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1=head1 NAME
2
3perlvar - Perl predefined variables
4
5=head1 DESCRIPTION
6
7=head2 Predefined Names
8
5a964f20 9The following names have special meaning to Perl. Most
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10punctuation names have reasonable mnemonics, or analogs in the
11shells. Nevertheless, if you wish to use long variable names,
12you need only say
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13
14 use English;
15
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16at the top of your program. This aliases all the short names to the long
17names in the current package. Some even have medium names, generally
18borrowed from B<awk>. In general, it's best to use the
a0d0e21e 19
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20 use English '-no_match_vars';
21
22invocation if you don't need $PREMATCH, $MATCH, or $POSTMATCH, as it avoids
23a certain performance hit with the use of regular expressions. See
24L<English>.
25
26Variables that depend on the currently selected filehandle may be set by
27calling an appropriate object method on the IO::Handle object, although
28this is less efficient than using the regular built-in variables. (Summary
29lines below for this contain the word HANDLE.) First you must say
a0d0e21e 30
19799a22 31 use IO::Handle;
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32
33after which you may use either
34
35 method HANDLE EXPR
36
5a964f20 37or more safely,
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38
39 HANDLE->method(EXPR)
40
14218588 41Each method returns the old value of the IO::Handle attribute.
a1ce9542 42The methods each take an optional EXPR, which, if supplied, specifies the
19799a22 43new value for the IO::Handle attribute in question. If not supplied,
14218588 44most methods do nothing to the current value--except for
a0d0e21e 45autoflush(), which will assume a 1 for you, just to be different.
a1ce9542 46
14218588 47Because loading in the IO::Handle class is an expensive operation, you should
19799a22 48learn how to use the regular built-in variables.
a0d0e21e 49
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50A few of these variables are considered "read-only". This means that if
51you try to assign to this variable, either directly or indirectly through
52a reference, you'll raise a run-time exception.
a0d0e21e 53
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54You should be very careful when modifying the default values of most
55special variables described in this document. In most cases you want
56to localize these variables before changing them, since if you don't,
57the change may affect other modules which rely on the default values
58of the special variables that you have changed. This is one of the
59correct 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
66But 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 $fh;
72
73since some other module, may want to read data from some file in the
74default "line mode", so if the code we have just presented has been
75executed, the global value of C<$/> is now changed for any other code
76running inside the same Perl interpreter.
77
78Usually when a variable is localized you want to make sure that this
79change affects the shortest scope possible. So unless you are already
80inside some short C<{}> block, you should create one yourself. For
81example:
82
83 my $content = '';
84 open my $fh, "foo" or die $!;
85 {
86 local $/;
87 $content = <$fh>;
88 }
89 close $fh;
90
91Here is an example of how your own code can go broken:
92
93 for (1..5){
94 nasty_break();
95 print "$_ ";
96 }
97 sub nasty_break {
98 $_ = 5;
99 # do something with $_
100 }
101
102You probably expect this code to print:
103
104 1 2 3 4 5
105
106but instead you get:
107
108 5 5 5 5 5
109
110Why? Because nasty_break() modifies C<$_> without localizing it
111first. The fix is to add local():
112
113 local $_ = 5;
114
115It's easy to notice the problem in such a short example, but in more
116complicated code you are looking for trouble if you don't localize
117changes to the special variables.
118
fb73857a 119The following list is ordered by scalar variables first, then the
87275199 120arrays, then the hashes.
fb73857a 121
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122=over 8
123
124=item $ARG
125
126=item $_
127
128The default input and pattern-searching space. The following pairs are
129equivalent:
130
19799a22 131 while (<>) {...} # equivalent only in while!
54310121 132 while (defined($_ = <>)) {...}
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133
134 /^Subject:/
135 $_ =~ /^Subject:/
136
137 tr/a-z/A-Z/
138 $_ =~ tr/a-z/A-Z/
139
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140 chomp
141 chomp($_)
a0d0e21e 142
54310121 143Here are the places where Perl will assume $_ even if you
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144don't use it:
145
146=over 3
147
148=item *
149
150Various unary functions, including functions like ord() and int(), as well
151as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
152STDIN.
153
154=item *
155
156Various list functions like print() and unlink().
157
158=item *
159
160The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
161without an C<=~> operator.
162
54310121 163=item *
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164
165The default iterator variable in a C<foreach> loop if no other
166variable is supplied.
167
54310121 168=item *
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169
170The implicit iterator variable in the grep() and map() functions.
171
54310121 172=item *
cb1a09d0 173
c47ff5f1 174The default place to put an input record when a C<< <FH> >>
cb1a09d0 175operation's result is tested by itself as the sole criterion of a C<while>
14218588 176test. 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
190Special package variables when using sort(), see L<perlfunc/sort>.
191Because of this specialness $a and $b don't need to be declared
192(using local(), use vars, or our()) even when using the strict
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193vars pragma. Don't lexicalize them with C<my $a> or C<my $b>
194if you want to be able to use them in the sort() comparison block
195or function.
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196
197=back
198
199=over 8
200
c47ff5f1 201=item $<I<digits>>
a0d0e21e 202
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203Contains the subpattern from the corresponding set of capturing
204parentheses from the last pattern match, not counting patterns
205matched in nested blocks that have been exited already. (Mnemonic:
206like \digits.) These variables are all read-only and dynamically
207scoped to the current BLOCK.
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208
209=item $MATCH
210
211=item $&
212
213The string matched by the last successful pattern match (not counting
214any matches hidden within a BLOCK or eval() enclosed by the current
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215BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
216and dynamically scoped to the current BLOCK.
a0d0e21e 217
19ddd453 218The use of this variable anywhere in a program imposes a considerable
667e1aea 219performance penalty on all regular expression matches. See L</BUGS>.
19ddd453 220
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221=item $PREMATCH
222
223=item $`
224
225The string preceding whatever was matched by the last successful
226pattern match (not counting any matches hidden within a BLOCK or eval
a8f8344d 227enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
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228string.) This variable is read-only.
229
19ddd453 230The use of this variable anywhere in a program imposes a considerable
667e1aea 231performance penalty on all regular expression matches. See L</BUGS>.
19ddd453 232
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233=item $POSTMATCH
234
235=item $'
236
237The string following whatever was matched by the last successful
238pattern match (not counting any matches hidden within a BLOCK or eval()
a8f8344d 239enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
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240string.) Example:
241
22d0716c 242 local $_ = 'abcdefghi';
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243 /def/;
244 print "$`:$&:$'\n"; # prints abc:def:ghi
245
19799a22 246This variable is read-only and dynamically scoped to the current BLOCK.
a0d0e21e 247
19ddd453 248The use of this variable anywhere in a program imposes a considerable
667e1aea 249performance penalty on all regular expression matches. See L</BUGS>.
