=head1 NAME
-perlmod - Perl modules (packages)
+perlmod - Perl modules (packages and symbol tables)
=head1 DESCRIPTION
=head2 Packages
-
-Perl provides a mechanism for alternative namespaces to protect packages
-from stomping on each other's variables. In fact, apart from certain
-magical variables, 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 (the same scope
-as the local() operator). 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<$main::sail>.
-
-(The old package delimiter was a single quote, but double colon
-is now the preferred delimiter, in part because it's more readable
-to humans, and in part because it's more readable to B<emacs> macros.
-It also makes C++ programmers feel like they know what's going on.)
-
-Packages may be nested inside other packages: C<$OUTER::INNER::var>. This
-implies nothing about the order of name lookups, however. All symbols
+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 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 ${owner}'s house">.
+X<::> X<'>
+
+Packages may themselves contain package separators, as in
+C<$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 $_. In addition, the
-identifiers STDIN, STDOUT, STDERR, ARGV, ARGVOUT, ENV, INC and SIG are
-forced to be in package C<main>, even when used for other purposes than
-their built-in one. Note also that, if you have a package called C<m>,
-C<s> or C<y>, then you can't use the qualified form of an identifier
-because it will be interpreted instead as a pattern match, a substitution,
-or a translation.
-
-(Variables beginning with underscore used to be forced into package
+within package C<OUTER> that C<$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>.
-See L<perlsub> for other scoping issues related to my() and local(),
-or L<perlref> regarding closures.
+The special symbol C<__PACKAGE__> contains the current package, but cannot
+(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 associative
-array of that name appended with two colons. The main symbol table's
-name is thus C<%main::>, or C<%::> for short. Likewise symbol table for
-the nested package mentioned earlier is named C<%OUTER::INNER::>.
+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 the symbol table for the nested
+package mentioned earlier is named C<%OUTER::INNER::>.
-The value in each entry of the associative array 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:
+The value in each entry of the hash is what you are referring to when you
+use the C<*name> typeglob notation.
- local(*main::foo) = *main::bar; local($main::{'foo'}) =
- $main::{'bar'};
+ local *main::foo = *main::bar;
You can use this to print out all the variables in a package, for
-instance. Here is F<dumpvar.pl> from the Perl library:
-
- package dumpvar;
- sub main::dumpvar {
- ($package) = @_;
- local(*stab) = eval("*${package}::");
- while (($key,$val) = each(%stab)) {
- local(*entry) = $val;
- if (defined $entry) {
- print "\$$key = '$entry'\n";
- }
-
- if (defined @entry) {
- print "\@$key = (\n";
- foreach $num ($[ .. $#entry) {
- print " $num\t'",$entry[$num],"'\n";
- }
- print ")\n";
- }
-
- if ($key ne "${package}::" && defined %entry) {
- print "\%$key = (\n";
- foreach $key (sort keys(%entry)) {
- print " $key\t'",$entry{$key},"'\n";
- }
- print ")\n";
- }
- }
- }
-
-Note that even though the subroutine is compiled in package C<dumpvar>,
-the name of the subroutine is qualified so that its name is inserted
-into package C<main>.
+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.,
*dick = *richard;
-causes variables, subroutines and file handles accessible via the
-identifier C<richard> to also be accessible via the identifier C<dick>. If
-you only want to alias a particular variable or subroutine, you can
-assign a reference instead:
+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, assign a reference instead:
*dick = \$richard;
-makes $richard and $dick the same variable, but leaves
+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
-thing.
+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<$bar> -- the one that was stuffed away by C<local()> and which will be
+restored when the block ends. Because variables are accessed through the
+typeglob, you can use C<*foo = *bar> to create an alias which can be
+localized. (But be aware that this means you can't have a separate
+C<@foo> and C<@bar>, etc.)
+
+What makes all of this important is that the Exporter module uses glob
+aliasing as the import/export mechanism. Whether or not you can properly
+localize a variable that has been exported from a module depends on how
+it was exported:
- %some_hash = ();
+ @EXPORT = qw($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.
+
+ %some_hash = (); # can't be my()
*some_hash = fn( \%another_hash );
sub fn {
local *hashsym = shift;
# now use %hashsym normally, and you
# will affect the caller's %another_hash
my %nhash = (); # do what you want
- return \%nhash;
+ return \%nhash;
}
-On return, the reference wil overwrite the hash slot in the
+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 refernces cheaply
-when you won't want to have to remember to dereference variables
+is a somewhat tricky way of passing around references cheaply
+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.
