On Sun, Dec 12, 2004 at 07:54:32AM +0000 Sisyphus wrote:
> Tassilo von Parseval wrote:
>
>
> >
> >PL_sv_yes looks like this:
> >
> > SV = PVNV(0x814d6b8) at 0x814c60c
> > REFCNT = 2147483647
> > FLAGS = (NOK,POK,READONLY,pNOK,pPOK)
> > IV = 0
> > NV = 1
> > PV = 0x814da80 "1"\0
> > CUR = 1
> > LEN = 2
> >
>
> Yes - I wrote a little module that uses overloading and did a
> Devel::Peek::Dump() of the third argument - and got essentially the same
> as that when the third overload argument was TRUE. I also found that
> PL_sv_no (which is the FALSE value that the third overload argument can
> take on) is very similar - except you get:
>
> PV = 0xblah ""\0
> CUR = 0
> LEN = 1
>
> (On occasion that third arg can be PL_sv_undef - I'll Dump() it, too,
> when I get around to it.)
>
> Ok - so I can assign PL_sv_yes in C-space by doing:
>
> sv = &PL_sv_yes; (should have realised.)
>
> How does it get done in perl-space ?
It's almost impossible. The problem is that the pointer to PL_sv_yes is
destroyed once you allow an assignment:
use Inline C => <<'EOC';
SV* yes () {
return &PL_sv_yes;
}
void get_yes (SV *a) {
if (a == &PL_sv_yes)
printf("Received PL_sv_yes\n");
else
printf("Something else\n");
}
EOC
get_yes(yes());
$a = yes();
get_yes($a);
__END__
Received PL_sv_yes
Something else
That's because an assignment in the end means that Perl_sv_setsv_flags
is called and that wont preserve the pointer.
> It seems that overload.pm does it reliably, though I can't see where and
> how it achieves it. There doesn't appear to be any shared object
> associated with the overload module, so I take it that
> PL_sv_yes/no/undef is being assigned from perl-space.
The overloading mechanism is deeply hooked into the perl source. Passing
around &PL_sv_yes is easy in XS and C. And that's what perl probably
does. It puts &PL_sv_yes onto the stack and so your overload handler
will receive an intact pointer to PL_sv_yes. You can only do that from
Perl-space if you create an XSUB that returns &PL_sv_yes (as shown
above).
Tassilo
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