integrate cfgperl changes into mainline

[perl5.git] / pod / perlvar.pod

=head1 NAME

perlvar - Perl predefined variables

=head1 DESCRIPTION

=head2 Predefined Names

The following names have special meaning to Perl.  Most 
punctuation names have reasonable mnemonics, or analogues in one of
the shells.  Nevertheless, if you wish to use long variable names,
you just need to say

    use English;

at the top of your program.  This will alias all the short names to the
long names in the current package.  Some even have medium names,
generally borrowed from B<awk>.

Due to an unfortunate accident of Perl's implementation, "C<use English>"
imposes a considerable performance penalty on all regular expression
matches in a program, regardless of whether they occur in the scope of
"C<use English>".  For that reason, saying "C<use English>" in
libraries is strongly discouraged.  See the Devel::SawAmpersand module
documentation from CPAN
(http://www.perl.com/CPAN/modules/by-module/Devel/Devel-SawAmpersand-0.10.readme)
for more information.

To go a step further, those variables that depend on the currently
selected filehandle may instead (and preferably) be set by calling an
object method on the FileHandle object.  (Summary lines below for this
contain the word HANDLE.)  First you must say

    use FileHandle;

after which you may use either

    method HANDLE EXPR

or more safely,

    HANDLE->method(EXPR)

Each of the methods returns the old value of the FileHandle attribute.
The methods each take an optional EXPR, which if supplied specifies the
new value for the FileHandle attribute in question.  If not supplied,
most of the methods do nothing to the current value, except for
autoflush(), which will assume a 1 for you, just to be different.

A few of these variables are considered "read-only".  This means that if
you try to assign to this variable, either directly or indirectly through
a reference, you'll raise a run-time exception.

The following list is ordered by scalar variables first, then the
arrays, then the hashes (except $^M was added in the wrong place).
This is somewhat obscured by the fact that %ENV and %SIG are listed as
$ENV{expr} and $SIG{expr}.


=over 8

=item $ARG

=item $_

The default input and pattern-searching space.  The following pairs are
equivalent:

    while (<>) {...}    # equivalent in only while!
    while (defined($_ = <>)) {...}

    /^Subject:/
    $_ =~ /^Subject:/

    tr/a-z/A-Z/
    $_ =~ tr/a-z/A-Z/

    chop
    chop($_)

Here are the places where Perl will assume $_ even if you
don't use it:

=over 3

=item *

Various unary functions, including functions like ord() and int(), as well
as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
STDIN.

=item *

Various list functions like print() and unlink().

=item *

The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
without an C<=~> operator.

=item *

The default iterator variable in a C<foreach> loop if no other
variable is supplied.

=item *

The implicit iterator variable in the grep() and map() functions.

=item *

The default place to put an input record when a C<E<lt>FHE<gt>>
operation's result is tested by itself as the sole criterion of a C<while>
test.  Note that outside of a C<while> test, this will not happen.

=back

(Mnemonic: underline is understood in certain operations.)

=back

=over 8

=item $E<lt>I<digits>E<gt>

Contains the subpattern from the corresponding set of parentheses in
the last pattern matched, not counting patterns matched in nested
blocks that have been exited already.  (Mnemonic: like \digits.)
These variables are all read-only.

=item $MATCH

=item $&

The string matched by the last successful pattern match (not counting
any matches hidden within a BLOCK or eval() enclosed by the current
BLOCK).  (Mnemonic: like & in some editors.)  This variable is read-only.

The use of this variable anywhere in a program imposes a considerable
performance penalty on all regular expression matches.  See the
Devel::SawAmpersand module from CPAN for more information.

=item $PREMATCH

=item $`

The string preceding whatever was matched by the last successful
pattern match (not counting any matches hidden within a BLOCK or eval
enclosed by the current BLOCK).  (Mnemonic: C<`> often precedes a quoted
string.)  This variable is read-only.

The use of this variable anywhere in a program imposes a considerable
performance penalty on all regular expression matches.  See the
Devel::SawAmpersand module from CPAN for more information.

=item $POSTMATCH

=item $'

The string following whatever was matched by the last successful
pattern match (not counting any matches hidden within a BLOCK or eval()
enclosed by the current BLOCK).  (Mnemonic: C<'> often follows a quoted
string.)  Example:

    $_ = 'abcdefghi';
    /def/;
    print "$`:$&:$'\n";         # prints abc:def:ghi

This variable is read-only.

