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1 | =head1 NAME |
2 | ||
3 | perlipc - Perl interprocess communication (signals, fifos, pipes, safe subprocesses, sockets, and semaphores) | |
4 | ||
5 | =head1 DESCRIPTION | |
6 | ||
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. | |
10 | ||
11 | =head1 Signals | |
12 | ||
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. | |
21 | ||
22 | For example, to trap an interrupt signal, set up a handler like this: | |
23 | ||
24 | sub catch_zap { | |
25 | my $signame = shift; | |
26 | $shucks++; | |
27 | die "Somebody sent me a SIG$signame"; | |
28 | } | |
29 | $SIG{INT} = 'catch_zap'; # could fail in modules | |
30 | $SIG{INT} = \&catch_zap; # best strategy | |
31 | ||
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 (Safe Signals)> below. | |
39 | ||
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: | |
44 | ||
45 | use Config; | |
46 | defined $Config{sig_name} || die "No sigs?"; | |
47 | foreach $name (split(' ', $Config{sig_name})) { | |
48 | $signo{$name} = $i; | |
49 | $signame[$i] = $name; | |
50 | $i++; | |
51 | } | |
52 | ||
53 | So to check whether signal 17 and SIGALRM were the same, do just this: | |
54 | ||
55 | print "signal #17 = $signame[17]\n"; | |
56 | if ($signo{ALRM}) { | |
57 | print "SIGALRM is $signo{ALRM}\n"; | |
58 | } | |
59 | ||
60 | You may also choose to assign the strings C<'IGNORE'> or C<'DEFAULT'> as | |
61 | the handler, in which case Perl will try to discard the signal or do the | |
62 | default thing. | |
63 | ||
64 | On most Unix platforms, the C<CHLD> (sometimes also known as C<CLD>) signal | |
65 | has special behavior with respect to a value of C<'IGNORE'>. | |
66 | Setting C<$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. | |
71 | ||
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.) | |
77 | ||
78 | sub precious { | |
79 | local $SIG{INT} = 'IGNORE'; | |
80 | &more_functions; | |
81 | } | |
82 | sub more_functions { | |
83 | # interrupts still ignored, for now... | |
84 | } | |
85 | ||
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): | |
90 | ||
91 | { | |
92 | local $SIG{HUP} = 'IGNORE'; | |
93 | kill HUP => -$$; | |
94 | # snazzy writing of: kill('HUP', -$$) | |
95 | } | |
96 | ||
97 | Another interesting signal to send is signal number zero. This doesn't | |
98 | actually affect a child process, but instead checks whether it's alive | |
99 | or has changed its UID. | |
100 | ||
101 | unless (kill 0 => $kid_pid) { | |
102 | warn "something wicked happened to $kid_pid"; | |
103 | } | |
104 | ||
105 | When directed at a process whose UID is not identical to that | |
106 | of the sending process, signal number zero may fail because | |
107 | you lack permission to send the signal, even though the process is alive. | |
108 | You may be able to determine the cause of failure using C<%!>. | |
109 | ||
110 | unless (kill 0 => $pid or $!{EPERM}) { | |
111 | warn "$pid looks dead"; | |
112 | } | |
113 | ||
114 | You might also want to employ anonymous functions for simple signal | |
115 | handlers: | |
116 | ||
117 | $SIG{INT} = sub { die "\nOutta here!\n" }; | |
118 | ||
119 | But that will be problematic for the more complicated handlers that need | |
120 | to reinstall themselves. Because Perl's signal mechanism is currently | |
121 | based on the signal(3) function from the C library, you may sometimes be so | |
122 | misfortunate as to run on systems where that function is "broken", that | |
123 | is, it behaves in the old unreliable SysV way rather than the newer, more | |
124 | reasonable BSD and POSIX fashion. So you'll see defensive people writing | |
125 | signal handlers like this: | |
126 | ||
127 | sub REAPER { | |
128 | $waitedpid = wait; | |
129 | # loathe sysV: it makes us not only reinstate | |
130 | # the handler, but place it after the wait | |
131 | $SIG{CHLD} = \&REAPER; | |
132 | } | |
133 | $SIG{CHLD} = \&REAPER; | |
134 | # now do something that forks... | |
135 | ||
136 | or better still: | |
137 | ||
138 | use POSIX ":sys_wait_h"; | |
139 | sub REAPER { | |
140 | my $child; | |
141 | # If a second child dies while in the signal handler caused by the | |
142 | # first death, we won't get another signal. So must loop here else | |
143 | # we will leave the unreaped child as a zombie. And the next time | |
144 | # two children die we get another zombie. And so on. | |
145 | while (($child = waitpid(-1,WNOHANG)) > 0) { | |
146 | $Kid_Status{$child} = $?; | |
147 | } | |
148 | $SIG{CHLD} = \&REAPER; # still loathe sysV | |
149 | } | |
150 | $SIG{CHLD} = \&REAPER; | |
151 | # do something that forks... | |
152 | ||
153 | Signal handling is also used for timeouts in Unix, While safely | |
154 | protected within an C<eval{}> block, you set a signal handler to trap | |
155 | alarm signals and then schedule to have one delivered to you in some | |
156 | number of seconds. Then try your blocking operation, clearing the alarm | |
157 | when it's done but not before you've exited your C<eval{}> block. If it | |
158 | goes off, you'll use die() to jump out of the block, much as you might | |
159 | using longjmp() or throw() in other languages. | |
160 | ||
161 | Here's an example: | |
162 | ||
163 | eval { | |
164 | local $SIG{ALRM} = sub { die "alarm clock restart" }; | |
165 | alarm 10; | |
166 | flock(FH, 2); # blocking write lock | |
167 | alarm 0; | |
168 | }; | |
169 | if ($@ and $@ !~ /alarm clock restart/) { die } | |
170 | ||
171 | If the operation being timed out is system() or qx(), this technique | |
172 | is liable to generate zombies. If this matters to you, you'll | |
173 | need to do your own fork() and exec(), and kill the errant child process. | |
174 | ||
175 | For more complex signal handling, you might see the standard POSIX | |
176 | module. Lamentably, this is almost entirely undocumented, but | |
177 | the F<t/lib/posix.t> file from the Perl source distribution has some | |
178 | examples in it. | |
179 | ||
180 | =head2 Handling the SIGHUP Signal in Daemons | |
181 | ||
182 | A process that usually starts when the system boots and shuts down | |
183 | when the system is shut down is called a daemon (Disk And Execution | |
184 | MONitor). If a daemon process has a configuration file which is | |
185 | modified after the process has been started, there should be a way to | |
186 | tell that process to re-read its configuration file, without stopping | |
187 | the process. Many daemons provide this mechanism using the C<SIGHUP> | |
188 | signal handler. When you want to tell the daemon to re-read the file | |
189 | you simply send it the C<SIGHUP> signal. | |
190 | ||
191 | Not all platforms automatically reinstall their (native) signal | |
192 | handlers after a signal delivery. This means that the handler works | |
193 | only the first time the signal is sent. The solution to this problem | |
194 | is to use C<POSIX> signal handlers if available, their behaviour | |
195 | is well-defined. | |
196 | ||
197 | The following example implements a simple daemon, which restarts | |
198 | itself every time the C<SIGHUP> signal is received. The actual code is | |
199 | located in the subroutine C<code()>, which simply prints some debug | |
200 | info to show that it works and should be replaced with the real code. | |
201 | ||
202 | #!/usr/bin/perl -w | |
203 | ||
204 | use POSIX (); | |
205 | use FindBin (); | |
206 | use File::Basename (); | |
207 | use File::Spec::Functions; | |
208 | ||
209 | $|=1; | |
210 | ||
211 | # make the daemon cross-platform, so exec always calls the script | |
212 | # itself with the right path, no matter how the script was invoked. | |
213 | my $script = File::Basename::basename($0); | |
214 | my $SELF = catfile $FindBin::Bin, $script; | |
215 | ||
216 | # POSIX unmasks the sigprocmask properly | |
217 | my $sigset = POSIX::SigSet->new(); | |
218 | my $action = POSIX::SigAction->new('sigHUP_handler', | |
219 | $sigset, | |
220 | &POSIX::SA_NODEFER); | |
221 | POSIX::sigaction(&POSIX::SIGHUP, $action); | |
222 | ||
223 | sub sigHUP_handler { | |
224 | print "got SIGHUP\n"; | |
225 | exec($SELF, @ARGV) or die "Couldn't restart: $!\n"; | |
226 | } | |
227 | ||
228 | code(); | |
229 | ||
230 | sub code { | |
231 | print "PID: $$\n"; | |
232 | print "ARGV: @ARGV\n"; | |
233 | my $c = 0; | |
234 | while (++$c) { | |
235 | sleep 2; | |
236 | print "$c\n"; | |
237 | } | |
238 | } | |
239 | __END__ | |
240 | ||
241 | ||
242 | =head1 Named Pipes | |
243 | ||
244 | A named pipe (often referred to as a FIFO) is an old Unix IPC | |
245 | mechanism for processes communicating on the same machine. It works | |
246 | just like a regular, connected anonymous pipes, except that the | |
247 | processes rendezvous using a filename and don't have to be related. | |
248 | ||
249 | To create a named pipe, use the C<POSIX::mkfifo()> function. | |
250 | ||
251 | use POSIX qw(mkfifo); | |
252 | mkfifo($path, 0700) or die "mkfifo $path failed: $!"; | |
253 | ||
254 | You can also use the Unix command mknod(1) or on some | |
255 | systems, mkfifo(1). These may not be in your normal path. | |
256 | ||
257 | # system return val is backwards, so && not || | |
258 | # | |
259 | $ENV{PATH} .= ":/etc:/usr/etc"; | |
