Question

I am trying to write a C++/WinAPI code to change a monitor brightness. The code must be compatible with Windows XP so I can’t use APIs like SetMonitorBrightness. So I thought to try out IOCTL_VIDEO_SET_DISPLAY_BRIGHTNESS but I can’t seem to even get a device handle.

HANDLE hDevice = ::CreateFile(_T("\\\\.\\LCD"),
    GENERIC_READ | GENERIC_WRITE,
    FILE_SHARE_READ | FILE_SHARE_WRITE,
    NULL,
    OPEN_EXISTING,
    0, NULL);
if(hDevice != INVALID_HANDLE_VALUE)
{
    //Do work here

    ::CloseHandle(hDevice);
}
else
{
    nOSErr = ::GetLastError();
    //Get code 2
}

I tried various combinations of GENERIC_READ, GENERIC_WRITE and FILE_SHARE_READ, FILE_SHARE_WRITE flags but I always get error code 2 when CreateFile is called.

So what am I doing wrong here?

Answer 1

You know that not all devices support this API, right? Your laptop probably supports it because it allows software adjustment of its screen brightness, perhaps even with function keys on the keyboard. Your other machine (the one running Windows 7) probably doesn’t support it, so calling CreateHandle with \\\\.\\LCD doesn’t get you anything useful. It has nothing to do with the operating system and everything to do with the hardware and/or the video drivers.