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Writing Documentation - Part 1: POD

By Christoph Spiel

The title, Writing Documentation, sounds somewhat formal. However, in this article I refer to documentation a broad sense, not only to documentation accompanying a particular piece of software, but to any related textual pieces of information. This textual information could be as short as a few lines and, for example, describe how to to start a program with all of its command line options and environment variables set correctly. On the other hand, the text could be several tens of thousands lines long, elaborating all the tricks a group of users has learned over the years while using a large software system.


With today's GNU/Linux distributions, the aspiring documentation writer immediately finds herself in fat city: there are several systems to chose from! Three documentation systems will be introduced in this article series. Here, I start with POD. Next month I'll address LaTeX in conjunction with latex2html, and in part 3 DocBook.

The systems cater different documentation needs and all have their highs and lows. But before assessing the pros and cons of the different systems, let me put up some requirements, which I want to impose on the documentation systems.

The sources of the documentation should be:

As of December 2001, writing a "portable" file almost implies using 7-bit ASCII to encode characters. Today, 7-bit ASCII is the only encoding that works on a huge number of computers. In the future, hopefully, it will mean Unicode. Unicode can represent many more characters than ASCII does and will be as portable.

Requiring portability ensures that the texts' sources can be read and modified on a wide variety of computer systems, thereby making the documentation accessible to other programmers, which is what Open Source Software is all about.

Data on a computer is only as good as the access to it. -- Sounds like a commonplace, but we easily forget that documentation ought to be maintained just like source code is. We want to be able to search existing documentation for, say, a particular term or identifier. Therefore, the source of the documentation should at least be amenable to standard searching tools like, for example, the grep family (grep, agrep, rgrep, sgrep) of tools.
Unless we write a short note, it is desirable that the document sources can be split into logical parts, for example sections, and the collection of all the source files is still processed as a whole by the documentation system.
Easy to Read
For documentation to be ``open'' (as in ``Open Source''), the source should be easy to read, and the system to generate the final output should be simple to learn. A documentation system will be better accepted if the writer -- and later possibly the maintainer -- can concentrate on contents rather than syntactic quirks or obscure tool chains.

Just as I require certain features in the documentation's source format, so I do with the output.

Multiple Output Formats
The documentation system must be capable of producing different output formats -- the more formats the better. At least HTML and PostScript (some users prefer PDF) must be supported, one for on-screen reading, the other for print outs.

HTML support in turn requires ``hyperlinks'', this is, references between documents or parts thereof that can be followed in a convenient way. References also help to implement the Modular Requirement in my list of source format features.

Automatic Reference Generation
All references should be resolved automatically as far as this is possible. For example, the system should resolve cross references within in the document and should allow for footnotes or sidenotes to be placed and numbered without the help of the writer. The index and bibliography also should be generated automatically.

Let us now look at a particularly easy to use format: Perl's Plain Old Documentation.

Perl's Plain Old Documentation (POD)

The ``Plain Old Documentation'' system that ships with every Perl distribution is simplest documentation system in my selection. It is simple to learn, simple to use, but -- and I hesitate to write therefore -- also the most limited of the three. Anyhow, the article you are currently reading (yes this one!) has been prepared with POD. If it is good for the goose, it can't be bad for the gander...

The big advantages of POD are


The POD format defines three different kinds of paragraphs. Paragraphs are separated from each other by one or more completely (!) empty lines.

Ordinary Paragraph.
Any line that does not start with at least four spaces or an equal sign is considered ordinary text. Paragraphs are separated by one or more empty lines. This means, the documenter simply writes one paragraph after the other with at least one blank line between each pair.

Ordinary paragraphs will be filled and justified (if the output format allows for justification) when output.

Verbatim Paragraph.
Lines indented by four or more spaces are considered verbatim text. They are output exactly as typed. All formatting instructions that we will see later, are disabled in verbatim paragraphs.
Command Paragraph.
Command paragraphs start with an equal sign ``='' in column zero, immediately followed by an identifier. Usually, command paragraphs consist of single lines. Yet they are syntactically paragraphs, because they are separated by blank lines before and after them.


