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 analogues in one of
11 the 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 Due to an unfortunate accident of Perl's implementation, "C<use English>"
21 imposes a considerable performance penalty on all regular expression
22 matches in a program, regardless of whether they occur in the scope of
23 "C<use English>". For that reason, saying "C<use English>" in
24 libraries is strongly discouraged. See the Devel::SawAmpersand module
25 documentation from CPAN
26 (http://www.perl.com/CPAN/modules/by-module/Devel/Devel-SawAmpersand-0.10.readme)
29 To go a step further, those variables that depend on the currently
30 selected filehandle may instead (and preferably) be set by calling an
31 object method on the FileHandle object. (Summary lines below for this
32 contain the word HANDLE.) First you must say
36 after which you may use either
44 Each of the methods returns the old value of the FileHandle attribute.
45 The methods each take an optional EXPR, which if supplied specifies the
46 new value for the FileHandle attribute in question. If not supplied,
47 most of the methods do nothing to the current value, except for
48 autoflush(), which will assume a 1 for you, just to be different.
50 A few of these variables are considered "read-only". This means that if
51 you try to assign to this variable, either directly or indirectly through
52 a reference, you'll raise a run-time exception.
54 The following list is ordered by scalar variables first, then the
55 arrays, then the hashes (except $^M was added in the wrong place).
56 This is somewhat obscured by the fact that %ENV and %SIG are listed as
57 $ENV{expr} and $SIG{expr}.
66 The default input and pattern-searching space. The following pairs are
69 while (<>) {...} # equivalent in only while!
70 while (defined($_ = <>)) {...}
81 Here are the places where Perl will assume $_ even if you
88 Various unary functions, including functions like ord() and int(), as well
89 as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
94 Various list functions like print() and unlink().
98 The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
99 without an C<=~> operator.
103 The default iterator variable in a C<foreach> loop if no other
104 variable is supplied.
108 The implicit iterator variable in the grep() and map() functions.
112 The default place to put an input record when a C<E<lt>FHE<gt>>
113 operation's result is tested by itself as the sole criterion of a C<while>
114 test. Note that outside of a C<while> test, this will not happen.
118 (Mnemonic: underline is understood in certain operations.)
124 =item $E<lt>I<digits>E<gt>
126 Contains the subpattern from the corresponding set of parentheses in
127 the last pattern matched, not counting patterns matched in nested
128 blocks that have been exited already. (Mnemonic: like \digits.)
129 These variables are all read-only.
135 The string matched by the last successful pattern match (not counting
136 any matches hidden within a BLOCK or eval() enclosed by the current
137 BLOCK). (Mnemonic: like & in some editors.) This variable is read-only.
139 The use of this variable anywhere in a program imposes a considerable
140 performance penalty on all regular expression matches. See the
141 Devel::SawAmpersand module from CPAN for more information.
147 The string preceding whatever was matched by the last successful
148 pattern match (not counting any matches hidden within a BLOCK or eval
149 enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
150 string.) This variable is read-only.
152 The use of this variable anywhere in a program imposes a considerable
153 performance penalty on all regular expression matches. See the
154 Devel::SawAmpersand module from CPAN for more information.
160 The string following whatever was matched by the last successful
161 pattern match (not counting any matches hidden within a BLOCK or eval()
162 enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
167 print "$`:$&:$'\n"; # prints abc:def:ghi
169 This variable is read-only.
171 The use of this variable anywhere in a program imposes a considerable
172 performance penalty on all regular expression matches. See the
173 Devel::SawAmpersand module from CPAN for more information.
175 =item $LAST_PAREN_MATCH
179 The last bracket matched by the last search pattern. This is useful if
180 you don't know which of a set of alternative patterns matched. For
183 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
185 (Mnemonic: be positive and forward looking.)
186 This variable is read-only.
190 $+[0] is the offset of the end of the last successfull match.
191 C<$+[>I<n>C<]> is the offset of the end of the substring matched by
192 I<n>-th subpattern, or undef if the subpattern did not match.
194 Thus after a match against $_, $& coincides with C<substr $_, $-[0],
195 $+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
196 $+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
197 C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#+> to find the number
198 of subgroups in the last successful match. Note the difference with
199 C<$#->, which is the last I<matched> subgroup. Compare with L<"@-">.
201 =item $MULTILINE_MATCHING
205 Set to 1 to do multi-line matching within a string, 0 to tell Perl
206 that it can assume that strings contain a single line, for the purpose
207 of optimizing pattern matches. Pattern matches on strings containing
208 multiple newlines can produce confusing results when "C<$*>" is 0. Default
209 is 0. (Mnemonic: * matches multiple things.) Note that this variable
210 influences the interpretation of only "C<^>" and "C<$>". A literal newline can
211 be searched for even when C<$* == 0>.
