C++多線程同步技巧（二） ---事件

簡介

Windows線上程控制方面提供了多種信号處理機制，其中一種便是使用 CreateEvent() 函數建立事件，然後使用信号控制線程運作。其中将事件變為有信号可使用 SetEvent() 函數，将事件信号複位（變為無信号）可使用 ResetEvent() 函數，信号可以配合 WaitForSingleObject() 函數對線程的同步進行控制，當有信号時，此函數便會放行；無信号時，此函數會将阻塞。

提示： CreateEvent() 函數的參數 bManualReset 的含義是信号是否由人工複位，如果選擇true，則信号必須手動采用ResetEvent() 函數進行複位操作。在這種情況下，可能會偶爾出現線程不同步的情況，問題出在可能同時會有多個線程穿過 WaitForSingleObject() 函數，導緻複位失效，是以在這種情況下，為確定萬無一失，我們一般會再添加一個限制條件，例如臨界區或互斥體；如果選擇的是false，則當一個信号經過 WaitForSingleObject() 函數的時候，函數會自動将事件信号複位。

代碼樣例

• bManualReset參數為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); // 重置事件為無信号狀态
    if (unCount < 100)
    {
      unCount++;
      Sleep(10);
      cout << "Count: " << unCount << endl;
      SetEvent(hEvent); // 設定事件為有信号狀态
    }
    else
    {
      SetEvent(hEvent);
      break;
    }
  }
  return 0;
}

int main(void)
{
  HANDLE hThread[2] = { NULL };
  hEvent = CreateEvent(NULL, false, false, NULL); //建立一個匿名事件,當參數bManualReset設定為false時
  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); //等待兩個線程運作結束
  CloseHandle(hThread[0]);
  CloseHandle(hThread[1]);
  CloseHandle(hEvent);
  system("pause");
  return 0;
}      

• bManualReset參數為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); //為互斥體上鎖
    ResetEvent(hEvent); // 重置事件為無信号狀态
    if (unCount < 100)
    {
      unCount++;
      Sleep(10);
      cout << "Count: " << unCount << endl;
      SetEvent(hEvent); // 設定事件為有信号狀态
      ReleaseMutex(hMutex); //互斥體解鎖
    }
    else
    {
      SetEvent(hEvent);
      ReleaseMutex(hMutex);
      break;
    }
  }
  return 0;
}

DWORD WINAPI func2(LPVOID lpParam)
{
  while (true)
  {
    WaitForSingleObject(hEvent, INFINITE);
    WaitForSingleObject(hMutex, INFINITE); //為互斥體上鎖
    ResetEvent(hEvent); // 重置事件為無信号狀态
    if (unCount < 100)
    {
      unCount++;
      Sleep(10);
      cout << "Count: " << unCount << endl;
      SetEvent(hEvent); // 設定事件為有信号狀态
      ReleaseMutex(hMutex);
    }
    else
    {
      SetEvent(hEvent);
      ReleaseMutex(hMutex);
      break;
    }
  }
  return 0;
}

int main(void)
{
  HANDLE hThread[2] = { NULL };
  hEvent = CreateEvent(NULL, true, false, NULL); //建立一個匿名事件,當參數bManualReset設定為true時
  hMutex = CreateMutex(NULL, false, NULL); //建立一個匿名互斥體
  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); // 設定事件為有信号狀态
  WaitForMultipleObjects(2, hThread, true, INFINITE); //等待兩個線程運作結束
  CloseHandle(hThread[0]);
  CloseHandle(hThread[1]);
  CloseHandle(hEvent);
  CloseHandle(hMutex);
  system("pause");
  return 0;
}