回声UDP服务器端/客户端

　　UDP是具有数据边界的协议，传输中调用I/O函数的次数非常重要。输入函数的调用次数要和输出函数的调用次数完全一致，这样才能保证接受全部已发送的数据。

　　TCP套接字中需注册待传输数据的目标IP和端口，而UDP中无需注册。通过sendto函数传递数据的过程大约分为3个阶段：
第1阶段：向udp套接字注册目标IP和端口号。
第2阶段：传输数据。
第3阶段：删除udp套接字中注册的目标地址信息。
　　每次调用sendto函数时都会重复上述过程。这种未注册目标地址信息的套接字成为未连接套接字，反之，注册了目标地址的套接字称为连接connected套接字。UDP默认属于未连接套接字，创建已连接UDP套接字只需调用connect函数，这样可以提高效率。

1、linux

服务器端：

 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
 #include <arpa/inet.h>
 #include <sys/socket.h>

 #define BUF_SIZE 1024
 void error_handling(char * messages);

 int main(int argc, char *argv[])
 {
     if(argc != )
     {
         printf("Usage : %s <port>\n", argv[]);
         exit();
     }
     int serverSock;
     struct sockaddr_in serverAddr, clientAddr;
     socklen_t clientAddrSize;

     char message[BUF_SIZE];
     int strLen;

     serverSock =socket(PF_INET, SOCK_DGRAM, );
     if(serverSock == -)
         error_handling("socket() error");

     memset(&serverAddr, , sizeof(serverAddr));
     serverAddr.sin_family = AF_INET;
     serverAddr.sin_addr.s_addr = htonl(INADDR_ANY);
     serverAddr.sin_port = htons(atoi(argv[]));

     if(bind(serverSock, (struct sockaddr*) &serverAddr, sizeof(serverAddr)) == -)
         error_handling("bind() error");

     puts("Server start...");
     while(){
         clientAddrSize = sizeof(clientAddr);
         strLen = recvfrom(serverSock, message, BUF_SIZE, , (struct sockaddr*) &clientAddr, &clientAddrSize);
         sendto(serverSock, message, strLen, , (struct sockaddr*)&clientAddr, clientAddrSize);
     }
     close(serverSock);
     puts("Server close...");
     return ;
 }

 void error_handling(char * messages)
 {
     puts(messages);
     exit();
 }

客户端：

 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
 #include <arpa/inet.h>
 #include <sys/socket.h>

 #define BUF_SIZE 1024
 void error_handling(char * messages);

 int main(int argc, char *argv[])
 {
     if(argc != )
     {
         printf("Usage : %s <IP> <port>\n", argv[]);
         exit();
     }
     int sock;
     struct sockaddr_in serv_addr, from_addr;
     socklen_t addr_size;
     char message[BUF_SIZE];
     int strLen;

     sock = socket(PF_INET, SOCK_DGRAM, );
     if(sock == -)
         error_handling("socket() error");

     memset(&serv_addr, , sizeof(serv_addr));
     serv_addr.sin_family = AF_INET;
     serv_addr.sin_addr.s_addr = inet_addr(argv[]);
     serv_addr.sin_port = htons(atoi(argv[]));

     while(){
         puts("Input message(Q to quit):");
         fgets(message, BUF_SIZE, stdin);
         if(!strcmp(message, "q\n") || !strcmp(message, "Q\n"))
             break;
         sendto(sock, message, strlen(message), , (struct sockaddr*)&serv_addr,sizeof(serv_addr));
         addr_size = sizeof(from_addr);
         strLen = recvfrom(sock, message, BUF_SIZE, , (struct sockaddr*)&from_addr, &addr_size);
         message[strLen]=;
         printf("Message from server: %s\n", message);
     }
     close(sock);
     return ;
 }

 void error_handling(char * messages)
 {
     puts(messages);
     exit();
 }

2、windows

服务器端：

 #include <stdio.h>
 #include <stdlib.h>
 #include <WinSock2.h>

 #define BUF_SIZE 1024
 void ErrorHandling(char *message);

 int main(int argc, char *argv[])
 {
     if (argc != ) {
         printf("Usage : %s <port>\n", argv[]);
         exit();
     }

