groff_diff(7)

Name
groff_diff - differences between GNU roff and AT&T troff

Description
The GNU roff text processing system, groff, is a reimplementation and extension of AT&T troff, the typesetting system originating in Unix systems of the 1970s. groff removes many arbitrary limitations and adds features, both to the input language and to the page description language output by the troff formatter. We also note here differences arising from groff's implementation of AT&T troff features. See roff(7) for background. GNU troff can operate in a manner that increases support for documents written for AT&T troff; see section “Compatibility mode” below.

Language
GNU troff features identifiers of arbitrary length; supports color output, non-integral type sizes, user-defined characters, and automatic hyphenation of languages other than English; adds more conditional expression operators; recognizes additional scaling units and arithmetic operators; enables general file I/O (in “unsafe mode” only); and exposes more formatter state.

  Long names
GNU troff introduces many new requests; with three exceptions (cp, do, rj), they have names longer than two characters. The names of registers, fonts, strings/macros/diversions, environments, special characters, character classes, streams, hyphenation language codes, and colors can be of any length. Anywhere AT&T troff supports a parameterized escape sequence that uses an opening parenthesis “(” to introduce a two-character argument, groff supports a square-bracketed form “[]” where the argument within can be of arbitrary length.

  Font families, abstract styles, and translation
GNU troff can group text typefaces into families. For example, groff ships with support for families containing each of the styles “R”, “I”, “B”, and “BI” (roman [upright], bold, italic [slanted], and bold-italic). So that a document need not be coupled to a specific font family, an output device can associate a style in the abstract sense with a mounting position. Thus the default family can combine with a style dynamically, producing a resolved font name. A document can translate, or remap, font names with the ftr request.

Applying the requests cs, bd, tkf, uf, or fspecial to an abstract style affects the member of the default family corresponding to that style. The default family can be set with the fam request or -f command-line option. The styles directive in the output device's DESC file controls which mounting positions (if any) are initially associated with abstract styles rather than fonts, and the sty request can update this association.

  Colors
groff supports color output with a variety of color spaces and up to 16 bits per channel. Some devices, particularly terminals, may be more limited. When color support is enabled, two colors are current at any given time: the stroke color, with which glyphs, rules (lines), and geometric figures are drawn, and the fill color, which paints the interior of filled geometric figures. The color, defcolor, gcolor, and fcolor requests; \m and \M escape sequences; and .color, .m, and .M registers exercise color support.

  Hyphenation
GNU troff uses a hyphenation algorithm and language-specific pattern files (based on TeX's) to decide which words can be hyphenated and where. AT&T troff's hyphenation system (“suftab”) was specific to English.

New requests permit finer control over hyphenation breaking; hyphenation of a word might be suppressed due to a limit on consecutive hyphenated lines (hlm), a minimum line length threshold (hym), or because the line can instead be adjusted with additional inter-word space (hys). The hla request selects a hyphenation language, whereas hpf and hpfa respectively load and append to the language's hyphenation patterns. If no hyphenation language is set or no patterns are loaded, GNU troff does not perform automatic hyphenation.

For automatic hyphenation to work, the formatter must know which letters are equivalent. For example, the letter “E” behaves like “e”; only the latter typically appears in hyphenation pattern files. GNU troff expects characters that participate in automatic hyphenation to be assigned hyphenation codes that define these equivalence classes. At startup, GNU troff assigns hyphenation codes to the letters “a”–“z”, applies the same codes to “A”–“Z” in one-to-one correspondence, and assigns a code of zero to all other characters.

The hcode request enables application of hyphenation codes to characters outside the Unicode basic Latin set; without doing so, words containing such letters won't hyphenate properly even if the corresponding hyphenation patterns contain them. Localization files for the input character set and language configure hyphenation codes; see groff_tmac(5).

GNU troff's \: escape sequence works like \% but produces no hyphen if the word breaks at that location.

  Fractional type sizes and new scaling units
When configuring the type size, AT&T troff ignored scaling units and intepreted all measurements in points. Combined with integer arithmetic, this design choice made it impossible to support, for instance, ten-and-a-half-point type. In GNU troff an output device can select a scaling factor that subdivides a point into “scaled points”. A type size expressed in scaled points can thus represent a non-integral size in points.

A scaled point, scaling unit s, is equal to 1/sizescale points, where the device description file, DESC, specifies sizescale and otherwise defaults to 1; see groff_font(5). GNU troff also defines the typographical point, scaling unit z, which explicitly specifies a type size of potentially non-integral measure. The program multiplies typographical points by sizescale and converts the value to an integer. Arguments GNU troff interprets in z units by default comprise those to the escape sequences \H and \s, to the request ps, the third argument to the cs request, and the second and fourth arguments to the tkf request. In GNU troff, the register \n[.s] interpolates the type size in typographical points (z), whereas the register \n[.ps] interpolates it in scaled points (s). “\n[.ps]s”, “\n[.s]z”, and “1m” are co-equal by definition.

For example, if sizescale is 1000, then a scaled point is one thousandth of a point. Consequently, “.ps 10.5” is synonymous with “.ps 10.5z”; both set the type size to 10,500 scaled points or 10.5 typographical points.

It makes no sense to use the “z” scaling unit in a numeric expression whose default scaling unit is neither “u” nor “z”, so GNU troff disallows this. Similarly, it is nonsensical to use scaling units other than “p”, “s”, “z”, or “u”, in a numeric expression whose default scaling unit is “z”, and so GNU troff disallows those as well.

Output devices may be limited in the type sizes they can employ. The .s and .ps registers represent the type size selected by the formatter as it understands a device's capability. the read-only registers .psr and (string-valued) .sr interpolate the last requested in scaled points and in points as a decimal fraction, respectively. Like the actual current and previous type size, the requested ones are properties of an environment.

For example, if a document requests a type size of 10.95 points, and the nearest size permitted by a sizes request (or by the sizes or sizescale directives in the device's DESC file) is 11 points, groff uses the latter value.

A further two new measurement units available in groff are “M”, which indicates hundredths of an em, and “f”, which multiplies by 65,536. The latter provides convenient fractions for color definitions with the defcolor request. For example, 0.5f equals 32768u.

  Special fonts
GNU troff's “special” and fspecial requests permit a document to supplement the set of fonts the device configures for glyph search without having to use the fp request to manipulate the list of mounting positions, which can be tedious—by default, GNU troff mounts 40 fonts at startup when using the ps device.

  Numeric expressions
GNU troff permits spaces in a numeric expression within parentheses, and offers three new operators.

e1>?e2
    Interpolate the greater of expressions e1 and e2.

e1<?e2
    Interpolate the lesser of expressions e1 and e2.

(c;e)
    Interpolate expression e using c as the default scaling unit, ignoring scaling units in e if c is empty.

Arithmetic in GNU troff saturates instead of wrapping.

  Conditional expressions
More conditions can be tested with the “if” and ie requests, as well as the new “while” request.

c chr
    True if a character chr is available; chr is an ordinary, special, or indexed character, whether defined by a font description file or a request.

d nam
    True if a string, macro, diversion, or request nam is defined.

F fnt
    True if a font fnt is available; fnt can be an abstract style or a font name. fnt is handled as if it were an argument to the ft request (that is, the default family is combined with an abstract style and font translation is applied), but fnt cannot be a mounting position, and no font is mounted.

m col
    True if a color col is defined.

r reg
    True if a register reg is defined.

S sty
    True if an abstract style sty is registered. Font translations apply.

v   Always false. This condition exists for compatibility with certain other troff implementations. (We refer to vtroff, a translator that converted the C/A/T command stream produced by early-vintage AT&T troff to a form suitable for Versatec and Benson-Varian plotters.)

  Drawing commands
GNU troff extends the \D escape sequence with drawing commands to create filled circles and ellipses, and polygons. Stroked (outlined) objects are drawn with the stroke color and filled (solid) ones shaded with the fill color. These are independent properties; if you want a filled, stroked figure, you must draw the same figure twice using each command. A filled figure is smaller than a stroked one using the same parameters because the former is drawn only within its defined area, whereas strokes have a line thickness, set with another new drawing command.

  Notational conventions
In the request and escape sequence descriptions below, GNU troff reads arguments named character-sequence, command, contents, file, and message in copy mode until the end of the input line. A character-sequence comprises one or more ordinary, special, or indexed characters; spaces; or escape sequences that interpolate only these. We name the remaining arguments for clarity; they are also character-sequences. A neutral double quote ‘"’ can optionally prefix a character-sequence; the formatter discards one if present, permitting initial embedded spaces in the argument.

input refers to arbitrary character sequences (up to a newline or delimiter) that GNU troff fully interprets, in contrast to copy mode.

  Escape sequences
groff introduces several new escape sequences, extends the syntax of the AT&T troff escape sequences \D, \f, \k, \n, \s, \$, and \*, and alters the behavior of \X. The following list collates escape sequences alphabetically at first, and then by symbol roughly in Unicode code point order. Neutral apostrophes ' illustrate escape sequences with a user-selectable delimiter. Many others are available; see subsection “Miscellaneous” and section “Compatibility Mode” below.

\A'input'
    Interpolate 1 if input is a valid identifier, and 0 otherwise. Because GNU troff ignores any input character with an invalid code when reading it, invalid identifiers are empty or contain spaces, tabs, newlines, or escape sequences that interpolate something other than a sequence of ordinary characters. You can employ \A to validate a macro argument before using it to construct another escape sequence or identifier.

\B'input'
    Interpolate 1 if input is a valid numeric expression, and 0 otherwise. You might use \B along with the “if” request to filter out invalid macro arguments.

\D'C d'
    Draw filled circle of diameter d with its leftmost point at the drawing position.

