«

Also sprach Sisyphus:

> Sisyphus wrote:
>
>>
>> use warnings;
>> use Inline C => Config =>
>> BUILD_NOISY => 1; # to make sure we get to
>> # see compiler warnings
>>
>> use Inline C => <<'END_OF_C_CODE';
>>
>> void set_undef(SV * a) {
>>
>> sv_setsv(a, &PL_sv_undef);
>
> Another thread, and another list - and it has just been drawn to my
> attention that's not the right way to assign PL_sv_undef to an SV. The
> above line should be replaced by:
>
> a = &PL_sv_undef

Wont help either. You cannot change 'a' in-place thusly. If you want
that you'd have to have something like:

void set_undef (SV **a) {
*a = &PL_sv_undef;
}

But I don't think that Inline::C or XS know about an SV** prototype. The
reason is again the difference between changing a C-structure internally
(this is what sv_setsv does) or having a variable point to something
else. If you want to do the latter, you need to pass the pointer by
reference. These are the usual C-semantics.

>> if(a == &PL_sv_undef)
>> printf("A valid means of testing for &PL_sv_undef\n");
>>
>> else printf("An INVALID means of testing for &PL_sv_undef\n");
>>
>> }
>>
>
> I haven't tested, but I expect one would then find that the script
> reports "A valid means of testing for &PL_sv_undef".

Only if the variable passed to set_undef() was a real PL_sv_undef in the
first place. What you are thinking of is a no-op.

Tassilo
--
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Tassilo v. Parseval wrote:
> Also sprach Sisyphus:
>
>
>>Sisyphus wrote:
>>
>>
>>>use warnings;
>>>use Inline C => Config =>
>>> BUILD_NOISY => 1; # to make sure we get to
>>> # see compiler warnings
>>>
>>>use Inline C => <<'END_OF_C_CODE';
>>>
>>>void set_undef(SV * a) {
>>>
>>>sv_setsv(a, &PL_sv_undef);
>>
>>Another thread, and another list - and it has just been drawn to my
>>attention that's not the right way to assign PL_sv_undef to an SV. The
>>above line should be replaced by:
>>
>>a = &PL_sv_undef
>
>
> Wont help either. You cannot change 'a' in-place thusly. If you want
> that you'd have to have something like:
>
> void set_undef (SV **a) {
> *a = &PL_sv_undef;
> }
>
> But I don't think that Inline::C or XS know about an SV** prototype. The
> reason is again the difference between changing a C-structure internally
> (this is what sv_setsv does) or having a variable point to something
> else. If you want to do the latter, you need to pass the pointer by
> reference. These are the usual C-semantics.
>
>
>>>if(a == &PL_sv_undef)
>>> printf("A valid means of testing for &PL_sv_undef\n");
>>>
>>>else printf("An INVALID means of testing for &PL_sv_undef\n");
>>>
>>>}
>>>
>>
>>I haven't tested, but I expect one would then find that the script
>>reports "A valid means of testing for &PL_sv_undef".
>
>
> Only if the variable passed to set_undef() was a real PL_sv_undef in the
> first place. What you are thinking of is a no-op.
>
> Tassilo

A tree ... and some rope .... that's all I *really* need

Cheers,
Rob

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