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 Garbage Collection). 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 allocates dynamic memory for
the part that didn’t fit. If Emacs will be dumped using the
pdump method (see Building Emacs), the pure-space overflow
is of no special importance (it just means some of the preloaded stuff
cannot be shared with other Emacs jobs). However, if Emacs will be
dumped using the now obsolete unexec method, the resulting
image will work, but garbage collection (see Garbage Collection)
is disabled in this situation, causing a memory leak. Such an
overflow normally won’t happen unless you try to preload additional
libraries or add features to the standard ones. Emacs will display a
warning about the overflow when it starts, if it was dumped using
unexec. If this happens, you should increase the compilation
parameter SYSTEM_PURESIZE_EXTRA in the file
src/puresize.h and rebuild Emacs.
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, but without text properties, 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 preloaded Lisp files.
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.
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 shareable 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.