Re^2: testing for undef in perl XS code (in DBD::ODBC)

I'm finding

  if(valuesv == &sv_undef)
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never tests true and

!SvOK(valuesv)
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tests true when valuesv is undef - of course. What I'm not sure is what the original author might have meant by the first test above - perhaps it should have been SvOK in which case it was bug, perhaps something else.

Comment on Re^2: testing for undef in perl XS code (in DBD::ODBC) Select or Download Code

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Re^3: testing for undef in perl XS code (in DBD::ODBC) by syphilis (Archbishop) on Nov 06, 2007 at 12:59 UTC
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);` [download] 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	[reply] [d/l] [select]
Re^4: testing for undef in perl XS code (in DBD::ODBC) by salva (Canon) on Nov 06, 2007 at 17:19 UTC
`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!	[reply] [d/l] [select]
Re^5: testing for undef in perl XS code (in DBD::ODBC) by syphilis (Archbishop) on Nov 07, 2007 at 06:39 UTC
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! Doh!! ... thanks salva. Cheers, Rob	[reply]
Re^3: testing for undef in perl XS code (in DBD::ODBC) by salva (Canon) on Nov 06, 2007 at 11:58 UTC
oh, I am pretty sure it is a bug!	[reply]