Comment 1 for bug 1272143

It works but the documentation is wrong.

The default socket name depends on the user name.

For non-root this will typically be $XDG_RUNTIME_DIR/mir_socket

For root (where XDG_RUNTIME_DIR isn't set) this will typically be /tmp/mir_socket

So you need "bin/mir_demo_client_egltriangle -m /tmp/mir_socket"