296 lines
9.7 KiB
Perl
296 lines
9.7 KiB
Perl
package AutoLoader;
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use vars qw(@EXPORT @EXPORT_OK);
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my $is_dosish;
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my $is_vms;
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BEGIN {
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require Exporter;
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@EXPORT = ();
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@EXPORT_OK = qw(AUTOLOAD);
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$is_dosish = $^O eq 'dos' || $^O eq 'os2' || $^O eq 'MSWin32';
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$is_vms = $^O eq 'VMS';
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}
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AUTOLOAD {
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my $name;
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# Braces used to preserve $1 et al.
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{
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# Try to find the autoloaded file from the package-qualified
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# name of the sub. e.g., if the sub needed is
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# Getopt::Long::GetOptions(), then $INC{Getopt/Long.pm} is
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# something like '/usr/lib/perl5/Getopt/Long.pm', and the
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# autoload file is '/usr/lib/perl5/auto/Getopt/Long/GetOptions.al'.
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#
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# However, if @INC is a relative path, this might not work. If,
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# for example, @INC = ('lib'), then $INC{Getopt/Long.pm} is
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# 'lib/Getopt/Long.pm', and we want to require
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# 'auto/Getopt/Long/GetOptions.al' (without the leading 'lib').
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# In this case, we simple prepend the 'auto/' and let the
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# C<require> take care of the searching for us.
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my ($pkg,$func) = $AUTOLOAD =~ /(.*)::([^:]+)$/;
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$pkg =~ s#::#/#g;
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if (defined($name=$INC{"$pkg.pm"})) {
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$name =~ s#^(.*)$pkg\.pm$#$1auto/$pkg/$func.al#;
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# if the file exists, then make sure that it is a
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# a fully anchored path (i.e either '/usr/lib/auto/foo/bar.al',
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# or './lib/auto/foo/bar.al'. This avoids C<require> searching
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# (and failing) to find the 'lib/auto/foo/bar.al' because it
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# looked for 'lib/lib/auto/foo/bar.al', given @INC = ('lib').
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if (-r $name) {
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unless ($name =~ m|^/|) {
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if ($is_dosish) {
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unless ($name =~ m{^([a-z]:)?[\\/]}i) {
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$name = "./$name";
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}
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}
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elsif ($is_vms) {
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# XXX todo by VMSmiths
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$name = "./$name";
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}
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else {
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$name = "./$name";
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}
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}
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}
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else {
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$name = undef;
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}
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}
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unless (defined $name) {
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# let C<require> do the searching
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$name = "auto/$AUTOLOAD.al";
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$name =~ s#::#/#g;
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}
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}
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my $save = $@;
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eval { local $SIG{__DIE__}; require $name };
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if ($@) {
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if (substr($AUTOLOAD,-9) eq '::DESTROY') {
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*$AUTOLOAD = sub {};
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} else {
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# The load might just have failed because the filename was too
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# long for some old SVR3 systems which treat long names as errors.
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# If we can succesfully truncate a long name then it's worth a go.
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# There is a slight risk that we could pick up the wrong file here
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# but autosplit should have warned about that when splitting.
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if ($name =~ s/(\w{12,})\.al$/substr($1,0,11).".al"/e){
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eval {local $SIG{__DIE__};require $name};
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}
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if ($@){
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$@ =~ s/ at .*\n//;
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my $error = $@;
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require Carp;
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Carp::croak($error);
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}
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}
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}
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$@ = $save;
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goto &$AUTOLOAD;
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}
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sub import {
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my $pkg = shift;
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my $callpkg = caller;
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#
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# Export symbols, but not by accident of inheritance.
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#
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Exporter::export $pkg, $callpkg, @_ if $pkg eq 'AutoLoader';
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#
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# Try to find the autosplit index file. Eg., if the call package
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# is POSIX, then $INC{POSIX.pm} is something like
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# '/usr/local/lib/perl5/POSIX.pm', and the autosplit index file is in
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# '/usr/local/lib/perl5/auto/POSIX/autosplit.ix', so we require that.
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#
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# However, if @INC is a relative path, this might not work. If,
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# for example, @INC = ('lib'), then
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# $INC{POSIX.pm} is 'lib/POSIX.pm', and we want to require
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# 'auto/POSIX/autosplit.ix' (without the leading 'lib').
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#
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(my $calldir = $callpkg) =~ s#::#/#g;
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my $path = $INC{$calldir . '.pm'};
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if (defined($path)) {
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# Try absolute path name.
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$path =~ s#^(.*)$calldir\.pm$#$1auto/$calldir/autosplit.ix#;
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eval { require $path; };
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# If that failed, try relative path with normal @INC searching.
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if ($@) {
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$path ="auto/$calldir/autosplit.ix";
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eval { require $path; };
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}
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if ($@) {
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my $error = $@;
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require Carp;
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Carp::carp($error);
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}
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}
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}
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1;
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__END__
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=head1 NAME
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AutoLoader - load subroutines only on demand
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=head1 SYNOPSIS
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package Foo;
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use AutoLoader 'AUTOLOAD'; # import the default AUTOLOAD subroutine
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package Bar;
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use AutoLoader; # don't import AUTOLOAD, define our own
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sub AUTOLOAD {
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...
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$AutoLoader::AUTOLOAD = "...";
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goto &AutoLoader::AUTOLOAD;
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}
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=head1 DESCRIPTION
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The B<AutoLoader> module works with the B<AutoSplit> module and the
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C<__END__> token to defer the loading of some subroutines until they are
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used rather than loading them all at once.
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To use B<AutoLoader>, the author of a module has to place the
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definitions of subroutines to be autoloaded after an C<__END__> token.
