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Date: Thu, 5 Feb 2026 15:27:35 +0100
To: cygwin AT cygwin DOT com
Subject: Re: Why does Cygwin always use absolute paths instead of
 OBJECT_ATTRIBUTES.RootDirectory?
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From: Corinna Vinschen via Cygwin <cygwin AT cygwin DOT com>
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Cc: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
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On Feb  4 23:50, Dan Shelton via Cygwin wrote:
> On Wed, 4 Feb 2026 at 22:37, Brian Inglis via Cygwin <cygwin AT cygwin DOT com> wrote:
> >
> > On 2026-02-04 14:17, Dan Shelton via Cygwin wrote:
> > > Why does Cygwin always use absolute paths instead of
> > > OBJECT_ATTRIBUTES.RootDirectory in NtOpenFile(),
> > > NtSetInformationFile() for rename and hardlinks
> > >
> > > This slows down Cygwin path lookups a lot, and with lots of path
> > > elements each lookup with a relative RootDirectory might be a lot
> > > faster, e.g. for openat(), linkat(), renameat().
> >
> > Because volunteers' spare time is limited, and performance is good enough for
> > those who might do the work:
> 
> Please do not slap me. That hurt.
> 
> My question is about whether there are any technical issues which
> prevent OBJECT_ATTRIBUTES.RootDirectory from being used in Cygwin for
> openat().

Yes.  Obviously it would be nice to be able to use relative paths
and a dir handle with openat and friends, but there are a few
technical reasons not to do that:

- Relative paths are length restricted.  They can only contain up to
  256 chars.

- Relative paths can't contain ".." and ".", so you can't do something
  like openat (dirfd, "../myfile", ...) without changing the path
  to an absolute path.

- Relative paths could cross Cygwin symlinks and Cygwin mount points.

- Relative paths could just as well cross virtual FS boundaries, like
  /proc, /dev/.

- Last but not least, the underlying path_conv class doesn't handle
  directory handle relative paths.  The history of this function is just
  as old as Cygwin itself, about 30 years.  I took a stab at rewriting
  the function a couple of times in the last couple of years but never
  got around to any workable code before giving up.  It's a pretty
  convoluted piece of code and writing a drop-in replacement is a
  multi-month job.  This very much collides with my day job, which isn't
  Cygwin-related at all.

> 2nd question: Is OBJECT_ATTRIBUTES.RootDirectory really a root, and
> you cannot do cd ..?

If I understand your question correctly, yes.  Because of the second
bullet point above.


Corinna

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