Commit | Line | Data |
---|---|---|
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 $signo{ALRM}\n"; |
54310121 | 55 | } |
4633a7c4 LW |
56 | |
57 | You may also choose to assign the strings C<'IGNORE'> or C<'DEFAULT'> as | |
58 | the handler, in which case Perl will try to discard the signal or do the | |
f648820c GS |
59 | default thing. |
60 | ||
19799a22 | 61 | On most Unix platforms, the C<CHLD> (sometimes also known as C<CLD>) signal |
f648820c GS |
62 | has special behavior with respect to a value of C<'IGNORE'>. |
63 | Setting C<$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 $child; |
1450d070 | 129 | while (($child = waitpid(-1,WNOHANG)) > 0) { |
4633a7c4 | 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 | ||
8a4f6ac2 GS |
155 | If the operation being timed out is system() or qx(), this technique |
156 | is liable to generate zombies. If this matters to you, you'll | |
157 | need to do your own fork() and exec(), and kill the errant child process. | |
158 | ||
4633a7c4 LW |
159 | For more complex signal handling, you might see the standard POSIX |
160 | module. Lamentably, this is almost entirely undocumented, but | |
161 | the F<t/lib/posix.t> file from the Perl source distribution has some | |
162 | examples in it. | |
163 | ||
164 | =head1 Named Pipes | |
165 | ||
166 | A named pipe (often referred to as a FIFO) is an old Unix IPC | |
167 | mechanism for processes communicating on the same machine. It works | |
54310121 | 168 | just like a regular, connected anonymous pipes, except that the |
4633a7c4 LW |
169 | processes rendezvous using a filename and don't have to be related. |
170 | ||
171 | To create a named pipe, use the Unix command mknod(1) or on some | |
172 | systems, mkfifo(1). These may not be in your normal path. | |
173 | ||
174 | # system return val is backwards, so && not || | |
175 | # | |
176 | $ENV{PATH} .= ":/etc:/usr/etc"; | |
54310121 | 177 | if ( system('mknod', $path, 'p') |
4633a7c4 LW |
178 | && system('mkfifo', $path) ) |
179 | { | |
5a964f20 | 180 | die "mk{nod,fifo} $path failed"; |
54310121 | 181 | } |
4633a7c4 LW |
182 | |
183 | ||
184 | A fifo is convenient when you want to connect a process to an unrelated | |
185 | one. When you open a fifo, the program will block until there's something | |
54310121 | 186 | on the other end. |
4633a7c4 LW |
187 | |
188 | For example, let's say you'd like to have your F<.signature> file be a | |
189 | named pipe that has a Perl program on the other end. Now every time any | |
6a3992aa | 190 | program (like a mailer, news reader, finger program, etc.) tries to read |
4633a7c4 | 191 | from that file, the reading program will block and your program will |
6a3992aa | 192 | supply the new signature. We'll use the pipe-checking file test B<-p> |
4633a7c4 LW |
193 | to find out whether anyone (or anything) has accidentally removed our fifo. |
194 | ||
195 | chdir; # go home | |
196 | $FIFO = '.signature'; | |
197 | $ENV{PATH} .= ":/etc:/usr/games"; | |
198 | ||
199 | while (1) { | |
200 | unless (-p $FIFO) { | |
201 | unlink $FIFO; | |
54310121 | 202 | system('mknod', $FIFO, 'p') |
4633a7c4 | 203 | && die "can't mknod $FIFO: $!"; |
54310121 | 204 | } |
4633a7c4 LW |
205 | |
206 | # next line blocks until there's a reader | |
207 | open (FIFO, "> $FIFO") || die "can't write $FIFO: $!"; | |
208 | print FIFO "John Smith (smith\@host.org)\n", `fortune -s`; | |
209 | close FIFO; | |
6a3992aa | 210 | sleep 2; # to avoid dup signals |
4633a7c4 | 211 | } |
a0d0e21e | 212 | |
5a964f20 TC |
213 | =head2 WARNING |
214 | ||
215 | By installing Perl code to deal with signals, you're exposing yourself | |
216 | to danger from two things. First, few system library functions are | |
217 | re-entrant. If the signal interrupts while Perl is executing one function | |
218 | (like malloc(3) or printf(3)), and your signal handler then calls the | |
219 | same function again, you could get unpredictable behavior--often, a | |
220 | core dump. Second, Perl isn't itself re-entrant at the lowest levels. | |
221 | If the signal interrupts Perl while Perl is changing its own internal | |
222 | data structures, similarly unpredictable behaviour may result. | |
223 | ||
224 | There are two things you can do, knowing this: be paranoid or be | |
225 | pragmatic. The paranoid approach is to do as little as possible in your | |
226 | signal handler. Set an existing integer variable that already has a | |
227 | value, and return. This doesn't help you if you're in a slow system call, | |
228 | which will just restart. That means you have to C<die> to longjump(3) out | |
229 | of the handler. Even this is a little cavalier for the true paranoiac, | |
230 | who avoids C<die> in a handler because the system I<is> out to get you. | |
231 | The pragmatic approach is to say ``I know the risks, but prefer the | |
232 | convenience'', and to do anything you want in your signal handler, | |
233 | prepared to clean up core dumps now and again. | |
234 | ||
235 | To forbid signal handlers altogether would bars you from | |
236 | many interesting programs, including virtually everything in this manpage, | |
f783e543 | 237 | since you could no longer even write SIGCHLD handlers. |
5a964f20 | 238 | |
a0d0e21e | 239 | |
4633a7c4 LW |
240 | =head1 Using open() for IPC |
241 | ||
242 | Perl's basic open() statement can also be used for unidirectional interprocess | |
243 | communication by either appending or prepending a pipe symbol to the second | |
a2eb9003 | 244 | argument to open(). Here's how to start something up in a child process you |
4633a7c4 LW |
245 | intend to write to: |
246 | ||
54310121 | 247 | open(SPOOLER, "| cat -v | lpr -h 2>/dev/null") |
4633a7c4 LW |
248 | || die "can't fork: $!"; |
249 | local $SIG{PIPE} = sub { die "spooler pipe broke" }; | |
250 | print SPOOLER "stuff\n"; | |
251 | close SPOOLER || die "bad spool: $! $?"; | |
252 | ||
253 | And here's how to start up a child process you intend to read from: | |
254 | ||
255 | open(STATUS, "netstat -an 2>&1 |") | |
256 | || die "can't fork: $!"; | |
257 | while (<STATUS>) { | |
258 | next if /^(tcp|udp)/; | |
259 | print; | |
54310121 | 260 | } |
a2eb9003 | 261 | close STATUS || die "bad netstat: $! $?"; |
4633a7c4 LW |
262 | |
263 | If one can be sure that a particular program is a Perl script that is | |
264 | expecting filenames in @ARGV, the clever programmer can write something | |
265 | like this: | |
266 | ||
5a964f20 | 267 | % program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile |
4633a7c4 LW |
268 | |
269 | and irrespective of which shell it's called from, the Perl program will | |
270 | read from the file F<f1>, the process F<cmd1>, standard input (F<tmpfile> | |
271 | in this case), the F<f2> file, the F<cmd2> command, and finally the F<f3> | |
272 | file. Pretty nifty, eh? | |
273 | ||
54310121 | 274 | You might notice that you could use backticks for much the |
4633a7c4 LW |
275 | same effect as opening a pipe for reading: |
276 | ||
277 | print grep { !/^(tcp|udp)/ } `netstat -an 2>&1`; | |
278 | die "bad netstat" if $?; | |
279 | ||
280 | While this is true on the surface, it's much more efficient to process the | |
281 | file one line or record at a time because then you don't have to read the | |
19799a22 | 282 | whole thing into memory at once. It also gives you finer control of the |
4633a7c4 LW |
283 | whole process, letting you to kill off the child process early if you'd |
284 | like. | |
285 | ||
286 | Be careful to check both the open() and the close() return values. If | |
287 | you're I<writing> to a pipe, you should also trap SIGPIPE. Otherwise, | |
288 | think of what happens when you start up a pipe to a command that doesn't | |
289 | exist: the open() will in all likelihood succeed (it only reflects the | |
290 | fork()'s success), but then your output will fail--spectacularly. Perl | |
291 | can't know whether the command worked because your command is actually | |
292 | running in a separate process whose exec() might have failed. Therefore, | |
