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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 | |
38 | dump - see L<Deferred 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 LW |
97 | Another interesting signal to send is signal number zero. This doesn't |
98 | actually affect another process, but instead checks whether it's alive | |
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 | |
4633a7c4 LW |
105 | You might also want to employ anonymous functions for simple signal |
106 | handlers: | |
a0d0e21e | 107 | |
4633a7c4 | 108 | $SIG{INT} = sub { die "\nOutta here!\n" }; |
a0d0e21e | 109 | |
4633a7c4 | 110 | But that will be problematic for the more complicated handlers that need |
54310121 | 111 | to reinstall themselves. Because Perl's signal mechanism is currently |
184e9718 | 112 | based on the signal(3) function from the C library, you may sometimes be so |
4633a7c4 LW |
113 | misfortunate as to run on systems where that function is "broken", that |
114 | is, it behaves in the old unreliable SysV way rather than the newer, more | |
115 | reasonable BSD and POSIX fashion. So you'll see defensive people writing | |
116 | signal handlers like this: | |
a0d0e21e | 117 | |
54310121 | 118 | sub REAPER { |
4633a7c4 | 119 | $waitedpid = wait; |
6a3992aa DL |
120 | # loathe sysV: it makes us not only reinstate |
121 | # the handler, but place it after the wait | |
54310121 | 122 | $SIG{CHLD} = \&REAPER; |
4633a7c4 LW |
123 | } |
124 | $SIG{CHLD} = \&REAPER; | |
125 | # now do something that forks... | |
126 | ||
816229cf | 127 | or better still: |
4633a7c4 | 128 | |
6a3992aa | 129 | use POSIX ":sys_wait_h"; |
54310121 | 130 | sub REAPER { |
4633a7c4 | 131 | my $child; |
816229cf NC |
132 | # If a second child dies while in the signal handler caused by the |
133 | # first death, we won't get another signal. So must loop here else | |
134 | # we will leave the unreaped child as a zombie. And the next time | |
135 | # two children die we get another zombie. And so on. | |
1450d070 | 136 | while (($child = waitpid(-1,WNOHANG)) > 0) { |
4633a7c4 | 137 | $Kid_Status{$child} = $?; |
54310121 | 138 | } |
6a3992aa | 139 | $SIG{CHLD} = \&REAPER; # still loathe sysV |
4633a7c4 LW |
140 | } |
141 | $SIG{CHLD} = \&REAPER; | |
142 | # do something that forks... | |
143 | ||
144 | Signal handling is also used for timeouts in Unix, While safely | |
145 | protected within an C<eval{}> block, you set a signal handler to trap | |
146 | alarm signals and then schedule to have one delivered to you in some | |
147 | number of seconds. Then try your blocking operation, clearing the alarm | |
148 | when it's done but not before you've exited your C<eval{}> block. If it | |
149 | goes off, you'll use die() to jump out of the block, much as you might | |
150 | using longjmp() or throw() in other languages. | |
151 | ||
152 | Here's an example: | |
153 | ||
54310121 | 154 | eval { |
4633a7c4 | 155 | local $SIG{ALRM} = sub { die "alarm clock restart" }; |
54310121 | 156 | alarm 10; |
4633a7c4 | 157 | flock(FH, 2); # blocking write lock |
54310121 | 158 | alarm 0; |
4633a7c4 LW |
159 | }; |
160 | if ($@ and $@ !~ /alarm clock restart/) { die } | |
161 | ||
8a4f6ac2 GS |
162 | If the operation being timed out is system() or qx(), this technique |
163 | is liable to generate zombies. If this matters to you, you'll | |
164 | need to do your own fork() and exec(), and kill the errant child process. | |
165 | ||
4633a7c4 LW |
166 | For more complex signal handling, you might see the standard POSIX |
167 | module. Lamentably, this is almost entirely undocumented, but | |
168 | the F<t/lib/posix.t> file from the Perl source distribution has some | |
169 | examples in it. | |
170 | ||
28494392 SB |
171 | =head2 Handling the SIGHUP Signal in Daemons |
172 | ||
173 | A process that usually starts when the system boots and shuts down | |
174 | when the system is shut down is called a daemon (Disk And Execution | |
175 | MONitor). If a daemon process has a configuration file which is | |
176 | modified after the process has been started, there should be a way to | |
177 | tell that process to re-read its configuration file, without stopping | |
178 | the process. Many daemons provide this mechanism using the C<SIGHUP> | |
179 | signal handler. When you want to tell the daemon to re-read the file | |
180 | you simply send it the C<SIGHUP> signal. | |
181 | ||
3031ea75 JH |
182 | Not all platforms automatically reinstall their (native) signal |
183 | handlers after a signal delivery. This means that the handler works | |
184 | only the first time the signal is sent. The solution to this problem | |
185 | is to use C<POSIX> signal handlers if available, their behaviour | |
186 | is well-defined. | |
28494392 SB |
187 | |
188 | The following example implements a simple daemon, which restarts | |
189 | itself every time the C<SIGHUP> signal is received. The actual code is | |
190 | located in the subroutine C<code()>, which simply prints some debug | |
191 | info to show that it works and should be replaced with the real code. | |
192 | ||
193 | #!/usr/bin/perl -w | |
d6fd60d6 | 194 | |
28494392 SB |
195 | use POSIX (); |
196 | use FindBin (); | |
197 | use File::Basename (); | |
198 | use File::Spec::Functions; | |
d6fd60d6 | 199 | |
28494392 | 200 | $|=1; |
d6fd60d6 | 201 | |
28494392 SB |
202 | # make the daemon cross-platform, so exec always calls the script |
203 | # itself with the right path, no matter how the script was invoked. | |
204 | my $script = File::Basename::basename($0); | |
205 | my $SELF = catfile $FindBin::Bin, $script; | |
d6fd60d6 | 206 | |
28494392 SB |
207 | # POSIX unmasks the sigprocmask properly |
208 | my $sigset = POSIX::SigSet->new(); | |
209 | my $action = POSIX::SigAction->new('sigHUP_handler', | |
210 | $sigset, | |
211 | &POSIX::SA_NODEFER); | |
212 | POSIX::sigaction(&POSIX::SIGHUP, $action); | |
d6fd60d6 | 213 | |
28494392 SB |
214 | sub sigHUP_handler { |
215 | print "got SIGHUP\n"; | |
216 | exec($SELF, @ARGV) or die "Couldn't restart: $!\n"; | |
217 | } | |
d6fd60d6 | 218 | |
28494392 | 219 | code(); |
d6fd60d6 | 220 | |
28494392 SB |
221 | sub code { |
222 | print "PID: $$\n"; | |
223 | print "ARGV: @ARGV\n"; | |
224 | my $c = 0; | |
225 | while (++$c) { | |
226 | sleep 2; | |
227 | print "$c\n"; | |
228 | } | |
229 | } | |
230 | __END__ | |
231 | ||
232 | ||
4633a7c4 LW |
233 | =head1 Named Pipes |
234 | ||
235 | A named pipe (often referred to as a FIFO) is an old Unix IPC | |
236 | mechanism for processes communicating on the same machine. It works | |
54310121 | 237 | just like a regular, connected anonymous pipes, except that the |
4633a7c4 LW |
238 | processes rendezvous using a filename and don't have to be related. |
239 | ||
240 | To create a named pipe, use the Unix command mknod(1) or on some | |
241 | systems, mkfifo(1). These may not be in your normal path. | |
242 | ||
243 | # system return val is backwards, so && not || | |
244 | # | |
245 | $ENV{PATH} .= ":/etc:/usr/etc"; | |
54310121 | 246 | if ( system('mknod', $path, 'p') |
4633a7c4 LW |
247 | && system('mkfifo', $path) ) |
248 | { | |
5a964f20 | 249 | die "mk{nod,fifo} $path failed"; |
54310121 | 250 | } |
4633a7c4 LW |
251 | |
252 | ||
253 | A fifo is convenient when you want to connect a process to an unrelated | |
254 | one. When you open a fifo, the program will block until there's something | |
54310121 | 255 | on the other end. |
4633a7c4 LW |
256 | |
257 | For example, let's say you'd like to have your F<.signature> file be a | |
258 | named pipe that has a Perl program on the other end. Now every time any | |
6a3992aa | 259 | program (like a mailer, news reader, finger program, etc.) tries to read |
4633a7c4 | 260 | from that file, the reading program will block and your program will |
6a3992aa | 261 | supply the new signature. We'll use the pipe-checking file test B<-p> |
4633a7c4 LW |
262 | to find out whether anyone (or anything) has accidentally removed our fifo. |
263 | ||
264 | chdir; # go home | |
265 | $FIFO = '.signature'; | |
266 | $ENV{PATH} .= ":/etc:/usr/games"; | |
267 | ||
268 | while (1) { | |
269 | unless (-p $FIFO) { | |
270 | unlink $FIFO; | |
54310121 | 271 | system('mknod', $FIFO, 'p') |
4633a7c4 | 272 | && die "can't mknod $FIFO: $!"; |
54310121 | 273 | } |
4633a7c4 LW |
274 | |
275 | # next line blocks until there's a reader | |
276 | open (FIFO, "> $FIFO") || die "can't write $FIFO: $!"; | |
277 | print FIFO "John Smith (smith\@host.org)\n", `fortune -s`; | |
278 | close FIFO; | |
6a3992aa | 279 | sleep 2; # to avoid dup signals |
4633a7c4 | 280 | } |
a0d0e21e | 281 | |
4ffa73a3 | 282 | =head2 Deferred Signals (Safe signals) |
5a964f20 | 283 | |
490f90af JH |
284 | In Perls before Perl 5.7.3 by installing Perl code to deal with |
285 | signals, you were exposing yourself to danger from two things. First, | |
286 | few system library functions are re-entrant. If the signal interrupts | |
287 | while Perl is executing one function (like malloc(3) or printf(3)), | |
288 | and your signal handler then calls the same function again, you could | |
289 | get unpredictable behavior--often, a core dump. Second, Perl isn't | |
290 | itself re-entrant at the lowest levels. If the signal interrupts Perl | |
291 | while Perl is changing its own internal data structures, similarly | |
292 | unpredictable behaviour may result. | |
5a964f20 | 293 | |
a11adca0 NIS |