19ddd453 250
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251=item $LAST_PAREN_MATCH
252
253=item $+
254
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255The text matched by the last bracket of the last successful search pattern.
256This is useful if you don't know which one of a set of alternative patterns
257matched. For example:
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258
259 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
260
261(Mnemonic: be positive and forward looking.)
19799a22 262This variable is read-only and dynamically scoped to the current BLOCK.
a0d0e21e 263
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264=item $^N
265
266The text matched by the used group most-recently closed (i.e. the group
267with the rightmost closing parenthesis) of the last successful search
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268pattern. (Mnemonic: the (possibly) Nested parenthesis that most
269recently closed.)
270
210b36aa 271This is primarily used inside C<(?{...})> blocks for examining text
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272recently matched. For example, to effectively capture text to a variable
273(in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
274
275 (?:(...)(?{ $var = $^N }))
276
277By setting and then using C<$var> in this way relieves you from having to
278worry about exactly which numbered set of parentheses they are.
279
280This variable is dynamically scoped to the current BLOCK.
281
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282=item @LAST_MATCH_END
283
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284=item @+
285
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286This array holds the offsets of the ends of the last successful
287submatches in the currently active dynamic scope. C<$+[0]> is
288the offset into the string of the end of the entire match. This
289is the same value as what the C<pos> function returns when called
290on the variable that was matched against. The I<n>th element
291of this array holds the offset of the I<n>th submatch, so
292C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset
293past where $2 ends, and so on. You can use C<$#+> to determine
294how many subgroups were in the last successful match. See the
295examples given for the C<@-> variable.
6cef1e77 296
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297=item $MULTILINE_MATCHING
298
299=item $*
300
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301Set to a non-zero integer value to do multi-line matching within a
302string, 0 (or undefined) to tell Perl that it can assume that strings
303contain a single line, for the purpose of optimizing pattern matches.
304Pattern matches on strings containing multiple newlines can produce
305confusing results when C<$*> is 0 or undefined. Default is undefined.
306(Mnemonic: * matches multiple things.) This variable influences the
307interpretation of only C<^> and C<$>. A literal newline can be searched
308for even when C<$* == 0>.
a0d0e21e 309
19799a22 310Use of C<$*> is deprecated in modern Perl, supplanted by
5a964f20 311the C</s> and C</m> modifiers on pattern matching.
a0d0e21e 312
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313Assigning a non-numerical value to C<$*> triggers a warning (and makes
314C<$*> act if C<$* == 0>), while assigning a numerical value to C<$*>
315makes that an implicit C<int> is applied on the value.
316
fcc7d916 317=item HANDLE->input_line_number(EXPR)
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318
319=item $INPUT_LINE_NUMBER
320
321=item $NR
322
323=item $.
324
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325Current line number for the last filehandle accessed.
326
327Each filehandle in Perl counts the number of lines that have been read
328from it. (Depending on the value of C<$/>, Perl's idea of what
329constitutes a line may not match yours.) When a line is read from a
330filehandle (via readline() or C<< <> >>), or when tell() or seek() is
331called on it, C<$.> becomes an alias to the line counter for that
332filehandle.
333
334You can adjust the counter by assigning to C<$.>, but this will not
335actually move the seek pointer. I<Localizing C<$.> will not localize
336the filehandle's line count>. Instead, it will localize perl's notion
337of which filehandle C<$.> is currently aliased to.
338
339C<$.> is reset when the filehandle is closed, but B<not> when an open
340filehandle is reopened without an intervening close(). For more
e48df184 341details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
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342an explicit close, line numbers increase across ARGV files (but see
343examples in L<perlfunc/eof>).
344
345You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
346line counter for a given filehandle without having to worry about
347which handle you last accessed.
348
349(Mnemonic: many programs use "." to mean the current line number.)
350
351=item IO::Handle->input_record_separator(EXPR)
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352
353=item $INPUT_RECORD_SEPARATOR
354
355=item $RS
356
357=item $/
358
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359The input record separator, newline by default. This
360influences Perl's idea of what a "line" is. Works like B<awk>'s RS
19799a22 361variable, including treating empty lines as a terminator if set to
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362the null string. (An empty line cannot contain any spaces
363or tabs.) You may set it to a multi-character string to match a
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364multi-character terminator, or to C<undef> to read through the end
365of file. Setting it to C<"\n\n"> means something slightly
366different than setting to C<"">, if the file contains consecutive
367empty lines. Setting to C<""> will treat two or more consecutive
368empty lines as a single empty line. Setting to C<"\n\n"> will
369blindly assume that the next input character belongs to the next
14218588 370paragraph, even if it's a newline. (Mnemonic: / delimits
19799a22 371line boundaries when quoting poetry.)
a0d0e21e 372
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373 local $/; # enable "slurp" mode
374 local $_ = <FH>; # whole file now here
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375 s/\n[ \t]+/ /g;
376
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377Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
378better for something. :-)
68dc0745 379
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380Setting C<$/> to a reference to an integer, scalar containing an integer, or
381scalar that's convertible to an integer will attempt to read records
5b2b9c68 382instead of lines, with the maximum record size being the referenced
19799a22 383integer. So this:
5b2b9c68 384
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385 local $/ = \32768; # or \"32768", or \$var_containing_32768
386 open my $fh, $myfile or die $!;
387 local $_ = <$fh>;
5b2b9c68 388
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389will read a record of no more than 32768 bytes from FILE. If you're
390not reading from a record-oriented file (or your OS doesn't have
391record-oriented files), then you'll likely get a full chunk of data
392with every read. If a record is larger than the record size you've
393set, you'll get the record back in pieces.
5b2b9c68 394
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395On VMS, record reads are done with the equivalent of C<sysread>,
396so it's best not to mix record and non-record reads on the same
397file. (This is unlikely to be a problem, because any file you'd
83763826 398want to read in record mode is probably unusable in line mode.)
14218588 399Non-VMS systems do normal I/O, so it's safe to mix record and
19799a22 400non-record reads of a file.
5b2b9c68 401
14218588 402See also L<perlport/"Newlines">. Also see C<$.>.
