diffmk can also be used in conjunction with the proper troff requests to produce program listings with marked changes. In the following command line:
example% diffmk old.c new.c marked.c ; nroff reqs marked.c | pr
the file reqs contains the following troff requests:
which eliminate page breaks, adjust the line length, set no-fill mode, ignore escape characters, and turn off hyphenation, respectively.
If the characters | and * are inappropriate, you might run markedfile through sed(1) to globally change them.