#define SER_PORT 69
#define SER_IP "192.168.125.71"
#define CLT_PORT 6666
#define CLT_IP "192.168.125.158"
int main(int argc, const char *argv[])
{
    //创建套接字文件描述符
    int cfd = socket(AF_INET,SOCK_DGRAM,0);
    if(cfd == -1)
    {
        perror("socket error");
        return -1;
    }
    printf("cfd = %d\n",cfd);

    //3数据收发
//  char rbuf[128] = "";

    //填充服务器的地址信息结构体
    struct sockaddr_in sin;
    sin.sin_family = AF_INET;
    sin.sin_port = htons(SER_PORT);
    sin.sin_addr.s_addr = inet_addr(SER_IP);

    //像服务器发送下请求
    char buf[516] = "";
    char rbuf[516] = "";

    //组装操作码
    short*p1 = (short*)buf;
    *p1 = htons(1);

    //组装文件名
    char *p2 = buf+2;
    strcpy(p2,"5.png");

    //组装传输模式
    char *p4 = p2+strlen(p2)+1;
    strcpy(p4,"octet");

    int size = 2+strlen(p2)+strlen(p4)+2;

    //将协议包发送给服务器
    sendto(cfd,buf,size,0,(struct sockaddr*)&sin,sizeof(sin));

    socklen_t addrlen = sizeof(sin);
    unsigned short  block_num = 1; //定义本地块编号

    while(1){
        //接收服务器传回来的信息
        int res = recvfrom(cfd,rbuf,sizeof(rbuf),0,(struct sockaddr*)&sin,&addrlen);
        if(res == -1){
            perror("recvfrom error");
            return -1;
        }
        if(rbuf[1] == 3){
            if(*(unsigned short*)(buf+2) == htons(block_num)){
                block_num++;//块编号自增
                int fd = open("./1",O_WRONLY|O_CREAT|O_TRUNC,0666);
                write(fd,rbuf+4,sizeof(rbuf+4);//将数据写到文件中



                //发送ACK
                rbuf[1] = 4;                                                                 
                sendto(cfd,rbuf,4,0,(struct sockaddr*)&sin,sizeof(sin));
                printf("%s 文件下载完毕");

            }
        }

        /*//将网络字节序转换为主机字节序
        unsigned short int value_net = htons(rbuf[1]);
        printf("%#x\n",value_net);*/

    }
    //4关闭套接字
    close(cfd);
    return 0;
}

没写完，后面会进行整改