Unexpected behavior using arrow keys on the terminal

George Prekas prekgeo@yahoo.com
Wed Nov 12 10:35:00 GMT 2014

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
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.
George Prekas 

Dipl. Electrical and Computer Engineer, National Technical University of Athens

P.S. To be honest, I am not sure how much unexpected is this behavior. I just thought it would be good to have a bug report for it.
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