-
-=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.
-
-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 is 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).
-
-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.
-
-=head2 Perl Classes
-
-There is no special class syntax in Perl, but a package may function
-as a class if it provides subroutines that function as methods. Such a
-package may also derive some of its methods from another class package
-by listing the other package name in its @ISA array.
-
-For more on this, see L<perlobj>.
-
-=head2 Perl Modules
-
-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.
-
-For example, to start a normal module called Fred, create
-a file called Fred.pm and put this at the start of it:
-
- package Fred;
- use Exporter ();
- @ISA = qw(Exporter);
- @EXPORT = qw(func1 func2);
- @EXPORT_OK = qw($sally @listabob %harry func3);
-
-Then go on to declare and use your variables in functions
-without any qualifications.
-See L<Exporter> and the I<Perl Modules File> for details on
-mechanics and style issues in module creation.
-
-Perl modules are included into your program by saying
-
- use Module;
-
-or
-
- use Module LIST;
-
-This is exactly equivalent to
-
- BEGIN { require "Module.pm"; import Module; }
-
-or
-
- BEGIN { require "Module.pm"; import Module LIST; }
-
-As a special case
-
- use Module ();
-
-is exactly equivalent to
-
- BEGIN { require "Module.pm"; }
-
-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).
-
-Because the C<use> statement implies a C<BEGIN> block, the importation
-of semantics happens at the moment 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
-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:
-
- require Cwd; # make Cwd:: accessible
- $here = Cwd::getcwd();
-
- use Cwd; # import names from Cwd::
- $here = getcwd();
-
- require Cwd; # make Cwd:: accessible
- $here = getcwd(); # oops! no main::getcwd()
-
-In general C<use Module ();> is recommended over C<require Module;>.
-
-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
-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
-just say C<use POSIX> to get it all.
-
-For more information on writing extension modules, see L<perlxs>
-and L<perlguts>.
-
-=head1 NOTE
-
-Perl does not enforce private and public parts of its modules as you may
-have been used to in other languages like C++, Ada, or Modula-17. Perl
-doesn't have an infatuation with enforced privacy. It would prefer
-that you stayed out of its living room because you weren't invited, not
-because it has a shotgun.
-
-The module and its user have a contract, part of which is common law,
-and part of which is "written". Part of the common law contract is
-that a module doesn't pollute any namespace it wasn't asked to. The
-written contract for the module (AKA documentation) may make other
-provisions. But then you know when you C<use RedefineTheWorld> that
-you're redefining the world and willing to take the consequences.
-
-=head1 THE PERL MODULE LIBRARY
-
-A number of modules are included the the Perl distribution. These are
-described below, and all end in F<.pm>. You may also discover files in
-the library directory that end in either F<.pl> or F<.ph>. These are old
-libraries supplied so that old programs that use them still run. The
-F<.pl> files will all eventually be converted into standard modules, and
-the F<.ph> files made by B<h2ph> will probably end up as extension modules
-made by B<h2xs>. (Some F<.ph> values may already be available through the
-POSIX module.) The B<pl2pm> file in the distribution may help in your
-conversion, but it's just a mechanical process, so is far from bulletproof.
-
-=head2 Pragmatic Modules
-
-They work somewhat like pragmas in that they tend to affect the compilation of
-your program, and thus will usually only work well when used within a
-C<use>, or C<no>. These are locally scoped, so an inner BLOCK
-may countermand any of these by saying
-
- no integer;
- no strict 'refs';
-
-which lasts until the end of that BLOCK.
-
-The following programs are defined (and have their own documentation).
-
-=over 12
-
-=item diagnostics
-
-Pragma to produce enhanced diagnostics
-
-=item integer
-
-Pragma to compute arithmetic in integer instead of double
-
-=item less
-
-Pragma to request less of something from the compiler
-
-=item ops
-
-Pragma to restrict use of unsafe opcodes
-
-=item overload
-
-Pragma for overloading operators
-
-=item sigtrap
-
-Pragma to enable stack backtrace on unexpected signals
-
-=item strict
-
-Pragma to restrict unsafe constructs
-
-=item subs
-
-Pragma to predeclare sub names
-
-=item vars
-
-Pragma to predeclare global symbols
-
-=back
-
-=head2 Standard Modules
-
-Standard, bundled modules are all expected to behave in a well-defined
-manner with respect to namespace pollution because they use the
-Exporter module. See their own documentation for details.
