Programming Language - Mentor of your Computer by Manthan Dave

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Programming Language Mentor of your Computer

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Contents Articles Programming language

Computer software

History

History of programming languages

Types of programming languages

Low-level programming language

High-level programming language

Software Programming Languages

Machine code

Assembly language

BASIC

C (programming language)

C++

Perl

Fortran

C Sharp (programming language)

116

Java (programming language)

127

.NET Framework

142

Some Software Programming Concepts

157

Computer programming

157

Algorithm

163

Computer data processing

180

Thread (computer science)

182

Parallel computing

188

Web Programming Languages

203

HTML

203

Web 2.0

219

PHP

228

Active Server Pages

240

References Article Sources and Contributors

243

Image Sources, Licenses and Contributors

252

Article Licenses License

254

Programming language

Programming language A programming language is an artificial language designed to express computations that can be performed by a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine, to express algorithms precisely, or as a mode of human communication. Many programming languages have some form of written specification of their syntax (form) and semantics (meaning). Some languages are defined by a specification document. For example, the C programming language is specified by an ISO Standard. Other languages, such as Perl, have a dominant implementation that is used as a reference. The earliest programming languages predate the invention of the computer, and were used to direct the behavior of machines such as Jacquard looms and player pianos. Thousands of different programming languages have been created, mainly in the computer field, with many more being created every year. Most programming languages describe computation in an imperative style, i.e., as a sequence of commands, although some languages, such as those that support functional programming or logic programming, use alternative forms of description.

Definitions A programming language is a notation for writing programs, which are specifications of a computation or algorithm.[1] Some, but not all, authors restrict the term "programming language" to those languages that can express all possible algorithms.[1] [2] Traits often considered important for what constitutes a programming language include: â&#x20AC;˘ Function and target: A computer programming language is a language[3] used to write computer programs, which involve a computer performing some kind of computation[4] or algorithm and possibly control external devices such as printers, disk drives, robots,[5] and so on. For example PostScript programs are frequently created by another program to control a computer printer or display. More generally, a programming language may describe computation on some, possibly abstract, machine. It is generally accepted that a complete specification for a programming language includes a description, possibly idealized, of a machine or processor for that language.[6] In most practical contexts, a programming language involves a computer; consequently programming languages are usually defined and studied this way.[7] Programming languages differ from natural languages in that natural languages are only used for interaction between people, while programming languages also allow humans to communicate instructions to machines. â&#x20AC;˘ Abstractions: Programming languages usually contain abstractions for defining and manipulating data structures or controlling the flow of execution. The practical necessity that a programming language support adequate abstractions is expressed by the abstraction principle;[8] this principle is sometimes formulated as recommendation to the programmer to make proper use of such abstractions.[9] â&#x20AC;˘ Expressive power: The theory of computation classifies languages by the computations they are capable of expressing. All Turing complete languages can implement the same set of algorithms. ANSI/ISO SQL and Charity are examples of languages that are not Turing complete, yet often called programming languages.[10] [11] Markup languages like XML, HTML or troff, which define structured data, are not generally considered programming languages.[12] [13] [14] Programming languages may, however, share the syntax with markup languages if a computational semantics is defined. XSLT, for example, is a Turing complete XML dialect.[15] [16] [17] Moreover, LaTeX, which is mostly used for structuring documents, also contains a Turing complete subset.[18] [19] The term computer language is sometimes used interchangeably with programming language.[20] However, the usage of both terms varies among authors, including the exact scope of each. One usage describes programming languages as a subset of computer languages.[21] In this vein, languages used in computing that have a different goal than expressing computer programs are generically designated computer languages. For instance, markup languages are sometimes referred to as computer languages to emphasize that they are not meant to be used for

Programming language programming.[22] Another usage regards programming languages as theoretical constructs for programming abstract machines, and computer languages as the subset thereof that runs on physical computers, which have finite hardware resources.[23] John C. Reynolds emphasizes that formal specification languages are just as much programming languages as are the languages intended for execution. He also argues that textual and even graphical input formats that affect the behavior of a computer are programming languages, despite the fact they are commonly not Turing-complete, and remarks that ignorance of programming language concepts is the reason for many flaws in input formats.[24]

Design and implementation Programming languages share properties with natural languages related to their purpose as vehicles for communication, having a syntactic form separate from its semantics, and showing language families of related languages branching one from another.[3] But as artificial constructs, they also differ in fundamental ways from languages that have evolved through usage. A significant difference is that a programming language can be fully described and studied in its entirety, since it has a precise and finite definition.[25] By contrast, natural languages have changing meanings given by their users in different communities. While constructed languages are also artificial languages designed from the ground up with a specific purpose, they lack the precise and complete semantic definition that a programming language has. Many languages have been designed from scratch, altered to meet new needs, combined with other languages, and eventually fallen into disuse. Although there have been attempts to design one "universal" programming language that serves all purposes, all of them have failed to be generally accepted as filling this role.[26] The need for diverse programming languages arises from the diversity of contexts in which languages are used: • Programs range from tiny scripts written by individual hobbyists to huge systems written by hundreds of programmers. • Programmers range in expertise from novices who need simplicity above all else, to experts who may be comfortable with considerable complexity. • Programs must balance speed, size, and simplicity on systems ranging from microcontrollers to supercomputers. • Programs may be written once and not change for generations, or they may undergo continual modification. • Finally, programmers may simply differ in their tastes: they may be accustomed to discussing problems and expressing them in a particular language. One common trend in the development of programming languages has been to add more ability to solve problems using a higher level of abstraction. The earliest programming languages were tied very closely to the underlying hardware of the computer. As new programming languages have developed, features have been added that let programmers express ideas that are more remote from simple translation into underlying hardware instructions. Because programmers are less tied to the complexity of the computer, their programs can do more computing with less effort from the programmer. This lets them write more functionality per time unit.[27] Natural language processors have been proposed as a way to eliminate the need for a specialized language for programming. However, this goal remains distant and its benefits are open to debate. Edsger W. Dijkstra took the position that the use of a formal language is essential to prevent the introduction of meaningless constructs, and dismissed natural language programming as "foolish".[28] Alan Perlis was similarly dismissive of the idea.[29] A language's designers and users must construct a number of artifacts that govern and enable the practice of programming. The most important of these artifacts are the language specification and implementation.

Programming language

Specification The specification of a programming language is intended to provide a definition that the language users and the implementors can use to determine whether the behavior of a program is correct, given its source code. A programming language specification can take several forms, including the following: â&#x20AC;˘ An explicit definition of the syntax, static semantics, and execution semantics of the language. While syntax is commonly specified using a formal grammar, semantic definitions may be written in natural language (e.g., as in the C language), or a formal semantics (e.g., as in Standard ML[30] and Scheme[31] specifications). â&#x20AC;˘ A description of the behavior of a translator for the language (e.g., the C++ and Fortran specifications). The syntax and semantics of the language have to be inferred from this description, which may be written in natural or a formal language. â&#x20AC;˘ A reference or model implementation, sometimes written in the language being specified (e.g., Prolog or ANSI REXX[32] ). The syntax and semantics of the language are explicit in the behavior of the reference implementation.

Implementation An implementation of a programming language provides a way to execute that program on one or more configurations of hardware and software. There are, broadly, two approaches to programming language implementation: compilation and interpretation. It is generally possible to implement a language using either technique. The output of a compiler may be executed by hardware or a program called an interpreter. In some implementations that make use of the interpreter approach there is no distinct boundary between compiling and interpreting. For instance, some implementations of BASIC compile and then execute the source a line at a time. Programs that are executed directly on the hardware usually run several orders of magnitude faster than those that are interpreted in software. One technique for improving the performance of interpreted programs is just-in-time compilation. Here the virtual machine, just before execution, translates the blocks of bytecode which are going to be used to machine code, for direct execution on the hardware.

Usage Thousands of different programming languages have been created, mainly in the computing field.[33] Programming languages differ from most other forms of human expression in that they require a greater degree of precision and completeness. When using a natural language to communicate with other people, human authors and speakers can be ambiguous and make small errors, and still expect their intent to be understood. However, figuratively speaking, computers "do exactly what they are told to do", and cannot "understand" what code the programmer intended to write. The combination of the language definition, a program, and the program's inputs must fully specify the external behavior that occurs when the program is executed, within the domain of control of that program. A programming language provides a structured mechanism for defining pieces of data, and the operations or transformations that may be carried out automatically on that data. A programmer uses the abstractions present in the language to represent the concepts involved in a computation. These concepts are represented as a collection of the simplest elements available (called primitives).[34] Programming is the process by which programmers combine these primitives to compose new programs, or adapt existing ones to new uses or a changing environment. Programs for a computer might be executed in a batch process without human interaction, or a user might type commands in an interactive session of an interpreter. In this case the "commands" are simply programs, whose execution is chained together. When a language is used to give commands to a software application (such as a shell) it is called a scripting language.

Programming language

Measuring language usage It is difficult to determine which programming languages are most widely used, and what usage means varies by context. One language may occupy the greater number of programmer hours, a different one have more lines of code, and a third utilize the most CPU time. Some languages are very popular for particular kinds of applications. For example, COBOL is still strong in the corporate data center, often on large mainframes; FORTRAN in engineering applications; C in embedded applications and operating systems; and other languages are regularly used to write many different kinds of applications. Various methods of measuring language popularity, each subject to a different bias over what is measured, have been proposed: • counting the number of job advertisements that mention the language[35] • the number of books sold that teach or describe the language[36] • estimates of the number of existing lines of code written in the language—which may underestimate languages not often found in public searches[37] • counts of language references (i.e., to the name of the language) found using a web search engine. Combining and averaging information from various internet sites, langpop.com claims that [38] in 2008 the 10 most cited programming languages are (in alphabetical order): C, C++, C#, Java, JavaScript, Perl, PHP, Python, Ruby, and SQL.

Taxonomies There is no overarching classification scheme for programming languages. A given programming language does not usually have a single ancestor language. Languages commonly arise by combining the elements of several predecessor languages with new ideas in circulation at the time. Ideas that originate in one language will diffuse throughout a family of related languages, and then leap suddenly across familial gaps to appear in an entirely different family. The task is further complicated by the fact that languages can be classified along multiple axes. For example, Java is both an object-oriented language (because it encourages object-oriented organization) and a concurrent language (because it contains built-in constructs for running multiple threads in parallel). Python is an object-oriented scripting language. In broad strokes, programming languages divide into programming paradigms and a classification by intended domain of use. Traditionally, programming languages have been regarded as describing computation in terms of imperative sentences, i.e. issuing commands. These are generally called imperative programming languages. A great deal of research in programming languages has been aimed at blurring the distinction between a program as a set of instructions and a program as an assertion about the desired answer, which is the main feature of declarative programming.[39] More refined paradigms include procedural programming, object-oriented programming, functional programming, and logic programming; some languages are hybrids of paradigms or multi-paradigmatic. An assembly language is not so much a paradigm as a direct model of an underlying machine architecture. By purpose, programming languages might be considered general purpose, system programming languages, scripting languages, domain-specific languages, or concurrent/distributed languages (or a combination of these).[40] Some general purpose languages were designed largely with educational goals.[41] A programming language may also be classified by factors unrelated to programming paradigm. For instance, most programming languages use English language keywords, while a minority do not. Other languages may be classified as being esoteric or not.

Programming language

History Early developments The first programming languages predate the modern computer. The 19th century had "programmable" looms and player piano scrolls which implemented what are today recognized as examples of domain-specific languages. By the beginning of the twentieth century, punch cards encoded data and directed mechanical processing. In the 1930s and 1940s, the formalisms of Alonzo Church's lambda calculus and Alan Turing's Turing machines provided mathematical abstractions for expressing algorithms; the lambda calculus remains influential in language design.[42]

A selection of textbooks that teach programming, in languages both popular and obscure. These are only a few of the thousands of programming languages and dialects that have been designed in history.

In the 1940s, the first electrically powered digital computers were created. The first high-level programming language to be designed for a computer was Plankalkül, developed for the German Z3 by Konrad Zuse between 1943 and 1945. However, it was not implemented until 1998 and 2000.[43] Programmers of early 1950s computers, notably UNIVAC I and IBM 701, used machine language programs, that is, the first generation language (1GL). 1GL programming was quickly superseded by similarly machine-specific, but mnemonic, second generation languages (2GL) known as assembly languages or "assembler". Later in the 1950s, assembly language programming, which had evolved to include the use of macro instructions, was followed by the development of "third generation" programming languages (3GL), such as FORTRAN, LISP, and COBOL.[44] 3GLs are more abstract and are "portable", or at least implemented similarly on computers that do not support the same native machine code. Updated versions of all of these 3GLs are still in general use, and each has strongly influenced the development of later languages.[45] At the end of the 1950s, the language formalized as ALGOL 60 was introduced, and most later programming languages are, in many respects, descendants of Algol.[45] The format and use of the early programming languages was heavily influenced by the constraints of the interface.[46]

Refinement The period from the 1960s to the late 1970s brought the development of the major language paradigms now in use, though many aspects were refinements of ideas in the very first Third-generation programming languages: • APL introduced array programming and influenced functional programming.[47] • PL/I (NPL) was designed in the early 1960s to incorporate the best ideas from FORTRAN and COBOL. • In the 1960s, Simula was the first language designed to support object-oriented programming; in the mid-1970s, Smalltalk followed with the first "purely" object-oriented language. • C was developed between 1969 and 1973 as a system programming language, and remains popular.[48] • Prolog, designed in 1972, was the first logic programming language. • In 1978, ML built a polymorphic type system on top of Lisp, pioneering statically typed functional programming languages. Each of these languages spawned an entire family of descendants, and most modern languages count at least one of them in their ancestry. The 1960s and 1970s also saw considerable debate over the merits of structured programming, and whether programming languages should be designed to support it.[49] Edsger Dijkstra, in a famous 1968 letter published in the Communications of the ACM, argued that GOTO statements should be eliminated from all "higher level" programming languages.[50]

Programming language The 1960s and 1970s also saw expansion of techniques that reduced the footprint of a program as well as improved productivity of the programmer and user. The card deck for an early 4GL was a lot smaller for the same functionality expressed in a 3GL deck.

Consolidation and growth The 1980s were years of relative consolidation. C++ combined object-oriented and systems programming. The United States government standardized Ada, a systems programming language derived from Pascal and intended for use by defense contractors. In Japan and elsewhere, vast sums were spent investigating so-called "fifth generation" languages that incorporated logic programming constructs.[51] The functional languages community moved to standardize ML and Lisp. Rather than inventing new paradigms, all of these movements elaborated upon the ideas invented in the previous decade. One important trend in language design during the 1980s was an increased focus on programming for large-scale systems through the use of modules, or large-scale organizational units of code. Modula-2, Ada, and ML all developed notable module systems in the 1980s, although other languages, such as PL/I, already had extensive support for modular programming. Module systems were often wedded to generic programming constructs.[52] The rapid growth of the Internet in the mid-1990s created opportunities for new languages. Perl, originally a Unix scripting tool first released in 1987, became common in dynamic websites. Java came to be used for server-side programming. These developments were not fundamentally novel, rather they were refinements to existing languages and paradigms, and largely based on the C family of programming languages. Programming language evolution continues, in both industry and research. Current directions include security and reliability verification, new kinds of modularity (mixins, delegates, aspects), and database integration such as Microsoft's LINQ. The 4GLs are examples of languages which are domain-specific, such as SQL, which manipulates and returns sets of data rather than the scalar values which are canonical to most programming languages. Perl, for example, with its 'here document' can hold multiple 4GL programs, as well as multiple JavaScript programs, in part of its own perl code and use variable interpolation in the 'here document' to support multi-language programming.[53]

See also • • • • • • • • • • • • • •

Comparison of programming languages (basic instructions) Comparison of programming languages Computer programming Computer science and Outline of computer science Educational programming language Invariant based programming Lists of programming languages List of programming language researchers Literate programming Dialect (computing) Programming language theory Pseudocode Scientific language Software engineering and List of software engineering topics

Programming language

Further reading • Daniel P. Friedman, Mitchell Wand, Christopher Thomas Haynes: Essentials of Programming Languages, The MIT Press 2001. • David Gelernter, Suresh Jagannathan: Programming Linguistics, The MIT Press 1990. • Shriram Krishnamurthi: Programming Languages: Application and Interpretation, online publication [54]. • Bruce J. MacLennan: Principles of Programming Languages: Design, Evaluation, and Implementation, Oxford University Press 1999. • John C. Mitchell: Concepts in Programming Languages, Cambridge University Press 2002. • Benjamin C. Pierce: Types and Programming Languages, The MIT Press 2002. • Ravi Sethi: Programming Languages: Concepts and Constructs, 2nd ed., Addison-Wesley 1996. • Michael L. Scott: Programming Language Pragmatics, Morgan Kaufmann Publishers 2005. • Richard L. Wexelblat (ed.): History of Programming Languages, Academic Press 1981.

External links • 99 Bottles of Beer [55] A collection of implementations in many languages. • Computer Programming Languages [56] at the Open Directory Project • Syntax Patterns for Various Languages [57]

References [1] Aaby, Anthony (2004). Introduction to Programming Languages (http:/ / burks. brighton. ac. uk/ burks/ pcinfo/ progdocs/ plbook/ index. htm). . [2] In mathematical terms, this means the programming language is Turing-complete MacLennan, Bruce J. (1987). Principles of Programming Languages. Oxford University Press. p. 1. ISBN 0-19-511306-3. [3] Steven R. Fischer, A history of language, Reaktion Books, 2003, ISBN 186189080X, p. 205 [4] ACM SIGPLAN (2003). "Bylaws of the Special Interest Group on Programming Languages of the Association for Computing Machinery" (http:/ / www. acm. org/ sigs/ sigplan/ sigplan_bylaws. htm). . Retrieved 2006-06-19., The scope of SIGPLAN is the theory, design, implementation, description, and application of computer programming languages - languages that permit the specification of a variety of different computations, thereby providing the user with significant control (immediate or delayed) over the computer's operation. [5] Dean, Tom (2002). "Programming Robots" (http:/ / www. cs. brown. edu/ people/ tld/ courses/ cs148/ 02/ programming. html). Building Intelligent Robots. Brown University Department of Computer Science. . Retrieved 2006-09-23. [6] R. Narasimahan, Programming Languages and Computers: A Unified Metatheory, pp. 189--247 in Franz Alt, Morris Rubinoff (eds.) Advances in computers, Volume 8, Academic Press, 1994, ISBN 012012108, p.193 : "a complete specification of a programming language must, by definition, include a specification of a processor--idealized, if you will--for that language." [the source cites many references to support this statement] [7] Ben Ari, Mordechai (1996). Understanding Programming Languages". John Wiley and Sons. "Programs and languages can be deﬁned as purely formal mathematical objects. However, more people are interested in programs than in other mathematical objects such as groups, precisely because it is possible to use the program—the sequence of symbols—to control the execution of a computer. While we highly recommend the study of the theory of programming, this text will generally limit itself to the study of programs as they are executed on a computer." [8] David A. Schmidt, The structure of typed programming languages, MIT Press, 1994, ISBN 0262193493, p. 32 [9] Pierce, Benjamin (2002). Types and Programming Languages. MIT Press. p. 339. ISBN 0-262-16209-1. [10] Digital Equipment Corporation. "Information Technology - Database Language SQL (Proposed revised text of DIS 9075)" (http:/ / www. contrib. andrew. cmu. edu/ ~shadow/ sql/ sql1992. txt). ISO/IEC 9075:1992, Database Language SQL. . Retrieved June 29, 2006. [11] The Charity Development Group (December 1996). "The CHARITY Home Page" (http:/ / pll. cpsc. ucalgary. ca/ charity1/ www/ home. html). . Retrieved 2006-06-29., Charity is a categorical programming language..., All Charity computations terminate. [12] XML in 10 points (http:/ / www. w3. org/ XML/ 1999/ XML-in-10-points. html) W3C, 1999, XML is not a programming language. [13] Powell, Thomas (2003). HTML & XHTML: the complete reference. McGraw-Hill. p. 25. ISBN 0-07-222-942-X. "HTML is not a programming language." [14] Dykes, Lucinda (2005). XML For Dummies, 4th Edition. Wiley. p. 20. ISBN 0-7645-8845-1. "...it's a markup language, not a programming language." [15] http:/ / www. ibm. com/ developerworks/ library/ x-xslt/ [16] http:/ / msdn. microsoft. com/ en-us/ library/ ms767587(VS. 85). aspx

Programming language [17] Scott, Michael (2006). Programming Language Pragmatics. Morgan Kaufmann. p. 802. ISBN 0-12-633951-1. "XSLT, though highly specialized to the transformation of XML, is a Turing-complete programming language." [18] http:/ / tobi. oetiker. ch/ lshort/ lshort. pdf [19] Syropoulos, Apostolos; Antonis Tsolomitis, Nick Sofroniou (2003). Digital typography using LaTeX. Springer-Verlag. p. 213. ISBN 0-387-95217-9. "TeX is not only an excellent typesetting engine but also a real programming language." [20] Robert A. Edmunds, The Prentice-Hall standard glossary of computer terminology, Prentice-Hall, 1985, p. 91 [21] Pascal Lando, Anne Lapujade, Gilles Kassel, and Frédéric Fürst, Towards a General Ontology of Computer Programs (http:/ / www. loa-cnr. it/ ICSOFT2007_final. pdf), ICSOFT 2007 (http:/ / dblp. uni-trier. de/ db/ conf/ icsoft/ icsoft2007-1. html), pp. 163-170 [22] S.K. Bajpai, Introduction To Computers And C Programming, New Age International, 2007, ISBN 812241379X, p. 346 [23] R. Narasimahan, Programming Languages and Computers: A Unified Metatheory, pp. 189--247 in Franz Alt, Morris Rubinoff (eds.) Advances in computers, Volume 8, Academic Press, 1994, ISBN 012012108, p.215: "[...] the model [...] for computer languages differs from that [...] for programming languages in only two respects. In a computer language, there are only finitely many names--or registers--which can assume only finitely many values--or states--and these states are not further distinguished in terms of any other attributes. [author's footnote:] This may sound like a truism but its implications are far reaching. For example, it would imply that any model for programming languages, by fixing certain of its parameters or features, should be reducible in a natural way to a model for computer languages." [24] John C. Reynolds, Some thoughts on teaching programming and programming languages, SIGPLAN Notices, Volume 43, Issue 11, November 2008, p.109 [25] Jing Huang. "Artificial Language vs. Natural Language" (http:/ / www. cs. cornell. edu/ info/ Projects/ Nuprl/ cs611/ fall94notes/ cn2/ subsection3_1_3. html). . [26] IBM in first publishing PL/I, for example, rather ambitiously titled its manual The universal programming language PL/I (IBM Library; 1966). The title reflected IBM's goals for unlimited subsetting capability: PL/I is designed in such a way that one can isolate subsets from it satisfying the requirements of particular applications. ( "Encyclopaedia of Mathematics » P » PL/I" (http:/ / eom. springer. de/ P/ p072885. htm). SpringerLink. . Retrieved June 29, 2006.). Ada and UNCOL had similar early goals. [27] Frederick P. Brooks, Jr.: The Mythical Man-Month, Addison-Wesley, 1982, pp. 93-94 [28] Dijkstra, Edsger W. On the foolishness of "natural language programming." (http:/ / www. cs. utexas. edu/ users/ EWD/ transcriptions/ EWD06xx/ EWD667. html) EWD667. [29] Perlis, Alan, Epigrams on Programming (http:/ / www-pu. informatik. uni-tuebingen. de/ users/ klaeren/ epigrams. html). SIGPLAN Notices Vol. 17, No. 9, September 1982, pp. 7-13 [30] Milner, R.; M. Tofte, R. Harper and D. MacQueen. (1997). The Definition of Standard ML (Revised). MIT Press. ISBN 0-262-63181-4. [31] Kelsey, Richard; William Clinger and Jonathan Rees (February 1998). "Section 7.2 Formal semantics" (http:/ / www. schemers. org/ Documents/ Standards/ R5RS/ HTML/ r5rs-Z-H-10. html#%_sec_7. 2). Revised5 Report on the Algorithmic Language Scheme. . Retrieved 2006-06-09. [32] ANSI — Programming Language Rexx, X3-274.1996 [33] "HOPL: an interactive Roster of Programming Languages" (http:/ / hopl. murdoch. edu. au/ ). Australia: Murdoch University. . Retrieved 2009-06-01. "This site lists 8512 languages." [34] Abelson, Sussman, and Sussman. "Structure and Interpretation of Computer Programs" (http:/ / mitpress. mit. edu/ sicp/ full-text/ book/ book-Z-H-10. html). . Retrieved 2009-03-03. [35] Survey of Job advertisements mentioning a given language (http:/ / www. computerweekly. com/ Articles/ 2007/ 09/ 11/ 226631/ sslcomputer-weekly-it-salary-survey-finance-boom-drives-it-job. htm) [36] Counting programming languages by book sales (http:/ / radar. oreilly. com/ archives/ 2006/ 08/ programming_language_trends_1. html) [37] Bieman, J.M.; Murdock, V., Finding code on the World Wide Web: a preliminary investigation, Proceedings First IEEE International Workshop on Source Code Analysis and Manipulation, 2001 [38] Programming Language Popularity (http:/ / www. langpop. com/ ) [39] Carl A. Gunter, Semantics of Programming Languages: Structures and Techniques, MIT Press, 1992, ISBN 0262570955, p. 1 [40] "TUNES: Programming Languages" (http:/ / tunes. org/ wiki/ programming_20languages. html). . [41] Wirth, Niklaus (1993). "Recollections about the development of Pascal" (http:/ / portal. acm. org/ citation. cfm?id=155378). Proc. 2nd ACM SIGPLAN conference on history of programming languages: 333–342. doi:10.1145/154766.155378. . Retrieved 2006-06-30. [42] Benjamin C. Pierce writes:

"... the lambda calculus has seen widespread use in the specification of programming language features, in language design and implementation, and in the study of type systems." Pierce, Benjamin C. (2002). Types and Programming Languages. MIT Press. p. 52. ISBN 0-262-16209-1. [43] Rojas, Raúl, et al. (2000). "Plankalkül: The First High-Level Programming Language and its Implementation". Institut für Informatik, Freie Universität Berlin, Technical Report B-3/2000. (full text) (http:/ / www. zib. de/ zuse/ Inhalt/ Programme/ Plankalkuel/ Plankalkuel-Report/ Plankalkuel-Report. htm) [44] Linda Null, Julia Lobur, The essentials of computer organization and architecture, Edition 2, Jones & Bartlett Publishers, 2006, ISBN 0763737690, p. 435 [45] O'Reilly Media. "History of programming languages" (http:/ / www. oreilly. com/ news/ graphics/ prog_lang_poster. pdf) (PDF). . Retrieved October 5, 2006.

Programming language [46] Frank da Cruz. IBM Punch Cards (http:/ / www. columbia. edu/ acis/ history/ cards. html) Columbia University Computing History (http:/ / www. columbia. edu/ acis/ history/ index. html). [47] Richard L. Wexelblat: History of Programming Languages, Academic Press, 1981, chapter XIV. [48] François Labelle. "Programming Language Usage Graph" (http:/ / www. cs. berkeley. edu/ ~flab/ languages. html). SourceForge. . Retrieved June 21, 2006.. This comparison analyzes trends in number of projects hosted by a popular community programming repository. During most years of the comparison, C leads by a considerable margin; in 2006, Java overtakes C, but the combination of C/C++ still leads considerably. [49] Hayes, Brian (2006), "The Semicolon Wars", American Scientist 94 (4): 299–303 [50] Dijkstra, Edsger W. (March 1968). "Go To Statement Considered Harmful" (http:/ / www. acm. org/ classics/ oct95/ ). Communications of the ACM 11 (3): 147–148. doi:10.1145/362929.362947. . Retrieved 2006-06-29. [51] Tetsuro Fujise, Takashi Chikayama Kazuaki Rokusawa, Akihiko Nakase (December 1994). "KLIC: A Portable Implementation of KL1" Proc. of FGCS '94, ICOT Tokyo, December 1994. KLIC is a portable implementation of a concurrent logic programming language [[KL1 (http:/ / www. icot. or. jp/ ARCHIVE/ HomePage-E. html)].] [52] Jim Bender (March 15, 2004). "Mini-Bibliography on Modules for Functional Programming Languages" (http:/ / readscheme. org/ modules/ ). ReadScheme.org. . Retrieved 2006-09-27. [53] Wall, Programming Perl ISBN 0-596-00027-8 p.66 [54] http:/ / www. cs. brown. edu/ ~sk/ Publications/ Books/ ProgLangs/ [55] http:/ / www. 99-bottles-of-beer. net/ [56] http:/ / www. dmoz. org/ Computers/ Programming/ Languages/ [57] http:/ / merd. sourceforge. net/ pixel/ language-study/ syntax-across-languages/

Computer software Computer software, or just software is a general term primarily used for digitally stored data such as computer programs and other kinds of information read and written by computers. Today, this includes data that has not traditionally been associated with computers, such as film, tapes and records.[1] The term was coined in order to contrast to the old term hardware (meaning physical devices); in contrast to hardware, software is intangible, meaning it "cannot be touched".[2] Software is also sometimes used in a more narrow sense, meaning application software only. Examples: • Application software, such as word processors which perform productive tasks for users. • Firmware, which is software programmed resident to electrically programmable memory devices on board mainboards or other types of integrated hardware carriers. • Middleware, which controls and co-ordinates distributed systems. • System software such as operating systems, which govern computing resources and provide convenience for users. • Software testing is a domain independent of development and programming. Software testing consists of various methods to test and declare a software product fit before it can be launched for use by either an individual or a group. • Testware, which is an umbrella term or container term for all utilities and application software that serve in combination for testing a software package but not necessarily may optionally contribute to operational purposes. As such, testware is not a standing configuration but merely a working environment for application software or subsets thereof. • Video games (except the hardware part) • Websites

Computer software

Overview Software includes all the various forms and roles that digitally stored data may have and play in a computer (or similar system), regardless of whether the data is used as code for a CPU, or other interpreter, or whether it represents other kinds of information. Software thus encompasses a wide array of products that may be developed using different techniques such as ordinary programming languages, scripting languages, microcode, or an FPGA configuration. The types of software include web pages developed in languages and frameworks like HTML, PHP, Perl, JSP, ASP.NET, XML, and desktop applications like OpenOffice, Microsoft Word developed in languages like C, C++, Java, C#, or Smalltalk. Application software usually runs on an underlying software operating systems such as Linux or Microsoft Windows. Software (or firmware) is also used in video games and for the configurable parts of the logic systems of automobiles, televisions, and other consumer electronics. Computer software is so called to distinguish it from computer A layer structure showing where operating hardware, which encompasses the physical interconnections and system is located on generally used software devices required to store and execute (or run) the software. At the systems on desktops lowest level, executable code consists of machine language instructions specific to an individual processor. A machine language consists of groups of binary values signifying processor instructions that change the state of the computer from its preceding state. Programs are an ordered sequence of instructions for changing the state of the computer in a particular sequence. It is usually written in high-level programming languages that are easier and more efficient for humans to use (closer to natural language) than machine language. High-level languages are compiled or interpreted into machine language object code. Software may also be written in an assembly language, essentially, a mnemonic representation of a machine language using a natural language alphabet. Assembly language must be assembled into object code via an assembler. The term "software" was first used in this sense by John W. Tukey in 1958.[3] In computer science and software engineering, computer software is all computer programs. The theory that is the basis for most modern software was first proposed by Alan Turing in his 1935 essay Computable numbers with an application to the Entscheidungsproblem (Decision problem).[4]

Types of software Practical computer systems divide software systems into three major classes: system software, programming software and application software, although the distinction is arbitrary, and often blurred.

System software System software helps run the computer hardware and computer system. It includes a combination of the following: • • • •

device drivers operating systems servers utilities

• windowing systems

Computer software The purpose of systems software is to unburden the applications programmer from the often complex details of the particular computer being used, including such accessories as communications devices, printers, device readers, displays and keyboards, and also to partition the computer's resources such as memory and processor time in a safe and stable manner. Examples are - Windows, Linux, and Mac OS X.

Programming software Programming software usually provides tools to assist a programmer in writing computer programs, and software using different programming languages in a more convenient way. The tools include: • • • • •

compilers debuggers interpreters linkers text editors

An Integrated development environment (IDE) is a single application that attempts to manage all these functions.

Application software Application software allows end users to accomplish one or more specific (not directly computer development related) tasks. Typical applications include: • • • • • • • • • • • • • • •

industrial automation business software video games quantum chemistry and solid state physics software telecommunications (i.e., the Internet and everything that flows on it) databases educational software medical software military software molecular modeling software image editing spreadsheet simulation software Word processing Decision making software

Application software exists for and has impacted a wide variety of topics.

Software topics Architecture Users often see things differently than programmers. People who use modern general purpose computers (as opposed to embedded systems, analog computers and supercomputers) usually see three layers of software performing a variety of tasks: platform, application, and user software. • Platform software: Platform includes the firmware, device drivers, an operating system, and typically a graphical user interface which, in total, allow a user to interact with the computer and its peripherals (associated equipment). Platform software often comes bundled with the computer. On a PC you will usually have the ability to change the platform software.

Computer software • Application software: Application software or Applications are what most people think of when they think of software. Typical examples include office suites and video games. Application software is often purchased separately from computer hardware. Sometimes applications are bundled with the computer, but that does not change the fact that they run as independent applications. Applications are usually independent programs from the operating system, though they are often tailored for specific platforms. Most users think of compilers, databases, and other "system software" as applications. • User-written software: End-user development tailors systems to meet users' specific needs. User software include spreadsheet templates, word processor [Platform software: Platform includes the firmware, device drivers, an operating system, and typically a graphical user interface which, in total, allow a user to interact with the computer and its peripherals (associated equipment). Platform software often comes bundled with the computer. On a PC you will usually have the ability to change the platform software. Even email filters are a kind of user software. Users create this software themselves and often overlook how important it is. Depending on how competently the user-written software has been integrated into default application packages, many users may not be aware of the distinction between the original packages, and what has been added by co-workers.

Documentation Most software has software documentation so that the end user can understand the program, what it does, and how to use it. Without a clear documentation, software can be hard to use—especially if it is a very specialized and relatively complex software like the Photoshop or AutoCAD. Developer documentation may also exist, either with the code as comments and/or as separate files, detailing how the programs works and can be modified.

Library An executable is almost always not sufficiently complete for direct execution. Software libraries include collections of functions and functionality that may be embedded in other applications. Operating systems include many standard Software libraries, and applications are often distributed with their own libraries.

Standard Since software can be designed using many different programming languages and in many different operating systems and operating environments, software standard is needed so that different software can understand and exchange information between each other. For instance, an email sent from a Microsoft Outlook should be readable from Yahoo! Mail and vice versa.

Execution Computer software has to be "loaded" into the computer's storage (such as a [hard drive], memory, or RAM). Once the software has loaded, the computer is able to execute the software. This involves passing instructions from the application software, through the system software, to the hardware which ultimately receives the instruction as machine code. Each instruction causes the computer to carry out an operation – moving data, carrying out a computation, or altering the control flow of instructions. Data movement is typically from one place in memory to another. Sometimes it involves moving data between memory and registers which enable high-speed data access in the CPU. Moving data, especially large amounts of it, can be costly. So, this is sometimes avoided by using "pointers" to data instead. Computations include simple operations such as incrementing the value of a variable data element. More complex computations may involve many operations and data elements together.

Computer software

Quality and reliability Software quality is very important, especially for commercial and system software like Microsoft Office, Microsoft Windows and Linux. If software is faulty (buggy), it can delete a person's work, crash the computer and do other unexpected things. Faults and errors are called "bugs." Many bugs are discovered and eliminated (debugged) through software testing. However, software testing rarely â&#x20AC;&#x201C; if ever â&#x20AC;&#x201C; eliminates every bug; some programmers say that "every program has at least one more bug" (Lubarsky's Law). All major software companies, such as Microsoft, Novell and Sun Microsystems, have their own software testing departments with the specific goal of just testing. Software can be tested through unit testing, regression testing and other methods, which are done manually, or most commonly, automatically, since the amount of code to be tested can be quite large. For instance, NASA has extremely rigorous software testing procedures for many operating systems and communication functions. Many NASA based operations interact and identify each other through command programs called software. This enables many people who work at NASA to check and evaluate functional systems overall. Programs containing command software enable hardware engineering and system operations to function much easier together.

License The software's license gives the user the right to use the software in the licensed environment. Some software comes with the license when purchased off the shelf, or an OEM license when bundled with hardware. Other software comes with a free software license, granting the recipient the rights to modify and redistribute the software. Software can also be in the form of freeware or shareware.

Patents Software can be patented; however, software patents can be controversial in the software industry with many people holding different views about it. The controversy over software patents is that a specific algorithm or technique that the software has may not be duplicated by others and is considered an intellectual property and copyright infringement depending on the severity.

Design and implementation Design and implementation of software varies depending on the complexity of the software. For instance, design and creation of Microsoft Word software will take much longer time than designing and developing Microsoft Notepad because of the difference in functionalities in each one. Software is usually designed and created (coded/written/programmed) in integrated development environments (IDE) like Eclipse, Emacs and Microsoft Visual Studio that can simplify the process and compile the program. As noted in different section, software is usually created on top of existing software and the application programming interface (API) that the underlying software provides like GTK+, JavaBeans or Swing. Libraries (APIs) are categorized for different purposes. For instance, JavaBeans library is used for designing enterprise applications, Windows Forms library is used for designing graphical user interface (GUI) applications like Microsoft Word, and Windows Communication Foundation is used for designing web services. Underlying computer programming concepts like quicksort, hashtable, array, and binary tree can be useful to creating software. When a program is designed, it relies on the API. For instance, if a user is designing a Microsoft Windows desktop application, he/she might use the .NET Windows Forms library to design the desktop application and call its APIs like Form1.Close() and Form1.Show()[5] to close or open the application and write the additional operations him/herself that it need to have. Without these APIs, the programmer needs to write these APIs him/herself. Companies like Sun Microsystems, Novell, and Microsoft provide their own APIs so that many applications are written using their software libraries that usually have numerous APIs in them.

Computer software Software has special economic characteristics that make its design, creation, and distribution different from most other economic goods.[6] [7] A person who creates software is called a programmer, software engineer, software developer, or code monkey, terms that all essentially have a same meaning.

Industry and organizations The software industry is made up of different entities and peoples that produce software, and as a result there are many software companies and programmers in the world. Because software is increasingly used in many different areas like in finance, web searching, data mining, mathematics, space exploration, gaming and mining and such, software companies and people usually specialize in certain areas. For instance, Electronic Arts primarily creates video games. Also selling software can be quite a profitable industry. For instance, Bill Gates, the founder of Microsoft is the richest person in the world in 2009 largely by selling the Microsoft Windows and Microsoft Office software programs. The same goes for Larry Ellison, largely through his Oracle database software. There are also many non-profit software organizations like the Free Software Foundation, GNU Project, Mozilla Foundation. Also there are many software standard organizations like the W3C, IETF and others that try to come up with a software standard so that many software can work and interoperate with each other like through standards such as XML, HTML, HTTP or FTP. Some of the well known software companies include Microsoft, Oracle, Novell, SAP, Symantec, Adobe Systems, and Corel. Many small companies provide innovation. This is particularly important in the Internet information age where individuals set up small websites that compete with big companies.

References [1] software..(n.d.). Dictionary.com Unabridged (v 1.1). Retrieved 2007-04-13, from Dictionary.com website: http:/ / dictionary. reference. com/ browse/ software [2] "Wordreference.com: WordNet 2.0" (http:/ / www. wordreference. com/ definition/ software). Princeton University, Princeton, NJ. . Retrieved 2007-08-19. [3] "John Tukey, 85, Statistician; Coined the Word 'Software'" (http:/ / query. nytimes. com/ gst/ fullpage. html?res=9500E4DA173DF93BA15754C0A9669C8B63). New York Times. 2000-07-28. . [4] Hally, Mike (2005:79). Electronic brains/Stories from the dawn of the computer age. British Broadcasting Corporation and Granta Books, London. ISBN 1-86207-663-4. [5] MSDN Library (http:/ / msdn. microsoft. com/ en-us/ library/ default. aspx) [6] v. Engelhardt, Sebastian (2008): "The Economic Properties of Software", Jena Economic Research Papers, Volume 2 (2008), Number 2008-045. (http:/ / ideas. repec. org/ p/ jrp/ jrpwrp/ 2008-045. html) (in Adobe pdf format) [7] "Why Open Source Is The Optimum Economic Paradigm for Software" (http:/ / www. doxpara. com/ read. php/ core. html) by Dan Kaminsky 1999

History History of programming languages This article discusses the major developments in the history of programming languages. For a detailed timeline of events, see the timeline of programming languages.

