I think there is a bug in CFFI with CCL's 32-bit version: (cffi:defcunion foo-union (int16 :int16 ) (uint64 :uint64 ) ) (cffi:defcstruct bar-struct (tag :uint8) (val foo-union)) (defun foo () (cffi:with-foreign-string (data "0123456789" :encoding :latin-1) (- (cffi:pointer-address (cffi:foreign-slot-value data 'bar-struct 'val)) (cffi:pointer-address data)))) FOO returns 4 in SBCL and Lispworks but with CCL it's 8. Helmut