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1 | package AutoLoader; |
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2 | |
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3 | use vars qw(@EXPORT @EXPORT_OK); |
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4 | |
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5 | BEGIN { |
6 | require Exporter; |
7 | @EXPORT = (); |
8 | @EXPORT_OK = qw(AUTOLOAD); |
9 | } |
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10 | |
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11 | AUTOLOAD { |
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12 | my $name; |
13 | # Braces used to preserve $1 et al. |
14 | { |
15 | my ($pkg,$func) = $AUTOLOAD =~ /(.*)::([^:]+)$/; |
16 | $pkg =~ s#::#/#g; |
17 | if (defined($name=$INC{"$pkg.pm"})) |
18 | { |
19 | $name =~ s#^(.*)$pkg\.pm$#$1auto/$pkg/$func.al#; |
20 | $name = undef unless (-r $name); |
21 | } |
22 | unless (defined $name) |
23 | { |
24 | $name = "auto/$AUTOLOAD.al"; |
25 | $name =~ s#::#/#g; |
26 | } |
27 | } |
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28 | my $save = $@; |
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29 | eval {local $SIG{__DIE__};require $name}; |
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30 | if ($@) { |
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31 | if (substr($AUTOLOAD,-9) eq '::DESTROY') { |
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32 | *$AUTOLOAD = sub {}; |
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33 | } else { |
34 | # The load might just have failed because the filename was too |
35 | # long for some old SVR3 systems which treat long names as errors. |
36 | # If we can succesfully truncate a long name then it's worth a go. |
37 | # There is a slight risk that we could pick up the wrong file here |
38 | # but autosplit should have warned about that when splitting. |
39 | if ($name =~ s/(\w{12,})\.al$/substr($1,0,11).".al"/e){ |
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40 | eval {local $SIG{__DIE__};require $name}; |
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41 | } |
42 | if ($@){ |
43 | $@ =~ s/ at .*\n//; |
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44 | my $error = $@; |
45 | require Carp; |
46 | Carp::croak($error); |
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47 | } |
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48 | } |
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49 | } |
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50 | $@ = $save; |
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51 | goto &$AUTOLOAD; |
52 | } |
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53 | |
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54 | sub import { |
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55 | my $pkg = shift; |
56 | my $callpkg = caller; |
57 | |
58 | # |
59 | # Export symbols, but not by accident of inheritance. |
60 | # |
61 | |
62 | Exporter::export $pkg, $callpkg, @_ if $pkg eq 'AutoLoader'; |
63 | |
64 | # |
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65 | # Try to find the autosplit index file. Eg., if the call package |
66 | # is POSIX, then $INC{POSIX.pm} is something like |
67 | # '/usr/local/lib/perl5/POSIX.pm', and the autosplit index file is in |
68 | # '/usr/local/lib/perl5/auto/POSIX/autosplit.ix', so we require that. |
69 | # |
70 | # However, if @INC is a relative path, this might not work. If, |
71 | # for example, @INC = ('lib'), then |
72 | # $INC{POSIX.pm} is 'lib/POSIX.pm', and we want to require |
73 | # 'auto/POSIX/autosplit.ix' (without the leading 'lib'). |
74 | # |
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75 | |
76 | (my $calldir = $callpkg) =~ s#::#/#; |
77 | my $path = $INC{$calldir . '.pm'}; |
78 | if (defined($path)) { |
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79 | # Try absolute path name. |
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80 | $path =~ s#^(.*)$calldir\.pm$#$1auto/$calldir/autosplit.ix#; |
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81 | eval { require $path; }; |
82 | # If that failed, try relative path with normal @INC searching. |
83 | if ($@) { |
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84 | $path ="auto/$calldir/autosplit.ix"; |
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85 | eval { require $path; }; |
86 | } |
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87 | if ($@) { |
88 | my $error = $@; |
89 | require Carp; |
90 | Carp::carp($error); |
91 | } |
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92 | } |
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93 | } |
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94 | |
95 | 1; |
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96 | |
97 | __END__ |
98 | |
99 | =head1 NAME |
100 | |
101 | AutoLoader - load subroutines only on demand |
102 | |
103 | =head1 SYNOPSIS |
104 | |
105 | package Foo; |
106 | use AutoLoader 'AUTOLOAD'; # import the default AUTOLOAD subroutine |
107 | |
108 | package Bar; |
109 | use AutoLoader; # don't import AUTOLOAD, define our own |
110 | sub AUTOLOAD { |
111 | ... |
112 | $AutoLoader::AUTOLOAD = "..."; |
113 | goto &AutoLoader::AUTOLOAD; |
114 | } |
115 | |
116 | =head1 DESCRIPTION |
117 | |
118 | The B<AutoLoader> module works with the B<AutoSplit> module and the |
119 | C<__END__> token to defer the loading of some subroutines until they are |
120 | used rather than loading them all at once. |
121 | |
122 | To use B<AutoLoader>, the author of a module has to place the |
123 | definitions of subroutines to be autoloaded after an C<__END__> token. |
124 | (See L<perldata>.) The B<AutoSplit> module can then be run manually to |
125 | extract the definitions into individual files F<auto/funcname.al>. |
126 | |
127 | B<AutoLoader> implements an AUTOLOAD subroutine. When an undefined |
