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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 | |
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16 | at the top of your program. This aliases all the short names to the long |
17 | names in the current package. Some even have medium names, generally |
18 | borrowed from B<awk>. In general, it's best to use the |
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19 | |
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20 | use English '-no_match_vars'; |
21 | |
22 | invocation if you don't need $PREMATCH, $MATCH, or $POSTMATCH, as it avoids |
23 | a certain performance hit with the use of regular expressions. See |
24 | L<English>. |
25 | |
26 | Variables that depend on the currently selected filehandle may be set by |
27 | calling an appropriate object method on the IO::Handle object, although |
28 | this is less efficient than using the regular built-in variables. (Summary |
29 | lines below for this contain the word HANDLE.) First you must say |
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30 | |
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31 | use IO::Handle; |
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32 | |
33 | after which you may use either |
34 | |
35 | method HANDLE EXPR |
36 | |
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37 | or more safely, |
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38 | |
39 | HANDLE->method(EXPR) |
40 | |
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41 | Each method returns the old value of the IO::Handle attribute. |
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42 | The methods each take an optional EXPR, which, if supplied, specifies the |
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43 | new value for the IO::Handle attribute in question. If not supplied, |
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44 | most methods do nothing to the current value--except for |
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45 | autoflush(), which will assume a 1 for you, just to be different. |
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46 | |
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47 | Because loading in the IO::Handle class is an expensive operation, you should |
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48 | learn how to use the regular built-in variables. |
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49 | |
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50 | A few of these variables are considered "read-only". This means that if |
51 | you try to assign to this variable, either directly or indirectly through |
52 | a reference, you'll raise a run-time exception. |
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53 | |
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54 | You should be very careful when modifying the default values of most |
55 | special variables described in this document. In most cases you want |
56 | to localize these variables before changing them, since if you don't, |
57 | the change may affect other modules which rely on the default values |
58 | of the special variables that you have changed. This is one of the |
59 | correct ways to read the whole file at once: |
60 | |
61 | open my $fh, "foo" or die $!; |
62 | local $/; # enable localized slurp mode |
63 | my $content = <$fh>; |
64 | close $fh; |
65 | |
66 | But the following code is quite bad: |
67 | |
68 | open my $fh, "foo" or die $!; |
69 | undef $/; # enable slurp mode |
70 | my $content = <$fh>; |
71 | close $fh; |
72 | |
73 | since some other module, may want to read data from some file in the |
74 | default "line mode", so if the code we have just presented has been |
75 | executed, the global value of C<$/> is now changed for any other code |
76 | running inside the same Perl interpreter. |
77 | |
78 | Usually when a variable is localized you want to make sure that this |
79 | change affects the shortest scope possible. So unless you are already |
80 | inside some short C<{}> block, you should create one yourself. For |
81 | example: |
82 | |
83 | my $content = ''; |
84 | open my $fh, "foo" or die $!; |
85 | { |
86 | local $/; |
87 | $content = <$fh>; |
88 | } |
89 | close $fh; |
90 | |
91 | Here is an example of how your own code can go broken: |
92 | |
93 | for (1..5){ |
94 | nasty_break(); |
95 | print "$_ "; |
96 | } |
97 | sub nasty_break { |
98 | $_ = 5; |
99 | # do something with $_ |
100 | } |
101 | |
102 | You probably expect this code to print: |
103 | |
104 | 1 2 3 4 5 |
105 | |
106 | but instead you get: |
107 | |
108 | 5 5 5 5 5 |
109 | |
110 | Why? Because nasty_break() modifies C<$_> without localizing it |
111 | first. The fix is to add local(): |
112 | |
113 | local $_ = 5; |
114 | |
115 | It's easy to notice the problem in such a short example, but in more |
116 | complicated code you are looking for trouble if you don't localize |
117 | changes to the special variables. |
118 | |
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119 | The following list is ordered by scalar variables first, then the |
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120 | arrays, then the hashes. |
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121 | |
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122 | =over 8 |
123 | |
124 | =item $ARG |
125 | |
126 | =item $_ |
127 | |
128 | The default input and pattern-searching space. The following pairs are |
129 | equivalent: |
130 | |
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131 | while (<>) {...} # equivalent only in while! |
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132 | while (defined($_ = <>)) {...} |
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133 | |
134 | /^Subject:/ |
135 | $_ =~ /^Subject:/ |
136 | |
137 | tr/a-z/A-Z/ |
138 | $_ =~ tr/a-z/A-Z/ |
139 | |
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140 | chomp |
141 | chomp($_) |
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142 | |
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143 | Here are the places where Perl will assume $_ even if you |
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144 | don't use it: |
145 | |
146 | =over 3 |
147 | |
148 | =item * |
149 | |
150 | Various unary functions, including functions like ord() and int(), as well |
151 | as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to |
152 | STDIN. |
153 | |
154 | =item * |
155 | |
156 | Various list functions like print() and unlink(). |
157 | |
158 | =item * |
159 | |
160 | The pattern matching operations C<m//>, C<s///>, and C<tr///> when used |
161 | without an C<=~> operator. |
162 | |
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163 | =item * |
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164 | |
165 | The default iterator variable in a C<foreach> loop if no other |
166 | variable is supplied. |
167 | |
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168 | =item * |
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169 | |
170 | The implicit iterator variable in the grep() and map() functions. |
171 | |
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172 | =item * |
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173 | |
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174 | The default place to put an input record when a C<< <FH> >> |
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175 | operation's result is tested by itself as the sole criterion of a C<while> |
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176 | test. Outside a C<while> test, this will not happen. |
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177 | |
178 | =back |
179 | |
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180 | (Mnemonic: underline is understood in certain operations.) |
181 | |
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182 | =back |
183 | |
184 | =over 8 |
185 | |
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186 | =item $a |
187 | |
188 | =item $b |
189 | |
190 | Special package variables when using sort(), see L<perlfunc/sort>. |
191 | Because of this specialness $a and $b don't need to be declared |
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192 | (using use vars, or our()) even when using the C<strict 'vars'> pragma. |
193 | Don't lexicalize them with C<my $a> or C<my $b> if you want to be |
194 | able to use them in the sort() comparison block or function. |
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195 | |
196 | =back |
197 | |
198 | =over 8 |
199 | |
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200 | =item $<I<digits>> |
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201 | |
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202 | Contains the subpattern from the corresponding set of capturing |
203 | parentheses from the last pattern match, not counting patterns |
204 | matched in nested blocks that have been exited already. (Mnemonic: |
205 | like \digits.) These variables are all read-only and dynamically |
206 | scoped to the current BLOCK. |
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207 | |
208 | =item $MATCH |
209 | |
210 | =item $& |
211 | |
212 | The string matched by the last successful pattern match (not counting |
213 | any matches hidden within a BLOCK or eval() enclosed by the current |