260 | if ( system('mknod', $path, 'p') | |
261 | && system('mkfifo', $path) ) | |
262 | { | |
263 | die "mk{nod,fifo} $path failed"; | |
264 | } | |
265 | ||
266 | ||
267 | A fifo is convenient when you want to connect a process to an unrelated | |
268 | one. When you open a fifo, the program will block until there's something | |
269 | on the other end. | |
270 | ||
271 | For example, let's say you'd like to have your F<.signature> file be a | |
272 | named pipe that has a Perl program on the other end. Now every time any | |
273 | program (like a mailer, news reader, finger program, etc.) tries to read | |
274 | from that file, the reading program will block and your program will | |
275 | supply the new signature. We'll use the pipe-checking file test B<-p> | |
276 | to find out whether anyone (or anything) has accidentally removed our fifo. | |
277 | ||
278 | chdir; # go home | |
279 | $FIFO = '.signature'; | |
280 | ||
281 | while (1) { | |
282 | unless (-p $FIFO) { | |
283 | unlink $FIFO; | |
284 | require POSIX; | |
285 | POSIX::mkfifo($FIFO, 0700) | |
286 | or die "can't mkfifo $FIFO: $!"; | |
287 | } | |
288 | ||
289 | # next line blocks until there's a reader | |
290 | open (FIFO, "> $FIFO") || die "can't write $FIFO: $!"; | |
291 | print FIFO "John Smith (smith\@host.org)\n", `fortune -s`; | |
292 | close FIFO; | |
293 | sleep 2; # to avoid dup signals | |
294 | } | |
295 | ||
296 | =head2 Deferred Signals (Safe Signals) | |
297 | ||
298 | In Perls before Perl 5.7.3 by installing Perl code to deal with | |
299 | signals, you were exposing yourself to danger from two things. First, | |
300 | few system library functions are re-entrant. If the signal interrupts | |
301 | while Perl is executing one function (like malloc(3) or printf(3)), | |
302 | and your signal handler then calls the same function again, you could | |
303 | get unpredictable behavior--often, a core dump. Second, Perl isn't | |
304 | itself re-entrant at the lowest levels. If the signal interrupts Perl | |
305 | while Perl is changing its own internal data structures, similarly | |
306 | unpredictable behaviour may result. | |
307 | ||
308 | There were two things you could do, knowing this: be paranoid or be | |
309 | pragmatic. The paranoid approach was to do as little as possible in your | |
310 | signal handler. Set an existing integer variable that already has a | |
311 | value, and return. This doesn't help you if you're in a slow system call, | |
312 | which will just restart. That means you have to C<die> to longjump(3) out | |
313 | of the handler. Even this is a little cavalier for the true paranoiac, | |
314 | who avoids C<die> in a handler because the system I<is> out to get you. | |
315 | The pragmatic approach was to say "I know the risks, but prefer the | |
316 | convenience", and to do anything you wanted in your signal handler, | |
317 | and be prepared to clean up core dumps now and again. | |
318 | ||
319 | In Perl 5.7.3 and later to avoid these problems signals are | |
320 | "deferred"-- that is when the signal is delivered to the process by | |
321 | the system (to the C code that implements Perl) a flag is set, and the | |
322 | handler returns immediately. Then at strategic "safe" points in the | |
323 | Perl interpreter (e.g. when it is about to execute a new opcode) the | |
324 | flags are checked and the Perl level handler from %SIG is | |
325 | executed. The "deferred" scheme allows much more flexibility in the | |
326 | coding of signal handler as we know Perl interpreter is in a safe | |
327 | state, and that we are not in a system library function when the | |
328 | handler is called. However the implementation does differ from | |
329 | previous Perls in the following ways: | |
330 | ||
331 | =over 4 | |
332 | ||
333 | =item Long running opcodes | |
334 | ||
335 | As Perl interpreter only looks at the signal flags when it about to | |
336 | execute a new opcode if a signal arrives during a long running opcode | |
337 | (e.g. a regular expression operation on a very large string) then | |
338 | signal will not be seen until operation completes. | |
339 | ||
340 | =item Interrupting IO | |
341 | ||
342 | When a signal is delivered (e.g. INT control-C) the operating system | |
343 | breaks into IO operations like C<read> (used to implement Perls | |
344 | E<lt>E<gt> operator). On older Perls the handler was called | |
345 | immediately (and as C<read> is not "unsafe" this worked well). With | |
346 | the "deferred" scheme the handler is not called immediately, and if | |
347 | Perl is using system's C<stdio> library that library may re-start the | |
348 | C<read> without returning to Perl and giving it a chance to call the | |
349 | %SIG handler. If this happens on your system the solution is to use | |
350 | C<:perlio> layer to do IO - at least on those handles which you want | |
351 | to be able to break into with signals. (The C<:perlio> layer checks | |
352 | the signal flags and calls %SIG handlers before resuming IO operation.) | |
353 | ||
354 | Note that the default in Perl 5.7.3 and later is to automatically use | |
355 | the C<:perlio> layer. | |
356 | ||
357 | Note that some networking library functions like gethostbyname() are | |
358 | known to have their own implementations of timeouts which may conflict | |
359 | with your timeouts. If you are having problems with such functions, | |
360 | you can try using the POSIX sigaction() function, which bypasses the | |
361 | Perl safe signals (note that this means subjecting yourself to | |
362 | possible memory corruption, as described above). Instead of setting | |
363 | C<$SIG{ALRM}>: | |
364 | ||
365 | local $SIG{ALRM} = sub { die "alarm" }; | |
366 | ||
367 | try something like the following: | |
368 | ||
369 | use POSIX qw(SIGALRM); | |
370 | POSIX::sigaction(SIGALRM, | |
371 | POSIX::SigAction->new(sub { die "alarm" })) | |
372 | or die "Error setting SIGALRM handler: $!\n"; | |
373 | ||
374 | =item Restartable system calls | |
375 | ||
376 | On systems that supported it, older versions of Perl used the | |
377 | SA_RESTART flag when installing %SIG handlers. This meant that | |
378 | restartable system calls would continue rather than returning when | |
379 | a signal arrived. In order to deliver deferred signals promptly, | |
380 | Perl 5.7.3 and later do I<not> use SA_RESTART. Consequently, | |
381 | restartable system calls can fail (with $! set to C<EINTR>) in places | |
382 | where they previously would have succeeded. | |
383 | ||
384 | Note that the default C<:perlio> layer will retry C<read>, C<write> | |
385 | and C<close> as described above and that interrupted C<wait> and | |
386 | C<waitpid> calls will always be retried. | |
387 | ||
388 | =item Signals as "faults" | |
389 | ||
390 | Certain signals e.g. SEGV, ILL, BUS are generated as a result of | |
391 | virtual memory or other "faults". These are normally fatal and there | |
392 | is little a Perl-level handler can do with them. (In particular the | |
393 | old signal scheme was particularly unsafe in such cases.) However if | |
394 | a %SIG handler is set the new scheme simply sets a flag and returns as | |
395 | described above. This may cause the operating system to try the | |
396 | offending machine instruction again and - as nothing has changed - it | |
397 | will generate the signal again. The result of this is a rather odd | |
398 | "loop". In future Perl's signal mechanism may be changed to avoid this | |
399 | - perhaps by simply disallowing %SIG handlers on signals of that | |
400 | type. Until then the work-round is not to set a %SIG handler on those | |
401 | signals. (Which signals they are is operating system dependent.) | |
402 | ||
403 | =item Signals triggered by operating system state | |
404 | ||
405 | On some operating systems certain signal handlers are supposed to "do | |
406 | something" before returning. One example can be CHLD or CLD which | |
407 | indicates a child process has completed. On some operating systems the | |
408 | signal handler is expected to C<wait> for the completed child | |
409 | process. On such systems the deferred signal scheme will not work for | |
410 | those signals (it does not do the C<wait>). Again the failure will | |
411 | look like a loop as the operating system will re-issue the signal as | |
412 | there are un-waited-for completed child processes. | |
413 | ||
414 | =back | |
415 | ||
416 | If you want the old signal behaviour back regardless of possible | |
417 | memory corruption, set the environment variable C<PERL_SIGNALS> to | |
418 | C<"unsafe"> (a new feature since Perl 5.8.1). | |
419 | ||
420 | =head1 Using open() for IPC | |
421 | ||
422 | Perl's basic open() statement can also be used for unidirectional | |
423 | interprocess communication by either appending or prepending a pipe | |
424 | symbol to the second argument to open(). Here's how to start | |
425 | something up in a child process you intend to write to: | |
426 | ||
427 | open(SPOOLER, "| cat -v | lpr -h 2>/dev/null") | |
428 | || die "can't fork: $!"; | |
429 | local $SIG{PIPE} = sub { die "spooler pipe broke" }; | |
430 | print SPOOLER "stuff\n"; | |
431 | close SPOOLER || die "bad spool: $! $?"; | |
432 | ||
433 | And here's how to start up a child process you intend to read from: | |
434 | ||
435 | open(STATUS, "netstat -an 2>&1 |") | |
436 | || die "can't fork: $!"; | |
437 | while (<STATUS>) { | |
438 | next if /^(tcp|udp)/; | |
439 | print; | |
440 | } | |
441 | close STATUS || die "bad netstat: $! $?"; | |
442 | ||
443 | If one can be sure that a particular program is a Perl script that is | |
444 | expecting filenames in @ARGV, the clever programmer can write something | |
445 | like this: | |
446 | ||
447 | % program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile | |
448 | ||
449 | and irrespective of which shell it's called from, the Perl program will | |
450 | read from the file F<f1>, the process F<cmd1>, standard input (F<tmpfile> | |
451 | in this case), the F<f2> file, the F<cmd2> command, and finally the F<f3> | |
452 | file. Pretty nifty, eh? | |
453 | ||
454 | You might notice that you could use backticks for much the | |
455 | same effect as opening a pipe for reading: | |
456 | ||
457 | print grep { !/^(tcp|udp)/ } `netstat -an 2>&1`; | |
458 | die "bad netstat" if $?; | |
459 | ||
460 | While this is true on the surface, it's much more efficient to process the | |
461 | file one line or record at a time because then you don't have to read the | |
462 | whole thing into memory at once. It also gives you finer control of the | |
463 | whole process, letting you to kill off the child process early if you'd | |
464 | like. | |
465 | ||
466 | Be careful to check both the open() and the close() return values. If | |
467 | you're I<writing> to a pipe, you should also trap SIGPIPE. Otherwise, | |
468 | think of what happens when you start up a pipe to a command that doesn't | |
469 | exist: the open() will in all likelihood succeed (it only reflects the | |
470 | fork()'s success), but then your output will fail--spectacularly. Perl | |
471 | can't know whether the command worked because your command is actually | |
472 | running in a separate process whose exec() might have failed. Therefore, | |
473 | while readers of bogus commands return just a quick end of file, writers | |
474 | to bogus command will trigger a signal they'd better be prepared to | |
475 | handle. Consider: | |
476 | ||
477 | open(FH, "|bogus") or die "can't fork: $!"; | |
478 | print FH "bang\n" or die "can't write: $!"; | |
479 | close FH or die "can't close: $!"; | |
480 | ||
481 | That won't blow up until the close, and it will blow up with a SIGPIPE. | |
482 | To catch it, you could use this: | |
483 | ||
484 | $SIG{PIPE} = 'IGNORE'; | |
485 | open(FH, "|bogus") or die "can't fork: $!"; | |
486 | print FH "bang\n" or die "can't write: $!"; | |
487 | close FH or die "can't close: status=$?"; | |
488 | ||
489 | =head2 Filehandles | |
490 | ||
491 | Both the main process and any child processes it forks share the same | |
492 | STDIN, STDOUT, and STDERR filehandles. If both processes try to access | |
493 | them at once, strange things can happen. You may also want to close | |
494 | or reopen the filehandles for the child. You can get around this by | |
495 | opening your pipe with open(), but on some systems this means that the | |
496 | child process cannot outlive the parent. | |
497 | ||
498 | =head2 Background Processes | |
499 | ||
500 | You can run a command in the background with: | |
501 | ||
502 | system("cmd &"); | |
503 | ||
504 | The command's STDOUT and STDERR (and possibly STDIN, depending on your | |
505 | shell) will be the same as the parent's. You won't need to catch | |
506 | SIGCHLD because of the double-fork taking place (see below for more | |
507 | details). | |
508 | ||
509 | =head2 Complete Dissociation of Child from Parent | |
510 | ||
511 | In some cases (starting server processes, for instance) you'll want to | |
512 | completely dissociate the child process from the parent. This is | |
513 | often called daemonization. A well behaved daemon will also chdir() | |
514 | to the root directory (so it doesn't prevent unmounting the filesystem | |
515 | containing the directory from which it was launched) and redirect its | |
516 | standard file descriptors from and to F</dev/null> (so that random | |
517 | output doesn't wind up on the user's terminal). | |
518 | ||
519 | use POSIX 'setsid'; | |
520 | ||
521 | sub daemonize { | |
522 | chdir '/' or die "Can't chdir to /: $!"; | |
523 | open STDIN, '/dev/null' or die "Can't read /dev/null: $!"; | |
524 | open STDOUT, '>/dev/null' | |
525 | or die "Can't write to /dev/null: $!"; | |
526 | defined(my $pid = fork) or die "Can't fork: $!"; | |
527 | exit if $pid; | |
528 | setsid or die "Can't start a new session: $!"; | |
529 | open STDERR, '>&STDOUT' or die "Can't dup stdout: $!"; | |
530 | } | |
531 | ||
532 | The fork() has to come before the setsid() to ensure that you aren't a | |
533 | process group leader (the setsid() will fail if you are). If your | |
534 | system doesn't have the setsid() function, open F</dev/tty> and use the | |
535 | C<TIOCNOTTY> ioctl() on it instead. See L<tty(4)> for details. | |
536 | ||
537 | Non-Unix users should check their Your_OS::Process module for other | |
538 | solutions. | |
539 | ||
540 | =head2 Safe Pipe Opens | |
541 | ||
542 | Another interesting approach to IPC is making your single program go | |
543 | multiprocess and communicate between (or even amongst) yourselves. The | |
544 | open() function will accept a file argument of either C<"-|"> or C<"|-"> | |
545 | to do a very interesting thing: it forks a child connected to the | |
546 | filehandle you've opened. The child is running the same program as the | |
547 | parent. This is useful for safely opening a file when running under an | |
548 | assumed UID or GID, for example. If you open a pipe I<to> minus, you can | |
549 | write to the filehandle you opened and your kid will find it in his | |
550 | STDIN. If you open a pipe I<from> minus, you can read from the filehandle | |
551 | you opened whatever your kid writes to his STDOUT. | |
552 | ||
553 | use English '-no_match_vars'; | |
554 | my $sleep_count = 0; | |
555 | ||
556 | do { | |
557 | $pid = open(KID_TO_WRITE, "|-"); | |
558 | unless (defined $pid) { | |
559 | warn "cannot fork: $!"; | |
560 | die "bailing out" if $sleep_count++ > 6; | |
561 | sleep 10; | |
562 | } | |
563 | } until defined $pid; | |
564 | ||
565 | if ($pid) { # parent | |
566 | print KID_TO_WRITE @some_data; | |
567 | close(KID_TO_WRITE) || warn "kid exited $?"; | |
568 | } else { # child | |
569 | ($EUID, $EGID) = ($UID, $GID); # suid progs only | |
570 | open (FILE, "> /safe/file") | |
571 | || die "can't open /safe/file: $!"; | |
572 | while (<STDIN>) { | |
573 | print FILE; # child's STDIN is parent's KID | |
574 | } | |
575 | exit; # don't forget this | |
576 | } | |
577 | ||
578 | Another common use for this construct is when you need to execute | |
579 | something without the shell's interference. With system(), it's | |
580 | straightforward, but you can't use a pipe open or backticks safely. | |
581 | That's because there's no way to stop the shell from getting its hands on | |
582 | your arguments. Instead, use lower-level control to call exec() directly. | |
583 | ||
584 | Here's a safe backtick or pipe open for read: | |
585 | ||
586 | # add error processing as above | |
587 | $pid = open(KID_TO_READ, "-|"); | |
588 | ||
589 | if ($pid) { # parent | |
590 | while (<KID_TO_READ>) { | |
591 | # do something interesting | |
592 | } | |
593 | close(KID_TO_READ) || warn "kid exited $?"; | |
594 | ||
595 | } else { # child | |
596 | ($EUID, $EGID) = ($UID, $GID); # suid only | |
597 | exec($program, @options, @args) | |
598 | || die "can't exec program: $!"; | |
599 | # NOTREACHED | |
600 | } | |
601 | ||
602 | ||
603 | And here's a safe pipe open for writing: | |
604 | ||
605 | # add error processing as above | |
606 | $pid = open(KID_TO_WRITE, "|-"); | |
607 | $SIG{PIPE} = sub { die "whoops, $program pipe broke" }; | |
608 | ||
609 | if ($pid) { # parent | |
610 | for (@data) { | |
611 | print KID_TO_WRITE; | |
612 | } | |
613 | close(KID_TO_WRITE) || warn "kid exited $?"; | |
614 | ||
615 | } else { # child | |
616 | ($EUID, $EGID) = ($UID, $GID); | |
617 | exec($program, @options, @args) | |
618 | || die "can't exec program: $!"; | |
619 | # NOTREACHED | |
620 | } | |
621 | ||
622 | Since Perl 5.8.0, you can also use the list form of C<open> for pipes : | |
623 | the syntax | |
624 | ||
625 | open KID_PS, "-|", "ps", "aux" or die $!; | |
626 | ||
627 | forks the ps(1) command (without spawning a shell, as there are more than | |
628 | three arguments to open()), and reads its standard output via the | |
629 | C<KID_PS> filehandle. The corresponding syntax to write to command | |
630 | pipes (with C<"|-"> in place of C<"-|">) is also implemented. | |
631 | ||
632 | Note that these operations are full Unix forks, which means they may not be | |
633 | correctly implemented on alien systems. Additionally, these are not true | |
634 | multithreading. If you'd like to learn more about threading, see the | |
635 | F<modules> file mentioned below in the SEE ALSO section. | |
636 | ||
637 | =head2 Bidirectional Communication with Another Process | |
638 | ||
639 | While this works reasonably well for unidirectional communication, what | |
640 | about bidirectional communication? The obvious thing you'd like to do | |
641 | doesn't actually work: | |
642 | ||
643 | open(PROG_FOR_READING_AND_WRITING, "| some program |") | |
644 | ||
645 | and if you forget to use the C<use warnings> pragma or the B<-w> flag, | |
646 | then you'll miss out entirely on the diagnostic message: | |
647 | ||
648 | Can't do bidirectional pipe at -e line 1. | |
649 | ||
650 | If you really want to, you can use the standard open2() library function | |
651 | to catch both ends. There's also an open3() for tridirectional I/O so you | |
652 | can also catch your child's STDERR, but doing so would then require an | |
653 | awkward select() loop and wouldn't allow you to use normal Perl input | |
654 | operations. | |
655 | ||
656 | If you look at its source, you'll see that open2() uses low-level | |
657 | primitives like Unix pipe() and exec() calls to create all the connections. | |
658 | While it might have been slightly more efficient by using socketpair(), it | |
659 | would have then been even less portable than it already is. The open2() | |
660 | and open3() functions are unlikely to work anywhere except on a Unix | |
661 | system or some other one purporting to be POSIX compliant. | |
662 | ||
663 | Here's an example of using open2(): | |
664 | ||
665 | use FileHandle; | |
666 | use IPC::Open2; | |
667 | $pid = open2(*Reader, *Writer, "cat -u -n" ); | |
668 | print Writer "stuff\n"; | |
669 | $got = <Reader>; | |
670 | ||
671 | The problem with this is that Unix buffering is really going to | |
672 | ruin your day. Even though your C<Writer> filehandle is auto-flushed, | |
673 | and the process on the other end will get your data in a timely manner, | |
674 | you can't usually do anything to force it to give it back to you | |
675 | in a similarly quick fashion. In this case, we could, because we | |
676 | gave I<cat> a B<-u> flag to make it unbuffered. But very few Unix | |
677 | commands are designed to operate over pipes, so this seldom works | |
678 | unless you yourself wrote the program on the other end of the | |
679 | double-ended pipe. | |
680 | ||
681 | A solution to this is the nonstandard F<Comm.pl> library. It uses | |
682 | pseudo-ttys to make your program behave more reasonably: | |
683 | ||
684 | require 'Comm.pl'; | |
685 | $ph = open_proc('cat -n'); | |
686 | for (1..10) { | |
687 | print $ph "a line\n"; | |
688 | print "got back ", scalar <$ph>; | |
689 | } | |
690 | ||
691 | This way you don't have to have control over the source code of the | |
692 | program you're using. The F<Comm> library also has expect() | |
693 | and interact() functions. Find the library (and we hope its | |
694 | successor F<IPC::Chat>) at your nearest CPAN archive as detailed | |
695 | in the SEE ALSO section below. | |
696 | ||
697 | The newer Expect.pm module from CPAN also addresses this kind of thing. | |
698 | This module requires two other modules from CPAN: IO::Pty and IO::Stty. | |
699 | It sets up a pseudo-terminal to interact with programs that insist on | |
700 | using talking to the terminal device driver. If your system is | |
701 | amongst those supported, this may be your best bet. | |
702 | ||
703 | =head2 Bidirectional Communication with Yourself | |
704 | ||
705 | If you want, you may make low-level pipe() and fork() | |
706 | to stitch this together by hand. This example only | |
707 | talks to itself, but you could reopen the appropriate | |
708 | handles to STDIN and STDOUT and call other processes. | |
709 | ||
710 | #!/usr/bin/perl -w | |
711 | # pipe1 - bidirectional communication using two pipe pairs | |
712 | # designed for the socketpair-challenged | |
713 | use IO::Handle; # thousands of lines just for autoflush :-( | |
714 | pipe(PARENT_RDR, CHILD_WTR); # XXX: failure? | |
715 | pipe(CHILD_RDR, PARENT_WTR); # XXX: failure? | |
716 | CHILD_WTR->autoflush(1); | |
717 | PARENT_WTR->autoflush(1); | |
718 | ||
719 | if ($pid = fork) { | |
720 | close PARENT_RDR; close PARENT_WTR; | |
721 | print CHILD_WTR "Parent Pid $$ is sending this\n"; | |
722 | chomp($line = <CHILD_RDR>); | |
723 | print "Parent Pid $$ just read this: `$line'\n"; | |
724 | close CHILD_RDR; close CHILD_WTR; | |
725 | waitpid($pid,0); | |
726 | } else { | |
727 | die "cannot fork: $!" unless defined $pid; | |
728 | close CHILD_RDR; close CHILD_WTR; | |
729 | chomp($line = <PARENT_RDR>); | |
730 | print "Child Pid $$ just read this: `$line'\n"; | |
731 | print PARENT_WTR "Child Pid $$ is sending this\n"; | |
732 | close PARENT_RDR; close PARENT_WTR; | |
733 | exit; | |
734 | } | |
735 | ||
736 | But you don't actually have to make two pipe calls. If you | |
737 | have the socketpair() system call, it will do this all for you. | |
738 | ||
739 | #!/usr/bin/perl -w | |
740 | # pipe2 - bidirectional communication using socketpair | |
741 | # "the best ones always go both ways" | |
742 | ||
743 | use Socket; | |
744 | use IO::Handle; # thousands of lines just for autoflush :-( | |
745 | # We say AF_UNIX because although *_LOCAL is the | |
746 | # POSIX 1003.1g form of the constant, many machines | |
747 | # still don't have it. | |
748 | socketpair(CHILD, PARENT, AF_UNIX, SOCK_STREAM, PF_UNSPEC) | |
749 | or die "socketpair: $!"; | |
750 | ||
751 | CHILD->autoflush(1); | |
752 | PARENT->autoflush(1); | |
753 | ||
754 | if ($pid = fork) { | |
755 | close PARENT; | |
756 | print CHILD "Parent Pid $$ is sending this\n"; | |
757 | chomp($line = <CHILD>); | |
758 | print "Parent Pid $$ just read this: `$line'\n"; | |
759 | close CHILD; | |
760 | waitpid($pid,0); | |
761 | } else { | |
762 | die "cannot fork: $!" unless defined $pid; | |
763 | close CHILD; | |
764 | chomp($line = <PARENT>); | |
765 | print "Child Pid $$ just read this: `$line'\n"; | |
766 | print PARENT "Child Pid $$ is sending this\n"; | |
767 | close PARENT; | |
768 | exit; | |
769 | } | |
770 | ||
771 | =head1 Sockets: Client/Server Communication | |
772 | ||
773 | While not limited to Unix-derived operating systems (e.g., WinSock on PCs | |
774 | provides socket support, as do some VMS libraries), you may not have | |
775 | sockets on your system, in which case this section probably isn't going to do | |
776 | you much good. With sockets, you can do both virtual circuits (i.e., TCP | |
777 | streams) and datagrams (i.e., UDP packets). You may be able to do even more | |
778 | depending on your system. | |
779 | ||
780 | The Perl function calls for dealing with sockets have the same names as | |
781 | the corresponding system calls in C, but their arguments tend to differ | |
782 | for two reasons: first, Perl filehandles work differently than C file | |
783 | descriptors. Second, Perl already knows the length of its strings, so you | |
784 | don't need to pass that information. | |
785 | ||
786 | One of the major problems with old socket code in Perl was that it used | |
787 | hard-coded values for some of the constants, which severely hurt | |
788 | portability. If you ever see code that does anything like explicitly | |
789 | setting C<$AF_INET = 2>, you know you're in for big trouble: An | |
790 | immeasurably superior approach is to use the C<Socket> module, which more | |
791 | reliably grants access to various constants and functions you'll need. | |
792 | ||
793 | If you're not writing a server/client for an existing protocol like | |
794 | NNTP or SMTP, you should give some thought to how your server will | |
795 | know when the client has finished talking, and vice-versa. Most | |
796 | protocols are based on one-line messages and responses (so one party | |
797 | knows the other has finished when a "\n" is received) or multi-line | |
798 | messages and responses that end with a period on an empty line | |
799 | ("\n.\n" terminates a message/response). | |
800 | ||
801 | =head2 Internet Line Terminators | |
802 | ||
803 | The Internet line terminator is "\015\012". Under ASCII variants of | |
804 | Unix, that could usually be written as "\r\n", but under other systems, | |
805 | "\r\n" might at times be "\015\015\012", "\012\012\015", or something | |
806 | completely different. The standards specify writing "\015\012" to be | |
807 | conformant (be strict in what you provide), but they also recommend | |
808 | accepting a lone "\012" on input (but be lenient in what you require). | |
809 | We haven't always been very good about that in the code in this manpage, | |
810 | but unless you're on a Mac, you'll probably be ok. | |
811 | ||
812 | =head2 Internet TCP Clients and Servers | |
813 | ||
814 | Use Internet-domain sockets when you want to do client-server | |
815 | communication that might extend to machines outside of your own system. | |
816 | ||
817 | Here's a sample TCP client using Internet-domain sockets: | |
818 | ||
819 | #!/usr/bin/perl -w | |
820 | use strict; | |
821 | use Socket; | |
822 | my ($remote,$port, $iaddr, $paddr, $proto, $line); | |
823 | ||
824 | $remote = shift || 'localhost'; | |
825 | $port = shift || 2345; # random port | |
826 | if ($port =~ /\D/) { $port = getservbyname($port, 'tcp') } | |
827 | die "No port" unless $port; | |
828 | $iaddr = inet_aton($remote) || die "no host: $remote"; | |
829 | $paddr = sockaddr_in($port, $iaddr); | |
830 | ||
831 | $proto = getprotobyname('tcp'); | |
832 | socket(SOCK, PF_INET, SOCK_STREAM, $proto) || die "socket: $!"; | |
833 | connect(SOCK, $paddr) || die "connect: $!"; | |
834 | while (defined($line = <SOCK>)) { | |
835 | print $line; | |
836 | } | |
837 | ||
838 | close (SOCK) || die "close: $!"; | |
839 | exit; | |
840 | ||
841 | And here's a corresponding server to go along with it. We'll | |
842 | leave the address as INADDR_ANY so that the kernel can choose | |
843 | the appropriate interface on multihomed hosts. If you want sit | |
844 | on a particular interface (like the external side of a gateway | |
845 | or firewall machine), you should fill this in with your real address | |
846 | instead. | |
847 | ||
848 | #!/usr/bin/perl -Tw | |
849 | use strict; | |
850 | BEGIN { $ENV{PATH} = '/usr/ucb:/bin' } | |
851 | use Socket; | |
852 | use Carp; | |
853 | my $EOL = "\015\012"; | |
854 | ||
855 | sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" } | |
856 | ||
857 | my $port = shift || 2345; | |
858 | my $proto = getprotobyname('tcp'); | |
859 | ||
860 | ($port) = $port =~ /^(\d+)$/ or die "invalid port"; | |
861 | ||
862 | socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!"; | |
863 | setsockopt(Server, SOL_SOCKET, SO_REUSEADDR, | |
864 | pack("l", 1)) || die "setsockopt: $!"; | |
865 | bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!"; | |
866 | listen(Server,SOMAXCONN) || die "listen: $!"; | |
867 | ||
868 | logmsg "server started on port $port"; | |
869 | ||
870 | my $paddr; | |
871 | ||
872 | $SIG{CHLD} = \&REAPER; | |
873 | ||
874 | for ( ; $paddr = accept(Client,Server); close Client) { | |
875 | my($port,$iaddr) = sockaddr_in($paddr); | |
876 | my $name = gethostbyaddr($iaddr,AF_INET); | |
877 | ||
878 | logmsg "connection from $name [", | |
879 | inet_ntoa($iaddr), "] | |
880 | at port $port"; | |
881 | ||
882 | print Client "Hello there, $name, it's now ", | |
883 | scalar localtime, $EOL; | |
884 | } | |
885 | ||
886 | And here's a multithreaded version. It's multithreaded in that | |
887 | like most typical servers, it spawns (forks) a slave server to | |
888 | handle the client request so that the master server can quickly | |
889 | go back to service a new client. | |
890 | ||
891 | #!/usr/bin/perl -Tw | |
892 | use strict; | |
893 | BEGIN { $ENV{PATH} = '/usr/ucb:/bin' } | |
894 | use Socket; | |
895 | use Carp; | |
896 | my $EOL = "\015\012"; | |
897 | ||
898 | sub spawn; # forward declaration | |
899 | sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" } | |
900 | ||
901 | my $port = shift || 2345; | |
902 | my $proto = getprotobyname('tcp'); | |
903 | ||
904 | ($port) = $port =~ /^(\d+)$/ or die "invalid port"; | |
905 | ||
906 | socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!"; | |
907 | setsockopt(Server, SOL_SOCKET, SO_REUSEADDR, | |
908 | pack("l", 1)) || die "setsockopt: $!"; | |
909 | bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!"; | |
910 | listen(Server,SOMAXCONN) || die "listen: $!"; | |
911 | ||
912 | logmsg "server started on port $port"; | |
913 | ||
914 | my $waitedpid = 0; | |
915 | my $paddr; | |
916 | ||
917 | use POSIX ":sys_wait_h"; | |
918 | sub REAPER { | |
919 | my $child; | |
920 | while (($waitedpid = waitpid(-1,WNOHANG)) > 0) { | |
921 | logmsg "reaped $waitedpid" . ($? ? " with exit $?" : ''); | |
922 | } | |
923 | $SIG{CHLD} = \&REAPER; # loathe sysV | |
924 | } | |
925 | ||
926 | $SIG{CHLD} = \&REAPER; | |
927 | ||
928 | for ( $waitedpid = 0; | |
929 | ($paddr = accept(Client,Server)) || $waitedpid; | |
930 | $waitedpid = 0, close Client) | |
931 | { | |
932 | next if $waitedpid and not $paddr; | |
933 | my($port,$iaddr) = sockaddr_in($paddr); | |
934 | my $name = gethostbyaddr($iaddr,AF_INET); | |
935 | ||
936 | logmsg "connection from $name [", | |
937 | inet_ntoa($iaddr), "] | |
938 | at port $port"; | |
939 | ||
940 | spawn sub { | |
941 | $|=1; | |
942 | print "Hello there, $name, it's now ", scalar localtime, $EOL; | |
943 | exec '/usr/games/fortune' # XXX: `wrong' line terminators | |
944 | or confess "can't exec fortune: $!"; | |
945 | }; | |
946 | ||
947 | } | |
948 | ||
949 | sub spawn { | |
950 | my $coderef = shift; | |
951 | ||
952 | unless (@_ == 0 && $coderef && ref($coderef) eq 'CODE') { | |
953 | confess "usage: spawn CODEREF"; | |
954 | } | |
955 | ||
956 | my $pid; | |
957 | if (!defined($pid = fork)) { | |
958 | logmsg "cannot fork: $!"; | |
959 | return; | |
960 | } elsif ($pid) { | |
961 | logmsg "begat $pid"; | |
962 | return; # I'm the parent | |
963 | } | |
964 | # else I'm the child -- go spawn | |
965 | ||
966 | open(STDIN, "<&Client") || die "can't dup client to stdin"; | |
967 | open(STDOUT, ">&Client") || die "can't dup client to stdout"; | |
968 | ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr"; | |
969 | exit &$coderef(); | |
970 | } | |
971 | ||
972 | This server takes the trouble to clone off a child version via fork() for | |
973 | each incoming request. That way it can handle many requests at once, | |
974 | which you might not always want. Even if you don't fork(), the listen() | |
975 | will allow that many pending connections. Forking servers have to be | |
976 | particularly careful about cleaning up their dead children (called | |
977 | "zombies" in Unix parlance), because otherwise you'll quickly fill up your | |
978 | process table. | |
979 | ||
980 | We suggest that you use the B<-T> flag to use taint checking (see L<perlsec>) | |
981 | even if we aren't running setuid or setgid. This is always a good idea | |
982 | for servers and other programs run on behalf of someone else (like CGI | |
983 | scripts), because it lessens the chances that people from the outside will | |
984 | be able to compromise your system. | |
985 | ||
986 | Let's look at another TCP client. This one connects to the TCP "time" | |
987 | service on a number of different machines and shows how far their clocks | |
988 | differ from the system on which it's being run: | |
989 | ||
990 | #!/usr/bin/perl -w | |
991 | use strict; | |
992 | use Socket; | |
993 | ||
994 | my $SECS_of_70_YEARS = 2208988800; | |
995 | sub ctime { scalar localtime(shift) } | |
996 | ||
997 | my $iaddr = gethostbyname('localhost'); | |
998 | my $proto = getprotobyname('tcp'); | |
999 | my $port = getservbyname('time', 'tcp'); | |
1000 | my $paddr = sockaddr_in(0, $iaddr); | |
1001 | my($host); | |
1002 | ||
1003 | $| = 1; | |
1004 | printf "%-24s %8s %s\n", "localhost", 0, ctime(time()); | |
1005 | ||
1006 | foreach $host (@ARGV) { | |
1007 | printf "%-24s ", $host; | |
1008 | my $hisiaddr = inet_aton($host) || die "unknown host"; | |
1009 | my $hispaddr = sockaddr_in($port, $hisiaddr); | |
1010 | socket(SOCKET, PF_INET, SOCK_STREAM, $proto) || die "socket: $!"; | |
1011 | connect(SOCKET, $hispaddr) || die "bind: $!"; | |
1012 | my $rtime = ' '; | |
1013 | read(SOCKET, $rtime, 4); | |
1014 | close(SOCKET); | |
1015 | my $histime = unpack("N", $rtime) - $SECS_of_70_YEARS; | |
1016 | printf "%8d %s\n", $histime - time, ctime($histime); | |
1017 | } | |
1018 | ||
1019 | =head2 Unix-Domain TCP Clients and Servers | |
1020 | ||
1021 | That's fine for Internet-domain clients and servers, but what about local | |
1022 | communications? While you can use the same setup, sometimes you don't | |
1023 | want to. Unix-domain sockets are local to the current host, and are often | |
1024 | used internally to implement pipes. Unlike Internet domain sockets, Unix | |
1025 | domain sockets can show up in the file system with an ls(1) listing. | |
1026 | ||
1027 | % ls -l /dev/log | |
1028 | srw-rw-rw- 1 root 0 Oct 31 07:23 /dev/log | |
1029 | ||
1030 | You can test for these with Perl's B<-S> file test: | |
1031 | ||
1032 | unless ( -S '/dev/log' ) { | |
1033 | die "something's wicked with the log system"; | |
1034 | } | |
1035 | ||
1036 | Here's a sample Unix-domain client: | |
1037 | ||
1038 | #!/usr/bin/perl -w | |
1039 | use Socket; | |
1040 | use strict; | |
1041 | my ($rendezvous, $line); | |
1042 | ||
1043 | $rendezvous = shift || 'catsock'; | |
1044 | socket(SOCK, PF_UNIX, SOCK_STREAM, 0) || die "socket: $!"; | |
1045 | connect(SOCK, sockaddr_un($rendezvous)) || die "connect: $!"; | |
1046 | while (defined($line = <SOCK>)) { | |
1047 | print $line; | |
1048 | } | |
1049 | exit; | |
1050 | ||
1051 | And here's a corresponding server. You don't have to worry about silly | |
1052 | network terminators here because Unix domain sockets are guaranteed | |
1053 | to be on the localhost, and thus everything works right. | |
1054 | ||
1055 | #!/usr/bin/perl -Tw | |
1056 | use strict; | |
1057 | use Socket; | |
1058 | use Carp; | |
1059 | ||
1060 | BEGIN { $ENV{PATH} = '/usr/ucb:/bin' } | |
1061 | sub spawn; # forward declaration | |
1062 | sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" } | |
1063 | ||
1064 | my $NAME = 'catsock'; | |
1065 | my $uaddr = sockaddr_un($NAME); | |
1066 | my $proto = getprotobyname('tcp'); | |
1067 | ||
1068 | socket(Server,PF_UNIX,SOCK_STREAM,0) || die "socket: $!"; | |
1069 | unlink($NAME); | |
1070 | bind (Server, $uaddr) || die "bind: $!"; | |
1071 | listen(Server,SOMAXCONN) || die "listen: $!"; | |
1072 | ||
1073 | logmsg "server started on $NAME"; | |
1074 | ||
1075 | my $waitedpid; | |
1076 | ||
1077 | use POSIX ":sys_wait_h"; | |
1078 | sub REAPER { | |
1079 | my $child; | |
1080 | while (($waitedpid = waitpid(-1,WNOHANG)) > 0) { | |
1081 | logmsg "reaped $waitedpid" . ($? ? " with exit $?" : ''); | |
1082 | } | |
1083 | $SIG{CHLD} = \&REAPER; # loathe sysV | |
1084 | } | |
1085 | ||
1086 | $SIG{CHLD} = \&REAPER; | |
1087 | ||
1088 | ||
1089 | for ( $waitedpid = 0; | |
1090 | accept(Client,Server) || $waitedpid; | |
1091 | $waitedpid = 0, close Client) | |
1092 | { | |
1093 | next if $waitedpid; | |
1094 | logmsg "connection on $NAME"; | |
1095 | spawn sub { | |
1096 | print "Hello there, it's now ", scalar localtime, "\n"; | |
1097 | exec '/usr/games/fortune' or die "can't exec fortune: $!"; | |
1098 | }; | |
1099 | } | |
1100 | ||
1101 | sub spawn { | |
1102 | my $coderef = shift; | |
1103 | ||
1104 | unless (@_ == 0 && $coderef && ref($coderef) eq 'CODE') { | |
1105 | confess "usage: spawn CODEREF"; | |
1106 | } | |
1107 | ||
1108 | my $pid; | |
1109 | if (!defined($pid = fork)) { | |
1110 | logmsg "cannot fork: $!"; | |
1111 | return; | |
1112 | } elsif ($pid) { | |
1113 | logmsg "begat $pid"; | |
1114 | return; # I'm the parent | |
1115 | } | |
1116 | # else I'm the child -- go spawn | |
1117 | ||
1118 | open(STDIN, "<&Client") || die "can't dup client to stdin"; | |
1119 | open(STDOUT, ">&Client") || die "can't dup client to stdout"; | |
1120 | ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr"; | |
1121 | exit &$coderef(); | |
1122 | } | |
1123 | ||
1124 | As you see, it's remarkably similar to the Internet domain TCP server, so | |
1125 | much so, in fact, that we've omitted several duplicate functions--spawn(), | |
1126 | logmsg(), ctime(), and REAPER()--which are exactly the same as in the | |
1127 | other server. | |
1128 | ||
1129 | So why would you ever want to use a Unix domain socket instead of a | |
1130 | simpler named pipe? Because a named pipe doesn't give you sessions. You | |
1131 | can't tell one process's data from another's. With socket programming, | |
1132 | you get a separate session for each client: that's why accept() takes two | |
1133 | arguments. | |
1134 | ||
1135 | For example, let's say that you have a long running database server daemon | |
1136 | that you want folks from the World Wide Web to be able to access, but only | |
1137 | if they go through a CGI interface. You'd have a small, simple CGI | |
1138 | program that does whatever checks and logging you feel like, and then acts | |
1139 | as a Unix-domain client and connects to your private server. | |
1140 | ||
1141 | =head1 TCP Clients with IO::Socket | |
1142 | ||
1143 | For those preferring a higher-level interface to socket programming, the | |
1144 | IO::Socket module provides an object-oriented approach. IO::Socket is | |
1145 | included as part of the standard Perl distribution as of the 5.004 | |
1146 | release. If you're running an earlier version of Perl, just fetch | |
1147 | IO::Socket from CPAN, where you'll also find modules providing easy | |
1148 | interfaces to the following systems: DNS, FTP, Ident (RFC 931), NIS and | |
1149 | NISPlus, NNTP, Ping, POP3, SMTP, SNMP, SSLeay, Telnet, and Time--just | |
1150 | to name a few. | |
1151 | ||
1152 | =head2 A Simple Client | |
1153 | ||
1154 | Here's a client that creates a TCP connection to the "daytime" | |
1155 | service at port 13 of the host name "localhost" and prints out everything | |
1156 | that the server there cares to provide. | |
1157 | ||
1158 | #!/usr/bin/perl -w | |
1159 | use IO::Socket; | |
1160 | $remote = IO::Socket::INET->new( | |
1161 | Proto => "tcp", | |
1162 | PeerAddr => "localhost", | |
1163 | PeerPort => "daytime(13)", | |
1164 | ) | |
1165 | or die "cannot connect to daytime port at localhost"; | |
1166 | while ( <$remote> ) { print } | |
1167 | ||
1168 | When you run this program, you should get something back that | |
1169 | looks like this: | |
1170 | ||
1171 | Wed May 14 08:40:46 MDT 1997 | |
1172 | ||
1173 | Here are what those parameters to the C<new> constructor mean: | |
1174 | ||
1175 | =over 4 | |
1176 | ||
1177 | =item C<Proto> | |
1178 | ||
1179 | This is which protocol to use. In this case, the socket handle returned | |
1180 | will be connected to a TCP socket, because we want a stream-oriented | |
1181 | connection, that is, one that acts pretty much like a plain old file. | |
1182 | Not all sockets are this of this type. For example, the UDP protocol | |
1183 | can be used to make a datagram socket, used for message-passing. | |
1184 | ||
1185 | =item C<PeerAddr> | |
1186 | ||
1187 | This is the name or Internet address of the remote host the server is | |
1188 | running on. We could have specified a longer name like C<"www.perl.com">, | |
1189 | or an address like C<"204.148.40.9">. For demonstration purposes, we've | |
1190 | used the special hostname C<"localhost">, which should always mean the | |
1191 | current machine you're running on. The corresponding Internet address | |
1192 | for localhost is C<"127.1">, if you'd rather use that. | |
1193 | ||
1194 | =item C<PeerPort> | |
1195 | ||
1196 | This is the service name or port number we'd like to connect to. | |
1197 | We could have gotten away with using just C<"daytime"> on systems with a | |
1198 | well-configured system services file,[FOOTNOTE: The system services file | |
1199 | is in I</etc/services> under Unix] but just in case, we've specified the | |
1200 | port number (13) in parentheses. Using just the number would also have | |
1201 | worked, but constant numbers make careful programmers nervous. | |
1202 | ||
1203 | =back | |
1204 | ||
1205 | Notice how the return value from the C<new> constructor is used as | |
1206 | a filehandle in the C<while> loop? That's what's called an indirect | |
1207 | filehandle, a scalar variable containing a filehandle. You can use | |
1208 | it the same way you would a normal filehandle. For example, you | |
1209 | can read one line from it this way: | |
1210 | ||
1211 | $line = <$handle>; | |
1212 | ||
1213 | all remaining lines from is this way: | |
1214 | ||
1215 | @lines = <$handle>; | |
1216 | ||
1217 | and send a line of data to it this way: | |
1218 | ||
1219 | print $handle "some data\n"; | |
1220 | ||
1221 | =head2 A Webget Client | |
1222 | ||
1223 | Here's a simple client that takes a remote host to fetch a document | |
1224 | from, and then a list of documents to get from that host. This is a | |
1225 | more interesting client than the previous one because it first sends | |
1226 | something to the server before fetching the server's response. | |
1227 | ||
1228 | #!/usr/bin/perl -w | |
1229 | use IO::Socket; | |
1230 | unless (@ARGV > 1) { die "usage: $0 host document ..." } | |
1231 | $host = shift(@ARGV); | |
1232 | $EOL = "\015\012"; | |
1233 | $BLANK = $EOL x 2; | |
1234 | foreach $document ( @ARGV ) { | |
1235 | $remote = IO::Socket::INET->new( Proto => "tcp", | |
1236 | PeerAddr => $host, | |
1237 | PeerPort => "http(80)", | |
1238 | ); | |
1239 | unless ($remote) { die "cannot connect to http daemon on $host" } | |
1240 | $remote->autoflush(1); | |
1241 | print $remote "GET $document HTTP/1.0" . $BLANK; | |
1242 | while ( <$remote> ) { print } | |
1243 | close $remote; | |
1244 | } | |
1245 | ||
1246 | The web server handing the "http" service, which is assumed to be at | |
1247 | its standard port, number 80. If the web server you're trying to | |
1248 | connect to is at a different port (like 1080 or 8080), you should specify | |
1249 | as the named-parameter pair, C<< PeerPort => 8080 >>. The C<autoflush> | |
1250 | method is used on the socket because otherwise the system would buffer | |
1251 | up the output we sent it. (If you're on a Mac, you'll also need to | |
1252 | change every C<"\n"> in your code that sends data over the network to | |
1253 | be a C<"\015\012"> instead.) | |
1254 | ||
1255 | Connecting to the server is only the first part of the process: once you | |
1256 | have the connection, you have to use the server's language. Each server | |
1257 | on the network has its own little command language that it expects as | |
1258 | input. The string that we send to the server starting with "GET" is in | |
1259 | HTTP syntax. In this case, we simply request each specified document. | |
1260 | Yes, we really are making a new connection for each document, even though | |
1261 | it's the same host. That's the way you always used to have to speak HTTP. | |
1262 | Recent versions of web browsers may request that the remote server leave | |
1263 | the connection open a little while, but the server doesn't have to honor | |
1264 | such a request. | |
1265 | ||
1266 | Here's an example of running that program, which we'll call I<webget>: | |
1267 | ||
1268 | % webget www.perl.com /guanaco.html | |
1269 | HTTP/1.1 404 File Not Found | |
1270 | Date: Thu, 08 May 1997 18:02:32 GMT | |
1271 | Server: Apache/1.2b6 | |
1272 | Connection: close | |
1273 | Content-type: text/html | |
1274 | ||
1275 | <HEAD><TITLE>404 File Not Found</TITLE></HEAD> | |
1276 | <BODY><H1>File Not Found</H1> | |
1277 | The requested URL /guanaco.html was not found on this server.<P> | |
1278 | </BODY> | |
1279 | ||
1280 | Ok, so that's not very interesting, because it didn't find that | |
1281 | particular document. But a long response wouldn't have fit on this page. | |
1282 | ||
1283 | For a more fully-featured version of this program, you should look to | |
1284 | the I<lwp-request> program included with the LWP modules from CPAN. | |
1285 | ||
1286 | =head2 Interactive Client with IO::Socket | |
1287 | ||
1288 | Well, that's all fine if you want to send one command and get one answer, | |
1289 | but what about setting up something fully interactive, somewhat like | |
1290 | the way I<telnet> works? That way you can type a line, get the answer, | |
1291 | type a line, get the answer, etc. | |
1292 | ||
1293 | This client is more complicated than the two we've done so far, but if | |
1294 | you're on a system that supports the powerful C<fork> call, the solution | |
1295 | isn't that rough. Once you've made the connection to whatever service | |
1296 | you'd like to chat with, call C<fork> to clone your process. Each of | |
1297 | these two identical process has a very simple job to do: the parent | |
1298 | copies everything from the socket to standard output, while the child | |
1299 | simultaneously copies everything from standard input to the socket. | |
1300 | To accomplish the same thing using just one process would be I<much> | |
1301 | harder, because it's easier to code two processes to do one thing than it | |
1302 | is to code one process to do two things. (This keep-it-simple principle | |
1303 | a cornerstones of the Unix philosophy, and good software engineering as | |
1304 | well, which is probably why it's spread to other systems.) | |
1305 | ||
1306 | Here's the code: | |
1307 | ||
1308 | #!/usr/bin/perl -w | |
1309 | use strict; | |
1310 | use IO::Socket; | |
1311 | my ($host, $port, $kidpid, $handle, $line); | |
1312 | ||
1313 | unless (@ARGV == 2) { die "usage: $0 host port" } | |
1314 | ($host, $port) = @ARGV; | |
1315 | ||
1316 | # create a tcp connection to the specified host and port | |
1317 | $handle = IO::Socket::INET->new(Proto => "tcp", | |
1318 | PeerAddr => $host, | |
1319 | PeerPort => $port) | |
1320 | or die "can't connect to port $port on $host: $!"; | |
1321 | ||
1322 | $handle->autoflush(1); # so output gets there right away | |
1323 | print STDERR "[Connected to $host:$port]\n"; | |
1324 | ||
1325 | # split the program into two processes, identical twins | |
1326 | die "can't fork: $!" unless defined($kidpid = fork()); | |
1327 | ||
1328 | # the if{} block runs only in the parent process | |
1329 | if ($kidpid) { | |
1330 | # copy the socket to standard output | |
1331 | while (defined ($line = <$handle>)) { | |
1332 | print STDOUT $line; | |
1333 | } | |
1334 | kill("TERM", $kidpid); # send SIGTERM to child | |
1335 | } | |
1336 | # the else{} block runs only in the child process | |
1337 | else { | |
1338 | # copy standard input to the socket | |
1339 | while (defined ($line = <STDIN>)) { | |
1340 | print $handle $line; | |
1341 | } | |
1342 | } | |
1343 | ||
1344 | The C<kill> function in the parent's C<if> block is there to send a | |
1345 | signal to our child process (current running in the C<else> block) | |
1346 | as soon as the remote server has closed its end of the connection. | |
1347 | ||
1348 | If the remote server sends data a byte at time, and you need that | |
1349 | data immediately without waiting for a newline (which might not happen), | |
1350 | you may wish to replace the C<while> loop in the parent with the | |
1351 | following: | |
1352 | ||
1353 | my $byte; | |
1354 | while (sysread($handle, $byte, 1) == 1) { | |
1355 | print STDOUT $byte; | |
1356 | } | |
1357 | ||
1358 | Making a system call for each byte you want to read is not very efficient | |
1359 | (to put it mildly) but is the simplest to explain and works reasonably | |
1360 | well. | |
1361 | ||
1362 | =head1 TCP Servers with IO::Socket | |
1363 | ||
1364 | As always, setting up a server is little bit more involved than running a client. | |
1365 | The model is that the server creates a special kind of socket that | |
1366 | does nothing but listen on a particular port for incoming connections. | |
1367 | It does this by calling the C<< IO::Socket::INET->new() >> method with | |
1368 | slightly different arguments than the client did. | |
1369 | ||
1370 | =over 4 | |
1371 | ||
1372 | =item Proto | |
1373 | ||
1374 | This is which protocol to use. Like our clients, we'll | |
1375 | still specify C<"tcp"> here. | |
1376 | ||
1377 | =item LocalPort | |
1378 | ||
1379 | We specify a local | |
1380 | port in the C<LocalPort> argument, which we didn't do for the client. | |
1381 | This is service name or port number for which you want to be the | |
1382 | server. (Under Unix, ports under 1024 are restricted to the | |
1383 | superuser.) In our sample, we'll use port 9000, but you can use | |
1384 | any port that's not currently in use on your system. If you try | |
1385 | to use one already in used, you'll get an "Address already in use" | |
1386 | message. Under Unix, the C<netstat -a> command will show | |
1387 | which services current have servers. | |
1388 | ||
1389 | =item Listen | |
1390 | ||
1391 | The C<Listen> parameter is set to the maximum number of | |
1392 | pending connections we can accept until we turn away incoming clients. | |
1393 | Think of it as a call-waiting queue for your telephone. | |
1394 | The low-level Socket module has a special symbol for the system maximum, which | |
1395 | is SOMAXCONN. | |
1396 | ||
1397 | =item Reuse | |
1398 | ||
1399 | The C<Reuse> parameter is needed so that we restart our server | |
1400 | manually without waiting a few minutes to allow system buffers to | |
1401 | clear out. | |
1402 | ||
1403 | =back | |
1404 | ||
1405 | Once the generic server socket has been created using the parameters | |
1406 | listed above, the server then waits for a new client to connect | |
1407 | to it. The server blocks in the C<accept> method, which eventually accepts a | |
1408 | bidirectional connection from the remote client. (Make sure to autoflush | |
1409 | this handle to circumvent buffering.) | |
1410 | ||
1411 | To add to user-friendliness, our server prompts the user for commands. | |
1412 | Most servers don't do this. Because of the prompt without a newline, | |
1413 | you'll have to use the C<sysread> variant of the interactive client above. | |
1414 | ||
1415 | This server accepts one of five different commands, sending output | |
1416 | back to the client. Note that unlike most network servers, this one | |
1417 | only handles one incoming client at a time. Multithreaded servers are | |
1418 | covered in Chapter 6 of the Camel. | |
1419 | ||
1420 | Here's the code. We'll | |
1421 | ||
1422 | #!/usr/bin/perl -w | |
1423 | use IO::Socket; | |
1424 | use Net::hostent; # for OO version of gethostbyaddr | |
1425 | ||
1426 | $PORT = 9000; # pick something not in use | |
1427 | ||
1428 | $server = IO::Socket::INET->new( Proto => 'tcp', | |
1429 | LocalPort => $PORT, | |
1430 | Listen => SOMAXCONN, | |
1431 | Reuse => 1); | |
1432 | ||
1433 | die "can't setup server" unless $server; | |
1434 | print "[Server $0 accepting clients]\n"; | |
1435 | ||
1436 | while ($client = $server->accept()) { | |
1437 | $client->autoflush(1); | |
1438 | print $client "Welcome to $0; type help for command list.\n"; | |
1439 | $hostinfo = gethostbyaddr($client->peeraddr); | |
1440 | printf "[Connect from %s]\n", $hostinfo ? $hostinfo->name : $client->peerhost; | |
1441 | print $client "Command? "; | |
1442 | while ( <$client>) { | |
1443 | next unless /\S/; # blank line | |
1444 | if (/quit|exit/i) { last; } | |
1445 | elsif (/date|time/i) { printf $client "%s\n", scalar localtime; } | |
1446 | elsif (/who/i ) { print $client `who 2>&1`; } | |
1447 | elsif (/cookie/i ) { print $client `/usr/games/fortune 2>&1`; } | |
1448 | elsif (/motd/i ) { print $client `cat /etc/motd 2>&1`; } | |
1449 | else { | |
1450 | print $client "Commands: quit date who cookie motd\n"; | |
1451 | } | |
1452 | } continue { | |
1453 | print $client "Command? "; | |
1454 | } | |
1455 | close $client; | |
1456 | } | |
1457 | ||
1458 | =head1 UDP: Message Passing | |
1459 | ||
1460 | Another kind of client-server setup is one that uses not connections, but | |
1461 | messages. UDP communications involve much lower overhead but also provide | |
1462 | less reliability, as there are no promises that messages will arrive at | |
1463 | all, let alone in order and unmangled. Still, UDP offers some advantages | |
1464 | over TCP, including being able to "broadcast" or "multicast" to a whole | |
1465 | bunch of destination hosts at once (usually on your local subnet). If you | |
1466 | find yourself overly concerned about reliability and start building checks | |
1467 | into your message system, then you probably should use just TCP to start | |
1468 | with. | |
1469 | ||
1470 | Note that UDP datagrams are I<not> a bytestream and should not be treated | |
1471 | as such. This makes using I/O mechanisms with internal buffering | |
1472 | like stdio (i.e. print() and friends) especially cumbersome. Use syswrite(), | |
