Best Tips for documenting code using doxygen?

Question

My team is starting to document our C code using doxygen, paying particular attention to our public API headers. There appears to be a lot of flexibility and different special commands in doxygen, which is great, but it's not clear what's a good thing and what's a bad thing without trial and error.

What are your favourite ways to mark up your code, what are your MUST DOs and DO NOTs?
Please provide your top tips, one per answer to facilitate voting.

I am looking to define our whole approach to API documentation, including providing a template to get the rest of the team started. So far I have something like this:

/**
 * @file   example_action.h
 * @Author Me (me@example.com)
 * @date   September, 2008
 * @brief  Brief description of file.
 *
 * Detailed description of file.
 */

/**
 * @name    Example API Actions
 * @brief   Example actions available.
 * @ingroup example
 *
 * This API provides certain actions as an example.
 *
 * @param [in] repeat  Number of times to do nothing.
 *
 * @retval TRUE   Successfully did nothing.
 * @retval FALSE  Oops, did something.
 *
 * Example Usage:
 * @code
 *    example_nada(3); // Do nothing 3 times.
 * @endcode
 */
boolean example(int repeat);

Answer 1

You don't need and should not write the name of the file in the @file directive, doxygen reads the name of the file automatically. The problem with writing the name of the file is that when you rename the file you will have to change the @file directive as well.

Providing @author and @date information is also useless most of the time since the source control system does it a lot better than someone editing the files manually.

You also don't have to write @brief if you use the following Doxygen syntax and enable JAVADOC_AUTOBRIEF in doxygen's configuration:

/*! Short Description on the first line

    Detailed description...
    ...
*/
void foo(void) {}

The @name directive for functions is also 100% redundant most of the time and completely useless. It only brings errors when someone modifies the name of the function and not the doxygen @name.

Answer 2

Write a descriptive home page using @mainpage (in a separate header file just for this purpose). Consider, as shown in my example, making it a guide to your main classes/functions and modules.

Another Sample

Whilst I was getting the above-linked main oofile doxygen content back online, here's an example from some current client work using Markdown format. Using Markdown you can refer to a mainpage in markdown (in the Doxygen settings) which is great for the typical readme.md file included in open-source projects.

Lingopal
========
Developer Documentation started when Andy Dent took over support in May 2014. 

There are a number of pages in Markdown format which explain key aspects:

- @ref doc/LingopalBuilding.md
- @ref doc/LingopalSigning.md
- @ref doc/LingopalDatabases.md
- @ref doc/LingopalExternals.md

See the <a href="pages.html">Related Pages list for more.</a>

-------------

_Note_

These pages, whilst readable by themselves, are designed to be run through the [Doxygen](http://www.doxygen.com) code documentation engine which builds an entire local cross-referenced set of docs. It uses a minor [extension of Markdown formatting.](http://www.stack.nl/~dimitri/doxygen/manual/markdown.html)

The settings to generate the documentation are `Lingopal.doxy` and `LingopalDocOnly.doxy`. The latter is used for quick re-generation of just these additional pages.

Answer 3

Use Groups to organise your code into modules.

Remember that you can put almost everything into multiple groups so they can be used to provide semantic tagging like the tags in Stack Overflow. For example, you might tag things as specific to a given platform.

You can also use groups to match a folder hierarchy within an IDE, as shown in my RB2Doxy sample output.

Groups work well when nested - I have a large example for the OOFILE source.

Answer 4

Some commands i use in my code :

• \todo { paragraph describing what is to be done } Useful to keep track of todos, a page will be created in final documentation containing your todo list.
• \c <word> Displays the argument using a typewriter font. Use this to refer to a word of code. I would use it before "TRUE" and "FALSE" in your example.
• \a , \warning , \see : see http://www.stack.nl/~dimitri/doxygen/commands.html#cmdc for description

Answer 5

A good "best practice" (though not always achievable) is to provide short, working examples for every API, and pull them into the help using \includelineno (or \include for no line numbers). These can be unit tests, if they're written so users can understand them (ie, not hooked into a larger test harness). As a nice side effect, changes to the API will break the samples, so they have to be kept up to date.