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214 | BLOCK). (Mnemonic: like & in some editors.) This variable is read-only |
215 | and dynamically scoped to the current BLOCK. |
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216 | |
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217 | The use of this variable anywhere in a program imposes a considerable |
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218 | performance penalty on all regular expression matches. See L</BUGS>. |
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219 | |
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220 | =item $PREMATCH |
221 | |
222 | =item $` |
223 | |
224 | The string preceding whatever was matched by the last successful |
225 | pattern match (not counting any matches hidden within a BLOCK or eval |
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226 | enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted |
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227 | string.) This variable is read-only. |
228 | |
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229 | The use of this variable anywhere in a program imposes a considerable |
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230 | performance penalty on all regular expression matches. See L</BUGS>. |
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231 | |
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232 | =item $POSTMATCH |
233 | |
234 | =item $' |
235 | |
236 | The string following whatever was matched by the last successful |
237 | pattern match (not counting any matches hidden within a BLOCK or eval() |
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238 | enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted |
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239 | string.) Example: |
240 | |
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241 | local $_ = 'abcdefghi'; |
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242 | /def/; |
243 | print "$`:$&:$'\n"; # prints abc:def:ghi |
244 | |
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245 | This variable is read-only and dynamically scoped to the current BLOCK. |
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246 | |
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247 | The use of this variable anywhere in a program imposes a considerable |
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248 | performance penalty on all regular expression matches. See L</BUGS>. |
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249 | |
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250 | =item $LAST_PAREN_MATCH |
251 | |
252 | =item $+ |
253 | |
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254 | The text matched by the last bracket of the last successful search pattern. |
255 | This is useful if you don't know which one of a set of alternative patterns |
256 | matched. For example: |
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257 | |
258 | /Version: (.*)|Revision: (.*)/ && ($rev = $+); |
259 | |
260 | (Mnemonic: be positive and forward looking.) |
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261 | This variable is read-only and dynamically scoped to the current BLOCK. |
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262 | |
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263 | =item $^N |
264 | |
265 | The text matched by the used group most-recently closed (i.e. the group |
266 | with the rightmost closing parenthesis) of the last successful search |
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267 | pattern. (Mnemonic: the (possibly) Nested parenthesis that most |
268 | recently closed.) |
269 | |
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270 | This is primarily used inside C<(?{...})> blocks for examining text |
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271 | recently matched. For example, to effectively capture text to a variable |
272 | (in addition to C<$1>, C<$2>, etc.), replace C<(...)> with |
273 | |
274 | (?:(...)(?{ $var = $^N })) |
275 | |
276 | By setting and then using C<$var> in this way relieves you from having to |
277 | worry about exactly which numbered set of parentheses they are. |
278 | |
279 | This variable is dynamically scoped to the current BLOCK. |
280 | |
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281 | =item @LAST_MATCH_END |
282 | |
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283 | =item @+ |
284 | |
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285 | This array holds the offsets of the ends of the last successful |
286 | submatches in the currently active dynamic scope. C<$+[0]> is |
287 | the offset into the string of the end of the entire match. This |
288 | is the same value as what the C<pos> function returns when called |
289 | on the variable that was matched against. The I<n>th element |
290 | of this array holds the offset of the I<n>th submatch, so |
291 | C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset |
292 | past where $2 ends, and so on. You can use C<$#+> to determine |
293 | how many subgroups were in the last successful match. See the |
294 | examples given for the C<@-> variable. |
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295 | |
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296 | =item HANDLE->input_line_number(EXPR) |
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297 | |
298 | =item $INPUT_LINE_NUMBER |
299 | |
300 | =item $NR |
301 | |
302 | =item $. |
303 | |
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304 | Current line number for the last filehandle accessed. |
305 | |
306 | Each filehandle in Perl counts the number of lines that have been read |
307 | from it. (Depending on the value of C<$/>, Perl's idea of what |
308 | constitutes a line may not match yours.) When a line is read from a |
309 | filehandle (via readline() or C<< <> >>), or when tell() or seek() is |
310 | called on it, C<$.> becomes an alias to the line counter for that |
311 | filehandle. |
312 | |
313 | You can adjust the counter by assigning to C<$.>, but this will not |
314 | actually move the seek pointer. I<Localizing C<$.> will not localize |
315 | the filehandle's line count>. Instead, it will localize perl's notion |
316 | of which filehandle C<$.> is currently aliased to. |
317 | |
318 | C<$.> is reset when the filehandle is closed, but B<not> when an open |
319 | filehandle is reopened without an intervening close(). For more |
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320 | details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does |
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321 | an explicit close, line numbers increase across ARGV files (but see |
322 | examples in L<perlfunc/eof>). |
323 | |
324 | You can also use C<< HANDLE->input_line_number(EXPR) >> to access the |
325 | line counter for a given filehandle without having to worry about |
326 | which handle you last accessed. |
327 | |
328 | (Mnemonic: many programs use "." to mean the current line number.) |
329 | |
330 | =item IO::Handle->input_record_separator(EXPR) |
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331 | |
332 | =item $INPUT_RECORD_SEPARATOR |
333 | |
334 | =item $RS |
335 | |
336 | =item $/ |
337 | |
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338 | The input record separator, newline by default. This |
339 | influences Perl's idea of what a "line" is. Works like B<awk>'s RS |
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340 | variable, including treating empty lines as a terminator if set to |
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341 | the null string. (An empty line cannot contain any spaces |
342 | or tabs.) You may set it to a multi-character string to match a |
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343 | multi-character terminator, or to C<undef> to read through the end |
344 | of file. Setting it to C<"\n\n"> means something slightly |
345 | different than setting to C<"">, if the file contains consecutive |
346 | empty lines. Setting to C<""> will treat two or more consecutive |
347 | empty lines as a single empty line. Setting to C<"\n\n"> will |
348 | blindly assume that the next input character belongs to the next |
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349 | paragraph, even if it's a newline. (Mnemonic: / delimits |
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350 | line boundaries when quoting poetry.) |
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351 | |
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352 | local $/; # enable "slurp" mode |
353 | local $_ = <FH>; # whole file now here |
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354 | s/\n[ \t]+/ /g; |
355 | |
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356 | Remember: the value of C<$/> is a string, not a regex. B<awk> has to be |
357 | better for something. :-) |
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358 | |
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359 | Setting C<$/> to a reference to an integer, scalar containing an integer, or |
360 | scalar that's convertible to an integer will attempt to read records |
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361 | instead of lines, with the maximum record size being the referenced |