\D'E h v'
    Draw filled ellipse of axis lengths h and v, with its leftmost point at the drawing position.

\D'p h1 v1
    ... hn vn ' Draw polygon with vertices at the drawing position and each point in sequence. GNU troff closes the polygon by drawing a line from (hn, vn) back to the initial drawing position; DWB and Heirloom troffs do not. Afterward, the drawing position is left at (hn, vn).

\D'P h1 v1
    ... hn vn ' As \D'p', but the polygon is filled. groff does not specify how the output device must fill concave or self-intersecting polygons.

\D't n'
    Set line thickness of geometric objects to n basic units. A zero n selects the minimum supported thickness. A negative n (the default) selects a thickness proportional to the type size.

\E  Embed an escape character that is not interpreted in copy mode (compare with \a and \t). You can use it to ease the writing of nested macro definitions. It is also convenient to define strings containing escape sequences that need to work when used in copy mode (for example, as macro arguments), or that will be interpolated at varying macro nesting depths.

\f[fnt]
    Select typeface fnt, which may be a mounting position, abstract style, or font name. \f[], \f[P], and \fP are synonyms; we recommend the first.

\Ff
\F(fm
\F[fml]
    Select default font family. \F[] makes the previous font family the default. \FP is unlike \fP; it selects font family “P” as the default. See the fam request below.

\k(rg
\k[reg]
    Store the horizontal drawing position, relative to that corresponding to the start of the input line (ignoring page offset and indentation), in two-character register name rg or arbitrary register name reg.

\mc
\m(cl
\m[col]
    Set stroke color to col. \m[] restores the previous stroke color, or the default if there is none.

\Mc
\M(cl
\M[col]
    Set fill color to col. \M[] restores the previous fill color, or the default if there is none.

\n[reg]
    Interpolate register reg.

\On
\O[n]
    Suppress troff output of glyphs and geometric objects. The sequences \O2, \O3, \O4, and \O5 are intended for internal use by grohtml(1).

    \O0
    \O1
        Disable and enable, respectively, the emission of glyphs and geometric objects to the output driver, provided that this sequence occurs at the outermost suppression level (see \O3 and \O4). Horizontal motions corresponding to non-overstruck glyph widths still occur. These sequences also reset the registers opminx, opminy, opmaxx, and opmaxy to -1. These four registers mark the top left and bottom right hand corners of a box encompassing all written or drawn output.

    \O2 At the outermost suppression level, enable emission of glyphs and geometric objects, and write to the standard error stream the page number and values of the four aforementioned registers encompassing glyphs written since the last interpolation of a \O sequence, as well as the page offset, line length, image file name (if any), horizontal and vertical device motion quanta, and input file name. Numeric values are in basic units.

    \O3
    \O4
        Begin and end a nested suppression level, respectively. grohtml uses this mechanism to create images of output preprocessed with pic, eqn, and tbl. At startup, troff is at the outermost suppression level. pre-grohtml generates these sequences when processing the document, using troff with the ps output device, Ghostscript, and the PNM tools to produce images in PNG format. These sequences start a new page if the device is not html or xhtml, to reduce the number of images crossing a page boundary.

    \O5[Pfile]
        At the outermost suppression level, write the name file to the standard error stream at position P, which must be one of l, r, c, or i, corresponding to left, right, centered, and inline alignments within the document, respectively. file is a name associated with the production of the next image.

\R'name ±n'
    Synonymous with “.nr name ±n”.

\s[0]
\s'0'
    Restore the previous type size; no operation if there is none.

\s[±n]
\s±[n]
\s'±n'
\s±'n'
    Set the type size to, or increment or decrement it by, n typographical points.

\Ve
\V(ev
\V[env]
    Interpolate contents of the system environment variable env (one-character name e, two-character name ev) as returned by getenv(3). \V is interpreted even in copy mode.

\X'character-sequence'
    Unlike AT&T troff, GNU troff performs some limited processing of the sequence of ordinary characters, special characters, and spaces in character-sequence.

    The formatter's special character repertoire is unknown to output drivers outside of glyphs named in a device's fonts, and even then they may not possess complete coverage of the names documented in groff_char(7). Further, escape sequences that produce horizontal or vertical motions, hyphenation breaks, or that are dummy characters may appear in strings or be converted to nodes, particularly in diversions. When they occur in a device extension command, they produce warnings in category “char”. These are not representable when interpolated directly into device-independent output, as might be done when writing out tag names for PDF bookmarks, which can appear in a viewer's navigation pane. This is also the case for a small number of special characters, such as \[ru], the baseline rule, that lack a Unicode definition.

    So that any Unicode code point can be represented in device extension commands, for example in an author's name in document metadata or as a usefully named bookmark or hyperlink anchor, GNU troff transforms its argument to represent characters outside the Unicode Basic Latin range as Unicode code points expressed in groff's notation for these, “\[uXXXX] ”; see groff_char(7). For these transformations, the formatter ignores character translations and definitions.

    GNU troff converts several ordinary characters that typeset as non-basic Latin code points to code points outside that range so that they are used consistently whether they are formatted as glyphs or used in a device extension command argument. These ordinary characters are “'”, “-”, “^”, “`”, and “~”; others are written as-is. Thus, “'” transforms to “\[u2019]”.

    Contrariwise, GNU troff translates special characters that typeset as Unicode basic Latin characters to basic Latin characters accordingly. Thus, “\[ga]” transforms to “`”, “\[Do]” to “$”, and so on.

\Ym
\Y(ma
\Y[mac]
    Interpolate a macro or string as a device extension command. As \X'\*[mac]' , except that GNU troff does not interpret the contents of mac; further, mac can be a macro and thus contain newlines, unlike the argument to \X . This inclusion of newlines requires an extension to the AT&T troff device-independent page description language, and their presence confuses drivers that do not know about it (see subsection “Device control commands” of groff_out(5)).

\Z'input'
    Save the drawing position, format input, then restore it. GNU troff ignores tabs and leaders in input with an error diagnostic.

\#  Read everything up to and including the next newline in copy mode and discard it. \# is like \", except that \" does not ignore a newline; the latter therefore cannot be used by itself for a whole-line comment—it leaves a blank line on the input stream.

\$0 Interpolate the name by which the macro being interpreted was called. In GNU troff this name can vary; see the als request.

\$(nn
\$[nnn]
    In a macro or string definition, interpolate the nnth or nnnth argument. In GNU troff, macros and strings can have an unlimited number of arguments.

\$* In a macro or string definition, interpolate the catenation of all arguments, separated by spaces.

\$@ In a macro or string definition, interpolate the catenation of all arguments, with each surrounded by double quotes and separated by spaces.

\$^ In a macro or string definition, interpolate the catenation of all arguments constructed in a form suitable for passage to the ds request.

\)  Interpolate a transparent dummy character—one that is ignored by end-of-sentence detection. It behaves as \&, except that \& is treated as letters and numerals normally are after “.”, “?”, and “!”; \& cancels end-of-sentence detection, and \) does not.

\*[string
    [arg ...] ] Interpolate string, passing it arg ... as arguments.

\/  Apply an italic correction: modify the spacing of the preceding glyph so that the distance between it and the following glyph is correct if the latter is of upright shape. For example, if an italic “f” is followed immediately by a roman right parenthesis, then in many fonts the top right portion of the “f” overlaps the top of the right parenthesis, which is ugly. Inserting \/ between them avoids this problem. Consider using \/ whenever a slanted glyph is immediately followed by an upright glyph without any intervening space.

\,  Apply a left italic correction: modify the spacing of the following glyph so that the distance between it and the preceding glyph is correct if the latter is of upright shape. For example, if a roman left parenthesis is immediately followed by an italic “f”, then in many fonts the bottom left portion of the “f” overlaps the bottom of the left parenthesis, which is ugly. Inserting \, between them avoids this problem. Consider using \, whenever an upright glyph is followed immediately by a slanted glyph without any intervening space.

\:  Insert a non-printing break point. That is, a word can break there, but the soft hyphen character does not mark the break point if it does (in contrast to “\%”). The remainder of the word is subject to hyphenation as normal.

\?character-sequence\?
    Suppress formatting of character-sequence. This feature has two applications.

    Surround operands to the output comparison operator with \? to compare them by character rather than as formatted output. Since GNU troff reads comparands protected with \? in copy mode, they need not even be valid groff syntax. The escape character is still lexically recognized, however, and consumes the next character.

    When used in a diversion, \? transparently embeds input, read in copy mode, until its own next occurrence on the input line. Use \! if you want to embed newlines in a diversion. Unlike \!, \? is interpreted even in copy mode, and a character-sequence in the top-level diversion is not sent to device-independent output.

\[char]
    Typeset the special character char. See groff_char(7).

\[base-char combining-component
    ...] Typeset a composite glyph consisting of base-char overlaid with one or more combining-components. For example, “\[A ho]” is a capital letter “A” with a “hook accent” (ogonek). See the composite request below; Groff: The GNU Implementation of troff, the groff Texinfo manual, for details of composite glyph name construction; and groff_char(7) for a list of components used in composite glyph names.

\~  Insert an adjustable, unbreakable space. As with ordinary spaces, GNU troff discards any sequence of these at the end of an output line if a break occurs.

  Restricted requests
To mitigate risks from untrusted input documents, GNU troff disables the cf, pi, and sy requests by default. Its -U option enables “unsafe mode”, restoring their function (and enabling additional groff extension requests, “open”, opena, and pso).

  Altered requests

.bd special-font font
    Stop emboldening special-font when font is selected. special-font must be a font name, not a mounting position.

