Check RX/TX streamers::sptr before flush/reset Current code triggers a segfault when user uses the stream arg setter before starting the flowgraph. Another surprising use case that segfaults is tb->start(); usrp_source->stop(); which is a clever way to allow streaming to start when the user calls usrp_source->issue_stream_cmd(...) rather than as soon as GNU Radio runs without using set_start_time (e.g. when the start time is unknown at FG initialization).