=head1 DESCRIPTION
=head2 Packages
-
-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 (although some identifiers default
-to the main package instead of the current one). 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>, C<sub>, or end of file, whichever comes first (the same scope
-as the my() and local() operators). All further unqualified dynamic
-identifiers will be in this namespace. A package statement only affects
-dynamic variables--including those you've used local() on--but
-I<not> lexical variables created with my(). Typically it would be
-the first declaration in a file to be included by the C<require> or
-C<use> operator. You can switch into a package in more than one place;
-it merely influences which symbol table is used by the compiler for the
-rest of that block. You can refer to variables and filehandles in other
-packages by prefixing the identifier with the package name and a double
-colon: C<$Package::Variable>. If the package name is null, the C<main>
-package is assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
+X<package> X<namespace> X<variable, global> X<global variable> X<global>
+
+Perl provides a mechanism for alternative namespaces to protect
+packages from stomping on each other's variables. In fact, there's
+really no such thing as a global variable in Perl. The package
+statement declares the compilation unit as being in the given
+namespace. The scope of the package declaration is from the
+declaration itself through the end of the enclosing block, C<eval>,
+or file, whichever comes first (the same scope as the my() and
+local() operators). Unqualified dynamic identifiers will be in
+this namespace, except for those few identifiers that if unqualified,
+default to the main package instead of the current one as described
+below. A package statement affects only dynamic variables--including
+those you've used local() on--but I<not> lexical variables created
+with my(). Typically it would be the first declaration in a file
+included by the C<do>, C<require>, or C<use> operators. You can
+switch into a package in more than one place; it merely influences
+which symbol table is used by the compiler for the rest of that
+block. You can refer to variables and filehandles in other packages
+by prefixing the identifier with the package name and a double
+colon: C<$Package::Variable>. If the package name is null, the
+C<main> package is assumed. That is, C<$::sail> is equivalent to
+C<$main::sail>.
The old package delimiter was a single quote, but double colon is now the
preferred delimiter, in part because it's more readable to humans, and
in part because it's more readable to B<emacs> macros. It also makes C++
programmers feel like they know what's going on--as opposed to using the
single quote as separator, which was there to make Ada programmers feel
-like they knew what's going on. Because the old-fashioned syntax is still
+like they knew what was going on. Because the old-fashioned syntax is still
supported for backwards compatibility, if you try to use a string like
C<"This is $owner's house">, you'll be accessing C<$owner::s>; that is,
the $s variable in package C<owner>, which is probably not what you meant.
Use braces to disambiguate, as in C<"This is ${owner}'s house">.
+X<::> X<'>
-Packages may be nested inside other packages: C<$OUTER::INNER::var>. This
-implies nothing about the order of name lookups, however. All symbols
+Packages may themselves contain package separators, as in
+C<$OUTER::INNER::var>. This implies nothing about the order of
+name lookups, however. There are no relative packages: all symbols
are either local to the current package, or must be fully qualified
from the outer package name down. For instance, there is nowhere
-within package C<OUTER> that C<$INNER::var> refers to C<$OUTER::INNER::var>.
-It would treat package C<INNER> as a totally separate global package.
-
-Only identifiers starting with letters (or underscore) are stored in a
-package's symbol table. All other symbols are kept in package C<main>,
-including all of the punctuation variables like $_. In addition, when
-unqualified, the identifiers STDIN, STDOUT, STDERR, ARGV, ARGVOUT, ENV,
-INC, and SIG are forced to be in package C<main>, even when used for other
-purposes than their builtin one. Note also that, if you have a package
-called C<m>, C<s>, or C<y>, then you can't use the qualified form of an
-identifier because it will be interpreted instead as a pattern match,
-a substitution, or a transliteration.
-
-(Variables beginning with underscore used to be forced into package
+within package C<OUTER> that C<$INNER::var> refers to
+C<$OUTER::INNER::var>. C<INNER> refers to a totally
+separate global package.
+
+Only identifiers starting with letters (or underscore) are stored
+in a package's symbol table. All other symbols are kept in package
+C<main>, including all punctuation variables, like $_. In addition,
+when unqualified, the identifiers STDIN, STDOUT, STDERR, ARGV,
+ARGVOUT, ENV, INC, and SIG are forced to be in package C<main>,
+even when used for other purposes than their built-in ones. If you
+have a package called C<m>, C<s>, or C<y>, then you can't use the
+qualified form of an identifier because it would be instead interpreted
+as a pattern match, a substitution, or a transliteration.