.cf ["]
    file Break and copy the contents of file as “throughput” to GNU troff's output. If a diversion is in use, GNU troff performs the copy only when the diversion is emitted. In AT&T troff, the contents of file are immediately copied to the output regardless of whether a diversion is being written to; this behavior is so anomalous that it must be considered a bug.

    GNU troff removes a leading neutral double quote ‘"’ from the argument, permitting initial embedded spaces in it, and reads it to the end of the input line in copy mode. If file does not exist or is not readable, a warning in category “file” is emitted and the request has no other effect.

.de name
    [end-name]
.am name
    [end-name]
.ds name
    [["] contents]
.as name
    [["] contents] In compatibility mode, these requests behave as de1, am1, ds1, and as1, respectively: GNU troff inserts a compatibility save token at the beginning of the macro, string, or appendment thereto as applicable and a compatibility restore token at its end, enabling compatibility mode during its interpolation. Thus they work as expected even if the interpolation context disables compatibility mode.

.hy n
    New values 16 and 32 are available; the former enables hyphenation before the last character in a word, and the latter enables hyphenation after the first character in a word. If invoked without an argument, the mode configured by the hydefault request is selected.

.lf input-line-number
    [["] file-identifier] In GNU troff the first argument becomes the input line number of the next line the formatter reads. It also removes a leading neutral double quote ‘"’ from file-identifier, permitting initial embedded spaces in it, and reads it to the end of the input line in copy mode.

.nx [["]
    file] GNU troff removes a leading neutral double quote ‘"’ from file, permitting initial embedded spaces in it, and reads it to the end of the input line in copy mode.

.pi ["]
    command GNU troff strips a leading neutral double quote from the argument, permitting initial embedded spaces in it.

.pm name
    ... GNU troff reports, to the standard error stream, the JSON-encoded name and contents of each macro, string, or diversion name.

.so ["]
    file GNU troff removes a leading neutral double quote ‘"’ from file, permitting initial embedded spaces in it, and reads it to the end of the input line in copy mode. GNU troff searches for file in any directories specified by -I command-line options, followed by the current working directory. If file does not exist or is not readable, GNU troff emits a warning in category “file”.

.ss word-space-size
    [additional-sentence-space-size] A second argument sets the amount of additional space separating sentences on the same output line. If omitted, this amount is set to word-space-size. Both arguments are in twelfths of current font's space width (typically one-fourth to one-third em for Western scripts; see groff_font(5)). The default for both parameters is 12. Negative values are erroneous.

.sy ["]
    command GNU troff strips a leading neutral double quote from the argument, permitting initial embedded spaces in it.

.ta [[
    n1 n2 ... nn ] T r1 r2 ... rn] GNU troff supports an extended syntax to specify repeating tab stops after the “T” mark. These values are always taken as relative distances from the previous tab stop. This is the idiomatic way to specify tab stops at equal intervals in groff. GNU troff's startup value is “T 0.5i”.

    The syntax summary above instructs groff to set tabs at positions n1, n2, ..., nn, then at nn+r1, nn+r2, ..., nn+rn, then at nn+rn+r1, nn+rn+r2, ..., nn+rn+rn, and so on.

  New requests
Several GNU troff requests work like AT&T troff's “as” and ds requests, accepting an optional leading neutral double quote, notated ‘["]’, in an argument that the formatter reads in copy mode to the end of the input line, permitting inclusion of leading spaces.

.aln new-register existing-register
    Create alias (additional name) new-register of existing-register, causing the names to refer to the same stored object. If existing-register is undefined, the formatter ignores the request. GNU troff produces a warning in category “reg”. See section “Warnings” of troff(1) regarding the enablement and suppression of warnings. To remove a register alias, invoke rr on its name. A register's contents do not become inaccessible until it has no more names.

.als new-name existing-name
    Create alias (additional name) new-name of request, string, macro, or diversion existing-name, causing the names to refer to the same stored object. If existing-name is undefined, the formatter ignores the request. GNU troff produces a warning in category “mac”. If new-name already exists, its contents are lost unless already aliased. See section “Warnings” of troff(1) regarding the enablement and suppression of warnings. To remove an alias, invoke rm on its name. The object itself is not destroyed until it has no more names.

    When a request, macro, string, or diversion is aliased, redefinitions and appendments “write through” alias names. To replace an alias with a separately defined object, remove its name first.

.am1 name
    [end-name] As “am”, but GNU troff disables compatibility mode while interpreting the appendment to name: it inserts a compatibility save token at the beginning of the appendment, and a compatibility restore token at its end. The requests “am”, am1, de, and de1 can thus be intermixed freely since the compatibility save/restore tokens affect only the parts of the macro populated by am1 and de1.

.ami name
    [end-name] Append to macro indirectly. See dei below.

.ami1 name
    [end-name] As ami, but GNU troff disables compatibility mode while interpreting the appendment to the macro named by the contents of string name; see am1 above.

.as1 name
    [["] contents] As “as”, but GNU troff disables compatibility mode while interpreting the appendment to the string name: it inserts a compatibility save token at the beginning of the appendment, and a compatibility restore token at its end. The requests “as”, as1, ds, and ds1 can thus be intermixed freely since the compatibility save/restore tokens affect only the portions of the string populated by as1 and ds1.

.asciify div
    Unformat the diversion div in a way such that Unicode basic Latin (US-ASCII) characters, characters translated with the trin request, space characters, and some escape sequences that were formatted in the diversion div are treated like ordinary input characters when div is interpolated. Doing so can be useful in conjunction with the writem request.

    When transforming a glyph node back into an input sequence that demands expression as a special character escape sequence, GNU troff uses the default escape character.

    asciify cannot return all nodes in a diversion to their source equivalents: those produced by indexed characters (\N), for example, remain nodes, so the result cannot be guaranteed to be a character sequence as a macro or string is. Give the diversion name as an argument to the pm request to inspect its contents and node list. Glyph parameters such as the type face and size are not preserved; use “unformat” to achieve that.

.backtrace
    Write backtrace of input stack to the standard error stream. See the -b option of troff(1).

.blm [
    name] Set a blank line macro (trap). If a blank line macro is thus defined, groff executes name when a blank line is encountered in the input, instead of the usual behavior. A line consisting only of spaces is also treated as blank and subject to this trap. If no argument is supplied, the default blank line behavior is (re-)established.

.box [
    name]
.boxa [
    name] Divert (or append) output to name, similarly to the di and da requests, respectively. Any pending output line is not included in the diversion. Without an argument, stop diverting output; any pending output line inside the diversion is discarded.

.break
    Exit a “while” loop. Do not confuse this request with a typographical break or the br request. See “continue”.

.brp
    Break and force adjustment of the output line per the current adjustment mode. Like br, it does nothing if invoked with the no-break control character.

.cflags n c
    ... Assign properties encoded by non-negative integer n to each character or class c. Spaces need not separate c arguments.

    n is the sum of any of the following. Some combinations are nonsensical, such as “33” (1 + 32).

    1   Recognize the character as ending a sentence if followed by a newline or two spaces. Initially, characters “.?!” have this property.

    2   Enable breaks before the character. A line is not broken at a character with this property unless the characters on each side both have non-zero hyphenation codes. This exception can be overridden by adding 64. Initially, no characters have this property.

    4   Enable breaks after the character. A line is not broken at a character with this property unless the characters on each side both have non-zero hyphenation codes. This exception can be overridden by adding 64. Initially, characters “-\[hy]\[em]” have this property.

    8   Mark the glyph associated with this character as overlapping other instances of itself horizontally. Initially, characters “\[ul]\[rn]\[ru]\[radicalex]\[sqrtex]” have this property.

    16  Mark the glyph associated with this character as overlapping other instances of itself vertically. Initially, the character “\[br]” has this property.

    32  Mark the character as transparent for the purpose of end-of-sentence recognition. In other words, an end-of-sentence character followed by any number of characters with this property is treated as the end of a sentence if followed by a newline or two spaces. This is the same as having a zero space factor in TeX. Initially, characters “'")]*\[dg]\[dd]\[rq]\[cq]” have this property.

    64  Ignore hyphenation codes of the surrounding characters. Use this value in combination with values 2 and 4. Initially, no characters have this property.

    The remaining values were implemented for East Asian language support; those who use alphabetic scripts exclusively can disregard them.

    128 Prohibit a break before the character, but allow a break after the character. This works only in combination with values 256 and 512 and has no effect otherwise. Initially, no characters have this property.

    256 Prohibit a break after the character, but allow a break before the character. This works only in combination with values 128 and 512 and has no effect otherwise. Initially, no characters have this property.

    512 Allow a break before or after the character. This works only in combination with values 128 and 256 and has no effect otherwise. Initially, no characters have this property.

    In contrast to values 2 and 4, the values 128, 256, and 512 work pairwise. If, for example, the left character has value 512, and the right character 128, no break will be automatically inserted between them. If we use value 6 instead for the left character, a break after the character can't be suppressed since the neighboring character on the right doesn't get examined.

.char c
    [["] contents] Define an ordinary, special, or indexed character c as contents. Omitting contents gives c an empty definition.

    Defining (or redefining) a character c creates a formatter object that GNU troff recognizes like any other ordinary, special, or indexed character on input, and produces contents on output. When formatting c, GNU troff processes contents in a temporary environment and encapsulates the result in a node (see section “Gtroff Internals” in Groff: The GNU Implementation of troff, the groff Texinfo manual); disabling compatibility mode and setting the escape character to to \ while interpreting contents. Any emboldening, constant spacing, or track kerning applies to this object rather than to individual glyphs resulting from the formatting of contents.

