Keywords: ISO C++, documentation, style, docbook, doxygen

Writing and Generating Documentation
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## Writing and Generating Documentation

### Introduction

Documentation for the GNU C++ Library is created from three independent sources: a manual, a FAQ, and an API reference.

The sub-directory `doc` within the main source directory contains `Makefile.am` and `Makefile.in`, which provide rules for generating documentation, described in excruciating detail below. The `doc` sub-directory also contains three directories: `doxygen`, which contains scripts and fragments for doxygen, `html`, which contains an html version of the manual, and `xml`, which contains an xml version of the manual.

Diverging from established documentation conventions in the rest of the GCC project, libstdc++ does not use Texinfo as a markup language. Instead, Docbook is used to create the manual and the FAQ, and Doxygen is used to construct the API reference. Although divergent, this conforms to the GNU Project recommendations as long as the output is of sufficient quality, as per GNU Manuals.

### Generating Documentation

Certain Makefile rules are required by the GNU Coding Standards. These standard rules generate HTML, PDF, XML, or man files. For each of the generative rules, there is an additional install rule that is used to install any generated documentation files into the prescribed installation directory. Files are installed into `share/doc` or `share/man` directories.

The standard Makefile rules are conditionally supported, based on the results of examining the host environment for prerequisites at configuration time. If requirements are not found, the rule is aliased to a dummy rule that does nothing, and produces no documentation. If the requirements are found, the rule forwards to a private rule that produces the requested documentation.

For more details on what prerequisites were found and where, please consult the file `config.log` in the libstdc++ build directory. Compare this log to what is expected for the relevant Makefile conditionals: `BUILD_INFO`, `BUILD_XML`, `BUILD_HTML`, `BUILD_MAN`, `BUILD_PDF`, and `BUILD_EPUB`.

Supported Makefile rules:

make html , make install-html

  Generates multi-page HTML documentation, and installs it in the following directories:

  `doc/libstdc++/libstdc++-api.html`

  `doc/libstdc++/libstdc++-manual.html`

make pdf , make install-pdf

  Generates indexed PDF documentation, and installs it as the following files:

  `doc/libstdc++/libstdc++-api.pdf`

  `doc/libstdc++/libstdc++-manual.pdf`

make man , make install-man

  Generates man pages, and installs it in the following directory:

  `man/man3/`

  The generated man pages are namespace-qualified, so to look at the man page for `vector`, one would use man std::vector.

make epub , make install-epub

  Generates documentation in the ebook/portable electronic reader format called Epub, and installs it as the following file.

  `doc/libstdc++/libstdc++-manual.epub`

make xml , make install-xml

  Generates single-file XML documentation, and installs it as the following files:

  `doc/libstdc++/libstdc++-api-single.xml`

  `doc/libstdc++/libstdc++-manual-single.xml`

Makefile rules for several other formats are explicitly not supported, and are always aliased to dummy rules. These unsupported formats are: info, ps, and dvi.

### Doxygen

#### Prerequisites

Table B.1. Doxygen Prerequisites

Tool | Version | Required By
--- | --- | ---
coreutils | 8.5 | all
bash | 4.1 | all
doxygen | 1.7.6.1 | all
graphviz | 2.26 | graphical hierarchies
pdflatex | 2007-59 | pdf output

Prerequisite tools are Bash 2.0 or later, Doxygen, and the GNU coreutils. (GNU versions of find, xargs, and possibly sed and grep are used, just because the GNU versions make things very easy.)

To generate the pretty pictures and hierarchy graphs, the Graphviz package will need to be installed. For PDF output, pdflatex is required as well as a number of TeX packages such as texlive-xtab and texlive-tocloft.

Be warned the PDF file generated via doxygen is extremely large. At last count, the PDF file is over three thousand pages. Generating this document taxes the underlying TeX formatting system, and will require the expansion of TeX's memory capacity. Specifically, the `pool_size` variable in the configuration file `texmf.cnf` may need to be increased by a minimum factor of two.

#### Generating the Doxygen Files

The following Makefile rules run Doxygen to generate HTML docs, XML docs, XML docs as a single file, PDF docs, and the man pages. These rules are not conditional! If the required tools are not found, or are the wrong versions, the rule may end in an error.

    make doc-html-doxygen

    make doc-xml-doxygen

    make doc-xml-single-doxygen

    make doc-pdf-doxygen

    make doc-man-doxygen

Generated files are output into separate sub directories of `doc/doxygen/` in the build directory, based on the output format. For instance, the HTML docs will be in `doc/doxygen/html`.

