Commit | Line | Data |
---|---|---|
a0d0e21e LW |
1 | =head1 NAME |
2 | ||
3 | perlvar - Perl predefined variables | |
4 | ||
5 | =head1 DESCRIPTION | |
6 | ||
7 | =head2 Predefined Names | |
8 | ||
5a964f20 | 9 | The following names have special meaning to Perl. Most |
14218588 GS |
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 | |
a0d0e21e LW |
13 | |
14 | use English; | |
15 | ||
16 | at the top of your program. This will alias all the short names to the | |
5a964f20 | 17 | long names in the current package. Some even have medium names, |
a0d0e21e LW |
18 | generally borrowed from B<awk>. |
19 | ||
19799a22 GS |
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 | |
a0d0e21e | 24 | |
19799a22 | 25 | use IO::Handle; |
a0d0e21e LW |
26 | |
27 | after which you may use either | |
28 | ||
29 | method HANDLE EXPR | |
30 | ||
5a964f20 | 31 | or more safely, |
a0d0e21e LW |
32 | |
33 | HANDLE->method(EXPR) | |
34 | ||
14218588 | 35 | Each method returns the old value of the IO::Handle attribute. |
a0d0e21e | 36 | The methods each take an optional EXPR, which if supplied specifies the |
19799a22 | 37 | new value for the IO::Handle attribute in question. If not supplied, |
14218588 | 38 | most methods do nothing to the current value--except for |
a0d0e21e | 39 | autoflush(), which will assume a 1 for you, just to be different. |
14218588 | 40 | Because loading in the IO::Handle class is an expensive operation, you should |
19799a22 | 41 | learn how to use the regular built-in variables. |
a0d0e21e | 42 | |
748a9306 LW |
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. | |
a0d0e21e | 46 | |
fb73857a | 47 | The following list is ordered by scalar variables first, then the |
87275199 | 48 | arrays, then the hashes. |
fb73857a | 49 | |
a0d0e21e LW |
50 | =over 8 |
51 | ||
52 | =item $ARG | |
53 | ||
54 | =item $_ | |
55 | ||
56 | The default input and pattern-searching space. The following pairs are | |
57 | equivalent: | |
58 | ||
19799a22 | 59 | while (<>) {...} # equivalent only in while! |
54310121 | 60 | while (defined($_ = <>)) {...} |
a0d0e21e LW |
61 | |
62 | /^Subject:/ | |
63 | $_ =~ /^Subject:/ | |
64 | ||
65 | tr/a-z/A-Z/ | |
66 | $_ =~ tr/a-z/A-Z/ | |
67 | ||
19799a22 GS |
68 | chomp |
69 | chomp($_) | |
a0d0e21e | 70 | |
54310121 | 71 | Here are the places where Perl will assume $_ even if you |
cb1a09d0 AD |
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 | ||
54310121 | 91 | =item * |
cb1a09d0 AD |
92 | |
93 | The default iterator variable in a C<foreach> loop if no other | |
94 | variable is supplied. | |
95 | ||
54310121 | 96 | =item * |
cb1a09d0 AD |
97 | |
98 | The implicit iterator variable in the grep() and map() functions. | |
99 | ||
54310121 | 100 | =item * |
cb1a09d0 | 101 | |
c47ff5f1 | 102 | The default place to put an input record when a C<< <FH> >> |
cb1a09d0 | 103 | operation's result is tested by itself as the sole criterion of a C<while> |
14218588 | 104 | test. Outside a C<while> test, this will not happen. |
cb1a09d0 AD |
105 | |
106 | =back | |
107 | ||
a0d0e21e LW |
108 | (Mnemonic: underline is understood in certain operations.) |
109 | ||
6e2995f4 | 110 | =back |
111 | ||
112 | =over 8 | |
113 | ||
c47ff5f1 | 114 | =item $<I<digits>> |
a0d0e21e | 115 | |
19799a22 GS |
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. | |
a0d0e21e LW |
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 | |
19799a22 GS |
128 | BLOCK). (Mnemonic: like & in some editors.) This variable is read-only |
129 | and dynamically scoped to the current BLOCK. | |
a0d0e21e | 130 | |
19ddd453 | 131 | The use of this variable anywhere in a program imposes a considerable |
19799a22 | 132 | performance penalty on all regular expression matches. See L<BUGS>. |
19ddd453 | 133 | |
a0d0e21e LW |
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 | |
a8f8344d | 140 | enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted |
a0d0e21e LW |
141 | string.) This variable is read-only. |
142 | ||
19ddd453 | 143 | The use of this variable anywhere in a program imposes a considerable |
19799a22 | 144 | performance penalty on all regular expression matches. See L<BUGS>. |
19ddd453 | 145 | |
a0d0e21e LW |
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() | |
a8f8344d | 152 | enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted |
a0d0e21e LW |
153 | string.) Example: |
154 | ||
155 | $_ = 'abcdefghi'; | |
156 | /def/; | |
157 | print "$`:$&:$'\n"; # prints abc:def:ghi | |
158 | ||
19799a22 | 159 | This variable is read-only and dynamically scoped to the current BLOCK. |
a0d0e21e | 160 | |
19ddd453 | 161 | The use of this variable anywhere in a program imposes a considerable |
19799a22 | 162 | performance penalty on all regular expression matches. See L<BUGS>. |
19ddd453 | 163 | |
a0d0e21e LW |
164 | =item $LAST_PAREN_MATCH |
165 | ||
166 | =item $+ | |
167 | ||
168 | The last bracket matched by the last search pattern. This is useful if | |
19799a22 | 169 | you don't know which one of a set of alternative patterns matched. For |
a0d0e21e LW |
170 | example: |
171 | ||
172 | /Version: (.*)|Revision: (.*)/ && ($rev = $+); | |
173 | ||
174 | (Mnemonic: be positive and forward looking.) | |
19799a22 | 175 | This variable is read-only and dynamically scoped to the current BLOCK. |
a0d0e21e | 176 | |
6cef1e77 IZ |
177 | =item @+ |
178 | ||
4ba05bdc GS |
179 | This array holds the offsets of the ends of the last successful |
180 | submatches in the currently active dynamic scope. C<$+[0]> is | |
181 | the offset into the string of the end of the entire match. This | |
182 | is the same value as what the C<pos> function returns when called | |
183 | on the variable that was matched against. The I<n>th element | |
184 | of this array holds the offset of the I<n>th submatch, so | |
185 | C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset | |
186 | past where $2 ends, and so on. You can use C<$#+> to determine | |
187 | how many subgroups were in the last successful match. See the | |
188 | examples given for the C<@-> variable. | |
6cef1e77 | 189 | |
a0d0e21e LW |
190 | =item $MULTILINE_MATCHING |
191 | ||
192 | =item $* | |
193 | ||
4a6725af | 194 | Set to 1 to do multi-line matching within a string, 0 to tell Perl |
a0d0e21e LW |
195 | that it can assume that strings contain a single line, for the purpose |
196 | of optimizing pattern matches. Pattern matches on strings containing | |
19799a22 GS |
197 | multiple newlines can produce confusing results when C<$*> is 0. Default |
198 | is 0. (Mnemonic: * matches multiple things.) This variable | |
199 | influences the interpretation of only C<^> and C<$>. A literal newline can | |
a0d0e21e LW |
200 | be searched for even when C<$* == 0>. |
201 | ||
19799a22 | 202 | Use of C<$*> is deprecated in modern Perl, supplanted by |