213 Use of "C<$*>" is deprecated in modern Perls, supplanted by
214 the C</s> and C</m> modifiers on pattern matching.
216 =item input_line_number HANDLE EXPR
218 =item $INPUT_LINE_NUMBER
224 The current input line number for the last file handle from
225 which you read (or performed a C<seek> or C<tell> on). The value
226 may be different from the actual physical line number in the file,
227 depending on what notion of "line" is in effect--see L<$/> on how
229 explicit close on a filehandle resets the line number. Because
230 "C<E<lt>E<gt>>" never does an explicit close, line numbers increase
231 across ARGV files (but see examples under eof()). Localizing C<$.> has
232 the effect of also localizing Perl's notion of "the last read
233 filehandle". (Mnemonic: many programs use "." to mean the current line
236 =item input_record_separator HANDLE EXPR
238 =item $INPUT_RECORD_SEPARATOR
244 The input record separator, newline by default. This is used to
245 influence Perl's idea of what a "line" is. Works like B<awk>'s RS
246 variable, including treating empty lines as delimiters if set to the
247 null string. (Note: An empty line cannot contain any spaces or tabs.)
248 You may set it to a multi-character string to match a multi-character
249 delimiter, or to C<undef> to read to end of file. Note that setting it
250 to C<"\n\n"> means something slightly different than setting it to
251 C<"">, if the file contains consecutive empty lines. Setting it to
252 C<""> will treat two or more consecutive empty lines as a single empty
253 line. Setting it to C<"\n\n"> will blindly assume that the next input
254 character belongs to the next paragraph, even if it's a newline.
255 (Mnemonic: / is used to delimit line boundaries when quoting poetry.)
257 undef $/; # enable "slurp" mode
258 $_ = <FH>; # whole file now here
261 Remember: the value of $/ is a string, not a regexp. AWK has to be
262 better for something :-)
264 Setting $/ to a reference to an integer, scalar containing an integer, or
265 scalar that's convertable to an integer will attempt to read records
266 instead of lines, with the maximum record size being the referenced
269 $/ = \32768; # or \"32768", or \$var_containing_32768
273 will read a record of no more than 32768 bytes from FILE. If you're not
274 reading from a record-oriented file (or your OS doesn't have
275 record-oriented files), then you'll likely get a full chunk of data with
276 every read. If a record is larger than the record size you've set, you'll
277 get the record back in pieces.
279 On VMS, record reads are done with the equivalent of C<sysread>, so it's
280 best not to mix record and non-record reads on the same file. (This is
281 likely not a problem, as any file you'd want to read in record mode is
282 probably usable in line mode) Non-VMS systems perform normal I/O, so
283 it's safe to mix record and non-record reads of a file.
287 =item autoflush HANDLE EXPR
289 =item $OUTPUT_AUTOFLUSH
293 If set to nonzero, forces a flush right away and after every write or print on the
294 currently selected output channel. Default is 0 (regardless of whether
295 the channel is actually buffered by the system or not; C<$|> tells you
296 only whether you've asked Perl explicitly to flush after each write).
297 Note that STDOUT will typically be line buffered if output is to the
298 terminal and block buffered otherwise. Setting this variable is useful
299 primarily when you are outputting to a pipe, such as when you are running
300 a Perl script under rsh and want to see the output as it's happening. This
301 has no effect on input buffering.
302 (Mnemonic: when you want your pipes to be piping hot.)
304 =item output_field_separator HANDLE EXPR
306 =item $OUTPUT_FIELD_SEPARATOR
312 The output field separator for the print operator. Ordinarily the
313 print operator simply prints out the comma-separated fields you
314 specify. To get behavior more like B<awk>, set this variable
315 as you would set B<awk>'s OFS variable to specify what is printed
316 between fields. (Mnemonic: what is printed when there is a , in your
319 =item output_record_separator HANDLE EXPR
321 =item $OUTPUT_RECORD_SEPARATOR
327 The output record separator for the print operator. Ordinarily the
328 print operator simply prints out the comma-separated fields you
329 specify, with no trailing newline or record separator assumed.
