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1 | package AutoLoader; |
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2 | |
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3 | use Carp; |
4 | use vars qw(@EXPORT @EXPORT_OK); |
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5 | |
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6 | BEGIN { |
7 | require Exporter; |
8 | @EXPORT = (); |
9 | @EXPORT_OK = qw(AUTOLOAD); |
10 | } |
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11 | |
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12 | AUTOLOAD { |
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13 | my $name; |
14 | # Braces used to preserve $1 et al. |
15 | { |
16 | my ($pkg,$func) = $AUTOLOAD =~ /(.*)::([^:]+)$/; |
17 | $pkg =~ s#::#/#g; |
18 | if (defined($name=$INC{"$pkg.pm"})) |
19 | { |
20 | $name =~ s#^(.*)$pkg\.pm$#$1auto/$pkg/$func.al#; |
21 | $name = undef unless (-r $name); |
22 | } |
23 | unless (defined $name) |
24 | { |
25 | $name = "auto/$AUTOLOAD.al"; |
26 | $name =~ s#::#/#g; |
27 | } |
28 | } |
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29 | my $save = $@; |
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30 | eval {local $SIG{__DIE__};require $name}; |
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31 | if ($@) { |
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32 | if (substr($AUTOLOAD,-9) eq '::DESTROY') { |
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33 | *$AUTOLOAD = sub {}; |
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34 | } else { |
35 | # The load might just have failed because the filename was too |
36 | # long for some old SVR3 systems which treat long names as errors. |
37 | # If we can succesfully truncate a long name then it's worth a go. |
38 | # There is a slight risk that we could pick up the wrong file here |
39 | # but autosplit should have warned about that when splitting. |
40 | if ($name =~ s/(\w{12,})\.al$/substr($1,0,11).".al"/e){ |
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41 | eval {local $SIG{__DIE__};require $name}; |
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42 | } |
43 | if ($@){ |
44 | $@ =~ s/ at .*\n//; |
45 | croak $@; |
46 | } |
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47 | } |
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48 | } |
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49 | $@ = $save; |
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50 | goto &$AUTOLOAD; |
51 | } |
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52 | |
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53 | sub import { |
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54 | my $pkg = shift; |
55 | my $callpkg = caller; |
56 | |
57 | # |
58 | # Export symbols, but not by accident of inheritance. |
59 | # |
60 | |
61 | Exporter::export $pkg, $callpkg, @_ if $pkg eq 'AutoLoader'; |
62 | |
63 | # |
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64 | # Try to find the autosplit index file. Eg., if the call package |
65 | # is POSIX, then $INC{POSIX.pm} is something like |
66 | # '/usr/local/lib/perl5/POSIX.pm', and the autosplit index file is in |
67 | # '/usr/local/lib/perl5/auto/POSIX/autosplit.ix', so we require that. |
68 | # |
69 | # However, if @INC is a relative path, this might not work. If, |
70 | # for example, @INC = ('lib'), then |
71 | # $INC{POSIX.pm} is 'lib/POSIX.pm', and we want to require |
72 | # 'auto/POSIX/autosplit.ix' (without the leading 'lib'). |
73 | # |
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74 | |
75 | (my $calldir = $callpkg) =~ s#::#/#; |
76 | my $path = $INC{$calldir . '.pm'}; |
77 | if (defined($path)) { |
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78 | # Try absolute path name. |
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79 | $path =~ s#^(.*)$calldir\.pm$#$1auto/$calldir/autosplit.ix#; |
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80 | eval { require $path; }; |
81 | # If that failed, try relative path with normal @INC searching. |
82 | if ($@) { |
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83 | $path ="auto/$calldir/autosplit.ix"; |
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84 | eval { require $path; }; |
85 | } |
86 | carp $@ if ($@); |
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87 | } |
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88 | } |
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89 | |
90 | 1; |
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91 | |
92 | __END__ |
93 | |
94 | =head1 NAME |
95 | |
96 | AutoLoader - load subroutines only on demand |
97 | |
98 | =head1 SYNOPSIS |
99 | |
100 | package Foo; |
101 | use AutoLoader 'AUTOLOAD'; # import the default AUTOLOAD subroutine |
102 | |
103 | package Bar; |
104 | use AutoLoader; # don't import AUTOLOAD, define our own |
105 | sub AUTOLOAD { |
106 | ... |
107 | $AutoLoader::AUTOLOAD = "..."; |
108 | goto &AutoLoader::AUTOLOAD; |
109 | } |
110 | |
111 | =head1 DESCRIPTION |
112 | |
113 | The B<AutoLoader> module works with the B<AutoSplit> module and the |
114 | C<__END__> token to defer the loading of some subroutines until they are |
115 | used rather than loading them all at once. |
116 | |
117 | To use B<AutoLoader>, the author of a module has to place the |
118 | definitions of subroutines to be autoloaded after an C<__END__> token. |
119 | (See L<perldata>.) The B<AutoSplit> module can then be run manually to |
120 | extract the definitions into individual files F<auto/funcname.al>. |
121 | |
122 | B<AutoLoader> implements an AUTOLOAD subroutine. When an undefined |