6a3992aa | 293 | while readers of bogus commands return just a quick end of file, writers |
4633a7c4 LW |
294 | to bogus command will trigger a signal they'd better be prepared to |
295 | handle. Consider: | |
296 | ||
5a964f20 TC |
297 | open(FH, "|bogus") or die "can't fork: $!"; |
298 | print FH "bang\n" or die "can't write: $!"; | |
299 | close FH or die "can't close: $!"; | |
300 | ||
301 | That won't blow up until the close, and it will blow up with a SIGPIPE. | |
302 | To catch it, you could use this: | |
303 | ||
304 | $SIG{PIPE} = 'IGNORE'; | |
305 | open(FH, "|bogus") or die "can't fork: $!"; | |
306 | print FH "bang\n" or die "can't write: $!"; | |
307 | close FH or die "can't close: status=$?"; | |
4633a7c4 | 308 | |
68dc0745 | 309 | =head2 Filehandles |
310 | ||
5a964f20 TC |
311 | Both the main process and any child processes it forks share the same |
312 | STDIN, STDOUT, and STDERR filehandles. If both processes try to access | |
45bc9206 | 313 | them at once, strange things can happen. You may also want to close |
5a964f20 TC |
314 | or reopen the filehandles for the child. You can get around this by |
315 | opening your pipe with open(), but on some systems this means that the | |
316 | child process cannot outlive the parent. | |
68dc0745 | 317 | |
318 | =head2 Background Processes | |
319 | ||
320 | You can run a command in the background with: | |
321 | ||
7b05b7e3 | 322 | system("cmd &"); |
68dc0745 | 323 | |
324 | The command's STDOUT and STDERR (and possibly STDIN, depending on your | |
325 | shell) will be the same as the parent's. You won't need to catch | |
326 | SIGCHLD because of the double-fork taking place (see below for more | |
327 | details). | |
328 | ||
329 | =head2 Complete Dissociation of Child from Parent | |
330 | ||
331 | In some cases (starting server processes, for instance) you'll want to | |
893af57a RS |
332 | completely dissociate the child process from the parent. This is |
333 | often called daemonization. A well behaved daemon will also chdir() | |
334 | to the root directory (so it doesn't prevent unmounting the filesystem | |
335 | containing the directory from which it was launched) and redirect its | |
336 | standard file descriptors from and to F</dev/null> (so that random | |
337 | output doesn't wind up on the user's terminal). | |
338 | ||
339 | use POSIX 'setsid'; | |
340 | ||
341 | sub daemonize { | |
342 | chdir '/' or die "Can't chdir to /: $!"; | |
343 | open STDIN, '/dev/null' or die "Can't read /dev/null: $!"; | |
344 | open STDOUT, '>/dev/null' | |
345 | or die "Can't write to /dev/null: $!"; | |
346 | defined(my $pid = fork) or die "Can't fork: $!"; | |
347 | exit if $pid; | |
348 | setsid or die "Can't start a new session: $!"; | |
349 | open STDERR, '>&STDOUT' or die "Can't dup stdout: $!"; | |
350 | } | |
5a964f20 | 351 | |
893af57a RS |
352 | The fork() has to come before the setsid() to ensure that you aren't a |
353 | process group leader (the setsid() will fail if you are). If your | |
354 | system doesn't have the setsid() function, open F</dev/tty> and use the | |
355 | C<TIOCNOTTY> ioctl() on it instead. See L<tty(4)> for details. | |
5a964f20 | 356 | |
893af57a RS |
357 | Non-Unix users should check their Your_OS::Process module for other |
358 | solutions. | |
68dc0745 | 359 | |
4633a7c4 LW |
360 | =head2 Safe Pipe Opens |
361 | ||
362 | Another interesting approach to IPC is making your single program go | |
363 | multiprocess and communicate between (or even amongst) yourselves. The | |
364 | open() function will accept a file argument of either C<"-|"> or C<"|-"> | |
365 | to do a very interesting thing: it forks a child connected to the | |
366 | filehandle you've opened. The child is running the same program as the | |
367 | parent. This is useful for safely opening a file when running under an | |
368 | assumed UID or GID, for example. If you open a pipe I<to> minus, you can | |
369 | write to the filehandle you opened and your kid will find it in his | |
370 | STDIN. If you open a pipe I<from> minus, you can read from the filehandle | |
371 | you opened whatever your kid writes to his STDOUT. | |
372 | ||
373 | use English; | |
374 | my $sleep_count = 0; | |
375 | ||
54310121 | 376 | do { |
c07a80fd | 377 | $pid = open(KID_TO_WRITE, "|-"); |
4633a7c4 LW |
378 | unless (defined $pid) { |
379 | warn "cannot fork: $!"; | |
380 | die "bailing out" if $sleep_count++ > 6; | |
381 | sleep 10; | |
54310121 | 382 | } |
4633a7c4 LW |
383 | } until defined $pid; |
384 | ||
385 | if ($pid) { # parent | |
c07a80fd | 386 | print KID_TO_WRITE @some_data; |
387 | close(KID_TO_WRITE) || warn "kid exited $?"; | |
4633a7c4 LW |
388 | } else { # child |
389 | ($EUID, $EGID) = ($UID, $GID); # suid progs only | |
54310121 | 390 | open (FILE, "> /safe/file") |
4633a7c4 LW |
391 | || die "can't open /safe/file: $!"; |
392 | while (<STDIN>) { | |
393 | print FILE; # child's STDIN is parent's KID | |
54310121 | 394 | } |
4633a7c4 | 395 | exit; # don't forget this |
54310121 | 396 | } |
4633a7c4 LW |
397 | |
398 | Another common use for this construct is when you need to execute | |
399 | something without the shell's interference. With system(), it's | |
54310121 | 400 | straightforward, but you can't use a pipe open or backticks safely. |
4633a7c4 LW |
401 | That's because there's no way to stop the shell from getting its hands on |
402 | your arguments. Instead, use lower-level control to call exec() directly. | |
403 | ||
54310121 | 404 | Here's a safe backtick or pipe open for read: |
4633a7c4 LW |
405 | |
406 | # add error processing as above | |
c07a80fd | 407 | $pid = open(KID_TO_READ, "-|"); |
4633a7c4 LW |
408 | |
409 | if ($pid) { # parent | |
c07a80fd | 410 | while (<KID_TO_READ>) { |
4633a7c4 | 411 | # do something interesting |
54310121 | 412 | } |
c07a80fd | 413 | close(KID_TO_READ) || warn "kid exited $?"; |
4633a7c4 LW |
414 | |
415 | } else { # child | |
416 | ($EUID, $EGID) = ($UID, $GID); # suid only | |
417 | exec($program, @options, @args) | |
418 | || die "can't exec program: $!"; | |
419 | # NOTREACHED | |
54310121 | 420 | } |
4633a7c4 LW |
421 | |
422 | ||
423 | And here's a safe pipe open for writing: | |
424 | ||
425 | # add error processing as above | |
c07a80fd | 426 | $pid = open(KID_TO_WRITE, "|-"); |
4633a7c4 LW |
427 | $SIG{ALRM} = sub { die "whoops, $program pipe broke" }; |
428 | ||
429 | if ($pid) { # parent | |
430 | for (@data) { | |
c07a80fd | 431 | print KID_TO_WRITE; |
54310121 | 432 | } |
c07a80fd | 433 | close(KID_TO_WRITE) || warn "kid exited $?"; |
4633a7c4 LW |
434 | |
435 | } else { # child | |
436 | ($EUID, $EGID) = ($UID, $GID); | |
437 | exec($program, @options, @args) | |
438 | || die "can't exec program: $!"; | |
439 | # NOTREACHED | |
54310121 | 440 | } |
4633a7c4 LW |
441 | |
442 | Note that these operations are full Unix forks, which means they may not be | |
443 | correctly implemented on alien systems. Additionally, these are not true | |
54310121 | 444 | multithreading. If you'd like to learn more about threading, see the |
184e9718 | 445 | F<modules> file mentioned below in the SEE ALSO section. |
4633a7c4 | 446 | |
7b05b7e3 | 447 | =head2 Bidirectional Communication with Another Process |
4633a7c4 LW |
448 | |
449 | While this works reasonably well for unidirectional communication, what | |
450 | about bidirectional communication? The obvious thing you'd like to do | |
451 | doesn't actually work: | |
452 | ||
c07a80fd | 453 | open(PROG_FOR_READING_AND_WRITING, "| some program |") |
4633a7c4 | 454 | |
9f1b1f2d GS |
455 | and if you forget to use the C<use warnings> pragma or the B<-w> flag, |
456 | then you'll miss out entirely on the diagnostic message: | |
4633a7c4 LW |
457 | |
458 | Can't do bidirectional pipe at -e line 1. | |
459 | ||
460 | If you really want to, you can use the standard open2() library function | |
7b05b7e3 | 461 | to catch both ends. There's also an open3() for tridirectional I/O so you |
4633a7c4 LW |
462 | can also catch your child's STDERR, but doing so would then require an |
