# Getting Involved with the Clang Project

Once you have checked out and built clang and played around with it, you might be wondering what you can do to make it better and contribute to its development. Alternatively, maybe you just want to follow the development of the project to see it progress.

## Follow what's going on

Clang is a subproject of the LLVM Project, but has its own mailing lists because the communities have people with different interests. The two clang lists are:

- cfe-commits - This list is for patch submission/discussion.
- cfe-dev - This list is for everything else Clang related (questions and answers, design discussions, etc).

If you are interested in clang only, these two lists should be all you need. If you are interested in the LLVM optimizer and code generator, please consider signing up for llvmdev and llvm-commits as well.

The best way to talk with other developers on the project is through the cfe-dev mailing list. The clang mailing list is a very friendly place and we welcome newcomers. In addition to the cfe-dev list, a significant amount of design discussion takes place on the cfe-commits mailing list. All of these lists have archives, so you can browse through previous discussions or follow the list development on the web if you prefer.

You can also follow the Planet Clang community news feed which offers a window into the world, work and lives of Clang developers, contributors and the standards they implement.

If you're looking for something to work on, check out our Open Projects page or look through the Bugzilla bug database.

## Contributing Extensions to Clang

Clang has always been designed as a platform for experimentation, allowing programmers to easily extend the compiler to support great new language features and tools. At some point, the authors of these extensions may propose that the extensions become a part of Clang itself, to benefit the whole Clang community. But not every idea--not even every great idea--should become part of Clang. Extensions (particularly language extensions) pose a long-term maintenance burden on Clang, and therefore the benefits of the extension must outweight those costs. Hence, these are the seven criteria used to evaluate the merits of a proposed extension:

1. Evidence of a significant user community: This is based on a number of factors, including an actual, existing user community, the perceived likelihood that users would adopt such a feature if it were available, and any "trickle-down" effects that come from, e.g., a library adopting the feature and providing benefits to its users.
2. A specific need to reside within the Clang tree: There are some extensions that would be better expressed as a separate tool, and should remain as separate tools even if they end up being hosted as part of the LLVM umbrella project.
3. A complete specification: The specification must be sufficient to understand the design of the feature as well as interpret the meaning of specific examples. The specification should be detailed enough that another compiler vendor could conceivably implement the feature.
4. Representation within the appropriate governing organization: For extensions to a language governed by a standards committee (C, C++, OpenCL), the extension itself must have an active proposal and proponent within that committee and have a reasonable chance of acceptance. Clang should drive the standard, not diverge from it. This criterion does not apply to all extensions, since some extensions fall outside of the realm of the standards bodies.
5. A long-term support plan: Contributing a non-trivial extension to Clang implies a commitment to supporting that extension, improving the implementation and specification as Clang evolves. The capacity of the contributor to make that commitment is as important as the commitment itself.
6. A high-quality implementation: The implementation must fit well into Clang's architecture, follow LLVM's coding conventions, and meet Clang's quality standards, including high-quality diagnostics and rich AST representations. This is particularly important for language extensions, because users will learn how those extensions work through the behavior of the compiler.
7. A proper test suite: Extensive testing is crucial to ensure that the language extension is not broken by ongoing maintenance in Clang. The test suite should be complete enough that another compiler vendor could conceivably validate their implementation of the feature against it.