-
-=over 12
-
-=item AnyDBM_File
-
-provide framework for multiple DBMs
-
-=item AutoLoader
-
-load functions only on demand
-
-=item AutoSplit
-
-split a package for autoloading
-
-=item Benchmark
-
-benchmark running times of code
-
-=item Carp
-
-warn of errors (from perspective of caller)
-
-=item Config
-
-access Perl configuration option
-
-=item Cwd
-
-get pathname of current working directory
-
-=item DB_File
-
-Perl access to Berkeley DB
-
-=item Devel::SelfStubber
-
-generate stubs for a SelfLoading module
-
-=item DynaLoader
-
-Dynamically load C libraries into Perl code
-
-=item English
-
-use nice English (or awk) names for ugly punctuation variables
-
-=item Env
-
-perl module that imports environment variables
-
-=item Exporter
-
-provide inport/export controls for Perl modules
-
-=item ExtUtils::Liblist
-
-determine libraries to use and how to use them
-
-=item ExtUtils::MakeMaker
-
-create an extension Makefile
-
-=item ExtUtils::Manifest
-
-utilities to write and check a MANIFEST file
-
-=item ExtUtils::Mkbootstrap
-
-make a bootstrap file for use by DynaLoader
-
-=item ExtUtils::Miniperl
-
-!!!GOOD QUESTION!!!
-
-=item Fcntl
-
-load the C Fcntl.h defines
-
-=item File::Basename
-
-parse file specifications
-
-=item File::CheckTree
-
-run many filetest checks on a tree
+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. 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.
-=item File::Find
+You can say C<*foo{PACKAGE}> and C<*foo{NAME}> to find out what name and
+package the *foo symbol table entry comes from. This may be useful
+in a subroutine that gets passed typeglobs as arguments:
-traverse a file tree
-
-=item FileHandle
-
-supply object methods for filehandles
-
-=item File::Path
-
-create or remove a series of directories
-
-=item Getopt::Long
-
-extended getopt processing
-
-=item Getopt::Std
-
-Process single-character switches with switch clustering
-
-=item I18N::Collate
-
-compare 8-bit scalar data according to the current locale
-
-=item IPC::Open2
-
-a process for both reading and writing
-
-=item IPC::Open3
-
-open a process for reading, writing, and error handling
-
-=item Net::Ping
-
-check a host for upness
-
-=item POSIX
-
-Perl interface to IEEE Std 1003.1
-
-=item SelfLoader
-
-load functions only on demand
-
-=item Safe
-
-Creation controlled compartments in which perl code can be evaluated.
-
-=item Socket
-
-load the C socket.h defines and structure manipulators
-
-=item Test::Harness
-
-run perl standard test scripts with statistics
-
-=item Text::Abbrev
-
-rceate an abbreviation table from a list
-
-=back
-
-To find out I<all> the modules installed on your system, including
-those without documentation or outside the standard release, do this:
-
- find `perl -e 'print "@INC"'` -name '*.pm' -print
-
-They should all have their own documentation installed and accessible via
-your system man(1) command. If that fails, try the I<perldoc> program.
-
-=head2 Extension Modules
-
-Extension modules are written in C (or a mix of Perl and C) and get
-dynamically loaded into Perl if and when you need them. Supported
-extension modules include the Socket, Fcntl, and POSIX modules.
-
-Many popular C extension modules do not come bundled (at least, not
-completely) due to their size, volatility, or simply lack of time for
-adequate testing and configuration across the multitude of platforms on
-which Perl was beta-tested. You are encouraged to look for them in
-archie(1L), the Perl FAQ or Meta-FAQ, the WWW page, and even with their
-authors before randomly posting asking for their present condition and
-disposition.