463 | awkward select() loop and wouldn't allow you to use normal Perl input | |
464 | operations. | |
465 | ||
466 | If you look at its source, you'll see that open2() uses low-level | |
5a964f20 | 467 | primitives like Unix pipe() and exec() calls to create all the connections. |
4633a7c4 LW |
468 | While it might have been slightly more efficient by using socketpair(), it |
469 | would have then been even less portable than it already is. The open2() | |
470 | and open3() functions are unlikely to work anywhere except on a Unix | |
471 | system or some other one purporting to be POSIX compliant. | |
472 | ||
473 | Here's an example of using open2(): | |
474 | ||
475 | use FileHandle; | |
476 | use IPC::Open2; | |
5a964f20 | 477 | $pid = open2(*Reader, *Writer, "cat -u -n" ); |
4633a7c4 LW |
478 | print Writer "stuff\n"; |
479 | $got = <Reader>; | |
480 | ||
6a3992aa DL |
481 | The problem with this is that Unix buffering is really going to |
482 | ruin your day. Even though your C<Writer> filehandle is auto-flushed, | |
4633a7c4 | 483 | and the process on the other end will get your data in a timely manner, |
6a3992aa | 484 | you can't usually do anything to force it to give it back to you |
54310121 | 485 | in a similarly quick fashion. In this case, we could, because we |
4633a7c4 LW |
486 | gave I<cat> a B<-u> flag to make it unbuffered. But very few Unix |
487 | commands are designed to operate over pipes, so this seldom works | |
54310121 | 488 | unless you yourself wrote the program on the other end of the |
4633a7c4 LW |
489 | double-ended pipe. |
490 | ||
54310121 | 491 | A solution to this is the nonstandard F<Comm.pl> library. It uses |
4633a7c4 LW |
492 | pseudo-ttys to make your program behave more reasonably: |
493 | ||
494 | require 'Comm.pl'; | |
495 | $ph = open_proc('cat -n'); | |
496 | for (1..10) { | |
497 | print $ph "a line\n"; | |
498 | print "got back ", scalar <$ph>; | |
499 | } | |
a0d0e21e | 500 | |
4633a7c4 | 501 | This way you don't have to have control over the source code of the |
54310121 | 502 | program you're using. The F<Comm> library also has expect() |
503 | and interact() functions. Find the library (and we hope its | |
4633a7c4 | 504 | successor F<IPC::Chat>) at your nearest CPAN archive as detailed |
184e9718 | 505 | in the SEE ALSO section below. |
a0d0e21e | 506 | |
c8db1d39 TC |
507 | The newer Expect.pm module from CPAN also addresses this kind of thing. |
508 | This module requires two other modules from CPAN: IO::Pty and IO::Stty. | |
509 | It sets up a pseudo-terminal to interact with programs that insist on | |
510 | using talking to the terminal device driver. If your system is | |
511 | amongst those supported, this may be your best bet. | |
512 | ||
5a964f20 TC |
513 | =head2 Bidirectional Communication with Yourself |
514 | ||
515 | If you want, you may make low-level pipe() and fork() | |
516 | to stitch this together by hand. This example only | |
517 | talks to itself, but you could reopen the appropriate | |
518 | handles to STDIN and STDOUT and call other processes. | |
519 | ||
520 | #!/usr/bin/perl -w | |
521 | # pipe1 - bidirectional communication using two pipe pairs | |
522 | # designed for the socketpair-challenged | |
523 | use IO::Handle; # thousands of lines just for autoflush :-( | |
524 | pipe(PARENT_RDR, CHILD_WTR); # XXX: failure? | |
525 | pipe(CHILD_RDR, PARENT_WTR); # XXX: failure? | |
526 | CHILD_WTR->autoflush(1); | |
527 | PARENT_WTR->autoflush(1); | |
528 | ||
529 | if ($pid = fork) { | |
530 | close PARENT_RDR; close PARENT_WTR; | |
531 | print CHILD_WTR "Parent Pid $$ is sending this\n"; | |
532 | chomp($line = <CHILD_RDR>); | |
533 | print "Parent Pid $$ just read this: `$line'\n"; | |
534 | close CHILD_RDR; close CHILD_WTR; | |
535 | waitpid($pid,0); | |
536 | } else { | |
537 | die "cannot fork: $!" unless defined $pid; | |
538 | close CHILD_RDR; close CHILD_WTR; | |
539 | chomp($line = <PARENT_RDR>); | |
540 | print "Child Pid $$ just read this: `$line'\n"; | |
541 | print PARENT_WTR "Child Pid $$ is sending this\n"; | |
542 | close PARENT_RDR; close PARENT_WTR; | |
543 | exit; | |
544 | } | |
545 | ||
546 | But you don't actually have to make two pipe calls. If you | |
547 | have the socketpair() system call, it will do this all for you. | |
548 | ||
549 | #!/usr/bin/perl -w | |
550 | # pipe2 - bidirectional communication using socketpair | |
551 | # "the best ones always go both ways" | |
552 | ||
553 | use Socket; | |
554 | use IO::Handle; # thousands of lines just for autoflush :-( | |
555 | # We say AF_UNIX because although *_LOCAL is the | |
556 | # POSIX 1003.1g form of the constant, many machines | |
557 | # still don't have it. | |
558 | socketpair(CHILD, PARENT, AF_UNIX, SOCK_STREAM, PF_UNSPEC) | |
559 | or die "socketpair: $!"; | |
560 | ||
561 | CHILD->autoflush(1); | |
562 | PARENT->autoflush(1); | |
563 | ||
564 | if ($pid = fork) { | |
565 | close PARENT; | |
566 | print CHILD "Parent Pid $$ is sending this\n"; | |
567 | chomp($line = <CHILD>); | |
568 | print "Parent Pid $$ just read this: `$line'\n"; | |
569 | close CHILD; | |
570 | waitpid($pid,0); | |
571 | } else { | |
572 | die "cannot fork: $!" unless defined $pid; | |
573 | close CHILD; | |
574 | chomp($line = <PARENT>); | |
575 | print "Child Pid $$ just read this: `$line'\n"; | |
576 | print PARENT "Child Pid $$ is sending this\n"; | |
577 | close PARENT; | |
578 | exit; | |
579 | } | |
580 | ||
4633a7c4 | 581 | =head1 Sockets: Client/Server Communication |
a0d0e21e | 582 | |
6a3992aa | 583 | While not limited to Unix-derived operating systems (e.g., WinSock on PCs |
4633a7c4 | 584 | provides socket support, as do some VMS libraries), you may not have |
184e9718 | 585 | sockets on your system, in which case this section probably isn't going to do |
6a3992aa DL |
586 | you much good. With sockets, you can do both virtual circuits (i.e., TCP |
587 | streams) and datagrams (i.e., UDP packets). You may be able to do even more | |
4633a7c4 LW |
588 | depending on your system. |
589 | ||
590 | The Perl function calls for dealing with sockets have the same names as | |
591 | the corresponding system calls in C, but their arguments tend to differ | |
592 | for two reasons: first, Perl filehandles work differently than C file | |
593 | descriptors. Second, Perl already knows the length of its strings, so you | |
594 | don't need to pass that information. | |
a0d0e21e | 595 | |
4633a7c4 LW |
596 | One of the major problems with old socket code in Perl was that it used |
597 | hard-coded values for some of the constants, which severely hurt | |
598 | portability. If you ever see code that does anything like explicitly | |
599 | setting C<$AF_INET = 2>, you know you're in for big trouble: An | |
600 | immeasurably superior approach is to use the C<Socket> module, which more | |
601 | reliably grants access to various constants and functions you'll need. | |
a0d0e21e | 602 | |
68dc0745 | 603 | If you're not writing a server/client for an existing protocol like |
604 | NNTP or SMTP, you should give some thought to how your server will | |
605 | know when the client has finished talking, and vice-versa. Most | |
606 | protocols are based on one-line messages and responses (so one party | |
4a6725af | 607 | knows the other has finished when a "\n" is received) or multi-line |
68dc0745 | 608 | messages and responses that end with a period on an empty line |
609 | ("\n.\n" terminates a message/response). | |
610 | ||
5a964f20 TC |
611 | =head2 Internet Line Terminators |
612 | ||
613 | The Internet line terminator is "\015\012". Under ASCII variants of | |
614 | Unix, that could usually be written as "\r\n", but under other systems, | |
615 | "\r\n" might at times be "\015\015\012", "\012\012\015", or something | |
616 | completely different. The standards specify writing "\015\012" to be | |
617 | conformant (be strict in what you provide), but they also recommend | |
618 | accepting a lone "\012" on input (but be lenient in what you require). | |
619 | We haven't always been very good about that in the code in this manpage, | |
