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 $INPUT_LINE_NUMBER
224 The current input line number for the last file handle from
225 which you read (or performed a C<seek> or C<tell> on). An
226 explicit close on a filehandle resets the line number. Because
227 "C<E<lt>E<gt>>" never does an explicit close, line numbers increase
228 across ARGV files (but see examples under eof()). Localizing C<$.> has
229 the effect of also localizing Perl's notion of "the last read
230 filehandle". (Mnemonic: many programs use "." to mean the current line
233 =item input_record_separator HANDLE EXPR
235 =item $INPUT_RECORD_SEPARATOR
241 The input record separator, newline by default. Works like B<awk>'s RS
242 variable, including treating empty lines as delimiters if set to the
243 null string. (Note: An empty line cannot contain any spaces or tabs.)
244 You may set it to a multi-character string to match a multi-character
245 delimiter, or to C<undef> to read to end of file. Note that setting it
246 to C<"\n\n"> means something slightly different than setting it to
247 C<"">, if the file contains consecutive empty lines. Setting it to
248 C<""> will treat two or more consecutive empty lines as a single empty
249 line. Setting it to C<"\n\n"> will blindly assume that the next input
250 character belongs to the next paragraph, even if it's a newline.
251 (Mnemonic: / is used to delimit line boundaries when quoting poetry.)
254 $_ = <FH>; # whole file now here
257 Remember: the value of $/ is a string, not a regexp. AWK has to be
258 better for something :-)
260 Setting $/ to a reference to an integer, scalar containing an integer, or
261 scalar that's convertable to an integer will attempt to read records
262 instead of lines, with the maximum record size being the referenced
265 $/ = \32768; # or \"32768", or \$var_containing_32768
269 will read a record of no more than 32768 bytes from FILE. If you're not
270 reading from a record-oriented file (or your OS doesn't have
271 record-oriented files), then you'll likely get a full chunk of data with
272 every read. If a record is larger than the record size you've set, you'll
273 get the record back in pieces.
275 On VMS, record reads are done with the equivalent of C<sysread>, so it's
276 best not to mix record and non-record reads on the same file. (This is
277 likely not a problem, as any file you'd want to read in record mode is
278 proably usable in line mode) Non-VMS systems perform normal I/O, so
279 it's safe to mix record and non-record reads of a file.
281 =item autoflush HANDLE EXPR
283 =item $OUTPUT_AUTOFLUSH
287 If set to nonzero, forces a flush right away and after every write or print on the
288 currently selected output channel. Default is 0 (regardless of whether
289 the channel is actually buffered by the system or not; C<$|> tells you
290 only whether you've asked Perl explicitly to flush after each write).
291 Note that STDOUT will typically be line buffered if output is to the
292 terminal and block buffered otherwise. Setting this variable is useful
293 primarily when you are outputting to a pipe, such as when you are running
294 a Perl script under rsh and want to see the output as it's happening. This
295 has no effect on input buffering.
296 (Mnemonic: when you want your pipes to be piping hot.)
298 =item output_field_separator HANDLE EXPR
300 =item $OUTPUT_FIELD_SEPARATOR
306 The output field separator for the print operator. Ordinarily the
307 print operator simply prints out the comma-separated fields you
308 specify. To get behavior more like B<awk>, set this variable
309 as you would set B<awk>'s OFS variable to specify what is printed
310 between fields. (Mnemonic: what is printed when there is a , in your
313 =item output_record_separator HANDLE EXPR
315 =item $OUTPUT_RECORD_SEPARATOR
321 The output record separator for the print operator. Ordinarily the
322 print operator simply prints out the comma-separated fields you
323 specify, with no trailing newline or record separator assumed.
324 To get behavior more like B<awk>, set this variable as you would
325 set B<awk>'s ORS variable to specify what is printed at the end of the
326 print. (Mnemonic: you set "C<$\>" instead of adding \n at the end of the
327 print. Also, it's just like C<$/>, but it's what you get "back" from
330 =item $LIST_SEPARATOR
334 This is like "C<$,>" except that it applies to array values interpolated
335 into a double-quoted string (or similar interpreted string). Default
336 is a space. (Mnemonic: obvious, I think.)
338 =item $SUBSCRIPT_SEPARATOR
344 The subscript separator for multidimensional array emulation. If you
345 refer to a hash element as
351 $foo{join($;, $a, $b, $c)}
355 @foo{$a,$b,$c} # a slice--note the @
359 ($foo{$a},$foo{$b},$foo{$c})
361 Default is "\034", the same as SUBSEP in B<awk>. Note that if your
362 keys contain binary data there might not be any safe value for "C<$;>".
