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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 | ||
13 | Perl uses a simple signal handling model: the %SIG hash contains names or | |
14 | references of user-installed signal handlers. These handlers will be called | |
15 | with an argument which is the name of the signal that triggered it. A | |
16 | signal may be generated intentionally from a particular keyboard sequence like | |
a2eb9003 | 17 | control-C or control-Z, sent to you from another process, or |
4633a7c4 | 18 | triggered automatically by the kernel when special events transpire, like |
54310121 | 19 | a child process exiting, your process running out of stack space, or |
4633a7c4 LW |
20 | hitting file size limit. |
21 | ||
22 | For example, to trap an interrupt signal, set up a handler like this. | |
7b05b7e3 TC |
23 | Do as little as you possibly can in your handler; notice how all we do is |
24 | set a global variable and then raise an exception. That's because on most | |
25 | systems, libraries are not re-entrant; particularly, memory allocation and | |
26 | I/O routines are not. That means that doing nearly I<anything> in your | |
27 | handler could in theory trigger a memory fault and subsequent core dump. | |
4633a7c4 LW |
28 | |
29 | sub catch_zap { | |
30 | my $signame = shift; | |
31 | $shucks++; | |
32 | die "Somebody sent me a SIG$signame"; | |
54310121 | 33 | } |
4633a7c4 LW |
34 | $SIG{INT} = 'catch_zap'; # could fail in modules |
35 | $SIG{INT} = \&catch_zap; # best strategy | |
36 | ||
37 | The names of the signals are the ones listed out by C<kill -l> on your | |
38 | system, or you can retrieve them from the Config module. Set up an | |
39 | @signame list indexed by number to get the name and a %signo table | |
40 | indexed by name to get the number: | |
41 | ||
42 | use Config; | |
43 | defined $Config{sig_name} || die "No sigs?"; | |
44 | foreach $name (split(' ', $Config{sig_name})) { | |
45 | $signo{$name} = $i; | |
46 | $signame[$i] = $name; | |
47 | $i++; | |
54310121 | 48 | } |
4633a7c4 | 49 | |
6a3992aa | 50 | So to check whether signal 17 and SIGALRM were the same, do just this: |
4633a7c4 LW |
51 | |
52 | print "signal #17 = $signame[17]\n"; | |
54310121 | 53 | if ($signo{ALRM}) { |
4633a7c4 | 54 | print "SIGALRM is $signo{ALRM}\n"; |
54310121 | 55 | } |
4633a7c4 LW |
56 | |
57 | You may also choose to assign the strings C<'IGNORE'> or C<'DEFAULT'> as | |
58 | the handler, in which case Perl will try to discard the signal or do the | |
f648820c GS |
59 | default thing. |
60 | ||
61 | On most UNIX platforms, the C<CHLD> (sometimes also known as C<CLD>) signal | |
62 | has special behavior with respect to a value of C<'IGNORE'>. | |
63 | Setting C<$SIG{CHLD}> to C<'IGNORE'> on such a platform has the effect of | |
64 | not creating zombie processes when the parent process fails to C<wait()> | |
65 | on its child processes (i.e. child processes are automatically reaped). | |
66 | Calling C<wait()> with C<$SIG{CHLD}> set to C<'IGNORE'> usually returns | |
67 | C<-1> on such platforms. | |
68 | ||
69 | Some signals can be neither trapped nor ignored, such as | |
4633a7c4 LW |
70 | the KILL and STOP (but not the TSTP) signals. One strategy for |
71 | temporarily ignoring signals is to use a local() statement, which will be | |
72 | automatically restored once your block is exited. (Remember that local() | |
73 | values are "inherited" by functions called from within that block.) | |
74 | ||
75 | sub precious { | |
76 | local $SIG{INT} = 'IGNORE'; | |
77 | &more_functions; | |
54310121 | 78 | } |
4633a7c4 LW |
79 | sub more_functions { |
80 | # interrupts still ignored, for now... | |
54310121 | 81 | } |
4633a7c4 LW |
82 | |
83 | Sending a signal to a negative process ID means that you send the signal | |
fb73857a | 84 | to the entire Unix process-group. This code sends a hang-up signal to all |
85 | processes in the current process group (and sets $SIG{HUP} to IGNORE so | |
86 | it doesn't kill itself): | |
4633a7c4 LW |
87 | |
88 | { | |
89 | local $SIG{HUP} = 'IGNORE'; | |
90 | kill HUP => -$$; | |
91 | # snazzy writing of: kill('HUP', -$$) | |
92 | } | |
a0d0e21e | 93 | |
4633a7c4 LW |
94 | Another interesting signal to send is signal number zero. This doesn't |
95 | actually affect another process, but instead checks whether it's alive | |
54310121 | 96 | or has changed its UID. |
a0d0e21e | 97 | |
4633a7c4 LW |
98 | unless (kill 0 => $kid_pid) { |
99 | warn "something wicked happened to $kid_pid"; | |
54310121 | 100 | } |
a0d0e21e | 101 | |
4633a7c4 LW |
102 | You might also want to employ anonymous functions for simple signal |
103 | handlers: | |
a0d0e21e | 104 | |
4633a7c4 | 105 | $SIG{INT} = sub { die "\nOutta here!\n" }; |
a0d0e21e | 106 | |
4633a7c4 | 107 | But that will be problematic for the more complicated handlers that need |
54310121 | 108 | to reinstall themselves. Because Perl's signal mechanism is currently |
184e9718 | 109 | based on the signal(3) function from the C library, you may sometimes be so |
4633a7c4 LW |
110 | misfortunate as to run on systems where that function is "broken", that |
111 | is, it behaves in the old unreliable SysV way rather than the newer, more | |
112 | reasonable BSD and POSIX fashion. So you'll see defensive people writing | |
113 | signal handlers like this: | |
a0d0e21e | 114 | |
54310121 | 115 | sub REAPER { |
4633a7c4 | 116 | $waitedpid = wait; |
6a3992aa DL |
117 | # loathe sysV: it makes us not only reinstate |
118 | # the handler, but place it after the wait | |
54310121 | 119 | $SIG{CHLD} = \&REAPER; |
4633a7c4 LW |
120 | } |
121 | $SIG{CHLD} = \&REAPER; | |
122 | # now do something that forks... | |
123 | ||
124 | or even the more elaborate: | |
125 | ||
6a3992aa | 126 | use POSIX ":sys_wait_h"; |
54310121 | 127 | sub REAPER { |
4633a7c4 | 128 | my $child; |
4633a7c4 LW |
129 | while ($child = waitpid(-1,WNOHANG)) { |
130 | $Kid_Status{$child} = $?; | |
54310121 | 131 | } |
6a3992aa | 132 | $SIG{CHLD} = \&REAPER; # still loathe sysV |
4633a7c4 LW |
133 | } |
134 | $SIG{CHLD} = \&REAPER; | |
135 | # do something that forks... | |
136 | ||
137 | Signal handling is also used for timeouts in Unix, While safely | |
138 | protected within an C<eval{}> block, you set a signal handler to trap | |
139 | alarm signals and then schedule to have one delivered to you in some | |
140 | number of seconds. Then try your blocking operation, clearing the alarm | |
141 | when it's done but not before you've exited your C<eval{}> block. If it | |
142 | goes off, you'll use die() to jump out of the block, much as you might | |
143 | using longjmp() or throw() in other languages. | |
144 | ||
145 | Here's an example: | |
146 | ||
54310121 | 147 | eval { |
4633a7c4 | 148 | local $SIG{ALRM} = sub { die "alarm clock restart" }; |
54310121 | 149 | alarm 10; |
4633a7c4 | 150 | flock(FH, 2); # blocking write lock |
54310121 | 151 | alarm 0; |
4633a7c4 LW |
152 | }; |
153 | if ($@ and $@ !~ /alarm clock restart/) { die } | |
154 | ||
155 | For more complex signal handling, you might see the standard POSIX | |
156 | module. Lamentably, this is almost entirely undocumented, but | |
157 | the F<t/lib/posix.t> file from the Perl source distribution has some | |
158 | examples in it. | |
159 | ||
160 | =head1 Named Pipes | |
161 | ||
162 | A named pipe (often referred to as a FIFO) is an old Unix IPC | |
163 | mechanism for processes communicating on the same machine. It works | |
54310121 | 164 | just like a regular, connected anonymous pipes, except that the |
4633a7c4 LW |
165 | processes rendezvous using a filename and don't have to be related. |
166 | ||
167 | To create a named pipe, use the Unix command mknod(1) or on some | |
168 | systems, mkfifo(1). These may not be in your normal path. | |
169 | ||
170 | # system return val is backwards, so && not || | |
171 | # | |
172 | $ENV{PATH} .= ":/etc:/usr/etc"; | |
54310121 | 173 | if ( system('mknod', $path, 'p') |
4633a7c4 LW |
174 | && system('mkfifo', $path) ) |
175 | { | |
5a964f20 | 176 | die "mk{nod,fifo} $path failed"; |
54310121 | 177 | } |
4633a7c4 LW |
178 | |
179 | ||
180 | A fifo is convenient when you want to connect a process to an unrelated | |
181 | one. When you open a fifo, the program will block until there's something | |
54310121 | 182 | on the other end. |
4633a7c4 LW |
183 | |
184 | For example, let's say you'd like to have your F<.signature> file be a | |
