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1 | =head1 NAME |
2 | |
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3 | perlfaq8 - System Interaction ($Revision: 1.39 $, $Date: 1999/05/23 18:37:57 $) |
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4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | This section of the Perl FAQ covers questions involving operating |
8 | system interaction. This involves interprocess communication (IPC), |
9 | control over the user-interface (keyboard, screen and pointing |
10 | devices), and most anything else not related to data manipulation. |
11 | |
12 | Read the FAQs and documentation specific to the port of perl to your |
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13 | operating system (eg, L<perlvms>, L<perlplan9>, ...). These should |
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14 | contain more detailed information on the vagaries of your perl. |
15 | |
16 | =head2 How do I find out which operating system I'm running under? |
17 | |
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18 | The $^O variable ($OSNAME if you use English) contains an indication of |
19 | the name of the operating system (not its release number) that your perl |
20 | binary was built for. |
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21 | |
22 | =head2 How come exec() doesn't return? |
23 | |
24 | Because that's what it does: it replaces your currently running |
25 | program with a different one. If you want to keep going (as is |
26 | probably the case if you're asking this question) use system() |
27 | instead. |
28 | |
29 | =head2 How do I do fancy stuff with the keyboard/screen/mouse? |
30 | |
31 | How you access/control keyboards, screens, and pointing devices |
32 | ("mice") is system-dependent. Try the following modules: |
33 | |
34 | =over 4 |
35 | |
36 | =item Keyboard |
37 | |
38 | Term::Cap Standard perl distribution |
39 | Term::ReadKey CPAN |
40 | Term::ReadLine::Gnu CPAN |
41 | Term::ReadLine::Perl CPAN |
42 | Term::Screen CPAN |
43 | |
44 | =item Screen |
45 | |
46 | Term::Cap Standard perl distribution |
47 | Curses CPAN |
48 | Term::ANSIColor CPAN |
49 | |
50 | =item Mouse |
51 | |
52 | Tk CPAN |
53 | |
54 | =back |
55 | |
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56 | Some of these specific cases are shown below. |
57 | |
58 | =head2 How do I print something out in color? |
59 | |
60 | In general, you don't, because you don't know whether |
61 | the recipient has a color-aware display device. If you |
62 | know that they have an ANSI terminal that understands |
63 | color, you can use the Term::ANSIColor module from CPAN: |
64 | |
65 | use Term::ANSIColor; |
66 | print color("red"), "Stop!\n", color("reset"); |
67 | print color("green"), "Go!\n", color("reset"); |
68 | |
69 | Or like this: |
70 | |
71 | use Term::ANSIColor qw(:constants); |
72 | print RED, "Stop!\n", RESET; |
73 | print GREEN, "Go!\n", RESET; |
74 | |
75 | =head2 How do I read just one key without waiting for a return key? |
76 | |
77 | Controlling input buffering is a remarkably system-dependent matter. |
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78 | On many systems, you can just use the B<stty> command as shown in |
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79 | L<perlfunc/getc>, but as you see, that's already getting you into |
80 | portability snags. |
81 | |
82 | open(TTY, "+</dev/tty") or die "no tty: $!"; |
83 | system "stty cbreak </dev/tty >/dev/tty 2>&1"; |
84 | $key = getc(TTY); # perhaps this works |
85 | # OR ELSE |
86 | sysread(TTY, $key, 1); # probably this does |
87 | system "stty -cbreak </dev/tty >/dev/tty 2>&1"; |
88 | |
89 | The Term::ReadKey module from CPAN offers an easy-to-use interface that |
90 | should be more efficient than shelling out to B<stty> for each key. |
91 | It even includes limited support for Windows. |
92 | |
93 | use Term::ReadKey; |
94 | ReadMode('cbreak'); |
95 | $key = ReadKey(0); |
96 | ReadMode('normal'); |
97 | |
98 | However, that requires that you have a working C compiler and can use it |
99 | to build and install a CPAN module. Here's a solution using |
100 | the standard POSIX module, which is already on your systems (assuming |
101 | your system supports POSIX). |
102 | |
103 | use HotKey; |
104 | $key = readkey(); |
105 | |
106 | And here's the HotKey module, which hides the somewhat mystifying calls |
107 | to manipulate the POSIX termios structures. |
108 | |
109 | # HotKey.pm |
110 | package HotKey; |
111 | |
112 | @ISA = qw(Exporter); |
113 | @EXPORT = qw(cbreak cooked readkey); |
114 | |
115 | use strict; |
116 | use POSIX qw(:termios_h); |
117 | my ($term, $oterm, $echo, $noecho, $fd_stdin); |
118 | |
119 | $fd_stdin = fileno(STDIN); |
120 | $term = POSIX::Termios->new(); |
121 | $term->getattr($fd_stdin); |
122 | $oterm = $term->getlflag(); |
123 | |
124 | $echo = ECHO | ECHOK | ICANON; |
125 | $noecho = $oterm & ~$echo; |
126 | |
127 | sub cbreak { |
128 | $term->setlflag($noecho); # ok, so i don't want echo either |
129 | $term->setcc(VTIME, 1); |
130 | $term->setattr($fd_stdin, TCSANOW); |
131 | } |
132 | |
133 | sub cooked { |
134 | $term->setlflag($oterm); |
135 | $term->setcc(VTIME, 0); |
136 | $term->setattr($fd_stdin, TCSANOW); |
137 | } |
138 | |
139 | sub readkey { |
140 | my $key = ''; |
141 | cbreak(); |
142 | sysread(STDIN, $key, 1); |
143 | cooked(); |
144 | return $key; |
145 | } |
146 | |
147 | END { cooked() } |
148 | |
149 | 1; |
150 | |
151 | =head2 How do I check whether input is ready on the keyboard? |
152 | |
153 | The easiest way to do this is to read a key in nonblocking mode with the |
154 | Term::ReadKey module from CPAN, passing it an argument of -1 to indicate |
155 | not to block: |
156 | |
157 | use Term::ReadKey; |
158 | |
159 | ReadMode('cbreak'); |
160 | |
161 | if (defined ($char = ReadKey(-1)) ) { |
162 | # input was waiting and it was $char |
163 | } else { |
164 | # no input was waiting |
165 | } |
166 | |
167 | ReadMode('normal'); # restore normal tty settings |
168 | |