-
-=head1 CPAN
-
-CPAN stands for the Comprehensive Perl Archive Network. This is a globally
-replicated collection of all known Perl materials, including hundreds
-of unbunded modules. Here are the major categories of modules:
-
-=over
-
-=item *
-Language Extensions and Documentation Tools
-
-=item *
-Development Support
-
-=item *
-Operating System Interfaces
-
-=item *
-Networking, Device Control (modems) and InterProcess Communication
-
-=item *
-Data Types and Data Type Utilities
-
-=item *
-Database Interfaces
-
-=item *
-User Interfaces
-
-=item *
-Interfaces to / Emulations of Other Programming Languages
-
-=item *
-File Names, File Systems and File Locking (see also File Handles)
-
-=item *
-String Processing, Language Text Processing, Parsing and Searching
-
-=item *
-Option, Argument, Parameter and Configuration File Processing
-
-=item *
-Internationalization and Locale
-
-=item *
-Authentication, Security and Encryption
-
-=item *
-World Wide Web, HTML, HTTP, CGI, MIME
-
-=item *
-Server and Daemon Utilities
-
-=item *
-Archiving and Compression
-
-=item *
-Images, Pixmap and Bitmap Manipulation, Drawing and Graphing
-
-=item *
-Mail and Usenet News
-
-=item *
-Control Flow Utilities (callbacks and exceptions etc)
-
-=item *
-File Handle and Input/Output Stream Utilities
-
-=item *
-Miscellaneous Modules
-
-=back
-
-The registered CPAN sites as of this writing include the following.
-You should try to choose one close to you:
-
-=over
-
-=item *
-ftp://ftp.sterling.com/programming/languages/perl/
-
-=item *
-ftp://ftp.sedl.org/pub/mirrors/CPAN/
-
-=item *
-ftp://ftp.uoknor.edu/mirrors/CPAN/
-
-=item *
-ftp://ftp.delphi.com/pub/mirrors/packages/perl/CPAN/
-
-=item *
-ftp://uiarchive.cso.uiuc.edu/pub/lang/perl/CPAN/
-
-=item *
-ftp://ftp.cis.ufl.edu/pub/perl/CPAN/
-
-=item *
-ftp://ftp.switch.ch/mirror/CPAN/
-
-=item *
-ftp://ftp.sunet.se/pub/lang/perl/CPAN/
-
-=item *
-ftp://ftp.ci.uminho.pt/pub/lang/perl/
-
-=item *
-ftp://ftp.cs.ruu.nl/pub/PERL/CPAN/
-
-=item *
-ftp://ftp.demon.co.uk/pub/mirrors/perl/CPAN/
-
-=item *
-ftp://ftp.rz.ruhr-uni-bochum.de/pub/programming/languages/perl/CPAN/
-
-=item *
-ftp://ftp.leo.org/pub/comp/programming/languages/perl/CPAN/
-
-=item *
-ftp://ftp.pasteur.fr/pub/computing/unix/perl/CPAN/
-
-=item *
-ftp://ftp.ibp.fr/pub/perl/CPAN/
-
-=item *
-ftp://ftp.funet.fi/pub/languages/perl/CPAN/
-
-=item *
-ftp://ftp.tekotago.ac.nz/pub/perl/CPAN/
-
-=item *
-ftp://ftp.mame.mu.oz.au/pub/perl/CPAN/
-
-=item *
-ftp://coombs.anu.edu.au/pub/perl/
-
-=item *
-ftp://dongpo.math.ncu.edu.tw/perl/CPAN/
-
-=item *
-ftp://ftp.lab.kdd.co.jp/lang/perl/CPAN/
-
-=item *
-ftp://ftp.is.co.za/programming/perl/CPAN/
-
-=back
-
-For an up-to-date listing of CPAN sites,
-see F<http://www.perl.com/perl/CPAN> or F<ftp://ftp.perl.com/perl/>.
-
-=head1 Modules: Creation, Use and Abuse
-
-(The following section is borrowed directly from Tim Bunce's modules
-file, available at your nearest CPAN site.)
-
-Perl 5 implements a class using a package, but the presence of a
-package doesn't imply the presence of a class. A package is just a
-namespace. A class is a package that provides subroutines that can be
-used as methods. A method is just a subroutine that expects, as its
-first argument, either the name of a package (for "static" methods),
-or a reference to something (for "virtual" methods).