    A character defined by char can be used just like a glyph provided by the output device. In particular, other characters can be translated to it with the tr request; it can be made the tab or leader fill character with the tc and lc requests; sequences of it can be drawn with the \l and \L escape sequences; and, if the hcode request is used on c, it is subject to automatic hyphenation.

    However, a user-defined character c does not participate at its boundaries in kerning adjustments or italic corrections.

    The formatter prevents infinite recursion by treating an occurrence of a character in its own definition as if it were undefined; when interpolating such a character, GNU troff emits a warning in category “char”. (Mutually recursive character definitions are handled similarly.)

    The tr and trin requests take precedence if char also applies to c. The rchar request removes character definitions.

.chop name
    Remove the last character from the macro, string, or diversion name. This is useful for removing the newline from the end of a diversion that is to be interpolated as a string. This request can be used repeatedly on the same name; see section “Gtroff Internals” in Groff: The GNU Implementation of troff, the groff Texinfo manual, for discussion of nodes inserted by groff.

.class ident c
    ... Define a character class (or simply “class”) ident comprising the members c..., where each c is an ordinary, special, or indexed character; or a range expression. A class thus defined can then be referred to in a cflags request in lieu of listing all the characters within it.

    Since class and special character names share the same name space, we recommend starting and ending the class name with “[” and “]”, respectively, to avoid collisions with existing special character names defined by GNU troff or the user (with char and related requests). This practice applies the presence of “]” in the class name to prevent the usage of the special character escape form “\[...]”, you must therefore access a class thus named via the \C escape sequence.

    An argument c can alternatively be a range expression consisting of a start character followed by “-” and then an end character. Internally, GNU troff converts these two character names to Unicode code points (according to the groff glyph list [GGL]), which determine the start and end values of the range. If that conversion fails, GNU troff skips the range expression and any remaining arguments.

.close stream
    Close the named stream, invalidating it as an argument to the “write” request. See “open”.

.composite c1 c2
    Map ordinary or special character c1 to c2 when c1 is a combining component in a composite character. Typically, composite is used to map a spacing character to a combining one. See groff_char(7).

.continue
    Skip the remainder of a “while” loop's body, immediately retesting its conditional expression. See “break” above.

.color [
    b] Enable or disable output of color-related device-independent output commands per Boolean expression b. It is enabled by default, and if b is omitted.

.cp [
    b] Enable or disable AT&T troff compatibility mode per Boolean expression b. It is disabled by default, and enabled if b is omitted. In compatibility mode, long names are not recognized, and the incompatibilities they cause do not arise.

.defcolor ident scheme color-component
    ... Define a color named ident. scheme identifies a color space and determines the number of required color-components; it must be one of “rgb” (three components), “cmy” (three components), “cmyk” (four components), or “gray” (one component). “grey” is accepted as a synonym of “gray”. Each color component can be encoded as a hexadecimal value starting with # or ##. The former indicates that each component is in the range 0–255 (0–FF), the latter the range 0–65535 (0–FFFF). Alternatively, a component can be specified as a decimal fraction in the range 0–1, interpreted using a default scaling unit of “f”, which multiplies its value by 65,536 (but clamps it at 65,535).

    Each output device has a color named “default”, which cannot be redefined. A device's default stroke and fill colors are not necessarily the same.

.de1 ident
    [end-name] As “de”, but GNU troff disables compatibility mode while interpreting name: it inserts a compatibility save token at the beginning of the macro definition, and a compatibility restore token at its end. See .am1 above.

.dei name
    [end-name] Define macro indirectly, with the name of the macro to be defined in string name and the name of the end macro terminating its definition in string end-name.

.dei1 name
    [end-name] As dei, but GNU troff disables compatibility mode while interpreting the macro named by the contents of string name. See am1 and de1 above.

.device [
    ["] character-sequence] Embed character-sequence into GNU troff output as parameters to an “xX” device extension command; see groff_out(5). The output driver or other postprocessor interprets character-sequence as it sees fit.

.devicem name
    Write contents of macro or string name to troff output as the argument to a device extension command.

.do name
    [argument ...] Interpret the string, request, diversion, or macro name (along with any further arguments) with compatibility mode disabled. Compatibility mode is restored (only if it was active) when the interpolation of name is interpreted; that is, the restored compatibility state applies to the request or contents of the macro, string, or diversion name, its arguments, and data read from files or pipes if name is the “so”, soquiet, mso, msoquiet, or pso request.

.ds1 name
    [["] contents] As ds, but GNU troff disables AT&T compatibility mode while interpreting name: it inserts a “compatibility save” token at the beginning of contents, and a “compatibility restore” token after it.

.ecr
    Restore the escape character saved with ecs, or set escape character to “\” if none has been saved.

.ecs
    Save the current escape character.

.evc env
    Copy the properties of environment env to the current environment, except for:

    •   a partially collected line, if present;

    •   the interruption status of the previous input line (due to use of the \c escape sequence);

    •   the count of remaining lines to center, to right-align, or to underline (with or without underlined spaces)—these are set to zero;

    •   the activation status of temporary indentation;

    •   input traps and their associated data;

    •   the activation status of line numbering (which can be reactivated with “.nm +0”); and

    •   the count of consecutive hyphenated lines (set to zero).

    Copying an environment to itself discards the foregoing data.

.fam [
    fml] Set default font family to fml. With no argument, the previous font family is selected, and if none, the formatter's default family. This default is “T” (Times), but can be overridden by the output device—see groff_font(5). The default font family is associated with the environment. See \F.

.fchar c
    [["] contents] Define fallback character c as contents. As char, but while that request hides a glyph with the same name in the selected font, fchar definitions are used only if the font lacks a glyph for c. GNU troff performs this test before searching special fonts.

.fcolor [
    col] Select col as the environment's fill color, or, without an argument, restore the previous fill color, or the default if there is none.

.fschar f c
    [["] contents] Define fallback special character c for font f as contents. As char, but GNU troff locates a character defined by fschar after any fonts named as arguments to the fspecial are searched and before those named as arguments to the “special” request.

.fspecial f
    [s ...] Declare each font s as special only when font f is selected. Initially, a font f's list of associated special fonts is empty for all f. GNU troff searches fonts specified as arguments to the “special” request after those given as arguments to the fspecial request. See “special”.

.ftr f
    [g] Translate font f to g. Whenever a font named f is referred to in an \f escape sequence, in the F and S conditional expression operators, or in the ft, ul, bd, cs, tkf, “special”, fspecial, fp, or sty requests, font g is used. If g is missing or identical to f, then font f is not translated.

.fzoom f
    [zoom] Set zoom factor zoom for font f. zoom must be a non-negative integer; it scales the magnification by thousandths with 1000 as a basis. If zoom is missing or equal to zero or 1000, font f is not magnified. f must be a resolved font name, not an abstract style or mounting position.

.gcolor [
    col] Select col as the environment's stroke color, or, without an argument, restore the previous stroke color, or the default if there is none.

.hcode dst1 src1
    [dst2 src2] ... Set the hyphenation code of character dst1 to that of src1, and so on. dst1 must be an ordinary character (other than a numeral) or a special character, and src1 must be an ordinary character (other than a numeral) or a special character to which a hyphenation code has already been applied. Assigning the code of an ordinary character to itself effectively creates a unique hyphenation code (which can then be copied to others). hcode ignores spaces between arguments. If any argument is invalid, hcode reports an error and stops reading them.

.hla [
    lang] Set the hyphenation language to lang, or clear it if there is no argument. Hyphenation exceptions specified with the hw request and hyphenation patterns and exceptions specified with the hpf and hpfa requests are associated with the hyphenation language. The hla request is usually invoked by a localization file, which is in turn loaded by the troffrc or troffrc-end file; see the hpf request below.

.hlm [
    n] Set the consecutive automatically hyphenated line limit to n. A negative value means “no limit”. Omitting n implies a limit of -1. This value is associated with the environment. Only lines output from a given environment count toward the maximum associated with that environment. Hyphens resulting from \% are counted; explicit hyphens are not.

.hpf ["]
    pattern-file Read hyphenation patterns from pattern-file. This file is sought in the same way that macro files are with the mso request.

    The pattern-file should have the same format as (simple) TeX pattern files. The following scanning rules are implemented.

    •   A percent sign starts a comment (up to the end of the line) even if preceded by a backslash.

    •   “Digraphs” like \$ are not supported.

    •   “^^ xx” (where each x is 0–9 or a–f) and ^^c (character c in the code point range 0–127 decimal) are recognized; other uses of ^ cause an error.

    •   No macro expansion is performed.

    •   hpf checks for the expression \patterns{...} (possibly with whitespace before or after the braces). Everything between the braces is taken as hyphenation patterns. Consequently, “{” and “}” are not allowed in patterns.

    •   Similarly, \hyphenation{...} gives a list of hyphenation exceptions.

    •   \endinput is recognized also.

    •   For backward compatibility, if \patterns is missing, the whole file is treated as a list of hyphenation patterns (but the “%” character is still recognized as the start of a comment).

    Use the hcode request (see above) to map the encoding used in hyphenation pattern files to groff's input encoding.

    GNU troff ties the set of hyphenation patterns to the hyphenation language code selected by the hla request. The hpf request is usually invoked by a localization file loaded by the troffrc file. groff provides localization files for several languages; See subsection “Localization packages” of groff_tmac(5). For Western languages, the localization file sets the default hyphenation mode and loads hyphenation patterns and exceptions. By default, troffrc loads the localization file for English.

    A second call to hpf (for the same language) replaces the old patterns with the new ones.

    Invoking hpf causes an error if there is no hyphenation language.

    If no hpf request is specified (either in the document, in a file loaded at startup, or in a macro package), GNU troff won't automatically hyphenate at all.

