Broken process substitution

Corinna Vinschen corinna-cygwin@cygwin.com
Sat Aug 14 11:17:00 GMT 2010


On Aug 14 12:32, Corinna Vinschen wrote:
> On Aug 13 14:25, Eric Blake wrote:
> > On 08/13/2010 02:17 PM, Eric Blake wrote:
> > > 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 again, cat should exist until something causes the input side of
> > its pipe to declare EOF; so I guess there's no race in this example
> > after all.  Rather, it looks like a limitation in cygwin1.dll.  I don't
> > know why bash is unable to duplicate the output end of the pipe to the
> > echo process, unless cygwin's /dev/fd handling doesn't work on pipes.
> 
> I think I found the culprit in Cygwin.  The problem occurs in writev.
> At the start of this function is a test which checks for the original
> open mode of the file descriptor.  If it's O_RDONLY, the writev function
> errno set to EBADF.
> 
> However, this doesn't make much sense, afaics.  For one thing, if the
> underlying handle is really only readable, the underlying NtWriteFile
> function will fail anyway.  And, for this pipe problem it seems to
> be in the way.  If I remove this test, I get
> 
>   bash-3.2$  echo hallo > >(cat)
>   bash-3.2$ hallo
>   [...hangs here until any key is pressed...]
>   bash-3.2$
> 
> That's the exact same behaviour as on Linux, afaics.
> 
> However, I'm not sure just removing the test is really the right
> solution.  I'll have to take a deeper look first.

Yep, it was another problem.  At one point the code missed to copy
over information about a file descriptor.  I applied a fix to CVS.


Corinna

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

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