groff
request names unrecognized by other troff
implementations will likely be ignored by them; escape sequences that
are groff
extensions are liable to be interpreted as if the
escape character were not present.
For example, the adjustable, non-breaking space escape sequence
\~
is also supported by Heirloom Doctools troff
050915 (September
2005), mandoc
1.9.5 (2009-09-21), neatroff
(commit
1c6ab0f6e, 2016-09-13), and Plan 9 from User Space troff
(commit 93f8143600, 2022-08-12), but not by Solaris or Documenter’s
Workbench troff
s.
See Manipulating Filling and Adjustment.
GNU troff
does not allow the use of the escape sequences
\|
, \^
, \&
, \{
, \}
,
\SPC
, \'
, \`
, \-
, \_
, \!
,
\%
, and \c
in identifiers; AT&T troff
does. The \A
escape sequence (see Identifiers) may be
helpful in avoiding use of these escape sequences in names.
AT&T troff
discards trailing spaces from input
lines, like GNU troff
, but when it does so, AT&T
troff
also cancels end-of-sentence detection. Use of the
dummy character escape sequence \&
is more portable.
When adjusting to both margins, AT&T troff
at first
adjusts spaces starting from the right; GNU troff
begins from
the left. Both implementations adjust spaces from opposite ends on
alternating output lines in this adjustment mode to prevent “rivers”
in the text.
GNU troff
does not always hyphenate words as AT&T
troff
does. The AT&T implementation uses a set of
hard-coded rules specific to English, while GNU troff
uses
language-specific hyphenation pattern files derived from TeX.
Furthermore, in old versions of troff
there was a limited
amount of space to store hyphenation exceptions (arguments to the
hw
request); GNU troff
has no such restriction. When
the hy
request is invoked without an argument, GNU
troff
sets the automatic hyphenation mode to the value of the
.hydefault
register; the AT&T implementation sets it to
‘1’, which is not suitable in GNU troff
for some
languages, including English.
GNU troff
handles the dummy character \&
differently
from AT&T troff
when it is followed by the
hyphenation control escape sequence \%
at the beginning of a
word. GNU troff
does not regard the dummy character as
“starting” the word; AT&T troff
does. Further, Heirloom Doctools troff
does not honor an
explicit hyphenation point marked with \%
after a word-initial
one.142
GNU troff
predefines a string .T
containing the argument
given to the -T command-line option, namely the current output
device (for example, ‘pdf’ or ‘utf8’). The existence of this
string is a common feature of device-independent
troff
s143 but valid values are specific
to each implementation.
AT&T troff
ignored attempts to remove read-only
registers; GNU troff
honors such requests. See Built-in Registers.
The (read-only) register .T
interpolates 1 if GNU
troff
is called with the -T command-line option, and
0 otherwise. This behavior differs from AT&T troff
, which
interpolated 1 only if nroff
was the formatter and was
called with -T.
AT&T troff
and other implementations handle the
lf
request differently. For them, its line argument
changes the line number of the current line.
AT&T troff
had only environments named ‘0’,
‘1’, and ‘2’. In GNU troff
, any number of environments
may exist, using any valid identifiers for their names
(see Identifiers.)
Fractional type size support causes an incompatibility. In
AT&T troff
, the ps
request ignores scaling units
and thus ‘.ps 10u’ sets the type size to 10 points, whereas in
GNU troff
it sets the type size to 10 scaled points.
See Using Fractional Type Sizes.
The ab
request differs from AT&T troff
: GNU
troff
writes no message to the standard error stream if no
arguments are given, and it exits with a failure status instead of a
successful one.
The bp
request differs from AT&T troff
: GNU
troff
does not accept a scaling unit on the argument, a page
number; the former (somewhat uselessly) does.
The pm
request differs from AT&T troff
: GNU
troff
reports the sizes of macros, strings, and diversions in
bytes and ignores an argument to report only the sum of the sizes.
Unlike AT&T troff
, GNU troff
does not ignore the
ss
request if the output is a terminal device; instead, the
values of minimum inter-word and additional inter-sentence space are
each rounded down to the nearest multiple of 12.
GNU
troff
strips a leading neutral double quote from the argument to the
cf
,
nx
,
pi
,
so
,
and
sy
requests,
allowing it to contain embedded leading spaces.
In GNU troff
there is a fundamental difference between
(unformatted) characters and (formatted) glyphs. Everything that
affects how a glyph is output is stored with the glyph node; once a
glyph node has been constructed, it is unaffected by any subsequent
requests that are executed, including bd
, cs
, tkf
,
tr
, or fp
requests. Normally, glyphs are constructed from
characters immediately before the glyph is added to an output line.
Macros, diversions, and strings are all, in fact, the same type of
object; they contain a sequence of intermixed character and glyph nodes.
Special characters transform from one to the other: before being added
to the output, they behave as characters; afterward, they are glyphs. A
glyph node does not behave like a character node when it is processed by
a macro: it does not inherit any of the special properties that the
character from which it was constructed might have had. For example,
the input
.di x \\\\ .br .di .x
produces ‘\\’ in GNU troff
. Each pair of backslashes
becomes one backslash glyph; the resulting backslashes are thus
not interpreted as escape characters when they are interpolated
as the diversion is output. AT&T troff
would
interpret them as escape characters when interpolating them and end up
printing one ‘\’.
One correct way to obtain a printable backslash in most documents is to
use the \e
escape sequence; this always prints a single instance
of the current escape character,144 regardless of whether it is used in a diversion; it also works
in both GNU troff
and AT&T troff
.
The other correct way, appropriate in contexts independent of the
backslash’s common use as a troff
escape character—perhaps in
discussion of character sets or other programming languages—is
the character escape \(rs
or \[rs]
, for “reverse
solidus”, from its name in the ECMA-6 and ISO 10646
standards.145
To store an escape sequence in a diversion that is interpreted when the
diversion is interpolated, either use the traditional \!
transparent output facility, or, if this is unsuitable,
the new \?
escape sequence. See Diversions and gtroff
Internals.
In the somewhat pathological case where a diversion exists containing a
partially collected line and a partially collected line at the top-level
diversion has never existed, AT&T troff
will output a
partially collected but otherwise empty line (as if ‘\c’ were in
the top-level diversion) at the end of input; GNU troff
will not.