Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages

  
 
  

home | help
AutoLoader(3)	       Perl Programmers	Reference Guide		 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.

   Forcing AutoLoader to Load a	Function
       Sometimes, it can be necessary or useful	to make	sure that a certain
       function	is fully loaded	by AutoLoader. This is the case, for example,
       when you	need to	wrap a function	to inject debugging code. It is	also
       helpful to force	early loading of code before forking to	make use of
       copy-on-write as	much as	possible.

       Starting	with AutoLoader	5.73, you can call the
       "AutoLoader::autoload_sub" function with	the fully-qualified name of
       the function to load from its .al file. The behaviour is	exactly	the
       same as if you called the function, triggering the regular "AUTOLOAD"
       mechanism, but it does not actually execute the autoloaded function.

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.

AUTHOR
       "AutoLoader" is maintained by the perl5-porters.	Please direct any
       questions to the	canonical mailing list.	Anything that is applicable to
       the CPAN	release	can be sent to its maintainer, though.

       Author and Maintainer: The Perl5-Porters	<perl5-porters@perl.org>

       Maintainer of the CPAN release: Steffen Mueller <smueller@cpan.org>

COPYRIGHT AND LICENSE
       This package has	been part of the perl core since the first release of
       perl5. It has been released separately to CPAN so older installations
       can benefit from	bug fixes.

       This package has	the same copyright and license as the perl core:

		    Copyright (C) 1993,	1994, 1995, 1996, 1997,	1998, 1999,
	       2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009,
	       2011, 2012, 2013
	       by Larry	Wall and others

				   All rights reserved.

	   This	program	is free	software; you can redistribute it and/or modify
	   it under the	terms of either:

	       a) the GNU General Public License as published by the Free
	       Software	Foundation; either version 1, or (at your option) any
	       later version, or

	       b) the "Artistic	License" which comes with this Kit.

	   This	program	is distributed in the hope that	it will	be useful,
	   but WITHOUT ANY WARRANTY; without even the implied warranty of
	   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.	 See either
	   the GNU General Public License or the Artistic License for more details.

	   You should have received a copy of the Artistic License with	this
	   Kit,	in the file named "Artistic".  If not, I'll be glad to provide one.

	   You should also have	received a copy	of the GNU General Public License
	   along with this program in the file named "Copying".	If not,	write to the
	   Free	Software Foundation, Inc., 51 Franklin St, Fifth Floor,	Boston,
	   MA 02110-1301, USA or visit their web page on the internet at
	   http://www.gnu.org/copyleft/gpl.html.

	   For those of	you that choose	to use the GNU General Public License,
	   my interpretation of	the GNU	General	Public License is that no Perl
	   script falls	under the terms	of the GPL unless you explicitly put
	   said	script under the terms of the GPL yourself.  Furthermore, any
	   object code linked with perl	does not automatically fall under the
	   terms of the	GPL, provided such object code only adds definitions
	   of subroutines and variables, and does not otherwise	impair the
	   resulting interpreter from executing	any standard Perl script.  I
	   consider linking in C subroutines in	this manner to be the moral
	   equivalent of defining subroutines in the Perl language itself.  You
	   may sell such an object file	as proprietary provided	that you provide
	   or offer to provide the Perl	source,	as specified by	the GNU	General
	   Public License.  (This is merely an alternate way of	specifying input
	   to the program.)  You may also sell a binary	produced by the	dumping	of
	   a running Perl script that belongs to you, provided that you	provide	or
	   offer to provide the	Perl source as specified by the	GPL.  (The
	   fact	that a Perl interpreter	and your code are in the same binary file
	   is, in this case, a form of mere aggregation.)  This	is my interpretation
	   of the GPL.	If you still have concerns or difficulties understanding
	   my intent, feel free	to contact me.	Of course, the Artistic	License
	   spells all this out for your	protection, so you may prefer to use that.

perl v5.28.3			  2020-05-14			 AutoLoader(3)

NAME | SYNOPSIS | DESCRIPTION | CAVEATS | SEE ALSO | AUTHOR | COPYRIGHT AND LICENSE

Want to link to this manual page? Use this URL:
<https://www.freebsd.org/cgi/man.cgi?query=AutoLoader&sektion=3&manpath=FreeBSD+13.0-RELEASE+and+Ports>

home | help