620 | but unless you're on a Mac, you'll probably be ok. | |
621 | ||
4633a7c4 | 622 | =head2 Internet TCP Clients and Servers |
a0d0e21e | 623 | |
4633a7c4 LW |
624 | Use Internet-domain sockets when you want to do client-server |
625 | communication that might extend to machines outside of your own system. | |
626 | ||
627 | Here's a sample TCP client using Internet-domain sockets: | |
628 | ||
629 | #!/usr/bin/perl -w | |
4633a7c4 LW |
630 | use strict; |
631 | use Socket; | |
632 | my ($remote,$port, $iaddr, $paddr, $proto, $line); | |
633 | ||
634 | $remote = shift || 'localhost'; | |
635 | $port = shift || 2345; # random port | |
636 | if ($port =~ /\D/) { $port = getservbyname($port, 'tcp') } | |
637 | die "No port" unless $port; | |
638 | $iaddr = inet_aton($remote) || die "no host: $remote"; | |
639 | $paddr = sockaddr_in($port, $iaddr); | |
640 | ||
641 | $proto = getprotobyname('tcp'); | |
642 | socket(SOCK, PF_INET, SOCK_STREAM, $proto) || die "socket: $!"; | |
643 | connect(SOCK, $paddr) || die "connect: $!"; | |
54310121 | 644 | while (defined($line = <SOCK>)) { |
4633a7c4 | 645 | print $line; |
54310121 | 646 | } |
4633a7c4 LW |
647 | |
648 | close (SOCK) || die "close: $!"; | |
649 | exit; | |
650 | ||
651 | And here's a corresponding server to go along with it. We'll | |
652 | leave the address as INADDR_ANY so that the kernel can choose | |
54310121 | 653 | the appropriate interface on multihomed hosts. If you want sit |
c07a80fd | 654 | on a particular interface (like the external side of a gateway |
655 | or firewall machine), you should fill this in with your real address | |
656 | instead. | |
657 | ||
658 | #!/usr/bin/perl -Tw | |
c07a80fd | 659 | use strict; |
660 | BEGIN { $ENV{PATH} = '/usr/ucb:/bin' } | |
661 | use Socket; | |
662 | use Carp; | |
5865a7df | 663 | my $EOL = "\015\012"; |
c07a80fd | 664 | |
54310121 | 665 | sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" } |
c07a80fd | 666 | |
667 | my $port = shift || 2345; | |
668 | my $proto = getprotobyname('tcp'); | |
51ee6500 | 669 | |
5865a7df | 670 | ($port) = $port =~ /^(\d+)$/ or die "invalid port"; |
6a3992aa | 671 | |
c07a80fd | 672 | socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!"; |
54310121 | 673 | setsockopt(Server, SOL_SOCKET, SO_REUSEADDR, |
c07a80fd | 674 | pack("l", 1)) || die "setsockopt: $!"; |
675 | bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!"; | |
676 | listen(Server,SOMAXCONN) || die "listen: $!"; | |
677 | ||
678 | logmsg "server started on port $port"; | |
679 | ||
680 | my $paddr; | |
681 | ||
682 | $SIG{CHLD} = \&REAPER; | |
683 | ||
684 | for ( ; $paddr = accept(Client,Server); close Client) { | |
685 | my($port,$iaddr) = sockaddr_in($paddr); | |
686 | my $name = gethostbyaddr($iaddr,AF_INET); | |
687 | ||
54310121 | 688 | logmsg "connection from $name [", |
689 | inet_ntoa($iaddr), "] | |
c07a80fd | 690 | at port $port"; |
691 | ||
54310121 | 692 | print Client "Hello there, $name, it's now ", |
5a964f20 | 693 | scalar localtime, $EOL; |
54310121 | 694 | } |
c07a80fd | 695 | |
54310121 | 696 | And here's a multithreaded version. It's multithreaded in that |
697 | like most typical servers, it spawns (forks) a slave server to | |
c07a80fd | 698 | handle the client request so that the master server can quickly |
699 | go back to service a new client. | |
4633a7c4 LW |
700 | |
701 | #!/usr/bin/perl -Tw | |
4633a7c4 LW |
702 | use strict; |
703 | BEGIN { $ENV{PATH} = '/usr/ucb:/bin' } | |
a0d0e21e | 704 | use Socket; |
4633a7c4 | 705 | use Carp; |
5865a7df | 706 | my $EOL = "\015\012"; |
a0d0e21e | 707 | |
4633a7c4 | 708 | sub spawn; # forward declaration |
54310121 | 709 | sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" } |
a0d0e21e | 710 | |
4633a7c4 LW |
711 | my $port = shift || 2345; |
712 | my $proto = getprotobyname('tcp'); | |
51ee6500 | 713 | |
5865a7df | 714 | ($port) = $port =~ /^(\d+)$/ or die "invalid port"; |
54310121 | 715 | |
c07a80fd | 716 | socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!"; |
54310121 | 717 | setsockopt(Server, SOL_SOCKET, SO_REUSEADDR, |
c07a80fd | 718 | pack("l", 1)) || die "setsockopt: $!"; |
719 | bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!"; | |
720 | listen(Server,SOMAXCONN) || die "listen: $!"; | |
a0d0e21e | 721 | |
4633a7c4 | 722 | logmsg "server started on port $port"; |
a0d0e21e | 723 | |
4633a7c4 LW |
724 | my $waitedpid = 0; |
725 | my $paddr; | |
a0d0e21e | 726 | |
54310121 | 727 | sub REAPER { |
4633a7c4 | 728 | $waitedpid = wait; |
6a3992aa | 729 | $SIG{CHLD} = \&REAPER; # loathe sysV |
4633a7c4 LW |
730 | logmsg "reaped $waitedpid" . ($? ? " with exit $?" : ''); |
731 | } | |
732 | ||
733 | $SIG{CHLD} = \&REAPER; | |
734 | ||
54310121 | 735 | for ( $waitedpid = 0; |
736 | ($paddr = accept(Client,Server)) || $waitedpid; | |
737 | $waitedpid = 0, close Client) | |
4633a7c4 | 738 | { |
6a3992aa | 739 | next if $waitedpid and not $paddr; |
4633a7c4 LW |
740 | my($port,$iaddr) = sockaddr_in($paddr); |
741 | my $name = gethostbyaddr($iaddr,AF_INET); | |
742 | ||
54310121 | 743 | logmsg "connection from $name [", |
744 | inet_ntoa($iaddr), "] | |
4633a7c4 | 745 | at port $port"; |
a0d0e21e | 746 | |
54310121 | 747 | spawn sub { |
b921b357 | 748 | $|=1; |
5a964f20 TC |
749 | print "Hello there, $name, it's now ", scalar localtime, $EOL; |
750 | exec '/usr/games/fortune' # XXX: `wrong' line terminators | |
4633a7c4 LW |
751 | or confess "can't exec fortune: $!"; |
752 | }; | |
a0d0e21e | 753 | |
54310121 | 754 | } |
a0d0e21e | 755 | |
4633a7c4 LW |
756 | sub spawn { |
757 | my $coderef = shift; | |
a0d0e21e | 758 | |
54310121 | 759 | unless (@_ == 0 && $coderef && ref($coderef) eq 'CODE') { |
4633a7c4 | 760 | confess "usage: spawn CODEREF"; |
a0d0e21e | 761 | } |
4633a7c4 LW |
762 | |
763 | my $pid; | |
764 | if (!defined($pid = fork)) { | |
765 | logmsg "cannot fork: $!"; | |
766 | return; | |
767 | } elsif ($pid) { | |
768 | logmsg "begat $pid"; | |
6a3992aa | 769 | return; # I'm the parent |
4633a7c4 | 770 | } |
6a3992aa | 771 | # else I'm the child -- go spawn |
4633a7c4 | 772 | |
c07a80fd | 773 | open(STDIN, "<&Client") || die "can't dup client to stdin"; |
774 | open(STDOUT, ">&Client") || die "can't dup client to stdout"; | |
4633a7c4 LW |
775 | ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr"; |
776 | exit &$coderef(); | |
54310121 | 777 | } |
4633a7c4 LW |
778 | |
779 | This server takes the trouble to clone off a child version via fork() for | |
780 | each incoming request. That way it can handle many requests at once, | |
781 | which you might not always want. Even if you don't fork(), the listen() | |
782 | will allow that many pending connections. Forking servers have to be | |
783 | particularly careful about cleaning up their dead children (called | |
784 | "zombies" in Unix parlance), because otherwise you'll quickly fill up your | |
785 | process table. | |
786 | ||
787 | We suggest that you use the B<-T> flag to use taint checking (see L<perlsec>) | |
788 | even if we aren't running setuid or setgid. This is always a good idea | |
789 | for servers and other programs run on behalf of someone else (like CGI | |
790 | scripts), because it lessens the chances that people from the outside will | |
791 | be able to compromise your system. | |
792 | ||
793 | Let's look at another TCP client. This one connects to the TCP "time" | |
794 | service on a number of different machines and shows how far their clocks | |
795 | differ from the system on which it's being run: | |
796 | ||
797 | #!/usr/bin/perl -w | |
4633a7c4 LW |
798 | use strict; |
799 | use Socket; | |
800 | ||
801 | my $SECS_of_70_YEARS = 2208988800; | |
54310121 | 802 | sub ctime { scalar localtime(shift) } |
4633a7c4 | 803 | |
54310121 | 804 | my $iaddr = gethostbyname('localhost'); |
805 | my $proto = getprotobyname('tcp'); | |
806 | my $port = getservbyname('time', 'tcp'); | |
4633a7c4 LW |
807 | my $paddr = sockaddr_in(0, $iaddr); |
808 | my($host); | |
809 | ||
810 | $| = 1; | |