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362 | integer. So this: |
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363 | |
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364 | local $/ = \32768; # or \"32768", or \$var_containing_32768 |
365 | open my $fh, $myfile or die $!; |
366 | local $_ = <$fh>; |
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367 | |
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368 | will read a record of no more than 32768 bytes from FILE. If you're |
369 | not reading from a record-oriented file (or your OS doesn't have |
370 | record-oriented files), then you'll likely get a full chunk of data |
371 | with every read. If a record is larger than the record size you've |
372 | set, you'll get the record back in pieces. |
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373 | |
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374 | On VMS, record reads are done with the equivalent of C<sysread>, |
375 | so it's best not to mix record and non-record reads on the same |
376 | file. (This is unlikely to be a problem, because any file you'd |
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377 | want to read in record mode is probably unusable in line mode.) |
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378 | Non-VMS systems do normal I/O, so it's safe to mix record and |
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379 | non-record reads of a file. |
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380 | |
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381 | See also L<perlport/"Newlines">. Also see C<$.>. |
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382 | |
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383 | =item HANDLE->autoflush(EXPR) |
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384 | |
385 | =item $OUTPUT_AUTOFLUSH |
386 | |
387 | =item $| |
388 | |
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389 | If set to nonzero, forces a flush right away and after every write |
390 | or print on the currently selected output channel. Default is 0 |
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391 | (regardless of whether the channel is really buffered by the |
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392 | system or not; C<$|> tells you only whether you've asked Perl |
393 | explicitly to flush after each write). STDOUT will |
394 | typically be line buffered if output is to the terminal and block |
395 | buffered otherwise. Setting this variable is useful primarily when |
396 | you are outputting to a pipe or socket, such as when you are running |
397 | a Perl program under B<rsh> and want to see the output as it's |
398 | happening. This has no effect on input buffering. See L<perlfunc/getc> |
399 | for that. (Mnemonic: when you want your pipes to be piping hot.) |
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400 | |
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401 | =item IO::Handle->output_field_separator EXPR |
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402 | |
403 | =item $OUTPUT_FIELD_SEPARATOR |
404 | |
405 | =item $OFS |
406 | |
407 | =item $, |
408 | |
409 | The output field separator for the print operator. Ordinarily the |
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410 | print operator simply prints out its arguments without further |
411 | adornment. To get behavior more like B<awk>, set this variable as |
412 | you would set B<awk>'s OFS variable to specify what is printed |
413 | between fields. (Mnemonic: what is printed when there is a "," in |
414 | your print statement.) |
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415 | |
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416 | =item IO::Handle->output_record_separator EXPR |
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417 | |
418 | =item $OUTPUT_RECORD_SEPARATOR |
419 | |
420 | =item $ORS |
421 | |
422 | =item $\ |
423 | |
424 | The output record separator for the print operator. Ordinarily the |
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425 | print operator simply prints out its arguments as is, with no |
426 | trailing newline or other end-of-record string added. To get |
427 | behavior more like B<awk>, set this variable as you would set |
428 | B<awk>'s ORS variable to specify what is printed at the end of the |
429 | print. (Mnemonic: you set C<$\> instead of adding "\n" at the |
430 | end of the print. Also, it's just like C<$/>, but it's what you |
431 | get "back" from Perl.) |
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432 | |
433 | =item $LIST_SEPARATOR |
434 | |
435 | =item $" |
436 | |
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437 | This is like C<$,> except that it applies to array and slice values |
438 | interpolated into a double-quoted string (or similar interpreted |
439 | string). Default is a space. (Mnemonic: obvious, I think.) |
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440 | |
441 | =item $SUBSCRIPT_SEPARATOR |
442 | |
443 | =item $SUBSEP |
444 | |
445 | =item $; |
446 | |
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447 | The subscript separator for multidimensional array emulation. If you |
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448 | refer to a hash element as |
449 | |
450 | $foo{$a,$b,$c} |
451 | |
452 | it really means |
453 | |
454 | $foo{join($;, $a, $b, $c)} |
455 | |
456 | But don't put |
457 | |
458 | @foo{$a,$b,$c} # a slice--note the @ |
459 | |
460 | which means |
461 | |
462 | ($foo{$a},$foo{$b},$foo{$c}) |
463 | |
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464 | Default is "\034", the same as SUBSEP in B<awk>. If your |
465 | keys contain binary data there might not be any safe value for C<$;>. |
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466 | (Mnemonic: comma (the syntactic subscript separator) is a |
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467 | semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already |
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468 | taken for something more important.) |
469 | |
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470 | Consider using "real" multidimensional arrays as described |
471 | in L<perllol>. |
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472 | |
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473 | =item $# |
474 | |
475 | The output format for printed numbers. This variable is a half-hearted |
476 | attempt to emulate B<awk>'s OFMT variable. There are times, however, |
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477 | when B<awk> and Perl have differing notions of what counts as |
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478 | numeric. The initial value is "%.I<n>g", where I<n> is the value |
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479 | of the macro DBL_DIG from your system's F<float.h>. This is different from |
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480 | B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#> |
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481 | explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.) |
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482 | |
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483 | Use of C<$#> is deprecated. |
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484 | |
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485 | =item HANDLE->format_page_number(EXPR) |
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486 | |
487 | =item $FORMAT_PAGE_NUMBER |
488 | |
489 | =item $% |
490 | |
491 | The current page number of the currently selected output channel. |
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492 | Used with formats. |
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493 | (Mnemonic: % is page number in B<nroff>.) |
494 | |
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495 | =item HANDLE->format_lines_per_page(EXPR) |
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496 | |
497 | =item $FORMAT_LINES_PER_PAGE |
498 | |
499 | =item $= |
500 | |
501 | The current page length (printable lines) of the currently selected |
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502 | output channel. Default is 60. |
503 | Used with formats. |
504 | (Mnemonic: = has horizontal lines.) |
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505 | |
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506 | =item HANDLE->format_lines_left(EXPR) |
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507 | |
508 | =item $FORMAT_LINES_LEFT |
509 | |
510 | =item $- |
511 | |
512 | The number of lines left on the page of the currently selected output |
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513 | channel. |
514 | Used with formats. |
515 | (Mnemonic: lines_on_page - lines_printed.) |
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516 | |
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517 | =item @LAST_MATCH_START |
518 | |
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519 | =item @- |
520 | |
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521 | $-[0] is the offset of the start of the last successful match. |
