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To: cygwin <cygwin AT cygwin DOT com>
From: Ken Brown <kbrown AT cornell DOT edu>
Subject: Problem with named pipes
Message-ID: <567DB7AB.5060606@cornell.edu>
Date: Fri, 25 Dec 2015 16:39:55 -0500
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I've noticed a difference between Cygwin and Linux involving named 
pipes.  I don't know if this a bug or simply a difference.

Consider the following two scripts:

$ cat fifo1.sh
#!/bin/sh
set -x
rm -f foo
mkfifo foo
exec 7>foo
echo blah > foo

$ cat fifo2.sh
#!/bin/sh
set -x
read bar < foo
echo $bar

I run fifo1.sh in Terminal 1 and get the following on both Cygwin and Linux:

[Terminal 1]
$ ./fifo1.sh
+ rm -f foo
+ mkfifo foo
+ exec

The call to echo in the next line blocks, because foo has not yet been 
opened for reading.  Now I run fifo2.sh in Terminal 2.  On Linux, the 
"read" in fifo2.sh unblocks fifo1.sh, and I see the following:

[Terminal 1, Linux]
$ ./fifo1.sh
+ rm -f foo
+ mkfifo foo
+ exec
+ echo blah

[Terminal 2, Linux]
$ ./fifo2.sh
+ read bar
+ echo blah
blah

On Cygwin, however, Terminal 1 remains blocked, and Terminal 2 is 
blocked when it tries to read:

[Terminal 2, Cygwin]
$ ./fifo2.sh
+ read bar

The problem disappears if I remove the line "exec 7>foo" from fifo1.sh. 
  The problem also disappears if I leave that line in, but change the 
last line to "echo blah >&7".

Ken

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