Careful observers will see that the Makefile rules simply call a script from the source tree, `run_doxygen`, which does the actual work of running Doxygen and then (most importantly) massaging the output files. If for some reason you prefer to not go through the Makefile, you can call this script directly. (Start by passing `--help`.)

If you wish to tweak the Doxygen settings, do so by editing `doc/doxygen/user.cfg.in`. Notes to fellow library hackers are written in triple-# comments.

#### Debugging Generation

Sometimes, mis-configuration of the pre-requisite tools can lead to errors when attempting to build the documentation. Here are some of the obvious errors, and ways to fix some common issues that may appear quite cryptic.

First, if using a rule like `make pdf`, try to narrow down the scope of the error to either docbook (`make doc-pdf-docbook`) or doxygen (`make doc-pdf-doxygen`).

Working on the doxygen path only, closely examine the contents of the following build directory: `build/target/libstdc++-v3/doc/doxygen/latex`. Pay attention to three files enclosed within, annotated as follows.

- refman.tex

  The actual latex file, or partial latex file. This is generated via doxygen, and is the LaTeX version of the Doxygen XML file `libstdc++-api.xml`. Go to a specific line, and look at the generated LaTeX, and try to deduce what markup in `libstdc++-api.xml` is causing it.

- refman.log

  A log created by latex as it processes the `refman.tex` file. If generating the PDF fails look at the end of this file for errors such as:

          ! LaTeX Error: File `xtab.sty' not found.

  This indicates a required TeX package is missing. For the example above the texlive-xtab package needs to be installed.

- refman.out

  A log of the compilation of the converted LaTeX form to PDF. This is a linear list, from the beginning of the `refman.tex` file: the last entry of this file should be the end of the LaTeX file. If it is truncated, then you know that the last entry is the last part of the generated LaTeX source file that is valid. Often this file contains an error with a specific line number of `refman.tex` that is incorrect, or will have clues at the end of the file with the dump of the memory usage of LaTeX.

If the error at hand is not obvious after examination, a fall-back strategy is to start commenting out the doxygen input sources, which can be found in `doc/doxygen/user.cfg.in`, look for the `INPUT` tag. Start by commenting out whole directories of header files, until the offending header is identified. Then, read the latex log files to try and find surround text, and look for that in the offending header.

#### Markup

In general, libstdc++ files should be formatted according to the rules found in the Coding Standard. Before any doxygen-specific formatting tweaks are made, please try to make sure that the initial formatting is sound.

Adding Doxygen markup to a file (informally called “doxygenating”) is very simple. The Doxygen manual can be found here. We try to use a very-recent version of Doxygen.

For classes, use `deque`/`vector`/`list` and `std::pair` as examples. For functions, see their member functions, and the free functions in `stl_algobase.h`. Member functions of other container-like types should read similarly to these member functions.

Some commentary to accompany the first list in the Special Documentation Blocks section of the Doxygen manual:

1. For longer comments, use the Javadoc style...

2. ...not the Qt style. The intermediate *'s are preferred.

3. Use the triple-slash style only for one-line comments (the “brief” mode).

4. This is disgusting. Don't do this.

Some specific guidelines:

Use the @-style of commands, not the !-style. Please be careful about whitespace in your markup comments. Most of the time it doesn't matter; doxygen absorbs most whitespace, and both HTML and *roff are agnostic about whitespace. However, in <pre> blocks and @code/@endcode sections, spacing can have “interesting” effects.

Use either kind of grouping, as appropriate. `doxygroups.cc` exists for this purpose. See `stl_iterator.h` for a good example of the “other” kind of grouping.

Please use markup tags like @p and @a when referring to things such as the names of function parameters. Use @e for emphasis when necessary. Use @c to refer to other standard names. (Examples of all these abound in the present code.)