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522 | C<$-[>I<n>C<]> is the offset of the start of the substring matched by |
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523 | I<n>-th subpattern, or undef if the subpattern did not match. |
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524 | |
525 | Thus after a match against $_, $& coincides with C<substr $_, $-[0], |
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526 | $+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<], |
527 | $+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with |
c47ff5f1 |
528 | C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last |
14218588 |
529 | matched subgroup in the last successful match. Contrast with |
530 | C<$#+>, the number of subgroups in the regular expression. Compare |
19799a22 |
531 | with C<@+>. |
6cef1e77 |
532 | |
4ba05bdc |
533 | This array holds the offsets of the beginnings of the last |
534 | successful submatches in the currently active dynamic scope. |
535 | C<$-[0]> is the offset into the string of the beginning of the |
536 | entire match. The I<n>th element of this array holds the offset |
0926d669 |
537 | of the I<n>th submatch, so C<$-[1]> is the offset where $1 |
538 | begins, C<$-[2]> the offset where $2 begins, and so on. |
4ba05bdc |
539 | |
540 | After a match against some variable $var: |
541 | |
542 | =over 5 |
543 | |
4375e838 |
544 | =item C<$`> is the same as C<substr($var, 0, $-[0])> |
4ba05bdc |
545 | |
4375e838 |
546 | =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])> |
4ba05bdc |
547 | |
4375e838 |
548 | =item C<$'> is the same as C<substr($var, $+[0])> |
4ba05bdc |
549 | |
550 | =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])> |
551 | |
552 | =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])> |
553 | |
4375e838 |
554 | =item C<$3> is the same as C<substr $var, $-[3], $+[3] - $-[3])> |
4ba05bdc |
555 | |
556 | =back |
557 | |
fcc7d916 |
558 | =item HANDLE->format_name(EXPR) |
a0d0e21e |
559 | |
560 | =item $FORMAT_NAME |
561 | |
562 | =item $~ |
563 | |
564 | The name of the current report format for the currently selected output |
14218588 |
565 | channel. Default is the name of the filehandle. (Mnemonic: brother to |
19799a22 |
566 | C<$^>.) |
a0d0e21e |
567 | |
fcc7d916 |
568 | =item HANDLE->format_top_name(EXPR) |
a0d0e21e |
569 | |
570 | =item $FORMAT_TOP_NAME |
571 | |
572 | =item $^ |
573 | |
574 | The name of the current top-of-page format for the currently selected |
14218588 |
575 | output channel. Default is the name of the filehandle with _TOP |
a0d0e21e |
576 | appended. (Mnemonic: points to top of page.) |
577 | |
46550894 |
578 | =item IO::Handle->format_line_break_characters EXPR |
a0d0e21e |
579 | |
580 | =item $FORMAT_LINE_BREAK_CHARACTERS |
581 | |
582 | =item $: |
583 | |
584 | The current set of characters after which a string may be broken to |
54310121 |
585 | fill continuation fields (starting with ^) in a format. Default is |
a0d0e21e |
586 | S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in |
587 | poetry is a part of a line.) |
588 | |
46550894 |
589 | =item IO::Handle->format_formfeed EXPR |
a0d0e21e |
590 | |
591 | =item $FORMAT_FORMFEED |
592 | |
593 | =item $^L |
594 | |
14218588 |
595 | What formats output as a form feed. Default is \f. |
a0d0e21e |
596 | |
597 | =item $ACCUMULATOR |
598 | |
599 | =item $^A |
600 | |
601 | The current value of the write() accumulator for format() lines. A format |
19799a22 |
602 | contains formline() calls that put their result into C<$^A>. After |
a0d0e21e |
603 | calling its format, write() prints out the contents of C<$^A> and empties. |
14218588 |
604 | So you never really see the contents of C<$^A> unless you call |
a0d0e21e |
605 | formline() yourself and then look at it. See L<perlform> and |
606 | L<perlfunc/formline()>. |
607 | |
608 | =item $CHILD_ERROR |
609 | |
610 | =item $? |
611 | |
54310121 |
612 | The status returned by the last pipe close, backtick (C<``>) command, |
19799a22 |
613 | successful call to wait() or waitpid(), or from the system() |
614 | operator. This is just the 16-bit status word returned by the |
615 | wait() system call (or else is made up to look like it). Thus, the |
c47ff5f1 |
616 | exit value of the subprocess is really (C<<< $? >> 8 >>>), and |
19799a22 |
617 | C<$? & 127> gives which signal, if any, the process died from, and |
618 | C<$? & 128> reports whether there was a core dump. (Mnemonic: |
619 | similar to B<sh> and B<ksh>.) |
a0d0e21e |
620 | |
7b8d334a |
621 | Additionally, if the C<h_errno> variable is supported in C, its value |
14218588 |
622 | is returned via $? if any C<gethost*()> function fails. |
7b8d334a |
623 | |
19799a22 |
624 | If you have installed a signal handler for C<SIGCHLD>, the |
aa689395 |
625 | value of C<$?> will usually be wrong outside that handler. |
626 | |
a8f8344d |
627 | Inside an C<END> subroutine C<$?> contains the value that is going to be |
628 | given to C<exit()>. You can modify C<$?> in an C<END> subroutine to |
19799a22 |
629 | change the exit status of your program. For example: |
630 | |
631 | END { |
632 | $? = 1 if $? == 255; # die would make it 255 |
633 | } |
a8f8344d |
634 | |
aa689395 |
635 | Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the |
ff0cee69 |
636 | actual VMS exit status, instead of the default emulation of POSIX |
9bc98430 |
637 | status; see L<perlvms/$?> for details. |
f86702cc |
638 | |
55602bd2 |
639 | Also see L<Error Indicators>. |
640 | |
0a378802 |
641 | =item ${^ENCODING} |
642 | |
740bd165 |
643 | The I<object reference> to the Encode object that is used to convert |
644 | the source code to Unicode. Thanks to this variable your perl script |
645 | does not have to be written in UTF-8. Default is I<undef>. The direct |
646 | manipulation of this variable is highly discouraged. See L<encoding> |
048c20cb |
647 | for more details. |
0a378802 |
648 | |
a0d0e21e |
649 | =item $OS_ERROR |
650 | |
651 | =item $ERRNO |
652 | |
653 | =item $! |
654 | |
19799a22 |
655 | If used numerically, yields the current value of the C C<errno> |
6ab308ee |
656 | variable, or in other words, if a system or library call fails, it |
657 | sets this variable. This means that the value of C<$!> is meaningful |
658 | only I<immediately> after a B<failure>: |
659 | |
660 | if (open(FH, $filename)) { |
661 | # Here $! is meaningless. |
662 | ... |
663 | } else { |
664 | # ONLY here is $! meaningful. |
665 | ... |
666 | # Already here $! might be meaningless. |
667 | } |
668 | # Since here we might have either success or failure, |
669 | # here $! is meaningless. |
670 | |
671 | In the above I<meaningless> stands for anything: zero, non-zero, |
672 | C<undef>. A successful system or library call does B<not> set |
673 | the variable to zero. |
674 | |
271df126 |
675 | If used as a string, yields the corresponding system error string. |
19799a22 |
676 | You can assign a number to C<$!> to set I<errno> if, for instance, |
677 | you want C<"$!"> to return the string for error I<n>, or you want |
678 | to set the exit value for the die() operator. (Mnemonic: What just |
679 | went bang?) |
a0d0e21e |
680 | |
55602bd2 |
681 | Also see L<Error Indicators>. |
682 | |
4c5cef9b |
683 | =item %! |
684 | |
685 | Each element of C<%!> has a true value only if C<$!> is set to that |
686 | value. For example, C<$!{ENOENT}> is true if and only if the current |
3be065a1 |
687 | value of C<$!> is C<ENOENT>; that is, if the most recent error was |
688 | "No such file or directory" (or its moral equivalent: not all operating |
689 | systems give that exact error, and certainly not all languages). |
690 | To check if a particular key is meaningful on your system, use |
691 | C<exists $!{the_key}>; for a list of legal keys, use C<keys %!>. |
692 | See L<Errno> for more information, and also see above for the |
693 | validity of C<$!>. |
4c5cef9b |
694 | |
5c055ba3 |
695 | =item $EXTENDED_OS_ERROR |
696 | |
697 | =item $^E |
698 | |
22fae026 |
699 | Error information specific to the current operating system. At |
700 | the moment, this differs from C<$!> under only VMS, OS/2, and Win32 |
701 | (and for MacPerl). On all other platforms, C<$^E> is always just |
702 | the same as C<$!>. |
703 | |
704 | Under VMS, C<$^E> provides the VMS status value from the last |
705 | system error. This is more specific information about the last |
706 | system error than that provided by C<$!>. This is particularly |
d516a115 |
707 | important when C<$!> is set to B<EVMSERR>. |
22fae026 |
708 | |
1c1c7f20 |
709 | Under OS/2, C<$^E> is set to the error code of the last call to |
710 | OS/2 API either via CRT, or directly from perl. |
22fae026 |
711 | |
712 | Under Win32, C<$^E> always returns the last error information |
713 | reported by the Win32 call C<GetLastError()> which describes |
714 | the last error from within the Win32 API. Most Win32-specific |
19799a22 |
715 | code will report errors via C<$^E>. ANSI C and Unix-like calls |
22fae026 |
716 | set C<errno> and so most portable Perl code will report errors |
717 | via C<$!>. |
718 | |
719 | Caveats mentioned in the description of C<$!> generally apply to |
720 | C<$^E>, also. (Mnemonic: Extra error explanation.) |
5c055ba3 |
721 | |
55602bd2 |
722 | Also see L<Error Indicators>. |
723 | |
a0d0e21e |
724 | =item $EVAL_ERROR |
725 | |
726 | =item $@ |
727 | |
