Debug Emacs Lisp major mode

Question

I'm developing a major mode for Emacs. Is there any way that I can set a break point in the source code when fontification happens, for example?

Answer 1

Find the Lisp source of the function you'd like to step through, and type M-x edebug-defun there. Then, whenever that function is executed, you'll automatically be placed into Edebug, where you can step through it if you wish.

Fontification functions can be a bit tricky though, as they can be invoked at odd times. You can use the message function to write messages into the *Messages* buffer. Another trick is to turn off Font Lock (so your function doesn't get invoked automatically), then prep the function you're debugging with edebug-defun and invoke it manually. (Note that you can use M-: (a.k.a. eval-expression) to invoke a non-interactive function.)

Answer 2

The manual for debugging elisp can be found here.

You can used edebug as mentioned, there's also M-x debug-on-entry and you can set (setq debug-on-quit t). Check out all the options in the link, it all depends on how you want to skin the cat.