bug#79497: fmt breaks lines when it does not need to

On 23/09/2025 10:46, Bruno Haible via GNU coreutils Bug Reports wrote:
> Hi,
> 
> With the attached input file and coreutils-9.8, I see these
> results from the 'fmt' command, in the en_US.UTF-8 locale
> on a GNU system:
> 
> $ fmt -w 12 -g 12 input.txt
> Grüß
> Gott.
> Здравствуйте!
> x=(-b±sqrt(b²-4ac))/(2a)
> 日本語,中文,한글
> $ fmt -w 13 -g 13 input.txt
> Grüß Gott.
> Здравствуйте!
> x=(-b±sqrt(b²-4ac))/(2a)
> 日本語,中文,한글
> 
> Why does it take a width of 13, in order to keep the first two words
> on a single line? That line has width 10.
> 
> I would expect that already 'fmt -w 10 -g 10 input.txt' keeps these
> two words together.

Well the 10 vs 13 is due to fmt not being multi-byte aware
(only fold(1) got the multi-byte treatment in coreutils 9.8).

But yes there are still confusing things here.

The man page says -w counts characters,
while the info page says -w counts columns.
The code seems to follow the man page and includes the \n in the char count.
Note fold -w does not count the newline which is what I'd expect.
We could change to that behavior without much compat issue I think.
   $ printf '%s' 'aafd as fd d' | wc -c  12

   $ printf '%s' 'aafd as fd d' | git/coreutils/src/fmt -w12 -g12
   aafd as
   fd d

   $ printf '%s' 'aafd as fd d' | git/coreutils/src/fmt -w13 -g13
   aafd as fd d

BTW I see fmt is not part of POSIX

cheers,
Padraig