C++多线程（三）

今天药忘吃喽~ 2022-08-14 00:43 121阅读 0赞

多线程同步之Critical Sections（功能与Mutex相同，保证某一时刻只有一个线程能够访问共享资源，但是不是内核对象，所以访问速度要比Mutex快，但是增没有等待超时的功能，所以有可能会导致死锁，使用时可以根据实际的情况选择其一）

一 Critical Sections

1） 因为Critical Sections不是内核对象，所以只能用来统一进程内线程间的同步，不能用来多个不同进程间的线程的同步。

2） 如果在Critical Sections中间突然程序crash或是exit而没有调用LeaveCriticalSection，则结果是改线程所对应的内核不能被释放，该线程成为死线程。

3） 要比其他的内核对象的速度要快。  
  
二 使用CriticalSections的简单实例，Stack在push的时候可以分为3个步骤，看下面的代码，但是如果在第2步后此线程中断切换到其他的线程，其他的线程push后再返回执行时，此线程继续执行，这样有可能刚才其他线程push就会被覆盖了，在stack里找不到了。（下面的代码在debug下使用了CriticalSection，release下可能有问题）

![None.gif][]\#include <windows.h> 
![None.gif][]\#include <process.h> 
![None.gif][]\#include <stdio.h> 
![ExpandedBlockStart.gif][]/ 
![None.gif][]//stack: 
![None.gif][]struct Node  
![ExpandedBlockStart.gif][]\{ 
![InBlock.gif][]    struct Node \*next;  
![InBlock.gif][]    int data;  
![ExpandedBlockEnd.gif][]\};  
![None.gif][]struct Stack  
![ExpandedBlockStart.gif][]\{  
![InBlock.gif][]    struct Node \*head;  
![InBlock.gif][]\#ifdef \_DEBUG 
![InBlock.gif][]    CRITICAL\_SECTION critical\_sec;  
![InBlock.gif][]\#endif 
![InBlock.gif][] 
![InBlock.gif][]    Stack() 
![ExpandedSubBlockStart.gif][]    \{ 
![InBlock.gif][]        head = NULL; 
![InBlock.gif][]\#ifdef \_DEBUG 
![InBlock.gif][]        InitializeCriticalSection(&critical\_sec); 
![InBlock.gif][]\#endif 
![ExpandedSubBlockEnd.gif][]    \} 
![InBlock.gif][]    ~Stack() 
![ExpandedSubBlockStart.gif][]    \{         
![InBlock.gif][]        if(head != NULL)         
![ExpandedSubBlockStart.gif][]        \{ 
![InBlock.gif][]            if(NULL == head->next) 
![ExpandedSubBlockStart.gif][]            \{                 
![InBlock.gif][]                delete head; 
![InBlock.gif][]                head = NULL; 
![ExpandedSubBlockEnd.gif][]            \} 
![InBlock.gif][]            else 
![ExpandedSubBlockStart.gif][]            \{ 
![InBlock.gif][]                Node \*p = head; 
![InBlock.gif][]                Node \*q = head->next; 
![InBlock.gif][] 
![InBlock.gif][]                while(q != NULL) 
![ExpandedSubBlockStart.gif][]                \{                     
![InBlock.gif][]                    delete p; 
![InBlock.gif][]                    p = q; 
![InBlock.gif][]                    q = q->next; 
![ExpandedSubBlockEnd.gif][]                \};                 
![InBlock.gif][]                delete p; 
![InBlock.gif][]                p = NULL; 
![ExpandedSubBlockEnd.gif][]            \}     
![ExpandedSubBlockEnd.gif][]        \} 
![InBlock.gif][]\#ifdef \_DEBUG 
![InBlock.gif][]        DeleteCriticalSection(&critical\_sec); 
![InBlock.gif][]\#endif 
![ExpandedSubBlockEnd.gif][]    \} 
![InBlock.gif][]    void Push (int num)  
![ExpandedSubBlockStart.gif][]    \{  
![InBlock.gif][]        //enter critical section, add a new node and then      
![InBlock.gif][]\#ifdef \_DEBUG 
![InBlock.gif][]        EnterCriticalSection (&critical\_sec); 
![InBlock.gif][]\#endif 
![InBlock.gif][]        Node \* node = new Node(); 
![InBlock.gif][]        node->next = head; 
![InBlock.gif][]        node->data = num; 
![InBlock.gif][]        head = node;   
![InBlock.gif][]        printf("Stack:%d\\n",num); 
![InBlock.gif][]        //leave critical section  
![InBlock.gif][]\#ifdef \_DEBUG 
![InBlock.gif][]        LeaveCriticalSection (&critical\_sec); 
![InBlock.gif][]\#endif 
![ExpandedSubBlockEnd.gif][]    \}  
![InBlock.gif][]    int Pop ()  
![ExpandedSubBlockStart.gif][]    \{  
![InBlock.gif][]\#ifdef \_DEBUG 
![InBlock.gif][]        EnterCriticalSection (&critical\_sec); 
![InBlock.gif][]\#endif 
![InBlock.gif][]        int result = 0; 
![InBlock.gif][]        if(head!= NULL) 
![ExpandedSubBlockStart.gif][]        \{ 
![InBlock.gif][]            result = head->data; 
![InBlock.gif][]            if(head->next != NULL) 
![ExpandedSubBlockStart.gif][]            \{ 
![InBlock.gif][]                Node \*temp = head->next;  
