| perlvar - Perl predefined variables |
perlvar - Perl predefined variables
The following names have special meaning to Perl. Most punctuation names have reasonable mnemonics, or analogs in the shells. Nevertheless, if you wish to use long variable names, you need only say
use English;
at the top of your program. This aliases all the short names to the long names in the current package. Some even have medium names, generally borrowed from awk. In general, it's best to use the
use English '-no_match_vars';
invocation if you don't need $PREMATCH, $MATCH, or $POSTMATCH, as it avoids a certain performance hit with the use of regular expressions. See the English manpage.
Variables that depend on the currently selected filehandle may be set by calling an appropriate object method on the IO::Handle object, although this is less efficient than using the regular built-in variables. (Summary lines below for this contain the word HANDLE.) First you must say
use IO::Handle;
after which you may use either
method HANDLE EXPR
or more safely,
HANDLE->method(EXPR)
Each method returns the old value of the IO::Handle attribute. The methods each take an optional EXPR, which, if supplied, specifies the new value for the IO::Handle attribute in question. If not supplied, most methods do nothing to the current value--except for autoflush(), which will assume a 1 for you, just to be different.
Because loading in the IO::Handle class is an expensive operation, you should learn how to use the regular built-in variables.
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.
You should be very careful when modifying the default values of most special variables described in this document. In most cases you want to localize these variables before changing them, since if you don't, the change may affect other modules which rely on the default values of the special variables that you have changed. This is one of the correct ways to read the whole file at once:
open my $fh, "foo" or die $!;
local $/; # enable localized slurp mode
my $content = <$fh>;
close $fh;
But the following code is quite bad:
open my $fh, "foo" or die $!;
undef $/; # enable slurp mode
my $content = <$fh>;
close $fh;
since some other module, may want to read data from some file in the
default ``line mode'', so if the code we have just presented has been
executed, the global value of $/ is now changed for any other code
running inside the same Perl interpreter.
Usually when a variable is localized you want to make sure that this
change affects the shortest scope possible. So unless you are already
inside some short {} block, you should create one yourself. For
example:
my $content = '';
open my $fh, "foo" or die $!;
{
local $/;
$content = <$fh>;
}
close $fh;
Here is an example of how your own code can go broken:
for (1..5){
nasty_break();
print "$_ ";
}
sub nasty_break {
$_ = 5;
# do something with $_
}
You probably expect this code to print:
1 2 3 4 5
but instead you get:
5 5 5 5 5
Why? Because nasty_break() modifies $_ without localizing it
first. The fix is to add local():
local $_ = 5;
It's easy to notice the problem in such a short example, but in more complicated code you are looking for trouble if you don't localize changes to the special variables.
The following list is ordered by scalar variables first, then the arrays, then the hashes.
while (<>) {...} # equivalent only in while!
while (defined($_ = <>)) {...}
/^Subject:/
$_ =~ /^Subject:/
tr/a-z/A-Z/
$_ =~ tr/a-z/A-Z/
chomp
chomp($_)
Here are the places where Perl will assume $_ even if you don't use it:
ord() and int(), as well
as the all file tests (-f, -d) except for -t, which defaults to
STDIN.
Various list functions like print() and unlink().
The pattern matching operations m//, s///, and tr/// when used
without an =~ operator.
The default iterator variable in a foreach loop if no other
variable is supplied.
The implicit iterator variable in the grep() and map() functions.
The default place to put an input record when a <FH>
operation's result is tested by itself as the sole criterion of a while
test. Outside a while test, this will not happen.
(Mnemonic: underline is understood in certain operations.)
our()) even when using the strict
vars pragma. Don't lexicalize them with my $a or my $b
if you want to be able to use them in the sort() comparison block
or function.
eval() enclosed by the current
BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
and dynamically scoped to the current BLOCK.
The use of this variable anywhere in a program imposes a considerable performance penalty on all regular expression matches. See BUGS.
` 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 BUGS.
eval()
enclosed by the current BLOCK). (Mnemonic: ' often follows a quoted
string.) Example:
local $_ = 'abcdefghi';
/def/;
print "$`:$&:$'\n"; # prints abc:def:ghi
This variable is read-only and dynamically scoped to the current BLOCK.
