Why terminfo entries do not follow convention?

Question

terminfo database contains entries, describing "application mode". By convention, entries which correspond to buttons, pressed on keyboard, start with \EO (this is done so that applications which do not perform initialization will know in which mode terminal is initialized by default - "normal" or "application") and are named with k prefix. But for example, on Debian 10 ( /lib/terminfo/x/xterm-256color) this is not true:

$ infocmp -1|grep 'kf[0-9]='
kf1=\EOP,
kf2=\EOQ,
kf3=\EOR,
kf4=\EOS,
kf5=\E[15~,
kf6=\E[17~,
kf7=\E[18~,
kf8=\E[19~,
kf9=\E[20~,

Is this a bug in terminfo database?

Answer 1

That's a broad topic...

Most (not all) terminals are configurable, with the ones you're familiar with responding to control sequences which provide application modes for the cursor, editing and numeric keypad keys. You may notice that I didn't list function keys.

The terminfo database describes a particular configuration of the terminal.

Your example shows a configuration used for xterm to provide a VT100-style numeric keypad. The typical PC keyboard hasn't enough keys to match the VT100 keypad (because although it has 4 keys in the "right" place, NumLock is treated specially), and (since VT100s never had function keys), it was convenient to assign that top row to the first 4 PC function keys.

With xterm, that's configurable anyway, but as noted, terminfo entries describe a particular configuration.

You'll find that configuration copied by terminal emulators which don't actually provide a VT100-compatible numeric keypad, just to imitate xterm.

That's summarized in the xterm FAQ (for other terminals, typically it isn't documented at all):

• Why doesn't my keypad work? (VT100 keypads discussed)
• Why can't I use the cursor keys in (whatever) shell? (application mode)
• Keypad and Function Keys (terminfo manual, which doesn't mention a "convention")