883faa13 403
fcc7d916 404=item HANDLE->autoflush(EXPR)
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405
406=item $OUTPUT_AUTOFLUSH
407
408=item $|
409
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410If set to nonzero, forces a flush right away and after every write
411or print on the currently selected output channel. Default is 0
14218588 412(regardless of whether the channel is really buffered by the
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413system or not; C<$|> tells you only whether you've asked Perl
414explicitly to flush after each write). STDOUT will
415typically be line buffered if output is to the terminal and block
416buffered otherwise. Setting this variable is useful primarily when
417you are outputting to a pipe or socket, such as when you are running
418a Perl program under B<rsh> and want to see the output as it's
419happening. This has no effect on input buffering. See L<perlfunc/getc>
420for that. (Mnemonic: when you want your pipes to be piping hot.)
a0d0e21e 421
46550894 422=item IO::Handle->output_field_separator EXPR
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423
424=item $OUTPUT_FIELD_SEPARATOR
425
426=item $OFS
427
428=item $,
429
430The output field separator for the print operator. Ordinarily the
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431print operator simply prints out its arguments without further
432adornment. To get behavior more like B<awk>, set this variable as
433you would set B<awk>'s OFS variable to specify what is printed
434between fields. (Mnemonic: what is printed when there is a "," in
435your print statement.)
a0d0e21e 436
46550894 437=item IO::Handle->output_record_separator EXPR
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438
439=item $OUTPUT_RECORD_SEPARATOR
440
441=item $ORS
442
443=item $\
444
445The output record separator for the print operator. Ordinarily the
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446print operator simply prints out its arguments as is, with no
447trailing newline or other end-of-record string added. To get
448behavior more like B<awk>, set this variable as you would set
449B<awk>'s ORS variable to specify what is printed at the end of the
450print. (Mnemonic: you set C<$\> instead of adding "\n" at the
451end of the print. Also, it's just like C<$/>, but it's what you
452get "back" from Perl.)
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453
454=item $LIST_SEPARATOR
455
456=item $"
457
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458This is like C<$,> except that it applies to array and slice values
459interpolated into a double-quoted string (or similar interpreted
460string). Default is a space. (Mnemonic: obvious, I think.)
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461
462=item $SUBSCRIPT_SEPARATOR
463
464=item $SUBSEP
465
466=item $;
467
54310121 468The subscript separator for multidimensional array emulation. If you
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469refer to a hash element as
470
471 $foo{$a,$b,$c}
472
473it really means
474
475 $foo{join($;, $a, $b, $c)}
476
477But don't put
478
479 @foo{$a,$b,$c} # a slice--note the @
480
481which means
482
483 ($foo{$a},$foo{$b},$foo{$c})
484
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485Default is "\034", the same as SUBSEP in B<awk>. If your
486keys contain binary data there might not be any safe value for C<$;>.
a0d0e21e 487(Mnemonic: comma (the syntactic subscript separator) is a
19799a22 488semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
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489taken for something more important.)
490
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491Consider using "real" multidimensional arrays as described
492in L<perllol>.
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493
494=item $OFMT
495
496=item $#
497
498The output format for printed numbers. This variable is a half-hearted
499attempt to emulate B<awk>'s OFMT variable. There are times, however,
14218588 500when B<awk> and Perl have differing notions of what counts as
19799a22 501numeric. The initial value is "%.I<n>g", where I<n> is the value
6e2995f4 502of the macro DBL_DIG from your system's F<float.h>. This is different from
19799a22 503B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#>
6e2995f4 504explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
a0d0e21e 505
19799a22 506Use of C<$#> is deprecated.
a0d0e21e 507
fcc7d916 508=item HANDLE->format_page_number(EXPR)
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509
510=item $FORMAT_PAGE_NUMBER
511
512=item $%
513
514The current page number of the currently selected output channel.
19799a22 515Used with formats.
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516(Mnemonic: % is page number in B<nroff>.)
517
fcc7d916 518=item HANDLE->format_lines_per_page(EXPR)
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519
520=item $FORMAT_LINES_PER_PAGE
521
522=item $=
523
524The current page length (printable lines) of the currently selected
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525output channel. Default is 60.
526Used with formats.
527(Mnemonic: = has horizontal lines.)
a0d0e21e 528
fcc7d916 529=item HANDLE->format_lines_left(EXPR)
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530
531=item $FORMAT_LINES_LEFT
532
533=item $-
534
535The number of lines left on the page of the currently selected output
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536channel.
537Used with formats.
538(Mnemonic: lines_on_page - lines_printed.)
a0d0e21e 539
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540=item @LAST_MATCH_START
541
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542=item @-
543
19799a22 544$-[0] is the offset of the start of the last successful match.
6cef1e77 545C<$-[>I<n>C<]> is the offset of the start of the substring matched by
8f580fb8 546I<n>-th subpattern, or undef if the subpattern did not match.
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547
548Thus after a match against $_, $& coincides with C<substr $_, $-[0],
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549$+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
550$+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
c47ff5f1 551C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last
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552matched subgroup in the last successful match. Contrast with
553C<$#+>, the number of subgroups in the regular expression. Compare
19799a22 554with C<@+>.
6cef1e77 555
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556This array holds the offsets of the beginnings of the last
557successful submatches in the currently active dynamic scope.
558C<$-[0]> is the offset into the string of the beginning of the
559entire match. The I<n>th element of this array holds the offset
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560of the I<n>th submatch, so C<$-[1]> is the offset where $1
561begins, C<$-[2]> the offset where $2 begins, and so on.
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562
563After a match against some variable $var:
564
565=over 5
566
4375e838 567=item C<$`> is the same as C<substr($var, 0, $-[0])>
4ba05bdc 568
4375e838 569=item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
4ba05bdc 570
4375e838 571=item C<$'> is the same as C<substr($var, $+[0])>
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572
573=item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
574
575=item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
576
4375e838 577=item C<$3> is the same as C<substr $var, $-[3], $+[3] - $-[3])>
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578
579=back
580
fcc7d916 581=item HANDLE->format_name(EXPR)
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582
583=item $FORMAT_NAME
584
585=item $~
586
587The name of the current report format for the currently selected output
14218588 588channel. Default is the name of the filehandle. (Mnemonic: brother to
19799a22 589C<$^>.)
a0d0e21e 590
fcc7d916 591=item HANDLE->format_top_name(EXPR)
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592
593=item $FORMAT_TOP_NAME
594
595=item $^
596
597The name of the current top-of-page format for the currently selected
14218588 598output channel. Default is the name of the filehandle with _TOP
a0d0e21e
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599appended. (Mnemonic: points to top of page.)
600
46550894 601=item IO::Handle->format_line_break_characters EXPR
a0d0e21e
LW
602
603=item $FORMAT_LINE_BREAK_CHARACTERS
604
605=item $:
606
607The current set of characters after which a string may be broken to
54310121 608fill continuation fields (starting with ^) in a format. Default is
a0d0e21e
LW
609S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
610poetry is a part of a line.)