+X<variable, punctuation>
+
+Variables beginning with underscore used to be forced into package
main, but we decided it was more useful for package writers to be able
to use leading underscore to indicate private variables and method names.
-$_ is still global though.)
+However, variables and functions named with a single C<_>, such as
+$_ and C<sub _>, are still forced into the package C<main>. See also
+L<perlvar/"Technical Note on the Syntax of Variable Names">.
-Eval()ed strings are compiled in the package in which the eval() was
+C<eval>ed strings are compiled in the package in which the eval() was
compiled. (Assignments to C<$SIG{}>, however, assume the signal
handler specified is in the C<main> package. Qualify the signal handler
name if you wish to have a signal handler in a package.) For an
example, examine F<perldb.pl> in the Perl library. It initially switches
to the C<DB> package so that the debugger doesn't interfere with variables
-in the script you are trying to debug. At various points, however, it
+in the program you are trying to debug. At various points, however, it
temporarily switches back to the C<main> package to evaluate various
expressions in the context of the C<main> package (or wherever you came
from). See L<perldebug>.
The special symbol C<__PACKAGE__> contains the current package, but cannot
-(easily) be used to construct variables.
+(easily) be used to construct variable names.
See L<perlsub> for other scoping issues related to my() and local(),
and L<perlref> regarding closures.
=head2 Symbol Tables
+X<symbol table> X<stash> X<%::> X<%main::> X<typeglob> X<glob> X<alias>
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
-use the C<*name> typeglob notation. In fact, the following have the same
-effect, though the first is more efficient because it does the symbol
-table lookups at compile time:
+use the C<*name> typeglob notation.
local *main::foo = *main::bar;
- local $main::{foo} = $main::{bar};
You can use this to print out all the variables in a package, for
-instance. The standard F<dumpvar.pl> library and the CPAN module
-Devel::Symdump make use of this.
+instance. The standard but antiquated F<dumpvar.pl> library and
+the CPAN module Devel::Symdump make use of this.
Assignment to a typeglob performs an aliasing operation, i.e.,
causes variables, subroutines, formats, and file and directory handles
accessible via the identifier C<richard> also to be accessible via the
identifier C<dick>. If you want to alias only a particular variable or
-subroutine, you can assign a reference instead:
+subroutine, assign a reference instead:
*dick = \$richard;
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.
+Another use of symbol tables is for making "constant" scalars.
+X<constant> X<scalar, constant>
*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 main::foo
You gave me bar::baz
-The *foo{THING} notation can also be used to obtain references to the
-individual elements of *foo, see L<perlref>.
-
-=head2 Package Constructors and Destructors
-
-There are two special subroutine definitions that function as package
-constructors and destructors. These are the C<BEGIN> and C<END>
-routines. The C<sub> is optional for these routines.
-
-A C<BEGIN> subroutine is executed as soon as possible, that is, the moment
-it is completely defined, even before the rest of the containing file
-is parsed. You may have multiple C<BEGIN> blocks within a file--they
-will execute in order of definition. Because a C<BEGIN> block executes
-immediately, it can pull in definitions of subroutines and such from other
-files in time to be visible to the rest of the file. Once a C<BEGIN>
-has run, it is immediately undefined and any code it used is returned to
-Perl's memory pool. This means you can't ever explicitly call a C<BEGIN>.
-
-An C<END> subroutine is executed as late as possible, that is, when
-the interpreter is being exited, even if it is exiting as a result of
-a die() function. (But not if it's polymorphing into another program
-via C<exec>, or being blown out of the water by a signal--you have to
-trap that yourself (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).
-
-Inside an C<END> subroutine, C<$?> contains the value that the script is
+The C<*foo{THING}> notation can also be used to obtain references to the
+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
+occupy. You can define a subroutine outside its package by
+explicitly qualifying the name of the subroutine:
+
+ package main;
+ sub Some_package::foo { ... } # &foo defined in Some_package
+
+This is just a shorthand for a typeglob assignment at compile time:
+
+ BEGIN { *Some_package::foo = sub { ... } }
+
+and is I<not> the same as writing:
+
+ {
+ package Some_package;
+ sub foo { ... }
+ }
+
+In the first two versions, the body of the subroutine is
+lexically in the main package, I<not> in Some_package. So
+something like this:
+
+ package main;
+
+ $Some_package::name = "fred";
+ $main::name = "barney";
+
+ sub Some_package::foo {
+ print "in ", __PACKAGE__, ": \$name is '$name'\n";
+ }
+
+ Some_package::foo();
+
+prints:
+
+ in main: $name is 'barney'
+
+rather than:
+
+ in Some_package: $name is 'fred'
+
+This also has implications for the use of the SUPER:: qualifier
+(see L<perlobj>).