169 | =head2 How do I clear the screen? |
170 | |
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171 | If you only have do so infrequently, use C<system>: |
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172 | |
173 | system("clear"); |
174 | |
175 | If you have to do this a lot, save the clear string |
176 | so you can print it 100 times without calling a program |
177 | 100 times: |
178 | |
179 | $clear_string = `clear`; |
180 | print $clear_string; |
181 | |
182 | If you're planning on doing other screen manipulations, like cursor |
183 | positions, etc, you might wish to use Term::Cap module: |
184 | |
185 | use Term::Cap; |
186 | $terminal = Term::Cap->Tgetent( {OSPEED => 9600} ); |
187 | $clear_string = $terminal->Tputs('cl'); |
188 | |
189 | =head2 How do I get the screen size? |
190 | |
191 | If you have Term::ReadKey module installed from CPAN, |
192 | you can use it to fetch the width and height in characters |
193 | and in pixels: |
194 | |
195 | use Term::ReadKey; |
196 | ($wchar, $hchar, $wpixels, $hpixels) = GetTerminalSize(); |
197 | |
198 | This is more portable than the raw C<ioctl>, but not as |
199 | illustrative: |
200 | |
201 | require 'sys/ioctl.ph'; |
202 | die "no TIOCGWINSZ " unless defined &TIOCGWINSZ; |
203 | open(TTY, "+</dev/tty") or die "No tty: $!"; |
204 | unless (ioctl(TTY, &TIOCGWINSZ, $winsize='')) { |
205 | die sprintf "$0: ioctl TIOCGWINSZ (%08x: $!)\n", &TIOCGWINSZ; |
206 | } |
207 | ($row, $col, $xpixel, $ypixel) = unpack('S4', $winsize); |
208 | print "(row,col) = ($row,$col)"; |
209 | print " (xpixel,ypixel) = ($xpixel,$ypixel)" if $xpixel || $ypixel; |
210 | print "\n"; |
211 | |
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212 | =head2 How do I ask the user for a password? |
213 | |
214 | (This question has nothing to do with the web. See a different |
215 | FAQ for that.) |
216 | |
217 | There's an example of this in L<perlfunc/crypt>). First, you put |
218 | the terminal into "no echo" mode, then just read the password |
219 | normally. You may do this with an old-style ioctl() function, POSIX |
220 | terminal control (see L<POSIX>, and Chapter 7 of the Camel), or a call |
221 | to the B<stty> program, with varying degrees of portability. |
222 | |
223 | You can also do this for most systems using the Term::ReadKey module |
224 | from CPAN, which is easier to use and in theory more portable. |
225 | |
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226 | use Term::ReadKey; |
227 | |
228 | ReadMode('noecho'); |
229 | $password = ReadLine(0); |
230 | |
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231 | =head2 How do I read and write the serial port? |
232 | |
233 | This depends on which operating system your program is running on. In |
234 | the case of Unix, the serial ports will be accessible through files in |
235 | /dev; on other systems, the devices names will doubtless differ. |
236 | Several problem areas common to all device interaction are the |
237 | following |
238 | |
239 | =over 4 |
240 | |
241 | =item lockfiles |
242 | |
243 | Your system may use lockfiles to control multiple access. Make sure |
244 | you follow the correct protocol. Unpredictable behaviour can result |
245 | from multiple processes reading from one device. |
246 | |
247 | =item open mode |
248 | |
249 | If you expect to use both read and write operations on the device, |
250 | you'll have to open it for update (see L<perlfunc/"open"> for |
251 | details). You may wish to open it without running the risk of |
252 | blocking by using sysopen() and C<O_RDWR|O_NDELAY|O_NOCTTY> from the |
253 | Fcntl module (part of the standard perl distribution). See |
254 | L<perlfunc/"sysopen"> for more on this approach. |
255 | |
256 | =item end of line |
257 | |
258 | Some devices will be expecting a "\r" at the end of each line rather |
259 | than a "\n". In some ports of perl, "\r" and "\n" are different from |
260 | their usual (Unix) ASCII values of "\012" and "\015". You may have to |
261 | give the numeric values you want directly, using octal ("\015"), hex |
262 | ("0x0D"), or as a control-character specification ("\cM"). |
263 | |
264 | print DEV "atv1\012"; # wrong, for some devices |
265 | print DEV "atv1\015"; # right, for some devices |
266 | |
267 | Even though with normal text files, a "\n" will do the trick, there is |
268 | still no unified scheme for terminating a line that is portable |
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269 | between Unix, DOS/Win, and Macintosh, except to terminate I<ALL> line |
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270 | ends with "\015\012", and strip what you don't need from the output. |
271 | This applies especially to socket I/O and autoflushing, discussed |
272 | next. |
273 | |
274 | =item flushing output |
275 | |
276 | If you expect characters to get to your device when you print() them, |
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277 | you'll want to autoflush that filehandle. You can use select() |
278 | and the C<$|> variable to control autoflushing (see L<perlvar/$|> |
279 | and L<perlfunc/select>): |
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280 | |
281 | $oldh = select(DEV); |
282 | $| = 1; |
283 | select($oldh); |
284 | |
285 | You'll also see code that does this without a temporary variable, as in |
286 | |
287 | select((select(DEV), $| = 1)[0]); |
288 | |
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289 | Or if you don't mind pulling in a few thousand lines |
290 | of code just because you're afraid of a little $| variable: |
291 | |
292 | use IO::Handle; |
293 | DEV->autoflush(1); |
294 | |
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295 | As mentioned in the previous item, this still doesn't work when using |
296 | socket I/O between Unix and Macintosh. You'll need to hardcode your |
297 | line terminators, in that case. |
298 | |
299 | =item non-blocking input |
300 | |
301 | If you are doing a blocking read() or sysread(), you'll have to |
302 | arrange for an alarm handler to provide a timeout (see |
