Comparison – API and ABI

They are both interfaces.

Interface API ABI
Between whom? Application/Library and library Application/Library and platform
Defined where? Headers of the library Specifications of the platform
Contains what? Definitions of e.g.
– functions
– variables
– classes
Specifications of e.g.
– binary formats
– function calling conventions
– data structure alignment
Examples libc++
libstdc++
System V A64 ABI
Windows x64 ABI

If application and library are build against a different ABI, they won’t work together.