363 (Mnemonic: comma (the syntactic subscript separator) is a
364 semi-semicolon. Yeah, I know, it's pretty lame, but "C<$,>" is already
365 taken for something more important.)
367 Consider using "real" multidimensional arrays.
373 The output format for printed numbers. This variable is a half-hearted
374 attempt to emulate B<awk>'s OFMT variable. There are times, however,
375 when B<awk> and Perl have differing notions of what is in fact
376 numeric. The initial value is %.I<n>g, where I<n> is the value
377 of the macro DBL_DIG from your system's F<float.h>. This is different from
378 B<awk>'s default OFMT setting of %.6g, so you need to set "C<$#>"
379 explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
381 Use of "C<$#>" is deprecated.
383 =item format_page_number HANDLE EXPR
385 =item $FORMAT_PAGE_NUMBER
389 The current page number of the currently selected output channel.
390 (Mnemonic: % is page number in B<nroff>.)
392 =item format_lines_per_page HANDLE EXPR
394 =item $FORMAT_LINES_PER_PAGE
398 The current page length (printable lines) of the currently selected
399 output channel. Default is 60. (Mnemonic: = has horizontal lines.)
401 =item format_lines_left HANDLE EXPR
403 =item $FORMAT_LINES_LEFT
407 The number of lines left on the page of the currently selected output
408 channel. (Mnemonic: lines_on_page - lines_printed.)
412 $-[0] is the offset of the start of the last successfull match.
413 C<$-[>I<n>C<]> is the offset of the start of the substring matched by
414 I<n>-th subpattern, or undef if the subpattern did not match.
416 Thus after a match against $_, $& coincides with C<substr $_, $-[0],
417 $+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
418 $+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
419 C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last
420 matched subgroup in the last successful match. Note the difference with
421 C<$#+>, which is the number of subgroups in the regular expression. Compare
424 =item format_name HANDLE EXPR
430 The name of the current report format for the currently selected output
431 channel. Default is name of the filehandle. (Mnemonic: brother to
434 =item format_top_name HANDLE EXPR
436 =item $FORMAT_TOP_NAME
440 The name of the current top-of-page format for the currently selected
441 output channel. Default is name of the filehandle with _TOP
442 appended. (Mnemonic: points to top of page.)
444 =item format_line_break_characters HANDLE EXPR
446 =item $FORMAT_LINE_BREAK_CHARACTERS
450 The current set of characters after which a string may be broken to
451 fill continuation fields (starting with ^) in a format. Default is
452 S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
453 poetry is a part of a line.)
455 =item format_formfeed HANDLE EXPR
457 =item $FORMAT_FORMFEED
461 What formats output to perform a form feed. Default is \f.
467 The current value of the write() accumulator for format() lines. A format
468 contains formline() commands that put their result into C<$^A>. After
469 calling its format, write() prints out the contents of C<$^A> and empties.
470 So you never actually see the contents of C<$^A> unless you call
471 formline() yourself and then look at it. See L<perlform> and
472 L<perlfunc/formline()>.
478 The status returned by the last pipe close, backtick (C<``>) command,
479 or system() operator. Note that this is the status word returned by the
480 wait() system call (or else is made up to look like it). Thus, the exit
481 value of the subprocess is actually (C<$? E<gt>E<gt> 8>), and C<$? & 127>
482 gives which signal, if any, the process died from, and C<$? & 128> reports
483 whether there was a core dump. (Mnemonic: similar to B<sh> and B<ksh>.)
485 Additionally, if the C<h_errno> variable is supported in C, its value
486 is returned via $? if any of the C<gethost*()> functions fail.
488 Note that if you have installed a signal handler for C<SIGCHLD>, the
489 value of C<$?> will usually be wrong outside that handler.
491 Inside an C<END> subroutine C<$?> contains the value that is going to be
492 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
493 change the exit status of the script.
495 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
496 actual VMS exit status, instead of the default emulation of POSIX
499 Also see L<Error Indicators>.
507 If used in a numeric context, yields the current value of errno, with
508 all the usual caveats. (This means that you shouldn't depend on the
509 value of C<$!> to be anything in particular unless you've gotten a
510 specific error return indicating a system error.) If used in a string
511 context, yields the corresponding system error string. You can assign
512 to C<$!> to set I<errno> if, for instance, you want C<"$!"> to return the
513 string for error I<n>, or you want to set the exit value for the die()
514 operator. (Mnemonic: What just went bang?)