303 | L<perlfunc/alarm>). If you have a non-blocking open, you'll likely |
304 | have a non-blocking read, which means you may have to use a 4-arg |
305 | select() to determine whether I/O is ready on that device (see |
306 | L<perlfunc/"select">. |
307 | |
308 | =back |
309 | |
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310 | While trying to read from his caller-id box, the notorious Jamie Zawinski |
311 | <jwz@netscape.com>, after much gnashing of teeth and fighting with sysread, |
312 | sysopen, POSIX's tcgetattr business, and various other functions that |
313 | go bump in the night, finally came up with this: |
314 | |
315 | sub open_modem { |
316 | use IPC::Open2; |
317 | my $stty = `/bin/stty -g`; |
318 | open2( \*MODEM_IN, \*MODEM_OUT, "cu -l$modem_device -s2400 2>&1"); |
319 | # starting cu hoses /dev/tty's stty settings, even when it has |
320 | # been opened on a pipe... |
321 | system("/bin/stty $stty"); |
322 | $_ = <MODEM_IN>; |
323 | chop; |
324 | if ( !m/^Connected/ ) { |
325 | print STDERR "$0: cu printed `$_' instead of `Connected'\n"; |
326 | } |
327 | } |
328 | |
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329 | =head2 How do I decode encrypted password files? |
330 | |
331 | You spend lots and lots of money on dedicated hardware, but this is |
332 | bound to get you talked about. |
333 | |
334 | Seriously, you can't if they are Unix password files - the Unix |
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335 | password system employs one-way encryption. It's more like hashing than |
336 | encryption. The best you can check is whether something else hashes to |
337 | the same string. You can't turn a hash back into the original string. |
338 | Programs like Crack |
339 | can forcibly (and intelligently) try to guess passwords, but don't |
340 | (can't) guarantee quick success. |
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341 | |
342 | If you're worried about users selecting bad passwords, you should |
343 | proactively check when they try to change their password (by modifying |
344 | passwd(1), for example). |
345 | |
346 | =head2 How do I start a process in the background? |
347 | |
348 | You could use |
349 | |
350 | system("cmd &") |
351 | |
352 | or you could use fork as documented in L<perlfunc/"fork">, with |
353 | further examples in L<perlipc>. Some things to be aware of, if you're |
354 | on a Unix-like system: |
355 | |
356 | =over 4 |
357 | |
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358 | =item STDIN, STDOUT, and STDERR are shared |
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359 | |
360 | Both the main process and the backgrounded one (the "child" process) |
361 | share the same STDIN, STDOUT and STDERR filehandles. If both try to |
362 | access them at once, strange things can happen. You may want to close |
363 | or reopen these for the child. You can get around this with |
364 | C<open>ing a pipe (see L<perlfunc/"open">) but on some systems this |
365 | means that the child process cannot outlive the parent. |
366 | |
367 | =item Signals |
368 | |
369 | You'll have to catch the SIGCHLD signal, and possibly SIGPIPE too. |
370 | SIGCHLD is sent when the backgrounded process finishes. SIGPIPE is |
371 | sent when you write to a filehandle whose child process has closed (an |
372 | untrapped SIGPIPE can cause your program to silently die). This is |
373 | not an issue with C<system("cmd&")>. |
374 | |
375 | =item Zombies |
376 | |
377 | You have to be prepared to "reap" the child process when it finishes |
378 | |
379 | $SIG{CHLD} = sub { wait }; |
380 | |
381 | See L<perlipc/"Signals"> for other examples of code to do this. |
382 | Zombies are not an issue with C<system("prog &")>. |
383 | |
384 | =back |
385 | |
386 | =head2 How do I trap control characters/signals? |
387 | |
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388 | You don't actually "trap" a control character. Instead, that character |
389 | generates a signal which is sent to your terminal's currently |
390 | foregrounded process group, which you then trap in your process. |
391 | Signals are documented in L<perlipc/"Signals"> and chapter 6 of the Camel. |
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392 | |
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393 | Be warned that very few C libraries are re-entrant. Therefore, if you |
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394 | attempt to print() in a handler that got invoked during another stdio |
395 | operation your internal structures will likely be in an |
396 | inconsistent state, and your program will dump core. You can |
397 | sometimes avoid this by using syswrite() instead of print(). |
398 | |
399 | Unless you're exceedingly careful, the only safe things to do inside a |
400 | signal handler are: set a variable and exit. And in the first case, |
401 | you should only set a variable in such a way that malloc() is not |
402 | called (eg, by setting a variable that already has a value). |
403 | |
404 | For example: |
405 | |
406 | $Interrupted = 0; # to ensure it has a value |
407 | $SIG{INT} = sub { |
408 | $Interrupted++; |
409 | syswrite(STDERR, "ouch\n", 5); |
410 | } |
411 | |
412 | However, because syscalls restart by default, you'll find that if |
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413 | you're in a "slow" call, such as <FH>, read(), connect(), or |
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414 | wait(), that the only way to terminate them is by "longjumping" out; |
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415 | that is, by raising an exception. See the time-out handler for a |
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416 | blocking flock() in L<perlipc/"Signals"> or chapter 6 of the Camel. |
417 | |
418 | =head2 How do I modify the shadow password file on a Unix system? |
419 | |
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420 | If perl was installed correctly, and your shadow library was written |
421 | properly, the getpw*() functions described in L<perlfunc> should in |
