ABI and API
# API and ABI
One easy way to understand "ABI" is to compare it to "API".
You are already familiar with the concept of an API. If you want to use the features of, say, some library or your OS, you will use an API. The API consists of data types/structures, constants, functions, etc that you can use in your code to access the functionality of that external component.
An ABI is very similar. Think of it as the compiled version of an API (or as an API on the machine-language level). When you write source code, you access the library though an API. Once the code is compiled, your application accesses the binary data in the library through the ABI. The ABI defines the structures and methods that your compiled application will use to access the external library (just like the API did), only on a lower level.
ABIs are important when it comes to applications that use external libraries. If a program is built to use a particular library and that library is later updated, you don't want to have to re-compile that application (and from the end-user's standpoint, you may not have the source). If the updated library uses the same ABI, then your program will not need to change. The interface to the library (which is all your program really cares about) is the same even though the internal workings may have changed. Two versions of a library that have the same ABI are sometimes called "binary-compatible" since they have the same low-level interface (you should be able to replace the old version with the new one and not have any major problems).