... ctx = modbus_new_tcp("127.0.0.1", 502); if (modbus_connect(ctx) == -1) { fprintf(stderr, "Connection failed: %s\n", modbus_strerror(errno)); modbus_free(ctx); return -1; } /* Handle until 10 established connections */ server_socket = modbus_tcp_listen(ctx, 10); /* Clear the reference set of socket */ FD_ZERO(&refset); /* Add the server socket */ FD_SET(server_socket, &refset); if (select(server_socket + 1, &refset, NULL, NULL, NULL) == -1) { } ... close(server_socket); modbus_free(ctx);