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Definition: API


(Application Programming Interface) A language and message format used by an application program to communicate with the operating system or some other control program such as a database management system (DBMS) or communications protocol. APIs are implemented by writing function calls in the program, which provide the linkage to the required subroutine for execution. Thus, an API implies that a driver or program module is available in the computer to perform the operation or that software must be linked into the existing program to perform the tasks.

Plenty of API Programming Is Done
Understanding an API is a major part of what a programmer does. Except for writing the business logic that performs the actual data processing, all the rest of the programming is writing the code to communicate with the functions in the operating system and other system software.

The APIs for operating systems can be daunting, especially the calls to the user interface routines to display the menus, buttons and windows on the screen. There are more than a thousand API calls in a full-blown operating system such as Windows, Mac or Unix, and the APIs in one platform are very different from another platform. See IDE, function and interface.

APIs Become Obsolete
Operating system vendors are upgrading their capabilities all the time with new functions in the latest OS version. However, after many years, in order to streamline their software, they discontinue support for older functions, which means those APIs in an existing app no longer work when run in the newer version of the OS.

The only choice programmers have is to update their apps to communicate with the new routines (new APIs). In the case of business applications, an organization can put off installing a newer OS version, at least for a while. However, with commercial apps, developers have little choice but to update their programs because they do not control their users' environment. In the case of mobile apps, unless they update their code, developers will be unable to deploy the app to the online store.