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 To go a step further, those variables that depend on the currently
21 selected filehandle may instead (and preferably) be set by calling an
22 object method on the FileHandle object. (Summary lines below for this
23 contain the word HANDLE.) First you must say
27 after which you may use either
35 Each of the methods returns the old value of the FileHandle attribute.
36 The methods each take an optional EXPR, which if supplied specifies the
37 new value for the FileHandle attribute in question. If not supplied,
38 most of the methods do nothing to the current value, except for
39 autoflush(), which will assume a 1 for you, just to be different.
41 A few of these variables are considered "read-only". This means that if
42 you try to assign to this variable, either directly or indirectly through
43 a reference, you'll raise a run-time exception.
45 The following list is ordered by scalar variables first, then the
46 arrays, then the hashes (except $^M was added in the wrong place).
47 This is somewhat obscured by the fact that %ENV and %SIG are listed as
48 $ENV{expr} and $SIG{expr}.
57 The default input and pattern-searching space. The following pairs are
60 while (<>) {...} # equivalent in only while!
61 while (defined($_ = <>)) {...}
72 Here are the places where Perl will assume $_ even if you
79 Various unary functions, including functions like ord() and int(), as well
80 as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
85 Various list functions like print() and unlink().
89 The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
90 without an C<=~> operator.
94 The default iterator variable in a C<foreach> loop if no other
99 The implicit iterator variable in the grep() and map() functions.
103 The default place to put an input record when a C<E<lt>FHE<gt>>
104 operation's result is tested by itself as the sole criterion of a C<while>
105 test. Note that outside of a C<while> test, this will not happen.
109 (Mnemonic: underline is understood in certain operations.)
115 =item $E<lt>I<digits>E<gt>
117 Contains the subpattern from the corresponding set of parentheses in
118 the last pattern matched, not counting patterns matched in nested
119 blocks that have been exited already. (Mnemonic: like \digits.)
120 These variables are all read-only.
126 The string matched by the last successful pattern match (not counting
127 any matches hidden within a BLOCK or eval() enclosed by the current
128 BLOCK). (Mnemonic: like & in some editors.) This variable is read-only.
134 The string preceding whatever was matched by the last successful
135 pattern match (not counting any matches hidden within a BLOCK or eval
136 enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
137 string.) This variable is read-only.
143 The string following whatever was matched by the last successful
144 pattern match (not counting any matches hidden within a BLOCK or eval()
145 enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
150 print "$`:$&:$'\n"; # prints abc:def:ghi
152 This variable is read-only.
154 =item $LAST_PAREN_MATCH
158 The last bracket matched by the last search pattern. This is useful if
159 you don't know which of a set of alternative patterns matched. For
162 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
164 (Mnemonic: be positive and forward looking.)
165 This variable is read-only.
169 $+[0] is the offset of the end of the last successfull match.
170 C<$+[>I<n>C<]> is the offset of the end of the substring matched by
173 Thus after a match against $_, $& coincides with C<substr $_, $-[0],
174 $+[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
175 $+[>I<0>C<]> if C<$-[>I<n>C<]> is defined, and $+ conincides with
176 C<substr $_, $-[-1], $+[-1]>. One can use C<$#+> to find the last
177 matched subgroup in the last successful match. Compare with L<"@-">.
179 =item $MULTILINE_MATCHING
183 Set to 1 to do multi-line matching within a string, 0 to tell Perl
184 that it can assume that strings contain a single line, for the purpose
185 of optimizing pattern matches. Pattern matches on strings containing
186 multiple newlines can produce confusing results when "C<$*>" is 0. Default
187 is 0. (Mnemonic: * matches multiple things.) Note that this variable
188 influences the interpretation of only "C<^>" and "C<$>". A literal newline can
189 be searched for even when C<$* == 0>.
191 Use of "C<$*>" is deprecated in modern Perls, supplanted by
192 the C</s> and C</m> modifiers on pattern matching.
194 =item input_line_number HANDLE EXPR
196 =item $INPUT_LINE_NUMBER
202 The current input line number for the last file handle from
203 which you read (or performed a C<seek> or C<tell> on). An
204 explicit close on a filehandle resets the line number. Because
205 "C<E<lt>E<gt>>" never does an explicit close, line numbers increase
206 across ARGV files (but see examples under eof()). Localizing C<$.> has
207 the effect of also localizing Perl's notion of "the last read
208 filehandle". (Mnemonic: many programs use "." to mean the current line
211 =item input_record_separator HANDLE EXPR
213 =item $INPUT_RECORD_SEPARATOR
219 The input record separator, newline by default. Works like B<awk>'s RS
220 variable, including treating empty lines as delimiters if set to the
221 null string. (Note: An empty line cannot contain any spaces or tabs.)
