GNU Info

(elisp)Unloading

---

Next: Hooks for Loading Prev: Named Features Up: Loading

---

Enter node , (file) or (file)node

Unloading
=========

   You can discard the functions and variables loaded by a library to
reclaim memory for other Lisp objects.  To do this, use the function
`unload-feature':

 - Command: unload-feature feature &optional force
     This command unloads the library that provided feature FEATURE.
     It undefines all functions, macros, and variables defined in that
     library with `defun', `defalias', `defsubst', `defmacro',
     `defconst', `defvar', and `defcustom'.  It then restores any
     autoloads formerly associated with those symbols.  (Loading saves
     these in the `autoload' property of the symbol.)

     Before restoring the previous definitions, `unload-feature' runs
     `remove-hook' to remove functions in the library from certain
     hooks.  These hooks include variables whose names end in `hook' or
     `-hooks', plus those listed in `loadhist-special-hooks'.  This is
     to prevent Emacs from ceasing to function because important hooks
     refer to functions that are no longer defined.

     If these measures are not sufficient to prevent malfunction, a
     library can define an explicit unload hook.  If
     `FEATURE-unload-hook' is defined, it is run as a normal hook
     before restoring the previous definitions, _instead of_ the usual
     hook-removing actions.  The unload hook ought to undo all the
     global state changes made by the library that might cease to work
     once the library is unloaded.  `unload-feature' can cause problems
     with libraries that fail to do this, so it should be used with
     caution.

     Ordinarily, `unload-feature' refuses to unload a library on which
     other loaded libraries depend.  (A library A depends on library B
     if A contains a `require' for B.)  If the optional argument FORCE
     is non-`nil', dependencies are ignored and you can unload any
     library.

   The `unload-feature' function is written in Lisp; its actions are
based on the variable `load-history'.

 - Variable: load-history
     This variable's value is an alist connecting library names with the
     names of functions and variables they define, the features they
     provide, and the features they require.

     Each element is a list and describes one library.  The CAR of the
     list is the name of the library, as a string.  The rest of the
     list is composed of these kinds of objects:

        * Symbols that were defined by this library.

        * Cons cells of the form `(require . FEATURE)' indicating
          features that were required.

        * Cons cells of the form `(provide . FEATURE)' indicating
          features that were provided.

     The value of `load-history' may have one element whose CAR is
     `nil'.  This element describes definitions made with `eval-buffer'
     on a buffer that is not visiting a file.

   The command `eval-region' updates `load-history', but does so by
adding the symbols defined to the element for the file being visited,
rather than replacing that element.  Note: Eval.

   Preloaded libraries don't contribute initially to `load-history'.
Instead, preloading writes information about preloaded libraries into a
file, which can be loaded later on to add information to `load-history'
describing the preloaded files.  This file is installed in
`exec-directory' and has a name of the form `fns-EMACSVERSION.el'.

   See the source for the function `symbol-file', for an example of
code that loads this file to find functions in preloaded libraries.

 - Variable: loadhist-special-hooks
     This variable holds a list of hooks to be scanned before unloading
     a library, to remove functions defined in the library.