if(valuesv == &sv_undef)
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!SvOK(valuesv)
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I have a question about this:

use warnings;
use strict;

use Inline C => Config =>
    BUILD_NOISY => 1;

use Inline C => <<'EOC';

SV * create_undef() {
     return newSVsv(&PL_sv_undef);
} 

void is_defined(SV * x) {
     //x = &PL_sv_undef;
     x == &PL_sv_undef ? printf("Not defined\n") : printf("Defined\n")
+;
}

EOC

my $undef = create_undef();
is_defined($undef);
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Note that $undef is created (by 'create_undef()') with the value &PL_sv_undef, and that the 'is_defined()' function then tests to see whether $undef == &PL_sv_undef. Amazingly, 'is_defined()' reports that $undef != &PL_sv_undef (by stating that $undef is defined).

But if you uncomment that one line of C code that is excluded in the above script, then you find that $undef == &PL_sv_undef (as 'is_defined()' now states that $undef is undefined).

I think we're dealing with the fact &PL_sv_undef is the value associated with a certain memory location, and that the value associated with a certain memory location changes from one function to the next. salva (or anyone else, for that matter), is that a valid approximation of what's happening ?

Cheers,
Rob

PL_sv_undef is a perl scalar that has the value undef.

There can be other perl scalars with the undef value. Actually, you can create then using SV *a = newSV(0).

With a == &PL_sv_undef you are not testing if they contain the same value but if they are both pointers to the same scalar!

oh, I am pretty sure it is a bug!