This document describes under which circumstances API and ABI incompatiblities may occur. As developer from elektra your mission is to avoid that. The tool icheck against the interfaces mentioned above may help you too.
In 0.8.* the API and ABI must be always forward-compatible, but not backwards-compatible. That means that a program written and compiled against 0.8.0 compiles and links against 0.8.1. But because it is not necessarily backendwards-compatible a program written for 0.8.1 may not link or compile against elektra 0.8.0 (but it may do when you use the compatible subset, maybe with #ifdefs).
Following points are allowed: When you add a new function you break ABI and API backward- compatibilty, but not forward, so you are allowed to do so.
In the signature you are only allowed to add const to any parameter. You are not allowed to use subtypes to the objects, in C means you are not allowed to call any functions of an object which appear new. C does not typecheck that, its your responsibility.
What C also does not check are the pre and postconditions. That means you are not allowed to demand more client code (e.g. first accept a NULL pointer and in the next version you crash on it) and you are not allowed to return values that the previous version did not return. It is a complex topic, so better don't underestimate it, but generally said the methods should behave on the same data the same way.
This document describes how to increment the KDB_VERSION. KDB_VERSION consists of a triplet integer current:revision:age.
Revision is something which will always incremented when there is a new bugfix release.
current and age will be incremented by one when you release a compatible but changed API. Revision is set back to zero then.
Note: All 3 Versioning infos are handled separately!
TODO write about SO_VERSION