The number of the current interface exported by the library. A current value of '1', means that you are calling the interface exported by this library interface 1.


The implementation number of the most recent interface exported by this library. In this case, a revision value of 0 means that this is the first implementation of the interface.

If the next release of this library exports the same interface, but has a different implementation (perhaps some bugs have been fixed), the revision number will be higher, but current number will be the same. In that case, when given a choice, the library with the highest revision will always be used by the runtime loader.


The number of previous additional interfaces supported by this library. If age were '2', then this library can be linked into executables which were built with a release of this library that exported the current interface number, current, or any of the previous two interfaces. By definition age must be less than or equal to current. At the outset, only the first ever interface is implemented, so age can only be `0'.

For every release of the library -version-info argument needs to be set correctly depending on any interface changes you have made.

This is quite straightforward when you understand what the three numbers mean:

If you have changed any of the sources for this library, the revision number must be incremented. This is a new revision of the current interface. If the interface has changed, then current must be incremented, and revision reset to '0'.

This is the first revision of a new interface. If the new interface is a superset of the previous interface (that is, if the previous interface has not been broken by the changes in this new release), then age must be incremented. This release is backwards compatible with the previous release.