From deforest@boulder.swri.edu

The AutoLoader documentation page recommends using "goto &$AUTOLOAD"
to enter the newly declared subroutine. This munges the stack if the
declaration code includes loading XS modules. In particular, if the
subroutine is defined via "bootstrap newmodule" (this most often
happens with declarations that use Inline to autocompile a new XS
module), then a stray '1' is left on the Perl stack by the
declaration step. By using 'goto', the autoload subroutine preserves
the 1, which is then returned as an extra value(!) when the newly
declared subroutine returns.

In scalar context, the '1' is dropped on the floor; in list context,
it is interpolated into the returned list and appears as the first
value in that list.

In this case, the simplest workaround is to use "return &$AUTOLOAD
(@​_)" rather than "goto &$AUTOLOAD". That costs an extra stack
frame, but is more bulletproof.

I have tested this bug in Perl 5.8.6 and 5.8.8 under MacOS X and
5.8.8 under Fedora Core 4.

The bug could be fixed by updating the AutoLoad man page to use the
safer construction, or by looking at the dynamic loader to identify
why it is leaving an extra 1 on the stack. The former is less likely
to stir up trouble in other parts of Perl.

From @smpeters

On Tue Nov 06 23​:28​:01 2007, zowie wrote​:

The AutoLoader documentation page recommends using "goto &$AUTOLOAD"
to enter the newly declared subroutine. This munges the stack if the
declaration code includes loading XS modules. In particular, if the
subroutine is defined via "bootstrap newmodule" (this most often
happens with declarations that use Inline to autocompile a new XS
module), then a stray '1' is left on the Perl stack by the
declaration step. By using 'goto', the autoload subroutine preserves
the 1, which is then returned as an extra value(!) when the newly
declared subroutine returns.

In scalar context, the '1' is dropped on the floor; in list context,
it is interpolated into the returned list and appears as the first
value in that list.

In this case, the simplest workaround is to use "return &$AUTOLOAD
(@​_)" rather than "goto &$AUTOLOAD". That costs an extra stack
frame, but is more bulletproof.

I have tested this bug in Perl 5.8.6 and 5.8.8 under MacOS X and
5.8.8 under Fedora Core 4.

The bug could be fixed by updating the AutoLoad man page to use the
safer construction, or by looking at the dynamic loader to identify
why it is leaving an extra 1 on the stack. The former is less likely
to stir up trouble in other parts of Perl.

Do you have some code that demonstrates the problem? Its much easier to
investigate and test the problem using existing code rather than attempt
to devise it ourselves.

Thanks,

Steve Peters

The RT System itself - Status changed from 'new' to 'open'

From deforest@boulder.swri.edu

Hi, Steve,

Happy to supply my test case. It uses the PDL autoloader and Inline,
so you'll have to install those to tickle it.

The below code prints "1, Tickled!" for me with Perl 5.8.6 and 5.8.8
under MacOSX, with the current releases of Inline and PDL available
from CPAN.

Cheers,
Craig

----- write the following to 'tickle_bug.pdl'
sub tickle_bug {
  return tickler();
}

use Inline C=><<'EOF';

char *tickler() {
  return "Tickled!\n";
}
EOF
------ write the following to 'test.pl'
#!/usr/bin/perl
use PDL;
@​a = tickle_bug();
print join(", ",@​a),"\n";

On Nov 15, 2007, at 2​:27 PM, Steve Peters via RT wrote​:

On Tue Nov 06 23​:28​:01 2007, zowie wrote​:

The AutoLoader documentation page recommends using "goto &$AUTOLOAD"
to enter the newly declared subroutine. This munges the stack if the
declaration code includes loading XS modules. In particular, if the
subroutine is defined via "bootstrap newmodule" (this most often
happens with declarations that use Inline to autocompile a new XS
module), then a stray '1' is left on the Perl stack by the
declaration step. By using 'goto', the autoload subroutine preserves
the 1, which is then returned as an extra value(!) when the newly
declared subroutine returns.

In scalar context, the '1' is dropped on the floor; in list context,
it is interpolated into the returned list and appears as the first
value in that list.

In this case, the simplest workaround is to use "return &$AUTOLOAD
(@​_)" rather than "goto &$AUTOLOAD". That costs an extra stack
frame, but is more bulletproof.

I have tested this bug in Perl 5.8.6 and 5.8.8 under MacOS X and
5.8.8 under Fedora Core 4.

The bug could be fixed by updating the AutoLoad man page to use the
safer construction, or by looking at the dynamic loader to identify
why it is leaving an extra 1 on the stack. The former is less likely
to stir up trouble in other parts of Perl.

Do you have some code that demonstrates the problem? Its much
easier to
investigate and test the problem using existing code rather than
attempt
to devise it ourselves.

Thanks,

Steve Peters