=head2 The Syntax of Variable Names
-Variable names in Perl can have several formats. Usually, they
+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
-C<::> or C<'>. In this case, the part before the last C<::> or
+C<::> or C<'>. In this case, the part before the last C<::> or
C<'> is taken to be a I<package qualifier>; see L<perlmod>.
Perl variable names may also be a sequence of digits or a single
-punctuation or control character. These names are all reserved for
+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
+match. Perl has a special syntax for the single-control-character
names: It understands C<^X> (caret C<X>) to mean the control-C<X>
-character. For example, the notation C<$^W> (dollar-sign caret
+character. For example, the notation C<$^W> (dollar-sign caret
C<W>) is the scalar variable whose name is the single character
-control-C<W>. This is better than typing a literal control-C<W>
+control-C<W>. This is better than typing a literal control-C<W>
into your program.
Since 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 C<${^Foo}>; the braces
-are not optional. C<${^Foo}> denotes the scalar variable whose
-name is a control-C<F> followed by two C<o>'s. These variables are
+are not optional. C<${^Foo}> denotes the scalar variable whose
+name is a control-C<F> followed by two C<o>'s. These variables are
reserved for future special uses by Perl, except for the ones that
-begin with C<^_> (control-underscore or caret-underscore). No
+begin with C<^_> (control-underscore or caret-underscore). No
control-character name that begins with C<^_> will acquire a special
meaning in any future version of Perl; such names may therefore be
-used safely in programs. C<$^_> itself, however, I<is> reserved.
+used safely in programs. C<$^_> itself, however, I<is> reserved.
Perl identifiers that begin with digits, control characters, or
punctuation characters are exempt from the effects of the C<package>
declaration and are always forced to be in package C<main>; they are
-also exempt from C<strict 'vars'> errors. A few other names are also
+also exempt from C<strict 'vars'> errors. A few other names are also
exempt in these ways:
- ENV STDIN
- INC STDOUT
- ARGV STDERR
- ARGVOUT _
+ ENV STDIN
+ INC STDOUT
+ ARGV STDERR
+ ARGVOUT
SIG
-In particular, the new special C<${^_XYZ}> variables are always taken
+In particular, the special C<${^_XYZ}> variables are always taken
to be in package C<main>, regardless of any C<package> declarations
presently in scope.
=head1 SPECIAL VARIABLES
-The following names have special meaning to Perl. Most
-punctuation names have reasonable mnemonics, or analogs in the
-shells. Nevertheless, if you wish to use long variable names,
-you need only say
+The following names have special meaning to Perl. Most punctuation
+names have reasonable mnemonics, or analogs in the shells.
+Nevertheless, if you wish to use long variable names, you need only say:
- use English;
+ use English;
at the top of your program. This aliases all the short names to the long
names in the current package. Some even have medium names, generally
-borrowed from B<awk>. In general, it's best to use the
+borrowed from B<awk>. To avoid a performance hit, if you don't need the
+C<$PREMATCH>, C<$MATCH>, or C<$POSTMATCH> it's best to use the C<English>
+module without them:
- use English '-no_match_vars';
+ use English '-no_match_vars';
-invocation if you don't need $PREMATCH, $MATCH, or $POSTMATCH, as it avoids
-a certain performance hit with the use of regular expressions. See
-L<English>.
+Before you continue, note the sort order for variables. In general, we
+first list the variables in case-insensitive, almost-lexigraphical
+order (ignoring the C<{> or C<^> preceding words, as in C<${^UNICODE}>
+or C<$^T>), although C<$_> and C<@_> move up to the top of the pile.
+For variables with the same identifier, we list it in order of scalar,
+array, hash, and bareword.
=head2 General Variables
+=over 8
+
=item $ARG
=item $_
The default input and pattern-searching space. The following pairs are
equivalent:
- while (<>) {...} # equivalent only in while!
- while (defined($_ = <>)) {...}
+ while (<>) {...} # equivalent only in while!
+ while (defined($_ = <>)) {...}
- /^Subject:/
- $_ =~ /^Subject:/
+ /^Subject:/
+ $_ =~ /^Subject:/
- tr/a-z/A-Z/
- $_ =~ tr/a-z/A-Z/
+ tr/a-z/A-Z/
+ $_ =~ tr/a-z/A-Z/
- chomp
- chomp($_)
+ chomp
+ chomp($_)
-Here are the places where Perl will assume C<$_> even if you
-don't use it:
+Here are the places where Perl will assume C<$_> even if you don't use it:
=over 3
=item *
-The following functions:
+The following functions use C<$_> as a default argument:
abs, alarm, chomp, chop, chr, chroot, cos, defined, eval, exp, glob,
hex, int, lc, lcfirst, length, log, lstat, mkdir, oct, ord, pos, print,
All file tests (C<-f>, C<-d>) except for C<-t>, which defaults to STDIN.
See L<perlfunc/-X>
-
=item *
The pattern matching operations C<m//>, C<s///> and C<tr///> (aka C<y///>)
Mnemonic: underline is understood in certain operations.
-=back
-
-=over 8
-
-=item $a
-
-=item $b
-X<$a> X<$b>
-
-Special package variables when using C<sort()>, see L<perlfunc/sort>.
-Because of this specialness C<$a> and C<$b> don't need to be declared
-(using C<use vars>, or C<our()>) even when using the C<strict 'vars'>
-pragma. Don't lexicalize them with C<my $a> or C<my $b> if you want to
-be able to use them in the C<sort()> comparison block or function.
-
-=item $SUBSCRIPT_SEPARATOR
-
-=item $SUBSEP
-
-=item $;
-X<$;> X<$SUBSEP> X<SUBSCRIPT_SEPARATOR>
-
-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, $c)}
+=item @ARG
-But don't put
+=item @_
+X<@_> X<@ARG>
- @foo{$a,$b,$c} # a slice--note the @
+Within a subroutine the array C<@_> contains the parameters passed to
+that subroutine. Inside a subroutine, C<@_> is the default array for
+the array operators C<push>, C<pop>, C<shift>, and C<unshift>.
-which means
+See L<perlsub>.
- ($foo{$a},$foo{$b},$foo{$c})
+=item $LIST_SEPARATOR
-Default is "\034", the same as SUBSEP in B<awk>. If your
-keys contain binary data there might not be any safe value for C<$;>.
+=item $"
+X<$"> X<$LIST_SEPARATOR>
-Consider using "real" multidimensional arrays as described
-in L<perllol>.
+When an array or an array slice is interpolated into a double-quoted
+string or a similar context such as C</.../>, its elements are
+separated by this value. Default is a space. For example, this:
-Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon.
+ print "The array is: @array\n";
-=item ${^ENCODING}
-X<$^ENCODING>
+is equivalent to this:
-The I<object reference> to the C<Encode> object that is used to convert
-the source code to Unicode. Thanks to this variable your Perl script
-does not have to be written in UTF-8. Default is I<undef>. The direct
-manipulation of this variable is highly discouraged.
+ print "The array is: " . join($", @array) . "\n";
-This variable was added in Perl 5.8.2.
+Mnemonic: works in double-quoted context.
=item $PROCESS_ID
you may use the CPAN module C<Linux::Pid>.
Mnemonic: same as shells.
-
-=item $REAL_USER_ID
-
-=item $UID
-
-=item $<
-X<< $< >> X<$UID> X<$REAL_USER_ID>
-
-The real uid of this process. You can change both the real uid and the
-effective uid at the same time by using C<POSIX::setuid()>. Since
-changes to C<< $< >> require a system call, check C<$!> after a change
-attempt to detect any possible errors.
-
-Mnemonic: it's the uid you came I<from>, if you're running setuid.
-
-=item $EFFECTIVE_USER_ID
-
-=item $EUID
-
-=item $>
-X<< $> >> X<$EUID> X<$EFFECTIVE_USER_ID>
-
-The effective uid of this process. For 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 C<POSIX::setuid()>. Changes to C<< $> >> require a check
-to C<$!> to detect any possible errors after an attempted change.
