用C语言编写一个UDP客户端程序，向服务器发送一个字符串，并接收服务器返回的响应。

在C语言中，我们可以使用`sys/socket.h`, `arpa/inet.h`, 和 `unistd.h` 等头文件来创建UDP客户端。以下是一个简单的示例，展示了如何连接到服务器、发送数据并接收响应：

#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

#define PORT 12345 // 假设服务器监听这个端口
#define BUFFER_SIZE 1024

int main() {
    struct sockaddr_in server_addr;
    char send_data[] = "Hello from client"; // 发送的数据
    char receive_buffer[BUFFER_SIZE];
    int sock, bytes_sent, bytes_received;

    // 创建套接字
    sock = socket(AF_INET, SOCK_DGRAM, 0);
    if (sock < 0) {
        perror("Error creating socket");
        return 1;
    }

    // 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);
    inet_pton(AF_INET, "localhost", &server_addr.sin_addr);

    // 发送数据
    bytes_sent = sendto(sock, send_data, strlen(send_data), 0, (struct sockaddr*)&server_addr, sizeof(server_addr));
    if (bytes_sent <= 0) {
        perror("Error sending data");
        return 1;
    }
    printf("Sent %d bytes of data\n", bytes_sent);

    // 接收数据
    bytes_received = recvfrom(sock, receive_buffer, BUFFER_SIZE, 0, NULL, NULL);
    if (bytes_received <= 0) {
        perror("Error receiving response");
        return 1;
    }
    printf("Received %d bytes of response: %s\n", bytes_received, receive_buffer);

    // 关闭套接字
    close(sock);

    return 0;
}