程式碼片：

    fd_set save_fds;
    int sockfd;
    struct sockaddr_in serv_addr;  //my address
    struct sockaddr_in client_addr;

    if((sockfd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP))== -1)
    {
        perror("socekt error");
        return -1;
    }
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_port = htons(SERVERPORT);
    serv_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    bzero(&(serv_addr.sin_zero),8);

    socklen_t sin_size = sizeof(struct sockaddr_in);

    assert(bind(sockfd,(struct sockaddr *)&serv_addr, sizeof(struct sockaddr_in)) == 0);

    FD_ZERO(&save_fds);
    FD_SET(sockfd,&save_fds);
    struct timeval timeout= {0,0}; //非阻塞就置0

    while(1)
    {
        <span style="color:#FF0000;"> fd_set read_fds = save_fds;</span>
         int result = select(sockfd + 1, &read_fds, 0, 0, &timeout);
         if(result >0 )
         {
             if(FD_ISSET(sockfd,&read_fds))
             {
                bzero(revbuf,BUFSIZE);
                int ret = recvfrom(sockfd, revbuf, BUFSIZE, 0, (struct sockaddr*) &client_addr, &sin_size);
                if (ret > 0)
                {

                    printf("receive buf is %s",revbuf);
                   
                }
             }
         }

         sleep(1);
         //.....

    }
 

FD_SET(sockfd,&save_fds);

新增到描述符集合中；然後通過

FD_ISSET(sockfd,&read_fds)

判斷哪個描述符可以讀取。

注意：

<span style="color:#FF0000;">fd_set read_fds = save_fds;</span>

好像沒有這段程式碼，程式執行之後不能成功監聽到傳送來的資料。暫時也不知道為什麼。