命名管道客户端提示句柄无效

/*
    两个进程用命名管道实现售票，我思路是服务端先ReadFile，再WriteFile。客户端先WriteFile，再ReadFile。这样子两边都能共享同一个票数(tickets)。不知道为什么传了一次之后句柄就无效。
*/
// 这是客户端的代码
#include <windows.h>
#include <iostream>

using namespace std;

int main(void)
{
    HANDLE hpip;

    char buf[] = "命名管道测试程序";
    DWORD readbuf;
    DWORD writebuf;
    int wait_time = 0;
    int tickets = 100;

    cout << "正在连接命名管道！" << endl;
    if (WaitNamedPipe("\\\\.\\pipe\\pipename", NMPWAIT_WAIT_FOREVER)) //连接命名管道
    {
        hpip = CreateFile("\\\\.\\pipe\\pipename", GENERIC_READ | GENERIC_WRITE, 0, NULL,
                          OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
        //打开指定命名管道
        if (hpip == INVALID_HANDLE_VALUE)
        {
            cout << "打开命名管道失败！" << endl;
        }
        else
        {
            cout << "连接成功！" << endl;
            while (1)
            {

            --tickets;
            itoa(tickets, buf, 10);

            if (WriteFile(hpip, buf, sizeof(buf), &writebuf, NULL))
            {

                cout << "writing..." << endl;
                wait_time = rand() % 4901 + 100;
                Sleep(wait_time);
                cout << "窗口2售出一张票，剩余票数" << tickets << "等待时间" << wait_time << "毫秒" << endl;
            }
            else
            {
                cout << "DEBUG: " << tickets << endl;
                cout << GetLastError() << endl;
                Sleep(5000);

            }

            if (ReadFile(hpip, buf, 200, &readbuf, NULL))
            {
                cout << "reading..." << endl;
                tickets = atoi(buf);
                cout << "tickets: " << tickets << endl;
            }
            if (tickets == 0)
                break;

            }
        }


    }
    else
    {
        cout << "连接命名管道失败！" << endl;
    }

    CloseHandle(hpip);

    return 0;
}

// 这是服务端的代码
#include <windows.h>
#include <iostream>
#include <stdlib.h>

using namespace std;

int main(void)
{
    HANDLE hpip;
    OVERLAPPED ovi = {0};
    char buf[200];
    DWORD readbuf;
    DWORD writebuf;
    int tickets = 0;
    int wait_time = 0;

    hpip = CreateNamedPipe("\\\\.\\pipe\\pipename", PIPE_ACCESS_DUPLEX,
                          PIPE_TYPE_BYTE, 10, 0, 0, 0, NULL);

    cout << "创建管道成功，正在等待客户连接！" << endl;

    if (ConnectNamedPipe(hpip, &ovi))
    {
        cout << "客户端连接成功！" << endl;
        cout << "正在读取数据！" << endl;


         while (1)
            {

            if (ReadFile(hpip, buf, 200, &readbuf, NULL))
            {
                cout << "Reading..." << endl;
                tickets = atoi(buf);
                cout << "tickets: " << tickets << endl;
            }
            else
            {
                cout << "DEBUG: " << tickets << endl;
                cout << GetLastError() << endl;
            }
            if (tickets == 0)
                break;
            tickets--;

            itoa(tickets, buf, 10);
            if (WriteFile(hpip, buf, sizeof(buf), &writebuf, NULL))
            {
                cout << "Writing..." << endl;
                wait_time = rand() % 4901 + 100;
                Sleep(wait_time);
                cout << "窗口1售出一张票，剩余票数" << tickets << "等待时间" << wait_time << "毫秒" << endl;
            }
            else
            {
                cout << "DEBUG: " << tickets << endl;
                cout << GetLastError() << endl;
            }

            }

    }
    CloseHandle(hpip);


    return 0;
}