5a964f20 | 203 | the C</s> and C</m> modifiers on pattern matching. |
a0d0e21e LW |
204 | |
205 | =item input_line_number HANDLE EXPR | |
206 | ||
207 | =item $INPUT_LINE_NUMBER | |
208 | ||
209 | =item $NR | |
210 | ||
211 | =item $. | |
212 | ||
19799a22 | 213 | The current input record number for the last file handle from which |
14218588 | 214 | you just read() (or called a C<seek> or C<tell> on). The value |
883faa13 | 215 | may be different from the actual physical line number in the file, |
19799a22 GS |
216 | depending on what notion of "line" is in effect--see C<$/> on how |
217 | to change that. An explicit close on a filehandle resets the line | |
c47ff5f1 | 218 | number. Because C<< <> >> never does an explicit close, line |
19799a22 GS |
219 | numbers increase across ARGV files (but see examples in L<perlfunc/eof>). |
220 | Consider this variable read-only: setting it does not reposition | |
1e374101 PJ |
221 | the seek pointer; you'll have to do that on your own. Localizing C<$.> |
222 | has the effect of also localizing Perl's notion of "the last read | |
223 | filehandle". (Mnemonic: many programs use "." to mean the current line | |
224 | number.) | |
a0d0e21e LW |
225 | |
226 | =item input_record_separator HANDLE EXPR | |
227 | ||
228 | =item $INPUT_RECORD_SEPARATOR | |
229 | ||
230 | =item $RS | |
231 | ||
232 | =item $/ | |
233 | ||
14218588 GS |
234 | The input record separator, newline by default. This |
235 | influences Perl's idea of what a "line" is. Works like B<awk>'s RS | |
19799a22 | 236 | variable, including treating empty lines as a terminator if set to |
14218588 GS |
237 | the null string. (An empty line cannot contain any spaces |
238 | or tabs.) You may set it to a multi-character string to match a | |
19799a22 GS |
239 | multi-character terminator, or to C<undef> to read through the end |
240 | of file. Setting it to C<"\n\n"> means something slightly | |
241 | different than setting to C<"">, if the file contains consecutive | |
242 | empty lines. Setting to C<""> will treat two or more consecutive | |
243 | empty lines as a single empty line. Setting to C<"\n\n"> will | |
244 | blindly assume that the next input character belongs to the next | |
14218588 | 245 | paragraph, even if it's a newline. (Mnemonic: / delimits |
19799a22 | 246 | line boundaries when quoting poetry.) |
a0d0e21e | 247 | |
fbad3eb5 GS |
248 | undef $/; # enable "slurp" mode |
249 | $_ = <FH>; # whole file now here | |
a0d0e21e LW |
250 | s/\n[ \t]+/ /g; |
251 | ||
19799a22 GS |
252 | Remember: the value of C<$/> is a string, not a regex. B<awk> has to be |
253 | better for something. :-) | |
68dc0745 | 254 | |
19799a22 GS |
255 | Setting C<$/> to a reference to an integer, scalar containing an integer, or |
256 | scalar that's convertible to an integer will attempt to read records | |
5b2b9c68 | 257 | instead of lines, with the maximum record size being the referenced |
19799a22 | 258 | integer. So this: |
5b2b9c68 HM |
259 | |
260 | $/ = \32768; # or \"32768", or \$var_containing_32768 | |
261 | open(FILE, $myfile); | |
262 | $_ = <FILE>; | |
263 | ||
19799a22 GS |
264 | will read a record of no more than 32768 bytes from FILE. If you're |
265 | not reading from a record-oriented file (or your OS doesn't have | |
266 | record-oriented files), then you'll likely get a full chunk of data | |
267 | with every read. If a record is larger than the record size you've | |
268 | set, you'll get the record back in pieces. | |
5b2b9c68 | 269 | |
19799a22 GS |
270 | On VMS, record reads are done with the equivalent of C<sysread>, |
271 | so it's best not to mix record and non-record reads on the same | |
272 | file. (This is unlikely to be a problem, because any file you'd | |
83763826 | 273 | want to read in record mode is probably unusable in line mode.) |
14218588 | 274 | Non-VMS systems do normal I/O, so it's safe to mix record and |
19799a22 | 275 | non-record reads of a file. |
5b2b9c68 | 276 | |
14218588 | 277 | See also L<perlport/"Newlines">. Also see C<$.>. |
883faa13 | 278 | |
a0d0e21e LW |
279 | =item autoflush HANDLE EXPR |
280 | ||
281 | =item $OUTPUT_AUTOFLUSH | |
282 | ||
283 | =item $| | |
284 | ||
19799a22 GS |
285 | If set to nonzero, forces a flush right away and after every write |
286 | or print on the currently selected output channel. Default is 0 | |
14218588 | 287 | (regardless of whether the channel is really buffered by the |
19799a22 GS |
288 | system or not; C<$|> tells you only whether you've asked Perl |
289 | explicitly to flush after each write). STDOUT will | |
290 | typically be line buffered if output is to the terminal and block | |
291 | buffered otherwise. Setting this variable is useful primarily when | |
292 | you are outputting to a pipe or socket, such as when you are running | |
293 | a Perl program under B<rsh> and want to see the output as it's | |
294 | happening. This has no effect on input buffering. See L<perlfunc/getc> | |
295 | for that. (Mnemonic: when you want your pipes to be piping hot.) | |
a0d0e21e LW |
296 | |
297 | =item output_field_separator HANDLE EXPR | |
298 | ||
299 | =item $OUTPUT_FIELD_SEPARATOR | |
300 | ||
301 | =item $OFS | |
302 | ||
303 | =item $, | |
304 | ||
305 | The output field separator for the print operator. Ordinarily the | |
19799a22 GS |
306 | print operator simply prints out its arguments without further |
307 | adornment. To get behavior more like B<awk>, set this variable as | |
308 | you would set B<awk>'s OFS variable to specify what is printed | |
309 | between fields. (Mnemonic: what is printed when there is a "," in | |
310 | your print statement.) | |
a0d0e21e LW |
311 | |
312 | =item output_record_separator HANDLE EXPR | |
313 | ||
314 | =item $OUTPUT_RECORD_SEPARATOR | |
315 | ||
316 | =item $ORS | |
317 | ||
318 | =item $\ | |
319 | ||
320 | The output record separator for the print operator. Ordinarily the | |
19799a22 GS |
321 | print operator simply prints out its arguments as is, with no |
322 | trailing newline or other end-of-record string added. To get | |
323 | behavior more like B<awk>, set this variable as you would set | |
324 | B<awk>'s ORS variable to specify what is printed at the end of the | |
325 | print. (Mnemonic: you set C<$\> instead of adding "\n" at the | |
326 | end of the print. Also, it's just like C<$/>, but it's what you | |
327 | get "back" from Perl.) | |
a0d0e21e LW |
328 | |
329 | =item $LIST_SEPARATOR | |
330 | ||
331 | =item $" | |
332 | ||
19799a22 GS |
333 | This is like C<$,> except that it applies to array and slice values |
334 | interpolated into a double-quoted string (or similar interpreted | |
335 | string). Default is a space. (Mnemonic: obvious, I think.) | |
a0d0e21e LW |
336 | |
337 | =item $SUBSCRIPT_SEPARATOR | |
338 | ||