-
-A module is a file that (by convention) provides a class of the same
-name (sans the .pm), plus an import method in that class that can be
-called to fetch exported symbols. This module may implement some of
-its methods by loading dynamic C or C++ objects, but that should be
-totally transparent to the user of the module. Likewise, the module
-might set up an AUTOLOAD function to slurp in subroutine definitions on
-demand, but this is also transparent. Only the .pm file is required to
-exist.
-
-=head2 Guidelines for Module Creation
-
-=over 4
-
-=item Do similar modules already exist in some form?
-
-If so, please try to reuse the existing modules either in whole or
-by inheriting useful features into a new class. If this is not
-practical try to get together with the module authors to work on
-extending or enhancing the functionality of the existing modules.
-A perfect example is the plethora of packages in perl4 for dealing
-with command line options.
-
-If you are writing a module to expand an already existing set of
-modules, please coordinate with the author of the package. It
-helps if you follow the same naming scheme and module interaction
-scheme as the original author.
-
-=item Try to design the new module to be easy to extend and reuse.
-
-Use blessed references. Use the two argument form of bless to bless
-into the class name given as the first parameter of the constructor,
-e.g.:
-
- sub new {
- my $class = shift;
- return bless {}, $class;
- }
-
-or even this if you'd like it to be used as either a static
-or a virtual method.
-
- sub new {
- my $self = shift;
- my $class = ref($self) || $self;
- return bless {}, $class;
- }
-
-Pass arrays as references so more parameters can be added later
-(it's also faster). Convert functions into methods where
-appropriate. Split large methods into smaller more flexible ones.
-Inherit methods from other modules if appropriate.
-
-Avoid class name tests like: die "Invalid" unless ref $ref eq 'FOO'.
-Generally you can delete the "eq 'FOO'" part with no harm at all.
-Let the objects look after themselves! Generally, avoid hardwired
-class names as far as possible.
-
-Avoid $r-E<gt>Class::func() where using @ISA=qw(... Class ...) and
-$r-E<gt>func() would work (see perlbot man page for more details).
-
-Use autosplit so little used or newly added functions won't be a
-burden to programs which don't use them. Add test functions to
-the module after __END__ either using AutoSplit or by saying:
-
- eval join('',<main::DATA>) || die $@ unless caller();
-
-Does your module pass the 'empty sub-class' test? If you say
-"@SUBCLASS::ISA = qw(YOURCLASS);" your applications should be able
-to use SUBCLASS in exactly the same way as YOURCLASS. For example,
-does your application still work if you change: $obj = new YOURCLASS;
-into: $obj = new SUBCLASS; ?
-
-Avoid keeping any state information in your packages. It makes it
-difficult for multiple other packages to use yours. Keep state
-information in objects.
-
-Always use C<-w>. Try to C<use strict;> (or C<use strict qw(...);>).
-Remember that you can add C<no strict qw(...);> to individual blocks
-of code which need less strictness. Always use C<-w>. Always use C<-w>!
-Follow the guidelines in the perlstyle(1) manual.
-
-=item Some simple style guidelines
-
-The perlstyle manual supplied with perl has many helpful points.
-
-Coding style is a matter of personal taste. Many people evolve their
-style over several years as they learn what helps them write and
-maintain good code. Here's one set of assorted suggestions that
-seem to be widely used by experienced developers:
-
-Use underscores to separate words. It is generally easier to read
-$var_names_like_this than $VarNamesLikeThis, especially for
-non-native speakers of English. It's also a simple rule that works
-consistently with VAR_NAMES_LIKE_THIS.
-
-Package/Module names are an exception to this rule. Perl informally
-reserves lowercase module names for 'pragma' modules like integer
-and strict. Other modules normally begin with a capital letter and
-use mixed case with no underscores (need to be short and portable).
-
-You may find it helpful to use letter case to indicate the scope
-or nature of a variable. For example:
-
- $ALL_CAPS_HERE constants only (beware clashes with perl vars)
- $Some_Caps_Here package-wide global/static
- $no_caps_here function scope my() or local() variables
-
-Function and method names seem to work best as all lowercase.
-E.g., $obj-E<gt>as_string().
-
-You can use a leading underscore to indicate that a variable or
-function should not be used outside the package that defined it.
+ sub identify_typeglob {
+ my $glob = shift;
+ print 'You gave me ', *{$glob}{PACKAGE}, '::', *{$glob}{NAME}, "\n";
+ }
+ identify_typeglob *foo;
+ identify_typeglob *bar::baz;
-=item Select what to export.
