用C语言写一个只接受一个客户端连接的服务器程序

在C语言中，创建一个简单的单客户端连接的TCP服务器程序需要用到`socket`、`bind`、`listen`、`accept`和`close`等系统调用。以下是一个基本的示例：

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

#define PORT 12345

int main() {
    int server_sock, client_sock;
    struct sockaddr_in server_addr, client_addr;
    socklen_t addr_len = sizeof(struct sockaddr_in);
    char buffer[1024] = {0};
    
    // 创建服务器套接字
    server_sock = socket(AF_INET, SOCK_STREAM, 0);
    if (server_sock == -1) {
        perror("Failed to create server socket");
        return 1;
    }

    // 设置服务器地址和端口
    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 = INADDR_ANY; // 对于所有网络接口

    // 绑定套接字到指定地址
    if (bind(server_sock, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
        perror("Failed to bind server socket");
        close(server_sock);
        return 1;
    }

    // 监听等待连接
    if (listen(server_sock, 1) < 0) {
        perror("Failed to listen server socket");
        close(server_sock);
        return 1;
    }

    printf("Server is listening on port %d...\n", PORT);

    // 接受客户端连接
    while (true) {
        client_sock = accept(server_sock, (struct sockaddr *)&client_addr, &addr_len);
        if (client_sock == -1) {
            perror("Failed to accept client connection");
            continue;
        }

        printf("Accepted connection from %s:%d\n", inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port));

        // 接收客户端数据
        ssize_t bytes_received = recv(client_sock, buffer, sizeof(buffer), 0);
        if (bytes_received > 0) {
            buffer[bytes_received] = '\0';
            printf("Received message: %s\n", buffer);

            // 发送响应
            send(client_sock, "Hello from server!", strlen("Hello from server!"), 0);
        } else if (bytes_received == 0) {
            printf("Client disconnected.\n");
        } else {
            perror("Failed to receive data");
        }

        // 关闭客户端连接
        close(client_sock);
    }

    // 关闭服务器套接字
    close(server_sock);
    return 0;
}

这个程序会在给定的端口（本例中是12345）上监听，当有客户端连接时，会接收客户端的消息并发送回复。注意，这个例子没有处理异常情况，实际应用中需要添加适当的错误检查。