extra code in pp_concat, Take 2
[p5sagit/p5-mst-13.2.git] / pod / perlvar.pod
CommitLineData
a0d0e21e 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
14218588 10punctuation names have reasonable mnemonics, or analogs in the
11shells. Nevertheless, if you wish to use long variable names,
12you need only say
a0d0e21e 13
14 use English;
15
a1ce9542 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
a1ce9542 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;
a0d0e21e 32
33after which you may use either
34
35 method HANDLE EXPR
36
5a964f20 37or more safely,
a0d0e21e 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
748a9306 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
22d0716c 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
a0d0e21e 122=over 8
123
124=item $ARG
125
126=item $_
127
128The default input and pattern-searching space. The following pairs are
129equivalent:
130
19799a22 131 while (<>) {...} # equivalent only in while!
54310121 132 while (defined($_ = <>)) {...}
a0d0e21e 133
134 /^Subject:/
135 $_ =~ /^Subject:/
136
137 tr/a-z/A-Z/
138 $_ =~ tr/a-z/A-Z/
139
19799a22 140 chomp
141 chomp($_)
a0d0e21e 142
54310121 143Here are the places where Perl will assume $_ even if you
cb1a09d0 144don't use it:
145
146=over 3
147
148=item *
149
150Various unary functions, including functions like ord() and int(), as well
151as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
152STDIN.
153
154=item *
155
156Various list functions like print() and unlink().
157
158=item *
159
160The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
161without an C<=~> operator.
162
54310121 163=item *
cb1a09d0 164
165The default iterator variable in a C<foreach> loop if no other
166variable is supplied.
167
54310121 168=item *
cb1a09d0 169
170The implicit iterator variable in the grep() and map() functions.
171
54310121 172=item *
cb1a09d0 173
c47ff5f1 174The default place to put an input record when a C<< <FH> >>
cb1a09d0 175operation's result is tested by itself as the sole criterion of a C<while>
14218588 176test. Outside a C<while> test, this will not happen.
cb1a09d0 177
178=back
179
59f00321 180As C<$_> is a global variable, this may lead in some cases to unwanted
181side-effects. As of perl 5.9.1, you can now use a lexical version of
182C<$_> by declaring it in a file or in a block with C<my>. Moreover,
183declaring C<our $> restores the global C<$_> in the current scope.
184
a0d0e21e 185(Mnemonic: underline is understood in certain operations.)
186
6e2995f4 187=back
188
189=over 8
190
a1db74c9 191=item $a
192
193=item $b
194
195Special package variables when using sort(), see L<perlfunc/sort>.
196Because of this specialness $a and $b don't need to be declared
f83912f2 197(using use vars, or our()) even when using the C<strict 'vars'> pragma.
198Don't lexicalize them with C<my $a> or C<my $b> if you want to be
199able to use them in the sort() comparison block or function.
a1db74c9 200
201=back
202
203=over 8
204
c47ff5f1 205=item $<I<digits>>
a0d0e21e 206
19799a22 207Contains the subpattern from the corresponding set of capturing
208parentheses from the last pattern match, not counting patterns
209matched in nested blocks that have been exited already. (Mnemonic:
210like \digits.) These variables are all read-only and dynamically
211scoped to the current BLOCK.
a0d0e21e 212
213=item $MATCH
214
215=item $&
216
217The string matched by the last successful pattern match (not counting
218any matches hidden within a BLOCK or eval() enclosed by the current
19799a22 219BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
220and dynamically scoped to the current BLOCK.
a0d0e21e 221
19ddd453 222The use of this variable anywhere in a program imposes a considerable
667e1aea 223performance penalty on all regular expression matches. See L</BUGS>.
19ddd453 224
a0d0e21e 225=item $PREMATCH
226
227=item $`
228
229The string preceding whatever was matched by the last successful
230pattern match (not counting any matches hidden within a BLOCK or eval
a8f8344d 231enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
a0d0e21e 232string.) This variable is read-only.
233
19ddd453 234The use of this variable anywhere in a program imposes a considerable
667e1aea 235performance penalty on all regular expression matches. See L</BUGS>.
19ddd453 236
a0d0e21e 237=item $POSTMATCH
238
239=item $'
240
241The string following whatever was matched by the last successful
242pattern match (not counting any matches hidden within a BLOCK or eval()
a8f8344d 243enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
a0d0e21e 244string.) Example:
245
22d0716c 246 local $_ = 'abcdefghi';
a0d0e21e 247 /def/;
248 print "$`:$&:$'\n"; # prints abc:def:ghi
249
19799a22 250This variable is read-only and dynamically scoped to the current BLOCK.
a0d0e21e 251
19ddd453 252The use of this variable anywhere in a program imposes a considerable
667e1aea 253performance penalty on all regular expression matches. See L</BUGS>.
19ddd453 254
a0d0e21e 255=item $LAST_PAREN_MATCH
256
257=item $+
258
a01268b5 259The text matched by the last bracket of the last successful search pattern.
260This is useful if you don't know which one of a set of alternative patterns
261matched. For example:
a0d0e21e 262
263 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
264
265(Mnemonic: be positive and forward looking.)
19799a22 266This variable is read-only and dynamically scoped to the current BLOCK.
a0d0e21e 267
a01268b5 268=item $^N
269
270The text matched by the used group most-recently closed (i.e. the group
271with the rightmost closing parenthesis) of the last successful search
ad83b128 272pattern. (Mnemonic: the (possibly) Nested parenthesis that most
273recently closed.)
274
210b36aa 275This is primarily used inside C<(?{...})> blocks for examining text
a01268b5 276recently matched. For example, to effectively capture text to a variable
277(in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
278
279 (?:(...)(?{ $var = $^N }))
280
281By setting and then using C<$var> in this way relieves you from having to
282worry about exactly which numbered set of parentheses they are.
283
284This variable is dynamically scoped to the current BLOCK.
285
fe307981 286=item @LAST_MATCH_END
287
6cef1e77 288=item @+
289
4ba05bdc 290This array holds the offsets of the ends of the last successful
291submatches in the currently active dynamic scope. C<$+[0]> is
292the offset into the string of the end of the entire match. This
293is the same value as what the C<pos> function returns when called
294on the variable that was matched against. The I<n>th element
295of this array holds the offset of the I<n>th submatch, so
296C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset
297past where $2 ends, and so on. You can use C<$#+> to determine
298how many subgroups were in the last successful match. See the
299examples given for the C<@-> variable.
6cef1e77 300
fcc7d916 301=item HANDLE->input_line_number(EXPR)
a0d0e21e 302
303=item $INPUT_LINE_NUMBER
304
305=item $NR
306
307=item $.
308
fcc7d916 309Current line number for the last filehandle accessed.
310
311Each filehandle in Perl counts the number of lines that have been read
312from it. (Depending on the value of C<$/>, Perl's idea of what
313constitutes a line may not match yours.) When a line is read from a
314filehandle (via readline() or C<< <> >>), or when tell() or seek() is
315called on it, C<$.> becomes an alias to the line counter for that
316filehandle.
