Question

This is a part of an open source project called JNotify. I am trying to fix the Win32 Implementation and it’s really driving me nuts. I already read everything there is to read in MSDN about this, and read every web post about this sucky API.
I am trying to receive file system notifications on windows using ReadDirectoryChangesW, using a completion port.

the behavior I am seeing is that normally it works, but some times the buffer I receive when GetQueuedCompletionStatus returns is corrupted in strange ways. eitehr FILE_NOTIFY_INFORMATION.NextEntryOffset points to the itself (resulting in an endless loop), or something else goes wrong and I receive a bogus file name length.
This only happens if I re-watch the directory, never in the first event (but re-watching is required otherwise you only get one event for that directory).

The test code that crashes every thing is trivial, it just watch many dirs and creates 2 files in each directory.

here is some relevant code, I can add all of it if you want (the whole thing is not too big), but feels too big for a question here.

This bit of code creates the completion port, it only runs once – and then I use this completion port for all directories.

_completionPort = CreateIoCompletionPort(INVALID_HANDLE_VALUE, NULL, 0, 1);

This is the WatchData constructor, that actually opens the directory handle and associates it with the completion port.

WatchData::WatchData(const WCHAR* path, int mask, bool watchSubtree, HANDLE completionPort)
    :
    _watchId(++_counter), 
    _mask(mask), 
    _watchSubtree(watchSubtree),
    _byteReturned(0),
    _completionPort(completionPort)
{
    _path = _wcsdup(path); 
    _hDir = CreateFileW(_path,
                         FILE_LIST_DIRECTORY | GENERIC_READ | GENERIC_WRITE,
                         FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE,
                         NULL, //security attributes
                         OPEN_EXISTING,
                         FILE_FLAG_BACKUP_SEMANTICS | FILE_FLAG_OVERLAPPED, NULL);
    if(_hDir == INVALID_HANDLE_VALUE )  
    {
        throw GetLastError();
    }

    if (NULL == CreateIoCompletionPort(_hDir, _completionPort, (ULONG_PTR)&_watchId, 0))
    {
        throw GetLastError();
    }
}

This is the code running (inside a WatchData object) when I start watching a directory:

int WatchData::watchDirectory()
{
    printf("(Re)watching %ls\n", _path);
    memset(_buffer, 0, sizeof(_buffer));
    memset(&_overLapped, 0, sizeof(_overLapped));
    if( !ReadDirectoryChangesW( _hDir,
                                _buffer,//<--FILE_NOTIFY_INFORMATION records are put into this buffer
                                sizeof(_buffer),
                                _watchSubtree,
                                _mask,
                                &_byteReturned,
                                &_overLapped,
                                NULL))



    {
        return GetLastError();
    }
    else
    {
        return 0;
    }
}

This is the main loop that run in it’s own thread, handling completion events.
Note that “This should not happen bit”, it actually happens a lot.

DWORD WINAPI Win32FSHook::mainLoop( LPVOID lpParam )
{
    debug("mainLoop starts");
    Win32FSHook* _this = (Win32FSHook*)lpParam;

    HANDLE hPort = _this->_completionPort;
    DWORD dwNoOfBytes = 0;
    ULONG_PTR ulKey = 0;
    OVERLAPPED* pov = NULL;
    WCHAR name[1024];

    while (_this->_isRunning)
    {
        pov = NULL;
        BOOL fSuccess = GetQueuedCompletionStatus(
                        hPort,         // Completion port handle
                        &dwNoOfBytes,  // Bytes transferred
                        &ulKey,
                        &pov,          // OVERLAPPED structure
                        INFINITE       // Notification time-out interval
                        );
        if (fSuccess)
        {
            if (dwNoOfBytes == 0)
            {
                // can happen after a watch is removed
                continue;
            }
            int wd = *(int*)ulKey;
            EnterCriticalSection(&_this->_cSection);
            WatchData *watchData = _this->find(wd);
            if (!watchData)
            {
                log("mainLoop : ignoring event for watch id %d, no longer in wid2WatchData map", wd);
                LeaveCriticalSection(&_this->_cSection);
                continue;
            }

            //const char* buffer = watchData->getBuffer();
            char buffer[watchData->getBufferSize()];
            memcpy(buffer, watchData->getBuffer(), watchData->getBufferSize());
            LeaveCriticalSection(&_this->_cSection);
            FILE_NOTIFY_INFORMATION *event;
            DWORD i=0;
            do
            {
                event = (FILE_NOTIFY_INFORMATION*)(buffer+i);
                int action = event->Action;
                DWORD len = event->FileNameLength / sizeof(WCHAR);
                for (DWORD k=0;k<len && k < (sizeof(name)-sizeof(WCHAR))/sizeof(WCHAR);k++)
                {
                    name[k] = event->FileName[k];
                }
                name[len] = 0;

                _this->_callback(watchData->getId(), action, watchData->getPath(), name);

                if (i != 0 && event->NextEntryOffset == i)
                {
                    log("should not happen!");
                    break;
                }

                i = event->NextEntryOffset;
            }
            while (i != 0);

            int res = watchData->watchDirectory();
            if (res != 0)
            {
                log("Error watching dir %s : %d",watchData->getPath(), res);
            }
        }
        else
        {
            log("GetQueuedCompletionStatus returned an error");
        }
    }
    debug("mainLoop exits");
    return 0;
}

Answer 1

I’m pretty sure NextEntryOffset is relative to the current record, not the first record.

...
char* current = buffer;
do
{
    event = (FILE_NOTIFY_INFORMATION*)current;
    ...
    i = event->NextEntryOffset;
    current += i;
}
while (i != 0);
...