Application binary interface (ABI)

As you may already know, every library or framework, regardless of the technologies or the programming language used, exposes a set of certain functionalities, which is known as its Application Programming Interface (API). If a library is supposed to be used by another code, then the consumer code should use the provided API. To be clear, nothing other than the API should be used in order to use a library because it is the public interface of the library and everything else is seen as a black box, hence cannot be used.

Now suppose after some time, the library's API undergoes some modifications. In order for the consumer code to continue using the newer versions of the library, the code must adapt itself to the new API; otherwise, it won't be able to use it anymore. The consumer code could stick to a certain version of the library (maybe an old one) and ignore the newer versions, but let's assume that there is a desire to upgrade...