16.9 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 the deprecated suffix ‘-hooks’), plus those listed in unload-feature-special-hooks, as well as auto-mode-alist. This is to prevent Emacs from ceasing to function because important hooks refer to functions that are no longer defined.

Standard unloading activities also undoes ELP profiling of functions in that library, unprovides any features provided by the library, and cancels timers held in variables defined by the library.

If these measures are not sufficient to prevent malfunction, a library can define an explicit unloader named feature-unload-function. If that symbol is defined as a function, unload-feature calls it with no arguments before doing anything else. It can do whatever is appropriate to unload the library. If it returns nil, unload-feature proceeds to take the normal unload actions. Otherwise it considers the job to be done.

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 unload-feature-special-hooks

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