294 | There were two things you could do, knowing this: be paranoid or be |
295 | pragmatic. The paranoid approach was to do as little as possible in your | |
5a964f20 TC |
296 | signal handler. Set an existing integer variable that already has a |
297 | value, and return. This doesn't help you if you're in a slow system call, | |
298 | which will just restart. That means you have to C<die> to longjump(3) out | |
299 | of the handler. Even this is a little cavalier for the true paranoiac, | |
300 | who avoids C<die> in a handler because the system I<is> out to get you. | |
a11adca0 NIS |
301 | The pragmatic approach was to say ``I know the risks, but prefer the |
302 | convenience'', and to do anything you wanted in your signal handler, | |
303 | and be prepared to clean up core dumps now and again. | |
304 | ||
490f90af JH |
305 | In Perl 5.7.3 and later to avoid these problems signals are |
306 | "deferred"-- that is when the signal is delivered to the process by | |
307 | the system (to the C code that implements Perl) a flag is set, and the | |
308 | handler returns immediately. Then at strategic "safe" points in the | |
309 | Perl interpreter (e.g. when it is about to execute a new opcode) the | |
310 | flags are checked and the Perl level handler from %SIG is | |
311 | executed. The "deferred" scheme allows much more flexibility in the | |
312 | coding of signal handler as we know Perl interpreter is in a safe | |
313 | state, and that we are not in a system library function when the | |
314 | handler is called. However the implementation does differ from | |
315 | previous Perls in the following ways: | |
5a964f20 | 316 | |
a11adca0 | 317 | =over 4 |
5a964f20 | 318 | |
a11adca0 NIS |
319 | =item Long running opcodes |
320 | ||
490f90af JH |
321 | As Perl interpreter only looks at the signal flags when it about to |
322 | execute a new opcode if a signal arrives during a long running opcode | |
323 | (e.g. a regular expression operation on a very large string) then | |
324 | signal will not be seen until operation completes. | |
a11adca0 NIS |
325 | |
326 | =item Interrupting IO | |
327 | ||
490f90af JH |
328 | When a signal is delivered (e.g. INT control-C) the operating system |
329 | breaks into IO operations like C<read> (used to implement Perls | |
330 | E<lt>E<gt> operator). On older Perls the handler was called | |
331 | immediately (and as C<read> is not "unsafe" this worked well). With | |
332 | the "deferred" scheme the handler is not called immediately, and if | |
333 | Perl is using system's C<stdio> library that library may re-start the | |
334 | C<read> without returning to Perl and giving it a chance to call the | |
335 | %SIG handler. If this happens on your system the solution is to use | |
336 | C<:perlio> layer to do IO - at least on those handles which you want | |
337 | to be able to break into with signals. (The C<:perlio> layer checks | |
338 | the signal flags and calls %SIG handlers before resuming IO operation.) | |
339 | ||
340 | Note that the default in Perl 5.7.3 and later is to automatically use | |
341 | the C<:perlio> layer. | |
a11adca0 | 342 | |
91d81acc JH |
343 | Note that some networking library functions like gethostbyname() are |
344 | known to have their own implementations of timeouts which may conflict | |
345 | with your timeouts. If you are having problems with such functions, | |
346 | you can try using the POSIX sigaction() function, which bypasses the | |
347 | Perl safe signals (note that this means subjecting yourself to | |
348 | possible memory corruption, as described above). Instead of setting | |
349 | C<$SIG{ALRM}> try something like the following: | |
350 | ||
351 | use POSIX; | |
352 | sigaction SIGALRM, new POSIX::SigAction sub { die "alarm\n" } | |
353 | or die "Error setting SIGALRM handler: $!\n"; | |
354 | ||
a11adca0 NIS |
355 | =item Signals as "faults" |
356 | ||
490f90af JH |
357 | Certain signals e.g. SEGV, ILL, BUS are generated as a result of |
358 | virtual memory or other "faults". These are normally fatal and there | |
359 | is little a Perl-level handler can do with them. (In particular the | |
360 | old signal scheme was particularly unsafe in such cases.) However if | |
361 | a %SIG handler is set the new scheme simply sets a flag and returns as | |
362 | described above. This may cause the operating system to try the | |
363 | offending machine instruction again and - as nothing has changed - it | |
364 | will generate the signal again. The result of this is a rather odd | |
365 | "loop". In future Perl's signal mechanism may be changed to avoid this | |
366 | - perhaps by simply disallowing %SIG handlers on signals of that | |
367 | type. Until then the work-round is not to set a %SIG handler on those | |
368 | signals. (Which signals they are is operating system dependant.) | |
a11adca0 NIS |
369 | |
370 | =item Signals triggered by operating system state | |
371 | ||
490f90af JH |
372 | On some operating systems certain signal handlers are supposed to "do |
373 | something" before returning. One example can be CHLD or CLD which | |
374 | indicates a child process has completed. On some operating systems the | |
375 | signal handler is expected to C<wait> for the completed child | |
376 | process. On such systems the deferred signal scheme will not work for | |
377 | those signals (it does not do the C<wait>). Again the failure will | |
378 | look like a loop as the operating system will re-issue the signal as | |
379 | there are un-waited-for completed child processes. | |
a11adca0 | 380 | |
818c4caa | 381 | =back |
a0d0e21e | 382 | |
4ffa73a3 JH |
383 | If you want the old signal behaviour back regardless of possible |
384 | memory corruption, set the environment variable C<PERL_SIGNALS> to | |
45c0772f | 385 | C<"unsafe"> (a new feature since Perl 5.8.1). |
4ffa73a3 | 386 | |
4633a7c4 LW |
387 | =head1 Using open() for IPC |
388 | ||
490f90af JH |
389 | Perl's basic open() statement can also be used for unidirectional |
390 | interprocess communication by either appending or prepending a pipe | |
391 | symbol to the second argument to open(). Here's how to start | |
392 | something up in a child process you intend to write to: | |
4633a7c4 | 393 | |
54310121 | 394 | open(SPOOLER, "| cat -v | lpr -h 2>/dev/null") |
4633a7c4 LW |
395 | || die "can't fork: $!"; |
396 | local $SIG{PIPE} = sub { die "spooler pipe broke" }; | |
397 | print SPOOLER "stuff\n"; | |
398 | close SPOOLER || die "bad spool: $! $?"; | |
399 | ||
400 | And here's how to start up a child process you intend to read from: | |
401 | ||
402 | open(STATUS, "netstat -an 2>&1 |") | |
403 | || die "can't fork: $!"; | |
404 | while (<STATUS>) { | |
405 | next if /^(tcp|udp)/; | |
406 | print; | |
54310121 | 407 | } |
a2eb9003 | 408 | close STATUS || die "bad netstat: $! $?"; |
4633a7c4 LW |
409 | |
410 | If one can be sure that a particular program is a Perl script that is | |
411 | expecting filenames in @ARGV, the clever programmer can write something | |
412 | like this: | |
413 | ||
5a964f20 | 414 | % program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile |
4633a7c4 LW |
415 | |
416 | and irrespective of which shell it's called from, the Perl program will | |
417 | read from the file F<f1>, the process F<cmd1>, standard input (F<tmpfile> | |
418 | in this case), the F<f2> file, the F<cmd2> command, and finally the F<f3> | |
419 | file. Pretty nifty, eh? | |
420 | ||
54310121 | 421 | You might notice that you could use backticks for much the |
4633a7c4 LW |
422 | same effect as opening a pipe for reading: |
423 | ||
424 | print grep { !/^(tcp|udp)/ } `netstat -an 2>&1`; | |
425 | die "bad netstat" if $?; | |
426 | ||
427 | While this is true on the surface, it's much more efficient to process the | |
428 | file one line or record at a time because then you don't have to read the | |
19799a22 | 429 | whole thing into memory at once. It also gives you finer control of the |
4633a7c4 LW |
430 | whole process, letting you to kill off the child process early if you'd |
431 | like. | |
432 | ||
433 | Be careful to check both the open() and the close() return values. If | |
434 | you're I<writing> to a pipe, you should also trap SIGPIPE. Otherwise, | |
435 | think of what happens when you start up a pipe to a command that doesn't | |
436 | exist: the open() will in all likelihood succeed (it only reflects the | |
437 | fork()'s success), but then your output will fail--spectacularly. Perl | |
438 | can't know whether the command worked because your command is actually | |
439 | running in a separate process whose exec() might have failed. Therefore, | |
6a3992aa | 440 | while readers of bogus commands return just a quick end of file, writers |
4633a7c4 LW |
441 | to bogus command will trigger a signal they'd better be prepared to |
442 | handle. Consider: | |
443 | ||
5a964f20 TC |
444 | open(FH, "|bogus") or die "can't fork: $!"; |
445 | print FH "bang\n" or die "can't write: $!"; | |
446 | close FH or die "can't close: $!"; | |
447 | ||
448 | That won't blow up until the close, and it will blow up with a SIGPIPE. | |
449 | To catch it, you could use this: | |
450 | ||
451 | $SIG{PIPE} = 'IGNORE'; | |
452 | open(FH, "|bogus") or die "can't fork: $!"; | |
453 | print FH "bang\n" or die "can't write: $!"; | |
454 | close FH or die "can't close: status=$?"; | |
4633a7c4 | 455 | |
68dc0745 | 456 | =head2 Filehandles |
