EnumDisplayDevices function not working for me

Question

I'm trying to get info on my monitors programmatically. The content of the loops is not important right now, they just contain debug statements that will be printed when the loop condition is satisfied. Right now, the outer loop code executes three times and the inner loop code is never accessed, meaning the while condition of the (inner) loop is never true, meaning that call fails.

My problem here is that the Windows API says, regarding this function:

To obtain information on a display monitor, first call EnumDisplayDevices with lpDevice >set to NULL. Then call EnumDisplayDevices with lpDevice set to DISPLAY_DEVICE.DeviceName >from the first call to EnumDisplayDevices and with iDevNum set to zero. Then >DISPLAY_DEVICE.DeviceString is the monitor name.

...but even after doing exactly what it says, the second EnumDisplayDevices call always fails? Any insight???

Also I'm doing this as a service level application on both windows xp and windows 7 and getting the same results. When I try to print out dd.DeviceName, it gives me an address (for example: 0x12cfa4), but this must be what the function is expecting on the second call as MSDN says just pass in your display device pointer and append .DeviceName to it...

C++ (using Qt), Windows API/MSDN calls are used.

    DISPLAY_DEVICE dd;

    dd.cb = sizeof(DISPLAY_DEVICE);

    DWORD deviceNum = 0;
    while( EnumDisplayDevices(NULL, deviceNum, &dd, 0) ){
        qWarning() << "We've entered the outer loop.";

        while( EnumDisplayDevices(dd.DeviceName, 0, &dd, 0)){
            qWarning() << "We've entered the inner loop.";
        }

        deviceNum++;
    }

Solution

The problem is passing 'dd' into the inner call while using a member of 'dd' as the input string. I know this doesn't make sense, but I suspect that since dd is an out parameter the API is writing to it, but then looking that the contents of the input parameter after it has scribbled on it. This could happen if they did something like memset'ing the output parm to 0 before execution for sanity.

Just make sure it wasn't something about sending in a non-empty dd, I made a second dd with the exact same bits in it and things still work fine. It's definitely the aliased memory.

#include <windows.h>
#include <stdio.h>

#pragma comment(lib, "user32.lib")

void DumpDevice(const DISPLAY_DEVICE& dd, size_t nSpaceCount )
{
    printf("%*sDevice Name: %s\n", nSpaceCount, "", dd.DeviceName );
    printf("%*sDevice String: %s\n", nSpaceCount, "", dd.DeviceString );
    printf("%*sState Flags: %x\n", nSpaceCount, "", dd.StateFlags );
    printf("%*sDeviceID: %s\n", nSpaceCount, "", dd.DeviceID );
    printf("%*sDeviceKey: ...%s\n\n", nSpaceCount, "", dd.DeviceKey+42 );
}

int main()
{
    DISPLAY_DEVICE dd;

    dd.cb = sizeof(DISPLAY_DEVICE);

    DWORD deviceNum = 0;
    while( EnumDisplayDevices(NULL, deviceNum, &dd, 0) ){
        DumpDevice( dd, 0 );
        DISPLAY_DEVICE newdd = {0};
        newdd.cb = sizeof(DISPLAY_DEVICE);
        DWORD monitorNum = 0;
        while ( EnumDisplayDevices(dd.DeviceName, monitorNum, &newdd, 0))
        {
            DumpDevice( newdd, 4 );
            monitorNum++;
        }
        puts("");
        deviceNum++;
    }

    return 0;
}

The box I'm on right now only has 1 monitor, so I can't verify the inner loop, but I suspect it's fine since 'newdd' never gets aliased into the call. You also say you are in a service context - I'm not sure if that winstation will restrict what you see about the displays - so that could also be a problem; but I suspect you should still be able to at least see the physical device. On my machine I get:

Device Name: \\.\DISPLAY1
Device String: NVIDIA GeForce GTX 580
State Flags: 8000005
DeviceID: PCI\VEN_10DE&DEV_1080&SUBSYS_15803842&REV_A1
DeviceKey: ...\Control\Video\{B0CDD262-FCFB-4FD4-A03C-54621896C9CD}\0000

   Device Name: \\.\DISPLAY1\Monitor0
   Device String: Generic PnP Monitor
   State Flags: 3
   DeviceID: MONITOR\DEL4016\{4d36e96e-e325-11ce-bfc1-08002be10318}\0002
   DeviceKey: ...\Control\Class\{4d36e96e-e325-11ce-bfc1-08002be10318}\0002

Device Name: \\.\DISPLAY2
Device String: NVIDIA GeForce GTX 580
State Flags: 0
DeviceID: PCI\VEN_10DE&DEV_1080&SUBSYS_15803842&REV_A1
DeviceKey: ...\Control\Video\{B0CDD262-FCFB-4FD4-A03C-54621896C9CD}\0001

Device Name: \\.\DISPLAYV1
Device String: RDPDD Chained DD
State Flags: 8
DeviceID:
DeviceKey: ...\Control\Video\{DEB039CC-B704-4F53-B43E-9DD4432FA2E9}\0000

Device Name: \\.\DISPLAYV2
Device String: RDP Encoder Mirror Driver
State Flags: 200008
DeviceID:
DeviceKey: ...\Control\Video\{42cf9257-1d96-4c9d-87f3-0d8e74595f78}\0000

Device Name: \\.\DISPLAYV3
Device String: RDP Reflector Display Driver
State Flags: 200008
DeviceID:
DeviceKey: ...\Control\Video\{b043b95c-5670-4f10-b934-8ed0c8eb59a8}\0000

Comments

[arx]

I'd just finished writing much the same thing. The aliasing is the problem.

[scone]

Not sure if this is something which has changed since you posted the code, but I found that in MSVC2015, I had to use %S instead of %s, because the DeviceName, DeviceString, etc. are WCHAR arrays, and I guess contain /0 data in every second character.