4a280ebe |
728 | The Perl syntax error message from the last eval() operator. |
729 | If $@ is the null string, the last eval() parsed and executed |
730 | correctly (although the operations you invoked may have failed in the |
731 | normal fashion). (Mnemonic: Where was the syntax error "at"?) |
a0d0e21e |
732 | |
19799a22 |
733 | Warning messages are not collected in this variable. You can, |
a8f8344d |
734 | however, set up a routine to process warnings by setting C<$SIG{__WARN__}> |
54310121 |
735 | as described below. |
748a9306 |
736 | |
55602bd2 |
737 | Also see L<Error Indicators>. |
738 | |
a0d0e21e |
739 | =item $PROCESS_ID |
740 | |
741 | =item $PID |
742 | |
743 | =item $$ |
744 | |
19799a22 |
745 | The process number of the Perl running this script. You should |
746 | consider this variable read-only, although it will be altered |
747 | across fork() calls. (Mnemonic: same as shells.) |
a0d0e21e |
748 | |
4d76a344 |
749 | Note for Linux users: on Linux, the C functions C<getpid()> and |
750 | C<getppid()> return different values from different threads. In order to |
751 | be portable, this behavior is not reflected by C<$$>, whose value remains |
752 | consistent across threads. If you want to call the underlying C<getpid()>, |
e3256f86 |
753 | you may use the CPAN module C<Linux::Pid>. |
4d76a344 |
754 | |
a0d0e21e |
755 | =item $REAL_USER_ID |
756 | |
757 | =item $UID |
758 | |
759 | =item $< |
760 | |
19799a22 |
761 | The real uid of this process. (Mnemonic: it's the uid you came I<from>, |
a043a685 |
762 | if you're running setuid.) You can change both the real uid and |
763 | the effective uid at the same time by using POSIX::setuid(). |
a0d0e21e |
764 | |
765 | =item $EFFECTIVE_USER_ID |
766 | |
767 | =item $EUID |
768 | |
769 | =item $> |
770 | |
771 | The effective uid of this process. Example: |
772 | |
773 | $< = $>; # set real to effective uid |
774 | ($<,$>) = ($>,$<); # swap real and effective uid |
775 | |
a043a685 |
776 | You can change both the effective uid and the real uid at the same |
777 | time by using POSIX::setuid(). |
778 | |
19799a22 |
779 | (Mnemonic: it's the uid you went I<to>, if you're running setuid.) |
c47ff5f1 |
780 | C<< $< >> and C<< $> >> can be swapped only on machines |
8cc95fdb |
781 | supporting setreuid(). |
a0d0e21e |
782 | |
783 | =item $REAL_GROUP_ID |
784 | |
785 | =item $GID |
786 | |
787 | =item $( |
788 | |
789 | The real gid of this process. If you are on a machine that supports |
790 | membership in multiple groups simultaneously, gives a space separated |
791 | list of groups you are in. The first number is the one returned by |
792 | getgid(), and the subsequent ones by getgroups(), one of which may be |
8cc95fdb |
793 | the same as the first number. |
794 | |
19799a22 |
795 | However, a value assigned to C<$(> must be a single number used to |
796 | set the real gid. So the value given by C<$(> should I<not> be assigned |
797 | back to C<$(> without being forced numeric, such as by adding zero. |
8cc95fdb |
798 | |
a043a685 |
799 | You can change both the real gid and the effective gid at the same |
800 | time by using POSIX::setgid(). |
801 | |
19799a22 |
802 | (Mnemonic: parentheses are used to I<group> things. The real gid is the |
803 | group you I<left>, if you're running setgid.) |
a0d0e21e |
804 | |
805 | =item $EFFECTIVE_GROUP_ID |
806 | |
807 | =item $EGID |
808 | |
809 | =item $) |
810 | |
811 | The effective gid of this process. If you are on a machine that |
812 | supports membership in multiple groups simultaneously, gives a space |
813 | separated list of groups you are in. The first number is the one |
814 | returned by getegid(), and the subsequent ones by getgroups(), one of |
8cc95fdb |
815 | which may be the same as the first number. |
816 | |
19799a22 |
817 | Similarly, a value assigned to C<$)> must also be a space-separated |
14218588 |
818 | list of numbers. The first number sets the effective gid, and |
8cc95fdb |
819 | the rest (if any) are passed to setgroups(). To get the effect of an |
820 | empty list for setgroups(), just repeat the new effective gid; that is, |
821 | to force an effective gid of 5 and an effectively empty setgroups() |
822 | list, say C< $) = "5 5" >. |
823 | |
a043a685 |
824 | You can change both the effective gid and the real gid at the same |
825 | time by using POSIX::setgid() (use only a single numeric argument). |
826 | |
19799a22 |
827 | (Mnemonic: parentheses are used to I<group> things. The effective gid |
828 | is the group that's I<right> for you, if you're running setgid.) |
a0d0e21e |
829 | |
c47ff5f1 |
830 | C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on |
19799a22 |
831 | machines that support the corresponding I<set[re][ug]id()> routine. C<$(> |
832 | and C<$)> can be swapped only on machines supporting setregid(). |
a0d0e21e |
833 | |
834 | =item $PROGRAM_NAME |
835 | |
836 | =item $0 |
837 | |
80bca1b4 |
838 | Contains the name of the program being executed. |
839 | |
840 | On some (read: not all) operating systems assigning to C<$0> modifies |
841 | the argument area that the C<ps> program sees. On some platforms you |
842 | may have to use special C<ps> options or a different C<ps> to see the |
843 | changes. Modifying the $0 is more useful as a way of indicating the |
844 | current program state than it is for hiding the program you're |
845 | running. (Mnemonic: same as B<sh> and B<ksh>.) |
f9cbb277 |
846 | |
847 | Note that there are platform specific limitations on the the maximum |
848 | length of C<$0>. In the most extreme case it may be limited to the |
849 | space occupied by the original C<$0>. |
a0d0e21e |
850 | |
80bca1b4 |
851 | In some platforms there may be arbitrary amount of padding, for |
852 | example space characters, after the modified name as shown by C<ps>. |
dda345b7 |
853 | In some platforms this padding may extend all the way to the original |
c80e2480 |
854 | length of the argument area, no matter what you do (this is the case |
855 | for example with Linux 2.2). |
80bca1b4 |
856 | |
4bc88a62 |
857 | Note for BSD users: setting C<$0> does not completely remove "perl" |
6a4647a3 |
858 | from the ps(1) output. For example, setting C<$0> to C<"foobar"> may |
859 | result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix |
860 | and the " (perl)" suffix are shown depends on your exact BSD variant |
861 | and version). This is an operating system feature, Perl cannot help it. |
4bc88a62 |
862 | |
e2975953 |
863 | In multithreaded scripts Perl coordinates the threads so that any |
864 | thread may modify its copy of the C<$0> and the change becomes visible |
80bca1b4 |
865 | to ps(1) (assuming the operating system plays along). Note that the |
866 | the view of C<$0> the other threads have will not change since they |
867 | have their own copies of it. |
e2975953 |
868 | |
a0d0e21e |
869 | =item $[ |
870 | |
871 | The index of the first element in an array, and of the first character |
19799a22 |
872 | in a substring. Default is 0, but you could theoretically set it |
873 | to 1 to make Perl behave more like B<awk> (or Fortran) when |
874 | subscripting and when evaluating the index() and substr() functions. |
875 | (Mnemonic: [ begins subscripts.) |
a0d0e21e |
876 | |
19799a22 |
877 | As of release 5 of Perl, assignment to C<$[> is treated as a compiler |
878 | directive, and cannot influence the behavior of any other file. |
f83ed198 |
879 | (That's why you can only assign compile-time constants to it.) |
19799a22 |
880 | Its use is highly discouraged. |
a0d0e21e |
881 | |
f83ed198 |
882 | Note that, unlike other compile-time directives (such as L<strict>), |
883 | assignment to $[ can be seen from outer lexical scopes in the same file. |
884 | However, you can use local() on it to strictly bound its value to a |
885 | lexical block. |
886 | |
a0d0e21e |
887 | =item $] |
888 | |
54310121 |
889 | The version + patchlevel / 1000 of the Perl interpreter. This variable |
890 | can be used to determine whether the Perl interpreter executing a |
891 | script is in the right range of versions. (Mnemonic: Is this version |
892 | of perl in the right bracket?) Example: |
a0d0e21e |
893 | |
894 | warn "No checksumming!\n" if $] < 3.019; |
895 | |
54310121 |
896 | See also the documentation of C<use VERSION> and C<require VERSION> |
19799a22 |
897 | for a convenient way to fail if the running Perl interpreter is too old. |
a0d0e21e |
898 | |
0c8d858b |
899 | The floating point representation can sometimes lead to inaccurate |
900 | numeric comparisons. See C<$^V> for a more modern representation of |
901 | the Perl version that allows accurate string comparisons. |
16070b82 |
902 | |
305aace0 |
903 | =item $COMPILING |
904 | |
905 | =item $^C |
906 | |
19799a22 |
907 | The current value of the flag associated with the B<-c> switch. |
908 | Mainly of use with B<-MO=...> to allow code to alter its behavior |
909 | when being compiled, such as for example to AUTOLOAD at compile |
910 | time rather than normal, deferred loading. See L<perlcc>. Setting |
911 | C<$^C = 1> is similar to calling C<B::minus_c>. |
305aace0 |
912 | |
a0d0e21e |
913 | =item $DEBUGGING |
914 | |
915 | =item $^D |
916 | |
917 | The current value of the debugging flags. (Mnemonic: value of B<-D> |
b4ab917c |
918 | switch.) May be read or set. Like its command-line equivalent, you can use |
