Re: Problems with nested structs and the newer cffi

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).

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.

Liam

On Sat, Apr 20, 2013 at 4:40 AM, Willem Rein Oudshoorn woudshoo@xs4all.nlwrote:

I am encountering some problems with trying to upgrade my old cffi code to the new version from quicklisp. If I use the old code, I get lots of warnings about my structs, that I should use either (:struct ...) or (:pointer (:struct ...))

However whateer I try, I do not seem to get it to work, and I am starting to think there is some issue between me and the new cffi.

The main issue is a nested struct definition like this:

(defcstruct timeval
  (time %time)
  (offset :int))

(defcstruct (git-signature)
  (name :string)
  (email :string)
  (time (:struct timeval)))

With type definitions

(define-foreign-type time-type ()
  nil
  (:actual-type :int64)
  (:simple-parser %time))

(define-foreign-type git-signature-type ()
  nil
  (:actual-type :pointer)
  (:simple-parser %git-signature))

I encounter problems when I am trying to update the `translate-to-foreign` for the `git-signature-type`. The implementation boils down to:

(defmethod translate-to-foreign ((value list) (type git-signature-type))
  (let ((signature (foreign-alloc '(:struct git-signature))))
    (with-foreign-slots ((name email time) signature (:struct
git-signature))
      (setf name (getf value :name (getenv "USER")))
      (setf email (getf value :email (default-email)))
      (with-foreign-slots ((time offset) time (:struct timeval))
        (let ((time-to-set (getf value :time (local-time:now))))
          (setf time time-to-set)
          (setf offset (/ (local-time:timestamp-subtimezone
                           time-to-set local-time:*default-timezone*)
                          60)))))
    signature)

The problem is in the inner `with-foreign-slots ((time offset)...`.

The error I get is:

; Evaluation aborted on #<TYPE-ERROR expected-type: SYSTEM-AREA-POINTER datum: (CL-GIT::OFFSET 0 TIME @1970-01-01T01:00:03.000000+01:00)>.

The reason is, as far as I can tell, that in the code:

(with-foreign-slots ((time offset) time (:struct timeval)) ...

the value of `time` is already a plist (due to the outer with-foreign-slots), and the inner with-foreign-slots expects a pointer.

So what is the best way of fixing this? I can fix this by replacing the inner `with-foreign-slots` by

      (with-foreign-slots ((time offset)
                           (foreign-slot-pointer signature '(:struct
git-signature) 'time)
                           (:struct timeval))

which seems a bit long.

An additional question is, when to use (:struct ...) and when to use (:pointer (:struct ..)))

I did expect that I needed to use (:pointer (:struct ..)) in the toplevel `with-foreign-slots`, because the argument is actually a pointer to the struct. But that did not work.

As you can tell, I am a bit confused. Hope that someone can give some hints to clear my head.

Kind regards, Wim Oudshoorn.