Re: Problem with named pipes

[Corinna Vinschen <corinna-cygwin at cygwin dot com>]

On Dec 25 16:39, Ken Brown wrote:
> 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".

Very tricky problem.  The FIFO code falls over its own feet trying to
handle more than one writer (exec 7> is the first, echo blah is the
second writer).  Sigh.  This code needs a thorough rewrite...


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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