Info Catalog elisp: Undo elisp: Text elisp: Filling

elisp: Maintaining Undo

 
 31.10 Maintaining Undo Lists
 ============================
 
 This section describes how to enable and disable undo information for a
 given buffer.  It also explains how the undo list is truncated
 automatically so it doesn’t get too big.
 
    Recording of undo information in a newly created buffer is normally
 enabled to start with; but if the buffer name starts with a space, the
 undo recording is initially disabled.  You can explicitly enable or
 disable undo recording with the following two functions, or by setting
 ‘buffer-undo-list’ yourself.
 
  -- Command: buffer-enable-undo &optional buffer-or-name
      This command enables recording undo information for buffer
      BUFFER-OR-NAME, so that subsequent changes can be undone.  If no
      argument is supplied, then the current buffer is used.  This
      function does nothing if undo recording is already enabled in the
      buffer.  It returns ‘nil’.
 
      In an interactive call, BUFFER-OR-NAME is the current buffer.  You
      cannot specify any other buffer.
 
  -- Command: buffer-disable-undo &optional buffer-or-name
      This function discards the undo list of BUFFER-OR-NAME, and
      disables further recording of undo information.  As a result, it is
      no longer possible to undo either previous changes or any
      subsequent changes.  If the undo list of BUFFER-OR-NAME is already
      disabled, this function has no effect.
 
      In an interactive call, BUFFER-OR-NAME is the current buffer.  You
      cannot specify any other buffer.  This function returns ‘nil’.
 
    As editing continues, undo lists get longer and longer.  To prevent
 them from using up all available memory space, garbage collection trims
 them back to size limits you can set.  (For this purpose, the size of an
 undo list measures the cons cells that make up the list, plus the
 strings of deleted text.)  Three variables control the range of
 acceptable sizes: ‘undo-limit’, ‘undo-strong-limit’ and
 ‘undo-outer-limit’.  In these variables, size is counted as the number
 of bytes occupied, which includes both saved text and other data.
 
  -- User Option: undo-limit
      This is the soft limit for the acceptable size of an undo list.
      The change group at which this size is exceeded is the last one
      kept.
 
  -- User Option: undo-strong-limit
      This is the upper limit for the acceptable size of an undo list.
      The change group at which this size is exceeded is discarded itself
      (along with all older change groups).  There is one exception: the
      very latest change group is only discarded if it exceeds
      ‘undo-outer-limit’.
 
  -- User Option: undo-outer-limit
      If at garbage collection time the undo info for the current command
      exceeds this limit, Emacs discards the info and displays a warning.
      This is a last ditch limit to prevent memory overflow.
 
  -- User Option: undo-ask-before-discard
      If this variable is non-‘nil’, when the undo info exceeds
      ‘undo-outer-limit’, Emacs asks in the echo area whether to discard
      the info.  The default value is ‘nil’, which means to discard it
      automatically.
 
      This option is mainly intended for debugging.  Garbage collection
      is inhibited while the question is asked, which means that Emacs
      might leak memory if the user waits too long before answering the
      question.
 

Info Catalog elisp: Undo elisp: Text elisp: Filling