457 | ||
5a964f20 TC |
458 | Both the main process and any child processes it forks share the same |
459 | STDIN, STDOUT, and STDERR filehandles. If both processes try to access | |
45bc9206 | 460 | them at once, strange things can happen. You may also want to close |
5a964f20 TC |
461 | or reopen the filehandles for the child. You can get around this by |
462 | opening your pipe with open(), but on some systems this means that the | |
463 | child process cannot outlive the parent. | |
68dc0745 | 464 | |
465 | =head2 Background Processes | |
466 | ||
467 | You can run a command in the background with: | |
468 | ||
7b05b7e3 | 469 | system("cmd &"); |
68dc0745 | 470 | |
471 | The command's STDOUT and STDERR (and possibly STDIN, depending on your | |
472 | shell) will be the same as the parent's. You won't need to catch | |
473 | SIGCHLD because of the double-fork taking place (see below for more | |
474 | details). | |
475 | ||
476 | =head2 Complete Dissociation of Child from Parent | |
477 | ||
478 | In some cases (starting server processes, for instance) you'll want to | |
893af57a RS |
479 | completely dissociate the child process from the parent. This is |
480 | often called daemonization. A well behaved daemon will also chdir() | |
481 | to the root directory (so it doesn't prevent unmounting the filesystem | |
482 | containing the directory from which it was launched) and redirect its | |
483 | standard file descriptors from and to F</dev/null> (so that random | |
484 | output doesn't wind up on the user's terminal). | |
485 | ||
486 | use POSIX 'setsid'; | |
487 | ||
488 | sub daemonize { | |
489 | chdir '/' or die "Can't chdir to /: $!"; | |
490 | open STDIN, '/dev/null' or die "Can't read /dev/null: $!"; | |
491 | open STDOUT, '>/dev/null' | |
492 | or die "Can't write to /dev/null: $!"; | |
493 | defined(my $pid = fork) or die "Can't fork: $!"; | |
494 | exit if $pid; | |
495 | setsid or die "Can't start a new session: $!"; | |
496 | open STDERR, '>&STDOUT' or die "Can't dup stdout: $!"; | |
497 | } | |
5a964f20 | 498 | |
893af57a RS |
499 | The fork() has to come before the setsid() to ensure that you aren't a |
500 | process group leader (the setsid() will fail if you are). If your | |
501 | system doesn't have the setsid() function, open F</dev/tty> and use the | |
502 | C<TIOCNOTTY> ioctl() on it instead. See L<tty(4)> for details. | |
5a964f20 | 503 | |
893af57a RS |
504 | Non-Unix users should check their Your_OS::Process module for other |
505 | solutions. | |
68dc0745 | 506 | |
4633a7c4 LW |
507 | =head2 Safe Pipe Opens |
508 | ||
509 | Another interesting approach to IPC is making your single program go | |
510 | multiprocess and communicate between (or even amongst) yourselves. The | |
511 | open() function will accept a file argument of either C<"-|"> or C<"|-"> | |
512 | to do a very interesting thing: it forks a child connected to the | |
513 | filehandle you've opened. The child is running the same program as the | |
514 | parent. This is useful for safely opening a file when running under an | |
515 | assumed UID or GID, for example. If you open a pipe I<to> minus, you can | |
516 | write to the filehandle you opened and your kid will find it in his | |
517 | STDIN. If you open a pipe I<from> minus, you can read from the filehandle | |
518 | you opened whatever your kid writes to his STDOUT. | |
519 | ||
a1ce9542 | 520 | use English '-no_match_vars'; |
4633a7c4 LW |
521 | my $sleep_count = 0; |
522 | ||
54310121 | 523 | do { |
c07a80fd | 524 | $pid = open(KID_TO_WRITE, "|-"); |
4633a7c4 LW |
525 | unless (defined $pid) { |
526 | warn "cannot fork: $!"; | |
527 | die "bailing out" if $sleep_count++ > 6; | |
528 | sleep 10; | |
54310121 | 529 | } |
4633a7c4 LW |
530 | } until defined $pid; |
531 | ||
532 | if ($pid) { # parent | |
c07a80fd | 533 | print KID_TO_WRITE @some_data; |
534 | close(KID_TO_WRITE) || warn "kid exited $?"; | |
4633a7c4 LW |
535 | } else { # child |
536 | ($EUID, $EGID) = ($UID, $GID); # suid progs only | |
54310121 | 537 | open (FILE, "> /safe/file") |
4633a7c4 LW |
538 | || die "can't open /safe/file: $!"; |
539 | while (<STDIN>) { | |
540 | print FILE; # child's STDIN is parent's KID | |
54310121 | 541 | } |
4633a7c4 | 542 | exit; # don't forget this |
54310121 | 543 | } |
4633a7c4 LW |
544 | |
545 | Another common use for this construct is when you need to execute | |
546 | something without the shell's interference. With system(), it's | |
54310121 | 547 | straightforward, but you can't use a pipe open or backticks safely. |
4633a7c4 LW |
548 | That's because there's no way to stop the shell from getting its hands on |
549 | your arguments. Instead, use lower-level control to call exec() directly. | |
550 | ||
54310121 | 551 | Here's a safe backtick or pipe open for read: |
4633a7c4 LW |
552 | |
553 | # add error processing as above | |
c07a80fd | 554 | $pid = open(KID_TO_READ, "-|"); |
4633a7c4 LW |
555 | |
556 | if ($pid) { # parent | |
c07a80fd | 557 | while (<KID_TO_READ>) { |
4633a7c4 | 558 | # do something interesting |
54310121 | 559 | } |
c07a80fd | 560 | close(KID_TO_READ) || warn "kid exited $?"; |
4633a7c4 LW |
561 | |
562 | } else { # child | |
563 | ($EUID, $EGID) = ($UID, $GID); # suid only | |
564 | exec($program, @options, @args) | |
565 | || die "can't exec program: $!"; | |
566 | # NOTREACHED | |
54310121 | 567 | } |
4633a7c4 LW |
568 | |
569 | ||
570 | And here's a safe pipe open for writing: | |
571 | ||
572 | # add error processing as above | |
c07a80fd | 573 | $pid = open(KID_TO_WRITE, "|-"); |
76c0e0db | 574 | $SIG{PIPE} = sub { die "whoops, $program pipe broke" }; |
4633a7c4 LW |
575 | |
576 | if ($pid) { # parent | |
577 | for (@data) { | |
c07a80fd | 578 | print KID_TO_WRITE; |
54310121 | 579 | } |
c07a80fd | 580 | close(KID_TO_WRITE) || warn "kid exited $?"; |
4633a7c4 LW |
581 | |
582 | } else { # child | |
583 | ($EUID, $EGID) = ($UID, $GID); | |
584 | exec($program, @options, @args) | |
585 | || die "can't exec program: $!"; | |
586 | # NOTREACHED | |
54310121 | 587 | } |
4633a7c4 | 588 | |
307eac13 RGS |
589 | Since Perl 5.8.0, you can also use the list form of C<open> for pipes : |
590 | the syntax | |
591 | ||
592 | open KID_PS, "-|", "ps", "aux" or die $!; | |
593 | ||
594 | forks the ps(1) command (without spawning a shell, as there are more than | |
595 | three arguments to open()), and reads its standard output via the | |
ca585e4d RGS |
596 | C<KID_PS> filehandle. The corresponding syntax to read from command |
597 | pipes (with C<"|-"> in place of C<"-|">) is also implemented. | |
307eac13 | 598 | |
4633a7c4 LW |
599 | Note that these operations are full Unix forks, which means they may not be |
600 | correctly implemented on alien systems. Additionally, these are not true | |
54310121 | 601 | multithreading. If you'd like to learn more about threading, see the |
184e9718 | 602 | F<modules> file mentioned below in the SEE ALSO section. |
4633a7c4 | 603 | |
7b05b7e3 | 604 | =head2 Bidirectional Communication with Another Process |
4633a7c4 LW |
605 | |
606 | While this works reasonably well for unidirectional communication, what | |
607 | about bidirectional communication? The obvious thing you'd like to do | |
608 | doesn't actually work: | |
609 | ||
c07a80fd | 610 | open(PROG_FOR_READING_AND_WRITING, "| some program |") |
4633a7c4 | 611 | |
9f1b1f2d GS |
612 | and if you forget to use the C<use warnings> pragma or the B<-w> flag, |
613 | then you'll miss out entirely on the diagnostic message: | |
4633a7c4 LW |
614 | |
615 | Can't do bidirectional pipe at -e line 1. | |
616 | ||
617 | If you really want to, you can use the standard open2() library function | |
7b05b7e3 | 618 | to catch both ends. There's also an open3() for tridirectional I/O so you |
4633a7c4 LW |
619 | can also catch your child's STDERR, but doing so would then require an |
620 | awkward select() loop and wouldn't allow you to use normal Perl input | |
621 | operations. | |
622 | ||
623 | If you look at its source, you'll see that open2() uses low-level | |
5a964f20 | 624 | primitives like Unix pipe() and exec() calls to create all the connections. |
4633a7c4 LW |
625 | While it might have been slightly more efficient by using socketpair(), it |
626 | would have then been even less portable than it already is. The open2() | |
627 | and open3() functions are unlikely to work anywhere except on a Unix | |
628 | system or some other one purporting to be POSIX compliant. | |
629 | ||
630 | Here's an example of using open2(): | |
631 | ||
632 | use FileHandle; | |
633 | use IPC::Open2; | |
5a964f20 | 634 | $pid = open2(*Reader, *Writer, "cat -u -n" ); |
4633a7c4 LW |
635 | print Writer "stuff\n"; |
636 | $got = <Reader>; | |
637 | ||
6a3992aa DL |
638 | The problem with this is that Unix buffering is really going to |
639 | ruin your day. Even though your C<Writer> filehandle is auto-flushed, | |
4633a7c4 | 640 | and the process on the other end will get your data in a timely manner, |
6a3992aa | 641 | you can't usually do anything to force it to give it back to you |
54310121 | 642 | in a similarly quick fashion. In this case, we could, because we |
4633a7c4 LW |
643 | gave I<cat> a B<-u> flag to make it unbuffered. But very few Unix |
644 | commands are designed to operate over pipes, so this seldom works | |
54310121 | 645 | unless you yourself wrote the program on the other end of the |
4633a7c4 LW |
