c++ - CreateFile，ReadDirectoryChanges 问题

Question

我正在使用 ReadDirectoryChangesW 来监视我用 CreateFile 打开的文件夹，添加文件后我调用一个函数 (OnFileChanged ) 读取大小并打开它进行阅读，我的应用程序适用于小文件，但当我尝试将大文件复制到我的文件夹 (7,24 M) 时出现问题，并且我得到 Permission denied 调用 fopen 读取它后出错。

watching过程是根据this

    void Init(const QString FullPath)
    {    ...
         hDir = CreateFile(
                dirPath, // pointer to the directory containing the tex files
                FILE_LIST_DIRECTORY|GENERIC_READ,                // access (read-write) mode
                FILE_SHARE_READ|FILE_SHARE_DELETE|FILE_SHARE_WRITE,  // share mode
                NULL, // security descriptor
                OPEN_EXISTING, // how to create
                FILE_FLAG_BACKUP_SEMANTICS  | FILE_FLAG_OVERLAPPED , // file attributes
                NULL); // file with attributes to copy


            SecureZeroMemory (&overl, sizeof(overl));
            SecureZeroMemory(buffer, sizeof(buffer));
            overl.hEvent = CreateEvent(NULL, FALSE, FALSE, NULL);

            // watch the directory
           BOOL res= ReadDirectoryChangesW(
                 hDir, /* handle to directory */
                 &buffer[curBuffer], /* read results buffer */
                 sizeof(buffer[curBuffer]), /* length of buffer */
                 FALSE, /* monitoring option */
                 FILE_NOTIFY_CHANGE_FILE_NAME ,
                 //FILE_NOTIFY_CHANGE_LAST_WRITE, /* filter conditions */
                 NULL, /* bytes returned */
                 &overl, /* overlapped buffer */
                 NULL); /* completion routine */

    }

void StartWatchThread()
{
    // if the thread already exists then stop it
    if (IsThreadRunning())
        SynchronousAbort();

    //CrashIf(!hDir);
    if(!hDir)
       { qDebug()<<" handle "<<hDir<<" last error"<<GetLastError();
        exit(-1);}

    else
    {// reset the hEvtStopWatching event so that it can be set if
    // some thread requires the watching thread to stop
    ResetEvent(hEvtStopWatching);

    DWORD watchingthreadID;
    qDebug()<<"befrore creating thread";
    hWatchingThread = CreateThread(NULL, 0, WatchingThread, this, 0, &watchingthreadID);
    qDebug()<<"watchingthreadID"<<watchingthreadID;
    }
}


DWORD WINAPI WatchingThread(void *param)
{
    //qDebug()<<"in WatchingThread";
    FileWatcher *fw = (FileWatcher *)param;

    HANDLE hp[2] = { fw->hEvtStopWatching, fw->overl.hEvent };
    for (;;)
    {

        DWORD dwObj = WaitForMultipleObjects((sizeof(hp)/(sizeof(hp[0])))
                                             , hp, FALSE, INFINITE);
        if (dwObj == WAIT_OBJECT_0) // the user asked to quit the program
        {
            qDebug()<<"in WatchingThread the user asked to quit the program";
            //exit(-1);
            break;
        }
        if (dwObj != WAIT_OBJECT_0 + 1)
        {
            // BUG!
            //assert(0);
            qDebug()<<"dwObj "<<dwObj<<" last error "<<GetLastError();
            break;
        }
        //qDebug()<<"WatchingThread fw->NotifyChange() ";

        //if (fw->wakeup)
            fw->NotifyChange();

    }
    return 0;
} 


bool NotifyChange()
{
    //qDebug()<<"in NotifyChange";

    GetOverlappedResult(hDir, &overl, &dwNumberbytes, FALSE);

    FILE_NOTIFY_INFORMATION *pFileNotify = (FILE_NOTIFY_INFORMATION *)buffer[curBuffer];
    // Switch the 2 buffers
    curBuffer = (curBuffer + 1) % (sizeof(buffer)/(sizeof(buffer[0])));
    SecureZeroMemory(buffer[curBuffer], sizeof(buffer[curBuffer]));
    // start a new asynchronous call to ReadDirectory in the alternate buffer
    ReadDirectoryChangesW(
         hDir, /* handle to directory */
         &buffer[curBuffer], /* read results buffer */
         sizeof(buffer[curBuffer]), /* length of buffer */
         FALSE, /* monitoring option */
         FILE_NOTIFY_CHANGE_FILE_NAME  ,
         //FILE_NOTIFY_CHANGE_LAST_WRITE, /* filter conditions */
         NULL, /* bytes returned */
        &overl, /* overlapped buffer */
         NULL); /* completion routine */

    // Note: the ReadDirectoryChangesW API fills the buffer with WCHAR strings.
    for (;;) {

        if (pFileNotify->Action == FILE_ACTION_ADDED)
        {
            qDebug()<<"in NotifyChange if ";
                char szAction[42];
                char szFilename[MAX_PATH] ;
                memset(szFilename,'\0',sizeof( szFilename));
                strcpy(szAction,"added");
                wcstombs( szFilename, pFileNotify->FileName, MAX_PATH);

                OnFileChanged(szFilename,szAction);
                qDebug()<<"in NotifyChange after OnFileChanged ";

        }

        // step to the next entry if there is one
        if (!pFileNotify->NextEntryOffset)
            return false;
        pFileNotify = (FILE_NOTIFY_INFORMATION *)((PBYTE)pFileNotify + pFileNotify->NextEntryOffset);
    }
    pFileNotify=NULL;
    return true;
}

我无法弄清楚如何处理该文件并且不让我阅读它？我试图在调用 OnFileChanged 后调用 Sleep() 并在它完成时唤醒但徒劳无功。请有任何想法。

Answer 1

I call a function (OnFileChanged) that read the size and open it for reading

这几乎肯定迟早会失败。文件更改通知是在进程写入文件时生成的。如果进程尚未完成写入或以其他方式保持文件打开，这将阻止您打开文件。并且不允许读取共享，这对于写入文件的程序来说很常见。这肯定与文件大小有关，文件越大，写入时间越长，因此您尝试打开它而其他进程尚未关闭它的可能性就越大。

您需要处理这种可能发生的事故。您只能将路径保留在队列中，以便稍后尝试。