I've extended with-foreign-slots to return slot pointers in this fashion (basically I took this macro with minor modifications but named it with-foreign-slots).

Liam

On Thu, Apr 25, 2013 at 10:23 PM, Liam Healy <lnp@healy.washington.dc.us> wrote:
On Thu, Apr 25, 2013 at 4:33 AM, Chris Bagley <chris.bagley@gmail.com> wrote:
I ran into similar issues to Willem and have taken to using the following macro:

(defmacro with-foreign-slots* ((vars ptr type) &body body)
  "Create local symbol macros for each var in VARS to reference
foreign slots in PTR of TYPE. Similar to WITH-SLOTS.
Each var can be of the form: slot-name - in which case slot-name will
be bound to the value of the slot or: (:pointer slot-name) - in which
case slot-name will be bound to the pointer to that slot."
  (let ((ptr-var (gensym "PTR")))
    `(let ((,ptr-var ,ptr))
       (symbol-macrolet
           ,(loop :for var :in vars :collect
                 (if (listp var)
                     (if (eq (first var) :pointer)
                         `(,(second var) (foreign-slot-pointer
                                          ,ptr-var ',type ',(second var)))
                         (error "Malformed var names must be:~%name~% -or- ~%(:pointer name)"))
                     `(,var (foreign-slot-value
                             ,ptr-var ',type ',var))))
         ,@body))))

This, for me, balances the needs quite nicely and feels in line with the style of cffi. Thoughts or  modifications?

I had thought about doing something like this when looking into Willem's problems. I like it, but I think the "*" in the name is unnecessary. It is upwardly compatible with the current with-foreign-slots, isn't it? That is, if it were to replace with-foreign-slots, it would expand correctly for current usage (to values).  I think we should replace that macro with this.

Liam