646 | double-ended pipe. |
647 | ||
54310121 | 648 | A solution to this is the nonstandard F<Comm.pl> library. It uses |
4633a7c4 LW |
649 | pseudo-ttys to make your program behave more reasonably: |
650 | ||
651 | require 'Comm.pl'; | |
652 | $ph = open_proc('cat -n'); | |
653 | for (1..10) { | |
654 | print $ph "a line\n"; | |
655 | print "got back ", scalar <$ph>; | |
656 | } | |
a0d0e21e | 657 | |
4633a7c4 | 658 | This way you don't have to have control over the source code of the |
54310121 | 659 | program you're using. The F<Comm> library also has expect() |
660 | and interact() functions. Find the library (and we hope its | |
4633a7c4 | 661 | successor F<IPC::Chat>) at your nearest CPAN archive as detailed |
184e9718 | 662 | in the SEE ALSO section below. |
a0d0e21e | 663 | |
c8db1d39 TC |
664 | The newer Expect.pm module from CPAN also addresses this kind of thing. |
665 | This module requires two other modules from CPAN: IO::Pty and IO::Stty. | |
666 | It sets up a pseudo-terminal to interact with programs that insist on | |
a11adca0 | 667 | using talking to the terminal device driver. If your system is |
c8db1d39 TC |
668 | amongst those supported, this may be your best bet. |
669 | ||
5a964f20 TC |
670 | =head2 Bidirectional Communication with Yourself |
671 | ||
672 | If you want, you may make low-level pipe() and fork() | |
673 | to stitch this together by hand. This example only | |
674 | talks to itself, but you could reopen the appropriate | |
675 | handles to STDIN and STDOUT and call other processes. | |
676 | ||
677 | #!/usr/bin/perl -w | |
678 | # pipe1 - bidirectional communication using two pipe pairs | |
679 | # designed for the socketpair-challenged | |
680 | use IO::Handle; # thousands of lines just for autoflush :-( | |
681 | pipe(PARENT_RDR, CHILD_WTR); # XXX: failure? | |
682 | pipe(CHILD_RDR, PARENT_WTR); # XXX: failure? | |
683 | CHILD_WTR->autoflush(1); | |
684 | PARENT_WTR->autoflush(1); | |
685 | ||
686 | if ($pid = fork) { | |
687 | close PARENT_RDR; close PARENT_WTR; | |
688 | print CHILD_WTR "Parent Pid $$ is sending this\n"; | |
689 | chomp($line = <CHILD_RDR>); | |
690 | print "Parent Pid $$ just read this: `$line'\n"; | |
691 | close CHILD_RDR; close CHILD_WTR; | |
692 | waitpid($pid,0); | |
693 | } else { | |
694 | die "cannot fork: $!" unless defined $pid; | |
695 | close CHILD_RDR; close CHILD_WTR; | |
696 | chomp($line = <PARENT_RDR>); | |
697 | print "Child Pid $$ just read this: `$line'\n"; | |
698 | print PARENT_WTR "Child Pid $$ is sending this\n"; | |
699 | close PARENT_RDR; close PARENT_WTR; | |
700 | exit; | |
701 | } | |
702 | ||
a11adca0 | 703 | But you don't actually have to make two pipe calls. If you |
5a964f20 TC |
704 | have the socketpair() system call, it will do this all for you. |
705 | ||
706 | #!/usr/bin/perl -w | |
707 | # pipe2 - bidirectional communication using socketpair | |
708 | # "the best ones always go both ways" | |
709 | ||
710 | use Socket; | |
711 | use IO::Handle; # thousands of lines just for autoflush :-( | |
712 | # We say AF_UNIX because although *_LOCAL is the | |
713 | # POSIX 1003.1g form of the constant, many machines | |
714 | # still don't have it. | |
715 | socketpair(CHILD, PARENT, AF_UNIX, SOCK_STREAM, PF_UNSPEC) | |
716 | or die "socketpair: $!"; | |
717 | ||
718 | CHILD->autoflush(1); | |
719 | PARENT->autoflush(1); | |
720 | ||
721 | if ($pid = fork) { | |
722 | close PARENT; | |
723 | print CHILD "Parent Pid $$ is sending this\n"; | |
724 | chomp($line = <CHILD>); | |
725 | print "Parent Pid $$ just read this: `$line'\n"; | |
726 | close CHILD; | |
727 | waitpid($pid,0); | |
728 | } else { | |
729 | die "cannot fork: $!" unless defined $pid; | |
730 | close CHILD; | |
731 | chomp($line = <PARENT>); | |
732 | print "Child Pid $$ just read this: `$line'\n"; | |
733 | print PARENT "Child Pid $$ is sending this\n"; | |
734 | close PARENT; | |
735 | exit; | |
736 | } | |
737 | ||
4633a7c4 | 738 | =head1 Sockets: Client/Server Communication |
a0d0e21e | 739 | |
6a3992aa | 740 | While not limited to Unix-derived operating systems (e.g., WinSock on PCs |
4633a7c4 | 741 | provides socket support, as do some VMS libraries), you may not have |
184e9718 | 742 | sockets on your system, in which case this section probably isn't going to do |
6a3992aa DL |
743 | you much good. With sockets, you can do both virtual circuits (i.e., TCP |
744 | streams) and datagrams (i.e., UDP packets). You may be able to do even more | |
4633a7c4 LW |
745 | depending on your system. |
746 | ||
747 | The Perl function calls for dealing with sockets have the same names as | |
748 | the corresponding system calls in C, but their arguments tend to differ | |
749 | for two reasons: first, Perl filehandles work differently than C file | |
750 | descriptors. Second, Perl already knows the length of its strings, so you | |
751 | don't need to pass that information. | |
a0d0e21e | 752 | |
4633a7c4 LW |
753 | One of the major problems with old socket code in Perl was that it used |
754 | hard-coded values for some of the constants, which severely hurt | |
755 | portability. If you ever see code that does anything like explicitly | |
756 | setting C<$AF_INET = 2>, you know you're in for big trouble: An | |
757 | immeasurably superior approach is to use the C<Socket> module, which more | |
758 | reliably grants access to various constants and functions you'll need. | |
a0d0e21e | 759 | |
68dc0745 | 760 | If you're not writing a server/client for an existing protocol like |
761 | NNTP or SMTP, you should give some thought to how your server will | |
762 | know when the client has finished talking, and vice-versa. Most | |
763 | protocols are based on one-line messages and responses (so one party | |
4a6725af | 764 | knows the other has finished when a "\n" is received) or multi-line |
68dc0745 | 765 | messages and responses that end with a period on an empty line |
766 | ("\n.\n" terminates a message/response). | |
767 | ||
5a964f20 TC |
768 | =head2 Internet Line Terminators |
769 | ||
770 | The Internet line terminator is "\015\012". Under ASCII variants of | |
771 | Unix, that could usually be written as "\r\n", but under other systems, | |
772 | "\r\n" might at times be "\015\015\012", "\012\012\015", or something | |
773 | completely different. The standards specify writing "\015\012" to be | |
774 | conformant (be strict in what you provide), but they also recommend | |
775 | accepting a lone "\012" on input (but be lenient in what you require). | |
776 | We haven't always been very good about that in the code in this manpage, | |
777 | but unless you're on a Mac, you'll probably be ok. | |
778 | ||
4633a7c4 | 779 | =head2 Internet TCP Clients and Servers |
a0d0e21e | 780 | |
4633a7c4 LW |
781 | Use Internet-domain sockets when you want to do client-server |
782 | communication that might extend to machines outside of your own system. | |
783 | ||
784 | Here's a sample TCP client using Internet-domain sockets: | |
785 | ||
786 | #!/usr/bin/perl -w | |
4633a7c4 LW |
787 | use strict; |
788 | use Socket; | |
789 | my ($remote,$port, $iaddr, $paddr, $proto, $line); | |
790 | ||
791 | $remote = shift || 'localhost'; | |
792 | $port = shift || 2345; # random port | |
793 | if ($port =~ /\D/) { $port = getservbyname($port, 'tcp') } | |
794 | die "No port" unless $port; | |
795 | $iaddr = inet_aton($remote) || die "no host: $remote"; | |
796 | $paddr = sockaddr_in($port, $iaddr); | |
797 | ||
798 | $proto = getprotobyname('tcp'); | |
799 | socket(SOCK, PF_INET, SOCK_STREAM, $proto) || die "socket: $!"; | |
800 | connect(SOCK, $paddr) || die "connect: $!"; | |
54310121 | 801 | while (defined($line = <SOCK>)) { |
4633a7c4 | 802 | print $line; |
54310121 | 803 | } |
4633a7c4 LW |
804 | |
805 | close (SOCK) || die "close: $!"; | |
806 | exit; | |
807 | ||
808 | And here's a corresponding server to go along with it. We'll | |
809 | leave the address as INADDR_ANY so that the kernel can choose | |
54310121 | 810 | the appropriate interface on multihomed hosts. If you want sit |
c07a80fd | 811 | on a particular interface (like the external side of a gateway |
812 | or firewall machine), you should fill this in with your real address | |
813 | instead. | |
814 | ||
815 | #!/usr/bin/perl -Tw | |
c07a80fd | 816 | use strict; |
817 | BEGIN { $ENV{PATH} = '/usr/ucb:/bin' } | |
818 | use Socket; | |
819 | use Carp; | |
5865a7df | 820 | my $EOL = "\015\012"; |
c07a80fd | 821 | |
54310121 | 822 | sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" } |
c07a80fd | 823 | |
824 | my $port = shift || 2345; | |
825 | my $proto = getprotobyname('tcp'); | |
51ee6500 | 826 | |
5865a7df | 827 | ($port) = $port =~ /^(\d+)$/ or die "invalid port"; |
6a3992aa | 828 | |
c07a80fd | 829 | socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!"; |
54310121 | 830 | setsockopt(Server, SOL_SOCKET, SO_REUSEADDR, |
c07a80fd | 831 | pack("l", 1)) || die "setsockopt: $!"; |
832 | bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!"; | |
833 | listen(Server,SOMAXCONN) || die "listen: $!"; | |
834 | ||
835 | logmsg "server started on port $port"; | |
836 | ||
837 | my $paddr; | |
838 | ||
839 | $SIG{CHLD} = \&REAPER; | |
840 | ||
841 | for ( ; $paddr = accept(Client,Server); close Client) { | |
842 | my($port,$iaddr) = sockaddr_in($paddr); | |
843 | my $name = gethostbyaddr($iaddr,AF_INET); | |
844 | ||
54310121 | 845 | logmsg "connection from $name [", |
