Just to make it easier for anyone who is reading this thread in the archives:
At the moment the library you need is libxrandr (on Ubuntu install with apt install libxrandr-dev), and read the man page Xrandr. It is also useful to read the paper describing Xrandr protocol: https://cgit.freedesktop.org/xorg/proto/randrproto/tree/randrproto.txt best Vladimir Dergachev
