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On ext file system in Linux, why can't we create hard links to directories?
What could be the reason?

thanks.

(Haven't tried with other FS or maybe it is with *NIX. No idea.)

Hard links can have multiple "parents". If you're in a hard linked directory and type "cd ..", where should the new current directory be?

It's clear now. Thanks.

Even if that "cd .." is managed (e.g. you keep the full path somewhere in the kernel), hard links to directories turn your filesystem (usually a tree) into a graph -- which may contain cycles.

/home: mkdir foo
/home/foo: ln /home bar
/home: find -type d
/home
/home/foo
/home/foo/bar
/home/foo/bar/foo
/home/foo/bar/foo/bar ...

that would break most directory-crawling software existing. Generally speaking, allowing hard links to directories while preventing cycles to appear require something looking like a mark-and-sweep garbage collector ... not the kind of thing you'll love to have in the kernel FS driver

(more about it in Tanenbaum books)

The only requirement is to check for the entire tree link to the top of the target location whether your item is in it. If not, you're not creating a cycle.

Of course, you have to add this to the move logic too.

Candy wrote:The only requirement is to check for the entire tree link to the top of the target location whether your item is in it.

Is that even deterministic?