perlvar.pod

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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 C<$SIG{__WARN__}>
as described below.

Also see L<Error Indicators>.

=item $PROCESS_ID

=item $PID

=item $$
X<$$> X<$PID> X<$PROCESS_ID>

The process number of the Perl running this script.  You should
consider this variable read-only, although it will be altered
across fork() calls.  (Mnemonic: same as shells.)

Note for Linux users: on Linux, the C functions C<getpid()> and
C<getppid()> return different values from different threads. In order to
be portable, this behavior is not reflected by C<$$>, whose value remains
consistent across threads. If you want to call the underlying C<getpid()>,
you may use the CPAN module C<Linux::Pid>.

=item $REAL_USER_ID

=item $UID

=item $<
X<< $< >> X<$UID> X<$REAL_USER_ID>

The real uid of this process.  (Mnemonic: it's the uid you came I<from>,
if you're running setuid.)  You can change both the real uid and
the effective uid at the same time by using POSIX::setuid().  Since
changes to $< require a system call, check $! after a change attempt to 
detect any possible errors.

=item $EFFECTIVE_USER_ID

=item $EUID

=item $>
X<< $> >> X<$EUID> X<$EFFECTIVE_USER_ID>

The effective uid of this process.  Example:

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

You can change both the effective uid and the real uid at the same
time by using POSIX::setuid().  Changes to $> require a check to $!
to detect any possible errors after an attempted change. 

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

=item $REAL_GROUP_ID

=item $GID

=item $(
X<$(> X<$GID> X<$REAL_GROUP_ID>

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<$(> without being forced numeric, such as by adding zero. Note
that this is different to the effective gid (C<$)>) which does take a
list.

You can change both the real gid and the effective gid at the same
time by using POSIX::setgid().  Changes to $( require a check to $!
to detect any possible errors after an attempted change.

(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 $)
X<$)> X<$EGID> X<$EFFECTIVE_GROUP_ID>

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 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 C< $) = "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).
Changes to $) require a check to $! to detect any possible errors
after an attempted change.

(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.)

C<< $< >>, C<< $> >>, C<$(> 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
X<$0> X<$PROGRAM_NAME>

Contains the name of the program being executed.

On some (read: not all) operating systems assigning to C<$0> modifies
the argument area that the C<ps> program sees.  On some platforms you
may have to use special C<ps> options or a different C<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 B<sh> and B<ksh>.)

Note that there are platform specific limitations on the maximum
length of C<$0>.  In the most extreme case it may be limited to the
space occupied by the original C<$0>.

In some platforms there may be arbitrary amount of padding, for
example space characters, after the modified name as shown by C<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 C<$0> does not completely remove "perl"
from the ps(1) output.  For example, setting C<$0> to C<"foobar"> may
result in C<"perl: foobar (perl)"> (whether both the C<"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 C<$0> and the change becomes visible
to ps(1) (assuming the operating system plays along).  Note that
the view of C<$0> the other threads have will not change since they
have their own copies of it.

=item $[
X<$[>

The index of the first element in an array, and of the first character
in a substring.  Default is 0, but you could theoretically 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 release 5 of Perl, assignment to C<$[> 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 L<strict>),
assignment to C<$[> can be seen from outer lexical scopes in the same file.
However, you can use local() on it to strictly bind its value to a
lexical block.

=item $]
X<$]>

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 running Perl interpreter is too old.

The floating point representation can sometimes lead to inaccurate
numeric comparisons.  See C<$^V> for a more modern representation of
the Perl version that allows accurate string comparisons.

=item $COMPILING

=item $^C
X<$^C> X<$COMPILING>

The current value of the flag associated with the B<-c> switch.
Mainly of use with B<-MO=...> to allow code to alter its behavior
when being compiled, such as for example to AUTOLOAD at compile
time rather than normal, deferred loading.  Setting
C<$^C = 1> is similar to calling C<B::minus_c>.