185 | named pipe that has a Perl program on the other end. Now every time any | |
6a3992aa | 186 | program (like a mailer, news reader, finger program, etc.) tries to read |
4633a7c4 | 187 | from that file, the reading program will block and your program will |
6a3992aa | 188 | supply the new signature. We'll use the pipe-checking file test B<-p> |
4633a7c4 LW |
189 | to find out whether anyone (or anything) has accidentally removed our fifo. |
190 | ||
191 | chdir; # go home | |
192 | $FIFO = '.signature'; | |
193 | $ENV{PATH} .= ":/etc:/usr/games"; | |
194 | ||
195 | while (1) { | |
196 | unless (-p $FIFO) { | |
197 | unlink $FIFO; | |
54310121 | 198 | system('mknod', $FIFO, 'p') |
4633a7c4 | 199 | && die "can't mknod $FIFO: $!"; |
54310121 | 200 | } |
4633a7c4 LW |
201 | |
202 | # next line blocks until there's a reader | |
203 | open (FIFO, "> $FIFO") || die "can't write $FIFO: $!"; | |
204 | print FIFO "John Smith (smith\@host.org)\n", `fortune -s`; | |
205 | close FIFO; | |
6a3992aa | 206 | sleep 2; # to avoid dup signals |
4633a7c4 | 207 | } |
a0d0e21e | 208 | |
5a964f20 TC |
209 | =head2 WARNING |
210 | ||
211 | By installing Perl code to deal with signals, you're exposing yourself | |
212 | to danger from two things. First, few system library functions are | |
213 | re-entrant. If the signal interrupts while Perl is executing one function | |
214 | (like malloc(3) or printf(3)), and your signal handler then calls the | |
215 | same function again, you could get unpredictable behavior--often, a | |
216 | core dump. Second, Perl isn't itself re-entrant at the lowest levels. | |
217 | If the signal interrupts Perl while Perl is changing its own internal | |
218 | data structures, similarly unpredictable behaviour may result. | |
219 | ||
220 | There are two things you can do, knowing this: be paranoid or be | |
221 | pragmatic. The paranoid approach is to do as little as possible in your | |
222 | signal handler. Set an existing integer variable that already has a | |
223 | value, and return. This doesn't help you if you're in a slow system call, | |
224 | which will just restart. That means you have to C<die> to longjump(3) out | |
225 | of the handler. Even this is a little cavalier for the true paranoiac, | |
226 | who avoids C<die> in a handler because the system I<is> out to get you. | |
227 | The pragmatic approach is to say ``I know the risks, but prefer the | |
228 | convenience'', and to do anything you want in your signal handler, | |
229 | prepared to clean up core dumps now and again. | |
230 | ||
231 | To forbid signal handlers altogether would bars you from | |
232 | many interesting programs, including virtually everything in this manpage, | |
233 | since you could no longer even write SIGCHLD handlers. Their dodginess | |
234 | is expected to be addresses in the 5.005 release. | |
235 | ||
a0d0e21e | 236 | |
4633a7c4 LW |
237 | =head1 Using open() for IPC |
238 | ||
239 | Perl's basic open() statement can also be used for unidirectional interprocess | |
240 | communication by either appending or prepending a pipe symbol to the second | |
a2eb9003 | 241 | argument to open(). Here's how to start something up in a child process you |
4633a7c4 LW |
242 | intend to write to: |
243 | ||
54310121 | 244 | open(SPOOLER, "| cat -v | lpr -h 2>/dev/null") |
4633a7c4 LW |
245 | || die "can't fork: $!"; |
246 | local $SIG{PIPE} = sub { die "spooler pipe broke" }; | |
247 | print SPOOLER "stuff\n"; | |
248 | close SPOOLER || die "bad spool: $! $?"; | |
249 | ||
250 | And here's how to start up a child process you intend to read from: | |
251 | ||
252 | open(STATUS, "netstat -an 2>&1 |") | |
253 | || die "can't fork: $!"; | |
254 | while (<STATUS>) { | |
255 | next if /^(tcp|udp)/; | |
256 | print; | |
54310121 | 257 | } |
a2eb9003 | 258 | close STATUS || die "bad netstat: $! $?"; |
4633a7c4 LW |
259 | |
260 | If one can be sure that a particular program is a Perl script that is | |
261 | expecting filenames in @ARGV, the clever programmer can write something | |
262 | like this: | |
263 | ||
5a964f20 | 264 | % program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile |
4633a7c4 LW |
265 | |
266 | and irrespective of which shell it's called from, the Perl program will | |
267 | read from the file F<f1>, the process F<cmd1>, standard input (F<tmpfile> | |
268 | in this case), the F<f2> file, the F<cmd2> command, and finally the F<f3> | |
269 | file. Pretty nifty, eh? | |
270 | ||
54310121 | 271 | You might notice that you could use backticks for much the |
4633a7c4 LW |
272 | same effect as opening a pipe for reading: |
273 | ||
274 | print grep { !/^(tcp|udp)/ } `netstat -an 2>&1`; | |
275 | die "bad netstat" if $?; | |
276 | ||
277 | While this is true on the surface, it's much more efficient to process the | |
278 | file one line or record at a time because then you don't have to read the | |
279 | whole thing into memory at once. It also gives you finer control of the | |
280 | whole process, letting you to kill off the child process early if you'd | |
281 | like. | |
282 | ||
283 | Be careful to check both the open() and the close() return values. If | |
284 | you're I<writing> to a pipe, you should also trap SIGPIPE. Otherwise, | |
285 | think of what happens when you start up a pipe to a command that doesn't | |
286 | exist: the open() will in all likelihood succeed (it only reflects the | |
287 | fork()'s success), but then your output will fail--spectacularly. Perl | |
288 | can't know whether the command worked because your command is actually | |
289 | running in a separate process whose exec() might have failed. Therefore, | |
6a3992aa | 290 | while readers of bogus commands return just a quick end of file, writers |
4633a7c4 LW |
291 | to bogus command will trigger a signal they'd better be prepared to |
292 | handle. Consider: | |
293 | ||
5a964f20 TC |
294 | open(FH, "|bogus") or die "can't fork: $!"; |
295 | print FH "bang\n" or die "can't write: $!"; | |
296 | close FH or die "can't close: $!"; | |
297 | ||
298 | That won't blow up until the close, and it will blow up with a SIGPIPE. | |
299 | To catch it, you could use this: | |
300 | ||
301 | $SIG{PIPE} = 'IGNORE'; | |
302 | open(FH, "|bogus") or die "can't fork: $!"; | |
303 | print FH "bang\n" or die "can't write: $!"; | |
304 | close FH or die "can't close: status=$?"; | |
4633a7c4 | 305 | |
68dc0745 | 306 | =head2 Filehandles |
307 | ||
5a964f20 TC |
308 | Both the main process and any child processes it forks share the same |
309 | STDIN, STDOUT, and STDERR filehandles. If both processes try to access | |
310 | them at once, strange things can happen. You'll certainly want to any | |
311 | stdio flush output buffers before forking. You may also want to close | |
312 | or reopen the filehandles for the child. You can get around this by | |
313 | opening your pipe with open(), but on some systems this means that the | |
314 | child process cannot outlive the parent. | |
68dc0745 | 315 | |
316 | =head2 Background Processes | |
317 | ||
318 | You can run a command in the background with: | |
319 | ||
7b05b7e3 | 320 | system("cmd &"); |
68dc0745 | 321 | |
322 | The command's STDOUT and STDERR (and possibly STDIN, depending on your | |
323 | shell) will be the same as the parent's. You won't need to catch | |
324 | SIGCHLD because of the double-fork taking place (see below for more | |
325 | details). | |
326 | ||
327 | =head2 Complete Dissociation of Child from Parent | |
328 | ||
329 | In some cases (starting server processes, for instance) you'll want to | |
893af57a RS |
330 | completely dissociate the child process from the parent. This is |
331 | often called daemonization. A well behaved daemon will also chdir() | |
332 | to the root directory (so it doesn't prevent unmounting the filesystem | |
333 | containing the directory from which it was launched) and redirect its | |
334 | standard file descriptors from and to F</dev/null> (so that random | |
335 | output doesn't wind up on the user's terminal). | |
336 | ||
337 | use POSIX 'setsid'; | |
338 | ||
339 | sub daemonize { | |
340 | chdir '/' or die "Can't chdir to /: $!"; | |
341 | open STDIN, '/dev/null' or die "Can't read /dev/null: $!"; | |
342 | open STDOUT, '>/dev/null' | |
343 | or die "Can't write to /dev/null: $!"; | |
344 | defined(my $pid = fork) or die "Can't fork: $!"; | |
345 | exit if $pid; | |
346 | setsid or die "Can't start a new session: $!"; | |
347 | open STDERR, '>&STDOUT' or die "Can't dup stdout: $!"; | |
