Message-ID: <4C65A83F.4010608@redhat.com>
Date: Fri, 13 Aug 2010 14:17:03 -0600
From: Eric Blake <eblake AT redhat DOT com>
User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.7) Gecko/20100720 Fedora/3.1.1-1.fc13 Lightning/1.0b2pre Mnenhy/0.8.3 Thunderbird/3.1.1
MIME-Version: 1.0
To: cygwin AT cygwin DOT com
Subject: Re: Broken process substitution
References: <00c301cb3b22$ae416800$0ac43800$@gmail.com>
In-Reply-To: <00c301cb3b22$ae416800$0ac43800$@gmail.com>
Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="------------enigB65460134C04D85A4979FF36"
Mailing-List: contact cygwin-help AT cygwin DOT com; run by ezmlm
Sender: cygwin-owner AT cygwin DOT com
Mail-Followup-To: cygwin AT cygwin DOT com

--------------enigB65460134C04D85A4979FF36
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable

On 08/13/2010 02:04 PM, Daniel Colascione wrote:
> Try "echo hello > >(cat)" -- that's supposed to output "hello".

What makes you think it's supposed to echo hello?  That's system
specific on what will happen.  According to the bash manual,

>(cat)

is evaluated first, and will result in a /dev/fd reference, or a named
pipe (it so happens that it is a /dev/fd reference in cygwin).  But this
pipe is tied to the subprocess, so it only exists as long as the
subprocess exists.

Then bash does the redirection.  If >(cat) has already gone away, then
the resulting string /dev/fd/63 no longer exists.  So redirecting to a
non-existent file fails.  But if cat hasn't finished executing yet, then
the redirection will succeed.

Classic data race.  Your expectations are wrong, and this is not a bug
in cygwin, per se.

On the other hand, a named fifo might be more persistent; bash might
keep the fifo around until the entire statement is complete, rather than
keeping the pipe and /dev/fd reference for the life of the subprocess.
But I'd have to recompile bash to force it to use a named fifo, and we
already know that named fifos have bugs in cygwin.

--=20
Eric Blake   eblake AT redhat DOT com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org


--------------enigB65460134C04D85A4979FF36
Content-Type: application/pgp-signature; name="signature.asc"
Content-Description: OpenPGP digital signature
Content-Disposition: attachment; filename="signature.asc"

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v2.0.14 (GNU/Linux)
Comment: Public key at http://people.redhat.com/eblake/eblake.gpg
Comment: Using GnuPG with Fedora - http://enigmail.mozdev.org/

iQEcBAEBCAAGBQJMZag/AAoJEKeha0olJ0NqKBsH/iOoPdArdPEt8p+ieXTXw39f
PbOxrqusG7wBBZpUdJzx9pVwextD4/6Uhl+ywusQxFn4aopP3930dm6r8MKP9KCC
3pxOPy9mGzzLEPGYR3qPsldgWQvfj6ldnnOr8r1znhNDYEDc6+1g9PSB8t5wG1uG
v1wgnwafRXCyOcmfgLebxKQAn/7z4hEi9X55hgkwE7WqFa1DtRvROP3XyjcX4OWO
myTnkokBsXKA2CMLWuVmS4wdg21Rb+KaTjaMC3beMinPge3Vi4Eipr9DlA54Ie2i
UvMsxcqhyvIChGb9CPQ9bOdyElKMuBJTqCKws7Ec8gNZLkfoZfPVI+p9QDsqI8M=
=5sDm
-----END PGP SIGNATURE-----

--------------enigB65460134C04D85A4979FF36--