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1 | =head1 NAME |
2 | ||
3 | perlvar - Perl predefined variables | |
4 | ||
5 | =head1 DESCRIPTION | |
6 | ||
7 | =head2 Predefined Names | |
8 | ||
9 | The following names have special meaning to Perl. Most | |
10 | punctuation names have reasonable mnemonics, or analogs in the | |
11 | shells. Nevertheless, if you wish to use long variable names, | |
12 | you need only say | |
13 | ||
14 | use English; | |
15 | ||
16 | at the top of your program. This will alias all the short names to the | |
17 | long names in the current package. Some even have medium names, | |
18 | generally borrowed from B<awk>. | |
19 | ||
20 | If you don't mind the performance hit, variables that depend on the | |
21 | currently selected filehandle may instead be set by calling an | |
22 | appropriate object method on the IO::Handle object. (Summary lines | |
23 | below for this contain the word HANDLE.) First you must say | |
24 | ||
25 | use IO::Handle; | |
26 | ||
27 | after which you may use either | |
28 | ||
29 | method HANDLE EXPR | |
30 | ||
31 | or more safely, | |
32 | ||
33 | HANDLE->method(EXPR) | |
34 | ||
35 | Each method returns the old value of the IO::Handle attribute. | |
36 | The methods each take an optional EXPR, which if supplied specifies the | |
37 | new value for the IO::Handle attribute in question. If not supplied, | |
38 | most methods do nothing to the current value--except for | |
39 | autoflush(), which will assume a 1 for you, just to be different. | |
40 | Because loading in the IO::Handle class is an expensive operation, you should | |
41 | learn how to use the regular built-in variables. | |
42 | ||
43 | A few of these variables are considered "read-only". This means that if | |
44 | you try to assign to this variable, either directly or indirectly through | |
45 | a reference, you'll raise a run-time exception. | |
46 | ||
47 | The following list is ordered by scalar variables first, then the | |
48 | arrays, then the hashes. | |
49 | ||
50 | =over 8 | |
51 | ||
52 | =item $ARG | |
53 | ||
54 | =item $_ | |
55 | ||
56 | The default input and pattern-searching space. The following pairs are | |
57 | equivalent: | |
58 | ||
59 | while (<>) {...} # equivalent only in while! | |
60 | while (defined($_ = <>)) {...} | |
61 | ||
62 | /^Subject:/ | |
63 | $_ =~ /^Subject:/ | |
64 | ||
65 | tr/a-z/A-Z/ | |
66 | $_ =~ tr/a-z/A-Z/ | |
67 | ||
68 | chomp | |
69 | chomp($_) | |
70 | ||
71 | Here are the places where Perl will assume $_ even if you | |
72 | don't use it: | |
73 | ||
74 | =over 3 | |
75 | ||
76 | =item * | |
77 | ||
78 | Various unary functions, including functions like ord() and int(), as well | |
79 | as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to | |
80 | STDIN. | |
81 | ||
82 | =item * | |
83 | ||
84 | Various list functions like print() and unlink(). | |
85 | ||
86 | =item * | |
87 | ||
88 | The pattern matching operations C<m//>, C<s///>, and C<tr///> when used | |
89 | without an C<=~> operator. | |
90 | ||
91 | =item * | |
92 | ||
93 | The default iterator variable in a C<foreach> loop if no other | |
94 | variable is supplied. | |
95 | ||
96 | =item * | |
97 | ||
98 | The implicit iterator variable in the grep() and map() functions. | |
99 | ||
100 | =item * | |
101 | ||
102 | The default place to put an input record when a C<< <FH> >> | |
103 | operation's result is tested by itself as the sole criterion of a C<while> | |
104 | test. Outside a C<while> test, this will not happen. | |
105 | ||
106 | =back | |
107 | ||
108 | (Mnemonic: underline is understood in certain operations.) | |
109 | ||
110 | =back | |
111 | ||
112 | =over 8 | |
113 | ||
114 | =item $<I<digits>> | |
115 | ||
116 | Contains the subpattern from the corresponding set of capturing | |
117 | parentheses from the last pattern match, not counting patterns | |
118 | matched in nested blocks that have been exited already. (Mnemonic: | |
119 | like \digits.) These variables are all read-only and dynamically | |
120 | scoped to the current BLOCK. | |
121 | ||
122 | =item $MATCH | |
123 | ||
124 | =item $& | |
125 | ||
126 | The string matched by the last successful pattern match (not counting | |
127 | any matches hidden within a BLOCK or eval() enclosed by the current | |
128 | BLOCK). (Mnemonic: like & in some editors.) This variable is read-only | |
129 | and dynamically scoped to the current BLOCK. | |
130 | ||
131 | The use of this variable anywhere in a program imposes a considerable | |
132 | performance penalty on all regular expression matches. See L<BUGS>. | |
133 | ||
134 | =item $PREMATCH | |
135 | ||
136 | =item $` | |
137 | ||
138 | The string preceding whatever was matched by the last successful | |
139 | pattern match (not counting any matches hidden within a BLOCK or eval | |
140 | enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted | |
141 | string.) This variable is read-only. | |
142 | ||
143 | The use of this variable anywhere in a program imposes a considerable | |
144 | performance penalty on all regular expression matches. See L<BUGS>. | |
145 | ||
146 | =item $POSTMATCH | |
147 | ||
148 | =item $' | |
149 | ||
150 | The string following whatever was matched by the last successful | |
151 | pattern match (not counting any matches hidden within a BLOCK or eval() | |
152 | enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted | |
153 | string.) Example: | |
154 | ||
155 | $_ = 'abcdefghi'; | |
156 | /def/; | |
157 | print "$`:$&:$'\n"; # prints abc:def:ghi | |
158 | ||
159 | This variable is read-only and dynamically scoped to the current BLOCK. | |
160 | ||
161 | The use of this variable anywhere in a program imposes a considerable | |
162 | performance penalty on all regular expression matches. See L<BUGS>. | |
163 | ||
164 | =item $LAST_PAREN_MATCH | |
165 | ||
166 | =item $+ | |
167 | ||
168 | The last bracket matched by the last search pattern. This is useful if | |
169 | you don't know which one of a set of alternative patterns matched. For | |
170 | example: | |
171 | ||
172 | /Version: (.*)|Revision: (.*)/ && ($rev = $+); | |
173 | ||
174 | (Mnemonic: be positive and forward looking.) | |
175 | This variable is read-only and dynamically scoped to the current BLOCK. | |
176 | ||
177 | =item @LAST_MATCH_END | |
178 | ||
179 | =item @+ | |
180 | ||
181 | This array holds the offsets of the ends of the last successful | |
182 | submatches in the currently active dynamic scope. C<$+[0]> is | |
183 | the offset into the string of the end of the entire match. This | |