348 | } | |
5a964f20 | 349 | |
893af57a RS |
350 | The fork() has to come before the setsid() to ensure that you aren't a |
351 | process group leader (the setsid() will fail if you are). If your | |
352 | system doesn't have the setsid() function, open F</dev/tty> and use the | |
353 | C<TIOCNOTTY> ioctl() on it instead. See L<tty(4)> for details. | |
5a964f20 | 354 | |
893af57a RS |
355 | Non-Unix users should check their Your_OS::Process module for other |
356 | solutions. | |
68dc0745 | 357 | |
4633a7c4 LW |
358 | =head2 Safe Pipe Opens |
359 | ||
360 | Another interesting approach to IPC is making your single program go | |
361 | multiprocess and communicate between (or even amongst) yourselves. The | |
362 | open() function will accept a file argument of either C<"-|"> or C<"|-"> | |
363 | to do a very interesting thing: it forks a child connected to the | |
364 | filehandle you've opened. The child is running the same program as the | |
365 | parent. This is useful for safely opening a file when running under an | |
366 | assumed UID or GID, for example. If you open a pipe I<to> minus, you can | |
367 | write to the filehandle you opened and your kid will find it in his | |
368 | STDIN. If you open a pipe I<from> minus, you can read from the filehandle | |
369 | you opened whatever your kid writes to his STDOUT. | |
370 | ||
371 | use English; | |
372 | my $sleep_count = 0; | |
373 | ||
54310121 | 374 | do { |
c07a80fd | 375 | $pid = open(KID_TO_WRITE, "|-"); |
4633a7c4 LW |
376 | unless (defined $pid) { |
377 | warn "cannot fork: $!"; | |
378 | die "bailing out" if $sleep_count++ > 6; | |
379 | sleep 10; | |
54310121 | 380 | } |
4633a7c4 LW |
381 | } until defined $pid; |
382 | ||
383 | if ($pid) { # parent | |
c07a80fd | 384 | print KID_TO_WRITE @some_data; |
385 | close(KID_TO_WRITE) || warn "kid exited $?"; | |
4633a7c4 LW |
386 | } else { # child |
387 | ($EUID, $EGID) = ($UID, $GID); # suid progs only | |
54310121 | 388 | open (FILE, "> /safe/file") |
4633a7c4 LW |
389 | || die "can't open /safe/file: $!"; |
390 | while (<STDIN>) { | |
391 | print FILE; # child's STDIN is parent's KID | |
54310121 | 392 | } |
4633a7c4 | 393 | exit; # don't forget this |
54310121 | 394 | } |
4633a7c4 LW |
395 | |
396 | Another common use for this construct is when you need to execute | |
397 | something without the shell's interference. With system(), it's | |
54310121 | 398 | straightforward, but you can't use a pipe open or backticks safely. |
4633a7c4 LW |
399 | That's because there's no way to stop the shell from getting its hands on |
400 | your arguments. Instead, use lower-level control to call exec() directly. | |
401 | ||
54310121 | 402 | Here's a safe backtick or pipe open for read: |
4633a7c4 LW |
403 | |
404 | # add error processing as above | |
c07a80fd | 405 | $pid = open(KID_TO_READ, "-|"); |
4633a7c4 LW |
406 | |
407 | if ($pid) { # parent | |
c07a80fd | 408 | while (<KID_TO_READ>) { |
4633a7c4 | 409 | # do something interesting |
54310121 | 410 | } |
c07a80fd | 411 | close(KID_TO_READ) || warn "kid exited $?"; |
4633a7c4 LW |
412 | |
413 | } else { # child | |
414 | ($EUID, $EGID) = ($UID, $GID); # suid only | |
415 | exec($program, @options, @args) | |
416 | || die "can't exec program: $!"; | |
417 | # NOTREACHED | |
54310121 | 418 | } |
4633a7c4 LW |
419 | |
420 | ||
421 | And here's a safe pipe open for writing: | |
422 | ||
423 | # add error processing as above | |
c07a80fd | 424 | $pid = open(KID_TO_WRITE, "|-"); |
4633a7c4 LW |
425 | $SIG{ALRM} = sub { die "whoops, $program pipe broke" }; |
426 | ||
427 | if ($pid) { # parent | |
428 | for (@data) { | |
c07a80fd | 429 | print KID_TO_WRITE; |
54310121 | 430 | } |
c07a80fd | 431 | close(KID_TO_WRITE) || warn "kid exited $?"; |
4633a7c4 LW |
432 | |
433 | } else { # child | |
434 | ($EUID, $EGID) = ($UID, $GID); | |
435 | exec($program, @options, @args) | |
436 | || die "can't exec program: $!"; | |
437 | # NOTREACHED | |
54310121 | 438 | } |
4633a7c4 LW |
439 | |
440 | Note that these operations are full Unix forks, which means they may not be | |
441 | correctly implemented on alien systems. Additionally, these are not true | |
54310121 | 442 | multithreading. If you'd like to learn more about threading, see the |
184e9718 | 443 | F<modules> file mentioned below in the SEE ALSO section. |
4633a7c4 | 444 | |
7b05b7e3 | 445 | =head2 Bidirectional Communication with Another Process |
4633a7c4 LW |
446 | |
447 | While this works reasonably well for unidirectional communication, what | |
448 | about bidirectional communication? The obvious thing you'd like to do | |
449 | doesn't actually work: | |
450 | ||
c07a80fd | 451 | open(PROG_FOR_READING_AND_WRITING, "| some program |") |
4633a7c4 | 452 | |
54310121 | 453 | and if you forget to use the B<-w> flag, then you'll miss out |
4633a7c4 LW |
454 | entirely on the diagnostic message: |
455 | ||
456 | Can't do bidirectional pipe at -e line 1. | |
457 | ||
458 | If you really want to, you can use the standard open2() library function | |
7b05b7e3 | 459 | to catch both ends. There's also an open3() for tridirectional I/O so you |
4633a7c4 LW |
460 | can also catch your child's STDERR, but doing so would then require an |
461 | awkward select() loop and wouldn't allow you to use normal Perl input | |
462 | operations. | |
463 | ||
464 | If you look at its source, you'll see that open2() uses low-level | |
5a964f20 | 465 | primitives like Unix pipe() and exec() calls to create all the connections. |
4633a7c4 LW |
466 | While it might have been slightly more efficient by using socketpair(), it |
467 | would have then been even less portable than it already is. The open2() | |
468 | and open3() functions are unlikely to work anywhere except on a Unix | |
469 | system or some other one purporting to be POSIX compliant. | |
470 | ||
471 | Here's an example of using open2(): | |
472 | ||
473 | use FileHandle; | |
474 | use IPC::Open2; | |
5a964f20 | 475 | $pid = open2(*Reader, *Writer, "cat -u -n" ); |
4633a7c4 LW |
476 | Writer->autoflush(); # default here, actually |
477 | print Writer "stuff\n"; | |
478 | $got = <Reader>; | |
479 | ||
6a3992aa DL |
480 | The problem with this is that Unix buffering is really going to |
481 | ruin your day. Even though your C<Writer> filehandle is auto-flushed, | |
4633a7c4 | 482 | and the process on the other end will get your data in a timely manner, |
6a3992aa | 483 | you can't usually do anything to force it to give it back to you |
54310121 | 484 | in a similarly quick fashion. In this case, we could, because we |
4633a7c4 LW |
485 | gave I<cat> a B<-u> flag to make it unbuffered. But very few Unix |
486 | commands are designed to operate over pipes, so this seldom works | |
54310121 | 487 | unless you yourself wrote the program on the other end of the |
4633a7c4 LW |
488 | double-ended pipe. |
489 | ||
54310121 | 490 | A solution to this is the nonstandard F<Comm.pl> library. It uses |
4633a7c4 LW |
491 | pseudo-ttys to make your program behave more reasonably: |
492 | ||
493 | require 'Comm.pl'; | |
494 | $ph = open_proc('cat -n'); | |
495 | for (1..10) { | |
496 | print $ph "a line\n"; | |
497 | print "got back ", scalar <$ph>; | |
498 | } | |
a0d0e21e | 499 | |
4633a7c4 | 500 | This way you don't have to have control over the source code of the |
54310121 | 501 | program you're using. The F<Comm> library also has expect() |
502 | and interact() functions. Find the library (and we hope its | |
4633a7c4 | 503 | successor F<IPC::Chat>) at your nearest CPAN archive as detailed |
184e9718 | 504 | in the SEE ALSO section below. |
a0d0e21e | 505 | |
c8db1d39 TC |
506 | The newer Expect.pm module from CPAN also addresses this kind of thing. |
507 | This module requires two other modules from CPAN: IO::Pty and IO::Stty. | |
508 | It sets up a pseudo-terminal to interact with programs that insist on | |
509 | using talking to the terminal device driver. If your system is | |
510 | amongst those supported, this may be your best bet. | |
511 | ||
5a964f20 TC |
512 | =head2 Bidirectional Communication with Yourself |
513 | ||
514 | If you want, you may make low-level pipe() and fork() | |
515 | to stitch this together by hand. This example only | |
516 | talks to itself, but you could reopen the appropriate | |
517 | handles to STDIN and STDOUT and call other processes. | |
518 | ||
519 | #!/usr/bin/perl -w | |
520 | # pipe1 - bidirectional communication using two pipe pairs | |
