Linux网络编程：UDP实现可靠的文件传输

      我们知道，用TCP实现文件传输很简单。相对于TCP，因为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。

     下面是我用linux C实现的一个Demo，定义一个包的结构体，其中包含数据和包头，包头里包含包的编号和数据大小，经过测试，传输一个视频文件，传送成功。

代码：

/*************************************************************************
    > File Name: server.c
    > Author: SongLee
    > E-mail: lisong.shine@qq.com
    > Created Time: 2014年05月11日 星期日 19时55分49秒
    > Personal Blog: http://songlee24.github.io
 ************************************************************************/
#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
", file_name);

		/* 打开文件 */
		FILE *fp = fopen(file_name, "r");
		if(NULL == fp)
		{
			printf("File:%s Not Found.
", 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!
", file_name);
		}
	}
	close(server_socket_fd);
	return 0;
}

/*************************************************************************
    > File Name: client.c
    > Author: SongLee
    > E-mail: lisong.shine@qq.com
    > Created Time: 2014年05月11日 星期日 21时53分39秒
    > Personal Blog: http://songlee24.github.io
 ************************************************************************/
#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:	%s Can Not Open To Write
", 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:	%s Write Failed
", 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;
				/* 重发数据包确认信息 */
				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!");
				}
			}
			else
			{

			}
		}
		else
		{
			break;
		}
	}

	printf("Receive File:	%s From Server IP Successful!
", file_name);
	fclose(fp);
	close(client_socket_fd);
	return 0;
}