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An Application Programming Interface (API) is a set of definitions of the ways in which one piece of computer software communicates with another. It is a method of achieving abstraction, usually (but not necessarily) between lower-level and higher-level software. One of the primary purposes of an API is to provide a set of commonly-used functions—for example, to draw windows or icons on the screen. Programmers can then take advantage of the API by making use of its functionality, saving them the task of programming everything from scratch. APIs themselves are abstract: software which provides a certain API is often called the implementation of that API.
For example, one may look at the task of writing "Hello World" on a screen at increasing levels of abstraction:
Obviously, the first option requires more steps, each of them much more complicated than the steps in the subsequent options. In addition, it is impractical to use the first approach to present a substantial amount of information, like an encyclopedia article, on the screen, whereas the second approach makes the task easier by an order, and third approach would basically just require you to type "Hello World". However, higher-level APIs often lose flexibility; for example, a it would be much more difficult in a Web browser to rotate text around a point with blinking outlines, something that could be done easily at the lower levels. This difference is a typical example of the trade-offs encountered by choosing to use an API.
APIs are as essential to computers as electrical standards are to the home. One can plug his toaster into the wall whether he is at home or at a neighbour's house, because both houses conform to the standard API for an electrical socket. If there wasn't an API standard, one would have to bring a power station along in order to make toast! Note that there is nothing stopping somebody else coming up with another standard; a European toaster will not work in the US without a transformer, just as a program written for Microsoft Windows will not work directly on a UNIX system without an intermediate API adapter.
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. A good API is able to provide a "black box" or abstraction layer[?], which prevents 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.
Two general lines of policies exist regarding publishing APIs:
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 developer's kit.
An API which does not require royalties for access and usage is called "open". The APIs provided by Free software (such as software distributed under the GNU General Public License), are open by definition, since anyone can look into the source of the software and figure out the API. Although usually authoritative "reference" implementations exist for an API (such as Microsoft Windows for the Win32 API), there's 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.
It is generally 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 may include patents that may not be used until the patent holder gives permission.
Examples of APIs are: