Perl provides a mechanism for alternative namespaces to protect
packages from stomping on each other's variables. In fact, there's
-really no such thing as a global variable in Perl . The package
+really no such thing as a global variable in Perl. The package
statement declares the compilation unit as being in the given
namespace. The scope of the package declaration is from the
declaration itself through the end of the enclosing block, C<eval>,
Variables beginning with underscore used to be forced into package
main, but we decided it was more useful for package writers to be able
to use leading underscore to indicate private variables and method names.
-$_ is still global though. See also L<perlvar/"Technical Note on the
-Syntax of Variable Names">.
+$_ is still global though. See also
+L<perlvar/"Technical Note on the Syntax of Variable Names">.
C<eval>ed strings are compiled in the package in which the eval() was
compiled. (Assignments to C<$SIG{}>, however, assume the signal
The symbol table for a package happens to be stored in the hash of that
name with two colons appended. The main symbol table's name is thus
-C<%main::>, or C<%::> for short. Likewise symbol table for the nested
+C<%main::>, or C<%::> for short. Likewise the symbol table for the nested
package mentioned earlier is named C<%OUTER::INNER::>.
The value in each entry of the hash is what you are referring to when you
local *main::foo = *main::bar;
local $main::{foo} = $main::{bar};
+(Be sure to note the B<vast> difference between the second line above
+and C<local $main::foo = $main::bar>. The former is accessing the hash
+C<%main::>, which is the symbol table of package C<main>. The latter is
+simply assigning scalar C<$bar> in package C<main> to scalar C<$foo> of
+the same package.)
+
You can use this to print out all the variables in a package, for
instance. The standard but antiquated F<dumpvar.pl> library and
the CPAN module Devel::Symdump make use of this.
Which makes $richard and $dick the same variable, but leaves
@richard and @dick as separate arrays. Tricky, eh?
-This mechanism may be used to pass and return cheap references
-into or from subroutines if you won't want to copy the whole
+There is one subtle difference between the following statements:
+
+ *foo = *bar;
+ *foo = \$bar;
+
+C<*foo = *bar> makes the typeglobs themselves synonymous while
+C<*foo = \$bar> makes the SCALAR portions of two distinct typeglobs
+refer to the same scalar value. This means that the following code:
+
+ $bar = 1;
+ *foo = \$bar; # Make $foo an alias for $bar
+
+ {
+ local $bar = 2; # Restrict changes to block
+ print $foo; # Prints '1'!
+ }
+
+Would print '1', because C<$foo> holds a reference to the I<original>
+C<$bar> -- the one that was stuffed away by C<local()> and which will be
+restored when the block ends. Because variables are accessed through the
+typeglob, you can use C<*foo = *bar> to create an alias which can be
+localized. (But be aware that this means you can't have a separate
+C<@foo> and C<@bar>, etc.)
+
+What makes all of this important is that the Exporter module uses glob
+aliasing as the import/export mechanism. Whether or not you can properly
+localize a variable that has been exported from a module depends on how
+it was exported:
+
+ @EXPORT = qw($FOO); # Usual form, can't be localized
+ @EXPORT = qw(*FOO); # Can be localized
+
+You can work around the first case by using the fully qualified name
+(C<$Package::FOO>) where you need a local value, or by overriding it
+by saying C<*FOO = *Package::FOO> in your script.
+
+The C<*x = \$y> mechanism may be used to pass and return cheap references
+into or from subroutines if you don't want to copy the whole
thing. It only works when assigning to dynamic variables, not
lexicals.
On return, the reference will overwrite the hash slot in the
symbol table specified by the *some_hash typeglob. This
is a somewhat tricky way of passing around references cheaply
-when you won't want to have to remember to dereference variables
+when you don't want to have to remember to dereference variables
explicitly.
Another use of symbol tables is for making "constant" scalars.
*PI = \3.14159265358979;
-Now you cannot alter $PI, which is probably a good thing all in all.
+Now you cannot alter C<$PI>, which is probably a good thing all in all.
This isn't the same as a constant subroutine, which is subject to
-optimization at compile-time. This isn't. A constant subroutine is one
-prototyped to take no arguments and to return a constant expression.