846 | inet_ntoa($iaddr), "] | |
c07a80fd | 847 | at port $port"; |
848 | ||
54310121 | 849 | print Client "Hello there, $name, it's now ", |
5a964f20 | 850 | scalar localtime, $EOL; |
54310121 | 851 | } |
c07a80fd | 852 | |
54310121 | 853 | And here's a multithreaded version. It's multithreaded in that |
854 | like most typical servers, it spawns (forks) a slave server to | |
c07a80fd | 855 | handle the client request so that the master server can quickly |
856 | go back to service a new client. | |
4633a7c4 LW |
857 | |
858 | #!/usr/bin/perl -Tw | |
4633a7c4 LW |
859 | use strict; |
860 | BEGIN { $ENV{PATH} = '/usr/ucb:/bin' } | |
a0d0e21e | 861 | use Socket; |
4633a7c4 | 862 | use Carp; |
5865a7df | 863 | my $EOL = "\015\012"; |
a0d0e21e | 864 | |
4633a7c4 | 865 | sub spawn; # forward declaration |
54310121 | 866 | sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" } |
a0d0e21e | 867 | |
4633a7c4 LW |
868 | my $port = shift || 2345; |
869 | my $proto = getprotobyname('tcp'); | |
51ee6500 | 870 | |
5865a7df | 871 | ($port) = $port =~ /^(\d+)$/ or die "invalid port"; |
54310121 | 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: $!"; | |
a0d0e21e | 878 | |
4633a7c4 | 879 | logmsg "server started on port $port"; |
a0d0e21e | 880 | |
4633a7c4 LW |
881 | my $waitedpid = 0; |
882 | my $paddr; | |
a0d0e21e | 883 | |
816229cf | 884 | use POSIX ":sys_wait_h"; |
54310121 | 885 | sub REAPER { |
816229cf NC |
886 | my $child; |
887 | while (($waitedpid = waitpid(-1,WNOHANG)) > 0) { | |
888 | logmsg "reaped $waitedpid" . ($? ? " with exit $?" : ''); | |
889 | } | |
6a3992aa | 890 | $SIG{CHLD} = \&REAPER; # loathe sysV |
4633a7c4 LW |
891 | } |
892 | ||
893 | $SIG{CHLD} = \&REAPER; | |
894 | ||
54310121 | 895 | for ( $waitedpid = 0; |
896 | ($paddr = accept(Client,Server)) || $waitedpid; | |
897 | $waitedpid = 0, close Client) | |
4633a7c4 | 898 | { |
6a3992aa | 899 | next if $waitedpid and not $paddr; |
4633a7c4 LW |
900 | my($port,$iaddr) = sockaddr_in($paddr); |
901 | my $name = gethostbyaddr($iaddr,AF_INET); | |
902 | ||
54310121 | 903 | logmsg "connection from $name [", |
904 | inet_ntoa($iaddr), "] | |
4633a7c4 | 905 | at port $port"; |
a0d0e21e | 906 | |
54310121 | 907 | spawn sub { |
b921b357 | 908 | $|=1; |
5a964f20 TC |
909 | print "Hello there, $name, it's now ", scalar localtime, $EOL; |
910 | exec '/usr/games/fortune' # XXX: `wrong' line terminators | |
4633a7c4 LW |
911 | or confess "can't exec fortune: $!"; |
912 | }; | |
a0d0e21e | 913 | |
54310121 | 914 | } |
a0d0e21e | 915 | |
4633a7c4 LW |
916 | sub spawn { |
917 | my $coderef = shift; | |
a0d0e21e | 918 | |
54310121 | 919 | unless (@_ == 0 && $coderef && ref($coderef) eq 'CODE') { |
4633a7c4 | 920 | confess "usage: spawn CODEREF"; |
a0d0e21e | 921 | } |
4633a7c4 LW |
922 | |
923 | my $pid; | |
924 | if (!defined($pid = fork)) { | |
925 | logmsg "cannot fork: $!"; | |
926 | return; | |
927 | } elsif ($pid) { | |
928 | logmsg "begat $pid"; | |
6a3992aa | 929 | return; # I'm the parent |
4633a7c4 | 930 | } |
6a3992aa | 931 | # else I'm the child -- go spawn |
4633a7c4 | 932 | |
c07a80fd | 933 | open(STDIN, "<&Client") || die "can't dup client to stdin"; |
934 | open(STDOUT, ">&Client") || die "can't dup client to stdout"; | |
4633a7c4 LW |
935 | ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr"; |
936 | exit &$coderef(); | |
54310121 | 937 | } |
4633a7c4 LW |
938 | |
939 | This server takes the trouble to clone off a child version via fork() for | |
940 | each incoming request. That way it can handle many requests at once, | |
941 | which you might not always want. Even if you don't fork(), the listen() | |
942 | will allow that many pending connections. Forking servers have to be | |
943 | particularly careful about cleaning up their dead children (called | |
944 | "zombies" in Unix parlance), because otherwise you'll quickly fill up your | |
945 | process table. | |
946 | ||
947 | We suggest that you use the B<-T> flag to use taint checking (see L<perlsec>) | |
948 | even if we aren't running setuid or setgid. This is always a good idea | |
949 | for servers and other programs run on behalf of someone else (like CGI | |
950 | scripts), because it lessens the chances that people from the outside will | |
951 | be able to compromise your system. | |
952 | ||
953 | Let's look at another TCP client. This one connects to the TCP "time" | |
954 | service on a number of different machines and shows how far their clocks | |
955 | differ from the system on which it's being run: | |
956 | ||
957 | #!/usr/bin/perl -w | |
4633a7c4 LW |
958 | use strict; |
959 | use Socket; | |
960 | ||
961 | my $SECS_of_70_YEARS = 2208988800; | |
54310121 | 962 | sub ctime { scalar localtime(shift) } |
4633a7c4 | 963 | |
54310121 | 964 | my $iaddr = gethostbyname('localhost'); |
965 | my $proto = getprotobyname('tcp'); | |
966 | my $port = getservbyname('time', 'tcp'); | |
4633a7c4 LW |
967 | my $paddr = sockaddr_in(0, $iaddr); |
968 | my($host); | |
969 | ||
970 | $| = 1; | |
971 | printf "%-24s %8s %s\n", "localhost", 0, ctime(time()); | |
972 | ||
973 | foreach $host (@ARGV) { | |
974 | printf "%-24s ", $host; | |
975 | my $hisiaddr = inet_aton($host) || die "unknown host"; | |
976 | my $hispaddr = sockaddr_in($port, $hisiaddr); | |
977 | socket(SOCKET, PF_INET, SOCK_STREAM, $proto) || die "socket: $!"; | |
978 | connect(SOCKET, $hispaddr) || die "bind: $!"; | |
979 | my $rtime = ' '; | |
980 | read(SOCKET, $rtime, 4); | |
981 | close(SOCKET); | |
982 | my $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ; | |
983 | printf "%8d %s\n", $histime - time, ctime($histime); | |
a0d0e21e LW |
984 | } |
985 | ||
4633a7c4 LW |
986 | =head2 Unix-Domain TCP Clients and Servers |
987 | ||
a2eb9003 | 988 | That's fine for Internet-domain clients and servers, but what about local |
4633a7c4 LW |
989 | communications? While you can use the same setup, sometimes you don't |
990 | want to. Unix-domain sockets are local to the current host, and are often | |
54310121 | 991 | used internally to implement pipes. Unlike Internet domain sockets, Unix |
4633a7c4 LW |
992 | domain sockets can show up in the file system with an ls(1) listing. |
993 | ||
5a964f20 | 994 | % ls -l /dev/log |
4633a7c4 | 995 | srw-rw-rw- 1 root 0 Oct 31 07:23 /dev/log |
a0d0e21e | 996 | |
4633a7c4 LW |
997 | You can test for these with Perl's B<-S> file test: |
998 | ||
999 | unless ( -S '/dev/log' ) { | |
3ba19564 | 1000 | die "something's wicked with the log system"; |
54310121 | 1001 | } |
4633a7c4 LW |
1002 | |
1003 | Here's a sample Unix-domain client: | |
1004 | ||
1005 | #!/usr/bin/perl -w | |
4633a7c4 LW |
1006 | use Socket; |
1007 | use strict; | |
1008 | my ($rendezvous, $line); | |
1009 | ||
1010 | $rendezvous = shift || '/tmp/catsock'; | |
1011 | socket(SOCK, PF_UNIX, SOCK_STREAM, 0) || die "socket: $!"; | |
9607fc9c | 1012 | connect(SOCK, sockaddr_un($rendezvous)) || die "connect: $!"; |
54310121 | 1013 | while (defined($line = <SOCK>)) { |
4633a7c4 | 1014 | print $line; |
54310121 | 1015 | } |
4633a7c4 LW |
1016 | exit; |
1017 | ||
5a964f20 TC |
1018 | And here's a corresponding server. You don't have to worry about silly |
1019 | network terminators here because Unix domain sockets are guaranteed | |
1020 | to be on the localhost, and thus everything works right. | |
4633a7c4 LW |
1021 | |
1022 | #!/usr/bin/perl -Tw | |
4633a7c4 LW |
1023 | use strict; |
1024 | use Socket; | |
1025 | use Carp; | |
1026 | ||
1027 | BEGIN { $ENV{PATH} = '/usr/ucb:/bin' } | |
5865a7df | 1028 | sub spawn; # forward declaration |
5a964f20 | 1029 | sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" } |
4633a7c4 LW |
1030 | |
1031 | my $NAME = '/tmp/catsock'; | |
1032 | my $uaddr = sockaddr_un($NAME); | |
1033 | my $proto = getprotobyname('tcp'); | |
1034 | ||
c07a80fd | 1035 | socket(Server,PF_UNIX,SOCK_STREAM,0) || die "socket: $!"; |
4633a7c4 | 1036 | unlink($NAME); |
c07a80fd | 1037 | bind (Server, $uaddr) || die "bind: $!"; |
1038 | listen(Server,SOMAXCONN) || die "listen: $!"; | |
4633a7c4 LW |
1039 | |
1040 | logmsg "server started on $NAME"; | |
1041 | ||
5a964f20 TC |
1042 | my $waitedpid; |
1043 | ||
816229cf | 1044 | use POSIX ":sys_wait_h"; |
5a964f20 | 1045 | sub REAPER { |
816229cf NC |
1046 | my $child; |
1047 | while (($waitedpid = waitpid(-1,WNOHANG)) > 0) { | |
1048 | logmsg "reaped $waitedpid" . ($? ? " with exit $?" : ''); | |
1049 | } | |
5a964f20 | 1050 | $SIG{CHLD} = \&REAPER; # loathe sysV |
5a964f20 TC |
1051 | } |
1052 | ||
4633a7c4 LW |
1053 | $SIG{CHLD} = \&REAPER; |
1054 | ||
5a964f20 | 1055 | |
54310121 | 1056 | for ( $waitedpid = 0; |
1057 | accept(Client,Server) || $waitedpid; | |
1058 | $waitedpid = 0, close Client) | |
4633a7c4 LW |
1059 | { |
1060 | next if $waitedpid; | |
1061 | logmsg "connection on $NAME"; | |
54310121 | 1062 | spawn sub { |
4633a7c4 LW |
1063 | print "Hello there, it's now ", scalar localtime, "\n"; |
1064 | exec '/usr/games/fortune' or die "can't exec fortune: $!"; | |
1065 | }; | |
54310121 | 1066 | } |
4633a7c4 | 1067 | |
5865a7df NC |
1068 | sub spawn { |
1069 | my $coderef = shift; | |
1070 | ||
1071 | unless (@_ == 0 && $coderef && ref($coderef) eq 'CODE') { | |
1072 | confess "usage: spawn CODEREF"; | |
1073 | } | |
1074 | ||
1075 | my $pid; | |
1076 | if (!defined($pid = fork)) { | |
1077 | logmsg "cannot fork: $!"; | |
1078 | return; | |
1079 | } elsif ($pid) { | |
1080 | logmsg "begat $pid"; | |
