ReadDirectoryChangesW

Does anyone know the proper way to use ReadDirectoryChangesW? Or, does anyone have an alternative function I can use? Ultimately, I want to keep track of the progress of a file writing operation, and I thought this was one possible way.

What I've tried is this:

Code:

---

HANDLE hDir = CreateFile(directory, GENERIC_READ, FILE_SHARE_READ|FILE_SHARE_WRITE|FILE_SHARE_DELETE, NULL, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS, NULL);
if(hDir==INVALID_HANDLE_VALUE)
{
    MessageBox(NULL,"Failed to get directory handle!","Error",MB_OK);
}
FILE_NOTIFY_INFORMATION* fni= (FILE_NOTIFY_INFORMATION*)new char[512]; //how do I know how large a buffer to allocate?
DWORD bytesret=0;
OVERLAPPED ol={0};
if(!ReadDirectoryChangesW(hDir, &fni, 512, 0, FILE_NOTIFY_CHANGE_CREATION|FILE_NOTIFY_CHANGE_SIZE, &bytesret, &ol, ProgressRoutine))
{
    char temp[10];
    MessageBox(NULL,itoa(GetLastError(),temp,10),"Error:",MB_OK);
}

---

I figured I was doing something wrong and would crash/freeze my program, but no such luck--I just receive error code 6: ERROR_INVALID_HANDLE. This makes me think that I'm somehow using CreateFile incorrectly, but I copied it exactly from the documentation :confused:

Edit: Just realized I was checking the return of CreateFile improperly. It returns INVALID_HANDLE_VALUE and GetLastError() returns 2: ERROR_FILE_NOT_FOUND, but I've checked the string and the directory does exist...

that's probably because you're passing it the name of a directory instead of a filename.

[edit]
ah, I was wrong. you can open a directory.
try adding the flag FILE_LIST_DIRECTORY.
[/edit]

on winNT you can open a handle to a logical drive with CreateFile using \\.\PHYSICALDRIVEx, where x is the drive number (0 being the first).

Heh, I lied. The directory string was actually wrong, so CreateFile works now, I'm just not sure how to use ReadDirectoryChangesW. The way I have it, my program just stall until I manually create a new file in the directory (actually I needed to change the first flag to FILE_NOTIFY_CHANGE_FILE_NAME), then says, "Run-Time Check Failure #2 - Stack around the variable 'fni' was corrupted."

fni is already a pointer to your buffer. You don't want to apply the addressof operator. I suggest you avoid asynchronous mode and use synchronous mode. If you need asynchronous functionality, I'd suggest the easiest method is to fire up a second thread and run the synchronous version in a loop. Using the asynchronous mode is quite complicated and probably not worth the bother unless you are doing other overlapped I/O.

So:

Code:

---

ReadDirectoryChangesW(hDir, fni, 512, 0, FILE_NOTIFY_CHANGE_CREATION|FILE_NOTIFY_CHANGE_SIZE, &bytesret, NULL, NULL);

---

Also, 512 is a small buffer, I'd suggest using something like 32KB.

That was a pretty stupid error. Anyways, creating my own thread sounds like a good idea, but I'm not sure how to handle the data I get from ReadDirectoryChangesW. MSDN says the file name will be in unicode format and isn't null-terminated, and I'm not too experienced with unicode. FileNameLength keeps giving me twice the length of the file name :confused:

Oh, and why do I need such a large buffer?

Edit: I guess this makes sense. A WCHAR must be two bytes in size. Since I'm not using unicode in the rest of my project, is something like this safe?

Code:

---

char* asciiFileName = new char[fni->FileNameLength/2+1];
  for (int i=0;i<fni->FileNameLength/2;i++)
  {
    if (fni->FileName[i]>=0x80)
      return 1; //contains characters out range
    asciiFileName[i] = fni->FileName[i];
  }
  asciiFileName[fni->FileNameLength/2] = '\0';

---

Ok I have a new problem. I don't know how to get the progress bar to update from the thread that I created. If I use SendMessage to send PBM_SETPOS, my app stalls. If I use SendNotifyMessage, the progress bar updates after the thread terminates.

This is turning out to be more complicated than I thought :rolleyes:

It is best to get Windows to do the unicode to multi-byte conversion. Here is some sample code. It works whether UNICODE is defined or not.

