nearest CPAN site, especially
1794       <http://www.cpan.org/modules/00modlist.long.html#ID5_Networking_>.  See
1795       perlmodlib or best yet, the Perl FAQ for a description of what CPAN is
1796       and where to get it if the previous link doesn't work for you.
1797
1798       Section 5 of CPAN's modules file is devoted to "Networking, Device
1799       Control (modems), and Interprocess Communication", and contains
1800       numerous unbundled modules numerous networking modules, Chat and Expect
1801       operations, CGI programming, DCE, FTP, IPC, NNTP, Proxy, Ptty, RPC,
1802       SNMP, SMTP, Telnet, Threads, and ToolTalk--to name just a few.
1803
1804
1805
1806perl v5.26.3                      2018-03-01                        PERLIPC(1)
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