The function null-pointer-p is used to determine if a pointer is a null pointer. A null pointer is a pointer pointing to address 0.
If pointer is a null pointer (that is, a pointer pointing to address 0) then result is true, otherwise null-pointer-p returns false.
In the following example a pointer to an :int is defined, and tested with null-pointer-p. The pointer is then freed, becoming a null pointer, and is once again tested using null-pointer-p.
(setq point (fli:allocate-foreign-object :type :int))
(fli:null-pointer-p point)
(fli:free-foreign-object point)
(fli:null-pointer-p point)
Pointer testing functions
Testing whether a function is defined
*null-pointer*
pointer-address
pointer-eq
LispWorks Foreign Language Interface User Guide and Reference Manual - 29 Sep 2017