317
318You can adjust the counter by assigning to C<$.>, but this will not
319actually move the seek pointer. I<Localizing C<$.> will not localize
320the filehandle's line count>. Instead, it will localize perl's notion
321of which filehandle C<$.> is currently aliased to.
322
323C<$.> is reset when the filehandle is closed, but B<not> when an open
324filehandle is reopened without an intervening close(). For more
e48df184 325details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
fcc7d916 326an explicit close, line numbers increase across ARGV files (but see
327examples in L<perlfunc/eof>).
328
329You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
330line counter for a given filehandle without having to worry about
331which handle you last accessed.
332
333(Mnemonic: many programs use "." to mean the current line number.)
334
335=item IO::Handle->input_record_separator(EXPR)
a0d0e21e 336
337=item $INPUT_RECORD_SEPARATOR
338
339=item $RS
340
341=item $/
342
14218588 343The input record separator, newline by default. This
344influences Perl's idea of what a "line" is. Works like B<awk>'s RS
19799a22 345variable, including treating empty lines as a terminator if set to
14218588 346the null string. (An empty line cannot contain any spaces
347or tabs.) You may set it to a multi-character string to match a
19799a22 348multi-character terminator, or to C<undef> to read through the end
349of file. Setting it to C<"\n\n"> means something slightly
350different than setting to C<"">, if the file contains consecutive
351empty lines. Setting to C<""> will treat two or more consecutive
352empty lines as a single empty line. Setting to C<"\n\n"> will
353blindly assume that the next input character belongs to the next
14218588 354paragraph, even if it's a newline. (Mnemonic: / delimits
19799a22 355line boundaries when quoting poetry.)
a0d0e21e 356
22d0716c 357 local $/; # enable "slurp" mode
358 local $_ = <FH>; # whole file now here
a0d0e21e 359 s/\n[ \t]+/ /g;
360
19799a22 361Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
362better for something. :-)
68dc0745 363
19799a22 364Setting C<$/> to a reference to an integer, scalar containing an integer, or
365scalar that's convertible to an integer will attempt to read records
5b2b9c68 366instead of lines, with the maximum record size being the referenced
19799a22 367integer. So this:
5b2b9c68 368
22d0716c 369 local $/ = \32768; # or \"32768", or \$var_containing_32768
370 open my $fh, $myfile or die $!;
371 local $_ = <$fh>;
5b2b9c68 372
19799a22 373will read a record of no more than 32768 bytes from FILE. If you're
374not reading from a record-oriented file (or your OS doesn't have
375record-oriented files), then you'll likely get a full chunk of data
376with every read. If a record is larger than the record size you've
377set, you'll get the record back in pieces.
5b2b9c68 378
19799a22 379On VMS, record reads are done with the equivalent of C<sysread>,
380so it's best not to mix record and non-record reads on the same
381file. (This is unlikely to be a problem, because any file you'd
83763826 382want to read in record mode is probably unusable in line mode.)
14218588 383Non-VMS systems do normal I/O, so it's safe to mix record and
19799a22 384non-record reads of a file.
5b2b9c68 385
14218588 386See also L<perlport/"Newlines">. Also see C<$.>.
883faa13 387
fcc7d916 388=item HANDLE->autoflush(EXPR)
a0d0e21e 389
390=item $OUTPUT_AUTOFLUSH
391
392=item $|
393
19799a22 394If set to nonzero, forces a flush right away and after every write
395or print on the currently selected output channel. Default is 0
14218588 396(regardless of whether the channel is really buffered by the
19799a22 397system or not; C<$|> tells you only whether you've asked Perl
398explicitly to flush after each write). STDOUT will
399typically be line buffered if output is to the terminal and block
400buffered otherwise. Setting this variable is useful primarily when
401you are outputting to a pipe or socket, such as when you are running
402a Perl program under B<rsh> and want to see the output as it's
403happening. This has no effect on input buffering. See L<perlfunc/getc>
404for that. (Mnemonic: when you want your pipes to be piping hot.)
a0d0e21e 405
46550894 406=item IO::Handle->output_field_separator EXPR
a0d0e21e 407
408=item $OUTPUT_FIELD_SEPARATOR
409
410=item $OFS
411
412=item $,
413
414The output field separator for the print operator. Ordinarily the
19799a22 415print operator simply prints out its arguments without further
416adornment. To get behavior more like B<awk>, set this variable as
417you would set B<awk>'s OFS variable to specify what is printed
418between fields. (Mnemonic: what is printed when there is a "," in
419your print statement.)
a0d0e21e 420
46550894 421=item IO::Handle->output_record_separator EXPR
a0d0e21e 422
423=item $OUTPUT_RECORD_SEPARATOR
424
425=item $ORS
426
427=item $\
428
429The output record separator for the print operator. Ordinarily the
19799a22 430print operator simply prints out its arguments as is, with no
431trailing newline or other end-of-record string added. To get
432behavior more like B<awk>, set this variable as you would set
433B<awk>'s ORS variable to specify what is printed at the end of the
434print. (Mnemonic: you set C<$\> instead of adding "\n" at the
435end of the print. Also, it's just like C<$/>, but it's what you
436get "back" from Perl.)
a0d0e21e 437
438=item $LIST_SEPARATOR
439
440=item $"
441
19799a22 442This is like C<$,> except that it applies to array and slice values
443interpolated into a double-quoted string (or similar interpreted
444string). Default is a space. (Mnemonic: obvious, I think.)
a0d0e21e 445
446=item $SUBSCRIPT_SEPARATOR
447
448=item $SUBSEP
449
450=item $;
451
54310121 452The subscript separator for multidimensional array emulation. If you
a0d0e21e 453refer to a hash element as
454
455 $foo{$a,$b,$c}
456
457it really means
458
459 $foo{join($;, $a, $b, $c)}
460
461But don't put
462
463 @foo{$a,$b,$c} # a slice--note the @
464
465which means
466
467 ($foo{$a},$foo{$b},$foo{$c})
468
19799a22 469Default is "\034", the same as SUBSEP in B<awk>. If your
470keys contain binary data there might not be any safe value for C<$;>.
a0d0e21e 471(Mnemonic: comma (the syntactic subscript separator) is a
19799a22 472semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
a0d0e21e 473taken for something more important.)
474
19799a22 475Consider using "real" multidimensional arrays as described
476in L<perllol>.
a0d0e21e 477
a0d0e21e 478=item $#
479
480The output format for printed numbers. This variable is a half-hearted
481attempt to emulate B<awk>'s OFMT variable. There are times, however,
14218588 482when B<awk> and Perl have differing notions of what counts as
19799a22 483numeric. The initial value is "%.I<n>g", where I<n> is the value
6e2995f4 484of the macro DBL_DIG from your system's F<float.h>. This is different from
19799a22 485B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#>
6e2995f4 486explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
a0d0e21e 487
19799a22 488Use of C<$#> is deprecated.
a0d0e21e 489
fcc7d916 490=item HANDLE->format_page_number(EXPR)
a0d0e21e 491
492=item $FORMAT_PAGE_NUMBER
493
494=item $%
495
496The current page number of the currently selected output channel.