184 | is the same value as what the C<pos> function returns when called | |
185 | on the variable that was matched against. The I<n>th element | |
186 | of this array holds the offset of the I<n>th submatch, so | |
187 | C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset | |
188 | past where $2 ends, and so on. You can use C<$#+> to determine | |
189 | how many subgroups were in the last successful match. See the | |
190 | examples given for the C<@-> variable. | |
191 | ||
192 | =item $MULTILINE_MATCHING | |
193 | ||
194 | =item $* | |
195 | ||
196 | Set to a non-zero integer value to do multi-line matching within a | |
197 | string, 0 (or undefined) to tell Perl that it can assume that strings | |
198 | contain a single line, for the purpose of optimizing pattern matches. | |
199 | Pattern matches on strings containing multiple newlines can produce | |
200 | confusing results when C<$*> is 0 or undefined. Default is undefined. | |
201 | (Mnemonic: * matches multiple things.) This variable influences the | |
202 | interpretation of only C<^> and C<$>. A literal newline can be searched | |
203 | for even when C<$* == 0>. | |
204 | ||
205 | Use of C<$*> is deprecated in modern Perl, supplanted by | |
206 | the C</s> and C</m> modifiers on pattern matching. | |
207 | ||
208 | Assigning a non-numerical value to C<$*> triggers a warning (and makes | |
209 | C<$*> act if C<$* == 0>), while assigning a numerical value to C<$*> | |
210 | makes that an implicit C<int> is applied on the value. | |
211 | ||
212 | =item input_line_number HANDLE EXPR | |
213 | ||
214 | =item $INPUT_LINE_NUMBER | |
215 | ||
216 | =item $NR | |
217 | ||
218 | =item $. | |
219 | ||
220 | The current input record number for the last file handle from which | |
221 | you just read() (or called a C<seek> or C<tell> on). The value | |
222 | may be different from the actual physical line number in the file, | |
223 | depending on what notion of "line" is in effect--see C<$/> on how | |
224 | to change that. An explicit close on a filehandle resets the line | |
225 | number. Because C<< <> >> never does an explicit close, line | |
226 | numbers increase across ARGV files (but see examples in L<perlfunc/eof>). | |
227 | Consider this variable read-only: setting it does not reposition | |
228 | the seek pointer; you'll have to do that on your own. Localizing C<$.> | |
229 | has the effect of also localizing Perl's notion of "the last read | |
230 | filehandle". (Mnemonic: many programs use "." to mean the current line | |
231 | number.) | |
232 | ||
233 | =item input_record_separator HANDLE EXPR | |
234 | ||
235 | =item $INPUT_RECORD_SEPARATOR | |
236 | ||
237 | =item $RS | |
238 | ||
239 | =item $/ | |
240 | ||
241 | The input record separator, newline by default. This | |
242 | influences Perl's idea of what a "line" is. Works like B<awk>'s RS | |
243 | variable, including treating empty lines as a terminator if set to | |
244 | the null string. (An empty line cannot contain any spaces | |
245 | or tabs.) You may set it to a multi-character string to match a | |
246 | multi-character terminator, or to C<undef> to read through the end | |
247 | of file. Setting it to C<"\n\n"> means something slightly | |
248 | different than setting to C<"">, if the file contains consecutive | |
249 | empty lines. Setting to C<""> will treat two or more consecutive | |
250 | empty lines as a single empty line. Setting to C<"\n\n"> will | |
251 | blindly assume that the next input character belongs to the next | |
252 | paragraph, even if it's a newline. (Mnemonic: / delimits | |
253 | line boundaries when quoting poetry.) | |
254 | ||
255 | undef $/; # enable "slurp" mode | |
256 | $_ = <FH>; # whole file now here | |
257 | s/\n[ \t]+/ /g; | |
258 | ||
259 | Remember: the value of C<$/> is a string, not a regex. B<awk> has to be | |
260 | better for something. :-) | |
261 | ||
262 | Setting C<$/> to a reference to an integer, scalar containing an integer, or | |
263 | scalar that's convertible to an integer will attempt to read records | |
264 | instead of lines, with the maximum record size being the referenced | |
265 | integer. So this: | |
266 | ||
267 | $/ = \32768; # or \"32768", or \$var_containing_32768 | |
268 | open(FILE, $myfile); | |
269 | $_ = <FILE>; | |
270 | ||
271 | will read a record of no more than 32768 bytes from FILE. If you're | |
272 | not reading from a record-oriented file (or your OS doesn't have | |
273 | record-oriented files), then you'll likely get a full chunk of data | |
274 | with every read. If a record is larger than the record size you've | |
275 | set, you'll get the record back in pieces. | |
276 | ||
277 | On VMS, record reads are done with the equivalent of C<sysread>, | |
278 | so it's best not to mix record and non-record reads on the same | |
279 | file. (This is unlikely to be a problem, because any file you'd | |
280 | want to read in record mode is probably unusable in line mode.) | |
281 | Non-VMS systems do normal I/O, so it's safe to mix record and | |
282 | non-record reads of a file. | |
283 | ||
284 | See also L<perlport/"Newlines">. Also see C<$.>. | |
285 | ||
286 | =item autoflush HANDLE EXPR | |
287 | ||
288 | =item $OUTPUT_AUTOFLUSH | |
289 | ||
290 | =item $| | |
291 | ||
292 | If set to nonzero, forces a flush right away and after every write | |
293 | or print on the currently selected output channel. Default is 0 | |
294 | (regardless of whether the channel is really buffered by the | |
295 | system or not; C<$|> tells you only whether you've asked Perl | |
296 | explicitly to flush after each write). STDOUT will | |
297 | typically be line buffered if output is to the terminal and block | |
298 | buffered otherwise. Setting this variable is useful primarily when | |
299 | you are outputting to a pipe or socket, such as when you are running | |
300 | a Perl program under B<rsh> and want to see the output as it's | |
301 | happening. This has no effect on input buffering. See L<perlfunc/getc> | |
302 | for that. (Mnemonic: when you want your pipes to be piping hot.) | |
303 | ||
304 | =item output_field_separator HANDLE EXPR | |
305 | ||
306 | =item $OUTPUT_FIELD_SEPARATOR | |
307 | ||
308 | =item $OFS | |
309 | ||
310 | =item $, | |
311 | ||
312 | The output field separator for the print operator. Ordinarily the | |
313 | print operator simply prints out its arguments without further | |
314 | adornment. To get behavior more like B<awk>, set this variable as | |
315 | you would set B<awk>'s OFS variable to specify what is printed | |
