DISPLAY_DEVICE main monitor StateFlags

Jeff Ibey 41

I am trying to draw on any monitor not the main monitor. I use the DISPLAY_DEVICE pointer and run through all 10 ports. I check
if (EnumDisplayDevices(0, [0-9],&DISPLAY_DEVICE,0)!=NULL)
If this is true, that means I found a connected display. I than check the StateFlags for main monitor by checking the third bit set.
If(!(DISPLAY_DEVICE.StateFlags & DISPLAY_DEVICE_PRIMARY_DEVICE))

If it's not set and a display, it's not the main monitor.
If it is set and a display, it is the main monitor.

I am running into the opposite in one scenario. The main display is identity number as 2, and it does not have the third bit set. So it's drawing on the main monitor when I am looking for ANYTHING but the main monitor. I know usually identity numbers are meaningless, however out of 70 machines only one acts this way. Am I understanding this wrong or does windows have a bug here the main monitor is set to identity 2 and does not set the 3rd bit in the state flags?

Accepted answer

Xiaopo Yang - MSFT 12,726 Reputation points Microsoft Vendor

2021-06-24T01:44:36.18+00:00

Thank for your attached code!
And according to the StateFlags of DISPLAY_DEVICEA structure,

DISPLAY_DEVICE_PRIMARY_DEVICE: The primary desktop is on the device. For a system with a single display card, this is always set. For a system with multiple display cards, only one device can have this set.
Please sign in to rate this answer.
Jeff Ibey 41 Reputation points

2021-06-24T02:28:37.743+00:00

I rechecked your documentation, seems someone has updated the thread haha. Based on my old code that is how it was explained, however they have now added these 2 new paragraphs:

This paragraph answers my current question, thank you guys.

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.

To query all monitor devices associated with an adapter, call EnumDisplayDevices in a loop with lpDevice set to the adapter name, iDevNum set to start at 0, and iDevNum set to increment until the function fails. Note that DISPLAY_DEVICE.DeviceName changes with each call for monitor information, so you must save the adapter name. The function fails when there are no more monitors for the adapter.

Thanks for your support!
Sign in to comment

Use comments to ask for clarification, additional information, or improvements to the question.

2 additional answers

Song Zhu - MSFT 906 Reputation points

2021-06-16T08:06:02.257+00:00

Please refer to the same thread you ask.
Please sign in to rate this answer.

0 comments No comments
Sign in to comment

Use comments to ask for clarification, additional information, or improvements to the question.
Jeff Ibey 41 Reputation points

2021-06-17T05:37:48.627+00:00

Same thread? This completely different scenario with usage of state flags and lpDisplayDevice pointer. If you don't understand the use case or the differences between them please do not lump them.
Please sign in to rate this answer.
Song Zhu - MSFT 906 Reputation points

2021-06-17T06:21:15.353+00:00

Maybe you can provide a sample to reproduce the problem so that we can understand your problem.

Jeff Ibey 41 Reputation points

2021-06-20T06:32:41.927+00:00

DISPLAY_DEVICE dd; ZeroMemory(&dd, sizeof(dd)); dd.cb = sizeof(dd); //check 10 ports int displayDevicePort=0; for(int i =0; i <10; i++) { //check ports are connected if(EnumDisplayDevices(0, i, &dd, 0)!=NULL) { //check if not main display if(!(dd.StateFlags & DISPLAY_DEVICE_PRIMARY_DEVICE)) { //found extended display break out //EnumDisplayDevices(0, 1, &dd, 0); break; } else { ZeroMemory(&dd, sizeof(dd)); dd.cb = sizeof(dd); } } }
Sign in to comment

Use comments to ask for clarification, additional information, or improvements to the question.

Share via

DISPLAY_DEVICE main monitor StateFlags

2 additional answers

Your answer