Contributed by Satoshi Asami, Peter
Wemm, and David O'Brien.
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:
-
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
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.
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 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).
Note: ld.so will always use the highest ``minor''
revision. For instance, it will use libc.so.2.2 in preference
to libc.so.2.0, even if the program was initially linked with
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 Makefiles ``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 3.0 to 4.0).
When you make a change to a system library that requires the version number to be bumped,
check the 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 Makefile to be updated,
and any subsequent changes will not.
