[bug #65837] [troff] input backspace character incorrectly documented

[G. Branden Robinson]

URL:
  <https://savannah.gnu.org/bugs/?65837>

                 Summary: [troff] input backspace character incorrectly
documented
                   Group: GNU roff
               Submitter: gbranden
               Submitted: Wed 05 Jun 2024 06:36:22 AM UTC
                Category: Core
                Severity: 3 - Normal
              Item Group: Documentation
                  Status: In Progress
                 Privacy: Public
             Assigned to: gbranden
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: None


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Follow-up Comments:


-------------------------------------------------------
Date: Wed 05 Jun 2024 06:36:22 AM UTC By: G. Branden Robinson <gbranden>
Our Texinfo manual and groff(7) make a false claim.


   On a machine using the ISO 646, 8859, or 10646 character encodings,
invalid input characters are '0x00', '0x08', '0x0B', '0x0D'-'0x1F', and
'0x80'-'0x9F'.  On an EBCDIC host, they are '0x00'-'0x01', '0x08',
'0x09', '0x0B', '0x0D'-'0x14', '0x17'-'0x1F', and '0x30'-'0x3F'.(1)
(*note Identifiers-Footnote-1::) Some of these code points are used by
GNU 'troff' internally, making it non-trivial to extend the program to
accept UTF-8 or other encodings that use characters from these
ranges.(2)  (*note Identifiers-Footnote-2::)


0x08 should not be in that list.

An input backspace character is tokenized as TOKEN_BACKSPACE and is a
first-class citizen.

As one might expect, it produces a reverse horizontal motion of one space
width.

https://git.savannah.gnu.org/cgit/groff.git/tree/src/roff/troff/input.cpp?h=1.23.0#n7375

Observe:


$ printf 'Romeo is banishe\010`d' | nroff -Tascii | hd
00000000  52 6f 6d 65 6f 20 69 73  20 62 61 6e 69 73 68 65  |Romeo is
banishe|
00000010  08 60 64 0a 0a 0a 0a 0a  0a 0a 0a 0a 0a 0a 0a 0a 
|.`d.............|
00000020  0a 0a 0a 0a 0a 0a 0a 0a  0a 0a 0a 0a 0a 0a 0a 0a 
|................|
*
00000050  0a 0a 0a 0a 0a                                    |.....|
00000055









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