316 | between fields. (Mnemonic: what is printed when there is a "," in | |
317 | your print statement.) | |
318 | ||
319 | =item output_record_separator HANDLE EXPR | |
320 | ||
321 | =item $OUTPUT_RECORD_SEPARATOR | |
322 | ||
323 | =item $ORS | |
324 | ||
325 | =item $\ | |
326 | ||
327 | The output record separator for the print operator. Ordinarily the | |
328 | print operator simply prints out its arguments as is, with no | |
329 | trailing newline or other end-of-record string added. To get | |
330 | behavior more like B<awk>, set this variable as you would set | |
331 | B<awk>'s ORS variable to specify what is printed at the end of the | |
332 | print. (Mnemonic: you set C<$\> instead of adding "\n" at the | |
333 | end of the print. Also, it's just like C<$/>, but it's what you | |
334 | get "back" from Perl.) | |
335 | ||
336 | =item $LIST_SEPARATOR | |
337 | ||
338 | =item $" | |
339 | ||
340 | This is like C<$,> except that it applies to array and slice values | |
341 | interpolated into a double-quoted string (or similar interpreted | |
342 | string). Default is a space. (Mnemonic: obvious, I think.) | |
343 | ||
344 | =item $SUBSCRIPT_SEPARATOR | |
345 | ||
346 | =item $SUBSEP | |
347 | ||
348 | =item $; | |
349 | ||
350 | The subscript separator for multidimensional array emulation. If you | |
351 | refer to a hash element as | |
352 | ||
353 | $foo{$a,$b,$c} | |
354 | ||
355 | it really means | |
356 | ||
357 | $foo{join($;, $a, $b, $c)} | |
358 | ||
359 | But don't put | |
360 | ||
361 | @foo{$a,$b,$c} # a slice--note the @ | |
362 | ||
363 | which means | |
364 | ||
365 | ($foo{$a},$foo{$b},$foo{$c}) | |
366 | ||
367 | Default is "\034", the same as SUBSEP in B<awk>. If your | |
368 | keys contain binary data there might not be any safe value for C<$;>. | |
369 | (Mnemonic: comma (the syntactic subscript separator) is a | |
370 | semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already | |
371 | taken for something more important.) | |
372 | ||
373 | Consider using "real" multidimensional arrays as described | |
374 | in L<perllol>. | |
375 | ||
376 | =item $OFMT | |
377 | ||
378 | =item $# | |
379 | ||
380 | The output format for printed numbers. This variable is a half-hearted | |
381 | attempt to emulate B<awk>'s OFMT variable. There are times, however, | |
382 | when B<awk> and Perl have differing notions of what counts as | |
383 | numeric. The initial value is "%.I<n>g", where I<n> is the value | |
384 | of the macro DBL_DIG from your system's F<float.h>. This is different from | |
385 | B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#> | |
386 | explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.) | |
387 | ||
388 | Use of C<$#> is deprecated. | |
389 | ||
390 | =item format_page_number HANDLE EXPR | |
391 | ||
392 | =item $FORMAT_PAGE_NUMBER | |
393 | ||
394 | =item $% | |
395 | ||
396 | The current page number of the currently selected output channel. | |
397 | Used with formats. | |
398 | (Mnemonic: % is page number in B<nroff>.) | |
399 | ||
400 | =item format_lines_per_page HANDLE EXPR | |
401 | ||
402 | =item $FORMAT_LINES_PER_PAGE | |
403 | ||
404 | =item $= | |
405 | ||
406 | The current page length (printable lines) of the currently selected | |
407 | output channel. Default is 60. | |
408 | Used with formats. | |
409 | (Mnemonic: = has horizontal lines.) | |
410 | ||
411 | =item format_lines_left HANDLE EXPR | |
412 | ||
413 | =item $FORMAT_LINES_LEFT | |
414 | ||
415 | =item $- | |
416 | ||
417 | The number of lines left on the page of the currently selected output | |
418 | channel. | |
419 | Used with formats. | |
420 | (Mnemonic: lines_on_page - lines_printed.) | |
421 | ||
422 | =item @LAST_MATCH_START | |
423 | ||
424 | =item @- | |
425 | ||
426 | $-[0] is the offset of the start of the last successful match. | |
427 | C<$-[>I<n>C<]> is the offset of the start of the substring matched by | |
428 | I<n>-th subpattern, or undef if the subpattern did not match. | |
429 | ||
430 | Thus after a match against $_, $& coincides with C<substr $_, $-[0], | |
431 | $+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<], | |
432 | $+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with | |
433 | C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last | |
434 | matched subgroup in the last successful match. Contrast with | |
435 | C<$#+>, the number of subgroups in the regular expression. Compare | |
436 | with C<@+>. | |
437 | ||
438 | This array holds the offsets of the beginnings of the last | |
439 | successful submatches in the currently active dynamic scope. | |
440 | C<$-[0]> is the offset into the string of the beginning of the | |
441 | entire match. The I<n>th element of this array holds the offset | |
442 | of the I<n>th submatch, so C<$+[1]> is the offset where $1 | |
443 | begins, C<$+[2]> the offset where $2 begins, and so on. | |
444 | You can use C<$#-> to determine how many subgroups were in the | |
445 | last successful match. Compare with the C<@+> variable. | |
446 | ||
447 | After a match against some variable $var: | |
448 | ||
449 | =over 5 | |
450 | ||
451 | =item C<$`> is the same as C<substr($var, 0, $-[0])> | |
452 | ||
453 | =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])> | |
454 | ||
455 | =item C<$'> is the same as C<substr($var, $+[0])> | |
456 | ||
457 | =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])> | |
458 | ||
459 | =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])> | |
460 | ||
461 | =item C<$3> is the same as C<substr $var, $-[3], $+[3] - $-[3])> | |
462 | ||
463 | =back | |
464 | ||
465 | =item format_name HANDLE EXPR | |
466 | ||
467 | =item $FORMAT_NAME | |
468 | ||
469 | =item $~ | |
470 | ||
471 | The name of the current report format for the currently selected output | |
472 | channel. Default is the name of the filehandle. (Mnemonic: brother to | |
473 | C<$^>.) | |
474 | ||
475 | =item format_top_name HANDLE EXPR | |
476 | ||
477 | =item $FORMAT_TOP_NAME | |
478 | ||
479 | =item $^ | |
480 | ||
481 | The name of the current top-of-page format for the currently selected | |
482 | output channel. Default is the name of the filehandle with _TOP | |
483 | appended. (Mnemonic: points to top of page.) | |
484 | ||
485 | =item format_line_break_characters HANDLE EXPR | |
486 | ||
487 | =item $FORMAT_LINE_BREAK_CHARACTERS | |
488 | ||
489 | =item $: | |
490 | ||
491 | The current set of characters after which a string may be broken to | |
492 | fill continuation fields (starting with ^) in a format. Default is | |