-
-C<< $< >> and C<< $> >> can be swapped only on machines
-supporting C<setreuid()>.
-
-Mnemonic: it's the uid you went I<to>, if you're running setuid.
=item $REAL_GROUP_ID
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
+On some (but 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 C<$0> is more useful as a way of indicating the
+changes. Modifying the C<$0> is more useful as a way of indicating the
current program state than it is for hiding the program you're
running.
-Note that there are platform specific limitations on the maximum
+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>.
C<$0> will contain the string C<"-e">.
On Linux as of perl 5.14 the legacy process name will be set with
-
L<prctl(2)>, in addition to altering the POSIX name via C<argv[0]> as
+
C<prctl(2)>, in addition to altering the POSIX name via C<argv[0]> as
perl has done since version 4.000. Now system utilities that read the
legacy process name such as ps, top and killall will recognize the
name you set when assigning to C<$0>. The string you supply will be
cut off at 16 bytes, this is a limitation imposed by Linux.
Mnemonic: same as B<sh> and B<ksh>.
-
+
+=item $SUBSCRIPT_SEPARATOR
+
+=item $SUBSEP
+
+=item $;
+X<$;> X<$SUBSEP> X<SUBSCRIPT_SEPARATOR>
+
+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>. If your keys contain
+binary data there might not be any safe value for C<$;>.
+
+Consider using "real" multidimensional arrays as described
+in L<perllol>.
+
+Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon.
+
+=item $REAL_USER_ID
+
+=item $UID
+
+=item $<
+X<< $< >> X<$UID> X<$REAL_USER_ID>
+
+The real uid of this process. You can change both the real uid and the
+effective uid at the same time by using C<POSIX::setuid()>. Since
+changes to C<< $< >> require a system call, check C<$!> after a change
+attempt to detect any possible errors.
+
+Mnemonic: it's the uid you came I<from>, if you're running setuid.
+
+=item $EFFECTIVE_USER_ID
+
+=item $EUID
+
+=item $>
+X<< $> >> X<$EUID> X<$EFFECTIVE_USER_ID>
+
+The effective uid of this process. For example:
+
+ $< = $>; # set real to effective uid
+ ($<,$>) = ($>,$<); # swap real and effective uids
+
+You can change both the effective uid and the real uid at the same
+time by using C<POSIX::setuid()>. Changes to C<< $> >> require a check
+to C<$!> to detect any possible errors after an attempted change.
+
+C<< $< >> and C<< $> >> can be swapped only on machines
+supporting C<setreuid()>.
+
+Mnemonic: it's the uid you went I<to>, if you're running setuid.
+
+=item $a
+
+=item $b
+X<$a> X<$b>
+
+Special package variables when using C<sort()>, see L<perlfunc/sort>.
+Because of this specialness C<$a> and C<$b> don't need to be declared
+(using C<use vars>, or C<our()>) even when using the C<strict 'vars'>
+pragma. Don't lexicalize them with C<my $a> or C<my $b> if you want to
+be able to use them in the C<sort()> comparison block or function.
+
=item $COMPILING
=item $^C
The current value of the flag associated with the B<-c> switch.
Mainly of use with B<-MO=...> to allow code to alter its behavior
when being compiled, such as for example to C<AUTOLOAD> at compile
-time rather than normal, deferred loading. Setting
+time rather than normal, deferred loading. Setting
C<$^C = 1> is similar to calling C<B::minus_c>.
This variable was added in Perl 5.6.
Mnemonic: value of B<-D> switch.
+=item ${^ENCODING}
+X<${^ENCODING}>
+
+The I<object reference> to the C<Encode> object that is used to convert
+the source code to Unicode. Thanks to this variable your Perl script
+does not have to be written in UTF-8. Default is I<undef>. The direct
+manipulation of this variable is highly discouraged.
+
+This variable was added in Perl 5.8.2.
+
+=item %ENV
+X<%ENV>
+
+The hash C<%ENV> contains your current environment. Setting a
+value in C<ENV> changes the environment for any child processes
+you subsequently C<fork()> off.
+
=item $SYSTEM_FD_MAX
=item $^F
C<$^F> when the corresponding file, pipe, or socket was opened, not the
time of the C<exec()>.
+=item @F
+X<@F>
+
+The array C<@F> contains the fields of each line read in when autosplit
+mode is turned on. See L<perlrun> for the B<-a> switch. This array
+is package-specific, and must be declared or given a full package name
+if not in package main when running under C<strict 'vars'>.
+
+=item ${^GLOBAL_PHASE}
+X<${^GLOBAL_PHASE}>
+
+The current phase of the perl interpreter.
+
+Possible values are:
+
+=over 8
+
+=item CONSTRUCT
+
+The C<PerlInterpreter*> is being constructed via C<perl_construct>. This
+value is mostly there for completeness and for use via the
+underlying C variable C<PL_phase>. It's not really possible for Perl
+code to be executed unless construction of the interpreter is
+finished.
+
+=item START
+
+This is the global compile-time. That includes, basically, every
+C<BEGIN> block executed directly or indirectly from during the
+compile-time of the top-level program.
+
+This phase is not called "BEGIN" to avoid confusion with
+C<BEGIN>-blocks, as those are executed during compile-time of any
+compilation unit, not just the top-level program. A new, localised
+compile-time entered at run-time, for example by constructs as
+C<eval "use SomeModule"> are not global interpreter phases, and
+therefore aren't reflected by C<${^GLOBAL_PHASE}>.
+
+=item CHECK
+
+Execution of any C<CHECK> blocks.
+
+=item INIT
+
+Similar to "CHECK", but for C<INIT>-blocks, not C<CHECK> blocks.
+
+=item RUN
+
+The main run-time, i.e. the execution of C<PL_main_root>.
+
+=item END
+
+Execution of any C<END> blocks.
+
+=item DESTRUCT
+
+Global destruction.
+
+=back
+
+Also note that there's no value for UNITCHECK-blocks. That's because
+those are run for each compilation unit individually, and therefore is
+not a global interpreter phase.
+
+Not every program has to go through each of the possible phases, but
+transition from one phase to another can only happen in the order
+described in the above list.
+
+An example of all of the phases Perl code can see:
+
+ BEGIN { print "compile-time: ${^GLOBAL_PHASE}\n" }
+
+ INIT { print "init-time: ${^GLOBAL_PHASE}\n" }
+
+ CHECK { print "check-time: ${^GLOBAL_PHASE}\n" }
+
+ {
+ package Print::Phase;
+
+ sub new {
+ my ($class, $time) = @_;
+ return bless \$time, $class;
+ }
+
+ sub DESTROY {
+ my $self = shift;
+ print "$$self: ${^GLOBAL_PHASE}\n";
+ }
+ }
+
+ print "run-time: ${^GLOBAL_PHASE}\n";
+
+ my $runtime = Print::Phase->new(
+ "lexical variables are garbage collected before END"
+ );
+
+ END { print "end-time: ${^GLOBAL_PHASE}\n" }
+
+ our $destruct = Print::Phase->new(
+ "package variables are garbage collected after END"
+ );
+
+This will print out
+
+ compile-time: START
+ check-time: CHECK
+ init-time: INIT
+ run-time: RUN
+ lexical variables are garbage collected before END: RUN
+ end-time: END
+ package variables are garbage collected after END: DESTRUCT
+
+This variable was added in Perl 5.14.0.
+
=item $^H
+X<$^H>
WARNING: This variable is strictly for internal use only. Its availability,
behavior, and contents are subject to change without notice.
