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groff_out (5)
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    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 to stack 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 [em] those for drawing and device controlling [em] 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 splitted by a line break. Empty lines, i.e., lines containing only space and/or a comment, can occur everywhere. They are just ignored.  

    Argument Units

    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 the groff info file for more on this topic. Most commands assume the scale indicator~ the basic unit of the device, some use~ 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. The names of characters and fonts can be of arbitrary length. A character 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.  

    Document Parts

    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 the section Device Control Commands. But 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
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