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了解网络传输协议的人都知道，采用TCP实现文件传输很简单。相对于TCP，由于UDP是面向无连接、不可靠的传输协议，所以我们需要考虑丢包和后发先至（包的顺序）的问题，所以我们想要实现UDP传输文件，则需要解决这两个问题。方法就是给数据包编号，按照包的顺序接收并存储，接收端接收到数据包后发送确认信息给发送端，发送端接收确认数据以后再继续发送下一个包，如果接收端收到的数据包的编号不是期望的编号，则要求发送端重新发送。
下面展示的是基于linux下C语言实现的一个示例程序，该程序定义一个包的结构体，其中包含数据和包头，包头里包含有包的编号和数据大小，经过测试后，该程序可以成功传输一个视频文件。
具体实现代码如下：
server端代码如下：
/*************************************************************************
> File Name: server.c
> Author: SongLee
************************************************************************/
#include<sys/types.h>
#include<sys/socket.h>
#include<unistd.h>
#include<netinet/in.h>
#include<arpa/inet.h>
#include<stdio.h>
#include<stdlib.h>
#include<errno.h>
#include<netdb.h>
#include<stdarg.h>
#include<string.h>
#define SERVER_PORT 8000
#define BUFFER_SIZE 1024
#define FILE_NAME_MAX_SIZE 512
/* 包头 */
typedef struct
{
int id;
int buf_size;
}PackInfo;
/* 接收包 */
struct SendPack
{
PackInfo head;
char buf[BUFFER_SIZE];
} data;
int main()
{
/* 发送id */
int send_id = 0;
/* 接收id */
int receive_id = 0;
/* 创建UDP套接口 */
struct sockaddr_in server_addr;
bzero(&server_addr, sizeof(server_addr));
server_addr.sin_family = AF_INET;
server_addr.sin_addr.s_addr = htonl(INADDR_ANY);
server_addr.sin_port = htons(SERVER_PORT);
/* 创建socket */
int server_socket_fd = socket(AF_INET, SOCK_DGRAM, 0);
if(server_socket_fd == -1)
{
perror("Create Socket Failed:");
exit(1);
}
/* 绑定套接口 */
if(-1 == (bind(server_socket_fd,(struct sockaddr*)&server_addr,sizeof(server_addr))))
{
perror("Server Bind Failed:");
exit(1);
}
/* 数据传输 */
while(1)
{
/* 定义一个地址，用于捕获客户端地址 */
struct sockaddr_in client_addr;
socklen_t client_addr_length = sizeof(client_addr);
/* 接收数据 */
char buffer[BUFFER_SIZE];
bzero(buffer, BUFFER_SIZE);
if(recvfrom(server_socket_fd, buffer, BUFFER_SIZE,0,(struct sockaddr*)&client_addr, &client_addr_length) == -1)
{
perror("Receive Data Failed:");
exit(1);
}
/* 从buffer中拷贝出file_name */
char file_name[FILE_NAME_MAX_SIZE+1];
bzero(file_name,FILE_NAME_MAX_SIZE+1);
strncpy(file_name, buffer, strlen(buffer)>FILE_NAME_MAX_SIZE?FILE_NAME_MAX_SIZE:strlen(buffer));
printf("%s\n", file_name);
/* 打开文件 */
FILE *fp = fopen(file_name, "r");
if(NULL == fp)
{
printf("File:%s Not Found.\n", file_name);
}
else
{
int len = 0;
/* 每读取一段数据，便将其发给客户端 */
while(1)
{
PackInfo pack_info;
if(receive_id == send_id)
{
++send_id;
if((len = fread(data.buf, sizeof(char), BUFFER_SIZE, fp)) > 0)
{
data.head.id = send_id; /* 发送id放进包头,用于标记顺序 */
data.head.buf_size = len; /* 记录数据长度 */
if(sendto(server_socket_fd, (char*)&data, sizeof(data), 0, (struct sockaddr*)&client_addr, client_addr_length) < 0)
{
perror("Send File Failed:");
break;
}
/* 接收确认消息 */
recvfrom(server_socket_fd, (char*)&pack_info, sizeof(pack_info), 0, (struct sockaddr*)&client_addr, &client_addr_length);
receive_id = pack_info.id;
}
else
{
break;
}
}
else
{
/* 如果接收的id和发送的id不相同,重新发送 */
