主线程作为server，子线程起client，用c++怎么写

可以使用C++中的socket编程来实现这个功能。以下是一个简单的示例代码：

#include <iostream>
#include <thread>
#include <winsock2.h> // Windows平台下的socket头文件

#pragma comment(lib, "ws2_32.lib") // 链接ws2_32库

using namespace std;

void clientThread()
{
    // 创建socket
    SOCKET clientSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (clientSocket == INVALID_SOCKET) {
        cerr << "Error creating socket: " << WSAGetLastError() << endl;
        return;
    }

    // 设置server地址
    sockaddr_in serverAddr = {};
    serverAddr.sin_family = AF_INET;
    serverAddr.sin_addr.s_addr = inet_addr("127.0.0.1"); // server的IP地址
    serverAddr.sin_port = htons(12345); // server的端口号

    // 连接server
    if (connect(clientSocket, (sockaddr*)&serverAddr, sizeof(serverAddr)) == SOCKET_ERROR) {
        cerr << "Error connecting to server: " << WSAGetLastError() << endl;
        closesocket(clientSocket);
        return;
    }

    // 发送数据
    char message[] = "Hello from client!";
    if (send(clientSocket, message, strlen(message), 0) == SOCKET_ERROR) {
        cerr << "Error sending data: " << WSAGetLastError() << endl;
        closesocket(clientSocket);
        return;
    }
    cout << "Data sent from client." << endl;

    // 关闭socket
    closesocket(clientSocket);
}

int main()
{
    // 初始化Winsock库
    WSADATA wsaData = {};
    if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0) {
        cerr << "Error initializing Winsock." << endl;
        return 1;
    }

    // 创建socket
    SOCKET serverSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (serverSocket == INVALID_SOCKET) {
        cerr << "Error creating socket: " << WSAGetLastError() << endl;
        WSACleanup();
        return 1;
    }

    // 设置server地址
    sockaddr_in serverAddr = {};
    serverAddr.sin_family = AF_INET;
    serverAddr.sin_addr.s_addr = INADDR_ANY;
    serverAddr.sin_port = htons(12345); // 指定端口号

    // 绑定socket和地址
    if (bind(serverSocket, (sockaddr*)&serverAddr, sizeof(serverAddr)) == SOCKET_ERROR) {
        cerr << "Error binding socket: " << WSAGetLastError() << endl;
        closesocket(serverSocket);
        WSACleanup();
        return 1;
    }

    // 监听socket
    if (listen(serverSocket, SOMAXCONN) == SOCKET_ERROR) {
        cerr << "Error listening on socket: " << WSAGetLastError() << endl;
        closesocket(serverSocket);
        WSACleanup();
        return 1;
    }
    cout << "Server started." << endl;

    // 接受client连接
    sockaddr_in clientAddr = {};
    int addrLen = sizeof(clientAddr);
    SOCKET clientSocket = accept(serverSocket, (sockaddr*)&clientAddr, &addrLen);
    if (clientSocket == INVALID_SOCKET) {
        cerr << "Error accepting client connection: " << WSAGetLastError() << endl;
        closesocket(serverSocket);
        WSACleanup();
        return 1;
    }
    cout << "Client connected." << endl;

    // 接收数据
    char buffer[1024];
    int recvLen = recv(clientSocket, buffer, sizeof(buffer), 0);
    if (recvLen == SOCKET_ERROR) {
        cerr << "Error receiving data: " << WSAGetLastError() << endl;
        closesocket(clientSocket);
        closesocket(serverSocket);
        WSACleanup();
        return 1;
    }
    buffer[recvLen] = '\0';
    cout << "Data received from client: " << buffer << endl;

    // 关闭socket
    closesocket(clientSocket);
    closesocket(serverSocket);
    WSACleanup();

    // 创建client线程
    thread client(clientThread);
    client.join();

    return 0;
}

代码中创建了一个主线程作为server，负责监听端口并接受client连接。当client连接成功后，主线程从client接收数据，并关闭连接。同时，主线程还创建了一个子线程，作为client向server发送数据。