19799a22 497Used with formats.
a0d0e21e 498(Mnemonic: % is page number in B<nroff>.)
499
fcc7d916 500=item HANDLE->format_lines_per_page(EXPR)
a0d0e21e 501
502=item $FORMAT_LINES_PER_PAGE
503
504=item $=
505
506The current page length (printable lines) of the currently selected
19799a22 507output channel. Default is 60.
508Used with formats.
509(Mnemonic: = has horizontal lines.)
a0d0e21e 510
fcc7d916 511=item HANDLE->format_lines_left(EXPR)
a0d0e21e 512
513=item $FORMAT_LINES_LEFT
514
515=item $-
516
517The number of lines left on the page of the currently selected output
19799a22 518channel.
519Used with formats.
520(Mnemonic: lines_on_page - lines_printed.)
a0d0e21e 521
fe307981 522=item @LAST_MATCH_START
523
6cef1e77 524=item @-
525
19799a22 526$-[0] is the offset of the start of the last successful match.
6cef1e77 527C<$-[>I<n>C<]> is the offset of the start of the substring matched by
8f580fb8 528I<n>-th subpattern, or undef if the subpattern did not match.
6cef1e77 529
530Thus after a match against $_, $& coincides with C<substr $_, $-[0],
8f580fb8 531$+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
532$+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
c47ff5f1 533C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last
14218588 534matched subgroup in the last successful match. Contrast with
535C<$#+>, the number of subgroups in the regular expression. Compare
19799a22 536with C<@+>.
6cef1e77 537
4ba05bdc 538This array holds the offsets of the beginnings of the last
539successful submatches in the currently active dynamic scope.
540C<$-[0]> is the offset into the string of the beginning of the
541entire match. The I<n>th element of this array holds the offset
0926d669 542of the I<n>th submatch, so C<$-[1]> is the offset where $1
543begins, C<$-[2]> the offset where $2 begins, and so on.
4ba05bdc 544
545After a match against some variable $var:
546
547=over 5
548
4375e838 549=item C<$`> is the same as C<substr($var, 0, $-[0])>
4ba05bdc 550
4375e838 551=item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
4ba05bdc 552
4375e838 553=item C<$'> is the same as C<substr($var, $+[0])>
4ba05bdc 554
555=item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
556
557=item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
558
4375e838 559=item C<$3> is the same as C<substr $var, $-[3], $+[3] - $-[3])>
4ba05bdc 560
561=back
562
fcc7d916 563=item HANDLE->format_name(EXPR)
a0d0e21e 564
565=item $FORMAT_NAME
566
567=item $~
568
569The name of the current report format for the currently selected output
14218588 570channel. Default is the name of the filehandle. (Mnemonic: brother to
19799a22 571C<$^>.)
a0d0e21e 572
fcc7d916 573=item HANDLE->format_top_name(EXPR)
a0d0e21e 574
575=item $FORMAT_TOP_NAME
576
577=item $^
578
579The name of the current top-of-page format for the currently selected
14218588 580output channel. Default is the name of the filehandle with _TOP
a0d0e21e 581appended. (Mnemonic: points to top of page.)
582
46550894 583=item IO::Handle->format_line_break_characters EXPR
a0d0e21e 584
585=item $FORMAT_LINE_BREAK_CHARACTERS
586
587=item $:
588
589The current set of characters after which a string may be broken to
54310121 590fill continuation fields (starting with ^) in a format. Default is
a0d0e21e 591S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
592poetry is a part of a line.)
593
46550894 594=item IO::Handle->format_formfeed EXPR
a0d0e21e 595
596=item $FORMAT_FORMFEED
597
598=item $^L
599
14218588 600What formats output as a form feed. Default is \f.
a0d0e21e 601
602=item $ACCUMULATOR
603
604=item $^A
605
606The current value of the write() accumulator for format() lines. A format
19799a22 607contains formline() calls that put their result into C<$^A>. After
a0d0e21e 608calling its format, write() prints out the contents of C<$^A> and empties.
14218588 609So you never really see the contents of C<$^A> unless you call
a0d0e21e 610formline() yourself and then look at it. See L<perlform> and
611L<perlfunc/formline()>.
612
613=item $CHILD_ERROR
614
615=item $?
616
54310121 617The status returned by the last pipe close, backtick (C<``>) command,
19799a22 618successful call to wait() or waitpid(), or from the system()
619operator. This is just the 16-bit status word returned by the
620wait() system call (or else is made up to look like it). Thus, the
c47ff5f1 621exit value of the subprocess is really (C<<< $? >> 8 >>>), and
19799a22 622C<$? & 127> gives which signal, if any, the process died from, and
623C<$? & 128> reports whether there was a core dump. (Mnemonic:
624similar to B<sh> and B<ksh>.)
a0d0e21e 625
7b8d334a 626Additionally, if the C<h_errno> variable is supported in C, its value
14218588 627is returned via $? if any C<gethost*()> function fails.
7b8d334a 628
19799a22 629If you have installed a signal handler for C<SIGCHLD>, the
aa689395 630value of C<$?> will usually be wrong outside that handler.
631
a8f8344d 632Inside an C<END> subroutine C<$?> contains the value that is going to be
633given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
19799a22 634change the exit status of your program. For example:
635
636 END {
637 $? = 1 if $? == 255; # die would make it 255
638 }
a8f8344d 639
aa689395 640Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
ff0cee69 641actual VMS exit status, instead of the default emulation of POSIX
9bc98430 642status; see L<perlvms/$?> for details.
f86702cc 643
55602bd2 644Also see L<Error Indicators>.
645
0a378802 646=item ${^ENCODING}
647
740bd165 648The I<object reference> to the Encode object that is used to convert
649the source code to Unicode. Thanks to this variable your perl script
650does not have to be written in UTF-8. Default is I<undef>. The direct
651manipulation of this variable is highly discouraged. See L<encoding>
048c20cb 652for more details.
0a378802 653
a0d0e21e 654=item $OS_ERROR
655
656=item $ERRNO
657
658=item $!
659
19799a22 660If used numerically, yields the current value of the C C<errno>
6ab308ee 661variable, or in other words, if a system or library call fails, it
662sets this variable. This means that the value of C<$!> is meaningful
663only I<immediately> after a B<failure>:
664
665 if (open(FH, $filename)) {
666 # Here $! is meaningless.
667 ...
668 } else {
669 # ONLY here is $! meaningful.
670 ...
671 # Already here $! might be meaningless.
672 }
673 # Since here we might have either success or failure,
674 # here $! is meaningless.
675
676In the above I<meaningless> stands for anything: zero, non-zero,
677C<undef>. A successful system or library call does B<not> set
678the variable to zero.
679
271df126 680If used as a string, yields the corresponding system error string.
19799a22 681You can assign a number to C<$!> to set I<errno> if, for instance,
682you want C<"$!"> to return the string for error I<n>, or you want
683to set the exit value for the die() operator. (Mnemonic: What just
684went bang?)
a0d0e21e 685
55602bd2 686Also see L<Error Indicators>.
