如何用C语言编写一个UDP客户端和服务器，实现双向数据通信的示例代码？

在C语言中，我们可以使用标准库中的socket API来编写UDP（用户数据报协议）客户端和服务器程序，这里提供一个简单的双向通信的示例。注意，这个示例假设你已经对网络编程有基本的理解。

首先，创建一个UDP服务器：

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

#define PORT 12345

int main() {
    int server_socket, client_socket;
    struct sockaddr_in server_addr, client_addr;
    socklen_t addr_len = sizeof(struct sockaddr_in);
    char buffer[1024];

    // 创建服务器套接字
    server_socket = socket(AF_INET, SOCK_DGRAM, 0);
    if (server_socket == -1) {
        perror("Failed to create socket");
        exit(EXIT_FAILURE);
    }

    // 设置服务器地址
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(PORT);
    server_addr.sin_addr.s_addr = htonl(INADDR_ANY);

    // 绑定到端口
    if (bind(server_socket, (struct sockaddr *)&server_addr, addr_len) == -1) {
        perror("Failed to bind to port");
        close(server_socket);
        exit(EXIT_FAILURE);
    }

    printf("Server started, listening on %d\n", PORT);

    while (1) {
        // 接收来自客户端的数据
        recvfrom(server_socket, buffer, sizeof(buffer), 0, (struct sockaddr *)&client_addr, &addr_len);

        // 发送回应给客户端
        sendto(server_socket, buffer, strlen(buffer), 0, (struct sockaddr *)&client_addr, addr_len);
        
        printf("Received message from %s: %s\n", inet_ntoa(client_addr.sin_addr), buffer);
    }

    close(server_socket);
    return 0;
}

接下来，创建一个UDP客户端：

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

#define SERVER_IP "127.0.0.1"
#define SERVER_PORT 12345

int main() {
    int client_socket;
    struct sockaddr_in server_addr;
    char message[] = "Hello, Server!";
    size_t msg_len = strlen(message);

    // 创建客户端套接字
    client_socket = socket(AF_INET, SOCK_DGRAM, 0);
    if (client_socket == -1) {
        perror("Failed to create socket");
        exit(EXIT_FAILURE);
    }

    // 设置服务器地址
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(SERVER_PORT);
    inet_pton(AF_INET, SERVER_IP, &server_addr.sin_addr);

    // 发送消息到服务器
    if (sendto(client_socket, message, msg_len, 0, (struct sockaddr *)&server_addr, sizeof(server_addr)) == -1) {
        perror("Failed to send data");
        close(client_socket);
        exit(EXIT_FAILURE);
    }

    printf("Message sent to %s:%d\n", SERVER_IP, SERVER_PORT);

    // 接收服务器响应
    char received_msg[1024];
    socklen_t addr_len = sizeof(server_addr);
    ssize_t bytes_received = recvfrom(client_socket, received_msg, sizeof(received_msg), 0, (struct sockaddr *)&server_addr, &addr_len);

    if (bytes_received > 0) {
        printf("Received message from %s: %s\n", inet_ntoa(server_addr.sin_addr), received_msg);
    } else {
        perror("Failed to receive response");
    }

    close(client_socket);
    return 0;
}

运行这两个程序，服务器会监听指定端口，当客户端连接并发送消息时，服务器接收并返回消息。同样，客户端也尝试连接服务器，发送消息，并接收服务器的回应。