互斥体

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简介

  互斥体实现了“互相排斥”(mutual exclusion)同步的简单形式(所以名为互斥体(mutex))。互斥体禁止多个线程同时进入受保护的代码“临界区”(critical section)。因此,在任意时刻,只有一个线程被允许进入这样的代码保护区。任何线程在进入临界区之前,必须获取(acquire)与此区域相关联的互斥体的所有权。如果已有另一线程拥有了临界区的互斥体,其他线程就不能再进入其中。这些线程必须等待,直到当前的属主线程释放(release)该互斥体。什么时候需要使用互斥体呢?互斥体用于保护共享的易变代码,也就是,全局或静态数据。这样的数据必须通过互斥体进行保护,以防止它们在多个线程同时访问时损坏。VC中的互斥体

创建

  在VC中,我们用 CreateMutex函数创建互斥体。   HANDLE WINAPI CreateMutex(   __in_opt LPSECURITY_ATTRIBUTES lpMutexAttributes,   __in BOOL bInitialOwner,   __in_opt LPCTSTR lpName   );

应用示例

  #include   #include   #define THREADCOUNT 2   HANDLE ghMutex;   DWORD WINAPI WriteToDatabase( LPVOID );   void main()   {   HANDLE aThread[THREADCOUNT];   DWORD ThreadID;   int i;   // Create a mutex with no initial owner   ghMutex = CreateMutex(   NULL, // default security attributes   FALSE, // initially not owned   NULL); // unnamed mutex   if (ghMutex == NULL)   {   printf("CreateMutex error: %d\n", GetLastError());   return;   }   // Create worker threads   for( i=0; i < THREADCOUNT; i++ )   {   aThread[i] = CreateThread(   NULL, // default security attributes   0, // default stack size   (LPTHREAD_START_ROUTINE) WriteToDatabase,   NULL, // no thread function arguments   0, // default creation flags   &ThreadID); // receive thread identifier   if( aThread[i] == NULL )   {   printf("CreateThread error: %d\n", GetLastError());   return;   }   }   // Wait for all threads to terminate   WaitForMultipleObjects(THREADCOUNT, aThread, TRUE, INFINITE);   // Close thread and mutex handles   for( i=0; i < THREADCOUNT; i++ )   CloseHandle(aThread[i]);   CloseHandle(ghMutex);   }   DWORD WINAPI WriteToDatabase( LPVOID lpParam )   {   DWORD dwCount=0, dwWaitResult;   // Request ownership of mutex.   while( dwCount < 20 )   {   dwWaitResult = WaitForSingleObject(   ghMutex, // handle to mutex   INFINITE); // no time-out interval   switch (dwWaitResult)   {   // The thread got ownership of the mutex   case WAIT_OBJECT_0:   __try {   // TODO: Write to the database   printf("Thread %d writing to database...\n",   GetCurrentThreadId());   dwCount++;   }   __finally {   // Release ownership of the mutex object   if (! ReleaseMutex(ghMutex))   {   // Handle error.   }   }   break;   // The thread got ownership of an abandoned mutex   // The database is in an indeterminate state   case WAIT_ABANDONED:   return FALSE;   }   }   return TRUE;   }

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