+
+=head2 BEGIN, UNITCHECK, CHECK, INIT and END
+X<BEGIN> X<UNITCHECK> X<CHECK> X<INIT> X<END>
+
+Five specially named code blocks are executed at the beginning and at
+the end of a running Perl program. These are the C<BEGIN>,
+C<UNITCHECK>, C<CHECK>, C<INIT>, and C<END> blocks.
+
+These code blocks can be prefixed with C<sub> to give the appearance of a
+subroutine (although this is not considered good style). One should note
+that these code blocks don't really exist as named subroutines (despite
+their appearance). The thing that gives this away is the fact that you can
+have B<more than one> of these code blocks in a program, and they will get
+B<all> executed at the appropriate moment. So you can't execute any of
+these code blocks by name.
+
+A C<BEGIN> code block is executed as soon as possible, that is, the moment
+it is completely defined, even before the rest of the containing file (or
+string) is parsed. You may have multiple C<BEGIN> blocks within a file (or
+eval'ed string) -- they will execute in order of definition. Because a C<BEGIN>
+code block executes immediately, it can pull in definitions of subroutines
+and such from other files in time to be visible to the rest of the compile
+and run time. Once a C<BEGIN> has run, it is immediately undefined and any
+code it used is returned to Perl's memory pool.
+
+It should be noted that C<BEGIN> and C<UNITCHECK> code blocks B<are>
+executed inside string C<eval()>'s. The C<CHECK> and C<INIT> code
+blocks are B<not> executed inside a string eval, which e.g. can be a
+problem in a mod_perl environment.
+
+An C<END> code block is executed as late as possible, that is, after
+perl has finished running the program and just before the interpreter
+is being exited, even if it is exiting as a result of a die() function.
+(But not if it's morphing into another program via C<exec>, or
+being blown out of the water by a signal--you have to trap that yourself
+(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, or if compilation fails.
+
+Note that C<END> code blocks are B<not> executed at the end of a string
+C<eval()>: if any C<END> code blocks are created in a string C<eval()>,
+they will be executed just as any other C<END> code block of that package
+in LIFO order just before the interpreter is being exited.
+
+Inside an C<END> code block, C<$?> contains the value that the program is
going to pass to C<exit()>. You can modify C<$?> to change the exit
-value of the script. Beware of changing C<$?> by accident (e.g. by
+value of the program. Beware of changing C<$?> by accident (e.g. by
running something via C<system>).
+X<$?>
+
+C<UNITCHECK>, C<CHECK> and C<INIT> code blocks are useful to catch the
+transition between the compilation phase and the execution phase of
+the main program.
+
+C<UNITCHECK> blocks are run just after the unit which defined them has
+been compiled. The main program file and each module it loads are
+compilation units, as are string C<eval>s, code compiled using the
+C<(?{ })> construct in a regex, calls to C<do FILE>, C<require FILE>,
+and code after the C<-e> switch on the command line.
+
+C<CHECK> code blocks are run just after the B<initial> Perl compile phase ends
+and before the run time begins, in LIFO order. C<CHECK> code blocks are used
+in the Perl compiler suite to save the compiled state of the program.
+
+C<INIT> blocks are run just before the Perl runtime begins execution, in
+"first in, first out" (FIFO) order.
+
+When you use the B<-n> and B<-p> switches to Perl, C<BEGIN> and
+C<END> work just as they do in B<awk>, as a degenerate case.
+Both C<BEGIN> and C<CHECK> blocks are run when you use the B<-c>
+switch for a compile-only syntax check, although your main code
+is not.
+
+The B<begincheck> program makes it all clear, eventually:
-Note that when you use the B<-n> and B<-p> switches to Perl, C<BEGIN> and
-C<END> work just as they do in B<awk>, as a degenerate case. As currently
-implemented (and subject to change, since its inconvenient at best),
-both C<BEGIN> I<and> C<END> blocks are run when you use the B<-c> switch
-for a compile-only syntax check, although your main code is not.