422 | theory provide (read-only) access to entries in the shadow password |
423 | file. To change the file, make a new shadow password file (the format |
424 | varies from system to system - see L<passwd(5)> for specifics) and use |
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425 | pwd_mkdb(8) to install it (see L<pwd_mkdb(8)> for more details). |
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426 | |
427 | =head2 How do I set the time and date? |
428 | |
429 | Assuming you're running under sufficient permissions, you should be |
430 | able to set the system-wide date and time by running the date(1) |
431 | program. (There is no way to set the time and date on a per-process |
432 | basis.) This mechanism will work for Unix, MS-DOS, Windows, and NT; |
433 | the VMS equivalent is C<set time>. |
434 | |
435 | However, if all you want to do is change your timezone, you can |
436 | probably get away with setting an environment variable: |
437 | |
438 | $ENV{TZ} = "MST7MDT"; # unixish |
439 | $ENV{'SYS$TIMEZONE_DIFFERENTIAL'}="-5" # vms |
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440 | system "trn comp.lang.perl.misc"; |
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441 | |
442 | =head2 How can I sleep() or alarm() for under a second? |
443 | |
444 | If you want finer granularity than the 1 second that the sleep() |
445 | function provides, the easiest way is to use the select() function as |
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446 | documented in L<perlfunc/"select">. Try the Time::HiRes and |
447 | the BSD::Itimer modules (available from CPAN). |
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448 | |
449 | =head2 How can I measure time under a second? |
450 | |
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451 | In general, you may not be able to. The Time::HiRes module (available |
452 | from CPAN) provides this functionality for some systems. |
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453 | |
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454 | If your system supports both the syscall() function in Perl as well as |
455 | a system call like gettimeofday(2), then you may be able to do |
456 | something like this: |
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457 | |
458 | require 'sys/syscall.ph'; |
459 | |
460 | $TIMEVAL_T = "LL"; |
461 | |
462 | $done = $start = pack($TIMEVAL_T, ()); |
463 | |
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464 | syscall(&SYS_gettimeofday, $start, 0) != -1 |
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465 | or die "gettimeofday: $!"; |
466 | |
467 | ########################## |
468 | # DO YOUR OPERATION HERE # |
469 | ########################## |
470 | |
471 | syscall( &SYS_gettimeofday, $done, 0) != -1 |
472 | or die "gettimeofday: $!"; |
473 | |
474 | @start = unpack($TIMEVAL_T, $start); |
475 | @done = unpack($TIMEVAL_T, $done); |
476 | |
477 | # fix microseconds |
478 | for ($done[1], $start[1]) { $_ /= 1_000_000 } |
479 | |
480 | $delta_time = sprintf "%.4f", ($done[0] + $done[1] ) |
481 | - |
482 | ($start[0] + $start[1] ); |
483 | |
484 | =head2 How can I do an atexit() or setjmp()/longjmp()? (Exception handling) |
485 | |
486 | Release 5 of Perl added the END block, which can be used to simulate |
487 | atexit(). Each package's END block is called when the program or |
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488 | thread ends (see L<perlmod> manpage for more details). |
489 | |
490 | For example, you can use this to make sure your filter program |
491 | managed to finish its output without filling up the disk: |
492 | |
493 | END { |
494 | close(STDOUT) || die "stdout close failed: $!"; |
495 | } |
496 | |
497 | The END block isn't called when untrapped signals kill the program, though, so if |
498 | you use END blocks you should also use |
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499 | |
500 | use sigtrap qw(die normal-signals); |
501 | |
502 | Perl's exception-handling mechanism is its eval() operator. You can |
503 | use eval() as setjmp and die() as longjmp. For details of this, see |
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504 | the section on signals, especially the time-out handler for a blocking |
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505 | flock() in L<perlipc/"Signals"> and chapter 6 of the Camel. |
506 | |
507 | If exception handling is all you're interested in, try the |
508 | exceptions.pl library (part of the standard perl distribution). |
509 | |
510 | If you want the atexit() syntax (and an rmexit() as well), try the |
511 | AtExit module available from CPAN. |
512 | |
513 | =head2 Why doesn't my sockets program work under System V (Solaris)? What does the error message "Protocol not supported" mean? |
514 | |
515 | Some Sys-V based systems, notably Solaris 2.X, redefined some of the |
516 | standard socket constants. Since these were constant across all |
517 | architectures, they were often hardwired into perl code. The proper |
518 | way to deal with this is to "use Socket" to get the correct values. |
519 | |
520 | Note that even though SunOS and Solaris are binary compatible, these |
521 | values are different. Go figure. |
522 | |
523 | =head2 How can I call my system's unique C functions from Perl? |
524 | |
525 | In most cases, you write an external module to do it - see the answer |
526 | to "Where can I learn about linking C with Perl? [h2xs, xsubpp]". |
527 | However, if the function is a system call, and your system supports |
528 | syscall(), you can use the syscall function (documented in |
529 | L<perlfunc>). |
530 | |
531 | Remember to check the modules that came with your distribution, and |
532 | CPAN as well - someone may already have written a module to do it. |
533 | |
534 | =head2 Where do I get the include files to do ioctl() or syscall()? |
535 | |
536 | Historically, these would be generated by the h2ph tool, part of the |
537 | standard perl distribution. This program converts cpp(1) directives |
538 | in C header files to files containing subroutine definitions, like |