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 add_100 { $^H |= 0x100 }
- sub foo {
- BEGIN { add_100() }
- bar->baz($boon);
- }
+ sub foo {
+ BEGIN { add_100() }
+ bar->baz($boon);
+ }
Consider what happens during execution of the BEGIN block. At this point
the BEGIN block has already been compiled, but the body of C<foo()> is still
being compiled. The new value of C<$^H> will therefore be visible only while
the body of C<foo()> is being compiled.
-Substitution of the above BEGIN block with:
+Substitution of C<BEGIN { add_100() }> block with:
- BEGIN { require strict; strict->import('vars') }
+ BEGIN { require strict; strict->import('vars') }
-demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
+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 }
+ BEGIN { require strict; strict->import('vars') if $condition }
This variable was added in Perl 5.003.
=item %^H
+X<%^H>
The C<%^H> hash provides the same scoping semantic as C<$^H>. This makes it
useful for implementation of lexically scoped pragmas. See L<perlpragma>.
This variable was added in Perl 5.6.
+=item @INC
+X<@INC>
+
+The array C<@INC> contains the list of places that the C<do EXPR>,
+C<require>, or C<use> constructs look for their library files. 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. ("." will not be appended if taint checks are enabled,
+either by C<-T> or by C<-t>.) 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;
+
+You can also insert hooks into the file inclusion system by putting Perl
+code directly into C<@INC>. Those hooks may be subroutine references, array
+references or blessed objects. See L<perlfunc/require> for details.
+
+=item %INC
+X<%INC>
+
+The hash C<%INC> contains entries for each filename included via the
+C<do>, C<require>, or C<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 C<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
+L<perlfunc/require> for a description of these hooks), this hook is
+by default inserted into C<%INC> in place of a filename. Note, however,
+that the hook may have set the C<%INC> entry by itself to provide some more
+specific info.
+
=item $INPLACE_EDIT
=item $^I
were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
Then
- $^M = 'a' x (1 << 16);
+ $^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
This variable was added in Perl 5.003.
=item ${^OPEN}
+X<${^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
=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.
+run-time only. This is a new mechanism and the details may change.
See also L<perldebguts>.
-=item @F
-X<@F>
+=item %SIG
+X<%SIG>
-The array C<@F> contains the fields of each line read in when autosplit
-mode is turned on. See L<perlrun> for the B<-a> switch. This array
-is package-specific, and must be declared or given a full package name
-if not in package main when running under C<strict 'vars'>.
+The hash C<%SIG> contains signal handlers for signals. For example:
-=item @INC
-X<@INC>
+ sub handler { # 1st argument is signal name
+ my($sig) = @_;
+ print "Caught a SIG$sig--shutting down\n";
+ close(LOG);
+ exit(0);
+ }
-The array C<@INC> contains the list of places that the C<do EXPR>,
-C<require>, or C<use> constructs look for their library files. 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. ("." will not be appended if taint checks are enabled,
-either by C<-T> or by C<-t>.) 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;
-
-You can also insert hooks into the file inclusion system by putting Perl
-code directly into C<@INC>. Those hooks may be subroutine references, array
-references or blessed objects. See L<perlfunc/require> for details.
-
-=item @ARG
-
-=item @_
-X<@_> X<@ARG>
-
-Within a subroutine the array C<@_> contains the parameters passed to that
-subroutine. See L<perlsub>.
-
-=item %INC
-X<%INC>
-
-The hash C<%INC> contains entries for each filename included via the
-C<do>, C<require>, or C<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 C<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
-L<perlfunc/require> for a description of these hooks), this hook is
-by default inserted into C<%INC> in place of a filename. Note, however,
-that the hook may have set the C<%INC> entry by itself to provide some more
-specific info.
-
-=item %ENV
-
-=item $ENV{expr}
-X<%ENV>
-
-The hash C<%ENV> contains your current environment. Setting a
-value in C<ENV> changes the environment for any child processes
-you subsequently C<fork()> off.
-
-=item %SIG
-
-=item $SIG{expr}
-X<%SIG>
-
-The hash C<%SIG> contains signal handlers for signals. For example:
-
- sub handler { # 1st argument is signal name
- my($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
+ $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
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??
+ $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
+ $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
+ $SIG{"PIPE"} = *Plumber; # somewhat esoteric
+ $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
Be sure not to use a bareword as the name of a signal handler,
lest you inadvertently call it.
The default delivery policy of signals changed in Perl 5.8.0 from
immediate (also known as "unsafe") to deferred, also known as "safe
-signals".
See L<perlipc> for more information.
+signals". See L<perlipc> for more information.
Certain internal hooks can be also set using the C<%SIG> hash. The
routine indicated by C<$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 C<__WARN__> hook causes the
+message is about to be printed. The warning message is passed as the
+first argument. The presence of a C<__WARN__> hook causes the
ordinary printing of warnings to C<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;
+ local $SIG{__WARN__} = sub { die $_[0] };
+ eval $proggie;
As the C<'IGNORE'> hook is not supported by C<__WARN__>, you can
disable warnings using the empty subroutine:
- local $SIG{__WARN__} = sub {};
+ local $SIG{__WARN__} = sub {};
The routine indicated by C<$SIG{__DIE__}> is called when a fatal
exception is about to be thrown. The error message is passed as the
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";
+ require Carp if defined $^S;
+ Carp::confess("Something wrong") if defined &Carp::confess;
+ die "Something wrong, but could not load Carp to give backtrace...
+ To see backtrace try starting Perl with -MCarp switch";
Here the first line will load C<Carp> I<unless> it is the parser who
called the handler. The second line will print backtrace and die if
C<Carp> was available. The third line will be executed only if C<Carp> was
not available.
+Having to even think about the C<$^S> variable in your exception
+handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
+invites grievous and difficult to track down errors. Avoid it
+and use an C<END{}> or CORE::GLOBAL::die override instead.
+
See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
L<warnings> for additional information.
-=back
-
-=head2 Names that are no longer special
-
-These variables had special meaning in prior versions of Perl but now
-have no effect and will cause warnings if used. They are included
-here for historical reference.
-
-=over 8
-
=item $BASETIME
=item $^T
and B<-C> filetests are based on this value.
=item ${^TAINT}
+X<${^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>).
+B<-t> or B<-TU>).
This variable is read-only.
This variable was added in Perl 5.8.
=item ${^UNICODE}
+X<${^UNICODE}>
-Reflects certain Unicode settings of Perl.
See L<perlrun>
+Reflects certain Unicode settings of Perl. See L<perlrun>
documentation for the C<-C> switch for more information about
-the possible values.
+the possible values.
This variable is set during Perl startup and is thereafter read-only.
This variable was added in Perl 5.8.2.
=item ${^UTF8CACHE}
+X<${^UTF8CACHE}>
This variable controls the state of the internal UTF-8 offset caching code.
1 for on (the default), 0 for off, -1 to debug the caching code by checking
This variable was added in Perl 5.8.9.
=item ${^UTF8LOCALE}
+X<${^UTF8LOCALE}>
This variable indicates whether a UTF-8 locale was detected by perl at
startup. This information is used by perl when it's in
C<$^V> can be used to determine whether the Perl interpreter executing
a script is in the right range of versions. For example:
- warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
+ warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
To convert C<$^V> into its string representation use C<sprintf()>'s
C<"%vd"> conversion:
- printf "version is v%vd\n", $^V; # Perl's version
+ printf "version is v%vd\n", $^V; # Perl's version
See the documentation of C<use VERSION> and C<require VERSION>
for a convenient way to fail if the running Perl interpreter is too old.
Mnemonic: use ^V for Version Control.
=item ${^WIN32_SLOPPY_STAT}
-X<sitecustomize> X<sitecustomize.pl>
+X<${^WIN32_SLOPPY_STAT}> X<sitecustomize> X<sitecustomize.pl>
If this variable is set to a true value, then C<stat()> on Windows will
not try to open the file. This means that the link count cannot be
You usually can use the value of C<$^X> to re-invoke an independent
copy of the same perl that is currently running, e.g.,
- @first_run = `$^X -le "print int rand 100 for 1..100"`;
+ @first_run = `$^X -le "print int rand 100 for 1..100"`;
But recall that not all operating systems support forking or
capturing of the output of commands, so this complex statement
a command. To convert the value of C<$^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;}
+ # Build up a set of file names (not command names).
