3 perlvar - Perl predefined variables
7 =head2 Predefined Names
9 The following names have special meaning to Perl. Most
10 punctuation names have reasonable mnemonics, or analogs in the
11 shells. Nevertheless, if you wish to use long variable names,
16 at the top of your program. This will alias all the short names to the
17 long names in the current package. Some even have medium names,
18 generally borrowed from B<awk>.
20 If you don't mind the performance hit, variables that depend on the
21 currently selected filehandle may instead be set by calling an
22 appropriate object method on the IO::Handle object. (Summary lines
23 below for this contain the word HANDLE.) First you must say
27 after which you may use either
35 Each method returns the old value of the IO::Handle attribute.
36 The methods each take an optional EXPR, which if supplied specifies the
37 new value for the IO::Handle attribute in question. If not supplied,
38 most methods do nothing to the current value--except for
39 autoflush(), which will assume a 1 for you, just to be different.
40 Because loading in the IO::Handle class is an expensive operation, you should
41 learn how to use the regular built-in variables.
43 A few of these variables are considered "read-only". This means that if
44 you try to assign to this variable, either directly or indirectly through
45 a reference, you'll raise a run-time exception.
47 The following list is ordered by scalar variables first, then the
48 arrays, then the hashes.
56 The default input and pattern-searching space. The following pairs are
59 while (<>) {...} # equivalent only in while!
60 while (defined($_ = <>)) {...}
71 Here are the places where Perl will assume $_ even if you
78 Various unary functions, including functions like ord() and int(), as well
79 as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
84 Various list functions like print() and unlink().
88 The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
89 without an C<=~> operator.
93 The default iterator variable in a C<foreach> loop if no other
98 The implicit iterator variable in the grep() and map() functions.
102 The default place to put an input record when a C<< <FH> >>
103 operation's result is tested by itself as the sole criterion of a C<while>
104 test. Outside a C<while> test, this will not happen.
108 (Mnemonic: underline is understood in certain operations.)
118 Special package variables when using sort(), see L<perlfunc/sort>.
119 Because of this specialness $a and $b don't need to be declared
120 (using local(), use vars, or our()) even when using the strict
121 vars pragma. Don't lexicalize them with C<my $a> or C<my $b>
122 if you want to be able to use them in the sort() comparison block
131 Contains the subpattern from the corresponding set of capturing
132 parentheses from the last pattern match, not counting patterns
133 matched in nested blocks that have been exited already. (Mnemonic:
134 like \digits.) These variables are all read-only and dynamically
135 scoped to the current BLOCK.
141 The string matched by the last successful pattern match (not counting
142 any matches hidden within a BLOCK or eval() enclosed by the current
143 BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
144 and dynamically scoped to the current BLOCK.
146 The use of this variable anywhere in a program imposes a considerable
147 performance penalty on all regular expression matches. See L<BUGS>.
153 The string preceding whatever was matched by the last successful
154 pattern match (not counting any matches hidden within a BLOCK or eval
155 enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
156 string.) This variable is read-only.
158 The use of this variable anywhere in a program imposes a considerable
159 performance penalty on all regular expression matches. See L<BUGS>.
165 The string following whatever was matched by the last successful
166 pattern match (not counting any matches hidden within a BLOCK or eval()
167 enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
172 print "$`:$&:$'\n"; # prints abc:def:ghi
174 This variable is read-only and dynamically scoped to the current BLOCK.
176 The use of this variable anywhere in a program imposes a considerable
177 performance penalty on all regular expression matches. See L<BUGS>.
179 =item $LAST_PAREN_MATCH
183 The last bracket matched by the last search pattern. This is useful if
184 you don't know which one of a set of alternative patterns matched. For
187 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
189 (Mnemonic: be positive and forward looking.)
190 This variable is read-only and dynamically scoped to the current BLOCK.
192 =item @LAST_MATCH_END
196 This array holds the offsets of the ends of the last successful
197 submatches in the currently active dynamic scope. C<$+[0]> is
198 the offset into the string of the end of the entire match. This
199 is the same value as what the C<pos> function returns when called
200 on the variable that was matched against. The I<n>th element
201 of this array holds the offset of the I<n>th submatch, so
202 C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset
203 past where $2 ends, and so on. You can use C<$#+> to determine
204 how many subgroups were in the last successful match. See the
205 examples given for the C<@-> variable.
207 =item $MULTILINE_MATCHING
211 Set to a non-zero integer value to do multi-line matching within a
212 string, 0 (or undefined) to tell Perl that it can assume that strings
213 contain a single line, for the purpose of optimizing pattern matches.