516 Also see L<Error Indicators>.
518 =item $EXTENDED_OS_ERROR
522 Error information specific to the current operating system. At
523 the moment, this differs from C<$!> under only VMS, OS/2, and Win32
524 (and for MacPerl). On all other platforms, C<$^E> is always just
527 Under VMS, C<$^E> provides the VMS status value from the last
528 system error. This is more specific information about the last
529 system error than that provided by C<$!>. This is particularly
530 important when C<$!> is set to B<EVMSERR>.
532 Under OS/2, C<$^E> is set to the error code of the last call to
533 OS/2 API either via CRT, or directly from perl.
535 Under Win32, C<$^E> always returns the last error information
536 reported by the Win32 call C<GetLastError()> which describes
537 the last error from within the Win32 API. Most Win32-specific
538 code will report errors via C<$^E>. ANSI C and UNIX-like calls
539 set C<errno> and so most portable Perl code will report errors
542 Caveats mentioned in the description of C<$!> generally apply to
543 C<$^E>, also. (Mnemonic: Extra error explanation.)
545 Also see L<Error Indicators>.
551 The Perl syntax error message from the last eval() command. If null, the
552 last eval() parsed and executed correctly (although the operations you
553 invoked may have failed in the normal fashion). (Mnemonic: Where was
554 the syntax error "at"?)
556 Note that warning messages are not collected in this variable. You can,
557 however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
560 Also see L<Error Indicators>.
568 The process number of the Perl running this script. (Mnemonic: same
577 The real uid of this process. (Mnemonic: it's the uid you came I<FROM>,
578 if you're running setuid.)
580 =item $EFFECTIVE_USER_ID
586 The effective uid of this process. Example:
588 $< = $>; # set real to effective uid
589 ($<,$>) = ($>,$<); # swap real and effective uid
591 (Mnemonic: it's the uid you went I<TO>, if you're running setuid.)
592 Note: "C<$E<lt>>" and "C<$E<gt>>" can be swapped only on machines
593 supporting setreuid().
601 The real gid of this process. If you are on a machine that supports
602 membership in multiple groups simultaneously, gives a space separated
603 list of groups you are in. The first number is the one returned by
604 getgid(), and the subsequent ones by getgroups(), one of which may be
605 the same as the first number.
607 However, a value assigned to "C<$(>" must be a single number used to
608 set the real gid. So the value given by "C<$(>" should I<not> be assigned
609 back to "C<$(>" without being forced numeric, such as by adding zero.
611 (Mnemonic: parentheses are used to I<GROUP> things. The real gid is the
612 group you I<LEFT>, if you're running setgid.)
614 =item $EFFECTIVE_GROUP_ID
620 The effective gid of this process. If you are on a machine that
621 supports membership in multiple groups simultaneously, gives a space
622 separated list of groups you are in. The first number is the one
623 returned by getegid(), and the subsequent ones by getgroups(), one of
624 which may be the same as the first number.
626 Similarly, a value assigned to "C<$)>" must also be a space-separated
627 list of numbers. The first number is used to set the effective gid, and
628 the rest (if any) are passed to setgroups(). To get the effect of an
629 empty list for setgroups(), just repeat the new effective gid; that is,
630 to force an effective gid of 5 and an effectively empty setgroups()
631 list, say C< $) = "5 5" >.
633 (Mnemonic: parentheses are used to I<GROUP> things. The effective gid
634 is the group that's I<RIGHT> for you, if you're running setgid.)
636 Note: "C<$E<lt>>", "C<$E<gt>>", "C<$(>" and "C<$)>" can be set only on
637 machines that support the corresponding I<set[re][ug]id()> routine. "C<$(>"
638 and "C<$)>" can be swapped only on machines supporting setregid().
644 Contains the name of the file containing the Perl script being
645 executed. On some operating systems
646 assigning to "C<$0>" modifies the argument area that the ps(1)
647 program sees. This is more useful as a way of indicating the
648 current program state than it is for hiding the program you're running.
649 (Mnemonic: same as B<sh> and B<ksh>.)
653 The index of the first element in an array, and of the first character
654 in a substring. Default is 0, but you could set it to 1 to make
655 Perl behave more like B<awk> (or Fortran) when subscripting and when
656 evaluating the index() and substr() functions. (Mnemonic: [ begins
659 As of Perl 5, assignment to "C<$[>" is treated as a compiler directive,
660 and cannot influence the behavior of any other file. Its use is
667 The version + patchlevel / 1000 of the Perl interpreter. This variable
668 can be used to determine whether the Perl interpreter executing a
669 script is in the right range of versions. (Mnemonic: Is this version
670 of perl in the right bracket?) Example:
672 warn "No checksumming!\n" if $] < 3.019;
674 See also the documentation of C<use VERSION> and C<require VERSION>
675 for a convenient way to fail if the Perl interpreter is too old.
