ReadDirectoryChangesW和GetOverlappedResult

Question

我异步调用ReadDirectoryChangesW来监视后台线程中的目录更改。

这是打开目录(basePath)和启动“读取”线程的方式：

    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);

我是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，我是“轮询”每一个框架的新变化。现在，当我调用GetOverlappedResult()时，它会失败并产生以下错误：

重叠I/O事件不处于信号状态。

我是不是遗漏了什么？ReadDirectoryChangesW是否意味着每一个“滴答”都被称为“滴答”？或者只是在检测到新的变化时？

注意:当我省略OVERLAPPED结构(和GetOverlappedResult)时，它可以工作，但是阻塞线程直到读取更改为止。这会阻止我的应用程序正确终止。(也就是说，我不能加入线程)

Answer 1

当调用GetOverlappedResult()时，如果您将bWait参数设置为FALSE，而I/O操作尚未完成，则GetOverlappedResult()将在ERROR_IO_INCOMPLETE错误代码中失败：

bWait in 如果此参数为TRUE，且lpOverlapped结构的Internal成员为STATUS_PENDING，则该函数在操作完成之前不会返回。如果此参数为FALSE ，且操作仍未完成，则函数返回 FALSE ， GetLastError 函数返回 ERROR_IO_INCOMPLETE.

这不是致命的错误，所以只需忽略该错误并继续前进。

是的，请确保在ReadDirectoryChangesW()报告之前，不要再次调用GetOverlappedResult()，因为之前的I/O操作已经完成。

现在，尽管如此，您的代码还有另一个问题。您的线程正在堆栈上分配一个FILE_NOTIFY_INFORMATION实例。如果您查看FILE_NOTIFY_INFORMATION的定义，它的FileName字段是可变长度：

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

FileName 包含相对于目录句柄的文件名的variable-length字段。文件名为Unicode字符格式，不以空结尾.

这意味着静态分配FILE_NOTIFY_INFORMATION将太小，而且dwBytes几乎总是为0，因为ReadDirectoryChangesW()无法向您返回完整的FILE_NOTIFY_INFORMATION (除非FileName长度正好为1个字符)：

当您第一次调用ReadDirectoryChangesW时，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相关联，直到其关闭，且其大小在其生存期内不发生变化。在调用此函数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，则会丢弃缓冲区的全部内容，lpBytesReturned ReadDirectoryChangesW 参数包含0，而错误代码 ERROR_NOTIFY_ENUM_DIR.导致ReadDirectoryChangesW函数失败。

​

ERROR_NOTIFY_ENUM_DIR 1022 (0x3FE) 正在完成通知更改请求，并且调用方的缓冲区中没有返回该信息。调用方现在需要枚举文件以查找更改。

因此，您需要动态分配一个大字节缓冲区来接收FILE_NOTIFY_INFORMATION数据，然后每当GetOverlappedResult()报告该数据可用时，您就可以遍历该缓冲区。

您的线程应该更像这样：

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);
}

另一种不定期轮询I/O状态的实现方法是摆脱m_Reading并使用可等待事件。让操作系统在应该调用GetOverlappedResult()或终止时向线程发出信号，这样它就可以在其余时间睡觉，而不是忙着做什么：

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);
}