Application programming interface

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"API" and "Api" redirect here. For other uses, see API (disambiguation).

An application programming interface (API) is a source code interface that a computer system or program library provides in order to support requests for services to be made of it by a computer program.

An API differs from an application binary interface in that it is specified in terms of a programming language that can be compiled when an application is built, rather than an explicit low level description of how data is laid out in memory.

The software that provides the functionality described by an API is said to be an implementation of the API. The API itself is abstract, in that it specifies an interface and does not get involved with implementation details.

A well known API is the Single UNIX Specification.

An API is often a part of a software development kit (SDK).

The term API is used in two related senses:

A coherent interface consisting of several classes or several sets of related functions or procedures.
A single entry point such as a method, function or procedure.

1 Design models
2 Release policies
3 Some example APIs
4 See also
5 External links

[edit] Design models

There are various design models for APIs. Interfaces intended for the fastest execution often consist of sets of functions, procedures, variables and data structures. However, other models exist as well - such as the interpreter used to evaluate expressions in ECMAScript/JavaScript or in the abstraction layer - which relieve the programmer from needing to know how the functions of the API relate to the lower levels of abstraction. This makes it possible to redesign or improve the functions within the API without breaking code that relies on it.

Some APIs, such as the ones standard to an operating system, are implemented as separate code libraries that are distributed with the operating system. Others require software publishers to integrate the API functionality directly into the application. This forms another distinction in the examples above. Microsoft Windows APIs come with the operating system for anyone to use. Software for embedded systems such as video game consoles generally falls into the application-integrated category. While an official PlayStation API document may be interesting to read, it is of little use without its corresponding implementation, in the form of a separate library or software development kit.

An API that does not require royalties for access and usage is called "open".^[1] Although usually authoritative "reference implementations" exist for an API (such as Microsoft Windows for the Win32 API), there is nothing that prevents the creation of additional implementations. For example, most of the Win32 API can be provided under a UNIX system using software called Wine.

In countries subject to software patents it is sometimes lawful to analyze API implementations in order to produce a compatible one. This technique is called reverse engineering for the purposes of interoperability. However, the legal situation is often ambiguous, so that care and legal counsel should be taken before the reverse engineering is carried out. For example, while APIs usually do not have an obvious legal status, they might include patents that may not be used until, and if, the patent holder gives permission. Many countries however do not grant software patents and within these countries only the protection of copyright applies to an API.

[edit] Release policies

Two general lines of API publishing policies:

Some companies protect information on their APIs from the general public. For example, Sony used to make its official PlayStation 2 API available only to licensed PlayStation developers. This enabled Sony to control who wrote PlayStation 2 games. Such control can have quality control benefits and potential license revenue.
Some companies make their APIs freely available. For example, Microsoft makes most of its API information public, so that software will be written for the Windows platform.

It is to be expected that companies base their choice of publishing policy on maximizing benefit to themselves.