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68dc0745 | 1 | =head1 NAME |
2 | ||
b68463f7 | 3 | perlfaq8 - System Interaction ($Revision: 1.25 $, $Date: 2005/06/04 04:12:40 $) |
68dc0745 | 4 | |
5 | =head1 DESCRIPTION | |
6 | ||
7 | This section of the Perl FAQ covers questions involving operating | |
a6dd486b | 8 | system interaction. Topics include interprocess communication (IPC), |
68dc0745 | 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 | |
46fc3d4c | 13 | operating system (eg, L<perlvms>, L<perlplan9>, ...). These should |
68dc0745 | 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 | ||
d92eb7b0 GS |
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. | |
68dc0745 | 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 | ||
c8db1d39 TC |
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. | |
d92eb7b0 | 78 | On many systems, you can just use the B<stty> command as shown in |
c8db1d39 | 79 | L<perlfunc/getc>, but as you see, that's already getting you into |
197aec24 | 80 | portability snags. |
c8db1d39 TC |
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 | ||
a6dd486b JB |
98 | However, using the code requires that you have a working C compiler |
99 | and can use it to build and install a CPAN module. Here's a solution | |
100 | using the standard POSIX module, which is already on your systems | |
101 | (assuming your system supports POSIX). | |
c8db1d39 TC |
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 | ||
d92eb7b0 | 171 | If you only have do so infrequently, use C<system>: |
c8db1d39 TC |
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 = $terminal->Tputs('cl'); | |
188 | ||
189 | =head2 How do I get the screen size? | |
190 | ||
197aec24 | 191 | If you have Term::ReadKey module installed from CPAN, |
c8db1d39 TC |
192 | you can use it to fetch the width and height in characters |
193 | and in pixels: | |
194 | ||
195 | use Term::ReadKey; | |
196 | ($wchar, $hchar, $wpixels, $hpixels) = GetTerminalSize(); | |
197 | ||
197aec24 | 198 | This is more portable than the raw C<ioctl>, but not as |
c8db1d39 TC |
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 | ||
68dc0745 | 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 | ||
a6dd486b JB |
217 | There's an example of this in L<perlfunc/crypt>). First, you put the |
218 | terminal into "no echo" mode, then just read the password normally. | |
219 | You may do this with an old-style ioctl() function, POSIX terminal | |
b73a15ae | 220 | control (see L<POSIX> or its documentation the Camel Book), or a call |
68dc0745 | 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 | ||
c8db1d39 TC |
226 | use Term::ReadKey; |
227 | ||
228 | ReadMode('noecho'); | |
229 | $password = ReadLine(0); | |
230 | ||
68dc0745 | 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 | |
a6dd486b | 235 | /dev; on other systems, device names will doubtless differ. |
68dc0745 | 236 | Several problem areas common to all device interaction are the |
a6dd486b | 237 | following: |
68dc0745 | 238 | |
239 | =over 4 | |
240 | ||
241 | =item lockfiles | |
242 | ||
243 | Your system may use lockfiles to control multiple access. Make sure | |
a6dd486b | 244 | you follow the correct protocol. Unpredictable behavior can result |
68dc0745 | 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 | ||
a6dd486b | 267 | Even though with normal text files a "\n" will do the trick, there is |
68dc0745 | 268 | still no unified scheme for terminating a line that is portable |
46fc3d4c | 269 | between Unix, DOS/Win, and Macintosh, except to terminate I<ALL> line |
68dc0745 | 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, | |
c8db1d39 | 277 | you'll want to autoflush that filehandle. You can use select() |
197aec24 | 278 | and the C<$|> variable to control autoflushing (see L<perlvar/$E<verbar>> |
b432a672 AL |
279 | and L<perlfunc/select>, or L<perlfaq5>, "How do I flush/unbuffer an |
280 | output filehandle? Why must I do this?"): | |
68dc0745 | 281 | |
282 | $oldh = select(DEV); | |
283 | $| = 1; | |
284 | select($oldh); | |
285 | ||
286 | You'll also see code that does this without a temporary variable, as in | |
287 | ||
288 | select((select(DEV), $| = 1)[0]); | |
289 | ||
c8db1d39 TC |
290 | Or if you don't mind pulling in a few thousand lines |
291 | of code just because you're afraid of a little $| variable: | |
292 | ||
293 | use IO::Handle; | |
294 | DEV->autoflush(1); | |
295 | ||
68dc0745 | 296 | As mentioned in the previous item, this still doesn't work when using |
8305e449 | 297 | socket I/O between Unix and Macintosh. You'll need to hard code your |
68dc0745 | 298 | line terminators, in that case. |
299 | ||
300 | =item non-blocking input | |
301 | ||
302 | If you are doing a blocking read() or sysread(), you'll have to | |
303 | arrange for an alarm handler to provide a timeout (see | |
304 | L<perlfunc/alarm>). If you have a non-blocking open, you'll likely | |
305 | have a non-blocking read, which means you may have to use a 4-arg | |
306 | select() to determine whether I/O is ready on that device (see | |
307 | L<perlfunc/"select">. | |
308 | ||
309 | =back | |
310 | ||
c8db1d39 TC |
311 | While trying to read from his caller-id box, the notorious Jamie Zawinski |
312 | <jwz@netscape.com>, after much gnashing of teeth and fighting with sysread, | |
313 | sysopen, POSIX's tcgetattr business, and various other functions that | |
314 | go bump in the night, finally came up with this: | |
315 | ||
316 | sub open_modem { | |
317 | use IPC::Open2; | |
318 | my $stty = `/bin/stty -g`; | |
319 | open2( \*MODEM_IN, \*MODEM_OUT, "cu -l$modem_device -s2400 2>&1"); | |
320 | # starting cu hoses /dev/tty's stty settings, even when it has | |
321 | # been opened on a pipe... | |
322 | system("/bin/stty $stty"); | |
323 | $_ = <MODEM_IN>; | |
5b3eff12 | 324 | chomp; |
c8db1d39 TC |
325 | if ( !m/^Connected/ ) { |
326 | print STDERR "$0: cu printed `$_' instead of `Connected'\n"; | |
327 | } | |
328 | } | |
329 | ||
68dc0745 | 330 | =head2 How do I decode encrypted password files? |
331 | ||
