MFC学习（24）线程锁的概念函数EnterCriticalSection和LeaveCriticalSection的用法

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线程锁的概念函数EnterCriticalSection和LeaveCriticalSection的用法

注：使用结构CRITICAL_SECTION 需加入头文件#include “afxmt.h”

定义一个全局的锁 CRITICAL_SECTION的实例
和一个静态全局变量

CRITICAL_SECTION cs;//可以理解为锁定一个资源
static int n_AddValue = 0;//定义一个静态的全部变量n_AddValue

创建两个线程函数，代码实现如下：

//第一个线程
UINT FirstThread(LPVOID lParam)
{
EnterCriticalSection(&cs);//加锁接下来的代码处理过程中不允许其他线程进行操作，除非遇到LeaveCriticalSection
for(int i = 0; i<10; i++){
n_AddValue ++;
cout << “n_AddValue in FirstThread is “<<n_AddValue <<endl;
}
LeaveCriticalSection(&cs);//解锁到EnterCriticalSection之间代码资源已经释放了，其他线程可以进行操作
return 0;
}
//第二个线程
UINT SecondThread(LPVOID lParam)
{
EnterCriticalSection(&cs);//加锁
for(int i = 0; i<10; i++){
n_AddValue ++;
cout << “n_AddValue in SecondThread is “<<n_AddValue <<endl;
}
LeaveCriticalSection(&cs);//解锁
return 0;
}

在主函数添加以下代码

int _tmain(int argc, TCHAR* argv[], TCHAR* envp[])
{
int nRetCode = 0;
// 初始化 MFC 并在失败时显示错误
if (!AfxWinInit(::GetModuleHandle(NULL), NULL, ::GetCommandLine(), 0))
{
// TODO: 更改错误代码以符合您的需要
_tprintf(_T(“错误: MFC 初始化失败/n“));
nRetCode = 1;
}
else
{
InitializeCriticalSection(&cs);//初始化结构CRITICAL_SECTION
CWinThread *pFirstThread,*pSecondThread;//存储函数AfxBeginThread返回的CWinThread指针
pFirstThread = AfxBeginThread(FirstThread,LPVOID(NULL));//启动第一个线程
pSecondThread = AfxBeginThread(SecondThread,LPVOID(NULL));//启动第二个线程
HANDLE hThreadHandle[2];//
hThreadHandle[0] = pFirstThread->m_hThread;
hThreadHandle[1] = pSecondThread->m_hThread;
//等待线程返回
WaitForMultipleObjects(2,hThreadHandle,TRUE,INFINITE);
}
return nRetCode;
}

输出：

n_AddValue in FirstThread is 1
n_AddValue in FirstThread is 2
n_AddValue in FirstThread is 3
n_AddValue in FirstThread is 4
n_AddValue in FirstThread is 5
n_AddValue in FirstThread is 6
n_AddValue in FirstThread is 7
n_AddValue in FirstThread is 8
n_AddValue in FirstThread is 9
n_AddValue in FirstThread is 10
n_AddValue in SecondThread is 11
n_AddValue in SecondThread is 12
n_AddValue in SecondThread is 13
n_AddValue in SecondThread is 14
n_AddValue in SecondThread is 15
n_AddValue in SecondThread is 16
n_AddValue in SecondThread is 17
n_AddValue in SecondThread is 18
n_AddValue in SecondThread is 19
n_AddValue in SecondThread is 20

如果把两个线程函数中的EnterCriticalSection和LeaveCriticalSection位置移到for循环中去，线程的执行顺序将会改变
输出也就跟着改变，如：

//第一个线程
UINT FirstThread(LPVOID lParam)
{
for(int i = 0; i<10; i++){
EnterCriticalSection(&cs);//加锁锁移到for循环内部里
n_AddValue ++;
cout << “n_AddValue in FirstThread is “<<n_AddValue <<endl;
LeaveCriticalSection(&cs);//解锁
}
return 0;
}
//第二个线程
UINT SecondThread(LPVOID lParam)
{
for(int i = 0; i<10; i++){
EnterCriticalSection(&cs);//加锁
n_AddValue ++;
cout << “n_AddValue in SecondThread is “<<n_AddValue <<endl;
LeaveCriticalSection(&cs);//解锁
}
return 0;
}

其他代码不变，输出的结果如下：

n_AddValue in FirstThread is 1
n_AddValue in SecondThread is 2
n_AddValue in FirstThread is 3
n_AddValue in SecondThread is 4
n_AddValue in FirstThread is 5
n_AddValue in SecondThread is 6
n_AddValue in FirstThread is 7
n_AddValue in SecondThread is 8
n_AddValue in FirstThread is 9
n_AddValue in SecondThread is 10
n_AddValue in FirstThread is 11
n_AddValue in SecondThread is 12
n_AddValue in FirstThread is 13
n_AddValue in SecondThread is 14
n_AddValue in FirstThread is 15
n_AddValue in SecondThread is 16
n_AddValue in FirstThread is 17
n_AddValue in SecondThread is 18
n_AddValue in FirstThread is 19
n_AddValue in SecondThread is 20