     WSADATA wsa_data;
     SOCKET server_sock;
     SOCKADDR_IN server_addr, client_addr;
     int sz_client_addr;
     char message[BUF_SIZE];
     int str_len;

     if (WSAStartup(MAKEWORD(, ), &wsa_data) != )
         ErrorHandling("WSAStartup() error.");

     server_sock = socket(PF_INET, SOCK_DGRAM, );
     if (server_sock == INVALID_SOCKET)
         ErrorHandling("socket() error.");

     memset(&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(atoi(argv[]));

     if (bind(server_sock, (SOCKADDR*)&server_addr, sizeof(server_addr)) == SOCKET_ERROR)
         ErrorHandling("bind() error.");
     printf("Server start...\n");
     while (true)
     {
         sz_client_addr = sizeof(client_addr);
         str_len = recvfrom(server_sock, message, BUF_SIZE, , (SOCKADDR*)&client_addr, &sz_client_addr);
         sendto(server_sock, message, str_len, , (SOCKADDR*)&client_addr, sizeof(message));
     }
     closesocket(server_sock);
     printf("Server end...\n");
     WSACleanup();
     return ;
 }

 void ErrorHandling(char *message) {
     fputs(message, stderr);
     fputc('\n', stderr);
 }

使用连接connected套接字的客户端：

　　针对UDP套接字调用connect函数并不意味着要与对方UDP套接字连接，这只是向UDP套接字注册目标IP和端口信息。

 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <winsock2.h>

 #define BUF_SIZE 1024
 void ErrorHandling(char *message);

 int main(int argc, char* argv[]) {
     if (argc != ) {
         printf("Usage : %s <IP> <port>\n", argv[]);
         exit();
     }

     WSADATA wsa_data;
     SOCKET h_socket;
     SOCKADDR_IN server_addr;

     char message[BUF_SIZE];
     int str_len;

     if (WSAStartup(MAKEWORD(, ), &wsa_data) != )
         ErrorHandling("WSAStartup() error.");

     h_socket = socket(PF_INET, SOCK_DGRAM, );
     if (h_socket == INVALID_SOCKET)
         ErrorHandling("socket() error.");

     memset(&server_addr, , sizeof(server_addr));
     server_addr.sin_family = AF_INET;
     server_addr.sin_addr.s_addr = inet_addr(argv[]);
     server_addr.sin_port = htons(atoi(argv[]));

     if (connect(h_socket, (SOCKADDR*)&server_addr, sizeof(server_addr)) == SOCKET_ERROR)
         ErrorHandling("connect() error.");

     while (true)
     {
         fputs("Insert message(q to quit): ", stdout);
         fgets(message, sizeof(message), stdin);
         if (!strcmp(message, "q\n") || !strcmp(message, "Q\n"))
             break;
         send(h_socket, message, strlen(message), );
         str_len = recv(h_socket, message, sizeof(message) - , );
         message[str_len] = ;
         printf("Message from server: %s", message);
     }
     closesocket(h_socket);
     WSACleanup();
     return ;
 }

 void ErrorHandling(char *message) {
     fputs(message, stderr);
     fputc('\n', stderr);
 }

使用未连接套接字的客户端：

 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <winsock2.h>

 #define BUF_SIZE 1024
 void ErrorHandling(char *message);

 int main(int argc, char* argv[]) {
     if (argc != ) {
         printf("Usage : %s <IP> <port>\n", argv[]);
         exit();
     }

     WSADATA wsa_data;
     SOCKET h_socket;
     SOCKADDR_IN server_addr, from_addr;

     char message[BUF_SIZE];
     int str_len;

     if (WSAStartup(MAKEWORD(, ), &wsa_data) != )
         ErrorHandling("WSAStartup() error.");

     h_socket = socket(PF_INET, SOCK_DGRAM, );
     if (h_socket == INVALID_SOCKET)
         ErrorHandling("socket() error.");

     memset(&server_addr, , sizeof(server_addr));
     server_addr.sin_family = AF_INET;
     server_addr.sin_addr.s_addr = inet_addr(argv[]);
     server_addr.sin_port = htons(atoi(argv[]));

     while (true)
     {
         fputs("Insert message(q to quit): ", stdout);
         fgets(message, sizeof(message), stdin);
         if (!strcmp(message, "q\n") || !strcmp(message, "Q\n"))
             break;
         sendto(h_socket, message, strlen(message), , (SOCKADDR*)&server_addr, sizeof(server_addr));
         int sz_from_addr = sizeof(from_addr);
         str_len = recvfrom(h_socket, message, BUF_SIZE, , (SOCKADDR*)&from_addr, &sz_from_addr);
         message[str_len] = ;
         printf("Message from server: %s", message);
     }
     closesocket(h_socket);
     WSACleanup();
     return ;
 }

 void ErrorHandling(char *message) {
     fputs(message, stderr);
     fputc('\n', stderr);
 }