Writing GNU Emacs Extensions

Modifystamps

Well, timestamps were marginally useful, and writestamps were somewhat more so, but modifystamps may be even better. A modifystamp is a writestamp that records the time the file was last modified, which may not be the same as the last time it was saved to disk. For instance, if you visit a file and save it under a new name without making any changes to it, you shouldn't cause the modifystamp to change.

In this section we'll briefly explore two very simple approaches to implementing modifystamps, and one clever one.

Simple Approach #1

Emacs has a hook variable called first-change-hook. Whenever a buffer is changed for the first time since it was last saved, the functions in first-change-hook get executed. Implementing modifystamps by using this hook merely entails moving our old update-writestamps function from local-write-file-hooks to first-change-hook. Of course, we'll also want to change its name to update-modifystamps, and introduce new variables—modifystamp-format, modifystamp-prefix, and modifystamp-suffix—that work like their writestamp counterparts without overloading the writestamp variables. Then update-modifystamps should be changed to use the new variables.

Before any of this happens, first-change-hook, which is normally global, should be made buffer-local. If we add update-modifystamps to first-change-hook while it is still global, update-modifystamps will be called every time any buffer is saved. Making it buffer-local in the current buffer causes changes ...