214 Pattern matches on strings containing multiple newlines can produce
215 confusing results when C<$*> is 0 or undefined. Default is undefined.
216 (Mnemonic: * matches multiple things.) This variable influences the
217 interpretation of only C<^> and C<$>. A literal newline can be searched
218 for even when C<$* == 0>.
220 Use of C<$*> is deprecated in modern Perl, supplanted by
221 the C</s> and C</m> modifiers on pattern matching.
223 Assigning a non-numerical value to C<$*> triggers a warning (and makes
224 C<$*> act if C<$* == 0>), while assigning a numerical value to C<$*>
225 makes that an implicit C<int> is applied on the value.
227 =item input_line_number HANDLE EXPR
229 =item $INPUT_LINE_NUMBER
235 The current input record number for the last file handle from which
236 you just read() (or called a C<seek> or C<tell> on). The value
237 may be different from the actual physical line number in the file,
238 depending on what notion of "line" is in effect--see C<$/> on how
239 to change that. An explicit close on a filehandle resets the line
240 number. Because C<< <> >> never does an explicit close, line
241 numbers increase across ARGV files (but see examples in L<perlfunc/eof>).
242 Consider this variable read-only: setting it does not reposition
243 the seek pointer; you'll have to do that on your own. Localizing C<$.>
244 has the effect of also localizing Perl's notion of "the last read
245 filehandle". (Mnemonic: many programs use "." to mean the current line
248 =item input_record_separator HANDLE EXPR
250 =item $INPUT_RECORD_SEPARATOR
256 The input record separator, newline by default. This
257 influences Perl's idea of what a "line" is. Works like B<awk>'s RS
258 variable, including treating empty lines as a terminator if set to
259 the null string. (An empty line cannot contain any spaces
260 or tabs.) You may set it to a multi-character string to match a
261 multi-character terminator, or to C<undef> to read through the end
262 of file. Setting it to C<"\n\n"> means something slightly
263 different than setting to C<"">, if the file contains consecutive
264 empty lines. Setting to C<""> will treat two or more consecutive
265 empty lines as a single empty line. Setting to C<"\n\n"> will
266 blindly assume that the next input character belongs to the next
267 paragraph, even if it's a newline. (Mnemonic: / delimits
268 line boundaries when quoting poetry.)
270 undef $/; # enable "slurp" mode
271 $_ = <FH>; # whole file now here
274 Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
275 better for something. :-)
277 Setting C<$/> to a reference to an integer, scalar containing an integer, or
278 scalar that's convertible to an integer will attempt to read records
279 instead of lines, with the maximum record size being the referenced
282 $/ = \32768; # or \"32768", or \$var_containing_32768
286 will read a record of no more than 32768 bytes from FILE. If you're
287 not reading from a record-oriented file (or your OS doesn't have
288 record-oriented files), then you'll likely get a full chunk of data
289 with every read. If a record is larger than the record size you've
290 set, you'll get the record back in pieces.
292 On VMS, record reads are done with the equivalent of C<sysread>,
293 so it's best not to mix record and non-record reads on the same
294 file. (This is unlikely to be a problem, because any file you'd
295 want to read in record mode is probably unusable in line mode.)
296 Non-VMS systems do normal I/O, so it's safe to mix record and
297 non-record reads of a file.
299 See also L<perlport/"Newlines">. Also see C<$.>.
301 =item autoflush HANDLE EXPR
303 =item $OUTPUT_AUTOFLUSH
307 If set to nonzero, forces a flush right away and after every write
308 or print on the currently selected output channel. Default is 0
309 (regardless of whether the channel is really buffered by the
310 system or not; C<$|> tells you only whether you've asked Perl
311 explicitly to flush after each write). STDOUT will
312 typically be line buffered if output is to the terminal and block
313 buffered otherwise. Setting this variable is useful primarily when
314 you are outputting to a pipe or socket, such as when you are running
315 a Perl program under B<rsh> and want to see the output as it's
316 happening. This has no effect on input buffering. See L<perlfunc/getc>
317 for that. (Mnemonic: when you want your pipes to be piping hot.)
319 =item output_field_separator HANDLE EXPR
321 =item $OUTPUT_FIELD_SEPARATOR
327 The output field separator for the print operator. Ordinarily the
328 print operator simply prints out its arguments without further
329 adornment. To get behavior more like B<awk>, set this variable as
330 you would set B<awk>'s OFS variable to specify what is printed
331 between fields. (Mnemonic: what is printed when there is a "," in
332 your print statement.)
