;; Create a signal handler that changes the global variable vtest to "b"
(defun sigusr-handler ()
            (interactive)
        (setq vtest "b")
            (message "Caught signal %S" last-input-event))

;; Register the signal handler
(define-key special-event-map [sigusr1] 'sigusr-handler)

;; Initially set vtest to "a". Wait until changed to "b".
(progn
 (setq vtest "a")
 (while (not (string= vtest "b")) (sleep-for 1))  ;; elisp hangs forever in this loop. Is only interruptible using Ctrl-G
 (message-box "after")
)