Re: ^G

[John Gardner]

Hi Branden,

> As I understand it, there is a tradition in GNU Emacs
> of using ^L as a page break character, and it is
> therefore an expectable character in an otherwise
> "plain text" stream.  If Version 7 Unix troff didn't
> support ^L in identifiers, it might have been injected
> into groff's set to placate the Emacs community.

That no doubt stems from the historical use of form feed (^L) characters to
indicate a literal page-break to an actual printing device. I've seen form
feeds used to demarcate sections within a file in the original Osannah
Troff's source code; even vi(1) considers form feeds to be section markers
(for the purposes of its { and } commands).

Most printers today still recognise form feeds as page-breaks when printing
plain-text output.

Cheers,
— John

On Mon, 15 Feb 2021, 4:30 pm G. Branden Robinson, <
g.branden.robinson@gmail.com> wrote:

> Hi Wim and John,
>
> At 2021-02-02T23:11:39+1100, John Gardner wrote:
> > Hi Wim,
> >
> > Roff syntax permits certain ASCII control characters in identifiers.
> > They're typically used by older macro-packages to avoid naming
> > conflicts, or in contexts when an arbitrary delimiter may appear in
> > legitimate texts (such as the .tl request).
>
> Since GNU troff's parser has a concept of "input level"--which is
> mentioned but unfortunately not actually defined in our Texinfo
> manual--.tl requests, among other things, are robust to this sort of
> confusability.
>
> Try the following with, say, "nroff":
>
>         .pl 2v
>         .de TL
>         .  tl 'left'\\*T'right'
>         ..
>         .ds T You won't get it up the steps.\"
>         .TL
>         .cp 1
>         .TL
>
> > Acceptable control characters are:
> >
> >    - ^B STX (Start of text)
> >    - ^C ETX (End of text)
> >    - ^E ENQ (Enquiry)
> >    - ^F ACK (Acknowledge)
> >    - ^G BEL (Alarm / bell)
> >    - ^? DEL (Delete)
> >
> > Groff also accepts ^D (End of transmission) and form feeds, although
> > the latter might be unintentional.
>
> As I understand it, there is a tradition in GNU Emacs of using ^L as a
> page break character, and it is therefore an expectable character in an
> otherwise "plain text" stream.  If Version 7 Unix troff didn't support
> ^L in identifiers, it might have been injected into groff's set to
> placate the Emacs community.
>
> Or one GNU Emacs user in particular...
>
> > On Tue, 2 Feb 2021 at 20:23, Wim Stockman <wim.stockman@gmail.com>
> wrote:
> >
> > > What is the purpose of this? ^G , control character.
> > > I see it a lot used in the hdtbl macro set.
>
> I'm not sure hdtbl's uses are necessary.  I see only a few usage
> patterns.  It's a point worth researching.  My _expectation_ is that
> arguments to the \A and \B escapes change the input level, and both of
> these are groff extensions anyway, so won't work in compatibility mode.
> If I'm right, then these uses of ^G can be replaced with ' or almost
> anything else.
>
> In hdmisc.tmac-u, an .index (like index(3)) string operator is defined.
> I haven't looked closely, but I think the algorithm uses the ^G as a
> marker that is not expected to occur in the string being searched.  But
> if it does, it won't work...
>
> In common.roff there is a third case, using \w, a portable escape.  It's
> like the \A and \B case except the context should be checked to see if
> it's possible that we're in compatibility mode.
>
> This is a superficial investigation as I'm pressed for time right this
> second.
>
> My preference would be to get rid of these control characters in our
> sources if possible.
>
> $ grep -r '^G' contrib/hdtbl | cat -v
> contrib/hdtbl/hdtbl.tmac-u:.    ie \B^G\\*[cols]^G \
> contrib/hdtbl/hdtbl.tmac-u:.    ie \B^G\\*[cpd]^G \
> contrib/hdtbl/hdtbl.tmac-u:.    ie \B^G\\*[csp]^G \
> contrib/hdtbl/hdtbl.tmac-u:.      ie \B^G\\*[border]^G \
> contrib/hdtbl/hdtbl.tmac-u:.    ie \B^G\\*[height]^G \
> contrib/hdtbl/hdtbl.tmac-u:.    ie \B^G\\*[height]^G \
> contrib/hdtbl/hdtbl.tmac-u:.    ie \B^G\\*[rowspan]^G \{\
> contrib/hdtbl/hdtbl.tmac-u:.    ie \B^G\\*[colspan]^G \
> contrib/hdtbl/hdtbl.tmac-u:.        ie \B^G\\*[**]^G \
> contrib/hdtbl/hdtbl.tmac-u:.        ie \B^G\\$[\\n[*]]^G \
> contrib/hdtbl/hdmisc.tmac-u:.  length ** \\$1^G
> contrib/hdtbl/hdmisc.tmac-u:.    ds * \\$1^G\"
> contrib/hdtbl/examples/common.roff:.  nr *w* (17 * \w^G\\$*^G / 10 + 4n)
> contrib/hdtbl/examples/common.roff:.  if !\B^G\\$1^G \{\
> contrib/hdtbl/examples/common.roff:.    ie !\B^G\\$2^G \
> contrib/hdtbl/examples/common.roff:.    ie !\A^G\\$3^G \
>
> Regards,
> Branden
>