1 Using Flymake

Flymake is only useful if at least one backend is configured to provide the buffer-checking service. This is done via the hook flymake-diagnostic-functions. See Hooks in The Emacs Editor.

It’s possible that some major modes or a third-party package has already setup this hook for you, by adding backend functions to flymake-diagnostic-functions. If you know Elisp you may also write your own Flymake backend functions. See Backend functions.