Code:

---

void FileCallback(LPTSTR szFile, DWORD action)
{
        MessageBox(NULL, szFile, NULL, 0);
}

...

while (TRUE)
{
    BYTE  fni[32 * 1024];
    DWORD offset = 0;
    TCHAR szFile[MAX_PATH];
    DWORD bytesret;
    PFILE_NOTIFY_INFORMATION pNotify;

    ReadDirectoryChangesW(hDir, fni, sizeof(fni), 0,
          FILE_NOTIFY_CHANGE_CREATION | FILE_NOTIFY_CHANGE_SIZE | FILE_NOTIFY_CHANGE_FILE_NAME,
          &bytesret, NULL, NULL);

    do
    {
        pNotify = (PFILE_NOTIFY_INFORMATION) &fni[offset];
        offset += pNotify->NextEntryOffset;

#      if defined(UNICODE)
        {
            lstrcpynW(szFile, pNotify->FileName,
                      min(MAX_PATH, pNotify->FileNameLength / sizeof(WCHAR) + 1));
        }
#      else
        {
            int count = WideCharToMultiByte(CP_ACP, 0, pNotify->FileName,
                                            pNotify->FileNameLength / sizeof(WCHAR),
                                            szFile, MAX_PATH - 1, NULL, NULL);
            szFile[count] = TEXT('\0');
        }
#      endif

        FileCallback(szFile, pNotify->Action);

    } while (pNotify->NextEntryOffset != 0);
}

---

Quote:

---

Oh, and why do I need such a large buffer?

---

Multiple events can be returned at the one time. If the buffer is too small you may miss events. This is unlikely, but memory is cheap.

Quote:

---

Ok I have a new problem. I don't know how to get the progress bar to update from the thread that I created. If I use SendMessage to send PBM_SETPOS, my app stalls. If I use SendNotifyMessage, the progress bar updates after the thread terminates.

---

It sounds like your GUI thread is frozen. Do you have something like:

Code:

---

hThread = CreateThread(...);
WaitForSingleObject(hThread, ...);

---

The GUI thread must continue to process messages.

Quote:

---

This is turning out to be more complicated than I thought

---

There is the FindFirstChangeNotification which is a little simpler.

I had a go at getting the asynchronous version of ReadDirectoryChangesW working. As I mentioned, it is pretty complicated. Note: APCs are used to call the callback function. These require a special message loop to be despatched. Therefore, calling functions that block and run their own message loop (such as MessageBox and DialogBox) will also block change notifications.

Code:

---

#define _WIN32_WINNT 0x0400
#include <windows.h>
#include <windowsx.h>
#include <commctrl.h>
#include <shlwapi.h>

#if defined(_MSC_VER)
#pragma comment(lib, "comctl32.lib")
#pragma comment(lib, "user32.lib")
#pragma comment(lib, "ole32.lib")
#endif

HINSTANCE g_hinst;


typedef void (CALLBACK *FileChangeCallback)(LPTSTR, DWORD, LPARAM);

typedef struct tagDIR_MONITOR
{
        OVERLAPPED ol;
        HANDLE    hDir;
        BYTE      buffer[32 * 1024];
        LPARAM    lParam;
        DWORD      notifyFilter;
        BOOL      fStop;
        FileChangeCallback callback;
} *HDIR_MONITOR;

/*
 * Unpacks events and passes them to a user defined callback.
 */
VOID CALLBACK MonitorCallback(DWORD dwErrorCode, DWORD dwNumberOfBytesTransfered, LPOVERLAPPED lpOverlapped)
{
        TCHAR                    szFile[MAX_PATH];
        PFILE_NOTIFY_INFORMATION pNotify;
        HDIR_MONITOR            pMonitor  = (HDIR_MONITOR) lpOverlapped;
        size_t                  offset    =  0;
        BOOL RefreshMonitoring(HDIR_MONITOR pMonitor);

        if (dwErrorCode == ERROR_SUCCESS)
        {
                do
                {
                        pNotify = (PFILE_NOTIFY_INFORMATION) &pMonitor->buffer[offset];
                        offset += pNotify->NextEntryOffset;

#                        if defined(UNICODE)
                        {
                            lstrcpynW(szFile, pNotify->FileName,
                                        min(MAX_PATH, pNotify->FileNameLength / sizeof(WCHAR) + 1));
                        }
#                        else
                        {
                            int count = WideCharToMultiByte(CP_ACP, 0, pNotify->FileName,
                                                            pNotify->FileNameLength / sizeof(WCHAR),
                                                            szFile, MAX_PATH - 1, NULL, NULL);
                            szFile[count] = TEXT('\0');
                        }
#                        endif

                        pMonitor->callback(szFile, pNotify->Action, pMonitor->lParam);

                } while (pNotify->NextEntryOffset != 0);
        }

        if (!pMonitor->fStop)
        {
                RefreshMonitoring(pMonitor);
        }
}

/*
 * Refreshes the directory monitoring.
 */
BOOL RefreshMonitoring(HDIR_MONITOR pMonitor)
{
        return
        ReadDirectoryChangesW(pMonitor->hDir, pMonitor->buffer, sizeof(pMonitor->buffer), FALSE,
                              pMonitor->notifyFilter, NULL, &pMonitor->ol, MonitorCallback);
}

/*
 * Stops monitoring a directory.
 */
void StopMonitoring(HDIR_MONITOR pMonitor)
{
        if (pMonitor)
        {
                pMonitor->fStop = TRUE;

                CancelIo(pMonitor->hDir);

                if (!HasOverlappedIoCompleted(&pMonitor->ol))
                {
                        SleepEx(5, TRUE);
                }

