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1 | =head1 NAME |
2 | |
3 | perlvar - Perl predefined variables |
4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | =head2 Predefined Names |
8 | |
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9 | The following names have special meaning to Perl. Most |
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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 |
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13 | |
14 | use English; |
15 | |
16 | at the top of your program. This will alias all the short names to the |
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17 | long names in the current package. Some even have medium names, |
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18 | generally borrowed from B<awk>. |
19 | |
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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 |
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24 | |
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25 | use IO::Handle; |
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26 | |
27 | after which you may use either |
28 | |
29 | method HANDLE EXPR |
30 | |
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31 | or more safely, |
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32 | |
33 | HANDLE->method(EXPR) |
34 | |
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35 | Each method returns the old value of the IO::Handle attribute. |
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36 | The methods each take an optional EXPR, which if supplied specifies the |
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37 | new value for the IO::Handle attribute in question. If not supplied, |
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38 | most methods do nothing to the current value--except for |
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39 | autoflush(), which will assume a 1 for you, just to be different. |
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40 | Because loading in the IO::Handle class is an expensive operation, you should |
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41 | learn how to use the regular built-in variables. |
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42 | |
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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. |
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46 | |
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47 | You should be very careful when modifying the default values of most |
48 | special variables described in this document. In most cases you want |
49 | to localize these variables before changing them, since if you don't, |
50 | the change may affect other modules which rely on the default values |
51 | of the special variables that you have changed. This is one of the |
52 | correct ways to read the whole file at once: |
53 | |
54 | open my $fh, "foo" or die $!; |
55 | local $/; # enable localized slurp mode |
56 | my $content = <$fh>; |
57 | close $fh; |
58 | |
59 | But the following code is quite bad: |
60 | |
61 | open my $fh, "foo" or die $!; |
62 | undef $/; # enable slurp mode |
63 | my $content = <$fh>; |
64 | close $fh; |
65 | |
66 | since some other module, may want to read data from some file in the |
67 | default "line mode", so if the code we have just presented has been |
68 | executed, the global value of C<$/> is now changed for any other code |
69 | running inside the same Perl interpreter. |
70 | |
71 | Usually when a variable is localized you want to make sure that this |
72 | change affects the shortest scope possible. So unless you are already |
73 | inside some short C<{}> block, you should create one yourself. For |
74 | example: |
75 | |
76 | my $content = ''; |
77 | open my $fh, "foo" or die $!; |
78 | { |
79 | local $/; |
80 | $content = <$fh>; |
81 | } |
82 | close $fh; |
83 | |
84 | Here is an example of how your own code can go broken: |
85 | |
86 | for (1..5){ |
87 | nasty_break(); |
88 | print "$_ "; |
89 | } |
90 | sub nasty_break { |
91 | $_ = 5; |
92 | # do something with $_ |
93 | } |
94 | |
95 | You probably expect this code to print: |
96 | |
97 | 1 2 3 4 5 |
98 | |
99 | but instead you get: |
100 | |
101 | 5 5 5 5 5 |
102 | |
103 | Why? Because nasty_break() modifies C<$_> without localizing it |
104 | first. The fix is to add local(): |
105 | |
106 | local $_ = 5; |
107 | |
108 | It's easy to notice the problem in such a short example, but in more |
109 | complicated code you are looking for trouble if you don't localize |
110 | changes to the special variables. |
111 | |
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112 | The following list is ordered by scalar variables first, then the |
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113 | arrays, then the hashes. |
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114 | |
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115 | =over 8 |
116 | |
117 | =item $ARG |
118 | |
119 | =item $_ |
120 | |
121 | The default input and pattern-searching space. The following pairs are |
122 | equivalent: |
123 | |
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124 | while (<>) {...} # equivalent only in while! |
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125 | while (defined($_ = <>)) {...} |
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126 | |
127 | /^Subject:/ |
128 | $_ =~ /^Subject:/ |
129 | |
130 | tr/a-z/A-Z/ |
131 | $_ =~ tr/a-z/A-Z/ |
132 | |
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133 | chomp |
134 | chomp($_) |
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135 | |
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136 | Here are the places where Perl will assume $_ even if you |
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137 | don't use it: |
138 | |
139 | =over 3 |
140 | |
141 | =item * |
142 | |
143 | Various unary functions, including functions like ord() and int(), as well |
144 | as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to |
145 | STDIN. |
146 | |
147 | =item * |
148 | |
149 | Various list functions like print() and unlink(). |
150 | |
151 | =item * |
152 | |
153 | The pattern matching operations C<m//>, C<s///>, and C<tr///> when used |
154 | without an C<=~> operator. |
155 | |
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156 | =item * |
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157 | |
158 | The default iterator variable in a C<foreach> loop if no other |
159 | variable is supplied. |
160 | |
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161 | =item * |
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162 | |
163 | The implicit iterator variable in the grep() and map() functions. |
164 | |
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165 | =item * |
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166 | |
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167 | The default place to put an input record when a C<< <FH> >> |
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168 | operation's result is tested by itself as the sole criterion of a C<while> |
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169 | test. Outside a C<while> test, this will not happen. |
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170 | |
171 | =back |
172 | |
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173 | (Mnemonic: underline is understood in certain operations.) |
174 | |
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175 | =back |
176 | |
177 | =over 8 |
178 | |
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179 | =item $a |
180 | |
181 | =item $b |
182 | |
183 | Special package variables when using sort(), see L<perlfunc/sort>. |
184 | Because of this specialness $a and $b don't need to be declared |
185 | (using local(), use vars, or our()) even when using the strict |
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186 | vars pragma. Don't lexicalize them with C<my $a> or C<my $b> |
187 | if you want to be able to use them in the sort() comparison block |
188 | or function. |
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189 | |
190 | =back |
191 | |
192 | =over 8 |
193 | |
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194 | =item $<I<digits>> |
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195 | |
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196 | Contains the subpattern from the corresponding set of capturing |