.hpfa ["]
    pattern-file As hpf, except that the hyphenation patterns and exceptions from pattern-file are appended to the patterns already applied to the hyphenation language of the environment.

.hpfcode a b
    [c d] ... Caution: This request will be withdrawn in a future groff release. Use hcode instead.

    Define mapping values for character codes in pattern files. hpf or hpfa apply the mapping after reading or appending to the active list of patterns. Its arguments are pairs of character codes—integers from 0 to 255. The request maps character code a to code b, code c to code d, and so on. Character codes that would otherwise be invalid in GNU troff can be used. By default, every code maps to itself except those for letters “A” to “Z”, which map to those for “a” to “z”.

.hydefault mode
    Set hyphenation mode default to mode. When the hy request is invoked without an argument, this mode is selected. The hyphenation mode default is associated with the environment. The formatter's default is 1 for AT&T troff compatibility. groff locale files generally set a more appropriate one; see groff_tmac(5).

.hym [
    length] Set the (right) hyphenation margin to length. If the adjustment mode is not “b” or “n”, the line is not hyphenated if it is shorter than length. Without an argument, the default hyphenation margin is reset to its default value, 0. The default scaling unit is “m”. The hyphenation margin is associated with the environment. A negative argument resets the hyphenation margin to zero, emitting a warning in category “range”.

.hys [
    hyphenation-space] Suppress hyphenation of the line in adjustment modes “b” or “n”, if that adjustment can be achieved by adding no more than hyphenation-space extra space to each inter-word space. Without an argument, the hyphenation space adjustment threshold is set to its default value, 0. The default scaling unit is “m”. The hyphenation space adjustment threshold is associated with the environment. A negative argument resets the hyphenation space adjustment threshold to zero, emitting a warning in category “range”.

.itc n
    [mac] As “it”, but lines interrupted with the \c escape sequence do not apply to the line count.

.kern [
    b] Enable or disable pairwise kerning of glyphs in the environment per Boolean expression b. It is enabled by default, and if b is omitted.

.length reg
    [["] contents] Compute the number of characters in contents and store the count in the register reg. If reg doesn't exist, GNU troff creates it.

    Caution: If you interpolate a macro or diversion in contents (see section “Punning Names” in groff(7)), the length request counts characters (or nodes) only up to the first newline, and leaves the rest on the input stream. In conventional circumstances, that means the remainder is interpreted, and may be formatted. To discover the length of any string, macro, or diversion, use the pm request. See section “Debugging” below.

.linetabs [
    b] Activate or deactivate line-tabs in the environment per Boolean expression b. They are inactive by default, and activated if b is omitted. When line-tabs are active, tab stops are computed relative to the start of the pending output line instead of the drawing position corresponding to the start of the input line.

.lsm [
    name] Set a leading space trap, calling the macro name when GNU troff encounters leading spaces on a text line; the implicit line break that normally happens in this case is suppressed. The formatter stores the count of leading spaces on the text line in register lsn, and the amount of corresponding horizontal motion in register lss, irrespective of whether a leading space trap is set. When it is, GNU troff removes the leading spaces from the input line and produces no motion before calling name.

    If no argument is supplied, GNU troff restablishes the default handling of leading spaces on text lines (breaking the line when filling, and formatting a horizontal motion of \n[lsn] word spaces).

.mso ["]
    file As “so”, except that GNU troff searches for the specified file in the same directories as macro files; see GROFF_TMAC_PATH in section “Environment” of groff(1) and -m in section “Options” of the same page. If file does not exist or is not readable, a warning in category “file” is emitted and the request has no other effect.

.msoquiet ["]
    file As mso, but no warning is emitted if file does not exist.

.nop [
    input] Interpret input as if it were an input line. nop resembles “.if 1”; it puts a break on the output if input is empty. Unlike “if”, it cannot govern conditional blocks. Its application is to maintain consistent indentation within macro definitions even when formatting output.

.nroff
    Make the n conditional expression evaluate true and t false. See troff.

.open ident
    ["] file Open file for writing and associate a stream named ident with it, making it available for “write” requests. Unsafe request; disabled by default. Also see writec and “close”.

.opena ident
    ["] file As “open”, but appends to file if it already exists instead of overwriting it.

.output ["]
    character-sequence Emit character-sequence, transparently to GNU troff output; this usage is similar to that of \! when it occurs in the top-level diversion.

.pchar c
    ... Report, to the standard error stream, information about each character (be it ordinary, special, or indexed) or character class c. A character defined by a request (char, fchar, fschar, or schar) reports its contents as a JSON-encoded string, but the output is not otherwise in JSON format.

.pcolor [
    col ...] Report, to the standard error stream, each defined color named col, its color space identifier, and channel value assignments, or, without arguments, those of all defined colors. A device's default stroke and/or fill colors, “default”, are not listed since they are immutable and their details unknown to the formatter.

.pcomposite
    Report, to the standard error stream, the list of configured composite character mappings. See “composite” above. The “from” code point is listed first, followed by its “to” mapping.

.pev
    Report the state of the current environment followed by that of all other environments to the standard error stream.

.pfp
    Report, to the standard error stream, the list of occupied font mounting positions. Occupied mounting positions are listed, one per line, in increasing order, followed by the typeface name; if the name corresponds to an abstract style, the entry ends there. Otherwise, the name of the font description file and the font's “internal name” datum, the meaning of which varies by output device, follow.

.pftr
    Report, to the standard error stream, the list of font translations. See pftr above. The “from” font identifier is listed first, followed by its “to” translation.

.phw
    Report, to the standard error stream, the list of hyphenation exception words associated with the hyphenation language selected by the hla request. A “-” marks each hyphenation point. A word prefixed with “-” is not hyphenated at all. The report suffixes words to which automatic hyphenation applies (meaning those defined in a hyphenation pattern file rather than with the hw request) with a tab and asterisk “*”.

.pline
    Report, in JSON syntax to the standard error stream, the list of output nodes corresponding to the pending output line. In JSON, a pair of empty brackets “[ ]” represents an empty list. A pending output line has not yet undergone adjustment, and lacks a line number and margin character (all as applicable).

.pnr [
    reg ...] Report the name and value and, if its type is numeric, the autoincrement amount and assigned format of each register reg, or, without arguments, those of all defined registers, to the standard error stream.

.psbb file
    Get the bounding box of a PostScript image file. This file must conform to Adobe's Document Structuring Conventions; the request attempts to extract the bounding box values from a %%BoundingBox comment. After invocation, the x and y coordinates (in PostScript units) of the lower left and upper right corners can be found in the registers \n[llx], \n[lly], \n[urx], and \n[ury], respectively. If an error occurs, these four registers are set to zero.

.pso ["]
    command As “so”, except that input comes from the standard output stream of command, which is passed to popen(3).

.pstream
    Report, in JSON syntax to the standard error stream, the list of open streams, including the name of each open stream, the name of the file backing it, and its mode (writing or appending).

.pwh
    Report the names and vertical positions, in basic units, of all page location traps to the standard error stream. GNU troff reports empty slots in the list, where a trap had been planted but subsequently (re)moved, because they can affect the visibility of subsequently planted traps.

.pvs ±n
    Set the post-vertical line spacing to n; default scaling unit is “p”. With no argument, the post-vertical line space is set to its previous value.

    In GNU troff, the distance between text baselines consists of the extra pre-vertical line spacing set by the most negative \x argument on the pending output line, the vertical spacing (vs), the extra post-vertical line spacing set by the most positive \x argument on the pending output line, and the post-vertical line spacing set by this request.

.rchar c
    ... Remove definition of each ordinary, special, or indexed character c, undoing the effect of a char, fchar, or schar request. Spaces need not separate c arguments. The character definition removed (if any) is the first encountered in the resolution process documented in section “Using Symbols” of Groff: The GNU Implementation of troff. Glyphs, which are defined by font description files, cannot be removed.

.return [
    input] Stop interpreting an interpolated macro, skipping to the end of its definition. Do not confuse “return” with rt. If called with an argument input, GNU troff performs the skip twice—once within the macro being interpreted and once in an enclosing macro, permitting a macro to wrap the request (as trace.tmac does).

.rfschar f c
    ... Remove each fallback special character c for font f. Spaces need not separate c arguments. See fschar.

.rj [
    n] Break, right-align the next n (default: 1) input lines, then break again. rj implies “.ce 0”, and ce implies “.rj 0”. Invoking the request with the no-break control character suppresses the first break.

.rnn r1 r2
    Rename register r1 to r2. If r1 doesn't exist, the request is ignored.

.schar c
    [["] contents] Define global fallback character c as contents. As char, but GNU troff locates a character defined with schar after any fonts named as arguments to the “special” request and before any mounted special fonts.

.shc
    [c] Set the soft hyphen character, inserted when a word is hyphenated automatically or at a hyphenation character, to c. If c is omitted, the soft hyphen character is set to the default, \[hy]. If the selected glyph does not exist in the font in use at a potential hyphenation point, then the line is not broken at that point. Neither character definitions (char and similar) nor translations (tr and similar) are considered when assigning the soft hyphen character.

.shift [
    n] Shift macro or string parameters n places (by 1 if n omitted): argument i becomes argument i-n; arguments 1 to n become unavailable. Shifting by a non-positive amount, or outside of a macro or string definition, performs no operation. The register .$ adjusts its value accordingly.

.sizes s1 s2
    ... sn [0] Set the available type sizes to s1, s2, ... sn scaled points. The list of sizes can be terminated by an optional “0”. Each si can also be a range m–n. In contrast to the device description file directive of the same name (see groff_font(5)), the argument list can't extend over more than one line.

.soquiet ["]
    file As “so”, but no warning is emitted if file does not exist.