811 | printf "%-24s %8s %s\n", "localhost", 0, ctime(time()); | |
812 | ||
813 | foreach $host (@ARGV) { | |
814 | printf "%-24s ", $host; | |
815 | my $hisiaddr = inet_aton($host) || die "unknown host"; | |
816 | my $hispaddr = sockaddr_in($port, $hisiaddr); | |
817 | socket(SOCKET, PF_INET, SOCK_STREAM, $proto) || die "socket: $!"; | |
818 | connect(SOCKET, $hispaddr) || die "bind: $!"; | |
819 | my $rtime = ' '; | |
820 | read(SOCKET, $rtime, 4); | |
821 | close(SOCKET); | |
822 | my $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ; | |
823 | printf "%8d %s\n", $histime - time, ctime($histime); | |
a0d0e21e LW |
824 | } |
825 | ||
4633a7c4 LW |
826 | =head2 Unix-Domain TCP Clients and Servers |
827 | ||
a2eb9003 | 828 | That's fine for Internet-domain clients and servers, but what about local |
4633a7c4 LW |
829 | communications? While you can use the same setup, sometimes you don't |
830 | want to. Unix-domain sockets are local to the current host, and are often | |
54310121 | 831 | used internally to implement pipes. Unlike Internet domain sockets, Unix |
4633a7c4 LW |
832 | domain sockets can show up in the file system with an ls(1) listing. |
833 | ||
5a964f20 | 834 | % ls -l /dev/log |
4633a7c4 | 835 | srw-rw-rw- 1 root 0 Oct 31 07:23 /dev/log |
a0d0e21e | 836 | |
4633a7c4 LW |
837 | You can test for these with Perl's B<-S> file test: |
838 | ||
839 | unless ( -S '/dev/log' ) { | |
3ba19564 | 840 | die "something's wicked with the log system"; |
54310121 | 841 | } |
4633a7c4 LW |
842 | |
843 | Here's a sample Unix-domain client: | |
844 | ||
845 | #!/usr/bin/perl -w | |
4633a7c4 LW |
846 | use Socket; |
847 | use strict; | |
848 | my ($rendezvous, $line); | |
849 | ||
850 | $rendezvous = shift || '/tmp/catsock'; | |
851 | socket(SOCK, PF_UNIX, SOCK_STREAM, 0) || die "socket: $!"; | |
9607fc9c | 852 | connect(SOCK, sockaddr_un($rendezvous)) || die "connect: $!"; |
54310121 | 853 | while (defined($line = <SOCK>)) { |
4633a7c4 | 854 | print $line; |
54310121 | 855 | } |
4633a7c4 LW |
856 | exit; |
857 | ||
5a964f20 TC |
858 | And here's a corresponding server. You don't have to worry about silly |
859 | network terminators here because Unix domain sockets are guaranteed | |
860 | to be on the localhost, and thus everything works right. | |
4633a7c4 LW |
861 | |
862 | #!/usr/bin/perl -Tw | |
4633a7c4 LW |
863 | use strict; |
864 | use Socket; | |
865 | use Carp; | |
866 | ||
867 | BEGIN { $ENV{PATH} = '/usr/ucb:/bin' } | |
5865a7df | 868 | sub spawn; # forward declaration |
5a964f20 | 869 | sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" } |
4633a7c4 LW |
870 | |
871 | my $NAME = '/tmp/catsock'; | |
872 | my $uaddr = sockaddr_un($NAME); | |
873 | my $proto = getprotobyname('tcp'); | |
874 | ||
c07a80fd | 875 | socket(Server,PF_UNIX,SOCK_STREAM,0) || die "socket: $!"; |
4633a7c4 | 876 | unlink($NAME); |
c07a80fd | 877 | bind (Server, $uaddr) || die "bind: $!"; |
878 | listen(Server,SOMAXCONN) || die "listen: $!"; | |
4633a7c4 LW |
879 | |
880 | logmsg "server started on $NAME"; | |
881 | ||
5a964f20 TC |
882 | my $waitedpid; |
883 | ||
884 | sub REAPER { | |
885 | $waitedpid = wait; | |
886 | $SIG{CHLD} = \&REAPER; # loathe sysV | |
887 | logmsg "reaped $waitedpid" . ($? ? " with exit $?" : ''); | |
888 | } | |
889 | ||
4633a7c4 LW |
890 | $SIG{CHLD} = \&REAPER; |
891 | ||
5a964f20 | 892 | |
54310121 | 893 | for ( $waitedpid = 0; |
894 | accept(Client,Server) || $waitedpid; | |
895 | $waitedpid = 0, close Client) | |
4633a7c4 LW |
896 | { |
897 | next if $waitedpid; | |
898 | logmsg "connection on $NAME"; | |
54310121 | 899 | spawn sub { |
4633a7c4 LW |
900 | print "Hello there, it's now ", scalar localtime, "\n"; |
901 | exec '/usr/games/fortune' or die "can't exec fortune: $!"; | |
902 | }; | |
54310121 | 903 | } |
4633a7c4 | 904 | |
5865a7df NC |
905 | sub spawn { |
906 | my $coderef = shift; | |
907 | ||
908 | unless (@_ == 0 && $coderef && ref($coderef) eq 'CODE') { | |
909 | confess "usage: spawn CODEREF"; | |
910 | } | |
911 | ||
912 | my $pid; | |
913 | if (!defined($pid = fork)) { | |
914 | logmsg "cannot fork: $!"; | |
915 | return; | |
916 | } elsif ($pid) { | |
917 | logmsg "begat $pid"; | |
918 | return; # I'm the parent | |
919 | } | |
920 | # else I'm the child -- go spawn | |
921 | ||
922 | open(STDIN, "<&Client") || die "can't dup client to stdin"; | |
923 | open(STDOUT, ">&Client") || die "can't dup client to stdout"; | |
924 | ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr"; | |
925 | exit &$coderef(); | |
926 | } | |
927 | ||
4633a7c4 LW |
928 | As you see, it's remarkably similar to the Internet domain TCP server, so |
929 | much so, in fact, that we've omitted several duplicate functions--spawn(), | |
930 | logmsg(), ctime(), and REAPER()--which are exactly the same as in the | |
931 | other server. | |
932 | ||
933 | So why would you ever want to use a Unix domain socket instead of a | |
934 | simpler named pipe? Because a named pipe doesn't give you sessions. You | |
935 | can't tell one process's data from another's. With socket programming, | |
936 | you get a separate session for each client: that's why accept() takes two | |
937 | arguments. | |
938 | ||
939 | For example, let's say that you have a long running database server daemon | |
940 | that you want folks from the World Wide Web to be able to access, but only | |
941 | if they go through a CGI interface. You'd have a small, simple CGI | |
942 | program that does whatever checks and logging you feel like, and then acts | |
943 | as a Unix-domain client and connects to your private server. | |
944 | ||
7b05b7e3 TC |
945 | =head1 TCP Clients with IO::Socket |
946 | ||
947 | For those preferring a higher-level interface to socket programming, the | |
948 | IO::Socket module provides an object-oriented approach. IO::Socket is | |
949 | included as part of the standard Perl distribution as of the 5.004 | |
950 | release. If you're running an earlier version of Perl, just fetch | |
106325ad | 951 | IO::Socket from CPAN, where you'll also find modules providing easy |
7b05b7e3 TC |
952 | interfaces to the following systems: DNS, FTP, Ident (RFC 931), NIS and |
953 | NISPlus, NNTP, Ping, POP3, SMTP, SNMP, SSLeay, Telnet, and Time--just | |
954 | to name a few. | |
955 | ||
956 | =head2 A Simple Client | |
957 | ||
958 | Here's a client that creates a TCP connection to the "daytime" | |
959 | service at port 13 of the host name "localhost" and prints out everything | |
960 | that the server there cares to provide. | |
961 | ||
962 | #!/usr/bin/perl -w | |
963 | use IO::Socket; | |
964 | $remote = IO::Socket::INET->new( | |
965 | Proto => "tcp", | |
966 | PeerAddr => "localhost", | |
967 | PeerPort => "daytime(13)", | |
968 | ) | |
969 | or die "cannot connect to daytime port at localhost"; | |
970 | while ( <$remote> ) { print } | |
971 | ||
972 | When you run this program, you should get something back that | |
973 | looks like this: | |
974 | ||
975 | Wed May 14 08:40:46 MDT 1997 | |
976 | ||
977 | Here are what those parameters to the C<new> constructor mean: | |
978 | ||
13a2d996 | 979 | =over 4 |
7b05b7e3 TC |
980 | |
981 | =item C<Proto> | |
982 | ||
983 | This is which protocol to use. In this case, the socket handle returned | |
984 | will be connected to a TCP socket, because we want a stream-oriented | |
985 | connection, that is, one that acts pretty much like a plain old file. | |
986 | Not all sockets are this of this type. For example, the UDP protocol | |
987 | can be used to make a datagram socket, used for message-passing. | |
988 | ||
989 | =item C<PeerAddr> | |
990 | ||
991 | This is the name or Internet address of the remote host the server is | |
992 | running on. We could have specified a longer name like C<"www.perl.com">, | |
993 | or an address like C<"204.148.40.9">. For demonstration purposes, we've | |
994 | used the special hostname C<"localhost">, which should always mean the | |
995 | current machine you're running on. The corresponding Internet address | |
996 | for localhost is C<"127.1">, if you'd rather use that. | |