197 | parentheses from the last pattern match, not counting patterns |
198 | matched in nested blocks that have been exited already. (Mnemonic: |
199 | like \digits.) These variables are all read-only and dynamically |
200 | scoped to the current BLOCK. |
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201 | |
202 | =item $MATCH |
203 | |
204 | =item $& |
205 | |
206 | The string matched by the last successful pattern match (not counting |
207 | any matches hidden within a BLOCK or eval() enclosed by the current |
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208 | BLOCK). (Mnemonic: like & in some editors.) This variable is read-only |
209 | and dynamically scoped to the current BLOCK. |
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210 | |
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211 | The use of this variable anywhere in a program imposes a considerable |
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212 | performance penalty on all regular expression matches. See L<BUGS>. |
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213 | |
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214 | =item $PREMATCH |
215 | |
216 | =item $` |
217 | |
218 | The string preceding whatever was matched by the last successful |
219 | pattern match (not counting any matches hidden within a BLOCK or eval |
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220 | enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted |
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221 | string.) This variable is read-only. |
222 | |
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223 | The use of this variable anywhere in a program imposes a considerable |
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224 | performance penalty on all regular expression matches. See L<BUGS>. |
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225 | |
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226 | =item $POSTMATCH |
227 | |
228 | =item $' |
229 | |
230 | The string following whatever was matched by the last successful |
231 | pattern match (not counting any matches hidden within a BLOCK or eval() |
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232 | enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted |
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233 | string.) Example: |
234 | |
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235 | local $_ = 'abcdefghi'; |
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236 | /def/; |
237 | print "$`:$&:$'\n"; # prints abc:def:ghi |
238 | |
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239 | This variable is read-only and dynamically scoped to the current BLOCK. |
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240 | |
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241 | The use of this variable anywhere in a program imposes a considerable |
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242 | performance penalty on all regular expression matches. See L<BUGS>. |
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243 | |
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244 | =item $LAST_PAREN_MATCH |
245 | |
246 | =item $+ |
247 | |
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248 | The text matched by the last bracket of the last successful search pattern. |
249 | This is useful if you don't know which one of a set of alternative patterns |
250 | matched. For example: |
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251 | |
252 | /Version: (.*)|Revision: (.*)/ && ($rev = $+); |
253 | |
254 | (Mnemonic: be positive and forward looking.) |
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255 | This variable is read-only and dynamically scoped to the current BLOCK. |
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256 | |
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257 | =item $^N |
258 | |
259 | The text matched by the used group most-recently closed (i.e. the group |
260 | with the rightmost closing parenthesis) of the last successful search |
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261 | pattern. (Mnemonic: the (possibly) Nested parenthesis that most |
262 | recently closed.) |
263 | |
264 | This is primarly used inside C<(?{...})> blocks for examining text |
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265 | recently matched. For example, to effectively capture text to a variable |
266 | (in addition to C<$1>, C<$2>, etc.), replace C<(...)> with |
267 | |
268 | (?:(...)(?{ $var = $^N })) |
269 | |
270 | By setting and then using C<$var> in this way relieves you from having to |
271 | worry about exactly which numbered set of parentheses they are. |
272 | |
273 | This variable is dynamically scoped to the current BLOCK. |
274 | |
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275 | =item @LAST_MATCH_END |
276 | |
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277 | =item @+ |
278 | |
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279 | This array holds the offsets of the ends of the last successful |
280 | submatches in the currently active dynamic scope. C<$+[0]> is |
281 | the offset into the string of the end of the entire match. This |
282 | is the same value as what the C<pos> function returns when called |
283 | on the variable that was matched against. The I<n>th element |
284 | of this array holds the offset of the I<n>th submatch, so |
285 | C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset |
286 | past where $2 ends, and so on. You can use C<$#+> to determine |
287 | how many subgroups were in the last successful match. See the |
288 | examples given for the C<@-> variable. |
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289 | |
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290 | =item $MULTILINE_MATCHING |
291 | |
292 | =item $* |
293 | |
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294 | Set to a non-zero integer value to do multi-line matching within a |
295 | string, 0 (or undefined) to tell Perl that it can assume that strings |
296 | contain a single line, for the purpose of optimizing pattern matches. |
297 | Pattern matches on strings containing multiple newlines can produce |
298 | confusing results when C<$*> is 0 or undefined. Default is undefined. |
299 | (Mnemonic: * matches multiple things.) This variable influences the |
300 | interpretation of only C<^> and C<$>. A literal newline can be searched |
301 | for even when C<$* == 0>. |
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302 | |
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303 | Use of C<$*> is deprecated in modern Perl, supplanted by |
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304 | the C</s> and C</m> modifiers on pattern matching. |
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305 | |
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306 | Assigning a non-numerical value to C<$*> triggers a warning (and makes |
307 | C<$*> act if C<$* == 0>), while assigning a numerical value to C<$*> |
308 | makes that an implicit C<int> is applied on the value. |
309 | |
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310 | =item HANDLE->input_line_number(EXPR) |
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311 | |
312 | =item $INPUT_LINE_NUMBER |
313 | |
314 | =item $NR |
315 | |
316 | =item $. |
317 | |
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318 | Current line number for the last filehandle accessed. |
319 | |
320 | Each filehandle in Perl counts the number of lines that have been read |
321 | from it. (Depending on the value of C<$/>, Perl's idea of what |
322 | constitutes a line may not match yours.) When a line is read from a |
323 | filehandle (via readline() or C<< <> >>), or when tell() or seek() is |
324 | called on it, C<$.> becomes an alias to the line counter for that |
325 | filehandle. |
326 | |
327 | You can adjust the counter by assigning to C<$.>, but this will not |
328 | actually move the seek pointer. I<Localizing C<$.> will not localize |
329 | the filehandle's line count>. Instead, it will localize perl's notion |
330 | of which filehandle C<$.> is currently aliased to. |
331 | |
332 | C<$.> is reset when the filehandle is closed, but B<not> when an open |
333 | filehandle is reopened without an intervening close(). For more |
334 | details, see L<perlop/"I/O Operators">. Because C<< <> >> never does |
335 | an explicit close, line numbers increase across ARGV files (but see |
336 | examples in L<perlfunc/eof>). |
