ctx = modbus_new_tcp("127.0.0.1", 1502); server_socket = modbus_tcp_listen(ctx, NB_CONNECTION); FD_ZERO(&rdset); FD_SET(server_socket, &rdset); /* .... */ if (FD_ISSET(master_socket, &rdset)) { modbus_set_socket(ctx, master_socket); rc = modbus_receive(ctx, query); if (rc != -1) { modbus_reply(ctx, query, rc, mb_mapping); } }