The use of this variable anywhere in a program imposes a considerable
performance penalty on all regular expression matches.  See the
Devel::SawAmpersand module from CPAN for more information.

=item $LAST_PAREN_MATCH

=item $+

The last bracket matched by the last search pattern.  This is useful if
you don't know which of a set of alternative patterns matched.  For
example:

    /Version: (.*)|Revision: (.*)/ && ($rev = $+);

(Mnemonic: be positive and forward looking.)
This variable is read-only.

=item @+

$+[0] is the offset of the end of the last successfull match.
C<$+[>I<n>C<]> is the offset of the end of the substring matched by
I<n>-th subpattern, or undef if the subpattern did not match.

Thus after a match against $_, $& coincides with C<substr $_, $-[0],
$+[0] - $-[0]>.  Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
$+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
C<substr $_, $-[$#-], $+[$#-]>.  One can use C<$#+> to find the number
of subgroups in the last successful match.  Note the difference with
C<$#->, which is the last I<matched> subgroup.  Compare with L<"@-">.

=item $MULTILINE_MATCHING

=item $*

Set to 1 to do multi-line matching within a string, 0 to tell Perl
that it can assume that strings contain a single line, for the purpose
of optimizing pattern matches.  Pattern matches on strings containing
multiple newlines can produce confusing results when "C<$*>" is 0.  Default
is 0.  (Mnemonic: * matches multiple things.)  Note that this variable
influences the interpretation of only "C<^>" and "C<$>".  A literal newline can
be searched for even when C<$* == 0>.

Use of "C<$*>" is deprecated in modern Perls, supplanted by 
the C</s> and C</m> modifiers on pattern matching.

=item input_line_number HANDLE EXPR

=item $INPUT_LINE_NUMBER

=item $NR

=item $.

The current input line number for the last file handle from
which you read (or performed a C<seek> or C<tell> on).  An
explicit close on a filehandle resets the line number.  Because
"C<E<lt>E<gt>>" never does an explicit close, line numbers increase
across ARGV files (but see examples under eof()).  Localizing C<$.> has
the effect of also localizing Perl's notion of "the last read
filehandle".  (Mnemonic: many programs use "." to mean the current line
number.)

=item input_record_separator HANDLE EXPR

=item $INPUT_RECORD_SEPARATOR

=item $RS

=item $/

The input record separator, newline by default.  Works like B<awk>'s RS
variable, including treating empty lines as delimiters if set to the
null string.  (Note: An empty line cannot contain any spaces or tabs.)
You may set it to a multi-character string to match a multi-character
delimiter, or to C<undef> to read to end of file.  Note that setting it
to C<"\n\n"> means something slightly different than setting it to
C<"">, if the file contains consecutive empty lines.  Setting it to
C<""> will treat two or more consecutive empty lines as a single empty
line.  Setting it to C<"\n\n"> will blindly assume that the next input
character belongs to the next paragraph, even if it's a newline.
(Mnemonic: / is used to delimit line boundaries when quoting poetry.)

    undef $/;
    $_ = <FH>;          # whole file now here
    s/\n[ \t]+/ /g;

Remember: the value of $/ is a string, not a regexp.  AWK has to be
better for something :-)

Setting $/ to a reference to an integer, scalar containing an integer, or
scalar that's convertable to an integer will attempt to read records
instead of lines, with the maximum record size being the referenced
integer. So this:

    $/ = \32768; # or \"32768", or \$var_containing_32768
    open(FILE, $myfile);
    $_ = <FILE>;

will read a record of no more than 32768 bytes from FILE. If you're not
reading from a record-oriented file (or your OS doesn't have
record-oriented files), then you'll likely get a full chunk of data with
every read. If a record is larger than the record size you've set, you'll
get the record back in pieces.

On VMS, record reads are done with the equivalent of C<sysread>, so it's
best not to mix record and non-record reads on the same file. (This is
likely not a problem, as any file you'd want to read in record mode is
proably usable in line mode) Non-VMS systems perform normal I/O, so
it's safe to mix record and non-record reads of a file.