Complicated math functions should use the multi-line format. An example from `random.h`:

/**
* @brief A model of a linear congruential random number generator.
*
* @f[
* x_{i+1}\leftarrow(ax_{i} + c) \bmod m
* @f]
*/

One area of note is the markup required for `@file` markup in header files. Two details are important: for filenames that have the same name in multiple directories, include part of the installed path to disambiguate. For example:

/** @file debug/vector
* This file is a GNU debug extension to the Standard C++ Library.
*/

The other relevant detail for header files is the use of a libstdc++-specific doxygen alias that helps distinguish between public header files (like `random`) from implementation or private header files (like `bits/c++config.h`.) This alias is spelled `@headername` and can take one or two arguments that detail the public header file or files that should be included to use the contents of the file. All header files that are not intended for direct inclusion must use `headername` in the `file` block. An example:

/** @file bits/basic_string.h
* This is an internal header file, included by other library headers.
* Do not attempt to use it directly. @headername{string}
*/

Be careful about using certain, special characters when writing Doxygen comments. Single and double quotes, and separators in filenames are two common trouble spots. When in doubt, consult the following table.

Table B.2. HTML to Doxygen Markup Comparison

HTML | Doxygen
--- | ---
\ | \\
" | \"
' | \'
<i> | @a word
<b> | @b word
<code> | @c word
<em> | @a word
<em> | <em>two words or more</em>

### Docbook

#### Prerequisites

Table B.3. Docbook Prerequisites

Tool | Version | Required By
--- | --- | ---
docbook5-style-xsl | 1.76.1 | all
xsltproc | 1.1.26 | all
xmllint | 2.7.7 | validation
dblatex | 0.3 | pdf output
pdflatex | 2007-59 | pdf output
docbook2X | 0.8.8 | info output
epub3 stylesheets | b3 | epub output

Editing the DocBook sources requires an XML editor. Many exist: some notable options include emacs, Kate, or Conglomerate.

Some editors support special “XML Validation” modes that can validate the file as it is produced. Recommended is the nXML Mode for emacs.

Besides an editor, additional DocBook files and XML tools are also required.

Access to the DocBook 5.0 stylesheets and schema is required. The stylesheets are usually packaged by vendor, in something like `docbook5-style-xsl`. To exactly match generated output, please use a version of the stylesheets equivalent to `docbook5-style-xsl-1.75.2-3`. The installation directory for this package corresponds to the `XSL_STYLE_DIR` in `doc/Makefile.am` and defaults to `/usr/share/sgml/docbook/xsl-ns-stylesheets`.

For processing XML, an XSLT processor and some style sheets are necessary. Defaults are xsltproc provided by `libxslt`.

For validating the XML document, you'll need something like xmllint and access to the relevant DocBook schema. These are provided by a vendor package like `libxml2` and `docbook5-schemas-5.0-4`

For PDF output, something that transforms valid Docbook XML to PDF is required. Possible solutions include dblatex, xmlto, or prince. Of these, dblatex is the default. Other options are listed on the DocBook web pages. Please consult the `<libstdc++@gcc.gnu.org>` list when preparing printed manuals for current best practice and suggestions.

For Texinfo output, something that transforms valid Docbook XML to Texinfo is required. The default choice is docbook2X.

For epub output, the stylesheets for EPUB3 are required. These stylesheets are still in development. To validate the created file, epubcheck is necessary.

#### Generating the DocBook Files

The following Makefile rules generate (in order): an HTML version of all the DocBook documentation, a PDF version of the same, and a single XML document. These rules are not conditional! If the required tools are not found, or are the wrong versions, the rule may end in an error.

    make doc-html-docbook

    make doc-pdf-docbook

    make doc-xml-single-docbook

Generated files are output into separate sub directores of `doc/docbook/` in the build directory, based on the output format. For instance, the HTML docs will be in `doc/docbook/html`.

If the Docbook stylesheets are installed in a custom location, one can use the variable `XSL_STYLE_DIR` to override the Makefile defaults. For example:

    make XSL_STYLE_DIR="/usr/share/xml/docbook/stylesheet/nwalsh" doc-html-docbook

#### Debugging Generation

Sometimes, mis-configuration of the pre-requisite tools can lead to errors when attempting to build the documentation. Here are some of the obvious errors, and ways to fix some common issues that may appear quite cryptic.

First, if using a rule like `make pdf`, try to narrow down the scope of the error to either docbook (`make doc-pdf-docbook`) or doxygen (`make doc-pdf-doxygen`).