Before 1940 The first programming languages predate the modern computer. At first, the languages were codes. The Jacquard loom, invented in 1801, used holes in punched cards to represent sewing loom arm movements in order to generate decorative patterns automatically. During a nine-month period in 1842-1843, Ada Lovelace translated the memoir of Italian mathematician Luigi Menabrea about Charles Babbage's newest proposed machine, the Analytical Engine. With the article, she appended a set of notes which specified in complete detail a method for calculating Bernoulli numbers with the Engine, recognized by some historians as the world's first computer program.[1] Herman Hollerith realized that he could encode information on punch cards when he observed that train conductors encode the appearance of the ticket holders on the train tickets using the position of punched holes on the tickets. Hollerith then encoded the 1890 census data on punch cards. The first computer codes were specialized for their applications. In the first decades of the 20th century, numerical calculations were based on decimal numbers. Eventually it was realized that logic could be represented with numbers, not only with words. For example, Alonzo Church was able to express the lambda calculus in a formulaic way. The Turing machine was an abstraction of the operation of a tape-marking machine, for example, in use at the telephone companies. Turing machines set the basis for storage of programs as data in the von Neumann architecture of computers by representing a machine through a finite number. However, unlike the lambda calculus, Turing's code does not serve well as a basis for higher-level languagesâ&#x20AC;&#x201D;its principal use is in rigorous analyses of algorithmic complexity. Like many "firsts" in history, the first modern programming language is hard to identify. From the start, the restrictions of the hardware defined the language. Punch cards allowed 80 columns, but some of the columns had to be used for a sorting number on each card. FORTRAN included some keywords which were the same as English words, such as "IF", "GOTO" (go to) and "CONTINUE". The use of a magnetic drum for memory meant that computer programs also had to be interleaved with the rotations of the drum. Thus the programs were more hardware-dependent than today. To some people, what was the first modern programming language depends on how much power and human-readability is required before the status of "programming language" is granted. Jacquard looms and Charles Babbage's Difference Engine both had simple, extremely limited languages for describing the actions that these machines should perform. One can even regard the punch holes on a player piano scroll as a limited domain-specific language, albeit not designed for human consumption.

History of programming languages

The 1940s In the 1940s the first recognizably modern, electrically powered computers were created. The limited speed and memory capacity forced programmers to write hand tuned assembly language programs. It was soon discovered that programming in assembly language required a great deal of intellectual effort and was error-prone. In 1948, Konrad Zuse published a paper about his programming language Plankalkül.[2] However, it was not implemented in his lifetime and his original contributions were isolated from other developments. Some important languages that were developed in this period include: • 1943 - Plankalkül (Konrad Zuse) • 1943 - ENIAC coding system • 1949 - C-10

The 1950s and 1960s In the 1950s the first three modern programming languages whose descendants are still in widespread use today were designed: • FORTRAN (1955), the "FORmula TRANslator", invented by John Backus et al.; • LISP, the "LISt Processor", invented by John McCarthy et al.; • COBOL, the COmmon Business Oriented Language, created by the Short Range Committee, heavily influenced by Grace Hopper. Another milestone in the late 1950s was the publication, by a committee of American and European computer scientists, of "a new language for algorithms"; the ALGOL 60 Report (the "ALGOrithmic Language"). This report consolidated many ideas circulating at the time and featured two key language innovations: • nested block structure: code sequences and associated declarations could be grouped into blocks without having to be turned into separate, explicitly named procedures; • lexical scoping: a block could have its own private variables, procedures and functions, invisible to code outside that block, i.e. information hiding. Another innovation, related to this, was in how the language was described: • a mathematically exact notation, Backus-Naur Form (BNF), was used to describe the language's syntax. Nearly all subsequent programming languages have used a variant of BNF to describe the context-free portion of their syntax. Algol 60 was particularly influential in the design of later languages, some of which soon became more popular. The Burroughs large systems were designed to be programmed in an extended subset of Algol. Algol's key ideas were continued, producing ALGOL 68: • syntax and semantics became even more orthogonal, with anonymous routines, a recursive typing system with higher-order functions, etc.; • not only the context-free part, but the full language syntax and semantics were defined formally, in terms of Van Wijngaarden grammar, a formalism designed specifically for this purpose. Algol 68's many little-used language features (e.g. concurrent and parallel blocks) and its complex system of syntactic shortcuts and automatic type coercions made it unpopular with implementers and gained it a reputation of being difficult. Niklaus Wirth actually walked out of the design committee to create the simpler Pascal language. Some important languages that were developed in this period include: • 1951 - Regional Assembly Language • 1952 - Autocode • 1954 - FORTRAN

History of programming languages • • • • • • • • • • • • • •

1954 - IPL (forerunner to LISP) 1955 - FLOW-MATIC (forerunner to COBOL) 1957 - COMTRAN (forerunner to COBOL) 1958 - LISP 1958 - ALGOL 58 1959 - FACT (forerunner to COBOL) 1959 - COBOL 1962 - APL 1962 - Simula 1962 - SNOBOL 1963 - CPL (forerunner to C) 1964 - BASIC 1964 - PL/I 1967 - BCPL (forerunner to C)

1967-1978: establishing fundamental paradigms The period from the late 1960s to the late 1970s brought a major flowering of programming languages. Most of the major language paradigms now in use were invented in this period: • Simula, invented in the late 1960s by Nygaard and Dahl as a superset of Algol 60, was the first language designed to support object-oriented programming. • C, an early systems programming language, was developed by Dennis Ritchie and Ken Thompson at Bell Labs between 1969 and 1973. • Smalltalk (mid 1970s) provided a complete ground-up design of an object-oriented language. • Prolog, designed in 1972 by Colmerauer, Roussel, and Kowalski, was the first logic programming language. • ML built a polymorphic type system (invented by Robin Milner in 1973) on top of Lisp, pioneering statically typed functional programming languages. Each of these languages spawned an entire family of descendants, and most modern languages count at least one of them in their ancestry. The 1960s and 1970s also saw considerable debate over the merits of "structured programming", which essentially meant programming without the use of Goto. This debate was closely related to language design: some languages did not include GOTO, which forced structured programming on the programmer. Although the debate raged hotly at the time, nearly all programmers now agree that, even in languages that provide GOTO, it is bad programming style to use it except in rare circumstances. As a result, later generations of language designers have found the structured programming debate tedious and even bewildering. Some important languages that were developed in this period include: • • • • • • • •

1968 - Logo 1970 - Pascal 1970 - Forth 1972 - C 1972 - Smalltalk 1972 - Prolog 1973 - ML 1975 - Scheme

• 1978 - SQL (initially only a query language, later extended with programming constructs)

History of programming languages

The 1980s: consolidation, modules, performance The 1980s were years of relative consolidation. C++ combined object-oriented and systems programming. The United States government standardized Ada, a systems programming language intended for use by defense contractors. In Japan and elsewhere, vast sums were spent investigating so-called fifth-generation programming languages that incorporated logic programming constructs. The functional languages community moved to standardize ML and Lisp. Rather than inventing new paradigms, all of these movements elaborated upon the ideas invented in the previous decade. However, one important new trend in language design was an increased focus on programming for large-scale systems through the use of modules, or large-scale organizational units of code. Modula, Ada, and ML all developed notable module systems in the 1980s. Module systems were often wedded to generic programming constructs---generics being, in essence, parameterized modules (see also polymorphism in object-oriented programming). Although major new paradigms for programming languages did not appear, many researchers expanded on the ideas of prior languages and adapted them to new contexts. For example, the languages of the Argus and Emerald systems adapted object-oriented programming to distributed systems. The 1980s also brought advances in programming language implementation. The RISC movement in computer architecture postulated that hardware should be designed for compilers rather than for human assembly programmers. Aided by processor speed improvements that enabled increasingly aggressive compilation techniques, the RISC movement sparked greater interest in compilation technology for high-level languages. Language technology continued along these lines well into the 1990s. Some important languages that were developed in this period include: • • • • • • • •

1980 - C++ (as C with classes) 1983 - Ada 1984 - Common Lisp 1985 - Eiffel 1986 - Erlang 1987 - Perl 1988 - Tcl 1989 - FL (Backus)

The 1990s: the Internet age The 1990s saw no fundamental novelty, but much recombination as well as maturation of old ideas. A big driving philosophy was programmer productivity. Many "rapid application development" (RAD) languages emerged, which usually came with an IDE, garbage collection, and were descendants of older languages. All such languages were object-oriented. These included Object Pascal, Visual Basic, and C#. Java was a more conservative language that also featured garbage collection and received much attention. More radical and innovative than the RAD languages were the new scripting languages. These did not directly descend from other languages and featured new syntaxes and more liberal incorporation of features. Many consider these scripting languages to be more productive than even the RAD languages, but often because of choices that make small programs simpler but large programs more difficult to write and maintain. Nevertheless, scripting languages came to be the most prominent ones used in connection with the Web. Some important languages that were developed in this period include: • 1990 - Haskell • 1991 - Python • 1991 - Java

History of programming languages • • • • • • •

1993 - Ruby 1993 - Lua 1994 - CLOS (part of ANSI Common Lisp) 1995 - Delphi (Object Pascal) 1995 - JavaScript 1995 - PHP 1997 - Rebol

Current trends Programming language evolution continues, in both industry and research. Some of the current trends include: • Mechanisms for adding security and reliability verification to the language: extended static checking, information flow control, static thread safety. • Alternative mechanisms for modularity: mixins, delegates, aspects. • Component-oriented software development. • Metaprogramming, reflection or access to the abstract syntax tree • Increased emphasis on distribution and mobility. • Integration with databases, including XML and relational databases. • Support for Unicode so that source code (program text) is not restricted to those characters contained in the ASCII character set; allowing, for example, use of non-Latin-based scripts or extended punctuation. • XML for graphical interface (XUL, XAML). Some important languages developed during this period include: • • • • • • • •

2001 - C# 2001 - Visual Basic .NET 2002 - F# 2003 - Scala 2003 - Factor 2006 - Windows Power Shell 2007 - Clojure 2009 - Go

Prominent people in the history of program languages • • • • • • • • • •

John Backus, inventor of Fortran. Alan Cooper, developer of Visual Basic. Edsger W. Dijkstra, developed the framework for proper programming. James Gosling, developer of Oak, the precursor of Java. Anders Hejlsberg, developer of Turbo Pascal, Delphi and C#. Grace Hopper, developer of Flow-Matic, influencing COBOL. Kenneth E. Iverson, developer of APL, and co-developer of J along with Roger Hui. Bill Joy, inventor of vi, early author of BSD Unix, and originator of SunOS, which became Solaris. Alan Kay, pioneering work on object-oriented programming, and originator of Smalltalk. Brian Kernighan, co-author of the first book on the C programming language with Dennis Ritchie, coauthor of the AWK and AMPL programming languages. • John McCarthy, inventor of LISP. • John von Neumann, originator of the operating system concept. • Dennis Ritchie, inventor of C. • Bjarne Stroustrup, developer of C++.

History of programming languages • • • •

Ken Thompson, inventor of Unix. Niklaus Wirth inventor of Pascal, Modula and Oberon. Larry Wall creator of Perl and Perl 6 Guido van Rossum creator of Python

See also • • • • • •

Compiler Disassembler Instruction set Little man computer – an educational computer model with a base-10 assembly language Microassembler Typed assembly language

Further reading • ASM Community Book [23] "An online book full of helpful ASM info, tutorials and code examples" by the ASM Community • Jonathan Bartlett: Programming from the Ground Up [24]. Bartlett Publishing, 2004. ISBN 0-9752838-4-7 Also available online as PDF [25] • Robert Britton: MIPS Assembly Language Programming. Prentice Hall, 2003. ISBN 0-13-142044-5 • Paul Carter: PC Assembly Language. Free ebook, 2001. Website [26] • Jeff Duntemann: Assembly Language Step-by-Step. Wiley, 2000. ISBN 0-471-37523-3 • Randall Hyde: The Art of Assembly Language. No Starch Press, 2003. ISBN 1-886411-97-2 Draft versions available online [27] as PDF and HTML • Peter Norton, John Socha, Peter Norton's Assembly Language Book for the IBM PC, Brady Books, NY: 1986.

Assembly language • Michael Singer, PDP-11. Assembler Language Programming and Machine Organization, John Wiley & Sons, NY: 1980. • Dominic Sweetman: See MIPS Run. Morgan Kaufmann Publishers, 1999. ISBN 1-55860-410-3 • John Waldron: Introduction to RISC Assembly Language Programming. Addison Wesley, 1998. ISBN 0-201-39828-1

External links • • • • • • • • • •

Randall Hyde's The Art of Assembly Language as HTML and PDF version [27] Machine language for beginners [28] Introduction to assembly language [29] The ASM Community [30], a programming resource about assembly including an ASM Book [23] Intel Assembly 80x86 CodeTable [31] (a cheat sheet reference) Unix Assembly Language Programming [32] IBM z/Architecture Principles of Operation [33] IBM manuals on mainframe machine language and internals. IBM High Level Assembler [34] IBM manuals on mainframe assembler language. PPR: Learning Assembly Language [35] An Introduction to Writing 32-bit Applications Using the x86 Assembly Language [36]

• • • • • • • • •

Assembly Language Programming Examples [37] Authoring Windows Applications In Assembly Language [38] Information on Linux assembly programming [39] x86 Instruction Set Reference [40] Iczelion's Win32 Assembly Tutorial [41] Assembly Optimization Tips [42] by Mark Larson NASM Manual [43] 8086 assembly coding [44] by F.A. Smit Microchip PIC assembly coding basics [45]

References [1] [2] [3] [4]

David Salomon (1993). Assemblers and Loaders (http:/ / www. davidsalomon. name/ assem. advertis/ asl. pdf) Beck, Leland L. (1996). "2". System Software: An Introduction to Systems Programming. Addison Wesley. "Concept 14 Macros" (http:/ / skycoast. us/ pscott/ software/ mvs/ concept14. html). MVS Software. . Retrieved May 25, 2009. Answers.com. "assembly language: Definition and Much More from Answers.com" (http:/ / www. answers. com/ topic/ assembly-language?cat=technology). . Retrieved 2008-06-19. [5] NESHLA: The High Level, Open Source, 6502 Assembler for the Nintendo Entertainment System (http:/ / neshla. sourceforge. net/ ) [6] Eidolon's Inn : SegaBase Saturn (http:/ / www. eidolons-inn. net/ tiki-index. php?page=SegaBase+ Saturn) [7] http:/ / www. theflamearrows. info/ homepage. html [8] Jim Lawless (2004-05-21). "Speaking with Don French : The Man Behind the French Silk Assembler Tools" (http:/ / www. radiks. net/ ~jimbo/ art/ int7. htm). . Retrieved 2008-07-25. [9] "Writing the Fastest Code, by Hand, for Fun: A Human Computer Keeps Speeding Up Chips" (http:/ / www. nytimes. com/ 2005/ 11/ 28/ technology/ 28super. html?_r=1). New York Times, John Markoff. 2005-11-28. . Retrieved 2010-03-04. [10] "Bit-field-badness" (http:/ / hardwarebug. org/ 2010/ 01/ 30/ bit-field-badness/ ). hardwarebug.org. 2010-01-30. . Retrieved 2010-03-04. [11] "GCC makes a mess" (http:/ / hardwarebug. org/ 2009/ 05/ 13/ gcc-makes-a-mess/ ). hardwarebug.org. 2009-05-13. . Retrieved 2010-03-04. [12] Randall Hyde. "The Great Debate" (http:/ / webster. cs. ucr. edu/ Page_TechDocs/ GreatDebate/ debate1. html). . Retrieved 2008-07-03. [13] "Code sourcery fails again" (http:/ / hardwarebug. org/ 2008/ 11/ 28/ codesourcery-fails-again/ ). hardwarebug.org. 2010-01-30. . Retrieved 2010-03-04. [14] "68K Programming in Fargo II" (http:/ / tifreakware. net/ tutorials/ 89/ a/ calc/ fargoii. htm). . Retrieved 2008-07-03. [15] Hyde, Randall (1996-09-30). "Foreword ("Why would anyone learn this stuff?"), op. cit." (http:/ / www. arl. wustl. edu/ ~lockwood/ class/ cs306/ books/ artofasm/ fwd. html). . Retrieved 2010-03-05. [16] "256bytes demos archives" (http:/ / web. archive. org/ web/ 20080211025322rn_1/ www. 256b. com/ home. php). . Retrieved 2008-07-03. [17] Stroustrup, Bjarne, The C++ Programming Language, Addison-Wesley, 1986, ISBN 0-201-12078-X: "C++ was primarily designed so that the author and his friends would not have to program in assembler, C, or various modern high-level languages. [use of the term assembler to

Assembly language mean assembly language]" [18] Saxon, James, and Plette, William, Programming the IBM 1401, Prentice-Hall, 1962, LoC 62-20615. [use of the term assembly program] [19] Randall Hyde. "Which Assembler is the Best?" (http:/ / webster. cs. ucr. edu/ AsmTools/ WhichAsm. html). . Retrieved 2007-10-19. [20] "vid". "FASMLIB: Features" (http:/ / fasmlib. x86asm. net/ features. html). . Retrieved 2007-10-19. [21] Murdocca, Miles J.; Vincent P. Heuring (2000). Principles of Computer Architecture. Prentice-Hall. ISBN 0-201-43664-7. [22] Principles of Computer Architecture (http:/ / iiusatech. com/ ~murdocca/ POCA) (POCA) – ARCTools virtual computer available for download to execute referenced code, accessed August 24, 2005 [23] http:/ / www. asmcommunity. net/ book/ [24] http:/ / programminggroundup. blogspot. com/ [25] http:/ / download. savannah. gnu. org/ releases-noredirect/ pgubook/ ProgrammingGroundUp-1-0-booksize. pdf [26] http:/ / drpaulcarter. com/ pcasm/ [27] http:/ / webster. cs. ucr. edu/ AoA/ index. html [28] http:/ / www. atariarchives. org/ mlb/ introduction. php [29] http:/ / www. swansontec. com/ sprogram. htm [30] http:/ / www. asmcommunity. net/ [31] http:/ / www. jegerlehner. ch/ intel/ IntelCodeTable. pdf [32] http:/ / www. int80h. org/ [33] http:/ / www-03. ibm. com/ systems/ z/ os/ zos/ bkserv/ r8pdf/ index. html#zarchpops [34] http:/ / www-03. ibm. com/ systems/ z/ os/ zos/ bkserv/ r8pdf/ index. html#hlasm [35] http:/ / c2. com/ cgi/ wiki?LearningAssemblyLanguage [36] http:/ / siyobik. info/ index. php?document=x86_32bit_asm [37] http:/ / www. azillionmonkeys. com/ qed/ asmexample. html [38] [39] [40] [41] [42] [43] [44] [45]

http:/ / www. grc. com/ smgassembly. htm http:/ / linuxassembly. org/ http:/ / siyobik. info/ index. php?module=x86 http:/ / win32assembly. online. fr/ tutorials. html http:/ / mark. masmcode. com/ http:/ / nasm. sourceforge. net/ doc/ nasmdoc0. html http:/ / www. xs4all. nl/ ~smit/ asm01001. htm#index1 http:/ / www. microautomate. com/ PIC/ lets-begin-assembly. html

BASIC

Screenshot of Atari BASIC, an early BASIC language for small computers

Paradigm

unstructured, later procedural, later object-oriented

Appeared in Designed by

John George Kemeny and Thomas Eugene Kurtz

Typing discipline

strong

Major implementations Apple BASIC, Commodore BASIC, Microsoft BASIC, BBC BASIC, TI-BASIC Influenced by

ALGOL 60, FORTRAN II, JOSS

Influenced

COMAL, Visual BASIC, Visual Basic .NET, Realbasic, REXX, Perl, GRASS

In computer programming, BASIC (an acronym for Beginner's All-purpose Symbolic Instruction Code[1] ) is a family of high-level programming languages. The original BASIC was designed in 1964 by John George Kemeny and Thomas Eugene Kurtz at Dartmouth College in New Hampshire, USA to provide computer access to non-science students. At the time, nearly all use of computers required writing custom software, which was something only scientists and mathematicians tended to be able to do. The language and its variants became widespread on microcomputers in the late 1970s and 1980s. BASIC remains popular to this day in a handful of highly modified dialects and new languages influenced by BASIC such as Microsoft Visual Basic. As of 2006, 59% of developers for the .NET platform used Visual Basic .NET as their only language.[2]

History Before the mid-1960s, computers were extremely expensive and used only for special-purpose tasks. A simple batch processing arrangement ran only a single "job" at a time, one after another. But during the 1960s faster and more affordable computers became available. With this extra processing power, computers would sometimes sit idle, without jobs to run. Programming languages in the batch programming era tended to be designed, like the machines on which they ran, for specific purposes (such as scientific formula calculations or business data processing or eventually for text editing). Since even the newer, less expensive machines were still major investments, there was a strong tendency to consider efficiency to be the most important feature of a language. In general, these specialized languages were difficult to use and had widely disparate syntax. As prices decreased, the possibility of sharing computer access began to move from research labs to commercial use. Newer computer systems supported time-sharing, a system which allows multiple users or processes to use the RAM and memory. In such a system the operating system alternates between running processes, giving each one running time on the RAM before switching to another. The machines had become fast enough that most users could feel they had the machine all to themselves. In theory, timesharing reduced the cost of computing tremendously, as a single

BASIC

machine could be shared among hundreds of users.

Early years: the mainframe and mini-computer era The original BASIC language was designed in 1963 by John Kemeny and Thomas Kurtz[3] and implemented by a team of Dartmouth students under their direction. BASIC was designed to allow students to write programs for the Dartmouth Time-Sharing System. It was intended to address the complexity issues of older languages with a new language design specifically for the new class of users that time-sharing systems allowedâ&#x20AC;&#x201D;that is, a less technical user who did not have the mathematical background of the more traditional users and was not interested in acquiring it. Being able to use a computer to support teaching and research was quite novel at the time. In the following years, as other dialects of BASIC appeared, Kemeny and Kurtz's original BASIC dialect became known as Dartmouth BASIC. The eight design principles of BASIC were: 1. 2. 3. 4. 5.

Be easy for beginners to use. Be a general-purpose programming language. Allow advanced features to be added for experts (while keeping the language simple for beginners). Be interactive. Provide clear and friendly error messages.

6. Respond quickly for small programs. 7. Not to require an understanding of computer hardware. 8. Shield the user from the operating system. The language was based partly on FORTRAN II and partly on ALGOL 60, with additions to make it suitable for timesharing. (The features of other time-sharing systems such as JOSS and CORC, and to a lesser extent LISP, were also considered.) It had been preceded by other teaching-language experiments at Dartmouth such as the DARSIMCO (1956) and DOPE (1962 implementations of SAP and DART (1963) which was a simplified FORTRAN II). Initially, BASIC concentrated on supporting straightforward mathematical work, with matrix arithmetic support from its initial implementation as a batch language and full string functionality being added by 1965. BASIC was first implemented on the GE-265 mainframe which supported multiple terminals. At the time of its introduction, it was a compiled language. It was also quite efficient, beating FORTRAN II and ALGOL 60 implementations on the 265 at several fairly computationally intensive (at the time) programming problems such as numerical integration by Simpson's Rule. The designers of the language decided to make the compiler available free of charge so that the language would become widespread. They also made it available to high schools in the Hanover, NH area and put a considerable amount of effort into promoting the language. As a result, knowledge of BASIC became relatively widespread (for a computer language) and BASIC was implemented by a number of manufacturers, becoming fairly popular on newer minicomputers like the DEC PDP series and the Data General Nova. The BASIC language was also central to the HP Time-Shared BASIC system in the late 1960s and early 1970s, and the Pick operating system. In these instances the language tended to be implemented as an interpreter, instead of (or in addition to) a compiler. Several years after its release, highly respected computer professionals, notably Edsger W. Dijkstra, expressed their opinions that the use of GOTO statements, which existed in many languages including BASIC, promoted poor programming practices.[4] Some have also derided BASIC as too slow (most interpreted versions are slower than equivalent compiled versions) or too simple (many versions, especially for small computers, left out important features and capabilities).

BASIC

Explosive growth: the home computer era Notwithstanding the language's use on several minicomputers, it was the introduction of the MITS Altair 8800 "kit" microcomputer in 1975 that provided BASIC a path to universality. Most programming languages required suitable text editors, large amounts of memory and available disk space, whereas the early microcomputers had no resident editors, limited memory and often substituted recordable audio tapes for disk space. All these issues allowed a language like BASIC, in its interpreted form with a built-in code editor, to operate within those constraints. BASIC also had the advantage that it was fairly MSX BASIC version 3.0 well-known to the young designers and computer hobbyists who took an interest in microcomputers, and generally worked in the electronics industries of the day. Kemeny and Kurtz's earlier proselytizing paid off in this respect and the few hobbyists' journals of the era were filled with columns that made mentions of the language or focused entirely on one version compared to others. One of the first to appear for the 8080 machines like the Altair was Tiny BASIC, a simple BASIC implementation originally written by Dr. Li-Chen Wang, and then ported onto the Altair by Dennis Allison at the request of Bob Albrecht (who later founded Dr. Dobb's Journal). The Tiny BASIC design and the full source code were published in 1976 in DDJ. In 1975, MITS released Altair BASIC, developed by college drop-outs Bill Gates and Paul Allen as the company Micro-Soft, which grew into today's corporate giant, Microsoft. The first Altair version was co-written by Gates, Allen, and Monte Davidoff in a burst of enthusiasm and neglect of studies. Versions of Microsoft BASIC (also known then, and most widely as M BASIC or MBASIC) was soon bundled with the original floppy disk-based CP/M computers, which became widespread in small business environments. As the popularity of BASIC on CP/M spread, newer computer designs also introduced their own version of the language, or had Micro-Soft port its version to their platform. When three major new computers were introduced in what Byte Magazine would later call the "1977 Trinity",[5] all three had BASIC as their primary programming language and operating environment. The Commodore PET licensed a version of Micro-Soft BASIC that was ported to the MOS 6502, while Apple II and TRS-80 both introduced new, largely similar versions of the language. As new companies entered the field, additional versions were added that subtly changed the BASIC family. The Atari 8-bit family had their own Atari BASIC that was modified in order to fit on an 8 kB ROM cartridge. The BBC published BBC BASIC, developed for them by Acorn Computers Ltd, incorporating many extra structuring keywords. Most of the home computers of the 1980s had a ROM-resident BASIC interpreter, allowing the machines to boot directly into BASIC. Because of this legacy, there are more dialects of BASIC than there are of any other programming language. As the popularity of BASIC grew in this period, magazines (such as Creative Computing Magazine in the US) published complete source code in BASIC for games, utilities, and other programs. Given BASIC's straightforward nature, it was a simple matter to type in the code from the magazine and execute the program. Different magazines were published featuring programs for specific computers, though some BASIC programs were considered universal and could be used in machines running any variant of BASIC (sometimes with minor adaptations). Correcting the publishing errors that frequently occurred in magazine listings was an educational exercise in itself. BASIC source code was also published in fully-fledged books: the seminal examples being David Ahl's BASIC Computer Games series.[6] [7] [8] Later packages, such as Learn to Program BASIC would also have gaming as an

BASIC

introductory focus.

Maturity: the personal computer era As early as 1979 Microsoft was in negotiations with IBM to supply their IBM PCs with an IBM Cassette BASIC (BASIC C) inside BIOS. Microsoft sold several versions of BASIC for MS-DOS/PC-DOS including IBM Disk BASIC (BASIC D), IBM BASICA (BASIC A), GW-BASIC (a BASICA-compatible version that did not need IBM's ROM) and QuickBASIC. Turbo Pascal-publisher Borland published Turbo BASIC 1.0 in 1985 (successor versions are still being marketed by the original author under the name PowerBASIC). Microsoft wrote the windowing based AmigaBASIC that was supplied with version 1.1 of the pre-emptive multitasking GUI Amiga computers (late 1985/ early 1986), although the product unusually did not bear any Microsoft marks. These languages introduced many extensions to the original home computer BASIC, such as improved string manipulation and graphics support, access to the file system and additional data types. More important were the facilities for structured programming, including additional control structures and proper subroutines supporting local variables. The new graphical features of these langauges also helped lay the groundwork for PC video gaming, with BASIC programs like DONKEY.BAS showing what the PC could do.

IBM Cassette BASIC 1.10

IBM Disk BASIC 1.10

However, by the latter half of the 1980s newer computers were far more capable IBM BASICA 1.10 with more resources. At the same time, computers had progressed from a hobbyist interest to tools used primarily for applications written by others, and programming became less important for most users. BASIC started to recede in

BASIC

importance, though numerous versions remained available. Compiled BASIC or CBASIC is still used in many IBM 4690 OS point of sale systems. BASIC's fortunes reversed once again with the introduction of Visual Basic by Microsoft. It is somewhat difficult to consider this language to be BASIC, because of the major shift in its orientation towards an object-oriented and event-driven perspective. The only significant similarity GW-BASIC 3.23 to older BASIC dialects was familiar syntax. Syntax itself no longer "fully defined" the language, since much development was done using "drag and drop" methods without exposing all code for commonly used objects such as buttons and scrollbars to the developer. While this could be considered an evolution of the language, few of the distinctive features of early Dartmouth BASIC, such as line numbers and the INPUT keyword, remain (although Visual Basic still uses INPUT to read data from files, and INPUTBOX is available for direct user input; line numbers can also optionally be used in all VB versions, even VB.NET, albeit they cannot be used in certain places, for instance before SUB). Ironically given the origin of BASIC as a "beginner's" language, and apparently even to the surprise of many at Microsoft who still initially marketed Visual Basic or "VB" as a language for hobbyists, the language had come into widespread use for small custom business applications shortly after the release of VB version 3.0, which is widely considered the first relatively stable version. While many advanced programmers still scoffed at its use, VB met the needs of small businesses efficiently wherever processing speed was less of a concern than easy development. By that time, computers running Windows 3.1 had become fast enough that many business-related processes could be completed "in the blink of an eye" even using a "slow" language, as long as massive amounts of data were not involved. Many small business owners found they could create their own small yet useful applications in a few evenings to meet their own specialized needs. Eventually, during the lengthy lifetime of VB3, knowledge of Visual Basic had become a marketable job skill. The language, like QBasic before it,[9] also became a favourite for amateur game development,[10] with one example being the Arthur Yahtzee trilogy of amateur adventure games. Many BASIC dialects have also sprung up in the last few years, including Bywater BASIC and True BASIC (the direct successor to Dartmouth BASIC from a company controlled by Kurtz). One notable variant is RealBasic which although first released in 1998 for Macintosh computers, has since 2005 fully compiled programs for Microsoft Windows, Mac OS X and 32-bit x86 Linux, from the same object-oriented source code. RealBasic compiled programs may execute natively on these platforms as services, consoles or windowed applications. However in keeping with BASIC tradition, single-platform hobbyist Three modern Basic variants: Mono Basic, OpenOffice.org Basic and Gambas

BASIC versions are also still maintained. Many other BASIC variants and adaptations have been written by hobbyists, equipment developers, and others, as it is a relatively simple language to develop translators for. An example of an open source interpreter, written in C, is MiniBasic [11]. More complex examples of free software BASIC implementations (development tools and compilers) include Gambas and FreeBASIC. The ubiquity of BASIC interpreters on personal computers was such that textbooks once included simple "Try It In BASIC" exercises that encouraged students to experiment with mathematical and computational concepts on classroom or home computers. Futurist and sci-fi writer David Brin mourned the loss of ubiquitous BASIC in a 2006 Salon article.[12]

Examples Unstructured BASIC New BASIC programmers on a home computer might start with a simple program similar to the Hello world program made famous by Kernighan and Ritchie. This generally involves simple use of the language's PRINT statement to display the message (such as the programmer's name) to the screen. Often an infinite loop was used to fill the display with the message. Most first generation BASIC languages such as MSX BASIC and GW-BASIC supported simple data types, loop cycles and arrays. The following example is written for GW-BASIC, but will work in most versions of BASIC with minimal changes: 10 INPUT "What is your name: ", U$ 20 PRINT "Hello "; U$ 30 INPUT "How many stars do you want: ", N 40 S$ = "" 50 FOR I = 1 TO N 60 S$ = S$ + "*" 70 NEXT I 80 PRINT S$ 90 INPUT "Do you want more stars? ", A$ 100 IF LEN(A$) = 0 THEN GOTO 90 110 A$ = LEFT$(A$, 1) 120 IF A$ = "Y" OR A$ = "y" THEN GOTO 30 130 PRINT "Goodbye "; U$ 140 END

Structured BASIC Second generation BASICs (for example QuickBASIC and PowerBASIC) introduced a number of features into the language, primarily related to structured and procedure-oriented programming. Usually, line numbering is omitted from the language and replaced with labels (for GOTO) and procedures to encourage easier and more flexible design.[13] INPUT "What is your name: ", UserName$ PRINT "Hello "; UserName$ DO INPUT "How many stars do you want: ", NumStars Stars$ = STRING$(NumStars, "*") PRINT Stars$ DO INPUT "Do you want more stars? ", Answer$

BASIC

LOOP UNTIL Answer$ <> "" Answer$ = LEFT$(Answer$, 1) LOOP WHILE UCASE$(Answer$) = "Y" PRINT "Goodbye "; UserName$

BASIC with object-oriented features Third generation BASIC dialects such as Visual Basic, REALbasic, StarOffice Basic and BlitzMax introduced features to support object-oriented and event-driven programming paradigm. Most built-in procedures and functions now represented as methods of standard objects rather than operators. The following example is in Visual Basic .NET:

Public Class StarsProgram Public Shared Sub Main() Dim UserName, Answer, stars As String, NumStars As Integer Console.Write("What is your name: ") UserName = Console.ReadLine() Console.WriteLine("Hello {0}", UserName) Do Console.Write("How many stars do you want: ") NumStars = CInt(Console.ReadLine()) stars = New String("*", NumStars) Console.WriteLine(stars) Do Console.Write("Do you want more stars? ") Answer = Console.ReadLine() Loop Until Answer <> "" Answer = Answer.Substring(0, 1) Loop While Answer.ToUpper() = "Y" Console.WriteLine("Goodbye {0}", UserName) End Sub End Class

List of BASIC programming commands/statements 1. Let Command - used to assign value/content to the variable. 2. Input Statement - is a conversational statement wherein the computer asks the user the value of the variable. Types of Input Statement a. Ordinary Input b. Prompted Input - (String Variable Name is used) 3. If ... Then Statement - is used in comparison or decision making. 4. Tab Function - allows the computer user to have complete control on the position where the next character will be shown on the screen or printed on paper. Argument - represents the position where printing will be made. 5. GOSUB Command - is used to avoid repetitive typing of the same set of instructions in the program.

BASIC

6. REM (Remarks) - is used to assign/give title to the program and to help identify the purpose of a given section of code. 7. ON ... GOTO Command - allows the program to choose from a list of line numbers where to go depending on certain conditions. It is the variation of If ... Then Statement and GOTO Command.

See also • List of BASIC dialects • List of BASIC dialects by platform

References • (PDF) A Manual for BASIC, the elementary algebraic language designed for use with the Dartmouth Time Sharing System [14]. Dartmouth College Computation Center. 1964.—The original Dartmouth BASIC manual. • Lien, David A. (1986). The Basic Handbook: Encyclopedia of the BASIC Computer Language (3rd ed. ed.). Compusoft Publishing. ISBN 0-932760-33-3.—Documents dialect variations for over 250 versions of BASIC. • Kemeny, John G.; Kurtz, Thomas E. (1985). Back To BASIC: The History, Corruption, and Future of the Language. Addison-Wesley. pp. 141 pp. ISBN 0-201-13433-0. • Sammet, Jean E. (1969). Programming languages: History and fundamentals. Englewood Cliffs, N.J.: Prentice-Hall. • The Encyclopedia of Computer Languages. BASIC - Beginners All-purpose Symbolic Instruction Code [15]. Murdoch University.

Standards • ANSI/ISO/IEC Standard for Minimal BASIC: • ANSI X3.60-1978 "FOR MINIMAL BASIC" • ISO/IEC 6373:1984 "DATA PROCESSING - PROGRAMMING LANGUAGES - MINIMAL BASIC" • ANSI/ISO/IEC Standard for Full BASIC: • ANSI X3.113-1987 "PROGRAMMING LANGUAGES FULL BASIC" • INCITS/ISO/IEC 10279-1991 (R2005) "Information Technology - Programming Languages - Full BASIC" [16]

• ANSI/ISO/IEC Addendum Defining Modules: • ANSI X3.113 INTERPRETATIONS-1992 "BASIC TECHNICAL INFORMATION BULLETIN # 1 INTERPRETATIONS OF ANSI 03.113-1987" • ISO/IEC 10279:1991/ Amd 1:1994 "MODULES AND SINGLE CHARACTER INPUT ENHANCEMENT" • ECMA-116 BASIC (withdrawn, similar to ANSI X3.113-1987)

BASIC

External links • BASIC [17] at the Open Directory Project • More Basic Computer Games [18] by David Ahl • Big Computer Games [19] by David Ahl

References [1] The acronym is tied to the name of an unpublished paper by Thomas Kurtz and is not a backronym, as is sometimes suggested in older versions of The Jargon File (http:/ / catb. org/ ~esr/ jargon/ html/ B/ BASIC. html) [2] Mono brings Visual Basic programs to Linux (http:/ / www. linux-watch. com/ news/ NS5656359853. html), By Steven J. Vaughan-Nichols, Feb. 19, 2007, Linux-Watch [3] Thomas E. Kurtz - History of Programming Languages (http:/ / cis-alumni. org/ TKurtz. html) [4] In a 1968 letter, Dutch computer scientist Edsger Dijkstra considered programming languages using GOTO statements for program structuring purposes harmful for the productivity of the programmer as well as the quality of the resulting code ( "Go To Statement Considered Harmful" (http:/ / www. acm. org/ classics/ oct95/ ), Communications of the ACM Volume 11, 147-148. 1968). The letter, which contributed the phrase considered harmful to programming jargon, did not mention any particular programming language; instead it states that the overuse of GOTO is damaging and gives technical reasons why this should be so. In a 1975 tongue-in-cheek article, "How do We Tell Truths that Might Hurt" (http:/ / www. cs. virginia. edu/ ~evans/ cs655/ readings/ ewd498. html), Sigplan Notices Volume 17 No. 5, Dijkstra gives a list of uncomfortable "truths", including his opinion of several programming languages of the time. Although BASIC is one of his targets ("It is practically impossible to teach good programming to students that have had a prior exposure to BASIC: as potential programmers they are mentally mutilated beyond hope of regeneration"), it receives no worse treatment in the piece than PL/I, COBOL or APL. [5] "Most Important Companies" (http:/ / www. byte. com/ art/ 9509/ sec7/ art15. htm). Byte Magazine. September 1995. . Retrieved 2008-06-10. [6] Table of Contents: BASIC Computer Games (http:/ / www. atariarchives. org/ basicgames) [7] Table of Contents: More BASIC Computer Games (http:/ / www. atariarchives. org/ morebasicgames) [8] Table of Contents: Big Computer Games (http:/ / www. atariarchives. org/ bigcomputergames) [9] QBasic Games (http:/ / members. chello. at/ theodor. lauppert/ games/ qbasic. htm) [10] Some examples include, Maxit (http:/ / members. chello. at/ theodor. lauppert/ games/ maxitwin. htm), Breme Counter (http:/ / members. chello. at/ theodor. lauppert/ games/ counter. htm), and a remake of Chuckie Egg (http:/ / members. chello. at/ theodor. lauppert/ games/ chuckie. htm) [11] http:/ / www. personal. leeds. ac. uk/ ~bgy1mm/ MiniBasic/ MiniBasicHome. html [12] Why Johnny Can't Code (http:/ / www. salon. com/ tech/ feature/ 2006/ 09/ 14/ basic/ index_np. html), By David Brin, Sept. 14, 2006, Salon Technology [13] "Differences Between GW-BASIC and QBasic" (http:/ / support. microsoft. com/ kb/ 73084). 2003-05-12. . Retrieved 2008-06-28. [14] http:/ / www. bitsavers. org/ pdf/ dartmouth/ BASIC_Oct64. pdf [15] http:/ / hopl. murdoch. edu. au/ showlanguage. prx?exp=176 [16] http:/ / www. iso. org/ iso/ catalogue_detail. htm?csnumber=18321 [17] http:/ / www. dmoz. org/ Computers/ Programming/ Languages/ BASIC/ / [18] http:/ / www. atariarchives. org/ morebasicgames [19] http:/ / www. atariarchives. org/ bigcomputergames

C (programming language)

The C Programming Language (also known as "K&R"), the seminal book on C Usual file extensions

.h .c

Paradigm

Imperative (procedural), structured

Appeared in

1972

Dennis Ritchie

Developer

Dennis Ritchie & Bell Labs

Stable release

C99 (March 2000)

Typing discipline

Static, weak, manifest

Major implementations

Clang, GCC, MSVC, Turbo C, Watcom C

Dialects

Cyclone, Unified Parallel C, Split-C, Cilk, C*

Influenced by

B (BCPL, CPL), ALGOL 68,

Influenced

AWK, csh, C++, C-- , C#, Objective-C, BitC, D, Go, Java, JavaScript, Limbo, Perl, PHP, Pike, Processing, Python

Cross-platform (multi-platform)

[1]

Assembly, PL/I, FORTRAN

C (pronounced /siË?/ see) is a general-purpose computer programming language developed in 1972 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system.[2] Although C was designed for implementing system software,[3] it is also widely used for developing portable application software. C is one of the most popular programming languages[4] [5] and there are very few computer architectures for which a C compiler does not exist. C has greatly influenced many other popular programming languages, most notably C++, which originally began as an extension to C.

C (programming language)

Design C is an imperative (procedural) systems implementation language. It was designed to be compiled using a relatively straightforward compiler, to provide low-level access to memory, to provide language constructs that map efficiently to machine instructions, and to require minimal run-time support. C was therefore useful for many applications that had formerly been coded in assembly language. Despite its low-level capabilities, the language was designed to encourage machine-independent programming. A standards-compliant and portably written C program can be compiled for a very wide variety of computer platforms and operating systems with little or no change to its source code. The language has become available on a very wide range of platforms, from embedded microcontrollers to supercomputers.

Minimalism C's design is tied to its intended use as a portable systems implementation language. It provides simple, direct access to any addressable object (for example, memory-mapped device control registers), and its source-code expressions can be translated in a straightforward manner to primitive machine operations in the executable code. Some early C compilers were comfortably implemented (as a few distinct passes communicating via intermediate files) on PDP-11 processors having only 16 address bits. C compilers for several common 8-bit platforms have been implemented as well.

