diff --git a/documentation/content/en/books/developers-handbook/policies/chapter.adoc b/documentation/content/en/books/developers-handbook/policies/chapter.adoc --- a/documentation/content/en/books/developers-handbook/policies/chapter.adoc +++ b/documentation/content/en/books/developers-handbook/policies/chapter.adoc @@ -323,21 +323,22 @@ If you are adding shared library support to a port or other piece of software that does not have one, the version numbers should follow these rules. Generally, the resulting numbers will have nothing to do with the release version of the software. -The three principles of shared library building are: +FreeBSD tries to maintain ABI stability. Most libraries in the system use symbol versioning to remain compatible with old versions of the library. When updating libraries with versioned symbols: -* Start from `1.0` -* If there is a change that is backwards compatible, bump minor number (note that ELF systems ignore the minor number) -* If there is an incompatible change, bump major number +* Create new symbols in the symbol map for otherwise incompatible changes. +* When deleting old functions, only remove visibility of symbols from newer versions (that is, do not delete functions). +* Avoid gratuitous rearrangement of structures. This will keep compatibility symbols to a minimum. +* If changes require a version bump, re-evaluate whether they are needed at all. -For instance, added functions and bugfixes result in the minor version number being bumped, while deleted functions, changed function call syntax, etc. will force the major version number to change. +The principles of shared library major numbers are: -Stick to version numbers of the form major.minor (`_x_._y_`). Our a.out dynamic linker does not handle version numbers of the form `_x_._y_._z_` well. Any version number after the `_y_` (i.e., the third digit) is totally ignored when comparing shared lib version numbers to decide which library to link with. Given two shared libraries that differ only in the "micro" revision, `ld.so` will link with the higher one. That is, if you link with [.filename]#libfoo.so.3.3.3#, the linker only records `3.3` in the headers, and will link with anything starting with `_libfoo.so.3_._(anything >= 3)_._(highest available)_`. +* Start from `1`. +* Create a version map for all exported symbols in new libraries. +* For non-symvers libraries, avoid changes that would require a version bump. +* For symvers libraries, changes requiring a version bump shouldn't be done. +** 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). +** Changes to function signatures often can be implemented by creating compatibility versions of the function (often calling the new version of the function in a specific way). +* 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. +* For libraries where the ABI cannot be guaranteed, make then FreeBSD private where possible. Third party libraries that have poor ABI guarantees are generally made private, though there are a few exceptions for historical reasons. -[NOTE] -==== -`ld.so` will always use the highest "minor" revision. For instance, it will use [.filename]#libc.so.2.2# in preference to [.filename]#libc.so.2.0#, even if the program was initially linked with [.filename]#libc.so.2.0#. -==== - -In addition, our ELF dynamic linker does not handle minor version numbers at all. However, one should still specify a major and minor version number as our [.filename]#Makefile#'s "do the right thing" based on the type of system. - -For non-port libraries, it is also our policy to change the shared library version number only once between releases. In addition, it is our policy to change the major shared library version number only once between major OS releases (i.e., from 6.0 to 7.0). When you make a change to a system library that requires the version number to be bumped, check the [.filename]#Makefile#'s commit logs. It is the responsibility of the committer to ensure that the first such change since the release will result in the shared library version number in the [.filename]#Makefile# to be updated, and any subsequent changes will not. +For non-ports libraries, it is our strong policy not to change the ABI of dynamicly linked libararies. Every reasonable effort should be made to make the changes in a binary compatible manner. When that cannot be avoided, it is also our policy to change the shared library version number at most once between major releases. In addition, it is our policy to change the major shared library version number only once between major OS releases (i.e., from 13.0 to 14.0). When you make a change to a system library that absolutely requires the version number to be bumped, check the [.filename]#Makefile#'s commit logs. It is the responsibility of the committer to ensure that the first such change since the release will result in the shared library version number in the [.filename]#Makefile# to be updated, and any subsequent changes will not. 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.