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<title>Other Differences (The GNU Troff Manual)</title>
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<p>
Previous: <a href="Compatibility-Mode.html" accesskey="p" rel="prev">Compatibility Mode</a>, Up: <a href="Implementation-Differences.html" accesskey="u" rel="up">Implementation Differences</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Request-Index.html" title="Index" rel="index">Index</a>]</p>
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<hr>
<h4 class="subsection" id="Other-Differences-1">5.38.3 Other Differences</h4>
<p><code class="code">groff</code> request names unrecognized by other <code class="code">troff</code>
implementations will likely be ignored by them; escape sequences that
are <code class="code">groff</code> extensions are liable to be interpreted as if the
escape character were not present.
<a class="index-entry-id" id="index-_005c_007e_002c-incompatibilities-with-AT_0026T-troff"></a>
For example, the adjustable, non-breaking escape sequence <code class="code">\~</code>
is also supported by Heirloom Doctools <code class="code">troff</code> 050915 (September
2005), <code class="code">mandoc</code> 1.9.5 (2009-09-21), <code class="code">neatroff</code> (commit
1c6ab0f6e, 2016-09-13), and Plan 9 from User Space <code class="code">troff</code>
(commit 93f8143600, 2022-08-12), but not by Solaris or Documenter’s
Workbench <code class="code">troff</code>s.
See <a class="xref" href="Manipulating-Filling-and-Adjustment.html">Manipulating Filling and Adjustment</a>.
</p>
<a class="index-entry-id" id="index-_005cA_002c-incompatibilities-with-AT_0026T-troff"></a>
<a class="index-entry-id" id="index-_005c_007c_002c-incompatibilities-with-AT_0026T-troff"></a>
<a class="index-entry-id" id="index-_005c_005e_002c-incompatibilities-with-AT_0026T-troff"></a>
<a class="index-entry-id" id="index-_005c_0026_002c-incompatibilities-with-AT_0026T-troff"></a>
<a class="index-entry-id" id="index-_005c_007b_002c-incompatibilities-with-AT_0026T-troff"></a>
<a class="index-entry-id" id="index-_005c_007d_002c-incompatibilities-with-AT_0026T-troff"></a>
<a class="index-entry-id" id="index-_005cSP_002c-incompatibilities-with-AT_0026T-troff"></a>
<a class="index-entry-id" id="index-_005c_0027_002c-incompatibilities-with-AT_0026T-troff"></a>
<a class="index-entry-id" id="index-_005c_0060_002c-incompatibilities-with-AT_0026T-troff"></a>
<a class="index-entry-id" id="index-_005c_002d_002c-incompatibilities-with-AT_0026T-troff"></a>
<a class="index-entry-id" id="index-_005c_005f_002c-incompatibilities-with-AT_0026T-troff"></a>
<a class="index-entry-id" id="index-_005c_0021_002c-incompatibilities-with-AT_0026T-troff"></a>
<a class="index-entry-id" id="index-_005c_0025_002c-incompatibilities-with-AT_0026T-troff"></a>
<a class="index-entry-id" id="index-_005cc_002c-incompatibilities-with-AT_0026T-troff"></a>
<p>GNU <code class="code">troff</code> does not allow the use of the escape sequences
<code class="code">\|</code>, <code class="code">\^</code>, <code class="code">\&</code>, <code class="code">\{</code>, <code class="code">\}</code>,
<code class="code">\<kbd class="key">SP</kbd></code>, <code class="code">\'</code>, <code class="code">\`</code>, <code class="code">\-</code>, <code class="code">\_</code>, <code class="code">\!</code>,
<code class="code">\%</code>, and <code class="code">\c</code> in identifiers; <abbr class="acronym">AT&T</abbr> <code class="code">troff</code>
does. The <code class="code">\A</code> escape sequence (see <a class="pxref" href="Identifiers.html">Identifiers</a>) may be
helpful in avoiding use of these escape sequences in names.
</p>
<a class="index-entry-id" id="index-adjustment-to-both-margins_002c-difference-from-AT_0026T-troff"></a>
<a class="index-entry-id" id="index-rivers"></a>
<p>When adjusting to both margins, <abbr class="acronym">AT&T</abbr> <code class="code">troff</code> at first
adjusts spaces starting from the right; GNU <code class="code">troff</code> 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.
</p>
<a class="index-entry-id" id="index-hyphenation_002c-incompatibilities-with-AT_0026T-troff"></a>
<p>GNU <code class="code">troff</code> does not always hyphenate words as <abbr class="acronym">AT&T</abbr>
<code class="code">troff</code> does. The <abbr class="acronym">AT&T</abbr> implementation uses a set of
hard-coded rules specific to English, while GNU <code class="code">troff</code> uses
language-specific hyphenation pattern files derived from TeX.
Furthermore, in old versions of <code class="code">troff</code> there was a limited amount
of space to store hyphenation exceptions (arguments to the <code class="code">hw</code>
request); GNU <code class="code">troff</code> has no such restriction.