The use of this variable anywhere in a program imposes a considerable performance penalty on all regular expression matches. See BUGS.
/Version: (.*)|Revision: (.*)/ && ($rev = $+);
(Mnemonic: be positive and forward looking.) This variable is read-only and dynamically scoped to the current BLOCK.
This is primarily used inside (?{...}) blocks for examining text
recently matched. For example, to effectively capture text to a variable
(in addition to $1, $2, etc.), replace (...) with
(?:(...)(?{ $var = $^N }))
By setting and then using $var in this way relieves you from having to
worry about exactly which numbered set of parentheses they are.
This variable is dynamically scoped to the current BLOCK.
$+[0] is
the offset into the string of the end of the entire match. This
is the same value as what the pos function returns when called
on the variable that was matched against. The nth element
of this array holds the offset of the nth submatch, so
$+[1] is the offset past where $1 ends, $+[2] the offset
past where $2 ends, and so on. You can use $#+ to determine
how many subgroups were in the last successful match. See the
examples given for the @- variable.
$* is 0 or undefined. Default is undefined.
(Mnemonic: * matches multiple things.) This variable influences the
interpretation of only ^ and $. A literal newline can be searched
for even when $* == 0.
Use of $* is deprecated in modern Perl, supplanted by
the /s and /m modifiers on pattern matching.
Assigning a non-numerical value to $* triggers a warning (and makes
$* act if $* == 0), while assigning a numerical value to $*
makes that an implicit int is applied on the value.
input_line_number(EXPR)Each filehandle in Perl counts the number of lines that have been read
from it. (Depending on the value of $/, Perl's idea of what
constitutes a line may not match yours.) When a line is read from a
filehandle (via readline() or <>), or when tell() or seek() is
called on it, $. becomes an alias to the line counter for that
filehandle.
You can adjust the counter by assigning to $., but this will not
actually move the seek pointer. Localizing $. will not localize
the filehandle's line count. Instead, it will localize perl's notion
of which filehandle $. is currently aliased to.
$. is reset when the filehandle is closed, but not when an open
filehandle is reopened without an intervening close(). For more
details, see I/O Operators in the perlop manpage. Because <> never does
an explicit close, line numbers increase across ARGV files (but see
examples in eof in the perlfunc manpage).
You can also use HANDLE->input_line_number(EXPR) to access the
line counter for a given filehandle without having to worry about
which handle you last accessed.
(Mnemonic: many programs use ``.'' to mean the current line number.)
input_record_separator(EXPR)undef to read through the end
of file. Setting it to "\n\n" means something slightly
different than setting to "", if the file contains consecutive
empty lines. Setting to "" will treat two or more consecutive
empty lines as a single empty line. Setting to "\n\n" will
blindly assume that the next input character belongs to the next
paragraph, even if it's a newline. (Mnemonic: / delimits
line boundaries when quoting poetry.)
local $/; # enable "slurp" mode
local $_ = <FH>; # whole file now here
s/\n[ \t]+/ /g;
Remember: the value of $/ is a string, not a regex. awk has to be
better for something. :-)
Setting $/ to a reference to an integer, scalar containing an integer, or
scalar that's convertible to an integer will attempt to read records
instead of lines, with the maximum record size being the referenced
integer. So this:
local $/ = \32768; # or \"32768", or \$var_containing_32768
open my $fh, $myfile or die $!;
local $_ = <$fh>;
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 sysread,
so it's best not to mix record and non-record reads on the same
file. (This is unlikely to be a problem, because any file you'd
want to read in record mode is probably unusable in line mode.)
Non-VMS systems do normal I/O, so it's safe to mix record and
non-record reads of a file.
See also Newlines in the perlport manpage. Also see $..
autoflush(EXPR)$| tells you only whether you've asked Perl
explicitly to flush after each write). 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 or socket, such as when you are running
a Perl program under rsh and want to see the output as it's
happening. This has no effect on input buffering. See getc in the perlfunc manpage
for that. (Mnemonic: when you want your pipes to be piping hot.)
$\ instead of adding ``\n'' at the
end of the print. Also, it's just like $/, but it's what you
get ``back'' from Perl.)
