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A useful source-file comment header starts by telling readers what the file does. Add only the context maintainers need—such as authorship, first-release date, revision notes, and licensing—and keep it accurate as the code changes. A header is documentation, not a place for boilerplate to hide the module’s purpose.
What should a source-file header contain?
Jack G. Ganssle’s 2016 article, “On Comment Headers”, recommends a compact set of information that helps someone understand a module and its history:
- A brief description: Put the module’s purpose in the first meaningful line. A reader scanning the file should not have to pass through legal language or unrelated prose to learn what it does.
- A fuller description: Explain the module’s role, important behavior, and relevant usage context. Include only information that helps a maintainer understand or use the file.
- Authorship and release context: Include the author’s name and the date of first release when those details are useful to the project.
- Revision notes: If the project tracks changes in headers, identify the developer, date, and description for each revision. Keep this concise; a long change history may belong in version control instead.
- Licensing: Add a short licensing note where appropriate. If a required notice is lengthy, preserve it as required but place the module description where it remains easy to find.
The right amount of detail depends on the file. A short header is not automatically better: it is useful only if it gives readers enough context to understand the module without reverse-engineering the implementation.
How should the header be organized?
Put purpose first, then supporting context. This order serves both a reader opening one file and someone scanning a directory or search result. Separate the brief summary from fuller detail so the first line can act as a clear headline.
#1 Best Overall
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Ganssle prefers block comments such as /* ... */ over a series of repeated // lines because block comments are easier to expand and reflow as prose. That is a style preference, not a universal rule: use the convention that fits the language and project. Readability and accuracy matter more than the delimiter.
Doxygen can help structure a summary followed by fuller documentation, making the first line function as a headline. Whatever format a project uses, avoid sales language and unexplained boilerplate that obscures what the module actually does.
Rank #2
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How much documentation is too much?
A header becomes counterproductive when it is so long that the useful description is hard to find, repeats information maintained elsewhere, or creates another history that can drift out of date. Ganssle’s examples illustrate distinct risks: Linux headers where licensing text can bury a module description, FreeRTOS openings that read more like sales copy than practical documentation, and a safety-critical project where duplicated headers described the wrong modules.
These examples point to a practical test: can a maintainer quickly identify the file’s purpose, and does every claim in the header match the code? Move extensive explanation to external documentation when the header would otherwise become disproportionately long. Keep legally required notices where the project requires them, but do not let them replace a clear module summary.
Rank #3
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How should teams keep headers accurate?
Treat the header as part of the source file’s maintenance, not as text to copy unchanged from another module. Update it when the module’s role or behavior changes, and review it when reorganizing or copying files. A stale description can be worse than no description because it leads maintainers toward the wrong understanding.
- Write the purpose specifically enough to distinguish this module from neighboring files.
- Check that the fuller description reflects current behavior and usage.
- Remove copied names, dates, and revision notes that no longer apply.
- Keep detailed design explanations and extensive histories in documentation or version control when they do not belong in the header.
Ganssle captures the long-term value this way: “The comments are a love letter to yourself and your successors.”
Quick Recap
Best Value
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Rank #4
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