</p>
<a class="index-entry-id" id="index-output-device-name-string-_0028_002eT_0029_002c-in-other-implementations"></a>
<p>GNU <code class="code">troff</code> predefines a string <code class="code">.T</code> containing the argument
given to the <samp class="option">-T</samp> command-line option, namely the current output
device (for example, ‘<samp class="samp">pdf</samp>’ or ‘<samp class="samp">utf8</samp>’). The existence of this
string is a common feature of post-CSTR #54
<code class="code">troff</code>s<a class="footnote" id="DOCF121" href="groff.html_fot.html#FOOT121"><sup>121</sup></a> but valid values are specific
to each implementation.
</p>
<a class="index-entry-id" id="index-removal-of-read_002donly-registers_002c-incompatibility-with-AT_0026T-troff"></a>
<a class="index-entry-id" id="index-register_002c-read_002donly_002c-removal_002c-incompatibility-with-AT_0026T-troff"></a>
<a class="index-entry-id" id="index-read_002donly-register-removal_002c-incompatibility-with-AT_0026T-troff"></a>
<p><abbr class="acronym">AT&T</abbr> <code class="code">troff</code> ignored attempts to remove read-only
registers; GNU <code class="code">troff</code> honors such requests. See <a class="xref" href="Built_002din-Registers.html">Built-in Registers</a>.
</p>
<a class="index-entry-id" id="index-output-device-usage-register-_0028_002eT_0029_002c-incompatibility-with-AT_0026T-troff"></a>
<p>The (read-only) register <code class="code">.T</code> interpolates 1 if GNU
<code class="code">troff</code> is called with the <samp class="option">-T</samp> command-line option, and
0 otherwise. This behavior differs from AT&T <code class="code">troff</code>, which
interpolated 1 only if <code class="code">nroff</code> was the formatter and was
called with <samp class="option">-T</samp>.
</p>
<a class="index-entry-id" id="index-lf-request_002c-incompatibilities-with-AT_0026T-troff"></a>
<p><abbr class="acronym">AT&T</abbr> <code class="code">troff</code> and other implementations handle the
<code class="code">lf</code> request differently. For them, its <var class="var">line</var> argument
changes the line number of the <em class="emph">current</em> line.
</p>
<a class="index-entry-id" id="index-environment-availability-and-naming_002c-incompatibilities-with"></a>
<p><abbr class="acronym">AT&T</abbr> <code class="code">troff</code> had only environments named ‘<samp class="samp">0</samp>’,
‘<samp class="samp">1</samp>’, and ‘<samp class="samp">2</samp>’. In GNU <code class="code">troff</code>, any number of environments
may exist, using any valid identifiers for their names
(see <a class="pxref" href="Identifiers.html">Identifiers</a>.)
</p>
<a class="index-entry-id" id="index-fractional-point-sizes-1"></a>
<a class="index-entry-id" id="index-fractional-type-sizes-1"></a>
<a class="index-entry-id" id="index-point-sizes_002c-fractional-1"></a>
<a class="index-entry-id" id="index-type-sizes_002c-fractional-1"></a>
<a class="index-entry-id" id="index-sizes_002c-fractional"></a>
<a class="index-entry-id" id="index-ps-request_002c-incompatibilities-with-AT_0026T-troff"></a>
<p>Fractional type sizes cause one noteworthy incompatibility. In
<abbr class="acronym">AT&T</abbr> <code class="code">troff</code> the <code class="code">ps</code> request ignores scaling units
and thus ‘<samp class="samp">.ps 10u</samp>’ sets the type size to 10 points, whereas in
GNU <code class="code">troff</code> it sets the type size to 10 <em class="emph">scaled</em> points.
See <a class="xref" href="Using-Fractional-Type-Sizes.html">Using Fractional Type Sizes</a>.
</p>
<a class="index-entry-id" id="index-ab-request_002c-incompatibilities-with-AT_0026T-troff"></a>
<p>The <code class="code">ab</code> request differs from <abbr class="acronym">AT&T</abbr> <code class="code">troff</code>:
GNU <code class="code">troff</code> 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.
</p>
<a class="index-entry-id" id="index-bp-request_002c-incompatibilities-with-AT_0026T-troff"></a>
<p>The <code class="code">bp</code> request differs from <abbr class="acronym">AT&T</abbr> <code class="code">troff</code>:
GNU <code class="code">troff</code> does not accept a scaling unit on the argument, a page
number; the former (somewhat uselessly) does.
</p>
<a class="index-entry-id" id="index-pm-request_002c-incompatibilities-with-AT_0026T-troff"></a>
<p>The <code class="code">pm</code> request differs from <abbr class="acronym">AT&T</abbr> <code class="code">troff</code>:
GNU <code class="code">troff</code> reports the sizes of macros, strings, and diversions in
bytes and ignores an argument to report only the sum of the sizes.
