Unexpected behavior using arrow keys on the terminal

Corinna Vinschen corinna-cygwin@cygwin.com
Wed Nov 12 15:46:00 GMT 2014

Hi George,

On Nov 12 10:35, George Prekas wrote:
> Using Cygwin 1.7.32, mintty 1.1.3 and OpenSSH_6.7p1 I am getting
> unexpected behavior regarding the use of arrow keys on the terminal.
> You can reproduce the behavior by doing the following:
> ssh linux
> cd /usr/src/linux/tools/perf
> make
> cd ~
> /usr/src/linux/tools/perf/perf record echo 42
> /usr/src/linux/tools/perf/perf report
> Pressing UP or DOWN should highlight one of the rows displayed. You
> can verify expected behavior by using either PuTTY or native Linux.
> Observation #1: You can fix perf's behavior by applying perf.patch
> (attached).
> Observation #2: Using Wireshark, I've observed that when I ssh to a
> host and press UP or DOWN on my terminal 3 packets are transmitted
> from the client. PuTTY on the other hand transmits only 1 packet
> (larger in size).
> Observation #3: I wrote the program test.c (attached). If I run it and
> press UP or DOWN:
> * on Windows from cmd.exe it says "Read 3 bytes. First is 27."
> * on Linux it says "Read 3 bytes. First is 27."
> * on Linux via PuTTY it says "Read 3 bytes. First is 27."
> * on Windows from mintty.exe it says "Read 1 bytes. First is 27. Read
>   1 bytes. First is 91. Read 1 bytes. First is 65."
> My understanding is that the unexpected behavior occurs because Cygwin
> sends the UP/DOWN sequence one byte at a time. Specifically:
> * winsup\cygwin\fhandler_tty.cc @ fhandler_pty_master::write
>     This is the function called by the write system call invoked by
>     mintty. Here len = 3. line_edit is invoked 3 times.
> * winsup\cygwin\fhandler_termios.cc @ fhandler_termios::line_edit
>     This is called by the previous and it calls accept_input.
> * winsup\cygwin\fhandler_tty.cc @ fhandler_pty_master::accept_input
>     This does the actual WriteFile to the pipe.
> I would have provided a patch to fix the problem, but I am not sure I
> completely understand the semantics of the above mentioned methods.

I have to admit that I'm not quite sure either.  In theory I'd say that
the perf tool is making some invalid assumption here.  You can't rely on
a set of bytes sent from any input source to be always sent or received
as a single package, unless you're working with a transport guaranteing

Your analyzes of the underlying mechanism in Cygwin is correct, though.
Despite what I said above, I take a look into this and perhaps I can fix
the code to send more than 1 byte at a time.  


Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat
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