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AutoLoader(3)
NAME
  AutoLoader - load subroutines only on demand

SYNOPSIS
      package Foo;
      use AutoLoader 'AUTOLOAD';   # import the default AUTOLOAD subroutine

      package Bar;
      use AutoLoader;		   # don't import AUTOLOAD, define our own
      sub AUTOLOAD {
	  ...
	  $AutoLoader::AUTOLOAD = "...";
	  goto &AutoLoader::AUTOLOAD;
      }

DESCRIPTION
  The AutoLoader module works with the AutoSplit module and the "__END__"
  token to defer the loading of some subroutines until they are used rather
  than loading them all at once.

  To use AutoLoader, the author of a module has to place the definitions of
  subroutines to be autoloaded after an "__END__" token.  (See perldata.)
  The AutoSplit module can then be run manually to extract the definitions
  into individual files auto/funcname.al.

  AutoLoader implements an AUTOLOAD subroutine.	 When an undefined subroutine
  in is called in a client module of AutoLoader, AutoLoader's AUTOLOAD
  subroutine attempts to locate the subroutine in a file with a name related
  to the location of the file from which the client module was read.  As an
  example, if POSIX.pm is located in /usr/local/lib/perl5/POSIX.pm,
  AutoLoader will look for perl subroutines POSIX in
  /usr/local/lib/perl5/auto/POSIX/*.al, where the ".al" file has the same
  name as the subroutine, sans package.	 If such a file exists, AUTOLOAD will
  read and evaluate it, thus (presumably) defining the needed subroutine.
  AUTOLOAD will then "goto" the newly defined subroutine.

  Once this process completes for a given function, it is defined, so future
  calls to the subroutine will bypass the AUTOLOAD mechanism.

  Subroutine Stubs

  In order for object method lookup and/or prototype checking to operate
  correctly even when methods have not yet been defined it is necessary to
  "forward declare" each subroutine (as in "sub NAME;").  See "SYNOPSIS" in
  perlsub.  Such forward declaration creates "subroutine stubs", which are
  place holders with no code.

  The AutoSplit and AutoLoader modules automate the creation of forward
  declarations.	 The AutoSplit module creates an 'index' file containing
  forward declarations of all the AutoSplit subroutines.  When the AutoLoader
  module is 'use'd it loads these declarations into its callers package.

  Because of this mechanism it is important that AutoLoader is always "use"d
  and not "require"d.

  Using AutoLoader's AUTOLOAD Subroutine

  In order to use AutoLoader's AUTOLOAD subroutine you must explicitly import
  it:

      use AutoLoader 'AUTOLOAD';

  Overriding AutoLoader's AUTOLOAD Subroutine

  Some modules, mainly extensions, provide their own AUTOLOAD subroutines.
  They typically need to check for some special cases (such as constants) and
  then fallback to AutoLoader's AUTOLOAD for the rest.

  Such modules should not import AutoLoader's AUTOLOAD subroutine.  Instead,
  they should define their own AUTOLOAD subroutines along these lines:

      use AutoLoader;
      use Carp;

      sub AUTOLOAD {
	  my $sub = $AUTOLOAD;
	  (my $constname = $sub) =~ s/.*:://;
	  my $val = constant($constname, @_ ? $_[0] : 0);
	  if ($! != 0) {
	      if ($! =~ /Invalid/ || $!{EINVAL}) {
		  $AutoLoader::AUTOLOAD = $sub;
		  goto &AutoLoader::AUTOLOAD;
	      }
	      else {
		  croak "Your vendor has not defined constant $constname";
	      }
	  }
	  *$sub = sub { $val }; # same as: eval "sub $sub { $val }";
	  goto &$sub;
      }

  If any module's own AUTOLOAD subroutine has no need to fallback to the
  AutoLoader's AUTOLOAD subroutine (because it doesn't have any AutoSplit
  subroutines), then that module should not use AutoLoader at all.

  Package Lexicals

  Package lexicals declared with "my" in the main block of a package using
  AutoLoader will not be visible to auto-loaded subroutines, due to the fact
  that the given scope ends at the "__END__" marker.  A module using such
  variables as package globals will not work properly under the AutoLoader.

  The "vars" pragma (see "vars" in perlmod) may be used in such situations as
  an alternative to explicitly qualifying all globals with the package
  namespace.  Variables pre-declared with this pragma will be visible to any
  autoloaded routines (but will not be invisible outside the package,
  unfortunately).

  Not Using AutoLoader

  You can stop using AutoLoader by simply

	  no AutoLoader;

  AutoLoader vs. SelfLoader

  The AutoLoader is similar in purpose to SelfLoader: both delay the loading
  of subroutines.

  SelfLoader uses the "__DATA__" marker rather than "__END__".	While this
  avoids the use of a hierarchy of disk files and the associated open/close
  for each routine loaded, SelfLoader suffers a startup speed disadvantage in
  the one-time parsing of the lines after "__DATA__", after which routines
  are cached.  SelfLoader can also handle multiple packages in a file.

  AutoLoader only reads code as it is requested, and in many cases should be
  faster, but requires a mechanism like AutoSplit be used to create the
  individual files.  ExtUtils::MakeMaker will invoke AutoSplit automatically
  if AutoLoader is used in a module source file.

CAVEATS
  AutoLoaders prior to Perl 5.002 had a slightly different interface.  Any
  old modules which use AutoLoader should be changed to the new calling
  style.  Typically this just means changing a require to a use, adding the
  explicit 'AUTOLOAD' import if needed, and removing AutoLoader from @ISA.

  On systems with restrictions on file name length, the file corresponding to
  a subroutine may have a shorter name that the routine itself.	 This can
  lead to conflicting file names.  The AutoSplit package warns of these
  potential conflicts when used to split a module.

  AutoLoader may fail to find the autosplit files (or even find the wrong
  ones) in cases where @INC contains relative paths, and the program does
  "chdir".

SEE ALSO
  SelfLoader - an autoloader that doesn't use external files.

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