Text is sectioned by =headN commands, like

=head1 primary_heading

=head2 secondary_heading

=head3 tertiary_heading

which also define the section headings primary_heading, etc.. How many heading levels (this is largest N permitted) actually are accepted, depends on the POD-to-something converter. For example, pod2man allows only two levels, pod2html allows up to six levels.

I have added line and column numbers to the source of the examples. The line numbers do not appear in the real source. They are included here to point out the empty lines that must separate the command paragraphs, this is, those starting with an equal sign in column 0. Additionally, I have added a column-number ruler at the beginning of the next example to clarify where column 0 starts.


                    1         2         3         4          5
     1    =head1 Hardware
     3    The physical parts of your computer are called "hardware".
     5    =head1 CPU
     7    The CPU is the most important part of your computer.
     9    =head1 Mass Storage
    11    Mass storage devices store data permanently.
    13    =head2 Hard Disk Drives
    15    Hard disk drives provide fast random access to data.
    17    =head2 Magnetic Tapes
    19    Magnetic tapes provide slow sequential access to data.
    21    =head1 Software
    23    This is where the trouble starts ...


Itemized, enumerated or description lists are produced with

=over N

=item label

=item label



where =over N starts a list that is indented at least N spaces, and extends until =back. Depending on the first label the POD-to-something translators generate an itemized list (label = *), a numbered list (label = 1) or a description list (label starts with a letter).

Example: itemized list

Again, I have added line numbers to alert the reader of the (many) empty lines used for separating the command paragraphs.


     1    =over 4
     3    =item *
     5    Fruit, particularly non-imported fruit like ...
     7    =item *
     9    Though not tasty, vegetables should make up a large part of your
    10    daily diet.
    12    =item *
    14    Fish is much easier digestible than meat.  Therefore, ...
    16    =back


Example: enumerated list


     1    =over 4
     3    =item 1.
     5    Ensure that the power switch is in position "OFF".
     7    =item 2.
     9    Plug in the power cord.
    11    =item 3.
    13    Switch the power switch in position "ON".
    15    =back


  1. Ensure that the power switch is in position ``OFF''.
  2. Plug in the power cord.
  3. Switch the power switch in position ``ON''.

Example: description list


     1    =over 8
     3    =item Robert
     5    Lead singer
     7    =item Jimmy
     9    Lead guitar
    11    =item John-Paul
    13    Bass guitar
    15    =item John
    17    Drums and percussion
    19    =back


Lead singer
Lead guitar
Base guitar
Drums and percussion

Inline Markup Commands

Within Ordinary Text, several markup commands are recognized. All markup commands start with a single capital letter and enclose their argument within angle brackets: LETTER<argument>. The argument can consist of multiple words, which can span more than one line.

Render argument in italics. I corresponds to the HTML tags em and var, thus it is primarily used for emphasizing words or marking up variables.


Render argument bold. B corresponds to the HTML tag b. It is used to emphasize in text and to mark up program names or switches.


C marks up code or anything else which is to be taken literally. The corresponding HTML tags is are code, samp, and tt.


L<reference> or L<description|reference>
Link to an existing reference. If description is omitted, the link's text is reference, otherwise it is description. Using L is a bit tricky. Therefore, I have devoted the next section to it.

Cross References

The L-command is distantly related to HTML's <a href = "reference">description</a>, however, in POD, reference is not a general unified resource locator (URL).

reference can only refer to (automatically by the POD-to-something translator) generated labels. These labels are inserted for every =head and =item. The label associated with =head heading is heading downcased, but otherwise unchanged, e.g.

    =head1 A Multi-Word Heading (MWH)

automatically gets assigned the label

    a multi-word heading (mwh)

The labels of =items are prefixed by item_, spaces are replaced by underscores, and non-alphanumeric characters are replaced by their hexadecimal ASCII code prefixed by a percent sign. Anybody expected an easy rule? So, one of the items in this article,

    =item Automatic Reference Generation.

has the label


because the ASCII number of the period is 46 in decimal or 2e in hexadecimal.



