+;;;; User data mechanism
+
+(internal *user-data* *user-data-count*)
+
+(declaim (fixnum *user-data-count*))
+
+(defvar *destroy-notify* (system:foreign-symbol-address "destroy_notify"))
+(defvar *user-data* (make-hash-table))
+(defvar *user-data-count* 0)
+
+(defun register-user-data (object &optional destroy-function)
+ (check-type destroy-function (or null symbol function))
+ (incf *user-data-count*)
+ (setf
+ (gethash *user-data-count* *user-data*)
+ (cons object destroy-function))
+ *user-data-count*)
+
+(defun find-user-data (id)
+ (check-type id fixnum)
+ (multiple-value-bind (user-data p) (gethash id *user-data*)
+ (values (car user-data) p)))
+
+(defun destroy-user-data (id)
+ (check-type id fixnum)
+ (let ((user-data (gethash id *user-data*)))
+ (when (cdr user-data)
+ (funcall (cdr user-data) (car user-data))))
+ (remhash id *user-data*))
+
+