客户端设计（仅供参考）：
下载本质：读取服务器发送的数据包，写入到本地文件
上传本质：读取本地文件内容，发送给服务器。
1、建立菜单选项，上传和下载。
2、上传功能函数：
1、只读打开本地文件
2、发送上传请求数据包
3、接收服务器应答并回复ACK包
3、1如果接收的是ACK确认包，将包修改为数据包填充本地文件内容并发送给服务器
3、2如果是错误包，上传结束，检查网络。
3、下载功能函数
1、发送下载请求包
2、循环接收服务器数据包
2.1判断收到的包如果是数据包并且块编号正确，就写入客户端文件，然后回复ACK确认包
2.2如果包的长度小于512+2+2，说明接收完毕
2.3如果是错误包。输出对应错误信息并停止接收。
TFTP下载模型：
TFTP上传模型
TFTP通信过程总结
1、服务器在69号端口等待客户端的请求
2、服务器若批准此请求,则使用临时端口与客户端进行通信
3、每个数据包的编号都有变化（从1开始）
4、每个数据包都要得到ACK的确认如果出现超时,则需要重新发送最后的包（数据或
ACK）
5、数据的长度以512Byte传输
6、小于512Byte的数据意味着传输结束
do_upload（上传文件）
void do_upload(int sockfd,struct sockaddr_in sin,const char *filename){char text[1024] = "";int text_len;socklen_t sinlen = sizeof(sin);int fd;int flags = 0;int num = 0;ssize_t bytes;text_len = sprintf(text,"%c%c%s%c%s%c",0,2,filename,0,"octet",0);if(sendto(sockfd,text,text_len,0,(struct sockaddr *)&sin,sinlen)<0){perror("sendto");exit(1);}while(1){if((bytes = recvfrom(sockfd,text,sizeof(text),0,(struct sockaddr *)&sin,&sinlen))<0){perror("recvfrom");exit(1);}printf("接收操作码：%d,块编号：%hd\n",text[1],ntohs(*(short *)(text+2)));if(text[1] == 5){printf("差错码是：%d,差错信息error:%s\n",ntohs(*(short *)(text+2)),text+4);return ;}else if(text[1] == 4){if(flags == 0){if((fd = open(filename,O_RDONLY))<0){perror("open");exit(1);}flags = 1;printf("文件打开成功:%d\n",fd);}int len = read(fd,text+4,512);if (len < 0) { //检查 read 函数的返回值perror("read");exit(1);}// printf("%s\n",text+4);if(len == 0){break;}text[1] = 3;*(short *)(text+2) = htons(++num);printf("发送操作码：%d,块编号：%hd\n",text[1],ntohs(*(short *)(text+2)));if(sendto(sockfd,text,len+4,0,(struct sockaddr *)&sin,sinlen)<0){perror("sendto");exit(1);}}}
}do_download（下载文件）
void do_download(int sockfd,struct sockaddr_in sin,const char *filename){char text[1024] = "";int text_len;socklen_t sinlen = sizeof(sin);int fd;int flags = 0;int num = 0;ssize_t bytes;text_len = sprintf(text,"%c%c%s%c%s%c",0,1,filename,0,"octet",0);if(sendto(sockfd,text,text_len,0,(struct sockaddr *)&sin,sinlen)<0){perror("sendto");exit(1);}while(1){if((bytes = recvfrom(sockfd,text,sizeof(text),0,(struct sockaddr *)&sin,&sinlen))<0){perror("recvfrom");exit(1);}printf("操作码:%d,块编号:%hd\n",text[1],ntohs(*(short *)(text+2)));if(text[1] == 5){printf("错误码是：%d,错误信息error：%s\n",ntohs(*(short *)(text+2)),text+4);return;}else if(text[1]==3){if(flags==0){if((fd = open(filename,O_WRONLY|O_CREAT|O_TRUNC,0664))<0){perror("open");exit(1);}flags = 1;}if((num+1 == ntohs(*(short *)(text+2))) && (bytes == 516)){num = ntohs(*(short *)(text+2));if(write(fd,text+4,bytes-4)<0){perror("write");exit(1);}text[1] = 4;if(sendto(sockfd,text,4,0,(struct sockaddr *)&sin,sinlen)<0){perror("sendto");exit(1);}}else if((num+1 == ntohs(*(short *)(text+2)))&&(bytes<516)){if(write(fd,text+4,bytes-4)<0){perror("write");exit(1);}text[1]=4;if(sendto(sockfd,text,4,0,(struct sockaddr *)&sin,sinlen)<0){perror("sendto");exit(1);}printf("文件下载完毕\n");return;}}}close(fd);
}运行结果
完整代码
#include <myhead.h>
#define SERPORT 69
#define SERIP "192.168.11.1"void do_download(int sockfd,struct sockaddr_in sin,const char *filename);
void do_upload(int sockfd,struct sockaddr_in sin,const char *filename);int main(int argc, const char *argv[])
