ReadDirectoryChangesW和GetOverlappedResult

Question

我ReadDirectoryChangesW异步调用以监视后台线程中的目录更改.

这是如何打开directory(basePath)并启动"读取"线程:

    m_hDIR = CreateFileW(
            basePath,
            FILE_LIST_DIRECTORY | GENERIC_READ,
            FILE_SHARE_WRITE | FILE_SHARE_READ,
            NULL,
            OPEN_EXISTING,
            FILE_FLAG_BACKUP_SEMANTICS | FILE_FLAG_OVERLAPPED,
            NULL);

        if (m_hDIR == INVALID_HANDLE_VALUE)
            throw CrException(CrWin32ErrorString());

        //Start reading changes in background thread
        m_Callback = std::move(a_Callback);
        m_Reading = true;
        m_ReadThread = std::thread(&CrDirectoryWatcher::StartRead, this);

这是StartRead():(注:m_Reading是atomic<bool>)

void StartRead()
        {
            DWORD dwBytes = 0;
            FILE_NOTIFY_INFORMATION fni{0};
            OVERLAPPED o{0};

            //Be sure to set the hEvent member of the OVERLAPPED structure to a unique event.
            o.hEvent = CreateEvent(0, 0, 0, 0);

            while(m_Reading)
            {
                if (!ReadDirectoryChangesW(m_hDIR,
                    &fni, sizeof(fni),
                    TRUE, FILE_NOTIFY_CHANGE_LAST_WRITE,
                    &dwBytes, &o, NULL))
                {
                    CrAssert(0, CrWin32ErrorString());
                }

                if (!GetOverlappedResult(m_hDIR, &o, &dwBytes, FALSE))
                    CrAssert(0, CrWin32ErrorString());

                if (fni.Action != 0)
                {
                    std::wstring fileName(fni.FileName, fni.FileNameLength);
                    m_Callback(fileName);
                    fni.Action = 0;
                }
            }
        }

基本上,我每次都在"轮询"新的变化.现在,当我调用GetOverlappedResult()它失败并产生以下错误:

重叠的I/O事件未处于信号状态.

我错过了什么吗？是ReadDirectoryChangesW指被称为每一个"滴答"？或者只是在检测到新的变化时？

注意:当我省略OVERLAPPEDstruct(和GetOverlappedResult)时,它可以工作,但是阻塞线程直到读取更改.这可以防止我的应用程序正确终止.(即我不能加入线程)

Answer 1

调用时GetOverlappedResult(),如果将bWait参数设置为FALSE且I/O操作尚未完成,GetOverlappedResult()则会失败并显示ERROR_IO_INCOMPLETE错误代码:

bWait [in]
如果此参数是TRUE,并且结构的Internal成员lpOverlapped是STATUS_PENDING,则该函数在操作完成之前不会返回.如果此参数为FALSE且操作仍处于挂起状态,则函数返回FALSE并GetLastError返回函数ERROR_IO_INCOMPLETE.

这不是致命的错误,所以请忽略该错误并继续前进.

是的,请确保你不叫ReadDirectoryChangesW(),直到再次GetOverlappedResult()报告之前的I/O操作已完成第一.

现在,话虽如此,您的代码还有另一个问题.您的线程正在FILE_NOTIFY_INFORMATION堆栈上分配单个实例.如果你看一下它的定义FILE_NOTIFY_INFORMATION,它的FileName字段是可变长度的:

typedef struct _FILE_NOTIFY_INFORMATION {
  DWORD NextEntryOffset;
  DWORD Action;
  DWORD FileNameLength;
  WCHAR FileName[1];
} FILE_NOTIFY_INFORMATION, *PFILE_NOTIFY_INFORMATION;

FileName
一个可变长度字段,包含相对于目录句柄的文件名.文件名采用Unicode字符格式,不以空值终止.

这意味着FILE_NOTIFY_INFORMATION静态分配将会太小,并且dwBytes几乎总是0,因为ReadDirectoryChangesW()无法FILE_NOTIFY_INFORMATION向您返回一个完整的(除非FileName长度恰好为1个字符):

第一次调用时ReadDirectoryChangesW,系统会分配一个缓冲区来存储更改信息.此缓冲区与目录句柄关联,直到它关闭并且其大小在其生命周期内不会更改.在对此函数的调用之间发生的目录更改将添加到缓冲区,然后在下一次调用时返回.如果缓冲区溢出,则丢弃缓冲区的全部内容,lpBytesReturned参数包含零,并且ReadDirectoryChangesW函数将失败并显示错误代码ERROR_NOTIFY_ENUM_DIR.

ERROR_NOTIFY_ENUM_DIR
1022(0x3FE)
正在完成通知更改请求,并且未在调用方缓冲区中返回信息.调用者现在需要枚举文件以查找更改.

因此,您需要动态分配大字节缓冲区以接收FILE_NOTIFY_INFORMATION数据,然后只要GetOverlappedResult()报告数据可用,您就可以遍历该缓冲区.

