mintty: Ctrl-Q does not work?

Corinna Vinschen
Fri Oct 19 12:17:00 GMT 2012

On Oct 19 12:26, Andy Koppe wrote:
> On 13 October 2012 16:38, Christopher Faylor wrote:
> > On Sat, Oct 06, 2012 at 05:48:44AM +0100, Andy Koppe wrote:
> >>The issue isn't specific to any of mintty, cat or Ctrl+S; for example,
> >>I've managed to reproduce it with xterm, hexdump, and just hitting
> >>Enter. Any other key that sends a keycode will do too. Ctrl+Q isn't
> >>needed for the freeze to happen. In xterm I've even managed it with
> >>find, by hitting Enter repeatedly.
> >>
> >>If you then look at the situation in ps, you'll see something like this:
> >>
> >>O    3396       1    3396       1472  ?       1004 05:11:07 /usr/bin/xterm
> >>O    3528    4460    3528        528  pty3    1004 05:25:01 /usr/bin/cat
> >>
> >>The interesting bit there is the two 'O's in the first column, which
> >>means both processes are waiting to output. I think what's happening
> >>is that both of them are trying to write to their side of the
> >>underlying pty device, but that those writes are blocking until data
> >>is read from the other side of the pty. Result: deadlock. If the cat
> >>is killed (possibly with -9, because of its nine lives), the terminal
> >>happily continues on its way.
> >>
> >>So why doesn't this happen more often? Not sure. The speed difference
> >>between the client process output and the terminal seems to play a
> >>role here. I can only guess that the issue occurs if a buffer in the
> >>pty's slave->master pipe overflows and something is written to the
> >>master->slave pipe at the same time (which is unbuffered?).
> >>
> >>I don't understand the pty implementation enough to verify any of
> >>that, so cgf would need to comment further. Note besides: I couldn't
> >>make this deadlock happen on Ubuntu.
> >
> > This should work in the latest snapshot.  I added a polling kludge for
> > 1.7.17 while I mull over the best way to handle this.
> Using snapshot 2012-10-16, I confirmed that Ctrl+S during a long cat
> no longer freezes the terminal and that Ctrl+Q
> However, I still see the deadlock described above when hitting any
> other key that sends something, e.g. just Enter.

Too bad.  Are you sure?  I tried really hard to get a deadlock and could
not reproduce it anymore under W7.  My Enter key is still on paracetamol.


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

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