diffmk compares two versions of a file and creates a third version that includes "change mark" (.mc) commands for nroff(1) and troff(1). oldfile and newfile are the old and new versions of the file. diffmk generates markedfile, which, contains the text from newfile with troff(1) "change mark" requests (.mc) inserted where newfile differs from oldfile. When markedfile is formatted, changed or inserted text is shown by | at the right margin of each line. The position of deleted text is shown by a single *.

See largefile(5) for the description of the behavior of diffmk when encountering files greater than or equal to 2 Gbyte ( 2³¹ bytes).

See attributes(5) for descriptions of the following attributes:

diff(1), nroff(1), sed(1), troff(1), attributes(5), largefile(5)

Aesthetic considerations may dictate manual adjustment of some output. File differences involving only formatting requests may produce undesirable output, that is, replacing .sp by .sp 2 will produce a "change mark" on the preceding or following line of output.