339 | =item $SUBSEP | |
340 | ||
341 | =item $; | |
342 | ||
54310121 | 343 | The subscript separator for multidimensional array emulation. If you |
a0d0e21e LW |
344 | refer to a hash element as |
345 | ||
346 | $foo{$a,$b,$c} | |
347 | ||
348 | it really means | |
349 | ||
350 | $foo{join($;, $a, $b, $c)} | |
351 | ||
352 | But don't put | |
353 | ||
354 | @foo{$a,$b,$c} # a slice--note the @ | |
355 | ||
356 | which means | |
357 | ||
358 | ($foo{$a},$foo{$b},$foo{$c}) | |
359 | ||
19799a22 GS |
360 | Default is "\034", the same as SUBSEP in B<awk>. If your |
361 | keys contain binary data there might not be any safe value for C<$;>. | |
a0d0e21e | 362 | (Mnemonic: comma (the syntactic subscript separator) is a |
19799a22 | 363 | semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already |
a0d0e21e LW |
364 | taken for something more important.) |
365 | ||
19799a22 GS |
366 | Consider using "real" multidimensional arrays as described |
367 | in L<perllol>. | |
a0d0e21e LW |
368 | |
369 | =item $OFMT | |
370 | ||
371 | =item $# | |
372 | ||
373 | The output format for printed numbers. This variable is a half-hearted | |
374 | attempt to emulate B<awk>'s OFMT variable. There are times, however, | |
14218588 | 375 | when B<awk> and Perl have differing notions of what counts as |
19799a22 | 376 | numeric. The initial value is "%.I<n>g", where I<n> is the value |
6e2995f4 | 377 | of the macro DBL_DIG from your system's F<float.h>. This is different from |
19799a22 | 378 | B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#> |
6e2995f4 | 379 | explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.) |
a0d0e21e | 380 | |
19799a22 | 381 | Use of C<$#> is deprecated. |
a0d0e21e LW |
382 | |
383 | =item format_page_number HANDLE EXPR | |
384 | ||
385 | =item $FORMAT_PAGE_NUMBER | |
386 | ||
387 | =item $% | |
388 | ||
389 | The current page number of the currently selected output channel. | |
19799a22 | 390 | Used with formats. |
a0d0e21e LW |
391 | (Mnemonic: % is page number in B<nroff>.) |
392 | ||
393 | =item format_lines_per_page HANDLE EXPR | |
394 | ||
395 | =item $FORMAT_LINES_PER_PAGE | |
396 | ||
397 | =item $= | |
398 | ||
399 | The current page length (printable lines) of the currently selected | |
19799a22 GS |
400 | output channel. Default is 60. |
401 | Used with formats. | |
402 | (Mnemonic: = has horizontal lines.) | |
a0d0e21e LW |
403 | |
404 | =item format_lines_left HANDLE EXPR | |
405 | ||
406 | =item $FORMAT_LINES_LEFT | |
407 | ||
408 | =item $- | |
409 | ||
410 | The number of lines left on the page of the currently selected output | |
19799a22 GS |
411 | channel. |
412 | Used with formats. | |
413 | (Mnemonic: lines_on_page - lines_printed.) | |
a0d0e21e | 414 | |
6cef1e77 IZ |
415 | =item @- |
416 | ||
19799a22 | 417 | $-[0] is the offset of the start of the last successful match. |
6cef1e77 | 418 | C<$-[>I<n>C<]> is the offset of the start of the substring matched by |
8f580fb8 | 419 | I<n>-th subpattern, or undef if the subpattern did not match. |
6cef1e77 IZ |
420 | |
421 | Thus after a match against $_, $& coincides with C<substr $_, $-[0], | |
8f580fb8 IZ |
422 | $+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<], |
423 | $+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with | |
c47ff5f1 | 424 | C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last |
14218588 GS |
425 | matched subgroup in the last successful match. Contrast with |
426 | C<$#+>, the number of subgroups in the regular expression. Compare | |
19799a22 | 427 | with C<@+>. |
6cef1e77 | 428 | |
4ba05bdc GS |
429 | This array holds the offsets of the beginnings of the last |
430 | successful submatches in the currently active dynamic scope. | |
431 | C<$-[0]> is the offset into the string of the beginning of the | |
432 | entire match. The I<n>th element of this array holds the offset | |
433 | of the I<n>th submatch, so C<$+[1]> is the offset where $1 | |
434 | begins, C<$+[2]> the offset where $2 begins, and so on. | |
435 | You can use C<$#-> to determine how many subgroups were in the | |
436 | last successful match. Compare with the C<@+> variable. | |
437 | ||
438 | After a match against some variable $var: | |
439 | ||
440 | =over 5 | |
441 | ||
442 | =item C<$`> is the same as C<substr($var, 0, $-[0]>) | |
443 | ||
444 | =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0]>) | |
445 | ||
446 | =item C<$'> is the same as C<substr($var, $+[0]>) | |
447 | ||
448 | =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])> | |
449 | ||
450 | =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])> | |
451 | ||
452 | =item C<$3> is the same as C<substr $var, $-[3], $+[3] - $-[3]>) | |
453 | ||
454 | =back | |
455 | ||
a0d0e21e LW |
456 | =item format_name HANDLE EXPR |
457 | ||
458 | =item $FORMAT_NAME | |
459 | ||
460 | =item $~ | |
461 | ||
462 | The name of the current report format for the currently selected output | |
14218588 | 463 | channel. Default is the name of the filehandle. (Mnemonic: brother to |
19799a22 | 464 | C<$^>.) |
a0d0e21e LW |
465 | |
466 | =item format_top_name HANDLE EXPR | |
467 | ||
468 | =item $FORMAT_TOP_NAME | |
469 | ||
470 | =item $^ | |
471 | ||
472 | The name of the current top-of-page format for the currently selected | |
14218588 | 473 | output channel. Default is the name of the filehandle with _TOP |
a0d0e21e LW |
474 | appended. (Mnemonic: points to top of page.) |
475 | ||
476 | =item format_line_break_characters HANDLE EXPR | |
477 | ||
478 | =item $FORMAT_LINE_BREAK_CHARACTERS | |
479 | ||
480 | =item $: | |
481 | ||
482 | The current set of characters after which a string may be broken to | |
54310121 | 483 | fill continuation fields (starting with ^) in a format. Default is |
a0d0e21e LW |
484 | S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in |
485 | poetry is a part of a line.) | |
486 | ||
487 | =item format_formfeed HANDLE EXPR | |
488 | ||
489 | =item $FORMAT_FORMFEED | |
490 | ||
491 | =item $^L | |
492 | ||
14218588 | 493 | What formats output as a form feed. Default is \f. |
a0d0e21e LW |
494 | |
495 | =item $ACCUMULATOR | |
496 | ||
497 | =item $^A | |
498 | ||
499 | The current value of the write() accumulator for format() lines. A format | |
19799a22 | 500 | contains formline() calls that put their result into C<$^A>. After |
a0d0e21e | 501 | calling its format, write() prints out the contents of C<$^A> and empties. |
14218588 | 502 | So you never really see the contents of C<$^A> unless you call |
a0d0e21e LW |
503 | formline() yourself and then look at it. See L<perlform> and |
504 | L<perlfunc/formline()>. | |
505 | ||
506 | =item $CHILD_ERROR | |
507 | ||
508 | =item $? | |
509 | ||
54310121 | 510 | The status returned by the last pipe close, backtick (C<``>) command, |
19799a22 GS |