997 | ||
998 | =item C<PeerPort> | |
999 | ||
1000 | This is the service name or port number we'd like to connect to. | |
1001 | We could have gotten away with using just C<"daytime"> on systems with a | |
1002 | well-configured system services file,[FOOTNOTE: The system services file | |
1003 | is in I</etc/services> under Unix] but just in case, we've specified the | |
1004 | port number (13) in parentheses. Using just the number would also have | |
1005 | worked, but constant numbers make careful programmers nervous. | |
1006 | ||
1007 | =back | |
1008 | ||
1009 | Notice how the return value from the C<new> constructor is used as | |
1010 | a filehandle in the C<while> loop? That's what's called an indirect | |
1011 | filehandle, a scalar variable containing a filehandle. You can use | |
1012 | it the same way you would a normal filehandle. For example, you | |
1013 | can read one line from it this way: | |
1014 | ||
1015 | $line = <$handle>; | |
1016 | ||
1017 | all remaining lines from is this way: | |
1018 | ||
1019 | @lines = <$handle>; | |
1020 | ||
1021 | and send a line of data to it this way: | |
1022 | ||
1023 | print $handle "some data\n"; | |
1024 | ||
1025 | =head2 A Webget Client | |
1026 | ||
1027 | Here's a simple client that takes a remote host to fetch a document | |
1028 | from, and then a list of documents to get from that host. This is a | |
1029 | more interesting client than the previous one because it first sends | |
1030 | something to the server before fetching the server's response. | |
1031 | ||
1032 | #!/usr/bin/perl -w | |
1033 | use IO::Socket; | |
1034 | unless (@ARGV > 1) { die "usage: $0 host document ..." } | |
1035 | $host = shift(@ARGV); | |
5a964f20 TC |
1036 | $EOL = "\015\012"; |
1037 | $BLANK = $EOL x 2; | |
7b05b7e3 TC |
1038 | foreach $document ( @ARGV ) { |
1039 | $remote = IO::Socket::INET->new( Proto => "tcp", | |
1040 | PeerAddr => $host, | |
1041 | PeerPort => "http(80)", | |
1042 | ); | |
1043 | unless ($remote) { die "cannot connect to http daemon on $host" } | |
1044 | $remote->autoflush(1); | |
5a964f20 | 1045 | print $remote "GET $document HTTP/1.0" . $BLANK; |
7b05b7e3 TC |
1046 | while ( <$remote> ) { print } |
1047 | close $remote; | |
1048 | } | |
1049 | ||
1050 | The web server handing the "http" service, which is assumed to be at | |
4375e838 | 1051 | its standard port, number 80. If the web server you're trying to |
7b05b7e3 | 1052 | connect to is at a different port (like 1080 or 8080), you should specify |
c47ff5f1 | 1053 | as the named-parameter pair, C<< PeerPort => 8080 >>. The C<autoflush> |
7b05b7e3 TC |
1054 | method is used on the socket because otherwise the system would buffer |
1055 | up the output we sent it. (If you're on a Mac, you'll also need to | |
1056 | change every C<"\n"> in your code that sends data over the network to | |
1057 | be a C<"\015\012"> instead.) | |
1058 | ||
1059 | Connecting to the server is only the first part of the process: once you | |
1060 | have the connection, you have to use the server's language. Each server | |
1061 | on the network has its own little command language that it expects as | |
1062 | input. The string that we send to the server starting with "GET" is in | |
1063 | HTTP syntax. In this case, we simply request each specified document. | |
1064 | Yes, we really are making a new connection for each document, even though | |
1065 | it's the same host. That's the way you always used to have to speak HTTP. | |
1066 | Recent versions of web browsers may request that the remote server leave | |
1067 | the connection open a little while, but the server doesn't have to honor | |
1068 | such a request. | |
1069 | ||
1070 | Here's an example of running that program, which we'll call I<webget>: | |
1071 | ||
5a964f20 | 1072 | % webget www.perl.com /guanaco.html |
7b05b7e3 TC |
1073 | HTTP/1.1 404 File Not Found |
1074 | Date: Thu, 08 May 1997 18:02:32 GMT | |
1075 | Server: Apache/1.2b6 | |
1076 | Connection: close | |
1077 | Content-type: text/html | |
1078 | ||
1079 | <HEAD><TITLE>404 File Not Found</TITLE></HEAD> | |
1080 | <BODY><H1>File Not Found</H1> | |
1081 | The requested URL /guanaco.html was not found on this server.<P> | |
1082 | </BODY> | |
1083 | ||
1084 | Ok, so that's not very interesting, because it didn't find that | |
1085 | particular document. But a long response wouldn't have fit on this page. | |
1086 | ||
1087 | For a more fully-featured version of this program, you should look to | |
1088 | the I<lwp-request> program included with the LWP modules from CPAN. | |
1089 | ||
1090 | =head2 Interactive Client with IO::Socket | |
1091 | ||
1092 | Well, that's all fine if you want to send one command and get one answer, | |
1093 | but what about setting up something fully interactive, somewhat like | |
1094 | the way I<telnet> works? That way you can type a line, get the answer, | |
1095 | type a line, get the answer, etc. | |
1096 | ||
1097 | This client is more complicated than the two we've done so far, but if | |
1098 | you're on a system that supports the powerful C<fork> call, the solution | |
1099 | isn't that rough. Once you've made the connection to whatever service | |
1100 | you'd like to chat with, call C<fork> to clone your process. Each of | |
1101 | these two identical process has a very simple job to do: the parent | |
1102 | copies everything from the socket to standard output, while the child | |
1103 | simultaneously copies everything from standard input to the socket. | |
1104 | To accomplish the same thing using just one process would be I<much> | |
1105 | harder, because it's easier to code two processes to do one thing than it | |
1106 | is to code one process to do two things. (This keep-it-simple principle | |
5a964f20 TC |
1107 | a cornerstones of the Unix philosophy, and good software engineering as |
1108 | well, which is probably why it's spread to other systems.) | |
7b05b7e3 TC |
1109 | |
1110 | Here's the code: | |
1111 | ||
1112 | #!/usr/bin/perl -w | |
1113 | use strict; | |
1114 | use IO::Socket; | |
1115 | my ($host, $port, $kidpid, $handle, $line); | |
1116 | ||
1117 | unless (@ARGV == 2) { die "usage: $0 host port" } | |
1118 | ($host, $port) = @ARGV; | |
1119 | ||
1120 | # create a tcp connection to the specified host and port | |
1121 | $handle = IO::Socket::INET->new(Proto => "tcp", | |
1122 | PeerAddr => $host, | |
1123 | PeerPort => $port) | |
1124 | or die "can't connect to port $port on $host: $!"; | |
1125 | ||
1126 | $handle->autoflush(1); # so output gets there right away | |
1127 | print STDERR "[Connected to $host:$port]\n"; | |
1128 | ||
1129 | # split the program into two processes, identical twins | |
1130 | die "can't fork: $!" unless defined($kidpid = fork()); | |
1131 | ||
1132 | # the if{} block runs only in the parent process | |
1133 | if ($kidpid) { | |
1134 | # copy the socket to standard output | |
1135 | while (defined ($line = <$handle>)) { | |
1136 | print STDOUT $line; | |
1137 | } | |
1138 | kill("TERM", $kidpid); # send SIGTERM to child | |
1139 | } | |
1140 | # the else{} block runs only in the child process | |
1141 | else { | |
1142 | # copy standard input to the socket | |
1143 | while (defined ($line = <STDIN>)) { | |
1144 | print $handle $line; | |
1145 | } | |
1146 | } | |
1147 | ||
1148 | The C<kill> function in the parent's C<if> block is there to send a | |
1149 | signal to our child process (current running in the C<else> block) | |
1150 | as soon as the remote server has closed its end of the connection. | |
1151 | ||
7b05b7e3 TC |
1152 | If the remote server sends data a byte at time, and you need that |
1153 | data immediately without waiting for a newline (which might not happen), | |
1154 | you may wish to replace the C<while> loop in the parent with the | |
1155 | following: | |
1156 | ||
1157 | my $byte; | |
1158 | while (sysread($handle, $byte, 1) == 1) { | |
1159 | print STDOUT $byte; | |
1160 | } | |
1161 | ||
1162 | Making a system call for each byte you want to read is not very efficient | |