=item $DEBUGGING

=item $^D
X<$^D> X<$DEBUGGING>

The current value of the debugging flags.  (Mnemonic: value of B<-D>
switch.) May be read or set. Like its command-line equivalent, you can use
numeric or symbolic values, eg C<$^D = 10> or C<$^D = "st">.

=item ${^RE_DEBUG_FLAGS}

The current value of the regex debugging flags. Set to 0 for no debug output
even when the re 'debug' module is loaded. See L<re> for details.

=item ${^RE_TRIE_MAXBUF}

Controls how certain regex optimisations are applied and how much memory they
utilize. This value by default is 65536 which corresponds to a 512kB temporary
cache. Set this to a higher value to trade memory for speed when matching
large alternations. Set it to a lower value if you want the optimisations to
be as conservative of memory as possible but still occur, and set it to a
negative value to prevent the optimisation and conserve the most memory.
Under normal situations this variable should be of no interest to you.

=item $SYSTEM_FD_MAX

=item $^F
X<$^F> X<$SYSTEM_FD_MAX>

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.)  The close-on-exec
status of a file descriptor will be decided according to the value of
C<$^F> when the corresponding file, pipe, or socket was opened, not the
time of the exec().

=item $^H

WARNING: This variable is strictly for internal use only.  Its availability,
behavior, and contents are subject to change without notice.

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 C<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 C<use strict 'vars'> is implemented.  Here's a conditional
version of the same lexical pragma:

    BEGIN { require strict; strict->import('vars') if $condition }

=item %^H

The %^H hash provides the same scoping semantic as $^H.  This makes it
useful for implementation of lexically scoped pragmas. See L<perlpragma>.

=item $INPLACE_EDIT

=item $^I
X<$^I> X<$INPLACE_EDIT>

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

=item $^M
X<$^M>

By default, running out of memory is an untrappable, fatal error.
However, if suitably built, Perl can use the contents of C<$^M>
as an emergency memory pool after die()ing.  Suppose that your Perl
were compiled with C<-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
F<INSTALL> file in the Perl distribution for information on how to
add custom C compilation flags when compiling perl.  To discourage casual
use of this advanced feature, there is no L<English|English> long name for
this variable.

=item $OSNAME

=item $^O
X<$^O> X<$OSNAME>

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'}>.  See also L<Config> and the 
B<-V> command-line switch documented in L<perlrun>.

In Windows platforms, $^O is not very helpful: since it is always
C<MSWin32>, it doesn't tell the difference between
95/98/ME/NT/2000/XP/CE/.NET.  Use Win32::GetOSName() or
Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
between the variants.

=item ${^OPEN}

An internal variable used by PerlIO.  A string in two parts, separated
by a C<\0> byte, the first part describes the input layers, the second
part describes the output layers.

=item $PERLDB

=item $^P
X<$^P> X<$PERLDB>

The internal variable for debugging support.  The meanings of the
various bits are subject to change, but currently indicate:

=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.

=item 0x40

Use subroutine address instead of name when reporting.

=item 0x80

Report C<goto &subroutine> as well.

=item 0x100

Provide informative "file" names for evals based on the place they were compiled.

=item 0x200

Provide informative names to anonymous subroutines based on the place they
were compiled.

=item 0x400

Debug assertion subroutines enter/exit.

=back

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

=item $LAST_REGEXP_CODE_RESULT

=item $^R
X<$^R> X<$LAST_REGEXP_CODE_RESULT>

The result of evaluation of the last successful C<(?{ code })>
regular expression assertion (see L<perlre>).  May be written to.

=item $EXCEPTIONS_BEING_CAUGHT

=item $^S
X<$^S> X<$EXCEPTIONS_BEING_CAUGHT>

Current state of the interpreter.

    $^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.

=item $BASETIME

=item $^T
X<$^T> X<$BASETIME>

The time at which the program 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 ${^TAINT}

Reflects if taint mode is on or off.  1 for on (the program was run with
B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
B<-t> or B<-TU>).  This variable is read-only.