你的主题应该看起来像这样:

void StartRead()
{
    DWORD dwBytes = 0;
    std::vector<BYTE> buffer(1024*64);
    OVERLAPPED o{0};
    bool bPending = false;

    //Be sure to set the hEvent member of the OVERLAPPED structure to a unique event.
    o.hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
    if (!o.hEvent) {
        CrAssert(0, CrWin32ErrorString());
    }

    while (m_Reading)
    {
        bPending = ReadDirectoryChangesW(m_hDIR,
            &buffer[0], buffer.size(),
            TRUE, FILE_NOTIFY_CHANGE_LAST_WRITE,
            &dwBytes, &o, NULL);
        if (!bPending)
        {
            CrAssert(0, CrWin32ErrorString());
        }

        while (m_Reading)
        {
            if (GetOverlappedResult(m_hDIR, &o, &dwBytes, FALSE))
            {
                bPending = false;

                if (dwBytes != 0)
                {
                    FILE_NOTIFY_INFORMATION *fni = reinterpret_cast<FILE_NOTIFY_INFORMATION*>(&buffer[0]);
                    do
                    {
                        if (fni->Action != 0)
                        {
                            std::wstring fileName(fni->FileName, fni->FileNameLength);
                            m_Callback(fileName);
                        }

                        if (fni->NextEntryOffset == 0)
                            break;

                        fni = reinterpret_cast<FILE_NOTIFY_INFORMATION*>(reinterpret_cast<BYTE*>(fni) + fni->NextEntryOffset);
                    }
                    while (true);
                }

                break;
            }

            if (GetLastError() != ERROR_IO_INCOMPLETE) {
                CrAssert(0, CrWin32ErrorString());
            }

            Sleep(10);
        }

        if (bPending)
        {
            CancelIo(m_hDIR);
            GetOverlappedResult(m_hDIR, &o, &dwBytes, TRUE);
        }
    }

    CloseHandle(o.hEvent);
}

在不定期轮询I/O状态的情况下实现此方法的另一种方法是取消m_Reading并使用等待事件.让操作系统在应该调用GetOverlappedResult()或终止时发出信号,这样它可以在剩下的时间内休息,它不忙于做某事:

m_hDIR = CreateFileW(
            basePath,
            FILE_LIST_DIRECTORY | GENERIC_READ,
            FILE_SHARE_WRITE | FILE_SHARE_READ,
            NULL,
            OPEN_EXISTING,
            FILE_FLAG_BACKUP_SEMANTICS | FILE_FLAG_OVERLAPPED,
            NULL);

if (m_hDIR == INVALID_HANDLE_VALUE)
    throw CrException(CrWin32ErrorString());

m_TermEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
if (!m_TermEvent)
    throw CrException(CrWin32ErrorString());

//Start reading changes in background thread
m_Callback = std::move(a_Callback);
m_ReadThread = std::thread(&CrDirectoryWatcher::StartRead, this);

...

SetEvent(m_TermEvent);
m_ReadThread.join();

void StartRead()
{
    DWORD dwBytes = 0;
    std::vector<BYTE> buffer(1024*64);
    OVERLAPPED o{0};
    bool bPending = false, bKeepRunning = true;

    //Be sure to set the hEvent member of the OVERLAPPED structure to a unique event.
    o.hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
    if (!o.hEvent) {
        CrAssert(0, CrWin32ErrorString());
    }

    HANDLE h[2] = {o.hEvent, h_TermEvent};

    do
    {
        bPending = ReadDirectoryChangesW(m_hDIR,
            &buffer[0], buffer.size(),
            TRUE, FILE_NOTIFY_CHANGE_LAST_WRITE,
            &dwBytes, &o, NULL);
        if (!bPending)
        {
            CrAssert(0, CrWin32ErrorString());
        }

        switch (WaitForMultipleObjects(2, h, FALSE, INFINITE))
        {
            case WAIT_OBJECT_0:
            {
                if (!GetOverlappedResult(m_hDIR, &o, &dwBytes, TRUE)) {
                    CrAssert(0, CrWin32ErrorString());
                }

                bPending = false;

                if (dwBytes == 0)
                    break;

                FILE_NOTIFY_INFORMATION *fni = reinterpret_cast<FILE_NOTIFY_INFORMATION*>(&buffer[0]);
                do
                {
                    if (fni->Action != 0)
                    {
                        std::wstring fileName(fni->FileName, fni->FileNameLength);
                        m_Callback(fileName);
                    }

                    if (fni->NextEntryOffset == 0)
                         break;

                    fni = reinterpret_cast<FILE_NOTIFY_INFORMATION*>(reinterpret_cast<BYTE*>(fni) + fni->NextEntryOffset);
                }
                while (true);

                break;
            }

            case WAIT_OBJECT_0+1:
                bKeepRunning = false;
                break;

            case WAIT_FAILED:
                CrAssert(0, CrWin32ErrorString());
                break;
        }
    }
    while (bKeepRunning);

    if (bPending)
    {
        CancelIo(m_hDIR);
        GetOverlappedResult(m_hDIR, &o, &dwBytes, TRUE);
    }

    CloseHandle(o.hEvent);
}