error using v4l2 with userptr and gstreamer

short: I wrote an app that captures frames from video device using v4l2. the app should support all i-o modes. I am mostly interested in userptr mode. when i run the code with physical camera it works (with userptr mode). when i run the same code with v4l2loopback camera and gstreamer, it fails. calling VIDIOC_REQBUFS with V4L2_MEMEORY_USERPTR returns not supported.

Questions:

1.has anyone got that scenario working ?

2.I have found this link: https://developer.ridgerun.com/wiki/index.php/GStreamer1.0_and_V4L2_UserPtr. it suggests calling " ./configure --without-libv4l2". I am nit sure how to call this " ./configure --without-libv4l2". any idea ? BTW, gstreamer indeed has the code metioned in the ink above (see: https://github.com/operasoftware/gstreamer/blob/master/gst-plugins-good/sys/v4l2/v4l2_calls.h)

3.does this link means the feature is not supported ? https://github.com/umlaeute/v4l2loopback/issues/146

4.any other alternatives to v4l2sink ?

5.any other alternatives for gstreamer or v4l2loopback ?

6.any idea why this feature is not supported ? what are the main challanged updating v4l2sink to supprot the userptr mode ?