Re: how to define a comment syntax for files having a specific extension?

[Stefan Monnier <monnier@iro.umontreal.ca>]

> and working on journal text files *.jou for which the commented lines
> must start with the character "/".

> While editing one of such files I need to run
> M-: (setq comment-start "/")
> in order to comment regions with the sequence C-c;

> I'd like to have emacs recognize the "*.jou" files and
> apply the corresponding comment syntax automatically,
> sparing me from having to type
> M-: (setq comment-start "/")
> each time I start working on a *.jou file.

You need to create a major mode for those types of files.
Check out sample-mode.el


        Stefan