521 | # designed for the socketpair-challenged | |
522 | use IO::Handle; # thousands of lines just for autoflush :-( | |
523 | pipe(PARENT_RDR, CHILD_WTR); # XXX: failure? | |
524 | pipe(CHILD_RDR, PARENT_WTR); # XXX: failure? | |
525 | CHILD_WTR->autoflush(1); | |
526 | PARENT_WTR->autoflush(1); | |
527 | ||
528 | if ($pid = fork) { | |
529 | close PARENT_RDR; close PARENT_WTR; | |
530 | print CHILD_WTR "Parent Pid $$ is sending this\n"; | |
531 | chomp($line = <CHILD_RDR>); | |
532 | print "Parent Pid $$ just read this: `$line'\n"; | |
533 | close CHILD_RDR; close CHILD_WTR; | |
534 | waitpid($pid,0); | |
535 | } else { | |
536 | die "cannot fork: $!" unless defined $pid; | |
537 | close CHILD_RDR; close CHILD_WTR; | |
538 | chomp($line = <PARENT_RDR>); | |
539 | print "Child Pid $$ just read this: `$line'\n"; | |
540 | print PARENT_WTR "Child Pid $$ is sending this\n"; | |
541 | close PARENT_RDR; close PARENT_WTR; | |
542 | exit; | |
543 | } | |
544 | ||
545 | But you don't actually have to make two pipe calls. If you | |
546 | have the socketpair() system call, it will do this all for you. | |
547 | ||
548 | #!/usr/bin/perl -w | |
549 | # pipe2 - bidirectional communication using socketpair | |
550 | # "the best ones always go both ways" | |
551 | ||
552 | use Socket; | |
553 | use IO::Handle; # thousands of lines just for autoflush :-( | |
554 | # We say AF_UNIX because although *_LOCAL is the | |
555 | # POSIX 1003.1g form of the constant, many machines | |
556 | # still don't have it. | |
557 | socketpair(CHILD, PARENT, AF_UNIX, SOCK_STREAM, PF_UNSPEC) | |
558 | or die "socketpair: $!"; | |
559 | ||
560 | CHILD->autoflush(1); | |
561 | PARENT->autoflush(1); | |
562 | ||
563 | if ($pid = fork) { | |
564 | close PARENT; | |
565 | print CHILD "Parent Pid $$ is sending this\n"; | |
566 | chomp($line = <CHILD>); | |
567 | print "Parent Pid $$ just read this: `$line'\n"; | |
568 | close CHILD; | |
569 | waitpid($pid,0); | |
570 | } else { | |
571 | die "cannot fork: $!" unless defined $pid; | |
572 | close CHILD; | |
573 | chomp($line = <PARENT>); | |
574 | print "Child Pid $$ just read this: `$line'\n"; | |
575 | print PARENT "Child Pid $$ is sending this\n"; | |
576 | close PARENT; | |
577 | exit; | |
578 | } | |
579 | ||
4633a7c4 | 580 | =head1 Sockets: Client/Server Communication |
a0d0e21e | 581 | |
6a3992aa | 582 | While not limited to Unix-derived operating systems (e.g., WinSock on PCs |
4633a7c4 | 583 | provides socket support, as do some VMS libraries), you may not have |
184e9718 | 584 | sockets on your system, in which case this section probably isn't going to do |
6a3992aa DL |
585 | you much good. With sockets, you can do both virtual circuits (i.e., TCP |
586 | streams) and datagrams (i.e., UDP packets). You may be able to do even more | |
4633a7c4 LW |
587 | depending on your system. |
588 | ||
589 | The Perl function calls for dealing with sockets have the same names as | |
590 | the corresponding system calls in C, but their arguments tend to differ | |
591 | for two reasons: first, Perl filehandles work differently than C file | |
592 | descriptors. Second, Perl already knows the length of its strings, so you | |
593 | don't need to pass that information. | |
a0d0e21e | 594 | |
4633a7c4 LW |
595 | One of the major problems with old socket code in Perl was that it used |
596 | hard-coded values for some of the constants, which severely hurt | |
597 | portability. If you ever see code that does anything like explicitly | |
598 | setting C<$AF_INET = 2>, you know you're in for big trouble: An | |
599 | immeasurably superior approach is to use the C<Socket> module, which more | |
600 | reliably grants access to various constants and functions you'll need. | |
a0d0e21e | 601 | |
68dc0745 | 602 | If you're not writing a server/client for an existing protocol like |
603 | NNTP or SMTP, you should give some thought to how your server will | |
604 | know when the client has finished talking, and vice-versa. Most | |
605 | protocols are based on one-line messages and responses (so one party | |
4a6725af | 606 | knows the other has finished when a "\n" is received) or multi-line |
68dc0745 | 607 | messages and responses that end with a period on an empty line |
608 | ("\n.\n" terminates a message/response). | |
609 | ||
5a964f20 TC |
610 | =head2 Internet Line Terminators |
611 | ||
612 | The Internet line terminator is "\015\012". Under ASCII variants of | |
613 | Unix, that could usually be written as "\r\n", but under other systems, | |
614 | "\r\n" might at times be "\015\015\012", "\012\012\015", or something | |
615 | completely different. The standards specify writing "\015\012" to be | |
616 | conformant (be strict in what you provide), but they also recommend | |
617 | accepting a lone "\012" on input (but be lenient in what you require). | |
618 | We haven't always been very good about that in the code in this manpage, | |
619 | but unless you're on a Mac, you'll probably be ok. | |
620 | ||
4633a7c4 | 621 | =head2 Internet TCP Clients and Servers |
a0d0e21e | 622 | |
4633a7c4 LW |
623 | Use Internet-domain sockets when you want to do client-server |
624 | communication that might extend to machines outside of your own system. | |
625 | ||
626 | Here's a sample TCP client using Internet-domain sockets: | |
627 | ||
628 | #!/usr/bin/perl -w | |
4633a7c4 LW |
629 | use strict; |
630 | use Socket; | |
631 | my ($remote,$port, $iaddr, $paddr, $proto, $line); | |
632 | ||
633 | $remote = shift || 'localhost'; | |
634 | $port = shift || 2345; # random port | |
635 | if ($port =~ /\D/) { $port = getservbyname($port, 'tcp') } | |
636 | die "No port" unless $port; | |
637 | $iaddr = inet_aton($remote) || die "no host: $remote"; | |
638 | $paddr = sockaddr_in($port, $iaddr); | |
639 | ||
640 | $proto = getprotobyname('tcp'); | |
641 | socket(SOCK, PF_INET, SOCK_STREAM, $proto) || die "socket: $!"; | |
642 | connect(SOCK, $paddr) || die "connect: $!"; | |
54310121 | 643 | while (defined($line = <SOCK>)) { |
4633a7c4 | 644 | print $line; |
54310121 | 645 | } |
4633a7c4 LW |
646 | |
647 | close (SOCK) || die "close: $!"; | |
648 | exit; | |
649 | ||
650 | And here's a corresponding server to go along with it. We'll | |
651 | leave the address as INADDR_ANY so that the kernel can choose | |
54310121 | 652 | the appropriate interface on multihomed hosts. If you want sit |
c07a80fd | 653 | on a particular interface (like the external side of a gateway |
654 | or firewall machine), you should fill this in with your real address | |
655 | instead. | |
656 | ||
657 | #!/usr/bin/perl -Tw | |
c07a80fd | 658 | use strict; |
659 | BEGIN { $ENV{PATH} = '/usr/ucb:/bin' } | |
660 | use Socket; | |
661 | use Carp; | |
5a964f20 | 662 | $EOL = "\015\012"; |
c07a80fd | 663 | |
54310121 | 664 | sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" } |
c07a80fd | 665 | |
666 | my $port = shift || 2345; | |
667 | my $proto = getprotobyname('tcp'); | |
6a3992aa DL |
668 | $port = $1 if $port =~ /(\d+)/; # untaint port number |
669 | ||
c07a80fd | 670 | socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!"; |
54310121 | 671 | setsockopt(Server, SOL_SOCKET, SO_REUSEADDR, |
c07a80fd | 672 | pack("l", 1)) || die "setsockopt: $!"; |
673 | bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!"; | |
674 | listen(Server,SOMAXCONN) || die "listen: $!"; | |
675 | ||
676 | logmsg "server started on port $port"; | |
677 | ||
678 | my $paddr; | |
679 | ||
680 | $SIG{CHLD} = \&REAPER; | |
681 | ||
682 | for ( ; $paddr = accept(Client,Server); close Client) { | |
683 | my($port,$iaddr) = sockaddr_in($paddr); | |
684 | my $name = gethostbyaddr($iaddr,AF_INET); | |
685 | ||
54310121 | 686 | logmsg "connection from $name [", |
687 | inet_ntoa($iaddr), "] | |
c07a80fd | 688 | at port $port"; |
689 | ||
54310121 | 690 | print Client "Hello there, $name, it's now ", |
5a964f20 | 691 | scalar localtime, $EOL; |
54310121 | 692 | } |
c07a80fd | 693 | |
54310121 | 694 | And here's a multithreaded version. It's multithreaded in that |
695 | like most typical servers, it spawns (forks) a slave server to | |
c07a80fd | 696 | handle the client request so that the master server can quickly |
697 | go back to service a new client. | |
4633a7c4 LW |
698 | |
699 | #!/usr/bin/perl -Tw | |
4633a7c4 LW |
700 | use strict; |
701 | BEGIN { $ENV{PATH} = '/usr/ucb:/bin' } | |
a0d0e21e | 702 | use Socket; |
4633a7c4 | 703 | use Carp; |
5a964f20 | 704 | $EOL = "\015\012"; |
a0d0e21e | 705 | |