330 To get behavior more like B<awk>, set this variable as you would
331 set B<awk>'s ORS variable to specify what is printed at the end of the
332 print. (Mnemonic: you set "C<$\>" instead of adding \n at the end of the
333 print. Also, it's just like C<$/>, but it's what you get "back" from
336 =item $LIST_SEPARATOR
340 This is like "C<$,>" except that it applies to array values interpolated
341 into a double-quoted string (or similar interpreted string). Default
342 is a space. (Mnemonic: obvious, I think.)
344 =item $SUBSCRIPT_SEPARATOR
350 The subscript separator for multidimensional array emulation. If you
351 refer to a hash element as
357 $foo{join($;, $a, $b, $c)}
361 @foo{$a,$b,$c} # a slice--note the @
365 ($foo{$a},$foo{$b},$foo{$c})
367 Default is "\034", the same as SUBSEP in B<awk>. Note that if your
368 keys contain binary data there might not be any safe value for "C<$;>".
369 (Mnemonic: comma (the syntactic subscript separator) is a
370 semi-semicolon. Yeah, I know, it's pretty lame, but "C<$,>" is already
371 taken for something more important.)
373 Consider using "real" multidimensional arrays.
379 The output format for printed numbers. This variable is a half-hearted
380 attempt to emulate B<awk>'s OFMT variable. There are times, however,
381 when B<awk> and Perl have differing notions of what is in fact
382 numeric. The initial value is %.I<n>g, where I<n> is the value
383 of the macro DBL_DIG from your system's F<float.h>. This is different from
384 B<awk>'s default OFMT setting of %.6g, so you need to set "C<$#>"
385 explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
387 Use of "C<$#>" is deprecated.
389 =item format_page_number HANDLE EXPR
391 =item $FORMAT_PAGE_NUMBER
395 The current page number of the currently selected output channel.
396 (Mnemonic: % is page number in B<nroff>.)
398 =item format_lines_per_page HANDLE EXPR
400 =item $FORMAT_LINES_PER_PAGE
404 The current page length (printable lines) of the currently selected
405 output channel. Default is 60. (Mnemonic: = has horizontal lines.)
407 =item format_lines_left HANDLE EXPR
409 =item $FORMAT_LINES_LEFT
413 The number of lines left on the page of the currently selected output
414 channel. (Mnemonic: lines_on_page - lines_printed.)
418 $-[0] is the offset of the start of the last successfull match.
419 C<$-[>I<n>C<]> is the offset of the start of the substring matched by
420 I<n>-th subpattern, or undef if the subpattern did not match.
422 Thus after a match against $_, $& coincides with C<substr $_, $-[0],
423 $+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
424 $+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
425 C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last
426 matched subgroup in the last successful match. Note the difference with
427 C<$#+>, which is the number of subgroups in the regular expression. Compare
430 =item format_name HANDLE EXPR
436 The name of the current report format for the currently selected output
437 channel. Default is name of the filehandle. (Mnemonic: brother to
440 =item format_top_name HANDLE EXPR
442 =item $FORMAT_TOP_NAME
446 The name of the current top-of-page format for the currently selected
447 output channel. Default is name of the filehandle with _TOP
448 appended. (Mnemonic: points to top of page.)
450 =item format_line_break_characters HANDLE EXPR
452 =item $FORMAT_LINE_BREAK_CHARACTERS
456 The current set of characters after which a string may be broken to
457 fill continuation fields (starting with ^) in a format. Default is
458 S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
459 poetry is a part of a line.)
461 =item format_formfeed HANDLE EXPR
463 =item $FORMAT_FORMFEED
467 What formats output to perform a form feed. Default is \f.
473 The current value of the write() accumulator for format() lines. A format
474 contains formline() commands that put their result into C<$^A>. After
475 calling its format, write() prints out the contents of C<$^A> and empties.
476 So you never actually see the contents of C<$^A> unless you call
477 formline() yourself and then look at it. See L<perlform> and
478 L<perlfunc/formline()>.
484 The status returned by the last pipe close, backtick (C<``>) command,
485 or system() operator. Note that this is the status word returned by the
486 wait() system call (or else is made up to look like it). Thus, the exit
487 value of the subprocess is actually (C<$? E<gt>E<gt> 8>), and C<$? & 127>
488 gives which signal, if any, the process died from, and C<$? & 128> reports
489 whether there was a core dump. (Mnemonic: similar to B<sh> and B<ksh>.)
