ULONG BOOL_TO_ERROR(BOOL fOk)
{
    return fOk ? NOERROR : GetLastError();
}

void CheckEventState(HANDLE hEvent, ULONG err, NTSTATUS status = RtlGetLastNtStatus())
{
    DbgPrint("error = %u(%x)", err, err ? status : STATUS_SUCCESS);

    switch (WaitForSingleObject(hEvent, 0))
    {
    case WAIT_OBJECT_0:
        DbgPrint("Signaled\n");
        break;
    case WAIT_TIMEOUT:
        DbgPrint("NON signaled\n");
        break;
    default:
        DbgPrint("error=%u\n", GetLastError());
    }
#if 1
    EVENT_BASIC_INFORMATION ebi;
    if (0 <= ZwQueryEvent(hEvent, EventBasicInformation, &ebi, sizeof(ebi), 0))
    {
        DbgPrint("EventState = %x\n", ebi.EventState);
    }
#endif
}

void demoIoEvent()
{
    WCHAR sz[MAX_PATH];
    GetSystemDirectoryW(sz, RTL_NUMBER_OF(sz));

    HANDLE hFile = CreateFileW(sz, 0, FILE_SHARE_VALID_FLAGS, 0, 
        OPEN_EXISTING, FILE_FLAG_OVERLAPPED|FILE_FLAG_BACKUP_SEMANTICS, 0);

    if (hFile != INVALID_HANDLE_VALUE)
    {
        FILESYSTEM_STATISTICS fs;

        OVERLAPPED ov = {};

        if (ov.hEvent = CreateEvent(0, TRUE, FALSE, 0))
        {
            FILE_NOTIFY_INFORMATION fni;
            IO_STATUS_BLOCK iosb;

            // STATUS_DATATYPE_MISALIGNMENT from I/O manager
            // event will be not set
            NTSTATUS status = ZwNotifyChangeDirectoryFile(hFile, ov.hEvent, 0, 0, &iosb, 
                (FILE_NOTIFY_INFORMATION*)(1 + (PBYTE)&fni), 1, FILE_NOTIFY_VALID_MASK, FALSE);

            CheckEventState(ov.hEvent, ERROR_NOACCESS, status);

            // windows bug ! ReadDirectoryChangesW return .. true and no last error
            // but really api fail. event will be not set and no notifications
            ULONG err = BOOL_TO_ERROR(ReadDirectoryChangesW(hFile,
                (FILE_NOTIFY_INFORMATION*)(1 + (PBYTE)&fni), 1, 0, FILE_NOTIFY_VALID_MASK, 0, &ov, 0));

            CheckEventState(ov.hEvent, err);

            // fail with ERROR_INSUFFICIENT_BUFFER (STATUS_BUFFER_TOO_SMALL)
            // NT_ERROR(c0000023) - event will be not set
            err = BOOL_TO_ERROR(DeviceIoControl(hFile, 
                FSCTL_FILESYSTEM_GET_STATISTICS, 0, 0, 0, 0, 0, &ov));

            CheckEventState(ov.hEvent, err);

            // ERROR_MORE_DATA (STATUS_BUFFER_OVERFLOW)
            // !NT_ERROR(80000005) - event will be set
            // note - win 32 api return false and error != ERROR_IO_PENDING
            err = BOOL_TO_ERROR(DeviceIoControl(hFile, 
                FSCTL_FILESYSTEM_GET_STATISTICS, 0, 0, &fs, sizeof(fs), 0, &ov));

            CheckEventState(ov.hEvent, err);

            if (err == ERROR_MORE_DATA)
            {
                SYSTEM_INFO si;
                GetSystemInfo(&si);

                ULONG cb = si.dwNumberOfProcessors * fs.SizeOfCompleteStructure;

                union {
                    PVOID pv;
                    PBYTE pb;
                    PFILESYSTEM_STATISTICS pfs;
                };

                pv = alloca(cb);

                // must be NOERROR(0) here
                // !NT_ERROR(0) - event will be set
                err = BOOL_TO_ERROR(DeviceIoControl(hFile, FSCTL_FILESYSTEM_GET_STATISTICS, 0, 0, 
                    pv, cb, 0, &ov));

                CheckEventState(ov.hEvent, err);

                if (!err && GetOverlappedResult(hFile, &ov, &cb, FALSE))
                {
                    do 
                    {
                        // use pfs here
                    } while (pb += fs.SizeOfCompleteStructure, --si.dwNumberOfProcessors);
                }
            }

            CloseHandle(ov.hEvent);
        }
        CloseHandle(hFile);
    }
}

error = 998(80000002)NON signaled
EventState = 0
error = 0(0)NON signaled
EventState = 0
error = 122(c0000023)NON signaled
EventState = 0
error = 234(80000005)Signaled
EventState = 1
error = 0(0)Signaled
EventState = 1