hard links to directories [LWN.net]

hard links to directories

Posted Sep 14, 2007 19:34 UTC (Fri) by nix (subscriber, #2304)
In reply to: hard links to directories by Duncan
Parent article: The many faces of fsck

. and .. appear as hard links to users of the VFS. That doesn't mean they
have any physical existence, especially not at filesystem roots.

The VFS and layers below manage . and .. entirely on their own: users just
call mkdir(2), rmdir(2), or rename(2) on the directory and the kernel does
all the rest.

The file-as-directory idea, while beguiling (especially for structured
file formats) has *many* more problems, not least that whether specific
programs were willing to treat a file as a directory or vice versa, or
both, would depend on currently-irrelevant details of how they were coded.
It is currently safe for a program to stat() something and then switch on
its d_type; but what d_type does a file-and-directory return? If it's new,
nothing much will support it unless you hack the S_ISREG() and S_ISDIR()
macros to return true for the new type too (and that's an ABI change: new
glibc major version!): if it's nailed to S_IFREG, very little will treat
it as a directory, and vice versa if it's nailed to S_IFDIR.

I wish it could be implemented, but unless we're willing to throw away our
existing software, I suspect we'll be stuck with kludges that expose both
as different faked-up-but-aliased inodes somehow, one a file, the other a
directory.

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hard links to directories

Posted Sep 15, 2007 14:59 UTC (Sat) by sflintham (subscriber, #47422) [Link]

For what it's worth, and assuming I haven't misunderstood, Acorn's RISC OS had/has the concept of thing-which-is-a-file-and-directory. There were programs that made things like zip files behave as if they were a directory, and IIRC when another program asked the OS 'is this a file or a directory', it returned a bitmap where (say) bit 1 signalled 'this is a file' and 2 'this is a directory', and both would be set. Naturally this slightly broke programs that were written before they added this feature which just checked for a 1 or 2 value, but I don't remember it causing too many problems.

Not trying to argue a case for or against making the change in Linux here, just thought I'd drop this in...