Callback Example (CMUCL User’s Manual)

8.7.5 Callback Example

Here is a simple example of using callbacks.

(use-package :alien)
(use-package :c-call)

(def-callback foo (int (arg1 int) (arg2 int))
  (format t "~&foo: ~S, ~S~%" arg1 arg2)
  (+ arg1 arg2))

(defun test-foo ()
  (callback-funcall foo 555 444444))

In this example, the callback function foo is defined which takes two C int parameters and returns a int. As this shows, we can use arbitrary Lisp inside the function.

The function test-foo shows how we can call this callback function from Lisp. The macro callback extracts the necessary information for the callback function foo which can be converted into a pointer which we can call via alien-funcall.

The following code is a more complete example where a foreign routine calls our Lisp routine.

(use-package :alien)
(use-package :c-call)

(def-alien-routine qsort void
  (base (* t))
  (nmemb int)
  (size int)
  (compar (* (function int (* t) (* t)))))

(def-callback my< (int (arg1 (* double))
                       (arg2 (* double)))
  (let ((a1 (deref arg1))
        (a2 (deref arg2)))
    (cond ((= a1 a2)  0)
          ((< a1 a2) -1)
          (t         +1))))

(defun test-qsort ()
  (let ((a (make-array 10 :element-type 'double-float
                       :initial-contents '(0.1d0 0.5d0 0.2d0 1.2d0 1.5d0
                                           2.5d0 0.0d0 0.1d0 0.2d0 0.3d0))))
    (print a)
    (qsort (sys:vector-sap a)
           (length a)
           (alien-size double :bytes)
           (alien:callback my<))
    (print a)))

We define the alien routine, qsort, and a callback, my<, to determine whether two double’s are less than, greater than or equal to each other.

The test function test-qsort shows how we can call the alien sort routine with our Lisp comparison routine to produce a sorted array.