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);
    ResetEvent(hEvent);
    if (unCount < 100)
    {
      unCount++;
      Sleep(10);
      cout << "Count: " << unCount << endl;
      SetEvent(hEvent);
    }
    else
    {
      SetEvent(hEvent);
      break;
    }
  }
  return 0;
}

DWORD WINAPI func2(LPVOID lpParam)
{
  while (true)
  {
    WaitForSingleObject(hEvent, INFINITE);
    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
    }
    else
    {
      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;
}

​