337 | |
338 | You can also use C<< HANDLE->input_line_number(EXPR) >> to access the |
339 | line counter for a given filehandle without having to worry about |
340 | which handle you last accessed. |
341 | |
342 | (Mnemonic: many programs use "." to mean the current line number.) |
343 | |
344 | =item IO::Handle->input_record_separator(EXPR) |
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345 | |
346 | =item $INPUT_RECORD_SEPARATOR |
347 | |
348 | =item $RS |
349 | |
350 | =item $/ |
351 | |
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352 | The input record separator, newline by default. This |
353 | influences Perl's idea of what a "line" is. Works like B<awk>'s RS |
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354 | variable, including treating empty lines as a terminator if set to |
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355 | the null string. (An empty line cannot contain any spaces |
356 | or tabs.) You may set it to a multi-character string to match a |
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357 | multi-character terminator, or to C<undef> to read through the end |
358 | of file. Setting it to C<"\n\n"> means something slightly |
359 | different than setting to C<"">, if the file contains consecutive |
360 | empty lines. Setting to C<""> will treat two or more consecutive |
361 | empty lines as a single empty line. Setting to C<"\n\n"> will |
362 | blindly assume that the next input character belongs to the next |
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363 | paragraph, even if it's a newline. (Mnemonic: / delimits |
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364 | line boundaries when quoting poetry.) |
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365 | |
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366 | local $/; # enable "slurp" mode |
367 | local $_ = <FH>; # whole file now here |
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368 | s/\n[ \t]+/ /g; |
369 | |
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370 | Remember: the value of C<$/> is a string, not a regex. B<awk> has to be |
371 | better for something. :-) |
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372 | |
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373 | Setting C<$/> to a reference to an integer, scalar containing an integer, or |
374 | scalar that's convertible to an integer will attempt to read records |
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375 | instead of lines, with the maximum record size being the referenced |
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376 | integer. So this: |
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377 | |
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378 | local $/ = \32768; # or \"32768", or \$var_containing_32768 |
379 | open my $fh, $myfile or die $!; |
380 | local $_ = <$fh>; |
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381 | |
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382 | will read a record of no more than 32768 bytes from FILE. If you're |
383 | not reading from a record-oriented file (or your OS doesn't have |
384 | record-oriented files), then you'll likely get a full chunk of data |
385 | with every read. If a record is larger than the record size you've |
386 | set, you'll get the record back in pieces. |
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387 | |
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388 | On VMS, record reads are done with the equivalent of C<sysread>, |
389 | so it's best not to mix record and non-record reads on the same |
390 | file. (This is unlikely to be a problem, because any file you'd |
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391 | want to read in record mode is probably unusable in line mode.) |
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392 | Non-VMS systems do normal I/O, so it's safe to mix record and |
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393 | non-record reads of a file. |
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394 | |
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395 | See also L<perlport/"Newlines">. Also see C<$.>. |
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396 | |
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397 | =item HANDLE->autoflush(EXPR) |
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398 | |
399 | =item $OUTPUT_AUTOFLUSH |
400 | |
401 | =item $| |
402 | |
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403 | If set to nonzero, forces a flush right away and after every write |
404 | or print on the currently selected output channel. Default is 0 |
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405 | (regardless of whether the channel is really buffered by the |
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406 | system or not; C<$|> tells you only whether you've asked Perl |
407 | explicitly to flush after each write). STDOUT will |
408 | typically be line buffered if output is to the terminal and block |
409 | buffered otherwise. Setting this variable is useful primarily when |
410 | you are outputting to a pipe or socket, such as when you are running |
411 | a Perl program under B<rsh> and want to see the output as it's |
412 | happening. This has no effect on input buffering. See L<perlfunc/getc> |
413 | for that. (Mnemonic: when you want your pipes to be piping hot.) |
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414 | |
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415 | =item IO::Handle->output_field_separator EXPR |
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416 | |
417 | =item $OUTPUT_FIELD_SEPARATOR |
418 | |
419 | =item $OFS |
420 | |
421 | =item $, |
422 | |
423 | The output field separator for the print operator. Ordinarily the |
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424 | print operator simply prints out its arguments without further |
425 | adornment. To get behavior more like B<awk>, set this variable as |
426 | you would set B<awk>'s OFS variable to specify what is printed |
427 | between fields. (Mnemonic: what is printed when there is a "," in |
428 | your print statement.) |
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429 | |
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430 | =item IO::Handle->output_record_separator EXPR |
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431 | |
432 | =item $OUTPUT_RECORD_SEPARATOR |
433 | |
434 | =item $ORS |
435 | |
436 | =item $\ |
437 | |
438 | The output record separator for the print operator. Ordinarily the |
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439 | print operator simply prints out its arguments as is, with no |
440 | trailing newline or other end-of-record string added. To get |
441 | behavior more like B<awk>, set this variable as you would set |
442 | B<awk>'s ORS variable to specify what is printed at the end of the |
443 | print. (Mnemonic: you set C<$\> instead of adding "\n" at the |
444 | end of the print. Also, it's just like C<$/>, but it's what you |
445 | get "back" from Perl.) |
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446 | |
447 | =item $LIST_SEPARATOR |
448 | |
449 | =item $" |
450 | |
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451 | This is like C<$,> except that it applies to array and slice values |
452 | interpolated into a double-quoted string (or similar interpreted |
453 | string). Default is a space. (Mnemonic: obvious, I think.) |
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454 | |
455 | =item $SUBSCRIPT_SEPARATOR |
456 | |
457 | =item $SUBSEP |
458 | |
459 | =item $; |
460 | |
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461 | The subscript separator for multidimensional array emulation. If you |
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462 | refer to a hash element as |
463 | |
464 | $foo{$a,$b,$c} |
465 | |
466 | it really means |
467 | |
468 | $foo{join($;, $a, $b, $c)} |
469 | |
470 | But don't put |
471 | |
472 | @foo{$a,$b,$c} # a slice--note the @ |
473 | |
474 | which means |
475 | |
476 | ($foo{$a},$foo{$b},$foo{$c}) |
477 | |
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478 | Default is "\034", the same as SUBSEP in B<awk>. If your |
479 | keys contain binary data there might not be any safe value for C<$;>. |
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480 | (Mnemonic: comma (the syntactic subscript separator) is a |