                CloseHandle(pMonitor->ol.hEvent);
                CloseHandle(pMonitor->hDir);
                HeapFree(GetProcessHeap(), 0, pMonitor);
        }
}

/*
 * Starts monitoring a directory.
 */
HDIR_MONITOR StartMonitoring(LPCTSTR szDirectory, DWORD notifyFilter, FileChangeCallback callback)
{
        HDIR_MONITOR pMonitor = HeapAlloc(GetProcessHeap(), HEAP_ZERO_MEMORY, sizeof(*pMonitor));

        pMonitor->hDir = CreateFile(szDirectory, FILE_LIST_DIRECTORY, FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE,
                                    NULL, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS | FILE_FLAG_OVERLAPPED, NULL);

        if (pMonitor->hDir != INVALID_HANDLE_VALUE)
        {
                pMonitor->ol.hEvent    = CreateEvent(NULL, TRUE, FALSE, NULL);
                pMonitor->notifyFilter = notifyFilter;
                pMonitor->callback    = callback;

                if (RefreshMonitoring(pMonitor))
                {
                        return pMonitor;
                }
                else
                {
                        CloseHandle(pMonitor->ol.hEvent);
                        CloseHandle(pMonitor->hDir);
                }
        }

        HeapFree(GetProcessHeap(), 0, pMonitor);
        return NULL;
}

/*
 * Runs a message loop that allows APCs to be despatched.
 */
int RunAPCMessageLoop(void)
{
        BOOL done = FALSE;
        MSG  msg  = { 0 };

        while (!done)
        {
                /* Wait for either an APC or a message. */
                while (WAIT_IO_COMPLETION ==
                        MsgWaitForMultipleObjectsEx(0, NULL, INFINITE, QS_ALLINPUT, MWMO_ALERTABLE)) /* Do nothing */;

                /* One or more messages have arrived. */
                while (PeekMessage(&msg, NULL, 0, 0, PM_REMOVE))
                {
                        if (msg.message == WM_QUIT)
                        {
                                done = TRUE;
                                break;
                        }

                        TranslateMessage(&msg);
                        DispatchMessage(&msg);
                }
        }

        return (int) msg.wParam;
}


 
void CALLBACK FileCallback(LPTSTR szFile, DWORD action, LPARAM lParam)
{
        /* Shouldn't call MessageBox as it will block new notifications coming in. */
        MessageBox(NULL, szFile, NULL, 0);
}

HDIR_MONITOR h;

BOOL OnCreate(HWND hwnd, LPCREATESTRUCT lpcs)
{
        h = StartMonitoring(TEXT("C:\\Documents and Settings\\USER\\My Documents"),
                        FILE_NOTIFY_CHANGE_CREATION | FILE_NOTIFY_CHANGE_SIZE | FILE_NOTIFY_CHANGE_FILE_NAME,
                        FileCallback);

        return TRUE;
}

void OnDestroy(HWND hwnd)
{
        if (h) StopMonitoring(h);

        PostQuitMessage(0);
}


LRESULT CALLBACK WndProc(HWND hwnd, UINT uiMsg, WPARAM wParam, LPARAM lParam)
{
    switch (uiMsg)
    {
        HANDLE_MSG(hwnd, WM_CREATE, OnCreate);
        HANDLE_MSG(hwnd, WM_DESTROY, OnDestroy);
    }

    return DefWindowProc(hwnd, uiMsg, wParam, lParam);
}

BOOL InitApp(void)
{
    WNDCLASS wc = { 0 };

    wc.style        = 0;
    wc.lpfnWndProc  = WndProc;
    wc.cbClsExtra    = 0;
    wc.cbWndExtra    = 0;
    wc.hInstance    = g_hinst;
    wc.hIcon        = NULL;
    wc.hCursor      = LoadCursor(NULL, IDC_ARROW);
    wc.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);
    wc.lpszMenuName  = NULL;
    wc.lpszClassName = TEXT("Scratch");

    if (!RegisterClass(&wc)) return FALSE;

    return TRUE;
}


 
int WINAPI WinMain(HINSTANCE hinst, HINSTANCE hinstPrev,
                  LPSTR lpCmdLine, int nShowCmd)
{
    HWND hwnd;
    int  result;

    g_hinst = hinst;

    if (!InitApp()) return 0;

    hwnd = CreateWindow(
            TEXT("Scratch"),                      /* Class Name */
            TEXT("Scratch"),                      /* Title */
            WS_OVERLAPPEDWINDOW,                  /* Style */
            CW_USEDEFAULT, CW_USEDEFAULT,        /* Position */
            CW_USEDEFAULT, CW_USEDEFAULT,        /* Size */
            NULL,                                /* Parent */
            NULL,                                /* No menu */
            hinst,                                /* Instance */
            0);                                  /* No special parameters */

    ShowWindow(hwnd, nShowCmd);

    result = RunAPCMessageLoop();

    return result;
}

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