if(sendto(server_socket_fd, (char*)&data, sizeof(data), 0, (struct sockaddr*)&client_addr, client_addr_length) < 0)
{
perror("Send File Failed:");
break;
}
/* 接收确认消息 */
recvfrom(server_socket_fd, (char*)&pack_info, sizeof(pack_info), 0, (struct sockaddr*)&client_addr, &client_addr_length);
receive_id = pack_info.id;
}
}
/* 关闭文件 */
fclose(fp);
printf("File:%s Transfer Successful!\n", file_name);
}
}
close(server_socket_fd);
return 0;
}
</div>
client端代码如下：
/*************************************************************************
> File Name: client.c
> Author: SongLee
************************************************************************/
#include<sys/types.h>
#include<sys/socket.h>
#include<unistd.h>
#include<netinet/in.h>
#include<arpa/inet.h>
#include<stdio.h>
#include<stdlib.h>
#include<errno.h>
#include<netdb.h>
#include<stdarg.h>
#include<string.h>
#define SERVER_PORT 8000
#define BUFFER_SIZE 1024
#define FILE_NAME_MAX_SIZE 512
/* 包头 */
typedef struct
{
int id;
int buf_size;
}PackInfo;
/* 接收包 */
struct RecvPack
{
PackInfo head;
char buf[BUFFER_SIZE];
} data;
int main()
{
int id = 1;
/* 服务端地址 */
struct sockaddr_in server_addr;
bzero(&server_addr, sizeof(server_addr));
server_addr.sin_family = AF_INET;
server_addr.sin_addr.s_addr = inet_addr("127.0.0.1");
server_addr.sin_port = htons(SERVER_PORT);
socklen_t server_addr_length = sizeof(server_addr);
/* 创建socket */
int client_socket_fd = socket(AF_INET, SOCK_DGRAM, 0);
if(client_socket_fd < 0)
{
perror("Create Socket Failed:");
exit(1);
}
/* 输入文件名到缓冲区 */
char file_name[FILE_NAME_MAX_SIZE+1];
bzero(file_name, FILE_NAME_MAX_SIZE+1);
printf("Please Input File Name On Server: ");
scanf("%s", file_name);
char buffer[BUFFER_SIZE];
bzero(buffer, BUFFER_SIZE);
strncpy(buffer, file_name, strlen(file_name)>BUFFER_SIZE?BUFFER_SIZE:strlen(file_name));
/* 发送文件名 */
if(sendto(client_socket_fd, buffer, BUFFER_SIZE,0,(struct sockaddr*)&server_addr,server_addr_length) < 0)
{
perror("Send File Name Failed:");
exit(1);
}
/* 打开文件，准备写入 */
FILE *fp = fopen(file_name, "w");
if(NULL == fp)
{
printf("File:\t%s Can Not Open To Write\n", file_name);
exit(1);
}
/* 从服务器接收数据，并写入文件 */
int len = 0;
while(1)
{
PackInfo pack_info;
if((len = recvfrom(client_socket_fd, (char*)&data, sizeof(data), 0, (struct sockaddr*)&server_addr,&server_addr_length)) > 0)
{
if(data.head.id == id)
{
pack_info.id = data.head.id;
pack_info.buf_size = data.head.buf_size;
++id;
/* 发送数据包确认信息 */
if(sendto(client_socket_fd, (char*)&pack_info, sizeof(pack_info), 0, (struct sockaddr*)&server_addr, server_addr_length) < 0)
{
printf("Send confirm information failed!");
}
/* 写入文件 */
if(fwrite(data.buf, sizeof(char), data.head.buf_size, fp) < data.head.buf_size)
{
printf("File:\t%s Write Failed\n", file_name);
break;
}
}
else if(data.head.id < id) /* 如果是重发的包 */
{
pack_info.id = data.head.id;
pack_info.buf_size = data.head.buf_size;