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
194 Thus after a match against $_, $& coincides with C<substr $_, $-[0],
195 $+[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
196 $+[>I<0>C<]> if C<$-[>I<n>C<]> is defined, and $+ conincides with
197 C<substr $_, $-[-1], $+[-1]>. One can use C<$#+> to find the last
198 matched subgroup in the last successful match. Compare with L<"@-">.
200 =item $MULTILINE_MATCHING
204 Set to 1 to do multi-line matching within a string, 0 to tell Perl
205 that it can assume that strings contain a single line, for the purpose
206 of optimizing pattern matches. Pattern matches on strings containing
207 multiple newlines can produce confusing results when "C<$*>" is 0. Default
208 is 0. (Mnemonic: * matches multiple things.) Note that this variable
209 influences the interpretation of only "C<^>" and "C<$>". A literal newline can
210 be searched for even when C<$* == 0>.
212 Use of "C<$*>" is deprecated in modern Perls, supplanted by
213 the C</s> and C</m> modifiers on pattern matching.
215 =item input_line_number HANDLE EXPR
217 =item $INPUT_LINE_NUMBER
223 The current input line number for the last file handle from
224 which you read (or performed a C<seek> or C<tell> on). An
225 explicit close on a filehandle resets the line number. Because
226 "C<E<lt>E<gt>>" never does an explicit close, line numbers increase
227 across ARGV files (but see examples under eof()). Localizing C<$.> has
228 the effect of also localizing Perl's notion of "the last read
229 filehandle". (Mnemonic: many programs use "." to mean the current line
232 =item input_record_separator HANDLE EXPR
234 =item $INPUT_RECORD_SEPARATOR
240 The input record separator, newline by default. Works like B<awk>'s RS
241 variable, including treating empty lines as delimiters if set to the
242 null string. (Note: An empty line cannot contain any spaces or tabs.)
243 You may set it to a multi-character string to match a multi-character
244 delimiter, or to C<undef> to read to end of file. Note that setting it
245 to C<"\n\n"> means something slightly different than setting it to
246 C<"">, if the file contains consecutive empty lines. Setting it to
247 C<""> will treat two or more consecutive empty lines as a single empty
248 line. Setting it to C<"\n\n"> will blindly assume that the next input
249 character belongs to the next paragraph, even if it's a newline.
250 (Mnemonic: / is used to delimit line boundaries when quoting poetry.)
253 $_ = <FH>; # whole file now here
256 Remember: the value of $/ is a string, not a regexp. AWK has to be
257 better for something :-)
259 Setting $/ to a reference to an integer, scalar containing an integer, or
260 scalar that's convertable to an integer will attempt to read records
261 instead of lines, with the maximum record size being the referenced
264 $/ = \32768; # or \"32768", or \$var_containing_32768
268 will read a record of no more than 32768 bytes from FILE. If you're not
269 reading from a record-oriented file (or your OS doesn't have
270 record-oriented files), then you'll likely get a full chunk of data with
271 every read. If a record is larger than the record size you've set, you'll
272 get the record back in pieces.
274 On VMS, record reads are done with the equivalent of C<sysread>, so it's
275 best not to mix record and non-record reads on the same file. (This is
276 likely not a problem, as any file you'd want to read in record mode is
277 proably usable in line mode) Non-VMS systems perform normal I/O, so
278 it's safe to mix record and non-record reads of a file.
280 =item autoflush HANDLE EXPR
282 =item $OUTPUT_AUTOFLUSH
286 If set to nonzero, forces a flush right away and after every write or print on the
287 currently selected output channel. Default is 0 (regardless of whether
288 the channel is actually buffered by the system or not; C<$|> tells you
289 only whether you've asked Perl explicitly to flush after each write).
290 Note that STDOUT will typically be line buffered if output is to the
291 terminal and block buffered otherwise. Setting this variable is useful
292 primarily when you are outputting to a pipe, such as when you are running
293 a Perl script under rsh and want to see the output as it's happening. This
294 has no effect on input buffering.