681 The current value of the debugging flags. (Mnemonic: value of B<-D>
688 The maximum system file descriptor, ordinarily 2. System file
689 descriptors are passed to exec()ed processes, while higher file
690 descriptors are not. Also, during an open(), system file descriptors are
691 preserved even if the open() fails. (Ordinary file descriptors are
692 closed before the open() is attempted.) Note that the close-on-exec
693 status of a file descriptor will be decided according to the value of
694 C<$^F> when the open() or pipe() was called, not the time of the exec().
698 The current set of syntax checks enabled by C<use strict> and other block
699 scoped compiler hints. See the documentation of C<strict> for more details.
705 The current value of the inplace-edit extension. Use C<undef> to disable
706 inplace editing. (Mnemonic: value of B<-i> switch.)
710 By default, running out of memory it is not trappable. However, if
711 compiled for this, Perl may use the contents of C<$^M> as an emergency
712 pool after die()ing with this message. Suppose that your Perl were
713 compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc. Then
717 would allocate a 64K buffer for use when in emergency. See the F<INSTALL>
718 file for information on how to enable this option. As a disincentive to
719 casual use of this advanced feature, there is no L<English> long name for
726 The name of the operating system under which this copy of Perl was
727 built, as determined during the configuration process. The value
728 is identical to C<$Config{'osname'}>.
734 The internal variable for debugging support. Different bits mean the
735 following (subject to change):
741 Debug subroutine enter/exit.
745 Line-by-line debugging.
749 Switch off optimizations.
753 Preserve more data for future interactive inspections.
757 Keep info about source lines on which a subroutine is defined.
761 Start with single-step on.
765 Note that some bits may be relevent at compile-time only, some at
766 run-time only. This is a new mechanism and the details may change.
770 The result of evaluation of the last successful L<perlre/C<(?{ code })>>
771 regular expression assertion. (Excluding those used as switches.) May
776 Current state of the interpreter. Undefined if parsing of the current
777 module/eval is not finished (may happen in $SIG{__DIE__} and
778 $SIG{__WARN__} handlers). True if inside an eval, otherwise false.
784 The time at which the script began running, in seconds since the
785 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
786 and B<-C> filetests are
793 The current value of the warning switch, either TRUE or FALSE.
794 (Mnemonic: related to the B<-w> switch.)
796 =item $EXECUTABLE_NAME
800 The name that the Perl binary itself was executed as, from C's C<argv[0]>.
804 contains the name of the current file when reading from E<lt>E<gt>.
808 The array @ARGV contains the command line arguments intended for the
809 script. Note that C<$#ARGV> is the generally number of arguments minus
810 one, because C<$ARGV[0]> is the first argument, I<NOT> the command name. See
811 "C<$0>" for the command name.
815 The array @INC contains the list of places to look for Perl scripts to
816 be evaluated by the C<do EXPR>, C<require>, or C<use> constructs. It
817 initially consists of the arguments to any B<-I> command line switches,
818 followed by the default Perl library, probably F</usr/local/lib/perl>,
819 followed by ".", to represent the current directory. If you need to
820 modify this at runtime, you should use the C<use lib> pragma
821 to get the machine-dependent library properly loaded also:
823 use lib '/mypath/libdir/';
828 Within a subroutine the array @_ contains the parameters passed to that
829 subroutine. See L<perlsub>.
833 The hash %INC contains entries for each filename that has
834 been included via C<do> or C<require>. The key is the filename you
835 specified, and the value is the location of the file actually found.
836 The C<require> command uses this array to determine whether a given file
837 has already been included.
839 =item %ENV $ENV{expr}
841 The hash %ENV contains your current environment. Setting a
842 value in C<ENV> changes the environment for child processes.
844 =item %SIG $SIG{expr}
846 The hash %SIG is used to set signal handlers for various
849 sub handler { # 1st argument is signal name
851 print "Caught a SIG$sig--shutting down\n";
856 $SIG{'INT'} = \&handler;
857 $SIG{'QUIT'} = \&handler;
859 $SIG{'INT'} = 'DEFAULT'; # restore default action
860 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
862 Using a value of C<'IGNORE'> usually has the effect of ignoring the
863 signal, except for the C<CHLD> signal. See L<perlipc> for more about
866 The %SIG array contains values for only the signals actually set within
867 the Perl script. Here are some other examples:
869 $SIG{"PIPE"} = Plumber; # SCARY!!