+ use Config;
+ my $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
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;}
+ use Config;
+ my $secure_perl_path = $Config{perlpath};
+ if ($^O ne 'VMS') {
+ $secure_perl_path .= $Config{_exe}
+ unless $secure_perl_path =~ m/$Config{_exe}$/i;
+ }
=back
print "I found $1 and $2\n";
}
-These variables are read-only and dynamically-scoped, unless we note
+These variables are read-only and dynamically-scoped, unless we note
otherwise.
-The dynamic nature of the regular expression variables means that their value
-is limited to the block that they are in, as demonstrated by this bit of code:
+The dynamic nature of the regular expression variables means that
+their value is limited to the block that they are in, as demonstrated
+by this bit of code:
my $outer = 'Wallace and Grommit';
my $inner = 'Mutt and Jeff';
-
+
my $pattern = qr/(\S+) and (\S+)/;
-
+
sub show_n { print "\$1 is $1; \$2 is $2\n" }
-
+
{
OUTER:
show_n() if $outer =~ m/$pattern/;
-
+
INNER: {
show_n() if $inner =~ m/$pattern/;
}
-
+
show_n();
}
-The output shows that while in the C<OUTER> block, the values of C<$1> and C<$2>
-are from the match against C<$outer>. Inside the C<INNER> block, the values of
-C<$1> and C<$2> are from the match against C<$inner>, but only until the end of the
-block (i.e. the dynamic scope). After the C<INNER> block completes, the values of
-C<$1> and C<$2> return to the values for the match against C<$outer> even though
+The output shows that while in the C<OUTER> block, the values of C<$1>
+and C<$2> are from the match against C<$outer>. Inside the C<INNER>
+block, the values of C<$1> and C<$2> are from the match against
+C<$inner>, but only until the end of the block (i.e. the dynamic
+scope). After the C<INNER> block completes, the values of C<$1> and
+C<$2> return to the values for the match against C<$outer> even though
we have not made another match:
$1 is Wallace; $2 is Grommit
$1 is Mutt; $2 is Jeff
$1 is Wallace; $2 is Grommit
+Due to an unfortunate accident of Perl's implementation, C<use
+English> imposes a considerable performance penalty on all regular
+expression matches in a program because it uses the C<$`>, C<$&>, and
+C<$'>, regardless of whether they occur in the scope of C<use
+English>. For that reason, saying C<use English> in libraries is
+strongly discouraged unless you import it without the match variables:
+
+ use English '-no_match_vars'
+
+The C<Devel::NYTProf> module can help you find uses of these
+problematic match variables in your code.
+
+Since Perl 5.10, you can use the C</p> match operator flag and the
+C<${^PREMATCH}>, C<${^MATCH}>, and C<${^POSTMATCH}> variables instead
+so you only suffer the performance penalties.
+
=over 8
=item $<I<digits>> ($1, $2, ...)
BLOCK).
The use of this variable anywhere in a program imposes a considerable
-performance penalty on all regular expression matches. See L</BUGS>.
-To avoid this penatly, you can extract the same substring by
-using L</@->. Starting with Perl 5.10, you can use the </p> match flag
-and the C<${^MATCH}> variable to do the same thing for particular
-match operations.
+performance penalty on all regular expression matches. To avoid this
+penalty, you can extract the same substring by using L</@->. Starting
+with Perl 5.10, you can use the </p> match flag and the C<${^MATCH}>
+variable to do the same thing for particular match operations.
This variable is read-only and dynamically-scoped.
Mnemonic: like C<&> in some editors.
-
+
=item ${^MATCH}
X<${^MATCH}>
=item $PREMATCH
=item $`
-X<$`> X<$PREMATCH>
+X<$`> X<$PREMATCH> X<${^PREMATCH}>
The string preceding whatever was matched by the last successful
pattern match, not counting any matches hidden within a BLOCK or C<eval>
-enclosed by the current BLOCK.
+enclosed by the current BLOCK.
The use of this variable anywhere in a program imposes a considerable
-performance penalty on all regular expression matches. See L</BUGS>.
-To avoid this penatly, you can extract the same substring by
-using L</@->. Starting with Perl 5.10, you can use the </p> match flag
-and the C<${^PREMATCH}> variable to do the same thing for particular
-match operations.
+performance penalty on all regular expression matches. To avoid this
+penalty, you can extract the same substring by using L</@->. Starting
+with Perl 5.10, you can use the </p> match flag and the
+C<${^PREMATCH}> variable to do the same thing for particular match
+operations.
This variable is read-only and dynamically-scoped.
Mnemonic: C<`> often precedes a quoted string.
=item ${^PREMATCH}
-X<${^PREMATCH}>
+X<$`> X<${^PREMATCH}>
This is similar to C<$`> ($PREMATCH) except that it does not incur the
performance penalty associated with that variable, and is only guaranteed
=item $POSTMATCH
=item $'
-X<$'> X<$POSTMATCH>
+X<$'> X<$POSTMATCH> X<${^POSTMATCH}> X<@->
The string following whatever was matched by the last successful
pattern match (not counting any matches hidden within a BLOCK or C<eval()>
enclosed by the current BLOCK). Example:
- local $_ = 'abcdefghi';
- /def/;
- print "$`:$&:$'\n"; # prints abc:def:ghi
+ local $_ = 'abcdefghi';
+ /def/;
+ print "$`:$&:$'\n"; # prints abc:def:ghi
The use of this variable anywhere in a program imposes a considerable
-performance penalty on all regular expression matches. See L</BUGS>.
-To avoid this penatly, you can extract the same substring by
+performance penalty on all regular expression matches.
+To avoid this penalty, you can extract the same substring by
using L</@->. Starting with Perl 5.10, you can use the </p> match flag
-and the C<${^POSTMATCH}> variable to do the same thing for particular
+and the C<${^POSTMATCH}> variable to do the same thing for particular
match operations.
This variable is read-only and dynamically-scoped.
Mnemonic: C<'> often follows a quoted string.
=item ${^POSTMATCH}
-X<${^POSTMATCH}>
+X<${^POSTMATCH}> X<$'> X<$POSTMATCH>
This is similar to C<$'> (C<$POSTMATCH>) except that it does not incur the
performance penalty associated with that variable, and is only guaranteed
This is useful if you don't know which one of a set of alternative patterns
matched. For example:
- /Version: (.*)|Revision: (.*)/ && ($rev = $+);
+ /Version: (.*)|Revision: (.*)/ && ($rev = $+);
This variable is read-only and dynamically-scoped.
=item $LAST_SUBMATCH_RESULT
=item $^N
-X<$^N>
+X<$^N> X<$LAST_SUBMATCH_RESULT>
The text matched by the used group most-recently closed (i.e. the group
with the rightmost closing parenthesis) of the last successful search
recently matched. For example, to effectively capture text to a variable
(in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
- (?:(...)(?{ $var = $^N }))
+ (?:(...)(?{ $var = $^N }))
By setting and then using C<$var> in this way relieves you from having to
worry about exactly which numbered set of parentheses they are.
on the variable that was matched against. The I<n>th element
of this array holds the offset of the I<n>th submatch, so
C<$+[1]> is the offset past where C<$1> ends, C<$+[2]> the offset
-past where C<$2> ends, and so on. You can use C<$#+> to determine
+past where C<$2> ends, and so on. You can use C<$#+> to determine
how many subgroups were in the last successful match. See the
examples given for the C<@-> variable.
