在windows 系统下编写程序，将客户端和服务器端都改为多线程程序，并实现一个服务器能同时 与多个客户端通信。

好的，下面是一个简单的示例程序，演示如何在Windows系统下使用多线程实现一个服务器能同时与多个客户端通信。该程序使用了Winsock2库来实现网络通信，使用了C++11的thread库来实现多线程。

服务端代码：

#include <iostream>
#include <thread>
#include <vector>
#include <WinSock2.h>

#pragma comment(lib, "ws2_32.lib")

const int MAX_CLIENTS = 10;
const int BUFFER_SIZE = 1024;

void handleClient(SOCKET clientSocket)
{
    char buffer[BUFFER_SIZE];
    int recvSize;

    while (true)
    {
        // 接收客户端发送的数据
        recvSize = recv(clientSocket, buffer, BUFFER_SIZE, 0);
        if (recvSize == 0) // 客户端已关闭连接
        {
            std::cout << "Client disconnected." << std::endl;
            break;
        }
        else if (recvSize < 0)
        {
            std::cerr << "Error: recv failed with error code " << WSAGetLastError() << std::endl;
            break;
        }

        // 发送数据回客户端
        if (send(clientSocket, buffer, recvSize, 0) < 0)
        {
            std::cerr << "Error: send failed with error code " << WSAGetLastError() << std::endl;
            break;
        }
    }

    // 关闭套接字
    closesocket(clientSocket);
}

int main()
{
    WSADATA wsaData;
    int result = WSAStartup(MAKEWORD(2, 2), &wsaData);
    if (result != 0)
    {
        std::cerr << "Error: WSAStartup failed with error code " << result << std::endl;
        return 1;
    }

    // 创建服务器套接字
    SOCKET serverSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (serverSocket == INVALID_SOCKET)
    {
        std::cerr << "Error: socket creation failed with error code " << WSAGetLastError() << std::endl;
        WSACleanup();
        return 1;
    }

    // 绑定端口
    sockaddr_in serverAddr;
    serverAddr.sin_family = AF_INET;
    serverAddr.sin_addr.s_addr = htonl(INADDR_ANY);
    serverAddr.sin_port = htons(12345);
    if (bind(serverSocket, (sockaddr*)&serverAddr, sizeof(serverAddr)) == SOCKET_ERROR)
    {
        std::cerr << "Error: bind failed with error code " << WSAGetLastError() << std::endl;
        closesocket(serverSocket);
        WSACleanup();
        return 1;
    }

    // 监听端口
    if (listen(serverSocket, MAX_CLIENTS) == SOCKET_ERROR)
    {
        std::cerr << "Error: listen failed with error code " << WSAGetLastError() << std::endl;
        closesocket(serverSocket);
        WSACleanup();
        return 1;
    }

    std::cout << "Server started, waiting for connections..." << std::endl;

    std::vector<std::thread> threads;
    SOCKET clientSocket;
    sockaddr_in clientAddr;
    int addrLen = sizeof(clientAddr);

    while (true)
    {
        // 接受客户端连接请求
        clientSocket = accept(serverSocket, (sockaddr*)&clientAddr, &addrLen);
        if (clientSocket == INVALID_SOCKET)
        {
            std::cerr << "Error: accept failed with error code " << WSAGetLastError() << std::endl;
            continue;
        }

        // 创建新线程来处理客户端连接
        threads.emplace_back(std::thread(handleClient, clientSocket));
    }

    // 关闭服务器套接字
    closesocket(serverSocket);
    WSACleanup();

    return 0;
}

客户端代码：

#include <iostream>
#include <WinSock2.h>

#pragma comment(lib, "ws2_32.lib")

const int BUFFER_SIZE = 1024;

int main()
{
    WSADATA wsaData;
    int result = WSAStartup(MAKEWORD(2, 2), &wsaData);
    if (result != 0)
    {
        std::cerr << "Error: WSAStartup failed with error code " << result << std::endl;
        return 1;
    }

    // 创建客户端套接字
    SOCKET clientSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (clientSocket == INVALID_SOCKET)
    {
        std::cerr << "Error: socket creation failed with error code " << WSAGetLastError() << std::endl;
        WSACleanup();
        return 1;
    }

    // 连接服务器
    sockaddr_in serverAddr;
    serverAddr.sin_family = AF_INET;
    serverAddr.sin_addr.s_addr = inet_addr("127.0.0.1");
    serverAddr.sin_port = htons(12345);
    if (connect(clientSocket, (sockaddr*)&serverAddr, sizeof(serverAddr)) == SOCKET_ERROR)
    {
        std::cerr << "Error: connect failed with error code " << WSAGetLastError() << std::endl;
        closesocket(clientSocket);
        WSACleanup();
        return 1;
    }

    std::cout << "Connected to server." << std::endl;

    char buffer[BUFFER_SIZE];
    int recvSize;

    while (true)
    {
        // 从标准输入读取数据
        std::cin.getline(buffer, BUFFER_SIZE);

        // 发送数据到服务器
        if (send(clientSocket, buffer, strlen(buffer), 0) < 0)
        {
            std::cerr << "Error: send failed with error code " << WSAGetLastError() << std::endl;
            break;
        }

        // 接收服务器返回的数据
        recvSize = recv(clientSocket, buffer, BUFFER_SIZE, 0);
        if (recvSize == 0) // 服务器已关闭连接
        {
            std::cout << "Server disconnected." << std::endl;
            break;
        }
        else if (recvSize < 0)
        {
            std::cerr << "Error: recv failed with error code " << WSAGetLastError() << std::endl;
            break;
        }

        // 在标准输出中显示服务器返回的数据
        buffer[recvSize] = '\0';
        std::cout << "Server: " << buffer << std::endl;
    }

    // 关闭客户端套接字
    closesocket(clientSocket);
    WSACleanup();

    return 0;
}

这个示例程序中，服务端使用一个while循环不断接受客户端连接请求，并为每个客户端连接创建一个新的线程来处理通信。客户端则在一个while循环中不断从标准输入读取数据，并将数据发送到服务器，然后等待服务器返回数据并在标准输出中显示。