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。
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