Writing GNU Emacs Extensions

Local Variables Lists

What we've described in this chapter so far is enough to set up a file of Lisp code and have it loaded on demand. But in the case of timestamp, things are a little different. We've already arranged for calling update-writestamps to autoload timestamp, but who or what is going to call update-writestamps and force timestamp to load? Recall that update-writestamps is supposed to get called from local-write-file-hooks. So how does update-writestamps get into local-write-file-hooks? Loading the file mustn't do that for the reasons mentioned in the section called Creating a Lisp File earlier in this chapter.

What we need is a way to get update-writestamps into local-write-file-hooks in buffers that need it, so that the first invocation of local-write-file-hooks can cause the autoloading of timestamp.

A good way to accomplish this is by using the local variables list that may appear near the end of any file. Whenever Emacs visits a new file, it scans near the end^[22] for a block of text that looks like this:

Local variables:
var1: value1
var2: value2
...
End:

When Emacs finds such a block, it assigns each value to the corresponding var, which is automatically made buffer-local. Emacs can recognize this block even if each line begins with a prefix, as long as they all begin with the same prefix. This is necessary in a file of Lisp code, for example, to comment out the lines so they're not interpreted as Lisp:

; Local variables:
; var1: value1
; var2: value2 ; ... ; End: ...