-
See L<perlsub> for details on these. The C<use constant> pragma is a
+optimization at compile-time. A constant subroutine is one prototyped
+to take no arguments and to return a constant expression. See
+L<perlsub> for details on these. The C<use constant> pragma is a
convenient shorthand for these.
You can say C<*foo{PACKAGE}> and C<*foo{NAME}> to find out what name and
You gave me bar::baz
The C<*foo{THING}> notation can also be used to obtain references to the
-individual elements of *foo
, see L<perlref>.
+individual elements of *foo
. See L<perlref>.
Subroutine definitions (and declarations, for that matter) need
not necessarily be situated in the package whose symbol table they
(if you can).) You may have multiple C<END> blocks within a file--they
will execute in reverse order of definition; that is: last in, first
out (LIFO). C<END> blocks are not executed when you run perl with the
-C<-c> switch.
+C<-c> switch, or if compilation fails.
Inside an C<END> subroutine, C<$?> contains the value that the program is
going to pass to C<exit()>. You can modify C<$?> to change the exit
=head2 Perl Modules
-A module is just a set of related function in a library file a Perl
-package with the same name as the file. It is specifically designed
-to be reusable by other modules or programs. It may do this by
-providing a mechanism for exporting some of its symbols into the
+A module is just a set of related functions in a library file, i.e.,
+a Perl package with the same name as the file. It is specifically
+designed to be reusable by other modules or programs. It may do this
+by providing a mechanism for exporting some of its symbols into the
symbol table of any package using it. Or it may function as a class
definition and make its semantics available implicitly through
method calls on the class and its objects, without explicitly
}
our @EXPORT_OK;
+ # exported package globals go here
+ our $Var1;
+ our %Hashit;
+
# non-exported package globals go here
our @more;
our $stuff;
Perl packages may be nested inside other package names, so we can have
package names containing C<::>. But if we used that package name
-directly as a filename it would makes for unwieldy or impossible
+directly as a filename it would make for unwieldy or impossible
filenames on some systems. Therefore, if a module's name is, say,
C<Text::Soundex>, then its definition is actually found in the library
file F<Text/Soundex.pm>.
Perl modules always have a F<.pm> file, but there may also be
dynamically linked executables (often ending in F<.so>) or autoloaded
-subroutine definitions (often ending in F<.al> associated with the
+subroutine definitions (often ending in F<.al>) associated with the
module. If so, these will be entirely transparent to the user of
the module. It is the responsibility of the F<.pm> file to load
(or arrange to autoload) any additional functionality. For example,
although the POSIX module happens to do both dynamic loading and
-autoloading, but the user can say just C<use POSIX> to get it all.
+autoloading, the user can say just C<use POSIX> to get it all.
+
+=head2 Making your module threadsafe
+
+Perl has since 5.6.0 support for a new type of threads called
+interpreter threads. These threads can be used explicitly and implicitly.
+
+Ithreads work by cloning the data tree so that no data is shared
+between different threads. These threads can be used using the threads
+module or by doing fork() on win32 (fake fork() support). When a thread is
+cloned all perl data is cloned, however non perl data cannot be cloned.
+Perl after 5.7.2 has support for the C<CLONE> keyword. C<CLONE> will be
+executed once for every package that has it defined (or inherits it).
+It will be called in the context of the new thread, so all modifications
+are made in the new area.
+
+If you want to CLONE all objects you will need to keep track of them per
+package. This is simply done using a hash and Scalar::Util::weaken().
=head1 SEE ALSO
See L<perlmodlib> for general style issues related to building Perl
modules and classes, as well as descriptions of the standard library
and CPAN, L<Exporter> for how Perl's standard import/export mechanism
-works, L<perltoot> and L<perltootc> for an in-depth tutorial on
+works, L<perltoot> and L<perltooc> for an in-depth tutorial on
creating classes, L<perlobj> for a hard-core reference document on
objects, L<perlsub> for an explanation of functions and scoping,
and L<perlxstut> and L<perlguts> for more information on writing
https://perl5.git.perl.org / perl5.git / blobdiff