![InBlock.gif][]                delete head; 
![InBlock.gif][]                head = temp; 
![ExpandedSubBlockEnd.gif][]            \} 
![InBlock.gif][]            else 
![InBlock.gif][]                head = NULL; 
![ExpandedSubBlockEnd.gif][]        \}     
![InBlock.gif][]\#ifdef \_DEBUG 
![InBlock.gif][]        LeaveCriticalSection (&critical\_sec);  
![InBlock.gif][]\#endif 
![InBlock.gif][]        return result;  
![ExpandedSubBlockEnd.gif][]    \} 
![ExpandedBlockEnd.gif][]\}; 
![None.gif][] 
![ExpandedBlockStart.gif][]// 
![None.gif][]//test: 
![None.gif][]unsigned  \_\_stdcall Thread1(void \* pVoid) 
![ExpandedBlockStart.gif][]\{ 
![InBlock.gif][]    Stack \*stack = ((Stack\*)pVoid); 
![InBlock.gif][]    for(int i = 200; i<220;++i) 
![ExpandedSubBlockStart.gif][]    \{ 
![InBlock.gif][]        stack->Push(i); 
![ExpandedSubBlockEnd.gif][]    \} 
![InBlock.gif][]    return 1;         
![ExpandedBlockEnd.gif][]\} 
![None.gif][]unsigned \_\_stdcall Thread2(void \*pVoid) 
![ExpandedBlockStart.gif][]\{ 
![InBlock.gif][]    Stack \*stack = ((Stack\*)pVoid); 
![InBlock.gif][]    for(int i = 0; i<20; ++i) 
![ExpandedSubBlockStart.gif][]    \{ 
![InBlock.gif][]        stack->Push(i);         
![ExpandedSubBlockEnd.gif][]    \} 
![InBlock.gif][]    return 1; 
![ExpandedBlockEnd.gif][]\} 
![None.gif][]int main() 
![ExpandedBlockStart.gif][]\{  
![InBlock.gif][]    Stack stack; 
![InBlock.gif][]    stack.Push(1000); 
![InBlock.gif][]    stack.Push(1000); 
![InBlock.gif][] 
![InBlock.gif][]    HANDLE   hth1; 
![InBlock.gif][]    unsigned  uiThread1ID; 
![InBlock.gif][] 
![InBlock.gif][]    hth1 = (HANDLE)\_beginthreadex( NULL,         // security 
![InBlock.gif][]        0,            // stack size 
![InBlock.gif][]        Thread1, 
![InBlock.gif][]        (void\*)&stack,           // arg list 
![InBlock.gif][]        CREATE\_SUSPENDED,  // so we can later call ResumeThread() 
![InBlock.gif][]        &uiThread1ID ); 
![InBlock.gif][] 
![InBlock.gif][]    if ( hth1 == 0 ) 
![InBlock.gif][]        printf("Failed to create thread 1\\n"); 
![InBlock.gif][] 
![InBlock.gif][]    DWORD   dwExitCode; 
![InBlock.gif][] 
![InBlock.gif][]    GetExitCodeThread( hth1, &dwExitCode );  // should be STILL\_ACTIVE = 0x00000103 = 259 
![InBlock.gif][]    printf( "initial thread 1 exit code = %u\\n", dwExitCode ); 
![InBlock.gif][] 
![InBlock.gif][] 
![InBlock.gif][] 
![InBlock.gif][]    HANDLE   hth2; 
![InBlock.gif][]    unsigned  uiThread2ID; 
![InBlock.gif][] 
![InBlock.gif][]    hth2 = (HANDLE)\_beginthreadex( NULL,         // security 
![InBlock.gif][]        0,            // stack size 
![InBlock.gif][]        Thread2, 
![InBlock.gif][]        (void\*)&stack,           // arg list 
![InBlock.gif][]        CREATE\_SUSPENDED,  // so we can later call ResumeThread() 
![InBlock.gif][]        &uiThread2ID ); 
![InBlock.gif][] 
![InBlock.gif][]    if ( hth2 == 0 ) 
![InBlock.gif][]        printf("Failed to create thread 2\\n"); 
![InBlock.gif][] 
![InBlock.gif][]    GetExitCodeThread( hth2, &dwExitCode );  // should be STILL\_ACTIVE = 0x00000103 = 259 
![InBlock.gif][]    printf( "initial thread 2 exit code = %u\\n", dwExitCode );   
![InBlock.gif][] 
![InBlock.gif][]    ResumeThread( hth1 );    
![InBlock.gif][]    ResumeThread( hth2 ); 
![InBlock.gif][] 
![InBlock.gif][]    WaitForSingleObject( hth1, INFINITE ); 
![InBlock.gif][]    WaitForSingleObject( hth2, INFINITE ); 
![InBlock.gif][] 
![InBlock.gif][]    GetExitCodeThread( hth1, &dwExitCode ); 
![InBlock.gif][]    printf( "thread 1 exited with code %u\\n", dwExitCode ); 
![InBlock.gif][] 
![InBlock.gif][]    GetExitCodeThread( hth2, &dwExitCode ); 
![InBlock.gif][]    printf( "thread 2 exited with code %u\\n", dwExitCode ); 
![InBlock.gif][] 
![InBlock.gif][]    CloseHandle( hth1 ); 
![InBlock.gif][]    CloseHandle( hth2 );     
![InBlock.gif][] 
![InBlock.gif][]    printf("Primary thread terminating.\\n"); 
![ExpandedBlockEnd.gif][]\}