Characteristics Like most imperative languages in the ALGOL tradition, C has facilities for structured programming and allows lexical variable scope and recursion, while a static type system prevents many unintended operations. In C, all executable code is contained within functions. Function parameters are always passed by value. Pass-by-reference is simulated in C by explicitly passing pointer values. Heterogeneous aggregate data types (struct) allow related data elements to be combined and manipulated as a unit. C program source text is free-format, using the semicolon as a statement terminator (not a delimiter). C also exhibits the following more specific characteristics: • • • • • • •

variables may be hidden in nested blocks partially weak typing; for instance, characters can be used as integers low-level access to computer memory by converting machine addresses to typed pointers function and data pointers supporting ad hoc run-time polymorphism array indexing as a secondary notion, defined in terms of pointer arithmetic a preprocessor for macro definition, source code file inclusion, and conditional compilation complex functionality such as I/O, string manipulation, and mathematical functions consistently delegated to library routines • A relatively small set of reserved keywords • A lexical structure that resembles B more than ALGOL, for example: • • • •

{ ... } rather than either of ALGOL 60's begin ... end or ALGOL 68's ( ... ) = is used for assignment (copying), like Fortran, rather than ALGOL's := == is used to test for equality (rather than .EQ. in Fortran, or = in BASIC and ALGOL) Logical "and" and "or" are represented with && and || in place of ALGOL's ∧ and ∨ operators; note that the doubled-up operators will never evaluate the right operand if the result can be determined from the left alone (this is called short-circuit evaluation), and are semantically distinct from the bit-wise operators & and | • However Unix Version 6 & 7 versions of C used ALGOL's /\ and \/ operators (in ASCII), but for determining the infimum and supremum respectively.[6]

C (programming language) • a large number of compound operators, such as +=, -=, *= and ++ etc. (Equivalent to the ALGOL 68 operators +:=, -:=, *:= and +:=1 )

Absent features The relatively low-level nature of the language affords the programmer close control over what the computer does, while allowing special tailoring and aggressive optimization for a particular platform. This allows the code to run efficiently on very limited hardware, such as embedded systems. C does not have some features that are available in some other programming languages: • No nested function definitions • No direct assignment of arrays or strings (copying can be done via standard functions; assignment of objects having struct or union type is supported) • No automatic garbage collection • No requirement for bounds checking of arrays • No operations on whole arrays • No syntax for ranges, such as the A..B notation used in several languages • Prior to C99, no separate Boolean type (zero/nonzero is used instead)[7] • No formal closures or functions as parameters (only function and variable pointers) • No generators or coroutines; intra-thread control flow consists of nested function calls, except for the use of the longjmp or setcontext library functions • No exception handling; standard library functions signify error conditions with the global errno variable and/or special return values, and library functions provide non-local gotos • Only rudimentary support for modular programming • No compile-time polymorphism in the form of function or operator overloading • Very limited support for object-oriented programming with regard to polymorphism and inheritance • Limited support for encapsulation • No native support for multithreading and networking • No standard libraries for computer graphics and several other application programming needs A number of these features are available as extensions in some compilers, or are provided in some operating environments (e.g., POSIX), or are supplied by third-party libraries, or can be simulated by adopting certain coding disciplines.

Undefined behavior Many operations in C that have undefined behavior are not required to be diagnosed at compile time. In the case of C, "undefined behavior" means that the exact behavior which arises is not specified by the standard, and exactly what will happen does not have to be documented by the C implementation. A famous, although misleading, expression in the newsgroups [news:comp.std.c comp.std.c] and [news:comp.lang.c comp.lang.c] is that the program could cause "demons to fly out of your nose".[8] Sometimes in practice what happens for an instance of undefined behavior is a bug that is hard to track down and which may corrupt the contents of memory. Sometimes a particular compiler generates reasonable and well-behaved actions that are completely different from those that would be obtained using a different C compiler. The reason some behavior has been left undefined is to allow compilers for a wide variety of instruction set architectures to generate more efficient executable code for well-defined behavior, which was deemed important for C's primary role as a systems implementation language; thus C makes it the programmer's responsibility to avoid undefined behavior, possibly using tools to find parts of a program whose behavior is undefined. Examples of undefined behavior are: • accessing outside the bounds of an array • overflowing a signed integer

C (programming language) • reaching the end of a non-void function without finding a return statement, when the return value is used • reading the value of a variable before initializing it These operations are all programming errors that could occur using many programming languages; C draws criticism because its standard explicitly identifies numerous cases of undefined behavior, including some where the behavior could have been made well defined, and does not specify any run-time error handling mechanism. Invoking fflush() on a stream opened for input is an example of a different kind of undefined behavior, not necessarily a programming error but a case for which some conforming implementations may provide well-defined, useful semantics (in this example, presumably discarding input through the next new-line) as an allowed extension. Use of such nonstandard extensions generally limits software portability.

History Early developments The initial development of C occurred at AT&T Bell Labs between 1969 and 1973; according to Ritchie, the most creative period occurred in 1972. It was named "C" because of its features were derived from an earlier language called "B", which according to Ken Thompson was a stripped-down version of the BCPL programming language. The origin of C is closely tied to the development of the Unix operating system, originally implemented in assembly language on a PDP-7 by Ritchie and Thompson, incorporating several ideas from colleagues. Eventually they decided to port the operating system to a PDP-11. B's lack of functionality taking advantage of some of the PDP-11's features, notably byte addressability, led to the development of an early version of C. The original PDP-11 version of the Unix system was developed in assembly language. By 1973, with the addition of struct types, the C language had become powerful enough that most of the Unix kernel was rewritten in C. This was one of the first operating system kernels implemented in a language other than assembly. (Earlier instances include the Multics system (written in PL/I), and MCP (Master Control Program) for the Burroughs B5000 written in ALGOL in 1961.)

K&R C In 1978, Brian Kernighan and Dennis Ritchie published the first edition of The C Programming Language. This book, known to C programmers as "K&R", served for many years as an informal specification of the language. The version of C that it describes is commonly referred to as K&R C. The second edition of the book covers the later ANSI C standard. K&R introduced several language features: • • • •

standard I/O library long int data type unsigned int data type compound assignment operators of the form =op (such as =-) were changed to the form op= to remove the semantic ambiguity created by such constructs as i=-10, which had been interpreted as i =- 10 instead of the possibly intended i = -10

Even after the publication of the 1989 C standard, for many years K&R C was still considered the "lowest common denominator" to which C programmers restricted themselves when maximum portability was desired, since many older compilers were still in use, and because carefully written K&R C code can be legal Standard C as well. In early versions of C, only functions that returned a non-integer value need to be declared if used before the function definition; a function used without any previous declaration was assumed to return an integer, if its value was used. For example:

C (programming language) long int SomeFunction(); /* int OtherFunction(); */ /* int */ CallingFunction() { long int test1; register /* int */ test2; test1 = SomeFunction(); if (test1 > 0) test2 = 0; else test2 = OtherFunction(); return test2; } All the above commented-out int declarations could be omitted in K&R C. Since K&R function declarations did not include any information about function arguments, function parameter type checks were not performed, although some compilers would issue a warning message if a local function was called with the wrong number of arguments, or if multiple calls to an external function used different numbers or types of arguments. Separate tools such as Unix's lint utility were developed that (among other things) could check for consistency of function use across multiple source files. In the years following the publication of K&R C, several unofficial features were added to the language, supported by compilers from AT&T and some other vendors. These included: • • • •

void functions functions returning struct or union types (rather than pointers) assignment for struct data types enumerated types

The large number of extensions and lack of agreement on a standard library, together with the language popularity and the fact that not even the Unix compilers precisely implemented the K&R specification, led to the necessity of standardization.

ANSI C and ISO C During the late 1970s and 1980s, versions of C were implemented for a wide variety of mainframe computers, minicomputers, and microcomputers, including the IBM PC, as its popularity began to increase significantly. In 1983, the American National Standards Institute (ANSI) formed a committee, X3J11, to establish a standard specification of C. In 1989, the standard was ratified as ANSI X3.159-1989 "Programming Language C". This version of the language is often referred to as ANSI C, Standard C, or sometimes C89. In 1990, the ANSI C standard (with formatting changes) was adopted by the International Organization for Standardization (ISO) as ISO/IEC 9899:1990, which is sometimes called C90. Therefore, the terms "C89" and "C90" refer to the same programming language. ANSI, like other national standards bodies, no longer develops the C standard independently, but defers to the ISO C standard. National adoption of updates to the international standard typically occurs within a year of ISO publication. One of the aims of the C standardization process was to produce a superset of K&R C, incorporating many of the unofficial features subsequently introduced. The standards committee also included several additional features such

C (programming language) as function prototypes (borrowed from C++), void pointers, support for international character sets and locales, and preprocessor enhancements. The syntax for parameter declarations was also augmented to include the style used in C++, although the K&R interface continued to be permitted, for compatibility with existing source code. C89 is supported by current C compilers, and most C code being written nowadays is based on it. Any program written only in Standard C and without any hardware-dependent assumptions will run correctly on any platform with a conforming C implementation, within its resource limits. Without such precautions, programs may compile only on a certain platform or with a particular compiler, due, for example, to the use of non-standard libraries, such as GUI libraries, or to a reliance on compiler- or platform-specific attributes such as the exact size of data types and byte endianness. In cases where code must be compilable by either standard-conforming or K&R C-based compilers, the __STDC__ macro can be used to split the code into Standard and K&R sections to take advantage of features available only in Standard C.

C99 After the ANSI/ISO standardization process, the C language specification remained relatively static for some time, whereas C++ continued to evolve, largely during its own standardization effort. In 1995 Normative Amendment 1 to the 1990 C standard was published, to correct some details and to add more extensive support for international character sets. The C standard was further revised in the late 1990s, leading to the publication of ISO/IEC 9899:1999 in 1999, which is commonly referred to as "C99". It has since been amended three times by Technical Corrigenda. The international C standard is maintained by the working group ISO/IEC JTC1/SC22/WG14. C99 introduced several new features, including inline functions, several new data types (including long long int and a complex type to represent complex numbers), variable-length arrays, support for variadic macros (macros of variable arity) and support for one-line comments beginning with //, as in BCPL or C++. Many of these had already been implemented as extensions in several C compilers. C99 is for the most part backward compatible with C90, but is stricter in some ways; in particular, a declaration that lacks a type specifier no longer has int implicitly assumed. A standard macro __STDC_VERSION__ is defined with value 199901L to indicate that C99 support is available. GCC, Sun Studio and other C compilers now support many or all of the new features of C99.

C1X In 2007, work began in anticipation of another revision of the C standard, informally called "C1X". The C standards committee has adopted guidelines to limit the adoption of new features that have not been tested by existing implementations.

Uses C's primary use is for "system programming", including implementing operating systems and embedded system applications, due to a combination of desirable characteristics such as code portability and efficiency, ability to access specific hardware addresses, ability to "pun" types to match externally imposed data access requirements, and low runtime demand on system resources. C can also be used for website programming using CGI as a "gateway" for information between the Web application, the server, and the browser.[9] Some factors to choose C over Interpreted languages are its speed, stability and less susceptibility to changes in operating environments due to its compiled nature.[10] One consequence of C's wide acceptance and efficiency is that compilers, libraries, and interpreters of other programming languages are often implemented in C.

C (programming language) C is sometimes used as an intermediate language by implementations of other languages. This approach may be used for portability or convenience; by using C as an intermediate language, it is not necessary to develop machine-specific code generators. Some compilers which use C this way are BitC, Gambit, the Glasgow Haskell Compiler, Squeak, and Vala. However, C was designed as a programming language, not as a compiler target language, and is thus less than ideal for use as an intermediate language. This has led to development of C-based intermediate languages such as C--. C has also been widely used to implement end-user applications, but as applications became larger, much of that development shifted to other languages.

Syntax Unlike languages such as FORTRAN 77, C source code is free-form which allows arbitrary use of whitespace to format code, rather than column-based or text-line-based restrictions. Comments may appear either between the delimiters /* and */, or (in C99) following // until the end of the line. Each source file contains declarations and function definitions. Function definitions, in turn, contain declarations and statements. Declarations either define new types using keywords such as struct, union, and enum, or assign types to and perhaps reserve storage for new variables, usually by writing the type followed by the variable name. Keywords such as char and int specify built-in types. Sections of code are enclosed in braces ({ and }, sometimes called "curly brackets") to limit the scope of declarations and to act as a single statement for control structures. As an imperative language, C uses statements to specify actions. The most common statement is an expression statement, consisting of an expression to be evaluated, followed by a semicolon; as a side effect of the evaluation, functions may be called and variables may be assigned new values. To modify the normal sequential execution of statements, C provides several control-flow statements identified by reserved keywords. Structured programming is supported by if(-else) conditional execution and by do-while, while, and for iterative execution (looping). The for statement has separate initialization, testing, and reinitialization expressions, any or all of which can be omitted. break and continue can be used to leave the innermost enclosing loop statement or skip to its reinitialization. There is also a non-structured goto statement which branches directly to the designated label within the function. switch selects a case to be executed based on the value of an integer expression. Expressions can use a variety of built-in operators (see below) and may contain function calls. The order in which arguments to functions and operands to most operators are evaluated is unspecified. The evaluations may even be interleaved. However, all side effects (including storage to variables) will occur before the next "sequence point"; sequence points include the end of each expression statement, and the entry to and return from each function call. Sequence points also occur during evaluation of expressions containing certain operators(&&, ||, ?: and the comma operator). This permits a high degree of object code optimization by the compiler, but requires C programmers to take more care to obtain reliable results than is needed for other programming languages. Although mimicked by many languages because of its widespread familiarity, C's syntax has often been criticized. For example, Kernighan and Ritchie say in the Introduction of The C Programming Language, "C, like any other language, has its blemishes. Some of the operators have the wrong precedence; some parts of the syntax could be better." Some specific problems worth noting are: â&#x20AC;˘ Not checking number and types of arguments when the function declaration has an empty parameter list. (This provides backward compatibility with K&R C, which lacked prototypes.) â&#x20AC;˘ Some questionable choices of operator precedence, as mentioned by Kernighan and Ritchie above, such as == binding more tightly than & and | in expressions like x & 1 == 0. â&#x20AC;˘ The use of the = operator, used in mathematics for equality, to indicate assignment, following the precedent of Fortran, PL/I, and BASIC, but unlike ALGOL and its derivatives. Ritchie made this syntax design decision

C (programming language) consciously, based primarily on the argument that assignment occurs more often than comparison. • Similarity of the assignment and equality operators (= and ==), making it easy to accidentally substitute one for the other. C's weak type system permits each to be used in the context of the other without a compilation error (although some compilers produce warnings). For example, the conditional expression in if (a=b) is only true if a is not zero after the assignment.[11] • A lack of infix operators for complex objects, particularly for string operations, making programs which rely heavily on these operations (implemented as functions instead) somewhat difficult to read. • A declaration syntax that some find unintuitive, particularly for function pointers. (Ritchie's idea was to declare identifiers in contexts resembling their use: "declaration reflects use".)

Operators C supports a rich set of operators, which are symbols used within an expression to specify the manipulations to be performed while evaluating that expression. C has operators for: • • • •

arithmetic (+, -, *, /, %) equality testing (==, !=) order relations (<, <=, >, >=) boolean logic (!, &&, ||)

• • • • • • • • • • • •

bitwise logic (~, &, |, ^) bitwise shifts (<<, >>) assignment (=, +=, -=, *=, /=, %=, &=, |=, ^=, <<=, >>=) increment and decrement (++, --) reference and dereference (&, *, [ ]) conditional evaluation (? :) member selection (., ->) type conversion (( )) object size (sizeof) function argument collection (( )) sequencing (,) subexpression grouping (( ))

C has a formal grammar, specified by the C standard. Integer-float conversion and rounding The type casting syntax can be used to convert values between an integer type and a floating-point type, or between two integer types or two float types with different sizes; e.g. (long int)sqrt(1000.0), (double)(256*256), or (float)sqrt(1000.0). Conversions are implicit in several contexts, e.g. when assigning a value to a variable or to a function parameter, when using a floating-point value as index to a vector, or in arithmetic operations on operand with different types. Unlike some other cases of type casting (where the bit encoding of the operands are simply re-interpreted according to the target type), conversions between integers and floating-point values generally change the bit encoding so as to preserve the numerical value of the operand, to the extent possible. In particular, conversion from an integer to a floating-point type will preserve its numeric value exactly—unless the number of fraction bits in the target type is insufficient, in which case the least-significant bits are lost. Conversion from a floating-point value to an integer type entails truncation of any fractional part (i.e. the value is rounded "towards zero"). For other kinds of rounding, the C99 standard specifies (in <math.h>) the following functions: • round(): round to nearest integer, halfway away from zero

C (programming language) • • • •

rint(), nearbyint(): round according to current floating-point rounding direction ceil(): smallest integral value not less than argument (round up) floor(): largest integral value (in double representation) not greater than argument (round down) trunc(): round towards zero (same as typecasting to an int)

All these functions take a double argument and return a double result, which may then be cast to an integer if necessary. The conversion of a float value to the double type preserves the numerical value exactly, while the opposite conversion rounds the value to fit in the smaller number of fraction bits, usually towards zero. (Since float also has a smaller exponent range, the conversion may yield an infinite value.) Some compilers will silently convert float values to double in some contexts, e.g. function parameters declared as float may be actually passed as double. In machines that comply with the IEEE floating point standard, some rounding events are affected by the current rounding mode (which includes round-to-even, round-down, round-up, and round-to-zero), which may be retrieved and set using the fegetround()/fesetround() functions defined in <fenv.h>.

"Hello, world" example The "hello, world" example which appeared in the first edition of K&R has become the model for an introductory program in most programming textbooks, regardless of programming language. The program prints "hello, world" to the standard output, which is usually a terminal or screen display. The original version was: main() { printf("hello, world\n"); }

A standard-conforming "hello, world" program is:[12] #include <stdio.h> int main(void) { printf("hello, world\n"); return 0; } The first line of the program contains a preprocessing directive, indicated by #include. This causes the preprocessor—the first tool to examine source code as it is compiled—to substitute the line with the entire text of the stdio.h standard header, which contains declarations for standard input and output functions such as printf. The angle brackets surrounding stdio.h indicate that stdio.h is located using a search strategy that prefers standard headers to other headers having the same name. Double quotes may also be used to include local or project-specific header files. The next line indicates that a function named main is being defined. The main function serves a special purpose in C programs: The run-time environment calls the main function to begin program execution. The type specifier int indicates that the return value, the value that is returned to the invoker (in this case the run-time environment) as a result of evaluating the main function, is an integer. The keyword void as a parameter list indicates that the main function takes no arguments.[13] The opening curly brace indicates the beginning of the definition of the main function.

C (programming language) The next line calls (diverts execution to) a function named printf, which was declared in stdio.h and is supplied from a system library. In this call, the printf function is passed (provided with) a single argument, the address of the first character in the string literal "hello, world\n". The string literal is an unnamed array with elements of type char, set up automatically by the compiler with a final 0-valued character to mark the end of the array (printf needs to know this). The \n is an escape sequence that C translates to a newline character, which on output signifies the end of the current line. The return value of the printf function is of type int, but it is silently discarded since it is not used. (A more careful program might test the return value to determine whether or not the printf function succeeded.) The semicolon ; terminates the statement. The return statement terminates the execution of the main function and causes it to return the integer value 0, which is interpreted by the run-time system as an exit code indicating successful execution. The closing curly brace indicates the end of the code for the main function.

Data structures C has a static weak typing type system that shares some similarities with that of other ALGOL descendants such as Pascal. There are built-in types for integers of various sizes, both signed and unsigned, floating-point numbers, characters, and enumerated types (enum). C99 added a boolean datatype. There are also derived types including arrays, pointers, records (struct), and untagged unions (union). C is often used in low-level systems programming where escapes from the type system may be necessary. The compiler attempts to ensure type correctness of most expressions, but the programmer can override the checks in various ways, either by using a type cast to explicitly convert a value from one type to another, or by using pointers or unions to reinterpret the underlying bits of a value in some other way. Pointers C supports the use of pointers, a very simple type of reference that records, in effect, the address or location of an object or function in memory. Pointers can be dereferenced to access data stored at the address pointed to, or to invoke a pointed-to function. Pointers can be manipulated using assignment and also pointer arithmetic. The run-time representation of a pointer value is typically a raw memory address (perhaps augmented by an offset-within-word field), but since a pointer's type includes the type of the thing pointed to, expressions including pointers can be type-checked at compile time. Pointer arithmetic is automatically scaled by the size of the pointed-to data type. (See Array-pointer interchangeability below.) Pointers are used for many different purposes in C. Text strings are commonly manipulated using pointers into arrays of characters. Dynamic memory allocation, which is described below, is performed using pointers. Many data types, such as trees, are commonly implemented as dynamically allocated struct objects linked together using pointers. Pointers to functions are useful for callbacks from event handlers. A null pointer is a pointer value that points to no valid location (it is often represented by address zero). Dereferencing a null pointer is therefore meaningless, typically resulting in a run-time error. Null pointers are useful for indicating special cases such as no next pointer in the final node of a linked list, or as an error indication from functions returning pointers. Void pointers (void *) point to objects of unknown type, and can therefore be used as "generic" data pointers. Since the size and type of the pointed-to object is not known, void pointers cannot be dereferenced, nor is pointer arithmetic on them allowed, although they can easily be (and in many contexts implicitly are) converted to and from any other object pointer type. Careless use of pointers is potentially dangerous. Because they are typically unchecked, a pointer variable can be made to point to any arbitrary location, which can cause undesirable effects. Although properly-used pointers point to safe places, they can be made to point to unsafe places by using invalid pointer arithmetic; the objects they point to may be deallocated and reused (dangling pointers); they may be used without having been initialized (wild

C (programming language) pointers); or they may be directly assigned an unsafe value using a cast, union, or through another corrupt pointer. In general, C is permissive in allowing manipulation of and conversion between pointer types, although compilers typically provide options for various levels of checking. Some other programming languages address these problems by using more restrictive reference types. Arrays Array types in C are traditionally of a fixed, static size specified at compile time. (The more recent C99 standard also allows a form of variable-length arrays.) However, it is also possible to allocate a block of memory (of arbitrary size) at run-time, using the standard library's malloc function, and treat it as an array. C's unification of arrays and pointers (see below) means that true arrays and these dynamically-allocated, simulated arrays are virtually interchangeable. Since arrays are always accessed (in effect) via pointers, array accesses are typically not checked against the underlying array size, although the compiler may provide bounds checking as an option. Array bounds violations are therefore possible and rather common in carelessly written code, and can lead to various repercussions, including illegal memory accesses, corruption of data, buffer overruns, and run-time exceptions. C does not have a special provision for declaring multidimensional arrays, but rather relies on recursion within the type system to declare arrays of arrays, which effectively accomplishes the same thing. The index values of the resulting "multidimensional array" can be thought of as increasing in row-major order. Although C supports static arrays, it is not required that array indices be validated (bounds checking). For example, one can try to write to the sixth element of an array with five elements, generally yielding undesirable results. This type of bug, called a buffer overflow or buffer overrun, is notorious for causing a number of security problems. Since bounds checking elimination technology was largely nonexistent when C was defined, bounds checking came with a severe performance penalty, particularly in numerical computation. A few years earlier, some Fortran compilers had a switch to toggle bounds checking on or off; however, this would have been much less useful for C, where array arguments are passed as simple pointers. Multidimensional arrays are commonly used in numerical algorithms (mainly from applied linear algebra) to store matrices. The structure of the C array is well suited to this particular task. However, since arrays are passed merely as pointers, the bounds of the array must be known fixed values or else explicitly passed to any subroutine that requires them, and dynamically sized arrays of arrays cannot be accessed using double indexing. (A workaround for this is to allocate the array with an additional "row vector" of pointers to the columns.) C99 introduced "variable-length arrays" which address some, but not all, of the issues with ordinary C arrays. Array-pointer interchangeability A distinctive (but potentially confusing) feature of C is its treatment of arrays and pointers. The array-subscript notation x[i] can also be used when x is a pointer; the interpretation (using pointer arithmetic) is to access the (i + 1)th object of several adjacent data objects pointed to by x, counting the object that x points to (which is x[0]) as the first element of the array. Formally, x[i] is equivalent to *(x + i). Since the type of the pointer involved is known to the compiler at compile time, the address that x + i points to is not the address pointed to by x incremented by i bytes, but rather incremented by i multiplied by the size of an element that x points to. The size of these elements can be determined with the operator sizeof by applying it to any dereferenced element of x, as in n = sizeof *x or n = sizeof x[0]. Furthermore, in most expression contexts (a notable exception is sizeof x), the name of an array is automatically converted to a pointer to the array's first element; this implies that an array is never copied as a whole when named as an argument to a function, but rather only the address of its first element is passed. Therefore, although function calls in C use pass-by-value semantics, arrays are in effect passed by reference. The number of elements in a declared array x can be determined as sizeof x / sizeof x[0].

C (programming language) An interesting demonstration of the interchangeability of pointers and arrays is shown below. The four assignments are equivalent and each is valid C code. /* x is an array and i is an integer */ x[i] = 1; /* equivalent to *(x + i) */ *(x + i) = 1; *(i + x) = 1; i[x] = 1; /* uncommon usage, but correct: i[x] is equivalent to *(i + x) */

Note that the last line contains the uncommon, but semantically correct, expression i[x], which appears to interchange the index variable i with the array variable x. This last line might be found in obfuscated C code, but its use is rare among C programmers. Despite this apparent equivalence between array and pointer variables, there is still a distinction to be made between them. Even though the name of an array is, in most expression contexts, converted into a pointer (to its first element), this pointer does not itself occupy any storage, unlike a pointer variable. Consequently, what an array "points to" cannot be changed, and it is impossible to assign a value to an array variable. (Array values may be copied, however, e.g., by using the memcpy function.)

Memory management One of the most important functions of a programming language is to provide facilities for managing memory and the objects that are stored in memory. C provides three distinct ways to allocate memory for objects: â&#x20AC;˘ Static memory allocation: space for the object is provided in the binary at compile-time; these objects have an extent (or lifetime) as long as the binary which contains them is loaded into memory â&#x20AC;˘ Automatic memory allocation: temporary objects can be stored on the stack, and this space is automatically freed and reusable after the block in which they are declared is exited â&#x20AC;˘ Dynamic memory allocation: blocks of memory of arbitrary size can be requested at run-time using library functions such as malloc from a region of memory called the heap; these blocks persist until subsequently freed for reuse by calling the library function free These three approaches are appropriate in different situations and have various tradeoffs. For example, static memory allocation has no allocation overhead, automatic allocation may involve a small amount of overhead, and dynamic memory allocation can potentially have a great deal of overhead for both allocation and deallocation. On the other hand, stack space is typically much more limited and transient than either static memory or heap space, and dynamic memory allocation allows allocation of objects whose size is known only at run-time. Most C programs make extensive use of all three. Where possible, automatic or static allocation is usually preferred because the storage is managed by the compiler, freeing the programmer of the potentially error-prone chore of manually allocating and releasing storage. However, many data structures can grow in size at runtime, and since static allocations (and automatic allocations in C89 and C90) must have a fixed size at compile-time, there are many situations in which dynamic allocation must be used. Prior to the C99 standard, variable-sized arrays were a common example of this (see malloc for an example of dynamically allocated arrays). Automatically and dynamically allocated objects are only initialized if an initial value is explicitly specified; otherwise they initially have indeterminate values (typically, whatever bit pattern happens to be present in the storage, which might not even represent a valid value for that type). If the program attempts to access an uninitialized value, the results are undefined. Many modern compilers try to detect and warn about this problem, but both false positives and false negatives occur.

C (programming language) Another issue is that heap memory allocation has to be manually synchronized with its actual usage in any program in order for it to be reused as much as possible. For example, if the only pointer to a heap memory allocation goes out of scope or has its value overwritten before free() has been called, then that memory cannot be recovered for later reuse and is essentially lost to the program, a phenomenon known as a memory leak. Conversely, it is possible to release memory too soon and continue to access it; however, since the allocation system can re-allocate or itself use the freed memory, unpredictable behavior is likely to occur when the multiple users corrupt each other's data. Typically, the symptoms will appear in a portion of the program far removed from the actual error. Such issues are ameliorated in languages with automatic garbage collection or RAII.

Libraries The C programming language uses libraries as its primary method of extension. In C, a library is a set of functions contained within a single "archive" file. Each library typically has a header file, which contains the prototypes of the functions contained within the library that may be used by a program, and declarations of special data types and macro symbols used with these functions. In order for a program to use a library, it must include the library's header file, and the library must be linked with the program, which in many cases requires compiler flags (e.g., -lm, shorthand for "math library"). The most common C library is the C standard library, which is specified by the ISO and ANSI C standards and comes with every C implementation (â&#x20AC;&#x153;freestandingâ&#x20AC;? [embedded] C implementations may provide only a subset of the standard library). This library supports stream input and output, memory allocation, mathematics, character strings, and time values. Another common set of C library functions are those used by applications specifically targeted for Unix and Unix-like systems, especially functions which provide an interface to the kernel. These functions are detailed in various standards such as POSIX and the Single UNIX Specification. Since many programs have been written in C, there are a wide variety of other libraries available. Libraries are often written in C because C compilers generate efficient object code; programmers then create interfaces to the library so that the routines can be used from higher-level languages like Java, Perl, and Python.

Language tools Tools have been created to help C programmers avoid some of the problems inherent in the language, such as statements with undefined behavior or statements that are not a good practice because they are more likely to result in unintended behavior or run-time errors. Automated source code checking and auditing are beneficial in any language, and for C many such tools exist, such as Lint. A common practice is to use Lint to detect questionable code when a program is first written. Once a program passes Lint, it is then compiled using the C compiler. Also, many compilers can optionally warn about syntactically valid constructs that are likely to actually be errors. MISRA C is a proprietary set of guidelines to avoid such questionable code, developed for embedded systems. There are also compilers, libraries and operating system level mechanisms for performing array bounds checking, buffer overflow detection, serialization and automatic garbage collection, that are not a standard part of C. Tools such as Purify, Valgrind, and linking with libraries containing special versions of the memory allocation functions can help uncover runtime memory errors.

C (programming language)

Related languages C has directly or indirectly influenced many later languages such as Java, Perl, PHP, JavaScript, LPC, C# and Unix's C Shell. The most pervasive influence has been syntactical: all of the languages mentioned combine the statement and (more or less recognizably) expression syntax of C with type systems, data models and/or large-scale program structures that differ from those of C, sometimes radically. When object-oriented languages became popular, C++ and Objective-C were two different extensions of C that provided object-oriented capabilities. Both languages were originally implemented as source-to-source compilers -source code was translated into C, and then compiled with a C compiler. Bjarne Stroustrup devised the C++ programming language as one approach to providing object-oriented functionality with C-like syntax. C++ adds greater typing strength, scoping and other tools useful in object-oriented programming and permits generic programming via templates. Nearly a superset of C, C++ now supports most of C, with a few exceptions (see Compatibility of C and C++ for an exhaustive list of differences). Unlike C++, which maintains nearly complete backwards compatibility with C, the D language makes a clean break with C while maintaining the same general syntax. It abandons a number of features of C which Walter Bright (the designer of D) considered undesirable, including the C preprocessor and trigraphs. Some, but not all, of D's extensions to C overlap with those of C++. Objective-C was originally a very "thin" layer on top of, and remains a strict superset of, C that permits object-oriented programming using a hybrid dynamic/static typing paradigm. Objective-C derives its syntax from both C and Smalltalk: syntax that involves preprocessing, expressions, function declarations and function calls is inherited from C, while the syntax for object-oriented features was originally taken from Smalltalk. Limbo is a language developed by the same team at Bell Labs that was responsible for C and Unix, and while retaining some of the syntax and the general style, introduced garbage collection, CSP based concurrency and other major innovations. Python has a different sort of C heritage. While the syntax and semantics of Python are radically different from C, the most widely used Python implementation, CPython, is an open source C program. This allows C users to extend Python with C, or embed Python into C programs. This close relationship is one of the key factors leading to Python's success as a general-use dynamic language. Perl is another example of a popular programming language rooted in C. However, unlike Python, Perl's syntax does closely follow C syntax. The standard Perl implementation is written in C and supports extensions written in C.

See also • • • • • •

Augmented assignment Comparison of Pascal and C Comparison of programming languages International Obfuscated C Code Contest List of articles with C programs List of compilers

C (programming language)

References • Brian Kernighan, Dennis Ritchie: The C Programming Language. Also known as K&R – The original book on C. • 1st, Prentice Hall 1978; ISBN 0-13-110163-3. Pre-ANSI C. • 2nd, Prentice Hall 1988; ISBN 0-13-110362-8. ANSI C. • ISO/IEC 9899 [14]. Official C99 documents, including technical corrigenda and a rationale. As of 2007 the latest version of the standard is ISO/IEC 9899:TC3 [15]PDF (3.61 MB). • Samuel P. Harbison, Guy L. Steele: C: A Reference Manual. This book is excellent as a definitive reference manual, and for those working on C compilers. The book contains a BNF grammar for C. • 5th, Prentice Hall 2002; ISBN 0-13-089592-X. • Derek M. Jones: The New C Standard: A Cultural and Economic Commentary, Addison-Wesley, ISBN 0-201-70917-1, online material [16] • Robert Sedgewick: Algorithms in C, Addison-Wesley, ISBN 0-201-31452-5 (Part 1–4) and ISBN 0-201-31663-3 (Part 5) • William H. Press, Saul A. Teukolsky, William T. Vetterling, Brian P. Flannery: Numerical Recipes in C (The Art of Scientific Computing), ISBN 0-521-43108-5

External links The current draft Standard (C99 with Technical corrigenda TC1, TC2, and TC3 included) [15]PDF (3.61 MB) Draft ANSI C Standard (ANSI X3J11/88-090) [17] (May 13, 1988), Third Public Review [18] Draft ANSI C Rationale (ANSI X3J11/88-151) [19] (Nov 18, 1988) ISO C Working Group [20] (official Web site) The Development of the C Language [21] by Dennis M. Ritchie comp.lang.c Frequently Asked Questions [22] The C Book [23] by M.Banahan-D.Brady-M.Doran (Addison-Wesley, 2nd ed.) – book for beginning and intermediate students, now out of print and free to download. • The New C Standard: An economic and cultural commentary [24]PDF (10.0 MB) – An unpublished book about "detailed analysis of the International Standard for the C language." • A New C Compiler [25] paper by Ken Thompson. • • • • • • •

References [1] Dennis M. Ritchie (January 1993). "The Development of the C Language" (http:/ / cm. bell-labs. com/ cm/ cs/ who/ dmr/ chist. html). . Retrieved Jan 1 2008. "The scheme of type composition adopted by C owes considerable debt to Algol 68, although it did not, perhaps, emerge in a form that Algol's adherents would approve of." [2] Stewart, Bill (January 7, 2000). "History of the C Programming Language" (http:/ / www. livinginternet. com/ i/ iw_unix_c. htm). Living Internet. . Retrieved 2006-10-31. [3] Patricia K. Lawlis, c.j. kemp systems, inc. (1997). "Guidelines for Choosing a Computer Language: Support for the Visionary Organization" (http:/ / archive. adaic. com/ docs/ reports/ lawlis/ k. htm). Ada Information Clearinghouse. . Retrieved 2006-07-18. [4] "Programming Language Popularity" (http:/ / www. langpop. com/ ). 2009. . Retrieved 2009-01-16. [5] "TIOBE Programming Community Index" (http:/ / www. tiobe. com/ index. php/ content/ paperinfo/ tpci/ index. html). 2009. . Retrieved 2009-05-06. [6] http:/ / stackoverflow. com/ questions/ 1540886/ what-did-the-c-operators-and-do [7] C99 added a _Bool type, but it was not retrofitted into the language's existing Boolean contexts. One can simulate a Boolean datatype, e.g. with enum { false, true } bool;, but this does not provide all of the features of a separate Boolean datatype. [8] "Jargon File entry for nasal demons" (http:/ / www. catb. org/ jargon/ html/ N/ nasal-demons. html). . [9] Dr. Dobb's Sourcebook. U.S.A.: Miller Freeman, Inc.. Nov/Dec 1995 issue. [10] "Using C for CGI Programming" (http:/ / www. linuxjournal. com/ article/ 6863). linuxjournal.com. 2005-03-01. . Retrieved 2010-01-04. [11] "10 Common Programming Mistakes in C" (http:/ / www. cs. ucr. edu/ ~nxiao/ cs10/ errors. htm). Cs.ucr.edu. . Retrieved 2009-06-26. [12] The original example code will compile on most modern compilers that are not in strict standard compliance mode, but it does not fully conform to the requirements of either C89 or C99. In fact, C99 requires that a diagnostic message be produced.

C (programming language) [13] The main function actually has two arguments, int argc and char *argv[], respectively, which can be used to handle command line arguments. The C standard requires that both forms of main be supported, which is special treatment not afforded any other function. [14] http:/ / www. open-std. org/ JTC1/ SC22/ WG14/ www/ standards [15] http:/ / www. open-std. org/ JTC1/ SC22/ WG14/ www/ docs/ n1256. pdf [16] http:/ / www. knosof. co. uk/ cbook/ cbook. html [17] http:/ / flash-gordon. me. uk/ ansi. c. txt [18] http:/ / groups. google. com/ group/ comp. lang. c/ msg/ 20b174b18cdd919d?hl=en [19] http:/ / www. scribd. com/ doc/ 16306895/ Draft-ANSI-C-Rationale [20] http:/ / www. open-std. org/ jtc1/ sc22/ wg14/ [21] http:/ / cm. bell-labs. com/ cm/ cs/ who/ dmr/ chist. html [22] http:/ / www. c-faq. com/ [23] http:/ / publications. gbdirect. co. uk/ c_book/ [24] http:/ / www. coding-guidelines. com/ cbook/ cbook1_2. pdf [25] http:/ / doc. cat-v. org/ bell_labs/ new_c_compilers/

C++

The C++ Programming Language, written by its architect, is the seminal book on the language. Usual file extensions

.hh .hpp .hxx .h++ .cc .cpp .cxx .c++

Paradigm

Multi-paradigm: procedural, object-oriented, generic

Appeared in

1983

Bjarne Stroustrup

Developer

Bjarne Stroustrup Bell Labs ISO/IEC JTC1/SC22/WG21

Preview release

C++0x

Typing discipline

Static, unsafe, nominative

Major implementations

Borland C++ Builder, GCC, Intel C++ Compiler, Microsoft Visual C++, Sun Studio, Turbo C++, Comeau C/C++

Dialects

ISO/IEC C++ 1998, ISO/IEC C++ 2003

Influenced by

C, Simula, Ada 83, ALGOL 68, CLU, ML

Influenced

Perl, Lua, Ada 95, Java, PHP, D, C99, C#, Aikido, Falcon, Dao

Cross-platform (multi-platform)

[1]

C++ (pronounced "see plus plus") is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as a middle-level language, as it comprises a combination of both high-level and low-level language features.[2] It was developed by Bjarne Stroustrup starting in 1979 at Bell Labs as an enhancement to the C programming language and originally named "C with Classes". It was renamed C++ in 1983.[3] As one of the most popular programming languages ever created,[4] [5] C++ is widely used in the software industry. Some of its application domains include systems software, application software, device drivers, embedded software, high-performance server and client applications, and entertainment software such as video games. Several groups provide both free and proprietary C++ compiler software, including the GNU Project, Microsoft, Intel and Borland. C++ has greatly influenced many other popular programming languages, most notably Java. C++ is also used for hardware design, where design is initially described in C++, then analyzed, architecturally constrained, and scheduled to create a register transfer level hardware description language via high-level synthesis. The language began as enhancements to C, first adding classes, then virtual functions, operator overloading, multiple inheritance, templates, and exception handling among other features. After years of development, the C++ programming language standard was ratified in 1998 as ISO/IEC 14882:1998. That standard is still current, but is amended by the 2003 technical corrigendum, ISO/IEC 14882:2003. The next standard version (known informally as C++0x) is in development.

C++

History Stroustrup began work on "C with Classes" in 1979. The idea of creating a new language originated from Stroustrup's experience in programming for his Ph.D. thesis. Stroustrup found that Simula had features that were very helpful for large software development, but the language was too slow for practical use, while BCPL was fast but too low-level to be suitable for large software development. When Stroustrup started working in AT&T Bell Labs, he had the problem of analyzing the UNIX kernel with respect to distributed computing. Remembering his Ph.D. experience, Stroustrup set out to enhance the Bjarne Stroustrup, creator of C++ C language with Simula-like features. C was chosen because it was general-purpose, fast, portable and widely used. Besides C and Simula, some other languages that inspired him were ALGOL 68, Ada, CLU and ML. At first, the class, derived class, strong type checking, inlining, and default argument features were added to C via Stroustrup's C++ to C compiler, Cfront. The first commercial implementation of C++ was released in October 1985.[6] In 1983, the name of the language was changed from C with Classes to C++ (++ being the increment operator in C and C++). New features were added including virtual functions, function name and operator overloading, references, constants, user-controlled free-store memory control, improved type checking, and BCPL style single-line comments with two forward slashes (//). In 1985, the first edition of The C++ Programming Language was released, providing an important reference to the language, since there was not yet an official standard. Release 2.0 of C++ came in 1989. New features included multiple inheritance, abstract classes, static member functions, const member functions, and protected members. In 1990, The Annotated C++ Reference Manual was published. This work became the basis for the future standard. Late addition of features included templates, exceptions, namespaces, new casts, and a Boolean type. As the C++ language evolved, a standard library also evolved with it. The first addition to the C++ standard library was the stream I/O library which provided facilities to replace the traditional C functions such as printf and scanf. Later, among the most significant additions to the standard library, was the Standard Template Library. C++ continues to be used and is one of the preferred programming languages to develop professional applications. The popularity of the language continues to grow.[7]

Language standard In 1998, the C++ standards committee (the ISO/IEC JTC1/SC22/WG21 working group) standardized C++ and published the international standard ISO/IEC 14882:1998 (informally known as C++98[8] ). For some years after the official release of the standard, the committee processed defect reports, and published a corrected version of the C++ standard, ISO/IEC 14882:2003, in 2003. In 2005, a technical report, called the "Library Technical Report 1" (often known as TR1 for short), was released. While not an official part of the standard, it specified a number of extensions to the standard library, which were expected to be included in the next version of C++. Support for TR1 is growing in almost all currently maintained C++ compilers. The standard for the next version of the language (known informally as C++0x) is in development.

