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(elisp)Pure Storage

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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.