687
4c5cef9b 688=item %!
689
690Each element of C<%!> has a true value only if C<$!> is set to that
691value. For example, C<$!{ENOENT}> is true if and only if the current
3be065a1 692value of C<$!> is C<ENOENT>; that is, if the most recent error was
693"No such file or directory" (or its moral equivalent: not all operating
694systems give that exact error, and certainly not all languages).
695To check if a particular key is meaningful on your system, use
696C<exists $!{the_key}>; for a list of legal keys, use C<keys %!>.
697See L<Errno> for more information, and also see above for the
698validity of C<$!>.
4c5cef9b 699
5c055ba3 700=item $EXTENDED_OS_ERROR
701
702=item $^E
703
22fae026 704Error information specific to the current operating system. At
705the moment, this differs from C<$!> under only VMS, OS/2, and Win32
706(and for MacPerl). On all other platforms, C<$^E> is always just
707the same as C<$!>.
708
709Under VMS, C<$^E> provides the VMS status value from the last
710system error. This is more specific information about the last
711system error than that provided by C<$!>. This is particularly
d516a115 712important when C<$!> is set to B<EVMSERR>.
22fae026 713
1c1c7f20 714Under OS/2, C<$^E> is set to the error code of the last call to
715OS/2 API either via CRT, or directly from perl.
22fae026 716
717Under Win32, C<$^E> always returns the last error information
718reported by the Win32 call C<GetLastError()> which describes
719the last error from within the Win32 API. Most Win32-specific
19799a22 720code will report errors via C<$^E>. ANSI C and Unix-like calls
22fae026 721set C<errno> and so most portable Perl code will report errors
722via C<$!>.
723
724Caveats mentioned in the description of C<$!> generally apply to
725C<$^E>, also. (Mnemonic: Extra error explanation.)
5c055ba3 726
55602bd2 727Also see L<Error Indicators>.
728
a0d0e21e 729=item $EVAL_ERROR
730
731=item $@
732
4a280ebe 733The Perl syntax error message from the last eval() operator.
734If $@ is the null string, the last eval() parsed and executed
735correctly (although the operations you invoked may have failed in the
736normal fashion). (Mnemonic: Where was the syntax error "at"?)
a0d0e21e 737
19799a22 738Warning messages are not collected in this variable. You can,
a8f8344d 739however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
54310121 740as described below.
748a9306 741
55602bd2 742Also see L<Error Indicators>.
743
a0d0e21e 744=item $PROCESS_ID
745
746=item $PID
747
748=item $$
749
19799a22 750The process number of the Perl running this script. You should
751consider this variable read-only, although it will be altered
752across fork() calls. (Mnemonic: same as shells.)
a0d0e21e 753
4d76a344 754Note for Linux users: on Linux, the C functions C<getpid()> and
755C<getppid()> return different values from different threads. In order to
756be portable, this behavior is not reflected by C<$$>, whose value remains
757consistent across threads. If you want to call the underlying C<getpid()>,
e3256f86 758you may use the CPAN module C<Linux::Pid>.
4d76a344 759
a0d0e21e 760=item $REAL_USER_ID
761
762=item $UID
763
764=item $<
765
19799a22 766The real uid of this process. (Mnemonic: it's the uid you came I<from>,
a043a685 767if you're running setuid.) You can change both the real uid and
a537debe 768the effective uid at the same time by using POSIX::setuid(). Since
769changes to $< require a system call, check $! after a change attempt to
770detect any possible errors.
a0d0e21e 771
772=item $EFFECTIVE_USER_ID
773
774=item $EUID
775
776=item $>
777
778The effective uid of this process. Example:
779
780 $< = $>; # set real to effective uid
781 ($<,$>) = ($>,$<); # swap real and effective uid
782
a043a685 783You can change both the effective uid and the real uid at the same
a537debe 784time by using POSIX::setuid(). Changes to $> require a check to $!
785to detect any possible errors after an attempted change.
a043a685 786
19799a22 787(Mnemonic: it's the uid you went I<to>, if you're running setuid.)
c47ff5f1 788C<< $< >> and C<< $> >> can be swapped only on machines
8cc95fdb 789supporting setreuid().
a0d0e21e 790
791=item $REAL_GROUP_ID
792
793=item $GID
794
795=item $(
796
797The real gid of this process. If you are on a machine that supports
798membership in multiple groups simultaneously, gives a space separated
799list of groups you are in. The first number is the one returned by
800getgid(), and the subsequent ones by getgroups(), one of which may be
8cc95fdb 801the same as the first number.
802
19799a22 803However, a value assigned to C<$(> must be a single number used to
804set the real gid. So the value given by C<$(> should I<not> be assigned
805back to C<$(> without being forced numeric, such as by adding zero.
8cc95fdb 806
a043a685 807You can change both the real gid and the effective gid at the same
a537debe 808time by using POSIX::setgid(). Changes to $( require a check to $!
809to detect any possible errors after an attempted change.
a043a685 810
19799a22 811(Mnemonic: parentheses are used to I<group> things. The real gid is the
812group you I<left>, if you're running setgid.)
a0d0e21e 813
814=item $EFFECTIVE_GROUP_ID
815
816=item $EGID
817
818=item $)
819
820The effective gid of this process. If you are on a machine that
821supports membership in multiple groups simultaneously, gives a space
822separated list of groups you are in. The first number is the one
823returned by getegid(), and the subsequent ones by getgroups(), one of
8cc95fdb 824which may be the same as the first number.
825
19799a22 826Similarly, a value assigned to C<$)> must also be a space-separated
14218588 827list of numbers. The first number sets the effective gid, and
8cc95fdb 828the rest (if any) are passed to setgroups(). To get the effect of an
829empty list for setgroups(), just repeat the new effective gid; that is,
830to force an effective gid of 5 and an effectively empty setgroups()
831list, say C< $) = "5 5" >.
832
a043a685 833You can change both the effective gid and the real gid at the same
834time by using POSIX::setgid() (use only a single numeric argument).
a537debe 835Changes to $) require a check to $! to detect any possible errors
836after an attempted change.
a043a685 837
19799a22 838(Mnemonic: parentheses are used to I<group> things. The effective gid
839is the group that's I<right> for you, if you're running setgid.)
a0d0e21e 840
c47ff5f1 841C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
19799a22 842machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
843and C<$)> can be swapped only on machines supporting setregid().
a0d0e21e 844
845=item $PROGRAM_NAME
846
847=item $0
848
80bca1b4 849Contains the name of the program being executed.
850
851On some (read: not all) operating systems assigning to C<$0> modifies
852the argument area that the C<ps> program sees. On some platforms you
853may have to use special C<ps> options or a different C<ps> to see the
854changes. Modifying the $0 is more useful as a way of indicating the
855current program state than it is for hiding the program you're
856running. (Mnemonic: same as B<sh> and B<ksh>.)
f9cbb277 857
858Note that there are platform specific limitations on the the maximum
859length of C<$0>. In the most extreme case it may be limited to the
860space occupied by the original C<$0>.
a0d0e21e 861
80bca1b4 862In some platforms there may be arbitrary amount of padding, for
863example space characters, after the modified name as shown by C<ps>.
dda345b7 864In some platforms this padding may extend all the way to the original
c80e2480 865length of the argument area, no matter what you do (this is the case
866for example with Linux 2.2).
