Libundo is a simple, easy-to-use library which manages recording and
playback of undo/redo information for application developers.  It is
designed to be simple to plug in to existing applications and require
only a minimal amount of support code be written to support
multi-level undo/redo.  Libundo handles all the details of determining
what has changed after an undoable action is performed, recording that
information and saving it for use when an undo is performed.  Libundo
makes it easy for application writers to give end users what they
want.

Libundo is available under the GNU GPL and is not tied to any GUI
libraries or application frameworks.  Libundo has a C language
interface.  Libundo does require the ability to do Unix-style 
memory mapping.