linux-kernel - Re: [RFC 0/5] pathconf(3) with _PC_LINK

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Message-ID: <20091206213930.GA10720@thunk.org>
Date:	Sun, 6 Dec 2009 16:39:30 -0500
From:	tytso@....edu
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	"J. R. Okajima" <hooanon05@...oo.co.jp>,
	linux-kernel@...r.kernel.org, stewb@...ux-foundation.org
Subject: Re: [RFC 0/5] pathconf(3) with _PC_LINK_MAX

On Sun, Dec 06, 2009 at 08:39:58AM +0000, Al Viro wrote:
> 
> Um...  Why do we need that, again?  Note that there is no way whatsoever
> for predicting whether link(2) will fail due to having too many existing
> links before you attempt the call - links can be created or removed between
> stat(2) and link(2).  So any uses of that value are heuristical.
> 
> Can you actually show any use cases of that thing?  Preferably - in existing
> code, but even a theoretical one would be interesting.

I think it's mainly a "if we're going to implement a POSIX interface,
it would be nice if it returned something based on reality instead of
a wild-assed guess".  :-)

The "real life" use case I could think of is that backup programs that
use hard links everywhere would be able to determine ahead of time in
advance when it might need to create a new file instead of using a
hard link, without needing to do the link and getting the EMLINK
error.  I agree that the only way you can know for sure is by actually
trying the link, so it's a pretty feeble use case.

I will note that without a functional, ext3 and ext4 (or ext3
filesystem with dir_nlink file system feature mounted with ext4) file
systems would be indistinguishable.

						- Ted
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