How to change the code such that it prints out whether it is an old socket or new socket accordingly?
if (FD_ISSET(sock_fd, &listen_fds)) {
            int client_fd = accept_connection(sock_fd, users);
            if (client_fd > max_fd) {
                max_fd = client_fd;
            }
            FD_SET(client_fd, &all_fds);
            write(client_socket, "What is your user name?\n", 24);
        }
    
