(emacs.info) Hooks

 
 Hooks
 -----
 
    A "hook" is a variable where you can store a function or functions
 to be called on a particular occasion by an existing program.  Emacs
 provides a number of hooks for the sake of customization.
 
    Most of the hooks in Emacs are "normal hooks".  These variables
 contain lists of functions to be called with no arguments.  The reason
 most hooks are normal hooks is so that you can use them in a uniform
 way.  Every variable in Emacs whose name ends in `-hook' is a normal
 hook.
 
    Most major modes run hooks as the last step of initialization.  This
 makes it easy for a user to customize the behavior of the mode, by
 overriding the local variable assignments already made by the mode.  But
 hooks may also be used in other contexts.  For example, the hook
 `suspend-hook' runs just before Emacs suspends itself ( Exiting).
 
    The recommended way to add a hook function to a normal hook is by
 calling `add-hook'.  You can use any valid Lisp function as the hook
 function.  For example, here's how to set up a hook to turn on Auto
 Fill mode when entering Text mode and other modes based on Text mode:
 
      (add-hook 'text-mode-hook 'turn-on-auto-fill)
 
    The next example shows how to use a hook to customize the indentation
 of C code.  (People often have strong personal preferences for one
 format compared to another.)  Here the hook function is an anonymous
 lambda expression.
 
      (setq my-c-style
        '((c-comment-only-line-offset . 4)
          (c-cleanup-list . (scope-operator
      		       empty-defun-braces
      		       defun-close-semi))
          (c-offsets-alist . ((arglist-close . c-lineup-arglist)
      			(substatement-open . 0)))))
      
      (add-hook 'c-mode-common-hook
        (function (lambda ()
          (c-add-style "my-style" my-c-style t))))
 
    It is best to design your hook functions so that the order in which
 they are executed does not matter.  Any dependence on the order is
 "asking for trouble."  However, the order is predictable: the most
 recently added hook functions are executed first.
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