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23.1 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 hooks for the sake of customization. Most often, hooks are set up in the init file (see Init File), but Lisp programs can set them also. See Standard Hooks, for a list of some standard hook variables.

Most of the hooks in Emacs are normal hooks. These variables contain lists of functions to be called with no arguments. By convention, whenever the hook name ends in ‘-hook’, that tells you it is normal. We try to make all hooks normal, as much as possible, so that you can use them in a uniform way.

Every major mode command is supposed to run a normal hook called the mode hook as one of the last steps of initialization. This makes it easy for a user to customize the behavior of the mode, by overriding the buffer-local variable assignments already made by the mode. Most minor mode functions also run a mode hook at the end. But hooks are used in other contexts too. For example, the hook suspend-hook runs just before Emacs suspends itself (see Suspending Emacs).

The recommended way to add a hook function to a hook is by calling add-hook (see Setting Hooks). The hook functions may be any of the valid kinds of functions that funcall accepts (see What Is a Function). Most normal hook variables are initially void; add-hook knows how to deal with this. You can add hooks either globally or buffer-locally with add-hook.

If the hook variable's name does not end with ‘-hook’, that indicates it is probably an abnormal hook. That means the hook functions are called with arguments, or their return values are used in some way. The hook's documentation says how the functions are called. You can use add-hook to add a function to an abnormal hook, but you must write the function to follow the hook's calling convention.

By convention, abnormal hook names end in ‘-functions’. If the variable's name ends in ‘-function’, then its value is just a single function, not a list of functions.