Adding and removing v4l2-loopback devices

Lines below create a loopback video device /dev/video5. After that ffmpeg is used to connect /dev/video0 to /dev/video5, but crop and hflip the stream on its way.

sudo apt-get install v4l2loopback-dkms 
sudo modprobe v4l2loopback video_nr=5
ffmpeg -i /dev/video0 -f v4l2 -pix_fmt yuv420p -filter:v "hflip,crop=400:400:0:0" /dev/video5

It's by design. First, let it be said that multiple processes can open the /dev/video0 device, but only one of them will be able to issue certain controls (ioctl()) until streaming starts.

Those V4L2 controls define things like bitrate. After you start streaming, the kernel will not let you change them and returns EBUSY (Device or resource busy) if you try. See this note in the kernel source. That effectively blocks other consumers, since you should set those before you start streaming.

What does v4l2loopback do differently? It adds logic and data structures for multiple openers and by default will not try to apply new controls by provinding its own setter.

Note that v4l2loopback needs to have multiple openers, at least two to be useful. One reader and one writer.