222 You may set it to a multi-character string to match a multi-character
223 delimiter, or to C<undef> to read to end of file. Note that setting it
224 to C<"\n\n"> means something slightly different than setting it to
225 C<"">, if the file contains consecutive empty lines. Setting it to
226 C<""> will treat two or more consecutive empty lines as a single empty
227 line. Setting it to C<"\n\n"> will blindly assume that the next input
228 character belongs to the next paragraph, even if it's a newline.
229 (Mnemonic: / is used to delimit line boundaries when quoting poetry.)
232 $_ = <FH>; # whole file now here
235 Remember: the value of $/ is a string, not a regexp. AWK has to be
236 better for something :-)
238 Setting $/ to a reference to an integer, scalar containing an integer, or
239 scalar that's convertable to an integer will attempt to read records
240 instead of lines, with the maximum record size being the referenced
243 $/ = \32768; # or \"32768", or \$var_containing_32768
247 will read a record of no more than 32768 bytes from FILE. If you're not
248 reading from a record-oriented file (or your OS doesn't have
249 record-oriented files), then you'll likely get a full chunk of data with
250 every read. If a record is larger than the record size you've set, you'll
251 get the record back in pieces.
253 On VMS, record reads are done with the equivalent of C<sysread>, so it's
254 best not to mix record and non-record reads on the same file. (This is
255 likely not a problem, as any file you'd want to read in record mode is
256 proably usable in line mode) Non-VMS systems perform normal I/O, so
257 it's safe to mix record and non-record reads of a file.
259 =item autoflush HANDLE EXPR
261 =item $OUTPUT_AUTOFLUSH
265 If set to nonzero, forces a flush right away and after every write or print on the
266 currently selected output channel. Default is 0 (regardless of whether
267 the channel is actually buffered by the system or not; C<$|> tells you
268 only whether you've asked Perl explicitly to flush after each write).
269 Note that STDOUT will typically be line buffered if output is to the
270 terminal and block buffered otherwise. Setting this variable is useful
271 primarily when you are outputting to a pipe, such as when you are running
272 a Perl script under rsh and want to see the output as it's happening. This
273 has no effect on input buffering.
274 (Mnemonic: when you want your pipes to be piping hot.)
276 =item output_field_separator HANDLE EXPR
278 =item $OUTPUT_FIELD_SEPARATOR
284 The output field separator for the print operator. Ordinarily the
285 print operator simply prints out the comma-separated fields you
286 specify. To get behavior more like B<awk>, set this variable
287 as you would set B<awk>'s OFS variable to specify what is printed
288 between fields. (Mnemonic: what is printed when there is a , in your
291 =item output_record_separator HANDLE EXPR
293 =item $OUTPUT_RECORD_SEPARATOR
299 The output record separator for the print operator. Ordinarily the
300 print operator simply prints out the comma-separated fields you
301 specify, with no trailing newline or record separator assumed.
302 To get behavior more like B<awk>, set this variable as you would
303 set B<awk>'s ORS variable to specify what is printed at the end of the
304 print. (Mnemonic: you set "C<$\>" instead of adding \n at the end of the
305 print. Also, it's just like C<$/>, but it's what you get "back" from
308 =item $LIST_SEPARATOR
312 This is like "C<$,>" except that it applies to array values interpolated
313 into a double-quoted string (or similar interpreted string). Default
314 is a space. (Mnemonic: obvious, I think.)
316 =item $SUBSCRIPT_SEPARATOR
322 The subscript separator for multidimensional array emulation. If you
323 refer to a hash element as
329 $foo{join($;, $a, $b, $c)}
333 @foo{$a,$b,$c} # a slice--note the @
337 ($foo{$a},$foo{$b},$foo{$c})
339 Default is "\034", the same as SUBSEP in B<awk>. Note that if your
340 keys contain binary data there might not be any safe value for "C<$;>".
341 (Mnemonic: comma (the syntactic subscript separator) is a
342 semi-semicolon. Yeah, I know, it's pretty lame, but "C<$,>" is already
343 taken for something more important.)