=item %LAST_PAREN_MATCH
=item %+
-X<%+>
+X<%+> X<%LAST_PAREN_MATCH>
Similar to C<@+>, the C<%+> hash allows access to the named capture
buffers, should they exist, in the last successful match in the
For example, C<$+{foo}> is equivalent to C<$1> after the following match:
- 'foo' =~ /(?<foo>foo)/;
+ 'foo' =~ /(?<foo>foo)/;
The keys of the C<%+> hash list only the names of buffers that have
captured (and that are thus associated to defined values).
This array holds the offsets of the beginnings of the last
successful submatches in the currently active dynamic scope.
C<$-[0]> is the offset into the string of the beginning of the
-entire match. The I<n>th element of this array holds the offset
+entire match. The I<n>th element of this array holds the offset
of the I<n>th submatch, so C<$-[1]> is the offset where C<$1>
begins, C<$-[2]> the offset where C<$2> begins, and so on.
This variable was added in Perl 5.6.
+=item %LAST_MATCH_START
+
=item %-
-X<%->
+X<%-> X<%LAST_MATCH_START>
Similar to C<%+>, this variable allows access to the named capture groups
in the last successful match in the currently active dynamic scope. To
would print out:
- $-{A}[0] : '1'
- $-{A}[1] : '3'
- $-{B}[0] : '2'
- $-{B}[1] : '4'
+ $-{A}[0] : '1'
+ $-{A}[1] : '3'
+ $-{B}[0] : '2'
+ $-{B}[1] : '4'
The keys of the C<%-> hash correspond to all buffer names found in
the regular expression.
This variable was added in Perl 5.005.
=item ${^RE_DEBUG_FLAGS}
+X<${^RE_DEBUG_FLAGS}>
The current value of the regex debugging flags. Set to 0 for no debug output
even when the C<re 'debug'> module is loaded. See L<re> for details.
This variable was added in Perl 5.10.
=item ${^RE_TRIE_MAXBUF}
+X<${^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
variables. (Summary lines below for this contain the word HANDLE.)
First you must say
- use IO::Handle;
+ use IO::Handle;
after which you may use either
- method HANDLE EXPR
+ method HANDLE EXPR
or more safely,
- HANDLE->method(EXPR)
+ HANDLE->method(EXPR)
Each method returns the old value of the C<IO::Handle> attribute. The
methods each take an optional EXPR, which, if supplied, specifies the
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;
+ 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;
+ 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
inside some short C<{}> block, you should create one yourself. For
example:
- my $content = '';
- open my $fh, "<", "foo" or die $!;
- {
- local $/;
- $content = <$fh>;
- }
- close $fh;
+ 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 $_
- }
+ for ( 1..3 ){
+ $\ = "\r\n";
+ nasty_break();
+ print "$_";
+ }
-You probably expect this code to print:
+ sub nasty_break {
+ $\ = "\f";
+ # do something with $_
+ }
+
+You probably expect this code to print the equivalent of
- 1 2 3 4 5
+ "1\r\n2\r\n3\r\n"
but instead you get:
- 5 5 5 5 5
+ "1\f2\f3\f"
-Why? Because C<nasty_break()> modifies C<$_> without localizing it
-first. The fix is to add C<local()>:
+Why? Because C<nasty_break()> modifies C<$\> without localizing it
+first. The value you set in C<nasty_break()> is still there when you
+return. The fix is to add C<local()> so the value doesn't leak out of
+C<nasty_break()>:
- local $_ = 5;
+ local $\ = "\f";
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
=over 8
+=item $ARGV
+X<$ARGV>
+
+Contains the name of the current file when reading from C<< <> >>.
+
+=item @ARGV
+X<@ARGV>
+
+The array C<@ARGV> contains the command-line arguments intended for
+the script. C<$#ARGV> is generally the number of arguments minus
+one, because C<$ARGV[0]> is the first argument, I<not> the program's
+command name itself. See C<$0> for the command name.
+
=item ARGV
X<ARGV>
may not cause your function to automatically read the contents of all the
files in C<@ARGV>.
-=item $ARGV
-X<$ARGV>
-
-contains the name of the current file when reading from <>.
-
-=item @ARGV
-X<@ARGV>
-
-The array @ARGV contains the command-line arguments intended for
-the script. C<$#ARGV> is generally the number of arguments minus
-one, because C<$ARGV[0]> is the first argument, I<not> the program's
-command name itself. See C<$0> for the command name.
-
=item ARGVOUT
X<ARGVOUT>
to do a lot of inserting and don't want to keep modifying C<$_>. See
L<perlrun> for the B<-i> switch.
-=item HANDLE->input_line_number(EXPR)
+=item Handle->output_field_separator( EXPR )
+
+=item $OUTPUT_FIELD_SEPARATOR
+
+=item $OFS
+
+=item $,
+X<$,> X<$OFS> X<$OUTPUT_FIELD_SEPARATOR>
+
+The output field separator for the print operator. If defined, this
+value is printed between each of print's arguments. Default is C<undef>.
+
+Mnemonic: what is printed when there is a "," in your print statement.
+
+=item HANDLE->input_line_number( EXPR )
=item $INPUT_LINE_NUMBER
Current line number for the last filehandle accessed.
Each filehandle in Perl counts the number of lines that have been read
-from it. (Depending on the value of C<$/>, Perl's idea of what
+from it. (Depending on the value of C<$/>, Perl's idea of what
constitutes a line may not match yours.) When a line is read from a
filehandle (via C<readline()> or C<< <> >>), or when C<tell()> or
C<seek()> is called on it, C<$.> becomes an alias to the line counter
Mnemonic: many programs use "." to mean the current line number.
-=item IO::Handle->input_record_separator(EXPR)
+=item HANDLE->input_record_separator( EXPR )
=item $INPUT_RECORD_SEPARATOR
=item $/
X<$/> X<$RS> X<$INPUT_RECORD_SEPARATOR>
-The input record separator, newline by default. This
-influences Perl's idea of what a "line" is. Works like B<awk>'s RS
-variable, including treating empty lines as a terminator if set to
-the null string (an empty line cannot contain any spaces
-or tabs). You may set it to a multi-character string to match a
-multi-character terminator, or to C<undef> to read through the end
-of file. Setting it to C<"\n\n"> means something slightly
-different than setting to C<"">, if the file contains consecutive
-empty lines. Setting to C<""> will treat two or more consecutive
-empty lines as a single empty line. Setting to C<"\n\n"> will
-blindly assume that the next input character belongs to the next
-paragraph, even if it's a newline.
+The input record separator, newline by default. This influences Perl's
+idea of what a "line" is. Works like B<awk>'s RS variable, including
+treating empty lines as a terminator if set to the null string (an
+empty line cannot contain any spaces or tabs). You may set it to a
+multi-character string to match a multi-character terminator, or to
+C<undef> to read through the end of file. Setting it to C<"\n\n">
+means something slightly different than setting to C<"">, if the file
+contains consecutive empty lines. Setting to C<""> will treat two or
+more consecutive empty lines as a single empty line. Setting to
+C<"\n\n"> will blindly assume that the next input character belongs to
+the next paragraph, even if it's a newline.
local $/; # enable "slurp" mode
local $_ = <FH>; # whole file now here
s/\n[ \t]+/ /g;
-Remember: the value of C<$/> is a string, not a regex.
B<awk> has to
+Remember: the value of C<$/> is a string, not a regex. B<awk> has to
be better for something. :-)
Setting C<$/> to a reference to an integer, scalar containing an
open my $fh, "<", $myfile or die $!;
local $_ = <$fh>;
-will read a record of no more than 32768 bytes from FILE. If you're
+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. Trying to set the record
+with every read. If a record is larger than the record size you've
+set, you'll get the record back in pieces. Trying to set the record
size to zero or less will cause reading in the (rest of the) whole file.
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 unlikely to be a problem, because any file you'd
+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 L<perlport/"Newlines">. Also see C<$.>.