491 Additionally, if the C<h_errno> variable is supported in C, its value
492 is returned via $? if any of the C<gethost*()> functions fail.
494 Note that if you have installed a signal handler for C<SIGCHLD>, the
495 value of C<$?> will usually be wrong outside that handler.
497 Inside an C<END> subroutine C<$?> contains the value that is going to be
498 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
499 change the exit status of the script.
501 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
502 actual VMS exit status, instead of the default emulation of POSIX
505 Also see L<Error Indicators>.
513 If used in a numeric context, yields the current value of errno, with
514 all the usual caveats. (This means that you shouldn't depend on the
515 value of C<$!> to be anything in particular unless you've gotten a
516 specific error return indicating a system error.) If used in a string
517 context, yields the corresponding system error string. You can assign
518 to C<$!> to set I<errno> if, for instance, you want C<"$!"> to return the
519 string for error I<n>, or you want to set the exit value for the die()
520 operator. (Mnemonic: What just went bang?)
522 Also see L<Error Indicators>.
524 =item $EXTENDED_OS_ERROR
528 Error information specific to the current operating system. At
529 the moment, this differs from C<$!> under only VMS, OS/2, and Win32
530 (and for MacPerl). On all other platforms, C<$^E> is always just
533 Under VMS, C<$^E> provides the VMS status value from the last
534 system error. This is more specific information about the last
535 system error than that provided by C<$!>. This is particularly
536 important when C<$!> is set to B<EVMSERR>.
538 Under OS/2, C<$^E> is set to the error code of the last call to
539 OS/2 API either via CRT, or directly from perl.
541 Under Win32, C<$^E> always returns the last error information
542 reported by the Win32 call C<GetLastError()> which describes
543 the last error from within the Win32 API. Most Win32-specific
544 code will report errors via C<$^E>. ANSI C and UNIX-like calls
545 set C<errno> and so most portable Perl code will report errors
548 Caveats mentioned in the description of C<$!> generally apply to
549 C<$^E>, also. (Mnemonic: Extra error explanation.)
551 Also see L<Error Indicators>.
557 The Perl syntax error message from the last eval() command. If null, the
558 last eval() parsed and executed correctly (although the operations you
559 invoked may have failed in the normal fashion). (Mnemonic: Where was
560 the syntax error "at"?)
562 Note that warning messages are not collected in this variable. You can,
563 however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
566 Also see L<Error Indicators>.
574 The process number of the Perl running this script. (Mnemonic: same
583 The real uid of this process. (Mnemonic: it's the uid you came I<FROM>,
584 if you're running setuid.)
586 =item $EFFECTIVE_USER_ID
592 The effective uid of this process. Example:
594 $< = $>; # set real to effective uid
595 ($<,$>) = ($>,$<); # swap real and effective uid
597 (Mnemonic: it's the uid you went I<TO>, if you're running setuid.)
598 Note: "C<$E<lt>>" and "C<$E<gt>>" can be swapped only on machines
599 supporting setreuid().
607 The real gid of this process. If you are on a machine that supports
608 membership in multiple groups simultaneously, gives a space separated
609 list of groups you are in. The first number is the one returned by
610 getgid(), and the subsequent ones by getgroups(), one of which may be
611 the same as the first number.
613 However, a value assigned to "C<$(>" must be a single number used to
614 set the real gid. So the value given by "C<$(>" should I<not> be assigned
615 back to "C<$(>" without being forced numeric, such as by adding zero.
617 (Mnemonic: parentheses are used to I<GROUP> things. The real gid is the
618 group you I<LEFT>, if you're running setgid.)
620 =item $EFFECTIVE_GROUP_ID
626 The effective gid of this process. If you are on a machine that
627 supports membership in multiple groups simultaneously, gives a space
628 separated list of groups you are in. The first number is the one
629 returned by getegid(), and the subsequent ones by getgroups(), one of
630 which may be the same as the first number.