295 (Mnemonic: when you want your pipes to be piping hot.)
297 =item output_field_separator HANDLE EXPR
299 =item $OUTPUT_FIELD_SEPARATOR
305 The output field separator for the print operator. Ordinarily the
306 print operator simply prints out the comma-separated fields you
307 specify. To get behavior more like B<awk>, set this variable
308 as you would set B<awk>'s OFS variable to specify what is printed
309 between fields. (Mnemonic: what is printed when there is a , in your
312 =item output_record_separator HANDLE EXPR
314 =item $OUTPUT_RECORD_SEPARATOR
320 The output record separator for the print operator. Ordinarily the
321 print operator simply prints out the comma-separated fields you
322 specify, with no trailing newline or record separator assumed.
323 To get behavior more like B<awk>, set this variable as you would
324 set B<awk>'s ORS variable to specify what is printed at the end of the
325 print. (Mnemonic: you set "C<$\>" instead of adding \n at the end of the
326 print. Also, it's just like C<$/>, but it's what you get "back" from
329 =item $LIST_SEPARATOR
333 This is like "C<$,>" except that it applies to array values interpolated
334 into a double-quoted string (or similar interpreted string). Default
335 is a space. (Mnemonic: obvious, I think.)
337 =item $SUBSCRIPT_SEPARATOR
343 The subscript separator for multidimensional array emulation. If you
344 refer to a hash element as
350 $foo{join($;, $a, $b, $c)}
354 @foo{$a,$b,$c} # a slice--note the @
358 ($foo{$a},$foo{$b},$foo{$c})
360 Default is "\034", the same as SUBSEP in B<awk>. Note that if your
361 keys contain binary data there might not be any safe value for "C<$;>".
362 (Mnemonic: comma (the syntactic subscript separator) is a
363 semi-semicolon. Yeah, I know, it's pretty lame, but "C<$,>" is already
364 taken for something more important.)
366 Consider using "real" multidimensional arrays.
372 The output format for printed numbers. This variable is a half-hearted
373 attempt to emulate B<awk>'s OFMT variable. There are times, however,
374 when B<awk> and Perl have differing notions of what is in fact
375 numeric. The initial value is %.I<n>g, where I<n> is the value
376 of the macro DBL_DIG from your system's F<float.h>. This is different from
377 B<awk>'s default OFMT setting of %.6g, so you need to set "C<$#>"
378 explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
380 Use of "C<$#>" is deprecated.
382 =item format_page_number HANDLE EXPR
384 =item $FORMAT_PAGE_NUMBER
388 The current page number of the currently selected output channel.
389 (Mnemonic: % is page number in B<nroff>.)
391 =item format_lines_per_page HANDLE EXPR
393 =item $FORMAT_LINES_PER_PAGE
397 The current page length (printable lines) of the currently selected
398 output channel. Default is 60. (Mnemonic: = has horizontal lines.)
400 =item format_lines_left HANDLE EXPR
402 =item $FORMAT_LINES_LEFT
406 The number of lines left on the page of the currently selected output
407 channel. (Mnemonic: lines_on_page - lines_printed.)
411 $-[0] is the offset of the start of the last successfull match.
412 C<$-[>I<n>C<]> is the offset of the start of the substring matched by
415 Thus after a match against $_, $& coincides with C<substr $_, $-[0],
416 $+[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
417 $+[>I<0>C<]> if C<$-[>I<n>C<]> is defined, and $+ conincides with
418 C<substr $_, $-[-1], $+[-1]>. One can use C<$#-> to find the last
419 matched subgroup in the last successful match. Compare with L<"@+">.
421 =item format_name HANDLE EXPR
427 The name of the current report format for the currently selected output
428 channel. Default is name of the filehandle. (Mnemonic: brother to
431 =item format_top_name HANDLE EXPR
433 =item $FORMAT_TOP_NAME
437 The name of the current top-of-page format for the currently selected
438 output channel. Default is name of the filehandle with _TOP
439 appended. (Mnemonic: points to top of page.)