332 | You spend lots and lots of money on dedicated hardware, but this is | |
333 | bound to get you talked about. | |
334 | ||
a6dd486b | 335 | Seriously, you can't if they are Unix password files--the Unix |
c8db1d39 TC |
336 | password system employs one-way encryption. It's more like hashing than |
337 | encryption. The best you can check is whether something else hashes to | |
338 | the same string. You can't turn a hash back into the original string. | |
339 | Programs like Crack | |
340 | can forcibly (and intelligently) try to guess passwords, but don't | |
341 | (can't) guarantee quick success. | |
68dc0745 | 342 | |
343 | If you're worried about users selecting bad passwords, you should | |
344 | proactively check when they try to change their password (by modifying | |
345 | passwd(1), for example). | |
346 | ||
347 | =head2 How do I start a process in the background? | |
348 | ||
49d635f9 RGS |
349 | Several modules can start other processes that do not block |
350 | your Perl program. You can use IPC::Open3, Parallel::Jobs, | |
351 | IPC::Run, and some of the POE modules. See CPAN for more | |
352 | details. | |
353 | ||
354 | You could also use | |
68dc0745 | 355 | |
356 | system("cmd &") | |
357 | ||
358 | or you could use fork as documented in L<perlfunc/"fork">, with | |
359 | further examples in L<perlipc>. Some things to be aware of, if you're | |
360 | on a Unix-like system: | |
361 | ||
362 | =over 4 | |
363 | ||
c8db1d39 | 364 | =item STDIN, STDOUT, and STDERR are shared |
68dc0745 | 365 | |
366 | Both the main process and the backgrounded one (the "child" process) | |
367 | share the same STDIN, STDOUT and STDERR filehandles. If both try to | |
368 | access them at once, strange things can happen. You may want to close | |
369 | or reopen these for the child. You can get around this with | |
370 | C<open>ing a pipe (see L<perlfunc/"open">) but on some systems this | |
371 | means that the child process cannot outlive the parent. | |
372 | ||
373 | =item Signals | |
374 | ||
375 | You'll have to catch the SIGCHLD signal, and possibly SIGPIPE too. | |
376 | SIGCHLD is sent when the backgrounded process finishes. SIGPIPE is | |
377 | sent when you write to a filehandle whose child process has closed (an | |
378 | untrapped SIGPIPE can cause your program to silently die). This is | |
379 | not an issue with C<system("cmd&")>. | |
380 | ||
381 | =item Zombies | |
382 | ||
49d635f9 | 383 | You have to be prepared to "reap" the child process when it finishes. |
68dc0745 | 384 | |
385 | $SIG{CHLD} = sub { wait }; | |
197aec24 | 386 | |
49d635f9 | 387 | $SIG{CHLD} = 'IGNORE'; |
197aec24 RGS |
388 | |
389 | You can also use a double fork. You immediately wait() for your | |
390 | first child, and the init daemon will wait() for your grandchild once | |
49d635f9 RGS |
391 | it exits. |
392 | ||
393 | unless ($pid = fork) { | |
394 | unless (fork) { | |
395 | exec "what you really wanna do"; | |
396 | die "exec failed!"; | |
397 | } | |
398 | exit 0; | |
399 | } | |
400 | waitpid($pid,0); | |
401 | ||
68dc0745 | 402 | |
403 | See L<perlipc/"Signals"> for other examples of code to do this. | |
404 | Zombies are not an issue with C<system("prog &")>. | |
405 | ||
406 | =back | |
407 | ||
408 | =head2 How do I trap control characters/signals? | |
409 | ||
c8db1d39 TC |
410 | You don't actually "trap" a control character. Instead, that character |
411 | generates a signal which is sent to your terminal's currently | |
412 | foregrounded process group, which you then trap in your process. | |
b73a15ae | 413 | Signals are documented in L<perlipc/"Signals"> and the |
b432a672 | 414 | section on "Signals" in the Camel. |
68dc0745 | 415 | |
6670e5e7 | 416 | You can set the values of the %SIG hash to be the functions you want |
c98c5709 RGS |
417 | to handle the signal. After perl catches the signal, it looks in %SIG |
418 | for a key with the same name as the signal, then calls the subroutine | |
419 | value for that key. | |
420 | ||
421 | # as an anonymous subroutine | |
6670e5e7 | 422 | |
c98c5709 | 423 | $SIG{INT} = sub { syswrite(STDERR, "ouch\n", 5 ) }; |
6670e5e7 | 424 | |
c98c5709 | 425 | # or a reference to a function |
6670e5e7 | 426 | |
c98c5709 | 427 | $SIG{INT} = \&ouch; |
6670e5e7 | 428 | |
c98c5709 | 429 | # or the name of the function as a string |
6670e5e7 RGS |
430 | |
431 | $SIG{INT} = "ouch"; | |
c98c5709 RGS |
432 | |
433 | Perl versions before 5.8 had in its C source code signal handlers which | |
434 | would catch the signal and possibly run a Perl function that you had set | |
435 | in %SIG. This violated the rules of signal handling at that level | |
436 | causing perl to dump core. Since version 5.8.0, perl looks at %SIG | |
437 | *after* the signal has been caught, rather than while it is being caught. | |
438 | Previous versions of this answer were incorrect. | |
68dc0745 | 439 | |
68dc0745 | 440 | |
441 | =head2 How do I modify the shadow password file on a Unix system? | |
442 | ||
a6dd486b | 443 | If perl was installed correctly and your shadow library was written |
c8db1d39 TC |
444 | properly, the getpw*() functions described in L<perlfunc> should in |
445 | theory provide (read-only) access to entries in the shadow password | |
446 | file. To change the file, make a new shadow password file (the format | |
197aec24 RGS |
447 | varies from system to system--see L<passwd> for specifics) and use |
448 | pwd_mkdb(8) to install it (see L<pwd_mkdb> for more details). | |
68dc0745 | 449 | |
450 | =head2 How do I set the time and date? | |
451 | ||
452 | Assuming you're running under sufficient permissions, you should be | |
453 | able to set the system-wide date and time by running the date(1) | |
454 | program. (There is no way to set the time and date on a per-process | |
455 | basis.) This mechanism will work for Unix, MS-DOS, Windows, and NT; | |
456 | the VMS equivalent is C<set time>. | |
457 | ||
8305e449 | 458 | However, if all you want to do is change your time zone, you can |
68dc0745 | 459 | probably get away with setting an environment variable: |
