Linux网络编程之基于UDP实现可靠的文件传输示例

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;
}

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;
        /* 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:\t%s From Server IP Successful!\n", file_name);
  fclose(fp);
  close(client_socket_fd);
  return 0;
}

感兴趣的朋友可以动手测试一下该程序，相信会对大家的Linux下C语言网络编程带来一定的帮助。