Drew Adams wrote:

(interactive  "sEnter number: ")


Consider using `read-number':

(interactive (list (read-number "Number: " 42)))

When hitting <Enter> without entering anything, emacs comes 
back to the input request. How can I find out, that nothing
has been entered so that I can use e.g. a default value?


I don't understand how that happens to you.
It should just return the empty string, "".

M-: (defun foo (arg) (interactive "sNum: ") (message "%S" arg))
M-x foo RET RET

That returns "" for me.

Here is my function

(defun mytest (n)
" Test"
; (interactive "nEnter number: ")
(interactive (list (read-number "Enter number: ")))
(message "%d was entered" n)
)

With both versions of the interactive statement I have the same effect: the prompt comes back
as long as I do not enter anything.

I am running emacs22 on Debian lenny.
Do I need a special setting to achive what I want?

K.D.J.