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Date: Sun, 16 Dec 2018 21:31:18 +0100
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: /dev/fd/N not synonymous with file descriptor N; it is on Linux
Message-ID: <20181216203118.GL28727@calimero.vinschen.de>
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References: <0f030e809f063f5a5e64ff7a7a0c3227 AT xs4all DOT nl> <20181216202847 DOT GK28727 AT calimero DOT vinschen DOT de>
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On Dec 16 21:28, Corinna Vinschen wrote:
> On Dec 16 17:31, Houder wrote:
> > L.S.,
> >=20
> > /dev/fd/N not synonymous with file descriptor N; it is on Linux
>=20
> Yes, it is.  Most of the time.  Try this:
>=20
> $ echo foo | cat /dev/fd/0
>=20
> The problem is that some of the concepts don't work as desired:
>=20
> > 64-@@ cat /dev/fd/0 <<\EOF
>=20
> If you observe what happens in tcsh in this situation you see that it
> doesn't even execute cat as long as you didn't type EOF.  What you type
> is written to a tmpfile:
>=20
> $ ls -l /proc/5980/fd
> total 0
> lrwxrwxrwx 1 corinna vinschen 0 Dec 16 21:15 0 -> /tmp/sh.lVQq04
> lrwxrwxrwx 1 corinna vinschen 0 Dec 16 21:15 15 -> /dev/pty0
> lrwxrwxrwx 1 corinna vinschen 0 Dec 16 21:15 16 -> /dev/pty0
> lrwxrwxrwx 1 corinna vinschen 0 Dec 16 21:15 17 -> /dev/pty0
> lrwxrwxrwx 1 corinna vinschen 0 Dec 16 21:15 18 -> /dev/pty0
> lrwxrwxrwx 1 corinna vinschen 0 Dec 16 21:15 19 -> /dev/pty0
>=20
> However, this tmpfile has been unlinked already, so it has been moved to =
the
> recycle bin:
>=20
> $ ls -l /tmp/sh.lVQq04
> ls: /tmp/sh.lVQq04: No such file or directory
>=20
> So the path in the fd subdir doesn't reflect the actual file path.
>=20
> But after starting cat, cat tries to open /proc/self/fd/0 which
> is in fact the non-existing path /tmp/sh.lVQq04.  Bad luck.
>=20
> In contrast to Linux the symlinks are not just faked symlinks with the
> underlying OS having direct access to the file descriptors.  The way
> it's implemented in Cygwin uses the actual file path resolution and then
> either works or fails as above.  I'm not sure how to fix that easily.
> I guess the fd/0 symlink would have to show the actual file path pointing
> to the recycle bin.  But that's often not what you want either since it
> hows a patch outside the POSIX namespace.
  shows a path

Sorry,
Corinna

--=20
Corinna Vinschen
Cygwin Maintainer

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