C++

Etymology According to Stroustrup: "the name signifies the evolutionary nature of the changes from C".[9] During C++'s development period, the language had been referred to as "new C", then "C with Classes". The final name is credited to Rick Mascitti (mid-1983) and was first used in December 1983. When Mascitti was questioned informally in 1992 about the naming, he indicated that it was given in a tongue-in-cheek spirit. It stems from C's "++" operator (which increments the value of a variable) and a common naming convention of using "+" to indicate an enhanced computer program. There is no language called "C plus". ABCL/c+ was the name of an earlier, unrelated programming language.

Philosophy In The Design and Evolution of C++ (1994), Bjarne Stroustrup describes some rules that he used for the design of C++: • C++ is designed to be a statically typed, general-purpose language that is as efficient and portable as C • C++ is designed to directly and comprehensively support multiple programming styles (procedural programming, data abstraction, object-oriented programming, and generic programming) • C++ is designed to give the programmer choice, even if this makes it possible for the programmer to choose incorrectly • C++ is designed to be as compatible with C as possible, therefore providing a smooth transition from C • C++ avoids features that are platform specific or not general purpose • C++ does not incur overhead for features that are not used (the "zero-overhead principle") • C++ is designed to function without a sophisticated programming environment Stroustrup also mentions that C++ was always intended to make programming more fun and that many of the double meanings in the language are intentional. Inside the C++ Object Model (Lippman, 1996) describes how compilers may convert C++ program statements into an in-memory layout. Compiler authors are, however, free to implement the standard in their own manner.

Standard library The 1998 ANSI/ISO C++ standard consists of two parts: the core language and the C++ Standard Library; the latter includes most of the Standard Template Library (STL) and a slightly modified version of the C standard library. Many C++ libraries exist which are not part of the standard, and, using linkage specification, libraries can even be written in languages such as C, Fortran, Pascal, or BASIC. Which of these are supported is compiler dependent. The C++ standard library incorporates the C standard library with some small modifications to make it optimized with the C++ language. Another large part of the C++ library is based on the STL. This provides such useful tools as containers (for example vectors and lists), iterators to provide these containers with array-like access and algorithms to perform operations such as searching and sorting. Furthermore (multi)maps (associative arrays) and (multi)sets are provided, all of which export compatible interfaces. Therefore it is possible, using templates, to write generic algorithms that work with any container or on any sequence defined by iterators. As in C, the features of the library are accessed by using the #include directive to include a standard header. C++ provides 69 standard headers, of which 19 are deprecated. The STL was originally a third-party library from HP and later SGI, before its incorporation into the C++ standard. The standard does not refer to it as "STL", as it is merely a part of the standard library, but many people still use that term to distinguish it from the rest of the library (input/output streams, internationalization, diagnostics, the C library subset, etc.). Most C++ compilers provide an implementation of the C++ standard library, including the STL. Compiler-independent implementations of the STL, such as STLPort,[10] also exist. Other projects also produce

C++

69 various custom implementations of the C++ standard library and the STL with various design goals.

Language features C++ inherits most of C's syntax and the C preprocessor. The following is Bjarne Stroustrup's version of the Hello world program which uses the C++ standard library stream facility to write a message to standard output:[11] [12] #include <iostream> int main() { std::cout << "Hello, world!\n"; } The C++ standard requires the main function to be defined with int as its return type, but it need not return a value with an explicit return statement, as an implicit return 0 is executed when the end of main is reached.[13] Such an implicit <return> rule does not apply to any other value-returning functions: If control reaches their closing } undefined behavior results.[14]

Operators and operator overloading C++ provides more than 30 operators, covering basic arithmetic, bit manipulation, indirection, comparisons, logical operations and others. Almost all operators can be overloaded for user-defined types, with a few notable exceptions such as member access (. and .*). The rich set of overloadable operators is central to using C++ as a domain specific language. The overloadable operators are also an essential part of many advanced C++ programming techniques, such as smart pointers. Overloading an operator does not change the precedence of calculations involving the operator, nor does it change the number of operands that the operator uses (any operand may however be ignored by the operator, though it will be evaluated prior to execution).

Templates C++ templates enable generic programming. C++ supports both function and class templates. Templates may be parameterized by types, compile-time constants, and other templates. C++ templates are implemented by instantiation at compile-time. To instantiate a template, compilers substitute specific arguments for a template's parameters to generate a concrete function or class instance. Some substitutions are not possible; these are eliminated by an overload resolution policy described by the phrase "Substitution failure is not an error" (SFINAE). Templates are a powerful tool that can be used for generic programming, template metaprogramming, and code optimization, but this power implies a cost. Template use may increase code size, since each template instantiation produces a copy of the template code: one for each set of template arguments. This is in contrast to run-time generics seen in other languages (e.g. Java) where at compile-time the type is erased and a single template body is preserved. Templates are different from macros: while both of these compile-time language features enable conditional compilation, templates are not restricted to lexical substitution. Templates are aware of the semantics and type system of their companion language, as well as all compile-time type definitions, and can perform high-level operations including programmatic flow control based on evaluation of strictly type-checked parameters. Macros are capable of conditional control over compilation based on predetermined criteria, but cannot instantiate new types, recurse, or perform type evaluation and in effect are limited to pre-compilation text-substitution and text-inclusion/exclusion. In other words, macros can control compilation flow based on pre-defined symbols but cannot, unlike templates, independently instantiate new symbols. Templates are a tool for static polymorphism (see below) and generic programming.

C++

70 In addition, templates are a compile time mechanism in C++ which is Turing-complete, meaning that any computation expressible by a computer program can be computed, in some form, by a template metaprogram prior to runtime. In summary, a template is a compile-time parameterized function or class written without knowledge of the specific arguments used to instantiate it. After instantiation the resulting code is equivalent to code written specifically for the passed arguments. In this manner, templates provide a way to decouple generic, broadly-applicable aspects of functions and classes (encoded in templates) from specific aspects (encoded in template parameters) without sacrificing performance due to abstraction.

Objects C++ introduces object-oriented (OO) features to C. It offers classes, which provide the four features commonly present in OO (and some non-OO) languages: abstraction, encapsulation, inheritance, and polymorphism. Objects are instances of classes created at runtime. The class can be thought of as a template from which many different individual objects may be generated as a program runs. Encapsulation Encapsulation is the hiding of information in order to ensure that data structures and operators are used as intended and to make the usage model more obvious to the developer. C++ provides the ability to define classes and functions as its primary encapsulation mechanisms. Within a class, members can be declared as either public, protected, or private in order to explicitly enforce encapsulation. A public member of the class is accessible to any function. A private member is accessible only to functions that are members of that class and to functions and classes explicitly granted access permission by the class ("friends"). A protected member is accessible to members of classes that inherit from the class in addition to the class itself and any friends. The OO principle is that all of the functions (and only the functions) that access the internal representation of a type should be encapsulated within the type definition. C++ supports this (via member functions and friend functions), but does not enforce it: the programmer can declare parts or all of the representation of a type to be public, and is allowed to make public entities that are not part of the representation of the type. Because of this, C++ supports not just OO programming, but other weaker decomposition paradigms, like modular programming. It is generally considered good practice to make all data private or protected, and to make public only those functions that are part of a minimal interface for users of the class. This hides all the details of data implementation, allowing the designer to later fundamentally change the implementation without changing the interface in any way.[15] [16] Inheritance Inheritance allows one data type to acquire properties of other data types. Inheritance from a base class may be declared as public, protected, or private. This access specifier determines whether unrelated and derived classes can access the inherited public and protected members of the base class. Only public inheritance corresponds to what is usually meant by "inheritance". The other two forms are much less frequently used. If the access specifier is omitted, a "class" inherits privately, while a "struct" inherits publicly. Base classes may be declared as virtual; this is called virtual inheritance. Virtual inheritance ensures that only one instance of a base class exists in the inheritance graph, avoiding some of the ambiguity problems of multiple inheritance. Multiple inheritance is a C++ feature sometimes considered controversial. Multiple inheritance allows a class to be derived from more than one base class; this can result in a complicated graph of inheritance relationships. For example, a "Flying Cat" class can inherit from both "Cat" and "Flying Mammal". Some other languages, such as Java or C#, accomplish something similar (although more limited) by allowing inheritance of multiple interfaces while restricting the number of base classes to one (interfaces, unlike classes, provide only declarations of member functions, no implementation or member data). Interfaces and abstract classes in Java and C# can be defined in C++

C++

71 as a class containing only pure virtual functions, often known as an abstract base class or "ABC." Programmers preferring the Java/C# model of inheritance can choose to inherit only one non-abstract class, although in this case the declared member functions of the abstract base classes must be explicitly defined and cannot be inherited.

Polymorphism Polymorphism enables one common interface for many implementations, and for objects to act differently under different circumstances. C++ supports several kinds of static (compile-time) and dynamic (run-time) polymorphisms. Compile-time polymorphism does not allow for certain run-time decisions, while run-time polymorphism typically incurs a performance penalty. Static polymorphism Function overloading allows programs to declare multiple functions having the same name (but with different arguments). The functions are distinguished by the number and/or types of their formal parameters. Thus, the same function name can refer to different functions depending on the context in which it is used. The type returned by the function is not used to distinguish overloaded functions. When declaring a function, a programmer can specify default arguments for one or more parameters. Doing so allows the parameters with defaults to optionally be omitted when the function is called, in which case the default arguments will be used. When a function is called with fewer arguments than there are declared parameters, explicit arguments are matched to parameters in left-to-right order, with any unmatched parameters at the end of the parameter list being assigned their default arguments. In many cases, specifying default arguments in a single function declaration is preferable to providing overloaded function definitions with different numbers of parameters. Templates in C++ provide a sophisticated mechanism for writing generic, polymorphic code. In particular, through the Curiously Recurring Template Pattern it's possible to implement a form of static polymorphism that closely mimics the syntax for overriding virtual functions. Since C++ templates are type-aware and Turing-complete they can also be used to let the compiler resolve recursive conditionals and generate substantial programs through template metaprogramming. Dynamic polymorphism Inheritance Variable pointers (and references) to a base class type in C++ can refer to objects of any derived classes of that type in addition to objects exactly matching the variable type. This allows arrays and other kinds of containers to hold pointers to objects of differing types. Because assignment of values to variables usually occurs at run-time, this is necessarily a run-time phenomenon. C++ also provides a dynamic_cast operator, which allows the program to safely attempt conversion of an object into an object of a more specific object type (as opposed to conversion to a more general type, which is always allowed). This feature relies on run-time type information (RTTI). Objects known to be of a certain specific type can also be cast to that type with static_cast, a purely compile-time construct which is faster and does not require RTTI. Virtual member functions Ordinarily when a function in a derived class overrides a function in a base class, the function to call is determined by the type of the object. A given function is overridden when there exists no difference, in the number or type of parameters, between two or more definitions of that function. Hence, at compile time it may not be possible to determine the type of the object and therefore the correct function to call, given only a base class pointer; the decision is therefore put off until runtime. This is called dynamic dispatch. Virtual member functions or methods[17] allow the most specific implementation of the function to be called, according to the actual run-time type of the

C++

72 object. In C++, this is commonly done using virtual function tables. If the object type is known, this may be bypassed by prepending a fully qualified class name before the function call, but in general calls to virtual functions are resolved at run time. In addition to standard member functions, operator overloads and destructors can be virtual. A general rule of thumb is that if any functions in the class are virtual, the destructor should be as well. As the type of an object at its creation is known at compile time, constructors, and by extension copy constructors, cannot be virtual. Nonetheless a situation may arise where a copy of an object needs to be created when a pointer to a derived object is passed as a pointer to a base object. In such a case a common solution is to create a clone() (or similar) function and declare that as virtual. The clone() method creates and returns a copy of the derived class when called. A member function can also be made "pure virtual" by appending it with = 0 after the closing parenthesis and before the semicolon. Objects cannot be created of a class with a pure virtual function and are called abstract data types. Such abstract data types can only be derived from. Any derived class inherits the virtual function as pure and must provide a non-pure definition of it (and all other pure virtual functions) before objects of the derived class can be created. A program that attempts to create an object of a class with a pure virtual member function or inherited pure virtual member function is ill-formed.

Parsing and processing C++ source code It is relatively difficult to write a good C++ parser with classic parsing algorithms such as LALR(1).[18] This is partly because the C++ grammar is not LALR. Because of this, there are very few tools for analyzing or performing non-trivial transformations (e.g., refactoring) of existing code. One way to handle this difficulty is to choose a different syntax, such as Significantly Prettier and Easier C++ Syntax, which is LALR(1) parsable. More powerful parsers, such as GLR parsers, can be substantially simpler (though slower). Parsing (in the literal sense of producing a syntax tree) is not the most difficult problem in building a C++ processing tool. Such tools must also have the same understanding of the meaning of the identifiers in the program as a compiler might have. Practical systems for processing C++ must then not only parse the source text, but be able to resolve for each identifier precisely which definition applies (e.g. they must correctly handle C++'s complex scoping rules) and what its type is, as well as the types of larger expressions. Finally, a practical C++ processing tool must be able to handle the variety of C++ dialects used in practice (such as that supported by the GNU Compiler Collection and that of Microsoft's Visual C++) and implement appropriate analyzers, source code transformers, and regenerate source text. Combining advanced parsing algorithms such as GLR with symbol table construction and program transformation machinery can enable the construction of arbitrary C++ tools.

Compatibility Producing a reasonably standards-compliant C++ compiler has proven to be a difficult task for compiler vendors in general. For many years, different C++ compilers implemented the C++ language to different levels of compliance to the standard, and their implementations varied widely in some areas such as partial template specialization. Recent releases of most popular C++ compilers support almost all of the C++ 1998 standard.[19] In order to give compiler vendors greater freedom, the C++ standards committee decided not to dictate the implementation of name mangling, exception handling, and other implementation-specific features. The downside of this decision is that object code produced by different compilers is expected to be incompatible. There are, however, third party standards for particular machines or operating systems which attempt to standardize compilers on those platforms (for example C++ ABI[20] ); some compilers adopt a secondary standard for these items.

C++

Exported templates One particular point of contention is the export keyword, intended to allow template definitions to be separated from their declarations. The first compiler to implement export was Comeau C/C++, in early 2003 (5 years after the release of the standard); in 2004, the beta compiler of Borland C++ Builder X was also released with export. Both of these compilers are based on the EDG C++ front end. Other compilers such as GCC do not support it at all. Many C++ books (such as Beginning ANSI C++ by Ivor Horton) provide example code with the keyword that will not compile in most compilers, without reference to this problem. Herb Sutter, former convener of the C++ standards committee, recommended that export be removed from future versions of the C++ standard.[21] During the March 2010 ISO C++ standards meeting, the C++ standards committee voted to remove exported templates entirely from C++0x but reserve the keyword for future use.[22]

With C C++ is often considered to be a superset of C, but this is not strictly true.[23] Most C code can easily be made to compile correctly in C++, but there are a few differences that cause some valid C code to be invalid in C++, or to behave differently in C++. One commonly encountered difference is that C allows implicit conversion from void* to other pointer types, but C++ does not. Another common portability issue is that C++ defines many new keywords, such as new and class, that may be used as identifiers (e.g. variable names) in a C program. Some incompatibilities have been removed by the latest (C99) C standard, which now supports C++ features such as // comments and mixed declarations and code. On the other hand, C99 introduced a number of new features that C++ does not support, such as variable-length arrays, native complex-number types, designated initializers and compound literals.[24] However, at least some of the new C99 features will likely be included in the next version of the C++ standard, C++0x. In order to intermix C and C++ code, any function declaration or definition that is to be called from/used both in C and C++ must be declared with C linkage by placing it within an extern "C" {/*...*/} block. Such a function may not rely on features depending on name mangling (i.e., function overloading).

Criticism Critics of the language raise several points. First, since C++ includes C as a subset, it inherits many of the criticisms leveled at C. For its large feature set, it is criticized as being "bloated", over-complicated, and difficult to fully master.[25] Bjarne Stroustrup points out that resultant executables do not support these claims of bloat: "I have even seen the C++ version of the 'hello world' program smaller than the C version."[26] An Embedded C++ standard was proposed to deal with part of this, but criticized for leaving out useful parts of the language that incur no runtime penalty.[27] C++ is more complex than some other programming languages. The ISO standard of the C++ language is about 310 pages (excluding the definitions of what is in the library). For comparison, the C programming language standard, written eight years earlier, is about 160 pages, and C#'s ECMA language definition document is about 440 pages. Bjarne Stroustrup points out that "The programming world is far more complex today than it was 30 years ago, and modern programming languages reflect that."[28] Other criticism stems from what is missing from C++. For example, the current version of Standard C++ provides no language features to create multi-threaded software. These facilities are present in some other languages including Java, Ada, and C# (see also Lock). It is possible to use operating system calls or third party libraries to do multi-threaded programming, but both approaches may create portability concerns. The new C++0x standard addresses this matter by extending the language with threading facilities.

C++

74 C++ is also sometimes compared unfavorably with languages such as Smalltalk, Java, or Eiffel on the basis that it enables programmers to "mix and match" object-oriented programming, procedural programming, generic programming, functional programming, declarative programming, and others, rather than strictly enforcing a single style, although this feature may also be considered an advantage. A fraudulent article was written wherein Bjarne Stroustrup is supposedly interviewed for a 1998 issue of IEEE's 'Computer' magazine[29] . In this article, the interviewer expects to discuss the successes of C++ now that several years had passed after its introduction. Instead, Stroustrup proceeds to confess that his invention of C++ was intended to create the most complex and difficult language possible to weed out amateur programmers and raise the salaries of the few programmers who could master the language. The article contains various criticisms of C++'s complexity and poor usability, most false or exaggerated. In reality, Stroustrup wrote no such article, and due to the pervasiveness of the hoax, was compelled to publish an official denial on his website.[30] . Some have criticized C++ for not having garbage collection, Dr. Stroustrup even discusses this on his website, he states that the memory management capabilities of C++ essentially perform the same function as garbage collection without an implicit garbage collector. [31]

See also • • • • • •

The C++ Programming Language C++0x, the planned new standard for C++ Comparison of integrated development environments for C/C++ Comparison of programming languages List of C++ compilers List of C++ template libraries

Further reading • Abrahams, David; Aleksey Gurtovoy. C++ Template Metaprogramming: Concepts, Tools, and Techniques from Boost and Beyond. Addison-Wesley. ISBN 0-321-22725-5. • Alexandrescu, Andrei (2001). Modern C++ Design: Generic Programming and Design Patterns Applied. Addison-Wesley. ISBN 0-201-70431-5. • Alexandrescu, Andrei; Herb Sutter (2004). C++ Design and Coding Standards: Rules and Guidelines for Writing Programs. Addison-Wesley. ISBN 0-321-11358-6. • Becker, Pete (2006). The C++ Standard Library Extensions : A Tutorial and Reference. Addison-Wesley. ISBN 0-321-41299-0. • Brokken, Frank (2010). C++ Annotations [32]. University of Groningen. ISBN 90 367 0470 7. • Coplien, James O. (1992, reprinted with corrections 1994). Advanced C++: Programming Styles and Idioms. ISBN 0-201-54855-0. • Dewhurst, Stephen C. (2005). C++ Common Knowledge: Essential Intermediate Programming. Addison-Wesley. ISBN 0-321-32192-8. • Information Technology Industry Council (15 October 2003). Programming languages — C++ (Second edition ed.). Geneva: ISO/IEC. 14882:2003(E). • Josuttis, Nicolai M. The C++ Standard Library. Addison-Wesley. ISBN 0-201-37926-0. • Koenig, Andrew; Barbara E. Moo (2000). Accelerated C++ - Practical Programming by Example. Addison-Wesley. ISBN 0-201-70353-X. • Lippman, Stanley B.; Josée Lajoie, Barbara E. Moo (2005). C++ Primer. Addison-Wesley. ISBN 0-201-72148-1. • Lippman, Stanley B. (1996). Inside the C++ Object Model. Addison-Wesley. ISBN 0-201-83454-5. • Stroustrup, Bjarne (2000). The C++ Programming Language (Special Edition ed.). Addison-Wesley. ISBN 0-201-70073-5.

C++

75 • Stroustrup, Bjarne (1994). The Design and Evolution of C++. Addison-Wesley. ISBN 0-201-54330-3. • Stroustrup, Bjarne. Programming Principles and Practice Using C++. Addison-Wesley. ISBN 0321543726. • Sutter, Herb (2001). More Exceptional C++: 40 New Engineering Puzzles, Programming Problems, and Solutions. Addison-Wesley. ISBN 0-201-70434-X. • Sutter, Herb (2004). Exceptional C++ Style. Addison-Wesley. ISBN 0-201-76042-8. • Vandevoorde, David; Nicolai M. Josuttis (2003). C++ Templates: The complete Guide. Addison-Wesley. ISBN 0-201-73484-2. • Scott Meyers (2005). Effective C++. Third Edition. Addison-Wesley. ISBN 0-321-33487-6

External links • JTC1/SC22/WG21 [33] - The ISO/IEC C++ Standard Working Group

• • • •

• n3092.pdf [34] - Final Committee Draft of "ISO/IEC IS 14882 - Programming Languages - C++" (26 March 2010) A paper by Stroustrup showing the timeline of C++ evolution (1979-1991) [35] Bjarne Stroustrup's C++ Style and Technique FAQ [36] Apache C++ Standard Library Documentation [37] C++ FAQ Lite by Marshall Cline [38]

• • • •

Computer World interview with Bjarne Stroustrup [39] CrazyEngineers.com interview with Bjarne Stroustrup [40] The State of the Language: An Interview with Bjarne Stroustrup (August 15, 2008) [41] Code practices for not breaking binary compatibility between releases of C++ libraries [42] (from KDE Techbase)

References [1] [2] [3] [4] [5]

Stroustrup, Bjarne (1997). "1". The C++ Programming Language (Third ed.). ISBN 0201889544. OCLC 59193992. C++ The Complete Reference Third Edition, Herbert Schildt, Publisher: Osborne McGraw-Hill. ATT.com (http:/ / www2. research. att. com/ ~bs/ bs_faq. html#invention) "Programming Language Popularity" (http:/ / www. langpop. com/ ). 2009. . Retrieved 2009-01-16. "TIOBE Programming Community Index" (http:/ / www. tiobe. com/ index. php/ content/ paperinfo/ tpci/ index. html). 2009. . Retrieved 2009-05-06. [6] "Bjarne Stroustrup's FAQ — When was C++ invented?" (http:/ / public. research. att. com/ ~bs/ bs_faq. html#invention). . Retrieved 30 May 2006. [7] "Trends on C++ Programmers, Developers & Engineers" (http:/ / www. odesk. com/ trends/ c+ + ). . Retrieved 1 December 2008. [8] Stroustrup, Bjarne. "C++ Glossary" (http:/ / www. research. att. com/ ~bs/ glossary. html). . Retrieved 8 June 2007. [9] "Bjarne Stroustrup's FAQ — Where did the name "C++" come from?" (http:/ / public. research. att. com/ ~bs/ bs_faq. html#name). . Retrieved 16 January 2008. [10] STLPort home page (http:/ / www. stlport. org/ ), quote from "The C++ Standard Library" by Nicolai M. Josuttis, p138., ISBN 0-201 37926-0, Addison-Wesley, 1999: "An exemplary version of STL is the STLport, which is available for free for any platform" [11] Stroustrup, Bjarne (2000). The C++ Programming Language (Special Edition ed.). Addison-Wesley. p. 46. ISBN 0-201-70073-5. [12] Open issues for The C++ Programming Language (3rd Edition) (http:/ / www. research. att. com/ ~bs/ 3rd_issues. html) - This code is copied directly from Bjarne Stroustrup's errata page (p. 633). He addresses the use of '\n' rather than std::endl. Also see www.research.att.com (http:/ / www. research. att. com/ ~bs/ bs_faq2. html#void-main) for an explanation of the implicit return 0; in the main function. This implicit return is not available in other functions. [13] ISO/IEC (2003). ISO/IEC 14882:2003(E): Programming Languages - C++ §3.6.1 Main function [basic.start.main] para. 5 [14] ISO/IEC (2003). ISO/IEC 14882:2003(E): Programming Languages - C++ §6.6.3 The return statement [stmt.return] para. 2 [15] Sutter, Herb; Alexandrescu, Andrei (2004). C++ Coding Standards: 101 Rules, Guidelines, and Best Practices. Addison-Wesley. [16] Henricson, Mats; Nyquist, Erik (1997). Industrial Strength C++. Prentice Hall. ISBN ISBN 0-13-120965-5. [17] Stroustrup, Bjarne (2000). The C++ Programming Language (Special Edition ed.). Addison-Wesley. p. 310. ISBN 0-201-70073-5. "A virtual member function is sometimes called a method." [18] Andrew Birkett. "Parsing C++ at nobugs.org" (http:/ / www. nobugs. org/ developer/ parsingcpp/ ). Nobugs.org. . Retrieved 3 July 2009. [19] Herb Sutter (15 April 2003). "C++ Conformance Roundup" (http:/ / www. ddj. com/ dept/ cpp/ 184401381). Dr. Dobb's Journal. . Retrieved 30 May 2006. [20] "C++ ABI" (http:/ / www. codesourcery. com/ cxx-abi/ ). . Retrieved 30 May 2006.

C++

76 [21] Why We Can’t Afford Export (http:/ / anubis. dkuug. dk/ jtc1/ sc22/ wg21/ docs/ papers/ 2003/ n1426. pdf)PDF (266 KB) [22] Herb Sutter (13 March 2010). "Trip Report: March 2010 ISO C++ Standards Meeting" (http:/ / herbsutter. com/ 2010/ 03/ 13/ trip-report-march-2010-iso-c-standards-meeting/ ). . Retrieved 8 April 2010. [23] "Bjarne Stroustrup's FAQ - Is C a subset of C++?" (http:/ / public. research. att. com/ ~bs/ bs_faq. html#C-is-subset). . Retrieved 18 January 2008. [24] "C9X -- The New C Standard" (http:/ / home. datacomm. ch/ t_wolf/ tw/ c/ c9x_changes. html). . Retrieved 27 December 2008. [25] Morris, Richard (July 2, 2009). "Niklaus Wirth: Geek of the Week" (http:/ / www. simple-talk. com/ opinion/ geek-of-the-week/ niklaus-wirth-geek-of-the-week/ ). . Retrieved 8 August 2009. "C++ is a language that was designed to cater to everybody’s perceived needs. As a result, the language and even more so its implementations have become complex and bulky, difficult to understand, and likely to contain errors for ever." [26] Why is the code generated for the "Hello world" program ten times larger for C++ than for C? (http:/ / www. research. att. com/ ~bs/ bs_faq. html#Hello-world) [27] What do you think of EC++? (http:/ / www. research. att. com/ ~bs/ bs_faq. html#EC+ + ) [28] Why is C++ so BIG? (http:/ / www. research. att. com/ ~bs/ bs_faq. html#big) [29] Unattributed. Previously unpublished interview with Bjarne Stroustroup, designer of C++ (http:/ / flinflon. brandonu. ca/ dueck/ 1997/ 62285/ stroustroup. html). [30] Stroustrup, Bjarne. Stroustrup FAQ: Did you really give an interview to IEEE? (http:/ / www2. research. att. com/ ~bs/ bs_faq. html#IEEE) [31] http:/ / www2. research. att. com/ ~bs/ bs_faq. html. [32] http:/ / www. icce. rug. nl/ documents/ cplusplus/ [33] http:/ / www. open-std. org/ jtc1/ sc22/ wg21/ [34] http:/ / www. open-std. org/ jtc1/ sc22/ wg21/ docs/ papers/ 2010/ n3092. pdf [35] http:/ / www. research. att. com/ ~bs/ hopl2. pdf [36] [37] [38] [39] [40] [41] [42]

http:/ / www. research. att. com/ ~bs/ bs_faq2. html http:/ / incubator. apache. org/ stdcxx/ doc http:/ / www. parashift. com/ c%2B%2B-faq-lite/ http:/ / www. computerworld. com. au/ index. php/ id;408408016;pp;1;fp;16;fpid;1 http:/ / www. crazyengineers. com/ small-talk/ 1-cover-story/ 24-small-talk-with-dr-bjarne-stroustrup http:/ / www. devx. com/ SpecialReports/ Article/ 38813/ 0/ page/ 1 http:/ / techbase. kde. org/ Policies/ Binary_Compatibility_Issues_With_C+ +

Perl

Usual file extensions

.pl

Paradigm

multi-paradigm: functional, imperative, object-oriented (class-based)

Appeared in

1987

Larry Wall

Developer

Larry Wall

Stable release

5.12.0 (April 12, 2010)

Preview release

5.13.0 (April 20, 2010)

Typing discipline

Dynamic

Influenced by

AWK, Smalltalk 80, Lisp, C, C++, sed, Unix shell, Pascal

Influenced

Python, PHP, Ruby, ECMAScript, Dao, Windows PowerShell, JavaScript, Falcon

Programming language C OS

Cross-platform

License

GNU General Public License, Artistic License

Website

www.perl.org

[1]

Perl is a high-level, general-purpose, interpreted, dynamic programming language. Perl was originally developed by Larry Wall in 1987 as a general-purpose Unix scripting language to make report processing easier.[2] [3] Since then, it has undergone many changes and revisions and become widely popular amongst programmers. Larry Wall continues to oversee development of the core language, and its upcoming version, Perl 6. Perl borrows features from other programming languages including C, shell scripting (sh), AWK, and sed.[4] The language provides powerful text processing facilities without the arbitrary data length limits of many contemporary Unix tools,[5] facilitating easy manipulation of text files. It is also used for graphics programming, system administration, network programming, applications that require database access and CGI programming on the Web. Perl is nicknamed "the Swiss Army chainsaw of programming languages" due to its flexibility and adaptability.[6]

History Early Perl Versions Larry Wall began work on Perl in 1987, while working as a programmer at Unisys,[7] and released version 1.0 to the comp.sources.misc newsgroup on December 18, 1987.[8] The language expanded rapidly over the next few years. Perl 2, released in 1988, featured a better regular expression engine. Perl 3, released in 1989, added support for binary data streams. Originally the only documentation for Perl was a single (increasingly lengthy) man page. In 1991, Programming Perl (known to many Perl programmers as the "Camel Book" because of its cover) was published and became the de facto reference for the language. At the same time, the Perl version number was bumped to 4窶馬ot to mark a major change in the language but to identify the version that was documented by the book.

Perl

Early Perl 5 Perl 4 went through a series of maintenance releases, culminating in Perl 4.036 in 1993. At that point, Wall abandoned Perl 4 to begin work on Perl 5. Initial design of Perl 5 continued into 1994. The perl5-porters mailing list was established in May 1994 to coordinate work on porting Perl 5 to different platforms. It remains the primary forum for development, maintenance, and porting of Perl 5.[9] Perl 5.000 was released on October 17, 1994.[10] It was a nearly complete rewrite of the interpreter, and it added many new features to the language, including objects, references, lexical (my) variables, and modules. Importantly, modules provided a mechanism for extending the language without modifying the interpreter. This allowed the core interpreter to stabilize, even as it enabled ordinary Perl programmers to add new language features. Perl 5 has been in active development since then. Perl 5.001 was released on March 13, 1995. Perl 5.002 was released on February 29, 1996 with the new prototypes feature. This allowed module authors to make subroutines that behaved like Perl builtins. Perl 5.003 was released June 25, 1996, as a security release. One of the most important events in Perl 5 history took place outside of the language proper and was a consequence of its module support. On October 26, 1995, the Comprehensive Perl Archive Network (CPAN) was established as a repository for Perl modules and Perl itself. At the time of writing, it carries almost 17,000 modules by more than 7,000 authors. CPAN is widely regarded as one of the greatest strengths of Perl in practice. Perl 5.004 was released on May 15, 1997, and included among other things the UNIVERSAL package, giving Perl a base object to which all classes were automatically derived and the ability to require versions of modules. In addition, Perl now supported running under Microsoft Windows and several other operating systems.[11] Perl 5.005 was released on July 22, 1998. This release included several enhancements to the Regex engine, new hooks into the backend through the B::* modules, the qr// regex quote operator, a large selection of other new core modules, and added support for several more operating systems, including BeOS.[12]

2000â&#x20AC;&#x201C;Present Perl 5.6 was released on March 22, 2000. Major changes included 64 bit support, unicode string representation, large file support (eg, files > 2 GiB) and the 'our' keyword.[13] [14] When developing Perl 5.6, the decision was made to switch the versioning scheme to one more similar to other open source projects; after 5.005_63, the next version became 5.5.640, with plans for development versions to have odd numbers and stable versions to have even numbers. In 2000, Larry Wall put forth a call for suggestions for a new version of Perl from the community. The process resulted in 361 RFCs (Request for Change) documents which were to be used in guiding development of Perl 6. In 2001,[15] work began on the apocalypses for Perl 6, a series of documents meant to summarize the change requests and present the design of the next generation of Perl. They were presented as a digest of the RFCs, rather than a formal document. At this point, Perl 6 simply existed as a description of a language. Perl 5.8 was first released on July 18, 2002, and had nearly yearly updates since then. The latest version of Perl 5.8 is 5.8.9, released December 14, 2008. Perl 5.8 improved unicode support, added a new IO implementation, added a new thread implementation, improved numeric accuracy, and added several new modules.[16] In 2004, work began on the Synopses â&#x20AC;&#x201C; originally documents that summarized the Apocalypes, but which became the specification for the Perl 6 language. In February 2005, Audrey Tang began work on Pugs, a Perl 6 interpreter written in Haskell[17] . This was the first real concerted effort towards making Perl 6 a reality. This effort stalled in 2006. On December 18, 2007, the 20th anniversary of Perl 1.0, Perl 5.10.0 was released. Perl 5.10.0 included notable new features, which brought it closer to Perl 6. Some of these new features were a new switch statement (called "given"/"when"), regular expressions updates, and the smart match operator, "~~".[18] [19]

Perl

79 Around this same time, development began in earnest on another implementation of Perl 6 known as Rakudo Perl, developed in tandem with the Parrot virtual machine. As of November 2009, Rakudo Perl has had regular monthly releases and now is the most complete implementation of Perl 6. A major change in the development process of Perl 5 occurred with Perl 5.11; the development community has switched to a monthly release cycle, with planned release dates three months ahead. On April 12, 2010, Perl 5.12.0 was released. Notable core enhancements include new package NAME VERSION syntax, the Yada Yada operator (intended to mark placeholder code that is not yet implemented), implicit strictures, full Y2038 compliance, regexp conversion overloading, DTrace support, and Unicode 5.2. [20] The latest development release of Perl 5 is 5.13.0, released by LĂŠon Brocard on April 20, 2010[21] .

Name Perl was originally named "Pearl," after the Parable of the Pearl from the Gospel of Matthew. Larry Wall wanted to give the language a short name with positive connotations; he claims that he considered (and rejected) every threeand four-letter word in the dictionary. He also considered naming it after his wife Gloria. Wall discovered the existing PEARL programming language before Perl's official release and changed the spelling of the name. When referring to the language, the name is normally capitalized (Perl) as a proper noun, as you would a spoken language (e.g. English or French). When referring to the interpreter program itself, the name is often uncapitalised (perl) because most Unix-like file systems are case-sensitive. Before the release of the first edition of Programming Perl, it was common to refer to the language as perl; Randal L. Schwartz, however, capitalised the language's name in the book to make it stand out better when typeset. This case distinction was subsequently documented as canonical.[22] There is some contention about the all-caps spelling "PERL," which the documentation declares incorrect[22] and which some core community members consider a sign of outsiders.[23] Although the name is occasionally taken as an acronym for Practical Extraction and Report Language (which appears at the top of the documentation[24] and in some printed literature[25] ), this expansion actually came after the name; several others have been suggested as equally canonical, including Wall's own humorous Pathologically Eclectic Rubbish Lister.[26] Indeed, Wall claims that the name was intended to inspire many different expansions.[27]

The camel symbol Programming Perl, published by O'Reilly Media, features a picture of a camel on the cover and is commonly referred to as The Camel Book.[7] This image of a camel has become a general symbol of Perl. It is also a hacker emblem, appearing on some T-shirts and other clothing items. O'Reilly owns the image as a trademark but claims to use their legal rights only to protect the "integrity and impact of that symbol".[28] O'Reilly allows non-commercial use of the symbol and provides Programming Republic of Perl logos and Powered by Perl buttons.[29] However, the Camel has never been meant to be an official Perl symbol, and if one is to be considered instead, it's an onion.[30]

Perl

Overview Perl is a general-purpose programming language originally developed for text manipulation and now used for a wide range of tasks including system administration, web development, network programming, games, bioinformatics, and GUI development. The language is intended to be practical (easy to use, efficient, complete) rather than beautiful (tiny, elegant, minimal).[31] Its major features include support for multiple programming paradigms (procedural, object-oriented, and functional styles), reference counting memory management (without a cycle-detecting garbage collector), built-in support for text processing, and a large collection of third-party modules. According to Larry Wall, Perl has two slogans. The first is "There's more than one way to do it," commonly known as TMTOWTDI. The second slogan is "Easy things should be easy and hard things should be possible."

Features The overall structure of Perl derives broadly from C. Perl is procedural in nature, with variables, expressions, assignment statements, brace-delimited blocks, control structures, and subroutines. Perl also takes features from shell programming. All variables are marked with leading sigils, which unambiguously identify the data type (for example, scalar, array, hash) of the variable in context. Importantly, sigils allow variables to be interpolated directly into strings. Perl has many built-in functions that provide tools often used in shell programming (although many of these tools are implemented by programs external to the shell) such as sorting, and calling on system facilities. Perl takes lists from Lisp, associative arrays (hashes) from AWK, and regular expressions from sed. These simplify and facilitate many parsing, text-handling, and data-management tasks. In Perl 5, features were added that support complex data structures, first-class functions (that is, closures as values), and an object-oriented programming model. These include references, packages, class-based method dispatch, and lexically scoped variables, along with compiler directives (for example, the strict pragma). A major additional feature introduced with Perl 5 was the ability to package code as reusable modules. Larry Wall later stated that "The whole intent of Perl 5's module system was to encourage the growth of Perl culture rather than the Perl core."[32] All versions of Perl do automatic data typing and memory management. The interpreter knows the type and storage requirements of every data object in the program; it allocates and frees storage for them as necessary using reference counting (so it cannot deallocate circular data structures without manual intervention). Legal type conversionsâ&#x20AC;&#x201D;for example, conversions from number to stringâ&#x20AC;&#x201D;are done automatically at run time; illegal type conversions are fatal errors.

Design The design of Perl can be understood as a response to three broad trends in the computer industry: falling hardware costs, rising labor costs, and improvements in compiler technology. Many earlier computer languages, such as Fortran and C, were designed to make efficient use of expensive computer hardware. In contrast, Perl is designed to make efficient use of expensive computer programmers. Perl has many features that ease the programmer's task at the expense of greater CPU and memory requirements. These include automatic memory management; dynamic typing; strings, lists, and hashes; regular expressions; introspection; and an eval() function. Wall was trained as a linguist, and the design of Perl is very much informed by linguistic principles. Examples include Huffman coding (common constructions should be short), good end-weighting (the important information should come first), and a large collection of language primitives. Perl favors language constructs that are concise and natural for humans to read and write, even where they complicate the Perl interpreter.

Perl

81 Perl syntax reflects the idea that "things that are different should look different." For example, scalars, arrays, and hashes have different leading sigils. Array indices and hash keys use different kinds of braces. Strings and regular expressions have different standard delimiters. This approach can be contrasted with languages such as Lisp, where the same S-expression construct and basic syntax are used for many different purposes. Perl does not enforce any particular programming paradigm (procedural, object-oriented, functional, and others) or even require the programmer to choose among them. There is a broad practical bent to both the Perl language and the community and culture that surround it. The preface to Programming Perl begins, "Perl is a language for getting your job done." One consequence of this is that Perl is not a tidy language. It includes many features, tolerates exceptions to its rules, and employs heuristics to resolve syntactical ambiguities. Because of the forgiving nature of the compiler, bugs can sometimes be hard to find. Discussing the variant behaviour of built-in functions in list and scalar contexts, the perlfunc(1) manual page says, "In general, they do what you want, unless you want consistency." In addition to Larry Wall's two slogans mentioned above, Perl has several mottos that convey aspects of its design and use, including "Perl: the Swiss Army Chainsaw of Programming Languages" and "No unnecessary limits". Perl has also been called "The Duct Tape of the Internet".[33] No written specification or standard for the Perl language exists for Perl versions through Perl 5, and there are no plans to create one for the current version of Perl. There has been only one implementation of the interpreter, and the language has evolved along with it. That interpreter, together with its functional tests, stands as a de facto specification of the language. Perl 6, however, started with a specification,[34] and several projects[35] aim to implement some or all of the specification.