1081 | return; # I'm the parent | |
1082 | } | |
1083 | # else I'm the child -- go spawn | |
1084 | ||
1085 | open(STDIN, "<&Client") || die "can't dup client to stdin"; | |
1086 | open(STDOUT, ">&Client") || die "can't dup client to stdout"; | |
1087 | ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr"; | |
1088 | exit &$coderef(); | |
1089 | } | |
1090 | ||
4633a7c4 LW |
1091 | As you see, it's remarkably similar to the Internet domain TCP server, so |
1092 | much so, in fact, that we've omitted several duplicate functions--spawn(), | |
1093 | logmsg(), ctime(), and REAPER()--which are exactly the same as in the | |
1094 | other server. | |
1095 | ||
1096 | So why would you ever want to use a Unix domain socket instead of a | |
1097 | simpler named pipe? Because a named pipe doesn't give you sessions. You | |
1098 | can't tell one process's data from another's. With socket programming, | |
1099 | you get a separate session for each client: that's why accept() takes two | |
1100 | arguments. | |
1101 | ||
1102 | For example, let's say that you have a long running database server daemon | |
1103 | that you want folks from the World Wide Web to be able to access, but only | |
1104 | if they go through a CGI interface. You'd have a small, simple CGI | |
1105 | program that does whatever checks and logging you feel like, and then acts | |
1106 | as a Unix-domain client and connects to your private server. | |
1107 | ||
7b05b7e3 TC |
1108 | =head1 TCP Clients with IO::Socket |
1109 | ||
1110 | For those preferring a higher-level interface to socket programming, the | |
1111 | IO::Socket module provides an object-oriented approach. IO::Socket is | |
1112 | included as part of the standard Perl distribution as of the 5.004 | |
1113 | release. If you're running an earlier version of Perl, just fetch | |
106325ad | 1114 | IO::Socket from CPAN, where you'll also find modules providing easy |
7b05b7e3 TC |
1115 | interfaces to the following systems: DNS, FTP, Ident (RFC 931), NIS and |
1116 | NISPlus, NNTP, Ping, POP3, SMTP, SNMP, SSLeay, Telnet, and Time--just | |
1117 | to name a few. | |
1118 | ||
1119 | =head2 A Simple Client | |
1120 | ||
1121 | Here's a client that creates a TCP connection to the "daytime" | |
1122 | service at port 13 of the host name "localhost" and prints out everything | |
1123 | that the server there cares to provide. | |
1124 | ||
1125 | #!/usr/bin/perl -w | |
1126 | use IO::Socket; | |
1127 | $remote = IO::Socket::INET->new( | |
1128 | Proto => "tcp", | |
1129 | PeerAddr => "localhost", | |
1130 | PeerPort => "daytime(13)", | |
1131 | ) | |
1132 | or die "cannot connect to daytime port at localhost"; | |
1133 | while ( <$remote> ) { print } | |
1134 | ||
1135 | When you run this program, you should get something back that | |
1136 | looks like this: | |
1137 | ||
1138 | Wed May 14 08:40:46 MDT 1997 | |
1139 | ||
1140 | Here are what those parameters to the C<new> constructor mean: | |
1141 | ||
13a2d996 | 1142 | =over 4 |
7b05b7e3 TC |
1143 | |
1144 | =item C<Proto> | |
1145 | ||
1146 | This is which protocol to use. In this case, the socket handle returned | |
1147 | will be connected to a TCP socket, because we want a stream-oriented | |
1148 | connection, that is, one that acts pretty much like a plain old file. | |
1149 | Not all sockets are this of this type. For example, the UDP protocol | |
1150 | can be used to make a datagram socket, used for message-passing. | |
1151 | ||
1152 | =item C<PeerAddr> | |
1153 | ||
1154 | This is the name or Internet address of the remote host the server is | |
1155 | running on. We could have specified a longer name like C<"www.perl.com">, | |
1156 | or an address like C<"204.148.40.9">. For demonstration purposes, we've | |
1157 | used the special hostname C<"localhost">, which should always mean the | |
1158 | current machine you're running on. The corresponding Internet address | |
1159 | for localhost is C<"127.1">, if you'd rather use that. | |
1160 | ||
1161 | =item C<PeerPort> | |
1162 | ||
1163 | This is the service name or port number we'd like to connect to. | |
1164 | We could have gotten away with using just C<"daytime"> on systems with a | |
1165 | well-configured system services file,[FOOTNOTE: The system services file | |
1166 | is in I</etc/services> under Unix] but just in case, we've specified the | |
1167 | port number (13) in parentheses. Using just the number would also have | |
1168 | worked, but constant numbers make careful programmers nervous. | |
1169 | ||
1170 | =back | |
1171 | ||
1172 | Notice how the return value from the C<new> constructor is used as | |
1173 | a filehandle in the C<while> loop? That's what's called an indirect | |
1174 | filehandle, a scalar variable containing a filehandle. You can use | |
1175 | it the same way you would a normal filehandle. For example, you | |
1176 | can read one line from it this way: | |
1177 | ||
1178 | $line = <$handle>; | |
1179 | ||
1180 | all remaining lines from is this way: | |
1181 | ||
1182 | @lines = <$handle>; | |
1183 | ||
1184 | and send a line of data to it this way: | |
1185 | ||
1186 | print $handle "some data\n"; | |
1187 | ||
1188 | =head2 A Webget Client | |
1189 | ||
1190 | Here's a simple client that takes a remote host to fetch a document | |
1191 | from, and then a list of documents to get from that host. This is a | |
1192 | more interesting client than the previous one because it first sends | |
1193 | something to the server before fetching the server's response. | |
1194 | ||
1195 | #!/usr/bin/perl -w | |
1196 | use IO::Socket; | |
1197 | unless (@ARGV > 1) { die "usage: $0 host document ..." } | |
1198 | $host = shift(@ARGV); | |
5a964f20 TC |
1199 | $EOL = "\015\012"; |
1200 | $BLANK = $EOL x 2; | |
7b05b7e3 TC |
1201 | foreach $document ( @ARGV ) { |
1202 | $remote = IO::Socket::INET->new( Proto => "tcp", | |
1203 | PeerAddr => $host, | |
1204 | PeerPort => "http(80)", | |
1205 | ); | |
1206 | unless ($remote) { die "cannot connect to http daemon on $host" } | |
1207 | $remote->autoflush(1); | |
5a964f20 | 1208 | print $remote "GET $document HTTP/1.0" . $BLANK; |
7b05b7e3 TC |
1209 | while ( <$remote> ) { print } |
1210 | close $remote; | |
1211 | } | |
1212 | ||
1213 | The web server handing the "http" service, which is assumed to be at | |
4375e838 | 1214 | its standard port, number 80. If the web server you're trying to |
7b05b7e3 | 1215 | connect to is at a different port (like 1080 or 8080), you should specify |
c47ff5f1 | 1216 | as the named-parameter pair, C<< PeerPort => 8080 >>. The C<autoflush> |
7b05b7e3 TC |
1217 | method is used on the socket because otherwise the system would buffer |
1218 | up the output we sent it. (If you're on a Mac, you'll also need to | |
1219 | change every C<"\n"> in your code that sends data over the network to | |
1220 | be a C<"\015\012"> instead.) | |
1221 | ||
1222 | Connecting to the server is only the first part of the process: once you | |
1223 | have the connection, you have to use the server's language. Each server | |
1224 | on the network has its own little command language that it expects as | |
1225 | input. The string that we send to the server starting with "GET" is in | |
1226 | HTTP syntax. In this case, we simply request each specified document. | |
1227 | Yes, we really are making a new connection for each document, even though | |
1228 | it's the same host. That's the way you always used to have to speak HTTP. | |
1229 | Recent versions of web browsers may request that the remote server leave | |
1230 | the connection open a little while, but the server doesn't have to honor | |
1231 | such a request. | |
1232 | ||
1233 | Here's an example of running that program, which we'll call I<webget>: | |
1234 | ||
5a964f20 | 1235 | % webget www.perl.com /guanaco.html |
7b05b7e3 TC |
1236 | HTTP/1.1 404 File Not Found |
1237 | Date: Thu, 08 May 1997 18:02:32 GMT | |
1238 | Server: Apache/1.2b6 | |
1239 | Connection: close | |
1240 | Content-type: text/html | |
1241 | ||
1242 | <HEAD><TITLE>404 File Not Found</TITLE></HEAD> | |
1243 | <BODY><H1>File Not Found</H1> | |
1244 | The requested URL /guanaco.html was not found on this server.<P> | |
1245 | </BODY> | |
1246 | ||
1247 | Ok, so that's not very interesting, because it didn't find that | |
1248 | particular document. But a long response wouldn't have fit on this page. | |
1249 | ||
1250 | For a more fully-featured version of this program, you should look to | |
1251 | the I<lwp-request> program included with the LWP modules from CPAN. | |
1252 | ||
1253 | =head2 Interactive Client with IO::Socket | |
1254 | ||
1255 | Well, that's all fine if you want to send one command and get one answer, | |
1256 | but what about setting up something fully interactive, somewhat like | |
1257 | the way I<telnet> works? That way you can type a line, get the answer, | |
1258 | type a line, get the answer, etc. | |
1259 | ||
1260 | This client is more complicated than the two we've done so far, but if | |
1261 | you're on a system that supports the powerful C<fork> call, the solution | |
1262 | isn't that rough. Once you've made the connection to whatever service | |
1263 | you'd like to chat with, call C<fork> to clone your process. Each of | |
1264 | these two identical process has a very simple job to do: the parent | |
1265 | copies everything from the socket to standard output, while the child | |
