5.39.3 Other Differences

In GNU troff, read-only registers are subject to removal like others; once removed, they can be recreated only as normal writable registers and will not otherwise reflect the configuration of the formatter.

GNU troff does not emit output if it has nothing to format. For example, it treats an input document consisting solely of nr and tm requests as empty, and produces nothing on its standard output stream. AT&T troff does, creating a blank page.

Use of C0 control characters in identifiers is not portable; Solaris, Plan 9, and Heirloom Doctools troffs accept Control+B, Control+C, Control+E, Control+F, and Control+G (only); DWB 3.3 troff does not. GNU troff rejects C0 controls in identifiers with an error diagnostic.

In DWB and Plan 9 from User Space troff, spaces or tabs between the ordinary control character and a ‘.’ or macro name that would normally end a macro definition prevent it from doing so.

GNU troff regards a failed attempt to select a different font as a meaningful action and updates the the identity of the previous font selection. Most AT&T-descended troffs do not. Consequently, given the input

\fBfoo\fZbar\fPbaz

and, assuming that the output device lacks a font named ‘Z’, GNU and Heirloom Doctools troffs set ‘baz’ in bold; DWB, Plan 9, and Solaris troffs set it in roman. The only way to determine if a font selection operation succeeded in the latter group of formatters is to save the value of the .f register prior to attempting the change, make the attempt, then compare the saved value to the current one; if they differ, the font selection presumably succeeded. Since doing so requires nr and if (or ie) requests, such a procedure cannot be performed amid \f sequences on an input line.

Formatters that don’t implement GNU troff extension request names tend to ignore them, and if they don’t support a GNU troff extension escape sequence, they are liable to format its function selector character as text. 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 troffs, which both render it as ‘~’. Recall Manipulating Filling and Adjustment.

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 output lines 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 U.S. English (“suftab”), while GNU troff uses language-specific hyphenation pattern files derived from TeX. Some versions of troff reserved meager storage for hyphenation exception words (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.³⁰⁷

GNU troff interprets a request argument representing a file name or system command in the same way it does the contents argument to the ds and as requests: it removes a leading neutral double quote ‘"’ from the argument to the cf, nx, pi, so, and sy requests, and the second argument (if present) to the lf request, permitting initial embedded spaces in it, and reads it to the end of the input line in copy mode. Recall Copy Mode. This difference permits the formatter to handle files with spaces in their names, but requires more care with trailing comments, and doubling of an initial neutral double quote ‘"’ if the file name has one.

GNU troff accepts multiple leading complementation operators ! in a conditional expression, inverting its truth value with each occurrence.

.ie !!0 .tm true
.el     .tm false
    error→ false

AT&T troff regards the foregoing conditional expression as malformed (but issues no diagnostic message) and takes neither branch.

The existence of the .T string is a common feature of device-independent troffs—DWB 3.3, Solaris, Heirloom Doctools, and Plan 9 troffs all support it—but valid values are specific to each implementation.

The (read-only) register .T interpolates 1 if GNU troff is run with the -T option, and 0 otherwise. In contrast, AT&T troff interpolated 1 only if nroff was the formatter and was run with -T.

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. Recall Identifiers.

As noted in Using Fractional Type Sizes, AT&T troff’s 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, possibly a much smaller measurement. AT&T’s behavior also means that ‘.ps 10p’ and ‘.ps 10z’ are portable.

AT&T troffs did not require a space between the arguments of the dt and wh requests. We know of no legacy documents that require such syntactical flexibility; GNU troff therefore does not support it. It may have been unknown because the numeric expression accepted by the requests could be of arbitrary length.

The ab request differs from that of 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 that of AT&T troff: GNU troff does not accept a scaling unit on the argument, a page number; the former does (uselessly).

When the dt request is given invalid arguments, AT&T troff behaves as if it were given no arguments at all, removing any existing diversion trap; GNU troff does nothing (besides issue diagnostics), more consistently with other requests given unintelligible arguments.

The lf request sets the number of the current input line in AT&T troff and the next in GNU troff.

In AT&T troff, the pm request reports macro, string, and diversion sizes in units of 128-byte blocks, and an argument reduces the report to a sum of the above in the same units. GNU troff reports their lengths in characters or nodes if given no arguments, and otherwise dumps the JSON-encoded name, contents, and other properties of each named argument.

The sp request differs from that of AT&T troff: invoking sp with the no-break control character while “not on a page”—that is, when writing output to the top-level diversion while the vertical drawing position reported by register nl is negative—provokes an error diagnostic and does not move the drawing position. AT&T troff sets the vertical drawing position to two basic units fewer than the specified argument, and to one vee minus one basic unit if no argument is present.

AT&T troff ignores the ss request if the output is a terminal device; GNU troff rounds down the values of minimum inter-word and additional inter-sentence space each to the nearest multiple of 12.

In AT&T troff, the sv request springs a page location trap if it is nearer to the drawing position than the amount of vertical space requested; GNU troff saves the space but does not spring the trap. To simulate AT&T behavior, consider the following.

.sv 3i \" or however much space you need
.if \n(.g .ne 3i

GNU troff distinguishes characters from glyphs. Characters can be ordinary, special, or indexed, and populate strings and macros. Characters per se have not (yet) been formatted. Glyphs represent graphemes (supplied by the output device) and populate diversions (recall Diversions). There is no such thing as a space character in GNU troff. Formatting converts characters into (sequences of) glyphs. GNU troff stores properties of the environment that affect how a glyph is rendered with the glyph node’s data. Thus, subsequent formatting operations do not affect it, including bd, cs, tkf, tr, and fp requests. Normally, a macro or string contains only a list of characters and a diversion contains only a list of nodes. However, applying the asciify or unformat requests to a diversion converts some of its nodes back into characters. Where the formatter cannot recover the character representation of a node, it stores a null character in the character list corresponding to a single node in the node list.

Consequently, a glyph node does not behave as a character does in macro interpolation: it does not inherit 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,³⁰⁸ 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 roff escape character—perhaps in discussion of character sets or other programming languages—is the special character escape sequence \(rs or \[rs], for “reverse solidus”, from its name in the ECMA-6 and ISO 10646 standards.³⁰⁹

To store in a diversion an escape sequence that is interpreted when the diversion is interpolated, either use the traditional \! transparent output facility, or, if this is unsuitable, the new \? escape sequence. Recall Diversions and GNU troff Internals.

Like AT&T troff, GNU troff maintains a buffer of device-independent output commands,³¹⁰ populating the buffer as formatted output accumulates. GNU troff always flushes this buffer when processing a break; AT&T troff does so according to no obvious schedule. Perhaps, if the buffer is of fixed size, the formatter performs the flush when the buffer runs out of room.

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 outputs a partially collected but otherwise empty line (as if ‘\c’ were in the top-level diversion) at the end of input; GNU troff does not.