Re: Problems with nested structs and the newer cffi

On Wed, Apr 24, 2013 at 4:35 AM, Willem Rein Oudshoorn woudshoo@xs4all.nlwrote:

Thank you for all your work and the time to answer my questions.

Liam Healy lnp@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-pointerhttp://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