mintty: Ctrl-Q does not work?

Andy Koppe
Sun Oct 21 04:38:00 GMT 2012

On 19 October 2012 16:00, Christopher Faylor wrote:
> On Fri, Oct 19, 2012 at 02:17:29PM +0200, Corinna Vinschen wrote:
>>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.
> I can't duplicate it either.

Sorry I didn't get round to have another look at this before the 1.7.17 release.

I found that it's the CYGWIN=pipe_byte option that makes the
difference. In 1.7.16, the deadlock occurs with and without that
option. It 1.7.17, it only occurs with pipe_byte enabled.


Problem reports:
Unsubscribe info:

More information about the Cygwin mailing list