GROFF_OUT(5) | File Formats Manual | GROFF_OUT(5) |
groff_out - groff intermediate output format
This manual page describes the intermediate output format of the GNU roff(7) text processing system groff(1). This output is produced by a run of the GNU troff(1) program. It contains already all device-specific information, but it is not yet fed into a device postprocessor program.
As the GNU roff processor groff(1) is a wrapper program around troff that automatically calls a postprocessor, this output does not show up normally. This is why it is called intermediate within the groff system. The groff program provides the option -Z to inhibit postprocessing, such that the produced intermediate output is sent to standard output just like calling troff manually.
In this document, the term troff output describes what is output by the GNU troff program, while intermediate output refers to the language that is accepted by the parser that prepares this output for the postprocessors. This parser is smarter on whitespace and implements obsolete elements for compatibility, otherwise both formats are the same. Both formats can be viewed directly with gxditview(1).
The main purpose of the intermediate output concept is to facilitate the development of postprocessors by providing a common programming interface for all devices. It has a language of its own that is completely different from the groff(7) language. While the groff language is a high-level programming language for text processing, the intermediate output language is a kind of low-level assembler language by specifying all positions on the page for writing and drawing.
The pre-groff roff versions are denoted as classical troff. The intermediate output produced by groff is fairly readable, while classical troff output was hard to understand because of strange habits that are still supported, but not used any longer by GNU troff.
During the run of troff, the roff input is cracked down to the information on what has to be printed at what position on the intended device. So the language of the intermediate output format can be quite small. Its only elements are commands with or without arguments. In this document, the term “command” always refers to the intermediate output language, never to the roff language used for document formatting. There are commands for positioning and text writing, for drawing, and for device controlling.
Classical troff output had strange requirements on whitespace. The groff output parser, however, is smart about whitespace by making it maximally optional. The whitespace characters, i.e., the tab, space, and newline characters, always have a syntactical meaning. They are never printable because spacing within the output is always done by positioning commands.
Any sequence of space or tab characters is treated as a single syntactical space. It separates commands and arguments, but is only required when there would occur a clashing between the command code and the arguments without the space. Most often, this happens when variable length command names, arguments, argument lists, or command clusters meet. Commands and arguments with a known, fixed length need not be separated by syntactical space.
A line break is a syntactical element, too. Every command argument can be followed by whitespace, a comment, or a newline character. Thus a syntactical line break is defined to consist of optional syntactical space that is optionally followed by a comment, and a newline character.
The normal commands, those for positioning and text, consist of a single letter taking a fixed number of arguments. For historical reasons, the parser allows stacking of such commands on the same line, but fortunately, in groff intermediate output, every command with at least one argument is followed by a line break, thus providing excellent readability.
The other commands — those for drawing and device controlling — have a more complicated structure; some recognize long command names, and some take a variable number of arguments. So all D and x commands were designed to request a syntactical line break after their last argument. Only one command, ‘x X’ has an argument that can stretch over several lines, all other commands must have all of their arguments on the same line as the command, i.e., the arguments may not be split by a line break.
Empty lines, i.e., lines containing only space and/or a comment, can occur everywhere. They are just ignored.
Some commands take integer arguments that are assumed to represent values in a measurement unit, but the letter for the corresponding scale indicator is not written with the output command arguments; see groff(7) and Groff: The GNU Implementation of troff, the groff Texinfo manual, for more on this topic. Most commands assume the scale indicator u, the basic unit of the device, some use z, the scaled point unit of the device, while others, such as the color commands expect plain integers. Note that these scale indicators are relative to the chosen device. They are defined by the parameters specified in the device's DESC file; see groff_font(5).
Note that single characters can have the eighth bit set, as can the names of fonts and special characters (this is, glyphs). The names of glyphs and fonts can be of arbitrary length. A glyph that is to be printed will always be in the current font.
A string argument is always terminated by the next whitespace character (space, tab, or newline); an embedded # character is regarded as part of the argument, not as the beginning of a comment command. An integer argument is already terminated by the next non-digit character, which then is regarded as the first character of the next argument or command.
A correct intermediate output document consists of two parts, the prologue and the body.
The task of the prologue is to set the general device parameters using three exactly specified commands. The groff prologue is guaranteed to consist of the following three lines (in that order):
x T device
x res n h v
x init
with the arguments set as outlined in subsection “Device Control Commands” below. However, the parser for the intermediate output format is able to swallow additional whitespace and comments as well.
The body is the main section for processing the document data. Syntactically, it is a sequence of any commands different from the ones used in the prologue. Processing is terminated as soon as the first x stop command is encountered; the last line of any groff intermediate output always contains such a command.
Semantically, the body is page oriented. A new page is started by a p command. Positioning, writing, and drawing commands are always done within the current page, so they cannot occur before the first p command. Absolute positioning (by the H and V commands) is done relative to the current page, all other positioning is done relative to the current location within this page.
This section describes all intermediate output commands, the classical commands as well as the groff extensions.
This command is the only possibility for commenting in the intermediate output. Each comment can be preceded by arbitrary syntactical space; every command can be terminated by a comment.
The commands in this subsection have a command code consisting of a single character, taking a fixed number of arguments. Most of them are commands for positioning and text writing. These commands are smart about whitespace. Optionally, syntactical space can be inserted before, after, and between the command letter and its arguments. All of these commands are stackable, i.e., they can be preceded by other simple commands or followed by arbitrary other commands on the same line. A separating syntactical space is only necessary when two integer arguments would clash or if the preceding argument ends with a string argument.