539 | &SYS_getitimer, which you can use as arguments to your functions. |
540 | It doesn't work perfectly, but it usually gets most of the job done. |
541 | Simple files like F<errno.h>, F<syscall.h>, and F<socket.h> were fine, |
542 | but the hard ones like F<ioctl.h> nearly always need to hand-edited. |
543 | Here's how to install the *.ph files: |
544 | |
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545 | 1. become super-user |
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546 | 2. cd /usr/include |
547 | 3. h2ph *.h */*.h |
548 | |
549 | If your system supports dynamic loading, for reasons of portability and |
550 | sanity you probably ought to use h2xs (also part of the standard perl |
551 | distribution). This tool converts C header files to Perl extensions. |
552 | See L<perlxstut> for how to get started with h2xs. |
553 | |
554 | If your system doesn't support dynamic loading, you still probably |
555 | ought to use h2xs. See L<perlxstut> and L<ExtUtils::MakeMaker> for |
556 | more information (in brief, just use B<make perl> instead of a plain |
557 | B<make> to rebuild perl with a new static extension). |
558 | |
559 | =head2 Why do setuid perl scripts complain about kernel problems? |
560 | |
561 | Some operating systems have bugs in the kernel that make setuid |
562 | scripts inherently insecure. Perl gives you a number of options |
563 | (described in L<perlsec>) to work around such systems. |
564 | |
565 | =head2 How can I open a pipe both to and from a command? |
566 | |
567 | The IPC::Open2 module (part of the standard perl distribution) is an |
c8db1d39 |
568 | easy-to-use approach that internally uses pipe(), fork(), and exec() to do |
569 | the job. Make sure you read the deadlock warnings in its documentation, |
570 | though (see L<IPC::Open2>). See L<perlipc/"Bidirectional Communication |
571 | with Another Process"> and L<perlipc/"Bidirectional Communication with |
572 | Yourself"> |
573 | |
574 | You may also use the IPC::Open3 module (part of the standard perl |
575 | distribution), but be warned that it has a different order of |
576 | arguments from IPC::Open2 (see L<IPC::Open3>). |
68dc0745 |
577 | |
3fe9a6f1 |
578 | =head2 Why can't I get the output of a command with system()? |
579 | |
46fc3d4c |
580 | You're confusing the purpose of system() and backticks (``). system() |
581 | runs a command and returns exit status information (as a 16 bit value: |
c8db1d39 |
582 | the low 7 bits are the signal the process died from, if any, and |
46fc3d4c |
583 | the high 8 bits are the actual exit value). Backticks (``) run a |
3fe9a6f1 |
584 | command and return what it sent to STDOUT. |
585 | |
46fc3d4c |
586 | $exit_status = system("mail-users"); |
587 | $output_string = `ls`; |
3fe9a6f1 |
588 | |
68dc0745 |
589 | =head2 How can I capture STDERR from an external command? |
590 | |
591 | There are three basic ways of running external commands: |
592 | |
593 | system $cmd; # using system() |
594 | $output = `$cmd`; # using backticks (``) |
595 | open (PIPE, "cmd |"); # using open() |
596 | |
597 | With system(), both STDOUT and STDERR will go the same place as the |
598 | script's versions of these, unless the command redirects them. |
599 | Backticks and open() read B<only> the STDOUT of your command. |
600 | |
601 | With any of these, you can change file descriptors before the call: |
602 | |
603 | open(STDOUT, ">logfile"); |
604 | system("ls"); |
605 | |
606 | or you can use Bourne shell file-descriptor redirection: |
607 | |
608 | $output = `$cmd 2>some_file`; |
609 | open (PIPE, "cmd 2>some_file |"); |
610 | |
611 | You can also use file-descriptor redirection to make STDERR a |
612 | duplicate of STDOUT: |
613 | |
614 | $output = `$cmd 2>&1`; |
615 | open (PIPE, "cmd 2>&1 |"); |
616 | |
617 | Note that you I<cannot> simply open STDERR to be a dup of STDOUT |
618 | in your Perl program and avoid calling the shell to do the redirection. |
619 | This doesn't work: |
620 | |
621 | open(STDERR, ">&STDOUT"); |
622 | $alloutput = `cmd args`; # stderr still escapes |
623 | |
624 | This fails because the open() makes STDERR go to where STDOUT was |
625 | going at the time of the open(). The backticks then make STDOUT go to |
626 | a string, but don't change STDERR (which still goes to the old |
627 | STDOUT). |
628 | |
629 | Note that you I<must> use Bourne shell (sh(1)) redirection syntax in |
630 | backticks, not csh(1)! Details on why Perl's system() and backtick |
631 | and pipe opens all use the Bourne shell are in |
632 | http://www.perl.com/CPAN/doc/FMTEYEWTK/versus/csh.whynot . |
c8db1d39 |
633 | To capture a command's STDERR and STDOUT together: |
68dc0745 |
634 | |
c8db1d39 |
635 | $output = `cmd 2>&1`; # either with backticks |
636 | $pid = open(PH, "cmd 2>&1 |"); # or with an open pipe |
637 | while (<PH>) { } # plus a read |
638 | |
639 | To capture a command's STDOUT but discard its STDERR: |
640 | |
641 | $output = `cmd 2>/dev/null`; # either with backticks |
642 | $pid = open(PH, "cmd 2>/dev/null |"); # or with an open pipe |
643 | while (<PH>) { } # plus a read |
644 | |
645 | To capture a command's STDERR but discard its STDOUT: |
646 | |
647 | $output = `cmd 2>&1 1>/dev/null`; # either with backticks |
648 | $pid = open(PH, "cmd 2>&1 1>/dev/null |"); # or with an open pipe |
649 | while (<PH>) { } # plus a read |
650 | |
651 | To exchange a command's STDOUT and STDERR in order to capture the STDERR |
652 | but leave its STDOUT to come out our old STDERR: |
653 | |
654 | $output = `cmd 3>&1 1>&2 2>&3 3>&-`; # either with backticks |
655 | $pid = open(PH, "cmd 3>&1 1>&2 2>&3 3>&-|");# or with an open pipe |
656 | while (<PH>) { } # plus a read |
657 | |
658 | To read both a command's STDOUT and its STDERR separately, it's easiest |
659 | and safest to redirect them separately to files, and then read from those |
660 | files when the program is done: |
661 | |
662 | system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr"); |
663 | |
664 | Ordering is important in all these examples. That's because the shell |
