Comment 6 for bug 1327139

Mir's default socket is $XDG_RUNTIME_DIR for reasons explained in LP #1236912. Mirscreencast reads the framebuffer contents from U-S-C, and currently U-S-C is hard-coded to override the default socket location with '/run/mir_socket'.

unity-system-compositor --spinner=/usr/bin/unity-system-compositor-spinner --file /run/mir_socket --from-dm-fd 9 --to-dm-fd 13 --vt 1

(Note that since '/var/run' is a symlink to ' /run' , '/run/mir_socket' == '/var/run/mir_socket'). So the right thing to do would be to change it in lightdm (responsible for launching the U-S-C) to not use an argument.

Or the easiest thing to do would be to not do anything and have mirscreencast continue providing the socket override.