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

[erik quanstrom <quanstro@quanstro.net>]

> 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