int sd_bus_open_user(sd_bus **ret);
int sd_bus_open_system(sd_bus **ret);
int sd_bus_open_system_remote(const char *host, sd_bus **ret);
int sd_bus_open_user(sd_bus **ret);
int sd_bus_open_system(sd_bus **ret);
int sd_bus_open_system_remote(const char *host, sd_bus **ret);