Re: DogCows or Polymorphism in the Hurd

[Thomas Schwinge]

Hello!

Something I just saw...

On Thu, Feb 02, 2006 at 01:56:50PM -0500, Marcus Brinkmann wrote:
> Take for example a tar.gz file object.  You can look at it as a binary
> file.  But you can also look at it as an archive, which provides many
> files in a directory structure.  In this case, the tar.gz object itself
> is the root directory of the archive.
> 
> [...]
>
> Now, the problem that now occurs is that a user of the object needs to
> make a decision about which facet of the object it wants to consider.
> This question is simple to answer if the user only knows one of the
> supported facets.  For example, a program that only knows about Files
> will treat all "DirFiles" in the Hurd as Files.  A program that only
> knows about Directories will treat all "DirFiles" as Directories.
> 
> However, it happens that in Unix, Directories and Files are not only
> very distinct objects, but they are also understand by a wide range of
> applications simultaneously.  Ie, many applications look at a node in
> the filesystem, decide if it is a file _or_ a directory, and then take
> an appropriate action.  All applications that can traverse a filesystem
> belong into this group, for example ls, rm, grep, find, etc.  This is
> the most prominent group, but I would expect there to be isolated cases
> of other applications that do this (maybe Apache?  Input welcome here).
> 
> These applications face a problem in the Hurd: They will see objects
> that look like Directories _and_ like Files.  This causes erratic
> behaviour.  For example, "grep *" will search through the binary content
> of directories (because it treats them as files).  One suggestion was
> that we add extra options to such programs to control how hybrid types
> should be treated by the application.

The Linux and FreeBSD FUSE (File system in USErspace) intend to implement
the same ``feature'':

<URL:http://sourceforge.net/potm/potm-2006-04.php>
#v+
[...]
What's on your project wish list?

Miklos: Maybe the most interesting feature would be allowing hybrid
objects -- for example, a file viewed as a directory. Not strictly
FUSE-dependent, this has been discussed many times on the Linux Kernel
Mailing List.
[...]
#v-


Regards,
 Thomas