Re: Can watermarking Unicode text using invisible differences sneak through Emacs, or can Emacs detect it?

[Eli Zaretskii]

> From: Tomas Hlavaty <tom@logand.com>
> Cc: psainty@orcon.net.nz, luangruo@yahoo.com,
>       kevin.legouguec@gmail.com, rms@gnu.org,
>       emacs-devel@gnu.org
> Date: Sun, 06 Feb 2022 02:29:18 +0100
> 
> On Sat 05 Feb 2022 at 16:12, Eli Zaretskii <eliz@gnu.org> wrote:
> >> If it's a console, does it support the ioctl issued by
> >> calculate_glyph_code_table?
> >
> > I guess the answer is NO, because if it did, you'd see t as the return
> > value, not 'unicode'.
> 
> ok, but is it rare or surprising, that my linux console does not support
> the discussed ioctl?

I don't know, I'm not an expert on that.  I thought the Linux console
always supports that.  Perhaps someone else could chime in.  Failing
that, how about asking about this on the forum dedicated your
GNU/Linux distribution, or the specific console you are using?

> I also found surprising you saying something about runtime test to
> detect supported characters and choosing replacements accordingly.
> That would be great if feasible.

That test is part of char-displayable-p.

> > So then it isn't surprising that you get false positives when your
> > terminal-coding-system is UTF-8:
> 
> There is no terminal-coding-system variable in my Emacs 27.2.

It's a function, not a variable.

> How can it be UTF-8?

Most probably because your locale specifies UTF-8 as the codeset.

> M-x occur on ~/.emacs shows me these utf-8 settings:
> current-language-environment "UTF-8"
> prefer-coding-system 'utf-8

These are evidence that your locale indeed specifies UTF-8.

> > that coding-system can encode any
> > character, and Emacs has no way of knowing which of them actually have
> > glyphs in the console font if the console doesn't support the
> > GIO_UNIMAP ioctl we use to find out which glyphs are actually
> > available.
> 
> Maybe the ioctl is not the sufficient source of information; emacs would
> need to query the current console font and then read the avalable glyphs
> from the font?

We don't know of any way of querying the console except via that
ioctl.  If someone can suggest a more reliable or more widely
supported method, let them please speak up.