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a0d0e21e LW |
1 | =head1 NAME |
2 | ||
184e9718 | 3 | perlipc - Perl interprocess communication (signals, fifos, pipes, safe subprocesses, sockets, and semaphores) |
a0d0e21e LW |
4 | |
5 | =head1 DESCRIPTION | |
6 | ||
4633a7c4 LW |
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 | ||
490f90af JH |
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. | |
4633a7c4 | 21 | |
a11adca0 | 22 | For example, to trap an interrupt signal, set up a handler like this: |
4633a7c4 LW |
23 | |
24 | sub catch_zap { | |
25 | my $signame = shift; | |
26 | $shucks++; | |
27 | die "Somebody sent me a SIG$signame"; | |
54310121 | 28 | } |
4633a7c4 LW |
29 | $SIG{INT} = 'catch_zap'; # could fail in modules |
30 | $SIG{INT} = \&catch_zap; # best strategy | |
31 | ||
490f90af JH |
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 | |
ec488bcf | 38 | dump - see L</Deferred Signals (Safe Signals)> below. |
a11adca0 | 39 | |
4633a7c4 LW |
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++; | |
54310121 | 51 | } |
4633a7c4 | 52 | |
6a3992aa | 53 | So to check whether signal 17 and SIGALRM were the same, do just this: |
4633a7c4 LW |
54 | |
55 | print "signal #17 = $signame[17]\n"; | |
54310121 | 56 | if ($signo{ALRM}) { |
4633a7c4 | 57 | print "SIGALRM is $signo{ALRM}\n"; |
54310121 | 58 | } |
4633a7c4 LW |
59 | |
60 | You may also choose to assign the strings C<'IGNORE'> or C<'DEFAULT'> as | |
61 | the handler, in which case Perl will try to discard the signal or do the | |
f648820c GS |
62 | default thing. |
63 | ||
19799a22 | 64 | On most Unix platforms, the C<CHLD> (sometimes also known as C<CLD>) signal |
f648820c GS |
65 | has special behavior with respect to a value of C<'IGNORE'>. |
66 | Setting C<$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 | |
4633a7c4 LW |
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; | |
54310121 | 81 | } |
4633a7c4 LW |
82 | sub more_functions { |
83 | # interrupts still ignored, for now... | |
54310121 | 84 | } |
4633a7c4 LW |
85 | |
86 | Sending a signal to a negative process ID means that you send the signal | |
fb73857a | 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): | |
4633a7c4 LW |
90 | |
91 | { | |
92 | local $SIG{HUP} = 'IGNORE'; | |
93 | kill HUP => -$$; | |
94 | # snazzy writing of: kill('HUP', -$$) | |
95 | } | |
a0d0e21e | 96 | |
4633a7c4 | 97 | Another interesting signal to send is signal number zero. This doesn't |
1e9c1022 | 98 | actually affect a child process, but instead checks whether it's alive |
54310121 | 99 | or has changed its UID. |
a0d0e21e | 100 | |
4633a7c4 LW |
101 | unless (kill 0 => $kid_pid) { |
102 | warn "something wicked happened to $kid_pid"; | |
54310121 | 103 | } |
a0d0e21e | 104 | |
1e9c1022 JL |
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. | |
bf003f36 | 108 | You may be able to determine the cause of failure using C<%!>. |
1e9c1022 | 109 | |
bf003f36 | 110 | unless (kill 0 => $pid or $!{EPERM}) { |
1e9c1022 JL |
111 | warn "$pid looks dead"; |
112 | } | |
113 | ||
4633a7c4 LW |
114 | You might also want to employ anonymous functions for simple signal |
115 | handlers: | |
a0d0e21e | 116 | |
4633a7c4 | 117 | $SIG{INT} = sub { die "\nOutta here!\n" }; |
a0d0e21e | 118 | |
4633a7c4 | 119 | But that will be problematic for the more complicated handlers that need |
54310121 | 120 | to reinstall themselves. Because Perl's signal mechanism is currently |
184e9718 | 121 | based on the signal(3) function from the C library, you may sometimes be so |
4633a7c4 LW |
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: | |
a0d0e21e | 126 | |
54310121 | 127 | sub REAPER { |
4633a7c4 | 128 | $waitedpid = wait; |
6a3992aa DL |
129 | # loathe sysV: it makes us not only reinstate |
130 | # the handler, but place it after the wait | |
54310121 | 131 | $SIG{CHLD} = \&REAPER; |
4633a7c4 LW |
132 | } |
133 | $SIG{CHLD} = \&REAPER; | |
134 | # now do something that forks... | |
135 | ||
816229cf | 136 | or better still: |
4633a7c4 | 137 | |
6a3992aa | 138 | use POSIX ":sys_wait_h"; |
54310121 | 139 | sub REAPER { |
4633a7c4 | 140 | my $child; |
816229cf NC |
141 | # If a second child dies while in the signal handler caused by the |
142 | # first death, we won't get another signal. So must loop here else | |
143 | # we will leave the unreaped child as a zombie. And the next time | |
144 | # two children die we get another zombie. And so on. | |
1450d070 | 145 | while (($child = waitpid(-1,WNOHANG)) > 0) { |
4633a7c4 | 146 | $Kid_Status{$child} = $?; |
54310121 | 147 | } |
6a3992aa | 148 | $SIG{CHLD} = \&REAPER; # still loathe sysV |
4633a7c4 LW |
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 | ||
54310121 | 163 | eval { |
4633a7c4 | 164 | local $SIG{ALRM} = sub { die "alarm clock restart" }; |
54310121 | 165 | alarm 10; |
4633a7c4 | 166 | flock(FH, 2); # blocking write lock |
54310121 | 167 | alarm 0; |
4633a7c4 LW |
168 | }; |
169 | if ($@ and $@ !~ /alarm clock restart/) { die } | |
170 | ||
8a4f6ac2 GS |
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 | ||
4633a7c4 LW |
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 | ||
28494392 SB |
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 | ||
3031ea75 JH |
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. | |
28494392 SB |
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 | |
d6fd60d6 | 203 | |
28494392 SB |
204 | use POSIX (); |
205 | use FindBin (); | |
206 | use File::Basename (); | |
207 | use File::Spec::Functions; | |
d6fd60d6 | 208 | |
28494392 | 209 | $|=1; |
d6fd60d6 | 210 | |
28494392 SB |
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; | |
d6fd60d6 | 215 | |
28494392 SB |
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, $action); | |
d6fd60d6 | 222 | |
28494392 SB |
223 | sub sigHUP_handler { |
224 | print "got SIGHUP\n"; | |
225 | exec($SELF, @ARGV) or die "Couldn't restart: $!\n"; | |
226 | } | |
d6fd60d6 | 227 | |
28494392 | 228 | code(); |
d6fd60d6 | 229 | |
28494392 SB |
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 | ||
4633a7c4 LW |
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 | |
54310121 | 246 | just like a regular, connected anonymous pipes, except that the |
4633a7c4 LW |
247 | processes rendezvous using a filename and don't have to be related. |
248 | ||
3341d187 AT |
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 | |
4633a7c4 LW |
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"; | |
54310121 | 260 | if ( system('mknod', $path, 'p') |
4633a7c4 LW |
261 | && system('mkfifo', $path) ) |
262 | { | |
5a964f20 | 263 | die "mk{nod,fifo} $path failed"; |
54310121 | 264 | } |
4633a7c4 LW |
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 | |
54310121 | 269 | on the other end. |
4633a7c4 LW |
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 | |
6a3992aa | 273 | program (like a mailer, news reader, finger program, etc.) tries to read |
4633a7c4 | 274 | from that file, the reading program will block and your program will |
6a3992aa | 275 | supply the new signature. We'll use the pipe-checking file test B<-p> |
4633a7c4 LW |
276 | to find out whether anyone (or anything) has accidentally removed our fifo. |
277 | ||
278 | chdir; # go home | |
279 | $FIFO = '.signature'; | |
4633a7c4 LW |
280 | |
281 | while (1) { | |
282 | unless (-p $FIFO) { | |
283 | unlink $FIFO; | |
3341d187 AT |
284 | require POSIX; |
285 | POSIX::mkfifo($FIFO, 0700) | |
286 | or die "can't mkfifo $FIFO: $!"; | |
54310121 | 287 | } |
4633a7c4 LW |
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; | |
6a3992aa | 293 | sleep 2; # to avoid dup signals |
4633a7c4 | 294 | } |
a0d0e21e | 295 | |
ffc145e8 | 296 | =head2 Deferred Signals (Safe Signals) |
5a964f20 | 297 | |
490f90af JH |
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. | |
5a964f20 | 307 | |
a11adca0 NIS |
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 | |
5a964f20 TC |
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, | |
7b34eba2 | 312 | which will just restart. That means you have to C<die> to longjmp(3) out |
5a964f20 TC |
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. | |
b432a672 AL |
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, | |
a11adca0 NIS |
317 | and be prepared to clean up core dumps now and again. |
318 | ||
490f90af JH |
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: | |
5a964f20 | 330 | |
a11adca0 | 331 | =over 4 |
5a964f20 | 332 | |
e188fdae CB |
333 | =item Long-running opcodes |
334 | ||
335 | As the Perl interpreter only looks at the signal flags when it is about | |
336 | to execute a new opcode, a signal that arrives during a long-running | |
337 | opcode (e.g. a regular expression operation on a very large string) will | |
338 | not be seen until the current opcode completes. | |
339 | ||
340 | N.B. If a signal of any given type fires multiple times during an opcode | |
341 | (such as from a fine-grained timer), the handler for that signal will | |
342 | only be called once after the opcode completes, and all the other | |
343 | instances will be discarded. Furthermore, if your system's signal queue | |
344 | gets flooded to the point that there are signals that have been raised | |
345 | but not yet caught (and thus not deferred) at the time an opcode | |
346 | completes, those signals may well be caught and deferred during | |
347 | subsequent opcodes, with sometimes surprising results. For example, you | |
