[[File:Linux kernel interfaces.svg|thumb|300px|A high-level comparison of in-kernel and kernel-to-userspace APIs and ABIs]]
[[File:Linux API and Linux ABI.svg|thumb|300px|[[Linux kernel]] and [[GNU C Library]] define the [[Linux kernel interfaces#Kernel–user space API|Linux API]]. After compilation, the binaries offer an ABI; keeping this ABI stable over a long time is important for [[Independent software vendor|ISVs]].]]
In [[computer software]], an '''application binary interface''' ('''ABI''') is the [[interface (computing)|interface]] between two program modules, one of which is often a [[Library (computing)|library]] and/or [[operating system]] and the other one is usually an application created by a regular programmer. In contrast to an [[Application programming interface|''API'']], which defines structures and methods one can use at software level, an ''ABI'' defines the structures and methods used to access external, ''already compiled'' libraries/code at the level of [[machine code]]. It does this by determining in which binary format information should be passed from one program component to the next, or to the operating system in the case of a [[system call]]. Thus it sets details such as the [[calling convention]].
Adhering to ABIs (which may or may not be officially standardized) is usually the job of the [[compiler]], OS or library writer, but application programmers may have to deal with ABIs directly when writing programs in a mix of programming languages, using [[foreign function call]] interfaces between them.
ABIs differ from [[application programming interface]]s (APIs), which similarly define interfaces between program components, but at the [[source code]] level.
