Writing GNU Emacs Extensions

Writestamps

Inserting the current date and time automatically and in such a configurable format is pretty neat and probably beyond the ken of most text editors, but its usefulness is limited. Undoubtedly more useful would be the ability to store a writestamp in a file; that is, the date and/or time the file was last written to disk. A writestamp updates itself each time the file is saved anew.

Updating Writestamps

The first thing we'll need is a way to run our writestamp-updating code each time the file is saved. As we discovered in the section Hooks in Chapter 2, the best way to associate some code with a common action (such as saving a file) is by adding a function to a hook variable, provided that a suitable hook variable exists. Using M-x apropos RET hook RET, we discover four promising hook variables: after-save-hook, local-write-file-hooks, write-contents-hooks, and write-file-hooks.

We can discard after-save-hook right away. We don't want our code executed, modifying writestamps, after the file is saved, because then it will be impossible to save an up-to-date version of the file!

The differences between the remaining candidates are subtle:

write-file-hooks

Code to execute for any buffer each time it is saved.

local-write-file-hooks

A buffer-local version of write-file-hooks. Recall from the Hooks section of Chapter 2 that a buffer-local variable is one that can have different values in different buffers. Whereas write-file-hooks pertains to every buffer, local-write-file-hooks