* @param target the target peer to which the stream has to be opened
* @param app_port the application port number which uniquely identifies this
* stream
- * @param open_cb this function will be called after stream has be established
+ * @param open_cb this function will be called after stream has be established;
+ * cannot be NULL
* @param open_cb_cls the closure for open_cb
* @param ... options to the stream, terminated by GNUNET_STREAM_OPTION_END
* @return if successful it returns the stream socket; NULL if stream cannot be