611
46550894 612=item IO::Handle->format_formfeed EXPR
a0d0e21e
LW
613
614=item $FORMAT_FORMFEED
615
616=item $^L
617
14218588 618What formats output as a form feed. Default is \f.
a0d0e21e
LW
619
620=item $ACCUMULATOR
621
622=item $^A
623
624The current value of the write() accumulator for format() lines. A format
19799a22 625contains formline() calls that put their result into C<$^A>. After
a0d0e21e 626calling its format, write() prints out the contents of C<$^A> and empties.
14218588 627So you never really see the contents of C<$^A> unless you call
a0d0e21e
LW
628formline() yourself and then look at it. See L<perlform> and
629L<perlfunc/formline()>.
630
631=item $CHILD_ERROR
632
633=item $?
634
54310121 635The status returned by the last pipe close, backtick (C<``>) command,
19799a22
GS
636successful call to wait() or waitpid(), or from the system()
637operator. This is just the 16-bit status word returned by the
638wait() system call (or else is made up to look like it). Thus, the
c47ff5f1 639exit value of the subprocess is really (C<<< $? >> 8 >>>), and
19799a22
GS
640C<$? & 127> gives which signal, if any, the process died from, and
641C<$? & 128> reports whether there was a core dump. (Mnemonic:
642similar to B<sh> and B<ksh>.)
a0d0e21e 643
7b8d334a 644Additionally, if the C<h_errno> variable is supported in C, its value
14218588 645is returned via $? if any C<gethost*()> function fails.
7b8d334a 646
19799a22 647If you have installed a signal handler for C<SIGCHLD>, the
aa689395
PP
648value of C<$?> will usually be wrong outside that handler.
649
a8f8344d
PP
650Inside an C<END> subroutine C<$?> contains the value that is going to be
651given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
19799a22
GS
652change the exit status of your program. For example:
653
654 END {
655 $? = 1 if $? == 255; # die would make it 255
656 }
a8f8344d 657
aa689395 658Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
ff0cee69 659actual VMS exit status, instead of the default emulation of POSIX
9bc98430 660status; see L<perlvms/$?> for details.
f86702cc 661
55602bd2
IZ
662Also see L<Error Indicators>.
663
0a378802
JH
664=item ${^ENCODING}
665
666The encoding used to interpret native eight-bit encodings to Unicode,
667see L<encode>. An opaque C<Encode::XS> object.
668
a0d0e21e
LW
669=item $OS_ERROR
670
671=item $ERRNO
672
673=item $!
674
19799a22
GS
675If used numerically, yields the current value of the C C<errno>
676variable, with all the usual caveats. (This means that you shouldn't
677depend on the value of C<$!> to be anything in particular unless
678you've gotten a specific error return indicating a system error.)
679If used an a string, yields the corresponding system error string.
680You can assign a number to C<$!> to set I<errno> if, for instance,
681you want C<"$!"> to return the string for error I<n>, or you want
682to set the exit value for the die() operator. (Mnemonic: What just
683went bang?)
a0d0e21e 684
55602bd2
IZ
685Also see L<Error Indicators>.
686
5c055ba3
PP
687=item $EXTENDED_OS_ERROR
688
689=item $^E
690
22fae026
TM
691Error information specific to the current operating system. At
692the moment, this differs from C<$!> under only VMS, OS/2, and Win32
693(and for MacPerl). On all other platforms, C<$^E> is always just
694the same as C<$!>.
695
696Under VMS, C<$^E> provides the VMS status value from the last
697system error. This is more specific information about the last
698system error than that provided by C<$!>. This is particularly
d516a115 699important when C<$!> is set to B<EVMSERR>.
22fae026 700
1c1c7f20
GS
701Under OS/2, C<$^E> is set to the error code of the last call to
702OS/2 API either via CRT, or directly from perl.
22fae026
TM
703
704Under Win32, C<$^E> always returns the last error information
705reported by the Win32 call C<GetLastError()> which describes
706the last error from within the Win32 API. Most Win32-specific
19799a22 707code will report errors via C<$^E>. ANSI C and Unix-like calls
22fae026
TM
708set C<errno> and so most portable Perl code will report errors
709via C<$!>.
710
711Caveats mentioned in the description of C<$!> generally apply to
712C<$^E>, also. (Mnemonic: Extra error explanation.)
5c055ba3 713
55602bd2
IZ
714Also see L<Error Indicators>.
715
a0d0e21e
LW
716=item $EVAL_ERROR
717
718=item $@
719
4a280ebe
JG
720The Perl syntax error message from the last eval() operator.
721If $@ is the null string, the last eval() parsed and executed
722correctly (although the operations you invoked may have failed in the
723normal fashion). (Mnemonic: Where was the syntax error "at"?)
a0d0e21e 724
19799a22 725Warning messages are not collected in this variable. You can,
a8f8344d 726however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
54310121 727as described below.
748a9306 728
55602bd2
IZ
729Also see L<Error Indicators>.
730
a0d0e21e
LW
731=item $PROCESS_ID
732
733=item $PID
734
735=item $$
736
19799a22
GS
737The process number of the Perl running this script. You should
738consider this variable read-only, although it will be altered
739across fork() calls. (Mnemonic: same as shells.)
a0d0e21e
LW
740
741=item $REAL_USER_ID
742
743=item $UID
744
745=item $<
746
19799a22 747The real uid of this process. (Mnemonic: it's the uid you came I<from>,
a043a685
GW
748if you're running setuid.) You can change both the real uid and
749the effective uid at the same time by using POSIX::setuid().
a0d0e21e
LW
750
751=item $EFFECTIVE_USER_ID
752
753=item $EUID
754
755=item $>
756
757The effective uid of this process. Example:
758
759 $< = $>; # set real to effective uid
760 ($<,$>) = ($>,$<); # swap real and effective uid
761
a043a685
GW
762You can change both the effective uid and the real uid at the same
763time by using POSIX::setuid().
764
19799a22 765(Mnemonic: it's the uid you went I<to>, if you're running setuid.)
c47ff5f1 766C<< $< >> and C<< $> >> can be swapped only on machines
8cc95fdb 767supporting setreuid().
a0d0e21e
LW
768
769=item $REAL_GROUP_ID
770
771=item $GID
772
773=item $(
774
775The real gid of this process. If you are on a machine that supports
776membership in multiple groups simultaneously, gives a space separated
777list of groups you are in. The first number is the one returned by
778getgid(), and the subsequent ones by getgroups(), one of which may be
8cc95fdb
PP
779the same as the first number.
