What am i doing wrong in my conversion of this c code to lisp I think the issue is with my callback function but not 100% when i run the c code and print with these 2 lines(from c callback): cout << "a = " << "(" << "" << "," << _a << ")" << endl ;           cout << "b = " << "(" << "" << "," << _b << ")" << endl;  i get 53 elements printed not icluding the other printf output and  in the lisp code i converterd when i print with this line(from lisp defcallback): (format t "~%x~a~%b~a" _a _b)  in the defcallback  i get 114 elements Hoping someone can look at my code and tell me if my coding is off the -> is a macro for with-foreign-slots (and if not a pointer just a getf for a plist) ;;;;;;;;C CODE;;;;;;;;;;;;;;;;;;;       static int cmp_func( const void* _a, const void* _b, void* userdata )       {           CvPoint* a = (CvPoint*)_a;           CvPoint* b = (CvPoint*)_b;           cout << "a = " << "(" << "" << "," << _a << ")" << endl ;           cout << "b = " << "(" << "" << "," << _b << ")" << endl;           int y_diff = a->y - b->y;           int x_diff = a->x - b->x;           return y_diff ? y_diff : x_diff;       }   ;;;;;;;;;;;;;;;;;;;;;;;;;LISP CODE;;;;;;;;;;;;;;;;;;;; (defcallback cmp-func :void  ((_a :pointer) (_b :pointer) (user-data :pointer) (a :int) (b :int) (y-diff :int) (x-diff :int))   (format t "~%x~a~%b~a" _a _b)   (setf a (mem-aref _a '(:struct cv-point)))   (setf b (mem-aref _b '(:struct cv-point)))                     ;(format t "~%a~a~%b~a" a b)   (setf y-diff (- (-> cv-point a y) (-> cv-point b y)))   (setf x-diff (- (-> cv-point a x) (-> cv-point b x)))   (if y-diff y-diff x-diff)) using callback with (callback cmp-func) in the OpenCV function cvSeqSort ....here is documentation for cvSeqSort http://docs.opencv.org/modules/core/doc/dynamic_structures.html?highlight=cv... the code the wrapper is used in is posted in the "Help with defcallback"  post below this one