c++ - 为什么线程同步不起作用？

Question

我写了一个多线程程序，其中三个线程试图将文本保存到同一个文件中。我应用了关键部分。在 windows 7 下完美运行，但在 CE 6.0 下不同步，即每个线程都在同时尝试保存:

现在可以了!!!感谢大家的帮助!

关键部分:

InitializeCriticalSection(&CriticalSection);

// Create worker threads
for( i=0; i < THREADCOUNT; i++ )
{
    aThread[i] = CreateThread( NULL, 0, (LPTHREAD_START_ROUTINE) WriteToFile, NULL, 0, &ThreadID);

    if( aThread[i] == NULL )
    {
        printf("CreateThread error: %d\n", GetLastError());
        return 1;
    }
}

// Wait for all threads to terminate
for( i=0; i < THREADCOUNT; i++ )
{
    WaitResult = WaitForSingleObject(aThread[i], INFINITE);

    switch(WaitResult)
    {
        case WAIT_OBJECT_0:
            printf("Thread %d has terminated...\n", i);
         break;

         // Time out
        case WAIT_TIMEOUT:
            printf("The waiting is timed out...\n");
            break;

        // Return value is invalid.
        default:
            printf("Waiting failed, error %d...\n", GetLastError());
            ExitProcess(0);
    }
}

// Close thread handles
for( i=0; i < THREADCOUNT; i++ )
    CloseHandle(aThread[i]);

// Release resources used by the critical section object.
DeleteCriticalSection(&CriticalSection);

线程调用的函数:

DWORD WINAPI WriteToFile( LPVOID lpParam )
{ 
// lpParam not used in this example
UNREFERENCED_PARAMETER(lpParam);

DWORD dwCount=1, dwWaitResult; 

HANDLE hFile; 
char DataBuffer[30];
DWORD dwBytesToWrite;
DWORD dwBytesWritten;

// Request ownership of the critical section.
EnterCriticalSection(&CriticalSection);

    // Write to the file
    printf("Thread %d writing to file...\n", GetCurrentThreadId());

    hFile = CreateFile(TEXT("file.txt"), GENERIC_WRITE, 0, NULL, OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL); 

    SetFilePointer(hFile, 0, NULL, FILE_END);

    while( dwCount <= 3 )
    {
        sprintf(DataBuffer, "Theard %d writing %d\n", GetCurrentThreadId(), dwCount);
        dwBytesToWrite = (DWORD)strlen(DataBuffer);

        WriteFile( hFile, DataBuffer, dwBytesToWrite, &dwBytesWritten, NULL);

            printf("Theard %d wrote %d successfully.\n", GetCurrentThreadId(), dwCount);
            }
        }

        dwCount++;
    }

CloseHandle(hFile);             

// Release ownership of the critical section.
LeaveCriticalSection(&CriticalSection);

return TRUE; 
}

Answer 1

问题是您将 TRUE 传递给 WaitForMultipleObjects 的 fWaitAll 标志。在 Windows CE 上，这是不支持的:documentation on MSDN表示此标志必须为 FALSE。 WaitForMultipleObjects 因此不是等待，而是返回一个错误，但您没有检查返回码。因此，主线程直接通过，关闭句柄并删除关键部分，同时“工作”线程仍在运行。一旦 DeleteCriticalSection 被调用，临界区“不能再用于同步”，所以 EnterCriticalSection 调用可能不再阻塞，你最终会遇到你想要的场景有这里。

在 Windows 7 上，一切正常，因为 WaitForMultipleObjects 调用确实等待所有线程完成。

与其使用 WaitForMultipleObjects，不如在循环中使用 WaitForSingleObject 依次等待每个线程。