780
19799a22
GS
781However, a value assigned to C<$(> must be a single number used to
782set the real gid. So the value given by C<$(> should I<not> be assigned
783back to C<$(> without being forced numeric, such as by adding zero.
8cc95fdb 784
a043a685
GW
785You can change both the real gid and the effective gid at the same
786time by using POSIX::setgid().
787
19799a22
GS
788(Mnemonic: parentheses are used to I<group> things. The real gid is the
789group you I<left>, if you're running setgid.)
a0d0e21e
LW
790
791=item $EFFECTIVE_GROUP_ID
792
793=item $EGID
794
795=item $)
796
797The effective gid of this process. If you are on a machine that
798supports membership in multiple groups simultaneously, gives a space
799separated list of groups you are in. The first number is the one
800returned by getegid(), and the subsequent ones by getgroups(), one of
8cc95fdb
PP
801which may be the same as the first number.
802
19799a22 803Similarly, a value assigned to C<$)> must also be a space-separated
14218588 804list of numbers. The first number sets the effective gid, and
8cc95fdb
PP
805the rest (if any) are passed to setgroups(). To get the effect of an
806empty list for setgroups(), just repeat the new effective gid; that is,
807to force an effective gid of 5 and an effectively empty setgroups()
808list, say C< $) = "5 5" >.
809
a043a685
GW
810You can change both the effective gid and the real gid at the same
811time by using POSIX::setgid() (use only a single numeric argument).
812
19799a22
GS
813(Mnemonic: parentheses are used to I<group> things. The effective gid
814is the group that's I<right> for you, if you're running setgid.)
a0d0e21e 815
c47ff5f1 816C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
19799a22
GS
817machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
818and C<$)> can be swapped only on machines supporting setregid().
a0d0e21e
LW
819
820=item $PROGRAM_NAME
821
822=item $0
823
19799a22
GS
824Contains the name of the program being executed. On some operating
825systems assigning to C<$0> modifies the argument area that the B<ps>
826program sees. This is more useful as a way of indicating the current
827program state than it is for hiding the program you're running.
a0d0e21e
LW
828(Mnemonic: same as B<sh> and B<ksh>.)
829
4bc88a62
PS
830Note for BSD users: setting C<$0> does not completely remove "perl"
831from the ps(1) output. For example, setting C<$0> to C<"foobar"> will
832result in C<"perl: foobar (perl)">. This is an operating system
833feature.
834
a0d0e21e
LW
835=item $[
836
837The index of the first element in an array, and of the first character
19799a22
GS
838in a substring. Default is 0, but you could theoretically set it
839to 1 to make Perl behave more like B<awk> (or Fortran) when
840subscripting and when evaluating the index() and substr() functions.
841(Mnemonic: [ begins subscripts.)
a0d0e21e 842
19799a22
GS
843As of release 5 of Perl, assignment to C<$[> is treated as a compiler
844directive, and cannot influence the behavior of any other file.
845Its use is highly discouraged.
a0d0e21e 846
a0d0e21e
LW
847=item $]
848
54310121
PP
849The version + patchlevel / 1000 of the Perl interpreter. This variable
850can be used to determine whether the Perl interpreter executing a
851script is in the right range of versions. (Mnemonic: Is this version
852of perl in the right bracket?) Example:
a0d0e21e
LW
853
854 warn "No checksumming!\n" if $] < 3.019;
855
54310121 856See also the documentation of C<use VERSION> and C<require VERSION>
19799a22 857for a convenient way to fail if the running Perl interpreter is too old.
a0d0e21e 858
0c8d858b
MS
859The floating point representation can sometimes lead to inaccurate
860numeric comparisons. See C<$^V> for a more modern representation of
861the Perl version that allows accurate string comparisons.
16070b82 862
305aace0
NIS
863=item $COMPILING
864
865=item $^C
866
19799a22
GS
867The current value of the flag associated with the B<-c> switch.
868Mainly of use with B<-MO=...> to allow code to alter its behavior
869when being compiled, such as for example to AUTOLOAD at compile
870time rather than normal, deferred loading. See L<perlcc>. Setting
871C<$^C = 1> is similar to calling C<B::minus_c>.
305aace0 872
a0d0e21e
LW
873=item $DEBUGGING
874
875=item $^D
876
877The current value of the debugging flags. (Mnemonic: value of B<-D>
878switch.)
879
880=item $SYSTEM_FD_MAX
881
882=item $^F
883
884The maximum system file descriptor, ordinarily 2. System file
885descriptors are passed to exec()ed processes, while higher file
886descriptors are not. Also, during an open(), system file descriptors are
887preserved even if the open() fails. (Ordinary file descriptors are
19799a22 888closed before the open() is attempted.) The close-on-exec
a0d0e21e 889status of a file descriptor will be decided according to the value of
8d2a6795
GS
890C<$^F> when the corresponding file, pipe, or socket was opened, not the
891time of the exec().
a0d0e21e 892
6e2995f4
PP
893=item $^H
894
0462a1ab
GS
895WARNING: This variable is strictly for internal use only. Its availability,
896behavior, and contents are subject to change without notice.
897
898This variable contains compile-time hints for the Perl interpreter. At the
899end of compilation of a BLOCK the value of this variable is restored to the
900value when the interpreter started to compile the BLOCK.
901
902When perl begins to parse any block construct that provides a lexical scope
903(e.g., eval body, required file, subroutine body, loop body, or conditional
904block), the existing value of $^H is saved, but its value is left unchanged.
905When the compilation of the block is completed, it regains the saved value.
906Between the points where its value is saved and restored, code that
907executes within BEGIN blocks is free to change the value of $^H.
908
909This behavior provides the semantic of lexical scoping, and is used in,
910for instance, the C<use strict> pragma.
911
912The contents should be an integer; different bits of it are used for
913different pragmatic flags. Here's an example:
914
915 sub add_100 { $^H |= 0x100 }
916
917 sub foo {
918 BEGIN { add_100() }
919 bar->baz($boon);
920 }
921
922Consider what happens during execution of the BEGIN block. At this point
923the BEGIN block has already been compiled, but the body of foo() is still
924being compiled. The new value of $^H will therefore be visible only while
925the body of foo() is being compiled.
926
927Substitution of the above BEGIN block with:
928
929 BEGIN { require strict; strict->import('vars') }
930
931demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
932version of the same lexical pragma:
933
934 BEGIN { require strict; strict->import('vars') if $condition }
935
936=item %^H
937
938WARNING: This variable is strictly for internal use only. Its availability,
939behavior, and contents are subject to change without notice.
