E.2 Pure Storage

Emacs Lisp uses two kinds of storage for user-created Lisp objects: normal storage and pure storage. Normal storage is where all the new data created during an Emacs session are kept; see the following section for information on normal storage. Pure storage is used for certain data in the preloaded standard Lisp files--data that should never change during actual use of Emacs.

Pure storage is allocated only while `temacs' is loading the standard preloaded Lisp libraries. In the file `emacs', it is marked as read-only (on operating systems that permit this), so that the memory space can be shared by all the Emacs jobs running on the machine at once. Pure storage is not expandable; a fixed amount is allocated when Emacs is compiled, and if that is not sufficient for the preloaded libraries, `temacs' crashes. If that happens, you must increase the compilation parameter PURESIZE in the file `src/puresize.h'. This normally won't happen unless you try to preload additional libraries or add features to the standard ones.

Function: purecopy object

This function makes a copy in pure storage of object, and returns it. It copies a string by simply making a new string with the same characters in pure storage. It recursively copies the contents of vectors and cons cells. It does not make copies of other objects such as symbols, but just returns them unchanged. It signals an error if asked to copy markers.

This function is a no-op except while Emacs is being built and dumped; it is usually called only in the file `emacs/lisp/loaddefs.el', but a few packages call it just in case you decide to preload them.

Variable: pure-bytes-used

The value of this variable is the number of bytes of pure storage allocated so far. Typically, in a dumped Emacs, this number is very close to the total amount of pure storage available--if it were not, we would preallocate less.

Variable: purify-flag

This variable determines whether defun should make a copy of the function definition in pure storage. If it is non-nil, then the function definition is copied into pure storage.

This flag is t while loading all of the basic functions for building Emacs initially (allowing those functions to be sharable and non-collectible). Dumping Emacs as an executable always writes nil in this variable, regardless of the value it actually has before and after dumping.

You should not change this flag in a running Emacs.