;; This seems to solve the problem above


;; callback used by our C-function. It returns itself, thus, the
;; result of C_callback is the callback function itself -
;; this is useful because sometimes this callback-function
;; is moved by the GC! So the original callback would point
;; to the old location and be invalid.
(define (callback-wrapper proc)
  (let* ([self #f]
         [cbwrap (lambda (cp-pointer) ; coming from C-land
                   (proc cp-pointer)
                   self)])
    (set! self cbwrap)
    cbwrap))

(define (cp:for-each/shape space callback) 
  ((foreign-safe-lambda void "forEachShape" (c-pointer "cpSpace") scheme-object)
   space (callback-wrapper callback)))