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 != 2)
    {
        printf("Usage : %s <port>
", argv[0]);
        exit(1);
    }
    int serverSock;
    struct sockaddr_in serverAddr, clientAddr;
    socklen_t clientAddrSize;

    char message[BUF_SIZE];
    int strLen;

    serverSock =socket(PF_INET, SOCK_DGRAM, 0);
    if(serverSock == -1)
        error_handling("socket() error");

    memset(&serverAddr, 0, sizeof(serverAddr));
    serverAddr.sin_family = AF_INET;
    serverAddr.sin_addr.s_addr = htonl(INADDR_ANY);
    serverAddr.sin_port = htons(atoi(argv[1]));

    if(bind(serverSock, (struct sockaddr*) &serverAddr, sizeof(serverAddr)) == -1)
        error_handling("bind() error");

    puts("Server start...");
    while(1){
        clientAddrSize = sizeof(clientAddr);
        strLen = recvfrom(serverSock, message, BUF_SIZE, 0, (struct sockaddr*) &clientAddr, &clientAddrSize);
        sendto(serverSock, message, strLen, 0, (struct sockaddr*)&clientAddr, clientAddrSize);
    }
    close(serverSock);
    puts("Server close...");
    return 0;
}

void error_handling(char * messages)
{
    puts(messages);
    exit(1);
}

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 != 3)
    {
        printf("Usage : %s <IP> <port>
", argv[0]);
        exit(1);
    }
    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, 0);
    if(sock == -1)
        error_handling("socket() error");

    memset(&serv_addr, 0, sizeof(serv_addr));
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = inet_addr(argv[1]);
    serv_addr.sin_port = htons(atoi(argv[2]));

    while(1){
        puts("Input message(Q to quit):");
        fgets(message, BUF_SIZE, stdin);
        if(!strcmp(message, "q
") || !strcmp(message, "Q
"))
            break;
        sendto(sock, message, strlen(message), 0, (struct sockaddr*)&serv_addr,sizeof(serv_addr));
        addr_size = sizeof(from_addr);
        strLen = recvfrom(sock, message, BUF_SIZE, 0, (struct sockaddr*)&from_addr, &addr_size);
        message[strLen]=0;
        printf("Message from server: %s
", message);
    }
    close(sock);
    return 0;
}

void error_handling(char * messages)
{
    puts(messages);
    exit(1);
}

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 != 2) {
        printf("Usage : %s <port>
", argv[0]);
        exit(1);
    }

    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(2, 2), &wsa_data) != 0)
        ErrorHandling("WSAStartup() error.");

    server_sock = socket(PF_INET, SOCK_DGRAM, 0);
    if (server_sock == INVALID_SOCKET)
        ErrorHandling("socket() error.");

    memset(&server_addr, 0, 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[1]));

    if (bind(server_sock, (SOCKADDR*)&server_addr, sizeof(server_addr)) == SOCKET_ERROR)
        ErrorHandling("bind() error.");
    printf("Server start...
");
    while (true)
    {
        sz_client_addr = sizeof(client_addr);
        str_len = recvfrom(server_sock, message, BUF_SIZE, 0, (SOCKADDR*)&client_addr, &sz_client_addr);
        sendto(server_sock, message, str_len, 0, (SOCKADDR*)&client_addr, sizeof(message));
    }
    closesocket(server_sock);
    printf("Server end...
");
    WSACleanup();
    return 0;
}

void ErrorHandling(char *message) {
    fputs(message, stderr);
    fputc('
', 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 != 3) {
        printf("Usage : %s <IP> <port>
", argv[0]);
        exit(1);
    }

    WSADATA wsa_data;
    SOCKET h_socket;
    SOCKADDR_IN server_addr;

    char message[BUF_SIZE];
    int str_len;

    if (WSAStartup(MAKEWORD(2, 2), &wsa_data) != 0)
        ErrorHandling("WSAStartup() error.");

    h_socket = socket(PF_INET, SOCK_DGRAM, 0);
    if (h_socket == INVALID_SOCKET)
        ErrorHandling("socket() error.");

    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = inet_addr(argv[1]);
    server_addr.sin_port = htons(atoi(argv[2]));

    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
") || !strcmp(message, "Q
"))
            break;
        send(h_socket, message, strlen(message), 0);
        str_len = recv(h_socket, message, sizeof(message) - 1, 0);
        message[str_len] = 0;
        printf("Message from server: %s", message);
    }
    closesocket(h_socket);
    WSACleanup();
    return 0;
}

void ErrorHandling(char *message) {
    fputs(message, stderr);
    fputc('
', 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 != 3) {
        printf("Usage : %s <IP> <port>
", argv[0]);
        exit(1);
    }

    WSADATA wsa_data;
    SOCKET h_socket;
    SOCKADDR_IN server_addr, from_addr;

    char message[BUF_SIZE];
    int str_len;

    if (WSAStartup(MAKEWORD(2, 2), &wsa_data) != 0)
        ErrorHandling("WSAStartup() error.");

    h_socket = socket(PF_INET, SOCK_DGRAM, 0);
    if (h_socket == INVALID_SOCKET)
        ErrorHandling("socket() error.");

    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = inet_addr(argv[1]);
    server_addr.sin_port = htons(atoi(argv[2]));

    while (true)
    {
        fputs("Insert message(q to quit): ", stdout);
        fgets(message, sizeof(message), stdin);
        if (!strcmp(message, "q
") || !strcmp(message, "Q
"))
            break;
        sendto(h_socket, message, strlen(message), 0, (SOCKADDR*)&server_addr, sizeof(server_addr));
        int sz_from_addr = sizeof(from_addr);
        str_len = recvfrom(h_socket, message, BUF_SIZE, 0, (SOCKADDR*)&from_addr, &sz_from_addr);
        message[str_len] = 0;
        printf("Message from server: %s", message);
    }
    closesocket(h_socket);
    WSACleanup();
    return 0;
}

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