4633a7c4 | 706 | sub spawn; # forward declaration |
54310121 | 707 | sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" } |
a0d0e21e | 708 | |
4633a7c4 LW |
709 | my $port = shift || 2345; |
710 | my $proto = getprotobyname('tcp'); | |
80aa6872 | 711 | $port = $1 if $port =~ /(\d+)/; # untaint port number |
54310121 | 712 | |
c07a80fd | 713 | socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!"; |
54310121 | 714 | setsockopt(Server, SOL_SOCKET, SO_REUSEADDR, |
c07a80fd | 715 | pack("l", 1)) || die "setsockopt: $!"; |
716 | bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!"; | |
717 | listen(Server,SOMAXCONN) || die "listen: $!"; | |
a0d0e21e | 718 | |
4633a7c4 | 719 | logmsg "server started on port $port"; |
a0d0e21e | 720 | |
4633a7c4 LW |
721 | my $waitedpid = 0; |
722 | my $paddr; | |
a0d0e21e | 723 | |
54310121 | 724 | sub REAPER { |
4633a7c4 | 725 | $waitedpid = wait; |
6a3992aa | 726 | $SIG{CHLD} = \&REAPER; # loathe sysV |
4633a7c4 LW |
727 | logmsg "reaped $waitedpid" . ($? ? " with exit $?" : ''); |
728 | } | |
729 | ||
730 | $SIG{CHLD} = \&REAPER; | |
731 | ||
54310121 | 732 | for ( $waitedpid = 0; |
733 | ($paddr = accept(Client,Server)) || $waitedpid; | |
734 | $waitedpid = 0, close Client) | |
4633a7c4 | 735 | { |
6a3992aa | 736 | next if $waitedpid and not $paddr; |
4633a7c4 LW |
737 | my($port,$iaddr) = sockaddr_in($paddr); |
738 | my $name = gethostbyaddr($iaddr,AF_INET); | |
739 | ||
54310121 | 740 | logmsg "connection from $name [", |
741 | inet_ntoa($iaddr), "] | |
4633a7c4 | 742 | at port $port"; |
a0d0e21e | 743 | |
54310121 | 744 | spawn sub { |
5a964f20 TC |
745 | print "Hello there, $name, it's now ", scalar localtime, $EOL; |
746 | exec '/usr/games/fortune' # XXX: `wrong' line terminators | |
4633a7c4 LW |
747 | or confess "can't exec fortune: $!"; |
748 | }; | |
a0d0e21e | 749 | |
54310121 | 750 | } |
a0d0e21e | 751 | |
4633a7c4 LW |
752 | sub spawn { |
753 | my $coderef = shift; | |
a0d0e21e | 754 | |
54310121 | 755 | unless (@_ == 0 && $coderef && ref($coderef) eq 'CODE') { |
4633a7c4 | 756 | confess "usage: spawn CODEREF"; |
a0d0e21e | 757 | } |
4633a7c4 LW |
758 | |
759 | my $pid; | |
760 | if (!defined($pid = fork)) { | |
761 | logmsg "cannot fork: $!"; | |
762 | return; | |
763 | } elsif ($pid) { | |
764 | logmsg "begat $pid"; | |
6a3992aa | 765 | return; # I'm the parent |
4633a7c4 | 766 | } |
6a3992aa | 767 | # else I'm the child -- go spawn |
4633a7c4 | 768 | |
c07a80fd | 769 | open(STDIN, "<&Client") || die "can't dup client to stdin"; |
770 | open(STDOUT, ">&Client") || die "can't dup client to stdout"; | |
4633a7c4 LW |
771 | ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr"; |
772 | exit &$coderef(); | |
54310121 | 773 | } |
4633a7c4 LW |
774 | |
775 | This server takes the trouble to clone off a child version via fork() for | |
776 | each incoming request. That way it can handle many requests at once, | |
777 | which you might not always want. Even if you don't fork(), the listen() | |
778 | will allow that many pending connections. Forking servers have to be | |
779 | particularly careful about cleaning up their dead children (called | |
780 | "zombies" in Unix parlance), because otherwise you'll quickly fill up your | |
781 | process table. | |
782 | ||
783 | We suggest that you use the B<-T> flag to use taint checking (see L<perlsec>) | |
784 | even if we aren't running setuid or setgid. This is always a good idea | |
785 | for servers and other programs run on behalf of someone else (like CGI | |
786 | scripts), because it lessens the chances that people from the outside will | |
787 | be able to compromise your system. | |
788 | ||
789 | Let's look at another TCP client. This one connects to the TCP "time" | |
790 | service on a number of different machines and shows how far their clocks | |
791 | differ from the system on which it's being run: | |
792 | ||
793 | #!/usr/bin/perl -w | |
4633a7c4 LW |
794 | use strict; |
795 | use Socket; | |
796 | ||
797 | my $SECS_of_70_YEARS = 2208988800; | |
54310121 | 798 | sub ctime { scalar localtime(shift) } |
4633a7c4 | 799 | |
54310121 | 800 | my $iaddr = gethostbyname('localhost'); |
801 | my $proto = getprotobyname('tcp'); | |
802 | my $port = getservbyname('time', 'tcp'); | |
4633a7c4 LW |
803 | my $paddr = sockaddr_in(0, $iaddr); |
804 | my($host); | |
805 | ||
806 | $| = 1; | |
807 | printf "%-24s %8s %s\n", "localhost", 0, ctime(time()); | |
808 | ||
809 | foreach $host (@ARGV) { | |
810 | printf "%-24s ", $host; | |
811 | my $hisiaddr = inet_aton($host) || die "unknown host"; | |
812 | my $hispaddr = sockaddr_in($port, $hisiaddr); | |
813 | socket(SOCKET, PF_INET, SOCK_STREAM, $proto) || die "socket: $!"; | |
814 | connect(SOCKET, $hispaddr) || die "bind: $!"; | |
815 | my $rtime = ' '; | |
816 | read(SOCKET, $rtime, 4); | |
817 | close(SOCKET); | |
818 | my $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ; | |
819 | printf "%8d %s\n", $histime - time, ctime($histime); | |
a0d0e21e LW |
820 | } |
821 | ||
4633a7c4 LW |
822 | =head2 Unix-Domain TCP Clients and Servers |
823 | ||
a2eb9003 | 824 | That's fine for Internet-domain clients and servers, but what about local |
4633a7c4 LW |
825 | communications? While you can use the same setup, sometimes you don't |
826 | want to. Unix-domain sockets are local to the current host, and are often | |
54310121 | 827 | used internally to implement pipes. Unlike Internet domain sockets, Unix |
4633a7c4 LW |
828 | domain sockets can show up in the file system with an ls(1) listing. |
829 | ||
5a964f20 | 830 | % ls -l /dev/log |
4633a7c4 | 831 | srw-rw-rw- 1 root 0 Oct 31 07:23 /dev/log |
a0d0e21e | 832 | |
4633a7c4 LW |
833 | You can test for these with Perl's B<-S> file test: |
834 | ||
835 | unless ( -S '/dev/log' ) { | |
836 | die "something's wicked with the print system"; | |
54310121 | 837 | } |
4633a7c4 LW |
838 | |
839 | Here's a sample Unix-domain client: | |
840 | ||
841 | #!/usr/bin/perl -w | |
4633a7c4 LW |
842 | use Socket; |
843 | use strict; | |
844 | my ($rendezvous, $line); | |
845 | ||
846 | $rendezvous = shift || '/tmp/catsock'; | |
847 | socket(SOCK, PF_UNIX, SOCK_STREAM, 0) || die "socket: $!"; | |
9607fc9c | 848 | connect(SOCK, sockaddr_un($rendezvous)) || die "connect: $!"; |
54310121 | 849 | while (defined($line = <SOCK>)) { |
4633a7c4 | 850 | print $line; |
54310121 | 851 | } |
4633a7c4 LW |
852 | exit; |
853 | ||
5a964f20 TC |
854 | And here's a corresponding server. You don't have to worry about silly |
855 | network terminators here because Unix domain sockets are guaranteed | |
856 | to be on the localhost, and thus everything works right. | |
4633a7c4 LW |
857 | |
858 | #!/usr/bin/perl -Tw | |
4633a7c4 LW |
859 | use strict; |
860 | use Socket; | |
861 | use Carp; | |
862 | ||
863 | BEGIN { $ENV{PATH} = '/usr/ucb:/bin' } | |
5a964f20 | 864 | sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" } |
4633a7c4 LW |
865 | |
866 | my $NAME = '/tmp/catsock'; | |
867 | my $uaddr = sockaddr_un($NAME); | |
868 | my $proto = getprotobyname('tcp'); | |
869 | ||
c07a80fd | 870 | socket(Server,PF_UNIX,SOCK_STREAM,0) || die "socket: $!"; |
4633a7c4 | 871 | unlink($NAME); |
c07a80fd | 872 | bind (Server, $uaddr) || die "bind: $!"; |
873 | listen(Server,SOMAXCONN) || die "listen: $!"; | |
4633a7c4 LW |
874 | |
875 | logmsg "server started on $NAME"; | |
876 | ||
5a964f20 TC |
877 | my $waitedpid; |
878 | ||
879 | sub REAPER { | |
880 | $waitedpid = wait; | |
881 | $SIG{CHLD} = \&REAPER; # loathe sysV | |
882 | logmsg "reaped $waitedpid" . ($? ? " with exit $?" : ''); | |
883 | } | |
884 | ||
4633a7c4 LW |
885 | $SIG{CHLD} = \&REAPER; |
886 | ||
5a964f20 | 887 | |
54310121 | 888 | for ( $waitedpid = 0; |
889 | accept(Client,Server) || $waitedpid; | |
890 | $waitedpid = 0, close Client) | |
4633a7c4 LW |
891 | { |
892 | next if $waitedpid; | |
893 | logmsg "connection on $NAME"; | |
54310121 | 894 | spawn sub { |
4633a7c4 LW |
895 | print "Hello there, it's now ", scalar localtime, "\n"; |
896 | exec '/usr/games/fortune' or die "can't exec fortune: $!"; | |
897 | }; | |
54310121 | 898 | } |
4633a7c4 LW |
899 | |
900 | As you see, it's remarkably similar to the Internet domain TCP server, so | |
901 | much so, in fact, that we've omitted several duplicate functions--spawn(), | |
902 | logmsg(), ctime(), and REAPER()--which are exactly the same as in the | |
903 | other server. | |
904 | ||
905 | So why would you ever want to use a Unix domain socket instead of a | |