Working on the docbook path only, closely examine the contents of the following build directory: `build/target/libstdc++-v3/doc/docbook/latex`. Pay attention to three files enclosed within, annotated as follows.

- spine.tex

  The actual latex file, or partial latex file. This is generated via dblatex, and is the LaTeX version of the DocBook XML file `spine.xml`. Go to a specific line, and look at the generated LaTeX, and try to deduce what markup in `spine.xml` is causing it.

- spine.out

  A log of the conversion from the XML form to the LaTeX form. This is a linear list, from the beginning of the `spine.xml` file: the last entry of this file should be the end of the DocBook file. If it is truncated, then you know that the last entry is the last part of the XML source file that is valid. The error is after this point.

- spine.log

  A log of the compilation of the converted LaTeX form to pdf. This is a linear list, from the beginning of the `spine.tex` file: the last entry of this file should be the end of the LaTeX file. If it is truncated, then you know that the last entry is the last part of the generated LaTeX source file that is valid. Often this file contains an error with a specific line number of `spine.tex` that is incorrect.

If the error at hand is not obvious after examination, or if one encounters the inscruitable “Incomplete \ifmmode” error, a fall-back strategy is to start commenting out parts of the XML document (regardless of what this does to over-all document validity). Start by commenting out each of the largest parts of the `spine.xml` file, section by section, until the offending section is identified.

#### Editing and Validation

After editing the xml sources, please make sure that the XML documentation and markup is still valid. This can be done easily, with the following validation rule:

    	make doc-xml-validate-docbook

This is equivalent to doing:

    	  xmllint --noout --valid xml/index.xml

Please note that individual sections and chapters of the manual can be validated by substituting the file desired for `xml/index.xml` in the command above. Reducing scope in this manner can be helpful when validation on the entire manual fails.

All Docbook xml sources should always validate. No excuses!

#### File Organization and Basics

Which files are important
All Docbook files are in the directory
libstdc++-v3/doc/xml
Inside this directory, the files of importance:
spine.xml - index to documentation set
manual/spine.xml - index to manual
manual/*.xml - individual chapters and sections of the manual
faq.xml - index to FAQ
api.xml - index to source level / API
All *.txml files are template xml files, i.e., otherwise empty files with
the correct structure, suitable for filling in with new information.
Canonical Writing Style
class template
function template
member function template
(via C++ Templates, Vandevoorde)
class in namespace std: allocator, not std::allocator
header file: iostream, not <iostream>
General structure
<set>
<book>
</book>
<book>
<chapter>
</chapter>
</book>
<book>
<part>
<chapter>
<section>
</section>
<sect1>
</sect1>
<sect1>
<sect2>
</sect2>
</sect1>
</chapter>
<chapter>
</chapter>
</part>
</book>
</set>

#### Markup By Example

Complete details on Docbook markup can be found in the DocBook Element Reference, online. An incomplete reference for HTML to Docbook conversion is detailed in the table below.

Table B.4. HTML to Docbook XML Markup Comparison

HTML | Docbook
--- | ---
<p> | <para>
<pre> | <computeroutput>, <programlisting>, <literallayout>
<ul> | <itemizedlist>
<ol> | <orderedlist>
<il> | <listitem>
<dl> | <variablelist>
<dt> | <term>
<dd> | <listitem>
<a href=""> | <ulink url="">
<code> | <literal>, <programlisting>
<strong> | <emphasis>
<em> | <emphasis>
" | <quote>

And examples of detailed markup for which there are no real HTML equivalents are listed in the table below.

Table B.5. Docbook XML Element Use

Element | Use
--- | ---
<structname> | <structname>char_traits</structname>
<classname> | <classname>string</classname>
<function> | <function>clear()</function> <function>fs.clear()</function>
<type> | <type>long long</type>
<varname> | <varname>fs</varname>
<literal> | <literal>-Weffc++</literal> <literal>rel_ops</literal>
<constant> | <constant>_GNU_SOURCE</constant> <constant>3.0</constant>
<command> | <command>g++</command>
<errortext> | <errortext>In instantiation of</errortext>
<filename> | <filename class="headerfile">ctype.h</filename> <filename class="directory">/home/gcc/build</filename> <filename class="libraryfile">libstdc++.so</filename>

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