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481 | semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already |
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482 | taken for something more important.) |
483 | |
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484 | Consider using "real" multidimensional arrays as described |
485 | in L<perllol>. |
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486 | |
487 | =item $OFMT |
488 | |
489 | =item $# |
490 | |
491 | The output format for printed numbers. This variable is a half-hearted |
492 | attempt to emulate B<awk>'s OFMT variable. There are times, however, |
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493 | when B<awk> and Perl have differing notions of what counts as |
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494 | numeric. The initial value is "%.I<n>g", where I<n> is the value |
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495 | of the macro DBL_DIG from your system's F<float.h>. This is different from |
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496 | B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#> |
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497 | explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.) |
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498 | |
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499 | Use of C<$#> is deprecated. |
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500 | |
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501 | =item HANDLE->format_page_number(EXPR) |
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502 | |
503 | =item $FORMAT_PAGE_NUMBER |
504 | |
505 | =item $% |
506 | |
507 | The current page number of the currently selected output channel. |
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508 | Used with formats. |
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509 | (Mnemonic: % is page number in B<nroff>.) |
510 | |
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511 | =item HANDLE->format_lines_per_page(EXPR) |
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512 | |
513 | =item $FORMAT_LINES_PER_PAGE |
514 | |
515 | =item $= |
516 | |
517 | The current page length (printable lines) of the currently selected |
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518 | output channel. Default is 60. |
519 | Used with formats. |
520 | (Mnemonic: = has horizontal lines.) |
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521 | |
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522 | =item HANDLE->format_lines_left(EXPR) |
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523 | |
524 | =item $FORMAT_LINES_LEFT |
525 | |
526 | =item $- |
527 | |
528 | The number of lines left on the page of the currently selected output |
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529 | channel. |
530 | Used with formats. |
531 | (Mnemonic: lines_on_page - lines_printed.) |
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532 | |
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533 | =item @LAST_MATCH_START |
534 | |
6cef1e77 |
535 | =item @- |
536 | |
19799a22 |
537 | $-[0] is the offset of the start of the last successful match. |
6cef1e77 |
538 | C<$-[>I<n>C<]> is the offset of the start of the substring matched by |
8f580fb8 |
539 | I<n>-th subpattern, or undef if the subpattern did not match. |
6cef1e77 |
540 | |
541 | Thus after a match against $_, $& coincides with C<substr $_, $-[0], |
8f580fb8 |
542 | $+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<], |
543 | $+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with |
c47ff5f1 |
544 | C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last |
14218588 |
545 | matched subgroup in the last successful match. Contrast with |
546 | C<$#+>, the number of subgroups in the regular expression. Compare |
19799a22 |
547 | with C<@+>. |
6cef1e77 |
548 | |
4ba05bdc |
549 | This array holds the offsets of the beginnings of the last |
550 | successful submatches in the currently active dynamic scope. |
551 | C<$-[0]> is the offset into the string of the beginning of the |
552 | entire match. The I<n>th element of this array holds the offset |
0926d669 |
553 | of the I<n>th submatch, so C<$-[1]> is the offset where $1 |
554 | begins, C<$-[2]> the offset where $2 begins, and so on. |
4ba05bdc |
555 | |
556 | After a match against some variable $var: |
557 | |
558 | =over 5 |
559 | |
4375e838 |
560 | =item C<$`> is the same as C<substr($var, 0, $-[0])> |
4ba05bdc |
561 | |
4375e838 |
562 | =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])> |
4ba05bdc |
563 | |
4375e838 |
564 | =item C<$'> is the same as C<substr($var, $+[0])> |
4ba05bdc |
565 | |
566 | =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])> |
567 | |
568 | =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])> |
569 | |
4375e838 |
570 | =item C<$3> is the same as C<substr $var, $-[3], $+[3] - $-[3])> |
4ba05bdc |
571 | |
572 | =back |
573 | |
fcc7d916 |
574 | =item HANDLE->format_name(EXPR) |
a0d0e21e |
575 | |
576 | =item $FORMAT_NAME |
577 | |
578 | =item $~ |
579 | |
580 | The name of the current report format for the currently selected output |
14218588 |
581 | channel. Default is the name of the filehandle. (Mnemonic: brother to |
19799a22 |
582 | C<$^>.) |
a0d0e21e |
583 | |
fcc7d916 |
584 | =item HANDLE->format_top_name(EXPR) |
a0d0e21e |
585 | |
586 | =item $FORMAT_TOP_NAME |
587 | |
588 | =item $^ |
589 | |
590 | The name of the current top-of-page format for the currently selected |
14218588 |
591 | output channel. Default is the name of the filehandle with _TOP |
a0d0e21e |
592 | appended. (Mnemonic: points to top of page.) |
593 | |
46550894 |
594 | =item IO::Handle->format_line_break_characters EXPR |
a0d0e21e |
595 | |
596 | =item $FORMAT_LINE_BREAK_CHARACTERS |
597 | |
598 | =item $: |
599 | |
600 | The current set of characters after which a string may be broken to |
54310121 |
601 | fill continuation fields (starting with ^) in a format. Default is |
a0d0e21e |
602 | S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in |
603 | poetry is a part of a line.) |
604 | |
46550894 |
605 | =item IO::Handle->format_formfeed EXPR |
a0d0e21e |
606 | |
607 | =item $FORMAT_FORMFEED |
608 | |
609 | =item $^L |
610 | |
14218588 |
611 | What formats output as a form feed. Default is \f. |
a0d0e21e |
612 | |
613 | =item $ACCUMULATOR |
614 | |
615 | =item $^A |
616 | |
617 | The current value of the write() accumulator for format() lines. A format |
19799a22 |
618 | contains formline() calls that put their result into C<$^A>. After |
a0d0e21e |
619 | calling its format, write() prints out the contents of C<$^A> and empties. |
14218588 |
620 | So you never really see the contents of C<$^A> unless you call |
a0d0e21e |
621 | formline() yourself and then look at it. See L<perlform> and |
622 | L<perlfunc/formline()>. |
623 | |
624 | =item $CHILD_ERROR |
625 | |
626 | =item $? |
627 | |
54310121 |
628 | The status returned by the last pipe close, backtick (C<``>) command, |
19799a22 |
629 | successful call to wait() or waitpid(), or from the system() |
630 | operator. This is just the 16-bit status word returned by the |
631 | wait() system call (or else is made up to look like it). Thus, the |
c47ff5f1 |
632 | exit value of the subprocess is really (C<<< $? >> 8 >>>), and |
19799a22 |
633 | C<$? & 127> gives which signal, if any, the process died from, and |
634 | C<$? & 128> reports whether there was a core dump. (Mnemonic: |
635 | similar to B<sh> and B<ksh>.) |
a0d0e21e |
636 | |
7b8d334a |
637 | Additionally, if the C<h_errno> variable is supported in C, its value |
14218588 |
638 | is returned via $? if any C<gethost*()> function fails. |
7b8d334a |
639 | |
19799a22 |
640 | If you have installed a signal handler for C<SIGCHLD>, the |
aa689395 |
641 | value of C<$?> will usually be wrong outside that handler. |
642 | |
a8f8344d |
643 | Inside an C<END> subroutine C<$?> contains the value that is going to be |
644 | given to C<exit()>. You can modify C<$?> in an C<END> subroutine to |
19799a22 |
645 | change the exit status of your program. For example: |
646 | |
647 | END { |
648 | $? = 1 if $? == 255; # die would make it 255 |
649 | } |
a8f8344d |
650 | |
aa689395 |
651 | Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the |
ff0cee69 |
652 | actual VMS exit status, instead of the default emulation of POSIX |
9bc98430 |
653 | status; see L<perlvms/$?> for details. |
f86702cc |
654 | |
55602bd2 |
655 | Also see L<Error Indicators>. |
656 | |
a0d0e21e |