三 对Critical Section的封装：  
//  
// 方法一： Lock中的CritSect成员变量必须是引用类型。

![None.gif][]class CritSect  
![ExpandedBlockStart.gif][]\{  
![InBlock.gif][]public:  
![InBlock.gif][]    friend class Lock;  
![ExpandedSubBlockStart.gif][]    CritSect() \{ InitializeCriticalSection(&\_critSection); \}  
![ExpandedSubBlockStart.gif][]    ~CritSect() \{ DeleteCriticalSection(&\_critSection); \}  
![InBlock.gif][]private:  
![ExpandedSubBlockStart.gif][]    void Acquire()\{EnterCriticalSection(&\_critSection);\}  
![ExpandedSubBlockStart.gif][]    void Release()\{LeaveCriticalSection(&\_critSection);\}  
![InBlock.gif][]  
![InBlock.gif][]    CRITICAL\_SECTION \_critSection;  
![ExpandedBlockEnd.gif][]\};  
![None.gif][]  
![None.gif][]class Lock  
![ExpandedBlockStart.gif][]\{  
![InBlock.gif][]public:  
![ExpandedSubBlockStart.gif][]     Lock(CritSect& critSect):\_critSect(critSect) \{    \_critSect.Acquire(); \}  
![ExpandedSubBlockStart.gif][]     ~Lock()\{\_critSect.Release();\}  
![InBlock.gif][]private:  
![InBlock.gif][]    CritSect& \_critSect;  
![ExpandedBlockEnd.gif][]\};