80bca1b4 867
4bc88a62 868Note for BSD users: setting C<$0> does not completely remove "perl"
6a4647a3 869from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
870result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
871and the " (perl)" suffix are shown depends on your exact BSD variant
872and version). This is an operating system feature, Perl cannot help it.
4bc88a62 873
e2975953 874In multithreaded scripts Perl coordinates the threads so that any
875thread may modify its copy of the C<$0> and the change becomes visible
80bca1b4 876to ps(1) (assuming the operating system plays along). Note that the
877the view of C<$0> the other threads have will not change since they
878have their own copies of it.
e2975953 879
a0d0e21e 880=item $[
881
882The index of the first element in an array, and of the first character
19799a22 883in a substring. Default is 0, but you could theoretically set it
884to 1 to make Perl behave more like B<awk> (or Fortran) when
885subscripting and when evaluating the index() and substr() functions.
886(Mnemonic: [ begins subscripts.)
a0d0e21e 887
19799a22 888As of release 5 of Perl, assignment to C<$[> is treated as a compiler
889directive, and cannot influence the behavior of any other file.
f83ed198 890(That's why you can only assign compile-time constants to it.)
19799a22 891Its use is highly discouraged.
a0d0e21e 892
f83ed198 893Note that, unlike other compile-time directives (such as L<strict>),
894assignment to $[ can be seen from outer lexical scopes in the same file.
895However, you can use local() on it to strictly bound its value to a
896lexical block.
897
a0d0e21e 898=item $]
899
54310121 900The version + patchlevel / 1000 of the Perl interpreter. This variable
901can be used to determine whether the Perl interpreter executing a
902script is in the right range of versions. (Mnemonic: Is this version
903of perl in the right bracket?) Example:
a0d0e21e 904
905 warn "No checksumming!\n" if $] < 3.019;
906
54310121 907See also the documentation of C<use VERSION> and C<require VERSION>
19799a22 908for a convenient way to fail if the running Perl interpreter is too old.
a0d0e21e 909
0c8d858b 910The floating point representation can sometimes lead to inaccurate
911numeric comparisons. See C<$^V> for a more modern representation of
912the Perl version that allows accurate string comparisons.
16070b82 913
305aace0 914=item $COMPILING
915
916=item $^C
917
19799a22 918The current value of the flag associated with the B<-c> switch.
919Mainly of use with B<-MO=...> to allow code to alter its behavior
920when being compiled, such as for example to AUTOLOAD at compile
921time rather than normal, deferred loading. See L<perlcc>. Setting
922C<$^C = 1> is similar to calling C<B::minus_c>.
305aace0 923
a0d0e21e 924=item $DEBUGGING
925
926=item $^D
927
928The current value of the debugging flags. (Mnemonic: value of B<-D>
b4ab917c 929switch.) May be read or set. Like its command-line equivalent, you can use
930numeric or symbolic values, eg C<$^D = 10> or C<$^D = "st">.
a0d0e21e 931
a3621e74 932=item ${^RE_DEBUG_FLAGS}
933
934The current value of the regex debugging flags. Set to 0 for no debug output
935even when the re 'debug' module is loaded. See L<re> for details.
936
0111c4fd 937=item ${^RE_TRIE_MAXBUF}
a3621e74 938
939Controls how certain regex optimisations are applied and how much memory they
940utilize. This value by default is 65536 which corresponds to a 512kB temporary
941cache. Set this to a higher value to trade memory for speed when matching
942large alternations. Set it to a lower value if you want the optimisations to
943be as conservative of memory as possible but still occur, and set it to a
944negative value to prevent the optimisation and conserve the most memory.
945Under normal situations this variable should be of no interest to you.
946
a0d0e21e 947=item $SYSTEM_FD_MAX
948
949=item $^F
950
951The maximum system file descriptor, ordinarily 2. System file
952descriptors are passed to exec()ed processes, while higher file
953descriptors are not. Also, during an open(), system file descriptors are
954preserved even if the open() fails. (Ordinary file descriptors are
19799a22 955closed before the open() is attempted.) The close-on-exec
a0d0e21e 956status of a file descriptor will be decided according to the value of
8d2a6795 957C<$^F> when the corresponding file, pipe, or socket was opened, not the
958time of the exec().
a0d0e21e 959
6e2995f4 960=item $^H
961
0462a1ab 962WARNING: This variable is strictly for internal use only. Its availability,
963behavior, and contents are subject to change without notice.
964
965This variable contains compile-time hints for the Perl interpreter. At the
966end of compilation of a BLOCK the value of this variable is restored to the
967value when the interpreter started to compile the BLOCK.
968
969When perl begins to parse any block construct that provides a lexical scope
970(e.g., eval body, required file, subroutine body, loop body, or conditional
971block), the existing value of $^H is saved, but its value is left unchanged.
972When the compilation of the block is completed, it regains the saved value.
973Between the points where its value is saved and restored, code that
974executes within BEGIN blocks is free to change the value of $^H.
975
976This behavior provides the semantic of lexical scoping, and is used in,
977for instance, the C<use strict> pragma.
978
979The contents should be an integer; different bits of it are used for
980different pragmatic flags. Here's an example:
981
982 sub add_100 { $^H |= 0x100 }
983
984 sub foo {
985 BEGIN { add_100() }
986 bar->baz($boon);
987 }
988
989Consider what happens during execution of the BEGIN block. At this point
990the BEGIN block has already been compiled, but the body of foo() is still
991being compiled. The new value of $^H will therefore be visible only while
992the body of foo() is being compiled.
993
994Substitution of the above BEGIN block with:
995
996 BEGIN { require strict; strict->import('vars') }
997
998demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
999version of the same lexical pragma:
1000
1001 BEGIN { require strict; strict->import('vars') if $condition }
1002
1003=item %^H
1004
1005WARNING: This variable is strictly for internal use only. Its availability,
1006behavior, and contents are subject to change without notice.
1007
1008The %^H hash provides the same scoping semantic as $^H. This makes it
1009useful for implementation of lexically scoped pragmas.
6e2995f4 1010
a0d0e21e 1011=item $INPLACE_EDIT
1012
1013=item $^I
1014
1015The current value of the inplace-edit extension. Use C<undef> to disable
1016inplace editing. (Mnemonic: value of B<-i> switch.)
1017
fb73857a 1018=item $^M
1019
19799a22 1020By default, running out of memory is an untrappable, fatal error.
1021However, if suitably built, Perl can use the contents of C<$^M>
1022as an emergency memory pool after die()ing. Suppose that your Perl
1023were compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc.