You can describe an API in words, but there's nothing like seeing the actual code to understand how to use it.

Answer 6

As I find myself editing code on higher-resolution screens I've moved from using the backslash to the @ prefix on Doxygen commands. Not so noisy backslash has found itself now too damned hard to make out the Doxygen commands.

Answer 7

If you are sure your team will follow such a heavyweight template, fine, use it as shown.

Otherwise, it looks like JavaDoc. One of the nice things about Doxygen is how good a job it does without having to use use such strong markup. You don't need to use @name and with the JAVADOC_AUTOBRIEF setting you can skip @brief - just make sure the first line of the comment is a reasonable brief description.

I prefer descriptive names over enforcing documentation and encouraging people to add comments only when they add significant value. That way, the valuable comments aren't drowned out by all the noise.

Answer 8

If you have bugs located in the code or you find bugs you can also tag in the code like this:

/** @bug The text explaining the bug */

When you then run doxygen you get a seperate Bug List alongside lists like Todo List

Answer 9

If you have a really, really big project -- big enough that Doxygen runs take over an hour -- you can cut it up into multiple modules that Doxygen later links together using tag files.

For example, if you have a big MSVC solution with twenty projects in it, you can make directory be its own Doxygen run, and then use tag-files to glue together the output the same way a linker glues together .libs to make an executable.

You can even take the linking metaphor more literally and make each Doxy config file correspond to a .vcproj file, so that each project (eg .lib or .dll) gets its own Doxy output.

Answer 10

I use a subversion post-commit hook to pull out the directories that have changed, write them to a file and then every night I automatically re-generate the doxygen html on our webserver so we always have up-to-date docco.

Every project I want documented has a little project.doxy file that contains the per-project settings and an include to the main doxygen settings - eg:

PROJECT_NAME           = "AlertServer"
PROJECT_NUMBER         = 8.1.2
INPUT                  = "C:/Dev/src/8.1.2/Common/AlertServer"
HTML_OUTPUT            = "AlertServer"
@INCLUDE = "c:\dev\CommonConfig.doxy"

For Windows SVN server, use the hook:

@echo off
for /F "eol=¬ delims=¬" %%A in ('svnlook dirs-changed %1 -r %2') do echo %%A >> c:\svn_exports\export.txt

and then run this nightly:

@echo off

rem ---------------
rem remove duplicates.
type nul> %TEMP%.\TEMP.txt

for /F "eol=¬ delims=¬" %%a in (c:\svn_exports\export.txt) do (
 findstr /L /C:"%%a" < %TEMP%.\TEMP.txt > nul
 if errorlevel=1 echo %%a>> %TEMP%.\TEMP.txt
)

copy /y %TEMP%.\TEMP.txt export_uniq.cmd >nul
if exist %TEMP%.\TEMP.txt del %TEMP%.\TEMP.txt


rem ---------------
rem fetch all the recently changed directories into the svn_exports directory

for /F "eol=¬ delims=¬" %%A in (c:\svn_exports\export_uniq.cmd) do (
  svn export "file:///d:/repos/MyRepo/%%A" "c:/svn_exports/%%A"  --force 
)


rem ---------------
rem search through all dirs for any config files, if found run doxygen

for /R c:\svn_exports %%i in (*.doxy) do c:\tools\doxygen\bin\doxygen.exe "%i"


rem ---------------
rem now remove the directories to be generated.
del /F c:\svn_exports

this removes duplicate entries, finds all projects that have a .doxy project file, and runs doxygen on them. Voila: fully documented, always up-to-date code on a webserver.