//  
//方法二：

![None.gif][]// MT-exclusive lock  
![ExpandedBlockStart.gif][]class CLock \{  
![InBlock.gif][]public:  
![ExpandedSubBlockStart.gif][]    CLock()             \{ InitializeCriticalSection (&m\_criticalSection); \}  
![ExpandedSubBlockStart.gif][]    void Lock ()        \{ EnterCriticalSection      (&m\_criticalSection); \}  
![ExpandedSubBlockStart.gif][]    void Unlock ()      \{ LeaveCriticalSection      (&m\_criticalSection); \}  
![ExpandedSubBlockStart.gif][]    virtual ~CLock()    \{ DeleteCriticalSection     (&m\_criticalSection); \}  
![InBlock.gif][]private:  
![InBlock.gif][]    CRITICAL\_SECTION                    m\_criticalSection;  
![ExpandedBlockEnd.gif][]\};  
![None.gif][]  
![None.gif][]  
![None.gif][]// Scoped MT-exclusive lock  
![ExpandedBlockStart.gif][]class CScopedLocker \{  
![InBlock.gif][]public:  
![ExpandedSubBlockStart.gif][]    CScopedLocker (CLock \* t) : m\_lock (t)      \{ m\_lock->Lock();   \}  
![ExpandedSubBlockStart.gif][]    ~CScopedLocker()                            \{ m\_lock->Unlock(); \}  
![InBlock.gif][]private:  
![InBlock.gif][]    CLock \*                             m\_lock;  
![ExpandedBlockEnd.gif][]\};

对上面的2中封装的调用都比较简单，都是只有2行代码。  
CritSect sect;  
Lock lock(sect);  
或  
CLock t；  
CSCopedLocker st(&t);

下面的对封装的测试代码，保证了对g\_n全局变量在线程1操作结束后线程2才可以操作。（下面的代码因为对全局变量同步，所以需要申明含有CRITICAL\_SECTION的类为全局）