493 | S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in | |
494 | poetry is a part of a line.) | |
495 | ||
496 | =item format_formfeed HANDLE EXPR | |
497 | ||
498 | =item $FORMAT_FORMFEED | |
499 | ||
500 | =item $^L | |
501 | ||
502 | What formats output as a form feed. Default is \f. | |
503 | ||
504 | =item $ACCUMULATOR | |
505 | ||
506 | =item $^A | |
507 | ||
508 | The current value of the write() accumulator for format() lines. A format | |
509 | contains formline() calls that put their result into C<$^A>. After | |
510 | calling its format, write() prints out the contents of C<$^A> and empties. | |
511 | So you never really see the contents of C<$^A> unless you call | |
512 | formline() yourself and then look at it. See L<perlform> and | |
513 | L<perlfunc/formline()>. | |
514 | ||
515 | =item $CHILD_ERROR | |
516 | ||
517 | =item $? | |
518 | ||
519 | The status returned by the last pipe close, backtick (C<``>) command, | |
520 | successful call to wait() or waitpid(), or from the system() | |
521 | operator. This is just the 16-bit status word returned by the | |
522 | wait() system call (or else is made up to look like it). Thus, the | |
523 | exit value of the subprocess is really (C<<< $? >> 8 >>>), and | |
524 | C<$? & 127> gives which signal, if any, the process died from, and | |
525 | C<$? & 128> reports whether there was a core dump. (Mnemonic: | |
526 | similar to B<sh> and B<ksh>.) | |
527 | ||
528 | Additionally, if the C<h_errno> variable is supported in C, its value | |
529 | is returned via $? if any C<gethost*()> function fails. | |
530 | ||
531 | If you have installed a signal handler for C<SIGCHLD>, the | |
532 | value of C<$?> will usually be wrong outside that handler. | |
533 | ||
534 | Inside an C<END> subroutine C<$?> contains the value that is going to be | |
535 | given to C<exit()>. You can modify C<$?> in an C<END> subroutine to | |
536 | change the exit status of your program. For example: | |
537 | ||
538 | END { | |
539 | $? = 1 if $? == 255; # die would make it 255 | |
540 | } | |
541 | ||
542 | Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the | |
543 | actual VMS exit status, instead of the default emulation of POSIX | |
544 | status. | |
545 | ||
546 | Also see L<Error Indicators>. | |
547 | ||
548 | =item $OS_ERROR | |
549 | ||
550 | =item $ERRNO | |
551 | ||
552 | =item $! | |
553 | ||
554 | If used numerically, yields the current value of the C C<errno> | |
555 | variable, with all the usual caveats. (This means that you shouldn't | |
556 | depend on the value of C<$!> to be anything in particular unless | |
557 | you've gotten a specific error return indicating a system error.) | |
558 | If used an a string, yields the corresponding system error string. | |
559 | You can assign a number to C<$!> to set I<errno> if, for instance, | |
560 | you want C<"$!"> to return the string for error I<n>, or you want | |
561 | to set the exit value for the die() operator. (Mnemonic: What just | |
562 | went bang?) | |
563 | ||
564 | Also see L<Error Indicators>. | |
565 | ||
566 | =item $EXTENDED_OS_ERROR | |
567 | ||
568 | =item $^E | |
569 | ||
570 | Error information specific to the current operating system. At | |
571 | the moment, this differs from C<$!> under only VMS, OS/2, and Win32 | |
572 | (and for MacPerl). On all other platforms, C<$^E> is always just | |
573 | the same as C<$!>. | |
574 | ||
575 | Under VMS, C<$^E> provides the VMS status value from the last | |
576 | system error. This is more specific information about the last | |
577 | system error than that provided by C<$!>. This is particularly | |
578 | important when C<$!> is set to B<EVMSERR>. | |
579 | ||
580 | Under OS/2, C<$^E> is set to the error code of the last call to | |
581 | OS/2 API either via CRT, or directly from perl. | |
582 | ||
583 | Under Win32, C<$^E> always returns the last error information | |
584 | reported by the Win32 call C<GetLastError()> which describes | |
585 | the last error from within the Win32 API. Most Win32-specific | |
586 | code will report errors via C<$^E>. ANSI C and Unix-like calls | |
587 | set C<errno> and so most portable Perl code will report errors | |
588 | via C<$!>. | |
589 | ||
590 | Caveats mentioned in the description of C<$!> generally apply to | |
591 | C<$^E>, also. (Mnemonic: Extra error explanation.) | |
592 | ||
593 | Also see L<Error Indicators>. | |
594 | ||
595 | =item $EVAL_ERROR | |
596 | ||
597 | =item $@ | |
598 | ||
599 | The Perl syntax error message from the last eval() operator. If null, the | |
600 | last eval() parsed and executed correctly (although the operations you | |
601 | invoked may have failed in the normal fashion). (Mnemonic: Where was | |
602 | the syntax error "at"?) | |
603 | ||
604 | Warning messages are not collected in this variable. You can, | |
605 | however, set up a routine to process warnings by setting C<$SIG{__WARN__}> | |
606 | as described below. | |
607 | ||
608 | Also see L<Error Indicators>. | |
609 | ||
610 | =item $PROCESS_ID | |
611 | ||
612 | =item $PID | |
613 | ||
614 | =item $$ | |
615 | ||
616 | The process number of the Perl running this script. You should | |
617 | consider this variable read-only, although it will be altered | |
618 | across fork() calls. (Mnemonic: same as shells.) | |
619 | ||
620 | =item $REAL_USER_ID | |
621 | ||
622 | =item $UID | |
623 | ||
624 | =item $< | |
625 | ||
626 | The real uid of this process. (Mnemonic: it's the uid you came I<from>, | |
627 | if you're running setuid.) | |
628 | ||
629 | =item $EFFECTIVE_USER_ID | |
630 | ||
631 | =item $EUID | |
632 | ||
633 | =item $> | |
634 | ||
635 | The effective uid of this process. Example: | |
636 | ||
637 | $< = $>; # set real to effective uid | |
638 | ($<,$>) = ($>,$<); # swap real and effective uid | |
639 | ||
640 | (Mnemonic: it's the uid you went I<to>, if you're running setuid.) | |
641 | C<< $< >> and C<< $> >> can be swapped only on machines | |
642 | supporting setreuid(). | |
643 | ||
644 | =item $REAL_GROUP_ID | |
645 | ||
646 | =item $GID | |
647 | ||
648 | =item $( | |
649 | ||
650 | The real gid of this process. If you are on a machine that supports | |
651 | membership in multiple groups simultaneously, gives a space separated | |
652 | list of groups you are in. The first number is the one returned by | |
653 | getgid(), and the subsequent ones by getgroups(), one of which may be | |
654 | the same as the first number. | |
655 | ||
656 | However, a value assigned to C<$(> must be a single number used to | |