1266 | simultaneously copies everything from standard input to the socket. | |
1267 | To accomplish the same thing using just one process would be I<much> | |
1268 | harder, because it's easier to code two processes to do one thing than it | |
1269 | is to code one process to do two things. (This keep-it-simple principle | |
5a964f20 TC |
1270 | a cornerstones of the Unix philosophy, and good software engineering as |
1271 | well, which is probably why it's spread to other systems.) | |
7b05b7e3 TC |
1272 | |
1273 | Here's the code: | |
1274 | ||
1275 | #!/usr/bin/perl -w | |
1276 | use strict; | |
1277 | use IO::Socket; | |
1278 | my ($host, $port, $kidpid, $handle, $line); | |
1279 | ||
1280 | unless (@ARGV == 2) { die "usage: $0 host port" } | |
1281 | ($host, $port) = @ARGV; | |
1282 | ||
1283 | # create a tcp connection to the specified host and port | |
1284 | $handle = IO::Socket::INET->new(Proto => "tcp", | |
1285 | PeerAddr => $host, | |
1286 | PeerPort => $port) | |
1287 | or die "can't connect to port $port on $host: $!"; | |
1288 | ||
1289 | $handle->autoflush(1); # so output gets there right away | |
1290 | print STDERR "[Connected to $host:$port]\n"; | |
1291 | ||
1292 | # split the program into two processes, identical twins | |
1293 | die "can't fork: $!" unless defined($kidpid = fork()); | |
1294 | ||
1295 | # the if{} block runs only in the parent process | |
1296 | if ($kidpid) { | |
1297 | # copy the socket to standard output | |
1298 | while (defined ($line = <$handle>)) { | |
1299 | print STDOUT $line; | |
1300 | } | |
1301 | kill("TERM", $kidpid); # send SIGTERM to child | |
1302 | } | |
1303 | # the else{} block runs only in the child process | |
1304 | else { | |
1305 | # copy standard input to the socket | |
1306 | while (defined ($line = <STDIN>)) { | |
1307 | print $handle $line; | |
1308 | } | |
1309 | } | |
1310 | ||
1311 | The C<kill> function in the parent's C<if> block is there to send a | |
1312 | signal to our child process (current running in the C<else> block) | |
1313 | as soon as the remote server has closed its end of the connection. | |
1314 | ||
7b05b7e3 TC |
1315 | If the remote server sends data a byte at time, and you need that |
1316 | data immediately without waiting for a newline (which might not happen), | |
1317 | you may wish to replace the C<while> loop in the parent with the | |
1318 | following: | |
1319 | ||
1320 | my $byte; | |
1321 | while (sysread($handle, $byte, 1) == 1) { | |
1322 | print STDOUT $byte; | |
1323 | } | |
1324 | ||
1325 | Making a system call for each byte you want to read is not very efficient | |
1326 | (to put it mildly) but is the simplest to explain and works reasonably | |
1327 | well. | |
1328 | ||
1329 | =head1 TCP Servers with IO::Socket | |
1330 | ||
5a964f20 | 1331 | As always, setting up a server is little bit more involved than running a client. |
7b05b7e3 TC |
1332 | The model is that the server creates a special kind of socket that |
1333 | does nothing but listen on a particular port for incoming connections. | |
c47ff5f1 | 1334 | It does this by calling the C<< IO::Socket::INET->new() >> method with |
7b05b7e3 TC |
1335 | slightly different arguments than the client did. |
1336 | ||
13a2d996 | 1337 | =over 4 |
7b05b7e3 TC |
1338 | |
1339 | =item Proto | |
1340 | ||
1341 | This is which protocol to use. Like our clients, we'll | |
1342 | still specify C<"tcp"> here. | |
1343 | ||
1344 | =item LocalPort | |
1345 | ||
1346 | We specify a local | |
1347 | port in the C<LocalPort> argument, which we didn't do for the client. | |
1348 | This is service name or port number for which you want to be the | |
1349 | server. (Under Unix, ports under 1024 are restricted to the | |
1350 | superuser.) In our sample, we'll use port 9000, but you can use | |
1351 | any port that's not currently in use on your system. If you try | |
1352 | to use one already in used, you'll get an "Address already in use" | |
19799a22 | 1353 | message. Under Unix, the C<netstat -a> command will show |
7b05b7e3 TC |
1354 | which services current have servers. |
1355 | ||
1356 | =item Listen | |
1357 | ||
1358 | The C<Listen> parameter is set to the maximum number of | |
1359 | pending connections we can accept until we turn away incoming clients. | |
1360 | Think of it as a call-waiting queue for your telephone. | |
1361 | The low-level Socket module has a special symbol for the system maximum, which | |
1362 | is SOMAXCONN. | |
1363 | ||
1364 | =item Reuse | |
1365 | ||
1366 | The C<Reuse> parameter is needed so that we restart our server | |
1367 | manually without waiting a few minutes to allow system buffers to | |
1368 | clear out. | |
1369 | ||
1370 | =back | |
1371 | ||
1372 | Once the generic server socket has been created using the parameters | |
1373 | listed above, the server then waits for a new client to connect | |
d1be9408 JF |
1374 | to it. The server blocks in the C<accept> method, which eventually accepts a |
1375 | bidirectional connection from the remote client. (Make sure to autoflush | |
7b05b7e3 TC |
1376 | this handle to circumvent buffering.) |
1377 | ||
1378 | To add to user-friendliness, our server prompts the user for commands. | |
1379 | Most servers don't do this. Because of the prompt without a newline, | |
1380 | you'll have to use the C<sysread> variant of the interactive client above. | |
1381 | ||
1382 | This server accepts one of five different commands, sending output | |
1383 | back to the client. Note that unlike most network servers, this one | |
1384 | only handles one incoming client at a time. Multithreaded servers are | |
f83494b9 | 1385 | covered in Chapter 6 of the Camel. |
7b05b7e3 TC |
1386 | |
1387 | Here's the code. We'll | |
1388 | ||
1389 | #!/usr/bin/perl -w | |
1390 | use IO::Socket; | |
1391 | use Net::hostent; # for OO version of gethostbyaddr | |
1392 | ||
1393 | $PORT = 9000; # pick something not in use | |
1394 | ||
1395 | $server = IO::Socket::INET->new( Proto => 'tcp', | |
1396 | LocalPort => $PORT, | |
1397 | Listen => SOMAXCONN, | |
1398 | Reuse => 1); | |
1399 | ||
1400 | die "can't setup server" unless $server; | |
1401 | print "[Server $0 accepting clients]\n"; | |
1402 | ||
1403 | while ($client = $server->accept()) { | |
1404 | $client->autoflush(1); | |
1405 | print $client "Welcome to $0; type help for command list.\n"; | |
1406 | $hostinfo = gethostbyaddr($client->peeraddr); | |
78fc38e1 | 1407 | printf "[Connect from %s]\n", $hostinfo ? $hostinfo->name : $client->peerhost; |
7b05b7e3 TC |
1408 | print $client "Command? "; |
1409 | while ( <$client>) { | |
1410 | next unless /\S/; # blank line | |
1411 | if (/quit|exit/i) { last; } | |
1412 | elsif (/date|time/i) { printf $client "%s\n", scalar localtime; } | |
1413 | elsif (/who/i ) { print $client `who 2>&1`; } | |
1414 | elsif (/cookie/i ) { print $client `/usr/games/fortune 2>&1`; } | |
1415 | elsif (/motd/i ) { print $client `cat /etc/motd 2>&1`; } | |
1416 | else { | |
1417 | print $client "Commands: quit date who cookie motd\n"; | |
1418 | } | |
1419 | } continue { | |
1420 | print $client "Command? "; | |
1421 | } | |
1422 | close $client; | |
1423 | } | |
1424 | ||
1425 | =head1 UDP: Message Passing | |
4633a7c4 LW |
1426 | |
1427 | Another kind of client-server setup is one that uses not connections, but | |
1428 | messages. UDP communications involve much lower overhead but also provide | |
1429 | less reliability, as there are no promises that messages will arrive at | |
1430 | all, let alone in order and unmangled. Still, UDP offers some advantages | |
1431 | over TCP, including being able to "broadcast" or "multicast" to a whole | |
1432 | bunch of destination hosts at once (usually on your local subnet). If you | |
1433 | find yourself overly concerned about reliability and start building checks | |
6a3992aa | 1434 | into your message system, then you probably should use just TCP to start |
4633a7c4 LW |
1435 | with. |
1436 | ||
90034919 LC |
1437 | Note that UDP datagrams are I<not> a bytestream and should not be treated |
1438 | as such. This makes using I/O mechanisms with internal buffering | |
1439 | like stdio (i.e. print() and friends) especially cumbersome. Use syswrite(), | |
1440 | or better send(), like in the example below. | |
1441 | ||
4633a7c4 | 1442 | Here's a UDP program similar to the sample Internet TCP client given |
7b05b7e3 | 1443 | earlier. However, instead of checking one host at a time, the UDP version |
4633a7c4 LW |
1444 | will check many of them asynchronously by simulating a multicast and then |
1445 | using select() to do a timed-out wait for I/O. To do something similar | |
1446 | with TCP, you'd have to use a different socket handle for each host. | |
1447 | ||
1448 | #!/usr/bin/perl -w | |
1449 | use strict; | |
4633a7c4 LW |
1450 | use Socket; |
1451 | use Sys::Hostname; | |
1452 | ||
54310121 | 1453 | my ( $count, $hisiaddr, $hispaddr, $histime, |
1454 | $host, $iaddr, $paddr, $port, $proto, | |
4633a7c4 LW |
1455 | $rin, $rout, $rtime, $SECS_of_70_YEARS); |
1456 | ||
1457 | $SECS_of_70_YEARS = 2208988800; | |
1458 | ||
1459 | $iaddr = gethostbyname(hostname()); | |
1460 | $proto = getprotobyname('udp'); | |
1461 | $port = getservbyname('time', 'udp'); | |
1462 | $paddr = sockaddr_in(0, $iaddr); # 0 means let kernel pick | |
1463 | ||
1464 | socket(SOCKET, PF_INET, SOCK_DGRAM, $proto) || die "socket: $!"; | |
1465 | bind(SOCKET, $paddr) || die "bind: $!"; | |
