What makes an elisp function available with M-x

Question

Why some functions in simple.el are invokable with M-x and some others aren't?
For example I can do M-x what-line but I can't do M-x line-number-at-pos.

Answer 1

Only functions which are declared to be interactive are callable with M-x. The declaration is done by putting an (interactive) form as the first thing in the body of the function. Like this:

(defun an-example ()
  "this is an example function"
  (interactive)
  (insert "42"))

There is a lot of other information you should know about interactive functions, so you should type C-h f and enter interactive to see more help for it.