128 | subroutine in is called in a client module of B<AutoLoader>, |
129 | B<AutoLoader>'s AUTOLOAD subroutine attempts to locate the subroutine in a |
130 | file with a name related to the location of the file from which the |
131 | client module was read. As an example, if F<POSIX.pm> is located in |
132 | F</usr/local/lib/perl5/POSIX.pm>, B<AutoLoader> will look for perl |
133 | subroutines B<POSIX> in F</usr/local/lib/perl5/auto/POSIX/*.al>, where |
134 | the C<.al> file has the same name as the subroutine, sans package. If |
135 | such a file exists, AUTOLOAD will read and evaluate it, |
136 | thus (presumably) defining the needed subroutine. AUTOLOAD will then |
137 | C<goto> the newly defined subroutine. |
138 | |
139 | Once this process completes for a given funtion, it is defined, so |
140 | future calls to the subroutine will bypass the AUTOLOAD mechanism. |
141 | |
142 | =head2 Subroutine Stubs |
143 | |
144 | In order for object method lookup and/or prototype checking to operate |
145 | correctly even when methods have not yet been defined it is necessary to |
146 | "forward declare" each subroutine (as in C<sub NAME;>). See |
147 | L<perlsub/"SYNOPSIS">. Such forward declaration creates "subroutine |
148 | stubs", which are place holders with no code. |
149 | |
150 | The AutoSplit and B<AutoLoader> modules automate the creation of forward |
151 | declarations. The AutoSplit module creates an 'index' file containing |
152 | forward declarations of all the AutoSplit subroutines. When the |
153 | AutoLoader module is 'use'd it loads these declarations into its callers |
154 | package. |
155 | |
156 | Because of this mechanism it is important that B<AutoLoader> is always |
157 | C<use>d and not C<require>d. |
158 | |
159 | =head2 Using B<AutoLoader>'s AUTOLOAD Subroutine |
160 | |
161 | In order to use B<AutoLoader>'s AUTOLOAD subroutine you I<must> |
162 | explicitly import it: |
163 | |
164 | use AutoLoader 'AUTOLOAD'; |
165 | |
166 | =head2 Overriding B<AutoLoader>'s AUTOLOAD Subroutine |
167 | |
168 | Some modules, mainly extensions, provide their own AUTOLOAD subroutines. |
169 | They typically need to check for some special cases (such as constants) |
170 | and then fallback to B<AutoLoader>'s AUTOLOAD for the rest. |
171 | |
172 | Such modules should I<not> import B<AutoLoader>'s AUTOLOAD subroutine. |
173 | Instead, they should define their own AUTOLOAD subroutines along these |
174 | lines: |
175 | |
176 | use AutoLoader; |
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177 | use Carp; |
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178 | |
179 | sub AUTOLOAD { |
180 | my $constname; |
181 | ($constname = $AUTOLOAD) =~ s/.*:://; |
182 | my $val = constant($constname, @_ ? $_[0] : 0); |
183 | if ($! != 0) { |
184 | if ($! =~ /Invalid/) { |
185 | $AutoLoader::AUTOLOAD = $AUTOLOAD; |
186 | goto &AutoLoader::AUTOLOAD; |
187 | } |
188 | else { |
189 | croak "Your vendor has not defined constant $constname"; |
190 | } |
191 | } |
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192 | *$AUTOLOAD = sub { $val }; # same as: eval "sub $AUTOLOAD { $val }"; |
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193 | goto &$AUTOLOAD; |
194 | } |
195 | |
196 | If any module's own AUTOLOAD subroutine has no need to fallback to the |
197 | AutoLoader's AUTOLOAD subroutine (because it doesn't have any AutoSplit |
198 | subroutines), then that module should not use B<AutoLoader> at all. |
199 | |
200 | =head2 Package Lexicals |
201 | |
202 | Package lexicals declared with C<my> in the main block of a package |
203 | using B<AutoLoader> will not be visible to auto-loaded subroutines, due to |
204 | the fact that the given scope ends at the C<__END__> marker. A module |
205 | using such variables as package globals will not work properly under the |
206 | B<AutoLoader>. |
207 | |
208 | The C<vars> pragma (see L<perlmod/"vars">) may be used in such |
209 | situations as an alternative to explicitly qualifying all globals with |
210 | the package namespace. Variables pre-declared with this pragma will be |
211 | visible to any autoloaded routines (but will not be invisible outside |
212 | the package, unfortunately). |
213 | |
214 | =head2 B<AutoLoader> vs. B<SelfLoader> |
215 | |
216 | The B<AutoLoader> is similar in purpose to B<SelfLoader>: both delay the |
217 | loading of subroutines. |
218 | |
219 | B<SelfLoader> uses the C<__DATA__> marker rather than C<__END__>. |
220 | While this avoids the use of a hierarchy of disk files and the |
221 | associated open/close for each routine loaded, B<SelfLoader> suffers a |
222 | startup speed disadvantage in the one-time parsing of the lines after |
223 | C<__DATA__>, after which routines are cached. B<SelfLoader> can also |
224 | handle multiple packages in a file. |
225 | |
226 | B<AutoLoader> only reads code as it is requested, and in many cases |
227 | should be faster, but requires a machanism like B<AutoSplit> be used to |
228 | create the individual files. L<ExtUtils::MakeMaker> will invoke |
229 | B<AutoSplit> automatically if B<AutoLoader> is used in a module source |
230 | file. |
231 | |
232 | =head1 CAVEATS |
233 | |
234 | AutoLoaders prior to Perl 5.002 had a slightly different interface. Any |
235 | old modules which use B<AutoLoader> should be changed to the new calling |
236 | style. Typically this just means changing a require to a use, adding |
237 | the explicit C<'AUTOLOAD'> import if needed, and removing B<AutoLoader> |
238 | from C<@ISA>. |
239 | |
240 | On systems with restrictions on file name length, the file corresponding |
241 | to a subroutine may have a shorter name that the routine itself. This |
242 | can lead to conflicting file names. The I<AutoSplit> package warns of |
243 | these potential conflicts when used to split a module. |
244 | |
245 | =head1 SEE ALSO |
246 | |
247 | L<SelfLoader> - an autoloader that doesn't use external files. |
248 | |
249 | =cut |