RE: Problems with data returned from Gtk2::SimpleList

it's definitely not a problem with list-vs-scalar context,
something deeper goes on. Can you try the attached minimal
example (untar, "perl Makefile.PL", "make", and then run
It mimicks a call to Gtk2::Image->new_from_file($n)
where $n is (1) a simple scalar and (2) a tied scalar.
You should see a "FETCH(...)" when the tied scalar's FETCH
method is invoked. simple() is a minimal XS sub that
just passes its only parameter down (as an SV*).
I get the following output for Perl 5.8.8 on Debian Linux
(same for Perl 5.8.7 from ActiveState and cygwin):

simple(red.png) at line 16.
FETCH(Foo=SCALAR(0x81da484)) at line 6.
        # OK: tied scalar's FETCH was invoked for simple($tied_scalar)
simple(blue.png) at line 17.
gperl_filename_from_sv: got "red.png" at line 22.
gperl_filename_from_sv: dup "red.png" at line 22.
gtk_image_new_from_file(red.png) at line 22.
        # now calling Bar->gtk_image_new_from_file($tied_scalar)
        # not OK: FETCH missing
gtk_image_new_from_file((null)) at line 23.
        # finally a NULL pointer was passed down

Further inspection shows that the problem is the
filename parameter to the XS sub. It's declared 
as a GPerlFilename_ornull. The latter has an input mapping of

$var = ($type)(($arg && SvOK ($arg))
                 ? gperl_filename_from_sv ($arg)
                 : NULL)

Turns out that SvOK($tied_scalar) is false (but
SvMAGICAL($tied_scalar) is true). That explains why NULL
is passed down.

I tried the same on an old Perl 5.8.0 on Solaris
and there I get the expected second FETCH and indeed
SvOK($tied_scalar) is true there.

AFAICT the check "$arg && SvOK ($arg)" is supposed to be
the XS equivalent of "defined $arg"? Apparently that's
broken for recent Perls when $arg is a tied variable.

Cheers, Roderich

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