The function execute-with-interface
is a useful way of operating on an interface owned by another process. It takes a top-level interface, a function and some arguments and queues the function to be run by that process when it next enters its event loop (for an interface owned by the current process, it calls the function immediately).
(setq a (capi:display (make-instance 'capi:interface)))
(capi:execute-with-interface
a 'break
"Break inside the interface process")