632 Similarly, a value assigned to "C<$)>" must also be a space-separated
633 list of numbers. The first number is used to set the effective gid, and
634 the rest (if any) are passed to setgroups(). To get the effect of an
635 empty list for setgroups(), just repeat the new effective gid; that is,
636 to force an effective gid of 5 and an effectively empty setgroups()
637 list, say C< $) = "5 5" >.
639 (Mnemonic: parentheses are used to I<GROUP> things. The effective gid
640 is the group that's I<RIGHT> for you, if you're running setgid.)
642 Note: "C<$E<lt>>", "C<$E<gt>>", "C<$(>" and "C<$)>" can be set only on
643 machines that support the corresponding I<set[re][ug]id()> routine. "C<$(>"
644 and "C<$)>" can be swapped only on machines supporting setregid().
650 Contains the name of the file containing the Perl script being
651 executed. On some operating systems
652 assigning to "C<$0>" modifies the argument area that the ps(1)
653 program sees. This is more useful as a way of indicating the
654 current program state than it is for hiding the program you're running.
655 (Mnemonic: same as B<sh> and B<ksh>.)
659 The index of the first element in an array, and of the first character
660 in a substring. Default is 0, but you could set it to 1 to make
661 Perl behave more like B<awk> (or Fortran) when subscripting and when
662 evaluating the index() and substr() functions. (Mnemonic: [ begins
665 As of Perl 5, assignment to "C<$[>" is treated as a compiler directive,
666 and cannot influence the behavior of any other file. Its use is
673 The version + patchlevel / 1000 of the Perl interpreter. This variable
674 can be used to determine whether the Perl interpreter executing a
675 script is in the right range of versions. (Mnemonic: Is this version
676 of perl in the right bracket?) Example:
678 warn "No checksumming!\n" if $] < 3.019;
680 See also the documentation of C<use VERSION> and C<require VERSION>
681 for a convenient way to fail if the Perl interpreter is too old.
687 The current value of the flag associated with the B<-c> switch. Mainly
688 of use with B<-MO=...> to allow code to alter its behaviour when being compiled.
689 (For example to automatically AUTOLOADing at compile time rather than normal
690 deferred loading.) Setting C<$^C = 1> is similar to calling C<B::minus_c>.
696 The current value of the debugging flags. (Mnemonic: value of B<-D>
703 The maximum system file descriptor, ordinarily 2. System file
704 descriptors are passed to exec()ed processes, while higher file
705 descriptors are not. Also, during an open(), system file descriptors are
706 preserved even if the open() fails. (Ordinary file descriptors are
707 closed before the open() is attempted.) Note that the close-on-exec
708 status of a file descriptor will be decided according to the value of
709 C<$^F> when the open() or pipe() was called, not the time of the exec().
713 The current set of syntax checks enabled by C<use strict> and other block
714 scoped compiler hints. See the documentation of C<strict> for more details.
720 The current value of the inplace-edit extension. Use C<undef> to disable
721 inplace editing. (Mnemonic: value of B<-i> switch.)
725 By default, running out of memory it is not trappable. However, if
726 compiled for this, Perl may use the contents of C<$^M> as an emergency
727 pool after die()ing with this message. Suppose that your Perl were
728 compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc. Then
732 would allocate a 64K buffer for use when in emergency. See the F<INSTALL>
733 file for information on how to enable this option. As a disincentive to
734 casual use of this advanced feature, there is no L<English> long name for
741 The name of the operating system under which this copy of Perl was
742 built, as determined during the configuration process. The value
743 is identical to C<$Config{'osname'}>.
749 The internal variable for debugging support. Different bits mean the
750 following (subject to change):
756 Debug subroutine enter/exit.
760 Line-by-line debugging.
764 Switch off optimizations.
768 Preserve more data for future interactive inspections.
772 Keep info about source lines on which a subroutine is defined.
776 Start with single-step on.
780 Note that some bits may be relevant at compile-time only, some at
781 run-time only. This is a new mechanism and the details may change.
785 The result of evaluation of the last successful L<perlre/C<(?{ code })>>
786 regular expression assertion. (Excluding those used as switches.) May
791 Current state of the interpreter. Undefined if parsing of the current
792 module/eval is not finished (may happen in $SIG{__DIE__} and
793 $SIG{__WARN__} handlers). True if inside an eval, otherwise false.
799 The time at which the script began running, in seconds since the
800 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
801 and B<-C> filetests are
808 The current value of the warning switch, either TRUE or FALSE.
809 (Mnemonic: related to the B<-w> switch.)