1163 | (to put it mildly) but is the simplest to explain and works reasonably | |
1164 | well. | |
1165 | ||
1166 | =head1 TCP Servers with IO::Socket | |
1167 | ||
5a964f20 | 1168 | As always, setting up a server is little bit more involved than running a client. |
7b05b7e3 TC |
1169 | The model is that the server creates a special kind of socket that |
1170 | does nothing but listen on a particular port for incoming connections. | |
c47ff5f1 | 1171 | It does this by calling the C<< IO::Socket::INET->new() >> method with |
7b05b7e3 TC |
1172 | slightly different arguments than the client did. |
1173 | ||
13a2d996 | 1174 | =over 4 |
7b05b7e3 TC |
1175 | |
1176 | =item Proto | |
1177 | ||
1178 | This is which protocol to use. Like our clients, we'll | |
1179 | still specify C<"tcp"> here. | |
1180 | ||
1181 | =item LocalPort | |
1182 | ||
1183 | We specify a local | |
1184 | port in the C<LocalPort> argument, which we didn't do for the client. | |
1185 | This is service name or port number for which you want to be the | |
1186 | server. (Under Unix, ports under 1024 are restricted to the | |
1187 | superuser.) In our sample, we'll use port 9000, but you can use | |
1188 | any port that's not currently in use on your system. If you try | |
1189 | to use one already in used, you'll get an "Address already in use" | |
19799a22 | 1190 | message. Under Unix, the C<netstat -a> command will show |
7b05b7e3 TC |
1191 | which services current have servers. |
1192 | ||
1193 | =item Listen | |
1194 | ||
1195 | The C<Listen> parameter is set to the maximum number of | |
1196 | pending connections we can accept until we turn away incoming clients. | |
1197 | Think of it as a call-waiting queue for your telephone. | |
1198 | The low-level Socket module has a special symbol for the system maximum, which | |
1199 | is SOMAXCONN. | |
1200 | ||
1201 | =item Reuse | |
1202 | ||
1203 | The C<Reuse> parameter is needed so that we restart our server | |
1204 | manually without waiting a few minutes to allow system buffers to | |
1205 | clear out. | |
1206 | ||
1207 | =back | |
1208 | ||
1209 | Once the generic server socket has been created using the parameters | |
1210 | listed above, the server then waits for a new client to connect | |
1211 | to it. The server blocks in the C<accept> method, which eventually an | |
1212 | bidirectional connection to the remote client. (Make sure to autoflush | |
1213 | this handle to circumvent buffering.) | |
1214 | ||
1215 | To add to user-friendliness, our server prompts the user for commands. | |
1216 | Most servers don't do this. Because of the prompt without a newline, | |
1217 | you'll have to use the C<sysread> variant of the interactive client above. | |
1218 | ||
1219 | This server accepts one of five different commands, sending output | |
1220 | back to the client. Note that unlike most network servers, this one | |
1221 | only handles one incoming client at a time. Multithreaded servers are | |
f83494b9 | 1222 | covered in Chapter 6 of the Camel. |
7b05b7e3 TC |
1223 | |
1224 | Here's the code. We'll | |
1225 | ||
1226 | #!/usr/bin/perl -w | |
1227 | use IO::Socket; | |
1228 | use Net::hostent; # for OO version of gethostbyaddr | |
1229 | ||
1230 | $PORT = 9000; # pick something not in use | |
1231 | ||
1232 | $server = IO::Socket::INET->new( Proto => 'tcp', | |
1233 | LocalPort => $PORT, | |
1234 | Listen => SOMAXCONN, | |
1235 | Reuse => 1); | |
1236 | ||
1237 | die "can't setup server" unless $server; | |
1238 | print "[Server $0 accepting clients]\n"; | |
1239 | ||
1240 | while ($client = $server->accept()) { | |
1241 | $client->autoflush(1); | |
1242 | print $client "Welcome to $0; type help for command list.\n"; | |
1243 | $hostinfo = gethostbyaddr($client->peeraddr); | |
1244 | printf "[Connect from %s]\n", $hostinfo->name || $client->peerhost; | |
1245 | print $client "Command? "; | |
1246 | while ( <$client>) { | |
1247 | next unless /\S/; # blank line | |
1248 | if (/quit|exit/i) { last; } | |
1249 | elsif (/date|time/i) { printf $client "%s\n", scalar localtime; } | |
1250 | elsif (/who/i ) { print $client `who 2>&1`; } | |
1251 | elsif (/cookie/i ) { print $client `/usr/games/fortune 2>&1`; } | |
1252 | elsif (/motd/i ) { print $client `cat /etc/motd 2>&1`; } | |
1253 | else { | |
1254 | print $client "Commands: quit date who cookie motd\n"; | |
1255 | } | |
1256 | } continue { | |
1257 | print $client "Command? "; | |
1258 | } | |
1259 | close $client; | |
1260 | } | |
1261 | ||
1262 | =head1 UDP: Message Passing | |
4633a7c4 LW |
1263 | |
1264 | Another kind of client-server setup is one that uses not connections, but | |
1265 | messages. UDP communications involve much lower overhead but also provide | |
1266 | less reliability, as there are no promises that messages will arrive at | |
1267 | all, let alone in order and unmangled. Still, UDP offers some advantages | |
1268 | over TCP, including being able to "broadcast" or "multicast" to a whole | |
1269 | bunch of destination hosts at once (usually on your local subnet). If you | |
1270 | find yourself overly concerned about reliability and start building checks | |
6a3992aa | 1271 | into your message system, then you probably should use just TCP to start |
4633a7c4 LW |
1272 | with. |
1273 | ||
90034919 LC |
1274 | Note that UDP datagrams are I<not> a bytestream and should not be treated |
1275 | as such. This makes using I/O mechanisms with internal buffering | |
1276 | like stdio (i.e. print() and friends) especially cumbersome. Use syswrite(), | |
1277 | or better send(), like in the example below. | |
1278 | ||
4633a7c4 | 1279 | Here's a UDP program similar to the sample Internet TCP client given |
7b05b7e3 | 1280 | earlier. However, instead of checking one host at a time, the UDP version |
4633a7c4 LW |
1281 | will check many of them asynchronously by simulating a multicast and then |
1282 | using select() to do a timed-out wait for I/O. To do something similar | |
1283 | with TCP, you'd have to use a different socket handle for each host. | |
1284 | ||
1285 | #!/usr/bin/perl -w | |
1286 | use strict; | |
4633a7c4 LW |
1287 | use Socket; |
1288 | use Sys::Hostname; | |
1289 | ||
54310121 | 1290 | my ( $count, $hisiaddr, $hispaddr, $histime, |
1291 | $host, $iaddr, $paddr, $port, $proto, | |
4633a7c4 LW |
1292 | $rin, $rout, $rtime, $SECS_of_70_YEARS); |
1293 | ||
1294 | $SECS_of_70_YEARS = 2208988800; | |
1295 | ||
1296 | $iaddr = gethostbyname(hostname()); | |
1297 | $proto = getprotobyname('udp'); | |
1298 | $port = getservbyname('time', 'udp'); | |
1299 | $paddr = sockaddr_in(0, $iaddr); # 0 means let kernel pick | |
1300 | ||
1301 | socket(SOCKET, PF_INET, SOCK_DGRAM, $proto) || die "socket: $!"; | |
1302 | bind(SOCKET, $paddr) || die "bind: $!"; | |
1303 | ||
1304 | $| = 1; | |
1305 | printf "%-12s %8s %s\n", "localhost", 0, scalar localtime time; | |
1306 | $count = 0; | |
1307 | for $host (@ARGV) { | |
1308 | $count++; | |
1309 | $hisiaddr = inet_aton($host) || die "unknown host"; | |
1310 | $hispaddr = sockaddr_in($port, $hisiaddr); | |
1311 | defined(send(SOCKET, 0, 0, $hispaddr)) || die "send $host: $!"; | |
1312 | } | |
1313 | ||
1314 | $rin = ''; | |
1315 | vec($rin, fileno(SOCKET), 1) = 1; | |
1316 | ||
1317 | # timeout after 10.0 seconds | |
1318 | while ($count && select($rout = $rin, undef, undef, 10.0)) { | |
1319 | $rtime = ''; | |
1320 | ($hispaddr = recv(SOCKET, $rtime, 4, 0)) || die "recv: $!"; | |
1321 | ($port, $hisiaddr) = sockaddr_in($hispaddr); | |
1322 | $host = gethostbyaddr($hisiaddr, AF_INET); | |
1323 | $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ; | |
1324 | printf "%-12s ", $host; | |
1325 | printf "%8d %s\n", $histime - time, scalar localtime($histime); | |
1326 | $count--; | |
1327 | } | |
1328 | ||
90034919 LC |
1329 | Note that this example does not include any retries and may consequently |
1330 | fail to contact a reachable host. The most prominent reason for this | |
1331 | is congestion of the queues on the sending host if the number of | |
a31a806a | 1332 | list of hosts to contact is sufficiently large. |
