Re: Passing C pointers through guile

["Kjetil S. Matheussen" <k.s.matheussen@notam02.no>]

Ludovic Court?s:
> Hi,
>
> "Maciek Godek" <pstrychuj@gmail.com> writes:
>
>> is there any portable and recommended way for passing C pointers around
>> in guile environment?
>> I think of something like scm_to_ptr (analogous to scm_to_int etc.).
>> Certainly such a value would be completely useless for the interpreter.
>> One can achieve simillar functionality using the aforementioned
>> scm_to_int (keeping track on the word length of the build target),
>> but it seems rather unnatural.
>
> If the goal is to make a C object pointed to by some pointer available
> to Scheme functions, the recommended (and simplest) way is to use a
> "SMOB" (see the manual for details).  In practice, SMOBs incur some
> storage overhead because they hold 4 machines words, while you may only
> need one.
>
> Another possibility is to use a "uo" field in a struct: it allows you to
> have a struct field holding a machine word inaccessible to Scheme
> functions (see the "Vtables" node of the manual).
>

I haven't heard of the "uo" field before, but at least using a SMOB
is really inconvenient, and using an unsigned long for storing
pointers is really convenient. (BTW. How does swig and gwrap handle
pointers?)

I agree with Maciek that it would at least be mind-comforting to have
functions like scm_to_ptr/etc, although not strictly necessary,
since using integers works just fine.