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Postings from May 2004
Part of one of XS API (let's say foo()) goes:
if (items == 1 && ST(0) == &PL_sv_undef)) { ... }
if that API is called as:
foo(undef);
then the if condition is true. However if I do:
$x = undef;
foo($x);
it's false, i.e. ST(0) is no longer == &PL_sv_undef. I did a quick check with
Devel::Peek:
% perl -le 'use Devel::Peek; Dump undef; $x = undef; Dump $x'
SV = NULL(0x0) at 0x804c6e4
REFCNT = 2147483624
FLAGS = (READONLY)
SV = NULL(0x0) at 0x8065690
REFCNT = 1
FLAGS = ()
which are different. And I've checked the C addresses, passing undef, gives
the same address as &PL_sv_undef, passing $x = undef, gives a different
address. It makes sense to me since in the latter case we have a real sv with
its own address, in the previous case we have a shared sv which always has the
same address.
So how do I check in C whether the passed variable (or a random sv) is undef
or not if comparing to &PL_sv_undef doesn't work? am I missing some obvious
RTFM section? I've read: 'perlguts:=head2 AVs, HVs and undefined values'
but it doesn't seem to be related. There were many other references to undef
in the perl manpages, but I didn't find anything useful.
Thanks
--
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Stas Bekman JAm_pH ------> Just Another mod_perl Hacker
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