906 | simpler named pipe? Because a named pipe doesn't give you sessions. You | |
907 | can't tell one process's data from another's. With socket programming, | |
908 | you get a separate session for each client: that's why accept() takes two | |
909 | arguments. | |
910 | ||
911 | For example, let's say that you have a long running database server daemon | |
912 | that you want folks from the World Wide Web to be able to access, but only | |
913 | if they go through a CGI interface. You'd have a small, simple CGI | |
914 | program that does whatever checks and logging you feel like, and then acts | |
915 | as a Unix-domain client and connects to your private server. | |
916 | ||
7b05b7e3 TC |
917 | =head1 TCP Clients with IO::Socket |
918 | ||
919 | For those preferring a higher-level interface to socket programming, the | |
920 | IO::Socket module provides an object-oriented approach. IO::Socket is | |
921 | included as part of the standard Perl distribution as of the 5.004 | |
922 | release. If you're running an earlier version of Perl, just fetch | |
923 | IO::Socket from CPAN, where you'll also find find modules providing easy | |
924 | interfaces to the following systems: DNS, FTP, Ident (RFC 931), NIS and | |
925 | NISPlus, NNTP, Ping, POP3, SMTP, SNMP, SSLeay, Telnet, and Time--just | |
926 | to name a few. | |
927 | ||
928 | =head2 A Simple Client | |
929 | ||
930 | Here's a client that creates a TCP connection to the "daytime" | |
931 | service at port 13 of the host name "localhost" and prints out everything | |
932 | that the server there cares to provide. | |
933 | ||
934 | #!/usr/bin/perl -w | |
935 | use IO::Socket; | |
936 | $remote = IO::Socket::INET->new( | |
937 | Proto => "tcp", | |
938 | PeerAddr => "localhost", | |
939 | PeerPort => "daytime(13)", | |
940 | ) | |
941 | or die "cannot connect to daytime port at localhost"; | |
942 | while ( <$remote> ) { print } | |
943 | ||
944 | When you run this program, you should get something back that | |
945 | looks like this: | |
946 | ||
947 | Wed May 14 08:40:46 MDT 1997 | |
948 | ||
949 | Here are what those parameters to the C<new> constructor mean: | |
950 | ||
951 | =over | |
952 | ||
953 | =item C<Proto> | |
954 | ||
955 | This is which protocol to use. In this case, the socket handle returned | |
956 | will be connected to a TCP socket, because we want a stream-oriented | |
957 | connection, that is, one that acts pretty much like a plain old file. | |
958 | Not all sockets are this of this type. For example, the UDP protocol | |
959 | can be used to make a datagram socket, used for message-passing. | |
960 | ||
961 | =item C<PeerAddr> | |
962 | ||
963 | This is the name or Internet address of the remote host the server is | |
964 | running on. We could have specified a longer name like C<"www.perl.com">, | |
965 | or an address like C<"204.148.40.9">. For demonstration purposes, we've | |
966 | used the special hostname C<"localhost">, which should always mean the | |
967 | current machine you're running on. The corresponding Internet address | |
968 | for localhost is C<"127.1">, if you'd rather use that. | |
969 | ||
970 | =item C<PeerPort> | |
971 | ||
972 | This is the service name or port number we'd like to connect to. | |
973 | We could have gotten away with using just C<"daytime"> on systems with a | |
974 | well-configured system services file,[FOOTNOTE: The system services file | |
975 | is in I</etc/services> under Unix] but just in case, we've specified the | |
976 | port number (13) in parentheses. Using just the number would also have | |
977 | worked, but constant numbers make careful programmers nervous. | |
978 | ||
979 | =back | |
980 | ||
981 | Notice how the return value from the C<new> constructor is used as | |
982 | a filehandle in the C<while> loop? That's what's called an indirect | |
983 | filehandle, a scalar variable containing a filehandle. You can use | |
984 | it the same way you would a normal filehandle. For example, you | |
985 | can read one line from it this way: | |
986 | ||
987 | $line = <$handle>; | |
988 | ||
989 | all remaining lines from is this way: | |
990 | ||
991 | @lines = <$handle>; | |
992 | ||
993 | and send a line of data to it this way: | |
994 | ||
995 | print $handle "some data\n"; | |
996 | ||
997 | =head2 A Webget Client | |
998 | ||
999 | Here's a simple client that takes a remote host to fetch a document | |
1000 | from, and then a list of documents to get from that host. This is a | |
1001 | more interesting client than the previous one because it first sends | |
1002 | something to the server before fetching the server's response. | |
1003 | ||
1004 | #!/usr/bin/perl -w | |
1005 | use IO::Socket; | |
1006 | unless (@ARGV > 1) { die "usage: $0 host document ..." } | |
1007 | $host = shift(@ARGV); | |
5a964f20 TC |
1008 | $EOL = "\015\012"; |
1009 | $BLANK = $EOL x 2; | |
7b05b7e3 TC |
1010 | foreach $document ( @ARGV ) { |
1011 | $remote = IO::Socket::INET->new( Proto => "tcp", | |
1012 | PeerAddr => $host, | |
1013 | PeerPort => "http(80)", | |
1014 | ); | |
1015 | unless ($remote) { die "cannot connect to http daemon on $host" } | |
1016 | $remote->autoflush(1); | |
5a964f20 | 1017 | print $remote "GET $document HTTP/1.0" . $BLANK; |
7b05b7e3 TC |
1018 | while ( <$remote> ) { print } |
1019 | close $remote; | |
1020 | } | |
1021 | ||
1022 | The web server handing the "http" service, which is assumed to be at | |
1023 | its standard port, number 80. If your the web server you're trying to | |
1024 | connect to is at a different port (like 1080 or 8080), you should specify | |
1025 | as the named-parameter pair, C<PeerPort =E<gt> 8080>. The C<autoflush> | |
1026 | method is used on the socket because otherwise the system would buffer | |
1027 | up the output we sent it. (If you're on a Mac, you'll also need to | |
1028 | change every C<"\n"> in your code that sends data over the network to | |
1029 | be a C<"\015\012"> instead.) | |
1030 | ||
1031 | Connecting to the server is only the first part of the process: once you | |
1032 | have the connection, you have to use the server's language. Each server | |
1033 | on the network has its own little command language that it expects as | |
1034 | input. The string that we send to the server starting with "GET" is in | |
1035 | HTTP syntax. In this case, we simply request each specified document. | |
1036 | Yes, we really are making a new connection for each document, even though | |
1037 | it's the same host. That's the way you always used to have to speak HTTP. | |
1038 | Recent versions of web browsers may request that the remote server leave | |
1039 | the connection open a little while, but the server doesn't have to honor | |
1040 | such a request. | |
1041 | ||
1042 | Here's an example of running that program, which we'll call I<webget>: | |
1043 | ||
5a964f20 | 1044 | % webget www.perl.com /guanaco.html |
7b05b7e3 TC |
1045 | HTTP/1.1 404 File Not Found |
1046 | Date: Thu, 08 May 1997 18:02:32 GMT | |
1047 | Server: Apache/1.2b6 | |
1048 | Connection: close | |
1049 | Content-type: text/html | |
1050 | ||
1051 | <HEAD><TITLE>404 File Not Found</TITLE></HEAD> | |
1052 | <BODY><H1>File Not Found</H1> | |
1053 | The requested URL /guanaco.html was not found on this server.<P> | |
1054 | </BODY> | |
1055 | ||
1056 | Ok, so that's not very interesting, because it didn't find that | |
1057 | particular document. But a long response wouldn't have fit on this page. | |
1058 | ||
1059 | For a more fully-featured version of this program, you should look to | |
1060 | the I<lwp-request> program included with the LWP modules from CPAN. | |
1061 | ||
1062 | =head2 Interactive Client with IO::Socket | |
1063 | ||
1064 | Well, that's all fine if you want to send one command and get one answer, | |
1065 | but what about setting up something fully interactive, somewhat like | |
1066 | the way I<telnet> works? That way you can type a line, get the answer, | |
1067 | type a line, get the answer, etc. | |
1068 | ||
1069 | This client is more complicated than the two we've done so far, but if | |
1070 | you're on a system that supports the powerful C<fork> call, the solution | |
1071 | isn't that rough. Once you've made the connection to whatever service | |
1072 | you'd like to chat with, call C<fork> to clone your process. Each of | |
1073 | these two identical process has a very simple job to do: the parent | |
