MAN page from CentOS Other perl-SelfLoader-1.23-420.el8.noarch.rpm


Section: Perl Programmers Reference Guide (3pm)
Updated: 2018-03-01


SelfLoader - load functions only on demand 


    package FOOBAR;    use SelfLoader;    ... (initializing code)    __DATA__    sub {....


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

The __DATA__ token

The "__DATA__" token tells the perl compiler that the perl codefor compilation is finished. Everything after the "__DATA__" tokenis 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 inpackage 'main', but for other modules data after "__END__" is notautomatically retrievable, whereas data after "__DATA__" is.The "__DATA__" token is not recognized in versions of perl prior to5.001m.

Note that it is possible to have "__DATA__" tokens in the same packagein multiple files, and that the last "__DATA__" token in a givenpackage that is encountered by the compiler is the one accessibleby the filehandle. This also applies to "__END__" and main, i.e. ifthe '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 andrun at 'require' time, but before subroutine declarationsthat can be loaded in later - usually because they may neverbe called.

The SelfLoader will read from the FOOBAR::DATA filehandle toload in the data after "__DATA__", and load in any subroutinewhen it is called. The costs are the one-time parsing of thedata 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 loadfunctions which are never used.

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

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

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_lexicallocal _only_ to the file up to the "__DATA__" token. Subroutinesdeclared elsewhere _cannot_ see these types of variables,just as if you declared subroutines in the package but in anotherfile, they cannot see these variables.

So specifically, autoloaded functions cannot see packagelexicals (this applies to both the SelfLoader and the Autoloader).The "vars" pragma provides an alternative to defining package-levelglobals that will be visible to autoloaded routines. See the documentationon 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 exportsthe AUTOLOAD function - but if you have your own AUTOLOAD andare using the AutoLoader too, you probably know what you're doing),and the "__END__" token to "__DATA__". You will need perl version 5.001mor 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 thesubs from after the "__DATA__" instead of in the 'lib/auto' directory.There is a maintenance gain in not needing to run AutoSplit on the moduleat installation, and a runtime gain in not needing to keep opening andclosing files to load subs. There is a runtime loss in needingto parse the code after the "__DATA__". Details of the AutoLoader andanother view of these distinctions can be found in that module'sdocumentation. 

__DATA__, __END__, and the FOOBAR::DATA filehandle.

This section is only relevant if you want to usethe "FOOBAR::DATA" together with the SelfLoader.

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

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

1. Put all your subroutine declarations immediately afterthe "__DATA__" token and put your own data after thosedeclarations, using the "__END__" token to mark the endof subroutine declarations. You must also ensure that the SelfLoaderreads first by calling 'SelfLoader->load_stubs();', or by using afunction which is selfloaded;


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

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 couldbe inherited.

A subroutine stub (or forward declaration) looks like

  sub stub;

i.e. it is a subroutine declaration without the body of thesubroutine. For modules which are not classes, there is no realneed 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 worksproperly. You can load the stubs into the module at 'require' time, byadding the statement 'SelfLoader->load_stubs();' to the module to dothis.

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

Multiple packages and fully qualified subroutine names

Subroutines in multiple packages within the same file are supported - but youshould note that this requires exporting the "SelfLoader::AUTOLOAD" toevery package which requires it. This is done automatically by theSelfLoader when it first loads the subs into the cache, but you shouldreally specify it in the initialization before the "__DATA__" by puttinga 'use SelfLoader' statement in each package.

Fully qualified subroutine names are also supported. For example,

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

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


"SelfLoader" is maintained by the perl5-porters. Please directany questions to the canonical mailing list. Anything thatis 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 releaseof perl5. It has been released separately to CPAN so older installationscan 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 modifyit under the terms of either:

the GNU General Public License as published by the Free Software Foundation;either version 1, or (at your option) any later version, or
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 ofMERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See eitherthe GNU General Public License or the Artistic License for more details.

You should have received a copy of the Artistic License with thisKit, 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 Licensealong with this program in the file named ``Copying''. If not, write to theFree 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 Perlscript falls under the terms of the GPL unless you explicitly putsaid script under the terms of the GPL yourself. Furthermore, anyobject code linked with perl does not automatically fall under theterms of the GPL, provided such object code only adds definitionsof subroutines and variables, and does not otherwise impair theresulting interpreter from executing any standard Perl script. Iconsider linking in C subroutines in this manner to be the moralequivalent of defining subroutines in the Perl language itself. Youmay sell such an object file as proprietary provided that you provideor offer to provide the Perl source, as specified by the GNU GeneralPublic License. (This is merely an alternate way of specifying inputto the program.) You may also sell a binary produced by the dumping ofa running Perl script that belongs to you, provided that you provide oroffer to provide the Perl source as specified by the GPL. (Thefact that a Perl interpreter and your code are in the same binary fileis, in this case, a form of mere aggregation.) This is my interpretationof the GPL. If you still have concerns or difficulties understandingmy intent, feel free to contact me. Of course, the Artistic Licensespells all this out for your protection, so you may prefer to use that.



The __DATA__ token
SelfLoader autoloading
Autoloading and package lexicals
SelfLoader and AutoLoader
__DATA__, __END__, and the FOOBAR::DATA filehandle.
Classes and inherited methods.
Multiple packages and fully qualified subroutine names

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