$, except that it applies to array and slice values
interpolated into a double-quoted string (or similar interpreted
string). Default is a space. (Mnemonic: obvious, I think.)
$foo{$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 awk. If your
keys contain binary data there might not be any safe value for $;.
(Mnemonic: comma (the syntactic subscript separator) is a
semi-semicolon. Yeah, I know, it's pretty lame, but $, is already
taken for something more important.)
Consider using ``real'' multidimensional arrays as described in the perllol manpage.
$#
explicitly to get awk's value. (Mnemonic: # is the number sign.)
Use of $# is deprecated.
format_page_number(EXPR)format_lines_per_page(EXPR)format_lines_left(EXPR)$-[n] is the offset of the start of the substring matched by
n-th subpattern, or undef if the subpattern did not match.
Thus after a match against $_, $& coincides with substr $_, $-[0],
$+[0] - $-[0]. Similarly, $n coincides with substr $_, $-[n],
$+[n] - $-[n] if $-[n] is defined, and $+ coincides with
substr $_, $-[$#-], $+[$#-]. One can use $#- to find the last
matched subgroup in the last successful match. Contrast with
$#+, the number of subgroups in the regular expression. Compare
with @+.
This array holds the offsets of the beginnings of the last
successful submatches in the currently active dynamic scope.
$-[0] is the offset into the string of the beginning of the
entire match. The nth element of this array holds the offset
of the nth submatch, so $-[1] is the offset where $1
begins, $-[2] the offset where $2 begins, and so on.
After a match against some variable $var:
$` is the same as substr($var, 0, $-[0])$& is the same as substr($var, $-[0], $+[0] - $-[0])$' is the same as substr($var, $+[0])$1 is the same as substr($var, $-[1], $+[1] - $-[1])$2 is the same as substr($var, $-[2], $+[2] - $-[2])$3 is the same as substr $var, $-[3], $+[3] - $-[3])format_name(EXPR)$^.)
format_top_name(EXPR)write() accumulator for format() lines. A format
contains formline() calls that put their result into $^A. After
calling its format, write() prints out the contents of $^A and empties.
So you never really see the contents of $^A unless you call
formline() yourself and then look at it. See the perlform manpage and
formline() in the perlfunc manpage.
``) command,
successful call to wait() or waitpid(), or from the system()
operator. This is just the 16-bit status word returned by the
wait() system call (or else is made up to look like it). Thus, the
exit value of the subprocess is really ($? >> 8), and
$? & 127 gives which signal, if any, the process died from, and
$? & 128 reports whether there was a core dump. (Mnemonic:
similar to sh and ksh.)
Additionally, if the h_errno variable is supported in C, its value
is returned via $? if any gethost*() function fails.
If you have installed a signal handler for SIGCHLD, the
value of $? will usually be wrong outside that handler.
Inside an END subroutine $? contains the value that is going to be
given to exit(). You can modify $? in an END subroutine to
change the exit status of your program. For example:
END {
$? = 1 if $? == 255; # die would make it 255
}
Under VMS, the pragma use vmsish 'status' makes $? reflect the
actual VMS exit status, instead of the default emulation of POSIX
status; see $? in the perlvms manpage for details.
Also see Error Indicators.
errno
variable, or in other words, if a system or library call fails, it
sets this variable. This means that the value of $! is meaningful
only immediately after a failure:
if (open(FH, $filename)) {
# Here $! is meaningless.
...
} else {
# ONLY here is $! meaningful.
...
# Already here $! might be meaningless.
}
# Since here we might have either success or failure,
# here $! is meaningless.
In the above meaningless stands for anything: zero, non-zero,
undef. A successful system or library call does not set
the variable to zero.
If used as a string, yields the corresponding system error string.
You can assign a number to $! to set errno if, for instance,
you want "$!" to return the string for error n, or you want
to set the exit value for the die() operator. (Mnemonic: What just
went bang?)
Also see Error Indicators.
%! has a true value only if $! is set to that
value. For example, $!{ENOENT} is true if and only if the current
value of $! is ENOENT; that is, if the most recent error was
``No such file or directory'' (or its moral equivalent: not all operating
systems give that exact error, and certainly not all languages).