=item autoflush HANDLE EXPR

=item $OUTPUT_AUTOFLUSH

=item $|

If set to nonzero, forces a flush right away and after every write or print on the
currently selected output channel.  Default is 0 (regardless of whether
the channel is actually buffered by the system or not; C<$|> tells you
only whether you've asked Perl explicitly to flush after each write).
Note that STDOUT will typically be line buffered if output is to the
terminal and block buffered otherwise.  Setting this variable is useful
primarily when you are outputting to a pipe, such as when you are running
a Perl script under rsh and want to see the output as it's happening.  This
has no effect on input buffering.
(Mnemonic: when you want your pipes to be piping hot.)

=item output_field_separator HANDLE EXPR

=item $OUTPUT_FIELD_SEPARATOR

=item $OFS

=item $,

The output field separator for the print operator.  Ordinarily the
print operator simply prints out the comma-separated fields you
specify.  To get behavior more like B<awk>, set this variable
as you would set B<awk>'s OFS variable to specify what is printed
between fields.  (Mnemonic: what is printed when there is a , in your
print statement.)

=item output_record_separator HANDLE EXPR

=item $OUTPUT_RECORD_SEPARATOR

=item $ORS

=item $\

The output record separator for the print operator.  Ordinarily the
print operator simply prints out the comma-separated fields you
specify, with no trailing newline or record separator assumed.
To get behavior more like B<awk>, set this variable as you would
set B<awk>'s ORS variable to specify what is printed at the end of the
print.  (Mnemonic: you set "C<$\>" instead of adding \n at the end of the
print.  Also, it's just like C<$/>, but it's what you get "back" from
Perl.)

=item $LIST_SEPARATOR

=item $"

This is like "C<$,>" except that it applies to array values interpolated
into a double-quoted string (or similar interpreted string).  Default
is a space.  (Mnemonic: obvious, I think.)

=item $SUBSCRIPT_SEPARATOR

=item $SUBSEP

=item $;

The subscript separator for multidimensional array emulation.  If you
refer to a hash element as

    $foo{$a,$b,$c}

it really means

    $foo{join($;, $a, $b, $c)}

But don't put

    @foo{$a,$b,$c}      # a slice--note the @

which means

    ($foo{$a},$foo{$b},$foo{$c})

Default is "\034", the same as SUBSEP in B<awk>.  Note that if your
keys contain binary data there might not be any safe value for "C<$;>".
(Mnemonic: comma (the syntactic subscript separator) is a
semi-semicolon.  Yeah, I know, it's pretty lame, but "C<$,>" is already
taken for something more important.)

Consider using "real" multidimensional arrays.

=item $OFMT

=item $#

The output format for printed numbers.  This variable is a half-hearted
attempt to emulate B<awk>'s OFMT variable.  There are times, however,
when B<awk> and Perl have differing notions of what is in fact
numeric.  The initial value is %.I<n>g, where I<n> is the value
of the macro DBL_DIG from your system's F<float.h>.  This is different from
B<awk>'s default OFMT setting of %.6g, so you need to set "C<$#>"
explicitly to get B<awk>'s value.  (Mnemonic: # is the number sign.)

Use of "C<$#>" is deprecated.

=item format_page_number HANDLE EXPR

=item $FORMAT_PAGE_NUMBER

=item $%

The current page number of the currently selected output channel.
(Mnemonic: % is page number in B<nroff>.)

=item format_lines_per_page HANDLE EXPR

=item $FORMAT_LINES_PER_PAGE

=item $=

The current page length (printable lines) of the currently selected
output channel.  Default is 60.  (Mnemonic: = has horizontal lines.)

=item format_lines_left HANDLE EXPR

=item $FORMAT_LINES_LEFT

=item $-

The number of lines left on the page of the currently selected output
channel.  (Mnemonic: lines_on_page - lines_printed.)

=item @-

$-[0] is the offset of the start of the last successfull match.
C<$-[>I<n>C<]> is the offset of the start of the substring matched by
I<n>-th subpattern, or undef if the subpattern did not match.

Thus after a match against $_, $& coincides with C<substr $_, $-[0],
$+[0] - $-[0]>.  Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
$+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
C<substr $_, $-[$#-], $+[$#-]>.  One can use C<$#-> to find the last
matched subgroup in the last successful match.  Note the difference with
C<$#+>, which is the number of subgroups in the regular expression.  Compare
with L<"@+">.

=item format_name HANDLE EXPR

=item $FORMAT_NAME

=item $~

The name of the current report format for the currently selected output
channel.  Default is name of the filehandle.  (Mnemonic: brother to
"C<$^>".)