1074 | copies everything from the socket to standard output, while the child | |
1075 | simultaneously copies everything from standard input to the socket. | |
1076 | To accomplish the same thing using just one process would be I<much> | |
1077 | harder, because it's easier to code two processes to do one thing than it | |
1078 | is to code one process to do two things. (This keep-it-simple principle | |
5a964f20 TC |
1079 | a cornerstones of the Unix philosophy, and good software engineering as |
1080 | well, which is probably why it's spread to other systems.) | |
7b05b7e3 TC |
1081 | |
1082 | Here's the code: | |
1083 | ||
1084 | #!/usr/bin/perl -w | |
1085 | use strict; | |
1086 | use IO::Socket; | |
1087 | my ($host, $port, $kidpid, $handle, $line); | |
1088 | ||
1089 | unless (@ARGV == 2) { die "usage: $0 host port" } | |
1090 | ($host, $port) = @ARGV; | |
1091 | ||
1092 | # create a tcp connection to the specified host and port | |
1093 | $handle = IO::Socket::INET->new(Proto => "tcp", | |
1094 | PeerAddr => $host, | |
1095 | PeerPort => $port) | |
1096 | or die "can't connect to port $port on $host: $!"; | |
1097 | ||
1098 | $handle->autoflush(1); # so output gets there right away | |
1099 | print STDERR "[Connected to $host:$port]\n"; | |
1100 | ||
1101 | # split the program into two processes, identical twins | |
1102 | die "can't fork: $!" unless defined($kidpid = fork()); | |
1103 | ||
1104 | # the if{} block runs only in the parent process | |
1105 | if ($kidpid) { | |
1106 | # copy the socket to standard output | |
1107 | while (defined ($line = <$handle>)) { | |
1108 | print STDOUT $line; | |
1109 | } | |
1110 | kill("TERM", $kidpid); # send SIGTERM to child | |
1111 | } | |
1112 | # the else{} block runs only in the child process | |
1113 | else { | |
1114 | # copy standard input to the socket | |
1115 | while (defined ($line = <STDIN>)) { | |
1116 | print $handle $line; | |
1117 | } | |
1118 | } | |
1119 | ||
1120 | The C<kill> function in the parent's C<if> block is there to send a | |
1121 | signal to our child process (current running in the C<else> block) | |
1122 | as soon as the remote server has closed its end of the connection. | |
1123 | ||
7b05b7e3 TC |
1124 | If the remote server sends data a byte at time, and you need that |
1125 | data immediately without waiting for a newline (which might not happen), | |
1126 | you may wish to replace the C<while> loop in the parent with the | |
1127 | following: | |
1128 | ||
1129 | my $byte; | |
1130 | while (sysread($handle, $byte, 1) == 1) { | |
1131 | print STDOUT $byte; | |
1132 | } | |
1133 | ||
1134 | Making a system call for each byte you want to read is not very efficient | |
1135 | (to put it mildly) but is the simplest to explain and works reasonably | |
1136 | well. | |
1137 | ||
1138 | =head1 TCP Servers with IO::Socket | |
1139 | ||
5a964f20 | 1140 | As always, setting up a server is little bit more involved than running a client. |
7b05b7e3 TC |
1141 | The model is that the server creates a special kind of socket that |
1142 | does nothing but listen on a particular port for incoming connections. | |
1143 | It does this by calling the C<IO::Socket::INET-E<gt>new()> method with | |
1144 | slightly different arguments than the client did. | |
1145 | ||
1146 | =over | |
1147 | ||
1148 | =item Proto | |
1149 | ||
1150 | This is which protocol to use. Like our clients, we'll | |
1151 | still specify C<"tcp"> here. | |
1152 | ||
1153 | =item LocalPort | |
1154 | ||
1155 | We specify a local | |
1156 | port in the C<LocalPort> argument, which we didn't do for the client. | |
1157 | This is service name or port number for which you want to be the | |
1158 | server. (Under Unix, ports under 1024 are restricted to the | |
1159 | superuser.) In our sample, we'll use port 9000, but you can use | |
1160 | any port that's not currently in use on your system. If you try | |
1161 | to use one already in used, you'll get an "Address already in use" | |
1162 | message. Under Unix, the C<netstat -a> command will show | |
1163 | which services current have servers. | |
1164 | ||
1165 | =item Listen | |
1166 | ||
1167 | The C<Listen> parameter is set to the maximum number of | |
1168 | pending connections we can accept until we turn away incoming clients. | |
1169 | Think of it as a call-waiting queue for your telephone. | |
1170 | The low-level Socket module has a special symbol for the system maximum, which | |
1171 | is SOMAXCONN. | |
1172 | ||
1173 | =item Reuse | |
1174 | ||
1175 | The C<Reuse> parameter is needed so that we restart our server | |
1176 | manually without waiting a few minutes to allow system buffers to | |
1177 | clear out. | |
1178 | ||
1179 | =back | |
1180 | ||
1181 | Once the generic server socket has been created using the parameters | |
1182 | listed above, the server then waits for a new client to connect | |
1183 | to it. The server blocks in the C<accept> method, which eventually an | |
1184 | bidirectional connection to the remote client. (Make sure to autoflush | |
1185 | this handle to circumvent buffering.) | |
1186 | ||
1187 | To add to user-friendliness, our server prompts the user for commands. | |
1188 | Most servers don't do this. Because of the prompt without a newline, | |
1189 | you'll have to use the C<sysread> variant of the interactive client above. | |
1190 | ||
1191 | This server accepts one of five different commands, sending output | |
1192 | back to the client. Note that unlike most network servers, this one | |
1193 | only handles one incoming client at a time. Multithreaded servers are | |
7b8d334a | 1194 | covered in Chapter 6 of the Camel as well as later in this manpage. |
7b05b7e3 TC |
1195 | |
1196 | Here's the code. We'll | |
1197 | ||
1198 | #!/usr/bin/perl -w | |
1199 | use IO::Socket; | |
1200 | use Net::hostent; # for OO version of gethostbyaddr | |
1201 | ||
1202 | $PORT = 9000; # pick something not in use | |
1203 | ||
1204 | $server = IO::Socket::INET->new( Proto => 'tcp', | |
1205 | LocalPort => $PORT, | |
1206 | Listen => SOMAXCONN, | |
1207 | Reuse => 1); | |
1208 | ||
1209 | die "can't setup server" unless $server; | |
1210 | print "[Server $0 accepting clients]\n"; | |
1211 | ||
1212 | while ($client = $server->accept()) { | |
1213 | $client->autoflush(1); | |
1214 | print $client "Welcome to $0; type help for command list.\n"; | |
1215 | $hostinfo = gethostbyaddr($client->peeraddr); | |
1216 | printf "[Connect from %s]\n", $hostinfo->name || $client->peerhost; | |
1217 | print $client "Command? "; | |
1218 | while ( <$client>) { | |
1219 | next unless /\S/; # blank line | |
1220 | if (/quit|exit/i) { last; } | |
1221 | elsif (/date|time/i) { printf $client "%s\n", scalar localtime; } | |
1222 | elsif (/who/i ) { print $client `who 2>&1`; } | |
1223 | elsif (/cookie/i ) { print $client `/usr/games/fortune 2>&1`; } | |
1224 | elsif (/motd/i ) { print $client `cat /etc/motd 2>&1`; } | |
1225 | else { | |
1226 | print $client "Commands: quit date who cookie motd\n"; | |
1227 | } | |
1228 | } continue { | |
1229 | print $client "Command? "; | |
1230 | } | |
1231 | close $client; | |
1232 | } | |
1233 | ||
1234 | =head1 UDP: Message Passing | |
4633a7c4 LW |
1235 | |
1236 | Another kind of client-server setup is one that uses not connections, but | |
1237 | messages. UDP communications involve much lower overhead but also provide | |
1238 | less reliability, as there are no promises that messages will arrive at | |
1239 | all, let alone in order and unmangled. Still, UDP offers some advantages | |
1240 | over TCP, including being able to "broadcast" or "multicast" to a whole | |
1241 | bunch of destination hosts at once (usually on your local subnet). If you | |
1242 | find yourself overly concerned about reliability and start building checks | |
6a3992aa | 1243 | into your message system, then you probably should use just TCP to start |
4633a7c4 LW |
1244 | with. |
1245 | ||
1246 | Here's a UDP program similar to the sample Internet TCP client given | |
7b05b7e3 | 1247 | earlier. However, instead of checking one host at a time, the UDP version |
4633a7c4 LW |
1248 | will check many of them asynchronously by simulating a multicast and then |
1249 | using select() to do a timed-out wait for I/O. To do something similar | |
1250 | with TCP, you'd have to use a different socket handle for each host. | |
1251 | ||
1252 | #!/usr/bin/perl -w | |
1253 | use strict; | |
4633a7c4 LW |
1254 | use Socket; |
1255 | use Sys::Hostname; | |
1256 | ||