919 | numeric or symbolic values, eg C<$^D = 10> or C<$^D = "st">. |
a0d0e21e |
920 | |
921 | =item $SYSTEM_FD_MAX |
922 | |
923 | =item $^F |
924 | |
925 | The maximum system file descriptor, ordinarily 2. System file |
926 | descriptors are passed to exec()ed processes, while higher file |
927 | descriptors are not. Also, during an open(), system file descriptors are |
928 | preserved even if the open() fails. (Ordinary file descriptors are |
19799a22 |
929 | closed before the open() is attempted.) The close-on-exec |
a0d0e21e |
930 | status of a file descriptor will be decided according to the value of |
8d2a6795 |
931 | C<$^F> when the corresponding file, pipe, or socket was opened, not the |
932 | time of the exec(). |
a0d0e21e |
933 | |
6e2995f4 |
934 | =item $^H |
935 | |
0462a1ab |
936 | WARNING: This variable is strictly for internal use only. Its availability, |
937 | behavior, and contents are subject to change without notice. |
938 | |
939 | This variable contains compile-time hints for the Perl interpreter. At the |
940 | end of compilation of a BLOCK the value of this variable is restored to the |
941 | value when the interpreter started to compile the BLOCK. |
942 | |
943 | When perl begins to parse any block construct that provides a lexical scope |
944 | (e.g., eval body, required file, subroutine body, loop body, or conditional |
945 | block), the existing value of $^H is saved, but its value is left unchanged. |
946 | When the compilation of the block is completed, it regains the saved value. |
947 | Between the points where its value is saved and restored, code that |
948 | executes within BEGIN blocks is free to change the value of $^H. |
949 | |
950 | This behavior provides the semantic of lexical scoping, and is used in, |
951 | for instance, the C<use strict> pragma. |
952 | |
953 | The contents should be an integer; different bits of it are used for |
954 | different pragmatic flags. Here's an example: |
955 | |
956 | sub add_100 { $^H |= 0x100 } |
957 | |
958 | sub foo { |
959 | BEGIN { add_100() } |
960 | bar->baz($boon); |
961 | } |
962 | |
963 | Consider what happens during execution of the BEGIN block. At this point |
964 | the BEGIN block has already been compiled, but the body of foo() is still |
965 | being compiled. The new value of $^H will therefore be visible only while |
966 | the body of foo() is being compiled. |
967 | |
968 | Substitution of the above BEGIN block with: |
969 | |
970 | BEGIN { require strict; strict->import('vars') } |
971 | |
972 | demonstrates how C<use strict 'vars'> is implemented. Here's a conditional |
973 | version of the same lexical pragma: |
974 | |
975 | BEGIN { require strict; strict->import('vars') if $condition } |
976 | |
977 | =item %^H |
978 | |
979 | WARNING: This variable is strictly for internal use only. Its availability, |
980 | behavior, and contents are subject to change without notice. |
981 | |
982 | The %^H hash provides the same scoping semantic as $^H. This makes it |
983 | useful for implementation of lexically scoped pragmas. |
6e2995f4 |
984 | |
a0d0e21e |
985 | =item $INPLACE_EDIT |
986 | |
987 | =item $^I |
988 | |
989 | The current value of the inplace-edit extension. Use C<undef> to disable |
990 | inplace editing. (Mnemonic: value of B<-i> switch.) |
991 | |
fb73857a |
992 | =item $^M |
993 | |
19799a22 |
994 | By default, running out of memory is an untrappable, fatal error. |
995 | However, if suitably built, Perl can use the contents of C<$^M> |
996 | as an emergency memory pool after die()ing. Suppose that your Perl |
997 | were compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc. |
998 | Then |
fb73857a |
999 | |
19799a22 |
1000 | $^M = 'a' x (1 << 16); |
fb73857a |
1001 | |
51ee6500 |
1002 | would allocate a 64K buffer for use in an emergency. See the |
19799a22 |
1003 | F<INSTALL> file in the Perl distribution for information on how to |
1004 | enable this option. To discourage casual use of this advanced |
4ec0190b |
1005 | feature, there is no L<English|English> long name for this variable. |
fb73857a |
1006 | |
5c055ba3 |
1007 | =item $OSNAME |
6e2995f4 |
1008 | |
5c055ba3 |
1009 | =item $^O |
1010 | |
1011 | The name of the operating system under which this copy of Perl was |
1012 | built, as determined during the configuration process. The value |
19799a22 |
1013 | is identical to C<$Config{'osname'}>. See also L<Config> and the |
1014 | B<-V> command-line switch documented in L<perlrun>. |
5c055ba3 |
1015 | |
443f6d01 |
1016 | In Windows platforms, $^O is not very helpful: since it is always |
7f510801 |
1017 | C<MSWin32>, it doesn't tell the difference between |
1018 | 95/98/ME/NT/2000/XP/CE/.NET. Use Win32::GetOSName() or |
1019 | Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish |
1020 | between the variants. |
916d64a3 |
1021 | |
e2e27056 |
1022 | =item ${^OPEN} |
1023 | |
1024 | An internal variable used by PerlIO. A string in two parts, separated |
fae2c0fb |
1025 | by a C<\0> byte, the first part describes the input layers, the second |
1026 | part describes the output layers. |
e2e27056 |
1027 | |
a0d0e21e |
1028 | =item $PERLDB |
1029 | |
1030 | =item $^P |
1031 | |
19799a22 |
1032 | The internal variable for debugging support. The meanings of the |
1033 | various bits are subject to change, but currently indicate: |
84902520 |
1034 | |
1035 | =over 6 |
1036 | |
1037 | =item 0x01 |
1038 | |
1039 | Debug subroutine enter/exit. |
1040 | |
1041 | =item 0x02 |
1042 | |
1043 | Line-by-line debugging. |
1044 | |
1045 | =item 0x04 |
1046 | |
1047 | Switch off optimizations. |
1048 | |
1049 | =item 0x08 |
1050 | |
1051 | Preserve more data for future interactive inspections. |
1052 | |
1053 | =item 0x10 |
1054 | |
1055 | Keep info about source lines on which a subroutine is defined. |
1056 | |
1057 | =item 0x20 |
1058 | |
1059 | Start with single-step on. |
1060 | |
83ee9e09 |
1061 | =item 0x40 |
1062 | |
1063 | Use subroutine address instead of name when reporting. |
1064 | |
1065 | =item 0x80 |
1066 | |
1067 | Report C<goto &subroutine> as well. |
1068 | |
1069 | =item 0x100 |
1070 | |
1071 | Provide informative "file" names for evals based on the place they were compiled. |
1072 | |
1073 | =item 0x200 |
1074 | |
1075 | Provide informative names to anonymous subroutines based on the place they |
1076 | were compiled. |
1077 | |
7619c85e |
1078 | =item 0x400 |
1079 | |
1080 | Debug assertion subroutines enter/exit. |
1081 | |
84902520 |
1082 | =back |
1083 | |
19799a22 |
1084 | Some bits may be relevant at compile-time only, some at |
1085 | run-time only. This is a new mechanism and the details may change. |
a0d0e21e |
1086 | |
66558a10 |
1087 | =item $LAST_REGEXP_CODE_RESULT |
1088 | |
b9ac3b5b |
1089 | =item $^R |
1090 | |
19799a22 |
1091 | The result of evaluation of the last successful C<(?{ code })> |
1092 | regular expression assertion (see L<perlre>). May be written to. |
b9ac3b5b |
1093 | |
66558a10 |
1094 | =item $EXCEPTIONS_BEING_CAUGHT |
1095 | |
fb73857a |
1096 | =item $^S |
1097 | |
fa05a9fd |
1098 | Current state of the interpreter. |
1099 | |
1100 | $^S State |
1101 | --------- ------------------- |
1102 | undef Parsing module/eval |
1103 | true (1) Executing an eval |
1104 | false (0) Otherwise |
1105 | |
1106 | The first state may happen in $SIG{__DIE__} and $SIG{__WARN__} handlers. |
fb73857a |
1107 | |
a0d0e21e |
1108 | =item $BASETIME |
1109 | |
1110 | =item $^T |
1111 | |
19799a22 |
1112 | The time at which the program began running, in seconds since the |
5f05dabc |
1113 | epoch (beginning of 1970). The values returned by the B<-M>, B<-A>, |
19799a22 |
1114 | and B<-C> filetests are based on this value. |
a0d0e21e |
1115 | |
7c36658b |
1116 | =item ${^TAINT} |
1117 | |
9aa05f58 |
1118 | Reflects if taint mode is on or off. 1 for on (the program was run with |
1119 | B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with |
18e8c5b0 |
1120 | B<-t> or B<-TU>). This variable is read-only. |
7c36658b |
1121 | |
a05d7ebb |
1122 | =item ${^UNICODE} |
1123 | |
ab9e1bb7 |
1124 | Reflects certain Unicode settings of Perl. See L<perlrun> |
1125 | documentation for the C<-C> switch for more information about |
1126 | the possible values. This variable is set during Perl startup |
1127 | and is thereafter read-only. |
fde18df1 |
1128 | |
44dcb63b |
1129 | =item $PERL_VERSION |
b459063d |
1130 | |
16070b82 |
1131 | =item $^V |
1132 | |
1133 | The revision, version, and subversion of the Perl interpreter, represented |
da2094fd |
1134 | as a string composed of characters with those ordinals. Thus in Perl v5.6.0 |
44dcb63b |
1135 | it equals C<chr(5) . chr(6) . chr(0)> and will return true for |
1136 | C<$^V eq v5.6.0>. Note that the characters in this string value can |
1137 | potentially be in Unicode range. |
16070b82 |
1138 | |
1139 | This can be used to determine whether the Perl interpreter executing a |
1140 | script is in the right range of versions. (Mnemonic: use ^V for Version |
44dcb63b |
1141 | Control.) Example: |
16070b82 |
1142 | |
3fd4402b |
1143 | warn "No \"our\" declarations!\n" if $^V and $^V lt v5.6.0; |
16070b82 |
1144 | |
aa2f2a36 |
1145 | To convert C<$^V> into its string representation use sprintf()'s |
1146 | C<"%vd"> conversion: |
1147 | |
1148 | printf "version is v%vd\n", $^V; # Perl's version |