+See also L<perlport/"Newlines">. Also see C<$.>.
Mnemonic: / delimits line boundaries when quoting poetry.
-=item HANDLE->autoflush(EXPR)
-
-=item $OUTPUT_AUTOFLUSH
-
-=item $|
-X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH>
-
-If set to nonzero, forces a flush right away and after every write
-or print on the currently selected output channel. Default is 0
-(regardless of whether the channel is really buffered by the
-system or not; C<$|> 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 B<rsh> and want to see the output as it's
-happening. This has no effect on input buffering. See L<perlfunc/getc>
-for that. See L<perldoc/select> on how to select the output channel.
-See also L<IO::Handle>.
-
-Mnemonic: when you want your pipes to be piping hot.
-
-=item IO::Handle->output_field_separator EXPR
-
-=item $OUTPUT_FIELD_SEPARATOR
-
-=item $OFS
-
-=item $,
-X<$,> X<$OFS> X<$OUTPUT_FIELD_SEPARATOR>
-
-The output field separator for the print operator. If defined, this
-value is printed between each of print's arguments. Default is C<undef>.
-
-Mnemonic: what is printed when there is a "," in your print statement.
-
-=item IO::Handle->output_record_separator EXPR
+=item Handle->output_record_separator( EXPR )
=item $OUTPUT_RECORD_SEPARATOR
Mnemonic: you set C<$\> 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.
-=back
+=item HANDLE->autoflush( EXPR )
+
+=item $OUTPUT_AUTOFLUSH
+
+=item $|
+X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH>
+
+If set to nonzero, forces a flush right away and after every write or
+print on the currently selected output channel. Default is 0
+(regardless of whether the channel is really buffered by the system or
+not; C<$|> 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 B<rsh> and
+want to see the output as it's happening. This has no effect on input
+buffering. See L<perlfunc/getc> for that. See L<perlfunc/select> on
+how to select the output channel. See also L<IO::Handle>.
+Mnemonic: when you want your pipes to be piping hot.
+
+=back
=head3 Variables related to formats
The special variables for formats are a subset of those for
-filehandles so they have
-
-See L<perlform> for more information about Perl's formats.
+filehandles. See L<perlform> for more information about Perl's
+formats.
=over 8
+=item $ACCUMULATOR
+
+=item $^A
+X<$^A> X<$ACCUMULATOR>
+
+The current value of the C<write()> accumulator for C<format()> lines.
+A format contains C<formline()> calls that put their result into
+C<$^A>. After calling its format, C<write()> prints out the contents
+of C<$^A> and empties. So you never really see the contents of C<$^A>
+unless you call C<formline()> yourself and then look at it. See
+L<perlform> and L<perlfunc/formline()>.
+
+=item HANDLE->format_formfeed(EXPR)
+
+=item $FORMAT_FORMFEED
+
+=item $^L
+X<$^L> X<$FORMAT_FORMFEED>
+
+What formats output as a form feed. The default is C<\f>.
+
=item HANDLE->format_page_number(EXPR)
=item $FORMAT_PAGE_NUMBER
Mnemonic: C<%> is page number in B<nroff>.
-=item HANDLE->format_lines_per_page(EXPR)
-
-=item $FORMAT_LINES_PER_PAGE
-
-=item $=
-X<$=> X<$FORMAT_LINES_PER_PAGE>
-
-The current page length (printable lines) of the currently selected
-output channel. The default is 60.
-
-Mnemonic: = has horizontal lines.
-
=item HANDLE->format_lines_left(EXPR)
=item $FORMAT_LINES_LEFT
Mnemonic: lines_on_page - lines_printed.
-=item HANDLE->format_name(EXPR)
+=item Handle->format_line_break_characters EXPR
-=item $FORMAT_NAME
+=item $FORMAT_LINE_BREAK_CHARACTERS
-=item $~
-X<$~> X<$FORMAT_NAME>
+=item $:
+X<$:> X<FORMAT_LINE_BREAK_CHARACTERS>
-The name of the current report format for the currently selected
-output channel. The default format name is the same as the filehandle
-name. For example, the default format name for the C<STDOUT>
-filehandle is just C<STDOUT>.
+The current set of characters after which a string may be broken to
+fill continuation fields (starting with C<^>) in a format. The default is
+S<" \n-">, to break on a space, newline, or a hyphen.
-Mnemonic: brother to C<$^>.
+Mnemonic: a "colon" in poetry is a part of a line.
+
+=item HANDLE->format_lines_per_page(EXPR)
+
+=item $FORMAT_LINES_PER_PAGE
+
+=item $=
+X<$=> X<$FORMAT_LINES_PER_PAGE>
+
+The current page length (printable lines) of the currently selected
+output channel. The default is 60.
+
+Mnemonic: = has horizontal lines.
=item HANDLE->format_top_name(EXPR)
Mnemonic: points to top of page.
-=item IO::Handle->format_line_break_characters EXPR
-
-=item $FORMAT_LINE_BREAK_CHARACTERS
-
-=item $:
-X<$:> X<FORMAT_LINE_BREAK_CHARACTERS>
-
-The current set of characters after which a string may be broken to
-fill continuation fields (starting with C<^>) in a format. The default is
-S<" \n-">, to break on a space, newline, or a hyphen.
-
-Mnemonic: a "colon" in poetry is a part of a line.
-
-=item IO::Handle->format_formfeed EXPR
-
-=item $FORMAT_FORMFEED
-
-=item $^L
-X<$^L> X<$FORMAT_FORMFEED>
+=item HANDLE->format_name(EXPR)
-What formats output as a form feed. The default is C<\f>.
+=item $FORMAT_NAME
-=item $ACCUMULATOR
+=item $~
+X<$~> X<$FORMAT_NAME>
-=item $^A
-X<$^A> X<$ACCUMULATOR>
+The name of the current report format for the currently selected
+output channel. The default format name is the same as the filehandle
+name. For example, the default format name for the C<STDOUT>
+filehandle is just C<STDOUT>.
-The current value of the C<write()> accumulator for C<format()> lines.
-A format contains C<formline()> calls that put their result into
-C<$^A>. After calling its format, C<write()> prints out the contents
-of C<$^A> and empties. So you never really see the contents of C<$^A>
-unless you call C<formline()> yourself and then look at it. See
-L<perlform> and L<perlfunc/formline()>.
+Mnemonic: brother to C<$^>.
=back
-=head2 Error Indicators
+=head2 Error Variables
X<error> X<exception>
The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
respectively.
To illustrate the differences between these variables, consider the
-following Perl expression, which uses a single-quoted string:
+following Perl expression, which uses a single-quoted string. After
+execution of this statement, perl may have set all four special error
+variables:
+
+ eval q{
+ open my $pipe, "/cdrom/install |" or die $!;
+ my @res = <$pipe>;
+ close $pipe or die "bad pipe: $?, $!";
+ };
+
+When perl executes the C<eval()> expression, it translates the
+C<open()>, C<< <PIPE> >>, and C<close> calls in the C run-time library
+and thence to the operating system kernel. perl sets C<$!> to
+the C library's C<errno> if one of these calls fails.
+
+C<$@> is set if the string to be C<eval>-ed did not compile (this may
+happen if C<open> or C<close> were imported with bad prototypes), or
+if Perl code executed during evaluation C<die()>d. In these cases the
+value of C<$@> is the compile error, or the argument to C<die> (which
+will interpolate C<$!> and C<$?>). (See also L<Fatal>, though.)
+
+Under a few operating systems, C<$^E> may contain a more verbose error
+indicator, such as in this case, "CDROM tray not closed." Systems that
+do not support extended error messages leave C<$^E> the same as C<$!>.