657 | set the real gid. So the value given by C<$(> should I<not> be assigned | |
658 | back to C<$(> without being forced numeric, such as by adding zero. | |
659 | ||
660 | (Mnemonic: parentheses are used to I<group> things. The real gid is the | |
661 | group you I<left>, if you're running setgid.) | |
662 | ||
663 | =item $EFFECTIVE_GROUP_ID | |
664 | ||
665 | =item $EGID | |
666 | ||
667 | =item $) | |
668 | ||
669 | The effective gid of this process. If you are on a machine that | |
670 | supports membership in multiple groups simultaneously, gives a space | |
671 | separated list of groups you are in. The first number is the one | |
672 | returned by getegid(), and the subsequent ones by getgroups(), one of | |
673 | which may be the same as the first number. | |
674 | ||
675 | Similarly, a value assigned to C<$)> must also be a space-separated | |
676 | list of numbers. The first number sets the effective gid, and | |
677 | the rest (if any) are passed to setgroups(). To get the effect of an | |
678 | empty list for setgroups(), just repeat the new effective gid; that is, | |
679 | to force an effective gid of 5 and an effectively empty setgroups() | |
680 | list, say C< $) = "5 5" >. | |
681 | ||
682 | (Mnemonic: parentheses are used to I<group> things. The effective gid | |
683 | is the group that's I<right> for you, if you're running setgid.) | |
684 | ||
685 | C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on | |
686 | machines that support the corresponding I<set[re][ug]id()> routine. C<$(> | |
687 | and C<$)> can be swapped only on machines supporting setregid(). | |
688 | ||
689 | =item $PROGRAM_NAME | |
690 | ||
691 | =item $0 | |
692 | ||
693 | Contains the name of the program being executed. On some operating | |
694 | systems assigning to C<$0> modifies the argument area that the B<ps> | |
695 | program sees. This is more useful as a way of indicating the current | |
696 | program state than it is for hiding the program you're running. | |
697 | (Mnemonic: same as B<sh> and B<ksh>.) | |
698 | ||
699 | Note for BSD users: setting C<$0> does not completely remove "perl" | |
700 | from the ps(1) output. For example, setting C<$0> to C<"foobar"> will | |
701 | result in C<"perl: foobar (perl)">. This is an operating system | |
702 | feature. | |
703 | ||
704 | =item $[ | |
705 | ||
706 | The index of the first element in an array, and of the first character | |
707 | in a substring. Default is 0, but you could theoretically set it | |
708 | to 1 to make Perl behave more like B<awk> (or Fortran) when | |
709 | subscripting and when evaluating the index() and substr() functions. | |
710 | (Mnemonic: [ begins subscripts.) | |
711 | ||
712 | As of release 5 of Perl, assignment to C<$[> is treated as a compiler | |
713 | directive, and cannot influence the behavior of any other file. | |
714 | Its use is highly discouraged. | |
715 | ||
716 | =item $] | |
717 | ||
718 | The version + patchlevel / 1000 of the Perl interpreter. This variable | |
719 | can be used to determine whether the Perl interpreter executing a | |
720 | script is in the right range of versions. (Mnemonic: Is this version | |
721 | of perl in the right bracket?) Example: | |
722 | ||
723 | warn "No checksumming!\n" if $] < 3.019; | |
724 | ||
725 | See also the documentation of C<use VERSION> and C<require VERSION> | |
726 | for a convenient way to fail if the running Perl interpreter is too old. | |
727 | ||
728 | The use of this variable is deprecated. The floating point representation | |
729 | can sometimes lead to inaccurate numeric comparisons. See C<$^V> for a | |
730 | more modern representation of the Perl version that allows accurate string | |
731 | comparisons. | |
732 | ||
733 | =item $COMPILING | |
734 | ||
735 | =item $^C | |
736 | ||
737 | The current value of the flag associated with the B<-c> switch. | |
738 | Mainly of use with B<-MO=...> to allow code to alter its behavior | |
739 | when being compiled, such as for example to AUTOLOAD at compile | |
740 | time rather than normal, deferred loading. See L<perlcc>. Setting | |
741 | C<$^C = 1> is similar to calling C<B::minus_c>. | |
742 | ||
743 | =item $DEBUGGING | |
744 | ||
745 | =item $^D | |
746 | ||
747 | The current value of the debugging flags. (Mnemonic: value of B<-D> | |
748 | switch.) | |
749 | ||
750 | =item $SYSTEM_FD_MAX | |
751 | ||
752 | =item $^F | |
753 | ||
754 | The maximum system file descriptor, ordinarily 2. System file | |
755 | descriptors are passed to exec()ed processes, while higher file | |
756 | descriptors are not. Also, during an open(), system file descriptors are | |
757 | preserved even if the open() fails. (Ordinary file descriptors are | |
758 | closed before the open() is attempted.) The close-on-exec | |
759 | status of a file descriptor will be decided according to the value of | |
760 | C<$^F> when the corresponding file, pipe, or socket was opened, not the | |
761 | time of the exec(). | |
762 | ||
763 | =item $^H | |
764 | ||
765 | WARNING: This variable is strictly for internal use only. Its availability, | |
766 | behavior, and contents are subject to change without notice. | |
767 | ||
768 | This variable contains compile-time hints for the Perl interpreter. At the | |
769 | end of compilation of a BLOCK the value of this variable is restored to the | |
770 | value when the interpreter started to compile the BLOCK. | |
771 | ||
772 | When perl begins to parse any block construct that provides a lexical scope | |
773 | (e.g., eval body, required file, subroutine body, loop body, or conditional | |
774 | block), the existing value of $^H is saved, but its value is left unchanged. | |
775 | When the compilation of the block is completed, it regains the saved value. | |
776 | Between the points where its value is saved and restored, code that | |
777 | executes within BEGIN blocks is free to change the value of $^H. | |
778 | ||
779 | This behavior provides the semantic of lexical scoping, and is used in, | |
780 | for instance, the C<use strict> pragma. | |
781 | ||
782 | The contents should be an integer; different bits of it are used for | |
783 | different pragmatic flags. Here's an example: | |
784 | ||
785 | sub add_100 { $^H |= 0x100 } | |
786 | ||
787 | sub foo { | |
788 | BEGIN { add_100() } | |
789 | bar->baz($boon); | |
790 | } | |
791 | ||
792 | Consider what happens during execution of the BEGIN block. At this point | |
793 | the BEGIN block has already been compiled, but the body of foo() is still | |
794 | being compiled. The new value of $^H will therefore be visible only while | |