1149 | |
44dcb63b |
1150 | See the documentation of C<use VERSION> and C<require VERSION> |
16070b82 |
1151 | for a convenient way to fail if the running Perl interpreter is too old. |
1152 | |
1153 | See also C<$]> for an older representation of the Perl version. |
1154 | |
a0d0e21e |
1155 | =item $WARNING |
1156 | |
1157 | =item $^W |
1158 | |
19799a22 |
1159 | The current value of the warning switch, initially true if B<-w> |
1160 | was used, false otherwise, but directly modifiable. (Mnemonic: |
4438c4b7 |
1161 | related to the B<-w> switch.) See also L<warnings>. |
1162 | |
6a818117 |
1163 | =item ${^WARNING_BITS} |
4438c4b7 |
1164 | |
1165 | The current set of warning checks enabled by the C<use warnings> pragma. |
1166 | See the documentation of C<warnings> for more details. |
a0d0e21e |
1167 | |
1168 | =item $EXECUTABLE_NAME |
1169 | |
1170 | =item $^X |
1171 | |
e71940de |
1172 | The name used to execute the current copy of Perl, from C's |
38e4f4ae |
1173 | C<argv[0]>. |
1174 | |
e71940de |
1175 | Depending on the host operating system, the value of $^X may be |
1176 | a relative or absolute pathname of the perl program file, or may |
1177 | be the string used to invoke perl but not the pathname of the |
1178 | perl program file. Also, most operating systems permit invoking |
1179 | programs that are not in the PATH environment variable, so there |
a10d74f3 |
1180 | is no guarantee that the value of $^X is in PATH. For VMS, the |
1181 | value may or may not include a version number. |
38e4f4ae |
1182 | |
e71940de |
1183 | You usually can use the value of $^X to re-invoke an independent |
1184 | copy of the same perl that is currently running, e.g., |
1185 | |
1186 | @first_run = `$^X -le "print int rand 100 for 1..100"`; |
1187 | |
1188 | But recall that not all operating systems support forking or |
1189 | capturing of the output of commands, so this complex statement |
1190 | may not be portable. |
38e4f4ae |
1191 | |
e71940de |
1192 | It is not safe to use the value of $^X as a path name of a file, |
1193 | as some operating systems that have a mandatory suffix on |
1194 | executable files do not require use of the suffix when invoking |
1195 | a command. To convert the value of $^X to a path name, use the |
1196 | following statements: |
1197 | |
1198 | # Build up a set of file names (not command names). |
1199 | use Config; |
68fb0eb7 |
1200 | $this_perl = $^X; |
1201 | if ($^O ne 'VMS') |
1202 | {$this_perl .= $Config{_exe} |
1203 | unless $this_perl =~ m/$Config{_exe}$/i;} |
e71940de |
1204 | |
1205 | Because many operating systems permit anyone with read access to |
1206 | the Perl program file to make a copy of it, patch the copy, and |
1207 | then execute the copy, the security-conscious Perl programmer |
1208 | should take care to invoke the installed copy of perl, not the |
1209 | copy referenced by $^X. The following statements accomplish |
1210 | this goal, and produce a pathname that can be invoked as a |
1211 | command or referenced as a file. |
38e4f4ae |
1212 | |
1213 | use Config; |
68fb0eb7 |
1214 | $secure_perl_path = $Config{perlpath}; |
1215 | if ($^O ne 'VMS') |
1216 | {$secure_perl_path .= $Config{_exe} |
1217 | unless $secure_perl_path =~ m/$Config{_exe}$/i;} |
a0d0e21e |
1218 | |
2d84a16a |
1219 | =item ARGV |
1220 | |
1221 | The special filehandle that iterates over command-line filenames in |
1222 | C<@ARGV>. Usually written as the null filehandle in the angle operator |
1223 | C<< <> >>. Note that currently C<ARGV> only has its magical effect |
1224 | within the C<< <> >> operator; elsewhere it is just a plain filehandle |
1225 | corresponding to the last file opened by C<< <> >>. In particular, |
1226 | passing C<\*ARGV> as a parameter to a function that expects a filehandle |
1227 | may not cause your function to automatically read the contents of all the |
1228 | files in C<@ARGV>. |
1229 | |
a0d0e21e |
1230 | =item $ARGV |
1231 | |
c47ff5f1 |
1232 | contains the name of the current file when reading from <>. |
a0d0e21e |
1233 | |
1234 | =item @ARGV |
1235 | |
19799a22 |
1236 | The array @ARGV contains the command-line arguments intended for |
14218588 |
1237 | the script. C<$#ARGV> is generally the number of arguments minus |
19799a22 |
1238 | one, because C<$ARGV[0]> is the first argument, I<not> the program's |
1239 | command name itself. See C<$0> for the command name. |
a0d0e21e |
1240 | |
5ccee41e |
1241 | =item ARGVOUT |
1242 | |
1243 | The special filehandle that points to the currently open output file |
1244 | when doing edit-in-place processing with B<-i>. Useful when you have |
1245 | to do a lot of inserting and don't want to keep modifying $_. See |
1246 | L<perlrun> for the B<-i> switch. |
1247 | |
9b0e6e7a |
1248 | =item @F |
1249 | |
1250 | The array @F contains the fields of each line read in when autosplit |
1251 | mode is turned on. See L<perlrun> for the B<-a> switch. This array |
1252 | is package-specific, and must be declared or given a full package name |
1253 | if not in package main when running under C<strict 'vars'>. |
1254 | |
a0d0e21e |
1255 | =item @INC |
1256 | |
19799a22 |
1257 | The array @INC contains the list of places that the C<do EXPR>, |
1258 | C<require>, or C<use> constructs look for their library files. It |
1259 | initially consists of the arguments to any B<-I> command-line |
1260 | switches, followed by the default Perl library, probably |
1261 | F</usr/local/lib/perl>, followed by ".", to represent the current |
e48df184 |
1262 | directory. ("." will not be appended if taint checks are enabled, either by |
1263 | C<-T> or by C<-t>.) If you need to modify this at runtime, you should use |
19799a22 |
1264 | the C<use lib> pragma to get the machine-dependent library properly |
1265 | loaded also: |
a0d0e21e |
1266 | |
cb1a09d0 |
1267 | use lib '/mypath/libdir/'; |
1268 | use SomeMod; |
303f2f76 |
1269 | |
d54b56d5 |
1270 | You can also insert hooks into the file inclusion system by putting Perl |
1271 | code directly into @INC. Those hooks may be subroutine references, array |
1272 | references or blessed objects. See L<perlfunc/require> for details. |
1273 | |
fb73857a |
1274 | =item @_ |
1275 | |
1276 | Within a subroutine the array @_ contains the parameters passed to that |
19799a22 |
1277 | subroutine. See L<perlsub>. |
fb73857a |
1278 | |
a0d0e21e |
1279 | =item %INC |
1280 | |
19799a22 |
1281 | The hash %INC contains entries for each filename included via the |
1282 | C<do>, C<require>, or C<use> operators. The key is the filename |
1283 | you specified (with module names converted to pathnames), and the |
14218588 |
1284 | value is the location of the file found. The C<require> |
87275199 |
1285 | operator uses this hash to determine whether a particular file has |
19799a22 |
1286 | already been included. |
a0d0e21e |
1287 | |
89ccab8c |
1288 | If the file was loaded via a hook (e.g. a subroutine reference, see |
1289 | L<perlfunc/require> for a description of these hooks), this hook is |
9ae8cd5b |
1290 | by default inserted into %INC in place of a filename. Note, however, |
1291 | that the hook may have set the %INC entry by itself to provide some more |
1292 | specific info. |
44f0be63 |
1293 | |
b687b08b |
1294 | =item %ENV |
1295 | |
1296 | =item $ENV{expr} |
a0d0e21e |
1297 | |
1298 | The hash %ENV contains your current environment. Setting a |
19799a22 |
1299 | value in C<ENV> changes the environment for any child processes |
1300 | you subsequently fork() off. |
a0d0e21e |
1301 | |
b687b08b |
1302 | =item %SIG |
1303 | |
1304 | =item $SIG{expr} |
a0d0e21e |
1305 | |
14218588 |
1306 | The hash %SIG contains signal handlers for signals. For example: |
a0d0e21e |
1307 | |
1308 | sub handler { # 1st argument is signal name |
fb73857a |
1309 | my($sig) = @_; |
a0d0e21e |
1310 | print "Caught a SIG$sig--shutting down\n"; |
1311 | close(LOG); |
1312 | exit(0); |
1313 | } |
1314 | |
fb73857a |
1315 | $SIG{'INT'} = \&handler; |
1316 | $SIG{'QUIT'} = \&handler; |
a0d0e21e |
1317 | ... |
19799a22 |
1318 | $SIG{'INT'} = 'DEFAULT'; # restore default action |
a0d0e21e |
1319 | $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT |
1320 | |
f648820c |
1321 | Using a value of C<'IGNORE'> usually has the effect of ignoring the |
1322 | signal, except for the C<CHLD> signal. See L<perlipc> for more about |
1323 | this special case. |
1324 | |
19799a22 |
1325 | Here are some other examples: |
a0d0e21e |
1326 | |
fb73857a |
1327 | $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended) |
a0d0e21e |
1328 | $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber |
19799a22 |
1329 | $SIG{"PIPE"} = *Plumber; # somewhat esoteric |
a0d0e21e |
1330 | $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return?? |
1331 | |
19799a22 |
1332 | Be sure not to use a bareword as the name of a signal handler, |
1333 | lest you inadvertently call it. |
748a9306 |
1334 | |
44a8e56a |
1335 | If your system has the sigaction() function then signal handlers are |
9ce5b4ad |
1336 | installed using it. This means you get reliable signal handling. |
44a8e56a |
1337 | |
9ce5b4ad |
1338 | The default delivery policy of signals changed in Perl 5.8.0 from |
1339 | immediate (also known as "unsafe") to deferred, also known as |
1340 | "safe signals". See L<perlipc> for more information. |
45c0772f |