![None.gif][]\#include<windows.h> 
![None.gif][]\#include<iostream> 
![None.gif][]using namespace std; 
![None.gif][] 
![ExpandedBlockStart.gif][]// 
![None.gif][]// ·½·¨Ò»£º 
![None.gif][] 
![None.gif][]class CritSect 
![ExpandedBlockStart.gif][]\{ 
![InBlock.gif][]public: 
![InBlock.gif][]    friend class Lock; 
![ExpandedSubBlockStart.gif][]    CritSect() \{ InitializeCriticalSection(&\_critSection); \} 
![ExpandedSubBlockStart.gif][]    ~CritSect() \{ DeleteCriticalSection(&\_critSection); \} 
![InBlock.gif][]private: 
![ExpandedSubBlockStart.gif][]    void Acquire()\{EnterCriticalSection(&\_critSection);\} 
![ExpandedSubBlockStart.gif][]    void Release()\{LeaveCriticalSection(&\_critSection);\} 
![InBlock.gif][] 
![InBlock.gif][]    CRITICAL\_SECTION \_critSection; 
![ExpandedBlockEnd.gif][]\}; 
![None.gif][] 
![None.gif][]class Lock 
![ExpandedBlockStart.gif][]\{ 
![InBlock.gif][]public: 
![ExpandedSubBlockStart.gif][]     Lock(CritSect& critSect):\_critSect(critSect) \{    \_critSect.Acquire(); \} 
![ExpandedSubBlockStart.gif][]     ~Lock()\{\_critSect.Release();\} 
![InBlock.gif][]private: 
![InBlock.gif][]    CritSect& \_critSect; 
![ExpandedBlockEnd.gif][]\}; 
![None.gif][] 
![ExpandedBlockStart.gif][]// 
![None.gif][]//·½·¨¶þ£º 
![None.gif][] 
![None.gif][]// MT-exclusive lock 
![ExpandedBlockStart.gif][]class CLock \{ 
![InBlock.gif][]public: 
![ExpandedSubBlockStart.gif][]    CLock()             \{ InitializeCriticalSection (&m\_criticalSection); \} 
![ExpandedSubBlockStart.gif][]    void Lock ()        \{ EnterCriticalSection      (&m\_criticalSection); \} 
![ExpandedSubBlockStart.gif][]    void Unlock ()      \{ LeaveCriticalSection      (&m\_criticalSection); \} 
![ExpandedSubBlockStart.gif][]    virtual ~CLock()    \{ DeleteCriticalSection     (&m\_criticalSection); \} 
![InBlock.gif][]private: 
![InBlock.gif][]    CRITICAL\_SECTION                    m\_criticalSection; 
![ExpandedBlockEnd.gif][]\}; 
![None.gif][] 
![None.gif][] 
![None.gif][]// Scoped MT-exclusive lock 
![ExpandedBlockStart.gif][]class CScopedLocker \{ 
![InBlock.gif][]public: 
![ExpandedSubBlockStart.gif][]    CScopedLocker (CLock \* t) : m\_lock (t)      \{ m\_lock->Lock();   \} 
![ExpandedSubBlockStart.gif][]    ~CScopedLocker()                            \{ m\_lock->Unlock(); \} 
![InBlock.gif][]private: 
![InBlock.gif][]    CLock \*                             m\_lock; 
![ExpandedBlockEnd.gif][]\}; 
![None.gif][] 
![None.gif][]// ¶ÔÈ«¾ÖµÄ±äÁ¿£¬Ê¹ÓÃCritical Section 
![None.gif][]// Declare the global variable 
![None.gif][]static int  g\_n; 
![None.gif][]CritSect sect; 
![None.gif][]//CLock t; 
![None.gif][] 
![None.gif][] 
![None.gif][] 
![ExpandedBlockStart.gif][]////Thread One Function/// 
![None.gif][]UINT ThreadOne(LPVOID lParam) 
![ExpandedBlockStart.gif][]\{ 
![InBlock.gif][]     
![InBlock.gif][]    Lock lock(sect);     
![InBlock.gif][]    //CScopedLocker st(&t); 
![InBlock.gif][]     
![InBlock.gif][]    for(int i=0;i<100;i++) 
![ExpandedSubBlockStart.gif][]    \{ 
![InBlock.gif][]        g\_n++; 
![InBlock.gif][]        cout << "Thread 1: " << g\_n << "\\n"; 
![ExpandedSubBlockEnd.gif][]    \}         
![InBlock.gif][]    // return the thread 
![InBlock.gif][]    return 0; 
![ExpandedBlockEnd.gif][]\} 
![None.gif][] 
![None.gif][] 
![ExpandedBlockStart.gif][]////Thread Two Function/// 
![None.gif][]UINT ThreadTwo(LPVOID lParam) 
![ExpandedBlockStart.gif][]\{ 
![InBlock.gif][] 
![InBlock.gif][] 
![InBlock.gif][]    Lock lock(sect); 
![InBlock.gif][]    //CScopedLocker st(&t); 
![InBlock.gif][] 
![InBlock.gif][]    for(int i=300;i<400;i++) 
![ExpandedSubBlockStart.gif][]    \{ 
![InBlock.gif][]        g\_n++; 
![InBlock.gif][]        cout << "Thread 2: "<< g\_n << "\\n"; 
![ExpandedSubBlockEnd.gif][]    \} 
![InBlock.gif][] 
![InBlock.gif][]    // return the thread 
![InBlock.gif][]    return 0; 
![ExpandedBlockEnd.gif][]\} 
![None.gif][] 
![None.gif][] 
![None.gif][]int main() 
![ExpandedBlockStart.gif][]\{ 
![InBlock.gif][] 
![InBlock.gif][]    // Create the array of Handle 
![InBlock.gif][]    HANDLE hThrd\[2\];     
![InBlock.gif][]    //Thread ID's 
![InBlock.gif][]    DWORD IDThread1, IDThread2; 
![InBlock.gif][] 
![InBlock.gif][] 
![InBlock.gif][]    // Create thredas use CreateThread function with NULL Security 
![InBlock.gif][]    hThrd\[0\] = CreateThread(NULL,0,(LPTHREAD\_START\_ROUTINE) ThreadOne,(LPVOID)NULL,0,&IDThread1);        
![InBlock.gif][]    hThrd\[1\] = CreateThread(NULL,0,(LPTHREAD\_START\_ROUTINE) ThreadTwo,(LPVOID)NULL,0,&IDThread2);  
![InBlock.gif][] 
![InBlock.gif][]    // Wait for the main thread  
![InBlock.gif][]    WaitForMultipleObjects(2,hThrd,TRUE,INFINITE); 
![InBlock.gif][]     
![InBlock.gif][]    return 0; 
![ExpandedBlockEnd.gif][]\}