657 | =item $OS_ERROR |
658 | |
659 | =item $ERRNO |
660 | |
661 | =item $! |
662 | |
19799a22 |
663 | If used numerically, yields the current value of the C C<errno> |
664 | variable, with all the usual caveats. (This means that you shouldn't |
665 | depend on the value of C<$!> to be anything in particular unless |
666 | you've gotten a specific error return indicating a system error.) |
667 | If used an a string, yields the corresponding system error string. |
668 | You can assign a number to C<$!> to set I<errno> if, for instance, |
669 | you want C<"$!"> to return the string for error I<n>, or you want |
670 | to set the exit value for the die() operator. (Mnemonic: What just |
671 | went bang?) |
a0d0e21e |
672 | |
55602bd2 |
673 | Also see L<Error Indicators>. |
674 | |
5c055ba3 |
675 | =item $EXTENDED_OS_ERROR |
676 | |
677 | =item $^E |
678 | |
22fae026 |
679 | Error information specific to the current operating system. At |
680 | the moment, this differs from C<$!> under only VMS, OS/2, and Win32 |
681 | (and for MacPerl). On all other platforms, C<$^E> is always just |
682 | the same as C<$!>. |
683 | |
684 | Under VMS, C<$^E> provides the VMS status value from the last |
685 | system error. This is more specific information about the last |
686 | system error than that provided by C<$!>. This is particularly |
d516a115 |
687 | important when C<$!> is set to B<EVMSERR>. |
22fae026 |
688 | |
1c1c7f20 |
689 | Under OS/2, C<$^E> is set to the error code of the last call to |
690 | OS/2 API either via CRT, or directly from perl. |
22fae026 |
691 | |
692 | Under Win32, C<$^E> always returns the last error information |
693 | reported by the Win32 call C<GetLastError()> which describes |
694 | the last error from within the Win32 API. Most Win32-specific |
19799a22 |
695 | code will report errors via C<$^E>. ANSI C and Unix-like calls |
22fae026 |
696 | set C<errno> and so most portable Perl code will report errors |
697 | via C<$!>. |
698 | |
699 | Caveats mentioned in the description of C<$!> generally apply to |
700 | C<$^E>, also. (Mnemonic: Extra error explanation.) |
5c055ba3 |
701 | |
55602bd2 |
702 | Also see L<Error Indicators>. |
703 | |
a0d0e21e |
704 | =item $EVAL_ERROR |
705 | |
706 | =item $@ |
707 | |
4a280ebe |
708 | The Perl syntax error message from the last eval() operator. |
709 | If $@ is the null string, the last eval() parsed and executed |
710 | correctly (although the operations you invoked may have failed in the |
711 | normal fashion). (Mnemonic: Where was the syntax error "at"?) |
a0d0e21e |
712 | |
19799a22 |
713 | Warning messages are not collected in this variable. You can, |
a8f8344d |
714 | however, set up a routine to process warnings by setting C<$SIG{__WARN__}> |
54310121 |
715 | as described below. |
748a9306 |
716 | |
55602bd2 |
717 | Also see L<Error Indicators>. |
718 | |
a0d0e21e |
719 | =item $PROCESS_ID |
720 | |
721 | =item $PID |
722 | |
723 | =item $$ |
724 | |
19799a22 |
725 | The process number of the Perl running this script. You should |
726 | consider this variable read-only, although it will be altered |
727 | across fork() calls. (Mnemonic: same as shells.) |
a0d0e21e |
728 | |
729 | =item $REAL_USER_ID |
730 | |
731 | =item $UID |
732 | |
733 | =item $< |
734 | |
19799a22 |
735 | The real uid of this process. (Mnemonic: it's the uid you came I<from>, |
a043a685 |
736 | if you're running setuid.) You can change both the real uid and |
737 | the effective uid at the same time by using POSIX::setuid(). |
a0d0e21e |
738 | |
739 | =item $EFFECTIVE_USER_ID |
740 | |
741 | =item $EUID |
742 | |
743 | =item $> |
744 | |
745 | The effective uid of this process. Example: |
746 | |
747 | $< = $>; # set real to effective uid |
748 | ($<,$>) = ($>,$<); # swap real and effective uid |
749 | |
a043a685 |
750 | You can change both the effective uid and the real uid at the same |
751 | time by using POSIX::setuid(). |
752 | |
19799a22 |
753 | (Mnemonic: it's the uid you went I<to>, if you're running setuid.) |
c47ff5f1 |
754 | C<< $< >> and C<< $> >> can be swapped only on machines |
8cc95fdb |
755 | supporting setreuid(). |
a0d0e21e |
756 | |
757 | =item $REAL_GROUP_ID |
758 | |
759 | =item $GID |
760 | |
761 | =item $( |
762 | |
763 | The real gid of this process. If you are on a machine that supports |
764 | membership in multiple groups simultaneously, gives a space separated |
765 | list of groups you are in. The first number is the one returned by |
766 | getgid(), and the subsequent ones by getgroups(), one of which may be |
8cc95fdb |
767 | the same as the first number. |
768 | |
19799a22 |
769 | However, a value assigned to C<$(> must be a single number used to |
770 | set the real gid. So the value given by C<$(> should I<not> be assigned |
771 | back to C<$(> without being forced numeric, such as by adding zero. |
8cc95fdb |
772 | |
a043a685 |
773 | You can change both the real gid and the effective gid at the same |
774 | time by using POSIX::setgid(). |
775 | |
19799a22 |
776 | (Mnemonic: parentheses are used to I<group> things. The real gid is the |
777 | group you I<left>, if you're running setgid.) |
a0d0e21e |
778 | |
779 | =item $EFFECTIVE_GROUP_ID |
780 | |
781 | =item $EGID |
782 | |
783 | =item $) |
784 | |
785 | The effective gid of this process. If you are on a machine that |
786 | supports membership in multiple groups simultaneously, gives a space |
787 | separated list of groups you are in. The first number is the one |
788 | returned by getegid(), and the subsequent ones by getgroups(), one of |
8cc95fdb |
789 | which may be the same as the first number. |
790 | |
19799a22 |
791 | Similarly, a value assigned to C<$)> must also be a space-separated |
14218588 |
792 | list of numbers. The first number sets the effective gid, and |
8cc95fdb |
793 | the rest (if any) are passed to setgroups(). To get the effect of an |
794 | empty list for setgroups(), just repeat the new effective gid; that is, |
795 | to force an effective gid of 5 and an effectively empty setgroups() |
796 | list, say C< $) = "5 5" >. |
797 | |
a043a685 |
798 | You can change both the effective gid and the real gid at the same |
799 | time by using POSIX::setgid() (use only a single numeric argument). |
800 | |
19799a22 |
801 | (Mnemonic: parentheses are used to I<group> things. The effective gid |
802 | is the group that's I<right> for you, if you're running setgid.) |
a0d0e21e |
803 | |
c47ff5f1 |
804 | C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on |
19799a22 |
805 | machines that support the corresponding I<set[re][ug]id()> routine. C<$(> |
806 | and C<$)> can be swapped only on machines supporting setregid(). |
a0d0e21e |
807 | |
808 | =item $PROGRAM_NAME |
809 | |
810 | =item $0 |
811 | |
19799a22 |
812 | Contains the name of the program being executed. On some operating |
813 | systems assigning to C<$0> modifies the argument area that the B<ps> |
814 | program sees. This is more useful as a way of indicating the current |
815 | program state than it is for hiding the program you're running. |
a0d0e21e |
816 | (Mnemonic: same as B<sh> and B<ksh>.) |
817 | |
4bc88a62 |
818 | Note for BSD users: setting C<$0> does not completely remove "perl" |
819 | from the ps(1) output. For example, setting C<$0> to C<"foobar"> will |
820 | result in C<"perl: foobar (perl)">. This is an operating system |
821 | feature. |
822 | |
a0d0e21e |
823 | =item $[ |
824 | |
825 | The index of the first element in an array, and of the first character |
19799a22 |
826 | in a substring. Default is 0, but you could theoretically set it |
827 | to 1 to make Perl behave more like B<awk> (or Fortran) when |
828 | subscripting and when evaluating the index() and substr() functions. |
829 | (Mnemonic: [ begins subscripts.) |
a0d0e21e |
830 | |
19799a22 |
831 | As of release 5 of Perl, assignment to C<$[> is treated as a compiler |
832 | directive, and cannot influence the behavior of any other file. |
833 | Its use is highly discouraged. |
a0d0e21e |
834 | |
a0d0e21e |
835 | =item $] |
836 | |
54310121 |
837 | The version + patchlevel / 1000 of the Perl interpreter. This variable |
838 | can be used to determine whether the Perl interpreter executing a |
839 | script is in the right range of versions. (Mnemonic: Is this version |
840 | of perl in the right bracket?) Example: |
a0d0e21e |
841 | |
842 | warn "No checksumming!\n" if $] < 3.019; |
843 | |
54310121 |
844 | See also the documentation of C<use VERSION> and C<require VERSION> |
19799a22 |