940
941The %^H hash provides the same scoping semantic as $^H. This makes it
942useful for implementation of lexically scoped pragmas.
6e2995f4 943
a0d0e21e
LW
944=item $INPLACE_EDIT
945
946=item $^I
947
948The current value of the inplace-edit extension. Use C<undef> to disable
949inplace editing. (Mnemonic: value of B<-i> switch.)
950
fb73857a
PP
951=item $^M
952
19799a22
GS
953By default, running out of memory is an untrappable, fatal error.
954However, if suitably built, Perl can use the contents of C<$^M>
955as an emergency memory pool after die()ing. Suppose that your Perl
956were compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc.
957Then
fb73857a 958
19799a22 959 $^M = 'a' x (1 << 16);
fb73857a 960
51ee6500 961would allocate a 64K buffer for use in an emergency. See the
19799a22
GS
962F<INSTALL> file in the Perl distribution for information on how to
963enable this option. To discourage casual use of this advanced
4ec0190b 964feature, there is no L<English|English> long name for this variable.
fb73857a 965
5c055ba3 966=item $OSNAME
6e2995f4 967
5c055ba3
PP
968=item $^O
969
970The name of the operating system under which this copy of Perl was
971built, as determined during the configuration process. The value
19799a22
GS
972is identical to C<$Config{'osname'}>. See also L<Config> and the
973B<-V> command-line switch documented in L<perlrun>.
5c055ba3 974
e2e27056
JH
975=item ${^OPEN}
976
977An internal variable used by PerlIO. A string in two parts, separated
978by a C<\0> byte, the first part is the input disciplines, the second
979part is the output disciplines.
980
a0d0e21e
LW
981=item $PERLDB
982
983=item $^P
984
19799a22
GS
985The internal variable for debugging support. The meanings of the
986various bits are subject to change, but currently indicate:
84902520
TB
987
988=over 6
989
990=item 0x01
991
992Debug subroutine enter/exit.
993
994=item 0x02
995
996Line-by-line debugging.
997
998=item 0x04
999
1000Switch off optimizations.
1001
1002=item 0x08
1003
1004Preserve more data for future interactive inspections.
1005
1006=item 0x10
1007
1008Keep info about source lines on which a subroutine is defined.
1009
1010=item 0x20
1011
1012Start with single-step on.
1013
83ee9e09
GS
1014=item 0x40
1015
1016Use subroutine address instead of name when reporting.
1017
1018=item 0x80
1019
1020Report C<goto &subroutine> as well.
1021
1022=item 0x100
1023
1024Provide informative "file" names for evals based on the place they were compiled.
1025
1026=item 0x200
1027
1028Provide informative names to anonymous subroutines based on the place they
1029were compiled.
1030
84902520
TB
1031=back
1032
19799a22
GS
1033Some bits may be relevant at compile-time only, some at
1034run-time only. This is a new mechanism and the details may change.
a0d0e21e 1035
66558a10
GS
1036=item $LAST_REGEXP_CODE_RESULT
1037
b9ac3b5b
GS
1038=item $^R
1039
19799a22
GS
1040The result of evaluation of the last successful C<(?{ code })>
1041regular expression assertion (see L<perlre>). May be written to.
b9ac3b5b 1042
66558a10
GS
1043=item $EXCEPTIONS_BEING_CAUGHT
1044
fb73857a
PP
1045=item $^S
1046
1047Current state of the interpreter. Undefined if parsing of the current
1048module/eval is not finished (may happen in $SIG{__DIE__} and
19799a22 1049$SIG{__WARN__} handlers). True if inside an eval(), otherwise false.
fb73857a 1050
a0d0e21e
LW
1051=item $BASETIME
1052
1053=item $^T
1054
19799a22 1055The time at which the program began running, in seconds since the
5f05dabc 1056epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
19799a22 1057and B<-C> filetests are based on this value.
a0d0e21e 1058
7c36658b
MS
1059=item ${^TAINT}
1060
c212f17f 1061Reflects if taint mode is on or off (i.e. if the program was run with
7c36658b
MS
1062B<-T> or not). True for on, false for off.
1063
44dcb63b 1064=item $PERL_VERSION
b459063d 1065
16070b82
GS
1066=item $^V
1067
1068The revision, version, and subversion of the Perl interpreter, represented
da2094fd 1069as a string composed of characters with those ordinals. Thus in Perl v5.6.0
44dcb63b
GS
1070it equals C<chr(5) . chr(6) . chr(0)> and will return true for
1071C<$^V eq v5.6.0>. Note that the characters in this string value can
1072potentially be in Unicode range.
16070b82
GS
1073
1074This can be used to determine whether the Perl interpreter executing a
1075script is in the right range of versions. (Mnemonic: use ^V for Version
44dcb63b 1076Control.) Example:
16070b82 1077
3fd4402b 1078 warn "No \"our\" declarations!\n" if $^V and $^V lt v5.6.0;
16070b82 1079
44dcb63b 1080See the documentation of C<use VERSION> and C<require VERSION>
16070b82
GS
1081for a convenient way to fail if the running Perl interpreter is too old.
1082
1083See also C<$]> for an older representation of the Perl version.
1084
a0d0e21e
LW
1085=item $WARNING
1086
1087=item $^W
1088
19799a22
GS
1089The current value of the warning switch, initially true if B<-w>
1090was used, false otherwise, but directly modifiable. (Mnemonic:
4438c4b7
JH
1091related to the B<-w> switch.) See also L<warnings>.
1092
6a818117 1093=item ${^WARNING_BITS}
4438c4b7
JH
1094
1095The current set of warning checks enabled by the C<use warnings> pragma.
1096See the documentation of C<warnings> for more details.
a0d0e21e 1097
46487f74
GS
1098=item ${^WIDE_SYSTEM_CALLS}
1099
1100Global flag that enables system calls made by Perl to use wide character
1101APIs native to the system, if available. This is currently only implemented
1102on the Windows platform.
1103
1104This can also be enabled from the command line using the C<-C> switch.
1105
1106The initial value is typically C<0> for compatibility with Perl versions
1107earlier than 5.6, but may be automatically set to C<1> by Perl if the system
1108provides a user-settable default (e.g., C<$ENV{LC_CTYPE}>).
1109
8058d7ab
GS
1110The C<bytes> pragma always overrides the effect of this flag in the current
1111lexical scope. See L<bytes>.
46487f74 1112
a0d0e21e
LW
1113=item $EXECUTABLE_NAME
1114
1115=item $^X
1116
1117The name that the Perl binary itself was executed as, from C's C<argv[0]>.