=item format_top_name HANDLE EXPR

=item $FORMAT_TOP_NAME

=item $^

The name of the current top-of-page format for the currently selected
output channel.  Default is name of the filehandle with _TOP
appended.  (Mnemonic: points to top of page.)

=item format_line_break_characters HANDLE EXPR

=item $FORMAT_LINE_BREAK_CHARACTERS

=item $:

The current set of characters after which a string may be broken to
fill continuation fields (starting with ^) in a format.  Default is
S<" \n-">, to break on whitespace or hyphens.  (Mnemonic: a "colon" in
poetry is a part of a line.)

=item format_formfeed HANDLE EXPR

=item $FORMAT_FORMFEED

=item $^L

What formats output to perform a form feed.  Default is \f.

=item $ACCUMULATOR

=item $^A

The current value of the write() accumulator for format() lines.  A format
contains formline() commands that put their result into C<$^A>.  After
calling its format, write() prints out the contents of C<$^A> and empties.
So you never actually see the contents of C<$^A> unless you call
formline() yourself and then look at it.  See L<perlform> and
L<perlfunc/formline()>.

=item $CHILD_ERROR

=item $?

The status returned by the last pipe close, backtick (C<``>) command,
or system() operator.  Note that this is the status word returned by the
wait() system call (or else is made up to look like it).  Thus, the exit
value of the subprocess is actually (C<$? E<gt>E<gt> 8>), and C<$? & 127>
gives which signal, if any, the process died from, and C<$? & 128> reports
whether there was a core dump.  (Mnemonic: similar to B<sh> and B<ksh>.)

Additionally, if the C<h_errno> variable is supported in C, its value
is returned via $? if any of the C<gethost*()> functions fail.

Note that if you have installed a signal handler for C<SIGCHLD>, the
value of C<$?> will usually be wrong outside that handler.

Inside an C<END> subroutine C<$?> contains the value that is going to be
given to C<exit()>.  You can modify C<$?> in an C<END> subroutine to
change the exit status of the script.

Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
actual VMS exit status, instead of the default emulation of POSIX
status.

Also see L<Error Indicators>.

=item $OS_ERROR

=item $ERRNO

=item $!

If used in a numeric context, yields the current value of errno, with
all the usual caveats.  (This means that you shouldn't depend on the
value of C<$!> to be anything in particular unless you've gotten a
specific error return indicating a system error.)  If used in a string
context, yields the corresponding system error string.  You can assign
to C<$!> to set I<errno> if, for instance, you want C<"$!"> to return the
string for error I<n>, or you want to set the exit value for the die()
operator.  (Mnemonic: What just went bang?)

Also see L<Error Indicators>.

=item $EXTENDED_OS_ERROR

=item $^E

Error information specific to the current operating system.  At
the moment, this differs from C<$!> under only VMS, OS/2, and Win32
(and for MacPerl).  On all other platforms, C<$^E> is always just
the same as C<$!>.

Under VMS, C<$^E> provides the VMS status value from the last
system error.  This is more specific information about the last
system error than that provided by C<$!>.  This is particularly
important when C<$!> is set to B<EVMSERR>.

Under OS/2, C<$^E> is set to the error code of the last call to
OS/2 API either via CRT, or directly from perl.

Under Win32, C<$^E> always returns the last error information
reported by the Win32 call C<GetLastError()> which describes
the last error from within the Win32 API.  Most Win32-specific
code will report errors via C<$^E>.  ANSI C and UNIX-like calls
set C<errno> and so most portable Perl code will report errors
via C<$!>. 

Caveats mentioned in the description of C<$!> generally apply to
C<$^E>, also.  (Mnemonic: Extra error explanation.)

Also see L<Error Indicators>.

=item $EVAL_ERROR

=item $@

The Perl syntax error message from the last eval() command.  If null, the
last eval() parsed and executed correctly (although the operations you
invoked may have failed in the normal fashion).  (Mnemonic: Where was
the syntax error "at"?)

Note that warning messages are not collected in this variable.  You can,
however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
as described below.

Also see L<Error Indicators>.

=item $PROCESS_ID

=item $PID

=item $$

The process number of the Perl running this script.  (Mnemonic: same
as shells.)

=item $REAL_USER_ID

=item $UID

=item $<

The real uid of this process.  (Mnemonic: it's the uid you came I<FROM>,
if you're running setuid.)

