Hi. I want to use the following C function: ;; void ClearBackground(Color color); Color is C Struct: ;; typedef struct Color { ;; unsigned char r; // Color red value ;; unsigned char g; // Color green value ;; unsigned char b; // Color blue value ;; unsigned char a; // Color alpha value ;; } Color; So I wrote: (cffi:defcstruct %color (r :uint8) (g :uint8) (b :uint8) (a :uint8)) (defvar *%lightgray* (cffi:foreign-alloc '(:struct %color))) (cffi:with-foreign-slots ((r g b a) *%lightgray* (:struct %color)) (setf (cffi:foreign-slot-value *%lightgray* '(:struct %color) 'r) 200) (setf (cffi:foreign-slot-value *%lightgray* '(:struct %color) 'g) 200) (setf (cffi:foreign-slot-value *%lightgray* '(:struct %color) 'b) 200) (setf (cffi:foreign-slot-value *%lightgray* '(:struct %color) 'a) 255) (format t "~s ~s ~s ~s" r g b a)) ;; void ClearBackground(Color color); (cffi:defcfun ("ClearBackground" %clear-background) :void (color (:struct %color))) (defun clear-background (color) (%clear-background color)) But when i run (clear-background *%lightgray*) That does not work, I have already added my .asd file ":cffi-libffi" as dependency. What is the problem? If this issue has no solution, is there an alternate way?