To check if a particular key is meaningful on your system, use
exists $!{the_key}; for a list of legal keys, use keys %!.
See the Errno manpage for more information, and also see above for the
validity of $!.
$! under only VMS, OS/2, and Win32
(and for MacPerl). On all other platforms, $^E is always just
the same as $!.
Under VMS, $^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 $!. This is particularly
important when $! is set to EVMSERR.
Under OS/2, $^E is set to the error code of the last call to
OS/2 API either via CRT, or directly from perl.
Under Win32, $^E always returns the last error information
reported by the Win32 call GetLastError() which describes
the last error from within the Win32 API. Most Win32-specific
code will report errors via $^E. ANSI C and Unix-like calls
set errno and so most portable Perl code will report errors
via $!.
Caveats mentioned in the description of $! generally apply to
$^E, also. (Mnemonic: Extra error explanation.)
Also see Error Indicators.
eval() operator.
If $@ is the null string, 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''?)
Warning messages are not collected in this variable. You can,
however, set up a routine to process warnings by setting $SIG{__WARN__}
as described below.
Also see Error Indicators.
fork() calls. (Mnemonic: same as shells.)
Note for Linux users: on Linux, the C functions getpid() and
getppid() return different values from different threads. In order to
be portable, this behavior is not reflected by $$, whose value remains
consistent across threads. If you want to call the underlying getpid(),
you may use the CPAN module Linux::Pid.
$< = $>; # set real to effective uid
($<,$>) = ($>,$<); # swap real and effective uid
You can change both the effective uid and the real uid at the same time by using POSIX::setuid().
(Mnemonic: it's the uid you went to, if you're running setuid.)
$< and $> can be swapped only on machines
supporting setreuid().
However, a value assigned to $( must be a single number used to
set the real gid. So the value given by $( should not be assigned
back to $( without being forced numeric, such as by adding zero.
You can change both the real gid and the effective gid at the same time by using POSIX::setgid().
(Mnemonic: parentheses are used to group things. The real gid is the group you left, if you're running setgid.)
Similarly, a value assigned to $) must also be a space-separated
list of numbers. The first number sets 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 $) = "5 5" .
You can change both the effective gid and the real gid at the same time by using POSIX::setgid() (use only a single numeric argument).
(Mnemonic: parentheses are used to group things. The effective gid is the group that's right for you, if you're running setgid.)
$<, $>, $( and $) can be set only on
machines that support the corresponding set[re][ug]id() routine. $(
and $) can be swapped only on machines supporting setregid().
On some (read: not all) operating systems assigning to $0 modifies
the argument area that the ps program sees. On some platforms you
may have to use special ps options or a different ps to see the
changes. Modifying the $0 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 sh and ksh.)
Note that there are platform specific limitations on the the maximum
length of $0. In the most extreme case it may be limited to the
space occupied by the original $0.
In some platforms there may be arbitrary amount of padding, for
example space characters, after the modified name as shown by ps.
In some platforms this padding may extend all the way to the original
length of the argument area, no matter what you do (this is the case
for example with Linux 2.2).
Note for BSD users: setting $0 does not completely remove ``perl''
from the ps(1) output. For example, setting $0 to "foobar" may
result in "perl: foobar (perl)" (whether both the "perl: " prefix
and the `` (perl)'' suffix are shown depends on your exact BSD variant
and version). This is an operating system feature, Perl cannot help it.
In multithreaded scripts Perl coordinates the threads so that any
thread may modify its copy of the $0 and the change becomes visible
to ps(1) (assuming the operating system plays along). Note that the
the view of $0 the other threads have will not change since they
have their own copies of it.
index() and substr() functions.
(Mnemonic: [ begins subscripts.)
As of release 5 of Perl, assignment to $[ is treated as a compiler
directive, and cannot influence the behavior of any other file.
(That's why you can only assign compile-time constants to it.)
Its use is highly discouraged.
Note that, unlike other compile-time directives (such as the strict manpage),
assignment to $[ can be seen from outer lexical scopes in the same file.