+This prints
-Do NOT export method names!
+ You gave me main::foo
+ You gave me bar::baz
-Do NOT export anything else by default without a good reason!
+The C<*foo{THING}> notation can also be used to obtain references to the
+individual elements of *foo. See L<perlref>.
-Exports pollute the namespace of the module user. If you must
-export try to use @EXPORT_OK in preference to @EXPORT and avoid
-short or common names to reduce the risk of name clashes.
+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:
-Generally anything not exported is still accessible from outside the
-module using the ModuleName::item_name (or $blessed_ref-E<gt>method)
-syntax. By convention you can use a leading underscore on names to
-informally indicate that they are 'internal' and not for public use.
+ package main;
+ sub Some_package::foo { ... } # &foo defined in Some_package
-(It is actually possible to get private functions by saying:
-my $subref = sub { ... }; &$subref; But there's no way to call that
-directly as a method, since a method must have a name in the symbol
-table.)
+This is just a shorthand for a typeglob assignment at compile time:
-As a general rule, if the module is trying to be object oriented
-then export nothing. If it's just a collection of functions then
-@EXPORT_OK anything but use @EXPORT with caution.
+ BEGIN { *Some_package::foo = sub { ... } }
-=item Select a name for the module.
+and is I<not> the same as writing:
-This name should be as descriptive, accurate and complete as
-possible. Avoid any risk of ambiguity. Always try to use two or
-more whole words. Generally the name should reflect what is special
-about what the module does rather than how it does it. Please use
-nested module names to informally group or categorise a module.
-A module should have a very good reason not to have a nested name.
-Module names should begin with a capital letter.
+ {
+ package Some_package;
+ sub foo { ... }
+ }
-Having 57 modules all called Sort will not make life easy for anyone
-(though having 23 called Sort::Quick is only marginally better :-).
-Imagine someone trying to install your module alongside many others.
-If in any doubt ask for suggestions in comp.lang.perl.misc.
+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:
-If you are developing a suite of related modules/classes it's good
-practice to use nested classes with a common prefix as this will
-avoid namespace clashes. For example: Xyz::Control, Xyz::View,
-Xyz::Model etc. Use the modules in this list as a naming guide.
+ package main;
-If adding a new module to a set, follow the original author's
-standards for naming modules and the interface to methods in
-those modules.
+ $Some_package::name = "fred";
+ $main::name = "barney";
-To be portable each component of a module name should be limited to
-11 characters. If it might be used on DOS then try to ensure each is
-unique in the first 8 characters. Nested modules make this easier.
+ sub Some_package::foo {
+ print "in ", __PACKAGE__, ": \$name is '$name'\n";
+ }
-=item Have you got it right?
+ Some_package::foo();
-How do you know that you've made the right decisions? Have you
-picked an interface design that will cause problems later? Have
-you picked the most appropriate name? Do you have any questions?
+prints:
-The best way to know for sure, and pick up many helpful suggestions,
-is to ask someone who knows. Comp.lang.perl.misc is read by just about
-all the people who develop modules and it's the best place to ask.
+ in main: $name is 'barney'
-All you need to do is post a short summary of the module, its
-purpose and interfaces. A few lines on each of the main methods is
-probably enough. (If you post the whole module it might be ignored
-by busy people - generally the very people you want to read it!)
+rather than:
-Don't worry about posting if you can't say when the module will be
-ready - just say so in the message. It might be worth inviting
-others to help you, they may be able to complete it for you!
+ in Some_package: $name is 'fred'
-=item README and other Additional Files.
+This also has implications for the use of the SUPER:: qualifier
+(see L<perlobj>).
-It's well known that software developers usually fully document the
-software they write. If, however, the world is in urgent need of
-your software and there is not enough time to write the full
-documentation please at least provide a README file containing:
+=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.
-=over 10
+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.
-=item *
-A description of the module/package/extension etc.
+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.
-=item *
-A copyright notice - see below.
+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.
-=item *
-Prerequisites - what else you may need to have.
+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.
-=item *
-How to build it - possible changes to Makefile.PL etc.
+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.
-=item *
-How to install it.
+Inside an C<END> code block, C<$?> contains the value that the program is
+going to pass to C<exit()>. You can modify C<$?> to change the exit
+value of the program. Beware of changing C<$?> by accident (e.g. by
+running something via C<system>).