334 =item output_record_separator HANDLE EXPR
336 =item $OUTPUT_RECORD_SEPARATOR
342 The output record separator for the print operator. Ordinarily the
343 print operator simply prints out its arguments as is, with no
344 trailing newline or other end-of-record string added. To get
345 behavior more like B<awk>, set this variable as you would set
346 B<awk>'s ORS variable to specify what is printed at the end of the
347 print. (Mnemonic: you set C<$\> instead of adding "\n" at the
348 end of the print. Also, it's just like C<$/>, but it's what you
349 get "back" from Perl.)
351 =item $LIST_SEPARATOR
355 This is like C<$,> except that it applies to array and slice values
356 interpolated into a double-quoted string (or similar interpreted
357 string). Default is a space. (Mnemonic: obvious, I think.)
359 =item $SUBSCRIPT_SEPARATOR
365 The subscript separator for multidimensional array emulation. If you
366 refer to a hash element as
372 $foo{join($;, $a, $b, $c)}
376 @foo{$a,$b,$c} # a slice--note the @
380 ($foo{$a},$foo{$b},$foo{$c})
382 Default is "\034", the same as SUBSEP in B<awk>. If your
383 keys contain binary data there might not be any safe value for C<$;>.
384 (Mnemonic: comma (the syntactic subscript separator) is a
385 semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
386 taken for something more important.)
388 Consider using "real" multidimensional arrays as described
395 The output format for printed numbers. This variable is a half-hearted
396 attempt to emulate B<awk>'s OFMT variable. There are times, however,
397 when B<awk> and Perl have differing notions of what counts as
398 numeric. The initial value is "%.I<n>g", where I<n> is the value
399 of the macro DBL_DIG from your system's F<float.h>. This is different from
400 B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#>
401 explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
403 Use of C<$#> is deprecated.
405 =item format_page_number HANDLE EXPR
407 =item $FORMAT_PAGE_NUMBER
411 The current page number of the currently selected output channel.
413 (Mnemonic: % is page number in B<nroff>.)
415 =item format_lines_per_page HANDLE EXPR
417 =item $FORMAT_LINES_PER_PAGE
421 The current page length (printable lines) of the currently selected
422 output channel. Default is 60.
424 (Mnemonic: = has horizontal lines.)
426 =item format_lines_left HANDLE EXPR
428 =item $FORMAT_LINES_LEFT
432 The number of lines left on the page of the currently selected output
435 (Mnemonic: lines_on_page - lines_printed.)
437 =item @LAST_MATCH_START
441 $-[0] is the offset of the start of the last successful match.
442 C<$-[>I<n>C<]> is the offset of the start of the substring matched by
443 I<n>-th subpattern, or undef if the subpattern did not match.
445 Thus after a match against $_, $& coincides with C<substr $_, $-[0],
446 $+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
447 $+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
448 C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last
449 matched subgroup in the last successful match. Contrast with
450 C<$#+>, the number of subgroups in the regular expression. Compare
453 This array holds the offsets of the beginnings of the last
454 successful submatches in the currently active dynamic scope.
455 C<$-[0]> is the offset into the string of the beginning of the
456 entire match. The I<n>th element of this array holds the offset
457 of the I<n>th submatch, so C<$+[1]> is the offset where $1
458 begins, C<$+[2]> the offset where $2 begins, and so on.
459 You can use C<$#-> to determine how many subgroups were in the
460 last successful match. Compare with the C<@+> variable.
462 After a match against some variable $var:
466 =item C<$`> is the same as C<substr($var, 0, $-[0])>
468 =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
470 =item C<$'> is the same as C<substr($var, $+[0])>
472 =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
474 =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
476 =item C<$3> is the same as C<substr $var, $-[3], $+[3] - $-[3])>
480 =item format_name HANDLE EXPR
486 The name of the current report format for the currently selected output
487 channel. Default is the name of the filehandle. (Mnemonic: brother to
490 =item format_top_name HANDLE EXPR
492 =item $FORMAT_TOP_NAME
496 The name of the current top-of-page format for the currently selected
497 output channel. Default is the name of the filehandle with _TOP
498 appended. (Mnemonic: points to top of page.)
500 =item format_line_break_characters HANDLE EXPR
502 =item $FORMAT_LINE_BREAK_CHARACTERS
506 The current set of characters after which a string may be broken to
507 fill continuation fields (starting with ^) in a format. Default is
508 S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
509 poetry is a part of a line.)
511 =item format_formfeed HANDLE EXPR
513 =item $FORMAT_FORMFEED
517 What formats output as a form feed. Default is \f.
523 The current value of the write() accumulator for format() lines. A format
524 contains formline() calls that put their result into C<$^A>. After
525 calling its format, write() prints out the contents of C<$^A> and empties.