441 =item format_line_break_characters HANDLE EXPR
443 =item $FORMAT_LINE_BREAK_CHARACTERS
447 The current set of characters after which a string may be broken to
448 fill continuation fields (starting with ^) in a format. Default is
449 S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
450 poetry is a part of a line.)
452 =item format_formfeed HANDLE EXPR
454 =item $FORMAT_FORMFEED
458 What formats output to perform a form feed. Default is \f.
464 The current value of the write() accumulator for format() lines. A format
465 contains formline() commands that put their result into C<$^A>. After
466 calling its format, write() prints out the contents of C<$^A> and empties.
467 So you never actually see the contents of C<$^A> unless you call
468 formline() yourself and then look at it. See L<perlform> and
469 L<perlfunc/formline()>.
475 The status returned by the last pipe close, backtick (C<``>) command,
476 or system() operator. Note that this is the status word returned by the
477 wait() system call (or else is made up to look like it). Thus, the exit
478 value of the subprocess is actually (C<$? E<gt>E<gt> 8>), and C<$? & 127>
479 gives which signal, if any, the process died from, and C<$? & 128> reports
480 whether there was a core dump. (Mnemonic: similar to B<sh> and B<ksh>.)
482 Additionally, if the C<h_errno> variable is supported in C, its value
483 is returned via $? if any of the C<gethost*()> functions fail.
485 Note that if you have installed a signal handler for C<SIGCHLD>, the
486 value of C<$?> will usually be wrong outside that handler.
488 Inside an C<END> subroutine C<$?> contains the value that is going to be
489 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
490 change the exit status of the script.
492 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
493 actual VMS exit status, instead of the default emulation of POSIX
496 Also see L<Error Indicators>.
504 If used in a numeric context, yields the current value of errno, with
505 all the usual caveats. (This means that you shouldn't depend on the
506 value of C<$!> to be anything in particular unless you've gotten a
507 specific error return indicating a system error.) If used in a string
508 context, yields the corresponding system error string. You can assign
509 to C<$!> to set I<errno> if, for instance, you want C<"$!"> to return the
510 string for error I<n>, or you want to set the exit value for the die()
511 operator. (Mnemonic: What just went bang?)
513 Also see L<Error Indicators>.
515 =item $EXTENDED_OS_ERROR
519 Error information specific to the current operating system. At
520 the moment, this differs from C<$!> under only VMS, OS/2, and Win32
521 (and for MacPerl). On all other platforms, C<$^E> is always just
524 Under VMS, C<$^E> provides the VMS status value from the last
525 system error. This is more specific information about the last
526 system error than that provided by C<$!>. This is particularly
527 important when C<$!> is set to B<EVMSERR>.
529 Under OS/2, C<$^E> is set to the error code of the last call to
530 OS/2 API either via CRT, or directly from perl.
532 Under Win32, C<$^E> always returns the last error information
533 reported by the Win32 call C<GetLastError()> which describes
534 the last error from within the Win32 API. Most Win32-specific
535 code will report errors via C<$^E>. ANSI C and UNIX-like calls
536 set C<errno> and so most portable Perl code will report errors
539 Caveats mentioned in the description of C<$!> generally apply to
540 C<$^E>, also. (Mnemonic: Extra error explanation.)
542 Also see L<Error Indicators>.
548 The Perl syntax error message from the last eval() command. If null, the
549 last eval() parsed and executed correctly (although the operations you
550 invoked may have failed in the normal fashion). (Mnemonic: Where was
551 the syntax error "at"?)
553 Note that warning messages are not collected in this variable. You can,
554 however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
557 Also see L<Error Indicators>.
565 The process number of the Perl running this script. (Mnemonic: same
574 The real uid of this process. (Mnemonic: it's the uid you came I<FROM>,
575 if you're running setuid.)
577 =item $EFFECTIVE_USER_ID
583 The effective uid of this process. Example:
585 $< = $>; # set real to effective uid
586 ($<,$>) = ($>,$<); # swap real and effective uid
588 (Mnemonic: it's the uid you went I<TO>, if you're running setuid.)
