-static ssize_t ac_read (struct file *, char *, size_t, loff_t *);
-static ssize_t ac_write (struct file *, const char *, size_t, loff_t *);
+static ssize_t ac_read (struct file *, char __user *, size_t, loff_t *);
+static ssize_t ac_write (struct file *, const char __user *, size_t, loff_t *);