</p>
<a class="index-entry-id" id="index-ss-request_002c-incompatibilities-with-AT_0026T-troff"></a>
<p>Unlike <abbr class="acronym">AT&T</abbr> <code class="code">troff</code>, GNU <code class="code">troff</code> does not ignore the
<code class="code">ss</code> request if the output is a terminal device; instead, the
values of minimal inter-word and additional inter-sentence space are
each rounded down to the nearest multiple of 12.
</p>
<a class="index-entry-id" id="index-bd-request_002c-incompatibilities-with-AT_0026T-troff"></a>
<a class="index-entry-id" id="index-cs-request_002c-incompatibilities-with-AT_0026T-troff"></a>
<a class="index-entry-id" id="index-tr-request_002c-incompatibilities-with-AT_0026T-troff"></a>
<a class="index-entry-id" id="index-fp-request_002c-incompatibilities-with-AT_0026T-troff"></a>
<a class="index-entry-id" id="index-input-characters-and-output-glyphs_002c-compatibility-with-AT_0026T-troff"></a>
<a class="index-entry-id" id="index-output-glyphs_002c-and-input-characters_002c-compatibility-with-AT_0026T-troff"></a>
<a class="index-entry-id" id="index-characters_002c-input_002c-and-output-glyphs_002c-compatibility-with-AT_0026T-troff"></a>
<a class="index-entry-id" id="index-glyphs_002c-output_002c-and-input-characters_002c-compatibility-with-AT_0026T-troff"></a>
<p>In GNU <code class="code">troff</code> 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 <code class="code">bd</code>, <code class="code">cs</code>, <code class="code">tkf</code>,
<code class="code">tr</code>, or <code class="code">fp</code> 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
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.di x
\\\\
.br
.di
.x
</pre></div></div>
<p>produces ‘<samp class="samp">\\</samp>’ in GNU <code class="code">troff</code>. Each pair of backslashes
becomes one backslash <em class="emph">glyph</em>; the resulting backslashes are thus
not interpreted as escape <em class="emph">characters</em> when they are reread as the
diversion is output. <abbr class="acronym">AT&T</abbr> <code class="code">troff</code> <em class="emph">would</em> interpret
them as escape characters when rereading them and end up printing one
‘<samp class="samp">\</samp>’.
</p>
<a class="index-entry-id" id="index-printing-backslash-_0028_005c_005c_002c-_005ce_002c-_005cE_002c-_005c_005brs_005d_0029"></a>
<a class="index-entry-id" id="index-backslash_002c-printing-_0028_005c_005c_002c-_005ce_002c-_005cE_002c-_005c_005brs_005d_0029"></a>
<a class="index-entry-id" id="index-_005ce_002c-incompatibilities-with-AT_0026T-troff"></a>
<a class="index-entry-id" id="index-_005c_0021_002c-incompatibilities-with-AT_0026T-troff-1"></a>
<a class="index-entry-id" id="index-_005c_003f_002c-incompatibilities-with-AT_0026T-troff"></a>
<a class="index-entry-id" id="index-transparent-output_002c-incompatibilities-with-AT_0026T-troff"></a>
<a class="index-entry-id" id="index-output_002c-transparent_002c-incompatibilities-with-AT_0026T-troff"></a>
<p>One correct way to obtain a printable backslash in most documents is to
use the <code class="code">\e</code> escape sequence; this always prints a single instance
of the current escape character,<a class="footnote" id="DOCF122" href="groff.html_fot.html#FOOT122"><sup>122</sup></a> regardless of whether or not it is used in a diversion; it
also works in both GNU <code class="code">troff</code> and <abbr class="acronym">AT&T</abbr> <code class="code">troff</code>.
</p>
<p>The other correct way, appropriate in contexts independent of the
backslash’s common use as a <code class="code">troff</code> escape character—perhaps in
discussion of character sets or other programming languages—is
the character escape <code class="code">\(rs</code> or <code class="code">\[rs]</code>, for “reverse
solidus”, from its name in the <abbr class="acronym">ECMA-6</abbr> (<abbr class="acronym">ISO/IEC</abbr> 646)
standard.<a class="footnote" id="DOCF123" href="groff.html_fot.html#FOOT123"><sup>123</sup></a>
</p>
<p>To store an escape sequence in a diversion that is interpreted when the
diversion is reread, either use the traditional <code class="code">\!</code> transparent
output facility, or, if this is unsuitable, the new <code class="code">\?</code> escape
sequence. See <a class="xref" href="Diversions.html">Diversions</a> and <a class="ref" href="Gtroff-Internals.html"><code class="code">gtroff</code> Internals</a>.
</p>
<p>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, <abbr class="acronym">AT&T</abbr> <code class="code">troff</code> will output the
partially collected line at the end of input; GNU <code class="code">troff</code> will not.
</p>
</div>
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