RE: What do 'hooks' do and how do they do it?

["Drew Adams" <drew.adams@oracle.com>]

> I know at a high level what a 'hook' is and how to use it an elisp
> statement.  And, I have seen hooks used in other programs 
> like SELinux.
> 
> But I am curious about what is going on at the kernel level with a
> 'hook'.  If someone can give me a brief overview in relatively plain
> language, I would appreciate it.
> 
> e.g. some of the kind of questions that spring to mind.
> Is it a process that is added to the task structure waiting to be
> called?
> How is it woken up?  And what kind of events might wake it?  etc.
> 
> As I said, an overview would be helpful.  If there is some 
> detail that I
> want clarified, I can dig deeper once I have a general idea of what is
> going on. 

See the Emacs manual, node `Hooks'.
See the Elisp manual, node `Hooks'.

A hook is a variable whose value is a list of functions. Typically, code invokes
the functions in such a list, one by one. Such invocation is done using a
function such as `run-mode-hooks' or `run-hooks'.

A typical example is the code for a major mode, such as `emacs-lisp-mode'. The
last thing such code does, when you turn on the mode, is to call
`run-mode-hooks' to run the hook provided specifically for that mode, e.g.
`emacs-lisp-mode-hook'. That is, it tries to invoke each of the functions on the
list `emacs-lisp-mode-hook', in turn. If there are no functions on the hook,
then nothing is done.

Because you can change the value of a hook variable, to include different
functions, you can in effect modify the behavior of the code that runs the hook.
If, for example, you add your own function `foo' to `emacs-lisp-mode-hook', then
whenever Emacs-Lisp mode is entered, `foo' will be invoked. In effect, you have
extended the definition of function `emacs-lisp-mode' so that it also does
`foo', after doing what it normally does.

Hooks are thus a way to allow easy code extension at predefined places.