460 | ||
461 | $ENV{TZ} = "MST7MDT"; # unixish | |
462 | $ENV{'SYS$TIMEZONE_DIFFERENTIAL'}="-5" # vms | |
c8db1d39 | 463 | system "trn comp.lang.perl.misc"; |
68dc0745 | 464 | |
465 | =head2 How can I sleep() or alarm() for under a second? | |
466 | ||
467 | If you want finer granularity than the 1 second that the sleep() | |
468 | function provides, the easiest way is to use the select() function as | |
0325b4c4 | 469 | documented in L<perlfunc/"select">. Try the Time::HiRes and |
83df6a1d JH |
470 | the BSD::Itimer modules (available from CPAN, and starting from |
471 | Perl 5.8 Time::HiRes is part of the standard distribution). | |
68dc0745 | 472 | |
473 | =head2 How can I measure time under a second? | |
474 | ||
65acb1b1 | 475 | In general, you may not be able to. The Time::HiRes module (available |
83df6a1d JH |
476 | from CPAN, and starting from Perl 5.8 part of the standard distribution) |
477 | provides this functionality for some systems. | |
68dc0745 | 478 | |
65acb1b1 TC |
479 | If your system supports both the syscall() function in Perl as well as |
480 | a system call like gettimeofday(2), then you may be able to do | |
481 | something like this: | |
68dc0745 | 482 | |
483 | require 'sys/syscall.ph'; | |
484 | ||
485 | $TIMEVAL_T = "LL"; | |
486 | ||
487 | $done = $start = pack($TIMEVAL_T, ()); | |
488 | ||
d92eb7b0 | 489 | syscall(&SYS_gettimeofday, $start, 0) != -1 |
68dc0745 | 490 | or die "gettimeofday: $!"; |
491 | ||
492 | ########################## | |
493 | # DO YOUR OPERATION HERE # | |
494 | ########################## | |
495 | ||
496 | syscall( &SYS_gettimeofday, $done, 0) != -1 | |
497 | or die "gettimeofday: $!"; | |
498 | ||
499 | @start = unpack($TIMEVAL_T, $start); | |
500 | @done = unpack($TIMEVAL_T, $done); | |
501 | ||
502 | # fix microseconds | |
503 | for ($done[1], $start[1]) { $_ /= 1_000_000 } | |
504 | ||
505 | $delta_time = sprintf "%.4f", ($done[0] + $done[1] ) | |
506 | - | |
507 | ($start[0] + $start[1] ); | |
508 | ||
509 | =head2 How can I do an atexit() or setjmp()/longjmp()? (Exception handling) | |
510 | ||
511 | Release 5 of Perl added the END block, which can be used to simulate | |
512 | atexit(). Each package's END block is called when the program or | |
197aec24 | 513 | thread ends (see L<perlmod> manpage for more details). |
c8db1d39 TC |
514 | |
515 | For example, you can use this to make sure your filter program | |
516 | managed to finish its output without filling up the disk: | |
517 | ||
518 | END { | |
519 | close(STDOUT) || die "stdout close failed: $!"; | |
197aec24 | 520 | } |
c8db1d39 | 521 | |
a6dd486b JB |
522 | The END block isn't called when untrapped signals kill the program, |
523 | though, so if you use END blocks you should also use | |
68dc0745 | 524 | |
525 | use sigtrap qw(die normal-signals); | |
526 | ||
527 | Perl's exception-handling mechanism is its eval() operator. You can | |
528 | use eval() as setjmp and die() as longjmp. For details of this, see | |
46fc3d4c | 529 | the section on signals, especially the time-out handler for a blocking |
b432a672 | 530 | flock() in L<perlipc/"Signals"> or the section on "Signals" in |
b73a15ae | 531 | the Camel Book. |
68dc0745 | 532 | |
533 | If exception handling is all you're interested in, try the | |
534 | exceptions.pl library (part of the standard perl distribution). | |
535 | ||
536 | If you want the atexit() syntax (and an rmexit() as well), try the | |
537 | AtExit module available from CPAN. | |
538 | ||
a6dd486b | 539 | =head2 Why doesn't my sockets program work under System V (Solaris)? What does the error message "Protocol not supported" mean? |
68dc0745 | 540 | |
541 | Some Sys-V based systems, notably Solaris 2.X, redefined some of the | |
542 | standard socket constants. Since these were constant across all | |
543 | architectures, they were often hardwired into perl code. The proper | |
544 | way to deal with this is to "use Socket" to get the correct values. | |
545 | ||
546 | Note that even though SunOS and Solaris are binary compatible, these | |
547 | values are different. Go figure. | |
548 | ||
549 | =head2 How can I call my system's unique C functions from Perl? | |
550 | ||
a6dd486b | 551 | In most cases, you write an external module to do it--see the answer |
68dc0745 | 552 | to "Where can I learn about linking C with Perl? [h2xs, xsubpp]". |
553 | However, if the function is a system call, and your system supports | |
554 | syscall(), you can use the syscall function (documented in | |
555 | L<perlfunc>). | |
556 | ||
557 | Remember to check the modules that came with your distribution, and | |
197aec24 RGS |
558 | CPAN as well---someone may already have written a module to do it. On |
559 | Windows, try Win32::API. On Macs, try Mac::Carbon. If no module | |
560 | has an interface to the C function, you can inline a bit of C in your | |
561 | Perl source with Inline::C. | |
68dc0745 | 562 | |
563 | =head2 Where do I get the include files to do ioctl() or syscall()? | |
564 | ||
565 | Historically, these would be generated by the h2ph tool, part of the | |
566 | standard perl distribution. This program converts cpp(1) directives | |
567 | in C header files to files containing subroutine definitions, like | |
568 | &SYS_getitimer, which you can use as arguments to your functions. | |
569 | It doesn't work perfectly, but it usually gets most of the job done. | |
570 | Simple files like F<errno.h>, F<syscall.h>, and F<socket.h> were fine, | |
571 | but the hard ones like F<ioctl.h> nearly always need to hand-edited. | |
572 | Here's how to install the *.ph files: | |
573 | ||
46fc3d4c | 574 | 1. become super-user |
68dc0745 | 575 | 2. cd /usr/include |
576 | 3. h2ph *.h */*.h | |
577 | ||
578 | If your system supports dynamic loading, for reasons of portability and | |
579 | sanity you probably ought to use h2xs (also part of the standard perl | |
580 | distribution). This tool converts C header files to Perl extensions. | |
581 | See L<perlxstut> for how to get started with h2xs. | |
582 | ||
583 | If your system doesn't support dynamic loading, you still probably | |
