signal-USR1-hook woes

[bjdouma@xs2.xs4all.nl (Bauke Jan Douma)]

Hi,

I am trying to get a list of currently open files in emacs, by
sending it SIGUSR1 (signo=10).  I have the following defines:

(defvar emacs-open-files "/root/emacs/.emacs-open-files" "List of files currently open in Emacs")

(defun list-emacs-open-files ()
	(interactive)
	
	(message "list-emacs-open-files: done")
	(let	(
		 (buf_list (buffer-list))
		)

		(with-temp-file emacs-open-files
			(while buf_list                                             	
				(cond	(
					 (buffer-file-name (car buf_list))
					 (insert (buffer-file-name (car buf_list)))
					 (insert "\n")
					)
				)
				(setq buf_list (cdr buf_list))
			)
		)
	)
)

(add-hook `signal-USR1-hook `list-emacs-open-files nil nil)

Alas, it doesn't work.  It does work from within emacs by
calling M-x list-emacs-open-files or by eval'ing
(signal-process (emacs-pid) 10).
When doing kill -USR1 <emacs-pid>, all that happens is I get a
visual bell, the file is not written.
An strace reveals that the signal is sent allright.

I am doing something wrong, but what?
Thanks!

bjd