(defun signal-usr1-f ()
  (interactive)
  (message "Got signal: USR1") )

(define-key special-event-map [sigusr1] #'signal-usr1-f)

;; test:   (signal-process (emacs-pid) 'sigusr1)
;;
;; shell:  kill -s usr1 `ps -e | grep emacs | cut -d " " -f 2`