584 | ought to use h2xs. See L<perlxstut> and L<ExtUtils::MakeMaker> for | |
585 | more information (in brief, just use B<make perl> instead of a plain | |
586 | B<make> to rebuild perl with a new static extension). | |
587 | ||
588 | =head2 Why do setuid perl scripts complain about kernel problems? | |
589 | ||
590 | Some operating systems have bugs in the kernel that make setuid | |
591 | scripts inherently insecure. Perl gives you a number of options | |
592 | (described in L<perlsec>) to work around such systems. | |
593 | ||
594 | =head2 How can I open a pipe both to and from a command? | |
595 | ||
596 | The IPC::Open2 module (part of the standard perl distribution) is an | |
c8db1d39 TC |
597 | easy-to-use approach that internally uses pipe(), fork(), and exec() to do |
598 | the job. Make sure you read the deadlock warnings in its documentation, | |
197aec24 RGS |
599 | though (see L<IPC::Open2>). See |
600 | L<perlipc/"Bidirectional Communication with Another Process"> and | |
13a2d996 | 601 | L<perlipc/"Bidirectional Communication with Yourself"> |
c8db1d39 TC |
602 | |
603 | You may also use the IPC::Open3 module (part of the standard perl | |
604 | distribution), but be warned that it has a different order of | |
605 | arguments from IPC::Open2 (see L<IPC::Open3>). | |
68dc0745 | 606 | |
3fe9a6f1 | 607 | =head2 Why can't I get the output of a command with system()? |
608 | ||
46fc3d4c | 609 | You're confusing the purpose of system() and backticks (``). system() |
610 | runs a command and returns exit status information (as a 16 bit value: | |
c8db1d39 | 611 | the low 7 bits are the signal the process died from, if any, and |
46fc3d4c | 612 | the high 8 bits are the actual exit value). Backticks (``) run a |
3fe9a6f1 | 613 | command and return what it sent to STDOUT. |
614 | ||
46fc3d4c | 615 | $exit_status = system("mail-users"); |
616 | $output_string = `ls`; | |
3fe9a6f1 | 617 | |
68dc0745 | 618 | =head2 How can I capture STDERR from an external command? |
619 | ||
620 | There are three basic ways of running external commands: | |
621 | ||
622 | system $cmd; # using system() | |
623 | $output = `$cmd`; # using backticks (``) | |
624 | open (PIPE, "cmd |"); # using open() | |
625 | ||
626 | With system(), both STDOUT and STDERR will go the same place as the | |
a6dd486b | 627 | script's STDOUT and STDERR, unless the system() command redirects them. |
68dc0745 | 628 | Backticks and open() read B<only> the STDOUT of your command. |
629 | ||
49d635f9 RGS |
630 | You can also use the open3() function from IPC::Open3. Benjamin |
631 | Goldberg provides some sample code: | |
632 | ||
633 | To capture a program's STDOUT, but discard its STDERR: | |
634 | ||
635 | use IPC::Open3; | |
636 | use File::Spec; | |
637 | use Symbol qw(gensym); | |
638 | open(NULL, ">", File::Spec->devnull); | |
639 | my $pid = open3(gensym, \*PH, ">&NULL", "cmd"); | |
640 | while( <PH> ) { } | |
641 | waitpid($pid, 0); | |
642 | ||
643 | To capture a program's STDERR, but discard its STDOUT: | |
644 | ||
645 | use IPC::Open3; | |
646 | use File::Spec; | |
647 | use Symbol qw(gensym); | |
648 | open(NULL, ">", File::Spec->devnull); | |
649 | my $pid = open3(gensym, ">&NULL", \*PH, "cmd"); | |
650 | while( <PH> ) { } | |
651 | waitpid($pid, 0); | |
652 | ||
653 | To capture a program's STDERR, and let its STDOUT go to our own STDERR: | |
654 | ||
655 | use IPC::Open3; | |
656 | use Symbol qw(gensym); | |
657 | my $pid = open3(gensym, ">&STDERR", \*PH, "cmd"); | |
658 | while( <PH> ) { } | |
659 | waitpid($pid, 0); | |
660 | ||
661 | To read both a command's STDOUT and its STDERR separately, you can | |
662 | redirect them to temp files, let the command run, then read the temp | |
663 | files: | |
664 | ||
665 | use IPC::Open3; | |
666 | use Symbol qw(gensym); | |
667 | use IO::File; | |
668 | local *CATCHOUT = IO::File->new_tempfile; | |
669 | local *CATCHERR = IO::File->new_tempfile; | |
670 | my $pid = open3(gensym, ">&CATCHOUT", ">&CATCHERR", "cmd"); | |
671 | waitpid($pid, 0); | |
672 | seek $_, 0, 0 for \*CATCHOUT, \*CATCHERR; | |
673 | while( <CATCHOUT> ) {} | |
674 | while( <CATCHERR> ) {} | |
675 | ||
676 | But there's no real need for *both* to be tempfiles... the following | |
677 | should work just as well, without deadlocking: | |
678 | ||
679 | use IPC::Open3; | |
680 | use Symbol qw(gensym); | |
681 | use IO::File; | |
682 | local *CATCHERR = IO::File->new_tempfile; | |
683 | my $pid = open3(gensym, \*CATCHOUT, ">&CATCHERR", "cmd"); | |
684 | while( <CATCHOUT> ) {} | |
685 | waitpid($pid, 0); | |
686 | seek CATCHERR, 0, 0; | |
687 | while( <CATCHERR> ) {} | |
688 | ||
689 | And it'll be faster, too, since we can begin processing the program's | |
690 | stdout immediately, rather than waiting for the program to finish. | |
691 | ||
68dc0745 | 692 | With any of these, you can change file descriptors before the call: |
693 | ||
694 | open(STDOUT, ">logfile"); | |
695 | system("ls"); | |
696 | ||
697 | or you can use Bourne shell file-descriptor redirection: | |
698 | ||
699 | $output = `$cmd 2>some_file`; | |
700 | open (PIPE, "cmd 2>some_file |"); | |
701 | ||
702 | You can also use file-descriptor redirection to make STDERR a | |
703 | duplicate of STDOUT: | |
704 | ||
705 | $output = `$cmd 2>&1`; | |
706 | open (PIPE, "cmd 2>&1 |"); | |
707 | ||
708 | Note that you I<cannot> simply open STDERR to be a dup of STDOUT | |
709 | in your Perl program and avoid calling the shell to do the redirection. | |
710 | This doesn't work: | |
711 | ||
712 | open(STDERR, ">&STDOUT"); | |
713 | $alloutput = `cmd args`; # stderr still escapes | |
714 | ||
715 | This fails because the open() makes STDERR go to where STDOUT was | |
716 | going at the time of the open(). The backticks then make STDOUT go to | |
717 | a string, but don't change STDERR (which still goes to the old | |
718 | STDOUT). | |
719 | ||
720 | Note that you I<must> use Bourne shell (sh(1)) redirection syntax in | |
721 | backticks, not csh(1)! Details on why Perl's system() and backtick | |
06a5f41f JH |
