Question: I'm attempting to detect if my capture camera gets unplugged. My assumption was that a call to `cvQueryFrame` would return `NULL`, however it continues to return the last valid frame. 

Does anyone know of how to detect camera plug/unplug events with OpenCV? This seems so rudimentary...what am I missing?

Answer: 
There is no API function to do that, unfortunately.

However, my suggestion is that you **create another thread** that simply calls cvCaptureFromCAM() and check it's result (inside a loop). If the camera get's disconnected then it should return NULL.

I'll paste some code just to illustrate my idea:

```
// This code should be executed on another thread!
while (1)
{
  CvCapture* capture = NULL;
  capture = cvCaptureFromCAM(-1); // or whatever parameter you are already using
  if (!capture)
  {
    std::cout << "!!! Camera got disconnected !!!!" << std::endl;
    break;
  }

  // I'm not sure if releasing it will have any affect on the other thread
  cvReleaseCapture(&capture); 
}

```
This still seems to be an issue.
Another solution would be to compare the returned data with the previous one. For a working camera there should always be flickering. if the data is identical you can be sure, the cam was unplugged.
Martin