19799a22 1118This may not be a full pathname, nor even necessarily in your path.
a0d0e21e 1119
2d84a16a
DM
1120=item ARGV
1121
1122The special filehandle that iterates over command-line filenames in
1123C<@ARGV>. Usually written as the null filehandle in the angle operator
1124C<< <> >>. Note that currently C<ARGV> only has its magical effect
1125within the C<< <> >> operator; elsewhere it is just a plain filehandle
1126corresponding to the last file opened by C<< <> >>. In particular,
1127passing C<\*ARGV> as a parameter to a function that expects a filehandle
1128may not cause your function to automatically read the contents of all the
1129files in C<@ARGV>.
1130
a0d0e21e
LW
1131=item $ARGV
1132
c47ff5f1 1133contains the name of the current file when reading from <>.
a0d0e21e
LW
1134
1135=item @ARGV
1136
19799a22 1137The array @ARGV contains the command-line arguments intended for
14218588 1138the script. C<$#ARGV> is generally the number of arguments minus
19799a22
GS
1139one, because C<$ARGV[0]> is the first argument, I<not> the program's
1140command name itself. See C<$0> for the command name.
a0d0e21e 1141
9b0e6e7a
JP
1142=item @F
1143
1144The array @F contains the fields of each line read in when autosplit
1145mode is turned on. See L<perlrun> for the B<-a> switch. This array
1146is package-specific, and must be declared or given a full package name
1147if not in package main when running under C<strict 'vars'>.
1148
a0d0e21e
LW
1149=item @INC
1150
19799a22
GS
1151The array @INC contains the list of places that the C<do EXPR>,
1152C<require>, or C<use> constructs look for their library files. It
1153initially consists of the arguments to any B<-I> command-line
1154switches, followed by the default Perl library, probably
1155F</usr/local/lib/perl>, followed by ".", to represent the current
e48df184
RGS
1156directory. ("." will not be appended if taint checks are enabled, either by
1157C<-T> or by C<-t>.) If you need to modify this at runtime, you should use
19799a22
GS
1158the C<use lib> pragma to get the machine-dependent library properly
1159loaded also:
a0d0e21e 1160
cb1a09d0
AD
1161 use lib '/mypath/libdir/';
1162 use SomeMod;
303f2f76 1163
d54b56d5
RGS
1164You can also insert hooks into the file inclusion system by putting Perl
1165code directly into @INC. Those hooks may be subroutine references, array
1166references or blessed objects. See L<perlfunc/require> for details.
1167
fb73857a
PP
1168=item @_
1169
1170Within a subroutine the array @_ contains the parameters passed to that
19799a22 1171subroutine. See L<perlsub>.
fb73857a 1172
a0d0e21e
LW
1173=item %INC
1174
19799a22
GS
1175The hash %INC contains entries for each filename included via the
1176C<do>, C<require>, or C<use> operators. The key is the filename
1177you specified (with module names converted to pathnames), and the
14218588 1178value is the location of the file found. The C<require>
87275199 1179operator uses this hash to determine whether a particular file has
19799a22 1180already been included.
a0d0e21e 1181
89ccab8c
RGS
1182If the file was loaded via a hook (e.g. a subroutine reference, see
1183L<perlfunc/require> for a description of these hooks), this hook is
9ae8cd5b
RGS
1184by default inserted into %INC in place of a filename. Note, however,
1185that the hook may have set the %INC entry by itself to provide some more
1186specific info.
44f0be63 1187
b687b08b
TC
1188=item %ENV
1189
1190=item $ENV{expr}
a0d0e21e
LW
1191
1192The hash %ENV contains your current environment. Setting a
19799a22
GS
1193value in C<ENV> changes the environment for any child processes
1194you subsequently fork() off.
a0d0e21e 1195
b687b08b
TC
1196=item %SIG
1197
1198=item $SIG{expr}
a0d0e21e 1199
14218588 1200The hash %SIG contains signal handlers for signals. For example:
a0d0e21e
LW
1201
1202 sub handler { # 1st argument is signal name
fb73857a 1203 my($sig) = @_;
a0d0e21e
LW
1204 print "Caught a SIG$sig--shutting down\n";
1205 close(LOG);
1206 exit(0);
1207 }
1208
fb73857a
PP
1209 $SIG{'INT'} = \&handler;
1210 $SIG{'QUIT'} = \&handler;
a0d0e21e 1211 ...
19799a22 1212 $SIG{'INT'} = 'DEFAULT'; # restore default action
a0d0e21e
LW
1213 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
1214
f648820c
GS
1215Using a value of C<'IGNORE'> usually has the effect of ignoring the
1216signal, except for the C<CHLD> signal. See L<perlipc> for more about
1217this special case.
1218
19799a22 1219Here are some other examples:
a0d0e21e 1220
fb73857a 1221 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
a0d0e21e 1222 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
19799a22 1223 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
a0d0e21e
LW
1224 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
1225
19799a22
GS
1226Be sure not to use a bareword as the name of a signal handler,
1227lest you inadvertently call it.
748a9306 1228
44a8e56a
PP
1229If your system has the sigaction() function then signal handlers are
1230installed using it. This means you get reliable signal handling. If
1231your system has the SA_RESTART flag it is used when signals handlers are
19799a22 1232installed. This means that system calls for which restarting is supported
44a8e56a
PP
1233continue rather than returning when a signal arrives. If you want your
1234system calls to be interrupted by signal delivery then do something like
1235this:
1236
1237 use POSIX ':signal_h';
1238
1239 my $alarm = 0;
1240 sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
1241 or die "Error setting SIGALRM handler: $!\n";
1242
1243See L<POSIX>.
1244
748a9306 1245Certain internal hooks can be also set using the %SIG hash. The
a8f8344d 1246routine indicated by C<$SIG{__WARN__}> is called when a warning message is
748a9306
LW
1247about to be printed. The warning message is passed as the first
1248argument. The presence of a __WARN__ hook causes the ordinary printing
1249of warnings to STDERR to be suppressed. You can use this to save warnings
1250in a variable, or turn warnings into fatal errors, like this:
1251
1252 local $SIG{__WARN__} = sub { die $_[0] };
1253 eval $proggie;
1254
a8f8344d 1255The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
748a9306
LW
1256is about to be thrown. The error message is passed as the first
1257argument. When a __DIE__ hook routine returns, the exception
1258processing continues as it would have in the absence of the hook,
cb1a09d0 1259unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
774d564b 1260The C<__DIE__> handler is explicitly disabled during the call, so that you
fb73857a
PP
1261can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
1262
19799a22
GS
1263Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
1264even inside an eval(). Do not use this to rewrite a pending exception
1265in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die().