1466 | ||
1467 | $| = 1; | |
1468 | printf "%-12s %8s %s\n", "localhost", 0, scalar localtime time; | |
1469 | $count = 0; | |
1470 | for $host (@ARGV) { | |
1471 | $count++; | |
1472 | $hisiaddr = inet_aton($host) || die "unknown host"; | |
1473 | $hispaddr = sockaddr_in($port, $hisiaddr); | |
1474 | defined(send(SOCKET, 0, 0, $hispaddr)) || die "send $host: $!"; | |
1475 | } | |
1476 | ||
1477 | $rin = ''; | |
1478 | vec($rin, fileno(SOCKET), 1) = 1; | |
1479 | ||
1480 | # timeout after 10.0 seconds | |
1481 | while ($count && select($rout = $rin, undef, undef, 10.0)) { | |
1482 | $rtime = ''; | |
1483 | ($hispaddr = recv(SOCKET, $rtime, 4, 0)) || die "recv: $!"; | |
1484 | ($port, $hisiaddr) = sockaddr_in($hispaddr); | |
1485 | $host = gethostbyaddr($hisiaddr, AF_INET); | |
1486 | $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ; | |
1487 | printf "%-12s ", $host; | |
1488 | printf "%8d %s\n", $histime - time, scalar localtime($histime); | |
1489 | $count--; | |
1490 | } | |
1491 | ||
90034919 LC |
1492 | Note that this example does not include any retries and may consequently |
1493 | fail to contact a reachable host. The most prominent reason for this | |
1494 | is congestion of the queues on the sending host if the number of | |
a31a806a | 1495 | list of hosts to contact is sufficiently large. |
90034919 | 1496 | |
4633a7c4 LW |
1497 | =head1 SysV IPC |
1498 | ||
1499 | While System V IPC isn't so widely used as sockets, it still has some | |
1500 | interesting uses. You can't, however, effectively use SysV IPC or | |
1501 | Berkeley mmap() to have shared memory so as to share a variable amongst | |
1502 | several processes. That's because Perl would reallocate your string when | |
1503 | you weren't wanting it to. | |
1504 | ||
54310121 | 1505 | Here's a small example showing shared memory usage. |
a0d0e21e | 1506 | |
41d6edb2 | 1507 | use IPC::SysV qw(IPC_PRIVATE IPC_RMID S_IRWXU); |
0ade1984 | 1508 | |
a0d0e21e | 1509 | $size = 2000; |
41d6edb2 JH |
1510 | $id = shmget(IPC_PRIVATE, $size, S_IRWXU) || die "$!"; |
1511 | print "shm key $id\n"; | |
a0d0e21e LW |
1512 | |
1513 | $message = "Message #1"; | |
41d6edb2 | 1514 | shmwrite($id, $message, 0, 60) || die "$!"; |
0ade1984 | 1515 | print "wrote: '$message'\n"; |
41d6edb2 | 1516 | shmread($id, $buff, 0, 60) || die "$!"; |
0ade1984 | 1517 | print "read : '$buff'\n"; |
a0d0e21e | 1518 | |
0ade1984 JH |
1519 | # the buffer of shmread is zero-character end-padded. |
1520 | substr($buff, index($buff, "\0")) = ''; | |
1521 | print "un" unless $buff eq $message; | |
1522 | print "swell\n"; | |
a0d0e21e | 1523 | |
41d6edb2 JH |
1524 | print "deleting shm $id\n"; |
1525 | shmctl($id, IPC_RMID, 0) || die "$!"; | |
a0d0e21e LW |
1526 | |
1527 | Here's an example of a semaphore: | |
1528 | ||
0ade1984 JH |
1529 | use IPC::SysV qw(IPC_CREAT); |
1530 | ||
a0d0e21e | 1531 | $IPC_KEY = 1234; |
41d6edb2 JH |
1532 | $id = semget($IPC_KEY, 10, 0666 | IPC_CREAT ) || die "$!"; |
1533 | print "shm key $id\n"; | |
a0d0e21e | 1534 | |
a2eb9003 | 1535 | Put this code in a separate file to be run in more than one process. |
a0d0e21e LW |
1536 | Call the file F<take>: |
1537 | ||
1538 | # create a semaphore | |
1539 | ||
1540 | $IPC_KEY = 1234; | |
41d6edb2 JH |
1541 | $id = semget($IPC_KEY, 0 , 0 ); |
1542 | die if !defined($id); | |
a0d0e21e LW |
1543 | |
1544 | $semnum = 0; | |
1545 | $semflag = 0; | |
1546 | ||
1547 | # 'take' semaphore | |
1548 | # wait for semaphore to be zero | |
1549 | $semop = 0; | |
41d6edb2 | 1550 | $opstring1 = pack("s!s!s!", $semnum, $semop, $semflag); |
a0d0e21e LW |
1551 | |
1552 | # Increment the semaphore count | |
1553 | $semop = 1; | |
41d6edb2 | 1554 | $opstring2 = pack("s!s!s!", $semnum, $semop, $semflag); |
a0d0e21e LW |
1555 | $opstring = $opstring1 . $opstring2; |
1556 | ||
41d6edb2 | 1557 | semop($id,$opstring) || die "$!"; |
a0d0e21e | 1558 | |
a2eb9003 | 1559 | Put this code in a separate file to be run in more than one process. |
a0d0e21e LW |
1560 | Call this file F<give>: |
1561 | ||
4633a7c4 | 1562 | # 'give' the semaphore |
a0d0e21e LW |
1563 | # run this in the original process and you will see |
1564 | # that the second process continues | |
1565 | ||
1566 | $IPC_KEY = 1234; | |
41d6edb2 JH |
1567 | $id = semget($IPC_KEY, 0, 0); |
1568 | die if !defined($id); | |
a0d0e21e LW |
1569 | |
1570 | $semnum = 0; | |
1571 | $semflag = 0; | |
1572 | ||
1573 | # Decrement the semaphore count | |
1574 | $semop = -1; | |
41d6edb2 | 1575 | $opstring = pack("s!s!s!", $semnum, $semop, $semflag); |
a0d0e21e | 1576 | |
41d6edb2 | 1577 | semop($id,$opstring) || die "$!"; |
a0d0e21e | 1578 | |
7b05b7e3 | 1579 | The SysV IPC code above was written long ago, and it's definitely |
0ade1984 JH |
1580 | clunky looking. For a more modern look, see the IPC::SysV module |
1581 | which is included with Perl starting from Perl 5.005. | |
4633a7c4 | 1582 | |
41d6edb2 JH |
1583 | A small example demonstrating SysV message queues: |
1584 | ||
1585 | use IPC::SysV qw(IPC_PRIVATE IPC_RMID IPC_CREAT S_IRWXU); | |
1586 | ||
1587 | my $id = msgget(IPC_PRIVATE, IPC_CREAT | S_IRWXU); | |
1588 | ||
1589 | my $sent = "message"; | |
1590 | my $type = 1234; | |
1591 | my $rcvd; | |
1592 | my $type_rcvd; | |
1593 | ||
1594 | if (defined $id) { | |
1595 | if (msgsnd($id, pack("l! a*", $type_sent, $sent), 0)) { | |
1596 | if (msgrcv($id, $rcvd, 60, 0, 0)) { | |
1597 | ($type_rcvd, $rcvd) = unpack("l! a*", $rcvd); | |
1598 | if ($rcvd eq $sent) { | |
1599 | print "okay\n"; | |
1600 | } else { | |
1601 | print "not okay\n"; | |
1602 | } | |
1603 | } else { | |
1604 | die "# msgrcv failed\n"; | |
1605 | } | |
1606 | } else { | |
1607 | die "# msgsnd failed\n"; | |
1608 | } | |
1609 | msgctl($id, IPC_RMID, 0) || die "# msgctl failed: $!\n"; | |
1610 | } else { | |
1611 | die "# msgget failed\n"; | |
1612 | } | |
1613 | ||
4633a7c4 LW |
1614 | =head1 NOTES |
1615 | ||
5a964f20 TC |
1616 | Most of these routines quietly but politely return C<undef> when they |
1617 | fail instead of causing your program to die right then and there due to | |
1618 | an uncaught exception. (Actually, some of the new I<Socket> conversion | |
1619 | functions croak() on bad arguments.) It is therefore essential to | |
1620 | check return values from these functions. Always begin your socket | |
1621 | programs this way for optimal success, and don't forget to add B<-T> | |
1622 | taint checking flag to the #! line for servers: | |
4633a7c4 | 1623 | |
5a964f20 | 1624 | #!/usr/bin/perl -Tw |
4633a7c4 LW |
1625 | use strict; |
1626 | use sigtrap; | |
1627 | use Socket; | |
1628 | ||
1629 | =head1 BUGS | |
1630 | ||
1631 | All these routines create system-specific portability problems. As noted | |
1632 | elsewhere, Perl is at the mercy of your C libraries for much of its system | |
1633 | behaviour. It's probably safest to assume broken SysV semantics for | |
6a3992aa | 1634 | signals and to stick with simple TCP and UDP socket operations; e.g., don't |
a2eb9003 | 1635 | try to pass open file descriptors over a local UDP datagram socket if you |
4633a7c4 LW |
1636 | want your code to stand a chance of being portable. |
1637 | ||
5a964f20 TC |
1638 | As mentioned in the signals section, because few vendors provide C |
1639 | libraries that are safely re-entrant, the prudent programmer will do | |
1640 | little else within a handler beyond setting a numeric variable that | |
1641 | already exists; or, if locked into a slow (restarting) system call, | |
1642 | using die() to raise an exception and longjmp(3) out. In fact, even | |
1643 | these may in some cases cause a core dump. It's probably best to avoid | |
a11adca0 | 1644 | signals except where they are absolutely inevitable. This |
5a964f20 | 1645 | will be addressed in a future release of Perl. |
4633a7c4 LW |
1646 | |
1647 | =head1 AUTHOR | |
1648 | ||
1649 | Tom Christiansen, with occasional vestiges of Larry Wall's original | |
7b05b7e3 | 1650 | version and suggestions from the Perl Porters. |
4633a7c4 LW |
1651 | |
1652 | =head1 SEE ALSO | |
1653 | ||
7b05b7e3 TC |
1654 | There's a lot more to networking than this, but this should get you |
1655 | started. | |
1656 | ||
c04e1326 AL |
1657 | For intrepid programmers, the indispensable textbook is I<Unix |
1658 | Network Programming, 2nd Edition, Volume 1> by W. Richard Stevens | |
1659 | (published by Prentice-Hall). Note that most books on networking | |
1660 | address the subject from the perspective of a C programmer; translation | |
1661 | to Perl is left as an exercise for the reader. | |
7b05b7e3 TC |
1662 | |
1663 | The IO::Socket(3) manpage describes the object library, and the Socket(3) | |
1664 | manpage describes the low-level interface to sockets. Besides the obvious | |
1665 | functions in L<perlfunc>, you should also check out the F<modules> file | |
1666 | at your nearest CPAN site. (See L<perlmodlib> or best yet, the F<Perl | |
1667 | FAQ> for a description of what CPAN is and where to get it.) | |
1668 | ||
4633a7c4 | 1669 | Section 5 of the F<modules> file is devoted to "Networking, Device Control |
6a3992aa | 1670 | (modems), and Interprocess Communication", and contains numerous unbundled |
4633a7c4 LW |
1671 | modules numerous networking modules, Chat and Expect operations, CGI |
1672 | programming, DCE, FTP, IPC, NNTP, Proxy, Ptty, RPC, SNMP, SMTP, Telnet, | |
1673 | Threads, and ToolTalk--just to name a few. |