Applications Perl has many and varied applications, compounded by the availability of many standard and third-party modules. Perl has been used since the early days of the Web to write CGI scripts. It is known as one of "the three Ps" (along with Python and PHP), the most popular dynamic languages for writing Web applications. It is also an integral component of the popular LAMP solution stack for web development. Large projects written in Perl include cPanel, Slash, Bugzilla, RT, TWiki, and Movable Type. Many high-traffic websites use Perl extensively. Examples include Amazon.com, bbc.co.uk, Priceline.com, Craigslist, IMDb,[36] LiveJournal, Slashdot and Ticketmaster. Perl is often used as a glue language, tying together systems and interfaces that were not specifically designed to interoperate, and for "data munging,"[37] that is, converting or processing large amounts of data for tasks such as creating reports. In fact, these strengths are intimately linked. The combination makes Perl a popular all-purpose language for system administrators, particularly because short programs can be entered and run on a single command line. With a degree of care, Perl code can be made portable across Windows and Unix. Portable Perl code is often used by suppliers of software (both COTS and bespoke) to simplify packaging and maintenance of software build and deployment scripts. Graphical user interfaces (GUIs) may be developed using Perl. For example, Perl/Tk is commonly used to enable user interaction with Perl scripts. Such interaction may be synchronous or asynchronous using callbacks to update the GUI. For more information about the technologies involved, see Tk, Tcl, WxPerl and Prima Perl. Perl is also widely used in finance and bioinformatics, where it is valued for rapid application development and deployment and for its capability to handle large data sets.

Perl

Implementation Perl is implemented as a core interpreter, written in C, together with a large collection of modules, written in Perl and C. The source distribution is, as of 2009, 13.5 MB when packaged in a tar file and compressed.[38] The interpreter is 150,000 lines of C code and compiles to a 1 MB executable on typical machine architectures. Alternatively, the interpreter can be compiled to a link library and embedded in other programs. There are nearly 500 modules in the distribution, comprising 200,000 lines of Perl and an additional 350,000 lines of C code. (Much of the C code in the modules consists of character-encoding tables.) The interpreter has an object-oriented architecture. All of the elements of the Perl language—scalars, arrays, hashes, coderefs, file handles—are represented in the interpreter by C structs. Operations on these structs are defined by a large collection of macros, typedefs, and functions; these constitute the Perl C API. The Perl API can be bewildering to the uninitiated, but its entry points follow a consistent naming scheme, which provides guidance to those who use it. The life of a Perl interpreter divides broadly into a compile phase and a run phase.[39] In Perl, the phases are the major stages in the interpreter's life cycle. Each interpreter goes through each phase only once, and the phases follow in a fixed sequence. Most of what happens in Perl's compile phase is compilation, and most of what happens in Perl's run phase is execution, but there are significant exceptions. Perl makes important use of its capability to execute Perl code during the compile phase. Perl will also delay compilation into the run phase. The terms that indicate the kind of processing that is actually occurring at any moment are compile time and run time. Perl is in compile time at most points during the compile phase, but compile time may also be entered during the run phase. The compile time for code in a string argument passed to the eval built-in occurs during the run phase. Perl is often in run time during the compile phase and spends most of the run phase in run time. Code in BEGIN blocks executes at run time but in the compile phase. At compile time, the interpreter parses Perl code into a syntax tree. At run time, it executes the program by walking the tree. Text is parsed only once, and the syntax tree is subject to optimization before it is executed, so that execution is relatively efficient. Compile-time optimizations on the syntax tree include constant folding and context propagation, but peephole optimization is also performed. Perl has a Turing-complete grammar because parsing can be affected by run-time code executed during the compile phase.[40] Therefore, Perl cannot be parsed by a straight Lex/Yacc lexer/parser combination. Instead, the interpreter implements its own lexer, which coordinates with a modified GNU bison parser to resolve ambiguities in the language. It is often said that "Only perl can parse Perl," meaning that only the Perl interpreter (perl) can parse the Perl language (Perl), but even this is not, in general, true. Because the Perl interpreter can simulate a Turing machine during its compile phase, it would need to decide the Halting Problem in order to complete parsing in every case. It's a long-standing result that the Halting Problem is undecidable, and therefore not even perl can always parse Perl. Perl makes the unusual choice of giving the user access to its full programming power in its own compile phase. The cost in terms of theoretical purity is high, but practical inconvenience seems to be rare. Other programs that undertake to parse Perl, such as source-code analyzers and auto-indenters, have to contend not only with ambiguous syntactic constructs but also with the undecidability of Perl parsing in the general case. Adam Kennedy's PPI project focused on parsing Perl code as a document (retaining its integrity as a document), instead of parsing Perl as executable code (which not even Perl itself can always do). It was Kennedy who first conjectured that, "parsing Perl suffers from the 'Halting Problem'."[41] and this was later proved.[42] Perl is distributed with some 120,000 functional tests. These run as part of the normal build process and extensively exercise the interpreter and its core modules. Perl developers rely on the functional tests to ensure that changes to the interpreter do not introduce bugs; conversely, Perl users who see that the interpreter passes its functional tests on

Perl

83 their system can have a high degree of confidence that it is working properly. Maintenance of the Perl interpreter has become increasingly difficult over the years. The code base has been in continuous development since 1994. The code has been optimized for performance at the expense of simplicity, clarity, and strong internal interfaces. New features have been added, yet virtually complete backward compatibility with earlier versions is maintained. Major releases of Perl were coordinated by Perl pumpkings which handled integrating patch submissions and bug fixes, but has since changed to a rotating, monthly release cycle. Development discussion takes place via the perl5_porters mailing list. As of Perl 5.11, development efforts have included refactoring certain core modules known as 'dual lifed' modules out of the Perl core[43] to help alleviate some of these problems.

Availability Perl is free software and is licensed under both the Artistic License and the GNU General Public License. Distributions are available for most operating systems. It is particularly prevalent on Unix and Unix-like systems, but it has been ported to most modern (and many obsolete) platforms. With only six reported exceptions, Perl can be compiled from source code on all Unix-like, POSIX-compliant, or otherwise-Unix-compatible platforms.[44] However, this is rarely necessary, because Perl is included in the default installation of many popular operating systems. Because of unusual changes required for the Mac OS Classic environment, a special port called MacPerl was shipped independently.[45] The Comprehensive Perl Archive Network (CPAN [46]) carries a complete list of supported platforms with links to the distributions available on each.[47] CPAN is also the source for publicly available Perl modules that are not part of the core Perl distribution. Windows Users of Microsoft Windows typically install one of the native binary distributions of Perl for Win32,[48] most commonly Strawberry Perl or ActivePerl. Compiling Perl from source code under Windows is possible, but most installations lack the requisite C compiler and build tools. This also makes it difficult to install modules from the CPAN, particularly those that are partially written in C. Users of the ActivePerl binary distribution are, therefore, dependent on the repackaged modules provided in ActiveStateâ&#x20AC;&#x2122;s module repository, which are precompiled and can be installed with PPM. Limited resources to maintain this repository have been cause for various long-standing problems.[49] [50] Strawberry Perl,[51] is an open source distribution for Windows. It has had regular, quarterly releases since January 2008, including new modules as feedback and requests come in. Strawberry Perl aims to be able to install modules like standard Perl distributions on other platforms, including compiling XS modules. Strawberry Perl started as a way in part to address the flaws in ActiveState's distribution and resolve other problems of Perl on the Windows platform. A community project[52] was launched by Adam Kennedy on behalf of The Perl Foundation in June 2006. A community website for "all things Windows and Perl." A major aim of this project is to provide production-quality alternative Perl distributions that include an embedded C compiler and build tools, so as to enable Windows users to install modules directly from the CPAN. A related version with research and experimental work was done in the Vanilla Perl distribution.[53] The Cygwin emulation layer is another popular way of running Perl under Windows. Cygwin provides a Unix-like environment on Windows, and both perl and cpan are conveniently available as standard pre-compiled packages in the Cygwin setup program. Because Cygwin also includes the gcc, compiling Perl from source is also possible.

Perl

Language structure In Perl, the minimal Hello world program may be written as follows: print "Hello, world!\n"; This prints the string Hello, world! and a newline, symbolically expressed by an n character whose interpretation is altered by the preceding escape character (a backslash). The canonical form of the program is slightly more verbose: #!/usr/bin/perl print "Hello, world!\n"; The hash mark character introduces a comment in Perl, which runs up to the end of the line of code and is ignored by the compiler. The comment used here is of a special kind: it’s called the shebang line. This tells Unix-like operating systems where to find the Perl interpreter, making it possible to invoke the program without explicitly mentioning perl. (Note that, on Microsoft Windows systems, Perl programs are typically invoked by associating the .pl extension with the Perl interpreter. In order to deal with such circumstances, perl detects the shebang line and parses it for switches;[54] therefore, it is not strictly true that the shebang line is ignored by the compiler.) The second line in the canonical form includes a semicolon, which is used to separate statements in Perl. With only a single statement in a block or file, a separator is unnecessary, so it can be omitted from the minimal form of the program—or more generally from the final statement in any block or file. The canonical form includes it because it is common to terminate every statement even when it is unnecessary to do so, as this makes editing easier: code can be added to, or moved away from, the end of a block or file without having to adjust semicolons. Version 5.10 of Perl introduces a say function that implicitly appends a newline character to its output, making the minimal "Hello world" program even shorter: use 5.010; # must be present to import the new 5.10 functions, notice that it is 5.010 not 5.10 say 'Hello, world!'

Data types Perl has a number of fundamental data types. The most commonly used and discussed are scalars, arrays, hashes, filehandles, and subroutines: • A scalar is a single value; it may be a number, a string, or a reference. • An array is an ordered collection of scalars. • A hash, or associative array, is a map from strings to scalars; the strings are called keys, and the scalars are called values. • A file handle is a map to a file, device, or pipe that is open for reading, writing, or both. • A subroutine is a piece of code that may be passed arguments, be executed, and return data Most variables are marked by a leading sigil, which identifies the data type being accessed (not the type of the variable itself), except filehandles, which don't have a sigil. The same name may be used for variables of different data types, without conflict.

Perl

Sigil Example Description $

$foo

a scalar

@foo

an array

%foo

a hash

none FOO

a file handle

&foo

a subroutine (the & is optional in some contexts)

*foo

a typeglob

File handles and constants need not be uppercase, but it is a common convention because there is no sigil to denote them. Both are global in scope, but file handles are interchangeable with references to file handles, which can be stored in scalars, which in turn permit lexical scoping. Doing so is encouraged in Damian Conway's Perl Best Practices. As a convenience, the open function in Perl 5.6 and newer will accept a scalar variable, which will be set (autovivified) to a reference to an anonymous file handle, in place of a named file handle. Scalar values String values (literals) must be enclosed by quotes. Enclosing a string in double quotes allows the values of variables whose names appear in the string to automatically replace the variable name (or be interpolated) in the string. Enclosing a string in single quotes prevents variable interpolation. If $name is "Jim", print("My name is $name") will print "My name is Jim", but print('My name is $name') will print "My name is $name". To include a double quotation mark in a string, precede it with a backslash or enclose the string in single quotes. To include a single quotation mark, precede it with a backslash or enclose the string in double quotes. Strings can also be quoted with the q and qq quote-like operators. 'this' is identical to q(this) and "$this" is identical to qq($this). Finally, multiline strings can be defined using here documents: $multilined_string = <<EOF; This is my multilined string note that I am terminating it with the word "EOF". EOF Numbers (numeric constants) do not require quotation. Perl will convert numbers into strings and vice versa depending on the context in which they are used. When strings are converted into numbers, trailing non-numeric parts of the strings are discarded. If no leading part of a string is numeric, the string will be converted to the number 0. In the following example, the strings $n and $m are treated as numbers. This code prints the number '5'. The values of the variables remain the same. Note that in Perl, + is always the numeric addition operator. The string concatenation operator is the period. $n = '3 apples'; $m = '2 oranges'; print $n + $m; Functions are provided for the rounding of fractional values to integer values: int chops off the fractional part, rounding towards zero; POSIX::ceil and POSIX::floor round always up and always down, respectively. The number-to-string conversion of printf "%f" or sprintf "%f" round out even, use bankers' rounding. Perl also has a boolean context that it uses in evaluating conditional statements. The following values all evaluate as false in Perl: $false = 0; # the number zero $false = 0.0; # the number zero as a float

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86 $false $false $false $false $false $false

= = = = = =

0b0; # the number zero in binary 0x0; # the number zero in hexadecimal '0'; # the string zero ""; # the empty string undef; # the return value from undef 2-3+1 # computes to 0 which is converted to "0" so it is false

All other (non-zero evaluating) values evaluate to true. This includes the odd self-describing literal string of "0 but true", which in fact is 0 as a number, but true when used as a boolean. All non-numeric strings also have this property, but this particular string is truncated by Perl without a numeric warning. A less explicit but more conceptually portable version of this string is '0E0' or '0e0', which does not rely on characters being evaluated as 0, because '0E0' is literally zero times ten to the power zero. Evaluated boolean expressions are also scalar values. The documentation does not promise which particular value of true or false is returned. Many boolean operators return 1 for true and the empty-string for false. The defined() function determines whether a variable has any value set. In the above examples, defined($false) is true for every value except undef. If either 1 or 0 are specifically needed, an explicit conversion can be done: my $real_result = $boolean_result ? 1 : 0; Array values An array value (or list) is specified by listing its elements, separated by commas, enclosed by parentheses (at least where required by operator precedence). @scores = (32, 45, 16, 5); The qw() quote-like operator allows the definition of a list of strings without typing of quotes and commas. Almost any delimiter can be used instead of parentheses. The following lines are equivalent: @names = ('Billy', 'Joe', 'Jim-Bob'); @names = qw(Billy Joe Jim-Bob); The split function returns a list of strings, which are split from a string expression using a delimiter string or regular expression. @scores = split(',', '32,45,16,5'); Individual elements of a list are accessed by providing a numerical index in square brackets. The scalar sigil must be used. Sublists (array slices) can also be specified, using a range or list of numeric indices in brackets. The array sigil is used in this case. For example, $month[3] is "March", and @month[4..6] is ("April", "May", "June"). Hash values A hash (or associative array) may be initialized from a list of key/value pairs. If the keys are separated from the values with the => operator (sometimes called a fat comma), rather than a comma, they may be unquoted (barewords). The following lines are equivalent: %favorite = ('joe', "red", 'sam', "blue"); %favorite = (joe => 'red', sam => 'blue'); Individual values in a hash are accessed by providing the corresponding key, in curly braces. The $ sigil identifies the accessed element as a scalar. For example, $favorite{joe} equals 'red'. A hash can also be initialized by setting its values individually:

Perl

87 $favorite{joe} = 'red'; $favorite{sam} = 'blue'; $favorite{oscar} = 'green'; Multiple elements may be accessed using the @ sigil instead (identifying the result as a list). For example, @favorite{'joe', 'sam'} equals ('red', 'blue'). Typeglob values A typeglob value is a symbol table entry. The main use of typeglobs is creating symbol table aliases. For example: *PI = \3.141592653; # creating constant scalar $PI *this = *that; # creating aliases for all data types 'this' to all data types 'that' Array functions The number of elements in an array can be determined either by evaluating the array in scalar context or with the help of the $# sigil. The latter gives the index of the last element in the array, not the number of elements. The expressions scalar(@array) and ($#arrayÂ +Â 1) are equivalent. Hash functions There are a few functions that operate on entire hashes. The keys function takes a hash and returns the list of its keys. Similarly, the values function returns a hash's values. Note that the keys and values are returned in a consistent but arbitrary order. # Every call to each returns the next key/value pair. # All values will be eventually returned, but their order # cannot be predicted. while (($name, $address) = each %addressbook) { print "$name lives at $address\n"; } # Similar to the above, but sorted alphabetically foreach my $next_name (sort keys %addressbook) { print "$next_name lives at $addressbook{$next_name}\n"; }

Control structures Perl has several kinds of control structures. It has block-oriented control structures, similar to those in the C, Javascript, and Java programming languages. Conditions are surrounded by parentheses, and controlled blocks are surrounded by braces: label while ( cond ) { ... } label while ( cond ) { ... } continue { ... } label for ( init-expr ; cond-expr ; incr-expr ) { ... } label foreach var ( list ) { ... } label foreach var ( list ) { ... } continue { ... } if ( cond ) { ... } if ( cond ) { ... } else { ... } if ( cond ) { ... } elsif ( cond ) { ... } else { ... }

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88 Where only a single statement is being controlled, statement modifiers provide a more-concise syntax: statement statement statement statement statement

if cond ; unless cond ; while cond ; until cond ; foreach list ;

Short-circuit logical operators are commonly used to affect control flow at the expression level: expr expr expr expr

and expr && expr or expr || expr

(The "and" and "or" operators are similar to && and || but have lower precedence, which makes it easier to use them to control entire statements.) The flow control keywords next (corresponding to C's continue), last (corresponding to C's break), return, and redo are expressions, so they can be used with short-circuit operators. Perl also has two implicit looping constructs, each of which has two forms: results results results results

= = = =

grep { ... } list grep expr, list map { ... } list map expr, list

grep returns all elements of list for which the controlled block or expression evaluates to true. map evaluates the controlled block or expression for each element of list and returns a list of the resulting values. These constructs enable a simple functional programming style. Up until the 5.10.0 release, there was no switch statement in Perl 5. From 5.10.0 onward, a multi-way branch statement called given/when is available, which takes the following form: use v5.10; # must be present to import the new 5.10 functions given ( expr ) { when ( cond ) { ... } default { ... } } Syntactically, this structure behaves similarly to switch statements found in other languages, but with a few important differences. The largest is that unlike switch/case structures, given/when statements break execution after the first successful branch, rather than waiting for explicitly defined break commands. Conversely, explicit continues are instead necessary to emulate switch behavior. For those not using Perl 5.10, the Perl documentation describes a half-dozen ways to achieve the same effect by using other control structures. There is also a Switch module, which provides functionality modeled on the forthcoming Perl 6 re-design. It is implemented using a source filter, so its use is unofficially discouraged.[55] Perl includes a goto label statement, but it is rarely used. Situations where a goto is called for in other languages don't occur as often in Perl because of its breadth of flow control options. There is also a goto &sub statement that performs a tail call. It terminates the current subroutine and immediately calls the specified sub. This is used in situations where a caller can perform more-efficient stack management than Perl itself (typically because no change to the current stack is required), and in deep recursion, tail calling can have substantial positive impact on performance because it avoids the overhead of scope/stack management on return.

Perl

Subroutines Subroutines are defined with the sub keyword and are invoked simply by naming them. If the subroutine in question has not yet been declared, invocation requires either parentheses after the function name or an ampersand (&) before it. But using & without parentheses will also implicitly pass the arguments of the current subroutine to the one called, and using & with parentheses will bypass prototypes. # Calling a subroutine # Parentheses are required here if the subroutine is defined later in the code foo(); &foo; # (this also works, but has other consequences regarding arguments passed to the subroutine) # Defining a subroutine sub foo { ... } foo; # Here parentheses are not required A list of arguments may be provided after the subroutine name. Arguments may be scalars, lists, or hashes. foo $x, @y, %z; The parameters to a subroutine do not need to be declared as to either number or type; in fact, they may vary from call to call. Any validation of parameters must be performed explicitly inside the subroutine. Arrays are expanded to their elements; hashes are expanded to a list of key/value pairs; and the whole lot is passed into the subroutine as one flat list of scalars. Whatever arguments are passed are available to the subroutine in the special array @_. The elements of @_ are references to the actual arguments; changing an element of @_ changes the corresponding argument. Elements of @_ may be accessed by subscripting it in the usual way. $_[0], $_[1] However, the resulting code can be difficult to read, and the parameters have pass-by-reference semantics, which may be undesirable. One common idiom is to assign @_ to a list of named variables. my ($x, $y, $z) = @_; This provides mnemonic parameter names and implements pass-by-value semantics. The my keyword indicates that the following variables are lexically scoped to the containing block. Another idiom is to shift parameters off of @_. This is especially common when the subroutine takes only one argument or for handling the $self argument in object-oriented modules. my $x = shift; Subroutines may assign @_ to a hash to simulate named arguments; this is recommended in Perl Best Practices for subroutines that are likely to ever have more than three parameters.[56] sub function1 { my %args = @_; print "'x' argument was '$args{x}'\n";

Perl

90 } function1( x => 23 ); Subroutines may return values. return 42, $x, @y, %z; If the subroutine does not exit via a return statement, then it returns the last expression evaluated within the subroutine body. Arrays and hashes in the return value are expanded to lists of scalars, just as they are for arguments. The returned expression is evaluated in the calling context of the subroutine; this can surprise the unwary. sub list { (4, 5, 6) } sub array { @x = (4, 5, 6); @x } $x $x @x @x

= = = =

list; # returns 6 - last element of list array; # returns 3 - number of elements in list list; # returns (4, 5, 6) array; # returns (4, 5, 6)

A subroutine can discover its calling context with the wantarray function. sub either { return wantarray ? (1, 2) : 'Oranges'; } $x = either; # returns "Oranges" @x = either; # returns (1, 2)

Regular expressions The Perl language includes a specialized syntax for writing regular expressions (RE, or regexes), and the interpreter contains an engine for matching strings to regular expressions. The regular-expression engine uses a backtracking algorithm, extending its capabilities from simple pattern matching to string capture and substitution. The regular-expression engine is derived from regex written by Henry Spencer. The Perl regular-expression syntax was originally taken from Unix Version 8 regular expressions. However, it diverged before the first release of Perl and has since grown to include far more features. Many other languages and applications are now adopting Perl compatible regular expressions over POSIX regular expressions, such as PHP, Ruby, Java, Microsoft's .NET Framework,[57] and the Apache HTTP server. Regular-expression syntax is extremely compact, owing to history. The first regular-expression dialects were only slightly more expressive than globs, and the syntax was designed so that an expression would resemble the text that it matches. This meant using no more than a single punctuation character or a pair of delimiting characters to express the few supported assertions. Over time, the expressiveness of regular expressions grew tremendously, but the syntax design was never revised and continues to rely on punctuation. As a result, regular expressions can be cryptic and extremely dense.

Perl

91 Uses The m// (match) operator introduces a regular-expression match. (If it is delimited by slashes, as in all of the examples here, then the leading m may be omitted for brevity. If the m is present, as in all of the following examples, other delimiters can be used in place of slashes.) In the simplest case, an expression such as $x =~ /abc/; evaluates to true if and only if the string $x matches the regular expression abc. The s/// (substitute) operator, on the other hand, specifies a search-and-replace operation: $x =~ s/abc/aBc/; # upcase the b Another use of regular expressions is to specify delimiters for the split function: @words = split /,/, $line; The split function creates a list of the parts of the string that are separated by matches of the regular expression. In this example, a line is divided into a list of its comma-separated parts, and this list is then assigned to the @words array. Syntax Modifiers Perl regular expressions can take modifiers. These are single-letter suffixes that modify the meaning of the expression: $x =~ /abc/i; # case-insensitive pattern match $x =~ s/abc/aBc/g; # global search and replace Because the compact syntax of regular expressions can make them dense and cryptic, the /x modifier was added in Perl to help programmers write more-legible regular expressions. It allows programmers to place whitespace and comments inside regular expressions: $x =~ / a # match 'a' . # followed by any character c # then followed by the 'c'character /x; Capturing Portions of a regular expression may be enclosed in parentheses; corresponding portions of a matching string are captured. Captured strings are assigned to the sequential built-in variables $1, $2, $3, ..., and a list of captured strings is returned as the value of the match. $x =~ /a(.)c/; # capture the character between 'a' and 'c' Captured strings $1, $2, $3, ... can be used later in the code. Perl regular expressions also allow built-in or user-defined functions apply to the captured match, by using the /e modifier: $x = "Oranges"; $x =~ s/(ge)/uc($1)/e; # OranGEs $x .= $1; # append $x with the contents of the match in the previous

Perl

92 statement: OranGEsge

Objects There are many ways to write object-oriented code in Perl. The most basic is using "blessed" references. Many modern Perl applications use the Moose object system. Examples An example of a class written using the MooseX::Declare[58] extension to Moose: use MooseX::Declare; class Point3D extends Point { has 'z' => (isa => 'Num', is => 'rw'); after clear { $self->z(0); } method set_to (Num $x, Num $y, Num $z) { $self->x($x); $self->y($y); $self->z($z); } } This is a class named Point3D that extends another class named Point explained in Moose examples. Id adds to its base class a new attribute z, redefines the method set_to and extends the method clear.

Database interfaces Perl is widely favored for database applications. Its text-handling facilities are useful for generating SQL queries; arrays, hashes, and automatic memory management make it easy to collect and process the returned data. In early versions of Perl, database interfaces were created by relinking the interpreter with a client-side database library. This was sufficiently difficult that it was done for only a few of the most-important and most widely used databases, and it restricted the resulting perl executable to using just one database interface at a time. In Perl 5, database interfaces are implemented by Perl DBI modules. The DBI (Database Interface) module presents a single, database-independent interface to Perl applications, while the DBD (Database Driver) modules handle the details of accessing some 50 different databases; there are DBD drivers for most ANSI SQL databases. DBI provides caching for database handles and queries, which can greatly improve performance in long-lived execution environments such as mod_perl,[59] helping high-volume systems avert load spikes as in the Slashdot effect. In modern Perl applications, especially those written using Web application frameworks such as Catalyst, the DBI module is often used indirectly via object-relational mappers such as DBIx::Class, Class::DBI or Rose::DB::Object which generate SQL queries and handle data transparently to the application author.

Perl

Comparative performance The Computer Language Benchmarks Game[60] compares the performance of implementations of typical programming problems in several programming languages. The submitted Perl implementations were typically toward the high end of the memory-usage spectrum and had varied speed results. Perl's performance in the benchmarks game is typical for interpreted languages. Large Perl programs start slower than similar programs in compiled languages because perl has to compile the source every time it runs. In a talk at the YAPC::Europe 2005 conference and subsequent article "A Timely Start," Jean-Louis Leroy found that his Perl programs took much longer to run than he expected because the perl interpreter spent much of the time finding modules because of his over-large include path.[61] Unlike Java, Python, and Ruby, Perl has only experimental support for pre-compiling.[62] Therefore Perl programs pay this overhead penalty on every execution. The run phase of typical programs is long enough that amortized startup time is not substantial, but results in benchmarks that measure very short execution times are likely to be skewed. A number of tools have been introduced to improve this situation. The first such tool was Apache's mod_perl, which sought to address one of the most-common reasons that small Perl programs were invoked rapidly: CGI Web development. ActivePerl, via Microsoft ISAPI, provides similar performance improvements. Once Perl code is compiled, there is additional overhead during the execution phase that typically isn't present for programs written in compiled languages such as C or C++. Examples of such overhead include bytecode interpretation, reference-counting memory management, and dynamic type checking.

Optimizing Like any code, Perl programs can be tuned for performance using benchmarks and profiles after a readable and correct implementation is finished. In part because of Perl's interpreted nature, writing more-efficient Perl will not always be enough to meet one's performance goals for a program. In such situations, the most-critical routines of a Perl program can be written in other languages such as C or Assembler, which can be connected to Perl via simple Inline modules or the more-complex-but-flexible XS mechanism.[63] Nicholas Clark, a Perl core developer, discusses some Perl design trade-offs and some solutions in When perl is not quite fast enough.[64] In extreme cases, optimizing Perl can require intimate knowledge of the interpreter's workings rather than skill with algorithms, the Perl language, or general principles of optimization.

Future At the 2000 Perl Conference, Jon Orwant made a case for a major new language initiative.[65] This led to a decision to begin work on a redesign of the language, to be called Perl 6. Proposals for new language features were solicited from the Perl community at large, and more than 300 RFCs were submitted. For clarity purposes: Perl6 and Perl5 are different languages, sharing a common ancestry. Larry Wall spent the next few years digesting the RFCs and synthesizing them into a coherent framework for Perl 6. He has presented his design for Perl 6 in a series of documents called "apocalypses," which are numbered to correspond to chapters in Programming Perl ("The Camel Book"). The current, not-yet-finalized specification of Perl 6 is encapsulated in design documents called Synopses, which are numbered to correspond to Apocalypses. Perl 6 is not intended to be backward compatible, although there will be a compatibility mode. Thesis work by Bradley M. Kuhn, overseen by Larry Wall, considered the possible use of the Java virtual machine as a runtime for Perl.[66] Kuhn's thesis showed this approach to be problematic, and in 2001, it was decided that Perl 6 would run on a cross-language virtual machine called Parrot. This will mean that other languages targeting the Parrot will gain native access to CPAN, allowing some level of cross-language development.

Perl

94 In 2005, Audrey Tang created the pugs project, an implementation of Perl 6 in Haskell. This was, and continues to act as, a test platform for the Perl 6 language (separate from the development of the actual implementation) allowing the language designers to explore. The pugs project spawned an active Perl/Haskell cross-language community centered around the freenode #perl6 irc channel. A number of features in the Perl 6 language now show similarities to Haskell. As of early 2009, Perl 6 development is primarily centred around Rakudo Perl 6, an implementation running on top of the Parrot virtual machine. Another implementation, Mildew, is also under active development and does not use Parrot. Development of Perl 5 is also continuing. Perl 5.12.0 was released in April 2010 influenced by the design of Perl 6.[68] [69]

[67]

, with some new features

The Perl community Perl's culture and community has developed alongside the language itself. Usenet was the first public venue in which Perl was introduced, but over the course of its evolution, Perl's community was shaped by the growth of broadening Internet-based services including the introduction of the World Wide Web. The community that surrounds Perl was, in fact, the topic of Larry Wall's first "State of the Onion" talk.[70]

State of the Onion State of the Onion is the name for Larry Wallâ&#x20AC;&#x2122;s yearly keynote-style summaries on the progress of Perl and its community. They are characterized by his hallmark humor, employing references to Perlâ&#x20AC;&#x2122;s culture, the wider hacker culture, Wallâ&#x20AC;&#x2122;s linguistic background, sometimes his family life, and occasionally even his Christian background. Each talk is first given at various Perl conferences and is eventually also published online.

Pastimes Perl's pastimes have become a defining element of the Perl community. They include both trivial and complex uses of the language. JAPHs In email, Usenet, and message-board postings, "Just another Perl hacker" (JAPH) programs have become a common trend, originated by Randal L. Schwartz, one of the earliest professional Perl trainers.[71] In the parlance of Perl culture, Perl programmers are known as Perl hackers, and from this derives the practice of writing short programs to print out the phrase "Just another Perl hacker,". In the spirit of the original concept, these programs are moderately obfuscated and short enough to fit into the signature of an email or Usenet message. The "canonical" JAPH includes the comma at the end, although this is often omitted. Perl golf Perl "golf" is the pastime of reducing the number of characters (key "strokes") used in a Perl program to the bare minimum, much as how golf players seek to take as few shots as possible in a round. This use of the word "golf" originally focused on the JAPHs used in signatures in Usenet postings and elsewhere, although the same stunts had been an unnamed pastime in the language APL in previous decades. The use of Perl to write a program that performed RSA encryption prompted a widespread and practical interest in this pastime.[72] In subsequent years, the term "code golf" has been applied to the pastime in other languages.[73] A Perl Golf Apocalypse was held at Perl Conference 4.0 in Monterey, California in July 2000.

Perl

95 Obfuscation As with C, obfuscated code competitions are a well-known pastime. The annual Obfuscated Perl contest made an arch virtue of Perl's syntactic flexibility. Poetry Similar to obfuscated code and golf, but with a different purpose, Perl poetry is the practice of writing poems that can actually be compiled as legal (although generally non-sensical) Perl code, for example the piece known as Black Perl. This hobby is more or less unique to Perl because of the large number of regular English words that are used in the language. New poems are regularly published in the Perl Monks site's Perl Poetry section.[74] Perl on IRC There are a couple of channels on IRC that offer free Perl support for the language and some modules. IRC Network

Channels

irc.freenode.net #perl #cbstream #perlcafe #poe irc.perl.org

#moose #poe #catalyst #dbix-class #perl-help #distzilla #epo #corehackers #sdl #win32 #toolchain #padre

irc.slashnet.org #perlmonks irc.oftc.net

#perl

irc.efnet.net

#perlhelp

irc.rizon.net

#perl

irc.debian.org

#debian-perl

CPAN Acme There are also many examples of code written purely for entertainment on the CPAN. Lingua::Romana::Perligata, for example, allows writing programs in Latin.[75] Upon execution of such a program, the module translates its source code into regular Perl and runs it. The Perl community has set aside the "Acme" namespace for modules that are fun in nature (but its scope has widened to include exploratory or experimental code or any other module that is not meant to ever be used in production). Some of the Acme modules are deliberately implemented in amusing ways. This includes Acme::Bleach, one of the first modules in the Acme:: namespace,[76] which allows the program's source code to be "whitened" (i.e., all characters replaced with whitespace) and yet still work.

See also • • • • • •

Comparison of programming languages Just another Perl hacker Perl Data Language Perl Object Environment PerlScript Plain Old Documentation

Perl

See also • • • • • •

Win32 Thread Information Block Hardware: Multithreading (computer hardware), Multi-core (computing), Simultaneous multithreading Theory: Communicating sequential processes, Computer multitasking, Message passing Problems: Thread safety, Priority inversion Techniques: Protothreads, Thread pool pattern, Lock-free and wait-free algorithms System Calls: clone (Linux system call)

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Thread (computer science)

References [1] http:/ / www. sun. com/ software/ whitepapers/ solaris9/ multithread. pdf [2] CreateFiber, MSDN (http:/ / msdn. microsoft. com/ en-us/ library/ ms682402(VS. 85). aspx)

• David R. Butenhof: Programming with POSIX Threads, Addison-Wesley, ISBN 0-201-63392-2 • Bradford Nichols, Dick Buttlar, Jacqueline Proulx Farell: Pthreads Programming, O'Reilly & Associates, ISBN 1-56592-115-1 • Charles J. Northrup: Programming with UNIX Threads, John Wiley & Sons, ISBN 0-471-13751-0 • Mark Walmsley: Multi-Threaded Programming in C++, Springer, ISBN 1-85233-146-1 • Paul Hyde: Java Thread Programming, Sams, ISBN 0-672-31585-8 • Bill Lewis: Threads Primer: A Guide to Multithreaded Programming, Prentice Hall, ISBN 0-13-443698-9 • Steve Kleiman, Devang Shah, Bart Smaalders: Programming With Threads, SunSoft Press, ISBN 0-13-172389-8 • Pat Villani: Advanced WIN32 Programming: Files, Threads, and Process Synchronization, Harpercollins Publishers, ISBN 0-87930-563-0 • Jim Beveridge, Robert Wiener: Multithreading Applications in Win32, Addison-Wesley, ISBN 0-201-44234-5 • Thuan Q. Pham, Pankaj K. Garg: Multithreaded Programming with Windows NT, Prentice Hall, ISBN 0-13-120643-5 • Len Dorfman, Marc J. Neuberger: Effective Multithreading in OS/2, McGraw-Hill Osborne Media, ISBN 0-07-017841-0 • Alan Burns, Andy Wellings: Concurrency in ADA, Cambridge University Press, ISBN 0-521-62911-X • Uresh Vahalia: Unix Internals: the New Frontiers, Prentice Hall, ISBN 0-13-101908-2 • Alan L. Dennis: .Net Multithreading , Manning Publications Company, ISBN 1-930110-54-5 • Tobin Titus, Fabio Claudio Ferracchiati, Srinivasa Sivakumar, Tejaswi Redkar, Sandra Gopikrishna: C# Threading Handbook, Peer Information Inc, ISBN 1-86100-829-5 • Tobin Titus, Fabio Claudio Ferracchiati, Srinivasa Sivakumar, Tejaswi Redkar, Sandra Gopikrishna: Visual Basic .Net Threading Handbook, Wrox Press Inc, ISBN 1-86100-713-2

External links • Article " Query by Slice, Parallel Execute, and Join: A Thread Pool Pattern in Java (http://today.java.net/pub/ a/today/2008/01/31/query-by-slice-parallel-execute-join-thread-pool-pattern.html)" by Binildas C. A. • Answers to frequently asked questions for comp.programming.threads (http://www.serpentine.com/~bos/ threads-faq/) • The C10K problem (http://www.kegel.com/c10k.html) • Article " The Free Lunch Is Over: A Fundamental Turn Toward Concurrency in Software (http://gotw.ca/ publications/concurrency-ddj.htm)" by Herb Sutter • Article " The Problem with Threads (http://www.computer.org/portal/site/computer/index. jsp?pageID=computer_level1_article&TheCat=1005&path=computer/homepage/0506&file=cover.xml& xsl=article.xsl)" by Edward Lee • parallel computing community (http://multicore.ning.com/) • POSIX threads explained (http://www.ibm.com/developerworks/library/l-posix1.html) by Daniel Robbins • Concepts of Multithreading (http://thekiransblog.blogspot.com/2010/02/multithreading.html)] • Multithreading in the Solaris Operating Environment (http://www.sun.com/software/whitepapers/solaris9/ multithread.pdf) • Debugging and Optimizing Multithreaded OpenMP Programs (http://www.ddj.com/215600207)

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Parallel computing Parallel computing is a form of computation in which many calculations are carried out simultaneously,[1] operating on the principle that large problems can often be divided into smaller ones, which are then solved concurrently ("in parallel"). There are several different forms of parallel computing: bit-level, instruction level, data, and task parallelism. Parallelism has been employed for many years, mainly in high-performance computing, but interest in it has grown lately due to the physical constraints preventing frequency scaling.[2] As power consumption (and consequently heat generation) by computers has become a concern in recent years,[3] parallel computing has become the dominant paradigm in computer architecture, mainly in the form of multicore processors.[4] Parallel computers can be roughly classified according to the level at which the hardware supports parallelism—with multi-core and multi-processor computers having multiple processing elements within a single machine, while clusters, MPPs, and grids use multiple computers to work on the same task. Specialized parallel computer architectures are sometimes used alongside traditional processors, for accelerating specific tasks. Parallel computer programs are more difficult to write than sequential ones,[5] because concurrency introduces several new classes of potential software bugs, of which race conditions are the most common. Communication and synchronization between the different subtasks are typically one of the greatest obstacles to getting good parallel program performance. The speed-up of a program as a result of parallelization is observed as Amdahl's law.

Background Traditionally, computer software has been written for serial computation. To solve a problem, an algorithm is constructed and implemented as a serial stream of instructions. These instructions are executed on a central processing unit on one computer. Only one instruction may execute at a time—after that instruction is finished, the next is executed.[6] Parallel computing, on the other hand, uses multiple processing elements simultaneously to solve a problem. This is accomplished by breaking the problem into independent parts so that each processing element can execute its part of the algorithm simultaneously with the others. The processing elements can be diverse and include resources such as a single computer with multiple processors, several networked computers, specialized hardware, or any combination of the above.[6] Frequency scaling was the dominant reason for improvements in computer performance from the mid-1980s until 2004. The runtime of a program is equal to the number of instructions multiplied by the average time per instruction. Maintaining everything else constant, increasing the clock frequency decreases the average time it takes to execute an instruction. An increase in frequency thus decreases runtime for all computation-bounded programs.[7] However, power consumption by a chip is given by the equation P = C × V2 × F, where P is power, C is the capacitance being switched per clock cycle (proportional to the number of transistors whose inputs change), V is voltage, and F is the processor frequency (cycles per second).[8] Increases in frequency increase the amount of power used in a processor. Increasing processor power consumption led ultimately to Intel's May 2004 cancellation of its Tejas and Jayhawk processors, which is generally cited as the end of frequency scaling as the dominant computer architecture paradigm.[9] Moore's Law is the empirical observation that transistor density in a microprocessor doubles every 18 to 24 months.[10] Despite power consumption issues, and repeated predictions of its end, Moore's law is still in effect. With the end of frequency scaling, these additional transistors (which are no longer used for frequency scaling) can be used to add extra hardware for parallel computing.

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Amdahl's law and Gustafson's law Optimally, the speed-up from parallelization would be linear—doubling the number of processing elements should halve the runtime, and doubling it a second time should again halve the runtime. However, very few parallel algorithms achieve optimal speed-up. Most of them have a near-linear speed-up for small numbers of processing elements, which flattens out into a constant value for large numbers of processing elements. The potential speed-up of an algorithm on a parallel computing platform is given by Amdahl's law, originally formulated by A graphical representation of Amdahl's law. The speed-up of a program from Gene Amdahl in the 1960s.[11] It states that parallelization is limited by how much of the program can be parallelized. For a small portion of the program which cannot example, if 90% of the program can be parallelized, the theoretical maximum speed-up using parallel computing would be 10x no matter how many processors be parallelized will limit the overall are used. speed-up available from parallelization. Any large mathematical or engineering problem will typically consist of several parallelizable parts and several non-parallelizable (sequential) parts. This relationship is given by the equation:

where S is the speed-up of the program (as a factor of its original sequential runtime), and P is the fraction that is parallelizable. If the sequential portion of a program is 10% of the runtime, we can get no more than a 10× speed-up, regardless of how many processors are added. This puts an upper limit on the usefulness of adding more parallel execution units. "When a task cannot be partitioned because of sequential constraints, the application of more effort has no effect on the schedule. The bearing of a child takes nine months, no matter how many women are assigned."[12] Gustafson's law is another law in computer engineering, closely related to Amdahl's law. It can be formulated as:

Assume that a task has two independent parts, A and B. B takes roughly 25% of the time of the whole computation. With effort, a programmer may be able to make this part five times faster, but this only reduces the time for the whole computation by a little. In contrast, one may need to perform less work to make part A twice as fast. This will make the computation much faster than by optimizing part B, even though B got a greater speed-up (5× versus 2×).

189

Parallel computing where P is the number of processors, S is the speed-up, and α the non-parallelizable part of the process.[13] Amdahl's law assumes a fixed problem size and that the size of the sequential section is independent of the number of processors, whereas Gustafson's law does not make these assumptions.

