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

FreeBSD Manual Pages


home | help
SelfLoader(3)	       Perl Programmers	Reference Guide		 SelfLoader(3)

       SelfLoader - load functions only	on demand

	   package FOOBAR;
	   use SelfLoader;

	   ... (initializing code)

	   sub {....

       This module tells its users that	functions in the FOOBAR	package	are to
       be autoloaded from after	the "__DATA__" token.  See also	"Autoloading"
       in perlsub.

   The __DATA__	token
       The "__DATA__" token tells the perl compiler that the perl code for
       compilation is finished.	Everything after the "__DATA__"	token is
       available for reading via the filehandle	FOOBAR::DATA, where FOOBAR is
       the name	of the current package when the	"__DATA__" token is reached.
       This works just the same	as "__END__" does in package 'main', but for
       other modules data after	"__END__" is not automatically retrievable,
       whereas data after "__DATA__" is.  The "__DATA__" token is not
       recognized in versions of perl prior to 5.001m.

       Note that it is possible	to have	"__DATA__" tokens in the same package
       in multiple files, and that the last "__DATA__" token in	a given
       package that is encountered by the compiler is the one accessible by
       the filehandle. This also applies to "__END__" and main,	i.e. if	the
       'main' program has an "__END__",	but a module 'require'd	(_not_ 'use'd)
       by that program has a 'package main;' declaration followed by an
       '"__DATA__"', then the "DATA" filehandle	is set to access the data
       after the "__DATA__" in the module, _not_ the data after	the "__END__"
       token in	the 'main' program, since the compiler encounters the
       'require'd file later.

   SelfLoader autoloading
       The SelfLoader works by the user	placing	the "__DATA__" token after
       perl code which needs to	be compiled and	run at 'require' time, but
       before subroutine declarations that can be loaded in later - usually
       because they may	never be called.

       The SelfLoader will read	from the FOOBAR::DATA filehandle to load in
       the data	after "__DATA__", and load in any subroutine when it is
       called. The costs are the one-time parsing of the data after
       "__DATA__", and a load delay for	the _first_ call of any	autoloaded
       function. The benefits (hopefully) are a	speeded	up compilation phase,
       with no need to load functions which are	never used.

       The SelfLoader will stop	reading	from "__DATA__"	if it encounters the
       "__END__" token - just as you would expect.  If the "__END__" token is
       present,	and is followed	by the token DATA, then	the SelfLoader leaves
       the FOOBAR::DATA	filehandle open	on the line after that token.

       The SelfLoader exports the "AUTOLOAD" subroutine	to the package using
       the SelfLoader, and this	loads the called subroutine when it is first

       There is	no advantage to	putting	subroutines which will _always_	be
       called after the	"__DATA__" token.

   Autoloading and package lexicals
       A 'my $pack_lexical' statement makes the	variable $pack_lexical local
       _only_ to the file up to	the "__DATA__" token. Subroutines declared
       elsewhere _cannot_ see these types of variables,	just as	if you
       declared	subroutines in the package but in another file,	they cannot
       see these variables.

       So specifically,	autoloaded functions cannot see	package	lexicals (this
       applies to both the SelfLoader and the Autoloader).  The	"vars" pragma
       provides	an alternative to defining package-level globals that will be
       visible to autoloaded routines. See the documentation on	vars in	the
       pragma section of perlmod.

   SelfLoader and AutoLoader
       The SelfLoader can replace the AutoLoader - just	change 'use
       AutoLoader' to 'use SelfLoader' (though note that the SelfLoader
       exports the AUTOLOAD function - but if you have your own	AUTOLOAD and
       are using the AutoLoader	too, you probably know what you're doing), and
       the "__END__" token to "__DATA__". You will need	perl version 5.001m or
       later to	use this (version 5.001	with all patches up to patch m).

       There is	no need	to inherit from	the SelfLoader.