+X<$?>
-=item *
-Recent changes in this release, especially incompatibilities
+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.
-=item *
-Changes / enhancements you plan to make in the future.
+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.
-=back
+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.
-If the README file seems to be getting too large you may wish to
-split out some of the sections into separate files: INSTALL,
-Copying, ToDo etc.
+C<INIT> blocks are run just before the Perl runtime begins execution, in
+"first in, first out" (FIFO) order.
-=over 4
+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.
-=item Adding a Copyright Notice.
+The B<begincheck> program makes it all clear, eventually:
-How you choose to licence your work is a personal decision.
-The general mechanism is to assert your Copyright and then make
-a declaration of how others may copy/use/modify your work.
+ #!/usr/bin/perl
-Perl, for example, is supplied with two types of licence: The GNU
-GPL and The Artistic License (see the files README, Copying and
-Artistic). Larry has good reasons for NOT just using the GNU GPL.
+ # begincheck
-My personal recommendation, out of respect for Larry, Perl and the
-perl community at large is to simply state something like:
+ print "10. Ordinary code runs at runtime.\n";
- Copyright (c) 1995 Your Name. All rights reserved.
- This program is free software; you can redistribute it and/or
- modify it under the same terms as Perl itself.
+ 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" }
-This statement should at least appear in the README file. You may
-also wish to include it in a Copying file and your source files.
-Remember to include the other words in addition to the Copyright.
+ print "11. It runs in order, of course.\n";
-=item Give the module a version/issue/release number.
+ 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__
-To be fully compatible with the Exporter and MakeMaker modules you
-should store your module's version number in a non-my package
-variable called $VERSION. This should be a valid floating point
-number with at least two digits after the decimal (ie hundredths,
-e.g, $VERSION = "0.01"). Don't use a "1.3.2" style version.
-See Exporter.pm in Perl5.001m or later for details.
+=head2 Perl Classes
+X<class> X<@ISA>
-It may be handy to add a function or method to retrieve the number.
-Use the number in announcements and archive file names when
-releasing the module (ModuleName-1.02.tar.Z).
-See perldoc ExtUtils::MakeMaker.pm for details.
+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(s) in its global @ISA array (which
+must be a package global, not a lexical).
-=item How to release and distribute a module.
+For more on this, see L<perltoot> and L<perlobj>.
-It's good idea to post an announcement of the availability of your
-module (or the module itself if small) to the comp.lang.perl.announce
-Usenet newsgroup. This will at least ensure very wide once-off
-distribution.
+=head2 Perl Modules
+X<module>
-If possible you should place the module into a major ftp archive and
-include details of it's location in your announcement.
+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.
-Some notes about ftp archives: Please use a long descriptive file
-name which includes the version number. Most incoming directories
-will not be readable/listable, i.e., you won't be able to see your
-file after uploading it. Remember to send your email notification
-message as soon as possible after uploading else your file may get
-deleted automatically. Allow time for the file to be processed
-and/or check the file has been processed before announcing its
-location.
+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:
-FTP Archives for Perl Modules:
+ package Some::Module; # assumes Some/Module.pm
-Follow the instructions and links on
+ use strict;
+ use warnings;
- http://franz.ww.tu-berlin.de/modulelist
+ BEGIN {
+ use Exporter ();
+ our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS);
-or upload to one of these sites:
+ # set the version for version checking
+ $VERSION = 1.00;
+ # if using RCS/CVS, this may be preferred
+ $VERSION = sprintf "%d.%03d", q$Revision: 1.1 $ =~ /(\d+)/g;
- ftp://franz.ww.tu-berlin.de/incoming
- ftp://ftp.cis.ufl.edu/incoming
+ @ISA = qw(Exporter);
+ @EXPORT = qw(&func1 &func2 &func4);
+ %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ],
-and notify upload@franz.ww.tu-berlin.de.
+ # your exported package globals go here,
+ # as well as any optionally exported functions
+ @EXPORT_OK = qw($Var1 %Hashit &func3);
+ }
+ our @EXPORT_OK;
-By using the WWW interface you can ask the Upload Server to mirror
-your modules from your ftp or WWW site into your own directory on
-CPAN!