511 | successful call to wait() or waitpid(), or from the system() |
512 | operator. This is just the 16-bit status word returned by the | |
513 | wait() system call (or else is made up to look like it). Thus, the | |
c47ff5f1 | 514 | exit value of the subprocess is really (C<<< $? >> 8 >>>), and |
19799a22 GS |
515 | C<$? & 127> gives which signal, if any, the process died from, and |
516 | C<$? & 128> reports whether there was a core dump. (Mnemonic: | |
517 | similar to B<sh> and B<ksh>.) | |
a0d0e21e | 518 | |
7b8d334a | 519 | Additionally, if the C<h_errno> variable is supported in C, its value |
14218588 | 520 | is returned via $? if any C<gethost*()> function fails. |
7b8d334a | 521 | |
19799a22 | 522 | If you have installed a signal handler for C<SIGCHLD>, the |
aa689395 | 523 | value of C<$?> will usually be wrong outside that handler. |
524 | ||
a8f8344d | 525 | Inside an C<END> subroutine C<$?> contains the value that is going to be |
526 | given to C<exit()>. You can modify C<$?> in an C<END> subroutine to | |
19799a22 GS |
527 | change the exit status of your program. For example: |
528 | ||
529 | END { | |
530 | $? = 1 if $? == 255; # die would make it 255 | |
531 | } | |
a8f8344d | 532 | |
aa689395 | 533 | Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the |
ff0cee69 | 534 | actual VMS exit status, instead of the default emulation of POSIX |
535 | status. | |
f86702cc | 536 | |
55602bd2 IZ |
537 | Also see L<Error Indicators>. |
538 | ||
a0d0e21e LW |
539 | =item $OS_ERROR |
540 | ||
541 | =item $ERRNO | |
542 | ||
543 | =item $! | |
544 | ||
19799a22 GS |
545 | If used numerically, yields the current value of the C C<errno> |
546 | variable, with all the usual caveats. (This means that you shouldn't | |
547 | depend on the value of C<$!> to be anything in particular unless | |
548 | you've gotten a specific error return indicating a system error.) | |
549 | If used an a string, yields the corresponding system error string. | |
550 | You can assign a number to C<$!> to set I<errno> if, for instance, | |
551 | you want C<"$!"> to return the string for error I<n>, or you want | |
552 | to set the exit value for the die() operator. (Mnemonic: What just | |
553 | went bang?) | |
a0d0e21e | 554 | |
55602bd2 IZ |
555 | Also see L<Error Indicators>. |
556 | ||
5c055ba3 | 557 | =item $EXTENDED_OS_ERROR |
558 | ||
559 | =item $^E | |
560 | ||
22fae026 TM |
561 | Error information specific to the current operating system. At |
562 | the moment, this differs from C<$!> under only VMS, OS/2, and Win32 | |
563 | (and for MacPerl). On all other platforms, C<$^E> is always just | |
564 | the same as C<$!>. | |
565 | ||
566 | Under VMS, C<$^E> provides the VMS status value from the last | |
567 | system error. This is more specific information about the last | |
568 | system error than that provided by C<$!>. This is particularly | |
d516a115 | 569 | important when C<$!> is set to B<EVMSERR>. |
22fae026 | 570 | |
1c1c7f20 GS |
571 | Under OS/2, C<$^E> is set to the error code of the last call to |
572 | OS/2 API either via CRT, or directly from perl. | |
22fae026 TM |
573 | |
574 | Under Win32, C<$^E> always returns the last error information | |
575 | reported by the Win32 call C<GetLastError()> which describes | |
576 | the last error from within the Win32 API. Most Win32-specific | |
19799a22 | 577 | code will report errors via C<$^E>. ANSI C and Unix-like calls |
22fae026 TM |
578 | set C<errno> and so most portable Perl code will report errors |
579 | via C<$!>. | |
580 | ||
581 | Caveats mentioned in the description of C<$!> generally apply to | |
582 | C<$^E>, also. (Mnemonic: Extra error explanation.) | |
5c055ba3 | 583 | |
55602bd2 IZ |
584 | Also see L<Error Indicators>. |
585 | ||
a0d0e21e LW |
586 | =item $EVAL_ERROR |
587 | ||
588 | =item $@ | |
589 | ||
19799a22 | 590 | The Perl syntax error message from the last eval() operator. If null, the |
a0d0e21e LW |
591 | last eval() parsed and executed correctly (although the operations you |
592 | invoked may have failed in the normal fashion). (Mnemonic: Where was | |
593 | the syntax error "at"?) | |
594 | ||
19799a22 | 595 | Warning messages are not collected in this variable. You can, |
a8f8344d | 596 | however, set up a routine to process warnings by setting C<$SIG{__WARN__}> |
54310121 | 597 | as described below. |
748a9306 | 598 | |
55602bd2 IZ |
599 | Also see L<Error Indicators>. |
600 | ||
a0d0e21e LW |
601 | =item $PROCESS_ID |
602 | ||
603 | =item $PID | |
604 | ||
605 | =item $$ | |
606 | ||
19799a22 GS |
607 | The process number of the Perl running this script. You should |
608 | consider this variable read-only, although it will be altered | |
609 | across fork() calls. (Mnemonic: same as shells.) | |
a0d0e21e LW |
610 | |
611 | =item $REAL_USER_ID | |
612 | ||
613 | =item $UID | |
614 | ||
615 | =item $< | |
616 | ||
19799a22 | 617 | The real uid of this process. (Mnemonic: it's the uid you came I<from>, |
a0d0e21e LW |
618 | if you're running setuid.) |
619 | ||
620 | =item $EFFECTIVE_USER_ID | |
621 | ||
622 | =item $EUID | |
623 | ||
624 | =item $> | |
625 | ||
626 | The effective uid of this process. Example: | |
627 | ||
628 | $< = $>; # set real to effective uid | |
629 | ($<,$>) = ($>,$<); # swap real and effective uid | |
630 | ||
19799a22 | 631 | (Mnemonic: it's the uid you went I<to>, if you're running setuid.) |
c47ff5f1 | 632 | C<< $< >> and C<< $> >> can be swapped only on machines |
8cc95fdb | 633 | supporting setreuid(). |
a0d0e21e LW |
634 | |
635 | =item $REAL_GROUP_ID | |
636 | ||
637 | =item $GID | |
638 | ||
639 | =item $( | |
640 | ||
641 | The real gid of this process. If you are on a machine that supports | |
642 | membership in multiple groups simultaneously, gives a space separated | |
643 | list of groups you are in. The first number is the one returned by | |
644 | getgid(), and the subsequent ones by getgroups(), one of which may be | |
8cc95fdb | 645 | the same as the first number. |
646 | ||
19799a22 GS |
647 | However, a value assigned to C<$(> must be a single number used to |
648 | set the real gid. So the value given by C<$(> should I<not> be assigned | |
649 | back to C<$(> without being forced numeric, such as by adding zero. | |
8cc95fdb | 650 | |
19799a22 GS |
651 | (Mnemonic: parentheses are used to I<group> things. The real gid is the |
652 | group you I<left>, if you're running setgid.) | |
a0d0e21e LW |
653 | |
654 | =item $EFFECTIVE_GROUP_ID | |
655 | ||
656 | =item $EGID | |
657 | ||
658 | =item $) | |
659 | ||
660 | The effective gid of this process. If you are on a machine that | |
661 | supports membership in multiple groups simultaneously, gives a space | |
662 | separated list of groups you are in. The first number is the one | |