       The SelfLoader works similarly to the AutoLoader, but picks up the subs
       from after the "__DATA__" instead of in the 'lib/auto' directory.
       There is	a maintenance gain in not needing to run AutoSplit on the
       module at installation, and a runtime gain in not needing to keep
       opening and closing files to load subs. There is	a runtime loss in
       needing to parse	the code after the "__DATA__". Details of the
       AutoLoader and another view of these distinctions can be	found in that
       module's	documentation.

   __DATA__, __END__, and the FOOBAR::DATA filehandle.
       This section is only relevant if	you want to use	the "FOOBAR::DATA"
       together	with the SelfLoader.

       Data after the "__DATA__" token in a module is read using the
       FOOBAR::DATA filehandle.	"__END__" can still be used to denote the end
       of the "__DATA__" section if followed by	the token DATA - this is
       supported by the	SelfLoader. The	"FOOBAR::DATA" filehandle is left open
       if an "__END__" followed	by a DATA is found, with the filehandle
       positioned at the start of the line after the "__END__" token. If no
       "__END__" token is present, or an "__END__" token with no DATA token on
       the same	line, then the filehandle is closed.

       The SelfLoader reads from wherever the current position of the
       "FOOBAR::DATA" filehandle is, until the EOF or "__END__". This means
       that if you want	to use that filehandle (and ONLY if you	want to), you
       should either

       1. Put all your subroutine declarations immediately after the
       "__DATA__" token	and put	your own data after those declarations,	using
       the "__END__" token to mark the end of subroutine declarations. You
       must also ensure	that the SelfLoader reads first	by  calling
       'SelfLoader->load_stubs();', or by using	a function which is


       2. You should read the "FOOBAR::DATA" filehandle	first, leaving the
       handle open and positioned at the first line of subroutine

       You could conceivably do	both.

   Classes and inherited methods.
       For modules which are not classes, this section is not relevant.	 This
       section is only relevant	if you have methods which could	be inherited.

       A subroutine stub (or forward declaration) looks	like

	 sub stub;

       i.e. it is a subroutine declaration without the body of the subroutine.
       For modules which are not classes, there	is no real need	for stubs as
       far as autoloading is concerned.

       For modules which ARE classes, and need to handle inherited methods,
       stubs are needed	to ensure that the method inheritance mechanism	works
       properly. You can load the stubs	into the module	at 'require' time, by
       adding the statement 'SelfLoader->load_stubs();'	to the module to do

       The alternative is to put the stubs in before the "__DATA__" token
       BEFORE releasing	the module, and	for this purpose the
       "Devel::SelfStubber" module is available.  However this does require
       the extra step of ensuring that the stubs are in	the module. If this is
       done I strongly recommend that this is done BEFORE releasing the	module
       - it should NOT be done at install time in general.

Multiple packages and fully qualified subroutine names
       Subroutines in multiple packages	within the same	file are supported -
       but you should note that	this requires exporting	the
       "SelfLoader::AUTOLOAD" to every package which requires it. This is done
       automatically by	the SelfLoader when it first loads the subs into the
       cache, but you should really specify it in the initialization before
       the "__DATA__" by putting a 'use	SelfLoader' statement in each package.

       Fully qualified subroutine names	are also supported. For	example,

	  sub foo::bar {23}
	  package baz;
	  sub dob {32}

       will all	be loaded correctly by the SelfLoader, and the SelfLoader will
       ensure that the packages	'foo' and 'baz'	correctly have the SelfLoader
       "AUTOLOAD" method when the data after "__DATA__"	is first parsed.

       "SelfLoader" 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	<>

       Maintainer of the CPAN release: Steffen Mueller <>

       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 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
       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

       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 v5.36.0			  2020-12-28			 SelfLoader(3)

NAME | SYNOPSIS | DESCRIPTION | Multiple packages and fully qualified subroutine names | AUTHOR | COPYRIGHT AND LICENSE

Want to link to this manual page? Use this URL:

home | help