.special [
    s ...] Declare each font s as special, irrespective of its description file, populating a list that GNU troff searches, in order, to find the glyph demanded. GNU troff mounts each font s. Invoking special without arguments empties the list. A font is not automatically unmounted if a subsequent special request removes it from the list. Initially, the list is empty. GNU troff searches fonts specified as arguments to the “special” request after those given as arguments to the fspecial request.

.spreadwarn [
    limit] Emit a “break” warning if the additional space inserted for each space between words in an output line adjusted to both margins with “.ad b” is larger than or equal to limit. A negative value is treated as zero; an absent argument toggles the warning on and off without changing limit. The default scaling unit is m. At startup, spreadwarn is inactive and limit is 3 m.

    For example, “.spreadwarn 0.2m” warns if troff must add 0.2 m or more to each inter-word space in a line.

.stringdown str
.stringup str
    Alter the string named str by replacing each of its bytes with its lowercase (down) or uppercase (up) version (if one exists). Special characters (see groff_char(7)) will often transform in the expected way due to the regular naming convention for accented characters. When they do not, use substrings and/or catenation.

.sty pos style
    Associate abstract style with non-negative font mounting position pos.

.substring str start
    [end] Replace the string named str with its substring bounded by the indices start and end, inclusively. The first character in the string has index 0. Negative indices count backward from the end of the string: the last character has index -1, the character before the last has index -2, and so on. If end is omitted, -1 is implied.

.tkf f s1 n1 s2 n2
    Enable track kerning for font f. When the current font is f, the width of every glyph is increased by an amount between n1 and n2; when the current type size is less than or equal to s1, the width is increased by n1; when it is greater than or equal to s2, the width is increased by n2; when the type size is greater than or equal to s1 and less than or equal to s2, the increase in width is a linear function of the type size.

.tm1 [["]
    message] As tm, but removes a leading neutral double quote ‘"’ from message, permitting initial embedded spaces in it.

.tmc [["]
    message] As tm1, but does not append a newline.

.trf file
    Break and copy file as “throughput” to GNU troff output, discarding characters that are invalid as input; contrast with cf. Each line of file is output as if preceded by \!, but is not interpreted by the formatter. If file does not end with a newline, trf appends one. Invoking the request with the no-break control character suppresses the break.

.trin abcd
    As the tr request, but the asciify request uses the character code (if any) before the character translation.

.trnt abcd
    As the tr request, but the translations do not apply to text that is transparently throughput into a diversion with \!.

.troff
    Make the t conditional expression evaluate true and n false. See nroff.

.unformat div
    Unformat the diversion div. Unlike asciify, “unformat” handles only tabs and spaces between words, the latter usually arising from spaces or newlines in the input. Tabs are treated as input tokens, and spaces become adjustable again. The vertical sizes of lines are not preserved, but glyph information (font, type size, space width, and so on) is retained.

.vpt [
    b] Enable or disable vertical position traps per Boolean expression b. They are enabled by default, and if b is omitted. Vertical position traps are those set by the ch, wh, and dt requests. Vertical position trap enablement is global.

.warn [
    n] Select the categories, or “types”, of reported warnings. n is the sum of the numeric codes associated with each warning category that is to be enabled; all other categories are disabled. The categories and their associated codes are listed in section “Warnings” of troff(1). For example, “.warn 0” disables all warnings, and “.warn 1” disables all warnings except those about missing glyphs. If no argument is given, all warning categories are enabled.

.warnscale scaling-unit
    Select scaling unit used in certain warnings (one of u, i, c, p, or P; default: i). Ignored on nroff-mode output devices, for which these diagnostics report the vertical page location in lines, and the horizontal page location in ens.

.while cond-expr input
    Evaluate the conditional expression cond-expr, and repeatedly execute input unless and until cond-expr evaluates false. input, which is often a conditional block, is referred to as the “while” request's body.

    GNU troff treats the body of a “while” request similarly to that of a de request (albeit one not read in copy mode), but stores it under an internal name and deletes it when the loop finishes. The operation of a macro containing a “while” request can slow significantly if its body is large. Each time GNU troff interpolates the macro, it parses and stores the “while” body again. An often better solution—and one that is more portable, since AT&T troff lacked the “while” request—is to instead write a recursive macro, which is parsed only once (unless you redefine it). To prevent infinite loops, GNU troff limits the default number of available recursion levels to 1,000 or somewhat less (because things other than macro calls can be on the input stack). You can disable this protective measure, or alter the limit, by setting the slimit register. See section “Debugging” below.

    If a “while” body begins with a conditional block, its closing brace must end an input line.

    The “break” and “continue” requests alter a “while” loop's flow of control.

.write stream
    [["] character-sequence] Write character-sequence, a sequence of ordinary characters, spaces, or tabs read in copy mode, to stream, which must previously have been the subject of an “open” (or opena) request, followed by a newline. GNU troff flushes the stream after writing to it.

.writec stream
    [["] character-sequence] As “write”, but does not append a newline to contents.

.writem stream name
    Write the contents of the macro or string name to stream, which must previously have been the subject of an “open” (or opena) request. GNU troff reads the contents of name in copy mode.

  Altered registers
GNU troff alters the semantics of two AT&T troff registers and imparts semantics to two others that CSTR #54 documented as “reserved”.

\n[.R]
    Because GNU troff dynamically manages register storage, it repurposes the .R register to interpolate the maximum integer representable in the formatter. Favor its use over numeric literals with many zeroes or nines to indicate an arbitrary large quantity.

\n[.s]
    In GNU troff, the .s register is string-valued; it interpolates the type size in typographical points, which can be represented as a decimal fraction.

\n[.x]
    Interpolate major version number of the running troff formatter. For example, if the version number is 1.23.0, then \n[.x] contains 1.

\n[.y]
    Interpolate minor version number of the running troff formatter. For example, if the version number is 1.23.0, then \n[.y] contains 23. Also see \n[.Y] below.

  New registers
GNU troff exposes more formatter state via many new read-only registers. Their names often correspond to the requests that affect them.

\n[.br]
    Within a macro definition, interpolate 1 if the macro is called with the “normal” control character (“.” by default), and 0 otherwise. This facility allows requests to be reliably wrapped by a macro. Interpolating the .br register outside of a macro definition makes no sense.

\n[.C]
    Interpolate 1 if AT&T troff compatibility mode is in effect, 0 otherwise. See cp.

\n[.cdp]
    Interpolate depth of last glyph added to the environment. It is positive if the glyph extends below the baseline.

\n[.ce]
    Interpolate count of input lines remaining to be centered in the environment.

\n[.cht]
    Interpolate height of last glyph added to the environment. It is positive if the glyph extends above the baseline.

\n[.color]
    Interpolate 1 if color output is enabled, 0 otherwise.

\n[.cp]
    Within a “do” request, interpolate the saved value of compatibility mode (see \n[.C] above).

\n[.csk]
    Interpolate skew of last glyph added to the environment. The skew of a glyph is how far to the right of the center of a glyph the center of an accent over that glyph is to be placed.

\n[.ev]
    Interpolate name of current environment. This is a string-valued register.

\n[.fam]
    Interpolate name of the environment's default font family. This is a string-valued register.

\n[.fn]
    Interpolate resolved name of the font selected in the environment. This is a string-valued register.

\n[.fp]
    Interpolate next free non-zero font mounting position.

\n[.g]
    Interpolate 1. Test with “if” or ie to check whether GNU troff is the formatter.

\n[.height]
    Interpolate the rescaled height of the environment's selected font, in scaled points. It is zero if the font height is not rescaled. See \H.

\n[.hla]
    Interpolate hyphenation language of the environment. This is a string-valued register.

\n[.hlc]
    Interpolate count of immediately preceding consecutive hyphenated lines in the environment.

\n[.hlm]
    Interpolate maximum number of consecutive hyphenated lines allowed in the environment.

\n[.hy]
    Interpolate automatic hyphenation mode of the environment.

\n[.hydefault]
    Interpolate hyphenation mode default of the environment.

\n[.hym]
    Inteprolate hyphenation margin of the environment.

\n[.hys]
    Interpolate hyphenation space adjustment threshold of the environment.

\n[.in]
    Interpolate indentation amount applicable to the output line pending in the environment.

\n[.int]
    Interpolate 1 if the text most recently formatted in the environment was “interrupted” or continued with \c, 0 otherwise.

\n[.it]
    Interpolate count of input lines remaining in the environment's pending input trap.

\n[.itc]
    Interpolate 1 if the environment's pending input trap honors the output line continuation escape sequence (\c), 0 otherwise.

\n[.itm]
    Interpolate the name of the macro associated with the environment's pending input trap. This is a string-valued register.

\n[.kern]
    Interpolate 1 if pairwise kerning is enabled, 0 otherwise.

\n[.lg]
    Interpolate ligature mode.

\n[.linetabs]
    Interpolate 1 if line-tabs mode is enabled in the environment, 0 otherwise.

\n[.ll]
    Interpolate line length applicable to the environment's pending output line.

\n[.lt]
    Interpolate the environment's title line length.

\n[.m]
    Interpolate name of the environment's selected stroke color. This is a string-valued register.

\n[.M]
    Interpolate name of the environment's selected fill color. This is a string-valued register.

\n[.ne]
    Interpolate amount of space demanded by the most recent ne request that sprang a page location trap. See \n[.trunc].

\n[.nm]
    Interpolate 1 if output line numbering is enabled in the environment (even if temporarily suppressed), 0 otherwise.

\n[.nn]
    Interpolate count of lines remaining in the environment for which numbering is suppressed while output line numbering is enabled.

\n[.ns]
    Interpolate 1 if no-space mode is enabled, 0 otherwise.

