CODEWITHSUNDEEP
c++winapiscreen-brightness
DoktorMerlin
DoktorMerlin

Reputation: 55

GetMonitorBrightness() crashes with Access Violation

I'm trying to set up a small programm to adjust Monitor Brightness by the current room brightness.

I followed the instructions from MSDN and set up this:

cout << "Legen Sie das Fenster bitte auf den zu steuernden Monitor.\n";
system("PAUSE");
HMONITOR hMon = NULL;
char OldConsoleTitle[1024];
char NewConsoleTitle[1024];
GetConsoleTitle(OldConsoleTitle, 1024);
SetConsoleTitle("CMDWindow7355608");
Sleep(40);
HWND hWnd = FindWindow(NULL, "CMDWindow7355608");
SetConsoleTitle(OldConsoleTitle);
hMon = MonitorFromWindow(hWnd, MONITOR_DEFAULTTOPRIMARY);


DWORD cPhysicalMonitors;
LPPHYSICAL_MONITOR pPhysicalMonitors = NULL;
BOOL bSuccess = GetNumberOfPhysicalMonitorsFromHMONITOR(
    hMon,
    &cPhysicalMonitors
    );

if(bSuccess)
{
    pPhysicalMonitors = (LPPHYSICAL_MONITOR)malloc(
        cPhysicalMonitors* sizeof(PHYSICAL_MONITOR));

    if(pPhysicalMonitors!=NULL)
    {
        LPDWORD min = NULL, max = NULL, current = NULL;
        GetPhysicalMonitorsFromHMONITOR(hMon, cPhysicalMonitors, pPhysicalMonitors);

        HANDLE pmh = pPhysicalMonitors[0].hPhysicalMonitor;

        if(!GetMonitorBrightness(pmh, min, current, max))
        {
            cout << "Fehler: " << GetLastError() << endl;
            system("PAUSE");
            return 0;
        }

        //cout << "Minimum: " << min << endl << "Aktuell: " << current << endl << "Maximum: " << max << endl;

        system("PAUSE");
    }

}

But the problem: everytime I try to use GetMonitorBrightness() the program crashes with Access Violation while writing at Position 0x00000000 (I translated this error from German)

While trying to debug I saw that pPhysicalMonitors actually contains the monitor I want to use, but pPhysicalMonitors[0].hPhysicalMonitor contains 0x0000000 only. Could this be part of the problem?

Upvotes: 1

Views: 222

Answers (1)

Remy Lebeau
Remy Lebeau

Reputation: 595711

everytime I try to use GetMonitorBrightness() the program crashes with Access Violation while writing at Position 0x00000000 (I translated this error from German)

You are passing NULL pointers to GetMonitorBrightness(), so it crashes when trying to write its output values to invalid memory.

Just like GetNumberOfPhysicalMonitorsFromHMONITOR(), GetMonitorBrightness() expects you to pass the address of actual variables, eg:

DWORD min, max, current;
if (!GetMonitorBrightness(pmh, &min, &current, &max))

While trying to debug I saw that pPhysicalMonitors actually contains the monitor I want to use, but pPhysicalMonitors[0].hPhysicalMonitor contains 0x0000000 only. Could this be part of the problem?

No. However, you are not checking to make sure that cPhysicalMonitors is > 0, and you are ignoring the return value of GetPhysicalMonitorsFromHMONITOR() to make sure it is actually populating the PHYSICAL_MONITOR array with data.

Upvotes: 2

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