However, you can use local() on it to strictly bound its value to a
lexical block.
warn "No checksumming!\n" if $] < 3.019;
See also the documentation of use VERSION and require VERSION
for a convenient way to fail if the running Perl interpreter is too old.
The floating point representation can sometimes lead to inaccurate
numeric comparisons. See $^V for a more modern representation of
the Perl version that allows accurate string comparisons.
$^C = 1 is similar to calling B::minus_c.
$^D = 10 or $^D = "st".
open() fails. (Ordinary file descriptors are
closed before the open() is attempted.) The close-on-exec
status of a file descriptor will be decided according to the value of
$^F when the corresponding file, pipe, or socket was opened, not the
time of the exec().
This variable contains compile-time hints for the Perl interpreter. At the end of compilation of a BLOCK the value of this variable is restored to the value when the interpreter started to compile the BLOCK.
When perl begins to parse any block construct that provides a lexical scope (e.g., eval body, required file, subroutine body, loop body, or conditional block), the existing value of $^H is saved, but its value is left unchanged. When the compilation of the block is completed, it regains the saved value. Between the points where its value is saved and restored, code that executes within BEGIN blocks is free to change the value of $^H.
This behavior provides the semantic of lexical scoping, and is used in,
for instance, the use strict pragma.
The contents should be an integer; different bits of it are used for different pragmatic flags. Here's an example:
sub add_100 { $^H |= 0x100 }
sub foo {
BEGIN { add_100() }
bar->baz($boon);
}
Consider what happens during execution of the BEGIN block. At this point
the BEGIN block has already been compiled, but the body of foo() is still
being compiled. The new value of $^H will therefore be visible only while
the body of foo() is being compiled.
Substitution of the above BEGIN block with:
BEGIN { require strict; strict->import('vars') }
demonstrates how use strict 'vars' is implemented. Here's a conditional
version of the same lexical pragma:
BEGIN { require strict; strict->import('vars') if $condition }
The %^H hash provides the same scoping semantic as $^H. This makes it useful for implementation of lexically scoped pragmas.
undef to disable
inplace editing. (Mnemonic: value of -i switch.)
$^M
as an emergency memory pool after die()ing. 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 in an emergency. See the INSTALL file in the Perl distribution for information on how to enable this option. To discourage casual use of this advanced feature, there is no English long name for this variable.
$Config{'osname'}. See also the Config manpage and the
-V command-line switch documented in the perlrun manpage.
In Windows platforms, $^O is not very helpful: since it is always
MSWin32, it doesn't tell the difference between
95/98/ME/NT/2000/XP/CE/.NET. Use Win32::GetOSName() or
Win32::GetOSVersion() (see the Win32 manpage and the perlport manpage) to distinguish
between the variants.
\0 byte, the first part describes the input layers, the second
part describes the output layers.
goto &subroutine as well.
Some bits may be relevant at compile-time only, some at run-time only. This is a new mechanism and the details may change.
(?{ code })
regular expression assertion (see the perlre manpage). May be written to.
$^S State
--------- -------------------
undef Parsing module/eval
true (1) Executing an eval
false (0) Otherwise
The first state may happen in $SIG{__DIE__} and $SIG{__WARN__} handlers.
-C switch for more information about
the possible values. This variable is set during Perl startup
and is thereafter read-only.
chr(5) . chr(6) . chr(0) and will return true for
$^V eq v5.6.0. Note that the characters in this string value can
potentially be in Unicode range.
This can be used to determine whether the Perl interpreter executing a script is in the right range of versions. (Mnemonic: use ^V for Version Control.) Example:
warn "No \"our\" declarations!\n" if $^V and $^V lt v5.6.0;
To convert $^V into its string representation use sprintf()'s
"%vd" conversion:
printf "version is v%vd\n", $^V; # Perl's version
See the documentation of use VERSION and require VERSION
for a convenient way to fail if the running Perl interpreter is too old.
See also $] for an older representation of the Perl version.
use warnings pragma.
See the documentation of warnings for more details.
argv[0].
Depending on the host operating system, the value of $^X may be a relative or absolute pathname of the perl program file, or may be the string used to invoke perl but not the pathname of the perl program file. Also, most operating systems permit invoking programs that are not in the PATH environment variable, so there is no guarantee that the value of $^X is in PATH. For VMS, the value may or may not include a version number.