663 | returned by getegid(), and the subsequent ones by getgroups(), one of | |
8cc95fdb | 664 | which may be the same as the first number. |
665 | ||
19799a22 | 666 | Similarly, a value assigned to C<$)> must also be a space-separated |
14218588 | 667 | list of numbers. The first number sets the effective gid, and |
8cc95fdb | 668 | the rest (if any) are passed to setgroups(). To get the effect of an |
669 | empty list for setgroups(), just repeat the new effective gid; that is, | |
670 | to force an effective gid of 5 and an effectively empty setgroups() | |
671 | list, say C< $) = "5 5" >. | |
672 | ||
19799a22 GS |
673 | (Mnemonic: parentheses are used to I<group> things. The effective gid |
674 | is the group that's I<right> for you, if you're running setgid.) | |
a0d0e21e | 675 | |
c47ff5f1 | 676 | C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on |
19799a22 GS |
677 | machines that support the corresponding I<set[re][ug]id()> routine. C<$(> |
678 | and C<$)> can be swapped only on machines supporting setregid(). | |
a0d0e21e LW |
679 | |
680 | =item $PROGRAM_NAME | |
681 | ||
682 | =item $0 | |
683 | ||
19799a22 GS |
684 | Contains the name of the program being executed. On some operating |
685 | systems assigning to C<$0> modifies the argument area that the B<ps> | |
686 | program sees. This is more useful as a way of indicating the current | |
687 | program state than it is for hiding the program you're running. | |
a0d0e21e LW |
688 | (Mnemonic: same as B<sh> and B<ksh>.) |
689 | ||
690 | =item $[ | |
691 | ||
692 | The index of the first element in an array, and of the first character | |
19799a22 GS |
693 | in a substring. Default is 0, but you could theoretically set it |
694 | to 1 to make Perl behave more like B<awk> (or Fortran) when | |
695 | subscripting and when evaluating the index() and substr() functions. | |
696 | (Mnemonic: [ begins subscripts.) | |
a0d0e21e | 697 | |
19799a22 GS |
698 | As of release 5 of Perl, assignment to C<$[> is treated as a compiler |
699 | directive, and cannot influence the behavior of any other file. | |
700 | Its use is highly discouraged. | |
a0d0e21e | 701 | |
a0d0e21e LW |
702 | =item $] |
703 | ||
54310121 | 704 | The version + patchlevel / 1000 of the Perl interpreter. This variable |
705 | can be used to determine whether the Perl interpreter executing a | |
706 | script is in the right range of versions. (Mnemonic: Is this version | |
707 | of perl in the right bracket?) Example: | |
a0d0e21e LW |
708 | |
709 | warn "No checksumming!\n" if $] < 3.019; | |
710 | ||
54310121 | 711 | See also the documentation of C<use VERSION> and C<require VERSION> |
19799a22 | 712 | for a convenient way to fail if the running Perl interpreter is too old. |
a0d0e21e | 713 | |
44dcb63b GS |
714 | The use of this variable is deprecated. The floating point representation |
715 | can sometimes lead to inaccurate numeric comparisons. See C<$^V> for a | |
716 | more modern representation of the Perl version that allows accurate string | |
717 | comparisons. | |
16070b82 | 718 | |
305aace0 NIS |
719 | =item $COMPILING |
720 | ||
721 | =item $^C | |
722 | ||
19799a22 GS |
723 | The current value of the flag associated with the B<-c> switch. |
724 | Mainly of use with B<-MO=...> to allow code to alter its behavior | |
725 | when being compiled, such as for example to AUTOLOAD at compile | |
726 | time rather than normal, deferred loading. See L<perlcc>. Setting | |
727 | C<$^C = 1> is similar to calling C<B::minus_c>. | |
305aace0 | 728 | |
a0d0e21e LW |
729 | =item $DEBUGGING |
730 | ||
731 | =item $^D | |
732 | ||
733 | The current value of the debugging flags. (Mnemonic: value of B<-D> | |
734 | switch.) | |
735 | ||
736 | =item $SYSTEM_FD_MAX | |
737 | ||
738 | =item $^F | |
739 | ||
740 | The maximum system file descriptor, ordinarily 2. System file | |
741 | descriptors are passed to exec()ed processes, while higher file | |
742 | descriptors are not. Also, during an open(), system file descriptors are | |
743 | preserved even if the open() fails. (Ordinary file descriptors are | |
19799a22 | 744 | closed before the open() is attempted.) The close-on-exec |
a0d0e21e | 745 | status of a file descriptor will be decided according to the value of |
8d2a6795 GS |
746 | C<$^F> when the corresponding file, pipe, or socket was opened, not the |
747 | time of the exec(). | |
a0d0e21e | 748 | |
6e2995f4 | 749 | =item $^H |
750 | ||
0462a1ab GS |
751 | WARNING: This variable is strictly for internal use only. Its availability, |
752 | behavior, and contents are subject to change without notice. | |
753 | ||
754 | This variable contains compile-time hints for the Perl interpreter. At the | |
755 | end of compilation of a BLOCK the value of this variable is restored to the | |
756 | value when the interpreter started to compile the BLOCK. | |
757 | ||
758 | When perl begins to parse any block construct that provides a lexical scope | |
759 | (e.g., eval body, required file, subroutine body, loop body, or conditional | |
760 | block), the existing value of $^H is saved, but its value is left unchanged. | |
761 | When the compilation of the block is completed, it regains the saved value. | |
762 | Between the points where its value is saved and restored, code that | |
763 | executes within BEGIN blocks is free to change the value of $^H. | |
764 | ||
765 | This behavior provides the semantic of lexical scoping, and is used in, | |
766 | for instance, the C<use strict> pragma. | |
767 | ||
768 | The contents should be an integer; different bits of it are used for | |
769 | different pragmatic flags. Here's an example: | |
770 | ||
771 | sub add_100 { $^H |= 0x100 } | |
772 | ||
773 | sub foo { | |
774 | BEGIN { add_100() } | |
775 | bar->baz($boon); | |
776 | } | |
777 | ||
778 | Consider what happens during execution of the BEGIN block. At this point | |
779 | the BEGIN block has already been compiled, but the body of foo() is still | |
780 | being compiled. The new value of $^H will therefore be visible only while | |
781 | the body of foo() is being compiled. | |
782 | ||
783 | Substitution of the above BEGIN block with: | |
784 | ||
785 | BEGIN { require strict; strict->import('vars') } | |
786 | ||
787 | demonstrates how C<use strict 'vars'> is implemented. Here's a conditional | |
788 | version of the same lexical pragma: | |
789 | ||
790 | BEGIN { require strict; strict->import('vars') if $condition } | |
791 | ||
792 | =item %^H | |
793 | ||
794 | WARNING: This variable is strictly for internal use only. Its availability, | |
795 | behavior, and contents are subject to change without notice. | |
796 | ||
797 | The %^H hash provides the same scoping semantic as $^H. This makes it | |
798 | useful for implementation of lexically scoped pragmas. | |
6e2995f4 | 799 | |
a0d0e21e LW |
