there's a problem with WITH-FOREIGN-SLOTS that shows up when a c-struct is defined in one package and used from another one. here's an example: (in-package :cl-user) (defpackage :internals (:use :cl :cffi)) (defpackage :mypackage (:use :cl :cffi)) (in-package :internals) (defcstruct foo (x :int)) (in-package :mypackage) (with-foreign-object (fooptr 'internals::foo) (with-foreign-slots ((x) fooptr internals::foo) x)) (with-foreign-slots ((x) fooptr internals::foo) x) gets expanded to (LET ((#:PTR2455 FOOPTR)) (SYMBOL-MACROLET ((X (FOREIGN-SLOT-VALUE #:PTR2455 'INTERNALS::FOO 'X))) X)) instead of (LET ((#:PTR2455 FOOPTR)) (SYMBOL-MACROLET ((X (FOREIGN-SLOT-VALUE #:PTR2455 'INTERNALS::FOO 'INTERNALS::X))) X)) ; ^^^^^^^^^^^ the attached patch fixes this -- (sign :name "Stelian Ionescu" :aka "fe[nl]ix" :quote "Quidquid latine dictum sit, altum sonatur.")