123 | subroutine in is called in a client module of B<AutoLoader>, |
124 | B<AutoLoader>'s AUTOLOAD subroutine attempts to locate the subroutine in a |
125 | file with a name related to the location of the file from which the |
126 | client module was read. As an example, if F<POSIX.pm> is located in |
127 | F</usr/local/lib/perl5/POSIX.pm>, B<AutoLoader> will look for perl |
128 | subroutines B<POSIX> in F</usr/local/lib/perl5/auto/POSIX/*.al>, where |
129 | the C<.al> file has the same name as the subroutine, sans package. If |
130 | such a file exists, AUTOLOAD will read and evaluate it, |
131 | thus (presumably) defining the needed subroutine. AUTOLOAD will then |
132 | C<goto> the newly defined subroutine. |
133 | |
134 | Once this process completes for a given funtion, it is defined, so |
135 | future calls to the subroutine will bypass the AUTOLOAD mechanism. |
136 | |
137 | =head2 Subroutine Stubs |
138 | |
139 | In order for object method lookup and/or prototype checking to operate |
140 | correctly even when methods have not yet been defined it is necessary to |
141 | "forward declare" each subroutine (as in C<sub NAME;>). See |
142 | L<perlsub/"SYNOPSIS">. Such forward declaration creates "subroutine |
143 | stubs", which are place holders with no code. |
144 | |
145 | The AutoSplit and B<AutoLoader> modules automate the creation of forward |
146 | declarations. The AutoSplit module creates an 'index' file containing |
147 | forward declarations of all the AutoSplit subroutines. When the |
148 | AutoLoader module is 'use'd it loads these declarations into its callers |
149 | package. |
150 | |
151 | Because of this mechanism it is important that B<AutoLoader> is always |
152 | C<use>d and not C<require>d. |
153 | |
154 | =head2 Using B<AutoLoader>'s AUTOLOAD Subroutine |
155 | |
156 | In order to use B<AutoLoader>'s AUTOLOAD subroutine you I<must> |
157 | explicitly import it: |
158 | |
159 | use AutoLoader 'AUTOLOAD'; |
160 | |
161 | =head2 Overriding B<AutoLoader>'s AUTOLOAD Subroutine |
162 | |
163 | Some modules, mainly extensions, provide their own AUTOLOAD subroutines. |
164 | They typically need to check for some special cases (such as constants) |
165 | and then fallback to B<AutoLoader>'s AUTOLOAD for the rest. |
166 | |
167 | Such modules should I<not> import B<AutoLoader>'s AUTOLOAD subroutine. |
168 | Instead, they should define their own AUTOLOAD subroutines along these |
169 | lines: |
170 | |
171 | use AutoLoader; |
172 | |
173 | sub AUTOLOAD { |
174 | my $constname; |
175 | ($constname = $AUTOLOAD) =~ s/.*:://; |
176 | my $val = constant($constname, @_ ? $_[0] : 0); |
177 | if ($! != 0) { |
178 | if ($! =~ /Invalid/) { |
179 | $AutoLoader::AUTOLOAD = $AUTOLOAD; |
180 | goto &AutoLoader::AUTOLOAD; |
181 | } |
182 | else { |
183 | croak "Your vendor has not defined constant $constname"; |
184 | } |
185 | } |
186 | eval "sub $AUTOLOAD { $val }"; |
187 | goto &$AUTOLOAD; |
188 | } |
189 | |
190 | If any module's own AUTOLOAD subroutine has no need to fallback to the |
191 | AutoLoader's AUTOLOAD subroutine (because it doesn't have any AutoSplit |
192 | subroutines), then that module should not use B<AutoLoader> at all. |
193 | |
194 | =head2 Package Lexicals |
195 | |
196 | Package lexicals declared with C<my> in the main block of a package |
197 | using B<AutoLoader> will not be visible to auto-loaded subroutines, due to |
198 | the fact that the given scope ends at the C<__END__> marker. A module |
199 | using such variables as package globals will not work properly under the |
200 | B<AutoLoader>. |
201 | |
202 | The C<vars> pragma (see L<perlmod/"vars">) may be used in such |
203 | situations as an alternative to explicitly qualifying all globals with |
204 | the package namespace. Variables pre-declared with this pragma will be |
205 | visible to any autoloaded routines (but will not be invisible outside |
206 | the package, unfortunately). |
207 | |
208 | =head2 B<AutoLoader> vs. B<SelfLoader> |
209 | |
210 | The B<AutoLoader> is similar in purpose to B<SelfLoader>: both delay the |
211 | loading of subroutines. |
212 | |
213 | B<SelfLoader> uses the C<__DATA__> marker rather than C<__END__>. |
214 | While this avoids the use of a hierarchy of disk files and the |
215 | associated open/close for each routine loaded, B<SelfLoader> suffers a |
216 | startup speed disadvantage in the one-time parsing of the lines after |
217 | C<__DATA__>, after which routines are cached. B<SelfLoader> can also |
218 | handle multiple packages in a file. |
219 | |
220 | B<AutoLoader> only reads code as it is requested, and in many cases |
221 | should be faster, but requires a machanism like B<AutoSplit> be used to |
222 | create the individual files. L<ExtUtils::MakeMaker> will invoke |
223 | B<AutoSplit> automatically if B<AutoLoader> is used in a module source |
224 | file. |
225 | |
226 | =head1 CAVEATS |
227 | |
228 | AutoLoaders prior to Perl 5.002 had a slightly different interface. Any |
229 | old modules which use B<AutoLoader> should be changed to the new calling |
230 | style. Typically this just means changing a require to a use, adding |
231 | the explicit C<'AUTOLOAD'> import if needed, and removing B<AutoLoader> |
232 | from C<@ISA>. |
233 | |
234 | On systems with restrictions on file name length, the file corresponding |
235 | to a subroutine may have a shorter name that the routine itself. This |
236 | can lead to conflicting file names. The I<AutoSplit> package warns of |
237 | these potential conflicts when used to split a module. |
238 | |
239 | =head1 SEE ALSO |
240 | |
241 | L<SelfLoader> - an autoloader that doesn't use external files. |
242 | |
243 | =cut |