- eval q{
- open my $pipe, "/cdrom/install |" or die $!;
- my @res = <$pipe>;
- close $pipe or die "bad pipe: $?, $!";
- };
+Finally, C<$?> may be set to non-0 value if the external program
+F</cdrom/install> fails. The upper eight bits reflect specific error
+conditions encountered by the program (the program's C<exit()> value).
+The lower eight bits reflect mode of failure, like signal death and
+core dump information. See C<wait(2)> for details. In contrast to
+C<$!> and C<$^E>, which are set only if error condition is detected,
+the variable C<$?> is set on each C<wait> or pipe C<close>,
+overwriting the old value. This is more like C<$@>, which on every
+C<eval()> is always set on failure and cleared on success.
-After execution of this statement all 4 variables may have been set.
+For more details, see the individual descriptions at C<$@>, C<$!>,
+C<$^E>, and C<$?>.
-C<$@> 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 . In these cases
-the value of $@ is the compile error, or the argument to C<die>
-(which will interpolate C<$!> and C<$?>). (See also L<Fatal>,
-though.)
+=over 8
-When the C<eval()> expression above is executed, C<open()>, C<< <PIPE> >>,
-and C<close> are translated to calls in the C run-time library and
-thence to the operating system kernel. C<$!> is set to the C library's
-C<errno> if one of these calls fails.
+=item ${^CHILD_ERROR_NATIVE}
+X<$^CHILD_ERROR_NATIVE>
-Under a few operating systems, C<$^E> may contain a more verbose
-error indicator, such as in this case, "CDROM tray not closed."
-Systems that do not support extended error messages leave C<$^E>
-the same as C<$!>.
+The native status returned by the last pipe close, backtick (C<``>)
+command, successful call to C<wait()> or C<waitpid()>, or from the
+C<system()> operator. On POSIX-like systems this value can be decoded
+with the WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED,
+WSTOPSIG and WIFCONTINUED functions provided by the L<POSIX> module.
-Finally, C<$?> may be set to non-0 value if the external program
-F</cdrom/install> fails. The upper eight bits reflect specific
-error conditions encountered by the program (the program's C<exit()>
-value). The lower eight bits reflect mode of failure, like signal
-death and core dump information. See C<wait(2)> for details. In
-contrast to C<$!> and C<$^E>, which are set only if error condition
-is detected, the variable C<$?> is set on each C<wait> or pipe
-C<close>, overwriting the old value. This is more like C<$@>, which
-on every C<eval()> is always set on failure and cleared on success.
+Under VMS this reflects the actual VMS exit status; i.e. it is the
+same as C<$?> when the pragma C<use vmsish 'status'> is in effect.
-For more details, see the individual descriptions at C<$@>, C<$!>,
-C<$^E>, and C<$?>.
+This variable was added in Perl 5.8.9.
-=item $CHILD_ERROR
+=item $EXTENDED_OS_ERROR
-=item $?
-X<$?> X<$CHILD_ERROR>
+=item $^E
+X<$^E> X<$EXTENDED_OS_ERROR>
-The status returned by the last pipe close, backtick (C<``>) command,
-successful call to C<wait()> or C<waitpid()>, or from the C<system()>
-operator. This is just the 16-bit status word returned by the
-traditional Unix C<wait()> system call (or else is made up to look
-like it). Thus, the exit value of the subprocess is really (C<<< $? >>
-8 >>>), and C<$? & 127> gives which signal, if any, the process died
-from, and C<$? & 128> reports whether there was a core dump.
+Error information specific to the current operating system. At the
+moment, this differs from C<$!> under only VMS, OS/2, and Win32 (and
+for MacPerl). On all other platforms, C<$^E> is always just the same
+as C<$!>.
-Additionally, if the C<h_errno> variable is supported in C, its value
-is returned via C<$?> if any C<gethost*()> function fails.
+Under VMS, C<$^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>.
-If you have installed a signal handler for C<SIGCHLD>, the
-value of C<$?> will usually be wrong outside that handler.
+Under OS/2, C<$^E> is set to the error code of the last call to OS/2
+API either via CRT, or directly from perl.
-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 your program. For example:
+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<$!>.
- END {
- $? = 1 if $? == 255; # die would make it 255
- }
+Caveats mentioned in the description of C<$!> generally apply to
+C<$^E>, also.
-Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
-actual VMS exit status, instead of the default emulation of POSIX
-status; see L<perlvms/$?> for details.
+This variable was added in Perl 5.003.
-Also see L<Error Indicators>.
+Mnemonic: Extra error explanation.
-Mnemonic: similar to B<sh> and B<ksh>.
+=item $EXCEPTIONS_BEING_CAUGHT
-=item ${^CHILD_ERROR_NATIVE}
-X<$^CHILD_ERROR_NATIVE>
+=item $^S
+X<$^S> X<$EXCEPTIONS_BEING_CAUGHT>
-The native status returned by the last pipe close, backtick (C<``>)
-command, successful call to C<wait()> or C<waitpid()>, or from the
-C<system()> operator. On POSIX-like systems this value can be decoded
-with the WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED,
-WSTOPSIG and WIFCONTINUED functions provided by the L<POSIX> module.
+Current state of the interpreter.
-Under VMS this reflects the actual VMS exit status; i.e. it is the
-same as C<$?> when the pragma C<use vmsish 'status'> is in effect.
+ $^S State
+ --------- -------------------
+ undef Parsing module/eval
+ true (1) Executing an eval
+ false (0) Otherwise
-This variable was added in Perl 5.8.9.
+The first state may happen in C<$SIG{__DIE__}> and C<$SIG{__WARN__}>
+handlers.
+
+This variable was added in Perl 5.004.
+
+=item $WARNING
+
+=item $^W
+X<$^W> X<$WARNING>
+
+The current value of the warning switch, initially true if B<-w> was
+used, false otherwise, but directly modifiable.
+
+See also L<warnings>.
+
+Mnemonic: related to the B<-w> switch.
+
+=item ${^WARNING_BITS}
+X<${^WARNING_BITS}>
+
+The current set of warning checks enabled by the C<use warnings> pragma.
+See the documentation of C<warnings> for more details.
+
+This variable was added in Perl 5.10.
=item $OS_ERROR
sets this variable. This means that the value of C<$!> is meaningful
only I<immediately> after a B<failure>:
- if (open my $fh, "<", $filename) {
- # Here $! is meaningless.
- ...
- } else {
- # ONLY here is $! meaningful.
- ...
- # Already here $! might be meaningless.
+ if (open my $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 $! is meaningless.
-In the above I<meaningless> stands for anything: zero, non-zero,
-C<undef>. A successful system or library call does B<not> set
-the variable to zero.
-
-If used as a string, yields the corresponding system error string.
-You can assign a number to C<$!> to set I<errno> if, for instance,
-you want C<"$!"> to return the string for error I<n>, or you want
-to set the exit value for the C<die()> operator.
+The I<meaningless> stands for anything: zero, non-zero,
+C<undef>. A successful system or library call does B<not> set the
+variable to zero.
-Also see L<Error Indicators>.
+If used as a string, yields the corresponding system error string. You
+can assign a number to C<$!> to set I<errno> if, for instance, you
+want C<"$!"> to return the string for error I<n>, or you want to set
+the exit value for the C<die()> operator.
Mnemonic: What just went bang?
=item %ERRNO
=item %!
-X<%!>
+X<%!> X<%OS_ERROR> X<%ERRNO>
Each element of C<%!> has a true value only if C<$!> is set to that
value. For example, C<$!{ENOENT}> is true if and only if the current
-value of C<$!> is C<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
-C<exists $!{the_key}>; for a list of legal keys, use C<keys %!>.
-See L<Errno> for more information, and also see above for the
-validity of C<$!>.
+value of C<$!> is C<ENOENT>; that is, if the most recent error was "No
+such file or directory" (or its moral equivalent: not all operating
+systems give that exact error, and certainly not all languages). To
+check if a particular key is meaningful on your system, use C<exists
+$!{the_key}>; for a list of legal keys, use C<keys %!>. See L<Errno>
+for more information, and also see L</$!>.