345 Consider using "real" multidimensional arrays.
351 The output format for printed numbers. This variable is a half-hearted
352 attempt to emulate B<awk>'s OFMT variable. There are times, however,
353 when B<awk> and Perl have differing notions of what is in fact
354 numeric. The initial value is %.I<n>g, where I<n> is the value
355 of the macro DBL_DIG from your system's F<float.h>. This is different from
356 B<awk>'s default OFMT setting of %.6g, so you need to set "C<$#>"
357 explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
359 Use of "C<$#>" is deprecated.
361 =item format_page_number HANDLE EXPR
363 =item $FORMAT_PAGE_NUMBER
367 The current page number of the currently selected output channel.
368 (Mnemonic: % is page number in B<nroff>.)
370 =item format_lines_per_page HANDLE EXPR
372 =item $FORMAT_LINES_PER_PAGE
376 The current page length (printable lines) of the currently selected
377 output channel. Default is 60. (Mnemonic: = has horizontal lines.)
379 =item format_lines_left HANDLE EXPR
381 =item $FORMAT_LINES_LEFT
385 The number of lines left on the page of the currently selected output
386 channel. (Mnemonic: lines_on_page - lines_printed.)
390 $-[0] is the offset of the start of the last successfull match.
391 C<$-[>I<n>C<]> is the offset of the start of the substring matched by
394 Thus after a match against $_, $& coincides with C<substr $_, $-[0],
395 $+[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
396 $+[>I<0>C<]> if C<$-[>I<n>C<]> is defined, and $+ conincides with
397 C<substr $_, $-[-1], $+[-1]>. One can use C<$#-> to find the last
398 matched subgroup in the last successful match. Compare with L<"@+">.
400 =item format_name HANDLE EXPR
406 The name of the current report format for the currently selected output
407 channel. Default is name of the filehandle. (Mnemonic: brother to
410 =item format_top_name HANDLE EXPR
412 =item $FORMAT_TOP_NAME
416 The name of the current top-of-page format for the currently selected
417 output channel. Default is name of the filehandle with _TOP
418 appended. (Mnemonic: points to top of page.)
420 =item format_line_break_characters HANDLE EXPR
422 =item $FORMAT_LINE_BREAK_CHARACTERS
426 The current set of characters after which a string may be broken to
427 fill continuation fields (starting with ^) in a format. Default is
428 S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
429 poetry is a part of a line.)
431 =item format_formfeed HANDLE EXPR
433 =item $FORMAT_FORMFEED
437 What formats output to perform a form feed. Default is \f.
443 The current value of the write() accumulator for format() lines. A format
444 contains formline() commands that put their result into C<$^A>. After
445 calling its format, write() prints out the contents of C<$^A> and empties.
446 So you never actually see the contents of C<$^A> unless you call
447 formline() yourself and then look at it. See L<perlform> and
448 L<perlfunc/formline()>.
454 The status returned by the last pipe close, backtick (C<``>) command,
455 or system() operator. Note that this is the status word returned by the
456 wait() system call (or else is made up to look like it). Thus, the exit
457 value of the subprocess is actually (C<$? E<gt>E<gt> 8>), and C<$? & 127>
458 gives which signal, if any, the process died from, and C<$? & 128> reports
459 whether there was a core dump. (Mnemonic: similar to B<sh> and B<ksh>.)
461 Additionally, if the C<h_errno> variable is supported in C, its value
462 is returned via $? if any of the C<gethost*()> functions fail.
464 Note that if you have installed a signal handler for C<SIGCHLD>, the
465 value of C<$?> will usually be wrong outside that handler.
467 Inside an C<END> subroutine C<$?> contains the value that is going to be
468 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
469 change the exit status of the script.
471 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
472 actual VMS exit status, instead of the default emulation of POSIX
475 Also see L<Error Indicators>.
483 If used in a numeric context, yields the current value of errno, with
484 all the usual caveats. (This means that you shouldn't depend on the
485 value of C<$!> to be anything in particular unless you've gotten a
486 specific error return indicating a system error.) If used in a string
487 context, yields the corresponding system error string. You can assign
488 to C<$!> to set I<errno> if, for instance, you want C<"$!"> to return the
489 string for error I<n>, or you want to set the exit value for the die()
490 operator. (Mnemonic: What just went bang?)
492 Also see L<Error Indicators>.
494 =item $EXTENDED_OS_ERROR
498 Error information specific to the current operating system. At
499 the moment, this differs from C<$!> under only VMS, OS/2, and Win32
500 (and for MacPerl). On all other platforms, C<$^E> is always just
503 Under VMS, C<$^E> provides the VMS status value from the last
504 system error. This is more specific information about the last
505 system error than that provided by C<$!>. This is particularly
506 important when C<$!> is set to B<EVMSERR>.
