Re: recognizing a file by scanning it

[Stefan Monnier]

>    file(1) contains a rather complete set of such heuristics and
>    has been stable for a long time.  AFAIK libmagic is free and
>    GPL-compatible.

> Yes, i believe so.  My concern w/ using libmagic would be
> portability, and handling the case where libmagic is not
> available.  I envision the feature to be non-optional.

Would could like we do for `ls': use libmagic if it's available and
fallback on an Elisp replacement if not.  The Elisp replacement would
need to work even in the absence of /etc/magic.

The higher-level functionality also needs adjusting: we want to bring
magic-mode-alist and auto-mode-alist to the same level (rather than give
absolute priority to one of the two).  Basically, try them both, each
one can return a set/list of potential modes, then try and figure out
which mode to use (I've done something similar in doc-view-mode).
Hopefully this can be used to disambiguate the .arc archives from the
.arc Lisp files as well.

>    Also, it might be nice if you could call this facility from
>    coding systems (after all, what else is a BOM but file magic?)
>    This kind of thing really doesn't need to be in Lisp, and might
>    benefit from being in C.

> Certainly, in C it can be much faster.  

If it's used just to choose the major-mode, speed is a non-issue.


        Stefan