665 | processes file descriptor redirections in strictly left to right order. |
666 | |
667 | system("prog args 1>tmpfile 2>&1"); |
668 | system("prog args 2>&1 1>tmpfile"); |
669 | |
670 | The first command sends both standard out and standard error to the |
671 | temporary file. The second command sends only the old standard output |
672 | there, and the old standard error shows up on the old standard out. |
68dc0745 |
673 | |
674 | =head2 Why doesn't open() return an error when a pipe open fails? |
675 | |
65acb1b1 |
676 | Because the pipe open takes place in two steps: first Perl calls |
677 | fork() to start a new process, then this new process calls exec() to |
678 | run the program you really wanted to open. The first step reports |
679 | success or failure to your process, so open() can only tell you |
680 | whether the fork() succeeded or not. |
681 | |
682 | To find out if the exec() step succeeded, you have to catch SIGCHLD |
683 | and wait() to get the exit status. You should also catch SIGPIPE if |
684 | you're writing to the child--you may not have found out the exec() |
3fe9a6f1 |
685 | failed by the time you write. This is documented in L<perlipc>. |
68dc0745 |
686 | |
65acb1b1 |
687 | In some cases, even this won't work. If the second argument to a |
688 | piped open() contains shell metacharacters, perl fork()s, then exec()s |
689 | a shell to decode the metacharacters and eventually run the desired |
690 | program. Now when you call wait(), you only learn whether or not the |
691 | I<shell> could be successfully started. Best to avoid shell |
692 | metacharacters. |
693 | |
68dc0745 |
694 | On systems that follow the spawn() paradigm, open() I<might> do what |
65acb1b1 |
695 | you expect--unless perl uses a shell to start your command. In this |
68dc0745 |
696 | case the fork()/exec() description still applies. |
697 | |
698 | =head2 What's wrong with using backticks in a void context? |
699 | |
700 | Strictly speaking, nothing. Stylistically speaking, it's not a good |
701 | way to write maintainable code because backticks have a (potentially |
d92eb7b0 |
702 | humongous) return value, and you're ignoring it. It's may also not be very |
68dc0745 |
703 | efficient, because you have to read in all the lines of output, allocate |
704 | memory for them, and then throw it away. Too often people are lulled |
705 | to writing: |
706 | |
707 | `cp file file.bak`; |
708 | |
709 | And now they think "Hey, I'll just always use backticks to run programs." |
710 | Bad idea: backticks are for capturing a program's output; the system() |
711 | function is for running programs. |
712 | |
713 | Consider this line: |
714 | |
715 | `cat /etc/termcap`; |
716 | |
717 | You haven't assigned the output anywhere, so it just wastes memory |
718 | (for a little while). Plus you forgot to check C<$?> to see whether |
719 | the program even ran correctly. Even if you wrote |
720 | |
721 | print `cat /etc/termcap`; |
722 | |
723 | In most cases, this could and probably should be written as |
724 | |
725 | system("cat /etc/termcap") == 0 |
726 | or die "cat program failed!"; |
727 | |
d92eb7b0 |
728 | Which will get the output quickly (as it is generated, instead of only |
c8db1d39 |
729 | at the end) and also check the return value. |
68dc0745 |
730 | |
731 | system() also provides direct control over whether shell wildcard |
732 | processing may take place, whereas backticks do not. |
733 | |
734 | =head2 How can I call backticks without shell processing? |
735 | |
736 | This is a bit tricky. Instead of writing |
737 | |
738 | @ok = `grep @opts '$search_string' @filenames`; |
739 | |
740 | You have to do this: |
741 | |
742 | my @ok = (); |
743 | if (open(GREP, "-|")) { |
744 | while (<GREP>) { |
745 | chomp; |
746 | push(@ok, $_); |
747 | } |
748 | close GREP; |
749 | } else { |
750 | exec 'grep', @opts, $search_string, @filenames; |
751 | } |
752 | |
753 | Just as with system(), no shell escapes happen when you exec() a list. |
d92eb7b0 |
754 | Further examples of this can be found in L<perlipc/"Safe Pipe Opens">. |
68dc0745 |
755 | |
d92eb7b0 |
756 | Note that if you're stuck on Microsoft, no solution to this vexing issue |
757 | is even possible. Even if Perl were to emulate fork(), you'd still |
758 | be hosed, because Microsoft gives no argc/argv-style API. Their API |
759 | always reparses from a single string, which is fundamentally wrong, |
760 | but you're not likely to get the Gods of Redmond to acknowledge this |
761 | and fix it for you. |
c8db1d39 |
762 | |
54310121 |
763 | =head2 Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)? |
68dc0745 |
764 | |
765 | Because some stdio's set error and eof flags that need clearing. The |
766 | POSIX module defines clearerr() that you can use. That is the |
767 | technically correct way to do it. Here are some less reliable |
768 | workarounds: |
769 | |
770 | =over 4 |
771 | |
772 | =item 1 |
773 | |
774 | Try keeping around the seekpointer and go there, like this: |
775 | |
776 | $where = tell(LOG); |
777 | seek(LOG, $where, 0); |
778 | |
779 | =item 2 |
780 | |
781 | If that doesn't work, try seeking to a different part of the file and |
782 | then back. |
783 | |
784 | =item 3 |
785 | |
786 | If that doesn't work, try seeking to a different part of |
787 | the file, reading something, and then seeking back. |
788 | |
789 | =item 4 |
790 | |
791 | If that doesn't work, give up on your stdio package and use sysread. |
792 | |
793 | =back |
794 | |
795 | =head2 How can I convert my shell script to perl? |
796 | |
797 | Learn Perl and rewrite it. Seriously, there's no simple converter. |
798 | Things that are awkward to do in the shell are easy to do in Perl, and |
799 | this very awkwardness is what would make a shell->perl converter |
800 | nigh-on impossible to write. By rewriting it, you'll think about what |
801 | you're really trying to do, and hopefully will escape the shell's |
46fc3d4c |
802 | pipeline datastream paradigm, which while convenient for some matters, |
68dc0745 |
803 | causes many inefficiencies. |