811 =item $EXECUTABLE_NAME
815 The name that the Perl binary itself was executed as, from C's C<argv[0]>.
819 contains the name of the current file when reading from E<lt>E<gt>.
823 The array @ARGV contains the command line arguments intended for the
824 script. Note that C<$#ARGV> is the generally number of arguments minus
825 one, because C<$ARGV[0]> is the first argument, I<NOT> the command name. See
826 "C<$0>" for the command name.
830 The array @INC contains the list of places to look for Perl scripts to
831 be evaluated by the C<do EXPR>, C<require>, or C<use> constructs. It
832 initially consists of the arguments to any B<-I> command line switches,
833 followed by the default Perl library, probably F</usr/local/lib/perl>,
834 followed by ".", to represent the current directory. If you need to
835 modify this at runtime, you should use the C<use lib> pragma
836 to get the machine-dependent library properly loaded also:
838 use lib '/mypath/libdir/';
843 Within a subroutine the array @_ contains the parameters passed to that
844 subroutine. See L<perlsub>.
848 The hash %INC contains entries for each filename that has
849 been included via C<do> or C<require>. The key is the filename you
850 specified, and the value is the location of the file actually found.
851 The C<require> command uses this array to determine whether a given file
852 has already been included.
858 The hash %ENV contains your current environment. Setting a
859 value in C<ENV> changes the environment for child processes.
865 The hash %SIG is used to set signal handlers for various
868 sub handler { # 1st argument is signal name
870 print "Caught a SIG$sig--shutting down\n";
875 $SIG{'INT'} = \&handler;
876 $SIG{'QUIT'} = \&handler;
878 $SIG{'INT'} = 'DEFAULT'; # restore default action
879 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
881 Using a value of C<'IGNORE'> usually has the effect of ignoring the
882 signal, except for the C<CHLD> signal. See L<perlipc> for more about
885 The %SIG array contains values for only the signals actually set within
886 the Perl script. Here are some other examples:
888 $SIG{"PIPE"} = Plumber; # SCARY!!
889 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
890 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
891 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
893 The one marked scary is problematic because it's a bareword, which means
894 sometimes it's a string representing the function, and sometimes it's
895 going to call the subroutine call right then and there! Best to be sure
896 and quote it or take a reference to it. *Plumber works too. See L<perlsub>.
898 If your system has the sigaction() function then signal handlers are
899 installed using it. This means you get reliable signal handling. If
900 your system has the SA_RESTART flag it is used when signals handlers are
901 installed. This means that system calls for which it is supported
902 continue rather than returning when a signal arrives. If you want your
903 system calls to be interrupted by signal delivery then do something like
906 use POSIX ':signal_h';
909 sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
910 or die "Error setting SIGALRM handler: $!\n";
914 Certain internal hooks can be also set using the %SIG hash. The
915 routine indicated by C<$SIG{__WARN__}> is called when a warning message is
916 about to be printed. The warning message is passed as the first
917 argument. The presence of a __WARN__ hook causes the ordinary printing
918 of warnings to STDERR to be suppressed. You can use this to save warnings
919 in a variable, or turn warnings into fatal errors, like this:
921 local $SIG{__WARN__} = sub { die $_[0] };
924 The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
925 is about to be thrown. The error message is passed as the first
926 argument. When a __DIE__ hook routine returns, the exception
927 processing continues as it would have in the absence of the hook,
928 unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
929 The C<__DIE__> handler is explicitly disabled during the call, so that you
930 can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
932 Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
933 blocks/strings. See L<perlfunc/die> and L<perlvar/$^S> for how to
936 Note that C<__DIE__>/C<__WARN__> handlers are very special in one
937 respect: they may be called to report (probable) errors found by the
938 parser. In such a case the parser may be in inconsistent state, so
939 any attempt to evaluate Perl code from such a handler will probably
940 result in a segfault. This means that calls which result/may-result
941 in parsing Perl should be used with extreme caution, like this:
943 require Carp if defined $^S;
944 Carp::confess("Something wrong") if defined &Carp::confess;
945 die "Something wrong, but could not load Carp to give backtrace...
