Info Catalog elisp: Building Emacs elisp: GNU Emacs Internals elisp: Garbage Collection

elisp: Pure Storage

 
 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 (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.  The resulting image will work, but garbage collection (
 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 this happens,
 you should increase the compilation parameter ‘SYSTEM_PURESIZE_EXTRA’ in
 the file ‘src/puresize.h’ and rebuild Emacs.
 
  -- 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, 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.
 
  -- 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 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.
 

Info Catalog elisp: Building Emacs elisp: GNU Emacs Internals elisp: Garbage Collection