The FLI :c-array type converts between FLI arrays and the C array type. In C, pointers are used to access the elements of an array. The implementation of the :c-array type takes this into account, by automatically dereferencing any pointers returned when accessing an array using foreign-aref.
When using the :c-array type in the specification of an argument to define-foreign-function, a pointer to the array is passed to the foreign function, as specified by the C language. You are allowed to call the foreign function with a FLI pointer pointing to an object of type type instead of a FLI array.
When using the :c-array type in other situations, it acts as an aggregate type like :foreign-array. In particular, :c-array with more than one dimension is an array containing embedded arrays, not an array of pointers.
:c-array uses the C convention that the first index value of an array is 0.:c-array type when the corresponding C code uses an array with a constant declared size. If you need a dynamically sized array, then use a pointer type, allocate the array using the nelems argument to allocate-foreign-object or with-dynamic-foreign-objects and use dereference to access the elements. The pointer type is more efficient than making :c-array types dynamically with different dimensions because the FLI caches information about every different FLI type descriptor that is used.The following code defines a 3 by 3 array of integers.
(setq aaa (fli:allocate-foreign-object
:type '(:c-array :int 3 3)))
The type of this is equivalent to the C declaration
int aaa[3][3];
The next example defines an array of arrays of bytes.
(setq bbb (fli:allocate-foreign-object
:type '(:c-array (:c-array :byte 3) 2)))
The type of this is equivalent to the C declaration
int bbb[2][3];
Note the reversal of the 3 and 2.
See foreign-aref and foreign-array-pointer for more examples on the use of arrays.
LispWorks Foreign Language Interface User Guide and Reference Manual - 29 Sep 2017