=item $EFFECTIVE_USER_ID

=item $EUID

=item $>

The effective uid of this process.  Example:

    $< = $>;            # set real to effective uid
    ($<,$>) = ($>,$<);  # swap real and effective uid

(Mnemonic: it's the uid you went I<TO>, if you're running setuid.)
Note: "C<$E<lt>>" and "C<$E<gt>>" can be swapped only on machines
supporting setreuid().

=item $REAL_GROUP_ID

=item $GID

=item $(

The real gid of this process.  If you are on a machine that supports
membership in multiple groups simultaneously, gives a space separated
list of groups you are in.  The first number is the one returned by
getgid(), and the subsequent ones by getgroups(), one of which may be
the same as the first number.

However, a value assigned to "C<$(>" must be a single number used to
set the real gid.  So the value given by "C<$(>" should I<not> be assigned
back to "C<$(>" without being forced numeric, such as by adding zero.

(Mnemonic: parentheses are used to I<GROUP> things.  The real gid is the
group you I<LEFT>, if you're running setgid.)

=item $EFFECTIVE_GROUP_ID

=item $EGID

=item $)

The effective gid of this process.  If you are on a machine that
supports membership in multiple groups simultaneously, gives a space
separated list of groups you are in.  The first number is the one
returned by getegid(), and the subsequent ones by getgroups(), one of
which may be the same as the first number.

Similarly, a value assigned to "C<$)>" must also be a space-separated
list of numbers.  The first number is used to set the effective gid, and
the rest (if any) are passed to setgroups().  To get the effect of an
empty list for setgroups(), just repeat the new effective gid; that is,
to force an effective gid of 5 and an effectively empty setgroups()
list, say C< $) = "5 5" >.

(Mnemonic: parentheses are used to I<GROUP> things.  The effective gid
is the group that's I<RIGHT> for you, if you're running setgid.)

Note: "C<$E<lt>>", "C<$E<gt>>", "C<$(>" and "C<$)>" can be set only on
machines that support the corresponding I<set[re][ug]id()> routine.  "C<$(>"
and "C<$)>" can be swapped only on machines supporting setregid().

=item $PROGRAM_NAME

=item $0

Contains the name of the file containing the Perl script being
executed.  On some operating systems
assigning to "C<$0>" modifies the argument area that the ps(1)
program sees.  This is more useful as a way of indicating the
current program state than it is for hiding the program you're running.
(Mnemonic: same as B<sh> and B<ksh>.)

=item $[

The index of the first element in an array, and of the first character
in a substring.  Default is 0, but you could set it to 1 to make
Perl behave more like B<awk> (or Fortran) when subscripting and when
evaluating the index() and substr() functions.  (Mnemonic: [ begins
subscripts.)

As of Perl 5, assignment to "C<$[>" is treated as a compiler directive,
and cannot influence the behavior of any other file.  Its use is
discouraged.

=item $PERL_VERSION

=item $]

The version + patchlevel / 1000 of the Perl interpreter.  This variable
can be used to determine whether the Perl interpreter executing a
script is in the right range of versions.  (Mnemonic: Is this version
of perl in the right bracket?)  Example:

    warn "No checksumming!\n" if $] < 3.019;

See also the documentation of C<use VERSION> and C<require VERSION>
for a convenient way to fail if the Perl interpreter is too old.

=item $DEBUGGING

=item $^D

The current value of the debugging flags.  (Mnemonic: value of B<-D>
switch.)

=item $SYSTEM_FD_MAX

=item $^F

The maximum system file descriptor, ordinarily 2.  System file
descriptors are passed to exec()ed processes, while higher file
descriptors are not.  Also, during an open(), system file descriptors are
preserved even if the open() fails.  (Ordinary file descriptors are
closed before the open() is attempted.)  Note that the close-on-exec
status of a file descriptor will be decided according to the value of
C<$^F> when the open() or pipe() was called, not the time of the exec().

=item $^H

The current set of syntax checks enabled by C<use strict> and other block
scoped compiler hints.  See the documentation of C<strict> for more details.

=item $INPLACE_EDIT

=item $^I

The current value of the inplace-edit extension.  Use C<undef> to disable
inplace editing.  (Mnemonic: value of B<-i> switch.)

=item $^M

By default, running out of memory it is not trappable.  However, if
compiled for this, Perl may use the contents of C<$^M> as an emergency
pool after die()ing with this message.  Suppose that your Perl were
compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc.  Then

    $^M = 'a' x (1<<16);

would allocate a 64K buffer for use when in emergency.  See the F<INSTALL>
file for information on how to enable this option.  As a disincentive to
casual use of this advanced feature, there is no L<English> long name for
this variable.