589 Note: "C<$E<lt>>" and "C<$E<gt>>" can be swapped only on machines
590 supporting setreuid().
598 The real gid of this process. If you are on a machine that supports
599 membership in multiple groups simultaneously, gives a space separated
600 list of groups you are in. The first number is the one returned by
601 getgid(), and the subsequent ones by getgroups(), one of which may be
602 the same as the first number.
604 However, a value assigned to "C<$(>" must be a single number used to
605 set the real gid. So the value given by "C<$(>" should I<not> be assigned
606 back to "C<$(>" without being forced numeric, such as by adding zero.
608 (Mnemonic: parentheses are used to I<GROUP> things. The real gid is the
609 group you I<LEFT>, if you're running setgid.)
611 =item $EFFECTIVE_GROUP_ID
617 The effective gid of this process. If you are on a machine that
618 supports membership in multiple groups simultaneously, gives a space
619 separated list of groups you are in. The first number is the one
620 returned by getegid(), and the subsequent ones by getgroups(), one of
621 which may be the same as the first number.
623 Similarly, a value assigned to "C<$)>" must also be a space-separated
624 list of numbers. The first number is used to set the effective gid, and
625 the rest (if any) are passed to setgroups(). To get the effect of an
626 empty list for setgroups(), just repeat the new effective gid; that is,
627 to force an effective gid of 5 and an effectively empty setgroups()
628 list, say C< $) = "5 5" >.
630 (Mnemonic: parentheses are used to I<GROUP> things. The effective gid
631 is the group that's I<RIGHT> for you, if you're running setgid.)
633 Note: "C<$E<lt>>", "C<$E<gt>>", "C<$(>" and "C<$)>" can be set only on
634 machines that support the corresponding I<set[re][ug]id()> routine. "C<$(>"
635 and "C<$)>" can be swapped only on machines supporting setregid().
641 Contains the name of the file containing the Perl script being
642 executed. On some operating systems
643 assigning to "C<$0>" modifies the argument area that the ps(1)
644 program sees. This is more useful as a way of indicating the
645 current program state than it is for hiding the program you're running.
646 (Mnemonic: same as B<sh> and B<ksh>.)
650 The index of the first element in an array, and of the first character
651 in a substring. Default is 0, but you could set it to 1 to make
652 Perl behave more like B<awk> (or Fortran) when subscripting and when
653 evaluating the index() and substr() functions. (Mnemonic: [ begins
656 As of Perl 5, assignment to "C<$[>" is treated as a compiler directive,
657 and cannot influence the behavior of any other file. Its use is
664 The version + patchlevel / 1000 of the Perl interpreter. This variable
665 can be used to determine whether the Perl interpreter executing a
666 script is in the right range of versions. (Mnemonic: Is this version
667 of perl in the right bracket?) Example:
669 warn "No checksumming!\n" if $] < 3.019;
671 See also the documentation of C<use VERSION> and C<require VERSION>
672 for a convenient way to fail if the Perl interpreter is too old.
678 The current value of the debugging flags. (Mnemonic: value of B<-D>
685 The maximum system file descriptor, ordinarily 2. System file
686 descriptors are passed to exec()ed processes, while higher file
687 descriptors are not. Also, during an open(), system file descriptors are
688 preserved even if the open() fails. (Ordinary file descriptors are
689 closed before the open() is attempted.) Note that the close-on-exec
690 status of a file descriptor will be decided according to the value of
691 C<$^F> when the open() or pipe() was called, not the time of the exec().
695 The current set of syntax checks enabled by C<use strict> and other block
696 scoped compiler hints. See the documentation of C<strict> for more details.
702 The current value of the inplace-edit extension. Use C<undef> to disable
703 inplace editing. (Mnemonic: value of B<-i> switch.)
