It works by running the diff program, using options taken from the variable diff-switches , whose value should be a string. See Compilation . If you specify the name of a backup file, diff-backup compares it with the source file that it is a backup of.
Comparison starts at point in each window.
Point moves forward in each window, a character at a time in each window, until the next characters in the two windows are different. Then the command is finished. For more information about windows in Emacs, Windows . With a numeric argument, compare-windows ignores changes in whitespace. See also Emerge , for convenient facilities for merging two similar files.