54310121 | 1257 | my ( $count, $hisiaddr, $hispaddr, $histime, |
1258 | $host, $iaddr, $paddr, $port, $proto, | |
4633a7c4 LW |
1259 | $rin, $rout, $rtime, $SECS_of_70_YEARS); |
1260 | ||
1261 | $SECS_of_70_YEARS = 2208988800; | |
1262 | ||
1263 | $iaddr = gethostbyname(hostname()); | |
1264 | $proto = getprotobyname('udp'); | |
1265 | $port = getservbyname('time', 'udp'); | |
1266 | $paddr = sockaddr_in(0, $iaddr); # 0 means let kernel pick | |
1267 | ||
1268 | socket(SOCKET, PF_INET, SOCK_DGRAM, $proto) || die "socket: $!"; | |
1269 | bind(SOCKET, $paddr) || die "bind: $!"; | |
1270 | ||
1271 | $| = 1; | |
1272 | printf "%-12s %8s %s\n", "localhost", 0, scalar localtime time; | |
1273 | $count = 0; | |
1274 | for $host (@ARGV) { | |
1275 | $count++; | |
1276 | $hisiaddr = inet_aton($host) || die "unknown host"; | |
1277 | $hispaddr = sockaddr_in($port, $hisiaddr); | |
1278 | defined(send(SOCKET, 0, 0, $hispaddr)) || die "send $host: $!"; | |
1279 | } | |
1280 | ||
1281 | $rin = ''; | |
1282 | vec($rin, fileno(SOCKET), 1) = 1; | |
1283 | ||
1284 | # timeout after 10.0 seconds | |
1285 | while ($count && select($rout = $rin, undef, undef, 10.0)) { | |
1286 | $rtime = ''; | |
1287 | ($hispaddr = recv(SOCKET, $rtime, 4, 0)) || die "recv: $!"; | |
1288 | ($port, $hisiaddr) = sockaddr_in($hispaddr); | |
1289 | $host = gethostbyaddr($hisiaddr, AF_INET); | |
1290 | $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ; | |
1291 | printf "%-12s ", $host; | |
1292 | printf "%8d %s\n", $histime - time, scalar localtime($histime); | |
1293 | $count--; | |
1294 | } | |
1295 | ||
1296 | =head1 SysV IPC | |
1297 | ||
1298 | While System V IPC isn't so widely used as sockets, it still has some | |
1299 | interesting uses. You can't, however, effectively use SysV IPC or | |
1300 | Berkeley mmap() to have shared memory so as to share a variable amongst | |
1301 | several processes. That's because Perl would reallocate your string when | |
1302 | you weren't wanting it to. | |
1303 | ||
54310121 | 1304 | Here's a small example showing shared memory usage. |
a0d0e21e | 1305 | |
0ade1984 JH |
1306 | use IPC::SysV qw(IPC_PRIVATE IPC_RMID S_IRWXU S_IRWXG S_IRWXO); |
1307 | ||
a0d0e21e | 1308 | $size = 2000; |
0ade1984 JH |
1309 | $key = shmget(IPC_PRIVATE, $size, S_IRWXU|S_IRWXG|S_IRWXO) || die "$!"; |
1310 | print "shm key $key\n"; | |
a0d0e21e LW |
1311 | |
1312 | $message = "Message #1"; | |
0ade1984 JH |
1313 | shmwrite($key, $message, 0, 60) || die "$!"; |
1314 | print "wrote: '$message'\n"; | |
1315 | shmread($key, $buff, 0, 60) || die "$!"; | |
1316 | print "read : '$buff'\n"; | |
a0d0e21e | 1317 | |
0ade1984 JH |
1318 | # the buffer of shmread is zero-character end-padded. |
1319 | substr($buff, index($buff, "\0")) = ''; | |
1320 | print "un" unless $buff eq $message; | |
1321 | print "swell\n"; | |
a0d0e21e | 1322 | |
0ade1984 JH |
1323 | print "deleting shm $key\n"; |
1324 | shmctl($key, IPC_RMID, 0) || die "$!"; | |
a0d0e21e LW |
1325 | |
1326 | Here's an example of a semaphore: | |
1327 | ||
0ade1984 JH |
1328 | use IPC::SysV qw(IPC_CREAT); |
1329 | ||
a0d0e21e | 1330 | $IPC_KEY = 1234; |
0ade1984 JH |
1331 | $key = semget($IPC_KEY, 10, 0666 | IPC_CREAT ) || die "$!"; |
1332 | print "shm key $key\n"; | |
a0d0e21e | 1333 | |
a2eb9003 | 1334 | Put this code in a separate file to be run in more than one process. |
a0d0e21e LW |
1335 | Call the file F<take>: |
1336 | ||
1337 | # create a semaphore | |
1338 | ||
1339 | $IPC_KEY = 1234; | |
1340 | $key = semget($IPC_KEY, 0 , 0 ); | |
1341 | die if !defined($key); | |
1342 | ||
1343 | $semnum = 0; | |
1344 | $semflag = 0; | |
1345 | ||
1346 | # 'take' semaphore | |
1347 | # wait for semaphore to be zero | |
1348 | $semop = 0; | |
1349 | $opstring1 = pack("sss", $semnum, $semop, $semflag); | |
1350 | ||
1351 | # Increment the semaphore count | |
1352 | $semop = 1; | |
1353 | $opstring2 = pack("sss", $semnum, $semop, $semflag); | |
1354 | $opstring = $opstring1 . $opstring2; | |
1355 | ||
1356 | semop($key,$opstring) || die "$!"; | |
1357 | ||
a2eb9003 | 1358 | Put this code in a separate file to be run in more than one process. |
a0d0e21e LW |
1359 | Call this file F<give>: |
1360 | ||
4633a7c4 | 1361 | # 'give' the semaphore |
a0d0e21e LW |
1362 | # run this in the original process and you will see |
1363 | # that the second process continues | |
1364 | ||
1365 | $IPC_KEY = 1234; | |
1366 | $key = semget($IPC_KEY, 0, 0); | |
1367 | die if !defined($key); | |
1368 | ||
1369 | $semnum = 0; | |
1370 | $semflag = 0; | |
1371 | ||
1372 | # Decrement the semaphore count | |
1373 | $semop = -1; | |
1374 | $opstring = pack("sss", $semnum, $semop, $semflag); | |
1375 | ||
1376 | semop($key,$opstring) || die "$!"; | |
1377 | ||
7b05b7e3 | 1378 | The SysV IPC code above was written long ago, and it's definitely |
0ade1984 JH |
1379 | clunky looking. For a more modern look, see the IPC::SysV module |
1380 | which is included with Perl starting from Perl 5.005. | |
4633a7c4 LW |
1381 | |
1382 | =head1 NOTES | |
1383 | ||
5a964f20 TC |
1384 | Most of these routines quietly but politely return C<undef> when they |
1385 | fail instead of causing your program to die right then and there due to | |
1386 | an uncaught exception. (Actually, some of the new I<Socket> conversion | |
1387 | functions croak() on bad arguments.) It is therefore essential to | |
1388 | check return values from these functions. Always begin your socket | |
1389 | programs this way for optimal success, and don't forget to add B<-T> | |
1390 | taint checking flag to the #! line for servers: | |
4633a7c4 | 1391 | |
5a964f20 | 1392 | #!/usr/bin/perl -Tw |
4633a7c4 LW |
1393 | use strict; |
1394 | use sigtrap; | |
1395 | use Socket; | |
1396 | ||
1397 | =head1 BUGS | |
1398 | ||
1399 | All these routines create system-specific portability problems. As noted | |
1400 | elsewhere, Perl is at the mercy of your C libraries for much of its system | |
1401 | behaviour. It's probably safest to assume broken SysV semantics for | |
6a3992aa | 1402 | signals and to stick with simple TCP and UDP socket operations; e.g., don't |
a2eb9003 | 1403 | try to pass open file descriptors over a local UDP datagram socket if you |
4633a7c4 LW |
1404 | want your code to stand a chance of being portable. |
1405 | ||
5a964f20 TC |
1406 | As mentioned in the signals section, because few vendors provide C |
1407 | libraries that are safely re-entrant, the prudent programmer will do | |
1408 | little else within a handler beyond setting a numeric variable that | |
1409 | already exists; or, if locked into a slow (restarting) system call, | |
1410 | using die() to raise an exception and longjmp(3) out. In fact, even | |
1411 | these may in some cases cause a core dump. It's probably best to avoid | |
1412 | signals except where they are absolutely inevitable. This | |
1413 | will be addressed in a future release of Perl. | |
4633a7c4 LW |
1414 | |
1415 | =head1 AUTHOR | |
1416 | ||
1417 | Tom Christiansen, with occasional vestiges of Larry Wall's original | |
7b05b7e3 | 1418 | version and suggestions from the Perl Porters. |
4633a7c4 LW |
1419 | |
1420 | =head1 SEE ALSO | |
1421 | ||
7b05b7e3 TC |
1422 | There's a lot more to networking than this, but this should get you |
1423 | started. | |
1424 | ||
5a964f20 TC |
1425 | For intrepid programmers, the indispensable textbook is I<Unix Network |
1426 | Programming> by W. Richard Stevens (published by Addison-Wesley). Note | |
1427 | that most books on networking address networking from the perspective of | |
1428 | a C programmer; translation to Perl is left as an exercise for the reader. | |
7b05b7e3 TC |
1429 | |
1430 | The IO::Socket(3) manpage describes the object library, and the Socket(3) | |
1431 | manpage describes the low-level interface to sockets. Besides the obvious | |
1432 | functions in L<perlfunc>, you should also check out the F<modules> file | |
1433 | at your nearest CPAN site. (See L<perlmodlib> or best yet, the F<Perl | |
1434 | FAQ> for a description of what CPAN is and where to get it.) | |
1435 | ||
4633a7c4 | 1436 | Section 5 of the F<modules> file is devoted to "Networking, Device Control |
6a3992aa | 1437 | (modems), and Interprocess Communication", and contains numerous unbundled |
4633a7c4 LW |
1438 | modules numerous networking modules, Chat and Expect operations, CGI |
1439 | programming, DCE, FTP, IPC, NNTP, Proxy, Ptty, RPC, SNMP, SMTP, Telnet, | |
1440 | Threads, and ToolTalk--just to name a few. |