707 By default, running out of memory it is not trappable. However, if
708 compiled for this, Perl may use the contents of C<$^M> as an emergency
709 pool after die()ing with this message. Suppose that your Perl were
710 compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc. Then
714 would allocate a 64K buffer for use when in emergency. See the F<INSTALL>
715 file for information on how to enable this option. As a disincentive to
716 casual use of this advanced feature, there is no L<English> long name for
723 The name of the operating system under which this copy of Perl was
724 built, as determined during the configuration process. The value
725 is identical to C<$Config{'osname'}>.
731 The internal variable for debugging support. Different bits mean the
732 following (subject to change):
738 Debug subroutine enter/exit.
742 Line-by-line debugging.
746 Switch off optimizations.
750 Preserve more data for future interactive inspections.
754 Keep info about source lines on which a subroutine is defined.
758 Start with single-step on.
762 Note that some bits may be relevent at compile-time only, some at
763 run-time only. This is a new mechanism and the details may change.
767 The result of evaluation of the last successful L<perlre/C<(?{ code })>>
768 regular expression assertion. (Excluding those used as switches.) May
773 Current state of the interpreter. Undefined if parsing of the current
774 module/eval is not finished (may happen in $SIG{__DIE__} and
775 $SIG{__WARN__} handlers). True if inside an eval, otherwise false.
781 The time at which the script began running, in seconds since the
782 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
783 and B<-C> filetests are
790 The current value of the warning switch, either TRUE or FALSE.
791 (Mnemonic: related to the B<-w> switch.)
793 =item $EXECUTABLE_NAME
797 The name that the Perl binary itself was executed as, from C's C<argv[0]>.
801 contains the name of the current file when reading from E<lt>E<gt>.
805 The array @ARGV contains the command line arguments intended for the
806 script. Note that C<$#ARGV> is the generally number of arguments minus
807 one, because C<$ARGV[0]> is the first argument, I<NOT> the command name. See
808 "C<$0>" for the command name.
812 The array @INC contains the list of places to look for Perl scripts to
813 be evaluated by the C<do EXPR>, C<require>, or C<use> constructs. It
814 initially consists of the arguments to any B<-I> command line switches,
815 followed by the default Perl library, probably F</usr/local/lib/perl>,
816 followed by ".", to represent the current directory. If you need to
817 modify this at runtime, you should use the C<use lib> pragma
818 to get the machine-dependent library properly loaded also:
820 use lib '/mypath/libdir/';
825 Within a subroutine the array @_ contains the parameters passed to that
826 subroutine. See L<perlsub>.
830 The hash %INC contains entries for each filename that has
831 been included via C<do> or C<require>. The key is the filename you
832 specified, and the value is the location of the file actually found.
833 The C<require> command uses this array to determine whether a given file
834 has already been included.
836 =item %ENV $ENV{expr}
838 The hash %ENV contains your current environment. Setting a
839 value in C<ENV> changes the environment for child processes.
841 =item %SIG $SIG{expr}
843 The hash %SIG is used to set signal handlers for various
846 sub handler { # 1st argument is signal name
848 print "Caught a SIG$sig--shutting down\n";
853 $SIG{'INT'} = \&handler;
854 $SIG{'QUIT'} = \&handler;
856 $SIG{'INT'} = 'DEFAULT'; # restore default action
857 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
859 Using a value of C<'IGNORE'> usually has the effect of ignoring the
860 signal, except for the C<CHLD> signal. See L<perlipc> for more about
863 The %SIG array contains values for only the signals actually set within
864 the Perl script. Here are some other examples:
866 $SIG{"PIPE"} = Plumber; # SCARY!!
867 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
868 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
869 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
871 The one marked scary is problematic because it's a bareword, which means
872 sometimes it's a string representing the function, and sometimes it's
873 going to call the subroutine call right then and there! Best to be sure
874 and quote it or take a reference to it. *Plumber works too. See L<perlsub>.
