如何检查客户端是否通过C++中的Winsock断开连接？

#include <winsock2.h>
#include<iostream>

int main()
{
    WSADATA wsaData;
    int iResult = WSAStartup(MAKEWORD(2, 2), &wsaData);
    if (iResult != NO_ERROR) {
        std::cout << "WSAStartup failed: " << iResult<< std::endl;
        return 1;
    }

    SOCKET sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (sock == INVALID_SOCKET) {
        std::cout << "socket failed: " << WSAGetLastError()<< std::endl;
        WSACleanup();
        return 1;
    }

    sockaddr_in addr;
    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = inet_addr("127.0.0.1");
    addr.sin_port = htons(8080);

    if (connect(sock, (sockaddr*)&addr, sizeof(addr)) == SOCKET_ERROR) {
        std::cout << "connect failed: " << WSAGetLastError()<< std::endl;
        closesocket(sock);
        WSACleanup();
        return 1;
    }

    char buf[1024];
    int recv_len = recv(sock, buf, sizeof(buf), 0);
    if (recv_len == 0) {
        std::cout << "Client disconnected"<< std::endl;
    } else if (recv_len == SOCKET_ERROR) {
        std::cout << "recv failed: " << WSAGetLastError()<< std::endl;
    } else {
        std::cout << "Received data: "<< std::string(buf, recv_len)<< std::endl;
    }

    closesocket(sock);
    WSACleanup();
    return 0;
}

#include <winsock2.h>
#include<iostream>

int main()
{
    WSADATA wsaData;
    int iResult = WSAStartup(MAKEWORD(2, 2), &wsaData);
    if (iResult != NO_ERROR) {
        std::cout << "WSAStartup failed: " << iResult<< std::endl;
        return 1;
    }

    SOCKET sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (sock == INVALID_SOCKET) {
        std::cout << "socket failed: " << WSAGetLastError()<< std::endl;
        WSACleanup();
        return 1;
    }

    sockaddr_in addr;
    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = inet_addr("127.0.0.1");
    addr.sin_port = htons(8080);

    if (connect(sock, (sockaddr*)&addr, sizeof(addr)) == SOCKET_ERROR) {
        std::cout << "connect failed: " << WSAGetLastError()<< std::endl;
        closesocket(sock);
        WSACleanup();
        return 1;
    }

    fd_set readfds;
    FD_ZERO(&readfds);
    FD_SET(sock, &readfds);

    timeval timeout;
    timeout.tv_sec = 5;
    timeout.tv_usec = 0;

    int select_result = select(0, &readfds, NULL, NULL, &timeout);
    if (select_result == SOCKET_ERROR) {
        std::cout << "select failed: " << WSAGetLastError()<< std::endl;
    } else if (select_result == 0) {
        std::cout << "Client disconnected"<< std::endl;
    } else {
        char buf[1024];
        int recv_len = recv(sock, buf, sizeof(buf), 0);
        if (recv_len == SOCKET_ERROR) {
            std::cout << "recv failed: " << WSAGetLastError()<< std::endl;
        } else {
            std::cout << "Received data: "<< std::string(buf, recv_len)<< std::endl;
        }
    }

    closesocket(sock);
    WSACleanup();
    return 0;
}