Delete sequences

guo333806

(June 2001)
This documents how to get your delete and backspace keys working
correctly under X by configuring it correctly.  There are many FAQs
out there that tell you to do stupid things like swap your Delete and
BackSpace keysyms or alter your stty erase character.  Ignore them all.
Delete sequences
There are three character sequences for deleting characters:
  character
  aka
  meaning
  ^H
  Ctrl-H aka C-h aka ASCII 8 aka Backspace
  delete left
  ^?
  Ctrl-? aka C-? aka ASCII 127 aka Delete
  delete left or right
  ESC [ 3 ~
  VT 220 remove
  delete right
The transition from punched tape to CRTs with keyboards resulted in
a number of decisions.  One decision was what to use to represent a newline,
and another was what to use to represent a delete.
On a punch card when you wanted to delete you would have gone left with
a backspace control character (^H) and then punched out the card with the
delete control character (^?).  The keyboard only needs to generate one
character for each key, so a choice had to be made as to which key to
output.
The unix community had many different terminals where some used ^H for
delete left and some used ^?.  However, the VT-terminals from DEC became so
popular that over time the unix community standarised on ^? to mean delete
left.
These days people come from a variety of backgrounds - many home computer
systems use the DOS method of ^H for the backspace key and ^? for the delete
key.  This is not appropriate in unix systems.  Many people try and
configure their systems to be like DOS, but this will only lead to
problems.  In unix ^H is not delete, infact many programs - emacs being
the most notable - use ^H to bring up a help screen.
  VT220
  Backspace key produces ^? (delete left)
      Delete key produces ESC [ 3 ~ (delete right)
  DOS
  Backspace key produces ^H (delete left)
      Delete key produces ^? (delete right)
X systems
We now live in a multi-national world where there are a wide variety
of keyboard layouts.  X deals with this by assigning each physical key on
the keyboard a unique keycode (which infact is derived from the kernel which
in turn by the keyboard itself).
X applications receive keycodes but they won't know what that means until
they ask X what that keycode represents.  This is managed by the
keycodekeysym table.  A keysym is supposed to represent a common symbol
that might appear on a keyboard whose function is well known.
There are two keysyms that interest us, they are defined as:

• BackSpace keysym means delete left
    
• Delete keysym means delete right
  

A properly setup system will generate the keysym BackSpace for
the physical backspace key and will generate the keysym Delete for
the physical delete key.  The BackSpace keysym means delete left
and the Delete keysym means delete right.  If you agree with that,
and most people do, then the above settings are what you want - ignore
any FAQ that tells you to swap them.
My system generates keycode 22 for the backspace key and 107 for the
delete key.  Run xmodmap -pk and look at key codes 22 and 107.
If you want your physical backspace key to delete left and your physical
delete key to delete right, then this is the correct mapping (and is the
default):

• xmodmap -e 'keycode 22 = BackSpace'
    
• xmodmap -e 'keycode 107 = Delete'
  

Netscape
Netscape is an X application.  When it receives keycode 22 it
will look it up in the keycodekeysym map and see that it is a
BackSpace keysym.  Netscape will then know you want to delete left.  If
it looks it up and sees a Delete keysym, it will know you want to delete
right.  All X applications should work this way.  If you press backspace
and you get a space then in all likelyhood the keycode that represents
your backspace has been configured incorrectly - FAQs sometimes (stupidly)
say that xmodmap -e 'keycode 22 = 0177' is a good idea (or other
such hard-coded number).  The reason behind them doing this is that it
forces xterm to send a ^? when the backspace is pressed, but by doing that
normal X applications won't know what the keys mean anymore.  You need
to get Netscape working first, and then fix xterm.  Your backspace keycode
should generate the BackSpace keysym, don't change that!
Xterm
Xterm is a normal X application and will receive keycodes just like
Netscape does.  It will then look up the corresponding keysym and find
BackSpace for your backspace key and Delete for your delete key.  So,
what does xterm send to the remote end?  There is no provision in X for
translating these codes into ASCII sequences.  Xterm has to handle this
itself...
Unfortunately xterm comes setup wrong (perhaps from only xfree86 4
onwards).  There is a X resource called
backarrowKey that specifies whether or not the BackSpace keysym
generates a backspace or not.  This is set to true by default but
it should be false.  To fix this add/create the following line in
your .Xdefaults file: xterm.*.backarrowKey: false.
This will make your backspace key generate the ^? code.  The Delete keysym
always generates the VT220 delete sequence, so that will be correct.
Emacs
Emacs receives your input via Xterm and expects ^? for delete left and
the VT220 delete sequence for delete right.  If you have followed the
above you will find emacs will work correctly - both locally and remotely.
There is no reason at all for altering emacs' configuration files.  If you
get the help file when you try and delete then xterm is sending the wrong
character.  If you find your backspace deletes right then this means
you've got your keysyms the wrong way around, more than likely this is
the cause of following some FAQ that tells you to swap them - don't.  The
backspace keycode should generate the BackSpace keysym. Do not alter
emacs' configuration files to fix keyboard problems!
Other notes
Your stty erase character should be ^?.  Type stty -a to see
what erase is currently set to, if it is wrong, type
stty erase '^?'.
               
               
               

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