C++-用关键段实现线程同步

Question

下面是一段用关键段实现线程同步的例子：

//关键段测试代码

#include <iostream>
#include <windows.h>

using namespace std;

CRITICAL_SECTION g_cs;//创建CRITICAL_SECTION结构全局变量

int g_x = 50;

//线程函数1
DWORD WINAPI ThreadFunc1(LPVOID lpParam)
{
    while(TRUE)
    {
        EnterCriticalSection(&g_cs);
        cout << "Thread1 get the access to resource" << endl;
        if(g_x > 25)
        {
           Sleep(1);
           cout<<"thread1 sell ticket : "<<g_x--<<endl;
        }
        else
           break;

        LeaveCriticalSection(&g_cs);
    }

    return 0;
}

//线程函数2
DWORD WINAPI ThreadFunc2(LPVOID lpParameter)
{
    while(TRUE)
    {
        EnterCriticalSection(&g_cs);
        cout << "Thread2 get the access to resource" << endl;
        if(g_x > 0)
        {
           Sleep(1);
           cout<<"thread2 sell ticket : "<<g_x--<<endl;
        }
        else
           break;
        LeaveCriticalSection(&g_cs);
    }

    return 0;
}

int main()
{
    HANDLE hThread1;
    HANDLE hThread2;
    InitializeCriticalSection(&g_cs);
    hThread1=CreateThread(NULL,0,ThreadFunc1,NULL,0,NULL);
    hThread2=CreateThread(NULL,0,ThreadFunc2,NULL,0,NULL);

    CloseHandle(hThread1);
    CloseHandle(hThread2);

    Sleep(4000);
    DeleteCriticalSection(&g_cs);

    return 0;
}

Answer 1

线程1 break后没有LeaveCriticalSection，导致了问题。
两个线程中都添加类似下面的处理

//线程函数1
DWORD WINAPI ThreadFunc1(LPVOID lpParam)
{
while(TRUE)
{
EnterCriticalSection(&g_cs);
cout << "Thread1 get the access to resource" << endl;
if(g_x > 25)
{
Sleep(10);
cout<<"thread1 sell ticket : "<<g_x--<<endl;
}
else
{
LeaveCriticalSection(&g_cs);//add
break;
}

LeaveCriticalSection(&g_cs);
}
cout << "Thread1 exit" << endl;
return 0;
}

Answer 2

哥哥：试着修改成这个哈，首先一点： sleep() 是要释放资源的,不然在你睡眠过程中你还是将资源锁住的哈。

cout<<"thread2 sell ticket : "<<g_x--<<endl;
LeaveCriticalSection(&g_cs);
Sleep(1);

Answer 3

你的代码存在几个细节问题：
1.睡眠的时候占用了资源，应该在睡眠前释放资源
2.else判断后跳出循环存在资源未释放的危险，造成死锁
3.CloseHandle(hThread1) 函数并没有关闭线程，DeleteCriticalSection(&g_cs)此时释放资源可能会造成正在运行的线程出现资源访问错误（主进程调用sleep进行睡眠是你程序没出错的原因，但这样并不可靠）。

下面是我修改过后的代码，你可以试试：

 #include <iostream>
#include <windows.h>

using namespace std;

CRITICAL_SECTION g_cs;//创建CRITICAL_SECTION结构全局变量

int g_x = 50;

//线程函数1
DWORD WINAPI ThreadFunc1(LPVOID lpParam)
{
while(TRUE)
{
EnterCriticalSection(&g_cs);
cout << "Thread1 get the access to resource" << endl;
if(g_x > 25)
{
cout<<"thread1 sell ticket : "<<g_x--<<endl;
}
else
break;

LeaveCriticalSection(&g_cs);
Sleep(1);
}

LeaveCriticalSection(&g_cs);
return 0;
}

//线程函数2
DWORD WINAPI ThreadFunc2(LPVOID lpParameter)
{
while(TRUE)
{
EnterCriticalSection(&g_cs);
cout << "Thread2 get the access to resource" << endl;
if(g_x > 0)
{
cout<<"thread2 sell ticket : "<<g_x--<<endl;
}
else
break;

LeaveCriticalSection(&g_cs);
Sleep(1);
}

LeaveCriticalSection(&g_cs);
return 0;
}

int main()
{
HANDLE hThread1;
HANDLE hThread2;
InitializeCriticalSection(&g_cs);
hThread1=CreateThread(NULL,0,ThreadFunc1,NULL,0,NULL);
hThread2=CreateThread(NULL,0,ThreadFunc2,NULL,0,NULL);

WaitForSingleObject(hThread1,INFINITE);
WaitForSingleObject(hThread2,INFINITE);

CloseHandle(hThread1);
CloseHandle(hThread2);

DeleteCriticalSection(&g_cs);

cout<<"Press Enter to end"<<endl;
getchar();

return 0;
}