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(elisp)Maintaining Undo

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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
 - Command: buffer-flush-undo &optional buffer
     This function discards the undo list of BUFFER, 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 is already disabled, this
     function has no effect.

     This function returns `nil'.

     The name `buffer-flush-undo' is not considered obsolete, but the
     preferred name is `buffer-disable-undo'.

   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.)  Two variables control the range of acceptable
sizes: `undo-limit' and `undo-strong-limit'.

 - Variable: 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.

 - Variable: 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 never discarded no
     matter how big it is.