四 API列表：

<table style="width:705px"> 
 <tbody> 
 <tr> 
 <th>Critical-section function</th> 
 <th>Description</th> 
 </tr> 
 <tr> 
 <td><a href="http://msdn2.microsoft.com/en-us/library/ms682552.aspx" style="color:rgb(0,102,170); text-decoration:none" rel="nofollow">DeleteCriticalSection</a></td> 
 <td>Releases all resources used by an unowned critical section object.</td> 
 </tr> 
 <tr> 
 <td><a href="http://msdn2.microsoft.com/en-us/library/ms682608.aspx" style="color:rgb(0,102,170); text-decoration:none" rel="nofollow">EnterCriticalSection</a></td> 
 <td>Waits for ownership of the specified critical section object.</td> 
 </tr> 
 <tr> 
 <td><a href="http://msdn2.microsoft.com/en-us/library/ms683472.aspx" style="color:rgb(0,102,170); text-decoration:none" rel="nofollow">InitializeCriticalSection</a></td> 
 <td>Initializes a critical section object.</td> 
 </tr> 
 <tr> 
 <td><a href="http://msdn2.microsoft.com/en-us/library/ms683476.aspx" style="color:rgb(0,102,170); text-decoration:none" rel="nofollow">InitializeCriticalSectionAndSpinCount</a></td> 
 <td>Initializes a critical section object and sets the spin count for the critical section.</td> 
 </tr> 
 <tr> 
 <td><a href="http://msdn2.microsoft.com/en-us/library/ms683477.aspx" style="color:rgb(0,102,170); text-decoration:none" rel="nofollow">InitializeCriticalSectionEx</a></td> 
 <td>Initializes a critical section object with a spin count and optional flags.</td> 
 </tr> 
 <tr> 
 <td><a href="http://msdn2.microsoft.com/en-us/library/ms684169.aspx" style="color:rgb(0,102,170); text-decoration:none" rel="nofollow">LeaveCriticalSection</a></td> 
 <td>Releases ownership of the specified critical section object.</td> 
 </tr> 
 <tr> 
 <td><a href="http://msdn2.microsoft.com/en-us/library/ms686197.aspx" style="color:rgb(0,102,170); text-decoration:none" rel="nofollow">SetCriticalSectionSpinCount</a></td> 
 <td>Sets the spin count for the specified critical section.</td> 
 </tr> 
 <tr> 
 <td><a href="http://msdn2.microsoft.com/en-us/library/ms686857.aspx" style="color:rgb(0,102,170); text-decoration:none" rel="nofollow">TryEnterCriticalSection</a></td> 
 <td>Attempts to enter a critical section without blocking. </td> 
 </tr> 
 </tbody> 
</table>

[None.gif]: /images/20220810/99012ef3dac24f9ea4b0aeae66475670.png
[ExpandedBlockStart.gif]: /images/20220810/e0cca251eb6f45c39264ce32a412e9c5.png
[InBlock.gif]: /images/20220810/73b3f4b097c4455c962fa1022bf631b5.png
[ExpandedBlockEnd.gif]: /images/20220810/1682c25c939a42e9afb172c63cfbbb47.png
[ExpandedSubBlockStart.gif]: /images/20220810/c5ac1e1996aa4141beca2a52c417ffdb.png
[ExpandedSubBlockEnd.gif]: /images/20220810/e19c611d798e4bf785db53e88ff72005.png