722 | and pipe opens all use the Bourne shell are in the |
723 | F<versus/csh.whynot> article in the "Far More Than You Ever Wanted To | |
49d635f9 | 724 | Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz . To |
06a5f41f | 725 | capture a command's STDERR and STDOUT together: |
68dc0745 | 726 | |
c8db1d39 TC |
727 | $output = `cmd 2>&1`; # either with backticks |
728 | $pid = open(PH, "cmd 2>&1 |"); # or with an open pipe | |
729 | while (<PH>) { } # plus a read | |
730 | ||
731 | To capture a command's STDOUT but discard its STDERR: | |
732 | ||
733 | $output = `cmd 2>/dev/null`; # either with backticks | |
734 | $pid = open(PH, "cmd 2>/dev/null |"); # or with an open pipe | |
735 | while (<PH>) { } # plus a read | |
736 | ||
737 | To capture a command's STDERR but discard its STDOUT: | |
738 | ||
739 | $output = `cmd 2>&1 1>/dev/null`; # either with backticks | |
740 | $pid = open(PH, "cmd 2>&1 1>/dev/null |"); # or with an open pipe | |
741 | while (<PH>) { } # plus a read | |
742 | ||
743 | To exchange a command's STDOUT and STDERR in order to capture the STDERR | |
744 | but leave its STDOUT to come out our old STDERR: | |
745 | ||
746 | $output = `cmd 3>&1 1>&2 2>&3 3>&-`; # either with backticks | |
747 | $pid = open(PH, "cmd 3>&1 1>&2 2>&3 3>&-|");# or with an open pipe | |
748 | while (<PH>) { } # plus a read | |
749 | ||
750 | To read both a command's STDOUT and its STDERR separately, it's easiest | |
2359510d SD |
751 | to redirect them separately to files, and then read from those files |
752 | when the program is done: | |
c8db1d39 | 753 | |
2359510d | 754 | system("program args 1>program.stdout 2>program.stderr"); |
c8db1d39 TC |
755 | |
756 | Ordering is important in all these examples. That's because the shell | |
757 | processes file descriptor redirections in strictly left to right order. | |
758 | ||
759 | system("prog args 1>tmpfile 2>&1"); | |
760 | system("prog args 2>&1 1>tmpfile"); | |
761 | ||
762 | The first command sends both standard out and standard error to the | |
763 | temporary file. The second command sends only the old standard output | |
764 | there, and the old standard error shows up on the old standard out. | |
68dc0745 | 765 | |
766 | =head2 Why doesn't open() return an error when a pipe open fails? | |
767 | ||
49d635f9 | 768 | If the second argument to a piped open() contains shell |
dfdf0730 MJD |
769 | metacharacters, perl fork()s, then exec()s a shell to decode the |
770 | metacharacters and eventually run the desired program. If the program | |
771 | couldn't be run, it's the shell that gets the message, not Perl. All | |
772 | your Perl program can find out is whether the shell itself could be | |
773 | successfully started. You can still capture the shell's STDERR and | |
774 | check it for error messages. See L<"How can I capture STDERR from an | |
775 | external command?"> elsewhere in this document, or use the | |
49d635f9 | 776 | IPC::Open3 module. |
dfdf0730 | 777 | |
49d635f9 | 778 | If there are no shell metacharacters in the argument of open(), Perl |
dfdf0730 MJD |
779 | runs the command directly, without using the shell, and can correctly |
780 | report whether the command started. | |
68dc0745 | 781 | |
782 | =head2 What's wrong with using backticks in a void context? | |
783 | ||
784 | Strictly speaking, nothing. Stylistically speaking, it's not a good | |
dfdf0730 MJD |
785 | way to write maintainable code. Perl has several operators for |
786 | running external commands. Backticks are one; they collect the output | |
787 | from the command for use in your program. The C<system> function is | |
197aec24 | 788 | another; it doesn't do this. |
68dc0745 | 789 | |
dfdf0730 MJD |
790 | Writing backticks in your program sends a clear message to the readers |
791 | of your code that you wanted to collect the output of the command. | |
792 | Why send a clear message that isn't true? | |
68dc0745 | 793 | |
794 | Consider this line: | |
795 | ||
796 | `cat /etc/termcap`; | |
797 | ||
dfdf0730 MJD |
798 | You forgot to check C<$?> to see whether the program even ran |
799 | correctly. Even if you wrote | |
68dc0745 | 800 | |
801 | print `cat /etc/termcap`; | |
802 | ||
a6dd486b | 803 | this code could and probably should be written as |
68dc0745 | 804 | |
805 | system("cat /etc/termcap") == 0 | |
806 | or die "cat program failed!"; | |
807 | ||
a6dd486b | 808 | which will get the output quickly (as it is generated, instead of only |
c8db1d39 | 809 | at the end) and also check the return value. |
68dc0745 | 810 | |
811 | system() also provides direct control over whether shell wildcard | |
812 | processing may take place, whereas backticks do not. | |
813 | ||
814 | =head2 How can I call backticks without shell processing? | |
815 | ||
49d635f9 RGS |
816 | This is a bit tricky. You can't simply write the command |
817 | like this: | |
68dc0745 | 818 | |
819 | @ok = `grep @opts '$search_string' @filenames`; | |
820 | ||
49d635f9 RGS |
821 | As of Perl 5.8.0, you can use open() with multiple arguments. |
822 | Just like the list forms of system() and exec(), no shell | |
823 | escapes happen. | |
824 | ||
825 | open( GREP, "-|", 'grep', @opts, $search_string, @filenames ); | |
826 | chomp(@ok = <GREP>); | |
827 | close GREP; | |
828 | ||
829 | You can also: | |
68dc0745 | 830 | |
831 | my @ok = (); | |
832 | if (open(GREP, "-|")) { | |
833 | while (<GREP>) { | |
834 | chomp; | |
835 | push(@ok, $_); | |
836 | } | |
837 | close GREP; | |
838 | } else { | |
839 | exec 'grep', @opts, $search_string, @filenames; | |
840 | } | |
841 | ||
842 | Just as with system(), no shell escapes happen when you exec() a list. | |
d92eb7b0 | 843 | Further examples of this can be found in L<perlipc/"Safe Pipe Opens">. |
68dc0745 | 844 | |
49d635f9 | 845 | Note that if you're use Microsoft, no solution to this vexing issue |
d92eb7b0 | 846 | is even possible. Even if Perl were to emulate fork(), you'd still |
49d635f9 | 847 | be stuck, because Microsoft does not have a argc/argv-style API. |
c8db1d39 | 848 | |
