C++多线程同步技巧（二）--- 事件

简介

Windows在线程控制方面提供了多种信号处理机制，其中一种便是使用 CreateEvent() 函数创建事件，然后使用信号控制线程运行。其中将事件变为有信号可使用 SetEvent() 函数，将事件信号复位（变为无信号）可使用 ResetEvent() 函数，信号可以配合 WaitForSingleObject() 函数对线程的同步进行控制，当有信号时，此函数便会放行；无信号时，此函数会将阻塞。

提示： CreateEvent() 函数的参数 bManualReset 的含义是信号是否由人工复位，如果选择true，则信号必须手动采用ResetEvent() 函数进行复位操作。在这种情况下，可能会偶尔出现线程不同步的情况，问题出在可能同时会有多个线程穿过 WaitForSingleObject() 函数，导致复位失效，所以在这种情况下，为确保万无一失，我们一般会再添加一个限制条件，例如临界区或互斥体；如果选择的是false，则当一个信号经过 WaitForSingleObject() 函数的时候，函数会自动将事件信号复位。

代码样例

• bManualReset参数为 false

//
// FileName : ThreadEventDemo.cpp
// Creator : PeterZheng
// Date : 2018/9/23 18:00
// Comment : The usage of "CreateEvent"
//


#pragma once

#include <cstdio>
#include <iostream>
#include <cstdlib>
#include <windows.h>

using namespace std;

DWORD WINAPI func1(LPVOID lpParam);
DWORD WINAPI func2(LPVOID lpParam);

HANDLE hEvent = NULL;
unsigned int unCount = 0;

DWORD WINAPI func1(LPVOID lpParam)
{
    while (true)
    {
        WaitForSingleObject(hEvent, INFINITE);
        if (unCount < 100)
        {
            unCount++;
            Sleep(10);
            cout << "Count: " << unCount << endl;
            SetEvent(hEvent);
            continue;
        }
        // 因为WaitForSingleObject函数会自动复位，可能导致另外一个线程始终等待不到信号，造成“假死”现象，所以这里需要使用SetEvent重置信号。
        SetEvent(hEvent);
        break;
    }
    return 0;
}

DWORD WINAPI func2(LPVOID lpParam)
{
    while (true)
    {
        WaitForSingleObject(hEvent, INFINITE);
        if (unCount < 100)
        {
            unCount++;
            Sleep(10);
            cout << "Count: " << unCount << endl;
            SetEvent(hEvent); // 设置事件为有信号状态
            continue;
        }
        SetEvent(hEvent);
        break;
    }
    return 0;
}

int main(void)
{
    HANDLE hThread[2] = { NULL };
    hEvent = CreateEvent(NULL, false, false, NULL); //创建一个匿名事件,当参数bManualReset设置为false时
    hThread[0] = CreateThread(NULL, 0, func1, NULL, 0, NULL);
    cout << "Thread-1 is RUNNING" << endl;
    hThread[1] = CreateThread(NULL, 0, func2, NULL, 0, NULL);
    cout << "Thread-2 is RUNNING" << endl;
    SetEvent(hEvent);
    WaitForMultipleObjects(2, hThread, true, INFINITE); //等待两个线程运行结束
    CloseHandle(hThread[0]);
    CloseHandle(hThread[1]);
    CloseHandle(hEvent);
    system("pause");
    return 0;
}

• bManualReset参数为 true

//
// FileName : ThreadEventDemo.cpp
// Creator : PeterZheng
// Date : 2018/9/23 18:00
// Comment : The usage of "CreateEvent"
//


#pragma once

#include <cstdio>
#include <iostream>
#include <cstdlib>
#include <windows.h>

using namespace std;

DWORD WINAPI func1(LPVOID lpParam);
DWORD WINAPI func2(LPVOID lpParam);

HANDLE hEvent = NULL;
HANDLE hMutex = NULL;
unsigned int unCount = 0;

DWORD WINAPI func1(LPVOID lpParam)
{
    while (true)
    {
        WaitForSingleObject(hEvent, INFINITE);
        WaitForSingleObject(hMutex, INFINITE); //为互斥体上锁
        ResetEvent(hEvent); // 重置事件为无信号状态
        if (unCount < 100)
        {
            unCount++;
            Sleep(10);
            cout << "Count: " << unCount << endl;
            SetEvent(hEvent); // 设置事件为有信号状态
            ReleaseMutex(hMutex); //互斥体解锁
        }
        else
        {
            SetEvent(hEvent);
            ReleaseMutex(hMutex);
            break;
        }
    }
    return 0;
}

DWORD WINAPI func2(LPVOID lpParam)
{
    while (true)
    {
        WaitForSingleObject(hEvent, INFINITE);
        WaitForSingleObject(hMutex, INFINITE); //为互斥体上锁
        ResetEvent(hEvent); // 重置事件为无信号状态
        if (unCount < 100)
        {
            unCount++;
            Sleep(10);
            cout << "Count: " << unCount << endl;
            SetEvent(hEvent); // 设置事件为有信号状态
            ReleaseMutex(hMutex);
        }
        else
        {
            SetEvent(hEvent);
            ReleaseMutex(hMutex);
            break;
        }
    }
    return 0;
}

int main(void)
{
    HANDLE hThread[2] = { NULL };
    hEvent = CreateEvent(NULL, true, false, NULL); //创建一个匿名事件,当参数bManualReset设置为true时
    hMutex = CreateMutex(NULL, false, NULL); //创建一个匿名互斥体
    hThread[0] = CreateThread(NULL, 0, func1, NULL, 0, NULL);
    cout << "Thread-1 is RUNNING" << endl;
    hThread[1] = CreateThread(NULL, 0, func2, NULL, 0, NULL);
    cout << "Thread-2 is RUNNING" << endl;
    SetEvent(hEvent); // 设置事件为有信号状态
    WaitForMultipleObjects(2, hThread, true, INFINITE); //等待两个线程运行结束
    CloseHandle(hThread[0]);
    CloseHandle(hThread[1]);
    CloseHandle(hEvent);
    CloseHandle(hMutex);
    system("pause");
    return 0;
}

参考文档

【1】https://blog.youkuaiyun.com/s_lisheng/article/details/74278765

转载于:https://www.cnblogs.com/PeterZ1997/p/9735331.html