526 So you never really see the contents of C<$^A> unless you call
527 formline() yourself and then look at it. See L<perlform> and
528 L<perlfunc/formline()>.
534 The status returned by the last pipe close, backtick (C<``>) command,
535 successful call to wait() or waitpid(), or from the system()
536 operator. This is just the 16-bit status word returned by the
537 wait() system call (or else is made up to look like it). Thus, the
538 exit value of the subprocess is really (C<<< $? >> 8 >>>), and
539 C<$? & 127> gives which signal, if any, the process died from, and
540 C<$? & 128> reports whether there was a core dump. (Mnemonic:
541 similar to B<sh> and B<ksh>.)
543 Additionally, if the C<h_errno> variable is supported in C, its value
544 is returned via $? if any C<gethost*()> function fails.
546 If you have installed a signal handler for C<SIGCHLD>, the
547 value of C<$?> will usually be wrong outside that handler.
549 Inside an C<END> subroutine C<$?> contains the value that is going to be
550 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
551 change the exit status of your program. For example:
554 $? = 1 if $? == 255; # die would make it 255
557 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
558 actual VMS exit status, instead of the default emulation of POSIX
561 Also see L<Error Indicators>.
569 If used numerically, yields the current value of the C C<errno>
570 variable, with all the usual caveats. (This means that you shouldn't
571 depend on the value of C<$!> to be anything in particular unless
572 you've gotten a specific error return indicating a system error.)
573 If used an a string, yields the corresponding system error string.
574 You can assign a number to C<$!> to set I<errno> if, for instance,
575 you want C<"$!"> to return the string for error I<n>, or you want
576 to set the exit value for the die() operator. (Mnemonic: What just
579 Also see L<Error Indicators>.
581 =item $EXTENDED_OS_ERROR
585 Error information specific to the current operating system. At
586 the moment, this differs from C<$!> under only VMS, OS/2, and Win32
587 (and for MacPerl). On all other platforms, C<$^E> is always just
590 Under VMS, C<$^E> provides the VMS status value from the last
591 system error. This is more specific information about the last
592 system error than that provided by C<$!>. This is particularly
593 important when C<$!> is set to B<EVMSERR>.
595 Under OS/2, C<$^E> is set to the error code of the last call to
596 OS/2 API either via CRT, or directly from perl.
598 Under Win32, C<$^E> always returns the last error information
599 reported by the Win32 call C<GetLastError()> which describes
600 the last error from within the Win32 API. Most Win32-specific
601 code will report errors via C<$^E>. ANSI C and Unix-like calls
602 set C<errno> and so most portable Perl code will report errors
605 Caveats mentioned in the description of C<$!> generally apply to
606 C<$^E>, also. (Mnemonic: Extra error explanation.)
608 Also see L<Error Indicators>.
614 The Perl syntax error message from the last eval() operator. If null, the
615 last eval() parsed and executed correctly (although the operations you
616 invoked may have failed in the normal fashion). (Mnemonic: Where was
617 the syntax error "at"?)
619 Warning messages are not collected in this variable. You can,
620 however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
623 Also see L<Error Indicators>.
631 The process number of the Perl running this script. You should
632 consider this variable read-only, although it will be altered
633 across fork() calls. (Mnemonic: same as shells.)
641 The real uid of this process. (Mnemonic: it's the uid you came I<from>,
642 if you're running setuid.) You can change both the real uid and
643 the effective uid at the same time by using POSIX::setuid().
645 =item $EFFECTIVE_USER_ID
651 The effective uid of this process. Example:
653 $< = $>; # set real to effective uid
654 ($<,$>) = ($>,$<); # swap real and effective uid
656 You can change both the effective uid and the real uid at the same
657 time by using POSIX::setuid().
659 (Mnemonic: it's the uid you went I<to>, if you're running setuid.)
660 C<< $< >> and C<< $> >> can be swapped only on machines
661 supporting setreuid().
669 The real gid of this process. If you are on a machine that supports
670 membership in multiple groups simultaneously, gives a space separated
671 list of groups you are in. The first number is the one returned by
672 getgid(), and the subsequent ones by getgroups(), one of which may be
673 the same as the first number.
675 However, a value assigned to C<$(> must be a single number used to
676 set the real gid. So the value given by C<$(> should I<not> be assigned
677 back to C<$(> without being forced numeric, such as by adding zero.
679 You can change both the real gid and the effective gid at the same
680 time by using POSIX::setgid().
