临界区锁和互斥对象锁对比介绍

  在Win32平台上进行多线程编程，常会用到锁。下边用C++实现了互斥对象（Mutex）锁和临界区（CRITICAL_SECTION）锁，以加深理解和今后方便使用。代码已在VS2005环境下编译测试通过。

Lock.h

#ifndef _Lock_H#define _Lock_H#include <windows.h>//锁接口类class ILock{public:virtual ~ILock() {}virtual void Lock() const = 0;virtual void Unlock() const = 0;};//互斥对象锁类class Mutex : public ILock{public:Mutex();~Mutex();virtual void Lock() const;virtual void Unlock() const;private:HANDLE m_mutex;};//临界区锁类class CriSection : public ILock{public:CriSection();~CriSection();virtual void Lock() const;virtual void Unlock() const;private:CRITICAL_SECTION m_critclSection;};//锁class CMyLock{public:CMyLock(const ILock&);~CMyLock();private:const ILock& m_lock;};#endif

Lock.cpp

#include "Lock.h"//---------------------------------------------------------------------------//创建一个匿名互斥对象Mutex::Mutex(){m_mutex = ::CreateMutex(NULL, FALSE, NULL);}//销毁互斥对象，释放资源Mutex::~Mutex(){::CloseHandle(m_mutex);}//确保拥有互斥对象的线程对被保护资源的独自访问void Mutex::Lock() const{DWORD d = WaitForSingleObject(m_mutex, INFINITE);}//释放当前线程拥有的互斥对象，以使其它线程可以拥有互斥对象，对被保护资源进行访问void Mutex::Unlock() const{::ReleaseMutex(m_mutex);}//---------------------------------------------------------------------------//初始化临界资源对象CriSection::CriSection(){::InitializeCriticalSection(&m_critclSection);}//释放临界资源对象CriSection::~CriSection(){::DeleteCriticalSection(&m_critclSection);}//进入临界区，加锁void CriSection::Lock() const{::EnterCriticalSection((LPCRITICAL_SECTION)&m_critclSection);}//离开临界区，解锁void CriSection::Unlock() const{::LeaveCriticalSection((LPCRITICAL_SECTION)&m_critclSection);}//---------------------------------------------------------------------------//利用C++特性，进行自动加锁CMyLock::CMyLock(const ILock& m) : m_lock(m){m_lock.Lock();}//利用C++特性，进行自动解锁CMyLock::~CMyLock(){m_lock.Unlock();}

 下边是测试代码

// MyLock.cpp : 定义控制台应用程序的入口点。//#include <iostream>#include <process.h>#include <time.h>#include "Lock.h"using namespace std;#define ENABLE_MUTEX#define ENABLE_CRITICAL_SECTION#if defined (ENABLE_MUTEX)//创建一个互斥对象类型锁Mutex g_Lock;#elif defined (ENABLE_CRITICAL_SECTION)//创建一个临界区类型锁CriSection g_Lock;#endifvoid LockCompare(int &iNum){CMyLock lock1(g_Lock);iNum++;}//线程函数unsigned int __stdcall StartThread(void *pParam){char *pMsg = (char *)pParam;if (!pMsg){return (unsigned int)1;}CMyLock lock2(g_Lock);clock_t tStart,tEnd;tStart = clock();int iNum = 0;for (int i = 0; i < 100000; i++){LockCompare(iNum);}tEnd = clock();#if defined (ENABLE_MUTEX)cout<<"The lock type is mutex, time = "<<(tEnd - tStart)<<" ms."<<endl;#elif defined (ENABLE_CRITICAL_SECTION)cout<<"The lock type is critical section, time = "<<(tEnd - tStart)<<" ms."<<endl;#endifreturn (unsigned int)0;}int main(int argc, char* argv[]){HANDLE hThread1, hThread2;unsigned int uiThreadId1, uiThreadId2;char *pMsg1 = "First print thread.";char *pMsg2 = "Second print thread.";//创建两个工作线程，分别打印不同的消息hThread1 = (HANDLE)_beginthreadex(NULL, 0, &StartThread, (void *)pMsg1, 0, &uiThreadId1);hThread2 = (HANDLE)_beginthreadex(NULL, 0, &StartThread, (void *)pMsg2, 0, &uiThreadId2);//等待线程结束DWORD dwRet = WaitForSingleObject(hThread1,INFINITE);if ( dwRet == WAIT_TIMEOUT ){TerminateThread(hThread1,0);}dwRet = WaitForSingleObject(hThread2,INFINITE);if ( dwRet == WAIT_TIMEOUT ){TerminateThread(hThread2,0);}//关闭线程句柄，释放资源::CloseHandle(hThread1);::CloseHandle(hThread2);system("pause");return 0;}

    在线程函数StartThread中，循环100000次，对保护资源“iNum ”反复加锁，解锁。编译，运行5次，将每次打印的线程锁切换耗时时间记录下来。之后，将测试代码中的宏 #define ENABLE_MUTEX 注释掉，禁掉互斥锁，启用临界区锁，重新编译代码，运行5次。下边是分别是互斥锁和临界区锁耗时记录（不同机器上耗时会不同）：

互斥锁

线程Id

耗时 / ms

总计

1

141

125

125

125

125

641

2

140

125

140

125

156

686

 

临界区锁

线程Id

耗时 / ms

总计

1

15

16

31

31

31

124

2

31

31

31

16

31

140

 

 

    互斥锁总共耗时：641+686=1327 ms，而临界区锁：124+140=264 ms。显而易见，临界区锁耗时比互斥锁耗时节约了大概5倍的时间。

    总结：1、在同一个进程的多线程同步锁，宜用临界区锁，它比较节约线程上下文切换带来的系统开销。但因临界区工作在用户模式下，所以不能对不同进程中的多线程进行同步。2、因互斥对象锁属于内核对象，所以在进行多线程同步时速度会比较慢，但是可以在不同进程的多个线程之间进行同步。