845 | for a convenient way to fail if the running Perl interpreter is too old. |
a0d0e21e |
846 | |
44dcb63b |
847 | The use of this variable is deprecated. The floating point representation |
848 | can sometimes lead to inaccurate numeric comparisons. See C<$^V> for a |
849 | more modern representation of the Perl version that allows accurate string |
850 | comparisons. |
16070b82 |
851 | |
305aace0 |
852 | =item $COMPILING |
853 | |
854 | =item $^C |
855 | |
19799a22 |
856 | The current value of the flag associated with the B<-c> switch. |
857 | Mainly of use with B<-MO=...> to allow code to alter its behavior |
858 | when being compiled, such as for example to AUTOLOAD at compile |
859 | time rather than normal, deferred loading. See L<perlcc>. Setting |
860 | C<$^C = 1> is similar to calling C<B::minus_c>. |
305aace0 |
861 | |
a0d0e21e |
862 | =item $DEBUGGING |
863 | |
864 | =item $^D |
865 | |
866 | The current value of the debugging flags. (Mnemonic: value of B<-D> |
867 | switch.) |
868 | |
869 | =item $SYSTEM_FD_MAX |
870 | |
871 | =item $^F |
872 | |
873 | The maximum system file descriptor, ordinarily 2. System file |
874 | descriptors are passed to exec()ed processes, while higher file |
875 | descriptors are not. Also, during an open(), system file descriptors are |
876 | preserved even if the open() fails. (Ordinary file descriptors are |
19799a22 |
877 | closed before the open() is attempted.) The close-on-exec |
a0d0e21e |
878 | status of a file descriptor will be decided according to the value of |
8d2a6795 |
879 | C<$^F> when the corresponding file, pipe, or socket was opened, not the |
880 | time of the exec(). |
a0d0e21e |
881 | |
6e2995f4 |
882 | =item $^H |
883 | |
0462a1ab |
884 | WARNING: This variable is strictly for internal use only. Its availability, |
885 | behavior, and contents are subject to change without notice. |
886 | |
887 | This variable contains compile-time hints for the Perl interpreter. At the |
888 | end of compilation of a BLOCK the value of this variable is restored to the |
889 | value when the interpreter started to compile the BLOCK. |
890 | |
891 | When perl begins to parse any block construct that provides a lexical scope |
892 | (e.g., eval body, required file, subroutine body, loop body, or conditional |
893 | block), the existing value of $^H is saved, but its value is left unchanged. |
894 | When the compilation of the block is completed, it regains the saved value. |
895 | Between the points where its value is saved and restored, code that |
896 | executes within BEGIN blocks is free to change the value of $^H. |
897 | |
898 | This behavior provides the semantic of lexical scoping, and is used in, |
899 | for instance, the C<use strict> pragma. |
900 | |
901 | The contents should be an integer; different bits of it are used for |
902 | different pragmatic flags. Here's an example: |
903 | |
904 | sub add_100 { $^H |= 0x100 } |
905 | |
906 | sub foo { |
907 | BEGIN { add_100() } |
908 | bar->baz($boon); |
909 | } |
910 | |
911 | Consider what happens during execution of the BEGIN block. At this point |
912 | the BEGIN block has already been compiled, but the body of foo() is still |
913 | being compiled. The new value of $^H will therefore be visible only while |
914 | the body of foo() is being compiled. |
915 | |
916 | Substitution of the above BEGIN block with: |
917 | |
918 | BEGIN { require strict; strict->import('vars') } |
919 | |
920 | demonstrates how C<use strict 'vars'> is implemented. Here's a conditional |
921 | version of the same lexical pragma: |
922 | |
923 | BEGIN { require strict; strict->import('vars') if $condition } |
924 | |
925 | =item %^H |
926 | |
927 | WARNING: This variable is strictly for internal use only. Its availability, |
928 | behavior, and contents are subject to change without notice. |
929 | |
930 | The %^H hash provides the same scoping semantic as $^H. This makes it |
931 | useful for implementation of lexically scoped pragmas. |
6e2995f4 |
932 | |
a0d0e21e |
933 | =item $INPLACE_EDIT |
934 | |
935 | =item $^I |
936 | |
937 | The current value of the inplace-edit extension. Use C<undef> to disable |
938 | inplace editing. (Mnemonic: value of B<-i> switch.) |
939 | |
fb73857a |
940 | =item $^M |
941 | |
19799a22 |
942 | By default, running out of memory is an untrappable, fatal error. |
943 | However, if suitably built, Perl can use the contents of C<$^M> |
944 | as an emergency memory pool after die()ing. Suppose that your Perl |
945 | were compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc. |
946 | Then |
fb73857a |
947 | |
19799a22 |
948 | $^M = 'a' x (1 << 16); |
fb73857a |
949 | |
51ee6500 |
950 | would allocate a 64K buffer for use in an emergency. See the |
19799a22 |
951 | F<INSTALL> file in the Perl distribution for information on how to |
952 | enable this option. To discourage casual use of this advanced |
4ec0190b |
953 | feature, there is no L<English|English> long name for this variable. |
fb73857a |
954 | |
5c055ba3 |
955 | =item $OSNAME |
6e2995f4 |
956 | |
5c055ba3 |
957 | =item $^O |
958 | |
959 | The name of the operating system under which this copy of Perl was |
960 | built, as determined during the configuration process. The value |
19799a22 |
961 | is identical to C<$Config{'osname'}>. See also L<Config> and the |
962 | B<-V> command-line switch documented in L<perlrun>. |
5c055ba3 |
963 | |
a0d0e21e |
964 | =item $PERLDB |
965 | |
966 | =item $^P |
967 | |
19799a22 |
968 | The internal variable for debugging support. The meanings of the |
969 | various bits are subject to change, but currently indicate: |
84902520 |
970 | |
971 | =over 6 |
972 | |
973 | =item 0x01 |
974 | |
975 | Debug subroutine enter/exit. |
976 | |
977 | =item 0x02 |
978 | |
979 | Line-by-line debugging. |
980 | |
981 | =item 0x04 |
982 | |
983 | Switch off optimizations. |
984 | |
985 | =item 0x08 |
986 | |
987 | Preserve more data for future interactive inspections. |
988 | |
989 | =item 0x10 |
990 | |
991 | Keep info about source lines on which a subroutine is defined. |
992 | |
993 | =item 0x20 |
994 | |
995 | Start with single-step on. |
996 | |
83ee9e09 |
997 | =item 0x40 |
998 | |
999 | Use subroutine address instead of name when reporting. |
1000 | |
1001 | =item 0x80 |
1002 | |
1003 | Report C<goto &subroutine> as well. |
1004 | |
1005 | =item 0x100 |
1006 | |
1007 | Provide informative "file" names for evals based on the place they were compiled. |
1008 | |
1009 | =item 0x200 |
1010 | |
1011 | Provide informative names to anonymous subroutines based on the place they |
1012 | were compiled. |
1013 | |
84902520 |
1014 | =back |
1015 | |
19799a22 |
1016 | Some bits may be relevant at compile-time only, some at |
1017 | run-time only. This is a new mechanism and the details may change. |
a0d0e21e |
1018 | |
66558a10 |
1019 | =item $LAST_REGEXP_CODE_RESULT |
1020 | |
b9ac3b5b |
1021 | =item $^R |
1022 | |
19799a22 |
1023 | The result of evaluation of the last successful C<(?{ code })> |
1024 | regular expression assertion (see L<perlre>). May be written to. |
b9ac3b5b |
1025 | |
66558a10 |
1026 | =item $EXCEPTIONS_BEING_CAUGHT |
1027 | |
fb73857a |
1028 | =item $^S |
1029 | |
1030 | Current state of the interpreter. Undefined if parsing of the current |
1031 | module/eval is not finished (may happen in $SIG{__DIE__} and |
19799a22 |
1032 | $SIG{__WARN__} handlers). True if inside an eval(), otherwise false. |
fb73857a |
1033 | |
a0d0e21e |
1034 | =item $BASETIME |
1035 | |
1036 | =item $^T |
1037 | |
19799a22 |
1038 | The time at which the program began running, in seconds since the |
5f05dabc |
1039 | epoch (beginning of 1970). The values returned by the B<-M>, B<-A>, |
19799a22 |
1040 | and B<-C> filetests are based on this value. |
a0d0e21e |
1041 | |
44dcb63b |
1042 | =item $PERL_VERSION |
b459063d |
1043 | |
16070b82 |
1044 | =item $^V |
1045 | |
1046 | The revision, version, and subversion of the Perl interpreter, represented |
da2094fd |
1047 | as a string composed of characters with those ordinals. Thus in Perl v5.6.0 |
44dcb63b |
1048 | it equals C<chr(5) . chr(6) . chr(0)> and will return true for |
1049 | C<$^V eq v5.6.0>. Note that the characters in this string value can |
1050 | potentially be in Unicode range. |
16070b82 |
1051 | |
1052 | This can be used to determine whether the Perl interpreter executing a |
1053 | script is in the right range of versions. (Mnemonic: use ^V for Version |
44dcb63b |
1054 | Control.) Example: |
16070b82 |
1055 | |
3fd4402b |
