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:

1. I stop the pre-installed Weston service: (systemctl stop weston-graphical-session.service, systemctl stop seatd-weston.service, systemctl stop weston-checkgpu.service)
2. 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
3. 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