800 | =item $INPLACE_EDIT |
801 | ||
802 | =item $^I | |
803 | ||
804 | The current value of the inplace-edit extension. Use C<undef> to disable | |
805 | inplace editing. (Mnemonic: value of B<-i> switch.) | |
806 | ||
fb73857a | 807 | =item $^M |
808 | ||
19799a22 GS |
809 | By default, running out of memory is an untrappable, fatal error. |
810 | However, if suitably built, Perl can use the contents of C<$^M> | |
811 | as an emergency memory pool after die()ing. Suppose that your Perl | |
812 | were compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc. | |
813 | Then | |
fb73857a | 814 | |
19799a22 | 815 | $^M = 'a' x (1 << 16); |
fb73857a | 816 | |
19799a22 GS |
817 | would allocate a 64K buffer for use when in emergency. See the |
818 | F<INSTALL> file in the Perl distribution for information on how to | |
819 | enable this option. To discourage casual use of this advanced | |
820 | feature, there is no L<English> long name for this variable. | |
fb73857a | 821 | |
5c055ba3 | 822 | =item $OSNAME |
6e2995f4 | 823 | |
5c055ba3 | 824 | =item $^O |
825 | ||
826 | The name of the operating system under which this copy of Perl was | |
827 | built, as determined during the configuration process. The value | |
19799a22 GS |
828 | is identical to C<$Config{'osname'}>. See also L<Config> and the |
829 | B<-V> command-line switch documented in L<perlrun>. | |
5c055ba3 | 830 | |
a0d0e21e LW |
831 | =item $PERLDB |
832 | ||
833 | =item $^P | |
834 | ||
19799a22 GS |
835 | The internal variable for debugging support. The meanings of the |
836 | various bits are subject to change, but currently indicate: | |
84902520 TB |
837 | |
838 | =over 6 | |
839 | ||
840 | =item 0x01 | |
841 | ||
842 | Debug subroutine enter/exit. | |
843 | ||
844 | =item 0x02 | |
845 | ||
846 | Line-by-line debugging. | |
847 | ||
848 | =item 0x04 | |
849 | ||
850 | Switch off optimizations. | |
851 | ||
852 | =item 0x08 | |
853 | ||
854 | Preserve more data for future interactive inspections. | |
855 | ||
856 | =item 0x10 | |
857 | ||
858 | Keep info about source lines on which a subroutine is defined. | |
859 | ||
860 | =item 0x20 | |
861 | ||
862 | Start with single-step on. | |
863 | ||
83ee9e09 GS |
864 | =item 0x40 |
865 | ||
866 | Use subroutine address instead of name when reporting. | |
867 | ||
868 | =item 0x80 | |
869 | ||
870 | Report C<goto &subroutine> as well. | |
871 | ||
872 | =item 0x100 | |
873 | ||
874 | Provide informative "file" names for evals based on the place they were compiled. | |
875 | ||
876 | =item 0x200 | |
877 | ||
878 | Provide informative names to anonymous subroutines based on the place they | |
879 | were compiled. | |
880 | ||
84902520 TB |
881 | =back |
882 | ||
19799a22 GS |
883 | Some bits may be relevant at compile-time only, some at |
884 | run-time only. This is a new mechanism and the details may change. | |
a0d0e21e | 885 | |
66558a10 GS |
886 | =item $LAST_REGEXP_CODE_RESULT |
887 | ||
b9ac3b5b GS |
888 | =item $^R |
889 | ||
19799a22 GS |
890 | The result of evaluation of the last successful C<(?{ code })> |
891 | regular expression assertion (see L<perlre>). May be written to. | |
b9ac3b5b | 892 | |
66558a10 GS |
893 | =item $EXCEPTIONS_BEING_CAUGHT |
894 | ||
fb73857a | 895 | =item $^S |
896 | ||
897 | Current state of the interpreter. Undefined if parsing of the current | |
898 | module/eval is not finished (may happen in $SIG{__DIE__} and | |
19799a22 | 899 | $SIG{__WARN__} handlers). True if inside an eval(), otherwise false. |
fb73857a | 900 | |
a0d0e21e LW |
901 | =item $BASETIME |
902 | ||
903 | =item $^T | |
904 | ||
19799a22 | 905 | The time at which the program began running, in seconds since the |
5f05dabc | 906 | epoch (beginning of 1970). The values returned by the B<-M>, B<-A>, |
19799a22 | 907 | and B<-C> filetests are based on this value. |
a0d0e21e | 908 | |
44dcb63b | 909 | =item $PERL_VERSION |
b459063d | 910 | |
16070b82 GS |
911 | =item $^V |
912 | ||
913 | The revision, version, and subversion of the Perl interpreter, represented | |
da2094fd | 914 | as a string composed of characters with those ordinals. Thus in Perl v5.6.0 |
44dcb63b GS |
915 | it equals C<chr(5) . chr(6) . chr(0)> and will return true for |
916 | C<$^V eq v5.6.0>. Note that the characters in this string value can | |
917 | potentially be in Unicode range. | |
16070b82 GS |
918 | |
919 | This can be used to determine whether the Perl interpreter executing a | |
920 | script is in the right range of versions. (Mnemonic: use ^V for Version | |
44dcb63b | 921 | Control.) Example: |
16070b82 | 922 | |
44dcb63b | 923 | warn "No "our" declarations!\n" if $^V and $^V lt v5.6.0; |
16070b82 | 924 | |
44dcb63b | 925 | See the documentation of C<use VERSION> and C<require VERSION> |
16070b82 GS |
926 | for a convenient way to fail if the running Perl interpreter is too old. |
927 | ||
928 | See also C<$]> for an older representation of the Perl version. | |
929 | ||
a0d0e21e LW |
930 | =item $WARNING |
931 | ||
932 | =item $^W | |
933 | ||
19799a22 GS |
934 | The current value of the warning switch, initially true if B<-w> |
935 | was used, false otherwise, but directly modifiable. (Mnemonic: | |
4438c4b7 JH |
936 | related to the B<-w> switch.) See also L<warnings>. |
937 | ||
6a818117 | 938 | =item ${^WARNING_BITS} |
4438c4b7 JH |
939 | |
940 | The current set of warning checks enabled by the C<use warnings> pragma. | |
941 | See the documentation of C<warnings> for more details. | |
a0d0e21e | 942 | |
46487f74 GS |
943 | =item ${^WIDE_SYSTEM_CALLS} |
944 | ||
945 | Global flag that enables system calls made by Perl to use wide character | |
946 | APIs native to the system, if available. This is currently only implemented | |
947 | on the Windows platform. | |
948 | ||
949 | This can also be enabled from the command line using the C<-C> switch. | |
950 | ||
951 | The initial value is typically C<0> for compatibility with Perl versions | |
952 | earlier than 5.6, but may be automatically set to C<1> by Perl if the system | |
953 | provides a user-settable default (e.g., C<$ENV{LC_CTYPE}>). | |
954 | ||
8058d7ab GS |
955 | The C<bytes> pragma always overrides the effect of this flag in the current |
956 | lexical scope. See L<bytes>. | |
46487f74 | 957 | |
a0d0e21e LW |
958 | =item $EXECUTABLE_NAME |
959 | ||
960 | =item $^X | |
961 | ||
962 | The name that the Perl binary itself was executed as, from C's C<argv[0]>. | |
19799a22 | 963 | This may not be a full pathname, nor even necessarily in your path. |
a0d0e21e LW |
964 | |
965 | =item $ARGV | |
966 | ||
c47ff5f1 | 967 | contains the name of the current file when reading from <>. |
a0d0e21e LW |
968 | |
969 | =item @ARGV | |
970 | ||
19799a22 | 971 | The array @ARGV contains the command-line arguments intended for |
