Problems with nested structs and the newer cffi

[Liam Healy]

On Wed, Apr 24, 2013 at 4:35 AM, Willem Rein Oudshoorn
<woudshoo at xs4all.nl>wrote:

>
> Thank you for all your work and the time to answer my questions.
>
> Liam Healy <lnp at healy.washington.dc.us> writes:
>
> > Willem,
> >
> > Thanks for the report.
> >
> > My thinking is that with-foreign-slots is intended to expose the value
> (and
> > not the pointer), and therefore, expands to foreign-slot-value, so the
> > behavior you're seeing is correct. Your fix to your code is the correct
> way
> > to access the pointer. I think with-foreign-slots is provided as a
> > convenient shortcut to get all the values; since it doesn't do what you
> > need, you need to use the actual access form (foreign-slot-pointer in
> your
> > case).
>
> Ah, ok, I am a bit struggling to convert the old way to the new way.  It
> probably has nothing to do with (with-foreign-slots ...), but just my
> mis understanding and trying to quickly convert old code to new.
>

Well, there's a bit of unwritten convention being used, so confusion is
understandable (and I had to think about it and infer what
with-foreign-slots is intended to do). Before we introduced
structures-by-value, this wasn't an issue.

>
> > For your second question: if the argument is actually a pointer to the
> > structure, :pointer is the right thing to use. Are you sure it is a
> pointer
> > argument? Check the .h file where it is defined.
> >
>


> Ah, I do not have an issue with passing it to the c library.
> What I meant was that in my mind the following confused me:
>
>      (let ((c-oid (foreing-alloc '(:struct git-oid))))
>         ;;; I think of c-oid, conceptually as type
>         ;;;  (:pointer (:struct git-oid))
>         ;;;; later:
>
>            (foreign-slot-pointer c-oid '(:struct git-oid) 'id)
>            ;;; I thought that because c-oid is of type
>            ;;; (:pointer (:struct git-oid))
>            ;;; I thought I needed to put here
>            ;;;   '(:pointer (:struct git-oid)) instead of
>            ;;;   '(:struct git-oid)
>
> Does this make sense?
>

The documentation to
foreign-slot-pointer<http://common-lisp.net/project/cffi/manual/html_node/foreign_002dslot_002dpointer.html>
 says
" ptr
A pointer to a structure.
type
A foreign structure type. "

so it makes sense to me the that first argument is a pointer to a
structure, and the second argument is the type of that structure (and not
the type of the first argument, i.e., the pointer type).


>
> Now I have a small additional question.
>
> I have a struct like:
>
> (defcstruct (git-index-time :class index-time-struct)
>             ....)
>
> Now the (translate-from-foreign value (type index-time-struct))
> works if I use as type:
>
>   (:struct git-index-time)
>
> So everyting works.   However, if I do:
>
> (defctype struct-index-time (:struct git-index-time))
>
> And use as type:
>
>   struct-index-time
>
> The `translate-from-foreign` is not called and I end up
> with untranslated values.
>
> I thought that (defctype ...) worked as a typedef and
> naively expected the type translation to still work.
>
> Is this as expected?
>
>
No, that seems like a CFFI error. Per Luis, a bug report would be helpful.

Liam
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