876 If your system has the sigaction() function then signal handlers are
877 installed using it. This means you get reliable signal handling. If
878 your system has the SA_RESTART flag it is used when signals handlers are
879 installed. This means that system calls for which it is supported
880 continue rather than returning when a signal arrives. If you want your
881 system calls to be interrupted by signal delivery then do something like
884 use POSIX ':signal_h';
887 sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
888 or die "Error setting SIGALRM handler: $!\n";
892 Certain internal hooks can be also set using the %SIG hash. The
893 routine indicated by C<$SIG{__WARN__}> is called when a warning message is
894 about to be printed. The warning message is passed as the first
895 argument. The presence of a __WARN__ hook causes the ordinary printing
896 of warnings to STDERR to be suppressed. You can use this to save warnings
897 in a variable, or turn warnings into fatal errors, like this:
899 local $SIG{__WARN__} = sub { die $_[0] };
902 The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
903 is about to be thrown. The error message is passed as the first
904 argument. When a __DIE__ hook routine returns, the exception
905 processing continues as it would have in the absence of the hook,
906 unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
907 The C<__DIE__> handler is explicitly disabled during the call, so that you
908 can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
910 Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
911 blocks/strings. See L<perlfunc/die> and L<perlvar/$^S> for how to
914 Note that C<__DIE__>/C<__WARN__> handlers are very special in one
915 respect: they may be called to report (probable) errors found by the
916 parser. In such a case the parser may be in inconsistent state, so
917 any attempt to evaluate Perl code from such a handler will probably
918 result in a segfault. This means that calls which result/may-result
919 in parsing Perl should be used with extreme causion, like this:
921 require Carp if defined $^S;
922 Carp::confess("Something wrong") if defined &Carp::confess;
923 die "Something wrong, but could not load Carp to give backtrace...
924 To see backtrace try starting Perl with -MCarp switch";
926 Here the first line will load Carp I<unless> it is the parser who
927 called the handler. The second line will print backtrace and die if
928 Carp was available. The third line will be executed only if Carp was
931 See L<perlfunc/die>, L<perlfunc/warn> and L<perlfunc/eval> for
936 =head2 Error Indicators
938 The variables L<$@>, L<$!>, L<$^E>, and L<$?> contain information about
939 different types of error conditions that may appear during execution of
940 Perl script. The variables are shown ordered by the "distance" between
941 the subsystem which reported the error and the Perl process, and
942 correspond to errors detected by the Perl interpreter, C library,
943 operating system, or an external program, respectively.
945 To illustrate the differences between these variables, consider the
946 following Perl expression:
949 open PIPE, "/cdrom/install |";
951 close PIPE or die "bad pipe: $?, $!";
954 After execution of this statement all 4 variables may have been set.
956 $@ is set if the string to be C<eval>-ed did not compile (this may happen if
957 C<open> or C<close> were imported with bad prototypes), or if Perl
958 code executed during evaluation die()d (either implicitly, say,
959 if C<open> was imported from module L<Fatal>, or the C<die> after
960 C<close> was triggered). In these cases the value of $@ is the compile
961 error, or C<Fatal> error (which will interpolate C<$!>!), or the argument
962 to C<die> (which will interpolate C<$!> and C<$?>!).
964 When the above expression is executed, open(), C<<PIPEE<gt>>, and C<close>
965 are translated to C run-time library calls. $! is set if one of these
966 calls fails. The value is a symbolic indicator chosen by the C run-time
967 library, say C<No such file or directory>.
969 On some systems the above C library calls are further translated
970 to calls to the kernel. The kernel may have set more verbose error
971 indicator that one of the handful of standard C errors. In such cases $^E
972 contains this verbose error indicator, which may be, say, C<CDROM tray not
973 closed>. On systems where C library calls are identical to system calls
974 $^E is a duplicate of $!.
976 Finally, $? may be set to non-C<0> value if the external program
977 C</cdrom/install> fails. Upper bits of the particular value may reflect
978 specific error conditions encountered by this program (this is
979 program-dependent), lower-bits reflect mode of failure (segfault, completion,
980 etc.). Note that in contrast to $@, $!, and $^E, which are set only
981 if error condition is detected, the variable $? is set on each C<wait> or
982 pipe C<close>, overwriting the old value.
984 For more details, see the individual descriptions at L<$@>, L<$!>, L<$^E>,