=item $OSNAME

=item $^O

The name of the operating system under which this copy of Perl was
built, as determined during the configuration process.  The value
is identical to C<$Config{'osname'}>.

=item $PERLDB

=item $^P

The internal variable for debugging support.  Different bits mean the
following (subject to change): 

=over 6

=item 0x01

Debug subroutine enter/exit.

=item 0x02

Line-by-line debugging.

=item 0x04

Switch off optimizations.

=item 0x08

Preserve more data for future interactive inspections.

=item 0x10

Keep info about source lines on which a subroutine is defined.

=item 0x20

Start with single-step on.

=back

Note that some bits may be relevent at compile-time only, some at
run-time only. This is a new mechanism and the details may change.

=item $^R

The result of evaluation of the last successful L<perlre/C<(?{ code })>> 
regular expression assertion.  (Excluding those used as switches.)  May
be written to.

=item $^S

Current state of the interpreter.  Undefined if parsing of the current
module/eval is not finished (may happen in $SIG{__DIE__} and
$SIG{__WARN__} handlers).  True if inside an eval, otherwise false.

=item $BASETIME

=item $^T

The time at which the script began running, in seconds since the
epoch (beginning of 1970).  The values returned by the B<-M>, B<-A>,
and B<-C> filetests are
based on this value.

=item $WARNING

=item $^W

The current value of the warning switch, either TRUE or FALSE.
(Mnemonic: related to the B<-w> switch.)

=item $EXECUTABLE_NAME

=item $^X

The name that the Perl binary itself was executed as, from C's C<argv[0]>.

=item $ARGV

contains the name of the current file when reading from E<lt>E<gt>.

=item @ARGV

The array @ARGV contains the command line arguments intended for the
script.  Note that C<$#ARGV> is the generally number of arguments minus
one, because C<$ARGV[0]> is the first argument, I<NOT> the command name.  See
"C<$0>" for the command name.

=item @INC

The array @INC contains the list of places to look for Perl scripts to
be evaluated by the C<do EXPR>, C<require>, or C<use> constructs.  It
initially consists of the arguments to any B<-I> command line switches,
followed by the default Perl library, probably F</usr/local/lib/perl>,
followed by ".", to represent the current directory.  If you need to
modify this at runtime, you should use the C<use lib> pragma
to get the machine-dependent library properly loaded also:

    use lib '/mypath/libdir/';
    use SomeMod;

=item @_

Within a subroutine the array @_ contains the parameters passed to that
subroutine. See L<perlsub>.

=item %INC

The hash %INC contains entries for each filename that has
been included via C<do> or C<require>.  The key is the filename you
specified, and the value is the location of the file actually found.
The C<require> command uses this array to determine whether a given file
has already been included.

=item %ENV  $ENV{expr}

The hash %ENV contains your current environment.  Setting a
value in C<ENV> changes the environment for child processes.

=item %SIG  $SIG{expr}

The hash %SIG is used to set signal handlers for various
signals.  Example:

    sub handler {       # 1st argument is signal name
        my($sig) = @_;
        print "Caught a SIG$sig--shutting down\n";
        close(LOG);
        exit(0);
    }

    $SIG{'INT'}  = \&handler;
    $SIG{'QUIT'} = \&handler;
    ...
    $SIG{'INT'} = 'DEFAULT';    # restore default action
    $SIG{'QUIT'} = 'IGNORE';    # ignore SIGQUIT

Using a value of C<'IGNORE'> usually has the effect of ignoring the
signal, except for the C<CHLD> signal.  See L<perlipc> for more about
this special case.

The %SIG array contains values for only the signals actually set within
the Perl script.  Here are some other examples:

    $SIG{"PIPE"} = Plumber;     # SCARY!!
    $SIG{"PIPE"} = "Plumber";   # assumes main::Plumber (not recommended)
    $SIG{"PIPE"} = \&Plumber;   # just fine; assume current Plumber
    $SIG{"PIPE"} = Plumber();   # oops, what did Plumber() return??

The one marked scary is problematic because it's a bareword, which means
sometimes it's a string representing the function, and sometimes it's
going to call the subroutine call right then and there!  Best to be sure
and quote it or take a reference to it.  *Plumber works too.  See L<perlsub>.