Each graphics or drawing command in the intermediate output starts with the letter D followed by one or two characters that specify a subcommand; this is followed by a fixed or variable number of integer arguments that are separated by a single space character. A D command may not be followed by another command on the same line (apart from a comment), so each D command is terminated by a syntactical line break.
troff output follows the classical spacing rules (no space between command and subcommand, all arguments are preceded by a single space character), but the parser allows optional space between the command letters and makes the space before the first argument optional. As usual, each space can be any sequence of tab and space characters.
Some graphics commands can take a variable number of arguments. In this case, they are integers representing a size measured in basic units u. The h arguments stand for horizontal distances where positive means right, negative left. The v arguments stand for vertical distances where positive means down, negative up. All these distances are offsets relative to the current location.
Unless indicated otherwise, each graphics command directly corresponds to a similar groff \D escape sequence; see groff(7).
Unknown D commands are assumed to be device-specific. Its arguments are parsed as strings; the whole information is then sent to the postprocessor.
In the following command reference, the syntax element ⟨line-break⟩ means a syntactical line break as defined in subsection “Separation” above.
mg 0 0 65536 Df -1
No position changing. This command is a groff extension.
Each device control command starts with the letter x followed by a space character (optional or arbitrary space/tab in groff) and a subcommand letter or word; each argument (if any) must be preceded by a syntactical space. All x commands are terminated by a syntactical line break; no device control command can be followed by another command on the same line (except a comment).
The subcommand is basically a single letter, but to increase readability, it can be written as a word, i.e., an arbitrary sequence of characters terminated by the next tab, space, or newline character. All characters of the subcommand word but the first are simply ignored. For example, troff outputs the initialization command x i as x init and the resolution command x r as x res. But writings like x i_like_groff and x roff_is_groff are accepted as well to mean the same commands.
In the following, the syntax element ⟨line-break⟩ means a syntactical line break as defined in subsection “Separation” above.
In classical troff output, emitting a single glyph was mostly done by a very strange command that combined a horizontal move and the printing of a glyph. It didn't have a command code, but is represented by a 3-character argument consisting of exactly 2 digits and a character.
In groff, arbitrary syntactical space around and within this command is allowed to be added. Only when a preceding command on the same line ends with an argument of variable length a separating space is obligatory. In classical troff, large clusters of these and other commands were used, mostly without spaces; this made such output almost unreadable.
For modern high-resolution devices, this command does not make sense because the width of the glyphs can become much larger than two decimal digits. In groff, this is only used for the devices X75, X75-12, X100, and X100-12. For other devices, the commands t and u provide a better functionality.
The roff postprocessors are programs that have the task to translate the intermediate output into actions that are sent to a device. A device can be some piece of hardware such as a printer, or a software file format suitable for graphical or text processing. The groff system provides powerful means that make the programming of such postprocessors an easy task.
There is a library function that parses the intermediate output and sends the information obtained to the device via methods of a class with a common interface for each device. So a groff postprocessor must only redefine the methods of this class. For details, see the reference in section “Files” below.
This section presents the intermediate output generated from the same input for three different devices. The input is the sentence hell world fed into groff on the command line.
shell> echo "hell world" | groff -Z -T ps
x T ps x res 72000 1 1 x init p1 x font 5 TR f5 s10000 V12000 H72000 thell wh2500 tw H96620 torld n12000 0 x trailer V792000 x stop
This output can be fed into the postprocessor grops(1) to get its representation as a PostScript file, or gropdf(1) to output directly to PDF.
This is similar to the high-resolution device except that the positioning is done at a minor scale. Some comments (lines starting with #) were added for clarification; they were not generated by the formatter.
shell> "hell world" | groff -Z -T latin1
# prologue x T latin1 x res 240 24 40 x init # begin a new page p1 # font setup x font 1 R f1 s10 # initial positioning on the page V40 H0 # write text ‘hell’ thell # inform about a space, and do it by a horizontal jump wh24 # write text ‘world’ tworld # announce line break, but do nothing because ... n40 0 # ... the end of the document has been reached x trailer V2640 x stop
This output can be fed into the postprocessor grotty(1) to get a formatted text document.
As a computer monitor has a very low resolution compared to modern printers the intermediate output for the X devices can use the jump-and-write command with its 2-digit displacements.
shell> "hell world" | groff -Z -T X100
x T X100 x res 100 1 1 x init p1 x font 5 TR f5 s10 V16 H100 # write text with old-style jump-and-write command ch07e07l03lw06w11o07r05l03dh7 n16 0 x trailer V1100 x stop
This output can be fed into the postprocessor xditview(1x) or gxditview(1) for displaying in X.
Due to the obsolete jump-and-write command, the text clusters in the classical output are almost unreadable.
The intermediate output language of the classical troff was first documented in [CSTR #97] . The groff intermediate output format is compatible with this specification except for the following features.
The differences between groff and classical troff are documented in groff_diff(7).
James Clark wrote an early version of this document, which described only the differences between ditroff(7)'s output format and that of GNU roff. The present version was completely rewritten in 2001 by Bernd Warken.
A reference like groff(7) refers to a manual page; here groff in section 7 of the man page documentation system. To read the example, look up section 7 in your desktop help system or call from the shell prompt
shell> man 7 groff
For more details, see man(1).
grodvi(1), grohtml(1), grolbp(1),
grolj4(1), grops(1), grotty(1)
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”.
The classical troff output language is described in two AT&T Bell Labs CSTR documents available on-line at Bell Labs CSTR site.
7 March 2023 | groff 1.22.4 |