A foreign object, a pointer to a foreign object, or a foreign structure or union type.
A symbol or a list of symbols identifying the slot to be accessed, as described for foreign-slot-value.
foreign-slot-offset returns the offset, in bytes, of a slot in a FLI object. The offset is the number of bytes from the beginning of the object to the start of the slot. For example, the offset of the first slot in any FLI object is 0.
(fli:foreign-slot-offset 'COMPASS 'north)
(setq dir (fli:allocate-foreign-object :type 'COMPASS))
(fli:foreign-slot-offset dir 'north)
LispWorks Foreign Language Interface User Guide and Reference Manual - 29 Sep 2017