:one-of type is used to allocate an object which can be one of a number of types. The types must have the same underlying structure, which means they must have the same size and must be referenced in the same manner. The FLI
:one-of type is useful when a foreign function returns a value whose underlying type is known, but whose exact type is not.
thing is set to be 100 using
dereference, it is taken to be an object of type
:int, as this is the first element in the sequence of types defined by
:one-of which matches the type of the number 100.
(setf (fli:dereference thing) 100)
thing is now dereferenced, it is returned as a pointer to the address 100 (Or hex address 64), as there is no method for determining the type of
thing, and therefore the first element in the list of
:one-of is used.
LispWorks Foreign Language Interface User Guide and Reference Manual - 29 Sep 2017