Dependencies Understanding data dependencies is fundamental in implementing parallel algorithms. No program can run more quickly than the longest chain of dependent calculations (known as the critical path), since calculations that depend upon prior calculations in the chain must be executed in order. However, most algorithms do not consist of just a long chain of dependent calculations; there are usually opportunities to execute independent calculations in parallel. Let Pi and Pj be two program fragments. Bernstein's conditions[14] describe when the two are independent and can be executed in parallel. For Pi, let Ii be all of the input variables and Oi the output variables, and likewise for Pj. P i and Pj are independent if they satisfy • • • Violation of the first condition introduces a flow dependency, corresponding to the first statement producing a result used by the second statement. The second condition represents an anti-dependency, when the first statement overwrites a variable needed by the second expression. The third and final condition represents an output dependency: When two statements write to the same location, the final result must come from the logically last executed statement.[15] Consider the following functions, which demonstrate several kinds of dependencies: 1: function Dep(a, b) 2: c := a·b 3: d := 2·c 4: end function Operation 3 in Dep(a, b) cannot be executed before (or even in parallel with) operation 2, because operation 3 uses a result from operation 2. It violates condition 1, and thus introduces a flow dependency. 1: function NoDep(a, b) 2: c := a·b 3: d := 2·b 4: e := a+b 5: end function In this example, there are no dependencies between the instructions, so they can all be run in parallel. Bernstein’s conditions do not allow memory to be shared between different processes. For that, some means of enforcing an ordering between accesses is necessary, such as semaphores, barriers or some other synchronization method.

Race conditions, mutual exclusion, synchronization, and parallel slowdown Subtasks in a parallel program are often called threads. Some parallel computer architectures use smaller, lightweight versions of threads known as fibers, while others use bigger versions known as processes. However, "threads" is generally accepted as a generic term for subtasks. Threads will often need to update some variable that is shared between them. The instructions between the two programs may be interleaved in any order. For example, consider the following program:

190

Parallel computing

191

Thread A

Thread B

1A: Read variable V

1B: Read variable V

2A: Add 1 to variable V

2B: Add 1 to variable V

3A Write back to variable V

3B: Write back to variable V

If instruction 1B is executed between 1A and 3A, or if instruction 1A is executed between 1B and 3B, the program will produce incorrect data. This is known as a race condition. The programmer must use a lock to provide mutual exclusion. A lock is a programming language construct that allows one thread to take control of a variable and prevent other threads from reading or writing it, until that variable is unlocked. The thread holding the lock is free to execute its critical section (the section of a program that requires exclusive access to some variable), and to unlock the data when it is finished. Therefore, to guarantee correct program execution, the above program can be rewritten to use locks: Thread A

Thread B

1A: Lock variable V

1B: Lock variable V

2A: Read variable V

2B: Read variable V

3A: Add 1 to variable V

3B: Add 1 to variable V

4A Write back to variable V

4B: Write back to variable V

5A: Unlock variable V

5B: Unlock variable V

One thread will successfully lock variable V, while the other thread will be locked outâ&#x20AC;&#x201D;unable to proceed until V is unlocked again. This guarantees correct execution of the program. Locks, while necessary to ensure correct program execution, can greatly slow a program. Locking multiple variables using non-atomic locks introduces the possibility of program deadlock. An atomic lock locks multiple variables all at once. If it cannot lock all of them, it does not lock any of them. If two threads each need to lock the same two variables using non-atomic locks, it is possible that one thread will lock one of them and the second thread will lock the second variable. In such a case, neither thread can complete, and deadlock results. Many parallel programs require that their subtasks act in synchrony. This requires the use of a barrier. Barriers are typically implemented using a software lock. One class of algorithms, known as lock-free and wait-free algorithms, altogether avoids the use of locks and barriers. However, this approach is generally difficult to implement and requires correctly designed data structures. Not all parallelization results in speed-up. Generally, as a task is split up into more and more threads, those threads spend an ever-increasing portion of their time communicating with each other. Eventually, the overhead from communication dominates the time spent solving the problem, and further parallelization (that is, splitting the workload over even more threads) increases rather than decreases the amount of time required to finish. This is known as parallel slowdown.

Fine-grained, coarse-grained, and embarrassing parallelism Applications are often classified according to how often their subtasks need to synchronize or communicate with each other. An application exhibits fine-grained parallelism if its subtasks must communicate many times per second; it exhibits coarse-grained parallelism if they do not communicate many times per second, and it is embarrassingly parallel if they rarely or never have to communicate. Embarrassingly parallel applications are considered the easiest to parallelize.

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Consistency models Parallel programming languages and parallel computers must have a consistency model (also known as a memory model). The consistency model defines rules for how operations on computer memory occur and how results are produced. One of the first consistency models was Leslie Lamport's sequential consistency model. Sequential consistency is the property of a parallel program that its parallel execution produces the same results as a sequential program. Specifically, a program is sequentially consistent if "... the results of any execution is the same as if the operations of all the processors were executed in some sequential order, and the operations of each individual processor appear in this sequence in the order specified by its program".[16] Software transactional memory is a common type of consistency model. Software transactional memory borrows from database theory the concept of atomic transactions and applies them to memory accesses. Mathematically, these models can be represented in several ways. Petri nets, which were introduced in Carl Adam Petri's 1962 doctoral thesis, were an early attempt to codify the rules of consistency models. Dataflow theory later built upon these, and Dataflow architectures were created to physically implement the ideas of dataflow theory. Beginning in the late 1970s, process calculi such as Calculus of Communicating Systems and Communicating Sequential Processes were developed to permit algebraic reasoning about systems composed of interacting components. More recent additions to the process calculus family, such as the π-calculus, have added the capability for reasoning about dynamic topologies. Logics such as Lamport's TLA+, and mathematical models such as traces and Actor event diagrams, have also been developed to describe the behavior of concurrent systems.

Flynn's taxonomy Michael J. Flynn created one of the earliest classification systems for parallel (and sequential) computers and programs, now known as Flynn's taxonomy. Flynn classified programs and computers by whether they were operating using a single set or multiple sets of instructions, whether or not those instructions were using a single or multiple sets of data.

Flynn's taxonomy Single Multiple Instruction Instruction Single Data

SISD

MISD

Multiple Data

SIMD

MIMD

The single-instruction-single-data (SISD) classification is equivalent to an entirely sequential program. The single-instruction-multiple-data (SIMD) classification is analogous to doing the same operation repeatedly over a large data set. This is commonly done in signal processing applications. Multiple-instruction-single-data (MISD) is a rarely used classification. While computer architectures to deal with this were devised (such as systolic arrays), few applications that fit this class materialized. Multiple-instruction-multiple-data (MIMD) programs are by far the most common type of parallel programs. According to David A. Patterson and John L. Hennessy, "Some machines are hybrids of these categories, of course, but this classic model has survived because it is simple, easy to understand, and gives a good first approximation. It is also—perhaps because of its understandability—the most widely used scheme."[17]

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Types of parallelism Bit-level parallelism From the advent of very-large-scale integration (VLSI) computer-chip fabrication technology in the 1970s until about 1986, speed-up in computer architecture was driven by doubling computer word size—the amount of information the processor can manipulate per cycle.[18] Increasing the word size reduces the number of instructions the processor must execute to perform an operation on variables whose sizes are greater than the length of the word. For example, where an 8-bit processor must add two 16-bit integers, the processor must first add the 8 lower-order bits from each integer using the standard addition instruction, then add the 8 higher-order bits using an add-with-carry instruction and the carry bit from the lower order addition; thus, an 8-bit processor requires two instructions to complete a single operation, where a 16-bit processor would be able to complete the operation with a single instruction. Historically, 4-bit microprocessors were replaced with 8-bit, then 16-bit, then 32-bit microprocessors. This trend generally came to an end with the introduction of 32-bit processors, which has been a standard in general-purpose computing for two decades. Not until recently (c. 2003–2004), with the advent of x86-64 architectures, have 64-bit processors become commonplace.

Instruction-level parallelism A computer program is, in essence, a stream of instructions executed by a processor. These instructions can be re-ordered and combined into groups which are then executed in parallel without changing the result of the program. This is known as instruction-level parallelism. Advances in instruction-level parallelism dominated computer architecture from the mid-1980s until the mid-1990s.[19]

A canonical five-stage pipeline in a RISC machine (IF = Instruction Fetch, ID = Instruction Decode, EX = Execute, MEM = Memory access, WB = Register write back)

Modern processors have multi-stage instruction pipelines. Each stage in the pipeline corresponds to a different action the processor performs on that instruction in that stage; a processor with an N-stage pipeline can have up to N different instructions at different stages of completion. The canonical example of a pipelined processor is a RISC processor, with five stages: instruction fetch, decode, execute, memory access, and write back. The Pentium 4 processor had a 35-stage pipeline.[20]

Parallel computing

In addition to instruction-level parallelism from pipelining, some processors can issue more than one instruction at a time. These are known as superscalar processors. Instructions can be grouped together only if there is no data dependency between them. Scoreboarding and the Tomasulo algorithm (which is similar to scoreboarding but makes use of register renaming) are two of the most common techniques for implementing out-of-order execution and instruction-level parallelism.

Data parallelism

194

A five-stage pipelined superscalar processor, capable of issuing two instructions per cycle. It can have two instructions in each stage of the pipeline, for a total of up to 10Â instructions (shown in green) being simultaneously executed.

Data parallelism is parallelism inherent in program loops, which focuses on distributing the data across different computing nodes to be processed in parallel. "Parallelizing loops often leads to similar (not necessarily identical) operation sequences or functions being performed on elements of a large data structure."[21] Many scientific and engineering applications exhibit data parallelism. A loop-carried dependency is the dependence of a loop iteration on the output of one or more previous iterations. Loop-carried dependencies prevent the parallelization of loops. For example, consider the following pseudocode that computes the first few Fibonacci numbers: 1: 2: 4: 5: 6: 7: 8:

PREV1 := 0 PREV2 := 1 do: CUR := PREV1 + PREV2 PREV1 := PREV2 PREV2 := CUR while (CUR < 10)

This loop cannot be parallelized because CUR depends on itself (PREV2) and PREV1, which are computed in each loop iteration. Since each iteration depends on the result of the previous one, they cannot be performed in parallel. As the size of a problem gets bigger, the amount of data-parallelism available usually does as well.[22]

Task parallelism Task parallelism is the characteristic of a parallel program that "entirely different calculations can be performed on either the same or different sets of data".[21] This contrasts with data parallelism, where the same calculation is performed on the same or different sets of data. Task parallelism does not usually scale with the size of a problem.[22]

Hardware Memory and communication Main memory in a parallel computer is either shared memory (shared between all processing elements in a single address space), or distributed memory (in which each processing element has its own local address space).[23] Distributed memory refers to the fact that the memory is logically distributed, but often implies that it is physically

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195

distributed as well. Distributed shared memory and memory virtualization combine the two approaches, where the processing element has its own local memory and access to the memory on non-local processors. Accesses to local memory are typically faster than accesses to non-local memory. Computer architectures in which each element of main memory can be accessed with equal latency and bandwidth are known as Uniform Memory Access (UMA) systems. Typically, that can be achieved only by a shared memory system, in which the memory is not physically distributed. A system that does not have this property is known as a Non-Uniform Memory Access (NUMA) architecture. Distributed memory systems have non-uniform memory access.

A logical view of a Non-Uniform Memory Access (NUMA) architecture. Processors in one directory can access that directory's memory with less latency than they can access memory in the other directory's memory.

Computer systems make use of caches—small, fast memories located close to the processor which store temporary copies of memory values (nearby in both the physical and logical sense). Parallel computer systems have difficulties with caches that may store the same value in more than one location, with the possibility of incorrect program execution. These computers require a cache coherency system, which keeps track of cached values and strategically purges them, thus ensuring correct program execution. Bus snooping is one of the most common methods for keeping track of which values are being accessed (and thus should be purged). Designing large, high-performance cache coherence systems is a very difficult problem in computer architecture. As a result, shared-memory computer architectures do not scale as well as distributed memory systems do.[23] Processor–processor and processor–memory communication can be implemented in hardware in several ways, including via shared (either multiported or multiplexed) memory, a crossbar switch, a shared bus or an interconnect network of a myriad of topologies including star, ring, tree, hypercube, fat hypercube (a hypercube with more than one processor at a node), or n-dimensional mesh. Parallel computers based on interconnect networks need to have some kind of routing to enable the passing of messages between nodes that are not directly connected. The medium used for communication between the processors is likely to be hierarchical in large multiprocessor machines.

Classes of parallel computers Parallel computers can be roughly classified according to the level at which the hardware supports parallelism. This classification is broadly analogous to the distance between basic computing nodes. These are not mutually exclusive; for example, clusters of symmetric multiprocessors are relatively common. Multicore computing A multicore processor is a processor that includes multiple execution units ("cores"). These processors differ from superscalar processors, which can issue multiple instructions per cycle from one instruction stream (thread); by contrast, a multicore processor can issue multiple instructions per cycle from multiple instruction streams. Each core in a multicore processor can potentially be superscalar as well—that is, on every cycle, each core can issue multiple instructions from one instruction stream. Simultaneous multithreading (of which Intel's HyperThreading is the best known) was an early form of pseudo-multicoreism. A processor capable of simultaneous multithreading has only one execution unit ("core"), but

Parallel computing when that execution unit is idling (such as during a cache miss), it uses that execution unit to process a second thread. IBM's Cell microprocessor, designed for use in the Sony PlayStation 3, is another prominent multicore processor. Symmetric multiprocessing A symmetric multiprocessor (SMP) is a computer system with multiple identical processors that share memory and connect via a bus.[24] Bus contention prevents bus architectures from scaling. As a result, SMPs generally do not comprise more than 32Â processors.[25] "Because of the small size of the processors and the significant reduction in the requirements for bus bandwidth achieved by large caches, such symmetric multiprocessors are extremely cost-effective, provided that a sufficient amount of memory bandwidth exists."[24] Distributed computing A distributed computer (also known as a distributed memory multiprocessor) is a distributed memory computer system in which the processing elements are connected by a network. Distributed computers are highly scalable. Cluster computing A cluster is a group of loosely coupled computers that work together closely, so that in some respects they can be regarded as a single computer.[26] Clusters are composed of multiple standalone machines connected by a network. While machines in a cluster do not have to be symmetric, load balancing is more difficult if they are not. The most common type of cluster is the Beowulf cluster, which is a cluster implemented on multiple identical commercial off-the-shelf computers connected with a TCP/IP Ethernet local area network.[27] Beowulf technology was originally developed by Thomas Sterling and Donald Becker. The vast majority of the TOP500 supercomputers are clusters.[28] Massive parallel processing A Beowulf cluster A massively parallel processor (MPP) is a single computer with many networked processors. MPPs have many of the same characteristics as clusters, but MPPs have specialized interconnect networks (whereas clusters use commodity hardware for networking). MPPs also tend to be larger than clusters, typically having "far more" than 100Â processors.[29] In an MPP, "each CPU contains its own memory and copy of the operating system and application. Each subsystem communicates with the others via a high-speed interconnect."[30]

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Blue Gene/L, the fifth fastest supercomputer in the world according to the June 2009 TOP500 ranking, is an MPP. Grid computing Grid computing is the most distributed form of parallel computing. It makes use of computers communicating over the Internet to work on a given problem. Because of the low bandwidth and extremely high latency available on the Internet, grid computing typically deals only with embarrassingly parallel problems. Many grid computing applications have been created, of which SETI@home and Folding@Home are the best-known examples.[31] Most grid computing applications use middleware, software that sits between the operating system and the application to manage network resources and standardize the software interface. The most common grid computing middleware is the Berkeley Open Infrastructure for Network Computing (BOINC). Often, grid computing software makes use of "spare cycles", performing computations at times when a computer is idling.

A cabinet from Blue Gene/L, ranked as the fourth fastest supercomputer in the world according to the 11/2008 TOP500 rankings. Blue Gene/L is a massively parallel processor.

Specialized parallel computers Within parallel computing, there are specialized parallel devices that remain niche areas of interest. While not domain-specific, they tend to be applicable to only a few classes of parallel problems. Reconfigurable computing with field-programmable gate arrays Reconfigurable computing is the use of a field-programmable gate array (FPGA) as a co-processor to a general-purpose computer. An FPGA is, in essence, a computer chip that can rewire itself for a given task. FPGAs can be programmed with hardware description languages such as VHDL or Verilog. However, programming in these languages can be tedious. Several vendors have created C to HDL languages that attempt to emulate the syntax and/or semantics of the C programming language, with which most programmers are familiar. The best known C to HDL languages are Mitrion-C, Impulse C, DIME-C, and Handel-C. Specific subsets of SystemC based on C++ can also be used for this purpose. AMD's decision to open its HyperTransport technology to third-party vendors has become the enabling technology for high-performance reconfigurable computing.[32] According to Michael R. D'Amour, Chief Operating Officer of DRC Computer Corporation, "when we first walked into AMD, they called us 'the socket stealers.' Now they call us their partners."[32] General-purpose computing on graphics processing units (GPGPU) General-purpose computing on graphics processing units (GPGPU) is a fairly recent trend in computer engineering research. GPUs are co-processors that have been heavily optimized for computer graphics processing.[33] Computer graphics processing is a field dominated by data parallel operationsâ&#x20AC;&#x201D;particularly linear algebra matrix operations.

Nvidia's Tesla GPGPU card

In the early days, GPGPU programs used the normal graphics APIs for executing programs. However, recently several new programming languages and platforms have been built to do general purpose computation on GPUs with both Nvidia and AMD releasing programming environments with CUDA and CTM respectively. Other GPU programming languages are BrookGPU, PeakStream, and RapidMind. Nvidia has also released specific products for computation in their Tesla series. Application-specific integrated circuits

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Several application-specific integrated circuit (ASIC) approaches have been devised for dealing with parallel applications.[34] [35] [36] Because an ASIC is (by definition) specific to a given application, it can be fully optimized for that application. As a result, for a given application, an ASIC tends to outperform a general-purpose computer. However, ASICs are created by X-ray lithography. This process requires a mask, which can be extremely expensive. A single mask can cost over a million US dollars.[37] (The smaller the transistors required for the chip, the more expensive the mask will be.) Meanwhile, performance increases in general-purpose computing over time (as described by Moore's Law) tend to wipe out these gains in only one or two chip generations.[32] High initial cost, and the tendency to be overtaken by Moore's-law-driven general-purpose computing, has rendered ASICs unfeasible for most parallel computing applications. However, some have been built. One example is the peta-flop RIKEN MDGRAPE-3 machine which uses custom ASICs for molecular dynamics simulation. Vector processors A vector processor is a CPU or computer system that can execute the same instruction on large sets of data. "Vector processors have high-level operations that work on linear arrays of numbers or vectors. An example vector operation is A = B × C, where A, B, and C are each 64-element vectors of 64-bit floating-point numbers."[38] They are closely related to Flynn's SIMD classification.[38] Cray computers became famous for their vector-processing computers in the 1970s and 1980s. However, vector processors—both as CPUs and as full computer systems—have generally disappeared. Modern processor instruction sets do include some vector processing instructions, such as with AltiVec and Streaming SIMD Extensions (SSE).

The Cray-1 is the most famous vector processor.

Software Parallel programming languages Concurrent programming languages, libraries, APIs, and parallel programming models have been created for programming parallel computers. These can generally be divided into classes based on the assumptions they make about the underlying memory architecture—shared memory, distributed memory, or shared distributed memory. Shared memory programming languages communicate by manipulating shared memory variables. Distributed memory uses message passing. POSIX Threads and OpenMP are two of most widely used shared memory APIs, whereas Message Passing Interface (MPI) is the most widely used message-passing system API.[39] One concept used in programming parallel programs is the future concept, where one part of a program promises to deliver a required datum to another part of a program at some future time.

Automatic parallelization Automatic parallelization of a sequential program by a compiler is the holy grail of parallel computing. Despite decades of work by compiler researchers, automatic parallelization has had only limited success.[40] Mainstream parallel programming languages remain either explicitly parallel or (at best) partially implicit, in which a programmer gives the compiler directives for parallelization. A few fully implicit parallel programming languages exist—SISAL, Parallel Haskell, and (for FPGAs) Mitrion-C.

Parallel computing

Application checkpointing The larger and more complex a computer is, the more that can go wrong and the shorter the mean time between failures. Application checkpointing is a technique whereby the computer system takes a "snapshot" of the application—a record of all current resource allocations and variable states, akin to a core dump; this information can be used to restore the program if the computer should fail. Application checkpointing means that the program has to restart from only its last checkpoint rather than the beginning. For an application that may run for months, that is critical. Application checkpointing may be used to facilitate process migration.

Applications As parallel computers become larger and faster, it becomes feasible to solve problems that previously took too long to run. Parallel computing is used in a wide range of fields, from bioinformatics (to do protein folding) to economics (to do simulation in mathematical finance). Common types of problems found in parallel computing applications are:[41] • Dense linear algebra • Sparse linear algebra • Spectral methods (such as Cooley–Tukey fast Fourier transform) • • • • • • • • • •

n-body problems (such as Barnes–Hut simulation) Structured grid problems (such as Lattice Boltzmann methods) Unstructured grid problems (such as found in finite element analysis) Monte Carlo simulation Combinational logic (such as brute-force cryptographic techniques) Graph traversal (such as sorting algorithms) Dynamic programming Branch and bound methods Graphical models (such as detecting hidden Markov models and constructing Bayesian networks) Finite-state machine simulation

History The origins of true (MIMD) parallelism go back to Federico Luigi, Conte Menabrea and his "Sketch of the Analytic Engine Invented by Charles Babbage".[42] [43] IBM introduced the 704 in 1954, through a project in which Gene Amdahl was one of the principal architects. It became the first commercially available computer to use fully automatic floating point arithmetic commands.[44] In April 1958, S. Gill (Ferranti) discussed parallel programming and the need for branching and waiting.[45] Also in 1958, IBM researchers John Cocke and Daniel Slotnick discussed the use of parallelism in ILLIAC IV, "perhaps the most infamous of numerical calculations for the first time.[46] Burroughs Corporation Supercomputers" introduced the D825 in 1962, a four-processor computer that accessed up to 16 memory modules through a crossbar switch.[47] In 1967, Amdahl and Slotnick published a debate about the feasibility of parallel processing at American Federation of Information Processing Societies Conference.[46] It was during this debate that Amdahl's Law was coined to define the limit of speed-up due to parallelism. In 1969, US company Honeywell introduced its first Multics system, a symmetric multiprocessor system capable of running up to eight processors in parallel.[46] C.mmp, a 1970s multi-processor project at Carnegie Mellon University,

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Parallel computing was "among the first multiprocessors with more than a few processors".[43] "The first bus-connected multi-processor with snooping caches was the Synapse N+1 in 1984."[43] SIMD parallel computers can be traced back to the 1970s. The motivation behind early SIMD computers was to amortize the gate delay of the processor's control unit over multiple instructions.[48] In 1964, Slotnick had proposed building a massively parallel computer for the Lawrence Livermore National Laboratory.[46] His design was funded by the US Air Force, which was the earliest SIMD parallel-computing effort, ILLIAC IV.[46] The key to its design was a fairly high parallelism, with up to 256 processors, which allowed the machine to work on large datasets in what would later be known as vector processing. However, ILLIAC IV was called "the most infamous of Supercomputers", because the project was only one fourth completed, but took 11 years and cost almost four times the original estimate.[49] When it was finally ready to run its first real application in 1976, it was outperformed by existing commercial supercomputers such as the Cray-1.

See also • List of important publications in concurrent, parallel, and distributed computing • List of distributed computing conferences

External links • • • • • • • • •

Parallel computing [50] at the Open Directory Project Lawrence Livermore National Laboratory: Introduction to Parallel Computing [51] Designing and Building Parallel Programs, by Ian Foster [52] Internet Parallel Computing Archive [53] Parallel processing topic area at IEEE Distributed Computing Online [54] Parallel Computing Works Free On-line Book [55] Frontiers of Supercomputing Free On-line Book Covering topics like algorithms and industrial applications [56] Universal Parallel Computing Research Center [57] Course in Parallel Programming at Columbia University (in collaboration with IBM T.J Watson X10 project) [58]

References [1] Almasi, G.S. and A. Gottlieb (1989). Highly Parallel Computing (http:/ / portal. acm. org/ citation. cfm?id=1011116. 1011127). Benjamin-Cummings publishers, Redwood City, CA. [2] S.V. Adve et al. (November 2008). "Parallel Computing Research at Illinois: The UPCRC Agenda" (http:/ / www. upcrc. illinois. edu/ documents/ UPCRC_Whitepaper. pdf) (PDF). Parallel@Illinois, University of Illinois at Urbana-Champaign. "The main techniques for these performance benefits – increased clock frequency and smarter but increasingly complex architectures – are now hitting the so-called power wall. The computer industry has accepted that future performance increases must largely come from increasing the number of processors (or cores) on a die, rather than making a single core go faster." [3] Asanovic et al. Old [conventional wisdom]: Power is free, but transistors are expensive. New [conventional wisdom] is [that] power is expensive, but transistors are "free". [4] Asanovic, Krste et al. (December 18, 2006). "The Landscape of Parallel Computing Research: A View from Berkeley" (http:/ / www. eecs. berkeley. edu/ Pubs/ TechRpts/ 2006/ EECS-2006-183. pdf) (PDF). University of California, Berkeley. Technical Report No. UCB/EECS-2006-183. "Old [conventional wisdom]: Increasing clock frequency is the primary method of improving processor performance. New [conventional wisdom]: Increasing parallelism is the primary method of improving processor performance ... Even representatives from Intel, a company generally associated with the 'higher clock-speed is better' position, warned that traditional approaches to maximizing performance through maximizing clock speed have been pushed to their limit." [5] Patterson, David A. and John L. Hennessy (1998). Computer Organization and Design, Second Edition, Morgan Kaufmann Publishers, p. 715. ISBN 1558604286. [6] Barney, Blaise. "Introduction to Parallel Computing" (http:/ / www. llnl. gov/ computing/ tutorials/ parallel_comp/ ). Lawrence Livermore National Laboratory. . Retrieved 2007-11-09. [7] Hennessy, John L. and David A. Patterson (2002). Computer Architecture: A Quantitative Approach. 3rd edition, Morgan Kaufmann, p. 43. ISBN 1558607242. [8] Rabaey, J. M. (1996). Digital Integrated Circuits. Prentice Hall, p. 235. ISBN 0131786091.

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Parallel computing [9] Flynn, Laurie J. "Intel Halts Development of 2 New Microprocessors" (http:/ / www. nytimes. com/ 2004/ 05/ 08/ business/ 08chip. html?ex=1399348800& en=98cc44ca97b1a562& ei=5007). The New York Times, May 8, 2004. Retrieved on April 22, 2008. [10] Moore, Gordon E. (1965). "Cramming more components onto integrated circuits" (ftp:/ / download. intel. com/ museum/ Moores_Law/ Articles-Press_Releases/ Gordon_Moore_1965_Article. pdf) (PDF). Electronics Magazine. pp. 4. . Retrieved 2006-11-11. [11] Amdahl, G. (April 1967) "The validity of the single processor approach to achieving large-scale computing capabilities". In Proceedings of AFIPS Spring Joint Computer Conference, Atlantic City, N.J., AFIPS Press, pp. 483–85. [12] Brooks, Frederick P. Jr. The Mythical Man-Month: Essays on Software Engineering. Chapter 2 – The Mythical Man Month. ISBN 0201835959 [13] Reevaluating Amdahl's Law (http:/ / www. scl. ameslab. gov/ Publications/ Gus/ AmdahlsLaw/ Amdahls. html) (1988). Communications of the ACM 31(5), pp. 532–33. [14] Bernstein, A. J. (October 1966). "Program Analysis for Parallel Processing,' IEEE Trans. on Electronic Computers". EC-15, pp. 757–62. [15] Roosta, Seyed H. (2000). "Parallel processing and parallel algorithms: theory and computation". Springer, p. 114. ISBN 0387987169. [16] Lamport, Leslie (September 1979). "How to Make a Multiprocessor Computer That Correctly Executes Multiprocess Programs", IEEE Transactions on Computers, C-28,9, pp. 690–91. [17] Patterson and Hennessy, p. 748. [18] Culler, David E.; Jaswinder Pal Singh and Anoop Gupta (1999). Parallel Computer Architecture - A Hardware/Software Approach. Morgan Kaufmann Publishers, p. 15. ISBN 1558603433. [19] Culler et al. p. 15. [20] Patt, Yale (April 2004). " The Microprocessor Ten Years From Now: What Are The Challenges, How Do We Meet Them? (http:/ / users. ece. utexas. edu/ ~patt/ Videos/ talk_videos/ cmu_04-29-04. wmv) (wmv). Distinguished Lecturer talk at Carnegie Mellon University. Retrieved on November 7, 2007. [21] Culler et al. p. 124. [22] Culler et al. p. 125. [23] Patterson and Hennessy, p. 713. [24] Hennessy and Patterson, p. 549. [25] Patterson and Hennessy, p. 714. [26] What is clustering? (http:/ / www. webopedia. com/ TERM/ c/ clustering. html) Webopedia computer dictionary. Retrieved on November 7, 2007. [27] Beowulf definition. (http:/ / www. pcmag. com/ encyclopedia_term/ 0,2542,t=Beowulf& i=38548,00. asp) PC Magazine. Retrieved on November 7, 2007. [28] Architecture share for 06/2007 (http:/ / www. top500. org/ stats/ list/ 29/ archtype). TOP500 Supercomputing Sites. Clusters make up 74.60% of the machines on the list. Retrieved on November 7, 2007. [29] Hennessy and Patterson, p. 537. [30] MPP Definition. (http:/ / www. pcmag. com/ encyclopedia_term/ 0,,t=mpp& i=47310,00. asp) PC Magazine. Retrieved on November 7, 2007. [31] Kirkpatrick, Scott (January 31, 2003). "Computer Science: Rough Times Ahead". Science, Vol. 299. No. 5607, pp. 668 - 669. DOI: 10.1126/science.1081623 [32] D'Amour, Michael R., Chief Operating Officer, DRC Computer Corporation. "Standard Reconfigurable Computing". Invited speaker at the University of Delaware, February 28, 2007. [33] Boggan, Sha'Kia and Daniel M. Pressel (August 2007). GPUs: An Emerging Platform for General-Purpose Computation (http:/ / www. arl. army. mil/ arlreports/ 2007/ ARL-SR-154. pdf) (PDF). ARL-SR-154, U.S. Army Research Lab. Retrieved on November 7, 2007. [34] Maslennikov, Oleg (2002). "Systematic Generation of Executing Programs for Processor Elements in Parallel ASIC or FPGA-Based Systems and Their Transformation into VHDL-Descriptions of Processor Element Control Units". (http:/ / www. springerlink. com/ content/ jjrdrb0lelyeu3e9/ ) Lecture Notes in Computer Science, 2328/2002: p. 272. [35] Shimokawa, Y.; Y. Fuwa and N. Aramaki (18–21 November 1991). A parallel ASIC VLSI neurocomputer for a large number of neurons and billion connections per second speed. (http:/ / ieeexplore. ieee. org/ xpl/ freeabs_all. jsp?arnumber=170708) IEEE International Joint Conference on Neural Networks. 3: pp. 2162–67. [36] Acken, K.P.; M.J. Irwin, R.M. Owens (July 1998). "A Parallel ASIC Architecture for Efficient Fractal Image Coding". (http:/ / www. ingentaconnect. com/ content/ klu/ vlsi/ 1998/ 00000019/ 00000002/ 00167697?crawler=true) The Journal of VLSI Signal Processing, 19(2):97–113(17) [37] Kahng, Andrew B. (June 21, 2004) " Scoping the Problem of DFM in the Semiconductor Industry (http:/ / www. future-fab. com/ documents. asp?grID=353& d_ID=2596)." University of California, San Diego. "Future design for manufacturing (DFM) technology must reduce design [non-recoverable expenditure] cost and directly address manufacturing [non-recoverable expenditures] – the cost of a mask set and probe card – which is well over $1 million at the 90 nm technology node and creates a significant damper on semiconductor-based innovation." [38] Patterson and Hennessy, p. 751. [39] The Sidney Fernbach Award given to MPI inventor Bill Gropp (http:/ / awards. computer. org/ ana/ award/ viewPastRecipients. action?id=16) refers to MPI as the "the dominant HPC communications interface"

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Parallel computing [40] Shen, John Paul and Mikko H. Lipasti (2005). Modern Processor Design: Fundamentals of Superscalar Processors. McGraw-Hill Professional. p. 561. ISBN 0070570647. "However, the holy grail of such research - automated parallelization of serial programs - has yet to materialize. While automated parallelization of certain classes of algorithms has been demonstrated, such success has largely been limited to scientific and numeric applications with predictable flow control (e.g., nested loop structures with statically determined iteration counts) and statically analyzable memory access patterns. (e.g., walks over large multidimensional arrays of float-point data)." [41] Asanovic, Krste, et al. (December 18, 2006). The Landscape of Parallel Computing Research: A View from Berkeley (http:/ / www. eecs. berkeley. edu/ Pubs/ TechRpts/ 2006/ EECS-2006-183. pdf) (PDF). University of California, Berkeley. Technical Report No. UCB/EECS-2006-183. See table on pages 17–19. [42] Menabrea, L. F. (1842). Sketch of the Analytic Engine Invented by Charles Babbage (http:/ / www. fourmilab. ch/ babbage/ sketch. html). Bibliothèque Universelle de Genève. Retrieved on November 7, 2007. [43] Patterson and Hennessy, p. 753. [44] da Cruz, Frank (2003). "Columbia University Computing History: The IBM 704" (http:/ / www. columbia. edu/ acis/ history/ 704. html). Columbia University. . Retrieved 2008-01-08. [45] Parallel Programming, S. Gill, The Computer Journal Vol. 1 #1, pp2-10, British Computer Society, April 1958. [46] Wilson, Gregory V (1994). "The History of the Development of Parallel Computing" (http:/ / ei. cs. vt. edu/ ~history/ Parallel. html). Virginia Tech/Norfolk State University, Interactive Learning with a Digital Library in Computer Science. . Retrieved 2008-01-08. [47] Anthes, Gry (November 19, 2001). "The Power of Parallelism" (http:/ / www. computerworld. com/ action/ article. do?command=viewArticleBasic& articleId=65878). Computerworld. . Retrieved 2008-01-08. [48] Patterson and Hennessy, p. 749. [49] Patterson and Hennessy, pp. 749–50: "Although successful in pushing several technologies useful in later projects, the ILLIAC IV failed as a computer. Costs escalated from the $8 million estimated in 1966 to $31 million by 1972, despite the construction of only a quarter of the planned machine ... It was perhaps the most infamous of supercomputers. The project started in 1965 and ran its first real application in 1976." [50] http:/ / www. dmoz. org/ Computers/ Parallel_Computing/ / [51] http:/ / www. llnl. gov/ computing/ tutorials/ parallel_comp/ [52] http:/ / www-unix. mcs. anl. gov/ dbpp/ [53] http:/ / wotug. ukc. ac. uk/ parallel/ [54] http:/ / dsonline. computer. org/ portal/ site/ dsonline/ index. jsp?pageID=dso_level1_home& path=dsonline/ topics/ parallel& file=index. xml& xsl=generic. xsl [55] http:/ / www. new-npac. org/ projects/ cdroms/ cewes-1998-05/ copywrite/ pcw/ book. html [56] http:/ / ark. cdlib. org/ ark:/ 13030/ ft0f59n73z/ [57] http:/ / www. upcrc. illinois. edu/ [58] http:/ / ppppcourse. ning. com/

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Web Programming Languages HTML Filename extension

.html, .htm

Internet media type

text/html

Type code

TEXT

Uniform Type Identifier public.html Developed by

World Wide Web Consortium & WHATWG

Type of format

Markup language

Extended from

SGML

Extended to

XHTML

Standard(s)

ISO/IEC 15445 [1] W3C HTML 4.01 [2] W3C HTML 5 (draft)

HTML, which stands for HyperText Markup Language, is the predominant markup language for web pages. It provides a means to create structured documents by denoting structural semantics for text such as headings, paragraphs, lists etc as well as for links, quotes, and other items. It allows images and objects to be embedded and can be used to create interactive forms. It is written in the form of HTML elements consisting of "tags" surrounded by angle brackets within the web page content. It can include or can load scripts in languages such as JavaScript which affect the behavior of HTML processors like Web browsers; and Cascading Style Sheets (CSS) to define the appearance and layout of text and other material. The W3C, maintainer of both HTML and CSS standards, encourages the use of CSS over explicit presentational markup.[3]

History Origins In 1980, physicist Tim Berners-Lee, who was a contractor at CERN, proposed and prototyped ENQUIRE, a system for CERN researchers to use and share documents. In 1989, Berners-Lee wrote a memo proposing an Internet-based hypertext system.[4] Berners-Lee specified HTML and wrote the browser and server software in the last part of 1990. In that year, Berners-Lee and CERN data systems engineer Robert Cailliau collaborated on a joint request for funding, but the project was not formally adopted by CERN. In his personal notes,[5] from 1990 he lists[6] "some of the many areas in which hypertext is used", and puts an encyclopedia first.

Tim Berners-Lee

HTML

First specifications The first publicly available description of HTML was a document called HTML Tags, first mentioned on the Internet by Berners-Lee in late 1991.[7] [8] It describes 20 elements comprising the initial, relatively simple design of HTML. Except for the hyperlink tag, these were strongly influenced by SGMLguid, an in-house SGML based documentation format at CERN. Thirteen of these elements still exist in HTML 4.[9] HTML is a text and image formatting language used by web browsers to dynamically format web pages. Many of the text elements are found in the 1988 ISO technical report TR 9537 Techniques for using SGML, which in turn covers the features of early text formatting languages such as that used by the RUNOFF command developed in the early 1960s for the CTSS (Compatible Time-Sharing System) operating system: these formatting commands were derived from the commands used by typesetters to manually format documents. However the SGML concept of generalized markup is based on elements (nested annotated ranges with attributes) rather than merely point effects, and also the separation of structure and processing: HTML has been progressively moved in this direction with CSS. Berners-Lee considered HTML to be an application of SGML, and it was formally defined as such by the Internet Engineering Task Force (IETF) with the mid-1993 publication of the first proposal for an HTML specification: "Hypertext Markup Language (HTML)" Internet-Draft [10] by Berners-Lee and Dan Connolly, which included an SGML Document Type Definition to define the grammar.[11] The draft expired after six months, but was notable for its acknowledgment of the NCSA Mosaic browser's custom tag for embedding in-line images, reflecting the IETF's philosophy of basing standards on successful prototypes.[12] Similarly, Dave Raggett's competing Internet-Draft, "HTML+ (Hypertext Markup Format)", from late 1993, suggested standardizing already-implemented features like tables and fill-out forms.[13] After the HTML and HTML+ drafts expired in early 1994, the IETF created an HTML Working Group, which in 1995 completed "HTML 2.0", the first HTML specification intended to be treated as a standard against which future implementations should be based.[12] Published as Request for Comments 1866, HTML 2.0 included ideas from the HTML and HTML+ drafts.[14] The 2.0 designation was intended to distinguish the new edition from previous drafts.[15] Further development under the auspices of the IETF was stalled by competing interests. Since 1996, the HTML specifications have been maintained, with input from commercial software vendors, by the World Wide Web Consortium (W3C).[16] However, in 2000, HTML also became an international standard (ISO/IEC 15445:2000). The last HTML specification published by the W3C is the HTML 4.01 Recommendation, published in late 1999. Its issues and errors were last acknowledged by errata published in 2001.

Version history of the standard HTML

• HTML and HTML5 • Dynamic HTML • XHTML

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HTML

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• XHTML Mobile Profile and C-HTML • Canvas element • Character encodings • Document Object Model • Font family • HTML editor • HTML element • HTML Frames • HTML5 video • HTML scripting • Layout engine • Quirks mode • Style sheets • Unicode and HTML • W3C and WHATWG • Web colors • WebGL • Web Storage • Comparison of • document markup languages • web browsers • layout engines for • • • • •

HTML HTML5 HTML5 Media Non-standard HTML XHTML

HTML version timeline November 24, 1995 HTML 2.0 was published as IETF RFC 1866. Supplemental RFCs added capabilities: • • • •

November 25, 1995: RFC 1867 (form-based file upload) May 1996: RFC 1942 (tables) August 1996: RFC 1980 (client-side image maps) January 1997: RFC 2070 (internationalization) In June 2000, all of these were declared obsolete/historic by RFC 2854.

January 1997 HTML 3.2[17] was published as a W3C Recommendation. It was the first version developed and standardized exclusively by the W3C, as the IETF had closed its HTML Working Group in September 1996.[18] HTML 3.2 dropped math formulas entirely, reconciled overlap among various proprietary extensions, and adopted most of Netscape's visual markup tags. Netscape's blink element and Microsoft's marquee element were omitted due to a mutual agreement between the two companies.[16] A markup for mathematical formulas

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206 similar to that in HTML wasn't standardized until 14 months later in MathML.

