Application programming interface

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 necesserily between lower-level and higher-level software). Generally, the main purpose of API is to spare from the programmer the need to write software from scratch. A set of programs that comply to a certain API are called an implementation.

For example, one may look at the task of writing "Hello World" on the screen:

1. Do it all yourself:
   1. Draw, on graph paper, the shapes of the letters H, e, l, o, W, r, d.
   2. Work out a matrix of black and white squares that look like those letters.
   3. Work out the sequence of electrical pulses that I need to send to the computer screen to draw that matrix. It might be 5 volts for black, 1 volt for white.
   4. Repeat the sequence every 60th of a second to keep the message displayed, exploiting persistence of vision.
2. Use somebody else's program to do all this hard work.

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, I would have to bring a power station with me to make my toast! Note that there is nothing stopping somebody else coming up with another standard; a UK toaster will not work in the US, just as a program written for Microsoft Windows will not work directly on DOS.

Two general lines of policies exist regarding publishing APIs:

• Some companies guard their APIs jealously. For example, Sony charges a large amount of money before you can even see the SDK documentation. This is because Sony wants to restrict the amount of people who are able to write a PlayStation game, and to profit from them as much as possible. This is typical of companies who do not profit from the sale of API implementations (in this case, PlayStation consoles which are sold at a loss for Sony).
• Other companies propagate their APIs freely. For example, Microsoft deliberately makes their API information public, so that software will be written for the Windows platform. The sale of the third party software sells copies of Windows. This is typical of companies who profit from the sale of API implementations (in this case, Microsoft Windows, which is sold at a gain for Microsoft).

An API which does 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 basing on it. Although usually authorative implementations exist for an API (such as Microsoft Windows for Windows API), there's nothing (besides legal constraints) that prevents the creation of additional implementations. For example, Windows API can be provided under Linux using software called [Wine].

Examples of APIs are:

• The PC BIOS call interface
• Document Object Models of applications such as the WWW
• The UNIX system call interface
• The Microsoft Win32? API