348 | may see alarms delivered even after calling C<alarm(0)> as the latter | |
349 | stops the raising of alarms but does not cancel the delivery of alarms | |
350 | raised but not yet caught. Do not depend on the behaviors described in | |
351 | this paragraph as they are side effects of the current implementation and | |
352 | may change in future versions of Perl. | |
a11adca0 | 353 | |
a11adca0 NIS |
354 | |
355 | =item Interrupting IO | |
356 | ||
490f90af JH |
357 | When a signal is delivered (e.g. INT control-C) the operating system |
358 | breaks into IO operations like C<read> (used to implement Perls | |
359 | E<lt>E<gt> operator). On older Perls the handler was called | |
360 | immediately (and as C<read> is not "unsafe" this worked well). With | |
361 | the "deferred" scheme the handler is not called immediately, and if | |
362 | Perl is using system's C<stdio> library that library may re-start the | |
363 | C<read> without returning to Perl and giving it a chance to call the | |
364 | %SIG handler. If this happens on your system the solution is to use | |
365 | C<:perlio> layer to do IO - at least on those handles which you want | |
366 | to be able to break into with signals. (The C<:perlio> layer checks | |
367 | the signal flags and calls %SIG handlers before resuming IO operation.) | |
368 | ||
369 | Note that the default in Perl 5.7.3 and later is to automatically use | |
370 | the C<:perlio> layer. | |
a11adca0 | 371 | |
91d81acc JH |
372 | Note that some networking library functions like gethostbyname() are |
373 | known to have their own implementations of timeouts which may conflict | |
374 | with your timeouts. If you are having problems with such functions, | |
375 | you can try using the POSIX sigaction() function, which bypasses the | |
376 | Perl safe signals (note that this means subjecting yourself to | |
377 | possible memory corruption, as described above). Instead of setting | |
e399c6ae | 378 | C<$SIG{ALRM}>: |
91d81acc | 379 | |
e399c6ae SB |
380 | local $SIG{ALRM} = sub { die "alarm" }; |
381 | ||
382 | try something like the following: | |
383 | ||
384 | use POSIX qw(SIGALRM); | |
385 | POSIX::sigaction(SIGALRM, | |
386 | POSIX::SigAction->new(sub { die "alarm" })) | |
387 | or die "Error setting SIGALRM handler: $!\n"; | |
91d81acc | 388 | |
a1966b02 RGS |
389 | Another way to disable the safe signal behavior locally is to use |
390 | the C<Perl::Unsafe::Signals> module from CPAN (which will affect | |
391 | all signals). | |
392 | ||
9ce5b4ad SG |
393 | =item Restartable system calls |
394 | ||
395 | On systems that supported it, older versions of Perl used the | |
396 | SA_RESTART flag when installing %SIG handlers. This meant that | |
397 | restartable system calls would continue rather than returning when | |
398 | a signal arrived. In order to deliver deferred signals promptly, | |
399 | Perl 5.7.3 and later do I<not> use SA_RESTART. Consequently, | |
400 | restartable system calls can fail (with $! set to C<EINTR>) in places | |
401 | where they previously would have succeeded. | |
402 | ||
403 | Note that the default C<:perlio> layer will retry C<read>, C<write> | |
404 | and C<close> as described above and that interrupted C<wait> and | |
405 | C<waitpid> calls will always be retried. | |
406 | ||
a11adca0 NIS |
407 | =item Signals as "faults" |
408 | ||
e188fdae CB |
409 | Certain signals, e.g. SEGV, ILL, and BUS, are generated as a result of |
410 | virtual memory or other "faults". These are normally fatal and there is | |
411 | little a Perl-level handler can do with them, so Perl now delivers them | |
412 | immediately rather than attempting to defer them. | |
a11adca0 NIS |
413 | |
414 | =item Signals triggered by operating system state | |
415 | ||
490f90af JH |
416 | On some operating systems certain signal handlers are supposed to "do |
417 | something" before returning. One example can be CHLD or CLD which | |
418 | indicates a child process has completed. On some operating systems the | |
419 | signal handler is expected to C<wait> for the completed child | |
420 | process. On such systems the deferred signal scheme will not work for | |
421 | those signals (it does not do the C<wait>). Again the failure will | |
422 | look like a loop as the operating system will re-issue the signal as | |
423 | there are un-waited-for completed child processes. | |
a11adca0 | 424 | |
818c4caa | 425 | =back |
a0d0e21e | 426 | |
4ffa73a3 JH |
427 | If you want the old signal behaviour back regardless of possible |
428 | memory corruption, set the environment variable C<PERL_SIGNALS> to | |
45c0772f | 429 | C<"unsafe"> (a new feature since Perl 5.8.1). |
4ffa73a3 | 430 | |
4633a7c4 LW |
431 | =head1 Using open() for IPC |
432 | ||
490f90af JH |
433 | Perl's basic open() statement can also be used for unidirectional |
434 | interprocess communication by either appending or prepending a pipe | |
435 | symbol to the second argument to open(). Here's how to start | |
436 | something up in a child process you intend to write to: | |
4633a7c4 | 437 | |
54310121 | 438 | open(SPOOLER, "| cat -v | lpr -h 2>/dev/null") |
4633a7c4 LW |
439 | || die "can't fork: $!"; |
440 | local $SIG{PIPE} = sub { die "spooler pipe broke" }; | |
441 | print SPOOLER "stuff\n"; | |
442 | close SPOOLER || die "bad spool: $! $?"; | |
443 | ||
444 | And here's how to start up a child process you intend to read from: | |
445 | ||
446 | open(STATUS, "netstat -an 2>&1 |") | |
447 | || die "can't fork: $!"; | |
448 | while (<STATUS>) { | |
449 | next if /^(tcp|udp)/; | |
450 | print; | |
54310121 | 451 | } |
a2eb9003 | 452 | close STATUS || die "bad netstat: $! $?"; |
4633a7c4 LW |
453 | |
454 | If one can be sure that a particular program is a Perl script that is | |
455 | expecting filenames in @ARGV, the clever programmer can write something | |
456 | like this: | |
457 | ||
5a964f20 | 458 | % program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile |
4633a7c4 LW |
459 | |
460 | and irrespective of which shell it's called from, the Perl program will | |
461 | read from the file F<f1>, the process F<cmd1>, standard input (F<tmpfile> | |
462 | in this case), the F<f2> file, the F<cmd2> command, and finally the F<f3> | |
463 | file. Pretty nifty, eh? | |
464 | ||
54310121 | 465 | You might notice that you could use backticks for much the |
4633a7c4 LW |
466 | same effect as opening a pipe for reading: |
467 | ||
468 | print grep { !/^(tcp|udp)/ } `netstat -an 2>&1`; | |
469 | die "bad netstat" if $?; | |
470 | ||
471 | While this is true on the surface, it's much more efficient to process the | |
472 | file one line or record at a time because then you don't have to read the | |
19799a22 | 473 | whole thing into memory at once. It also gives you finer control of the |
4633a7c4 LW |
474 | whole process, letting you to kill off the child process early if you'd |
475 | like. | |
476 | ||
477 | Be careful to check both the open() and the close() return values. If | |
478 | you're I<writing> to a pipe, you should also trap SIGPIPE. Otherwise, | |
479 | think of what happens when you start up a pipe to a command that doesn't | |
480 | exist: the open() will in all likelihood succeed (it only reflects the | |
481 | fork()'s success), but then your output will fail--spectacularly. Perl | |
482 | can't know whether the command worked because your command is actually | |
483 | running in a separate process whose exec() might have failed. Therefore, | |
6a3992aa | 484 | while readers of bogus commands return just a quick end of file, writers |
4633a7c4 LW |
485 | to bogus command will trigger a signal they'd better be prepared to |
486 | handle. Consider: | |
487 | ||
5a964f20 TC |
488 | open(FH, "|bogus") or die "can't fork: $!"; |
489 | print FH "bang\n" or die "can't write: $!"; | |
490 | close FH or die "can't close: $!"; | |
491 | ||
492 | That won't blow up until the close, and it will blow up with a SIGPIPE. | |
493 | To catch it, you could use this: | |
494 | ||
495 | $SIG{PIPE} = 'IGNORE'; | |
496 | open(FH, "|bogus") or die "can't fork: $!"; | |
497 | print FH "bang\n" or die "can't write: $!"; | |
498 | close FH or die "can't close: status=$?"; | |
4633a7c4 | 499 | |
68dc0745 | 500 | =head2 Filehandles |
501 | ||
5a964f20 TC |
502 | Both the main process and any child processes it forks share the same |
503 | STDIN, STDOUT, and STDERR filehandles. If both processes try to access | |
45bc9206 | 504 | them at once, strange things can happen. You may also want to close |
5a964f20 TC |
505 | or reopen the filehandles for the child. You can get around this by |
506 | opening your pipe with open(), but on some systems this means that the | |
507 | child process cannot outlive the parent. | |
68dc0745 | 508 | |
509 | =head2 Background Processes | |
510 | ||
511 | You can run a command in the background with: | |
512 | ||
7b05b7e3 | 513 | system("cmd &"); |
68dc0745 | 514 | |
515 | The command's STDOUT and STDERR (and possibly STDIN, depending on your | |
516 | shell) will be the same as the parent's. You won't need to catch | |
517 | SIGCHLD because of the double-fork taking place (see below for more | |
518 | details). | |
519 | ||
520 | =head2 Complete Dissociation of Child from Parent | |
521 | ||
522 | In some cases (starting server processes, for instance) you'll want to | |
893af57a RS |
523 | completely dissociate the child process from the parent. This is |
524 | often called daemonization. A well behaved daemon will also chdir() | |
525 | to the root directory (so it doesn't prevent unmounting the filesystem | |
526 | containing the directory from which it was launched) and redirect its | |
527 | standard file descriptors from and to F</dev/null> (so that random | |
528 | output doesn't wind up on the user's terminal). | |
529 | ||
530 | use POSIX 'setsid'; | |
531 | ||
532 | sub daemonize { | |
533 | chdir '/' or die "Can't chdir to /: $!"; | |
534 | open STDIN, '/dev/null' or die "Can't read /dev/null: $!"; | |
535 | open STDOUT, '>/dev/null' | |
536 | or die "Can't write to /dev/null: $!"; | |
537 | defined(my $pid = fork) or die "Can't fork: $!"; | |
538 | exit if $pid; | |
539 | setsid or die "Can't start a new session: $!"; | |
540 | open STDERR, '>&STDOUT' or die "Can't dup stdout: $!"; | |
541 | } | |
5a964f20 | 542 | |
893af57a RS |
543 | The fork() has to come before the setsid() to ensure that you aren't a |
544 | process group leader (the setsid() will fail if you are). If your | |
545 | system doesn't have the setsid() function, open F</dev/tty> and use the | |
f979aebc | 546 | C<TIOCNOTTY> ioctl() on it instead. See tty(4) for details. |
5a964f20 | 547 | |
893af57a RS |
548 | Non-Unix users should check their Your_OS::Process module for other |
549 | solutions. | |
68dc0745 | 550 | |
4633a7c4 LW |
551 | =head2 Safe Pipe Opens |
552 | ||
553 | Another interesting approach to IPC is making your single program go | |
554 | multiprocess and communicate between (or even amongst) yourselves. The | |
555 | open() function will accept a file argument of either C<"-|"> or C<"|-"> | |
556 | to do a very interesting thing: it forks a child connected to the | |
557 | filehandle you've opened. The child is running the same program as the | |
558 | parent. This is useful for safely opening a file when running under an | |
559 | assumed UID or GID, for example. If you open a pipe I<to> minus, you can | |
560 | write to the filehandle you opened and your kid will find it in his | |
561 | STDIN. If you open a pipe I<from> minus, you can read from the filehandle | |
562 | you opened whatever your kid writes to his STDOUT. | |
563 | ||
a1ce9542 | 564 | use English '-no_match_vars'; |
4633a7c4 LW |
565 | my $sleep_count = 0; |
566 | ||
54310121 | 567 | do { |
c07a80fd | 568 | $pid = open(KID_TO_WRITE, "|-"); |
4633a7c4 LW |
569 | unless (defined $pid) { |
570 | warn "cannot fork: $!"; | |
571 | die "bailing out" if $sleep_count++ > 6; | |
572 | sleep 10; | |
54310121 | 573 | } |
4633a7c4 LW |
574 | } until defined $pid; |
575 | ||
576 | if ($pid) { # parent | |
c07a80fd | 577 | print KID_TO_WRITE @some_data; |
578 | close(KID_TO_WRITE) || warn "kid exited $?"; | |
4633a7c4 LW |
579 | } else { # child |
580 | ($EUID, $EGID) = ($UID, $GID); # suid progs only | |
54310121 | 581 | open (FILE, "> /safe/file") |
4633a7c4 LW |
582 | || die "can't open /safe/file: $!"; |
583 | while (<STDIN>) { | |
584 | print FILE; # child's STDIN is parent's KID | |
54310121 | 585 | } |
4633a7c4 | 586 | exit; # don't forget this |
54310121 | 587 | } |
4633a7c4 LW |
588 | |
589 | Another common use for this construct is when you need to execute | |
590 | something without the shell's interference. With system(), it's | |
54310121 | 591 | straightforward, but you can't use a pipe open or backticks safely. |
4633a7c4 LW |
592 | That's because there's no way to stop the shell from getting its hands on |
593 | your arguments. Instead, use lower-level control to call exec() directly. | |
594 | ||
54310121 | 595 | Here's a safe backtick or pipe open for read: |
4633a7c4 LW |
596 | |
597 | # add error processing as above | |
c07a80fd | 598 | $pid = open(KID_TO_READ, "-|"); |
4633a7c4 LW |
599 | |
600 | if ($pid) { # parent | |
c07a80fd | 601 | while (<KID_TO_READ>) { |
4633a7c4 | 602 | # do something interesting |
54310121 | 603 | } |
c07a80fd | 604 | close(KID_TO_READ) || warn "kid exited $?"; |
4633a7c4 LW |
605 | |
606 | } else { # child | |
607 | ($EUID, $EGID) = ($UID, $GID); # suid only | |
608 | exec($program, @options, @args) | |
609 | || die "can't exec program: $!"; | |
610 | # NOTREACHED | |
54310121 | 611 | } |
4633a7c4 LW |
612 | |
613 | ||
614 | And here's a safe pipe open for writing: | |
615 | ||
616 | # add error processing as above | |
c07a80fd | 617 | $pid = open(KID_TO_WRITE, "|-"); |
76c0e0db | 618 | $SIG{PIPE} = sub { die "whoops, $program pipe broke" }; |
4633a7c4 LW |
619 | |
620 | if ($pid) { # parent | |
621 | for (@data) { | |
c07a80fd | 622 | print KID_TO_WRITE; |
54310121 | 623 | } |
c07a80fd | 624 | close(KID_TO_WRITE) || warn "kid exited $?"; |
4633a7c4 LW |
625 | |
626 | } else { # child | |
627 | ($EUID, $EGID) = ($UID, $GID); | |
628 | exec($program, @options, @args) | |
629 | || die "can't exec program: $!"; | |
630 | # NOTREACHED | |
54310121 | 631 | } |
4633a7c4 | 632 | |
307eac13 RGS |
633 | Since Perl 5.8.0, you can also use the list form of C<open> for pipes : |
634 | the syntax | |
635 | ||
636 | open KID_PS, "-|", "ps", "aux" or die $!; | |
637 | ||
638 | forks the ps(1) command (without spawning a shell, as there are more than | |
639 | three arguments to open()), and reads its standard output via the | |
8a2485f8 | 640 | C<KID_PS> filehandle. The corresponding syntax to write to command |
ca585e4d | 641 | pipes (with C<"|-"> in place of C<"-|">) is also implemented. |
307eac13 | 642 | |
4633a7c4 LW |
643 | Note that these operations are full Unix forks, which means they may not be |
644 | correctly implemented on alien systems. Additionally, these are not true | |
54310121 | 645 | multithreading. If you'd like to learn more about threading, see the |
184e9718 | 646 | F<modules> file mentioned below in the SEE ALSO section. |
4633a7c4 | 647 | |
7b05b7e3 | 648 | =head2 Bidirectional Communication with Another Process |
4633a7c4 LW |
649 | |
650 | While this works reasonably well for unidirectional communication, what | |
651 | about bidirectional communication? The obvious thing you'd like to do | |
652 | doesn't actually work: | |
653 | ||
c07a80fd | 654 | open(PROG_FOR_READING_AND_WRITING, "| some program |") |
4633a7c4 | 655 | |
9f1b1f2d GS |
656 | and if you forget to use the C<use warnings> pragma or the B<-w> flag, |
657 | then you'll miss out entirely on the diagnostic message: | |
4633a7c4 LW |
658 | |
659 | Can't do bidirectional pipe at -e line 1. | |
660 | ||
661 | If you really want to, you can use the standard open2() library function | |
7b05b7e3 | 662 | to catch both ends. There's also an open3() for tridirectional I/O so you |
4633a7c4 LW |
663 | can also catch your child's STDERR, but doing so would then require an |
664 | awkward select() loop and wouldn't allow you to use normal Perl input | |
665 | operations. | |
666 | ||
667 | If you look at its source, you'll see that open2() uses low-level | |
5a964f20 | 668 | primitives like Unix pipe() and exec() calls to create all the connections. |
4633a7c4 LW |
669 | While it might have been slightly more efficient by using socketpair(), it |
670 | would have then been even less portable than it already is. The open2() | |
671 | and open3() functions are unlikely to work anywhere except on a Unix | |
672 | system or some other one purporting to be POSIX compliant. | |
673 | ||
674 | Here's an example of using open2(): | |
675 | ||
676 | use FileHandle; | |
677 | use IPC::Open2; | |
5a964f20 | 678 | $pid = open2(*Reader, *Writer, "cat -u -n" ); |
4633a7c4 LW |
679 | print Writer "stuff\n"; |
680 | $got = <Reader>; | |
681 | ||
6a3992aa DL |
682 | The problem with this is that Unix buffering is really going to |
683 | ruin your day. Even though your C<Writer> filehandle is auto-flushed, | |
4633a7c4 | 684 | and the process on the other end will get your data in a timely manner, |
6a3992aa | 685 | you can't usually do anything to force it to give it back to you |
54310121 | 686 | in a similarly quick fashion. In this case, we could, because we |
4633a7c4 LW |
687 | gave I<cat> a B<-u> flag to make it unbuffered. But very few Unix |
688 | commands are designed to operate over pipes, so this seldom works | |
54310121 | 689 | unless you yourself wrote the program on the other end of the |
4633a7c4 LW |
690 | double-ended pipe. |
691 | ||
54310121 | 692 | A solution to this is the nonstandard F<Comm.pl> library. It uses |
4633a7c4 LW |
693 | pseudo-ttys to make your program behave more reasonably: |
694 | ||
695 | require 'Comm.pl'; | |
696 | $ph = open_proc('cat -n'); | |
697 | for (1..10) { | |
698 | print $ph "a line\n"; | |
699 | print "got back ", scalar <$ph>; | |
700 | } | |
a0d0e21e | 701 | |
4633a7c4 | 702 | This way you don't have to have control over the source code of the |
54310121 | 703 | program you're using. The F<Comm> library also has expect() |
704 | and interact() functions. Find the library (and we hope its | |
4633a7c4 | 705 | successor F<IPC::Chat>) at your nearest CPAN archive as detailed |
184e9718 | 706 | in the SEE ALSO section below. |
a0d0e21e | 707 | |
c8db1d39 TC |
708 | The newer Expect.pm module from CPAN also addresses this kind of thing. |
709 | This module requires two other modules from CPAN: IO::Pty and IO::Stty. | |
710 | It sets up a pseudo-terminal to interact with programs that insist on | |
a11adca0 | 711 | using talking to the terminal device driver. If your system is |
c8db1d39 TC |
712 | amongst those supported, this may be your best bet. |
713 | ||
5a964f20 TC |
714 | =head2 Bidirectional Communication with Yourself |
715 | ||
716 | If you want, you may make low-level pipe() and fork() | |
717 | to stitch this together by hand. This example only | |
718 | talks to itself, but you could reopen the appropriate | |
719 | handles to STDIN and STDOUT and call other processes. | |
720 | ||
721 | #!/usr/bin/perl -w | |
722 | # pipe1 - bidirectional communication using two pipe pairs | |
723 | # designed for the socketpair-challenged | |
724 | use IO::Handle; # thousands of lines just for autoflush :-( | |
725 | pipe(PARENT_RDR, CHILD_WTR); # XXX: failure? | |
726 | pipe(CHILD_RDR, PARENT_WTR); # XXX: failure? | |
727 | CHILD_WTR->autoflush(1); | |
728 | PARENT_WTR->autoflush(1); | |
729 | ||
730 | if ($pid = fork) { | |
731 | close PARENT_RDR; close PARENT_WTR; | |
732 | print CHILD_WTR "Parent Pid $$ is sending this\n"; | |
733 | chomp($line = <CHILD_RDR>); | |
734 | print "Parent Pid $$ just read this: `$line'\n"; | |
735 | close CHILD_RDR; close CHILD_WTR; | |
736 | waitpid($pid,0); | |
737 | } else { | |
738 | die "cannot fork: $!" unless defined $pid; | |
739 | close CHILD_RDR; close CHILD_WTR; | |
740 | chomp($line = <PARENT_RDR>); | |
741 | print "Child Pid $$ just read this: `$line'\n"; | |
742 | print PARENT_WTR "Child Pid $$ is sending this\n"; | |
743 | close PARENT_RDR; close PARENT_WTR; | |
744 | exit; | |
745 | } | |
746 | ||
a11adca0 | 747 | But you don't actually have to make two pipe calls. If you |
5a964f20 TC |
748 | have the socketpair() system call, it will do this all for you. |
749 | ||
750 | #!/usr/bin/perl -w | |
751 | # pipe2 - bidirectional communication using socketpair | |
752 | # "the best ones always go both ways" | |
753 | ||
754 | use Socket; | |
755 | use IO::Handle; # thousands of lines just for autoflush :-( | |
756 | # We say AF_UNIX because although *_LOCAL is the | |
757 | # POSIX 1003.1g form of the constant, many machines | |
758 | # still don't have it. | |
759 | socketpair(CHILD, PARENT, AF_UNIX, SOCK_STREAM, PF_UNSPEC) | |
760 | or die "socketpair: $!"; | |
761 | ||
762 | CHILD->autoflush(1); | |
763 | PARENT->autoflush(1); | |
764 | ||
765 | if ($pid = fork) { | |
766 | close PARENT; | |
767 | print CHILD "Parent Pid $$ is sending this\n"; | |
768 | chomp($line = <CHILD>); | |
769 | print "Parent Pid $$ just read this: `$line'\n"; | |
770 | close CHILD; | |
771 | waitpid($pid,0); | |
772 | } else { | |
773 | die "cannot fork: $!" unless defined $pid; | |
774 | close CHILD; | |
775 | chomp($line = <PARENT>); | |
776 | print "Child Pid $$ just read this: `$line'\n"; | |
777 | print PARENT "Child Pid $$ is sending this\n"; | |
778 | close PARENT; | |
779 | exit; | |
780 | } | |
781 | ||
4633a7c4 | 782 | =head1 Sockets: Client/Server Communication |
a0d0e21e | 783 | |
6a3992aa | 784 | While not limited to Unix-derived operating systems (e.g., WinSock on PCs |
4633a7c4 | 785 | provides socket support, as do some VMS libraries), you may not have |
184e9718 | 786 | sockets on your system, in which case this section probably isn't going to do |
6a3992aa DL |
787 | you much good. With sockets, you can do both virtual circuits (i.e., TCP |
788 | streams) and datagrams (i.e., UDP packets). You may be able to do even more | |
4633a7c4 LW |
789 | depending on your system. |
790 | ||
791 | The Perl function calls for dealing with sockets have the same names as | |
792 | the corresponding system calls in C, but their arguments tend to differ | |
793 | for two reasons: first, Perl filehandles work differently than C file | |
794 | descriptors. Second, Perl already knows the length of its strings, so you | |
795 | don't need to pass that information. | |
a0d0e21e | 796 | |
4633a7c4 LW |
797 | One of the major problems with old socket code in Perl was that it used |
798 | hard-coded values for some of the constants, which severely hurt | |
799 | portability. If you ever see code that does anything like explicitly | |
800 | setting C<$AF_INET = 2>, you know you're in for big trouble: An | |
801 | immeasurably superior approach is to use the C<Socket> module, which more | |
802 | reliably grants access to various constants and functions you'll need. | |
a0d0e21e | 803 | |
68dc0745 | 804 | If you're not writing a server/client for an existing protocol like |
805 | NNTP or SMTP, you should give some thought to how your server will | |
806 | know when the client has finished talking, and vice-versa. Most | |
807 | protocols are based on one-line messages and responses (so one party | |
4a6725af | 808 | knows the other has finished when a "\n" is received) or multi-line |
68dc0745 | 809 | messages and responses that end with a period on an empty line |
810 | ("\n.\n" terminates a message/response). | |
811 | ||
5a964f20 TC |
812 | =head2 Internet Line Terminators |
813 | ||
814 | The Internet line terminator is "\015\012". Under ASCII variants of | |
815 | Unix, that could usually be written as "\r\n", but under other systems, | |
816 | "\r\n" might at times be "\015\015\012", "\012\012\015", or something | |
817 | completely different. The standards specify writing "\015\012" to be | |
818 | conformant (be strict in what you provide), but they also recommend | |
819 | accepting a lone "\012" on input (but be lenient in what you require). | |
820 | We haven't always been very good about that in the code in this manpage, | |
821 | but unless you're on a Mac, you'll probably be ok. | |
822 | ||
4633a7c4 | 823 | =head2 Internet TCP Clients and Servers |
a0d0e21e | 824 | |
4633a7c4 LW |
825 | Use Internet-domain sockets when you want to do client-server |
826 | communication that might extend to machines outside of your own system. | |
827 | ||
828 | Here's a sample TCP client using Internet-domain sockets: | |
829 | ||
830 | #!/usr/bin/perl -w | |
4633a7c4 LW |
831 | use strict; |
832 | use Socket; | |
833 | my ($remote,$port, $iaddr, $paddr, $proto, $line); | |
834 | ||
835 | $remote = shift || 'localhost'; | |
836 | $port = shift || 2345; # random port | |
837 | if ($port =~ /\D/) { $port = getservbyname($port, 'tcp') } | |
838 | die "No port" unless $port; | |
839 | $iaddr = inet_aton($remote) || die "no host: $remote"; | |
840 | $paddr = sockaddr_in($port, $iaddr); | |
841 | ||
842 | $proto = getprotobyname('tcp'); | |
843 | socket(SOCK, PF_INET, SOCK_STREAM, $proto) || die "socket: $!"; | |
844 | connect(SOCK, $paddr) || die "connect: $!"; | |
54310121 | 845 | while (defined($line = <SOCK>)) { |
4633a7c4 | 846 | print $line; |
54310121 | 847 | } |
4633a7c4 LW |
848 | |
849 | close (SOCK) || die "close: $!"; | |
850 | exit; | |
851 | ||
852 | And here's a corresponding server to go along with it. We'll | |
853 | leave the address as INADDR_ANY so that the kernel can choose | |
54310121 | 854 | the appropriate interface on multihomed hosts. If you want sit |
c07a80fd | 855 | on a particular interface (like the external side of a gateway |
856 | or firewall machine), you should fill this in with your real address | |
857 | instead. | |
858 | ||
859 | #!/usr/bin/perl -Tw | |
c07a80fd | 860 | use strict; |
861 | BEGIN { $ENV{PATH} = '/usr/ucb:/bin' } | |
862 | use Socket; | |
863 | use Carp; | |
5865a7df | 864 | my $EOL = "\015\012"; |
c07a80fd | 865 | |
54310121 | 866 | sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" } |
c07a80fd | 867 | |
868 | my $port = shift || 2345; | |
869 | my $proto = getprotobyname('tcp'); | |
51ee6500 | 870 | |
5865a7df | 871 | ($port) = $port =~ /^(\d+)$/ or die "invalid port"; |
6a3992aa | 872 | |
c07a80fd | 873 | socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!"; |
54310121 | 874 | setsockopt(Server, SOL_SOCKET, SO_REUSEADDR, |
c07a80fd | 875 | pack("l", 1)) || die "setsockopt: $!"; |
876 | bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!"; | |
877 | listen(Server,SOMAXCONN) || die "listen: $!"; | |
878 | ||
879 | logmsg "server started on port $port"; | |
880 | ||
881 | my $paddr; | |
882 | ||
883 | $SIG{CHLD} = \&REAPER; | |
884 | ||
885 | for ( ; $paddr = accept(Client,Server); close Client) { | |
886 | my($port,$iaddr) = sockaddr_in($paddr); | |
887 | my $name = gethostbyaddr($iaddr,AF_INET); | |
888 | ||
54310121 | 889 | logmsg "connection from $name [", |
890 | inet_ntoa($iaddr), "] | |
c07a80fd | 891 | at port $port"; |
892 | ||
54310121 | 893 | print Client "Hello there, $name, it's now ", |
5a964f20 | 894 | scalar localtime, $EOL; |
54310121 | 895 | } |
c07a80fd | 896 | |
54310121 | 897 | And here's a multithreaded version. It's multithreaded in that |
898 | like most typical servers, it spawns (forks) a slave server to | |
c07a80fd | 899 | handle the client request so that the master server can quickly |
900 | go back to service a new client. | |
4633a7c4 LW |
901 | |
902 | #!/usr/bin/perl -Tw | |
4633a7c4 LW |
903 | use strict; |
904 | BEGIN { $ENV{PATH} = '/usr/ucb:/bin' } | |
a0d0e21e | 905 | use Socket; |
4633a7c4 | 906 | use Carp; |
5865a7df | 907 | my $EOL = "\015\012"; |
a0d0e21e | 908 | |
4633a7c4 | 909 | sub spawn; # forward declaration |
54310121 | 910 | sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" } |
a0d0e21e | 911 | |
4633a7c4 LW |
912 | my $port = shift || 2345; |
913 | my $proto = getprotobyname('tcp'); | |
51ee6500 | 914 | |
5865a7df | 915 | ($port) = $port =~ /^(\d+)$/ or die "invalid port"; |
54310121 | 916 | |
c07a80fd | 917 | socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!"; |
54310121 | 918 | setsockopt(Server, SOL_SOCKET, SO_REUSEADDR, |
c07a80fd | 919 | pack("l", 1)) || die "setsockopt: $!"; |
920 | bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!"; | |
921 | listen(Server,SOMAXCONN) || die "listen: $!"; | |
a0d0e21e | 922 | |
4633a7c4 | 923 | logmsg "server started on port $port"; |
a0d0e21e | 924 | |
4633a7c4 LW |
925 | my $waitedpid = 0; |
926 | my $paddr; | |
a0d0e21e | 927 | |
816229cf | 928 | use POSIX ":sys_wait_h"; |
c5ae6365 AW |
929 | use Errno; |
930 | ||
54310121 | 931 | sub REAPER { |
c5ae6365 AW |
932 | local $!; # don't let waitpid() overwrite current error |
933 | while ((my $pid = waitpid(-1,WNOHANG)) > 0 && WIFEXITED($?)) { | |
934 | logmsg "reaped $waitedpid" . ($? ? " with exit $?" : ''); | |
935 | } | |
936 | $SIG{CHLD} = \&REAPER; # loathe sysV | |
4633a7c4 LW |
937 | } |
938 | ||
939 | $SIG{CHLD} = \&REAPER; | |
940 | ||
c5ae6365 AW |
941 | while(1) { |
942 | $paddr = accept(Client, Server) || do { | |
943 | # try again if accept() returned because a signal was received | |
944 | next if $!{EINTR}; | |
945 | die "accept: $!"; | |
946 | }; | |
947 | my ($port, $iaddr) = sockaddr_in($paddr); | |
948 | my $name = gethostbyaddr($iaddr, AF_INET); | |
949 | ||
950 | logmsg "connection from $name [", | |
951 | inet_ntoa($iaddr), | |
952 | "] at port $port"; | |
953 | ||
954 | spawn sub { | |
955 | $|=1; | |
956 | print "Hello there, $name, it's now ", scalar localtime, $EOL; | |
957 | exec '/usr/games/fortune' # XXX: `wrong' line terminators | |
958 | or confess "can't exec fortune: $!"; | |
959 | }; | |
960 | close Client; | |
54310121 | 961 | } |
a0d0e21e | 962 | |
4633a7c4 | 963 | sub spawn { |
c5ae6365 AW |
964 | my $coderef = shift; |
965 | ||
966 | unless (@_ == 0 && $coderef && ref($coderef) eq 'CODE') { | |
967 | confess "usage: spawn CODEREF"; | |
968 | } | |
969 | ||
970 | my $pid; | |
971 | if (! defined($pid = fork)) { | |
972 | logmsg "cannot fork: $!"; | |
973 | return; | |
974 | } | |
975 | elsif ($pid) { | |
976 | logmsg "begat $pid"; | |
977 | return; # I'm the parent | |
978 | } | |
979 | # else I'm the child -- go spawn | |
980 | ||
981 | open(STDIN, "<&Client") || die "can't dup client to stdin"; | |
982 | open(STDOUT, ">&Client") || die "can't dup client to stdout"; | |
983 | ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr"; | |
984 | exit &$coderef(); | |
54310121 | 985 | } |
4633a7c4 | 986 | |
c5ae6365 AW |
987 | This server takes the trouble to clone off a child version via fork() |
988 | for each incoming request. That way it can handle many requests at | |
989 | once, which you might not always want. Even if you don't fork(), the | |
990 | listen() will allow that many pending connections. Forking servers | |
991 | have to be particularly careful about cleaning up their dead children | |
992 | (called "zombies" in Unix parlance), because otherwise you'll quickly | |
993 | fill up your process table. The REAPER subroutine is used here to | |
994 | call waitpid() for any child processes that have finished, thereby | |
995 | ensuring that they terminate cleanly and don't join the ranks of the | |
996 | living dead. | |
997 | ||
998 | Within the while loop we call accept() and check to see if it returns | |
999 | a false value. This would normally indicate a system error that needs | |
1000 | to be reported. However the introduction of safe signals (see | |
1001 | L</Deferred Signals (Safe Signals)> above) in Perl 5.7.3 means that | |
1002 | accept() may also be interrupted when the process receives a signal. | |
1003 | This typically happens when one of the forked sub-processes exits and | |
1004 | notifies the parent process with a CHLD signal. | |
1005 | ||
1006 | If accept() is interrupted by a signal then $! will be set to EINTR. | |
1007 | If this happens then we can safely continue to the next iteration of | |
1008 | the loop and another call to accept(). It is important that your | |
1009 | signal handling code doesn't modify the value of $! or this test will | |
1010 | most likely fail. In the REAPER subroutine we create a local version | |
1011 | of $! before calling waitpid(). When waitpid() sets $! to ECHILD (as | |
1012 | it inevitably does when it has no more children waiting), it will | |
1013 | update the local copy leaving the original unchanged. | |
4633a7c4 LW |
1014 | |
1015 | We suggest that you use the B<-T> flag to use taint checking (see L<perlsec>) | |
1016 | even if we aren't running setuid or setgid. This is always a good idea | |
1017 | for servers and other programs run on behalf of someone else (like CGI | |
1018 | scripts), because it lessens the chances that people from the outside will | |
1019 | be able to compromise your system. | |
1020 | ||
1021 | Let's look at another TCP client. This one connects to the TCP "time" | |
1022 | service on a number of different machines and shows how far their clocks | |
1023 | differ from the system on which it's being run: | |
1024 | ||
1025 | #!/usr/bin/perl -w | |
4633a7c4 LW |
1026 | use strict; |
1027 | use Socket; | |
1028 | ||
1029 | my $SECS_of_70_YEARS = 2208988800; | |
54310121 | 1030 | sub ctime { scalar localtime(shift) } |
4633a7c4 | 1031 | |
54310121 | 1032 | my $iaddr = gethostbyname('localhost'); |
1033 | my $proto = getprotobyname('tcp'); | |
1034 | my $port = getservbyname('time', 'tcp'); | |
4633a7c4 LW |
1035 | my $paddr = sockaddr_in(0, $iaddr); |
1036 | my($host); | |
1037 | ||
1038 | $| = 1; | |
1039 | printf "%-24s %8s %s\n", "localhost", 0, ctime(time()); | |
1040 | ||
1041 | foreach $host (@ARGV) { | |
1042 | printf "%-24s ", $host; | |
1043 | my $hisiaddr = inet_aton($host) || die "unknown host"; | |
1044 | my $hispaddr = sockaddr_in($port, $hisiaddr); | |
1045 | socket(SOCKET, PF_INET, SOCK_STREAM, $proto) || die "socket: $!"; | |
1046 | connect(SOCKET, $hispaddr) || die "bind: $!"; | |
1047 | my $rtime = ' '; | |
1048 | read(SOCKET, $rtime, 4); | |
1049 | close(SOCKET); | |
4358a253 | 1050 | my $histime = unpack("N", $rtime) - $SECS_of_70_YEARS; |
4633a7c4 | 1051 | printf "%8d %s\n", $histime - time, ctime($histime); |
a0d0e21e LW |
1052 | } |
1053 | ||
4633a7c4 LW |
1054 | =head2 Unix-Domain TCP Clients and Servers |
1055 | ||
a2eb9003 | 1056 | That's fine for Internet-domain clients and servers, but what about local |
4633a7c4 LW |
1057 | communications? While you can use the same setup, sometimes you don't |
1058 | want to. Unix-domain sockets are local to the current host, and are often | |
54310121 | 1059 | used internally to implement pipes. Unlike Internet domain sockets, Unix |
4633a7c4 LW |
1060 | domain sockets can show up in the file system with an ls(1) listing. |
1061 | ||
5a964f20 | 1062 | % ls -l /dev/log |
4633a7c4 | 1063 | srw-rw-rw- 1 root 0 Oct 31 07:23 /dev/log |
a0d0e21e | 1064 | |
4633a7c4 LW |
1065 | You can test for these with Perl's B<-S> file test: |
1066 | ||
1067 | unless ( -S '/dev/log' ) { | |
3ba19564 | 1068 | die "something's wicked with the log system"; |
54310121 | 1069 | } |
4633a7c4 LW |
1070 | |
1071 | Here's a sample Unix-domain client: | |
1072 | ||
1073 | #!/usr/bin/perl -w | |
4633a7c4 LW |
1074 | use Socket; |
1075 | use strict; | |
1076 | my ($rendezvous, $line); | |
1077 | ||
2359510d | 1078 | $rendezvous = shift || 'catsock'; |
4633a7c4 | 1079 | socket(SOCK, PF_UNIX, SOCK_STREAM, 0) || die "socket: $!"; |
9607fc9c | 1080 | connect(SOCK, sockaddr_un($rendezvous)) || die "connect: $!"; |
54310121 | 1081 | while (defined($line = <SOCK>)) { |
4633a7c4 | 1082 | print $line; |
54310121 | 1083 | } |
4633a7c4 LW |
1084 | exit; |
1085 | ||
5a964f20 TC |
1086 | And here's a corresponding server. You don't have to worry about silly |
1087 | network terminators here because Unix domain sockets are guaranteed | |
1088 | to be on the localhost, and thus everything works right. | |
4633a7c4 LW |
1089 | |
1090 | #!/usr/bin/perl -Tw | |
4633a7c4 LW |
1091 | use strict; |
1092 | use Socket; | |
1093 | use Carp; | |
1094 | ||
1095 | BEGIN { $ENV{PATH} = '/usr/ucb:/bin' } | |
5865a7df | 1096 | sub spawn; # forward declaration |
5a964f20 | 1097 | sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" } |
4633a7c4 | 1098 | |
2359510d | 1099 | my $NAME = 'catsock'; |
4633a7c4 LW |
1100 | my $uaddr = sockaddr_un($NAME); |
1101 | my $proto = getprotobyname('tcp'); | |
1102 | ||
c07a80fd | 1103 | socket(Server,PF_UNIX,SOCK_STREAM,0) || die "socket: $!"; |
4633a7c4 | 1104 | unlink($NAME); |
c07a80fd | 1105 | bind (Server, $uaddr) || die "bind: $!"; |
1106 | listen(Server,SOMAXCONN) || die "listen: $!"; | |
4633a7c4 LW |
1107 | |
1108 | logmsg "server started on $NAME"; | |
1109 | ||
5a964f20 TC |
1110 | my $waitedpid; |
1111 | ||
816229cf | 1112 | use POSIX ":sys_wait_h"; |
5a964f20 | 1113 | sub REAPER { |
816229cf NC |
1114 | my $child; |
1115 | while (($waitedpid = waitpid(-1,WNOHANG)) > 0) { | |
1116 | logmsg "reaped $waitedpid" . ($? ? " with exit $?" : ''); | |
1117 | } | |
5a964f20 | 1118 | $SIG{CHLD} = \&REAPER; # loathe sysV |
5a964f20 TC |
1119 | } |
1120 | ||
4633a7c4 LW |
1121 | $SIG{CHLD} = \&REAPER; |
1122 | ||
5a964f20 | 1123 | |
54310121 | 1124 | for ( $waitedpid = 0; |
1125 | accept(Client,Server) || $waitedpid; | |
1126 | $waitedpid = 0, close Client) | |
4633a7c4 LW |
1127 | { |
1128 | next if $waitedpid; | |
1129 | logmsg "connection on $NAME"; | |
54310121 | 1130 | spawn sub { |
4633a7c4 LW |
1131 | print "Hello there, it's now ", scalar localtime, "\n"; |
1132 | exec '/usr/games/fortune' or die "can't exec fortune: $!"; | |
1133 | }; | |
54310121 | 1134 | } |
4633a7c4 | 1135 | |
5865a7df NC |
1136 | sub spawn { |
1137 | my $coderef = shift; | |
1138 | ||
1139 | unless (@_ == 0 && $coderef && ref($coderef) eq 'CODE') { | |
1140 | confess "usage: spawn CODEREF"; | |
1141 | } | |
1142 | ||
1143 | my $pid; | |
1144 | if (!defined($pid = fork)) { | |
1145 | logmsg "cannot fork: $!"; | |
1146 | return; | |
1147 | } elsif ($pid) { | |
1148 | logmsg "begat $pid"; | |
1149 | return; # I'm the parent | |
1150 | } | |
1151 | # else I'm the child -- go spawn | |
1152 | ||
1153 | open(STDIN, "<&Client") || die "can't dup client to stdin"; | |
1154 | open(STDOUT, ">&Client") || die "can't dup client to stdout"; | |
1155 | ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr"; | |
1156 | exit &$coderef(); | |
1157 | } | |
1158 | ||
4633a7c4 LW |
1159 | As you see, it's remarkably similar to the Internet domain TCP server, so |
1160 | much so, in fact, that we've omitted several duplicate functions--spawn(), | |
1161 | logmsg(), ctime(), and REAPER()--which are exactly the same as in the | |
1162 | other server. | |
1163 | ||
1164 | So why would you ever want to use a Unix domain socket instead of a | |
1165 | simpler named pipe? Because a named pipe doesn't give you sessions. You | |
1166 | can't tell one process's data from another's. With socket programming, | |
1167 | you get a separate session for each client: that's why accept() takes two | |
1168 | arguments. | |
1169 | ||
1170 | For example, let's say that you have a long running database server daemon | |
1171 | that you want folks from the World Wide Web to be able to access, but only | |
1172 | if they go through a CGI interface. You'd have a small, simple CGI | |
1173 | program that does whatever checks and logging you feel like, and then acts | |
1174 | as a Unix-domain client and connects to your private server. | |
1175 | ||
7b05b7e3 TC |
1176 | =head1 TCP Clients with IO::Socket |
1177 | ||
1178 | For those preferring a higher-level interface to socket programming, the | |
1179 | IO::Socket module provides an object-oriented approach. IO::Socket is | |
1180 | included as part of the standard Perl distribution as of the 5.004 | |
1181 | release. If you're running an earlier version of Perl, just fetch | |
106325ad | 1182 | IO::Socket from CPAN, where you'll also find modules providing easy |
7b05b7e3 TC |
1183 | interfaces to the following systems: DNS, FTP, Ident (RFC 931), NIS and |
1184 | NISPlus, NNTP, Ping, POP3, SMTP, SNMP, SSLeay, Telnet, and Time--just | |
1185 | to name a few. | |
1186 | ||
1187 | =head2 A Simple Client | |
1188 | ||
1189 | Here's a client that creates a TCP connection to the "daytime" | |
1190 | service at port 13 of the host name "localhost" and prints out everything | |
1191 | that the server there cares to provide. | |
1192 | ||
1193 | #!/usr/bin/perl -w | |
1194 | use IO::Socket; | |
1195 | $remote = IO::Socket::INET->new( | |
1196 | Proto => "tcp", | |
1197 | PeerAddr => "localhost", | |
1198 | PeerPort => "daytime(13)", | |
1199 | ) | |
1200 | or die "cannot connect to daytime port at localhost"; | |
1201 | while ( <$remote> ) { print } | |
1202 | ||
1203 | When you run this program, you should get something back that | |
1204 | looks like this: | |
1205 | ||
1206 | Wed May 14 08:40:46 MDT 1997 | |
1207 | ||
1208 | Here are what those parameters to the C<new> constructor mean: | |
1209 | ||
13a2d996 | 1210 | =over 4 |
7b05b7e3 TC |
1211 | |
1212 | =item C<Proto> | |
1213 | ||
1214 | This is which protocol to use. In this case, the socket handle returned | |
1215 | will be connected to a TCP socket, because we want a stream-oriented | |
1216 | connection, that is, one that acts pretty much like a plain old file. | |
1217 | Not all sockets are this of this type. For example, the UDP protocol | |
1218 | can be used to make a datagram socket, used for message-passing. | |
1219 | ||
1220 | =item C<PeerAddr> | |
1221 | ||
1222 | This is the name or Internet address of the remote host the server is | |
1223 | running on. We could have specified a longer name like C<"www.perl.com">, | |
1224 | or an address like C<"204.148.40.9">. For demonstration purposes, we've | |
1225 | used the special hostname C<"localhost">, which should always mean the | |
1226 | current machine you're running on. The corresponding Internet address | |
1227 | for localhost is C<"127.1">, if you'd rather use that. | |
1228 | ||
1229 | =item C<PeerPort> | |
1230 | ||
1231 | This is the service name or port number we'd like to connect to. | |
1232 | We could have gotten away with using just C<"daytime"> on systems with a | |
1233 | well-configured system services file,[FOOTNOTE: The system services file | |
1234 | is in I</etc/services> under Unix] but just in case, we've specified the | |
1235 | port number (13) in parentheses. Using just the number would also have | |
1236 | worked, but constant numbers make careful programmers nervous. | |
1237 | ||
1238 | =back | |
1239 | ||
1240 | Notice how the return value from the C<new> constructor is used as | |
1241 | a filehandle in the C<while> loop? That's what's called an indirect | |
1242 | filehandle, a scalar variable containing a filehandle. You can use | |
1243 | it the same way you would a normal filehandle. For example, you | |
1244 | can read one line from it this way: | |
1245 | ||
1246 | $line = <$handle>; | |
1247 | ||
1248 | all remaining lines from is this way: | |
1249 | ||
1250 | @lines = <$handle>; | |
1251 | ||
1252 | and send a line of data to it this way: | |
1253 | ||
1254 | print $handle "some data\n"; | |
1255 | ||
1256 | =head2 A Webget Client | |
1257 | ||
1258 | Here's a simple client that takes a remote host to fetch a document | |
1259 | from, and then a list of documents to get from that host. This is a | |
1260 | more interesting client than the previous one because it first sends | |
1261 | something to the server before fetching the server's response. | |
1262 | ||
1263 | #!/usr/bin/perl -w | |
1264 | use IO::Socket; | |
1265 | unless (@ARGV > 1) { die "usage: $0 host document ..." } | |
1266 | $host = shift(@ARGV); | |
5a964f20 TC |
1267 | $EOL = "\015\012"; |
1268 | $BLANK = $EOL x 2; | |
7b05b7e3 TC |
1269 | foreach $document ( @ARGV ) { |
1270 | $remote = IO::Socket::INET->new( Proto => "tcp", | |
1271 | PeerAddr => $host, | |
1272 | PeerPort => "http(80)", | |
1273 | ); | |
1274 | unless ($remote) { die "cannot connect to http daemon on $host" } | |
1275 | $remote->autoflush(1); | |
5a964f20 | 1276 | print $remote "GET $document HTTP/1.0" . $BLANK; |
7b05b7e3 TC |
1277 | while ( <$remote> ) { print } |
1278 | close $remote; | |
1279 | } | |
1280 | ||
1281 | The web server handing the "http" service, which is assumed to be at | |
4375e838 | 1282 | its standard port, number 80. If the web server you're trying to |
7b05b7e3 | 1283 | connect to is at a different port (like 1080 or 8080), you should specify |
c47ff5f1 | 1284 | as the named-parameter pair, C<< PeerPort => 8080 >>. The C<autoflush> |
7b05b7e3 TC |
1285 | method is used on the socket because otherwise the system would buffer |
1286 | up the output we sent it. (If you're on a Mac, you'll also need to | |
1287 | change every C<"\n"> in your code that sends data over the network to | |
1288 | be a C<"\015\012"> instead.) | |
1289 | ||
1290 | Connecting to the server is only the first part of the process: once you | |
1291 | have the connection, you have to use the server's language. Each server | |
1292 | on the network has its own little command language that it expects as | |
1293 | input. The string that we send to the server starting with "GET" is in | |
1294 | HTTP syntax. In this case, we simply request each specified document. | |
1295 | Yes, we really are making a new connection for each document, even though | |
1296 | it's the same host. That's the way you always used to have to speak HTTP. | |
1297 | Recent versions of web browsers may request that the remote server leave | |
1298 | the connection open a little while, but the server doesn't have to honor | |
1299 | such a request. | |
1300 | ||
1301 | Here's an example of running that program, which we'll call I<webget>: | |
1302 | ||
5a964f20 | 1303 | % webget www.perl.com /guanaco.html |
7b05b7e3 TC |
1304 | HTTP/1.1 404 File Not Found |
1305 | Date: Thu, 08 May 1997 18:02:32 GMT | |
1306 | Server: Apache/1.2b6 | |
1307 | Connection: close | |
1308 | Content-type: text/html | |
1309 | ||
1310 | <HEAD><TITLE>404 File Not Found</TITLE></HEAD> | |
1311 | <BODY><H1>File Not Found</H1> | |
1312 | The requested URL /guanaco.html was not found on this server.<P> | |
1313 | </BODY> | |
1314 | ||
1315 | Ok, so that's not very interesting, because it didn't find that | |
1316 | particular document. But a long response wouldn't have fit on this page. | |
1317 | ||
1318 | For a more fully-featured version of this program, you should look to | |
1319 | the I<lwp-request> program included with the LWP modules from CPAN. | |
1320 | ||
1321 | =head2 Interactive Client with IO::Socket | |
1322 | ||
1323 | Well, that's all fine if you want to send one command and get one answer, | |
1324 | but what about setting up something fully interactive, somewhat like | |
1325 | the way I<telnet> works? That way you can type a line, get the answer, | |
1326 | type a line, get the answer, etc. | |
1327 | ||
1328 | This client is more complicated than the two we've done so far, but if | |
1329 | you're on a system that supports the powerful C<fork> call, the solution | |
1330 | isn't that rough. Once you've made the connection to whatever service | |
1331 | you'd like to chat with, call C<fork> to clone your process. Each of | |
1332 | these two identical process has a very simple job to do: the parent | |
1333 | copies everything from the socket to standard output, while the child | |
1334 | simultaneously copies everything from standard input to the socket. | |
1335 | To accomplish the same thing using just one process would be I<much> | |
1336 | harder, because it's easier to code two processes to do one thing than it | |
1337 | is to code one process to do two things. (This keep-it-simple principle | |
5a964f20 TC |
1338 | a cornerstones of the Unix philosophy, and good software engineering as |
1339 | well, which is probably why it's spread to other systems.) | |
7b05b7e3 TC |
1340 | |
1341 | Here's the code: | |
1342 | ||
1343 | #!/usr/bin/perl -w | |
1344 | use strict; | |
1345 | use IO::Socket; | |
1346 | my ($host, $port, $kidpid, $handle, $line); | |
1347 | ||
1348 | unless (@ARGV == 2) { die "usage: $0 host port" } | |
1349 | ($host, $port) = @ARGV; | |
1350 | ||
1351 | # create a tcp connection to the specified host and port | |
1352 | $handle = IO::Socket::INET->new(Proto => "tcp", | |
1353 | PeerAddr => $host, | |
1354 | PeerPort => $port) | |
1355 | or die "can't connect to port $port on $host: $!"; | |
1356 | ||
1357 | $handle->autoflush(1); # so output gets there right away | |
1358 | print STDERR "[Connected to $host:$port]\n"; | |
1359 | ||
1360 | # split the program into two processes, identical twins | |
1361 | die "can't fork: $!" unless defined($kidpid = fork()); | |
1362 | ||
1363 | # the if{} block runs only in the parent process | |
1364 | if ($kidpid) { | |
1365 | # copy the socket to standard output | |
1366 | while (defined ($line = <$handle>)) { | |
1367 | print STDOUT $line; | |
1368 | } | |
1369 | kill("TERM", $kidpid); # send SIGTERM to child | |
1370 | } | |
1371 | # the else{} block runs only in the child process | |
1372 | else { | |
1373 | # copy standard input to the socket | |
1374 | while (defined ($line = <STDIN>)) { | |
1375 | print $handle $line; | |
1376 | } | |
1377 | } | |
1378 | ||
1379 | The C<kill> function in the parent's C<if> block is there to send a | |
1380 | signal to our child process (current running in the C<else> block) | |
1381 | as soon as the remote server has closed its end of the connection. | |
1382 | ||
7b05b7e3 TC |
1383 | If the remote server sends data a byte at time, and you need that |
1384 | data immediately without waiting for a newline (which might not happen), | |
1385 | you may wish to replace the C<while> loop in the parent with the | |
1386 | following: | |
1387 | ||
1388 | my $byte; | |
1389 | while (sysread($handle, $byte, 1) == 1) { | |
1390 | print STDOUT $byte; | |
1391 | } | |
1392 | ||
1393 | Making a system call for each byte you want to read is not very efficient | |
1394 | (to put it mildly) but is the simplest to explain and works reasonably | |
1395 | well. | |
1396 | ||
1397 | =head1 TCP Servers with IO::Socket | |
1398 | ||
5a964f20 | 1399 | As always, setting up a server is little bit more involved than running a client. |
7b05b7e3 TC |
1400 | The model is that the server creates a special kind of socket that |
1401 | does nothing but listen on a particular port for incoming connections. | |
c47ff5f1 | 1402 | It does this by calling the C<< IO::Socket::INET->new() >> method with |
7b05b7e3 TC |
1403 | slightly different arguments than the client did. |
1404 | ||
13a2d996 | 1405 | =over 4 |
7b05b7e3 TC |
1406 | |
1407 | =item Proto | |
1408 | ||
1409 | This is which protocol to use. Like our clients, we'll | |
1410 | still specify C<"tcp"> here. | |
1411 | ||
1412 | =item LocalPort | |
1413 | ||
1414 | We specify a local | |
1415 | port in the C<LocalPort> argument, which we didn't do for the client. | |
1416 | This is service name or port number for which you want to be the | |
1417 | server. (Under Unix, ports under 1024 are restricted to the | |
1418 | superuser.) In our sample, we'll use port 9000, but you can use | |
1419 | any port that's not currently in use on your system. If you try | |
1420 | to use one already in used, you'll get an "Address already in use" | |
19799a22 | 1421 | message. Under Unix, the C<netstat -a> command will show |
7b05b7e3 TC |
1422 | which services current have servers. |
1423 | ||
1424 | =item Listen | |
1425 | ||
1426 | The C<Listen> parameter is set to the maximum number of | |
1427 | pending connections we can accept until we turn away incoming clients. | |
1428 | Think of it as a call-waiting queue for your telephone. | |
1429 | The low-level Socket module has a special symbol for the system maximum, which | |
1430 | is SOMAXCONN. | |
1431 | ||
1432 | =item Reuse | |
1433 | ||
1434 | The C<Reuse> parameter is needed so that we restart our server | |
1435 | manually without waiting a few minutes to allow system buffers to | |
1436 | clear out. | |
1437 | ||
1438 | =back | |
1439 | ||
1440 | Once the generic server socket has been created using the parameters | |
1441 | listed above, the server then waits for a new client to connect | |
d1be9408 JF |
1442 | to it. The server blocks in the C<accept> method, which eventually accepts a |
1443 | bidirectional connection from the remote client. (Make sure to autoflush | |
7b05b7e3 TC |
1444 | this handle to circumvent buffering.) |
1445 | ||
1446 | To add to user-friendliness, our server prompts the user for commands. | |
1447 | Most servers don't do this. Because of the prompt without a newline, | |
1448 | you'll have to use the C<sysread> variant of the interactive client above. | |
1449 | ||
1450 | This server accepts one of five different commands, sending output | |
1451 | back to the client. Note that unlike most network servers, this one | |
1452 | only handles one incoming client at a time. Multithreaded servers are | |
f83494b9 | 1453 | covered in Chapter 6 of the Camel. |
7b05b7e3 TC |
1454 | |
1455 | Here's the code. We'll | |
1456 | ||
1457 | #!/usr/bin/perl -w | |
1458 | use IO::Socket; | |
1459 | use Net::hostent; # for OO version of gethostbyaddr | |
1460 | ||
1461 | $PORT = 9000; # pick something not in use | |
1462 | ||
1463 | $server = IO::Socket::INET->new( Proto => 'tcp', | |
1464 | LocalPort => $PORT, | |
1465 | Listen => SOMAXCONN, | |
1466 | Reuse => 1); | |
1467 | ||
1468 | die "can't setup server" unless $server; | |
1469 | print "[Server $0 accepting clients]\n"; | |
1470 | ||
1471 | while ($client = $server->accept()) { | |
1472 | $client->autoflush(1); | |
1473 | print $client "Welcome to $0; type help for command list.\n"; | |
1474 | $hostinfo = gethostbyaddr($client->peeraddr); | |
78fc38e1 | 1475 | printf "[Connect from %s]\n", $hostinfo ? $hostinfo->name : $client->peerhost; |
7b05b7e3 TC |
1476 | print $client "Command? "; |
1477 | while ( <$client>) { | |
1478 | next unless /\S/; # blank line | |
1479 | if (/quit|exit/i) { last; } | |
1480 | elsif (/date|time/i) { printf $client "%s\n", scalar localtime; } | |
1481 | elsif (/who/i ) { print $client `who 2>&1`; } | |
1482 | elsif (/cookie/i ) { print $client `/usr/games/fortune 2>&1`; } | |
1483 | elsif (/motd/i ) { print $client `cat /etc/motd 2>&1`; } | |
1484 | else { | |
1485 | print $client "Commands: quit date who cookie motd\n"; | |
1486 | } | |
1487 | } continue { | |
1488 | print $client "Command? "; | |
1489 | } | |
1490 | close $client; | |
1491 | } | |
1492 | ||
1493 | =head1 UDP: Message Passing | |
4633a7c4 LW |
1494 | |
1495 | Another kind of client-server setup is one that uses not connections, but | |
1496 | messages. UDP communications involve much lower overhead but also provide | |
1497 | less reliability, as there are no promises that messages will arrive at | |
1498 | all, let alone in order and unmangled. Still, UDP offers some advantages | |
1499 | over TCP, including being able to "broadcast" or "multicast" to a whole | |
1500 | bunch of destination hosts at once (usually on your local subnet). If you | |
1501 | find yourself overly concerned about reliability and start building checks | |
6a3992aa | 1502 | into your message system, then you probably should use just TCP to start |
4633a7c4 LW |
1503 | with. |
1504 | ||
90034919 LC |
1505 | Note that UDP datagrams are I<not> a bytestream and should not be treated |
1506 | as such. This makes using I/O mechanisms with internal buffering | |
1507 | like stdio (i.e. print() and friends) especially cumbersome. Use syswrite(), | |
1508 | or better send(), like in the example below. | |
1509 | ||
4633a7c4 | 1510 | Here's a UDP program similar to the sample Internet TCP client given |
7b05b7e3 | 1511 | earlier. However, instead of checking one host at a time, the UDP version |
4633a7c4 LW |
1512 | will check many of them asynchronously by simulating a multicast and then |
1513 | using select() to do a timed-out wait for I/O. To do something similar | |
1514 | with TCP, you'd have to use a different socket handle for each host. | |
1515 | ||
1516 | #!/usr/bin/perl -w | |
1517 | use strict; | |
4633a7c4 LW |
1518 | use Socket; |
1519 | use Sys::Hostname; | |
1520 | ||
54310121 | 1521 | my ( $count, $hisiaddr, $hispaddr, $histime, |
1522 | $host, $iaddr, $paddr, $port, $proto, | |
4633a7c4 LW |
1523 | $rin, $rout, $rtime, $SECS_of_70_YEARS); |
1524 | ||
1525 | $SECS_of_70_YEARS = 2208988800; | |
1526 | ||
1527 | $iaddr = gethostbyname(hostname()); | |
1528 | $proto = getprotobyname('udp'); | |
1529 | $port = getservbyname('time', 'udp'); | |
1530 | $paddr = sockaddr_in(0, $iaddr); # 0 means let kernel pick | |
1531 | ||
1532 | socket(SOCKET, PF_INET, SOCK_DGRAM, $proto) || die "socket: $!"; | |
1533 | bind(SOCKET, $paddr) || die "bind: $!"; | |
1534 | ||
1535 | $| = 1; | |
1536 | printf "%-12s %8s %s\n", "localhost", 0, scalar localtime time; | |
1537 | $count = 0; | |
1538 | for $host (@ARGV) { | |
1539 | $count++; | |
1540 | $hisiaddr = inet_aton($host) || die "unknown host"; | |
1541 | $hispaddr = sockaddr_in($port, $hisiaddr); | |
1542 | defined(send(SOCKET, 0, 0, $hispaddr)) || die "send $host: $!"; | |
1543 | } | |
1544 | ||
1545 | $rin = ''; | |
1546 | vec($rin, fileno(SOCKET), 1) = 1; | |
1547 | ||
1548 | # timeout after 10.0 seconds | |
1549 | while ($count && select($rout = $rin, undef, undef, 10.0)) { | |
1550 | $rtime = ''; | |
1551 | ($hispaddr = recv(SOCKET, $rtime, 4, 0)) || die "recv: $!"; | |
1552 | ($port, $hisiaddr) = sockaddr_in($hispaddr); | |
1553 | $host = gethostbyaddr($hisiaddr, AF_INET); | |
4358a253 | 1554 | $histime = unpack("N", $rtime) - $SECS_of_70_YEARS; |
4633a7c4 LW |
1555 | printf "%-12s ", $host; |
1556 | printf "%8d %s\n", $histime - time, scalar localtime($histime); | |
1557 | $count--; | |
1558 | } | |
1559 | ||
90034919 LC |
1560 | Note that this example does not include any retries and may consequently |
1561 | fail to contact a reachable host. The most prominent reason for this | |
1562 | is congestion of the queues on the sending host if the number of | |
a31a806a | 1563 | list of hosts to contact is sufficiently large. |
90034919 | 1564 | |
4633a7c4 LW |
1565 | =head1 SysV IPC |
1566 | ||
1567 | While System V IPC isn't so widely used as sockets, it still has some | |
1568 | interesting uses. You can't, however, effectively use SysV IPC or | |
1569 | Berkeley mmap() to have shared memory so as to share a variable amongst | |
1570 | several processes. That's because Perl would reallocate your string when | |
1571 | you weren't wanting it to. | |
1572 | ||
54310121 | 1573 | Here's a small example showing shared memory usage. |
a0d0e21e | 1574 | |
7b34eba2 | 1575 | use IPC::SysV qw(IPC_PRIVATE IPC_RMID S_IRUSR S_IWUSR); |
0ade1984 | 1576 | |
a0d0e21e | 1577 | $size = 2000; |
7b34eba2 | 1578 | $id = shmget(IPC_PRIVATE, $size, S_IRUSR|S_IWUSR) || die "$!"; |
41d6edb2 | 1579 | print "shm key $id\n"; |
a0d0e21e LW |
1580 | |
1581 | $message = "Message #1"; | |
41d6edb2 | 1582 | shmwrite($id, $message, 0, 60) || die "$!"; |
0ade1984 | 1583 | print "wrote: '$message'\n"; |
41d6edb2 | 1584 | shmread($id, $buff, 0, 60) || die "$!"; |
0ade1984 | 1585 | print "read : '$buff'\n"; |
a0d0e21e | 1586 | |
0ade1984 JH |
1587 | # the buffer of shmread is zero-character end-padded. |
1588 | substr($buff, index($buff, "\0")) = ''; | |
1589 | print "un" unless $buff eq $message; | |
1590 | print "swell\n"; | |
a0d0e21e | 1591 | |
41d6edb2 JH |
1592 | print "deleting shm $id\n"; |
1593 | shmctl($id, IPC_RMID, 0) || die "$!"; | |
a0d0e21e LW |
1594 | |
1595 | Here's an example of a semaphore: | |
1596 | ||
0ade1984 JH |
1597 | use IPC::SysV qw(IPC_CREAT); |
1598 | ||
a0d0e21e | 1599 | $IPC_KEY = 1234; |
41d6edb2 JH |
1600 | $id = semget($IPC_KEY, 10, 0666 | IPC_CREAT ) || die "$!"; |
1601 | print "shm key $id\n"; | |
a0d0e21e | 1602 | |
a2eb9003 | 1603 | Put this code in a separate file to be run in more than one process. |
a0d0e21e LW |
1604 | Call the file F<take>: |
1605 | ||
1606 | # create a semaphore | |
1607 | ||
1608 | $IPC_KEY = 1234; | |
41d6edb2 JH |
1609 | $id = semget($IPC_KEY, 0 , 0 ); |
1610 | die if !defined($id); | |
a0d0e21e LW |
1611 | |
1612 | $semnum = 0; | |
1613 | $semflag = 0; | |
1614 | ||
1615 | # 'take' semaphore | |
1616 | # wait for semaphore to be zero | |
1617 | $semop = 0; | |
41d6edb2 | 1618 | $opstring1 = pack("s!s!s!", $semnum, $semop, $semflag); |
a0d0e21e LW |
1619 | |
1620 | # Increment the semaphore count | |
1621 | $semop = 1; | |
41d6edb2 | 1622 | $opstring2 = pack("s!s!s!", $semnum, $semop, $semflag); |
a0d0e21e LW |
1623 | $opstring = $opstring1 . $opstring2; |
1624 | ||
41d6edb2 | 1625 | semop($id,$opstring) || die "$!"; |
a0d0e21e | 1626 | |
a2eb9003 | 1627 | Put this code in a separate file to be run in more than one process. |
a0d0e21e LW |
1628 | Call this file F<give>: |
1629 | ||
4633a7c4 | 1630 | # 'give' the semaphore |
a0d0e21e LW |
1631 | # run this in the original process and you will see |
1632 | # that the second process continues | |
1633 | ||
1634 | $IPC_KEY = 1234; | |
41d6edb2 JH |
1635 | $id = semget($IPC_KEY, 0, 0); |
1636 | die if !defined($id); | |
a0d0e21e LW |
1637 | |
1638 | $semnum = 0; | |
1639 | $semflag = 0; | |
1640 | ||
1641 | # Decrement the semaphore count | |
1642 | $semop = -1; | |
41d6edb2 | 1643 | $opstring = pack("s!s!s!", $semnum, $semop, $semflag); |
a0d0e21e | 1644 | |
41d6edb2 | 1645 | semop($id,$opstring) || die "$!"; |
a0d0e21e | 1646 | |
7b05b7e3 | 1647 | The SysV IPC code above was written long ago, and it's definitely |
0ade1984 JH |
1648 | clunky looking. For a more modern look, see the IPC::SysV module |
1649 | which is included with Perl starting from Perl 5.005. | |
4633a7c4 | 1650 | |
41d6edb2 JH |
1651 | A small example demonstrating SysV message queues: |
1652 | ||
7b34eba2 | 1653 | use IPC::SysV qw(IPC_PRIVATE IPC_RMID IPC_CREAT S_IRUSR S_IWUSR); |
41d6edb2 | 1654 | |
7b34eba2 | 1655 | my $id = msgget(IPC_PRIVATE, IPC_CREAT | S_IRUSR | S_IWUSR); |
41d6edb2 JH |
1656 | |
1657 | my $sent = "message"; | |
e343e2e2 | 1658 | my $type_sent = 1234; |
41d6edb2 JH |
1659 | my $rcvd; |
1660 | my $type_rcvd; | |
1661 | ||
1662 | if (defined $id) { | |
1663 | if (msgsnd($id, pack("l! a*", $type_sent, $sent), 0)) { | |
1664 | if (msgrcv($id, $rcvd, 60, 0, 0)) { | |
1665 | ($type_rcvd, $rcvd) = unpack("l! a*", $rcvd); | |
1666 | if ($rcvd eq $sent) { | |
1667 | print "okay\n"; | |
1668 | } else { | |
1669 | print "not okay\n"; | |
1670 | } | |
1671 | } else { | |
1672 | die "# msgrcv failed\n"; | |
1673 | } | |
1674 | } else { | |
1675 | die "# msgsnd failed\n"; | |
1676 | } | |
1677 | msgctl($id, IPC_RMID, 0) || die "# msgctl failed: $!\n"; | |
1678 | } else { | |
1679 | die "# msgget failed\n"; | |
1680 | } | |
1681 | ||
4633a7c4 LW |
1682 | =head1 NOTES |
1683 | ||
5a964f20 TC |
1684 | Most of these routines quietly but politely return C<undef> when they |
1685 | fail instead of causing your program to die right then and there due to | |
1686 | an uncaught exception. (Actually, some of the new I<Socket> conversion | |
1687 | functions croak() on bad arguments.) It is therefore essential to | |
1688 | check return values from these functions. Always begin your socket | |
1689 | programs this way for optimal success, and don't forget to add B<-T> | |
1690 | taint checking flag to the #! line for servers: | |
4633a7c4 | 1691 | |
5a964f20 | 1692 | #!/usr/bin/perl -Tw |
4633a7c4 LW |
1693 | use strict; |
1694 | use sigtrap; | |
1695 | use Socket; | |
1696 | ||
1697 | =head1 BUGS | |
1698 | ||
1699 | All these routines create system-specific portability problems. As noted | |
1700 | elsewhere, Perl is at the mercy of your C libraries for much of its system | |
1701 | behaviour. It's probably safest to assume broken SysV semantics for | |
6a3992aa | 1702 | signals and to stick with simple TCP and UDP socket operations; e.g., don't |
a2eb9003 | 1703 | try to pass open file descriptors over a local UDP datagram socket if you |
4633a7c4 LW |
1704 | want your code to stand a chance of being portable. |
1705 | ||
4633a7c4 LW |
1706 | =head1 AUTHOR |
1707 | ||
1708 | Tom Christiansen, with occasional vestiges of Larry Wall's original | |
7b05b7e3 | 1709 | version and suggestions from the Perl Porters. |
4633a7c4 LW |
1710 | |
1711 | =head1 SEE ALSO | |
1712 | ||
7b05b7e3 TC |
1713 | There's a lot more to networking than this, but this should get you |
1714 | started. | |
1715 | ||
c04e1326 AL |
1716 | For intrepid programmers, the indispensable textbook is I<Unix |
1717 | Network Programming, 2nd Edition, Volume 1> by W. Richard Stevens | |
1718 | (published by Prentice-Hall). Note that most books on networking | |
1719 | address the subject from the perspective of a C programmer; translation | |
1720 | to Perl is left as an exercise for the reader. | |
7b05b7e3 TC |
1721 | |
1722 | The IO::Socket(3) manpage describes the object library, and the Socket(3) | |
1723 | manpage describes the low-level interface to sockets. Besides the obvious | |
1724 | functions in L<perlfunc>, you should also check out the F<modules> file | |
1725 | at your nearest CPAN site. (See L<perlmodlib> or best yet, the F<Perl | |
1726 | FAQ> for a description of what CPAN is and where to get it.) | |
1727 | ||
4633a7c4 | 1728 | Section 5 of the F<modules> file is devoted to "Networking, Device Control |
6a3992aa | 1729 | (modems), and Interprocess Communication", and contains numerous unbundled |
4633a7c4 LW |
1730 | modules numerous networking modules, Chat and Expect operations, CGI |
1731 | programming, DCE, FTP, IPC, NNTP, Proxy, Ptty, RPC, SNMP, SMTP, Telnet, | |
1732 | Threads, and ToolTalk--just to name a few. |