:void

The FLI :void type represents the C void type. It can only be used in a few limited circumstances, as the:

result-type of a define-foreign-function, define-foreign-funcallable or define-foreign-callable form. In this case, it means that no values are generated.
element type of a :pointer type, that is (:pointer :void) . Any FLI pointer can be converted to this type, for example when used like this as the argument type in define-foreign-function.
element type of a FLI pointer when memory is not being allocated, for example in a call to make-pointer. It is an error to dereference a FLI pointer with element type :void (but with-coerced-pointer can be used).
expansion of a define-c-typedef or define-foreign-type form. The type defined in this way can only be used in situations where :void is allowed.

LispWorks Foreign Language Interface User Guide and Reference Manual - 7 Dec 2011