mintty: Ctrl-Q does not work?

Andy Koppe
Fri Oct 19 11:26:00 GMT 2012

On 13 October 2012 16:38, Christopher Faylor wrote:
> 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.

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.


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