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Date: Sun, 4 Dec 2011 21:00:27 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: George Spelvin <linux@horizon.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Is there a reason hard links from /proc/$PID/fd/$NUM are
 disallowed?
Message-ID: <20111204210027.GA2203@ZenIV.linux.org.uk>
References: <20111204204044.10290.qmail@science.horizon.com>
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On Sun, Dec 04, 2011 at 03:40:44PM -0500, George Spelvin wrote:
> I was trying to save a large file that was in mid-download that I had
> accidentally deleted (as part of making space in /tmp to hold the file!).
> 
> Since it was being held open by the download process, I tried
> 
> ln /proc/$PID/fd/$FD /tmp/bigfile.mp4

You do realize that link(2) does *NOT* follow links, do you?  linkat(2)
does, if you explicitly ask for that:

linkat(AT_FDCWD, "/proc/42/fd/1", AT_FDCWD, "/tmp/foo", AT_SYMLINK_FOLLOW)

will do it just fine.