1AutoLoader(3pm) Perl Programmers Reference Guide AutoLoader(3pm)
2
6 AutoLoader - load subroutines only on demand
7
9 package Foo;
10 use AutoLoader 'AUTOLOAD'; # import the default AUTOLOAD subroutine
11 package Bar;
13 use AutoLoader; # don't import AUTOLOAD, define our own
14 sub AUTOLOAD {
15 ...
16 $AutoLoader::AUTOLOAD = "...";
17 goto &AutoLoader::AUTOLOAD;
18 }
19
21 The AutoLoader module works with the AutoSplit module and the "__END__"
22 token to defer the loading of some subroutines until they are used
23 rather than loading them all at once.
24 To use AutoLoader, the author of a module has to place the definitions
26 of subroutines to be autoloaded after an "__END__" token. (See
27 perldata.) The AutoSplit module can then be run manually to extract
28 the definitions into individual files auto/funcname.al.
29 AutoLoader implements an AUTOLOAD subroutine. When an undefined
31 subroutine in is called in a client module of AutoLoader, AutoLoader's
32 AUTOLOAD subroutine attempts to locate the subroutine in a file with a
33 name related to the location of the file from which the client module
34 was read. As an example, if POSIX.pm is located in
35 /usr/local/lib/perl5/POSIX.pm, AutoLoader will look for perl
36 subroutines POSIX in /usr/local/lib/perl5/auto/POSIX/*.al, where the
37 ".al" file has the same name as the subroutine, sans package. If such
38 a file exists, AUTOLOAD will read and evaluate it, thus (presumably)
39 defining the needed subroutine. AUTOLOAD will then "goto" the newly
40 defined subroutine.
41 Once this process completes for a given function, it is defined, so
43 future calls to the subroutine will bypass the AUTOLOAD mechanism.
44 Subroutine Stubs
46 In order for object method lookup and/or prototype checking to operate
47 correctly even when methods have not yet been defined it is necessary
48 to "forward declare" each subroutine (as in "sub NAME;"). See
49 "SYNOPSIS" in perlsub. Such forward declaration creates "subroutine
50 stubs", which are place holders with no code.
51 The AutoSplit and AutoLoader modules automate the creation of forward
53 declarations. The AutoSplit module creates an 'index' file containing
54 forward declarations of all the AutoSplit subroutines. When the
55 AutoLoader module is 'use'd it loads these declarations into its
56 callers package.
57 Because of this mechanism it is important that AutoLoader is always
59 "use"d and not "require"d.
60 Using AutoLoader's AUTOLOAD Subroutine
62 In order to use AutoLoader's AUTOLOAD subroutine you must explicitly
63 import it:
64 use AutoLoader 'AUTOLOAD';
66 Overriding AutoLoader's AUTOLOAD Subroutine
68 Some modules, mainly extensions, provide their own AUTOLOAD
69 subroutines. They typically need to check for some special cases (such
70 as constants) and then fallback to AutoLoader's AUTOLOAD for the rest.
71 Such modules should not import AutoLoader's AUTOLOAD subroutine.
73 Instead, they should define their own AUTOLOAD subroutines along these
74 lines:
75 use AutoLoader;
77 use Carp;
78 sub AUTOLOAD {
80 my $sub = $AUTOLOAD;
81 (my $constname = $sub) =~ s/.*:://;
82 my $val = constant($constname, @_ ? $_[0] : 0);
83 if ($! != 0) {
84 if ($! =~ /Invalid/ || $!{EINVAL}) {
85 $AutoLoader::AUTOLOAD = $sub;
86 goto &AutoLoader::AUTOLOAD;
87 }
88 else {
89 croak "Your vendor has not defined constant $constname";
90 }
91 }
92 *$sub = sub { $val }; # same as: eval "sub $sub { $val }";
93 goto &$sub;
94 }
95 If any module's own AUTOLOAD subroutine has no need to fallback to the
97 AutoLoader's AUTOLOAD subroutine (because it doesn't have any AutoSplit
98 subroutines), then that module should not use AutoLoader at all.
99 Package Lexicals
101 Package lexicals declared with "my" in the main block of a package
102 using AutoLoader will not be visible to auto-loaded subroutines, due to
103 the fact that the given scope ends at the "__END__" marker. A module
104 using such variables as package globals will not work properly under
105 the AutoLoader.
106 The "vars" pragma (see "vars" in perlmod) may be used in such
108 situations as an alternative to explicitly qualifying all globals with
109 the package namespace. Variables pre-declared with this pragma will be
110 visible to any autoloaded routines (but will not be invisible outside
111 the package, unfortunately).
112 Not Using AutoLoader
114 You can stop using AutoLoader by simply
115 no AutoLoader;
117 AutoLoader vs. SelfLoader
119 The AutoLoader is similar in purpose to SelfLoader: both delay the
120 loading of subroutines.