804 | |
805 | =head2 Can I use perl to run a telnet or ftp session? |
806 | |
46fc3d4c |
807 | Try the Net::FTP, TCP::Client, and Net::Telnet modules (available from |
808 | CPAN). http://www.perl.com/CPAN/scripts/netstuff/telnet.emul.shar |
809 | will also help for emulating the telnet protocol, but Net::Telnet is |
810 | quite probably easier to use.. |
811 | |
812 | If all you want to do is pretend to be telnet but don't need |
813 | the initial telnet handshaking, then the standard dual-process |
814 | approach will suffice: |
815 | |
816 | use IO::Socket; # new in 5.004 |
817 | $handle = IO::Socket::INET->new('www.perl.com:80') |
818 | || die "can't connect to port 80 on www.perl.com: $!"; |
819 | $handle->autoflush(1); |
820 | if (fork()) { # XXX: undef means failure |
821 | select($handle); |
822 | print while <STDIN>; # everything from stdin to socket |
823 | } else { |
824 | print while <$handle>; # everything from socket to stdout |
825 | } |
826 | close $handle; |
827 | exit; |
68dc0745 |
828 | |
829 | =head2 How can I write expect in Perl? |
830 | |
831 | Once upon a time, there was a library called chat2.pl (part of the |
c8db1d39 |
832 | standard perl distribution), which never really got finished. If you |
833 | find it somewhere, I<don't use it>. These days, your best bet is to |
834 | look at the Expect module available from CPAN, which also requires two |
835 | other modules from CPAN, IO::Pty and IO::Stty. |
68dc0745 |
836 | |
837 | =head2 Is there a way to hide perl's command line from programs such as "ps"? |
838 | |
839 | First of all note that if you're doing this for security reasons (to |
840 | avoid people seeing passwords, for example) then you should rewrite |
841 | your program so that critical information is never given as an |
842 | argument. Hiding the arguments won't make your program completely |
843 | secure. |
844 | |
845 | To actually alter the visible command line, you can assign to the |
846 | variable $0 as documented in L<perlvar>. This won't work on all |
847 | operating systems, though. Daemon programs like sendmail place their |
848 | state there, as in: |
849 | |
850 | $0 = "orcus [accepting connections]"; |
851 | |
852 | =head2 I {changed directory, modified my environment} in a perl script. How come the change disappeared when I exited the script? How do I get my changes to be visible? |
853 | |
854 | =over 4 |
855 | |
856 | =item Unix |
857 | |
858 | In the strictest sense, it can't be done -- the script executes as a |
859 | different process from the shell it was started from. Changes to a |
860 | process are not reflected in its parent, only in its own children |
861 | created after the change. There is shell magic that may allow you to |
862 | fake it by eval()ing the script's output in your shell; check out the |
92c2ed05 |
863 | comp.unix.questions FAQ for details. |
68dc0745 |
864 | |
68dc0745 |
865 | =back |
866 | |
867 | =head2 How do I close a process's filehandle without waiting for it to complete? |
868 | |
869 | Assuming your system supports such things, just send an appropriate signal |
870 | to the process (see L<perlfunc/"kill">. It's common to first send a TERM |
871 | signal, wait a little bit, and then send a KILL signal to finish it off. |
872 | |
873 | =head2 How do I fork a daemon process? |
874 | |
875 | If by daemon process you mean one that's detached (disassociated from |
876 | its tty), then the following process is reported to work on most |
877 | Unixish systems. Non-Unix users should check their Your_OS::Process |
878 | module for other solutions. |
879 | |
880 | =over 4 |
881 | |
882 | =item * |
883 | |
b5a41e52 |
884 | Open /dev/tty and use the TIOCNOTTY ioctl on it. See L<tty(4)> |
c8db1d39 |
885 | for details. Or better yet, you can just use the POSIX::setsid() |
886 | function, so you don't have to worry about process groups. |
68dc0745 |
887 | |
888 | =item * |
889 | |
890 | Change directory to / |
891 | |
892 | =item * |
893 | |
894 | Reopen STDIN, STDOUT, and STDERR so they're not connected to the old |
895 | tty. |
896 | |
897 | =item * |
898 | |
899 | Background yourself like this: |
900 | |
901 | fork && exit; |
902 | |
903 | =back |
904 | |
1a91aff4 |
905 | The Proc::Daemon module, available from CPAN, provides a function to |
906 | perform these actions for you. |
907 | |
68dc0745 |
908 | =head2 How do I find out if I'm running interactively or not? |
909 | |
910 | Good question. Sometimes C<-t STDIN> and C<-t STDOUT> can give clues, |
911 | sometimes not. |
912 | |
913 | if (-t STDIN && -t STDOUT) { |
914 | print "Now what? "; |
915 | } |
916 | |
917 | On POSIX systems, you can test whether your own process group matches |
918 | the current process group of your controlling terminal as follows: |
919 | |
920 | use POSIX qw/getpgrp tcgetpgrp/; |
921 | open(TTY, "/dev/tty") or die $!; |
65acb1b1 |
922 | $tpgrp = tcgetpgrp(fileno(*TTY)); |
68dc0745 |
923 | $pgrp = getpgrp(); |
924 | if ($tpgrp == $pgrp) { |
925 | print "foreground\n"; |
926 | } else { |
927 | print "background\n"; |
928 | } |
929 | |
930 | =head2 How do I timeout a slow event? |
931 | |
932 | Use the alarm() function, probably in conjunction with a signal |
b350dd2f |
933 | handler, as documented in L<perlipc/"Signals"> and chapter 6 of the |
68dc0745 |
934 | Camel. You may instead use the more flexible Sys::AlarmCall module |
935 | available from CPAN. |
936 | |
937 | =head2 How do I set CPU limits? |
938 | |
939 | Use the BSD::Resource module from CPAN. |
940 | |
941 | =head2 How do I avoid zombies on a Unix system? |
942 | |
943 | Use the reaper code from L<perlipc/"Signals"> to call wait() when a |
944 | SIGCHLD is received, or else use the double-fork technique described |
945 | in L<perlfunc/fork>. |
946 | |
947 | =head2 How do I use an SQL database? |
948 | |
949 | There are a number of excellent interfaces to SQL databases. See the |
6cecdcac |
950 | DBD::* modules available from http://www.perl.com/CPAN/modules/DBD . |
c8db1d39 |
951 | A lot of information on this can be found at |
6cecdcac |
