4.2.5 Calling a C function that takes an array of strings

Suppose you have a C function declared like this:

To call this from Lisp you need to first allocate the foreign memory for each piece of data, that is the array itself and each string. Assuming that foo does not capture any of the pointers, you can give this memory dynamic extent as follows:

(defun convert-to-dynamic-foreign-array (strings)

  (let* ((count (length strings))

         (array

          (fli:allocate-foreign-object

           :nelems (1+ count) ; assume NULL terminated

           :type '(:pointer :char))))

    (dotimes (index count)

      (setf (fli:dereference array :index index)

            (fli:convert-to-dynamic-foreign-string

             (elt strings index))))

    (setf (fli:dereference array :index count) nil)

    array))

(fli:define-foreign-function (%foo foo)

    ((string-array (:pointer (:pointer :char)))))

(defun foo (strings)

  (fli:with-dynamic-foreign-objects ()  ; provide a dynamic scope

    (%foo (convert-to-dynamic-foreign-array strings))))