1024Then
fb73857a 1025
19799a22 1026 $^M = 'a' x (1 << 16);
fb73857a 1027
51ee6500 1028would allocate a 64K buffer for use in an emergency. See the
19799a22 1029F<INSTALL> file in the Perl distribution for information on how to
1030enable this option. To discourage casual use of this advanced
4ec0190b 1031feature, there is no L<English|English> long name for this variable.
fb73857a 1032
5c055ba3 1033=item $OSNAME
6e2995f4 1034
5c055ba3 1035=item $^O
1036
1037The name of the operating system under which this copy of Perl was
1038built, as determined during the configuration process. The value
19799a22 1039is identical to C<$Config{'osname'}>. See also L<Config> and the
1040B<-V> command-line switch documented in L<perlrun>.
5c055ba3 1041
443f6d01 1042In Windows platforms, $^O is not very helpful: since it is always
7f510801 1043C<MSWin32>, it doesn't tell the difference between
104495/98/ME/NT/2000/XP/CE/.NET. Use Win32::GetOSName() or
1045Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
1046between the variants.
916d64a3 1047
e2e27056 1048=item ${^OPEN}
1049
1050An internal variable used by PerlIO. A string in two parts, separated
fae2c0fb 1051by a C<\0> byte, the first part describes the input layers, the second
1052part describes the output layers.
e2e27056 1053
a0d0e21e 1054=item $PERLDB
1055
1056=item $^P
1057
19799a22 1058The internal variable for debugging support. The meanings of the
1059various bits are subject to change, but currently indicate:
84902520 1060
1061=over 6
1062
1063=item 0x01
1064
1065Debug subroutine enter/exit.
1066
1067=item 0x02
1068
1069Line-by-line debugging.
1070
1071=item 0x04
1072
1073Switch off optimizations.
1074
1075=item 0x08
1076
1077Preserve more data for future interactive inspections.
1078
1079=item 0x10
1080
1081Keep info about source lines on which a subroutine is defined.
1082
1083=item 0x20
1084
1085Start with single-step on.
1086
83ee9e09 1087=item 0x40
1088
1089Use subroutine address instead of name when reporting.
1090
1091=item 0x80
1092
1093Report C<goto &subroutine> as well.
1094
1095=item 0x100
1096
1097Provide informative "file" names for evals based on the place they were compiled.
1098
1099=item 0x200
1100
1101Provide informative names to anonymous subroutines based on the place they
1102were compiled.
1103
7619c85e 1104=item 0x400
1105
1106Debug assertion subroutines enter/exit.
1107
84902520 1108=back
1109
19799a22 1110Some bits may be relevant at compile-time only, some at
1111run-time only. This is a new mechanism and the details may change.
a0d0e21e 1112
66558a10 1113=item $LAST_REGEXP_CODE_RESULT
1114
b9ac3b5b 1115=item $^R
1116
19799a22 1117The result of evaluation of the last successful C<(?{ code })>
1118regular expression assertion (see L<perlre>). May be written to.
b9ac3b5b 1119
66558a10 1120=item $EXCEPTIONS_BEING_CAUGHT
1121
fb73857a 1122=item $^S
1123
fa05a9fd 1124Current state of the interpreter.
1125
1126 $^S State
1127 --------- -------------------
1128 undef Parsing module/eval
1129 true (1) Executing an eval
1130 false (0) Otherwise
1131
1132The first state may happen in $SIG{__DIE__} and $SIG{__WARN__} handlers.
fb73857a 1133
a0d0e21e 1134=item $BASETIME
1135
1136=item $^T
1137
19799a22 1138The time at which the program began running, in seconds since the
5f05dabc 1139epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
19799a22 1140and B<-C> filetests are based on this value.
a0d0e21e 1141
7c36658b 1142=item ${^TAINT}
1143
9aa05f58 1144Reflects if taint mode is on or off. 1 for on (the program was run with
1145B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
18e8c5b0 1146B<-t> or B<-TU>). This variable is read-only.
7c36658b 1147
a05d7ebb 1148=item ${^UNICODE}
1149
ab9e1bb7 1150Reflects certain Unicode settings of Perl. See L<perlrun>
1151documentation for the C<-C> switch for more information about
1152the possible values. This variable is set during Perl startup
1153and is thereafter read-only.
fde18df1 1154
44dcb63b 1155=item $PERL_VERSION
b459063d 1156
16070b82 1157=item $^V
1158
1159The revision, version, and subversion of the Perl interpreter, represented
da2094fd 1160as a string composed of characters with those ordinals. Thus in Perl v5.6.0
44dcb63b 1161it equals C<chr(5) . chr(6) . chr(0)> and will return true for
1162C<$^V eq v5.6.0>. Note that the characters in this string value can
1163potentially be in Unicode range.
16070b82 1164
1165This can be used to determine whether the Perl interpreter executing a
1166script is in the right range of versions. (Mnemonic: use ^V for Version
44dcb63b 1167Control.) Example:
16070b82 1168
3fd4402b 1169 warn "No \"our\" declarations!\n" if $^V and $^V lt v5.6.0;
16070b82 1170
aa2f2a36 1171To convert C<$^V> into its string representation use sprintf()'s
1172C<"%vd"> conversion:
1173
1174 printf "version is v%vd\n", $^V; # Perl's version
1175
44dcb63b 1176See the documentation of C<use VERSION> and C<require VERSION>
16070b82 1177for a convenient way to fail if the running Perl interpreter is too old.
1178
1179See also C<$]> for an older representation of the Perl version.
1180
a0d0e21e 1181=item $WARNING
1182
1183=item $^W
1184
19799a22 1185The current value of the warning switch, initially true if B<-w>
1186was used, false otherwise, but directly modifiable. (Mnemonic:
4438c4b7 1187related to the B<-w> switch.) See also L<warnings>.
1188
6a818117 1189=item ${^WARNING_BITS}
4438c4b7 1190
1191The current set of warning checks enabled by the C<use warnings> pragma.
1192See the documentation of C<warnings> for more details.
a0d0e21e 1193
1194=item $EXECUTABLE_NAME
1195
1196=item $^X
1197
e71940de 1198The name used to execute the current copy of Perl, from C's
38e4f4ae 1199C<argv[0]>.
1200
e71940de 1201Depending on the host operating system, the value of $^X may be
1202a relative or absolute pathname of the perl program file, or may
1203be the string used to invoke perl but not the pathname of the
1204perl program file. Also, most operating systems permit invoking
1205programs that are not in the PATH environment variable, so there
a10d74f3 1206is no guarantee that the value of $^X is in PATH. For VMS, the
1207value may or may not include a version number.
38e4f4ae 1208
e71940de 1209You usually can use the value of $^X to re-invoke an independent
1210copy of the same perl that is currently running, e.g.,
1211
1212 @first_run = `$^X -le "print int rand 100 for 1..100"`;
1213
1214But recall that not all operating systems support forking or
1215capturing of the output of commands, so this complex statement
1216may not be portable.