508 Under OS/2, C<$^E> is set to the error code of the last call to
509 OS/2 API either via CRT, or directly from perl.
511 Under Win32, C<$^E> always returns the last error information
512 reported by the Win32 call C<GetLastError()> which describes
513 the last error from within the Win32 API. Most Win32-specific
514 code will report errors via C<$^E>. ANSI C and UNIX-like calls
515 set C<errno> and so most portable Perl code will report errors
518 Caveats mentioned in the description of C<$!> generally apply to
519 C<$^E>, also. (Mnemonic: Extra error explanation.)
521 Also see L<Error Indicators>.
527 The Perl syntax error message from the last eval() command. If null, the
528 last eval() parsed and executed correctly (although the operations you
529 invoked may have failed in the normal fashion). (Mnemonic: Where was
530 the syntax error "at"?)
532 Note that warning messages are not collected in this variable. You can,
533 however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
536 Also see L<Error Indicators>.
544 The process number of the Perl running this script. (Mnemonic: same
553 The real uid of this process. (Mnemonic: it's the uid you came I<FROM>,
554 if you're running setuid.)
556 =item $EFFECTIVE_USER_ID
562 The effective uid of this process. Example:
564 $< = $>; # set real to effective uid
565 ($<,$>) = ($>,$<); # swap real and effective uid
567 (Mnemonic: it's the uid you went I<TO>, if you're running setuid.)
568 Note: "C<$E<lt>>" and "C<$E<gt>>" can be swapped only on machines
569 supporting setreuid().
577 The real gid of this process. If you are on a machine that supports
578 membership in multiple groups simultaneously, gives a space separated
579 list of groups you are in. The first number is the one returned by
580 getgid(), and the subsequent ones by getgroups(), one of which may be
581 the same as the first number.
583 However, a value assigned to "C<$(>" must be a single number used to
584 set the real gid. So the value given by "C<$(>" should I<not> be assigned
585 back to "C<$(>" without being forced numeric, such as by adding zero.
587 (Mnemonic: parentheses are used to I<GROUP> things. The real gid is the
588 group you I<LEFT>, if you're running setgid.)
590 =item $EFFECTIVE_GROUP_ID
596 The effective gid of this process. If you are on a machine that
597 supports membership in multiple groups simultaneously, gives a space
598 separated list of groups you are in. The first number is the one
599 returned by getegid(), and the subsequent ones by getgroups(), one of
600 which may be the same as the first number.
602 Similarly, a value assigned to "C<$)>" must also be a space-separated
603 list of numbers. The first number is used to set the effective gid, and
604 the rest (if any) are passed to setgroups(). To get the effect of an
605 empty list for setgroups(), just repeat the new effective gid; that is,
606 to force an effective gid of 5 and an effectively empty setgroups()
607 list, say C< $) = "5 5" >.
609 (Mnemonic: parentheses are used to I<GROUP> things. The effective gid
610 is the group that's I<RIGHT> for you, if you're running setgid.)
612 Note: "C<$E<lt>>", "C<$E<gt>>", "C<$(>" and "C<$)>" can be set only on
613 machines that support the corresponding I<set[re][ug]id()> routine. "C<$(>"
614 and "C<$)>" can be swapped only on machines supporting setregid().
620 Contains the name of the file containing the Perl script being
621 executed. On some operating systems
622 assigning to "C<$0>" modifies the argument area that the ps(1)
623 program sees. This is more useful as a way of indicating the
624 current program state than it is for hiding the program you're running.
625 (Mnemonic: same as B<sh> and B<ksh>.)
629 The index of the first element in an array, and of the first character
630 in a substring. Default is 0, but you could set it to 1 to make
631 Perl behave more like B<awk> (or Fortran) when subscripting and when
632 evaluating the index() and substr() functions. (Mnemonic: [ begins
635 As of Perl 5, assignment to "C<$[>" is treated as a compiler directive,
636 and cannot influence the behavior of any other file. Its use is
643 The version + patchlevel / 1000 of the Perl interpreter. This variable
644 can be used to determine whether the Perl interpreter executing a
645 script is in the right range of versions. (Mnemonic: Is this version
646 of perl in the right bracket?) Example:
648 warn "No checksumming!\n" if $] < 3.019;
650 See also the documentation of C<use VERSION> and C<require VERSION>
651 for a convenient way to fail if the Perl interpreter is too old.