54310121 | 849 | =head2 Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)? |
68dc0745 | 850 | |
a6dd486b | 851 | Some stdio's set error and eof flags that need clearing. The |
68dc0745 | 852 | POSIX module defines clearerr() that you can use. That is the |
853 | technically correct way to do it. Here are some less reliable | |
854 | workarounds: | |
855 | ||
856 | =over 4 | |
857 | ||
858 | =item 1 | |
859 | ||
860 | Try keeping around the seekpointer and go there, like this: | |
861 | ||
862 | $where = tell(LOG); | |
863 | seek(LOG, $where, 0); | |
864 | ||
865 | =item 2 | |
866 | ||
867 | If that doesn't work, try seeking to a different part of the file and | |
868 | then back. | |
869 | ||
870 | =item 3 | |
871 | ||
872 | If that doesn't work, try seeking to a different part of | |
873 | the file, reading something, and then seeking back. | |
874 | ||
875 | =item 4 | |
876 | ||
877 | If that doesn't work, give up on your stdio package and use sysread. | |
878 | ||
879 | =back | |
880 | ||
881 | =head2 How can I convert my shell script to perl? | |
882 | ||
883 | Learn Perl and rewrite it. Seriously, there's no simple converter. | |
884 | Things that are awkward to do in the shell are easy to do in Perl, and | |
885 | this very awkwardness is what would make a shell->perl converter | |
886 | nigh-on impossible to write. By rewriting it, you'll think about what | |
887 | you're really trying to do, and hopefully will escape the shell's | |
46fc3d4c | 888 | pipeline datastream paradigm, which while convenient for some matters, |
68dc0745 | 889 | causes many inefficiencies. |
890 | ||
891 | =head2 Can I use perl to run a telnet or ftp session? | |
892 | ||
46fc3d4c | 893 | Try the Net::FTP, TCP::Client, and Net::Telnet modules (available from |
a93751fa | 894 | CPAN). http://www.cpan.org/scripts/netstuff/telnet.emul.shar |
46fc3d4c | 895 | will also help for emulating the telnet protocol, but Net::Telnet is |
896 | quite probably easier to use.. | |
897 | ||
898 | If all you want to do is pretend to be telnet but don't need | |
899 | the initial telnet handshaking, then the standard dual-process | |
900 | approach will suffice: | |
901 | ||
902 | use IO::Socket; # new in 5.004 | |
903 | $handle = IO::Socket::INET->new('www.perl.com:80') | |
904 | || die "can't connect to port 80 on www.perl.com: $!"; | |
905 | $handle->autoflush(1); | |
906 | if (fork()) { # XXX: undef means failure | |
907 | select($handle); | |
908 | print while <STDIN>; # everything from stdin to socket | |
909 | } else { | |
910 | print while <$handle>; # everything from socket to stdout | |
911 | } | |
912 | close $handle; | |
913 | exit; | |
68dc0745 | 914 | |
915 | =head2 How can I write expect in Perl? | |
916 | ||
917 | Once upon a time, there was a library called chat2.pl (part of the | |
c8db1d39 TC |
918 | standard perl distribution), which never really got finished. If you |
919 | find it somewhere, I<don't use it>. These days, your best bet is to | |
920 | look at the Expect module available from CPAN, which also requires two | |
921 | other modules from CPAN, IO::Pty and IO::Stty. | |
68dc0745 | 922 | |
923 | =head2 Is there a way to hide perl's command line from programs such as "ps"? | |
924 | ||
925 | First of all note that if you're doing this for security reasons (to | |
926 | avoid people seeing passwords, for example) then you should rewrite | |
927 | your program so that critical information is never given as an | |
928 | argument. Hiding the arguments won't make your program completely | |
929 | secure. | |
930 | ||
931 | To actually alter the visible command line, you can assign to the | |
932 | variable $0 as documented in L<perlvar>. This won't work on all | |
933 | operating systems, though. Daemon programs like sendmail place their | |
934 | state there, as in: | |
935 | ||
936 | $0 = "orcus [accepting connections]"; | |
937 | ||
938 | =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? | |
939 | ||
940 | =over 4 | |
941 | ||
942 | =item Unix | |
943 | ||
a6dd486b | 944 | In the strictest sense, it can't be done--the script executes as a |
68dc0745 | 945 | different process from the shell it was started from. Changes to a |
a6dd486b | 946 | process are not reflected in its parent--only in any children |
68dc0745 | 947 | created after the change. There is shell magic that may allow you to |
948 | fake it by eval()ing the script's output in your shell; check out the | |
197aec24 | 949 | comp.unix.questions FAQ for details. |
68dc0745 | 950 | |
68dc0745 | 951 | =back |
952 | ||
953 | =head2 How do I close a process's filehandle without waiting for it to complete? | |
954 | ||
955 | Assuming your system supports such things, just send an appropriate signal | |
a6dd486b | 956 | to the process (see L<perlfunc/"kill">). It's common to first send a TERM |
68dc0745 | 957 | signal, wait a little bit, and then send a KILL signal to finish it off. |
958 | ||
959 | =head2 How do I fork a daemon process? | |
960 | ||
961 | If by daemon process you mean one that's detached (disassociated from | |
962 | its tty), then the following process is reported to work on most | |
963 | Unixish systems. Non-Unix users should check their Your_OS::Process | |
964 | module for other solutions. | |
965 | ||
966 | =over 4 | |
967 | ||
968 | =item * | |
969 | ||
197aec24 | 970 | Open /dev/tty and use the TIOCNOTTY ioctl on it. See L<tty> |
c8db1d39 TC |
971 | for details. Or better yet, you can just use the POSIX::setsid() |
972 | function, so you don't have to worry about process groups. | |
68dc0745 | 973 | |
974 | =item * | |
975 | ||
976 | Change directory to / | |
977 | ||
978 | =item * | |
979 | ||
980 | Reopen STDIN, STDOUT, and STDERR so they're not connected to the old | |
981 | tty. | |
982 | ||
983 | =item * | |
984 | ||
985 | Background yourself like this: | |
986 | ||
987 | fork && exit; | |
988 | ||
989 | =back | |
990 | ||
1a91aff4 GS |
991 | The Proc::Daemon module, available from CPAN, provides a function to |
992 | perform these actions for you. | |
993 | ||
68dc0745 | 994 | =head2 How do I find out if I'm running interactively or not? |