795 | the body of foo() is being compiled. | |
796 | ||
797 | Substitution of the above BEGIN block with: | |
798 | ||
799 | BEGIN { require strict; strict->import('vars') } | |
800 | ||
801 | demonstrates how C<use strict 'vars'> is implemented. Here's a conditional | |
802 | version of the same lexical pragma: | |
803 | ||
804 | BEGIN { require strict; strict->import('vars') if $condition } | |
805 | ||
806 | =item %^H | |
807 | ||
808 | WARNING: This variable is strictly for internal use only. Its availability, | |
809 | behavior, and contents are subject to change without notice. | |
810 | ||
811 | The %^H hash provides the same scoping semantic as $^H. This makes it | |
812 | useful for implementation of lexically scoped pragmas. | |
813 | ||
814 | =item $INPLACE_EDIT | |
815 | ||
816 | =item $^I | |
817 | ||
818 | The current value of the inplace-edit extension. Use C<undef> to disable | |
819 | inplace editing. (Mnemonic: value of B<-i> switch.) | |
820 | ||
821 | =item $^M | |
822 | ||
823 | By default, running out of memory is an untrappable, fatal error. | |
824 | However, if suitably built, Perl can use the contents of C<$^M> | |
825 | as an emergency memory pool after die()ing. Suppose that your Perl | |
826 | were compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc. | |
827 | Then | |
828 | ||
829 | $^M = 'a' x (1 << 16); | |
830 | ||
831 | would allocate a 64K buffer for use in an emergency. See the | |
832 | F<INSTALL> file in the Perl distribution for information on how to | |
833 | enable this option. To discourage casual use of this advanced | |
834 | feature, there is no L<English|English> long name for this variable. | |
835 | ||
836 | =item $OSNAME | |
837 | ||
838 | =item $^O | |
839 | ||
840 | The name of the operating system under which this copy of Perl was | |
841 | built, as determined during the configuration process. The value | |
842 | is identical to C<$Config{'osname'}>. See also L<Config> and the | |
843 | B<-V> command-line switch documented in L<perlrun>. | |
844 | ||
845 | =item $PERLDB | |
846 | ||
847 | =item $^P | |
848 | ||
849 | The internal variable for debugging support. The meanings of the | |
850 | various bits are subject to change, but currently indicate: | |
851 | ||
852 | =over 6 | |
853 | ||
854 | =item 0x01 | |
855 | ||
856 | Debug subroutine enter/exit. | |
857 | ||
858 | =item 0x02 | |
859 | ||
860 | Line-by-line debugging. | |
861 | ||
862 | =item 0x04 | |
863 | ||
864 | Switch off optimizations. | |
865 | ||
866 | =item 0x08 | |
867 | ||
868 | Preserve more data for future interactive inspections. | |
869 | ||
870 | =item 0x10 | |
871 | ||
872 | Keep info about source lines on which a subroutine is defined. | |
873 | ||
874 | =item 0x20 | |
875 | ||
876 | Start with single-step on. | |
877 | ||
878 | =item 0x40 | |
879 | ||
880 | Use subroutine address instead of name when reporting. | |
881 | ||
882 | =item 0x80 | |
883 | ||
884 | Report C<goto &subroutine> as well. | |
885 | ||
886 | =item 0x100 | |
887 | ||
888 | Provide informative "file" names for evals based on the place they were compiled. | |
889 | ||
890 | =item 0x200 | |
891 | ||
892 | Provide informative names to anonymous subroutines based on the place they | |
893 | were compiled. | |
894 | ||
895 | =back | |
896 | ||
897 | Some bits may be relevant at compile-time only, some at | |
898 | run-time only. This is a new mechanism and the details may change. | |
899 | ||
900 | =item $LAST_REGEXP_CODE_RESULT | |
901 | ||
902 | =item $^R | |
903 | ||
904 | The result of evaluation of the last successful C<(?{ code })> | |
905 | regular expression assertion (see L<perlre>). May be written to. | |
906 | ||
907 | =item $EXCEPTIONS_BEING_CAUGHT | |
908 | ||
909 | =item $^S | |
910 | ||
911 | Current state of the interpreter. Undefined if parsing of the current | |
912 | module/eval is not finished (may happen in $SIG{__DIE__} and | |
913 | $SIG{__WARN__} handlers). True if inside an eval(), otherwise false. | |
914 | ||
915 | =item $BASETIME | |
916 | ||
917 | =item $^T | |
918 | ||
919 | The time at which the program began running, in seconds since the | |
920 | epoch (beginning of 1970). The values returned by the B<-M>, B<-A>, | |
921 | and B<-C> filetests are based on this value. | |
922 | ||
923 | =item $PERL_VERSION | |
924 | ||
925 | =item $^V | |
926 | ||
927 | The revision, version, and subversion of the Perl interpreter, represented | |
928 | as a string composed of characters with those ordinals. Thus in Perl v5.6.0 | |
929 | it equals C<chr(5) . chr(6) . chr(0)> and will return true for | |
930 | C<$^V eq v5.6.0>. Note that the characters in this string value can | |
931 | potentially be in Unicode range. | |
932 | ||
933 | This can be used to determine whether the Perl interpreter executing a | |
934 | script is in the right range of versions. (Mnemonic: use ^V for Version | |
935 | Control.) Example: | |
936 | ||
937 | warn "No \"our\" declarations!\n" if $^V and $^V lt v5.6.0; | |
938 | ||
939 | See the documentation of C<use VERSION> and C<require VERSION> | |
940 | for a convenient way to fail if the running Perl interpreter is too old. | |
941 | ||
942 | See also C<$]> for an older representation of the Perl version. | |
943 | ||
944 | =item $WARNING | |
945 | ||
946 | =item $^W | |
947 | ||
948 | The current value of the warning switch, initially true if B<-w> | |
949 | was used, false otherwise, but directly modifiable. (Mnemonic: | |
950 | related to the B<-w> switch.) See also L<warnings>. | |
951 | ||
952 | =item ${^WARNING_BITS} | |
953 | ||
954 | The current set of warning checks enabled by the C<use warnings> pragma. | |
955 | See the documentation of C<warnings> for more details. | |
956 | ||
957 | =item ${^WIDE_SYSTEM_CALLS} | |
958 | ||
959 | Global flag that enables system calls made by Perl to use wide character | |
960 | APIs native to the system, if available. This is currently only implemented | |
961 | on the Windows platform. | |
962 | ||
963 | This can also be enabled from the command line using the C<-C> switch. | |
964 | ||
965 | The initial value is typically C<0> for compatibility with Perl versions | |
966 | earlier than 5.6, but may be automatically set to C<1> by Perl if the system | |
967 | provides a user-settable default (e.g., C<$ENV{LC_CTYPE}>). | |
968 | ||
969 | The C<bytes> pragma always overrides the effect of this flag in the current | |
970 | lexical scope. See L<bytes>. | |
971 | ||
972 | =item $EXECUTABLE_NAME | |
973 | ||
974 | =item $^X | |
975 | ||