682 (Mnemonic: parentheses are used to I<group> things. The real gid is the
683 group you I<left>, if you're running setgid.)
685 =item $EFFECTIVE_GROUP_ID
691 The effective gid of this process. If you are on a machine that
692 supports membership in multiple groups simultaneously, gives a space
693 separated list of groups you are in. The first number is the one
694 returned by getegid(), and the subsequent ones by getgroups(), one of
695 which may be the same as the first number.
697 Similarly, a value assigned to C<$)> must also be a space-separated
698 list of numbers. The first number sets the effective gid, and
699 the rest (if any) are passed to setgroups(). To get the effect of an
700 empty list for setgroups(), just repeat the new effective gid; that is,
701 to force an effective gid of 5 and an effectively empty setgroups()
702 list, say C< $) = "5 5" >.
704 You can change both the effective gid and the real gid at the same
705 time by using POSIX::setgid() (use only a single numeric argument).
707 (Mnemonic: parentheses are used to I<group> things. The effective gid
708 is the group that's I<right> for you, if you're running setgid.)
710 C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
711 machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
712 and C<$)> can be swapped only on machines supporting setregid().
718 Contains the name of the program being executed. On some operating
719 systems assigning to C<$0> modifies the argument area that the B<ps>
720 program sees. This is more useful as a way of indicating the current
721 program state than it is for hiding the program you're running.
722 (Mnemonic: same as B<sh> and B<ksh>.)
724 Note for BSD users: setting C<$0> does not completely remove "perl"
725 from the ps(1) output. For example, setting C<$0> to C<"foobar"> will
726 result in C<"perl: foobar (perl)">. This is an operating system
731 The index of the first element in an array, and of the first character
732 in a substring. Default is 0, but you could theoretically set it
733 to 1 to make Perl behave more like B<awk> (or Fortran) when
734 subscripting and when evaluating the index() and substr() functions.
735 (Mnemonic: [ begins subscripts.)
737 As of release 5 of Perl, assignment to C<$[> is treated as a compiler
738 directive, and cannot influence the behavior of any other file.
739 Its use is highly discouraged.
743 The version + patchlevel / 1000 of the Perl interpreter. This variable
744 can be used to determine whether the Perl interpreter executing a
745 script is in the right range of versions. (Mnemonic: Is this version
746 of perl in the right bracket?) Example:
748 warn "No checksumming!\n" if $] < 3.019;
750 See also the documentation of C<use VERSION> and C<require VERSION>
751 for a convenient way to fail if the running Perl interpreter is too old.
753 The use of this variable is deprecated. The floating point representation
754 can sometimes lead to inaccurate numeric comparisons. See C<$^V> for a
755 more modern representation of the Perl version that allows accurate string
762 The current value of the flag associated with the B<-c> switch.
763 Mainly of use with B<-MO=...> to allow code to alter its behavior
764 when being compiled, such as for example to AUTOLOAD at compile
765 time rather than normal, deferred loading. See L<perlcc>. Setting
766 C<$^C = 1> is similar to calling C<B::minus_c>.
772 The current value of the debugging flags. (Mnemonic: value of B<-D>
779 The maximum system file descriptor, ordinarily 2. System file
780 descriptors are passed to exec()ed processes, while higher file
781 descriptors are not. Also, during an open(), system file descriptors are
782 preserved even if the open() fails. (Ordinary file descriptors are
783 closed before the open() is attempted.) The close-on-exec
784 status of a file descriptor will be decided according to the value of
785 C<$^F> when the corresponding file, pipe, or socket was opened, not the
790 WARNING: This variable is strictly for internal use only. Its availability,
791 behavior, and contents are subject to change without notice.
793 This variable contains compile-time hints for the Perl interpreter. At the
794 end of compilation of a BLOCK the value of this variable is restored to the
795 value when the interpreter started to compile the BLOCK.
797 When perl begins to parse any block construct that provides a lexical scope
798 (e.g., eval body, required file, subroutine body, loop body, or conditional
799 block), the existing value of $^H is saved, but its value is left unchanged.
800 When the compilation of the block is completed, it regains the saved value.
801 Between the points where its value is saved and restored, code that
802 executes within BEGIN blocks is free to change the value of $^H.
804 This behavior provides the semantic of lexical scoping, and is used in,
805 for instance, the C<use strict> pragma.
807 The contents should be an integer; different bits of it are used for
808 different pragmatic flags. Here's an example:
810 sub add_100 { $^H |= 0x100 }
817 Consider what happens during execution of the BEGIN block. At this point
818 the BEGIN block has already been compiled, but the body of foo() is still
819 being compiled. The new value of $^H will therefore be visible only while
820 the body of foo() is being compiled.