\n[.O]
    Interpolate output suppression level. See \O.

\n[.P]
    Interpolate 1 if the current page is selected for output, 0 otherwise. See -o command-line option to troff(1).

\n[.pe]
    Interpolate 1 during page ejection, 0 otherwise.

\n[.pn]
    Interpolate next page number (either that set by pn, or that of the current page plus 1).

\n[.ps]
    Interpolate the environment's type size in scaled points.

\n[.psr]
    Interpolate the environment's most recently requested type size in scaled points.

\n[.pvs]
    Interpolate the environment's post-vertical line spacing amount.

\n[.rj]
    Interpolate count of input lines remaining to be right-aligned in the environment.

\n[.slant]
    Interpolate slant in degrees of the environment's selected font. See \S.

\n[.sr]
    Interpolate the environment's most recently requested type size in typographical points. This is a string-valued register.

\n[.ss]
\n[.sss]
    Interpolate values of the environment's minimum inter-word space and supplemental inter-sentence space, respectively, in twelfths of the space width of the selected font.

\n[.sty]
    Interpolate the environment's selected abstract font style, if any. This is a string-valued register.

\n[.tabs]
    Interpolate the environment's tab stop settings (if any) in a form suitable for passage to the ta request. This is a string-valued register.

\n[.trap]
    Interpolate the name of the next vertical position trap after the vertical drawing position. This is a string-valued register.

\n[.trunc]
    Interpolate amount of vertical space truncated by the most recently sprung page location trap, or, if the trap was sprung by an ne request, minus the amount of vertical motion produced by the ne request. In other words, at the point a trap is sprung, \n[.trunc] represents the difference of what the vertical position would have been but for the trap, and what the vertical position actually is. See \n[.ne].

\n[.U]
    Interpolate 1 if in unsafe mode, 0 otherwise. See -U command-line option to troff(1).

\n[.vpt]
    Interpolate 1 if vertical position traps are enabled, 0 otherwise.

\n[.warn]
    Interpolate warning mask. See section “Warnings” of troff(1).

\n[.Y]
    Interpolate revision number of the running troff formatter. For example, if the version number is 1.23.0, then \n[.Y] contains 0. Also see \n[.x] and \n[.y] above.

\n[.zoom]
    Interpolate magnification of the environment's selected font, in thousandths, or 0 if magnification unused. See fzoom.

The following (writable) registers are set by the psbb request.

\n[llx]
\n[lly]
\n[urx]
\n[ury]
    Interpolate the (upper, lower, left, right) bounding box values (in PostScript units) of the most recently processed PostScript image.

The following (writable) registers are set by the \w escape sequence.

\n[rst]
\n[rsb]
    Like \n[st] and \n[sb], but taking account of the heights and depths of glyphs. In other words, these registers store the highest and lowest vertical positions attained by the argument formatted by the \w escape sequence, doing what AT&T troff documented \n[st] and \n[sb] as doing.

\n[ssc]
    The amount of (possibly negative) horizontal space to add to the last glyph before a subscript.

\n[skw]
    How far to right of the center of the last glyph in the \w argument, to place the center of an accent from a roman font over that glyph.

Other writable registers are as follows. Those relating to date and time are initialized using localtime(3) at formatter startup.

\n[c.]
    Interpolate input line number. \n[.c] is a read-only alias of this register.

\n[hours]
    Interpolate number of hours elapsed since midnight.

\n[hp]
    Interpolate horizontal position relative to that at the start of the input line.

\n[lsn]
\n[lss]
    Interpolate count of leading spaces on input line and amount of corresponding horizontal motion, respectively.

\n[minutes]
    Interpolate number of minutes elapsed in the hour.

\n[seconds]
    Interpolate number of seconds elapsed in the minute.

\n[systat]
    Interpolate return value of system(3) function executed by most recent sy request.

\n[slimit]
    Interpolates maximum quantity of objects on troff's internal input stack (default: 1000). If non-positive, there is no limit: recursion can continue until program memory is exhausted.

\n[year]
    Interpolate Gregorian year. AT&T troff's \n[yr] interpolates the Gregorian year minus 1900.

  Delimiters
AT&T troff recognized slightly varying sets of delimiters when expecting numerical expressions (as with the \h escape sequence), string expressions (as with the \w escape sequence and tl request), and output comparisons (as in “.if #foo#bar# .tm match”). GNU troff, when not in compatibility mode, recognizes a single consistent set of delimiters. Compatibility mode emulates AT&T troff only up to a point. GNU troff accepts leaders and tabs as delimiters, as well as Control+D (EOT or EOF), Control+H (BS or backspace), and Control+L (FF or form feed), all of which, when used as delimiters, cause AT&T troff to behave in ways difficult to predict.

  Miscellaneous
A font not listed in the output device's DESC file's fonts directive is automatically mounted at the next available font position when it is selected. If you mount a font explicitly with the fp request, you should do so on the first unused position, which can be found in the .fp register.

Unparameterized string interpolation does not conceal the arguments to a macro being interpreted. Thus, in a macro definition, the call of another macro with the existing argument list,
        .xx \\$@
is more efficiently done with
        \\*[xx]\\
(that is, with string interpolation). The trailing backslashes prevent the final newline in the macro definition from being interpolated, potentially putting an unwanted blank line on the output. See section “Punning Names” in groff(7).

If a font description file contains pairwise kerning information, glyphs from that font are kerned. Kerning between two glyphs can be inhibited by placing a dummy character \& between them.

GNU troff keeps track of the nesting depth of escape sequence interpolations and other uses of delimiters, as in the tl request and the output comparison operator (that is, input like 'foo'bar' as a conditional expression), so the only characters you need to avoid using as delimiters are those that appear in the arguments you input, not any that result from interpolation. Typically, ' works fine. Use visible characters as delimiters in GNU troff, not US-ASCII controls like BEL (Control+G). The implementation of \$@ ensures that the double quotes surrounding an argument appear at an interpolation depth different from that of the arguments themselves. Similarly, in bracket-form escape sequences like \f[ZCMI], a right bracket ] does not end the sequence unless it occurs at the same interpolation depth as the opening [. In compatibility mode, no attention is paid to the interpolation depth.

In GNU troff, the tr request can map characters to the unbreakable space escape sequence \~ as a special case (tr normally operates only on characters). This feature replaces the odd-parity tr mapping trick used in AT&T troff documents, where a character, often ~, was “sacrificed” by mapping it to “nothing”, drafting it into use as an unadjustable, unbreakable space. (This feature was gratuitous even in early AT&T troff, which supported the \space escape sequence by 1976.) Often, it makes more sense to use GNU troff's \~ escape sequence instead, which has been adopted by every other active troff implementation except that of Illumos, as well as by the non-troff mandoc. Translation of a character to \~ is generally unnecessary, but might be employed to obtain an unbreakable space when the escape character will subsequently be disabled.

GNU troff permits tabs and spaces after the first dot on a control line that ends a macro definition.

Formatter output
The page description language output by GNU troff is modeled after that used by AT&T troff once the latter adopted a device-independent approach in the early 1980s. Only the differences are documented here. For a fuller discussion, see groff_out(5).

Glyph and font names can be of arbitrary length; postprocessors should not assume that they are at most two characters. A glyph to be formatted is always drawn from the current font; in contrast to AT&T device-independent troff, drivers need not search special fonts to find a glyph.

  Units
The argument to the s command is in scaled points (units of points/n, where n is the argument to the sizescale command in the DESC file). The argument to the “x H” command is also in scaled points.

  Simple commands

If the tcommand directive is present in the output device's DESC file, GNU troff employs the following two commands.

t xyz
    ... Typeset word xyz; that is, set a sequence of ordinary glyphs named x, y, z, ..., terminated by a space or newline; an optional second integer argument is ignored (this allows the formatter to generate an even number of arguments). Each glyph is set at the current drawing position, and the position is then advanced horizontally by the glyph's width. A glyph's width is read from its metrics in the font description file, scaled to the current type size, and rounded to a multiple of the horizontal motion quantum. Use the C command to emplace glyphs of special characters.

u n xyz
    ... Typeset word xyz with track kerning. As t, but after placing each glyph, the drawing position is further advanced horizontally by n basic units.

New commands implement color support.

mc cyan magenta yellow
md
mg gray
mk cyan magenta yellow black
mr red green blue
    Set the components of the stroke color with respect to various color spaces. md resets the stroke color to the default value. The arguments are integers in the range 0 to 65535.

A new device control subcommand is available.

x u n
    If n is 1, start underlining of spaces. If n is 0, stop underlining of spaces. This facility is needed for the cu request in nroff mode and is ignored otherwise.

  Extended drawing commands
GNU pic does not produce troff escape sequences employing these extensions if its -n option is given.

Df n
    Set the shade of gray used to fill geometric objects to n, which must be an integer. 0 corresponds to white and 1000 to black. A grayscale ramp spans the two. A value outside this range uses the stroke color as the fill color. The fill color is opaque. Normally the default is black, but some drivers may provide a way of changing this. Df is obsolete since 2002, superseded by DFg below.

    The corresponding \D'f' escape sequence should not be used: its argument is rounded to an integer multiple of the horizontal motion quantum, which can limit the precision of n.

DC d
    Draw a filled circle of diameter d with its leftmost point at the drawing position.

DE h v
    Draw a filled ellipse, of horizontal axis h and vertical axis v, with its leftmost point at the drawing position.

