A sequence of one or more slots making up the union.
A symbol naming the slot.
The slot type. If no type is given, it defaults to an
type is an aggregate type, and converts between a FLI union and a C
type. The FLI union consists of a collection of one or more slots, only one of which can be active at any one time. The size of the whole union structure is therefore equal to the size of the largest slot. Each slot has a name and a type.
The foreign-slot-names, foreign-slot-type, and foreign-slot-value functions can be used to access and change the slots of the union. The convenience FLI function define-c-union is provided to simplify the definition of unions.
(setf (fli:foreign-slot-value length 'small))
LispWorks Foreign Language Interface User Guide and Reference Manual - 7 Dec 2011