[groff] 03/21: [docs]: Discuss portability of \*(.T, \n(.T more.

[G. Branden Robinson]

gbranden pushed a commit to branch master
in repository groff.

commit d4d3e632c83d4542f02870a2730e5f635cc06dca
Author: G. Branden Robinson <g.branden.robinson@gmail.com>
AuthorDate: Sat Jun 5 15:54:52 2021 +1000

    [docs]: Discuss portability of \*(.T, \n(.T more.
---
 doc/groff.texi       | 16 ++++++++++++++++
 man/groff_diff.7.man | 34 ++++++++++++++++++++++++++--------
 2 files changed, 42 insertions(+), 8 deletions(-)

diff --git a/doc/groff.texi b/doc/groff.texi
index 2a1bd47..c3e8539 100644
--- a/doc/groff.texi
+++ b/doc/groff.texi
@@ -15583,6 +15583,22 @@ name space, so choose a register name that is unlikely 
to collide with
 other uses.
 @endDefreq
 
+@cindex output device name string (@code{.T}), in other implementations
+GNU @code{troff} predefines a string @code{.T} containing the argument
+given to the @option{-T} command-line option, namely the current output
+device (for example, @samp{pdf} or @samp{utf8}).  The existence of this
+string is a common feature of post-CSTR@tie{}#54
+@code{troff}s@footnote{For example, DWB@tie{}3.3, Solaris, Heirloom
+Doctools, and Plan@tie{}9 @code{troff} all support it.} but valid values
+are specific to each implementation.
+
+@cindex output device usage register (@code{.T}), incompatibility with 
@acronym{AT&T} @code{troff}
+The (read-only) register @code{.T} is defined to be@tie{}1 if GNU
+@code{troff} is called with the @option{-T} command-line option, and
+zero otherwise.  This behavior differs from AT&T @code{troff}, which set
+this register to@tie{}1 when @code{nroff} was the formatter, was the
+formatter, and@tie{}0 otherwise.
+
 @cindex @code{lf} request, incompatibilities with @acronym{AT&T} @code{troff}
 @acronym{AT&T} @code{troff} and other implementations handle the
 @code{lf} request differently.  For them, its @var{line} argument
diff --git a/man/groff_diff.7.man b/man/groff_diff.7.man
index 2ef55c9..35f0a46 100644
--- a/man/groff_diff.7.man
+++ b/man/groff_diff.7.man
@@ -4051,10 +4051,11 @@ register stores the current year minus 1900.
 .SS Miscellaneous
 .\" ====================================================================
 .
-.I \%@g@troff
+GNU
+.I troff \" GNU
 predefines a string,
-.BR \[rs]*[.T] ;
-it contains the argument given to the
+.B \[rs]*[.T]
+containing the argument given to the
 .B \-T
 command-line option,
 namely the current output device
@@ -4063,17 +4064,34 @@ namely the current output device
 or
 .BR utf8 ).
 .
-This is not the same as the (read-only) register
-.BR \[rs]n[.T] ,
-which is defined to be\~1 if
+The existence of this string is a common feature of post-CSTR\~#54
+.IR troff s \" generic
+(for example,
+DWB\~3.3,
+Solaris,
+Heirloom Doctools,
+and Plan\~9
+.I troff \" foreign
+all support it)
+but valid values are specific to each implementation.
+.
+.
+.P
+The (read-only) register
+.B \[rs]n[.T]
+is defined to be\~1 if
 .I \%@g@troff
 is called with the
 .B \-T
 command-line option,
-and zero otherwise.
+and\~0 otherwise.
 .
 This behavior differs from AT&T
-.IR troff . \" AT&T
+.IR troff , \" AT&T
+which set this register to\~1 when
+.I nroff \" AT&T
+was the formatter,
+and\~0 otherwise.
 .
 .
 .P