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(See L<perldata>.) The B<AutoSplit> module can then be run manually to
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extract the definitions into individual files F<auto/funcname.al>.
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B<AutoLoader> implements an AUTOLOAD subroutine. When an undefined
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subroutine in is called in a client module of B<AutoLoader>,
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B<AutoLoader>'s AUTOLOAD subroutine attempts to locate the subroutine in a
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file with a name related to the location of the file from which the
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client module was read. As an example, if F<POSIX.pm> is located in
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F</usr/local/lib/perl5/POSIX.pm>, B<AutoLoader> will look for perl
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subroutines B<POSIX> in F</usr/local/lib/perl5/auto/POSIX/*.al>, where
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the C<.al> file has the same name as the subroutine, sans package. If
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such a file exists, AUTOLOAD will read and evaluate it,
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thus (presumably) defining the needed subroutine. AUTOLOAD will then
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C<goto> the newly defined subroutine.
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Once this process completes for a given function, it is defined, so
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future calls to the subroutine will bypass the AUTOLOAD mechanism.
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=head2 Subroutine Stubs
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In order for object method lookup and/or prototype checking to operate
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correctly even when methods have not yet been defined it is necessary to
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"forward declare" each subroutine (as in C<sub NAME;>). See
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L<perlsub/"SYNOPSIS">. Such forward declaration creates "subroutine
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stubs", which are place holders with no code.
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The AutoSplit and B<AutoLoader> modules automate the creation of forward
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declarations. The AutoSplit module creates an 'index' file containing
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forward declarations of all the AutoSplit subroutines. When the
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AutoLoader module is 'use'd it loads these declarations into its callers
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package.
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Because of this mechanism it is important that B<AutoLoader> is always
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C<use>d and not C<require>d.
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=head2 Using B<AutoLoader>'s AUTOLOAD Subroutine
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In order to use B<AutoLoader>'s AUTOLOAD subroutine you I<must>
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explicitly import it:
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use AutoLoader 'AUTOLOAD';
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=head2 Overriding B<AutoLoader>'s AUTOLOAD Subroutine
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Some modules, mainly extensions, provide their own AUTOLOAD subroutines.
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They typically need to check for some special cases (such as constants)
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and then fallback to B<AutoLoader>'s AUTOLOAD for the rest.
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Such modules should I<not> import B<AutoLoader>'s AUTOLOAD subroutine.
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Instead, they should define their own AUTOLOAD subroutines along these
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lines:
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use AutoLoader;
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use Carp;
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sub AUTOLOAD {
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my $constname;
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($constname = $AUTOLOAD) =~ s/.*:://;
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my $val = constant($constname, @_ ? $_[0] : 0);
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if ($! != 0) {
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if ($! =~ /Invalid/) {
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$AutoLoader::AUTOLOAD = $AUTOLOAD;
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goto &AutoLoader::AUTOLOAD;
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}
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else {
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croak "Your vendor has not defined constant $constname";
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}
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}
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*$AUTOLOAD = sub { $val }; # same as: eval "sub $AUTOLOAD { $val }";
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goto &$AUTOLOAD;
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}
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If any module's own AUTOLOAD subroutine has no need to fallback to the
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AutoLoader's AUTOLOAD subroutine (because it doesn't have any AutoSplit
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subroutines), then that module should not use B<AutoLoader> at all.
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=head2 Package Lexicals
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Package lexicals declared with C<my> in the main block of a package
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using B<AutoLoader> will not be visible to auto-loaded subroutines, due to
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the fact that the given scope ends at the C<__END__> marker. A module
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using such variables as package globals will not work properly under the
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B<AutoLoader>.
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The C<vars> pragma (see L<perlmod/"vars">) may be used in such
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situations as an alternative to explicitly qualifying all globals with
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the package namespace. Variables pre-declared with this pragma will be
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visible to any autoloaded routines (but will not be invisible outside
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the package, unfortunately).
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=head2 B<AutoLoader> vs. B<SelfLoader>
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The B<AutoLoader> is similar in purpose to B<SelfLoader>: both delay the
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loading of subroutines.
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B<SelfLoader> uses the C<__DATA__> marker rather than C<__END__>.
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While this avoids the use of a hierarchy of disk files and the
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associated open/close for each routine loaded, B<SelfLoader> suffers a
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startup speed disadvantage in the one-time parsing of the lines after
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C<__DATA__>, after which routines are cached. B<SelfLoader> can also
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handle multiple packages in a file.
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B<AutoLoader> only reads code as it is requested, and in many cases
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should be faster, but requires a mechanism like B<AutoSplit> be used to
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create the individual files. L<ExtUtils::MakeMaker> will invoke
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B<AutoSplit> automatically if B<AutoLoader> is used in a module source
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file.
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=head1 CAVEATS
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AutoLoaders prior to Perl 5.002 had a slightly different interface. Any
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old modules which use B<AutoLoader> should be changed to the new calling
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style. Typically this just means changing a require to a use, adding
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the explicit C<'AUTOLOAD'> import if needed, and removing B<AutoLoader>
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from C<@ISA>.
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On systems with restrictions on file name length, the file corresponding
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to a subroutine may have a shorter name that the routine itself. This
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can lead to conflicting file names. The I<AutoSplit> package warns of
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these potential conflicts when used to split a module.
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AutoLoader may fail to find the autosplit files (or even find the wrong
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ones) in cases where C<@INC> contains relative paths, B<and> the program
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does C<chdir>.
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=head1 SEE ALSO
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L<SelfLoader> - an autoloader that doesn't use external files.
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=cut
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