Application program interface

An application program interface (API) is a standard set of computer instructions that a software developer can use to perform certain functions. For example, in connection with the Microsoft Windows operating system:


 * APIs perform many functions, including allocating computer memory and controlling peripherals such as printers and keyboards. Operating systems also function as platforms for software applications. They do this by 'exposing' &mdash; i.e., making available to software developers &mdash; routines or protocols that perform certain widely-used functions. These are known as Application Programming Interfaces, or 'APIs.' . . . Software developers wishing to include that function in an application need not duplicate it in their own code. Instead, they can 'call' &mdash; i.e., use &mdash; the Windows API. Windows contains thousands of APIs, controlling everything from data storage to font display.
 * Every operating system has different APIs. Accordingly, a developer who writes an application for one operating system and wishes to sell the application to users of another must modify, or 'port,' the application to the second operating system. This process is both time-consuming and expensive.