+ # exported package globals go here
+ our $Var1;
+ our %Hashit;
-Please remember to send me an updated entry for the Module list!
+ # non-exported package globals go here
+ our @more;
+ our $stuff;
-=item Take care when changing a released module.
+ # initialize package globals, first exported ones
+ $Var1 = '';
+ %Hashit = ();
-Always strive to remain compatible with previous released versions
-(see 2.2 above) Otherwise try to add a mechanism to revert to the
-old behaviour if people rely on it. Document incompatible changes.
+ # then the others (which are still accessible as $Some::Module::stuff)
+ $stuff = '';
+ @more = ();
-=back
+ # all file-scoped lexicals must be created before
+ # the functions below that use them.
-=back
+ # file-private lexicals go here
+ my $priv_var = '';
+ my %secret_hash = ();
-=head2 Guidelines for Converting Perl 4 Library Scripts into Modules
+ # here's a file-private function as a closure,
+ # callable as &$priv_func; it cannot be prototyped.
+ my $priv_func = sub {
+ # stuff goes here.
+ };
-=over 4
+ # make all your functions, whether exported or not;
+ # remember to put something interesting in the {} stubs
+ sub func1 {} # no prototype
+ sub func2() {} # proto'd void
+ sub func3($$) {} # proto'd to 2 scalars
-=item There is no requirement to convert anything.
+ # this one isn't exported, but could be called!
+ sub func4(\%) {} # proto'd to 1 hash ref
-If it ain't broke, don't fix it! Perl 4 library scripts should
-continue to work with no problems. You may need to make some minor
-changes (like escaping non-array @'s in double quoted strings) but
-there is no need to convert a .pl file into a Module for just that.
+ END { } # module clean-up code here (global destructor)
-=item Consider the implications.
+ ## YOUR CODE GOES HERE
-All the perl applications which make use of the script will need to
-be changed (slightly) if the script is converted into a module. Is
-it worth it unless you plan to make other changes at the same time?
+ 1; # don't forget to return a true value from the file
-=item Make the most of the opportunity.
+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.
-If you are going to convert the script to a module you can use the
-opportunity to redesign the interface. The 'Guidelines for Module
-Creation' above include many of the issues you should consider.
+Perl modules are included into your program by saying
-=item The pl2pm utility will get you started.
+ use Module;
-This utility will read *.pl files (given as parameters) and write
-corresponding *.pm files. The pl2pm utilities does the following:
+or
-=over 10
+ use Module LIST;
-=item *
-Adds the standard Module prologue lines
+This is exactly equivalent to
-=item *
-Converts package specifiers from ' to ::
+ BEGIN { require Module; import Module; }
-=item *
-Converts die(...) to croak(...)
+or
-=item *
-Several other minor changes
+ BEGIN { require Module; import Module LIST; }
-=back
+As a special case
-Being a mechanical process pl2pm is not bullet proof. The converted
-code will need careful checking, especially any package statements.
-Don't delete the original .pl file till the new .pm one works!
+ use Module ();
-=back
+is exactly equivalent to
-=head2 Guidelines for Reusing Application Code
+ BEGIN { require Module; }
-=over 4
+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).
-=item Complete applications rarely belong in the Perl Module Library.
+The two statements:
-=item Many applications contain some perl code which could be reused.
+ require SomeModule;
+ require "SomeModule.pm";
-Help save the world! Share your code in a form that makes it easy
-to reuse.
+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.)
-=item Break-out the reusable code into one or more separate module files.
+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 or unary operators for
+the rest of the current file. This will not work if you use C<require>
+instead of C<use>. With C<require> you can get into this problem:
-=item Take the opportunity to reconsider and redesign the interfaces.
+ require Cwd; # make Cwd:: accessible
+ $here = Cwd::getcwd();
-=item In some cases the 'application' can then be reduced to a small
+ use Cwd; # import names from Cwd::
+ $here = getcwd();
-fragment of code built on top of the reusable modules. In these cases
-the application could invoked as:
+ require Cwd; # make Cwd:: accessible
+ $here = getcwd(); # oops! no main::getcwd()
- perl -e 'use Module::Name; method(@ARGV)' ...
-or
- perl -mModule::Name ... (in perl5.002)
+In general, C<use Module ()> is recommended over C<require Module>,
+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> instead.
-=back
+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 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
+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> 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