121 SelfLoader uses the "__DATA__" marker rather than "__END__". While
123 this avoids the use of a hierarchy of disk files and the associated
124 open/close for each routine loaded, SelfLoader suffers a startup speed
125 disadvantage in the one-time parsing of the lines after "__DATA__",
126 after which routines are cached. SelfLoader can also handle multiple
127 packages in a file.
128 AutoLoader only reads code as it is requested, and in many cases should
130 be faster, but requires a mechanism like AutoSplit be used to create
131 the individual files. ExtUtils::MakeMaker will invoke AutoSplit
132 automatically if AutoLoader is used in a module source file.
133 Forcing AutoLoader to Load a Function
135 Sometimes, it can be necessary or useful to make sure that a certain
136 function is fully loaded by AutoLoader. This is the case, for example,
137 when you need to wrap a function to inject debugging code. It is also
138 helpful to force early loading of code before forking to make use of
139 copy-on-write as much as possible.
140 Starting with AutoLoader 5.73, you can call the
142 "AutoLoader::autoload_sub" function with the fully-qualified name of
143 the function to load from its .al file. The behaviour is exactly the
144 same as if you called the function, triggering the regular "AUTOLOAD"
145 mechanism, but it does not actually execute the autoloaded function.
146
148 AutoLoaders prior to Perl 5.002 had a slightly different interface.
149 Any old modules which use AutoLoader should be changed to the new
150 calling style. Typically this just means changing a require to a use,
151 adding the explicit 'AUTOLOAD' import if needed, and removing
152 AutoLoader from @ISA.
153 On systems with restrictions on file name length, the file
155 corresponding to a subroutine may have a shorter name that the routine
156 itself. This can lead to conflicting file names. The AutoSplit
157 package warns of these potential conflicts when used to split a module.
158 AutoLoader may fail to find the autosplit files (or even find the wrong
160 ones) in cases where @INC contains relative paths, and the program does
161 "chdir".
162
164 SelfLoader - an autoloader that doesn't use external files.
165
167 "AutoLoader" is maintained by the perl5-porters. Please direct any
168 questions to the canonical mailing list. Anything that is applicable to
169 the CPAN release can be sent to its maintainer, though.
170 Author and Maintainer: The Perl5-Porters <perl5-porters@perl.org>
172 Maintainer of the CPAN release: Steffen Mueller <smueller@cpan.org>
174
176 This package has been part of the perl core since the first release of
177 perl5. It has been released separately to CPAN so older installations
178 can benefit from bug fixes.
179 This package has the same copyright and license as the perl core:
181 Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999,
183 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009,
184 2011, 2012, 2013
185 by Larry Wall and others
186 All rights reserved.
188 This program is free software; you can redistribute it and/or modify
190 it under the terms of either:
191 a) the GNU General Public License as published by the Free
193 Software Foundation; either version 1, or (at your option) any
194 later version, or
195 b) the "Artistic License" which comes with this Kit.
197 This program is distributed in the hope that it will be useful,
199 but WITHOUT ANY WARRANTY; without even the implied warranty of
200 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See either
201 the GNU General Public License or the Artistic License for more details.
202 You should have received a copy of the Artistic License with this
204 Kit, in the file named "Artistic". If not, I'll be glad to provide one.
205 You should also have received a copy of the GNU General Public License
207 along with this program in the file named "Copying". If not, write to the
208 Free Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston,
209 MA 02110-1301, USA or visit their web page on the internet at
210 http://www.gnu.org/copyleft/gpl.html.
211 For those of you that choose to use the GNU General Public License,
213 my interpretation of the GNU General Public License is that no Perl
214 script falls under the terms of the GPL unless you explicitly put
215 said script under the terms of the GPL yourself. Furthermore, any
216 object code linked with perl does not automatically fall under the
217 terms of the GPL, provided such object code only adds definitions
218 of subroutines and variables, and does not otherwise impair the
219 resulting interpreter from executing any standard Perl script. I
220 consider linking in C subroutines in this manner to be the moral
221 equivalent of defining subroutines in the Perl language itself. You
222 may sell such an object file as proprietary provided that you provide
223 or offer to provide the Perl source, as specified by the GNU General
224 Public License. (This is merely an alternate way of specifying input
225 to the program.) You may also sell a binary produced by the dumping of
226 a running Perl script that belongs to you, provided that you provide or
227 offer to provide the Perl source as specified by the GPL. (The
228 fact that a Perl interpreter and your code are in the same binary file
229 is, in this case, a form of mere aggregation.) This is my interpretation
230 of the GPL. If you still have concerns or difficulties understanding
231 my intent, feel free to contact me. Of course, the Artistic License
232 spells all this out for your protection, so you may prefer to use that.
233perl v5.28.2 2018-03-01 AutoLoader(3pm)