946 To see backtrace try starting Perl with -MCarp switch";
948 Here the first line will load Carp I<unless> it is the parser who
949 called the handler. The second line will print backtrace and die if
950 Carp was available. The third line will be executed only if Carp was
953 See L<perlfunc/die>, L<perlfunc/warn> and L<perlfunc/eval> for
958 =head2 Error Indicators
960 The variables L<$@>, L<$!>, L<$^E>, and L<$?> contain information about
961 different types of error conditions that may appear during execution of
962 Perl script. The variables are shown ordered by the "distance" between
963 the subsystem which reported the error and the Perl process, and
964 correspond to errors detected by the Perl interpreter, C library,
965 operating system, or an external program, respectively.
967 To illustrate the differences between these variables, consider the
968 following Perl expression:
971 open PIPE, "/cdrom/install |";
973 close PIPE or die "bad pipe: $?, $!";
976 After execution of this statement all 4 variables may have been set.
978 $@ is set if the string to be C<eval>-ed did not compile (this may happen if
979 C<open> or C<close> were imported with bad prototypes), or if Perl
980 code executed during evaluation die()d (either implicitly, say,
981 if C<open> was imported from module L<Fatal>, or the C<die> after
982 C<close> was triggered). In these cases the value of $@ is the compile
983 error, or C<Fatal> error (which will interpolate C<$!>!), or the argument
984 to C<die> (which will interpolate C<$!> and C<$?>!).
986 When the above expression is executed, open(), C<<PIPEE<gt>>, and C<close>
987 are translated to C run-time library calls. $! is set if one of these
988 calls fails. The value is a symbolic indicator chosen by the C run-time
989 library, say C<No such file or directory>.
991 On some systems the above C library calls are further translated
992 to calls to the kernel. The kernel may have set more verbose error
993 indicator that one of the handful of standard C errors. In such cases $^E
994 contains this verbose error indicator, which may be, say, C<CDROM tray not
995 closed>. On systems where C library calls are identical to system calls
996 $^E is a duplicate of $!.
998 Finally, $? may be set to non-C<0> value if the external program
999 C</cdrom/install> fails. Upper bits of the particular value may reflect
1000 specific error conditions encountered by this program (this is
1001 program-dependent), lower-bits reflect mode of failure (segfault, completion,
1002 etc.). Note that in contrast to $@, $!, and $^E, which are set only
1003 if error condition is detected, the variable $? is set on each C<wait> or
1004 pipe C<close>, overwriting the old value.
1006 For more details, see the individual descriptions at L<$@>, L<$!>, L<$^E>,
1010 =head2 Technical Note on the Syntax of Variable Names
1012 Variable names in Perl can have several formats. Usually, they must
1013 begin with a letter or underscore, in which case they can be
1014 arbitrarily long (up to an internal limit of 256 characters) and may
1015 contain letters, digits, underscores, or the special sequence C<::>.
1016 In this case the part before the last C<::> is taken to be a I<package
1017 qualifier>; see L<perlmod>.
1019 Perl variable names may also be a sequence of digits or a single
1020 punctuation or control character. These names are all reserved for
1021 special uses by Perl; for example, the all-digits names are used to
1022 hold backreferences after a regulare expression match. Perl has a
1023 special syntax for the single-control-character names: It understands
1024 C<^X> (caret C<X>) to mean the control-C<X> character. For example,
1025 the notation C<$^W> (dollar-sign caret C<W>) is the scalar variable
1026 whose name is the single character control-C<W>. This is better than
1027 typing a literal control-C<W> into your program.
1029 Finally, new in Perl 5.006, Perl variable names may be alphanumeric
1030 strings that begin with control characters. These variables must be
1031 written in the form C<${^Foo}>; the braces are not optional.
1032 C<${^Foo}> denotes the scalar variable whose name is a control-C<F>
1033 followed by two C<o>'s. These variables are reserved for future
1034 special uses by Perl, except for the ones that begin with C<^_>
1035 (control-underscore). No control-character name that begins with
1036 C<^_> will acquire a special meaning in any future version of Perl;
1037 such names may therefore be used safely in programs. C<^_> itself,
1038 however, I<is> reserved.
1040 All Perl variables that begin with digits, control characters, or
1041 punctuation characters are exempt from the effects of the C<package>
1042 declaration and are always forced to be in package C<main>. A few
1043 other names are also exempt:
1051 In particular, the new special C<${^_XYZ}> variables are always taken
1052 to be in package C<main> regardless of any C<package> declarations