December 1997 HTML 4.0[19] was published as a W3C Recommendation. It offers three variations: • Strict, in which deprecated elements are forbidden, • Transitional, in which deprecated elements are allowed, • Frameset, in which mostly only frame related elements are allowed; Initially code-named "Cougar",[20] HTML 4.0 adopted many browser-specific element types and attributes, but at the same time sought to phase out Netscape's visual markup features by marking them as deprecated in favor of style sheets. HTML 4 is an SGML application conforming to ISO 8879 - SGML.[21] April 1998 HTML 4.0[22] was reissued with minor edits without incrementing the version number. December 1999 HTML 4.01[23] was published as a W3C Recommendation. It offers the same three variations as HTML 4.0, and its last errata [24] were published May 12, 2001. May 2000 ISO/IEC 15445:2000[25] [26] ("ISO HTML", based on HTML 4.01 Strict) was published as an ISO/IEC international standard. In the ISO this standard falls in the domain of the ISO/IEC JTC1/SC34 (ISO/IEC Joint Technical Committee 1, Subcommittee 34 - Document description and processing languages).[25] As of mid-2008, HTML 4.01 and ISO/IEC 15445:2000 are the most recent versions of HTML. Development of the parallel, XML-based language XHTML occupied the W3C's HTML Working Group through the early and mid-2000s. HTML draft version timeline October 1991 HTML Tags,[7] an informal CERN document listing twelve HTML tags, was first mentioned in public. July 1992 First informal draft of the HTML DTD,[27] with six subsequent revisions November 1992 HTML DTD 1.1 (the first with a version number, based on RCS revisions, which start with 1.1 rather than 1.0), an informal draft June 1993 Hypertext Markup Language[28] was published by the IETF IIIR Working Group as an Internet-Draft (a rough proposal for a standard). It was replaced by a second version [10] one month later, followed by six further drafts published by IETF itself [29] that finally led to HTML 2.0 in RFC1866 November 1993 HTML+ was published by the IETF as an Internet-Draft and was a competing proposal to the Hypertext Markup Language draft. It expired in May 1994. April 1995 (authored March 1995) HTML 3.0[30] was proposed as a standard to the IETF, but the proposal expired five months later without further action. It included many of the capabilities that were in Raggett's HTML+ proposal, such as support for tables, text flow around figures, and the display of complex mathematical formulas.[31] W3C began development of its own Arena browser for testing support for HTML 3 and Cascading Style Sheets, but HTML 3.0 did not succeed for several reasons. The draft was considered very large at 150 pages

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207 and the pace of browser development, as well as the number of interested parties, had outstripped the resources of the IETF.[16] Browser vendors, including Microsoft and Netscape at the time, chose to implement different subsets of HTML 3's draft features as well as to introduce their own extensions to it.[16] (See Browser wars) These included extensions to control stylistic aspects of documents, contrary to the "belief [of the academic engineering community] that such things as text color, background texture, font size and font face were definitely outside the scope of a language when their only intent was to specify how a document would be organized."[16] Dave Raggett, who has been a W3C Fellow for many years has commented for example, "To a certain extent, Microsoft built its business on the Web by extending HTML features."[16]

January 2008 HTML 5[32] was published as a Working Draft by the W3C. Although its syntax closely resembles that of SGML, HTML 5 has abandoned any attempt to be an SGML application, and has explicitly defined its own "html" serialization, in addition to an alternative XML-based XHTML 5 serialization.[33] XHTML versions XHTML is a separate language that began as a reformulation of HTML 4.01 using XML 1.0. It continues to be developed: • XHTML 1.0,[34] published January 26, 2000 as a W3C Recommendation, later revised and republished August 1, 2002. It offers the same three variations as HTML 4.0 and 4.01, reformulated in XML, with minor restrictions. • XHTML 1.1,[35] published May 31, 2001 as a W3C Recommendation. It is based on XHTML 1.0 Strict, but includes minor changes, can be customized, and is reformulated using modules from Modularization of XHTML [36] , which was published April 10, 2001 as a W3C Recommendation. • XHTML 2.0,[37] . There is no XHTML 2.0 standard. XHTML 2.0 is incompatible with XHTML 1.x and, therefore, would be more accurate to characterize as an XHTML-inspired new language than an update to XHTML 1.x. • XHTML 5, which is an update to XHTML 1.x, is being defined alongside HTML 5 in the HTML 5 draft.[38]

Markup HTML markup consists of several key components, including elements (and their attributes), character-based data types, and character references and entity references. Another important component is the document type declaration, which specifies the Document Type Definition. As of HTML 5, no Document Type Definition will need to be specified, and will only determine the layout mode [39]. The Hello world program, a common computer program employed for comparing programming languages, scripting languages, and markup languages is made of 9 lines of code in HTML, albeit Newlines are optional: <!doctype html> <html> <head> <title>Hello HTML</title> </head> <body> <p>Hello World!</p> </body> </html> This Document Type Declaration is for HTML 5. If the <!doctype html> declaration is not included, most browsers will render using "quirks mode."[40]

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Elements HTML documents are composed entirely of HTML elements that, in their most general form have three components: a pair of element tags with a "start tag" and "end tag"; some element attributes given to the element within the tags; and finally, all the actual, textual and graphical, information content that will be rendered on the display. An HTML element is everything between and including the tags. A tag is a keyword enclosed in angle brackets. A common form of an HTML element is: <tag>content to be rendered</tag> The name of the HTML element is also the name of the tag. Note that the end tag's name starts with a slash character, "/". The most general form of an HTML element is: <tag attribute1="value1" attribute2="value2">content to be rendered</tag> By not assigning attributes most start tags default their attribute values. There are some basic types of tags: Heading of the HTML:<head>...</head>. Usually the title should be included in the head, for example: <head> <title>The title</title> </head> <source lang="html4strict"> <h1>Heading1</h1> <h2>Heading2</h2> <h3>Heading3</h3> <h4>Heading4</h4> <h5>Heading5</h5> <h6>Heading6</h6> Paragraph Partition: <p>Paragraph 1</p>

<p>Paragraph 2</p>

Newline:<br>. The difference between <br> and <p> is that 'br' breaks a line without altering the semantic structure of the page, whereas 'p' sections the page into paragraphs. Here is an example: <code><p>This <br> is a paragraph <br> with <br> line breaks</p></code> Annotation:  Annotations can help to understand the coding and do not display in the webpage. There are several types of markup elements used in HTML. â&#x20AC;˘ Structural markup describes the purpose of text. For example, <h2>Golf</h2> establishes "Golf" as a second-level heading, which would be rendered in a browser in a manner similar to the "HTML markup" title at the start of this section. Structural markup does not denote any specific rendering, but most Web browsers have

HTML standardized default styles for element formatting. Text may be further styled with Cascading Style Sheets (CSS). • Presentational markup describes the appearance of the text, regardless of its function. For example <b>boldface</b> indicates that visual output devices should render "boldface" in bold text, but gives no indication what devices which are unable to do this (such as aural devices that read the text aloud) should do. In the case of both <b>bold</b> and <i>italic</i>, there are elements which usually have an equivalent visual rendering but are more semantic in nature, namely <strong>strong emphasis</strong> and <em>emphasis</em> respectively. It is easier to see how an aural user agent should interpret the latter two elements. However, they are not equivalent to their presentational counterparts: it would be undesirable for a screen-reader to emphasize the name of a book, for instance, but on a screen such a name would be italicized. Most presentational markup elements have become deprecated under the HTML 4.0 specification, in favor of CSS based style design. • Hypertext markup makes parts of a document into links to other documents. HTML up through version XHTML 1.1 requires the use of an anchor element to create a hyperlink in the flow of text: <a>Wikipedia</a>. In addition, the href attribute must be set to a valid URL. For example, the HTML markup, <a href="http://en.wikipedia.org/">Wikipedia</a>, will render the word "Wikipedia [41]" as a hyperlink. An example to render an image as a hyperlink is: <a href="http://example.org"><img src="image.gif" alt="alternative text" width="50" height="50"></a>. Attributes Most of the attributes of an element are name-value pairs, separated by "=", and written within the start tag of an element, after the element's name. The value may be enclosed in single or double quotes, although values consisting of certain characters can be left unquoted in HTML (but not XHTML).[42] [43] Leaving attribute values unquoted is considered unsafe.[44] In contrast with name-value pair attributes, there are some attributes that affect the element simply by their presence in the start tag of the element[7] (like the ismap attribute for the img element[45] ). Most elements can take any of several common attributes: • The id attribute provides a document-wide unique identifier for an element. This can be used by stylesheets to provide presentational properties, by browsers to focus attention on the specific element, or by scripts to alter the contents or presentation of an element. Appended to the URL of the page, it provides a globally-unique identifier for an element; typically a sub-section of the page. For example, the ID "Attributes" in http://en.wikipedia.org/wiki/HTML#Attributes • The class attribute provides a way of classifying similar elements. This can be used for semantic or presentation purposes. Semantically, for example, classes are used in microformats. Presentationally, for example, an HTML document might use the designation class="notation" to indicate that all elements with this class value are subordinate to the main text of the document. Such elements might be gathered together and presented as footnotes on a page instead of appearing in the place where they occur in the HTML source. • An author may use the style non-attributal codes presentational properties to a particular element. It is considered better practice to use an element’s id or class attributes to select the element with a stylesheet, though sometimes this can be too cumbersome for a simple and specific or ad hoc application of styled properties. • The title attribute is used to attach subtextual explanation to an element. In most browsers this attribute is displayed as what is often referred to as a tooltip. The abbreviation element, abbr, can be used to demonstrate these various attributes: <abbr id="anId" class="aClass" style="color:blue;" title="Hypertext Markup Language">HTML</abbr> This example displays as HTML; in most browsers, pointing the cursor at the abbreviation should display the title text "Hypertext Markup Language." Most elements also take the language-related attributes lang and dir.

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Character and entity references As of version 4.0, HTML defines a set of 252 character entity references and a set of 1,114,050 numeric character references, both of which allow individual characters to be written via simple markup, rather than literally. A literal character and its markup counterpart are considered equivalent and are rendered identically. The ability to "escape" characters in this way allows for the characters < and & (when written as &lt; and &amp;, respectively) to be interpreted as character data, rather than markup. For example, a literal < normally indicates the start of a tag, and & normally indicates the start of a character entity reference or numeric character reference; writing it as &amp; or &#x26; or &#38; allows & to be included in the content of elements or the values of attributes. The double-quote character ("), when used to quote an attribute value, must also be escaped as &quot; or &#x22; or &#34; when it appears within the attribute value itself. The single-quote character ('), when used to quote an attribute value, must also be escaped as &#x27; or &#39; (should NOT be escaped as &apos; except in XHTML documents) when it appears within the attribute value itself. However, since document authors often overlook the need to escape these characters, browsers tend to be very forgiving, treating them as markup only when subsequent text appears to confirm that intent. Escaping also allows for characters that are not easily typed or that aren't even available in the document's character encoding to be represented within the element and attribute content. For example, the acute-accented e (é), a character typically found only on Western European keyboards, can be written in any HTML document as the entity reference &eacute; or as the numeric references &#233; or &#xE9;. The characters comprising those references (that is, the &, the ;, the letters in eacute, and so on) are available on all keyboards and are supported in all character encodings, whereas the literal é is not.

Data types HTML defines several data types for element content, such as script data and stylesheet data, and a plethora of types for attribute values, including IDs, names, URIs, numbers, units of length, languages, media descriptors, colors, character encodings, dates and times, and so on. All of these data types are specializations of character data.

Document type declaration HTML documents are required to start with a Document Type Declaration (informally, a "doctype"). In browsers, the function of the doctype is to indicate the rendering mode—particularly to avoid quirks mode. The original purpose of the doctype was to enable parsing and validation of HTML documents by SGML tools based on the Document Type Definition (DTD). The DTD to which the DOCTYPE refers contains machine-readable grammar specifying the permitted and prohibited content for a document conforming to such a DTD. Browsers, on the other hand, do not implement HTML as an application of SGML and by consequence do not read the DTD. HTML 5 does not define a DTD, because of the technology's inherent limitations, so in HTML 5 the doctype declaration, <!doctype html>, does not refer to a DTD. An example of an HTML 4 doctype is <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> This declaration references the DTD for the Strict version of HTML 4.01, which does not include presentational elements like font, leaving formatting to Cascading Style Sheets and the span and div element. SGML-based validators read the DTD in order to properly parse the document and to perform validation. In modern browsers, this doctype activates standards mode as opposed to quirks mode. In addition, HTML 4.01 provides Transitional and Frameset DTDs, as explained below.

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Semantic HTML Semantic HTML is a way of writing HTML that emphasizes the meaning of the encoded information over its presentation (look). HTML has included semantic markup from its inception,[46] but has also included presentational markup such as <font>, <i> and <center> tags. There are also the semantically neutral span and div tags. Since the late 1990s when Cascading Style Sheets were beginning to work in most browsers, web authors have been encouraged to avoid the use of presentational HTML markup with a view to the separation of presentation and content.[47] In a 2001 discussion of the Semantic Web, Tim Berners-Lee and others gave examples of ways in which intelligent software 'agents' may one day automatically trawl the Web and find, filter and correlate previously unrelated, published facts for the benefit of human users.[48] Such agents are not commonplace even now, but some of the ideas of Web 2.0, mashups and price comparison websites may be coming close. The main difference between these web application hybrids and Berners-Lee's semantic agents lies in the fact that the current aggregation and hybridisation of information is usually designed in by web developers, who already know the web locations and the API semantics of the specific data they wish to mash, compare and combine. An important type of web agent that does trawl and read web pages automatically, without prior knowledge of what it might find, is the Web crawler or search-engine spider. These software agents are dependent on the semantic clarity of web pages they find as they use various techniques and algorithms to read and index millions of web pages a day and provide web users with search facilities without which the World Wide Web would be only a fraction of its current usefulness. In order for search-engine spiders to be able to rate the significance of pieces of text they find in HTML documents, and also for those creating mashups and other hybrids, as well as for more automated agents as they are developed, the semantic structures that exist in HTML need to be widely and uniformly applied to bring out the meaning of published text.[49] Presentational markup tags are deprecated in current HTML and XHTML recommendations and are illegal in HTML 5. Good semantic HTML also improves the accessibility of web documents (see also Web Content Accessibility Guidelines). For example, when a screen reader or audio browser can correctly ascertain the structure of a document, it will not waste the visually impaired user's time by reading out repeated or irrelevant information when it has been marked up correctly.

Delivery HTML documents can be delivered by the same means as any other computer file; however, they are most often delivered either by HTTP from a Web server or by e-mail.

HTTP The World Wide Web is composed primarily of HTML documents transmitted from Web servers to Web browsers using the Hypertext Transfer Protocol (HTTP). However, HTTP is used to serve images, sound, and other content in addition to HTML. To allow the Web browser to know how to handle each document it receives, other information is transmitted along with the document. This meta data usually includes the MIME type (e.g. text/html or application/xhtml+xml) and the character encoding (see Character encoding in HTML). In modern browsers, the MIME type that is sent with the HTML document may affect how the document is initially interpreted. A document sent with the XHTML MIME type is expected to be well-formed XML, and syntax errors may cause the browser to fail to render it. The same document sent with the HTML MIME type might be displayed successfully, since some browsers are more lenient with HTML.

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HTML The W3C recommendations state that XHTML 1.0 documents that follow guidelines set forth in the recommendation's Appendix C may be labeled with either MIME Type.[50] The current XHTML 1.1 Working Draft also states that XHTML 1.1 documents should[51] be labeled with either MIME type.[52]

HTML e-mail Most graphical e-mail clients allow the use of a subset of HTML (often ill-defined) to provide formatting and semantic markup not available with plain text. This may include typographic information like coloured headings, emphasized and quoted text, inline images and diagrams. Many such clients include both a GUI editor for composing HTML e-mail messages and a rendering engine for displaying them. Use of HTML in e-mail is controversial because of compatibility issues, because it can help disguise phishing attacks, because it can confuse spam filters and because the message size is larger than plain text.

Naming conventions The most common filename extension for files containing HTML is .html. A common abbreviation of this is .htm, which originated because some early operating systems and file systems, such as DOS and FAT, limited file extensions to three letters.

HTML Application An HTML Application (HTA; file extension ".hta") is a Microsoft Windows application that uses HTML and Dynamic HTML in a browser to provide the application's graphical interface. A regular HTML file is confined to the security model of the web browser, communicating only to web servers and manipulating only webpage objects and site cookies. An HTA runs as a fully trusted application and therefore has more privileges, like creation/editing/removal of files and Windows Registry entries. Because they operate outside the browser's security model, HTAs cannot be executed via HTTP, but must be downloaded (just like an EXE file) and executed from local file system.

Current variations Since its inception, HTML and its associated protocols gained acceptance relatively quickly. However, no clear standards existed in the early years of the language. Though its creators originally conceived of HTML as a semantic language devoid of presentation details[53], practical uses pushed many presentational elements and attributes into the language, driven largely by the various browser vendors. The latest standards surrounding HTML reflect efforts to overcome the sometimes chaotic development of the language[54] and to create a rational foundation for building both meaningful and well-presented documents. To return HTML to its role as a semantic language, the W3C has developed style languages such as CSS and XSL to shoulder the burden of presentation. In conjunction, the HTML specification has slowly reined in the presentational elements. There are two axes differentiating various variations of HTML as currently specified: SGML-based HTML versus XML-based HTML (referred to as XHTML) on one axis, and strict versus transitional (loose) versus frameset on the other axis.

SGML-based versus XML-based HTML One difference in the latest HTML specifications lies in the distinction between the SGML-based specification and the XML-based specification. The XML-based specification is usually called XHTML to distinguish it clearly from the more traditional definition; however, the root element name continues to be 'html' even in the XHTML-specified HTML. The W3C intended XHTML 1.0 to be identical to HTML 4.01 except where limitations of XML over the more complex SGML require workarounds. Because XHTML and HTML are closely related, they are sometimes

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HTML documented in parallel. In such circumstances, some authors conflate the two names as (X)HTML or X(HTML). Like HTML 4.01, XHTML 1.0 has three sub-specifications: strict, loose, and frameset. Aside from the different opening declarations for a document, the differences between an HTML 4.01 and XHTML 1.0 document—in each of the corresponding DTDs—are largely syntactic. The underlying syntax of HTML allows many shortcuts that XHTML does not, such as elements with optional opening or closing tags, and even EMPTY elements which must not have an end tag. By contrast, XHTML requires all elements to have an opening tag or a closing tag. XHTML, however, also introduces a new shortcut: an XHTML tag may be opened and closed within the same tag, by including a slash before the end of the tag like this: <br/>. The introduction of this shorthand, which is not used in the SGML declaration for HTML 4.01, may confuse earlier software unfamiliar with this new convention. A fix for this is to include a space before closing the tag, as such: <br />.[55] To understand the subtle differences between HTML and XHTML, consider the transformation of a valid and well-formed XHTML 1.0 document that adheres to Appendix C (see below) into a valid HTML 4.01 document. To make this translation requires the following steps: 1. The language for an element should be specified with a lang attribute rather than the XHTML xml:lang attribute. XHTML uses XML's built in language-defining functionality attribute. 2. Remove the XML namespace (xmlns=URI). HTML has no facilities for namespaces. 3. Change the document type declaration from XHTML 1.0 to HTML 4.01. (see DTD section for further explanation). 4. If present, remove the XML declaration. (Typically this is: <?xml version="1.0" encoding="utf-8"?>). 5. Ensure that the document’s MIME type is set to text/html. For both HTML and XHTML, this comes from the HTTP Content-Type header sent by the server. 6. Change the XML empty-element syntax to an HTML style empty element (<br/> to <br>). Those are the main changes necessary to translate a document from XHTML 1.0 to HTML 4.01. To translate from HTML to XHTML would also require the addition of any omitted opening or closing tags. Whether coding in HTML or XHTML it may just be best to always include the optional tags within an HTML document rather than remembering which tags can be omitted. A well-formed XHTML document adheres to all the syntax requirements of XML. A valid document adheres to the content specification for XHTML, which describes the document structure. The W3C recommends several conventions to ensure an easy migration between HTML and XHTML (see HTML Compatibility Guidelines [56]). The following steps can be applied to XHTML 1.0 documents only: • • • •

Include both xml:lang and lang attributes on any elements assigning language. Use the empty-element syntax only for elements specified as empty in HTML. Include an extra space in empty-element tags: for example <br /> instead of <br/>. Include explicit close tags for elements that permit content but are left empty (for example, <div></div>, not <div />). • Omit the XML declaration. By carefully following the W3C’s compatibility guidelines, a user agent should be able to interpret the document equally as HTML or XHTML. For documents that are XHTML 1.0 and have been made compatible in this way, the W3C permits them to be served either as HTML (with a text/html MIME type), or as XHTML (with an application/xhtml+xml or application/xml MIME type). When delivered as XHTML, browsers should use an XML parser, which adheres strictly to the XML specifications for parsing the document's contents.

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Transitional versus strict HTML 4 defined three different versions of the language: Strict, Transitional (once called Loose), and Frameset. The Strict version is intended for new documents and is considered best practice, while the Transitional and Frameset versions were developed to make it easier to transition documents that conformed to older HTML specification or didn't conform to any specification to a version of HTML 4. The Transitional and Frameset versions allow for presentational markup, which is omitted in the Strict version. Instead, cascading style sheets are encouraged to improve the presentation of HTML documents. Because XHTML 1 only defines an XML syntax for the language defined by HTML 4, the same differences apply to XHTML 1 as well. The Transitional version allows the following parts of the vocabulary, which are not included in the Strict version: • A looser content model • Inline elements and plain text are allowed directly in: body, blockquote, form, noscript and noframes • Presentation related elements • underline (u) • strike-through (s) • center • font • basefont • Presentation related attributes • background and bgcolor attributes for body element. • align attribute on div, form, paragraph (p), and heading (h1...h6) elements • align, noshade, size, and width attributes on hr element • align, border, vspace, and hspace attributes on img and object elements • align attribute on legend and caption elements • align and bgcolor on table element • nowrap, bgcolor, width, height on td and th elements • bgcolor attribute on tr element • clear attribute on br element • compact attribute on dl, dir and menu elements • type, compact, and start attributes on ol and ul elements • type and value attributes on li element • width attribute on pre element • Additional elements in Transitional specification • menu list (no substitute, though unordered list is recommended) • dir list (no substitute, though unordered list is recommended) • isindex (element requires server-side support and is typically added to documents server-side, form and input elements can be used as a substitute) • applet (deprecated in favor of object element) • The language attribute on script element (redundant with the type attribute). • Frame related entities • iframe • noframes • target attribute on anchor, client-side image-map (imagemap), link, form, and base elements The Frameset version includes everything in the Transitional version, as well as the frameset element (used instead of body) and the frame element.

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Frameset versus transitional In addition to the above transitional differences, the frameset specifications (whether XHTML 1.0 or HTML 4.01) specifies a different content model, with frameset replacing body, containing frame elements, and optionally noframes, with a body.

Summary of specification versions As this list demonstrates, the loose versions of the specification are maintained for legacy support. However, contrary to popular misconceptions, the move to XHTML does not imply a removal of this legacy support. Rather the X in XML stands for extensible and the W3C is modularizing the entire specification and opening it up to independent extensions. The primary achievement in the move from XHTML 1.0 to XHTML 1.1 is the modularization of the entire specification. The strict version of HTML is deployed in XHTML 1.1 through a set of modular extensions to the base XHTML 1.1 specification. Likewise someone looking for the loose (transitional) or frameset specifications will find similar extended XHTML 1.1 support (much of it is contained in the legacy or frame modules). The modularization also allows for separate features to develop on their own timetable. So for example XHTML 1.1 will allow quicker migration to emerging XML standards such as MathML (a presentational and semantic math language based on XML) and XForms—a new highly advanced web-form technology to replace the existing HTML forms. In summary, the HTML 4.01 specification primarily reined in all the various HTML implementations into a single clear written specification based on SGML. XHTML 1.0, ported this specification, as is, to the new XML defined specification. Next, XHTML 1.1 takes advantage of the extensible nature of XML and modularizes the whole specification. XHTML 2.0 will be the first step in adding new features to the specification in a standards-body-based approach.

Hypertext features not in HTML HTML lacks some of the features found in earlier hypertext systems, such as typed links, source tracking, fat links, and more.[57] Even some hypertext features that were in early versions of HTML have been ignored by most popular web browsers until recently, such as the link element and in-browser Web page editing. Sometimes Web services or browser manufacturers remedy these shortcomings. For instance, wikis and content management systems allow surfers to edit the Web pages they visit.

WYSIWYG Editors There are some WYSIWYG editors in which the user lays out everything as it is to appear in the HTML document using a graphical user interface, and the editor renders this as an HTML document, no longer requiring the author to have extensive knowledge of HTML. Web page editing is clearly dominated by the WYSIWYG editing model. But, this model has been criticized,[58] [59] primarily because of the low quality of the generated code, and there are voices advocating a change to the WYSIWYM model. WYSIWYG editors remains a controversial topic because of their perceived flaws such as: • Relying mainly on layout as opposed to meaning, often using markup that does not convey the intended meaning but simply copies the layout.[60] • Often producing extremely verbose and redundant code that fails to make use of the cascading nature of HTML and CSS. • Often producing ingrammatal markup often called tag soup. • As a great deal of information of HTML documents is not in the layout, the model has been criticized for its 'what you see is all you get'-nature. [61]

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Nevertheless, since WYSIWYG editors offer convenience over hand-coded pages as well as not requiring the author to know the finer details of HTML, they still dominate web authoring.

See also • • • • • • • •

Breadcrumb (navigation) HTML decimal character rendering HTML element JHTML List of computer standards List of document markup languages Microformat The HTML Sourcebook: The Complete Guide to HTML (historical reference from 1995)

External links • HTML 4.01, the most recent finished specification (1999) [62] • HTML 5, the upcoming version of HTML [63] • Dave Raggett's Introduction to HTML [64] • Empty elements in SGML, HTML, XML, and XHTML [65]

HTML tutorials • • • •

HTML Dog [66] HTML.net [67] Your HTML Source [68] HTML tutorial [69]

References [1] [2] [3] [4]

http:/ / www. w3. org/ TR/ 1999/ REC-html401-19991224/ http:/ / dev. w3. org/ html5/ spec/ HTML 4 — Conformance: requirements and recommendations (http:/ / www. w3. org/ TR/ html401/ conform. html#deprecated) Tim Berners-Lee, "Information Management: A Proposal." CERN (March 1989, May 1990). W3.org (http:/ / www. w3. org/ History/ 1989/ proposal. html) [5] Tim Berners-Lee, "Design Issues" (http:/ / www. w3. org/ DesignIssues/ ) [6] Tim Berners-Lee, "Design Issues" (http:/ / www. w3. org/ DesignIssues/ Uses. html) [7] "Tags used in HTML" (http:/ / www. w3. org/ History/ 19921103-hypertext/ hypertext/ WWW/ MarkUp/ Tags. html). World Wide Web Consortium. November 3, 1992. . Retrieved November 16, 2008. [8] "First mention of HTML Tags on the www-talk mailing list" (http:/ / lists. w3. org/ Archives/ Public/ www-talk/ 1991SepOct/ 0003. html). World Wide Web Consortium. October 29, 1991. . Retrieved April 8, 2007. [9] "Index of elements in HTML 4" (http:/ / www. w3. org/ TR/ 1999/ REC-html401-19991224/ index/ elements). World Wide Web Consortium. December 24, 1999. . Retrieved April 8, 2007. [10] http:/ / www. w3. org/ MarkUp/ draft-ietf-iiir-html-01. txt [11] Tim Berners-Lee (December 9, 1991). "Re: SGML/HTML docs, X Browser (archived www-talk mailing list post)" (http:/ / lists. w3. org/ Archives/ Public/ www-talk/ 1991NovDec/ 0020. html). . Retrieved June 16, 2007. "SGML is very general. HTML is a specific application of the SGML basic syntax applied to hypertext documents with simple structure." [12] Raymond, Eric. "IETF and the RFC Standards Process" (http:/ / www. faqs. org/ docs/ artu/ ietf_process. html). [[The Art of Unix Programming (http:/ / www. faqs. org/ docs/ artu/ )]]. . "In IETF tradition, standards have to arise from experience with a working prototype implementation — but once they become standards, code that does not conform to them is considered broken and mercilessly scrapped. …Internet-Drafts are not specifications, and software implementers and vendors are specifically barred from claiming compliance with them as if they were specifications. Internet-Drafts are focal points for discussion, usually in a working group… Once an Internet-Draft has been published with an RFC number, it is a specification to which implementers may claim conformance. It is expected that the authors of the RFC and the community at large will begin correcting the specification with field experience."

HTML [13] "HTML+ Internet-Draft - Abstract" (https:/ / datatracker. ietf. org/ public/ idindex. cgi?command=id_detail& id=789). . "Browser writers are experimenting with extensions to HTML and it is now appropriate to draw these ideas together into a revised document format. The new format is designed to allow a gradual roll over from HTML, adding features like tables, captioned figures and fill-out forms for querying remote databases or mailing questionnaires." [14] "RFC 1866: Hypertext Markup Language - 2.0 - Acknowledgments" (http:/ / www. ietf. org/ rfc/ rfc1866. txt). Internet Engineering Task Force. September 22, 2005. . Retrieved June 16, 2007. "Since 1993, a wide variety of Internet participants have contributed to the evolution of HTML, which has included the addition of in-line images introduced by the NCSA Mosaic software for WWW. Dave Raggett played an important role in deriving the forms material from the HTML+ specification. Dan Connolly and Karen Olson Muldrow rewrote the HTML Specification in 1994. The document was then edited by the HTML working group as a whole, with updates being made by Eric Schieler, Mike Knezovich, and Eric W. Sink at Spyglass, Inc. Finally, Roy Fielding restructured the entire draft into its current form." [15] "RFC 1866: Hypertext Markup Language - 2.0 - Introduction" (http:/ / www. ietf. org/ rfc/ rfc1866. txt). Internet Engineering Task Force. September 22, 2005. . Retrieved June 16, 2007. "This document thus defines an HTML 2.0 (to distinguish it from the previous informal specifications). Future (generally upwardly compatible) versions of HTML with new features will be released with higher version numbers." [16] Raggett, Dave (1998). Raggett on HTML 4 (http:/ / www. w3. org/ People/ Raggett/ book4/ ch02. html). . Retrieved July 9, 2007. [17] "HTML 3.2 Reference Specification" (http:/ / www. w3. org/ TR/ REC-html32). World Wide Web Consortium. January 14, 1997. . Retrieved November 16, 2008. [18] "IETF HTML WG" (http:/ / www. w3. org/ MarkUp/ HTML-WG/ ). . Retrieved June 16, 2007. "Note: This working group is closed" [19] "HTML 4.0 Specification" (http:/ / www. w3. org/ TR/ REC-html40-971218/ ). World Wide Web Consortium. December 18,1997. . Retrieved November 16, 2008. [20] Arnoud Engelfriet. "Introduction to Wilbur" (http:/ / htmlhelp. com/ reference/ wilbur/ intro. html). Web Design Group. . Retrieved June 16, 2007. [21] "HTML 4 - 4 Conformance: requirements and recommendations" (http:/ / www. w3. org/ TR/ html4/ conform. html#h-4. 2). . Retrieved December 30, 2009. [22] "HTML 4.0 Specification" (http:/ / www. w3. org/ TR/ 1998/ REC-html40-19980424/ ). World Wide Web Consortium. April 24, 1998. . Retrieved November 16, 2008. [23] "HTML 4.01 Specification" (http:/ / www. w3. org/ TR/ html401/ ). World Wide Web Consortium. December 24, 1999. . Retrieved November 16, 2008. [24] http:/ / www. w3. org/ MarkUp/ html4-updates/ errata [25] ISO (2000). "ISO/IEC 15445:2000 - Information technology -- Document description and processing languages -- HyperText Markup Language (HTML)" (http:/ / www. iso. org/ iso/ iso_catalogue/ catalogue_tc/ catalogue_detail. htm?csnumber=27688). . Retrieved December 26, 2009. [26] CS.TCD.ie (https:/ / www. cs. tcd. ie/ 15445/ 15445. HTML) [27] http:/ / lists. w3. org/ Archives/ Public/ www-talk/ 1992JulAug/ 0020. html [28] Hypertext Markup Language: A Representation of Textual Information and MetaInformation for Retrieval and Interchange (http:/ / tools. ietf. org/ html/ draft-ietf-iiir-html-00) [29] http:/ / tools. ietf. org/ html/ draft-ietf-html-spec-00 [30] "HTML 3.0 Draft (Expired!) Materials" (http:/ / www. w3. org/ MarkUp/ html3/ ). World Wide Web Consortium. December 21, 1995. . Retrieved November 16, 2008. [31] "HyperText Markup Language Specification Version 3.0" (http:/ / www. w3. org/ MarkUp/ html3/ CoverPage). . Retrieved June 16, 2007. [32] "HTML 5" (http:/ / www. w3. org/ TR/ html5/ ). World Wide Web Consortium. June 10, 2008. . Retrieved November 16, 2008. [33] "HTML 5, one vocabulary, two serializations" (http:/ / www. w3. org/ QA/ 2008/ 01/ html5-is-html-and-xml. html). . Retrieved February 25, 2009. [34] "XHTML 1.0: The Extensible HyperText Markup Language (Second Edition)" (http:/ / www. w3. org/ TR/ xhtml1/ ). World Wide Web Consortium. January 26, 2000. . Retrieved November 16, 2008. [35] "XHTML 1.1 - Module-based XHTML - Second Edition" (http:/ / www. w3. org/ TR/ xhtml11/ ). World Wide Web Consortium. February 16, 2007. . Retrieved November 16, 2008. [36] http:/ / www. w3. org/ TR/ xhtml-modularization/ [37] "XHTM 2.0" (http:/ / www. w3. org/ TR/ xhtml2/ ). World Wide Web Consortium. July 26, 2006. . Retrieved November 16, 2008. "XHTML 2 Working Group Expected to Stop Work End of 2009, W3C to Increase Resources on HTML 5" (http:/ / www. w3. org/ News/ 2009#item119). World Wide Web Consortium. July 17, 2009. . Retrieved November 16, 2008. [38] "HTML 5" (http:/ / www. w3. org/ html/ wg/ html5/ ). World Wide Web Consortium. October 24, 2008. . Retrieved November 16, 2008. [39] http:/ / www. w3. org/ 2008/ Talks/ 04-24-smith/ index. html [40] Activating Browser Modes with Doctype (http:/ / hsivonen. iki. fi/ doctype/ ) [41] http:/ / en. wikipedia. org/ [42] "On SGML and HTML" (http:/ / www. w3. org/ TR/ html401/ intro/ sgmltut. html#h-3. 2. 2). World Wide Web Consortium. . Retrieved November 16, 2008. [43] "XHTML 1.0 - Differences with HTMLÂ 4" (http:/ / www. w3. org/ TR/ xhtml1/ diffs. html#h-4. 4). World Wide Web Consortium. . Retrieved November 16, 2008.

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HTML [44] Korpela, Jukka (July 6, 1998). "Why attribute values should always be quoted in HTML" (http:/ / www. cs. tut. fi/ ~jkorpela/ qattr. html). Cs.tut.fi. . Retrieved November 16, 2008. [45] "Objects, Images, and Applets in HTML documents" (http:/ / www. w3. org/ TR/ 1999/ REC-html401-19991224/ struct/ objects. html#adef-ismap). World Wide Web Consortium. December 24, 1999. . Retrieved November 16, 2008. [46] Berners-Lee, Tim; Fischetti, Mark (2000). Weaving the Web: The Original Design and Ultimate Destiny of the World Wide Web by Its Inventor. San Francisco: Harper. ISBNÂ 978-0-06-251587-X. [47] Raggett, Dave (2002). "Adding a touch of style" (http:/ / www. w3. org/ MarkUp/ Guide/ Style. html). W3C. . Retrieved October 2, 2009. This article notes that presentational HTML markup may be useful when targeting browsers "before Netscape 4.0 and Internet Explorer 4.0". See the list of web browsers to confirm that these were both released in 1997. [48] Tim Berners-Lee, James Hendler and Ora Lassila (2001). "The Semantic Web" (http:/ / www. scientificamerican. com/ article. cfm?id=the-semantic-web). Scientific American. . Retrieved October 2, 2009. [49] Nigel Shadbolt, Wendy Hall and Tim Berners-Lee (2006). "The Semantic Web Revisited" (http:/ / eprints. ecs. soton. ac. uk/ 12614/ 1/ Semantic_Web_Revisted. pdf). IEEE Intelligent Systems. . Retrieved October 2, 2009. [50] "XHTML 1.0 The Extensible HyperText Markup Language (Second Edition)" (http:/ / www. w3. org/ TR/ xhtml1/ #media). World Wide Web Consortium. 2000, revised 2002. . Retrieved December 7, 2008. "XHTML Documents which follow the guidelines set forth in Appendix C, "HTML Compatibility Guidelines" may be labeled with the Internet Media Type "text/html" [RFC2854], as they are compatible with most HTML browsers. Those documents, and any other document conforming to this specification, may also be labeled with the Internet Media Type "application/xhtml+xml" as defined in [RFC3236]." [51] "RFC 2119: Key words for use in RFCs to Indicate Requirement Levels" (http:/ / www. ietf. org/ rfc/ rfc2119. txt). Harvard University. 1997. . Retrieved December 7, 2008. "3. SHOULD This word, or the adjective "RECOMMENDED", mean that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course." [52] "XHTML 1.1 - Module-based XHTML - Second Edition" (http:/ / www. w3. org/ TR/ xhtml11/ conformance. html#strict). World Wide Web Consortium. 2007. . Retrieved December 7, 2008. "XHTML 1.1 documents SHOULD be labeled with the Internet Media Type text/html as defined in [RFC2854] or application/xhtml+xml as defined in [RFC3236]." [53] http:/ / www. w3. org/ History/ 19921103-hypertext/ hypertext/ WWW/ MarkUp/ HTMLConstraints. html [54] http:/ / ei. cs. vt. edu/ ~wwwbtb/ book/ chap13/ who. html [55] Freeman, E (2005). Head First HTML. O'Reilly. [56] http:/ / www. w3. org/ TR/ xhtml1/ #guidelines [57] Jakob Nielsen (January 3, 2005). "Reviving Advanced Hypertext" (http:/ / www. useit. com/ alertbox/ 20050103. html). . Retrieved June 16, 2007. [58] Sauer, C.: WYSIWIKI - Questioning WYSIWYG in the Internet Age. In: Wikimania (2006) [59] Spiesser, J., Kitchen, L.: Optimization of html automatically generated by WYSIWYG programs. In: 13th International Conference on World Wide Web, pp. 355--364. WWW '04. ACM, New York, NY (New York, NY, USA, May 17-20, 2004) [60] http:/ / xhtml. com/ en/ xhtml/ reference/ blockquote/ [61] http:/ / www. invisiblerevolution. net/ [62] http:/ / www. w3. org/ TR/ html401/ [63] http:/ / dev. w3. org/ html5/ spec/ spec. html [64] http:/ / www. w3. org/ MarkUp/ Guide/ [65] http:/ / www. cs. tut. fi/ ~jkorpela/ html/ empty. html [66] http:/ / htmldog. com/ guides/ [67] http:/ / www. html. net/ tutorials/ html/ introduction. asp [68] http:/ / www. yourhtmlsource. com/ [69] http:/ / programming-guides. com/ html

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Web 2.0

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Web 2.0 The term "Web 2.0" (2004–present) is commonly associated with web applications that facilitate interactive information sharing, interoperability, user-centered design,[1] and collaboration on the World Wide Web. Examples of Web 2.0 include web-based communities, hosted services, web applications, social-networking sites, video-sharing sites, wikis, blogs, mashups, and folksonomies. A Web 2.0 site allows its users to interact with each other as contributors to the website's content, in contrast to non-interactive websites where users are limited to the passive viewing of information that is provided to them.

A tag cloud (a typical Web 2.0 phenomenon in itself) presenting Web 2.0 themes

The term is closely associated with Tim O'Reilly because of the O'Reilly Media Web 2.0 conference in 2004.[2] [3] Although the term suggests a new version of the World Wide Web, it does not refer to an update to any technical specifications, but rather to cumulative changes in the ways software developers and end-users use the Web. Whether Web 2.0 is qualitatively different from prior web technologies has been challenged by World Wide Web inventor Tim Berners-Lee, who called the term a "piece of jargon"[4] — precisely because he specifically intended the Web to embody these values in the first place.

History: From Web 1.0 to 2.0 The term "Web 2.0" was coined in 1999 by Darcy DiNucci. In her article, "Fragmented Future," DiNucci writes:[5] The Web we know now, which loads into a browser window in essentially static screenfulls, is only an embryo of the Web to come. The first glimmerings of Web 2.0 are beginning to appear, and we are just starting to see how that embryo might develop. The Web will be understood not as screenfulls of text and graphics but as a transport mechanism, the ether through which interactivity happens. It will [...] appear on your computer screen, [...] on your TV set [...] your car dashboard [...] your cell phone [...] hand-held game machines [...] maybe even your microwave oven. Her use of the term deals mainly with Web design and aesthetics; she argues that the Web is "fragmenting" due to the widespread use of portable Web-ready devices. Her article is aimed at designers, reminding them to code for an ever-increasing variety of hardware. As such, her use of the term hints at – but does not directly relate to – the current uses of the term. The term did not resurface until 2003.[6] [7] [8] These authors focus on the concepts currently associated with the term where, as Scott Dietzen puts it, "the Web becomes a universal, standards-based integration platform".[9] In 2004, the term began its rise in popularity when O'Reilly Media and MediaLive hosted the first Web 2.0 conference. In their opening remarks, John Battelle and Tim O'Reilly outlined their definition of the "Web as Platform", where software applications are built upon the Web as opposed to upon the desktop. The unique aspect of this migration, they argued, is that "customers are building your business for you".[10] They argued that the activities of users generating content (in the form of ideas, text, videos, or pictures) could be "harnessed" to create value.