You usually can use the value of $^X to re-invoke an independent copy of the same perl that is currently running, e.g.,
@first_run = `$^X -le "print int rand 100 for 1..100"`;
But recall that not all operating systems support forking or capturing of the output of commands, so this complex statement may not be portable.
It is not safe to use the value of $^X as a path name of a file, as some operating systems that have a mandatory suffix on executable files do not require use of the suffix when invoking a command. To convert the value of $^X to a path name, use the following statements:
# Build up a set of file names (not command names). use Config; $this_perl = $^X; if ($^O ne 'VMS') {$this_perl .= $Config{_exe} unless $this_perl =~ m/$Config{_exe}$/i;}
Because many operating systems permit anyone with read access to the Perl program file to make a copy of it, patch the copy, and then execute the copy, the security-conscious Perl programmer should take care to invoke the installed copy of perl, not the copy referenced by $^X. The following statements accomplish this goal, and produce a pathname that can be invoked as a command or referenced as a file.
use Config;
$secure_perl_path = $Config{perlpath};
if ($^O ne 'VMS')
{$secure_perl_path .= $Config{_exe}
unless $secure_perl_path =~ m/$Config{_exe}$/i;}
@ARGV. Usually written as the null filehandle in the angle operator
<>. Note that currently ARGV only has its magical effect
within the <> operator; elsewhere it is just a plain filehandle
corresponding to the last file opened by <>. In particular,
passing \*ARGV as a parameter to a function that expects a filehandle
may not cause your function to automatically read the contents of all the
files in @ARGV.
$#ARGV is generally the number of arguments minus
one, because $ARGV[0] is the first argument, not the program's
command name itself. See $0 for the command name.
strict 'vars'.
do EXPR,
require, or use constructs look for their library files. It
initially consists of the arguments to any -I command-line
switches, followed by the default Perl library, probably
/usr/local/lib/perl, followed by ``.'', to represent the current
directory. (``.'' will not be appended if taint checks are enabled, either by
-T or by -t.) If you need to modify this at runtime, you should use
the use lib pragma to get the machine-dependent library properly
loaded also:
use lib '/mypath/libdir/';
use SomeMod;
You can also insert hooks into the file inclusion system by putting Perl code directly into @INC. Those hooks may be subroutine references, array references or blessed objects. See require in the perlfunc manpage for details.
do, require, or use operators. The key is the filename
you specified (with module names converted to pathnames), and the
value is the location of the file found. The require
operator uses this hash to determine whether a particular file has
already been included.
If the file was loaded via a hook (e.g. a subroutine reference, see require in the perlfunc manpage for a description of these hooks), this hook is by default inserted into %INC in place of a filename. Note, however, that the hook may have set the %INC entry by itself to provide some more specific info.
ENV changes the environment for any child processes
you subsequently fork() off.
sub handler { # 1st argument is signal name
my($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 'IGNORE' usually has the effect of ignoring the
signal, except for the CHLD signal. See the perlipc manpage for more about
this special case.
Here are some other examples:
$SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
$SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
$SIG{"PIPE"} = *Plumber; # somewhat esoteric
$SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
Be sure not to use a bareword as the name of a signal handler, lest you inadvertently call it.
If your system has the sigaction() function then signal handlers are
installed using it. This means you get reliable signal handling.
The default delivery policy of signals changed in Perl 5.8.0 from immediate (also known as ``unsafe'') to deferred, also known as ``safe signals''. See the perlipc manpage for more information.
Certain internal hooks can be also set using the %SIG hash. The
routine indicated by $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 $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 goto, a loop exit, or a die().
The __DIE__ handler is explicitly disabled during the call, so that you
can die from a __DIE__ handler. Similarly for __WARN__.
Due to an implementation glitch, the $SIG{__DIE__} hook is called
even inside an eval(). Do not use this to rewrite a pending exception
in $@, or as a bizarre substitute for overriding CORE::GLOBAL::die().
