Failed to create wl_display when starting Qt wayland compositor

SchroedingersCat

I am trying to cross-compile the Qt wayland compositor example 'fancy compositor' for an embedded linux on the stm32mp1.
When trying to start the compositor I get the error:

Failed to create wl_display (No such file or directory)

My procedure is the following:

I stop the pre-installed Weston service: (systemctl stop weston-graphical-session.service, systemctl stop seatd-weston.service, systemctl stop weston-checkgpu.service)
I set the required env. variables: XDG_RUNTIME_DIR=/home/root/tmp-dir, QT_WAYLAND_CLIENT_BUFFER_INTEGRATION=wayland-egl, QT_XCB_GL_INTEGRATION=xcb_egl, QT_QPA_PLATFORM=wayland
I deploy and run the compositor ./fancy-compositor

My main question is if the fancy compositor requires a Weston display server already running or should it be able to launch by its own and provide a display server for apps launched on the device?
My build does not include eglfs since this leads to conflicts with the wayland layers.

Thank you for any help.

SchroedingersCat

From what I have found out the fancy compositor can either be run

with the eglfs backend not requiring another running wayland display server
using another running wayland display server and attaching to it to run on top of it.

Note: check what wayland servers are already running and then must create a new one when launching the custom Qt compositor. On the stm32mp1 for example a wayland server 'wayland-0' was started once but is now now working correctly anymore so do not attach to it. We also have a 'wayland-1' server running and a 'wayland-1.lock' in the /home/Weston folder. Therefore you should attach your custom compositor to wayland-1 but not name it wayland-1.

Either pass it on the command line when starting

./fancy-compositor --wayland-socket-name 'nested-compositor'

Or specify in the QML code

WaylandCompositor {
    id: waylandCompositor
    socketName: "wayland-nested"

And then for running it use the wayland-1 server

WAYLAND_DISPLAY=wayland-1