822 Substitution of the above BEGIN block with:
824 BEGIN { require strict; strict->import('vars') }
826 demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
827 version of the same lexical pragma:
829 BEGIN { require strict; strict->import('vars') if $condition }
833 WARNING: This variable is strictly for internal use only. Its availability,
834 behavior, and contents are subject to change without notice.
836 The %^H hash provides the same scoping semantic as $^H. This makes it
837 useful for implementation of lexically scoped pragmas.
843 The current value of the inplace-edit extension. Use C<undef> to disable
844 inplace editing. (Mnemonic: value of B<-i> switch.)
848 By default, running out of memory is an untrappable, fatal error.
849 However, if suitably built, Perl can use the contents of C<$^M>
850 as an emergency memory pool after die()ing. Suppose that your Perl
851 were compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc.
854 $^M = 'a' x (1 << 16);
856 would allocate a 64K buffer for use in an emergency. See the
857 F<INSTALL> file in the Perl distribution for information on how to
858 enable this option. To discourage casual use of this advanced
859 feature, there is no L<English|English> long name for this variable.
865 The name of the operating system under which this copy of Perl was
866 built, as determined during the configuration process. The value
867 is identical to C<$Config{'osname'}>. See also L<Config> and the
868 B<-V> command-line switch documented in L<perlrun>.
874 The internal variable for debugging support. The meanings of the
875 various bits are subject to change, but currently indicate:
881 Debug subroutine enter/exit.
885 Line-by-line debugging.
889 Switch off optimizations.
893 Preserve more data for future interactive inspections.
897 Keep info about source lines on which a subroutine is defined.
901 Start with single-step on.
905 Use subroutine address instead of name when reporting.
909 Report C<goto &subroutine> as well.
913 Provide informative "file" names for evals based on the place they were compiled.
917 Provide informative names to anonymous subroutines based on the place they
922 Some bits may be relevant at compile-time only, some at
923 run-time only. This is a new mechanism and the details may change.
925 =item $LAST_REGEXP_CODE_RESULT
929 The result of evaluation of the last successful C<(?{ code })>
930 regular expression assertion (see L<perlre>). May be written to.
932 =item $EXCEPTIONS_BEING_CAUGHT
936 Current state of the interpreter. Undefined if parsing of the current
937 module/eval is not finished (may happen in $SIG{__DIE__} and
938 $SIG{__WARN__} handlers). True if inside an eval(), otherwise false.
944 The time at which the program began running, in seconds since the
945 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
946 and B<-C> filetests are based on this value.
952 The revision, version, and subversion of the Perl interpreter, represented
953 as a string composed of characters with those ordinals. Thus in Perl v5.6.0
954 it equals C<chr(5) . chr(6) . chr(0)> and will return true for
955 C<$^V eq v5.6.0>. Note that the characters in this string value can
956 potentially be in Unicode range.
958 This can be used to determine whether the Perl interpreter executing a
959 script is in the right range of versions. (Mnemonic: use ^V for Version
962 warn "No \"our\" declarations!\n" if $^V and $^V lt v5.6.0;
964 See the documentation of C<use VERSION> and C<require VERSION>
965 for a convenient way to fail if the running Perl interpreter is too old.
967 See also C<$]> for an older representation of the Perl version.
973 The current value of the warning switch, initially true if B<-w>
974 was used, false otherwise, but directly modifiable. (Mnemonic:
975 related to the B<-w> switch.) See also L<warnings>.
977 =item ${^WARNING_BITS}
979 The current set of warning checks enabled by the C<use warnings> pragma.
980 See the documentation of C<warnings> for more details.
982 =item ${^WIDE_SYSTEM_CALLS}
984 Global flag that enables system calls made by Perl to use wide character
985 APIs native to the system, if available. This is currently only implemented
986 on the Windows platform.
988 This can also be enabled from the command line using the C<-C> switch.
990 The initial value is typically C<0> for compatibility with Perl versions
991 earlier than 5.6, but may be automatically set to C<1> by Perl if the system
992 provides a user-settable default (e.g., C<$ENV{LC_CTYPE}>).
994 The C<bytes> pragma always overrides the effect of this flag in the current
995 lexical scope. See L<bytes>.
997 =item $EXECUTABLE_NAME
1001 The name that the Perl binary itself was executed as, from C's C<argv[0]>.
1002 This may not be a full pathname, nor even necessarily in your path.
1006 contains the name of the current file when reading from <>.
