Please disregard this query. I see that I was clueless and apologize.
I had forgotten my old C knowledge, and in particular, had forgotten
that the desire to have multiple values, combined with call-by-value,
means that one often must pass pointers to pointers as arguments, and
then to get the pointer, one dereferences....
Never mind!
Cheers,
R
Robert P. Goldman wrote:
> I wonder if anyone who's familiar with SBCL aliens can give me some
> advice in my ongoing translation of the freetype code into CFFI. IIUC
> (I quite possibly don't), a difference between the two is that SBCL
> has much more "understanding" of the foreign data structures, and that
> it is able to work directly with foreign structures. CFFI works more
> with pointers, and requires more explicit reference to the foreign
> types when dereferencing.
>
> I think I have mostly managed to translate the foreign data
> structures, and a good bit of the code. However, I am still having
> some trouble understanding a bit of the SBCL alien code, particularly
> the use of derefs when returning foreign values. For example, here's
> one of the original defuns:
>
> (defun make-concrete-font (vague-font size &key (dpi *dpi*))
> (with-slots (lib filename) vague-font
> (let* ((key (cons lib filename))
> (val (gethash key *concrete-font-hash*)))
> (unless val
> (let ((facef (make-alien freetype:face)))
> (declare (type (alien (* freetype:face)) facef))
> (if (zerop (freetype:new-face lib filename 0 facef))
> (setf val (setf (gethash key *concrete-font-hash*)
> (deref facef)))
> (error "Freetype error in make-concrete-font"))))
> (let ((face val))
> (declare (type (alien freetype:face) face))
> (freetype:set-char-size face 0 (round (* size 64)) (round dpi)
> (round dpi))
> face))))
>
> I'm not at all sure, moving to CFFI, how to translate the DEREF
> calls. CFFI is essentially always working with pointers, IIUC, so I'm
> wondering if I should just eliminate deref calls like this (i.e., just
> set the hash table entry to facef)...
>
> Any suggestions would be more than welcome.
>
>
> Thanks,
> R
>
>