995 | ||
996 | Good question. Sometimes C<-t STDIN> and C<-t STDOUT> can give clues, | |
997 | sometimes not. | |
998 | ||
999 | if (-t STDIN && -t STDOUT) { | |
1000 | print "Now what? "; | |
1001 | } | |
1002 | ||
1003 | On POSIX systems, you can test whether your own process group matches | |
1004 | the current process group of your controlling terminal as follows: | |
1005 | ||
1006 | use POSIX qw/getpgrp tcgetpgrp/; | |
1007 | open(TTY, "/dev/tty") or die $!; | |
65acb1b1 | 1008 | $tpgrp = tcgetpgrp(fileno(*TTY)); |
68dc0745 | 1009 | $pgrp = getpgrp(); |
1010 | if ($tpgrp == $pgrp) { | |
1011 | print "foreground\n"; | |
1012 | } else { | |
1013 | print "background\n"; | |
1014 | } | |
1015 | ||
1016 | =head2 How do I timeout a slow event? | |
1017 | ||
1018 | Use the alarm() function, probably in conjunction with a signal | |
b73a15ae | 1019 | handler, as documented in L<perlipc/"Signals"> and the section on |
b432a672 | 1020 | "Signals" in the Camel. You may instead use the more flexible |
b73a15ae | 1021 | Sys::AlarmCall module available from CPAN. |
68dc0745 | 1022 | |
49d635f9 RGS |
1023 | The alarm() function is not implemented on all versions of Windows. |
1024 | Check the documentation for your specific version of Perl. | |
1025 | ||
68dc0745 | 1026 | =head2 How do I set CPU limits? |
1027 | ||
1028 | Use the BSD::Resource module from CPAN. | |
1029 | ||
1030 | =head2 How do I avoid zombies on a Unix system? | |
1031 | ||
1032 | Use the reaper code from L<perlipc/"Signals"> to call wait() when a | |
1033 | SIGCHLD is received, or else use the double-fork technique described | |
49d635f9 | 1034 | in L<perlfaq8/"How do I start a process in the background?">. |
68dc0745 | 1035 | |
1036 | =head2 How do I use an SQL database? | |
1037 | ||
04d666b1 RGS |
1038 | The DBI module provides an abstract interface to most database |
1039 | servers and types, including Oracle, DB2, Sybase, mysql, Postgresql, | |
1040 | ODBC, and flat files. The DBI module accesses each database type | |
1041 | through a database driver, or DBD. You can see a complete list of | |
1042 | available drivers on CPAN: http://www.cpan.org/modules/by-module/DBD/ . | |
1043 | You can read more about DBI on http://dbi.perl.org . | |
1044 | ||
1045 | Other modules provide more specific access: Win32::ODBC, Alzabo, iodbc, | |
1046 | and others found on CPAN Search: http://search.cpan.org . | |
68dc0745 | 1047 | |
1048 | =head2 How do I make a system() exit on control-C? | |
1049 | ||
1050 | You can't. You need to imitate the system() call (see L<perlipc> for | |
1051 | sample code) and then have a signal handler for the INT signal that | |
c8db1d39 TC |
1052 | passes the signal on to the subprocess. Or you can check for it: |
1053 | ||
1054 | $rc = system($cmd); | |
197aec24 | 1055 | if ($rc & 127) { die "signal death" } |
68dc0745 | 1056 | |
1057 | =head2 How do I open a file without blocking? | |
1058 | ||
1059 | If you're lucky enough to be using a system that supports | |
1060 | non-blocking reads (most Unixish systems do), you need only to use the | |
1061 | O_NDELAY or O_NONBLOCK flag from the Fcntl module in conjunction with | |
1062 | sysopen(): | |
1063 | ||
1064 | use Fcntl; | |
2359510d SD |
1065 | sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644) |
1066 | or die "can't open /foo/somefile: $!": | |
68dc0745 | 1067 | |
c98c5709 RGS |
1068 | =head2 How do I tell the difference between errors from the shell and perl? |
1069 | ||
1070 | (answer contributed by brian d foy, C<< <bdfoy@cpan.org> >> | |
1071 | ||
1072 | When you run a Perl script, something else is running the script for you, | |
1073 | and that something else may output error messages. The script might | |
1074 | emit its own warnings and error messages. Most of the time you cannot | |
1075 | tell who said what. | |
1076 | ||
1077 | You probably cannot fix the thing that runs perl, but you can change how | |
1078 | perl outputs its warnings by defining a custom warning and die functions. | |
1079 | ||
1080 | Consider this script, which has an error you may not notice immediately. | |
1081 | ||
1082 | #!/usr/locl/bin/perl | |
1083 | ||
1084 | print "Hello World\n"; | |
1085 | ||
1086 | I get an error when I run this from my shell (which happens to be | |
1087 | bash). That may look like perl forgot it has a print() function, | |
1088 | but my shebang line is not the path to perl, so the shell runs the | |
1089 | script, and I get the error. | |
1090 | ||
1091 | $ ./test | |
1092 | ./test: line 3: print: command not found | |
1093 | ||
1094 | A quick and dirty fix involves a little bit of code, but this may be all | |
1095 | you need to figure out the problem. | |
1096 | ||
1097 | #!/usr/bin/perl -w | |
6670e5e7 | 1098 | |
c98c5709 RGS |
1099 | BEGIN { |
1100 | $SIG{__WARN__} = sub{ print STDERR "Perl: ", @_; }; | |
1101 | $SIG{__DIE__} = sub{ print STDERR "Perl: ", @_; exit 1}; | |
1102 | } | |
6670e5e7 | 1103 | |
c98c5709 RGS |
1104 | $a = 1 + undef; |
1105 | $x / 0; | |
1106 | __END__ | |
1107 | ||
1108 | The perl message comes out with "Perl" in front. The BEGIN block | |
1109 | works at compile time so all of the compilation errors and warnings | |
1110 | get the "Perl:" prefix too. | |
1111 | ||
1112 | Perl: Useless use of division (/) in void context at ./test line 9. | |
1113 | Perl: Name "main::a" used only once: possible typo at ./test line 8. | |
1114 | Perl: Name "main::x" used only once: possible typo at ./test line 9. | |
1115 | Perl: Use of uninitialized value in addition (+) at ./test line 8. | |
1116 | Perl: Use of uninitialized value in division (/) at ./test line 9. | |
1117 | Perl: Illegal division by zero at ./test line 9. | |
1118 | Perl: Illegal division by zero at -e line 3. | |
1119 | ||
1120 | If I don't see that "Perl:", it's not from perl. | |
1121 | ||
1122 | You could also just know all the perl errors, and although there are | |
1123 | some people who may know all of them, you probably don't. However, they | |
1124 | all should be in the perldiag manpage. If you don't find the error in | |