657 The current value of the debugging flags. (Mnemonic: value of B<-D>
664 The maximum system file descriptor, ordinarily 2. System file
665 descriptors are passed to exec()ed processes, while higher file
666 descriptors are not. Also, during an open(), system file descriptors are
667 preserved even if the open() fails. (Ordinary file descriptors are
668 closed before the open() is attempted.) Note that the close-on-exec
669 status of a file descriptor will be decided according to the value of
670 C<$^F> at the time of the open, not the time of the exec.
674 The current set of syntax checks enabled by C<use strict> and other block
675 scoped compiler hints. See the documentation of C<strict> for more details.
681 The current value of the inplace-edit extension. Use C<undef> to disable
682 inplace editing. (Mnemonic: value of B<-i> switch.)
686 By default, running out of memory it is not trappable. However, if
687 compiled for this, Perl may use the contents of C<$^M> as an emergency
688 pool after die()ing with this message. Suppose that your Perl were
689 compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc. Then
693 would allocate a 64K buffer for use when in emergency. See the F<INSTALL>
694 file for information on how to enable this option. As a disincentive to
695 casual use of this advanced feature, there is no L<English> long name for
702 The name of the operating system under which this copy of Perl was
703 built, as determined during the configuration process. The value
704 is identical to C<$Config{'osname'}>.
710 The internal variable for debugging support. Different bits mean the
711 following (subject to change):
717 Debug subroutine enter/exit.
721 Line-by-line debugging.
725 Switch off optimizations.
729 Preserve more data for future interactive inspections.
733 Keep info about source lines on which a subroutine is defined.
737 Start with single-step on.
741 Note that some bits may be relevent at compile-time only, some at
742 run-time only. This is a new mechanism and the details may change.
746 The result of evaluation of the last successful L<perlre/C<(?{ code })>>
747 regular expression assertion. (Excluding those used as switches.) May
752 Current state of the interpreter. Undefined if parsing of the current
753 module/eval is not finished (may happen in $SIG{__DIE__} and
754 $SIG{__WARN__} handlers). True if inside an eval, otherwise false.
760 The time at which the script began running, in seconds since the
761 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
762 and B<-C> filetests are
769 The current value of the warning switch, either TRUE or FALSE.
770 (Mnemonic: related to the B<-w> switch.)
772 =item $EXECUTABLE_NAME
776 The name that the Perl binary itself was executed as, from C's C<argv[0]>.
780 contains the name of the current file when reading from E<lt>E<gt>.
784 The array @ARGV contains the command line arguments intended for the
785 script. Note that C<$#ARGV> is the generally number of arguments minus
786 one, because C<$ARGV[0]> is the first argument, I<NOT> the command name. See
787 "C<$0>" for the command name.
791 The array @INC contains the list of places to look for Perl scripts to
792 be evaluated by the C<do EXPR>, C<require>, or C<use> constructs. It
793 initially consists of the arguments to any B<-I> command line switches,
794 followed by the default Perl library, probably F</usr/local/lib/perl>,
795 followed by ".", to represent the current directory. If you need to
796 modify this at runtime, you should use the C<use lib> pragma
797 to get the machine-dependent library properly loaded also:
799 use lib '/mypath/libdir/';
804 Within a subroutine the array @_ contains the parameters passed to that
805 subroutine. See L<perlsub>.
809 The hash %INC contains entries for each filename that has
810 been included via C<do> or C<require>. The key is the filename you
811 specified, and the value is the location of the file actually found.
812 The C<require> command uses this array to determine whether a given file
813 has already been included.
815 =item %ENV $ENV{expr}
817 The hash %ENV contains your current environment. Setting a
818 value in C<ENV> changes the environment for child processes.
820 =item %SIG $SIG{expr}
822 The hash %SIG is used to set signal handlers for various
825 sub handler { # 1st argument is signal name
827 print "Caught a SIG$sig--shutting down\n";
832 $SIG{'INT'} = \&handler;
833 $SIG{'QUIT'} = \&handler;
835 $SIG{'INT'} = 'DEFAULT'; # restore default action
836 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
838 The %SIG array contains values for only the signals actually set within
839 the Perl script. Here are some other examples:
841 $SIG{"PIPE"} = Plumber; # SCARY!!
842 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
843 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
844 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
846 The one marked scary is problematic because it's a bareword, which means
847 sometimes it's a string representing the function, and sometimes it's
848 going to call the subroutine call right then and there! Best to be sure
849 and quote it or take a reference to it. *Plumber works too. See L<perlsub>.
