C++ 线程同步之临界区CRITICAL_SECTION

一、临界区

临界区又称关键代码段，指的是一小段代码在代码执行前，他需要独占一些资源。程序中通常将多线程同时访问的某个资源作为临界区，需要定义一个CRITICAL_SECTION类型的变量，然后调用InitializeCriticalSection函数对变量进行初始化；

函数声明：

VOID InitializeCriticalSection(LPCRITICAL_SECTION lpCriticalSection )；

lpCriticalSection：一个CRITICAL_SECTION结构指针，表示用于初始化的临界区；

InitializeCriticalSection函数在内部设置了CRITICAL_SECTION结构的某些成员变量，所以他不会失败。

为了将某一段代码定义为临界区，需要调用EnterCriticalSection函数；

VOID WINAPI EnterCriticalSection(__inout LPCRITICAL_SECTION lpCriticalSection);

该函数的作用是判断是否有线程访问临界区资源，如果没有，就改变CRITICAL_SECTION结构的成员变量的值，赋予当前线程访问权，函数立即返回；如果有线程正在访问资源，则进入等待状态，直到没有线程访问。

释放资源函数：

void WINAPI LeaveCriticalSection( _Inout_LPCRITICAL_SECTION lpCriticalSection);

释放CRITICAL_SECTION结构指针

void WINAPI DeleteCriticalSection(_Inout_ LPCRITICAL_SECTION lpCriticalSection);

二、学习案例

用三个线程同时运行将一个变量增加到30；

临界区对象类

#ifndef CAUTO_LOCK_H__
#define CAUTO_LOCK_H__

class CAutoLock
{
public:
	CAutoLock();
	~CAutoLock();

	void Lock();
	void UnLock();

private:
	CRITICAL_SECTION m_Section;
};

#endif

#include "stdafx.h"
#include "CAutoLock.h"

CAutoLock::CAutoLock()
{
	InitializeCriticalSection(&m_Section);
	//Lock();如果是用的时候只定义锁对象，可以不手动进入临界区和退出临界区
}

CAutoLock::~CAutoLock()
{
	DeleteCriticalSection(&m_Section);
	//UnLock();
}

void CAutoLock::Lock()
{
	EnterCriticalSection(&m_Section);
}

void CAutoLock::UnLock()
{
	LeaveCriticalSection(&m_Section);
}

三个线程创建类

#ifndef _TEST_CRITICAL_SECTION_H__
#define _TEST_CRITICAL_SECTION_H__
#include "CAutoLock.h"
class TestCriticalSection
{
public:
	TestCriticalSection();
	~TestCriticalSection();

	void StartThread();//开始线程函数

	
	static DWORD __stdcall ThreadFun1(LPVOID lParam);//线程回调函数1
	static DWORD __stdcall ThreadFun2(LPVOID lParam);//线程回调函数2
	static DWORD __stdcall ThreadFun3(LPVOID lParam);//线程回调函数3

private:
	HANDLE m_hThread1;
        HANDLE m_hThread2;
	HANDLE m_hThread3;

	CAutoLock m_lock;//三个线程公用的临界区锁

	static int m_nTotals;
};

#endif

#include "stdafx.h"
#include "CCriticalSection.h"
#include <iostream>
using namespace std;

int TestCriticalSection::m_nTotals = 0;//初始化静态成员变量

TestCriticalSection::TestCriticalSection()
{
	m_nTotals = 0;
        m_hThread1 = INVALID_HANDLE_VALUE;
	m_hThread2 = INVALID_HANDLE_VALUE;
}

TestCriticalSection::~TestCriticalSection()
{
	if (m_hThread1 != NULL)
	{
		CloseHandle(m_hThread1);
		m_hThread1 = NULL;
	}

	if (m_hThread2 != NULL)
	{
		CloseHandle(m_hThread2);
		m_hThread2 = NULL;
	}

	if (m_hThread3 != NULL)
	{
		CloseHandle(m_hThread3);
		m_hThread3 = NULL;
	}
}

DWORD __stdcall TestCriticalSection::ThreadFun1(LPVOID lParam) //static只需要加在类定义里，类定义外面的函数定义前不能写static
{
	DWORD dRet = TRUE;
	TestCriticalSection * pThis = static_cast<TestCriticalSection*>(lParam);
	while(1)
	{
		pThis->m_lock.Lock();
		pThis->m_nTotals ++;
		cout<<"ThreadFun1: m_nTotals "<<pThis->m_nTotals<<endl;
		pThis->m_lock.UnLock();
		Sleep(10);
		if (pThis->m_nTotals == 30)
		{
			break;
		}
	}

	return dRet;
}

DWORD __stdcall TestCriticalSection::ThreadFun2(LPVOID lParam)
{
	DWORD dRet = TRUE;
	TestCriticalSection * pThis = static_cast<TestCriticalSection*>(lParam);
	while(1)
	{
		pThis->m_lock.Lock();
		pThis->m_nTotals ++;
		cout<<"ThreadFun2: m_nTotals "<<pThis->m_nTotals<<endl;
		pThis->m_lock.UnLock();
		Sleep(10);
		if (pThis->m_nTotals == 30)
		{
			break;
		}
	}
	return dRet;
}

