DeviceIoControl失败，GetLastError()回到87，求高手解答！

DWORD dwNum, dwOut = 4, dwOutRet;    HANDLE hDriver = CreateFile(".//EnumProcess.sys",        GENERIC_READ | GENERIC_WRITE,        0,         NULL,         OPEN_EXISTING,        FILE_ATTRIBUTE_NORMAL,        NULL);    if(INVALID_HANDLE_VALUE == hDriver)    {        MessageBox("CreateFile failed!");        return FALSE;    }    //获取进程个数    BOOL bOk = DeviceIoControl(hDriver,        IOCTL_NUM,        NULL, 0,        &dwNum,        dwOut,        &dwOutRet,        NULL);