1056 | warn "No \"our\" declarations!\n" if $^V and $^V lt v5.6.0; |
16070b82 |
1057 | |
44dcb63b |
1058 | See the documentation of C<use VERSION> and C<require VERSION> |
16070b82 |
1059 | for a convenient way to fail if the running Perl interpreter is too old. |
1060 | |
1061 | See also C<$]> for an older representation of the Perl version. |
1062 | |
a0d0e21e |
1063 | =item $WARNING |
1064 | |
1065 | =item $^W |
1066 | |
19799a22 |
1067 | The current value of the warning switch, initially true if B<-w> |
1068 | was used, false otherwise, but directly modifiable. (Mnemonic: |
4438c4b7 |
1069 | related to the B<-w> switch.) See also L<warnings>. |
1070 | |
6a818117 |
1071 | =item ${^WARNING_BITS} |
4438c4b7 |
1072 | |
1073 | The current set of warning checks enabled by the C<use warnings> pragma. |
1074 | See the documentation of C<warnings> for more details. |
a0d0e21e |
1075 | |
46487f74 |
1076 | =item ${^WIDE_SYSTEM_CALLS} |
1077 | |
1078 | Global flag that enables system calls made by Perl to use wide character |
1079 | APIs native to the system, if available. This is currently only implemented |
1080 | on the Windows platform. |
1081 | |
1082 | This can also be enabled from the command line using the C<-C> switch. |
1083 | |
1084 | The initial value is typically C<0> for compatibility with Perl versions |
1085 | earlier than 5.6, but may be automatically set to C<1> by Perl if the system |
1086 | provides a user-settable default (e.g., C<$ENV{LC_CTYPE}>). |
1087 | |
8058d7ab |
1088 | The C<bytes> pragma always overrides the effect of this flag in the current |
1089 | lexical scope. See L<bytes>. |
46487f74 |
1090 | |
a0d0e21e |
1091 | =item $EXECUTABLE_NAME |
1092 | |
1093 | =item $^X |
1094 | |
1095 | The name that the Perl binary itself was executed as, from C's C<argv[0]>. |
19799a22 |
1096 | This may not be a full pathname, nor even necessarily in your path. |
a0d0e21e |
1097 | |
2d84a16a |
1098 | =item ARGV |
1099 | |
1100 | The special filehandle that iterates over command-line filenames in |
1101 | C<@ARGV>. Usually written as the null filehandle in the angle operator |
1102 | C<< <> >>. Note that currently C<ARGV> only has its magical effect |
1103 | within the C<< <> >> operator; elsewhere it is just a plain filehandle |
1104 | corresponding to the last file opened by C<< <> >>. In particular, |
1105 | passing C<\*ARGV> as a parameter to a function that expects a filehandle |
1106 | may not cause your function to automatically read the contents of all the |
1107 | files in C<@ARGV>. |
1108 | |
a0d0e21e |
1109 | =item $ARGV |
1110 | |
c47ff5f1 |
1111 | contains the name of the current file when reading from <>. |
a0d0e21e |
1112 | |
1113 | =item @ARGV |
1114 | |
19799a22 |
1115 | The array @ARGV contains the command-line arguments intended for |
14218588 |
1116 | the script. C<$#ARGV> is generally the number of arguments minus |
19799a22 |
1117 | one, because C<$ARGV[0]> is the first argument, I<not> the program's |
1118 | command name itself. See C<$0> for the command name. |
a0d0e21e |
1119 | |
9b0e6e7a |
1120 | =item @F |
1121 | |
1122 | The array @F contains the fields of each line read in when autosplit |
1123 | mode is turned on. See L<perlrun> for the B<-a> switch. This array |
1124 | is package-specific, and must be declared or given a full package name |
1125 | if not in package main when running under C<strict 'vars'>. |
1126 | |
a0d0e21e |
1127 | =item @INC |
1128 | |
19799a22 |
1129 | The array @INC contains the list of places that the C<do EXPR>, |
1130 | C<require>, or C<use> constructs look for their library files. It |
1131 | initially consists of the arguments to any B<-I> command-line |
1132 | switches, followed by the default Perl library, probably |
1133 | F</usr/local/lib/perl>, followed by ".", to represent the current |
1134 | directory. If you need to modify this at runtime, you should use |
1135 | the C<use lib> pragma to get the machine-dependent library properly |
1136 | loaded also: |
a0d0e21e |
1137 | |
cb1a09d0 |
1138 | use lib '/mypath/libdir/'; |
1139 | use SomeMod; |
303f2f76 |
1140 | |
d54b56d5 |
1141 | You can also insert hooks into the file inclusion system by putting Perl |
1142 | code directly into @INC. Those hooks may be subroutine references, array |
1143 | references or blessed objects. See L<perlfunc/require> for details. |
1144 | |
fb73857a |
1145 | =item @_ |
1146 | |
1147 | Within a subroutine the array @_ contains the parameters passed to that |
19799a22 |
1148 | subroutine. See L<perlsub>. |
fb73857a |
1149 | |
a0d0e21e |
1150 | =item %INC |
1151 | |
19799a22 |
1152 | The hash %INC contains entries for each filename included via the |
1153 | C<do>, C<require>, or C<use> operators. The key is the filename |
1154 | you specified (with module names converted to pathnames), and the |
14218588 |
1155 | value is the location of the file found. The C<require> |
87275199 |
1156 | operator uses this hash to determine whether a particular file has |
19799a22 |
1157 | already been included. |
a0d0e21e |
1158 | |
89ccab8c |
1159 | If the file was loaded via a hook (e.g. a subroutine reference, see |
1160 | L<perlfunc/require> for a description of these hooks), this hook is |
9ae8cd5b |
1161 | by default inserted into %INC in place of a filename. Note, however, |
1162 | that the hook may have set the %INC entry by itself to provide some more |
1163 | specific info. |
44f0be63 |
1164 | |
b687b08b |
1165 | =item %ENV |
1166 | |
1167 | =item $ENV{expr} |
a0d0e21e |
1168 | |
1169 | The hash %ENV contains your current environment. Setting a |
19799a22 |
1170 | value in C<ENV> changes the environment for any child processes |
1171 | you subsequently fork() off. |
a0d0e21e |
1172 | |
b687b08b |
1173 | =item %SIG |
1174 | |
1175 | =item $SIG{expr} |
a0d0e21e |
1176 | |
14218588 |
1177 | The hash %SIG contains signal handlers for signals. For example: |
a0d0e21e |
1178 | |
1179 | sub handler { # 1st argument is signal name |
fb73857a |
1180 | my($sig) = @_; |
a0d0e21e |
1181 | print "Caught a SIG$sig--shutting down\n"; |
1182 | close(LOG); |
1183 | exit(0); |
1184 | } |
1185 | |
fb73857a |
1186 | $SIG{'INT'} = \&handler; |
1187 | $SIG{'QUIT'} = \&handler; |
a0d0e21e |
1188 | ... |
19799a22 |
1189 | $SIG{'INT'} = 'DEFAULT'; # restore default action |
a0d0e21e |
1190 | $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT |
1191 | |
f648820c |
1192 | Using a value of C<'IGNORE'> usually has the effect of ignoring the |
1193 | signal, except for the C<CHLD> signal. See L<perlipc> for more about |
1194 | this special case. |
1195 | |
19799a22 |
1196 | Here are some other examples: |
a0d0e21e |
1197 | |
fb73857a |
1198 | $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended) |
a0d0e21e |
1199 | $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber |
19799a22 |
1200 | $SIG{"PIPE"} = *Plumber; # somewhat esoteric |
a0d0e21e |
1201 | $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return?? |
1202 | |
19799a22 |
1203 | Be sure not to use a bareword as the name of a signal handler, |
1204 | lest you inadvertently call it. |
748a9306 |
1205 | |
44a8e56a |
1206 | If your system has the sigaction() function then signal handlers are |
1207 | installed using it. This means you get reliable signal handling. If |
1208 | your system has the SA_RESTART flag it is used when signals handlers are |
19799a22 |
1209 | installed. This means that system calls for which restarting is supported |
44a8e56a |
1210 | continue rather than returning when a signal arrives. If you want your |
1211 | system calls to be interrupted by signal delivery then do something like |
1212 | this: |
1213 | |
1214 | use POSIX ':signal_h'; |
1215 | |
1216 | my $alarm = 0; |
1217 | sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 } |
1218 | or die "Error setting SIGALRM handler: $!\n"; |
1219 | |
1220 | See L<POSIX>. |
1221 | |
748a9306 |
1222 | Certain internal hooks can be also set using the %SIG hash. The |
a8f8344d |
1223 | routine indicated by C<$SIG{__WARN__}> is called when a warning message is |
748a9306 |
1224 | about to be printed. The warning message is passed as the first |
1225 | argument. The presence of a __WARN__ hook causes the ordinary printing |
1226 | of warnings to STDERR to be suppressed. You can use this to save warnings |
1227 | in a variable, or turn warnings into fatal errors, like this: |
1228 | |
1229 | local $SIG{__WARN__} = sub { die $_[0] }; |
1230 | eval $proggie; |
1231 | |
a8f8344d |
1232 | The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception |
748a9306 |
1233 | is about to be thrown. The error message is passed as the first |
1234 | argument. When a __DIE__ hook routine returns, the exception |
1235 | processing continues as it would have in the absence of the hook, |
cb1a09d0 |
1236 | unless the hook routine itself exits via a C<goto>, a loop exit, or a die(). |
774d564b |
1237 | The C<__DIE__> handler is explicitly disabled during the call, so that you |
fb73857a |
1238 | can die from a C<__DIE__> handler. Similarly for C<__WARN__>. |
1239 | |
19799a22 |
1240 | Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called |
1241 | even inside an eval(). Do not use this to rewrite a pending exception |
1242 | in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die(). |
1243 | This strange action at a distance may be fixed in a future release |
1244 | so that C<$SIG{__DIE__}> is only called if your program is about |
1245 | to exit, as was the original intent. Any other use is deprecated. |
1246 | |
1247 | C<__DIE__>/C<__WARN__> handlers are very special in one respect: |
1248 | they may be called to report (probable) errors found by the parser. |
1249 | In such a case the parser may be in inconsistent state, so any |
1250 | attempt to evaluate Perl code from such a handler will probably |
1251 | result in a segfault. This means that warnings or errors that |
1252 | result from parsing Perl should be used with extreme caution, like |
1253 | this: |
fb73857a |
1254 | |
1255 | require Carp if defined $^S; |
1256 | Carp::confess("Something wrong") if defined &Carp::confess; |
1257 | die "Something wrong, but could not load Carp to give backtrace... |
1258 | To see backtrace try starting Perl with -MCarp switch"; |
1259 | |
1260 | Here the first line will load Carp I<unless> it is the parser who |
1261 | called the handler. The second line will print backtrace and die if |
1262 | Carp was available. The third line will be executed only if Carp was |
1263 | not available. |
1264 | |
19799a22 |
1265 | See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and |
4438c4b7 |
1266 | L<warnings> for additional information. |
68dc0745 |
1267 | |
a0d0e21e |
1268 | =back |
55602bd2 |
1269 | |
1270 | =head2 Error Indicators |
1271 | |
19799a22 |
1272 | The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information |
1273 | about different types of error conditions that may appear during |
1274 | execution of a Perl program. The variables are shown ordered by |
1275 | the "distance" between the subsystem which reported the error and |
1276 | the Perl process. They correspond to errors detected by the Perl |
1277 | interpreter, C library, operating system, or an external program, |
1278 | respectively. |
55602bd2 |
1279 | |
1280 | To illustrate the differences between these variables, consider the |
19799a22 |
1281 | following Perl expression, which uses a single-quoted string: |
55602bd2 |
1282 | |
19799a22 |
1283 | eval q{ |
22d0716c |
1284 | open my $pipe, "/cdrom/install |" or die $!; |
1285 | my @res = <$pipe>; |
1286 | close $pipe or die "bad pipe: $?, $!"; |
19799a22 |
1287 | }; |
55602bd2 |
1288 | |
1289 | After execution of this statement all 4 variables may have been set. |
1290 | |
19799a22 |
1291 | C<$@> is set if the string to be C<eval>-ed did not compile (this |
1292 | may happen if C<open> or C<close> were imported with bad prototypes), |
1293 | or if Perl code executed during evaluation die()d . In these cases |
1294 | the value of $@ is the compile error, or the argument to C<die> |
1295 | (which will interpolate C<$!> and C<$?>!). (See also L<Fatal>, |
1296 | though.) |
1297 | |
c47ff5f1 |
1298 | When the eval() expression above is executed, open(), C<< <PIPE> >>, |
19799a22 |
1299 | and C<close> are translated to calls in the C run-time library and |
1300 | thence to the operating system kernel. C<$!> is set to the C library's |
1301 | C<errno> if one of these calls fails. |
1302 | |
1303 | Under a few operating systems, C<$^E> may contain a more verbose |
1304 | error indicator, such as in this case, "CDROM tray not closed." |
14218588 |
1305 | Systems that do not support extended error messages leave C<$^E> |
19799a22 |
1306 | the same as C<$!>. |
1307 | |
1308 | Finally, C<$?> may be set to non-0 value if the external program |
1309 | F</cdrom/install> fails. The upper eight bits reflect specific |
1310 | error conditions encountered by the program (the program's exit() |
1311 | value). The lower eight bits reflect mode of failure, like signal |
1312 | death and core dump information See wait(2) for details. In |
1313 | contrast to C<$!> and C<$^E>, which are set only if error condition |
1314 | is detected, the variable C<$?> is set on each C<wait> or pipe |
1315 | C<close>, overwriting the old value. This is more like C<$@>, which |
1316 | on every eval() is always set on failure and cleared on success. |
2b92dfce |
1317 | |
19799a22 |
1318 | For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>, |
1319 | and C<$?>. |
2b92dfce |
1320 | |
1321 | =head2 Technical Note on the Syntax of Variable Names |
1322 | |
19799a22 |
1323 | Variable names in Perl can have several formats. Usually, they |
1324 | must begin with a letter or underscore, in which case they can be |
1325 | arbitrarily long (up to an internal limit of 251 characters) and |
1326 | may contain letters, digits, underscores, or the special sequence |
1327 | C<::> or C<'>. In this case, the part before the last C<::> or |
1328 | C<'> is taken to be a I<package qualifier>; see L<perlmod>. |
2b92dfce |
1329 | |
1330 | Perl variable names may also be a sequence of digits or a single |
1331 | punctuation or control character. These names are all reserved for |
19799a22 |
1332 | special uses by Perl; for example, the all-digits names are used |
1333 | to hold data captured by backreferences after a regular expression |
1334 | match. Perl has a special syntax for the single-control-character |
1335 | names: It understands C<^X> (caret C<X>) to mean the control-C<X> |
1336 | character. For example, the notation C<$^W> (dollar-sign caret |
1337 | C<W>) is the scalar variable whose name is the single character |
1338 | control-C<W>. This is better than typing a literal control-C<W> |
1339 | into your program. |
2b92dfce |
1340 | |
87275199 |
1341 | Finally, new in Perl 5.6, Perl variable names may be alphanumeric |
19799a22 |
1342 | strings that begin with control characters (or better yet, a caret). |
1343 | These variables must be written in the form C<${^Foo}>; the braces |
1344 | are not optional. C<${^Foo}> denotes the scalar variable whose |
1345 | name is a control-C<F> followed by two C<o>'s. These variables are |
1346 | reserved for future special uses by Perl, except for the ones that |
1347 | begin with C<^_> (control-underscore or caret-underscore). No |
1348 | control-character name that begins with C<^_> will acquire a special |
1349 | meaning in any future version of Perl; such names may therefore be |
1350 | used safely in programs. C<$^_> itself, however, I<is> reserved. |
1351 | |
1352 | Perl identifiers that begin with digits, control characters, or |
2b92dfce |
1353 | punctuation characters are exempt from the effects of the C<package> |
1354 | declaration and are always forced to be in package C<main>. A few |
1355 | other names are also exempt: |
1356 | |
1357 | ENV STDIN |
1358 | INC STDOUT |
1359 | ARGV STDERR |
1360 | ARGVOUT |
1361 | SIG |
1362 | |
1363 | In particular, the new special C<${^_XYZ}> variables are always taken |
19799a22 |
1364 | to be in package C<main>, regardless of any C<package> declarations |
2b92dfce |
1365 | presently in scope. |
1366 | |
19799a22 |
1367 | =head1 BUGS |
1368 | |
1369 | Due to an unfortunate accident of Perl's implementation, C<use |
1370 | English> imposes a considerable performance penalty on all regular |
1371 | expression matches in a program, regardless of whether they occur |
1372 | in the scope of C<use English>. For that reason, saying C<use |
1373 | English> in libraries is strongly discouraged. See the |
1374 | Devel::SawAmpersand module documentation from CPAN |
6cecdcac |
1375 | (http://www.perl.com/CPAN/modules/by-module/Devel/) |
19799a22 |
1376 | for more information. |
2b92dfce |
1377 | |
19799a22 |
1378 | Having to even think about the C<$^S> variable in your exception |
1379 | handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented |
1380 | invites grievous and difficult to track down errors. Avoid it |
1381 | and use an C<END{}> or CORE::GLOBAL::die override instead. |