Types of white box testing

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The following types of white box testing exist:

API testing (application programming interface)

An Application Programming Interface (API) is a particular set of rules and specifications that a software program can follow to access and make use of the services and resources provided by another particular software program that implements that API. It serves as an interface between different software programs and facilitates their interaction, similar to the way the user interface facilitates interaction between humans and computers.

Concept

An API can be created for applications, libraries, operating systems, etc, as a way to define their “vocabularies” and resources request conventions (e.g. functions calling conventions). It may include specifications for routines, data structures, object classes, and protocols used to communicate between the consumer program and the implementer program of the API.

An API is an abstraction that describes an interface for the interaction with a set of functions used by components of a software system. The software providing the functions described by an API is said to be an implementation of the API.

An API can be:

• general, the full set of an API that is bundled in the libraries of a programming language, e.g. Standard Template Library in C++ or Java API.
• specific, meant to address a specific problem, e.g. Google Maps API or Java API for XML Web Services.
• language-dependent, meaning it is only available by using the syntax and elements of a particular language, which makes the API more convenient to use.
• language-independent, written so that it can be called from several programming languages. This is a desirable feature for a service-oriented API that is not bound to a specific process or system and may be provided as remote procedure calls or web services. For example, a website that allows users to review local restaurants is able to layer their reviews over maps taken from Google Maps, because Google Maps has an API that facilitates this functionality. Google Maps’ API controls what information a third-party site can use and how they can use it.
• API may be used to refer to a complete interface, a single function, or even a set of APIs provided by an organization. Thus, the scope of meaning is usually determined by the context of usage.

Detailed Explanation

An API may describe the ways in which a particular task is performed. For example, in Unix systems, the math.h include file for the C language contains the definition of the mathematical functions available in the C language library for mathematical processing (usually called libm). This file would describe how to use these functions and the expected result. For example, on a Unix system the command man 3 sqrt will present the signature of the function sqrt in the form:

SYNOPSIS
            #include <math.h>
            double sqrt(double X);
            float  sqrtf(float X);
DESCRIPTION
       DESCRIPTION
       sqrt computes the positive square root of the argument. ...
NOTE: using OOP languages like java and C++, you can override the above function simply as sqrt..:)
RETURNS

On success, the square root is returned. If X is real and positive…

That means that the function returns the square root of a positive floating point number (single or double precision) as another floating point number. Hence the API in this case can be interpreted as the collection of the included files used by the C language and its human readable description provided by the man pages.

Web APIs

When used in the context of web development, an API is typically a defined set of Hypertext Transfer Protocol (HTTP) request messages, along with a definition of the structure of response messages, which is usually in an Extensible Markup Language (XML) or JavaScript Object Notation (JSON) format. While “Web API” is virtually a synonym for web service, the recent trend (so-called Web 2.0) has been moving away from Simple Object Access Protocol (SOAP) based services towards more direct Representational State Transfer (REST) style communications. Web APIs allow the combination of multiple services into new applications known as mashups.

Use of APIs to share content

The practice of publishing APIs has allowed web communities to create an open architecture for sharing content and data between communities and applications. In this way, content that is created in one place can be dynamically posted and updated in multiple locations on the web.

Photos can be shared from sites like Flickr and Photobucket to social network sites like Facebook and MySpace.

Content can be embedded, e.g. embedding a presentation from SlideShare on a LinkedIn profile.

Content can be dynamically posted. Sharing live comments made on Twitter with a Facebook account, for example, is enabled by their APIs.

Video content can be embedded on sites which are served by another host.

User information can be shared from web communities to outside applications, delivering new functionality to the web community that shares its user data via an open API. One of the best examples of this is the Facebook Application platform. Another is the Open Social platform.

Implementations

The POSIX standard defines an API that allows a wide range of common computing functions to be written in a way such that they may operate on many different systems (Mac OS X, and various Berkeley Software Distributions (BSDs) implement this interface); however, making use of this requires re-compiling for each platform. A compatible API, on the other hand, allows compiled object code to function without any changes to the system implementing that API. This is beneficial to both software providers (where they may distribute existing software on new systems without producing and distributing upgrades) and users (where they may install older software on their new systems without purchasing upgrades), although this generally requires that various software libraries implement the necessary APIs as well.

Microsoft has shown a strong commitment to a backward compatible API, particularly within their Windows API (Win32) library, such that older applications may run on newer versions of Windows using an executable-specific setting called “Compatibility Mode”.

Apple Inc. has shown less concern, breaking compatibility or implementing an API in a slower “emulation mode”; this allows greater freedom in development, at the cost of making older software obsolete.

Among Unix-like operating systems, there are many related but incompatible operating systems running on a common hardware platform (particularly Intel 80386-compatible systems). There have been several attempts to standardize the API such that software vendors may distribute one binary application for all these systems; however, to date, none of these have met with much success. The Linux Standard Base is attempting to do this for the Linux platform, while many of the BSD Unixes, such as FreeBSD, NetBSD, and OpenBSD, implement various levels of API compatibility for both backward compatibility (allowing programs written for older versions to run on newer distributions of the system) and cross-platform compatibility (allowing execution of foreign code without recompiling).

Release policies

The two options for releasing API are:

Protecting information on APIs from the general public. For example, Sony used to make its official PlayStation 2 API available only to licensed PlayStation developers. This enabled Sony to control who wrote PlayStation 2 games. This gives companies quality control privileges and can provide them with potential licensing revenue streams.

Making APIs freely available. For example, Microsoft makes the Microsoft Windows API public, and Apple releases its APIs Carbon and Cocoa, so that software can be written for their platforms.

A mix of the two behaviors can be used as well.

Language bindings and interface generators

APIs that are intended to be used by more than one high-level programming language often provide, or are augmented with, facilities to automatically map the API to features (syntactic or semantic) that are more natural in those languages. This is known as language binding, and is itself an API. The aim is to encapsulate most of the required functionality of the API, leaving a “thin” layer appropriate to each language.

Below are listed some interface generator tools which bind languages to APIs at compile time.

SWIG opensource interfaces bindings generator from many languages to many languages (Typically Compiled->Scripted)

F2PY: Fortran to Python interface generator.

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