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A file which contains downloadable binary data for use by the hardware is not encumbered, unless (1) or (2) apply to it. It must be stored in an architecture neutral ASCII format (file2c or uuencoding is recommended).
All new libraries should either be symvers or private unless there's some compelling reason to deviate.
Any changes to major libraries must still allow build from source upgrades to succeed
Any encumbered file requires specific approval from the link:https://www.FreeBSD.org/administration/#t-core[Core Team] before it is added to the repository.
Any file which is interpreted or executed by the system CPU(s) and not in source format is encumbered.
Any file with a license more restrictive than BSD or GNU is encumbered.
Avoid gratuitous rearrangement of structures. This will keep compatibility symbols to a minimum.
Because it makes it harder to import future versions minor, trivial and/or cosmetic changes are _strongly discouraged_ on files that are still tracking the vendor branch.
Carefully consider the impact of a major version bump. When a major version change cannot be avoided, add a version map to avoid future bumps.
Changes to directories which have a maintainer defined shall be sent to the maintainer for review before being committed. Only if the maintainer does not respond for an unacceptable period of time, to several emails, will it be acceptable to commit changes without review by the maintainer. However, it is suggested that you try to have the changes reviewed by someone else if at all possible.
Changes to function signatures can be implemented by creating compatibility versions of the function (often calling the new version of the function in a specific way).
Changes to structures can be worked around by providing the old structure and old functions to access it (if the layout of the structure is embedded in its clients).
Changing the ABI is allowed, otherwise FreeBSD cannot evolve. However, the project has generally chosen to pay the added cost of retaining ABI backward-compatibility.
Chapter 5. Source Tree Guidelines and Policies
Consistent coding style is extremely important, particularly with large projects like FreeBSD. Code should follow the FreeBSD coding styles described in man:style[9] and man:style.Makefile[5].
Contributed Software
Create new symbols in the symbol map for otherwise incompatible changes.
Encumbered Files
Encumbered files go in [.filename]#src/contrib# or [.filename]#src/sys/contrib#.
Ensure that the compatibility ports / packages are updated as appropriate