1125 | there, it probably isn't a perl error. | |
1126 | ||
1127 | Looking up every message is not the easiest way, so let perl to do it | |
1128 | for you. Use the diagnostics pragma with turns perl's normal messages | |
1129 | into longer discussions on the topic. | |
1130 | ||
1131 | use diagnostics; | |
1132 | ||
1133 | If you don't get a paragraph or two of expanded discussion, it | |
1134 | might not be perl's message. | |
1135 | ||
d92eb7b0 GS |
1136 | =head2 How do I install a module from CPAN? |
1137 | ||
1138 | The easiest way is to have a module also named CPAN do it for you. | |
197aec24 | 1139 | This module comes with perl version 5.004 and later. |
76817d6d JH |
1140 | |
1141 | $ perl -MCPAN -e shell | |
1142 | ||
1143 | cpan shell -- CPAN exploration and modules installation (v1.59_54) | |
1144 | ReadLine support enabled | |
1145 | ||
197aec24 | 1146 | cpan> install Some::Module |
76817d6d | 1147 | |
197aec24 | 1148 | To manually install the CPAN module, or any well-behaved CPAN module |
76817d6d | 1149 | for that matter, follow these steps: |
68dc0745 | 1150 | |
1151 | =over 4 | |
1152 | ||
1153 | =item 1 | |
1154 | ||
1155 | Unpack the source into a temporary area. | |
1156 | ||
1157 | =item 2 | |
1158 | ||
1159 | perl Makefile.PL | |
1160 | ||
1161 | =item 3 | |
1162 | ||
1163 | make | |
1164 | ||
1165 | =item 4 | |
1166 | ||
1167 | make test | |
1168 | ||
1169 | =item 5 | |
1170 | ||
1171 | make install | |
1172 | ||
1173 | =back | |
1174 | ||
1175 | If your version of perl is compiled without dynamic loading, then you | |
1176 | just need to replace step 3 (B<make>) with B<make perl> and you will | |
1177 | get a new F<perl> binary with your extension linked in. | |
1178 | ||
c8db1d39 | 1179 | See L<ExtUtils::MakeMaker> for more details on building extensions. |
b432a672 AL |
1180 | See also the next question, "What's the difference between require |
1181 | and use?". | |
c8db1d39 TC |
1182 | |
1183 | =head2 What's the difference between require and use? | |
1184 | ||
1185 | Perl offers several different ways to include code from one file into | |
1186 | another. Here are the deltas between the various inclusion constructs: | |
1187 | ||
a6dd486b | 1188 | 1) do $file is like eval `cat $file`, except the former |
5e3006a4 | 1189 | 1.1: searches @INC and updates %INC. |
c8db1d39 TC |
1190 | 1.2: bequeaths an *unrelated* lexical scope on the eval'ed code. |
1191 | ||
a6dd486b | 1192 | 2) require $file is like do $file, except the former |
c8db1d39 TC |
1193 | 2.1: checks for redundant loading, skipping already loaded files. |
1194 | 2.2: raises an exception on failure to find, compile, or execute $file. | |
1195 | ||
a6dd486b | 1196 | 3) require Module is like require "Module.pm", except the former |
c8db1d39 TC |
1197 | 3.1: translates each "::" into your system's directory separator. |
1198 | 3.2: primes the parser to disambiguate class Module as an indirect object. | |
1199 | ||
a6dd486b | 1200 | 4) use Module is like require Module, except the former |
c8db1d39 TC |
1201 | 4.1: loads the module at compile time, not run-time. |
1202 | 4.2: imports symbols and semantics from that package to the current one. | |
1203 | ||
1204 | In general, you usually want C<use> and a proper Perl module. | |
46fc3d4c | 1205 | |
1206 | =head2 How do I keep my own module/library directory? | |
1207 | ||
e8c8d959 | 1208 | When you build modules, use the PREFIX and LIB options when generating |
46fc3d4c | 1209 | Makefiles: |
1210 | ||
e8c8d959 | 1211 | perl Makefile.PL PREFIX=/mydir/perl LIB=/mydir/perl/lib |
46fc3d4c | 1212 | |
1213 | then either set the PERL5LIB environment variable before you run | |
1214 | scripts that use the modules/libraries (see L<perlrun>) or say | |
1215 | ||
e8c8d959 | 1216 | use lib '/mydir/perl/lib'; |
46fc3d4c | 1217 | |
a6dd486b | 1218 | This is almost the same as |
65acb1b1 TC |
1219 | |
1220 | BEGIN { | |
e8c8d959 | 1221 | unshift(@INC, '/mydir/perl/lib'); |
65acb1b1 TC |
1222 | } |
1223 | ||
1224 | except that the lib module checks for machine-dependent subdirectories. | |
46fc3d4c | 1225 | See Perl's L<lib> for more information. |
1226 | ||
1227 | =head2 How do I add the directory my program lives in to the module/library search path? | |
1228 | ||
1229 | use FindBin; | |
7b8d334a | 1230 | use lib "$FindBin::Bin"; |
46fc3d4c | 1231 | use your_own_modules; |
1232 | ||
f0d19b68 | 1233 | =head2 How do I add a directory to my include path (@INC) at runtime? |
46fc3d4c | 1234 | |
1235 | Here are the suggested ways of modifying your include path: | |
1236 | ||
1237 | the PERLLIB environment variable | |
1238 | the PERL5LIB environment variable | |
c2611fb3 | 1239 | the perl -Idir command line flag |
46fc3d4c | 1240 | the use lib pragma, as in |
1241 | use lib "$ENV{HOME}/myown_perllib"; | |
1242 | ||
1243 | The latter is particularly useful because it knows about machine | |
1244 | dependent architectures. The lib.pm pragmatic module was first | |
1245 | included with the 5.002 release of Perl. | |
68dc0745 | 1246 | |
65acb1b1 TC |
1247 | =head2 What is socket.ph and where do I get it? |
1248 | ||
1249 | It's a perl4-style file defining values for system networking | |
1250 | constants. Sometimes it is built using h2ph when Perl is installed, | |
1251 | but other times it is not. Modern programs C<use Socket;> instead. | |
1252 | ||
fc36a67e | 1253 | =head1 AUTHOR AND COPYRIGHT |
1254 | ||
7678cced RGS |
1255 | Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and |
1256 | other authors as noted. All rights reserved. | |
5a964f20 | 1257 | |
5a7beb56 JH |
1258 | This documentation is free; you can redistribute it and/or modify it |
1259 | under the same terms as Perl itself. | |
5a964f20 TC |
1260 | |
1261 | Irrespective of its distribution, all code examples in this file | |
1262 | are hereby placed into the public domain. You are permitted and | |
1263 | encouraged to use this code in your own programs for fun | |
1264 | or for profit as you see fit. A simple comment in the code giving | |
1265 | credit would be courteous but is not required. |