1341 | |
748a9306 |
1342 | Certain internal hooks can be also set using the %SIG hash. The |
a8f8344d |
1343 | routine indicated by C<$SIG{__WARN__}> is called when a warning message is |
748a9306 |
1344 | about to be printed. The warning message is passed as the first |
1345 | argument. The presence of a __WARN__ hook causes the ordinary printing |
1346 | of warnings to STDERR to be suppressed. You can use this to save warnings |
1347 | in a variable, or turn warnings into fatal errors, like this: |
1348 | |
1349 | local $SIG{__WARN__} = sub { die $_[0] }; |
1350 | eval $proggie; |
1351 | |
a8f8344d |
1352 | The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception |
748a9306 |
1353 | is about to be thrown. The error message is passed as the first |
1354 | argument. When a __DIE__ hook routine returns, the exception |
1355 | processing continues as it would have in the absence of the hook, |
cb1a09d0 |
1356 | unless the hook routine itself exits via a C<goto>, a loop exit, or a die(). |
774d564b |
1357 | The C<__DIE__> handler is explicitly disabled during the call, so that you |
fb73857a |
1358 | can die from a C<__DIE__> handler. Similarly for C<__WARN__>. |
1359 | |
19799a22 |
1360 | Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called |
1361 | even inside an eval(). Do not use this to rewrite a pending exception |
1362 | in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die(). |
1363 | This strange action at a distance may be fixed in a future release |
1364 | so that C<$SIG{__DIE__}> is only called if your program is about |
1365 | to exit, as was the original intent. Any other use is deprecated. |
1366 | |
1367 | C<__DIE__>/C<__WARN__> handlers are very special in one respect: |
1368 | they may be called to report (probable) errors found by the parser. |
1369 | In such a case the parser may be in inconsistent state, so any |
1370 | attempt to evaluate Perl code from such a handler will probably |
1371 | result in a segfault. This means that warnings or errors that |
1372 | result from parsing Perl should be used with extreme caution, like |
1373 | this: |
fb73857a |
1374 | |
1375 | require Carp if defined $^S; |
1376 | Carp::confess("Something wrong") if defined &Carp::confess; |
1377 | die "Something wrong, but could not load Carp to give backtrace... |
1378 | To see backtrace try starting Perl with -MCarp switch"; |
1379 | |
1380 | Here the first line will load Carp I<unless> it is the parser who |
1381 | called the handler. The second line will print backtrace and die if |
1382 | Carp was available. The third line will be executed only if Carp was |
1383 | not available. |
1384 | |
19799a22 |
1385 | See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and |
4438c4b7 |
1386 | L<warnings> for additional information. |
68dc0745 |
1387 | |
a0d0e21e |
1388 | =back |
55602bd2 |
1389 | |
1390 | =head2 Error Indicators |
1391 | |
19799a22 |
1392 | The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information |
1393 | about different types of error conditions that may appear during |
1394 | execution of a Perl program. The variables are shown ordered by |
1395 | the "distance" between the subsystem which reported the error and |
1396 | the Perl process. They correspond to errors detected by the Perl |
1397 | interpreter, C library, operating system, or an external program, |
1398 | respectively. |
55602bd2 |
1399 | |
1400 | To illustrate the differences between these variables, consider the |
19799a22 |
1401 | following Perl expression, which uses a single-quoted string: |
55602bd2 |
1402 | |
19799a22 |
1403 | eval q{ |
22d0716c |
1404 | open my $pipe, "/cdrom/install |" or die $!; |
1405 | my @res = <$pipe>; |
1406 | close $pipe or die "bad pipe: $?, $!"; |
19799a22 |
1407 | }; |
55602bd2 |
1408 | |
1409 | After execution of this statement all 4 variables may have been set. |
1410 | |
19799a22 |
1411 | C<$@> is set if the string to be C<eval>-ed did not compile (this |
1412 | may happen if C<open> or C<close> were imported with bad prototypes), |
1413 | or if Perl code executed during evaluation die()d . In these cases |
1414 | the value of $@ is the compile error, or the argument to C<die> |
1415 | (which will interpolate C<$!> and C<$?>!). (See also L<Fatal>, |
1416 | though.) |
1417 | |
c47ff5f1 |
1418 | When the eval() expression above is executed, open(), C<< <PIPE> >>, |
19799a22 |
1419 | and C<close> are translated to calls in the C run-time library and |
1420 | thence to the operating system kernel. C<$!> is set to the C library's |
1421 | C<errno> if one of these calls fails. |
1422 | |
1423 | Under a few operating systems, C<$^E> may contain a more verbose |
1424 | error indicator, such as in this case, "CDROM tray not closed." |
14218588 |
1425 | Systems that do not support extended error messages leave C<$^E> |
19799a22 |
1426 | the same as C<$!>. |
1427 | |
1428 | Finally, C<$?> may be set to non-0 value if the external program |
1429 | F</cdrom/install> fails. The upper eight bits reflect specific |
1430 | error conditions encountered by the program (the program's exit() |
1431 | value). The lower eight bits reflect mode of failure, like signal |
1432 | death and core dump information See wait(2) for details. In |
1433 | contrast to C<$!> and C<$^E>, which are set only if error condition |
1434 | is detected, the variable C<$?> is set on each C<wait> or pipe |
1435 | C<close>, overwriting the old value. This is more like C<$@>, which |
1436 | on every eval() is always set on failure and cleared on success. |
2b92dfce |
1437 | |
19799a22 |
1438 | For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>, |
1439 | and C<$?>. |
2b92dfce |
1440 | |
1441 | =head2 Technical Note on the Syntax of Variable Names |
1442 | |
19799a22 |
1443 | Variable names in Perl can have several formats. Usually, they |
1444 | must begin with a letter or underscore, in which case they can be |
1445 | arbitrarily long (up to an internal limit of 251 characters) and |
1446 | may contain letters, digits, underscores, or the special sequence |
1447 | C<::> or C<'>. In this case, the part before the last C<::> or |
1448 | C<'> is taken to be a I<package qualifier>; see L<perlmod>. |
2b92dfce |
1449 | |
1450 | Perl variable names may also be a sequence of digits or a single |
1451 | punctuation or control character. These names are all reserved for |
19799a22 |
1452 | special uses by Perl; for example, the all-digits names are used |
1453 | to hold data captured by backreferences after a regular expression |
1454 | match. Perl has a special syntax for the single-control-character |
1455 | names: It understands C<^X> (caret C<X>) to mean the control-C<X> |
1456 | character. For example, the notation C<$^W> (dollar-sign caret |
1457 | C<W>) is the scalar variable whose name is the single character |
1458 | control-C<W>. This is better than typing a literal control-C<W> |
1459 | into your program. |
2b92dfce |
1460 | |
87275199 |
1461 | Finally, new in Perl 5.6, Perl variable names may be alphanumeric |
19799a22 |
1462 | strings that begin with control characters (or better yet, a caret). |
1463 | These variables must be written in the form C<${^Foo}>; the braces |
1464 | are not optional. C<${^Foo}> denotes the scalar variable whose |
1465 | name is a control-C<F> followed by two C<o>'s. These variables are |
1466 | reserved for future special uses by Perl, except for the ones that |
1467 | begin with C<^_> (control-underscore or caret-underscore). No |
1468 | control-character name that begins with C<^_> will acquire a special |
1469 | meaning in any future version of Perl; such names may therefore be |
1470 | used safely in programs. C<$^_> itself, however, I<is> reserved. |
1471 | |
1472 | Perl identifiers that begin with digits, control characters, or |
2b92dfce |
1473 | punctuation characters are exempt from the effects of the C<package> |
747fafda |
1474 | declaration and are always forced to be in package C<main>; they are |
1475 | also exempt from C<strict 'vars'> errors. A few other names are also |
1476 | exempt in these ways: |
2b92dfce |
1477 | |
1478 | ENV STDIN |
1479 | INC STDOUT |
1480 | ARGV STDERR |
5b88253b |
1481 | ARGVOUT _ |
2b92dfce |
1482 | SIG |
1483 | |
1484 | In particular, the new special C<${^_XYZ}> variables are always taken |
19799a22 |
1485 | to be in package C<main>, regardless of any C<package> declarations |
747fafda |
1486 | presently in scope. |
2b92dfce |
1487 | |
19799a22 |
1488 | =head1 BUGS |
1489 | |
1490 | Due to an unfortunate accident of Perl's implementation, C<use |
1491 | English> imposes a considerable performance penalty on all regular |
1492 | expression matches in a program, regardless of whether they occur |
1493 | in the scope of C<use English>. For that reason, saying C<use |
1494 | English> in libraries is strongly discouraged. See the |
1495 | Devel::SawAmpersand module documentation from CPAN |
1577cd80 |
1496 | ( http://www.cpan.org/modules/by-module/Devel/ ) |
19799a22 |
1497 | for more information. |
2b92dfce |
1498 | |
19799a22 |
1499 | Having to even think about the C<$^S> variable in your exception |
1500 | handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented |
1501 | invites grievous and difficult to track down errors. Avoid it |
1502 | and use an C<END{}> or CORE::GLOBAL::die override instead. |