-callback(b, BIO_CB_CTRL, parg, cmd, larg, 1L) is called before the call and
-callback(b, BIO_CB_CTRL|BIO_CB_RETURN, parg, cmd, larg, ret) after.
+ callback_ex(b, BIO_CB_CTRL, parg, 0, cmd, larg, 1L, NULL)
+
+or
+
+ callback(b, BIO_CB_CTRL, parg, cmd, larg, 1L)
+
+is called before the call and
+
+ callback_ex(b, BIO_CB_CTRL | BIO_CB_RETURN, parg, 0, cmd, larg, ret, NULL)
+
+or
+
+ callback(b, BIO_CB_CTRL|BIO_CB_RETURN, parg, cmd, larg, ret)
+
+after.
+
+Note: B<cmd> == B<BIO_CTRL_SET_CALLBACK> is special, because B<parg> is not the
+argument of type B<BIO_info_cb> itself. In this case B<parg> is a pointer to
+the actual call parameter, see B<BIO_callback_ctrl>.