efaq: Turning on auto-fill by default

 
 5.16 How do I turn on ‘auto-fill-mode’ by default?
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 To turn on ‘auto-fill-mode’ just once for one buffer, use ‘M-x
 auto-fill-mode’.
 
    To turn it on for every buffer in a certain mode, you must use the
 hook for that mode.  For example, to turn on ‘auto-fill’ mode for all
 text buffers, including the following in your ‘.emacs’ file:
 
      (add-hook 'text-mode-hook 'turn-on-auto-fill)
 
    If you want ‘auto-fill’ mode on in all major modes, do this:
 
      (setq-default auto-fill-function 'do-auto-fill)