14218588 | 972 | the script. C<$#ARGV> is generally the number of arguments minus |
19799a22 GS |
973 | one, because C<$ARGV[0]> is the first argument, I<not> the program's |
974 | command name itself. See C<$0> for the command name. | |
a0d0e21e LW |
975 | |
976 | =item @INC | |
977 | ||
19799a22 GS |
978 | The array @INC contains the list of places that the C<do EXPR>, |
979 | C<require>, or C<use> constructs look for their library files. It | |
980 | initially consists of the arguments to any B<-I> command-line | |
981 | switches, followed by the default Perl library, probably | |
982 | F</usr/local/lib/perl>, followed by ".", to represent the current | |
983 | directory. If you need to modify this at runtime, you should use | |
984 | the C<use lib> pragma to get the machine-dependent library properly | |
985 | loaded also: | |
a0d0e21e | 986 | |
cb1a09d0 AD |
987 | use lib '/mypath/libdir/'; |
988 | use SomeMod; | |
303f2f76 | 989 | |
fb73857a | 990 | =item @_ |
991 | ||
992 | Within a subroutine the array @_ contains the parameters passed to that | |
19799a22 | 993 | subroutine. See L<perlsub>. |
fb73857a | 994 | |
a0d0e21e LW |
995 | =item %INC |
996 | ||
19799a22 GS |
997 | The hash %INC contains entries for each filename included via the |
998 | C<do>, C<require>, or C<use> operators. The key is the filename | |
999 | you specified (with module names converted to pathnames), and the | |
14218588 | 1000 | value is the location of the file found. The C<require> |
87275199 | 1001 | operator uses this hash to determine whether a particular file has |
19799a22 | 1002 | already been included. |
a0d0e21e | 1003 | |
b687b08b TC |
1004 | =item %ENV |
1005 | ||
1006 | =item $ENV{expr} | |
a0d0e21e LW |
1007 | |
1008 | The hash %ENV contains your current environment. Setting a | |
19799a22 GS |
1009 | value in C<ENV> changes the environment for any child processes |
1010 | you subsequently fork() off. | |
a0d0e21e | 1011 | |
b687b08b TC |
1012 | =item %SIG |
1013 | ||
1014 | =item $SIG{expr} | |
a0d0e21e | 1015 | |
14218588 | 1016 | The hash %SIG contains signal handlers for signals. For example: |
a0d0e21e LW |
1017 | |
1018 | sub handler { # 1st argument is signal name | |
fb73857a | 1019 | my($sig) = @_; |
a0d0e21e LW |
1020 | print "Caught a SIG$sig--shutting down\n"; |
1021 | close(LOG); | |
1022 | exit(0); | |
1023 | } | |
1024 | ||
fb73857a | 1025 | $SIG{'INT'} = \&handler; |
1026 | $SIG{'QUIT'} = \&handler; | |
a0d0e21e | 1027 | ... |
19799a22 | 1028 | $SIG{'INT'} = 'DEFAULT'; # restore default action |
a0d0e21e LW |
1029 | $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT |
1030 | ||
f648820c GS |
1031 | Using a value of C<'IGNORE'> usually has the effect of ignoring the |
1032 | signal, except for the C<CHLD> signal. See L<perlipc> for more about | |
1033 | this special case. | |
1034 | ||
19799a22 | 1035 | Here are some other examples: |
a0d0e21e | 1036 | |
fb73857a | 1037 | $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended) |
a0d0e21e | 1038 | $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber |
19799a22 | 1039 | $SIG{"PIPE"} = *Plumber; # somewhat esoteric |
a0d0e21e LW |
1040 | $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return?? |
1041 | ||
19799a22 GS |
1042 | Be sure not to use a bareword as the name of a signal handler, |
1043 | lest you inadvertently call it. | |
748a9306 | 1044 | |
44a8e56a | 1045 | If your system has the sigaction() function then signal handlers are |
1046 | installed using it. This means you get reliable signal handling. If | |
1047 | your system has the SA_RESTART flag it is used when signals handlers are | |
19799a22 | 1048 | installed. This means that system calls for which restarting is supported |
44a8e56a | 1049 | continue rather than returning when a signal arrives. If you want your |
1050 | system calls to be interrupted by signal delivery then do something like | |
1051 | this: | |
1052 | ||
1053 | use POSIX ':signal_h'; | |
1054 | ||
1055 | my $alarm = 0; | |
1056 | sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 } | |
1057 | or die "Error setting SIGALRM handler: $!\n"; | |
1058 | ||
1059 | See L<POSIX>. | |
1060 | ||
748a9306 | 1061 | Certain internal hooks can be also set using the %SIG hash. The |
a8f8344d | 1062 | routine indicated by C<$SIG{__WARN__}> is called when a warning message is |
748a9306 LW |
1063 | about to be printed. The warning message is passed as the first |
1064 | argument. The presence of a __WARN__ hook causes the ordinary printing | |
1065 | of warnings to STDERR to be suppressed. You can use this to save warnings | |
1066 | in a variable, or turn warnings into fatal errors, like this: | |
1067 | ||
1068 | local $SIG{__WARN__} = sub { die $_[0] }; | |
1069 | eval $proggie; | |
1070 | ||
a8f8344d | 1071 | The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception |
748a9306 LW |
1072 | is about to be thrown. The error message is passed as the first |
1073 | argument. When a __DIE__ hook routine returns, the exception | |
1074 | processing continues as it would have in the absence of the hook, | |
cb1a09d0 | 1075 | unless the hook routine itself exits via a C<goto>, a loop exit, or a die(). |
774d564b | 1076 | The C<__DIE__> handler is explicitly disabled during the call, so that you |
fb73857a | 1077 | can die from a C<__DIE__> handler. Similarly for C<__WARN__>. |
1078 | ||
19799a22 GS |
1079 | Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called |
1080 | even inside an eval(). Do not use this to rewrite a pending exception | |
1081 | in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die(). | |
1082 | This strange action at a distance may be fixed in a future release | |
1083 | so that C<$SIG{__DIE__}> is only called if your program is about | |
1084 | to exit, as was the original intent. Any other use is deprecated. | |
1085 | ||
1086 | C<__DIE__>/C<__WARN__> handlers are very special in one respect: | |
1087 | they may be called to report (probable) errors found by the parser. | |
1088 | In such a case the parser may be in inconsistent state, so any | |
1089 | attempt to evaluate Perl code from such a handler will probably | |
1090 | result in a segfault. This means that warnings or errors that | |
1091 | result from parsing Perl should be used with extreme caution, like | |
1092 | this: | |
fb73857a | 1093 | |
1094 | require Carp if defined $^S; | |
1095 | Carp::confess("Something wrong") if defined &Carp::confess; | |
1096 | die "Something wrong, but could not load Carp to give backtrace... | |
1097 | To see backtrace try starting Perl with -MCarp switch"; | |
1098 | ||
1099 | Here the first line will load Carp I<unless> it is the parser who | |
1100 | called the handler. The second line will print backtrace and die if | |
1101 | Carp was available. The third line will be executed only if Carp was | |