38e4f4ae 1217
e71940de 1218It is not safe to use the value of $^X as a path name of a file,
1219as some operating systems that have a mandatory suffix on
1220executable files do not require use of the suffix when invoking
1221a command. To convert the value of $^X to a path name, use the
1222following statements:
1223
1224# Build up a set of file names (not command names).
1225 use Config;
68fb0eb7 1226 $this_perl = $^X;
1227 if ($^O ne 'VMS')
1228 {$this_perl .= $Config{_exe}
1229 unless $this_perl =~ m/$Config{_exe}$/i;}
e71940de 1230
1231Because many operating systems permit anyone with read access to
1232the Perl program file to make a copy of it, patch the copy, and
1233then execute the copy, the security-conscious Perl programmer
1234should take care to invoke the installed copy of perl, not the
1235copy referenced by $^X. The following statements accomplish
1236this goal, and produce a pathname that can be invoked as a
1237command or referenced as a file.
38e4f4ae 1238
1239 use Config;
68fb0eb7 1240 $secure_perl_path = $Config{perlpath};
1241 if ($^O ne 'VMS')
1242 {$secure_perl_path .= $Config{_exe}
1243 unless $secure_perl_path =~ m/$Config{_exe}$/i;}
a0d0e21e 1244
2d84a16a 1245=item ARGV
1246
1247The special filehandle that iterates over command-line filenames in
1248C<@ARGV>. Usually written as the null filehandle in the angle operator
1249C<< <> >>. Note that currently C<ARGV> only has its magical effect
1250within the C<< <> >> operator; elsewhere it is just a plain filehandle
1251corresponding to the last file opened by C<< <> >>. In particular,
1252passing C<\*ARGV> as a parameter to a function that expects a filehandle
1253may not cause your function to automatically read the contents of all the
1254files in C<@ARGV>.
1255
a0d0e21e 1256=item $ARGV
1257
c47ff5f1 1258contains the name of the current file when reading from <>.
a0d0e21e 1259
1260=item @ARGV
1261
19799a22 1262The array @ARGV contains the command-line arguments intended for
14218588 1263the script. C<$#ARGV> is generally the number of arguments minus
19799a22 1264one, because C<$ARGV[0]> is the first argument, I<not> the program's
1265command name itself. See C<$0> for the command name.
a0d0e21e 1266
5ccee41e 1267=item ARGVOUT
1268
1269The special filehandle that points to the currently open output file
1270when doing edit-in-place processing with B<-i>. Useful when you have
1271to do a lot of inserting and don't want to keep modifying $_. See
1272L<perlrun> for the B<-i> switch.
1273
9b0e6e7a 1274=item @F
1275
1276The array @F contains the fields of each line read in when autosplit
1277mode is turned on. See L<perlrun> for the B<-a> switch. This array
1278is package-specific, and must be declared or given a full package name
1279if not in package main when running under C<strict 'vars'>.
1280
a0d0e21e 1281=item @INC
1282
19799a22 1283The array @INC contains the list of places that the C<do EXPR>,
1284C<require>, or C<use> constructs look for their library files. It
1285initially consists of the arguments to any B<-I> command-line
1286switches, followed by the default Perl library, probably
1287F</usr/local/lib/perl>, followed by ".", to represent the current
e48df184 1288directory. ("." will not be appended if taint checks are enabled, either by
1289C<-T> or by C<-t>.) If you need to modify this at runtime, you should use
19799a22 1290the C<use lib> pragma to get the machine-dependent library properly
1291loaded also:
a0d0e21e 1292
cb1a09d0 1293 use lib '/mypath/libdir/';
1294 use SomeMod;
303f2f76 1295
d54b56d5 1296You can also insert hooks into the file inclusion system by putting Perl
1297code directly into @INC. Those hooks may be subroutine references, array
1298references or blessed objects. See L<perlfunc/require> for details.
1299
fb73857a 1300=item @_
1301
1302Within a subroutine the array @_ contains the parameters passed to that
19799a22 1303subroutine. See L<perlsub>.
fb73857a 1304
a0d0e21e 1305=item %INC
1306
19799a22 1307The hash %INC contains entries for each filename included via the
1308C<do>, C<require>, or C<use> operators. The key is the filename
1309you specified (with module names converted to pathnames), and the
14218588 1310value is the location of the file found. The C<require>
87275199 1311operator uses this hash to determine whether a particular file has
19799a22 1312already been included.
a0d0e21e 1313
89ccab8c 1314If the file was loaded via a hook (e.g. a subroutine reference, see
1315L<perlfunc/require> for a description of these hooks), this hook is
9ae8cd5b 1316by default inserted into %INC in place of a filename. Note, however,
1317that the hook may have set the %INC entry by itself to provide some more
1318specific info.
44f0be63 1319
b687b08b 1320=item %ENV
1321
1322=item $ENV{expr}
a0d0e21e 1323
1324The hash %ENV contains your current environment. Setting a
19799a22 1325value in C<ENV> changes the environment for any child processes
1326you subsequently fork() off.
a0d0e21e 1327
b687b08b 1328=item %SIG
1329
1330=item $SIG{expr}
a0d0e21e 1331
14218588 1332The hash %SIG contains signal handlers for signals. For example:
a0d0e21e 1333
1334 sub handler { # 1st argument is signal name
fb73857a 1335 my($sig) = @_;
a0d0e21e 1336 print "Caught a SIG$sig--shutting down\n";
1337 close(LOG);
1338 exit(0);
1339 }
1340
fb73857a 1341 $SIG{'INT'} = \&handler;
1342 $SIG{'QUIT'} = \&handler;
a0d0e21e 1343 ...
19799a22 1344 $SIG{'INT'} = 'DEFAULT'; # restore default action
a0d0e21e 1345 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
1346
f648820c 1347Using a value of C<'IGNORE'> usually has the effect of ignoring the
1348signal, except for the C<CHLD> signal. See L<perlipc> for more about
1349this special case.
1350
19799a22 1351Here are some other examples:
a0d0e21e 1352
fb73857a 1353 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
a0d0e21e 1354 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
19799a22 1355 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
a0d0e21e 1356 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
1357
19799a22 1358Be sure not to use a bareword as the name of a signal handler,
1359lest you inadvertently call it.
748a9306 1360
44a8e56a 1361If your system has the sigaction() function then signal handlers are
9ce5b4ad 1362installed using it. This means you get reliable signal handling.
44a8e56a 1363
9ce5b4ad 1364The default delivery policy of signals changed in Perl 5.8.0 from
1365immediate (also known as "unsafe") to deferred, also known as
1366"safe signals". See L<perlipc> for more information.
45c0772f 1367
748a9306 1368Certain internal hooks can be also set using the %SIG hash. The
a8f8344d 1369routine indicated by C<$SIG{__WARN__}> is called when a warning message is
748a9306 1370about to be printed. The warning message is passed as the first
1371argument. The presence of a __WARN__ hook causes the ordinary printing
1372of warnings to STDERR to be suppressed. You can use this to save warnings
1373in a variable, or turn warnings into fatal errors, like this:
1374
1375 local $SIG{__WARN__} = sub { die $_[0] };
1376 eval $proggie;
1377
a8f8344d 1378The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
748a9306 1379is about to be thrown. The error message is passed as the first
1380argument. When a __DIE__ hook routine returns, the exception
1381processing continues as it would have in the absence of the hook,
cb1a09d0 1382unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
774d564b 1383The C<__DIE__> handler is explicitly disabled during the call, so that you
fb73857a 1384can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
1385
19799a22 1386Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
1387even inside an eval(). Do not use this to rewrite a pending exception
1388in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die().
