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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 APIs 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 operating system.

APIs can be daunting, especially the calls to the user interface 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, macOS or Unix, and APIs are very different between platforms. See IDE, function and interface.

APIs Become Obsolete
Operating system vendors upgrade their capabilities all the time with new functions in the latest version. However, after many years, in order to streamline their software, vendors discontinue support for older routines. The old APIs no longer work when run in the newer version of the OS, which means the app fails to operate.

Business organizations can put off installing the latest OS for a while to keep their existing apps running. However, commercial developers may have little choice because they need to be compatible with the OS versions in use. In addition, they might be denied access to the vendor's online store if their apps are not current.