20.5 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 defconst, defvar, defun, defmacro, defsubst, define-function and defalias. It then restores any autoloads formerly associated with those symbols. (Loading saves these in the autoload property of the symbol.)

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.
• Lists of the form (require . feature) indicating features that were required.
• Lists 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.