90034919 | 1333 | |
4633a7c4 LW |
1334 | =head1 SysV IPC |
1335 | ||
1336 | While System V IPC isn't so widely used as sockets, it still has some | |
1337 | interesting uses. You can't, however, effectively use SysV IPC or | |
1338 | Berkeley mmap() to have shared memory so as to share a variable amongst | |
1339 | several processes. That's because Perl would reallocate your string when | |
1340 | you weren't wanting it to. | |
1341 | ||
54310121 | 1342 | Here's a small example showing shared memory usage. |
a0d0e21e | 1343 | |
41d6edb2 | 1344 | use IPC::SysV qw(IPC_PRIVATE IPC_RMID S_IRWXU); |
0ade1984 | 1345 | |
a0d0e21e | 1346 | $size = 2000; |
41d6edb2 JH |
1347 | $id = shmget(IPC_PRIVATE, $size, S_IRWXU) || die "$!"; |
1348 | print "shm key $id\n"; | |
a0d0e21e LW |
1349 | |
1350 | $message = "Message #1"; | |
41d6edb2 | 1351 | shmwrite($id, $message, 0, 60) || die "$!"; |
0ade1984 | 1352 | print "wrote: '$message'\n"; |
41d6edb2 | 1353 | shmread($id, $buff, 0, 60) || die "$!"; |
0ade1984 | 1354 | print "read : '$buff'\n"; |
a0d0e21e | 1355 | |
0ade1984 JH |
1356 | # the buffer of shmread is zero-character end-padded. |
1357 | substr($buff, index($buff, "\0")) = ''; | |
1358 | print "un" unless $buff eq $message; | |
1359 | print "swell\n"; | |
a0d0e21e | 1360 | |
41d6edb2 JH |
1361 | print "deleting shm $id\n"; |
1362 | shmctl($id, IPC_RMID, 0) || die "$!"; | |
a0d0e21e LW |
1363 | |
1364 | Here's an example of a semaphore: | |
1365 | ||
0ade1984 JH |
1366 | use IPC::SysV qw(IPC_CREAT); |
1367 | ||
a0d0e21e | 1368 | $IPC_KEY = 1234; |
41d6edb2 JH |
1369 | $id = semget($IPC_KEY, 10, 0666 | IPC_CREAT ) || die "$!"; |
1370 | print "shm key $id\n"; | |
a0d0e21e | 1371 | |
a2eb9003 | 1372 | Put this code in a separate file to be run in more than one process. |
a0d0e21e LW |
1373 | Call the file F<take>: |
1374 | ||
1375 | # create a semaphore | |
1376 | ||
1377 | $IPC_KEY = 1234; | |
41d6edb2 JH |
1378 | $id = semget($IPC_KEY, 0 , 0 ); |
1379 | die if !defined($id); | |
a0d0e21e LW |
1380 | |
1381 | $semnum = 0; | |
1382 | $semflag = 0; | |
1383 | ||
1384 | # 'take' semaphore | |
1385 | # wait for semaphore to be zero | |
1386 | $semop = 0; | |
41d6edb2 | 1387 | $opstring1 = pack("s!s!s!", $semnum, $semop, $semflag); |
a0d0e21e LW |
1388 | |
1389 | # Increment the semaphore count | |
1390 | $semop = 1; | |
41d6edb2 | 1391 | $opstring2 = pack("s!s!s!", $semnum, $semop, $semflag); |
a0d0e21e LW |
1392 | $opstring = $opstring1 . $opstring2; |
1393 | ||
41d6edb2 | 1394 | semop($id,$opstring) || die "$!"; |
a0d0e21e | 1395 | |
a2eb9003 | 1396 | Put this code in a separate file to be run in more than one process. |
a0d0e21e LW |
1397 | Call this file F<give>: |
1398 | ||
4633a7c4 | 1399 | # 'give' the semaphore |
a0d0e21e LW |
1400 | # run this in the original process and you will see |
1401 | # that the second process continues | |
1402 | ||
1403 | $IPC_KEY = 1234; | |
41d6edb2 JH |
1404 | $id = semget($IPC_KEY, 0, 0); |
1405 | die if !defined($id); | |
a0d0e21e LW |
1406 | |
1407 | $semnum = 0; | |
1408 | $semflag = 0; | |
1409 | ||
1410 | # Decrement the semaphore count | |
1411 | $semop = -1; | |
41d6edb2 | 1412 | $opstring = pack("s!s!s!", $semnum, $semop, $semflag); |
a0d0e21e | 1413 | |
41d6edb2 | 1414 | semop($id,$opstring) || die "$!"; |
a0d0e21e | 1415 | |
7b05b7e3 | 1416 | The SysV IPC code above was written long ago, and it's definitely |
0ade1984 JH |
1417 | clunky looking. For a more modern look, see the IPC::SysV module |
1418 | which is included with Perl starting from Perl 5.005. | |
4633a7c4 | 1419 | |
41d6edb2 JH |
1420 | A small example demonstrating SysV message queues: |
1421 | ||
1422 | use IPC::SysV qw(IPC_PRIVATE IPC_RMID IPC_CREAT S_IRWXU); | |
1423 | ||
1424 | my $id = msgget(IPC_PRIVATE, IPC_CREAT | S_IRWXU); | |
1425 | ||
1426 | my $sent = "message"; | |
1427 | my $type = 1234; | |
1428 | my $rcvd; | |
1429 | my $type_rcvd; | |
1430 | ||
1431 | if (defined $id) { | |
1432 | if (msgsnd($id, pack("l! a*", $type_sent, $sent), 0)) { | |
1433 | if (msgrcv($id, $rcvd, 60, 0, 0)) { | |
1434 | ($type_rcvd, $rcvd) = unpack("l! a*", $rcvd); | |
1435 | if ($rcvd eq $sent) { | |
1436 | print "okay\n"; | |
1437 | } else { | |
1438 | print "not okay\n"; | |
1439 | } | |
1440 | } else { | |
1441 | die "# msgrcv failed\n"; | |
1442 | } | |
1443 | } else { | |
1444 | die "# msgsnd failed\n"; | |
1445 | } | |
1446 | msgctl($id, IPC_RMID, 0) || die "# msgctl failed: $!\n"; | |
1447 | } else { | |
1448 | die "# msgget failed\n"; | |
1449 | } | |
1450 | ||
4633a7c4 LW |
1451 | =head1 NOTES |
1452 | ||
5a964f20 TC |
1453 | Most of these routines quietly but politely return C<undef> when they |
1454 | fail instead of causing your program to die right then and there due to | |
1455 | an uncaught exception. (Actually, some of the new I<Socket> conversion | |
1456 | functions croak() on bad arguments.) It is therefore essential to | |
1457 | check return values from these functions. Always begin your socket | |
1458 | programs this way for optimal success, and don't forget to add B<-T> | |
1459 | taint checking flag to the #! line for servers: | |
4633a7c4 | 1460 | |
5a964f20 | 1461 | #!/usr/bin/perl -Tw |
4633a7c4 LW |
1462 | use strict; |
1463 | use sigtrap; | |
1464 | use Socket; | |
1465 | ||
1466 | =head1 BUGS | |
1467 | ||
1468 | All these routines create system-specific portability problems. As noted | |
1469 | elsewhere, Perl is at the mercy of your C libraries for much of its system | |
1470 | behaviour. It's probably safest to assume broken SysV semantics for | |
6a3992aa | 1471 | signals and to stick with simple TCP and UDP socket operations; e.g., don't |
a2eb9003 | 1472 | try to pass open file descriptors over a local UDP datagram socket if you |
4633a7c4 LW |
1473 | want your code to stand a chance of being portable. |
1474 | ||
5a964f20 TC |
1475 | As mentioned in the signals section, because few vendors provide C |
1476 | libraries that are safely re-entrant, the prudent programmer will do | |
1477 | little else within a handler beyond setting a numeric variable that | |
1478 | already exists; or, if locked into a slow (restarting) system call, | |
1479 | using die() to raise an exception and longjmp(3) out. In fact, even | |
1480 | these may in some cases cause a core dump. It's probably best to avoid | |
1481 | signals except where they are absolutely inevitable. This | |
1482 | will be addressed in a future release of Perl. | |
4633a7c4 LW |
1483 | |
1484 | =head1 AUTHOR | |
1485 | ||
1486 | Tom Christiansen, with occasional vestiges of Larry Wall's original | |
7b05b7e3 | 1487 | version and suggestions from the Perl Porters. |
4633a7c4 LW |
1488 | |
1489 | =head1 SEE ALSO | |
1490 | ||
7b05b7e3 TC |
1491 | There's a lot more to networking than this, but this should get you |
1492 | started. | |
1493 | ||
5a964f20 TC |
1494 | For intrepid programmers, the indispensable textbook is I<Unix Network |
1495 | Programming> by W. Richard Stevens (published by Addison-Wesley). Note | |
1496 | that most books on networking address networking from the perspective of | |
1497 | a C programmer; translation to Perl is left as an exercise for the reader. | |
7b05b7e3 TC |
1498 | |
1499 | The IO::Socket(3) manpage describes the object library, and the Socket(3) | |
1500 | manpage describes the low-level interface to sockets. Besides the obvious | |
1501 | functions in L<perlfunc>, you should also check out the F<modules> file | |
1502 | at your nearest CPAN site. (See L<perlmodlib> or best yet, the F<Perl | |
1503 | FAQ> for a description of what CPAN is and where to get it.) | |
1504 | ||
4633a7c4 | 1505 | Section 5 of the F<modules> file is devoted to "Networking, Device Control |
6a3992aa | 1506 | (modems), and Interprocess Communication", and contains numerous unbundled |
4633a7c4 LW |
1507 | modules numerous networking modules, Chat and Expect operations, CGI |
1508 | programming, DCE, FTP, IPC, NNTP, Proxy, Ptty, RPC, SNMP, SMTP, Telnet, | |
1509 | Threads, and ToolTalk--just to name a few. |