If your system has the sigaction() function then signal handlers are
installed using it.  This means you get reliable signal handling.  If
your system has the SA_RESTART flag it is used when signals handlers are
installed.  This means that system calls for which it is supported
continue rather than returning when a signal arrives.  If you want your
system calls to be interrupted by signal delivery then do something like
this:

    use POSIX ':signal_h';

    my $alarm = 0;
    sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
        or die "Error setting SIGALRM handler: $!\n";

See L<POSIX>.

Certain internal hooks can be also set using the %SIG hash.  The
routine indicated by C<$SIG{__WARN__}> is called when a warning message is
about to be printed.  The warning message is passed as the first
argument.  The presence of a __WARN__ hook causes the ordinary printing
of warnings to STDERR to be suppressed.  You can use this to save warnings
in a variable, or turn warnings into fatal errors, like this:

    local $SIG{__WARN__} = sub { die $_[0] };
    eval $proggie;

The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
is about to be thrown.  The error message is passed as the first
argument.  When a __DIE__ hook routine returns, the exception
processing continues as it would have in the absence of the hook,
unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
The C<__DIE__> handler is explicitly disabled during the call, so that you
can die from a C<__DIE__> handler.  Similarly for C<__WARN__>.

Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
blocks/strings.  See L<perlfunc/die> and L<perlvar/$^S> for how to
circumvent this.

Note that C<__DIE__>/C<__WARN__> handlers are very special in one
respect: they may be called to report (probable) errors found by the
parser.  In such a case the parser may be in inconsistent state, so
any attempt to evaluate Perl code from such a handler will probably
result in a segfault.  This means that calls which result/may-result
in parsing Perl should be used with extreme causion, like this:

    require Carp if defined $^S;
    Carp::confess("Something wrong") if defined &Carp::confess;
    die "Something wrong, but could not load Carp to give backtrace...
         To see backtrace try starting Perl with -MCarp switch";

Here the first line will load Carp I<unless> it is the parser who
called the handler.  The second line will print backtrace and die if
Carp was available.  The third line will be executed only if Carp was
not available.

See L<perlfunc/die>, L<perlfunc/warn> and L<perlfunc/eval> for
additional info.

=back

=head2 Error Indicators

The variables L<$@>, L<$!>, L<$^E>, and L<$?> contain information about
different types of error conditions that may appear during execution of
Perl script.  The variables are shown ordered by the "distance" between
the subsystem which reported the error and the Perl process, and
correspond to errors detected by the Perl interpreter, C library,
operating system, or an external program, respectively.

To illustrate the differences between these variables, consider the 
following Perl expression:

   eval '
         open PIPE, "/cdrom/install |";
         @res = <PIPE>;
         close PIPE or die "bad pipe: $?, $!";
        ';

After execution of this statement all 4 variables may have been set.  

$@ is set if the string to be C<eval>-ed did not compile (this may happen if 
C<open> or C<close> were imported with bad prototypes), or if Perl 
code executed during evaluation die()d (either implicitly, say, 
if C<open> was imported from module L<Fatal>, or the C<die> after 
C<close> was triggered).  In these cases the value of $@ is the compile 
error, or C<Fatal> error (which will interpolate C<$!>!), or the argument
to C<die> (which will interpolate C<$!> and C<$?>!).

When the above expression is executed, open(), C<<PIPEE<gt>>, and C<close> 
are translated to C run-time library calls.  $! is set if one of these 
calls fails.  The value is a symbolic indicator chosen by the C run-time 
library, say C<No such file or directory>.

On some systems the above C library calls are further translated 
to calls to the kernel.  The kernel may have set more verbose error 
indicator that one of the handful of standard C errors.  In such cases $^E 
contains this verbose error indicator, which may be, say, C<CDROM tray not
closed>.  On systems where C library calls are identical to system calls
$^E is a duplicate of $!.

Finally, $? may be set to non-C<0> value if the external program 
C</cdrom/install> fails.  Upper bits of the particular value may reflect 
specific error conditions encountered by this program (this is 
program-dependent), lower-bits reflect mode of failure (segfault, completion,
etc.).  Note that in contrast to $@, $!, and $^E, which are set only 
if error condition is detected, the variable $? is set on each C<wait> or
pipe C<close>, overwriting the old value.

For more details, see the individual descriptions at L<$@>, L<$!>, L<$^E>,
and L<$?>.