7.3 Updating library version information

If you want to use libtool’s versioning system, then you must specify the version information to libtool using the ‘-version-info’ flag during link mode (see section Link mode).

This flag accepts an argument of the form ‘current[:revision[:age]]’. So, passing ‘-version-info 3:12:1’ sets current to 3, revision to 12, and age to 1.

If either revision or age are omitted, they default to 0. Also note that age must be less than or equal to the current interface number.

Here are a set of rules to help you update your library version information:

1. Start with version information of ‘0:0:0’ for each libtool library.
2. Update the version information only immediately before a public release of your software. More frequent updates are unnecessary, and only guarantee that the current interface number gets larger faster.
3. If the library source code has changed at all since the last update, then increment revision (‘c:r:a’ becomes ‘c:r+1:a’).
4. If any interfaces have been added, removed, or changed since the last update, increment current, and set revision to 0.
5. If any interfaces have been added since the last public release, then increment age.
6. If any interfaces have been removed or changed since the last public release, then set age to 0.

Never try to set the interface numbers so that they correspond to the release number of your package. This is an abuse that only fosters misunderstanding of the purpose of library versions. Instead, use the ‘-release’ flag (see section Managing release information), but be warned that every release of your package will not be binary compatible with any other release.

The following explanation may help to understand the above rules a bit better: consider that there are three possible kinds of reactions from users of your library to changes in a shared library:

1. Programs using the previous version may use the new version as drop-in replacement, and programs using the new version can also work with the previous one. In other words, no recompiling nor relinking is needed. In this case, bump revision only, don’t touch current nor age.
2. Programs using the previous version may use the new version as drop-in replacement, but programs using the new version may use APIs not present in the previous one. In other words, a program linking against the new version may fail with “unresolved symbols” if linking against the old version at runtime: set revision to 0, bump current and age.
3. Programs may need to be changed, recompiled, relinked in order to use the new version. Bump current, set revision and age to 0.

In the above description, programs using the library in question may also be replaced by other libraries using it.

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