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Date: Fri, 12 Jan 2024 06:03:00 +0100
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Subject: Re: Cygwin tools to read/write NTFS alternate data streams?
To: cygwin AT cygwin DOT com
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From: Roland Mainz via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Roland Mainz <roland DOT mainz AT nrubsig DOT org>
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On Thu, Jan 11, 2024 at 3:00 PM Martin Wege via Cygwin
<cygwin AT cygwin DOT com> wrote:
> On Mon, Jan 8, 2024 at 3:11 PM Corinna Vinschen via Cygwin
> <cygwin AT cygwin DOT com> wrote:
> > On Dec 18 18:47, Martin Wege via Cygwin wrote:
> > > On Fri, Dec 1, 2023 at 10:52 AM Corinna Vinschen via Cygwin
> > > <cygwin AT cygwin DOT com> wrote:
[snip]
> > Apart from that, this sounds like a nice idea for Cygwin 3.6,
> > provided somebody implements it, https://cygwin.com/acronyms/#SHTDI
> >
> > Assuming we can live without actually having a subdir and just
> > allowing to open and create a file with the O_XATTR flag, it might be
> > pretty simple to implement.  The path handling code would just have to
> > drop the colon from the list of characters converted to the private-use
> > Unicode area.
> >
> > Implementing the subdir is a bit more complicated, especially when
> > taking opendir/readdir of that virtual subdir into account, but it
> > would certainly be doable.
>
> How do other OSes implement the O_XATTR subdir?

Basically you open a random file or dir with |openat(filename,
O_XATTR)| and get a fd to that (virtual) attribute directory.
The "." in that dir is virtual, but the ".." refers to the underlying
file, so a |open("..", ...)| will return a fd to a file/dir to which
the attributes belong to.

https://illumos.org/man/7/fsattr has the whole API description.

A Windows implementation should be therefore quite easy, as alternate
data streams show up as "foo:stream1", "foo:stream2" etc., so just the
directory lookup needs to be modified to skip the files with ':' in
general, and only show the streams for an O_XATTR subdir, e.g.
|openat("foo", O_XATTR)| would give a dir which lists only Windows
file names which match "foo:*"

I can support such an effort with testing, and a cleanroom
implementation of https://illumos.org/man/1/runat (see
https://github.com/illumos/illumos-gate/blob/master/usr/src/cmd/runat/runat.c
for the Solaris/Illumos code)

----

Bye,
Roland
-- 
  __ .  . __
 (o.\ \/ /.o) roland DOT mainz AT nrubsig DOT org
  \__\/\/__/  MPEG specialist, C&&JAVA&&Sun&&Unix programmer
  /O /==\ O\  TEL +49 641 3992797
 (;O/ \/ \O;)

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