DWORD __stdcall TestCriticalSection::ThreadFun3(LPVOID lParam)
{
	DWORD dRet = TRUE;
	TestCriticalSection * pThis = static_cast<TestCriticalSection*>(lParam);
	while(1)
	{
		pThis->m_lock.Lock();
		pThis->m_nTotals ++;
		cout<<"ThreadFun3: m_nTotals "<<pThis->m_nTotals<<endl;
		pThis->m_lock.UnLock();
		Sleep(10);
		if (pThis->m_nTotals == 30)
		{
			break;
		}
	}

	return dRet;
}

void TestCriticalSection::StartThread()
{
	m_hThread1 = CreateThread(NULL, 0, &TestCriticalSection::ThreadFun1, this,  0, NULL);
	m_hThread2 = CreateThread(NULL, 0, &TestCriticalSection::ThreadFun2, this, 0, NULL);
	m_hThread3 = CreateThread(NULL, 0, &TestCriticalSection::ThreadFun3, this, 0, NULL);
}

主函数：

// CriticalSection.cpp : 定义控制台应用程序的入口点。
//

#include "stdafx.h"
#include "CCriticalSection.h"

int _tmain(int argc, _TCHAR* argv[])
{
	TestCriticalSection  CriticalSectionObj;
	CriticalSectionObj.StartThread();
	Sleep(5000);
	system("pause");
	return 0;
}

结果：

如果代码改成下面这样，不加临界区；资源访问会冲突

#include "stdafx.h"
#include "CCriticalSection.h"
#include <iostream>
using namespace std;

int TestCriticalSection::m_nTotals = 0;

TestCriticalSection::TestCriticalSection()
{
	m_nTotals = 0;
    m_hThread1 = INVALID_HANDLE_VALUE;
	m_hThread2 = INVALID_HANDLE_VALUE;
}

TestCriticalSection::~TestCriticalSection()
{
	if (m_hThread1 != NULL)
	{
		CloseHandle(m_hThread1);
		m_hThread1 = NULL;
	}

	if (m_hThread2 != NULL)
	{
		CloseHandle(m_hThread2);
		m_hThread2 = NULL;
	}

	if (m_hThread3 != NULL)
	{
		CloseHandle(m_hThread3);
		m_hThread3 = NULL;
	}
}

DWORD __stdcall TestCriticalSection::ThreadFun1(LPVOID lParam) //static只需要加在类定义里，类定义外面的函数定义前不能写static
{
	DWORD dRet = TRUE;
	TestCriticalSection * pThis = static_cast<TestCriticalSection*>(lParam);
	while(1)
	{
		//pThis->m_lock.Lock();
		pThis->m_nTotals ++;
		cout<<"ThreadFun1: m_nTotals "<<pThis->m_nTotals<<endl;
		//pThis->m_lock.UnLock();
		Sleep(10);
		if (pThis->m_nTotals == 30)
		{
			break;
		}
	}

	return dRet;
}

DWORD __stdcall TestCriticalSection::ThreadFun2(LPVOID lParam)
{
	DWORD dRet = TRUE;
	TestCriticalSection * pThis = static_cast<TestCriticalSection*>(lParam);
	while(1)
	{
		//pThis->m_lock.Lock();
		pThis->m_nTotals ++;
		cout<<"ThreadFun2: m_nTotals "<<pThis->m_nTotals<<endl;
		//pThis->m_lock.UnLock();
		Sleep(10);
		if (pThis->m_nTotals == 30)
		{
			break;
		}
	}
	return dRet;
}

DWORD __stdcall TestCriticalSection::ThreadFun3(LPVOID lParam)
{
	DWORD dRet = TRUE;
	TestCriticalSection * pThis = static_cast<TestCriticalSection*>(lParam);
	while(1)
	{
		//pThis->m_lock.Lock();
		pThis->m_nTotals ++;
		cout<<"ThreadFun3: m_nTotals "<<pThis->m_nTotals<<endl;
		//pThis->m_lock.UnLock();
		Sleep(10);
		if (pThis->m_nTotals == 30)
		{
			break;
		}
	}

	return dRet;
}

void TestCriticalSection::StartThread()
{
	m_hThread1 = CreateThread(NULL, 0, &TestCriticalSection::ThreadFun1, this,  0, NULL);
	m_hThread2 = CreateThread(NULL, 0, &TestCriticalSection::ThreadFun2, this, 0, NULL);
	m_hThread3 = CreateThread(NULL, 0, &TestCriticalSection::ThreadFun3, this, 0, NULL);
}

结果可能会出现下面这种状况