This strange action at a distance may be fixed in a future release
so that $SIG{__DIE__} is only called if your program is about
to exit, as was the original intent. Any other use is deprecated.
__DIE__/__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 warnings or errors that
result from parsing Perl should be used with extreme caution, 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 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 die in the perlfunc manpage, warn in the perlfunc manpage, eval in the perlfunc manpage, and the warnings manpage for additional information.
The variables $@, $!, $^E, and $? contain information
about different types of error conditions that may appear during
execution of a Perl program. The variables are shown ordered by
the ``distance'' between the subsystem which reported the error and
the Perl process. They 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, which uses a single-quoted string:
eval q{
open my $pipe, "/cdrom/install |" or die $!;
my @res = <$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 eval-ed did not compile (this
may happen if open or close were imported with bad prototypes),
or if Perl code executed during evaluation die()d . In these cases
the value of $@ is the compile error, or the argument to die
(which will interpolate $! and $?!). (See also the Fatal manpage,
though.)
When the eval() expression above is executed, open(), <PIPE>,
and close are translated to calls in the C run-time library and
thence to the operating system kernel. $! is set to the C library's
errno if one of these calls fails.
Under a few operating systems, $^E may contain a more verbose
error indicator, such as in this case, ``CDROM tray not closed.''
Systems that do not support extended error messages leave $^E
the same as $!.
Finally, $? may be set to non-0 value if the external program
/cdrom/install fails. The upper eight bits reflect specific
error conditions encountered by the program (the program's exit()
value). The lower eight bits reflect mode of failure, like signal
death and core dump information See wait(2) for details. In
contrast to $! and $^E, which are set only if error condition
is detected, the variable $? is set on each wait or pipe
close, overwriting the old value. This is more like $@, which
on every eval() is always set on failure and cleared on success.
For more details, see the individual descriptions at $@, $!, $^E,
and $?.
Variable names in Perl can have several formats. Usually, they
must begin with a letter or underscore, in which case they can be
arbitrarily long (up to an internal limit of 251 characters) and
may contain letters, digits, underscores, or the special sequence
:: or '. In this case, the part before the last :: or
' is taken to be a package qualifier; see the perlmod manpage.
Perl variable names may also be a sequence of digits or a single
punctuation or control character. These names are all reserved for
special uses by Perl; for example, the all-digits names are used
to hold data captured by backreferences after a regular expression
match. Perl has a special syntax for the single-control-character
names: It understands ^X (caret X) to mean the control-X
character. For example, the notation $^W (dollar-sign caret
W) is the scalar variable whose name is the single character
control-W. This is better than typing a literal control-W
into your program.
Finally, new in Perl 5.6, Perl variable names may be alphanumeric
strings that begin with control characters (or better yet, a caret).
These variables must be written in the form ${^Foo}; the braces
are not optional. ${^Foo} denotes the scalar variable whose
name is a control-F followed by two o's. These variables are
reserved for future special uses by Perl, except for the ones that
begin with ^_ (control-underscore or caret-underscore). No
control-character name that begins with ^_ will acquire a special
meaning in any future version of Perl; such names may therefore be
used safely in programs. $^_ itself, however, is reserved.
Perl identifiers that begin with digits, control characters, or
punctuation characters are exempt from the effects of the package
declaration and are always forced to be in package main; they are
also exempt from strict 'vars' errors. A few other names are also
exempt in these ways:
ENV STDIN
INC STDOUT
ARGV STDERR
ARGVOUT _
SIG
In particular, the new special ${^_XYZ} variables are always taken
to be in package main, regardless of any package declarations
presently in scope.
Due to an unfortunate accident of Perl's implementation, use
English imposes a considerable performance penalty on all regular
expression matches in a program, regardless of whether they occur
in the scope of use English. For that reason, saying use
English in libraries is strongly discouraged. See the
Devel::SawAmpersand module documentation from CPAN
( http://www.cpan.org/modules/by-module/Devel/ )
for more information.
Having to even think about the $^S variable in your exception
handlers is simply wrong. $SIG{__DIE__} as currently implemented
invites grievous and difficult to track down errors. Avoid it
and use an END{} or CORE::GLOBAL::die override instead.
| perlvar - Perl predefined variables |