1010 The array @ARGV contains the command-line arguments intended for
1011 the script. C<$#ARGV> is generally the number of arguments minus
1012 one, because C<$ARGV[0]> is the first argument, I<not> the program's
1013 command name itself. See C<$0> for the command name.
1017 The array @F contains the fields of each line read in when autosplit
1018 mode is turned on. See L<perlrun> for the B<-a> switch. This array
1019 is package-specific, and must be declared or given a full package name
1020 if not in package main when running under C<strict 'vars'>.
1024 The array @INC contains the list of places that the C<do EXPR>,
1025 C<require>, or C<use> constructs look for their library files. It
1026 initially consists of the arguments to any B<-I> command-line
1027 switches, followed by the default Perl library, probably
1028 F</usr/local/lib/perl>, followed by ".", to represent the current
1029 directory. If you need to modify this at runtime, you should use
1030 the C<use lib> pragma to get the machine-dependent library properly
1033 use lib '/mypath/libdir/';
1038 Within a subroutine the array @_ contains the parameters passed to that
1039 subroutine. See L<perlsub>.
1043 The hash %INC contains entries for each filename included via the
1044 C<do>, C<require>, or C<use> operators. The key is the filename
1045 you specified (with module names converted to pathnames), and the
1046 value is the location of the file found. The C<require>
1047 operator uses this hash to determine whether a particular file has
1048 already been included.
1054 The hash %ENV contains your current environment. Setting a
1055 value in C<ENV> changes the environment for any child processes
1056 you subsequently fork() off.
1062 The hash %SIG contains signal handlers for signals. For example:
1064 sub handler { # 1st argument is signal name
1066 print "Caught a SIG$sig--shutting down\n";
1071 $SIG{'INT'} = \&handler;
1072 $SIG{'QUIT'} = \&handler;
1074 $SIG{'INT'} = 'DEFAULT'; # restore default action
1075 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
1077 Using a value of C<'IGNORE'> usually has the effect of ignoring the
1078 signal, except for the C<CHLD> signal. See L<perlipc> for more about
1081 Here are some other examples:
1083 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
1084 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
1085 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
1086 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
1088 Be sure not to use a bareword as the name of a signal handler,
1089 lest you inadvertently call it.
1091 If your system has the sigaction() function then signal handlers are
1092 installed using it. This means you get reliable signal handling. If
1093 your system has the SA_RESTART flag it is used when signals handlers are
1094 installed. This means that system calls for which restarting is supported
1095 continue rather than returning when a signal arrives. If you want your
1096 system calls to be interrupted by signal delivery then do something like
1099 use POSIX ':signal_h';
1102 sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
1103 or die "Error setting SIGALRM handler: $!\n";
1107 Certain internal hooks can be also set using the %SIG hash. The
1108 routine indicated by C<$SIG{__WARN__}> is called when a warning message is
1109 about to be printed. The warning message is passed as the first
1110 argument. The presence of a __WARN__ hook causes the ordinary printing
1111 of warnings to STDERR to be suppressed. You can use this to save warnings
1112 in a variable, or turn warnings into fatal errors, like this:
1114 local $SIG{__WARN__} = sub { die $_[0] };
1117 The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
1118 is about to be thrown. The error message is passed as the first
1119 argument. When a __DIE__ hook routine returns, the exception
1120 processing continues as it would have in the absence of the hook,
1121 unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
1122 The C<__DIE__> handler is explicitly disabled during the call, so that you
1123 can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
1125 Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
1126 even inside an eval(). Do not use this to rewrite a pending exception
1127 in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die().
1128 This strange action at a distance may be fixed in a future release
1129 so that C<$SIG{__DIE__}> is only called if your program is about
1130 to exit, as was the original intent. Any other use is deprecated.
1132 C<__DIE__>/C<__WARN__> handlers are very special in one respect:
1133 they may be called to report (probable) errors found by the parser.
1134 In such a case the parser may be in inconsistent state, so any
1135 attempt to evaluate Perl code from such a handler will probably
1136 result in a segfault. This means that warnings or errors that
1137 result from parsing Perl should be used with extreme caution, like
1140 require Carp if defined $^S;
1141 Carp::confess("Something wrong") if defined &Carp::confess;
1142 die "Something wrong, but could not load Carp to give backtrace...
