C - Generating Documentation with Doxygen for C Projects

Documentation is an essential part of software development because it explains how a program works, how functions should be used, and how different parts of the code interact. In C programming, manually creating documentation can be time-consuming and difficult to maintain as the project grows. Doxygen is a documentation generation tool that automatically creates well-structured documentation from specially formatted comments written inside the source code. It helps developers produce professional documentation without maintaining separate documents.

Doxygen supports C, C++, Java, Python, and several other programming languages. For C projects, it reads source files, extracts comments written in a specific format, and generates documentation in HTML, PDF, LaTeX, XML, and other formats. This allows developers to maintain documentation alongside the code, ensuring that it stays updated whenever the source code changes.

Why Documentation Matters

As software projects become larger, multiple developers work on the same codebase. Without proper documentation, understanding the purpose of functions, variables, structures, and modules becomes difficult. Good documentation provides several benefits:

  • Makes code easier to understand.

  • Reduces the learning curve for new developers.

  • Simplifies maintenance and debugging.

  • Helps users understand how to use APIs.

  • Improves collaboration among development teams.

  • Serves as a reference for future enhancements.

Instead of maintaining separate documentation files, Doxygen keeps documentation close to the actual source code.

What is Doxygen?

Doxygen is an open-source documentation generator. It scans source files, recognizes specially formatted comments, and creates organized documentation automatically.

It can document:

  • Functions

  • Variables

  • Structures

  • Enumerations

  • Macros

  • Typedefs

  • Global variables

  • Header files

  • Source files

  • Entire modules

It also generates diagrams showing relationships between different components when Graphviz is installed.

Installing Doxygen

Doxygen is available for Windows, Linux, and macOS.

Windows

Download the installer from the official Doxygen website and complete the installation using the setup wizard.

Linux

Use the package manager.

Ubuntu:

sudo apt install doxygen

Fedora:

sudo dnf install doxygen

Arch Linux:

sudo pacman -S doxygen

macOS

Using Homebrew:

brew install doxygen

Verify installation:

doxygen --version

Creating a Doxygen Configuration File

Every project typically contains a configuration file called Doxyfile.

Generate it using:

doxygen -g

This creates a file named:

Doxyfile

The configuration file contains various settings such as:

  • Project name

  • Version number

  • Source directory

  • Output directory

  • HTML generation

  • PDF generation

  • Recursive folder scanning

  • File patterns

Example:

PROJECT_NAME = Student Management System

OUTPUT_DIRECTORY = docs

INPUT = src include

RECURSIVE = YES

GENERATE_HTML = YES

GENERATE_LATEX = YES

Writing Documentation Comments

Doxygen recognizes specially formatted comments.

A simple documentation comment looks like this:

/**
 * Adds two integers.
 */

More detailed comments use tags.

Example:

/**
 * @brief Adds two numbers.
 *
 * This function calculates the sum
 * of two integer values.
 *
 * @param a First number
 * @param b Second number
 *
 * @return Sum of both numbers
 */
int add(int a, int b)
{
    return a + b;
}

Doxygen extracts these comments and includes them in the generated documentation.

Common Doxygen Tags

Several tags help organize documentation.

@brief

Provides a short description.

/**
 * @brief Calculates area.
 */

@param

Explains function parameters.

@param radius Radius of circle

@return

Describes the return value.

@return Area of circle

@author

Specifies the author's name.

@author John

@version

Shows version information.

@version 1.0

@date

Mentions the creation date.

@date July 2026

@note

Provides additional information.

@note Radius should be positive.

@warning

Highlights important warnings.

@warning Do not pass NULL pointer.

@see

Creates references to related functions.

@see calculateVolume()

Documenting Header Files

Header files contain declarations that other programs use.

Example:

/**
 * @file math_utils.h
 * @brief Mathematical utility functions.
 */

#ifndef MATH_UTILS_H
#define MATH_UTILS_H

int add(int a, int b);

int subtract(int a, int b);

#endif

This helps users understand the purpose of each header file.