1266This strange action at a distance may be fixed in a future release
1267so that C<$SIG{__DIE__}> is only called if your program is about
1268to exit, as was the original intent. Any other use is deprecated.
1269
1270C<__DIE__>/C<__WARN__> handlers are very special in one respect:
1271they may be called to report (probable) errors found by the parser.
1272In such a case the parser may be in inconsistent state, so any
1273attempt to evaluate Perl code from such a handler will probably
1274result in a segfault. This means that warnings or errors that
1275result from parsing Perl should be used with extreme caution, like
1276this:
fb73857a
PP
1277
1278 require Carp if defined $^S;
1279 Carp::confess("Something wrong") if defined &Carp::confess;
1280 die "Something wrong, but could not load Carp to give backtrace...
1281 To see backtrace try starting Perl with -MCarp switch";
1282
1283Here the first line will load Carp I<unless> it is the parser who
1284called the handler. The second line will print backtrace and die if
1285Carp was available. The third line will be executed only if Carp was
1286not available.
1287
19799a22 1288See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
4438c4b7 1289L<warnings> for additional information.
68dc0745 1290
a0d0e21e 1291=back
55602bd2
IZ
1292
1293=head2 Error Indicators
1294
19799a22
GS
1295The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1296about different types of error conditions that may appear during
1297execution of a Perl program. The variables are shown ordered by
1298the "distance" between the subsystem which reported the error and
1299the Perl process. They correspond to errors detected by the Perl
1300interpreter, C library, operating system, or an external program,
1301respectively.
55602bd2
IZ
1302
1303To illustrate the differences between these variables, consider the
19799a22 1304following Perl expression, which uses a single-quoted string:
55602bd2 1305
19799a22 1306 eval q{
22d0716c
SB
1307 open my $pipe, "/cdrom/install |" or die $!;
1308 my @res = <$pipe>;
1309 close $pipe or die "bad pipe: $?, $!";
19799a22 1310 };
55602bd2
IZ
1311
1312After execution of this statement all 4 variables may have been set.
1313
19799a22
GS
1314C<$@> is set if the string to be C<eval>-ed did not compile (this
1315may happen if C<open> or C<close> were imported with bad prototypes),
1316or if Perl code executed during evaluation die()d . In these cases
1317the value of $@ is the compile error, or the argument to C<die>
1318(which will interpolate C<$!> and C<$?>!). (See also L<Fatal>,
1319though.)
1320
c47ff5f1 1321When the eval() expression above is executed, open(), C<< <PIPE> >>,
19799a22
GS
1322and C<close> are translated to calls in the C run-time library and
1323thence to the operating system kernel. C<$!> is set to the C library's
1324C<errno> if one of these calls fails.
1325
1326Under a few operating systems, C<$^E> may contain a more verbose
1327error indicator, such as in this case, "CDROM tray not closed."
14218588 1328Systems that do not support extended error messages leave C<$^E>
19799a22
GS
1329the same as C<$!>.
1330
1331Finally, C<$?> may be set to non-0 value if the external program
1332F</cdrom/install> fails. The upper eight bits reflect specific
1333error conditions encountered by the program (the program's exit()
1334value). The lower eight bits reflect mode of failure, like signal
1335death and core dump information See wait(2) for details. In
1336contrast to C<$!> and C<$^E>, which are set only if error condition
1337is detected, the variable C<$?> is set on each C<wait> or pipe
1338C<close>, overwriting the old value. This is more like C<$@>, which
1339on every eval() is always set on failure and cleared on success.
2b92dfce 1340
19799a22
GS
1341For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1342and C<$?>.
2b92dfce
GS
1343
1344=head2 Technical Note on the Syntax of Variable Names
1345
19799a22
GS
1346Variable names in Perl can have several formats. Usually, they
1347must begin with a letter or underscore, in which case they can be
1348arbitrarily long (up to an internal limit of 251 characters) and
1349may contain letters, digits, underscores, or the special sequence
1350C<::> or C<'>. In this case, the part before the last C<::> or
1351C<'> is taken to be a I<package qualifier>; see L<perlmod>.
2b92dfce
GS
1352
1353Perl variable names may also be a sequence of digits or a single
1354punctuation or control character. These names are all reserved for
19799a22
GS
1355special uses by Perl; for example, the all-digits names are used
1356to hold data captured by backreferences after a regular expression
1357match. Perl has a special syntax for the single-control-character
1358names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1359character. For example, the notation C<$^W> (dollar-sign caret
1360C<W>) is the scalar variable whose name is the single character
1361control-C<W>. This is better than typing a literal control-C<W>
1362into your program.
2b92dfce 1363
87275199 1364Finally, new in Perl 5.6, Perl variable names may be alphanumeric
19799a22
GS
1365strings that begin with control characters (or better yet, a caret).
1366These variables must be written in the form C<${^Foo}>; the braces
1367are not optional. C<${^Foo}> denotes the scalar variable whose
1368name is a control-C<F> followed by two C<o>'s. These variables are
1369reserved for future special uses by Perl, except for the ones that
1370begin with C<^_> (control-underscore or caret-underscore). No
1371control-character name that begins with C<^_> will acquire a special
1372meaning in any future version of Perl; such names may therefore be
1373used safely in programs. C<$^_> itself, however, I<is> reserved.
1374
1375Perl identifiers that begin with digits, control characters, or
2b92dfce
GS
1376punctuation characters are exempt from the effects of the C<package>
1377declaration and are always forced to be in package C<main>. A few
1378other names are also exempt:
1379
1380 ENV STDIN
1381 INC STDOUT
1382 ARGV STDERR
1383 ARGVOUT
1384 SIG
1385
1386In particular, the new special C<${^_XYZ}> variables are always taken
19799a22 1387to be in package C<main>, regardless of any C<package> declarations
2b92dfce
GS
1388presently in scope.
1389
19799a22
GS
1390=head1 BUGS
1391
1392Due to an unfortunate accident of Perl's implementation, C<use
1393English> imposes a considerable performance penalty on all regular
1394expression matches in a program, regardless of whether they occur
1395in the scope of C<use English>. For that reason, saying C<use
1396English> in libraries is strongly discouraged. See the
1397Devel::SawAmpersand module documentation from CPAN
a93751fa 1398(http://www.cpan.org/modules/by-module/Devel/)
19799a22 1399for more information.
2b92dfce 1400
19799a22
GS
1401Having to even think about the C<$^S> variable in your exception
1402handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1403invites grievous and difficult to track down errors. Avoid it
1404and use an C<END{}> or CORE::GLOBAL::die override instead.