870 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
871 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
872 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
874 The one marked scary is problematic because it's a bareword, which means
875 sometimes it's a string representing the function, and sometimes it's
876 going to call the subroutine call right then and there! Best to be sure
877 and quote it or take a reference to it. *Plumber works too. See L<perlsub>.
879 If your system has the sigaction() function then signal handlers are
880 installed using it. This means you get reliable signal handling. If
881 your system has the SA_RESTART flag it is used when signals handlers are
882 installed. This means that system calls for which it is supported
883 continue rather than returning when a signal arrives. If you want your
884 system calls to be interrupted by signal delivery then do something like
887 use POSIX ':signal_h';
890 sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
891 or die "Error setting SIGALRM handler: $!\n";
895 Certain internal hooks can be also set using the %SIG hash. The
896 routine indicated by C<$SIG{__WARN__}> is called when a warning message is
897 about to be printed. The warning message is passed as the first
898 argument. The presence of a __WARN__ hook causes the ordinary printing
899 of warnings to STDERR to be suppressed. You can use this to save warnings
900 in a variable, or turn warnings into fatal errors, like this:
902 local $SIG{__WARN__} = sub { die $_[0] };
905 The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
906 is about to be thrown. The error message is passed as the first
907 argument. When a __DIE__ hook routine returns, the exception
908 processing continues as it would have in the absence of the hook,
909 unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
910 The C<__DIE__> handler is explicitly disabled during the call, so that you
911 can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
913 Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
914 blocks/strings. See L<perlfunc/die> and L<perlvar/$^S> for how to
917 Note that C<__DIE__>/C<__WARN__> handlers are very special in one
918 respect: they may be called to report (probable) errors found by the
919 parser. In such a case the parser may be in inconsistent state, so
920 any attempt to evaluate Perl code from such a handler will probably
921 result in a segfault. This means that calls which result/may-result
922 in parsing Perl should be used with extreme causion, like this:
924 require Carp if defined $^S;
925 Carp::confess("Something wrong") if defined &Carp::confess;
926 die "Something wrong, but could not load Carp to give backtrace...
927 To see backtrace try starting Perl with -MCarp switch";
929 Here the first line will load Carp I<unless> it is the parser who
930 called the handler. The second line will print backtrace and die if
931 Carp was available. The third line will be executed only if Carp was
934 See L<perlfunc/die>, L<perlfunc/warn> and L<perlfunc/eval> for
939 =head2 Error Indicators
941 The variables L<$@>, L<$!>, L<$^E>, and L<$?> contain information about
942 different types of error conditions that may appear during execution of
943 Perl script. The variables are shown ordered by the "distance" between
944 the subsystem which reported the error and the Perl process, and
945 correspond to errors detected by the Perl interpreter, C library,
946 operating system, or an external program, respectively.
948 To illustrate the differences between these variables, consider the
949 following Perl expression:
952 open PIPE, "/cdrom/install |";
954 close PIPE or die "bad pipe: $?, $!";
957 After execution of this statement all 4 variables may have been set.
959 $@ is set if the string to be C<eval>-ed did not compile (this may happen if
960 C<open> or C<close> were imported with bad prototypes), or if Perl
961 code executed during evaluation die()d (either implicitly, say,
962 if C<open> was imported from module L<Fatal>, or the C<die> after
963 C<close> was triggered). In these cases the value of $@ is the compile
964 error, or C<Fatal> error (which will interpolate C<$!>!), or the argument
965 to C<die> (which will interpolate C<$!> and C<$?>!).
967 When the above expression is executed, open(), C<<PIPEE<gt>>, and C<close>
968 are translated to C run-time library calls. $! is set if one of these
969 calls fails. The value is a symbolic indicator chosen by the C run-time
970 library, say C<No such file or directory>.
972 On some systems the above C library calls are further translated
973 to calls to the kernel. The kernel may have set more verbose error
974 indicator that one of the handful of standard C errors. In such cases $^E
975 contains this verbose error indicator, which may be, say, C<CDROM tray not
976 closed>. On systems where C library calls are identical to system calls
977 $^E is a duplicate of $!.
979 Finally, $? may be set to non-C<0> value if the external program
980 C</cdrom/install> fails. Upper bits of the particular value may reflect
981 specific error conditions encountered by this program (this is
982 program-dependent), lower-bits reflect mode of failure (segfault, completion,
983 etc.). Note that in contrast to $@, $!, and $^E, which are set only
984 if error condition is detected, the variable $? is set on each C<wait> or
985 pipe C<close>, overwriting the old value.
987 For more details, see the individual descriptions at L<$@>, L<$!>, L<$^E>,