int bus_connect_system_polkit(DBusConnection **_bus, DBusError *error);
const char *bus_error_message(const DBusError *error);
+const char *bus_error_message_or_strerror(const DBusError *error, int err);
typedef int (*BusPropertyCallback)(DBusMessageIter *iter, const char *property, void *data);
typedef int (*BusPropertySetCallback)(DBusMessageIter *iter, const char *property, void *data);