    =head1 Introduction
    Section L<"concepts"> introduces the basics of the field.
    =head1 Concepts
    =head1 Synchronization
    =over 4
    =item Deadlocks
    =item Race Conditions
    =item Recovering from Deadlocks
    How to cope with deadlocks was already discussed in
    L<Deadlocks|"item_Deadlocks">, and L<Recovering from



Section concepts introduces the basics of the field.




Race Conditions
Recovering from Deadlocks

How to cope with deadlocks was discussed in Deadlocks, and Recovering from Deadlocks.

The L-command is very limited in its use, for the writer cannot insert places to refer to with an L-command; HTML-like ``anchors'' are missing.

A second limiting factor are some POD translators trying to be smart and decorate link with additional text. For example, pod2latex mangles both references to items in the above example:

How to cope with deadlocks was discussed in the \textsf{Deadlocks$|$"item\_Deadlocks"} entry elsewhere in this document, and the \textsf{Recovering from Deadlocks$|$"item\_Recovering\_from\_Deadlocks"} entry elsewhere in this document.

where I have underlined the words added by pod2latex. Clearly, we want a better mechanism. The mechanism exists in format-specific paragraphs.

Format-Specific Paragraphs

We have just seen that the L-command is somewhat difficult to control. Why can't we simply use a HTML-reference? The terse answer, ``because POD is not HTML'', leads to the solution. If we had a way to say ``this text is for HTML, this line is for LaTeX, and this paragraph is for ''SnaFoo``, we could use the specific markup provided by these formats.

The special command

=for format paragraph_of_text

tells a translator to look at format before processing paragraph_of_text. If the translator feels responsible for handling format, it transforms paragraph_of_text according to its own rules, otherwise it completely ignores the paragraph. The second part of the translator's name usually specifies which format it takes care of. For example, pod2man transforms =for man paragraphs, pod2html processes =for html paragraphs, and so on.

As all command paragraphs, a =for format paragraph ends at the first completly empty line that follows the introducing =for.

A consistent document structure will show ``forks'' whenever specific formats are used, because a =for format clause ought to appear for each desired output format, otherwise we punch a logical holes into the document.

    This is an ordinary paragraph, which is processed by all
    =for html <p>This paragraph only appears if the file is processed
    with <b>pod2html</b>.</p>
    =for latex This very paragraph is only treated by {\bf pod2latex}.
    =for text I am a paragraph for the *pod2text* formatter.
    We now continue with the ordinary text for all formatters.

The translators ignore unknown formats, which means we can invent special paragraphs for our own purposes! For example, to ``comment out'' a paragraph, write

    =for comment Can someone clarify the next section?

Another popular use is the emacs format :-) To switch emacs into text-mode when preparing a POD-file, start the file with

    =for emacs -*- text -*-

or end it with

    =for emacs
    Local Variables:
    mode: text

The emacs-users who are using the hyperbole add-on can convert their "dumb" POD-files into hyper-linked collections (well -- hyperbole can do a lot more than that, but hyperlinks are a beginning) of files with

    =for hyperbole <(std-reference)>

where <(std-reference)> is a hyperbole button taking you to another file which holds the reference documentation of std when you click the button in emacs.

Programs That Work With POD

Pros And Cons of POD


Further Reading

Manual pages of perlpod(1), pod2man(1), pod2html(1), pod2latex(1), pod2text(1), and podchecker(1).

Next month: LaTeX in conjuction with latex2html.

Christoph Spiel

Chris runs an Open Source Software consulting company in Upper Bavaria/Germany. Despite being trained as a physicist -- he holds a PhD in physics from Munich University of Technology -- his main interests revolve around numerics, heterogenous programming environments, and software engineering. He can be reached at

Copyright © 2001, Christoph Spiel.
Copying license
Published in Issue 73 of Linux Gazette, December 2001

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