1143 To see backtrace try starting Perl with -MCarp switch";
1145 Here the first line will load Carp I<unless> it is the parser who
1146 called the handler. The second line will print backtrace and die if
1147 Carp was available. The third line will be executed only if Carp was
1150 See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
1151 L<warnings> for additional information.
1155 =head2 Error Indicators
1157 The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1158 about different types of error conditions that may appear during
1159 execution of a Perl program. The variables are shown ordered by
1160 the "distance" between the subsystem which reported the error and
1161 the Perl process. They correspond to errors detected by the Perl
1162 interpreter, C library, operating system, or an external program,
1165 To illustrate the differences between these variables, consider the
1166 following Perl expression, which uses a single-quoted string:
1169 open PIPE, "/cdrom/install |";
1171 close PIPE or die "bad pipe: $?, $!";
1174 After execution of this statement all 4 variables may have been set.
1176 C<$@> is set if the string to be C<eval>-ed did not compile (this
1177 may happen if C<open> or C<close> were imported with bad prototypes),
1178 or if Perl code executed during evaluation die()d . In these cases
1179 the value of $@ is the compile error, or the argument to C<die>
1180 (which will interpolate C<$!> and C<$?>!). (See also L<Fatal>,
1183 When the eval() expression above is executed, open(), C<< <PIPE> >>,
1184 and C<close> are translated to calls in the C run-time library and
1185 thence to the operating system kernel. C<$!> is set to the C library's
1186 C<errno> if one of these calls fails.
1188 Under a few operating systems, C<$^E> may contain a more verbose
1189 error indicator, such as in this case, "CDROM tray not closed."
1190 Systems that do not support extended error messages leave C<$^E>
1193 Finally, C<$?> may be set to non-0 value if the external program
1194 F</cdrom/install> fails. The upper eight bits reflect specific
1195 error conditions encountered by the program (the program's exit()
1196 value). The lower eight bits reflect mode of failure, like signal
1197 death and core dump information See wait(2) for details. In
1198 contrast to C<$!> and C<$^E>, which are set only if error condition
1199 is detected, the variable C<$?> is set on each C<wait> or pipe
1200 C<close>, overwriting the old value. This is more like C<$@>, which
1201 on every eval() is always set on failure and cleared on success.
1203 For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1206 =head2 Technical Note on the Syntax of Variable Names
1208 Variable names in Perl can have several formats. Usually, they
1209 must begin with a letter or underscore, in which case they can be
1210 arbitrarily long (up to an internal limit of 251 characters) and
1211 may contain letters, digits, underscores, or the special sequence
1212 C<::> or C<'>. In this case, the part before the last C<::> or
1213 C<'> is taken to be a I<package qualifier>; see L<perlmod>.
1215 Perl variable names may also be a sequence of digits or a single
1216 punctuation or control character. These names are all reserved for
1217 special uses by Perl; for example, the all-digits names are used
1218 to hold data captured by backreferences after a regular expression
1219 match. Perl has a special syntax for the single-control-character
1220 names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1221 character. For example, the notation C<$^W> (dollar-sign caret
1222 C<W>) is the scalar variable whose name is the single character
1223 control-C<W>. This is better than typing a literal control-C<W>
1226 Finally, new in Perl 5.6, Perl variable names may be alphanumeric
1227 strings that begin with control characters (or better yet, a caret).
1228 These variables must be written in the form C<${^Foo}>; the braces
1229 are not optional. C<${^Foo}> denotes the scalar variable whose
1230 name is a control-C<F> followed by two C<o>'s. These variables are
1231 reserved for future special uses by Perl, except for the ones that
1232 begin with C<^_> (control-underscore or caret-underscore). No
1233 control-character name that begins with C<^_> will acquire a special
1234 meaning in any future version of Perl; such names may therefore be
1235 used safely in programs. C<$^_> itself, however, I<is> reserved.
1237 Perl identifiers that begin with digits, control characters, or
1238 punctuation characters are exempt from the effects of the C<package>
1239 declaration and are always forced to be in package C<main>. A few
1240 other names are also exempt:
1248 In particular, the new special C<${^_XYZ}> variables are always taken
1249 to be in package C<main>, regardless of any C<package> declarations
1254 Due to an unfortunate accident of Perl's implementation, C<use
1255 English> imposes a considerable performance penalty on all regular
1256 expression matches in a program, regardless of whether they occur
1257 in the scope of C<use English>. For that reason, saying C<use
1258 English> in libraries is strongly discouraged. See the
1259 Devel::SawAmpersand module documentation from CPAN
1260 (http://www.perl.com/CPAN/modules/by-module/Devel/)
1261 for more information.
1263 Having to even think about the C<$^S> variable in your exception
1264 handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1265 invites grievous and difficult to track down errors. Avoid it
1266 and use an C<END{}> or CORE::GLOBAL::die override instead.