The FLI :boolean type converts between a Lisp boolean value and a C representation of a boolean value. The encapsulates option is used to specify the size of the value from which the boolean value is obtained. For example, if a byte is used in C to represent a boolean, the size to map across for the FLI will be one byte, but if an int is used, then the size will be four bytes.
A value of 0 in C represents a nil boolean value in Lisp, and a non-zero value in C represents a t boolean value in Lisp.
In the following three examples, the size of a :boolean, a (:boolean :int) and a (:boolean :byte) are returned.
(fli:size-of :boolean)
(fli:size-of '(:boolean :int))
(fli:size-of '(:boolean :byte))
LispWorks Foreign Language Interface User Guide and Reference Manual - 16 Feb 2015