1473 | or better send(), like in the example below. | |
1474 | ||
1475 | Here's a UDP program similar to the sample Internet TCP client given | |
1476 | earlier. However, instead of checking one host at a time, the UDP version | |
1477 | will check many of them asynchronously by simulating a multicast and then | |
1478 | using select() to do a timed-out wait for I/O. To do something similar | |
1479 | with TCP, you'd have to use a different socket handle for each host. | |
1480 | ||
1481 | #!/usr/bin/perl -w | |
1482 | use strict; | |
1483 | use Socket; | |
1484 | use Sys::Hostname; | |
1485 | ||
1486 | my ( $count, $hisiaddr, $hispaddr, $histime, | |
1487 | $host, $iaddr, $paddr, $port, $proto, | |
1488 | $rin, $rout, $rtime, $SECS_of_70_YEARS); | |
1489 | ||
1490 | $SECS_of_70_YEARS = 2208988800; | |
1491 | ||
1492 | $iaddr = gethostbyname(hostname()); | |
1493 | $proto = getprotobyname('udp'); | |
1494 | $port = getservbyname('time', 'udp'); | |
1495 | $paddr = sockaddr_in(0, $iaddr); # 0 means let kernel pick | |
1496 | ||
1497 | socket(SOCKET, PF_INET, SOCK_DGRAM, $proto) || die "socket: $!"; | |
1498 | bind(SOCKET, $paddr) || die "bind: $!"; | |
1499 | ||
1500 | $| = 1; | |
1501 | printf "%-12s %8s %s\n", "localhost", 0, scalar localtime time; | |
1502 | $count = 0; | |
1503 | for $host (@ARGV) { | |
1504 | $count++; | |
1505 | $hisiaddr = inet_aton($host) || die "unknown host"; | |
1506 | $hispaddr = sockaddr_in($port, $hisiaddr); | |
1507 | defined(send(SOCKET, 0, 0, $hispaddr)) || die "send $host: $!"; | |
1508 | } | |
1509 | ||
1510 | $rin = ''; | |
1511 | vec($rin, fileno(SOCKET), 1) = 1; | |
1512 | ||
1513 | # timeout after 10.0 seconds | |
1514 | while ($count && select($rout = $rin, undef, undef, 10.0)) { | |
1515 | $rtime = ''; | |
1516 | ($hispaddr = recv(SOCKET, $rtime, 4, 0)) || die "recv: $!"; | |
1517 | ($port, $hisiaddr) = sockaddr_in($hispaddr); | |
1518 | $host = gethostbyaddr($hisiaddr, AF_INET); | |
1519 | $histime = unpack("N", $rtime) - $SECS_of_70_YEARS; | |
1520 | printf "%-12s ", $host; | |
1521 | printf "%8d %s\n", $histime - time, scalar localtime($histime); | |
1522 | $count--; | |
1523 | } | |
1524 | ||
1525 | Note that this example does not include any retries and may consequently | |
1526 | fail to contact a reachable host. The most prominent reason for this | |
1527 | is congestion of the queues on the sending host if the number of | |
1528 | list of hosts to contact is sufficiently large. | |
1529 | ||
1530 | =head1 SysV IPC | |
1531 | ||
1532 | While System V IPC isn't so widely used as sockets, it still has some | |
1533 | interesting uses. You can't, however, effectively use SysV IPC or | |
1534 | Berkeley mmap() to have shared memory so as to share a variable amongst | |
1535 | several processes. That's because Perl would reallocate your string when | |
1536 | you weren't wanting it to. | |
1537 | ||
1538 | Here's a small example showing shared memory usage. | |
1539 | ||
1540 | use IPC::SysV qw(IPC_PRIVATE IPC_RMID S_IRWXU); | |
1541 | ||
1542 | $size = 2000; | |
1543 | $id = shmget(IPC_PRIVATE, $size, S_IRWXU) || die "$!"; | |
1544 | print "shm key $id\n"; | |
1545 | ||
1546 | $message = "Message #1"; | |
1547 | shmwrite($id, $message, 0, 60) || die "$!"; | |
1548 | print "wrote: '$message'\n"; | |
1549 | shmread($id, $buff, 0, 60) || die "$!"; | |
1550 | print "read : '$buff'\n"; | |
1551 | ||
1552 | # the buffer of shmread is zero-character end-padded. | |
1553 | substr($buff, index($buff, "\0")) = ''; | |
1554 | print "un" unless $buff eq $message; | |
1555 | print "swell\n"; | |
1556 | ||
1557 | print "deleting shm $id\n"; | |
1558 | shmctl($id, IPC_RMID, 0) || die "$!"; | |
1559 | ||
1560 | Here's an example of a semaphore: | |
1561 | ||
1562 | use IPC::SysV qw(IPC_CREAT); | |
1563 | ||
1564 | $IPC_KEY = 1234; | |
1565 | $id = semget($IPC_KEY, 10, 0666 | IPC_CREAT ) || die "$!"; | |
1566 | print "shm key $id\n"; | |
1567 | ||
1568 | Put this code in a separate file to be run in more than one process. | |
1569 | Call the file F<take>: | |
1570 | ||
1571 | # create a semaphore | |
1572 | ||
1573 | $IPC_KEY = 1234; | |
1574 | $id = semget($IPC_KEY, 0 , 0 ); | |
1575 | die if !defined($id); | |
1576 | ||
1577 | $semnum = 0; | |
1578 | $semflag = 0; | |
1579 | ||
1580 | # 'take' semaphore | |
1581 | # wait for semaphore to be zero | |
1582 | $semop = 0; | |
1583 | $opstring1 = pack("s!s!s!", $semnum, $semop, $semflag); | |
1584 | ||
1585 | # Increment the semaphore count | |
1586 | $semop = 1; | |
1587 | $opstring2 = pack("s!s!s!", $semnum, $semop, $semflag); | |
1588 | $opstring = $opstring1 . $opstring2; | |
1589 | ||
1590 | semop($id,$opstring) || die "$!"; | |
1591 | ||
1592 | Put this code in a separate file to be run in more than one process. | |
1593 | Call this file F<give>: | |
1594 | ||
1595 | # 'give' the semaphore | |
1596 | # run this in the original process and you will see | |
1597 | # that the second process continues | |
1598 | ||
1599 | $IPC_KEY = 1234; | |
1600 | $id = semget($IPC_KEY, 0, 0); | |
1601 | die if !defined($id); | |
1602 | ||
1603 | $semnum = 0; | |
1604 | $semflag = 0; | |
1605 | ||
1606 | # Decrement the semaphore count | |
1607 | $semop = -1; | |
1608 | $opstring = pack("s!s!s!", $semnum, $semop, $semflag); | |
1609 | ||
1610 | semop($id,$opstring) || die "$!"; | |
1611 | ||
1612 | The SysV IPC code above was written long ago, and it's definitely | |
1613 | clunky looking. For a more modern look, see the IPC::SysV module | |
1614 | which is included with Perl starting from Perl 5.005. | |
1615 | ||
1616 | A small example demonstrating SysV message queues: | |
1617 | ||
1618 | use IPC::SysV qw(IPC_PRIVATE IPC_RMID IPC_CREAT S_IRWXU); | |
1619 | ||
1620 | my $id = msgget(IPC_PRIVATE, IPC_CREAT | S_IRWXU); | |
1621 | ||
1622 | my $sent = "message"; | |
1623 | my $type_sent = 1234; | |
1624 | my $rcvd; | |
1625 | my $type_rcvd; | |
1626 | ||
1627 | if (defined $id) { | |
1628 | if (msgsnd($id, pack("l! a*", $type_sent, $sent), 0)) { | |
1629 | if (msgrcv($id, $rcvd, 60, 0, 0)) { | |
1630 | ($type_rcvd, $rcvd) = unpack("l! a*", $rcvd); | |
1631 | if ($rcvd eq $sent) { | |
1632 | print "okay\n"; | |
1633 | } else { | |
1634 | print "not okay\n"; | |
1635 | } | |
1636 | } else { | |
1637 | die "# msgrcv failed\n"; | |
1638 | } | |
1639 | } else { | |
1640 | die "# msgsnd failed\n"; | |
1641 | } | |
1642 | msgctl($id, IPC_RMID, 0) || die "# msgctl failed: $!\n"; | |
1643 | } else { | |
1644 | die "# msgget failed\n"; | |
1645 | } | |
1646 | ||
1647 | =head1 NOTES | |
1648 | ||
1649 | Most of these routines quietly but politely return C<undef> when they | |
1650 | fail instead of causing your program to die right then and there due to | |
1651 | an uncaught exception. (Actually, some of the new I<Socket> conversion | |
1652 | functions croak() on bad arguments.) It is therefore essential to | |
1653 | check return values from these functions. Always begin your socket | |
1654 | programs this way for optimal success, and don't forget to add B<-T> | |
1655 | taint checking flag to the #! line for servers: | |
1656 | ||
1657 | #!/usr/bin/perl -Tw | |
1658 | use strict; | |
1659 | use sigtrap; | |
1660 | use Socket; | |
1661 | ||
1662 | =head1 BUGS | |
1663 | ||
1664 | All these routines create system-specific portability problems. As noted | |
1665 | elsewhere, Perl is at the mercy of your C libraries for much of its system | |
1666 | behaviour. It's probably safest to assume broken SysV semantics for | |
1667 | signals and to stick with simple TCP and UDP socket operations; e.g., don't | |
1668 | try to pass open file descriptors over a local UDP datagram socket if you | |
1669 | want your code to stand a chance of being portable. | |
1670 | ||
1671 | =head1 AUTHOR | |
1672 | ||
1673 | Tom Christiansen, with occasional vestiges of Larry Wall's original | |
1674 | version and suggestions from the Perl Porters. | |
1675 | ||
1676 | =head1 SEE ALSO | |
1677 | ||
1678 | There's a lot more to networking than this, but this should get you | |
1679 | started. | |
1680 | ||
1681 | For intrepid programmers, the indispensable textbook is I<Unix | |
1682 | Network Programming, 2nd Edition, Volume 1> by W. Richard Stevens | |
1683 | (published by Prentice-Hall). Note that most books on networking | |
1684 | address the subject from the perspective of a C programmer; translation | |
1685 | to Perl is left as an exercise for the reader. | |
1686 | ||
1687 | The IO::Socket(3) manpage describes the object library, and the Socket(3) | |
1688 | manpage describes the low-level interface to sockets. Besides the obvious | |
1689 | functions in L<perlfunc>, you should also check out the F<modules> file | |
1690 | at your nearest CPAN site. (See L<perlmodlib> or best yet, the F<Perl | |
1691 | FAQ> for a description of what CPAN is and where to get it.) | |
1692 | ||
1693 | Section 5 of the F<modules> file is devoted to "Networking, Device Control | |
1694 | (modems), and Interprocess Communication", and contains numerous unbundled | |
1695 | modules numerous networking modules, Chat and Expect operations, CGI | |
1696 | programming, DCE, FTP, IPC, NNTP, Proxy, Ptty, RPC, SNMP, SMTP, Telnet, | |
1697 | Threads, and ToolTalk--just to name a few. |