851 If your system has the sigaction() function then signal handlers are
852 installed using it. This means you get reliable signal handling. If
853 your system has the SA_RESTART flag it is used when signals handlers are
854 installed. This means that system calls for which it is supported
855 continue rather than returning when a signal arrives. If you want your
856 system calls to be interrupted by signal delivery then do something like
859 use POSIX ':signal_h';
862 sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
863 or die "Error setting SIGALRM handler: $!\n";
867 Certain internal hooks can be also set using the %SIG hash. The
868 routine indicated by C<$SIG{__WARN__}> is called when a warning message is
869 about to be printed. The warning message is passed as the first
870 argument. The presence of a __WARN__ hook causes the ordinary printing
871 of warnings to STDERR to be suppressed. You can use this to save warnings
872 in a variable, or turn warnings into fatal errors, like this:
874 local $SIG{__WARN__} = sub { die $_[0] };
877 The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
878 is about to be thrown. The error message is passed as the first
879 argument. When a __DIE__ hook routine returns, the exception
880 processing continues as it would have in the absence of the hook,
881 unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
882 The C<__DIE__> handler is explicitly disabled during the call, so that you
883 can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
885 Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
886 blocks/strings. See L<perlfunc/die> and L<perlvar/$^S> for how to
889 Note that C<__DIE__>/C<__WARN__> handlers are very special in one
890 respect: they may be called to report (probable) errors found by the
891 parser. In such a case the parser may be in inconsistent state, so
892 any attempt to evaluate Perl code from such a handler will probably
893 result in a segfault. This means that calls which result/may-result
894 in parsing Perl should be used with extreme causion, like this:
896 require Carp if defined $^S;
897 Carp::confess("Something wrong") if defined &Carp::confess;
898 die "Something wrong, but could not load Carp to give backtrace...
899 To see backtrace try starting Perl with -MCarp switch";
901 Here the first line will load Carp I<unless> it is the parser who
902 called the handler. The second line will print backtrace and die if
903 Carp was available. The third line will be executed only if Carp was
906 See L<perlfunc/die>, L<perlfunc/warn> and L<perlfunc/eval> for
911 =head2 Error Indicators
913 The variables L<$@>, L<$!>, L<$^E>, and L<$?> contain information about
914 different types of error conditions that may appear during execution of
915 Perl script. The variables are shown ordered by the "distance" between
916 the subsystem which reported the error and the Perl process, and
917 correspond to errors detected by the Perl interpreter, C library,
918 operating system, or an external program, respectively.
920 To illustrate the differences between these variables, consider the
921 following Perl expression:
924 open PIPE, "/cdrom/install |";
926 close PIPE or die "bad pipe: $?, $!";
929 After execution of this statement all 4 variables may have been set.
931 $@ is set if the string to be C<eval>-ed did not compile (this may happen if
932 C<open> or C<close> were imported with bad prototypes), or if Perl
933 code executed during evaluation die()d (either implicitly, say,
934 if C<open> was imported from module L<Fatal>, or the C<die> after
935 C<close> was triggered). In these cases the value of $@ is the compile
936 error, or C<Fatal> error (which will interpolate C<$!>!), or the argument
937 to C<die> (which will interpolate C<$!> and C<$?>!).
939 When the above expression is executed, open(), C<<PIPEE<gt>>, and C<close>
940 are translated to C run-time library calls. $! is set if one of these
941 calls fails. The value is a symbolic indicator chosen by the C run-time
942 library, say C<No such file or directory>.
944 On some systems the above C library calls are further translated
945 to calls to the kernel. The kernel may have set more verbose error
946 indicator that one of the handful of standard C errors. In such cases $^E
947 contains this verbose error indicator, which may be, say, C<CDROM tray not
948 closed>. On systems where C library calls are identical to system calls
949 $^E is a duplicate of $!.
951 Finally, $? may be set to non-C<0> value if the external program
952 C</cdrom/install> fails. Upper bits of the particular value may reflect
953 specific error conditions encountered by this program (this is
954 program-dependent), lower-bits reflect mode of failure (segfault, completion,
955 etc.). Note that in contrast to $@, $!, and $^E, which are set only
956 if error condition is detected, the variable $? is set on each C<wait> or
957 pipe C<close>, overwriting the old value.
959 For more details, see the individual descriptions at L<$@>, L<$!>, L<$^E>,