1102 | not available. | |
1103 | ||
19799a22 | 1104 | See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and |
4438c4b7 | 1105 | L<warnings> for additional information. |
68dc0745 | 1106 | |
a0d0e21e | 1107 | =back |
55602bd2 IZ |
1108 | |
1109 | =head2 Error Indicators | |
1110 | ||
19799a22 GS |
1111 | The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information |
1112 | about different types of error conditions that may appear during | |
1113 | execution of a Perl program. The variables are shown ordered by | |
1114 | the "distance" between the subsystem which reported the error and | |
1115 | the Perl process. They correspond to errors detected by the Perl | |
1116 | interpreter, C library, operating system, or an external program, | |
1117 | respectively. | |
55602bd2 IZ |
1118 | |
1119 | To illustrate the differences between these variables, consider the | |
19799a22 | 1120 | following Perl expression, which uses a single-quoted string: |
55602bd2 | 1121 | |
19799a22 GS |
1122 | eval q{ |
1123 | open PIPE, "/cdrom/install |"; | |
1124 | @res = <PIPE>; | |
1125 | close PIPE or die "bad pipe: $?, $!"; | |
1126 | }; | |
55602bd2 IZ |
1127 | |
1128 | After execution of this statement all 4 variables may have been set. | |
1129 | ||
19799a22 GS |
1130 | C<$@> is set if the string to be C<eval>-ed did not compile (this |
1131 | may happen if C<open> or C<close> were imported with bad prototypes), | |
1132 | or if Perl code executed during evaluation die()d . In these cases | |
1133 | the value of $@ is the compile error, or the argument to C<die> | |
1134 | (which will interpolate C<$!> and C<$?>!). (See also L<Fatal>, | |
1135 | though.) | |
1136 | ||
c47ff5f1 | 1137 | When the eval() expression above is executed, open(), C<< <PIPE> >>, |
19799a22 GS |
1138 | and C<close> are translated to calls in the C run-time library and |
1139 | thence to the operating system kernel. C<$!> is set to the C library's | |
1140 | C<errno> if one of these calls fails. | |
1141 | ||
1142 | Under a few operating systems, C<$^E> may contain a more verbose | |
1143 | error indicator, such as in this case, "CDROM tray not closed." | |
14218588 | 1144 | Systems that do not support extended error messages leave C<$^E> |
19799a22 GS |
1145 | the same as C<$!>. |
1146 | ||
1147 | Finally, C<$?> may be set to non-0 value if the external program | |
1148 | F</cdrom/install> fails. The upper eight bits reflect specific | |
1149 | error conditions encountered by the program (the program's exit() | |
1150 | value). The lower eight bits reflect mode of failure, like signal | |
1151 | death and core dump information See wait(2) for details. In | |
1152 | contrast to C<$!> and C<$^E>, which are set only if error condition | |
1153 | is detected, the variable C<$?> is set on each C<wait> or pipe | |
1154 | C<close>, overwriting the old value. This is more like C<$@>, which | |
1155 | on every eval() is always set on failure and cleared on success. | |
2b92dfce | 1156 | |
19799a22 GS |
1157 | For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>, |
1158 | and C<$?>. | |
2b92dfce GS |
1159 | |
1160 | =head2 Technical Note on the Syntax of Variable Names | |
1161 | ||
19799a22 GS |
1162 | Variable names in Perl can have several formats. Usually, they |
1163 | must begin with a letter or underscore, in which case they can be | |
1164 | arbitrarily long (up to an internal limit of 251 characters) and | |
1165 | may contain letters, digits, underscores, or the special sequence | |
1166 | C<::> or C<'>. In this case, the part before the last C<::> or | |
1167 | C<'> is taken to be a I<package qualifier>; see L<perlmod>. | |
2b92dfce GS |
1168 | |
1169 | Perl variable names may also be a sequence of digits or a single | |
1170 | punctuation or control character. These names are all reserved for | |
19799a22 GS |
1171 | special uses by Perl; for example, the all-digits names are used |
1172 | to hold data captured by backreferences after a regular expression | |
1173 | match. Perl has a special syntax for the single-control-character | |
1174 | names: It understands C<^X> (caret C<X>) to mean the control-C<X> | |
1175 | character. For example, the notation C<$^W> (dollar-sign caret | |
1176 | C<W>) is the scalar variable whose name is the single character | |
1177 | control-C<W>. This is better than typing a literal control-C<W> | |
1178 | into your program. | |
2b92dfce | 1179 | |
87275199 | 1180 | Finally, new in Perl 5.6, Perl variable names may be alphanumeric |
19799a22 GS |
1181 | strings that begin with control characters (or better yet, a caret). |
1182 | These variables must be written in the form C<${^Foo}>; the braces | |
1183 | are not optional. C<${^Foo}> denotes the scalar variable whose | |
1184 | name is a control-C<F> followed by two C<o>'s. These variables are | |
1185 | reserved for future special uses by Perl, except for the ones that | |
1186 | begin with C<^_> (control-underscore or caret-underscore). No | |
1187 | control-character name that begins with C<^_> will acquire a special | |
1188 | meaning in any future version of Perl; such names may therefore be | |
1189 | used safely in programs. C<$^_> itself, however, I<is> reserved. | |
1190 | ||
1191 | Perl identifiers that begin with digits, control characters, or | |
2b92dfce GS |
1192 | punctuation characters are exempt from the effects of the C<package> |
1193 | declaration and are always forced to be in package C<main>. A few | |
1194 | other names are also exempt: | |
1195 | ||
1196 | ENV STDIN | |
1197 | INC STDOUT | |
1198 | ARGV STDERR | |
1199 | ARGVOUT | |
1200 | SIG | |
1201 | ||
1202 | In particular, the new special C<${^_XYZ}> variables are always taken | |
19799a22 | 1203 | to be in package C<main>, regardless of any C<package> declarations |
2b92dfce GS |
1204 | presently in scope. |
1205 | ||
19799a22 GS |
1206 | =head1 BUGS |
1207 | ||
1208 | Due to an unfortunate accident of Perl's implementation, C<use | |
1209 | English> imposes a considerable performance penalty on all regular | |
1210 | expression matches in a program, regardless of whether they occur | |
1211 | in the scope of C<use English>. For that reason, saying C<use | |
1212 | English> in libraries is strongly discouraged. See the | |
1213 | Devel::SawAmpersand module documentation from CPAN | |
6cecdcac | 1214 | (http://www.perl.com/CPAN/modules/by-module/Devel/) |
19799a22 | 1215 | for more information. |
2b92dfce | 1216 | |
19799a22 GS |
1217 | Having to even think about the C<$^S> variable in your exception |
1218 | handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented | |
1219 | invites grievous and difficult to track down errors. Avoid it | |
1220 | and use an C<END{}> or CORE::GLOBAL::die override instead. |