+ #!/usr/bin/perl
+
+ # begincheck
+
+ print "10. Ordinary code runs at runtime.\n";
+
+ END { print "16. So this is the end of the tale.\n" }
+ INIT { print " 7. INIT blocks run FIFO just before runtime.\n" }
+ UNITCHECK {
+ print " 4. And therefore before any CHECK blocks.\n"
+ }
+ CHECK { print " 6. So this is the sixth line.\n" }
+
+ print "11. It runs in order, of course.\n";
+
+ BEGIN { print " 1. BEGIN blocks run FIFO during compilation.\n" }
+ END { print "15. Read perlmod for the rest of the story.\n" }
+ CHECK { print " 5. CHECK blocks run LIFO after all compilation.\n" }
+ INIT { print " 8. Run this again, using Perl's -c switch.\n" }
+
+ print "12. This is anti-obfuscated code.\n";
+
+ END { print "14. END blocks run LIFO at quitting time.\n" }
+ BEGIN { print " 2. So this line comes out second.\n" }
+ UNITCHECK {
+ print " 3. UNITCHECK blocks run LIFO after each file is compiled.\n"
+ }
+ INIT { print " 9. You'll see the difference right away.\n" }
+
+ print "13. It merely _looks_ like it should be confusing.\n";
+
+ __END__
=head2 Perl Classes
+X<class> X<@ISA>
-There is no special class syntax in Perl, but a package may function
+There is no special class syntax in Perl, but a package may act
as a class if it provides subroutines to act as methods. Such a
package may also derive some of its methods from another class (package)
-by listing the other package name in its global @ISA array (which
+by listing the other package name(s) in its global @ISA array (which
must be a package global, not a lexical).
For more on this, see L<perltoot> and L<perlobj>.
=head2 Perl Modules
+X<module>
-A module is just a package that is defined in a library file of
-the same name, and is designed to be reusable. 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 explicit exportation of any
-symbols. Or it can do a little of both.
+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
+exporting anything. Or it can do a little of both.
-For example, to start a normal module called Some::Module, create
-a file called Some/Module.pm and start with this template:
+For example, to start a traditional, non-OO module called Some::Module,
+create a file called F<Some/Module.pm> and start with this template:
package Some::Module; # assumes Some/Module.pm
use strict;
+ use warnings;
BEGIN {
use Exporter ();
- use vars qw($VERSION @ISA @EXPORT @EXPORT_OK %EXPORT_TAGS);
+ our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS);
# set the version for version checking
$VERSION = 1.00;
# if using RCS/CVS, this may be preferred
- $VERSION = do { my @r = (q$Revision: 2.21 $ =~ /\d+/g); sprintf "%d."."%02d" x $#r, @r }; # must be all one line, for MakeMaker
+ $VERSION = sprintf "%d.%03d", q$Revision: 1.1 $ =~ /(\d+)/g;
@ISA = qw(Exporter);
@EXPORT = qw(&func1 &func2 &func4);
# as well as any optionally exported functions
@EXPORT_OK = qw($Var1 %Hashit &func3);
}
- use vars @EXPORT_OK;
+ our @EXPORT_OK;
+
+ # exported package globals go here
+ our $Var1;
+ our %Hashit;
# non-exported package globals go here
- use vars qw(@more $stuff);
+ our @more;
+ our $stuff;
- # initalize package globals, first exported ones
+ # initialize package globals, first exported ones
$Var1 = '';
%Hashit = ();
END { } # module clean-up code here (global destructor)
-Then go on to declare and use your variables in functions
-without any qualifications.
-See L<Exporter> and the L<perlmodlib> for details on
-mechanics and style issues in module creation.
+ ## YOUR CODE GOES HERE
+
+ 1; # don't forget to return a true value from the file
+
+Then go on to declare and use your variables in functions without
+any qualifications. See L<Exporter> and the L<perlmodlib> for
+details on mechanics and style issues in module creation.
Perl modules are included into your program by saying
BEGIN { require Module; }
-All Perl module files have the extension F<.pm>. C<use> assumes this so
-that you don't have to spell out "F<Module.pm>" in quotes. This also
-helps to differentiate new modules from old F<.pl> and F<.ph> files.
-Module names are also capitalized unless they're functioning as pragmas,
-"Pragmas" are in effect compiler directives, and are sometimes called
-"pragmatic modules" (or even "pragmata" if you're a classicist).
+All Perl module files have the extension F<.pm>. The C<use> operator
+assumes this so you don't have to spell out "F<Module.pm>" in quotes.
