Question about XkbToControl results

[hyoputer]

Hi, I have a question about the control modifier in XkbToControl.

When xkb_state_key_get_utf32 is called and the control key is pressed, the return value would be modified by the function above. However, I couldn't find any documentation about the rules. Actually, I did come across information about the control modifiers, but I couldn't understand why the result is 0 when ctrl+2 is pressed. Could you please explain it to me?

[wismill]

So we are talking about these lines:

libxkbcommon/src/state.c

Lines 897 to 914 in c1b6c79

	/* Verbatim from libX11:src/xkb/XKBBind.c */
	static char
	XkbToControl(char ch)
	{
	char c = ch;
	if ((c >= '@' && c < '\177') || c == ' ')
	c &= 0x1F;
	else if (c == '2')
	c = '\000';
	else if (c >= '3' && c <= '7')
	c -= ('3' - '\033');
	else if (c == '8')
	c = '\177';
	else if (c == '/')
	c = '_' & 0x1F;
	return c;
	}

This code does implement the control transformation described in the XKB document you linked, which is a standard way to input control characters. See also the caret notation.

My understanding is that this implementation also goes much beyond:

• The range of ASCII characters @ … _ (caret notation) produces the same characters as the range ` … ~, i.e. the caret notation extended to lower-case letters.
• 2, 6 and / correspond respectively to @, ^ and ? when the former keys are pressed on a US keyboard with Shift. As for lower-case letters, this is just a handy way to avoid having to press Shift.
• 3, 4, 5, 7 and 8 look like a kind of universal way to input non-letter characters needed in the caret notation, as non-US keyboards may lack theses characters or have them in difficult positions.

However, I cannot find a source for this. The original code has no doc either.

[hyoputer]

Thank you for the explanation! This helped me a lot!
After reviewing the Wikipedia page you provided, I understand that Control + 2 is equivalent to Control + @. Does this mean the function "XkbToControl" should produce the same results in those cases?

[wismill]

Yes, this is my understanding. Documentation is scarce! It took me a while.

I do not know if this feature is much use these days, appart from some subset such as: ctrl+c, ctrl+d, etc.

I think it would be good to document it. @whot @bluetech explanations welcome!

[wismill]

Some findings that seem to validate my understanding:

• https://www.vt100.net/shuford/terminal/dec_keyboards_news.txt

  On DEC VT-series terminals CTRL-3 gives escape. No, there is NO ANSI
  STANDARD for these key sequences! They are all proprietary. (The
  VT200-series, though, does form an informal standard of sorts.)

  CTRL-2 gives NUL, CTRL-4 gives FS, CTRL-5 gives GS, CTRL-6 gives RS,
  CTRL-7 gives RS, and CTRL-8 gives US. These are all on DEC terminals.

  While these sequences are not true for 7 and 8, they seem to give some origin.

• https://misc.openbsd.narkive.com/NvSWf6ax/which-key-shortcuts-are-safe-to-bind-and-some-q-s-about-history-and-os-diffs-re-ctrl-4-means

  In particular:

  And I'll venture a guess why DEC added those combinations: In order
  to type ^[ ^\ ^] to produce the ESC, FS, GS characters, you need
  keys for [ \ ]. If you look at non-English keyboard layouts, you'll
  see that the corresponding keys have been re-purposed for other
  characters. In the old days of national ASCII variants, even the
  characters [ \ ] didn't exist in many national encodings. Later,
  when extended 8-bit character sets were introduced, [ \ ] were only
  made available in a secondary mapping reachable with an extra
  modifier key (AltGr or such). And that's the situation right into
  the present.

  By contrast, combinations like ^3, ^4, ^5 were readily available on keyboards.

• VT220: Table 3-5 Keys Used to Generate 7-Bit Control Characters

[wismill]

I consider this question answered. Let's continue the discussion in the PR for the details.