976 | The name that the Perl binary itself was executed as, from C's C<argv[0]>. | |
977 | This may not be a full pathname, nor even necessarily in your path. | |
978 | ||
979 | =item $ARGV | |
980 | ||
981 | contains the name of the current file when reading from <>. | |
982 | ||
983 | =item @ARGV | |
984 | ||
985 | The array @ARGV contains the command-line arguments intended for | |
986 | the script. C<$#ARGV> is generally the number of arguments minus | |
987 | one, because C<$ARGV[0]> is the first argument, I<not> the program's | |
988 | command name itself. See C<$0> for the command name. | |
989 | ||
990 | =item @INC | |
991 | ||
992 | The array @INC contains the list of places that the C<do EXPR>, | |
993 | C<require>, or C<use> constructs look for their library files. It | |
994 | initially consists of the arguments to any B<-I> command-line | |
995 | switches, followed by the default Perl library, probably | |
996 | F</usr/local/lib/perl>, followed by ".", to represent the current | |
997 | directory. If you need to modify this at runtime, you should use | |
998 | the C<use lib> pragma to get the machine-dependent library properly | |
999 | loaded also: | |
1000 | ||
1001 | use lib '/mypath/libdir/'; | |
1002 | use SomeMod; | |
1003 | ||
1004 | =item @_ | |
1005 | ||
1006 | Within a subroutine the array @_ contains the parameters passed to that | |
1007 | subroutine. See L<perlsub>. | |
1008 | ||
1009 | =item %INC | |
1010 | ||
1011 | The hash %INC contains entries for each filename included via the | |
1012 | C<do>, C<require>, or C<use> operators. The key is the filename | |
1013 | you specified (with module names converted to pathnames), and the | |
1014 | value is the location of the file found. The C<require> | |
1015 | operator uses this hash to determine whether a particular file has | |
1016 | already been included. | |
1017 | ||
1018 | =item %ENV | |
1019 | ||
1020 | =item $ENV{expr} | |
1021 | ||
1022 | The hash %ENV contains your current environment. Setting a | |
1023 | value in C<ENV> changes the environment for any child processes | |
1024 | you subsequently fork() off. | |
1025 | ||
1026 | =item %SIG | |
1027 | ||
1028 | =item $SIG{expr} | |
1029 | ||
1030 | The hash %SIG contains signal handlers for signals. For example: | |
1031 | ||
1032 | sub handler { # 1st argument is signal name | |
1033 | my($sig) = @_; | |
1034 | print "Caught a SIG$sig--shutting down\n"; | |
1035 | close(LOG); | |
1036 | exit(0); | |
1037 | } | |
1038 | ||
1039 | $SIG{'INT'} = \&handler; | |
1040 | $SIG{'QUIT'} = \&handler; | |
1041 | ... | |
1042 | $SIG{'INT'} = 'DEFAULT'; # restore default action | |
1043 | $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT | |
1044 | ||
1045 | Using a value of C<'IGNORE'> usually has the effect of ignoring the | |
1046 | signal, except for the C<CHLD> signal. See L<perlipc> for more about | |
1047 | this special case. | |
1048 | ||
1049 | Here are some other examples: | |
1050 | ||
1051 | $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended) | |
1052 | $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber | |
1053 | $SIG{"PIPE"} = *Plumber; # somewhat esoteric | |
1054 | $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return?? | |
1055 | ||
1056 | Be sure not to use a bareword as the name of a signal handler, | |
1057 | lest you inadvertently call it. | |
1058 | ||
1059 | If your system has the sigaction() function then signal handlers are | |
1060 | installed using it. This means you get reliable signal handling. If | |
1061 | your system has the SA_RESTART flag it is used when signals handlers are | |
1062 | installed. This means that system calls for which restarting is supported | |
1063 | continue rather than returning when a signal arrives. If you want your | |
1064 | system calls to be interrupted by signal delivery then do something like | |
1065 | this: | |
1066 | ||
1067 | use POSIX ':signal_h'; | |
1068 | ||
1069 | my $alarm = 0; | |
1070 | sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 } | |
1071 | or die "Error setting SIGALRM handler: $!\n"; | |
1072 | ||
1073 | See L<POSIX>. | |
1074 | ||
1075 | Certain internal hooks can be also set using the %SIG hash. The | |
1076 | routine indicated by C<$SIG{__WARN__}> is called when a warning message is | |
1077 | about to be printed. The warning message is passed as the first | |
1078 | argument. The presence of a __WARN__ hook causes the ordinary printing | |
1079 | of warnings to STDERR to be suppressed. You can use this to save warnings | |
1080 | in a variable, or turn warnings into fatal errors, like this: | |
1081 | ||
1082 | local $SIG{__WARN__} = sub { die $_[0] }; | |
1083 | eval $proggie; | |
1084 | ||
1085 | The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception | |
1086 | is about to be thrown. The error message is passed as the first | |
1087 | argument. When a __DIE__ hook routine returns, the exception | |
1088 | processing continues as it would have in the absence of the hook, | |
1089 | unless the hook routine itself exits via a C<goto>, a loop exit, or a die(). | |
1090 | The C<__DIE__> handler is explicitly disabled during the call, so that you | |
1091 | can die from a C<__DIE__> handler. Similarly for C<__WARN__>. | |
1092 | ||
1093 | Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called | |
1094 | even inside an eval(). Do not use this to rewrite a pending exception | |
1095 | in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die(). | |
1096 | This strange action at a distance may be fixed in a future release | |
1097 | so that C<$SIG{__DIE__}> is only called if your program is about | |
1098 | to exit, as was the original intent. Any other use is deprecated. | |
1099 | ||
1100 | C<__DIE__>/C<__WARN__> handlers are very special in one respect: | |
1101 | they may be called to report (probable) errors found by the parser. | |
1102 | In such a case the parser may be in inconsistent state, so any | |
1103 | attempt to evaluate Perl code from such a handler will probably | |
1104 | result in a segfault. This means that warnings or errors that | |
1105 | result from parsing Perl should be used with extreme caution, like | |
1106 | this: | |
1107 | ||
1108 | require Carp if defined $^S; | |
1109 | Carp::confess("Something wrong") if defined &Carp::confess; | |
1110 | die "Something wrong, but could not load Carp to give backtrace... | |
1111 | To see backtrace try starting Perl with -MCarp switch"; | |
1112 | ||
1113 | Here the first line will load Carp I<unless> it is the parser who | |