Answer 11

For complex projects it may be useful to have a separate file for module management, which controls the groups and subgroups. The whole hierarchy can be in one place and then each file can simply stuff to the child groups. e.g.:

/**
 * @defgroup example Top Level Example Group
 * @brief    The Example module.
 *
 * @{
 */

/**
 * @defgroup example_child1 First Child of Example
 * @brief    1st of 2 example children.
 */

/**
 * @defgroup example_child2 Second Child of Example
 * @brief    2nd of 2 example children.
 */

// @}

Simply including the definition of a group within the { } of another group makes it a child of that group. Then in the code and header files functions can just be tagged as part of whatever group they are in and it all just works in the finished documentation. It makes refactoring the documentation to match the refactor code much easier.

Answer 12

Uses lots and lots of links. This can be done using see also links (\see or @see if you prefer), and making sure that you use any references to other class names in documentation by their correct class name. For example if you refer to class FUZZYObject as an "object", then write immediately after it the name of the class (e.g. "frazzle the objects (FUZZYObject)").

Answer 13

Automatically build and publish your documentation. As part of automatically building the documentation, pay attention to the warnings, its very easy to write badly structure doxygen comments.

Answer 14

Use \example as much as you can. It auto-links API elements to example code.

Answer 15

Don't bother with @author or @date (@date was mentioned in another post). These are both handled by a revision control system.

Answer 16

Always include a description with your classes. Try to say how a class is used, or why it is used, not just what it is (which usually just reflects the name anyway).

Answer 17

Group your member functions and fields if it makes sense to do so with \defgroup. This is very helpful, even if you don't say much.

Answer 18

If you are worried that some team members will avoid documenting or you just want a working minimal sets of documentation, you can enable these in your doxygen configuration.

WARNINGS               = YES
WARN_IF_UNDOCUMENTED   = YES
WARN_IF_DOC_ERROR      = YES

As part of your doxygen build process save the warnings to a file and try to get and keep the warning count as low as possible (0 if that is reasonable). If you do this, every public and protected class member will need at least an @brief, @param for each function argument and an @return. This is good enough to describe most APIs and not too much to encumber other living codebases.

You should, of course, encourage people to document as much as they feel is required on a case by case basis, as long as they meet the minimum project standards. Don't set the minimum too high though, then you may not get useful documentation in the end.

For example, in our project, everything another coder is likely to touch should be documented. Enabling the warnings let see how close that goal we are. We also try to use @internal to describe what/why we do what we do with some of our private members.

Answer 19

If you find that the configuration directive INLINE_SOURCES puts too much code in the documentation, you can manually quote specific portions of the code using the \snippet command.

  /**
   * Requirment XYZ is implemented by the following code.
   * 
   * \snippet file.c CODE_LABEL
   */
  int D() 
  {
     //[CODE_LABEL]
     if( A )
     {
        B= C();
     }
     //[CODE_LABEL]
  }

note: snippet gets its files from the EXAMPLE_PATH, not where the source path is. You will have to put the same list of files and paths from INPUT directive on the EXAMPLE_PATH directive.

Answer 20

For larger projects taking 5+min to generate, I found it useful to quicly be able to generate doxygen for a single file and view it in a web browser.

While references to anything outside the file won't resolve, it can still useful to see the basic formatting is ok.

This script takes a single file and the projects doxy config and runs doxygen, I've set this up to run from my IDE.

#!/usr/bin/env python3
"""
This script takes 2-3 args: [--browse] <Doxyfile> <sourcefile>

  --browse will open the resulting docs in a web browser.
"""
import sys
import os
import subprocess
import tempfile

doxyfile, sourcefile = sys.argv[-2:]

tempfile = tempfile.NamedTemporaryFile(mode='w+b')
doxyfile_tmp = tempfile.name
tempfile.write(open(doxyfile, "r+b").read())
tempfile.write(b'\n\n')
tempfile.write(b'INPUT=' + os.fsencode(sourcefile) + b'\n')
tempfile.flush()

subprocess.call(("doxygen", doxyfile_tmp))
del tempfile

# Maybe handy, but also annoying as default.
if "--browse" in sys.argv:
    import webbrowser
    webbrowser.open("html/files.html")