+This also helps to differentiate new modules from old F<.pl> and
+F<.ph> files. Module names are also capitalized unless they're
+functioning as pragmas; pragmas are in effect compiler directives,
+and are sometimes called "pragmatic modules" (or even "pragmata"
+if you're a classicist).
The two statements:
require SomeModule;
- require "SomeModule.pm";
+ require "SomeModule.pm";
differ from each other in two ways. In the first case, any double
colons in the module name, such as C<Some::Module>, are translated
into your system's directory separator, usually "/". The second
-case does not, and would have to be specified literally. The other difference
-is that seeing the first C<require> clues in the compiler that uses of
-indirect object notation involving "SomeModule", as in C<$ob = purge SomeModule>,
-are method calls, not function calls. (Yes, this really can make a difference.)
-
-Because the C<use> statement implies a C<BEGIN> block, the importation
-of semantics happens at the moment the C<use> statement is compiled,
+case does not, and would have to be specified literally. The other
+difference is that seeing the first C<require> clues in the compiler
+that uses of indirect object notation involving "SomeModule", as
+in C<$ob = purge SomeModule>, are method calls, not function calls.
+(Yes, this really can make a difference.)
+
+Because the C<use> statement implies a C<BEGIN> block, the importing
+of semantics happens as soon as the C<use> statement is compiled,
before the rest of the file is compiled. This is how it is able
to function as a pragma mechanism, and also how modules are able to
-declare subroutines that are then visible as list operators for
+declare subroutines that are then visible as list or unary operators for
the rest of the current file. This will not work if you use C<require>
-instead of C<use>. With require you can get into this problem:
+instead of C<use>. With C<require> you can get into this problem:
require Cwd; # make Cwd:: accessible
$here = Cwd::getcwd();
because it determines module availability at compile time, not in the
middle of your program's execution. An exception would be if two modules
each tried to C<use> each other, and each also called a function from
-that other module. In that case, it's easy to use C<require>s instead.
+that other module. In that case, it's easy to use C<require> instead.
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 or autoloaded subroutine definitions 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. The POSIX module
-happens to do both dynamic loading and autoloading, but the user can
-say just C<use POSIX> to get it all.
-
-For more information on writing extension modules, see L<perlxstut>
-and L<perlguts>.
+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
+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, the user can say just C<use POSIX> to get it all.
+
+=head2 Making your module threadsafe
+X<threadsafe> X<thread safe>
+X<module, threadsafe> X<module, thread safe>
+X<CLONE> X<CLONE_SKIP> X<thread> X<threads> X<ithread>
+
+Since 5.6.0, Perl has had support for a new type of threads called
+interpreter threads (ithreads). 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 by using the C<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 automatically. Perl after 5.7.2 has support for the C<CLONE>
+special subroutine. In C<CLONE> you can do whatever
+you need to do,
+like for example handle the cloning of non-Perl data, if necessary.
+C<CLONE> will be called once as a class method 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. Currently CLONE is called with
+no parameters other than the invocant package name, but code should not assume
+that this will remain unchanged, as it is likely that in future extra parameters
+will be passed in to give more information about the state of cloning.
+
+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().
+
+Perl after 5.8.7 has support for the C<CLONE_SKIP> special subroutine.
+Like C<CLONE>, C<CLONE_SKIP> is called once per package; however, it is
+called just before cloning starts, and in the context of the parent
+thread. If it returns a true value, then no objects of that class will
+be cloned; or rather, they will be copied as unblessed, undef values.
+For example: if in the parent there are two references to a single blessed
+hash, then in the child there will be two references to a single undefined
+scalar value instead.
+This provides a simple mechanism for making a module threadsafe; just add
+C<sub CLONE_SKIP { 1 }> at the top of the class, and C<DESTROY()> will be
+now only be called once per object. Of course, if the child thread needs
+to make use of the objects, then a more sophisticated approach is
+needed.
+
+Like C<CLONE>, C<CLONE_SKIP> is currently called with no parameters other
+than the invocant package name, although that may change. Similarly, to
+allow for future expansion, the return value should be a single C<0> or
+C<1> value.
=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> for an in-depth tutorial on creating classes, L<perlobj>
-for a hard-core reference document on objects, and L<perlsub> for an
-explanation of functions and scoping.
+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<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
+extension modules.
https://perl5.git.perl.org / perl5.git / blobdiff