1389This strange action at a distance may be fixed in a future release
1390so that C<$SIG{__DIE__}> is only called if your program is about
1391to exit, as was the original intent. Any other use is deprecated.
1392
1393C<__DIE__>/C<__WARN__> handlers are very special in one respect:
1394they may be called to report (probable) errors found by the parser.
1395In such a case the parser may be in inconsistent state, so any
1396attempt to evaluate Perl code from such a handler will probably
1397result in a segfault. This means that warnings or errors that
1398result from parsing Perl should be used with extreme caution, like
1399this:
fb73857a 1400
1401 require Carp if defined $^S;
1402 Carp::confess("Something wrong") if defined &Carp::confess;
1403 die "Something wrong, but could not load Carp to give backtrace...
1404 To see backtrace try starting Perl with -MCarp switch";
1405
1406Here the first line will load Carp I<unless> it is the parser who
1407called the handler. The second line will print backtrace and die if
1408Carp was available. The third line will be executed only if Carp was
1409not available.
1410
19799a22 1411See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
4438c4b7 1412L<warnings> for additional information.
68dc0745 1413
a0d0e21e 1414=back
55602bd2 1415
1416=head2 Error Indicators
1417
19799a22 1418The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1419about different types of error conditions that may appear during
1420execution of a Perl program. The variables are shown ordered by
1421the "distance" between the subsystem which reported the error and
1422the Perl process. They correspond to errors detected by the Perl
1423interpreter, C library, operating system, or an external program,
1424respectively.
55602bd2 1425
1426To illustrate the differences between these variables, consider the
19799a22 1427following Perl expression, which uses a single-quoted string:
55602bd2 1428
19799a22 1429 eval q{
22d0716c 1430 open my $pipe, "/cdrom/install |" or die $!;
1431 my @res = <$pipe>;
1432 close $pipe or die "bad pipe: $?, $!";
19799a22 1433 };
55602bd2 1434
1435After execution of this statement all 4 variables may have been set.
1436
19799a22 1437C<$@> is set if the string to be C<eval>-ed did not compile (this
1438may happen if C<open> or C<close> were imported with bad prototypes),
1439or if Perl code executed during evaluation die()d . In these cases
1440the value of $@ is the compile error, or the argument to C<die>
1441(which will interpolate C<$!> and C<$?>!). (See also L<Fatal>,
1442though.)
1443
c47ff5f1 1444When the eval() expression above is executed, open(), C<< <PIPE> >>,
19799a22 1445and C<close> are translated to calls in the C run-time library and
1446thence to the operating system kernel. C<$!> is set to the C library's
1447C<errno> if one of these calls fails.
1448
1449Under a few operating systems, C<$^E> may contain a more verbose
1450error indicator, such as in this case, "CDROM tray not closed."
14218588 1451Systems that do not support extended error messages leave C<$^E>
19799a22 1452the same as C<$!>.
1453
1454Finally, C<$?> may be set to non-0 value if the external program
1455F</cdrom/install> fails. The upper eight bits reflect specific
1456error conditions encountered by the program (the program's exit()
1457value). The lower eight bits reflect mode of failure, like signal
1458death and core dump information See wait(2) for details. In
1459contrast to C<$!> and C<$^E>, which are set only if error condition
1460is detected, the variable C<$?> is set on each C<wait> or pipe
1461C<close>, overwriting the old value. This is more like C<$@>, which
1462on every eval() is always set on failure and cleared on success.
2b92dfce 1463
19799a22 1464For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1465and C<$?>.
2b92dfce 1466
1467=head2 Technical Note on the Syntax of Variable Names
1468
19799a22 1469Variable names in Perl can have several formats. Usually, they
1470must begin with a letter or underscore, in which case they can be
1471arbitrarily long (up to an internal limit of 251 characters) and
1472may contain letters, digits, underscores, or the special sequence
1473C<::> or C<'>. In this case, the part before the last C<::> or
1474C<'> is taken to be a I<package qualifier>; see L<perlmod>.
2b92dfce 1475
1476Perl variable names may also be a sequence of digits or a single
1477punctuation or control character. These names are all reserved for
19799a22 1478special uses by Perl; for example, the all-digits names are used
1479to hold data captured by backreferences after a regular expression
1480match. Perl has a special syntax for the single-control-character
1481names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1482character. For example, the notation C<$^W> (dollar-sign caret
1483C<W>) is the scalar variable whose name is the single character
1484control-C<W>. This is better than typing a literal control-C<W>
1485into your program.
2b92dfce 1486
87275199 1487Finally, new in Perl 5.6, Perl variable names may be alphanumeric
19799a22 1488strings that begin with control characters (or better yet, a caret).
1489These variables must be written in the form C<${^Foo}>; the braces
1490are not optional. C<${^Foo}> denotes the scalar variable whose
1491name is a control-C<F> followed by two C<o>'s. These variables are
1492reserved for future special uses by Perl, except for the ones that
1493begin with C<^_> (control-underscore or caret-underscore). No
1494control-character name that begins with C<^_> will acquire a special
1495meaning in any future version of Perl; such names may therefore be
1496used safely in programs. C<$^_> itself, however, I<is> reserved.
1497
1498Perl identifiers that begin with digits, control characters, or
2b92dfce 1499punctuation characters are exempt from the effects of the C<package>
747fafda 1500declaration and are always forced to be in package C<main>; they are
1501also exempt from C<strict 'vars'> errors. A few other names are also
1502exempt in these ways:
2b92dfce 1503
1504 ENV STDIN
1505 INC STDOUT
1506 ARGV STDERR
5b88253b 1507 ARGVOUT _
2b92dfce 1508 SIG
1509
1510In particular, the new special C<${^_XYZ}> variables are always taken
19799a22 1511to be in package C<main>, regardless of any C<package> declarations
747fafda 1512presently in scope.
2b92dfce 1513
19799a22 1514=head1 BUGS
1515
1516Due to an unfortunate accident of Perl's implementation, C<use
1517English> imposes a considerable performance penalty on all regular
1518expression matches in a program, regardless of whether they occur
1519in the scope of C<use English>. For that reason, saying C<use
1520English> in libraries is strongly discouraged. See the
1521Devel::SawAmpersand module documentation from CPAN
1577cd80 1522( http://www.cpan.org/modules/by-module/Devel/ )
19799a22 1523for more information.
2b92dfce 1524
19799a22 1525Having to even think about the C<$^S> variable in your exception
1526handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1527invites grievous and difficult to track down errors. Avoid it
1528and use an C<END{}> or CORE::GLOBAL::die override instead.