This variable was added in Perl 5.005.
-=item $EXTENDED_OS_ERROR
+=item $CHILD_ERROR
-=item $^E
-X<$^E> X<$EXTENDED_OS_ERROR>
+=item $?
+X<$?> X<$CHILD_ERROR>
-Error information specific to the current operating system. At
-the moment, this differs from C<$!> under only VMS, OS/2, and Win32
-(and for MacPerl). On all other platforms, C<$^E> is always just
-the same as C<$!>.
+The status returned by the last pipe close, backtick (C<``>) command,
+successful call to C<wait()> or C<waitpid()>, or from the C<system()>
+operator. This is just the 16-bit status word returned by the
+traditional Unix C<wait()> system call (or else is made up to look
+like it). Thus, the exit value of the subprocess is really (C<<< $? >>
+8 >>>), and C<$? & 127> gives which signal, if any, the process died
+from, and C<$? & 128> reports whether there was a core dump.
-Under VMS, C<$^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>.
+Additionally, if the C<h_errno> variable is supported in C, its value
+is returned via C<$?> if any C<gethost*()> function fails.
-Under OS/2, C<$^E> is set to the error code of the last call to
-OS/2 API either via CRT, or directly from perl.
+If you have installed a signal handler for C<SIGCHLD>, the
+value of C<$?> will usually be wrong outside that handler.
-Under Win32, C<$^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<$^E>. ANSI C and Unix-like calls
-set C<errno> and so most portable Perl code will report errors
-via C<$!>.
+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 your program. For example:
-Caveats mentioned in the description of C<$!> generally apply to
-C<$^E>, also.
+ END {
+ $? = 1 if $? == 255; # die would make it 255
+ }
-This variable was added in Perl 5.003.
+Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
+actual VMS exit status, instead of the default emulation of POSIX
+status; see L<perlvms/$?> for details.
+
+Mnemonic: similar to B<sh> and B<ksh>.
-Mnemonic: Extra error explanation.
-
=item $EVAL_ERROR
=item $@
X<$@> X<$EVAL_ERROR>
-The Perl syntax error message from the last eval() operator. If $@ is
-the null string, the last eval() parsed and executed correctly
+The Perl syntax error message from the last C<eval()> operator. If C<$@> is
+the null string, the last C<eval()> parsed and executed correctly
(although the operations you invoked may have failed in the normal
fashion).
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>.
+described in L</%SIG>.
Mnemonic: Where was the syntax error "at"?
-
-=item $EXCEPTIONS_BEING_CAUGHT
-=item $^S
-X<$^S> X<$EXCEPTIONS_BEING_CAUGHT>
+=back
-Current state of the interpreter.
+=head2 Deprecated and removed variables
- $^S State
- --------- -------------------
- undef Parsing module/eval
- true (1) Executing an eval
- false (0) Otherwise
+Deprecating a variable announces the intent of the perl maintainers to
+eventually remove the variable from the langauge. It may still be
+available despite its status. Using a deprecated variable triggers
+a warning.
-The first state may happen in C<$SIG{__DIE__}> and C<$SIG{__WARN__}> handlers.
+Once a variable is removed, its use triggers an error telling you
+the variable is unsupported.
-This variable was added in Perl 5.004.
+See L<perldiag> for details about error messages.
-=item $WARNING
+=over 8
-=item $^W
-X<$^W> X<$WARNING>
+=item $OFMT
-The current value of the warning switch, initially true if B<-w> was
-used, false otherwise, but directly modifiable.
+=item $#
+X<$#> X<$OFMT>
-See also L<warnings>.
+C<$#> was a variable that you could be use to format printed numbers.
+After a deprecation cycle, its magic was removed in Perl 5.10 and
+using it now triggers a warning: C<$# is no longer supported>.
-Mnemonic: related to the B<-w> switch.
+This is not the sigil you use in front of an array name to get the
+last index, like C<$#array>. That's still how you get the last index
+of an array in Perl. The two have nothing to do with each other.
-=item ${^WARNING_BITS}
+Deprecated in Perl 5.
-The current set of warning checks enabled by the C<use warnings> pragma.
-See the documentation of C<warnings> for more details.
+Removed in Perl 5.10.
-This variable was added in Perl 5.10.
+=item $*
+X<$*>
-=back
+C<$*> was a variable that you could use to enable multiline matching.
+After a deprecation cycle, its magic was removed in Perl 5.10.
+Using it now triggers a warning: C<$* is no longer supported>.
+You should use the C</s> and C</m> regexp modifiers instead.
-=head2 Deprecated and removed variables
+Deprecated in Perl 5.
-Deprecating a variable announces the perl maintainers intent to
-eventually remove the varaible from the langauge. It may still be
-available despite its status. Using a deprecated variable triggers
-a warning.
+Removed in Perl 5.10.
-Once the variable is removed, its use triggers an error telling you
-the variable is unsupported.
+=item $ARRAY_BASE
-See L<perldiag> for details about the error messages.
+=item $[
+X<$[> X<$ARRAY_BASE>
-=over 8
+This variable stores the index of the first element in an array, and
+of the first character in a substring. You used to be able to assign to
+this variable, but you can't do that anymore. It's now always 0, like
+it should be.
-=item $*
-X<$*>
+Mnemonic: [ begins subscripts.
-C<$*> used to be a variable that enabled multiline matching.
-After a deprecation cycle, its magic was removed in Perl 5.10.
-Using it now triggers a warning: C<$* is no longer supported>.
-Use the C</s> and C</m> regexp modifiers instead.
+This variable is read-only.
-Deprecated in Perl 5.
+Deprecated in Perl 5.12.
-Removed in Perl 5.10.
+=item $OLD_PERL_VERSION
=item $]
-X<$]>
+X<$]> X<$OLD_PERL_VERSION>
+
+See C<$^V> for a more modern representation of the Perl version that allows
+accurate string comparisons.
The version + patchlevel / 1000 of the Perl interpreter. This variable
can be used to determine whether the Perl interpreter executing a
warn "No checksumming!\n" if $] < 3.019;
+The floating point representation can sometimes lead to inaccurate
+numeric comparisons.
+
See also the documentation of C<use VERSION> and C<require VERSION>
for a convenient way to fail if the running Perl interpreter is too old.
-The floating point representation can sometimes lead to inaccurate
-numeric comparisons. See C<$^V> for a more modern representation of
-the Perl version that allows accurate string comparisons.
-
Mnemonic: Is this version of perl in the right bracket?
Deprecated in Perl 5.6.
-=item $#
-X<$#>
-
-C<$#> used to be a variable that could be used to format printed numbers.
-After a deprecation cycle, its magic was removed in Perl 5.10 and using it
-now triggers a warning: C<$# is no longer supported>.
-
-This is not the sigil you use in front of an array name to get the
-last index, like C<$#array>. That's still how you get the last index
-of an array in Perl. The two have nothing to do with each other.
-
-Deprecated in Perl 5.
-
-Removed in Perl 5.10.
-
-=item $[
-X<$[>
-
-The index of the first element in an array, and of the first character
-in a substring. You use to be able to assign to this variable, but you
-can't do that anymore. It's always 0, like God intended.
-
-Mnemonic: [ begins subscripts.
-
-Deprecated in Perl 5.12.
-
=back
-=head1 BUGS
-
-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.cpan.org/modules/by-module/Devel/ )
-for more information. Writing C<use English '-no_match_vars';>
-avoids the performance penalty.
-
-Having to even think about the C<$^S> variable in your exception
-handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
-invites grievous and difficult to track down errors. Avoid it
-and use an C<END{}> or CORE::GLOBAL::die override instead.
+=cut
https://perl5.git.perl.org / perl5.git / blobdiff