952 | http://www.symbolstone.org/technology/perl/DBI/ |
68dc0745 |
953 | |
954 | =head2 How do I make a system() exit on control-C? |
955 | |
956 | You can't. You need to imitate the system() call (see L<perlipc> for |
957 | sample code) and then have a signal handler for the INT signal that |
c8db1d39 |
958 | passes the signal on to the subprocess. Or you can check for it: |
959 | |
960 | $rc = system($cmd); |
961 | if ($rc & 127) { die "signal death" } |
68dc0745 |
962 | |
963 | =head2 How do I open a file without blocking? |
964 | |
965 | If you're lucky enough to be using a system that supports |
966 | non-blocking reads (most Unixish systems do), you need only to use the |
967 | O_NDELAY or O_NONBLOCK flag from the Fcntl module in conjunction with |
968 | sysopen(): |
969 | |
970 | use Fcntl; |
971 | sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644) |
972 | or die "can't open /tmp/somefile: $!": |
973 | |
c47ff5f1 |
974 | |
68dc0745 |
975 | |
d92eb7b0 |
976 | |
977 | =head2 How do I install a module from CPAN? |
978 | |
979 | The easiest way is to have a module also named CPAN do it for you. |
980 | This module comes with perl version 5.004 and later. To manually install |
981 | the CPAN module, or any well-behaved CPAN module for that matter, follow |
982 | these steps: |
68dc0745 |
983 | |
984 | =over 4 |
985 | |
986 | =item 1 |
987 | |
988 | Unpack the source into a temporary area. |
989 | |
990 | =item 2 |
991 | |
992 | perl Makefile.PL |
993 | |
994 | =item 3 |
995 | |
996 | make |
997 | |
998 | =item 4 |
999 | |
1000 | make test |
1001 | |
1002 | =item 5 |
1003 | |
1004 | make install |
1005 | |
1006 | =back |
1007 | |
1008 | If your version of perl is compiled without dynamic loading, then you |
1009 | just need to replace step 3 (B<make>) with B<make perl> and you will |
1010 | get a new F<perl> binary with your extension linked in. |
1011 | |
c8db1d39 |
1012 | See L<ExtUtils::MakeMaker> for more details on building extensions. |
1013 | See also the next question. |
1014 | |
1015 | =head2 What's the difference between require and use? |
1016 | |
1017 | Perl offers several different ways to include code from one file into |
1018 | another. Here are the deltas between the various inclusion constructs: |
1019 | |
1020 | 1) do $file is like eval `cat $file`, except the former: |
5e3006a4 |
1021 | 1.1: searches @INC and updates %INC. |
c8db1d39 |
1022 | 1.2: bequeaths an *unrelated* lexical scope on the eval'ed code. |
1023 | |
1024 | 2) require $file is like do $file, except the former: |
1025 | 2.1: checks for redundant loading, skipping already loaded files. |
1026 | 2.2: raises an exception on failure to find, compile, or execute $file. |
1027 | |
1028 | 3) require Module is like require "Module.pm", except the former: |
1029 | 3.1: translates each "::" into your system's directory separator. |
1030 | 3.2: primes the parser to disambiguate class Module as an indirect object. |
1031 | |
1032 | 4) use Module is like require Module, except the former: |
1033 | 4.1: loads the module at compile time, not run-time. |
1034 | 4.2: imports symbols and semantics from that package to the current one. |
1035 | |
1036 | In general, you usually want C<use> and a proper Perl module. |
46fc3d4c |
1037 | |
1038 | =head2 How do I keep my own module/library directory? |
1039 | |
1040 | When you build modules, use the PREFIX option when generating |
1041 | Makefiles: |
1042 | |
1043 | perl Makefile.PL PREFIX=/u/mydir/perl |
1044 | |
1045 | then either set the PERL5LIB environment variable before you run |
1046 | scripts that use the modules/libraries (see L<perlrun>) or say |
1047 | |
1048 | use lib '/u/mydir/perl'; |
1049 | |
65acb1b1 |
1050 | This is almost the same as: |
1051 | |
1052 | BEGIN { |
1053 | unshift(@INC, '/u/mydir/perl'); |
1054 | } |
1055 | |
1056 | except that the lib module checks for machine-dependent subdirectories. |
46fc3d4c |
1057 | See Perl's L<lib> for more information. |
1058 | |
1059 | =head2 How do I add the directory my program lives in to the module/library search path? |
1060 | |
1061 | use FindBin; |
7b8d334a |
1062 | use lib "$FindBin::Bin"; |
46fc3d4c |
1063 | use your_own_modules; |
1064 | |
1065 | =head2 How do I add a directory to my include path at runtime? |
1066 | |
1067 | Here are the suggested ways of modifying your include path: |
1068 | |
1069 | the PERLLIB environment variable |
1070 | the PERL5LIB environment variable |
c2611fb3 |
1071 | the perl -Idir command line flag |
46fc3d4c |
1072 | the use lib pragma, as in |
1073 | use lib "$ENV{HOME}/myown_perllib"; |
1074 | |
1075 | The latter is particularly useful because it knows about machine |
1076 | dependent architectures. The lib.pm pragmatic module was first |
1077 | included with the 5.002 release of Perl. |
68dc0745 |
1078 | |
65acb1b1 |
1079 | =head2 What is socket.ph and where do I get it? |
1080 | |
1081 | It's a perl4-style file defining values for system networking |
1082 | constants. Sometimes it is built using h2ph when Perl is installed, |
1083 | but other times it is not. Modern programs C<use Socket;> instead. |
1084 | |
fc36a67e |
1085 | =head1 AUTHOR AND COPYRIGHT |
1086 | |
65acb1b1 |
1087 | Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington. |
5a964f20 |
1088 | All rights reserved. |
1089 | |
1090 | When included as part of the Standard Version of Perl, or as part of |
1091 | its complete documentation whether printed or otherwise, this work |
d92eb7b0 |
1092 | may be distributed only under the terms of Perl's Artistic License. |
5a964f20 |
1093 | Any distribution of this file or derivatives thereof I<outside> |
1094 | of that package require that special arrangements be made with |
1095 | copyright holder. |
1096 | |
1097 | Irrespective of its distribution, all code examples in this file |
1098 | are hereby placed into the public domain. You are permitted and |
1099 | encouraged to use this code in your own programs for fun |
1100 | or for profit as you see fit. A simple comment in the code giving |
1101 | credit would be courteous but is not required. |