{//1.创建用于通信的套接字文件描述符int clienfd = socket(AF_INET, SOCK_DGRAM, 0);if (clienfd == -1) {perror("socket");return -1;}//2.填充服务器的地址信息结构体struct sockaddr_in sin = {.sin_family = AF_INET,.sin_port = htons(SERPORT),.sin_addr.s_addr = inet_addr(SERIP)};//3.菜单栏选择int choice;char filename[100];while (1) { //3.菜单栏printf("请选择操作：\n");printf("1.上传文件\n");printf("2.下载文件\n");printf("3.退出\n");scanf("%d", &choice);bzero(filename, sizeof(filename));if (choice == 1) {printf("请输入要上传的文件名：");scanf("%s", filename);do_upload(clienfd, sin, filename);} else if (choice == 2) {printf("请输入要下载的文件名：");scanf("%s", filename);do_download(clienfd, sin, filename);} else if (choice == 3) {break;} else {printf("无效选项，请重新选择。\n");}}close(clienfd);return 0;
}void do_download(int sockfd,struct sockaddr_in sin,const char *filename){char text[1024] = "";int text_len;socklen_t sinlen = sizeof(sin);int fd;int flags = 0;int num = 0;ssize_t bytes;text_len = sprintf(text,"%c%c%s%c%s%c",0,1,filename,0,"octet",0);if(sendto(sockfd,text,text_len,0,(struct sockaddr *)&sin,sinlen)<0){perror("sendto");exit(1);}while(1){if((bytes = recvfrom(sockfd,text,sizeof(text),0,(struct sockaddr *)&sin,&sinlen))<0){perror("recvfrom");exit(1);}printf("操作码:%d,块编号:%hd\n",text[1],ntohs(*(short *)(text+2)));if(text[1] == 5){printf("错误码是：%d,错误信息error：%s\n",ntohs(*(short *)(text+2)),text+4);return;}else if(text[1]==3){if(flags==0){if((fd = open(filename,O_WRONLY|O_CREAT|O_TRUNC,0664))<0){perror("open");exit(1);}flags = 1;}if((num+1 == ntohs(*(short *)(text+2))) && (bytes == 516)){num = ntohs(*(short *)(text+2));if(write(fd,text+4,bytes-4)<0){perror("write");exit(1);}text[1] = 4;if(sendto(sockfd,text,4,0,(struct sockaddr *)&sin,sinlen)<0){perror("sendto");exit(1);}}else if((num+1 == ntohs(*(short *)(text+2)))&&(bytes<516)){if(write(fd,text+4,bytes-4)<0){perror("write");exit(1);}text[1]=4;if(sendto(sockfd,text,4,0,(struct sockaddr *)&sin,sinlen)<0){perror("sendto");exit(1);}printf("文件下载完毕\n");return;}}}close(fd);
}void do_upload(int sockfd,struct sockaddr_in sin,const char *filename){char text[1024] = "";int text_len;socklen_t sinlen = sizeof(sin);int fd;int flags = 0;int num = 0;ssize_t bytes;text_len = sprintf(text,"%c%c%s%c%s%c",0,2,filename,0,"octet",0);if(sendto(sockfd,text,text_len,0,(struct sockaddr *)&sin,sinlen)<0){perror("sendto");exit(1);}while(1){if((bytes = recvfrom(sockfd,text,sizeof(text),0,(struct sockaddr *)&sin,&sinlen))<0){perror("recvfrom");exit(1);}printf("接收操作码：%d,块编号：%hd\n",text[1],ntohs(*(short *)(text+2)));if(text[1] == 5){printf("差错码是：%d,差错信息error:%s\n",ntohs(*(short *)(text+2)),text+4);return ;}else if(text[1] == 4){if(flags == 0){if((fd = open(filename,O_RDONLY))<0){perror("open");exit(1);}flags = 1;printf("文件打开成功:%d\n",fd);}int len = read(fd,text+4,512);if (len < 0) { //检查 read 函数的返回值perror("read");exit(1);}// printf("%s\n",text+4);if(len == 0){break;}text[1] = 3;*(short *)(text+2) = htons(++num);printf("发送操作码：%d,块编号：%hd\n",text[1],ntohs(*(short *)(text+2)));if(sendto(sockfd,text,len+4,0,(struct sockaddr *)&sin,sinlen)<0){perror("sendto");exit(1);}}}
}
