Re: [9fans] a question of file and the history of magic

[Brantley Coile <brantley@coraid.com>]

I remember the day I first saw a file magic file.  I welcomed it because
for the first time I didn't have access to the source code.  Those were
the days when you had to have $45k to get the source.  A hard thing to
ask for.  Today a separate magic file is just a leftover vestige of the
past.  There are a lot of things like that.  Do we still need to
compress man pages on 1TB disk driver? :)

erik quanstrom wrote:
>>In a sense, the question is more about the historical change and/or
>>adoption of a new file command for Plan 9 that doesn't use a magic
>>file for references.  Why opt out of a magic file other than the
>>obvious performance hit of scanning it each run?  Is it worth
>>repeating the old forms that used magic, or has anyone in the Plan 9
>>community already improved upon the idea and introduced a new, more
>>adaptable tool?
>
>
> what is the upside to an external magic file?  as you've shown, you
> can add a file type in 1 line of code.  while the external magic file
> isn't c, i would argue that it's still code.
>
> the disadvantage is that you need to write a parser for yet another
> file format.  it turns out that linux file's maintainers felt that a text file
> wasn't good enough so they implemented a magic compiler.  i really
> don't understand the logic behind the compiler, since it would seem
> to trade reduced cpu cycles for increased i/o.  that would seem to be
> a terrible trade off these days.
>
> ; wc magic magic.mgc
>   13469   69850  484372 magic
>    1301   17997 1062400 magic.mgc		# compiled version
>
> the source is pretty big, too:
>
> ; wc -l ffile-4.20/src/*.[ch]|grep total
>   9273 total
>
> according to wikipedia (http://en.wikipedia.org/wiki/File_(Unix)),
> system v introduced the external magic file.  i don't think that system v
> was in anyway an ancestor of plan 9.  but i don't know anything of
> the history of plan 9 file.
>
> - erik
>