1114 | called the handler. The second line will print backtrace and die if | |
1115 | Carp was available. The third line will be executed only if Carp was | |
1116 | not available. | |
1117 | ||
1118 | See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and | |
1119 | L<warnings> for additional information. | |
1120 | ||
1121 | =back | |
1122 | ||
1123 | =head2 Error Indicators | |
1124 | ||
1125 | The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information | |
1126 | about different types of error conditions that may appear during | |
1127 | execution of a Perl program. The variables are shown ordered by | |
1128 | the "distance" between the subsystem which reported the error and | |
1129 | the Perl process. They correspond to errors detected by the Perl | |
1130 | interpreter, C library, operating system, or an external program, | |
1131 | respectively. | |
1132 | ||
1133 | To illustrate the differences between these variables, consider the | |
1134 | following Perl expression, which uses a single-quoted string: | |
1135 | ||
1136 | eval q{ | |
1137 | open PIPE, "/cdrom/install |"; | |
1138 | @res = <PIPE>; | |
1139 | close PIPE or die "bad pipe: $?, $!"; | |
1140 | }; | |
1141 | ||
1142 | After execution of this statement all 4 variables may have been set. | |
1143 | ||
1144 | C<$@> is set if the string to be C<eval>-ed did not compile (this | |
1145 | may happen if C<open> or C<close> were imported with bad prototypes), | |
1146 | or if Perl code executed during evaluation die()d . In these cases | |
1147 | the value of $@ is the compile error, or the argument to C<die> | |
1148 | (which will interpolate C<$!> and C<$?>!). (See also L<Fatal>, | |
1149 | though.) | |
1150 | ||
1151 | When the eval() expression above is executed, open(), C<< <PIPE> >>, | |
1152 | and C<close> are translated to calls in the C run-time library and | |
1153 | thence to the operating system kernel. C<$!> is set to the C library's | |
1154 | C<errno> if one of these calls fails. | |
1155 | ||
1156 | Under a few operating systems, C<$^E> may contain a more verbose | |
1157 | error indicator, such as in this case, "CDROM tray not closed." | |
1158 | Systems that do not support extended error messages leave C<$^E> | |
1159 | the same as C<$!>. | |
1160 | ||
1161 | Finally, C<$?> may be set to non-0 value if the external program | |
1162 | F</cdrom/install> fails. The upper eight bits reflect specific | |
1163 | error conditions encountered by the program (the program's exit() | |
1164 | value). The lower eight bits reflect mode of failure, like signal | |
1165 | death and core dump information See wait(2) for details. In | |
1166 | contrast to C<$!> and C<$^E>, which are set only if error condition | |
1167 | is detected, the variable C<$?> is set on each C<wait> or pipe | |
1168 | C<close>, overwriting the old value. This is more like C<$@>, which | |
1169 | on every eval() is always set on failure and cleared on success. | |
1170 | ||
1171 | For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>, | |
1172 | and C<$?>. | |
1173 | ||
1174 | =head2 Technical Note on the Syntax of Variable Names | |
1175 | ||
1176 | Variable names in Perl can have several formats. Usually, they | |
1177 | must begin with a letter or underscore, in which case they can be | |
1178 | arbitrarily long (up to an internal limit of 251 characters) and | |
1179 | may contain letters, digits, underscores, or the special sequence | |
1180 | C<::> or C<'>. In this case, the part before the last C<::> or | |
1181 | C<'> is taken to be a I<package qualifier>; see L<perlmod>. | |
1182 | ||
1183 | Perl variable names may also be a sequence of digits or a single | |
1184 | punctuation or control character. These names are all reserved for | |
1185 | special uses by Perl; for example, the all-digits names are used | |
1186 | to hold data captured by backreferences after a regular expression | |
1187 | match. Perl has a special syntax for the single-control-character | |
1188 | names: It understands C<^X> (caret C<X>) to mean the control-C<X> | |
1189 | character. For example, the notation C<$^W> (dollar-sign caret | |
1190 | C<W>) is the scalar variable whose name is the single character | |
1191 | control-C<W>. This is better than typing a literal control-C<W> | |
1192 | into your program. | |
1193 | ||
1194 | Finally, new in Perl 5.6, Perl variable names may be alphanumeric | |
1195 | strings that begin with control characters (or better yet, a caret). | |
1196 | These variables must be written in the form C<${^Foo}>; the braces | |
1197 | are not optional. C<${^Foo}> denotes the scalar variable whose | |
1198 | name is a control-C<F> followed by two C<o>'s. These variables are | |
1199 | reserved for future special uses by Perl, except for the ones that | |
1200 | begin with C<^_> (control-underscore or caret-underscore). No | |
1201 | control-character name that begins with C<^_> will acquire a special | |
1202 | meaning in any future version of Perl; such names may therefore be | |
1203 | used safely in programs. C<$^_> itself, however, I<is> reserved. | |
1204 | ||
1205 | Perl identifiers that begin with digits, control characters, or | |
1206 | punctuation characters are exempt from the effects of the C<package> | |
1207 | declaration and are always forced to be in package C<main>. A few | |
1208 | other names are also exempt: | |
1209 | ||
1210 | ENV STDIN | |
1211 | INC STDOUT | |
1212 | ARGV STDERR | |
1213 | ARGVOUT | |
1214 | SIG | |
1215 | ||
1216 | In particular, the new special C<${^_XYZ}> variables are always taken | |
1217 | to be in package C<main>, regardless of any C<package> declarations | |
1218 | presently in scope. | |
1219 | ||
1220 | =head1 BUGS | |
1221 | ||
1222 | Due to an unfortunate accident of Perl's implementation, C<use | |
1223 | English> imposes a considerable performance penalty on all regular | |
1224 | expression matches in a program, regardless of whether they occur | |
1225 | in the scope of C<use English>. For that reason, saying C<use | |
1226 | English> in libraries is strongly discouraged. See the | |
1227 | Devel::SawAmpersand module documentation from CPAN | |
1228 | (http://www.perl.com/CPAN/modules/by-module/Devel/) | |
1229 | for more information. | |
1230 | ||
1231 | Having to even think about the C<$^S> variable in your exception | |
1232 | handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented | |
1233 | invites grievous and difficult to track down errors. Avoid it | |
1234 | and use an C<END{}> or CORE::GLOBAL::die override instead. |