mintty: Ctrl-Q does not work?

Christopher Faylor
Sat Oct 13 15:38:00 GMT 2012

On Sat, Oct 06, 2012 at 05:48:44AM +0100, Andy Koppe wrote:
>On 25 September 2012 14:32, Ryan Johnson wrote:
>> On 25/09/2012 6:05 AM, Helmut Karlowski wrote:
>>> I type
>>> cat [some long ascii-file]
>>> then Ctrl-S (output stops), then Ctrl-Q (terminal hangs, can only be
>>> terminated by closing the window).
>>> Using minty 1.1.2 and  1.7.16(0.262/5/3) 2012-07-20 22:55 i686 Cygwin
>>> Can anybody reproduce this?
>> Confirmed with mintty 1.1.1 and the same version of cygwin (w7-64). FWIW,
>> doing it with `find .' rather than `cat' works fine.
>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.


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