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GNU troff
has a number of features that cause incompatibilities
with documents written with old versions of troff
.
Long names cause some incompatibilities. UNIX troff
interprets
.dsabcd
as defining a string ‘ab’ with contents ‘cd’. Normally, GNU
troff
interprets this as a call of a macro named dsabcd
.
Also UNIX troff
interprets \*[
or \n[
as
references to a string or number register called ‘[’. In GNU
troff
, however, this is normally interpreted as the start of a
long name. In compatibility mode GNU troff
interprets long names
in the traditional way (which means that they are not recognized as
names).
If n is missing or non-zero, turn on compatibility mode; otherwise, turn it off.
The read-only number register .C
is 1 if compatibility mode
is on, 0 otherwise.
Compatibility mode can be also turned on with the -C command line option.
The do
request turns off compatibility mode while executing its
arguments as a gtroff
command. However, it does not turn off
compatibility mode while processing the macro itself. To do that, use
the de1
request (or manipulate the .C
register manually).
See Writing Macros.
.do fam T
executes the fam
request when compatibility mode is enabled.
gtroff
restores the previous compatibility setting before
interpreting any files sourced by the cmd.
Two other features are controlled by -C. If not in
compatibility mode, GNU troff
preserves the input level in
delimited arguments:
.ds xx ' \w'abc\*(xxdef'
In compatibility mode, the string ‘72def'’ is returned; without -C the resulting string is ‘168’ (assuming a TTY output device).
Finally, the escapes \f
, \H
, \m
, \M
,
\R
, \s
, and \S
are transparent for recognizing the
beginning of a line only in compatibility mode (this is a rather obscure
feature). For example, the code
.de xx Hallo! .. \fB.xx\fP
prints ‘Hallo!’ in bold face if in compatibility mode, and ‘.xx’ in bold face otherwise.
GNU troff
does not allow the use of the escape sequences
\|
, \^
, \&
, \{
, \}
,
\SP
, \'
, \`
, \-
, \_
, \!
,
\%
, and \c
in names of strings, macros, diversions, number
registers, fonts or environments; UNIX troff
does. The
\A
escape sequence (see Identifiers) may be helpful in
avoiding use of these escape sequences in names.
Fractional point sizes cause one noteworthy incompatibility. In
UNIX troff
the ps
request ignores scale
indicators and thus
.ps 10u
sets the point size to 10 points, whereas in GNU troff
it
sets the point size to 10 scaled points. See Fractional Type Sizes, for more information.
In GNU troff
there is a fundamental difference between
(unformatted) input characters and (formatted) output 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 input characters at the moment immediately before the
glyph is added to the current output line. Macros, diversions and
strings are all, in fact, the same type of object; they contain lists of
input characters and glyph nodes in any combination. A glyph node does
not behave like an input character for the purposes of macro processing;
it does not inherit any of the special properties that the input
character from which it was constructed might have had. For example,
.di x \\\\ .br .di .x
prints ‘\\’ in GNU troff
; each pair of input backslashes is
turned into one output backslash and the resulting output backslashes
are not interpreted as escape characters when they are reread.
UNIX troff
would interpret them as escape characters
when they were reread and would end up printing one ‘\’. The
correct way to obtain a printable backslash is to use the \e
escape sequence: This always prints a single instance of the current
escape character, regardless of whether or not it is used in a
diversion; it also works in both GNU troff
and UNIX
troff
.28 To store, for some reason, an escape sequence in a
diversion that is interpreted when the diversion is reread, either use
the traditional \!
transparent output facility, or, if this is
unsuitable, the new \?
escape sequence.
See Diversions, and Gtroff Internals, for more information.
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