Documenting Structures

Structures should also include descriptions.

Example:

/**
 * @brief Stores student information.
 */
struct Student
{
    int id;
    char name[50];
    float marks;
};

Each member can also be documented.

/**
 * @brief Student information.
 */
struct Student
{
    /** Student ID */
    int id;

    /** Student name */
    char name[50];

    /** Percentage marks */
    float marks;
};

Documenting Enumerations

Example:

/**
 * @brief Traffic light states.
 */
enum TrafficLight
{
    RED,
    YELLOW,
    GREEN
};

This explains the purpose of each enumeration.

Documenting Macros

Example:

/**
 * Maximum number of students.
 */
#define MAX_STUDENTS 100

Documentation generated will explain the macro.

Documenting Global Variables

Example:

/**
 * Stores total number of students.
 */
int totalStudents;

Global variables should always include their purpose.

Grouping Related Functions

Doxygen supports grouping multiple functions into modules.

Example:

/**
 * @defgroup MathFunctions Mathematical Functions
 *
 * Collection of arithmetic operations.
 *
 * @{
 */

int add(int a,int b);

int subtract(int a,int b);

/** @} */

Generated documentation displays these functions together under one category.

Running Doxygen

After writing comments, execute:

doxygen Doxyfile

Doxygen scans the project and generates documentation.

Typical output folders include:

docs/

html/

latex/

The HTML folder contains the website version of the documentation.

Viewing HTML Documentation

Open:

docs/html/index.html

or

html/index.html

The documentation appears in a web browser with sections such as:

  • Main Page

  • Files

  • Functions

  • Structures

  • Enumerations

  • Macros

  • Variables

  • Modules

Users can navigate easily using hyperlinks.

Generating PDF Documentation

Enable LaTeX generation in the configuration file.

GENERATE_LATEX = YES

After running Doxygen:

cd latex

make

A PDF document is generated containing all project documentation.

Generating Call Graphs

When Graphviz is installed, Doxygen can generate diagrams.

Configuration:

HAVE_DOT = YES

CALL_GRAPH = YES

CALLER_GRAPH = YES

These diagrams illustrate:

  • Which functions call other functions.

  • Function dependencies.

  • Overall program flow.

Such visualizations make it easier to understand complex codebases.

Best Practices for Using Doxygen

  • Write documentation as you develop the code rather than after completing the project.

  • Keep descriptions concise but informative.

  • Document every public function and header file.

  • Explain parameters and return values clearly.

  • Update comments whenever code changes.

  • Avoid documenting obvious implementation details; focus on behavior and purpose.

  • Use consistent formatting and tag usage across the project.

  • Group related functions into logical modules.

  • Review generated documentation periodically to ensure accuracy.

Advantages of Using Doxygen

  • Automatically generates professional documentation.

  • Reduces manual documentation effort.

  • Improves code readability and maintainability.

  • Keeps documentation synchronized with the source code.

  • Produces searchable HTML documentation.

  • Supports multiple output formats, including HTML and PDF.

  • Generates diagrams that clarify relationships between components.

  • Helps developers onboard more quickly to existing projects.

  • Facilitates API documentation for libraries and reusable modules.

  • Encourages consistent documentation practices across teams.

Limitations of Doxygen

  • Documentation quality depends on the quality of comments provided by developers.

  • Initial setup and configuration may require some effort.

  • Large projects can take longer to process.

  • Some advanced formatting features require familiarity with Doxygen syntax.

  • Diagram generation depends on additional tools such as Graphviz.

Conclusion

Doxygen is a valuable tool for documenting C projects because it transforms source code comments into organized, professional documentation. By documenting functions, structures, macros, and modules directly within the code, developers ensure that documentation remains accurate and easy to maintain. Its ability to generate HTML, PDF, and graphical representations of code relationships makes it especially useful for individual programmers, educational projects, open-source software, and large development teams. Integrating Doxygen into the development workflow promotes better code quality, simplifies maintenance, and enhances collaboration by providing clear and up-to-date technical documentation.