Dp $dx sub 1 ~ dy sub 1 ~ ldots ~ dx sub n ~ dy sub n$
    Draw a polygon with, for $i = 1 , ldots , n + 1$, its ith vertex at the drawing position $+ sum from { j = 1 } to { i - 1 } ( dx sub j , dy sub j )$. groff output drivers automatically close polygons, drawing a line from $( dx sub n , dy sub n )$ back to $( dx sub 1 , dy sub 1 )$. The drawing position is left at the last specified vertex, but this may change in a future version of GNU troff. Heirloom Doctools troff, like DWB troff, by default does not close the polygon. In its groff compatibility mode, Heirloom closes the polygon but leaves the drawing position unchanged—that is, at the polygon's initial drawing position.

DP $dx sub 1 ~ dy sub 1 ~ ldots ~ dx sub n ~ dy sub n$
    As Dp, but draw a filled rather than a stroked polygon.

Dt n
    Set the line thickness to n basic units. AT&T troff output drivers use a thickness proportional to the type size; this is the GNU troff default. A negative n requests this explicitly. An n of zero selects the smallest available line thickness.

A difficulty arises in how the drawing position should be changed after the execution of these commands. This has little importance to most users, since the output of GNU grn and pic does not depend on it. Given a drawing command of the form Dz $x sub 1 ~ y sub 1 ~ ldots ~ x sub n ~ y sub n$, where z is not c or e, AT&T troff treats each $x sub i$ as a horizontal motion, each $y sub i$ as a vertical one, and therefore assumes that the width of the drawn object is $sum from { i = 1 } to n x sub i$, and its height is $sum from { i = 1 } to n y sub i$. (Verify its assumption about height by examining the st and sb registers after using such a drawing command in a \w escape sequence). Thus after executing a D command of the form Dz $x sub 1 ~ y sub 1 ~ ldots ~ x sub n ~ y sub n$, the drawing position increases by $( sum from { i = 1 } to n x sub i , sum from { i = 1 } to n y sub i )$. For the sake of compatibility, GNU troff follows this rule, even though it frustrates extensions to the D command that set drawing parameters rather than rendering objects, producing ugly results in the case of Dt and Df, or otherwise don't parameterize objects as a series of vertices, as with GNU troff's filled ellipse, DE. In a future release, GNU troff and its output drivers may abandon the application of this assumption to drawing commands not explicitly specified in the AT&T “Troff User's Manual”. You can ensure predictable output by enclosing drawing commands in the zero-motion escape sequence \Z.

GNU troff implements fill color selection with another set of extensions.

DFc cyan magenta yellow
DFd
DFg gray
DFk cyan magenta yellow black
DFr red green blue
    Set the components of the fill color as described under the \M escape sequence above. DFd restores the device's default fill color. The drawing position is not updated, in contrast to Df.

  Device control syntax extension
GNU troff introduces a line continuation convention, permitting the argument to the x X command to contain newlines. A newline in the input is transformed to the sequence “newline +”. When interpreting an x X command, a postprocessor should therefore be prepared for a plus sign after a newline; if it occurs, preserve the newline, discard the plus sign, and continue to collect the input into the argument of the x X command. A newline not followed by a plus sign terminates the x X command. An application of this feature is the embedding of PostScript or PDF language command streams into troff output.

GNU troff guarantees that the first three output commands it emits are as follows.

        x T device
        x res n h v
        x init

Debugging
In addition to AT&T troff's debugging features, GNU troff emits more error diagnostics when syntactical or semantic nonsense is encountered and supports several warning categories; the output of these can be selected with “warn”. Also see the -E, -w, and -W options of troff(1).

A trace of the formatter's input processing stack can be emitted when errors or warnings occur by means of GNU troff -b option, or produced on demand with the backtrace request.

groff also adds more flexible diagnostic output requests (tmc and tm1). Examine the state of the formatter with requests that write lists of defined colors (pcolor), composite character mappings (pcomposite), environments (pev), font translations (pftr), automatic hyphenation codes (pchar) and exceptions (phw), registers (pnr), open streams (pstream), and page location traps (pwh). Requests can also disclose to the standard error stream the internal properties and representations of characters and classes (pchar), macros (and strings and diversions) (pm), and the list of output nodes corresponding to the pending input line (pline).

Compatibility mode
Some syntactical and behavioral differences between AT&T and GNU troffs are thought too important to neglect; GNU troff therefore makes available a compatibility mode in an effort to keep documents prepared for AT&T troff rendering well.

Identifiers of arbitrary length may be GNU troff's most obvious innovation. AT&T troff interprets “.dsabcd” as defining a string “ab” with contents “cd”. Normally, GNU troff interprets this input as calling a macro named “dsabcd”. AT&T troff also interprets \*[ and \n[ as interpolating a string or register, respectively, named “[”. GNU troff, however, normally interprets “[” as bracketing a long name (with “]” at the distal end). In compatibility mode, GNU troff interprets names in the traditional way, they thus can be two characters long at most. See the -C option in troff(1) and, above, the .C and .cp registers, and cp and “do” requests, for more on compatibility mode.

The register \n[.cp] is specialized and may require a statement of rationale. When writing macro packages or documents that use GNU troff features and which may be mixed with other packages or documents that do not—common scenarios include serial processing of man pages or use of the “so” or mso requests—you may desire correct operation regardless of compatibility mode enablement in the surrounding context. It may occur to you to save the existing value of \n(.C into a register, say, _C, at the beginning of your file, turn compatibility mode off with “.cp 0”, then restore it from that register at the end with “.cp \n(_C”. At the same time, a modular design of a document or macro package may lead you to multiple layers of inclusion. You cannot use the same register name everywhere lest you “clobber” the value from a preceding or enclosing context. The two-character register name space of AT&T troff is confining, but employing GNU troff's more capacious one, as with “.nr _my_saved_C \n(.C” does not work in compatibility mode; the register name is too long. Employing the “do” request is no help: “.do nr _my_saved_C \n(.C” always saves zero to the register, because “do” turns compatibility mode off while it interprets its argument list.

GNU troff normally tracks the interpolation depth of escape sequence parameters and other delimited structures, but not in compatibility mode. See section “Miscellaneous” above.

The escape sequences \f, \H, \m, \M, \R, \s, and \S are transparent to control character recognition at the beginning of an input line, or after the conditional expression of an “if” or ie request, only in compatibility mode. That is, upon interpreting them, GNU troff normally no longer recognizes a control character on the input line; but in compatibility mode, it does, just like AT&T troff.

Normally, the syntax form \sn accepts only a single character (a digit) for n, consistently with other forms that originated in AT&T troff, like \*, \$, \f, \g, \k, \n, and \z. In compatibility mode only, a non-zero n must be in the range 4–39. Legacy documents relying upon this quirk of parsing should migrate to another \s form. [Background: The Graphic Systems C/A/T phototypesetter (the original device target for AT&T troff) supported only a few discrete type sizes in the range 6–36 points, so Ossanna contrived a special case in the parser to do what the user must have meant. Kernighan warned of this in the 1992 revision of CSTR #54 (§2.3), and more recently, McIlroy referred to it as a “living fossil”.]

Other differences
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.

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 “~”. The \A escape sequence (see subsection “Escape sequences” above) may be helpful in avoiding their use.

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, 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.

Unlike GNU troff, AT&T troff does not recognize an occurrence of \% at the beginning of a word as suppressing its hyphenation; instead, it (uselessly) marks the start of the word as a potential hyphenation point, permitting output lines to end with hyphens that are not interior to a word.

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.

GNU troff interprets request arguments representing file names and system commands 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. 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.

The existence of the .T string is a common feature of device-independent troffs—DWB 3.3, Solaris, Heirloom Doctools, and Plan 9 troff 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 ignored attempts to remove read-only registers; GNU troff honors such requests.

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

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.

As noted above in “Fractional type sizes and new scaling units”, 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.

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 does (uselessly).

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.

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.

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. 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.

One way to format a backslash in most documents is with the \e escape sequence; this formats the glyph of the current escape character, regardless of whether it is used in a diversion; it also works in both GNU troff and AT&T troff. (Naturally, if you've changed the escape character, you need to prefix the “e” with whatever it is—and you'll likely get something other than a backslash in the output.)

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. [AT&T troff 's font description files did not define the rs special character, but those of its descendant Heirloom Doctools troff do, as of its 060716 release (July 2006).]

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 subsection “Escape sequences” above and sections “Diversions” and “Gtroff Internals” in Groff: The GNU Implementation of troff, the groff Texinfo manual.

Like AT&T troff, GNU troff maintains a buffer of device-independent output commands, 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.

  Formatter output incompatibilities
Its extensions notwithstanding, GNU troff's page description language has some incompatibilities with that of AT&T troff, but better compatibility is sought; problem reports and patches are welcome. The following incompatibilities are known.

•   The drawing position after rendering polygons is inconsistent with AT&T troff practice. Other implementations have diverged on this point as well.

•   The output cannot be easily rescaled to other devices as AT&T troff's could.

Authors
This document was written by James Clark (jjc@jclark.com), Werner Lemberg (wl@gnu.org), Bernd Warken (groff-bernd.warken-72@web.de), and G. Branden Robinson (g.branden.robinson@gmail.com).

See also
Groff: The GNU Implementation of troff, by Trent A. Fisher and Werner Lemberg, is the primary groff manual. You can browse it interactively with “info groff”.

“Troff User's Manual” by Joseph F. Ossanna, 1976 (revised by Brian W. Kernighan, 1992), AT&T Bell Laboratories Computing Science Technical Report No. 54, widely called simply “CSTR #54”, documents the language, device and font description file formats, and page description language referred to collectively in groff documentation as AT&T troff.

“A Typesetter-independent TROFF” by Brian W. Kernighan, 1982, AT&T Bell Laboratories Computing Science Technical Report No. 97, provides additional insights into the device and font description file formats and page description language.

groff(1), groff(7), roff(7)