[dev] Backspace (was: st stutter and freeze ...)

From: Roberto E. Vargas Caballero <k0ga_AT_shike2.com>
Date: Sat, 5 Apr 2014 12:08:00 +0200

> well, the information got updated to the latest stable release, and IMHO the
> system default only matters for
> * people using st from their repositories (which is the latest stable, 0.4.1),
> so the def. matches now
> * people using openbsd, which relies entirely on entries in the terminfo db

Or people that has to connect to systems where they are not administrators
and that are not connected to internet (it is my case when I am in the job).
In some machines, the first thing I have to do is "export TERM=xterm",
because the definition of st is so old, that I cannot do anything.

But don;t worry, it always better have a more recent definition ;).

> People compiling from git on non-openbsd systems should always get the latest
> st.info. If you want to change the behaviour of backspace, do it and push for
> a new stable release, then we can ask upstream again to incorporate these
> changes :)

Well, I am going to comment why I want to change the behaviour of
this key. When ascii was defined in 1968 communication with computers
were done using punched cards, or hardcopy terminals (basically a
typewritter machine connected with the computer using a serial
port). Due to this, ascii defines DELETE as 7F, because in the
puched cards, it means all the holes of the card punched, so it is
a kind of 'phisical delete'. In the same way, BACKSPACE key was a
non destructive back space, as in typewriter machines. So, if you
wanted to delete a character, you had to BACKSPACE and then DELETE.
Other use of BACKSPACE was accented characters, for example 'a
BACKSPACE `'. The VT100 had no BACKSPACE key, it was generated
using the CONTROL key as another control character (CONTROL key sets
to 0 b7 b6 b5, so it converts H (code 0x48) into BACKSPACE (code
0x08)), but it had a DELETE key in a similar position where BACKSPACE
key is located today in common PC keyboards. All the terminal
emulators emulated correctly the difference between these keys, and
backspace key generated a BACKSPACE (^H) and delete key generated
a DELETE (^?).

But the problem arised when Linus Torvald wrote Linux, and he did
that the virtual terminal (the terminal emulator integrated in the
kernel) returns a DELETE when backspace was pressed, due to the
fact of the key in that position in VT100 was a delete key. This
created a lot of problems (you can see it in [1] and [2]), and how
Linux became the king, a lot of terminal emulators today generate
a DELETE when backspace key is pressed in order to avoid problems
with linux. It causes that the only way of generating a BACKSPACE
in these systems is using CONTROL + H. I also think that emacs had
an important point here because CONTROL + H prefix is used in emacs
in some commands (help commands).

From point of view of the kernel, you can change the key for deleting
a previous character with stty erase. When you connect a real terminal
into a machine you describe the type of terminal, so getty configure
the correct value of stty erase for this terminal, but in the case of
terminal emulators you don't have any getty that can set the correct
value of stty erase, so you always get the default value. So it means
that in case of changing the value of the backspace keyboard, you have
to add a 'stty erase ^H' into your profile. Of course, other solution
can be that st itself modify the value of stty erase.

I have usually the inverse problem, when I connect with non Unix machines,
and I have to press control + h to get a BACKSPACE, or the inverse,
when a user connects to my unix machines from a different system with
a correct backspace key.

I would like listen the opinion of st users about this topic before
of doing this kind of modifications.


[1] http://www.ibb.net/~anne/keyboard.html
[2] http://www.tldp.org/HOWTO/Keyboard-and-Console-HOWTO-5.html
Roberto E. Vargas Caballero
Received on Sat Apr 05 2014 - 12:08:00 CEST

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