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O'Reilly et al. contrasted Web 2.0 with what they called "Web 1.0". They associated Web 1.0 with the business models of Netscape and the Encyclopedia Britannica Online. For example, Netscape framed "the web as platform" in terms of the old software paradigm: their flagship product was the web browser, a desktop application, and their strategy was to use their dominance in the browser market to establish a market for high-priced server products. Control over standards for displaying content and applications in the browser would, in theory, give Netscape the kind of market power enjoyed by Microsoft in the PC market. Much like the "horseless carriage" framed the automobile as an extension of the familiar, Netscape promoted a "webtop" to replace the desktop, and planned to populate that webtop with information updates and applets pushed to the webtop by information providers who would purchase Netscape servers.[11] In short, Netscape focused on creating software, updating it on occasion, and distributing it to the end users. O'Reilly contrasted this with Google, a company which did not at the time focus on producing software, such as a browser, but instead focused on providing a service based on data. The data being the links Web page authors make between sites. Google exploits this user-generated content to offer Web search based on reputation through its "page rank" algorithm. Unlike software, which undergoes scheduled releases, such services are constantly updated, a process called "the perpetual beta". A similar difference can be seen between the Encyclopedia Britannica Online and Wikipedia: while the Britannica relies upon experts to create articles and releases them periodically in publications, Wikipedia relies on trust in anonymous users to constantly and quickly build content. Wikipedia is not based on expertise but rather an adaptation of the open source software adage "given enough eyeballs, all bugs are shallow", and it produces and updates articles constantly. O'Reilly's Web 2.0 conferences have been held every year since 2004, attracting entrepreneurs, large companies, and technology reporters. In terms of the lay public, the term Web 2.0 was largely championed by bloggers and by technology journalists, culminating in the 2006 TIME magazine Person of The Year â&#x20AC;&#x201C; "You".[12] That is, TIME selected the masses of users who were participating in content creation on social networks, blogs, wikis, and media sharing sites. The cover story author Lev Grossman explains: It's a story about community and collaboration on a scale never seen before. It's about the cosmic compendium of knowledge Wikipedia and the million-channel people's network YouTube and the online metropolis MySpace. It's about the many wresting power from the few and helping one another for nothing and how that will not only change the world, but also change the way the world changes. Since that time, Web 2.0 has found a place in the lexicon; the Global Language Monitor recently declared it to be the one-millionth English word.[13]

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Characteristics Web 2.0 websites allow users to do more than just retrieve information. They can build on the interactive facilities of "Web 1.0" to provide "Network as platform" computing, allowing users to run software-applications entirely through a browser.[3] Users can own the data on a Web 2.0 site and exercise control over that data.[3] [14] These sites may have an "Architecture of participation" that encourages users to add value to the application as they use it.[2] [3] The concept of Web-as-participation-platform captures many of these characteristics. Bart Decrem, a founder and former CEO of Flock, calls Web 2.0 the "participatory Web"[15] and regards the Flickr, a Web 2.0 web site that allows its users to Web-as-information-source as Web 1.0. The impossibility of excluding upload and share photos group-members who donâ&#x20AC;&#x2122;t contribute to the provision of goods from sharing profits gives rise to the possibility that rational members will prefer to withhold their contribution of effort and free-ride on the contribution of others.[16] This requires what is sometimes called Radical Trust by the management of the website. According to Best,[17] the characteristics of Web 2.0 are: rich user experience, user participation, dynamic content, metadata, web standards and scalability. Further characteristics, such as openness, freedom[18] and collective intelligence[19] by way of user participation, can also be viewed as essential attributes of Web 2.0.

Technology overview Web 2.0 draws together the capabilities of client- and server-side software, content syndication and the use of network protocols. Standards-oriented web browsers may use plug-ins and software extensions to handle the content and the user interactions. Web 2.0 sites provide users with information storage, creation, and dissemination capabilities that were not possible in the environment now known as "Web 1.0". Web 2.0 websites typically include some of the following features and techniques. Andrew McAfee used the acronym SLATES to refer to them:[20] Search Finding information through keyword search. Links Connects information together into a meaningful information ecosystem using the model of the Web, and provides low-barrier social tools. Authoring The ability to create and update content leads to the collaborative work of many rather than just a few web authors. In wikis, users may extend, undo and redo each other's work. In blogs, posts and the comments of individuals build up over time. Tags Categorization of content by users adding "tags" - short, usually one-word descriptions = to facilitate searching, without dependence on pre-made categories. Collections of tags created by many users within a single system may be referred to as "folksonomies" (i.e., folk taxonomies). Extensions Software that makes the Web an application platform as well as a document server. Signals

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222 The use of syndication technology such as RSS to notify users of content changes.

While SLATES forms the basic framework of Enterprise 2.0, it does not contradict all of the higher level Web 2.0 design patterns and business models. And in this way, the new Web 2.0 report from O'Reilly is quite effective and diligent in interweaving the story of Web 2.0 with the specific aspects of Enterprise 2.0. It includes discussions of self-service IT, the long tail of enterprise IT demand, and many other consequences of the Web 2.0 era in the enterprise. The report also makes many sensible recommendations around starting small with pilot projects and measuring results, among a fairly long list. [21]

How it works The client-side/web browser technologies typically used in Web 2.0 development are Asynchronous JavaScript and XML (Ajax), Adobe Flash and the Adobe Flex framework, and JavaScript/Ajax frameworks such as Yahoo! UI Library, Dojo Toolkit, MooTools, and jQuery. Ajax programming uses JavaScript to upload and download new data from the web server without undergoing a full page reload. To permit the user to continue to interact with the page, communications such as data requests going to the server are separated from data coming back to the page (asynchronously). Otherwise, the user would have to routinely wait for the data to come back before they can do anything else on that page, just as a user has to wait for a page to complete the reload. This also increases overall performance of the site, as the sending of requests can complete quicker independent of blocking and queueing required to send data back to the client. The data fetched by an Ajax request is typically formatted in XML or JSON (JavaScript Object Notation) format, two widely used structured data formats. Since both of these formats are natively understood by JavaScript, a programmer can easily use them to transmit structured data in their web application. When this data is received via Ajax, the JavaScript program then uses the Document Object Model (DOM) to dynamically update the web page based on the new data, allowing for a rapid and interactive user experience. In short, using these techniques, Web designers can make their pages function like desktop applications. For example, Google Docs uses this technique to create a Web-based word processor. Adobe Flex is another technology often used in Web 2.0 applications. Compared to JavaScript libraries like jQuery, Flex makes it easier for programmers to populate large data grids, charts, and other heavy user interactions.[22] Applications programmed in Flex, are compiled and displayed as Flash within the browser. As a widely available plugin independent of W3C (World Wide Web Consortium, the governing body of web standards and protocols), standards, Flash is capable of doing many things which are not currently possible in HTML, the language used to construct web pages. Of Flash's many capabilities, the most commonly used in Web 2.0 is its ability to play audio and video files. This has allowed for the creation of Web 2.0 sites where video media is seamlessly integrated with standard HTML. In addition to Flash and Ajax, JavaScript/Ajax frameworks have recently become a very popular means of creating Web 2.0 sites. At their core, these frameworks do not use technology any different from JavaScript, Ajax, and the DOM. What frameworks do is smooth over inconsistencies between web browsers and extend the functionality available to developers. Many of them also come with customizable, prefabricated 'widgets' that accomplish such common tasks as picking a date from a calendar, displaying a data chart, or making a tabbed panel. On the server side, Web 2.0 uses many of the same technologies as Web 1.0. Languages such as PHP, Ruby, ColdFusion, Perl, Python, JSP and ASP are used by developers to dynamically output data using information from files and databases. What has begun to change in Web 2.0 is the way this data is formatted. In the early days of the Internet, there was little need for different websites to communicate with each other and share data. In the new "participatory web", however, sharing data between sites has become an essential capability. To share its data with other sites, a web site must be able to generate output in machine-readable formats such as XML, RSS, and JSON. When a site's data is available in one of these formats, another website can use it to integrate a portion of that site's functionality into itself, linking the two together. When this design pattern is implemented, it ultimately leads to data

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that is both easier to find and more thoroughly categorized, a hallmark of the philosophy behind the Web 2.0 movement.

Usage The popularity of the term Web 2.0, along with the increasing use of blogs, wikis, and social networking technologies, has led many in academia and business to coin a flurry of 2.0s,[23] including Library 2.0,[24] Social Work 2.0,[25] Enterprise 2.0, PR 2.0,[26] Classroom 2.0, Publishing 2.0, Medicine 2.0, Telco 2.0, Travel 2.0, Government 2.0,[27] and even Porn 2.0.[28] Many of these 2.0s refer to Web 2.0 technologies as the source of the new version in their respective disciplines and areas. For example, in the Talis white paper "Library 2.0: The Challenge of Disruptive Innovation", Paul Miller argues Blogs, wikis and RSS are often held up as exemplary manifestations of Web 2.0. A reader of a blog or a wiki is provided with tools to add a comment or even, in the case of the wiki, to edit the content. This is what we call the Read/Write web.Talis believes that Library 2.0 means harnessing this type of participation so that libraries can benefit from increasingly rich collaborative cataloguing efforts, such as including contributions from partner libraries as well as adding rich enhancements, such as book jackets or movie files, to records from publishers and others.[29] Here, Miller links Web 2.0 technologies and the culture of participation that they engender to the field of library science, supporting his claim that there is now a "Library 2.0". Many of the other proponents of new 2.0s mentioned here use similar methods.

Web 3.0 Not much time passed before "Web 3.0" was coined. Definitions of Web 3.0 vary greatly. Amit Agarwal states that Web 3.0 is, among other things, about the Semantic Web and personalization.[30] Andrew Keen, author of The Cult of the Amateur [31], considers the Semantic Web an "unrealisable abstraction" and sees Web 3.0 as the return of experts and authorities to the Web. For example, he points to Bertelsman's deal with the German Wikipedia to produce an edited print version of that encyclopedia. Others still such as Manoj Sharma, an organization strategist, in the keynote "A Brave New World Of Web 3.0" proposes that Web 3.0 will be a "Totally Integrated World" cradle-to-grave experience of being always plugged onto the net.[32] CNN Money's Jessi Hempel expects Web 3.0 to emerge from new and innovative Web 2.0 services with a profitable business model.[33] Conrad Wolfram has argued that Web 3.0 is where "the computer is generating new information", rather than humans. [34]

Web-based applications and desktops Ajax has prompted the development of websites that mimic desktop applications, such as word processing, the spreadsheet, and slide-show presentation. WYSIWYG wiki sites replicate many features of PC authoring applications. In 2006 Google, Inc. acquired one of the best-known sites of this broad class, Writely.[35] Several browser-based "operating systems" have emerged, including EyeOS[36] and YouOS.[37] Although coined as such, many of these services function less like a traditional operating system and more as an application platform. They mimic the user experience of desktop operating-systems, offering features and applications similar to a PC environment, as well as the added ability of being able to run within any modern browser. However, these operating systems do not control the hardware on the client's computer. Numerous web-based application services appeared during the dot-com bubble of 1997â&#x20AC;&#x201C;2001 and then vanished, having failed to gain a critical mass of customers. In 2005, WebEx acquired one of the better-known of these, Intranets.com, for $45 million.[38]

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XML and RSS Advocates of "Web 2.0" may regard syndication of site content as a Web 2.0 feature, involving as it does standardized protocols, which permit end-users to make use of a site's data in another context (such as another website, a browser plugin, or a separate desktop application). Protocols which permit syndication include RSS (Really Simple Syndication — also known as "web syndication"), RDF (as in RSS 1.1), and Atom, all of them XML-based formats. Observers have started to refer to these technologies as "Web feed" as the usability of Web 2.0 evolves and the more user-friendly Feeds icon supplants the RSS icon. Specialized protocols Specialized protocols such as FOAF and XFN (both for social networking) extend the functionality of sites or permit end-users to interact without centralized websites. Other protocols, like XMPP enables services to users like Services over the Messenger

Web APIs Web 2.0 often uses machine-based interactions such as REST and SOAP. Often servers use proprietary APIs, but standard APIs (for example, for posting to a blog or notifying a blog update) have also come into wide use. Most communications through APIs involve XML or JSON payloads. Web Services Description Language (WSDL) is the standard way of publishing a SOAP API and there are a range of Web Service specifications. See also EMML by the Open Mashup Alliance for enterprise mashups.

Criticism Critics of the term claim that "Web 2.0" does not represent a new version of the World Wide Web at all, but merely continues to use so-called "Web 1.0" technologies and concepts. First, techniques such as AJAX do not replace underlying protocols like HTTP, but add an additional layer of abstraction on top of them. Second, many of the ideas of Web 2.0 had already been featured in implementations on networked systems well before the term "Web 2.0" emerged. Amazon.com, for instance, has allowed users to write reviews and consumer guides since its launch in 1995, in a form of self-publishing. Amazon also opened its API to outside developers in 2002.[39] Previous developments also came from research in computer-supported collaborative learning and computer-supported cooperative work and from established products like Lotus Notes and Lotus Domino, all phenomena which precede Web 2.0. But perhaps the most common criticism is that the term is unclear or simply a buzzword. For example, in a podcast interview,[4] Tim Berners-Lee described the term "Web 2.0" as a "piece of jargon": "Nobody really knows what it means...If Web 2.0 for you is blogs and wikis, then that is people to people. But that was what the Web was supposed to be all along."[4] Other critics labeled Web 2.0 “a second bubble” (referring to the Dot-com bubble of circa 1995–2001), suggesting that too many Web 2.0 companies attempt to develop the same product with a lack of business models. For example, The Economist has dubbed the mid- to late-2000s focus on Web companies "Bubble 2.0".[40] Venture capitalist Josh Kopelman noted that Web 2.0 had excited only 53,651 people (the number of subscribers at that time to TechCrunch, a Weblog covering Web 2.0 startups and technology news), too few users to make them an economically viable target for consumer applications.[41] Although Bruce Sterling reports he's a fan of Web 2.0, he thinks it is now dead as a rallying concept.[42] Critics have cited the language used to describe the hype cycle of Web 2.0[43] as an example of Techno-utopianist rhetoric.[44]

Web 2.0 In terms of Web 2.0's social impact, critics such as Andrew Keen argue that Web 2.0 has created a cult of digital narcissism and amateurism, which undermines the notion of expertise by allowing anybody, anywhere to share – and place undue value upon – their own opinions about any subject and post any kind of content, regardless of their particular talents, knowledgeability, credentials, biases or possible hidden agendas. He states that the core assumption of Web 2.0, that all opinions and user-generated content are equally valuable and relevant, is misguided and is instead "creating an endless digital forest of mediocrity: uninformed political commentary, unseemly home videos, embarrassingly amateurish music, unreadable poems, essays and novels", also stating that Wikipedia is full of "mistakes, half truths and misunderstandings".[45]

Trademark In November 2004, CMP Media applied to the USPTO for a service mark on the use of the term "WEB 2.0" for live events.[46] On the basis of this application, CMP Media sent a cease-and-desist demand to the Irish non-profit organization IT@Cork on May 24, 2006,[47] but retracted it two days later.[48] The "WEB 2.0" service mark registration passed final PTO Examining Attorney review on May 10, 2006, and was registered on June 27, 2006.[46] The European Union application (application number 004972212, which would confer unambiguous status in Ireland) was refused [49] on May 23, 2007.

See also • • • • • • • • • • • • • • • • •

Cloud computing Collective intelligence Consumer-generated media Enterprise social software Mashups New Media Office suite Open Mashup Alliance Open source governance Radical Trust Social commerce Social media Social shopping User-generated content Web 1.0 Web 2.0 for development (web2fordev) You (Time Person of the Year)

Application Domains • • • • • •

Business 2.0 E-learning 2.0 Government 2.0 Health 2.0 Library 2.0 Porn 2.0

225

Web 2.0

Resources Books Handbook of Research on Web 2.0, 3.0, and X.0: Technologies, Business, and Social Applications", San Murugesan (Editor)'http:/ / www. igi-global. com/ reference/ details. asp?id=34850'', Information Science Research, Hershey – New York, October 2009, ISBN 978-1-60566-384-5 Articles Understanding Web 2.0, San Murugesan, IEEE IT Professional, 2007, http:/ / www. computer. org/ portal/ web/ buildyourcareer/fa009

References [1] "Core Characteristics of Web 2.0 Services" (http:/ / www. techpluto. com/ web-20-services/ ). . [2] Paul Graham (November 2005). "Web 2.0" (http:/ / www. paulgraham. com/ web20. html). . Retrieved 2006-08-02. ""I first heard the phrase 'Web 2.0' in the name of the Web 2.0 conference in 2004."" [3] Tim O'Reilly (2005-09-30). "What Is Web 2.0" (http:/ / www. oreillynet. com/ pub/ a/ oreilly/ tim/ news/ 2005/ 09/ 30/ what-is-web-20. html). O'Reilly Network. . Retrieved 2006-08-06. [4] "DeveloperWorks Interviews: Tim Berners-Lee" (http:/ / www. ibm. com/ developerworks/ podcast/ dwi/ cm-int082206txt. html). 2006-07-28. . Retrieved 2007-02-07. [5] DiNucci, D. (1999). "Fragmented Future" (http:/ / www. cdinucci. com/ Darcy2/ articles/ Print/ Printarticle7. html). Print 53 (4): 32. . [6] Idehen, Kingsley. 2003. RSS: INJAN (It's not just about news). Blog. Blog Data Space. August 21 OpenLinksW.com (http:/ / www. openlinksw. com/ dataspace/ kidehen@openlinksw. com/ weblog/ kidehen@openlinksw. com's BLOG [127]/ 241) [7] Idehen, Kingsley. 2003. Jeff Bezos Comments about Web Services. Blog. Blog Data Space. September 25. OpenLinksW.com (http:/ / www. openlinksw. com/ blog/ ~kidehen/ index. vspx?id=373) [8] Knorr, Eric. 2003. The year of Web services. CIO, December 15. [9] ibid [10] O'Reilly, Tim, and John Battelle. 2004. Opening Welcome: State of the Internet Industry. In . San Francisco, CA, October 5. [11] O’Reilly, T., 2005. [12] Grossman, Lev. 2006. Person of the Year: You. December 25. Time.com (http:/ / www. time. com/ time/ covers/ 0,16641,20061225,00. html) [13] "'Millionth English Word' declared". NEWS.BBC.co.uk (http:/ / news. bbc. co. uk/ 1/ hi/ world/ americas/ 8092549. stm) [14] Dion Hinchcliffe (2006-04-02). "The State of Web 2.0" (http:/ / web2. wsj2. com/ the_state_of_web_20. htm). Web Services Journal. . Retrieved 2006-08-06. [15] Bart Decrem (2006-06-13). "Introducing Flock Beta 1" (http:/ / www. flock. com/ node/ 4500). Flock official blog. . Retrieved 2007-01-13. [16] Gerald Marwell and Ruth E. Ames: "Experiments on the Provision of Public Goods. I. Resources, Interest, Group Size, and the Free-Rider Problem". The American Journal of Sociology, Vol. 84, No. 6 (May, 1979), pp. 1335–1360 [17] Best, D., 2006. Web 2.0 Next Big Thing or Next Big Internet Bubble? Lecture Web Information Systems. Techni sche Universiteit Eindhoven. [18] Greenmeier, Larry and Gaudin, Sharon. "Amid The Rush To Web 2.0, Some Words Of Warning – Web 2.0 – InformationWeek" (http:/ / www. informationweek. com/ news/ management/ showArticle. jhtml;jsessionid=EWRPGLVJ53OW2QSNDLPCKHSCJUNN2JVN?articleID=199702353& _requestid=494050). www.informationweek.com. . Retrieved 2008-04-04. [19] O’Reilly, T., 2005. What is Web 2.0. Design Patterns and Business Models for the Next Generation of Software, 30, p.2005 [20] McAfee, A. (2006). Enterprise 2.0: The Dawn of Emergent Collaboration. MIT Sloan Management review. Vol. 47, No. 3, p. 21–28. [21] Blogs.ZDnet.com (http:/ / blogs. zdnet. com/ Hinchcliffe/ ?p=71) [22] Maraksquires.com (http:/ / maraksquires. com/ articles/ 2009/ 11/ 16/ javascript-jquery-versus-actionscript-flex-take-1/ ) [23] Schick, S., 2005. I second that emotion. IT Business.ca (Canada). [24] Miller, P., 2008. Library 2.0: The Challenge of Disruptive Innovation. Available at: Google.com (http:/ / www. google. com/ url?sa=t& source=web& ct=res& cd=1& url=http:/ / www. talis. com/ resources/ documents/ 447_Library_2_prf1. pdf& ei=qTUWSsWsH5SJtgeAlNTiDA& usg=AFQjCNER9YGaOBSyJFzz4lQ_GszwcQVqlw& sig2=_TunJ0srmZ8pio8Qzkzq-Q) [25] Singer, Jonathan B. (2009). The Role and Regulations for Technology in Social Work Practice and E-Therapy: Social Work 2.0. In A. R. Roberts (Ed). (http:/ / www. us. oup. com/ us/ catalog/ general/ subject/ SocialWork/ ~~/ dmlldz11c2EmY2k9OTc4MDE5NTM2OTM3Mw==). New York, U.S.A.: Oxford University Press. ISBN 978-0195369373. . [26] Breakenridge, D., 2008. PR 2.0: New Media, New Tools, New Audiences 1st ed., FT Press. [27] Eggers, William D. (2005). Government 2.0: Using Technology to Improve Education, Cut Red Tape, Reduce Gridlock, and Enhance Democracy (http:/ / www. manhattan-institute. org/ government2. 0/ ). Lanham MD, U.S.A.: Rowman & Littlefield Publishers, Inc.. ISBN 978-0742541757. .

226

Web 2.0 [28] Rusak, Sergey (2009). Web 2.0 Becoming An Outdated Term (http:/ / www. progressiveadvertiser. com/ web-2-0-becoming-an-outdated-term/ ). Boston, MA, U.S.A.: Progressive Advertiser. . [29] Miller 10–11 [30] Agarwal, Amit. "Web 3.0 concepts explained in plain English". Labnol.org (http:/ / www. labnol. org/ internet/ web-3-concepts-explained/ 8908/ ) [31] http:/ / www. amazon. com/ dp/ 0385520816 [32] Keen, Andrew. "Web 1.0 + Web 2.0 = Web 3.0." TypePad.com (http:/ / andrewkeen. typepad. com/ the_great_seduction/ 2008/ 04/ web-10-web-20-w. html) [33] Hempel, Jessi. "Web 2.0 is so over. Welcome to Web 3.0." CNN Money. CNN.com (http:/ / money. cnn. com/ 2009/ 01/ 07/ technology/ hempel_threepointo. fortune/ index. htm) [34] Conrad Wolfram on Communicating with apps in web 3.0 (http:/ / www. itpro. co. uk/ 621535/ q-a-conrad-wolfram-on-communicating-with-apps-in-web-3-0) IT PRO, 17 Mar 2010 [35] "Google buys Web word-processing technology" (http:/ / www. news. com/ 2100-1032_3-6048136. html). www.news.com. . Retrieved 2007-12-12. [36] "Can eyeOS Succeed Where Desktop.com Failed?" (http:/ / www. techcrunch. com/ 2006/ 11/ 27/ eyeos-open-source-webos-for-the-masses/ ). www.techcrunch.com. . Retrieved 2007-12-12. [37] "Tech Beat Hey YouOS! – BusinessWeek" (http:/ / www. businessweek. com/ the_thread/ techbeat/ archives/ 2006/ 03/ hey_youos. html). www.businessweek.com. . Retrieved 2007-12-12. [38] "PC World — WebEx Snaps Up Intranets.com" (http:/ / www. pcworld. com/ article/ id,122068-page,1/ article. html). www.pcworld.com. . Retrieved 2007-12-12. [39] Tim O'Reilly (2002-06-18). "Amazon Web Services API" (http:/ / www. oreillynet. com/ pub/ wlg/ 1707?wlg=yes). O'Reilly Network. . Retrieved 2006-05-27. [40] "Bubble 2.0" (http:/ / www. economist. com/ business/ displaystory. cfm?story_id=E1_QQNVDDS). The Economist. 2005-12-22. . Retrieved 2006-12-20. [41] Josh Kopelman (2006-05-11). "53,651" (http:/ / redeye. firstround. com/ 2006/ 05/ 53651. html). Redeye VC. . Retrieved 2006-12-21. [42] ""Bruce Sterling presenta il web 2.0"" (http:/ / www. lastampa. it/ multimedia/ multimedia. asp?p=1& IDmsezione=29& IDalbum=8558& tipo=VIDEO#mpos). "LASTAMPA.it". . [43] ""Gartner 2006 Emerging Technologies Hype Cycle" (http:/ / www. gartner. com/ it/ page. jsp?id=495475). . [44] ""Critical Perspectives on Web 2.0", Special issue of First Monday, 13(3), 2008. UIC.edu (http:/ / www. uic. edu/ htbin/ cgiwrap/ bin/ ojs/ index. php/ fm/ issue/ view/ 263/ showToc)". [45] ""Thinking is so over" (http:/ / technology. timesonline. co. uk/ tol/ news/ tech_and_web/ personal_tech/ article1874668. ece). London. . [46] USPTO serial number 78322306 (http:/ / tarr. uspto. gov/ servlet/ tarr?regser=serial& entry=78322306) [47] "O'Reilly and CMP Exercise Trademark on 'Web 2.0'" (http:/ / yro. slashdot. org/ article. pl?sid=06/ 05/ 26/ 1238245). Slashdot. 2006-05-26. . Retrieved 2006-05-27. [48] Nathan Torkington (2006-05-26). "O'Reilly's coverage of Web 2.0 as a service mark" (http:/ / radar. oreilly. com/ archives/ 2006/ 05/ more_on_our_web_20_service_mar. html). O'Reilly Radar. . Retrieved 2006-06-01. [49] http:/ / oami. europa. eu/ CTMOnline/ RequestManager/ en_Result?transition=ResultsDetailed& ntmark=& application=CTMOnline& bAdvanced=0& language=en& deno=& source=search_basic. jsp& idappli=004972212#

227

PHP

228

PHP

Usual file extensions

.php, .phtml .php5 .phps

Paradigm

imperative, object-oriented

Appeared in

1995

Rasmus Lerdorf

Developer

The PHP Group

Stable release

5.2.13 / 5.3.2 (5.2.13 February 25, 2010 / 5.3.2: March 4, 2010)

Typing discipline

Dynamic, weak

[1]

Major implementations Zend Engine, Roadsend PHP, Phalanger, Quercus [2], Project Zero, HipHop [1]

Influenced by

C, Perl, Java, C++, Tcl

Influenced

PHP4Delphi

Programming language C OS

Cross-platform

License

PHP License

Website

http:/ / www. php. net/

PHP: Hypertext Preprocessor is a widely used, general-purpose scripting language that was originally designed for web development to produce dynamic web pages. For this purpose, PHP code is embedded into the HTML source document and interpreted by a web server with a PHP processor module, which generates the web page document. As a general-purpose programming language, PHP code is processed by an interpreter application in command-line mode performing desired operating system operations and producing program output on its standard output channel. It may also function as a graphical application. PHP is available as a processor for most modern web servers and as standalone interpreter on most operating systems and computing platforms. PHP was originally created by Rasmus Lerdorf in 1995[1] and has been in continuous development ever since. The main implementation of PHP is now produced by The PHP Group and serves as the de facto standard for PHP as there is no formal specification.[3] PHP is free software released under the PHP License.

History

Rasmus Lerdorf, who wrote the original Common Gateway Interface component, Andi Gutmans and Zeev Suraski, who rewrote the parser that formed PHP 3

PHP

229 PHP originally stood for personal home page.[3] It began in 1994 as a set of Common Gateway Interface (CGI) binaries written in the C programming language by the Danish/Greenlandic programmer Rasmus Lerdorf.[4] [5] Lerdorf initially created these Personal Home Page Tools to replace a small set of Perl scripts he had been using to maintain his personal homepage. The tools were used to perform tasks such as displaying his rĂŠsumĂŠ and recording how much traffic his page was receiving.[3] He combined these binaries with his Form Interpreter to create PHP/FI, which had more functionality. PHP/FI included a larger implementation for the C programming language and could communicate with databases, enabling the building of simple, dynamic web applications. Lerdorf released PHP publicly on June 8, 1995, to accelerate bug location and improve the code.[6] This release was named PHP version 2 and already had the basic functionality that PHP has today. This included Perl-like variables, form handling, and the ability to embed HTML. The syntax was similar to Perl but was more limited, simpler, and less consistent.[3] Zeev Suraski and Andi Gutmans, two Israeli developers at the Technion IIT, rewrote the parser in 1997 and formed the base of PHP 3, changing the language's name to the recursive initialism PHP: Hypertext Preprocessor.[3] The development team officially released PHP/FI 2 in November 1997 after months of beta testing. Afterwards, public testing of PHP 3 began, and the official launch came in June 1998. Suraski and Gutmans then started a new rewrite of PHP's core, producing the Zend Engine in 1999.[7] They also founded Zend Technologies in Ramat Gan, Israel.[3] On May 22, 2000, PHP 4, powered by the Zend Engine 1.0, was released.[3] As of August 2008 this branch is up to version 4.4.9. PHP 4 is no longer under development nor will any security updates be released.[8] [9] On July 13, 2004, PHP 5 was released, powered by the new Zend Engine II.[3] PHP 5 included new features such as improved support for object-oriented programming, the PHP Data Objects extension (which defines a lightweight and consistent interface for accessing databases), and numerous performance enhancements.[10] In 2008 PHP 5 became the only stable version under development. Late static binding has been missing from PHP and has been added in version 5.3.[11] [12] A new major version has been under development alongside PHP 5 for several years. This version was originally planned to be released as PHP 6 as a result of its significant changes, which included plans for full Unicode support. However, Unicode support took developers much longer to implement than originally thought, and the decision was made in March 2010[13] to move the project to a branch, with features still under development moved to a trunk. Changes in the new code include the removal of register_globals,[14] magic quotes, and safe mode.[8] [15] The reason for the removals was that register_globals had given way to security holes, and magic quotes had an unpredictable nature, and was best avoided. Instead, to escape characters, magic quotes may be substituted with the addslashes() function, or more appropriately an escape mechanism specific to the database vendor itself like mysql_real_escape_string() for MySQL. Functions that will be removed in future versions and have been deprecated in PHP 5.3, will produce a warning if used.[16] Many high-profile open-source projects ceased to support PHP 4 in new code as of February 5, 2008, because of the GoPHP5 initiative[17] , provided by a consortium of PHP developers promoting the transition from PHP 4 to PHP 5.[18] [19] PHP currently does not have native support for Unicode or multibyte strings; Unicode support is under development for a future verson of PHP and will allow strings as well as class, method, and function names to contain non-ASCII characters.[20] [21] PHP interpreters are available on both 32-bit and 64-bit operating systems, but on Microsoft Windows the only official distribution is a 32-bit implementation, requiring Windows 32-bit compatibility mode while using Internet Information Services (IIS) on a 64-bit Windows platform. As of PHP 5.3.0, experimental 64-bit versions are available for MS Windows.[22]

PHP

230

Licensing PHP is free software released under the PHP License, which insists that: â&#x20AC;˘ The name "PHP" must not be used to endorse or promote products derived from this software without prior written permission.[23] This makes it incompatible with the GNU General Public License (GPL) because restrictions exist regarding the use of the term PHP.[24]

Release history Meaning Red

Release no longer supported

Green Release still supported Blue

Major version

Minor version

Release date

Future release

Notes

1.0.0

1995-06-08 Officially called "Personal Home Page Tools (PHP Tools)". This is the first use of the name "PHP".[3]

2.0.0

1997-11-01 Considered by its creator as the "fastest and simplest tool" for creating dynamic web pages.[3]

3.0.0

1998-06-06 Development moves from one person to multiple developers. Zeev Suraski and Andi Gutmans rewrite the [3] base for this version.

4.0.0

2000-05-22 Added more advanced two-stage parse/execute tag-parsing system called the Zend engine.[25]

4.1.0

2001-12-10 Introduced 'superglobals' ($_GET, $_POST, $_SESSION, etc.)[25]

4.2.0

2002-04-22 Disabled register_globals by default. Data received over the network is not inserted directly into the [25] global namespace anymore, closing possible security holes in applications.

4.3.0

2002-12-27 Introduced the CLI, in addition to the CGI.[25] [26]

4.4.0

2005-07-11 Added man pages for phpize and php-config scripts.[25]

4.4.8

2008-01-03 Several security enhancements and bug fixes. Was to be the end of life release for PHP 4. Security [27] updates only until 2008-08-08, if necessary.

4.4.9

2008-08-07 More security enhancements and bug fixes. The last release of the PHP 4.4 series.[28] [29]

PHP

231 5

php-trunk-dev

5.0.0

2004-07-13 Zend Engine II with a new object model.[30]

5.1.0

2005-11-24 Performance improvements with introduction of compiler variables in re-engineered PHP Engine.[30]

5.2.0

2006-11-02 Enabled the filter extension by default. Native JSON support.[30]

5.2.11

2009-09-16 Bug and security fixes.

5.2.12

2009-12-17 Over 60 bug fixes, including 5 security fixes.

5.2.13

2010-02-25 Bug and security fixes.

5.3.0

2009-06-30 Namespace support; Late static bindings, Jump label (limited goto), Native closures, Native PHP archives (phar), garbage collection for circular references, improved Windows support, sqlite3, mysqlnd as a replacement for libmysql as underlying library for the extensions that work with MySQL, fileinfo as a replacement for mime_magic for better MIME support, the Internationalization extension, and deprecation of ereg extension.

5.3.1

2009-11-19 Over 100 bug fixes [31], some of which were security fixes as well.

5.3.2

2010-03-04 Includes a large number of bug fixes. No date set

Unicode support; removal of 'register_globals', 'magic_quotes' and 'safe_mode'; Alternative PHP Cache

Usage PHP is a general-purpose scripting language that is especially suited to server-side web development where PHP generally runs on a web server. Any PHP code in a requested file is executed by the PHP runtime, usually to create dynamic web page content. It can also be used for command-line scripting and client-side GUI applications. PHP can be deployed on most web servers, many operating systems and platforms, and can be used with many relational database management systems. It is available free of charge, and the PHP Group provides the complete source code for users to build, customize and extend for their own use.[32] PHP primarily acts as a filter,[33] taking input from a file or stream containing text and/or PHP instructions and outputs another stream of data; most commonly the output will be HTML. Since PHP 4, the PHP parser compiles input to produce bytecode for processing by the Zend Engine, giving improved performance over its interpreter predecessor.[34] Originally designed to create dynamic web pages, PHP now focuses mainly on server-side scripting,[35] and it is similar to other server-side scripting languages that provide dynamic content from a web server to a client, such as Microsoft's Active Server Pages, Sun Microsystems' JavaServer Pages,[36] and mod_perl. PHP has also attracted the development of many frameworks that provide building blocks and a design structure to promote rapid application development (RAD). Some of these include CakePHP, Symfony, CodeIgniter, and Zend Framework, offering features similar to other web application frameworks. The LAMP architecture has become popular in the web industry as a way of deploying web applications. PHP is commonly used as the P in this bundle alongside Linux, Apache and MySQL, although the P may also refer to Python or Perl or some combination of the three. As of April 2007, over 20 million Internet domains had web services hosted on servers with PHP installed and mod_php was recorded as the most popular Apache HTTP Server module.[37] Significant websites are written in PHP including the user-facing portion of Facebook,[38] Wikipedia (MediaWiki),[39] Yahoo!, MyYearbook, Digg, Joomla, eZ Publish, WordPress,[40] YouTube in its early stages, Drupal, Tagged and Moodle[41] .

PHP

232

Security The National Vulnerability Database stores all vulnerabilities found in computer software. The overall proportion of PHP-related vulnerabilities on the database amounted to: 20% in 2004, 28% in 2005, 43% in 2006, 36% in 2007, 35% in 2008, and 30% in 2009.[42] Most of these PHP-related vulnerabilities can be exploited remotely: they allow crackers to steal or destroy data from data sources linked to the webserver (such as an SQL database), send spam or contribute to DoS attacks using malware, which itself can be installed on the vulnerable servers. These vulnerabilities are caused mostly by not following best practice programming rules: technical security flaws of the language itself or of its core libraries are not frequent (23 in 2008, about 1% of the total). [43] [44] Recognizing that programmers cannot be trusted, some languages include taint checking to detect automatically the lack of input validation which induces many issues. Such a feature is being developed for PHP,[45] but its inclusion in a release has been rejected several times in the past.[46] [47] Hosting PHP applications on a server requires a careful and constant attention to deal with these security risks.[48] There are advanced protection patches such as Suhosin and Hardening-Patch, especially designed for web hosting environments.[49]

Syntax <html> <head> <title>PHP Test</title> </head> <body> <?php echo "Hello World"; /* echo("Hello World"); works as well, although echo isn't a function (it's a language construct). In some cases, such as when multiple parameters are passed to echo, parameters cannot be enclosed in parentheses */ ?> </body> </html> PHP code embedded within HTML PHP only parses code within its delimiters. Anything outside its delimiters is sent directly to the output and is not processed by PHP (although non-PHP text is still subject to control structures described within PHP code). The most common delimiters are <?php to open and ?> to close PHP sections. <script language="php"> and </script> delimiters are also available, as are the shortened forms <? or <?= (which is used to echo back a string or variable) and ?> as well as ASP-style short forms <% or <%= and %>. While short delimiters are used, they make script files less portable as their purpose can be disabled in the PHP configuration [50], and so they are discouraged.[51] The purpose of all these delimiters is to separate PHP code from non-PHP code, including HTML.[52] The first form of delimiters, <?php and ?>, in XHTML and other XML documents, creates correctly formed XML 'processing instructions'.[53] This means that the resulting mixture of PHP code and other markup in the server-side file is itself well-formed XML.

PHP

233 Variables are prefixed with a dollar symbol and a type does not need to be specified in advance. Unlike function and class names, variable names are case sensitive. Both double-quoted ("") and heredoc strings allow the ability to embed a variable's value into the string.[54] PHP treats newlines as whitespace in the manner of a free-form language (except when inside string quotes), and statements are terminated by a semicolon.[55] PHP has three types of comment syntax: /* */ marks block and inline comments; // as well as # are used for one-line comments.[56] The echo statement is one of several facilities PHP provides to output text (e.g. to a web browser). In terms of keywords and language syntax, PHP is similar to most high level languages that follow the C style syntax. If conditions, for and while loops, and function returns are similar in syntax to languages such as C, C++, Java and Perl.

Data types PHP stores whole numbers in a platform-dependent range. This range is typically that of 32-bit signed integers. Unsigned integers are converted to signed values in certain situations; this behavior is different from other programming languages.[57] Integer variables can be assigned using decimal (positive and negative), octal, and hexadecimal notations. Floating point numbers are also stored in a platform-specific range. They can be specified using floating point notation, or two forms of scientific notation.[58] PHP has a native Boolean type that is similar to the native Boolean types in Java and C++. Using the Boolean type conversion rules, non-zero values are interpreted as true and zero as false, as in Perl and C++.[58] The null data type represents a variable that has no value. The only value in the null data type is NULL.[58] Variables of the "resource" type represent references to resources from external sources. These are typically created by functions from a particular extension, and can only be processed by functions from the same extension; examples include file, image, and database resources.[58] Arrays can contain elements of any type that PHP can handle, including resources, objects, and even other arrays. Order is preserved in lists of values and in hashes with both keys and values, and the two can be intermingled.[58] PHP also supports strings, which can be used with single quotes, double quotes, or heredoc syntax.[59] The Standard PHP Library (SPL) attempts to solve standard problems and implements efficient data access interfaces and classes.[60]

Functions PHP has hundreds of base functions and thousands more via extensions. These functions are well documented on the PHP site; however, the built-in library has a wide variety of naming conventions and inconsistencies. PHP currently has no functions for thread programming, although it does support multiprocess programming on POSIX systems.[61] 5.2 and earlier Functions are not first-class functions and can only be referenced by their name, directly or dynamically by a variable containing the name of the function.[62] User-defined functions can be created at any time without being prototyped.[62] Functions can be defined inside code blocks, permitting a run-time decision as to whether or not a function should be defined. Function calls must use parentheses, with the exception of zero argument class constructor functions called with the PHP new operator, where parentheses are optional. PHP supports quasi-anonymous functions through the create_function() function, although they are not true anonymous functions because anonymous functions are nameless, but functions can only be referenced by name, or indirectly through a variable $function_name();, in PHP.[62]

PHP

234 5.3 and newer PHP gained support for closures. True anonymous functions are supported using the following syntax: function getAdder($x) { return function ($y) use ($x) { return $x + $y; }; } $adder = getAdder(8); echo $adder(2); // prints "10"

Here, getAdder() function creates a closure using parameter $x (keyword "use" forces getting variable from context), which takes additional argument $y and returns it to the caller. Such a function can be stored, given as the parameter to other functions, etc. For more details see Lambda functions and closures RFC [63]. The goto flow control device was made available in PHP 5.3 and is used as follows: function lock() { $file = fopen("file.txt","r+"); retry: if(flock($file,LOCK_EX)) { fwrite($file, "Success!"); fclose($file); return 0; } else goto retry; }

When lock() is called, PHP opens a file and tries to lock it. retry:, the target label, defines the point to which execution should return if flock() is unsuccessful and the goto retry; is called. goto is not unrestricted and requires that the target label be in the same file and context.

Objects Basic object-oriented programming functionality was added in PHP 3 and improved in PHP 4.[3] Object handling was completely rewritten for PHP 5, expanding the feature set and enhancing performance.[64] In previous versions of PHP, objects were handled like value types.[64] The drawback of this method was that the whole object was copied when a variable was assigned or passed as a parameter to a method. In the new approach, objects are referenced by handle, and not by value. PHP 5 introduced private and protected member variables and methods, along with abstract classes and final classes as well as abstract methods and final methods. It also introduced a standard way of declaring constructors and destructors, similar to that of other object-oriented languages such as C++, and a standard exception handling model. Furthermore, PHP 5 added interfaces and allowed for multiple interfaces to be implemented. There are special interfaces that allow objects to interact with the runtime system. Objects implementing ArrayAccess can be used with array syntax and objects implementing Iterator or

PHP

235 IteratorAggregate can be used with the foreach language construct. There is no virtual table feature in the engine, so static variables are bound with a name instead of a reference at compile time.[65] If the developer creates a copy of an object using the reserved word clone, the Zend engine will check if a __clone() method has been defined or not. If not, it will call a default __clone() which will copy the object's properties. If a __clone() method is defined, then it will be responsible for setting the necessary properties in the created object. For convenience, the engine will supply a function that imports the properties of the source object, so that the programmer can start with a by-value replica of the source object and only override properties that need to be changed.[66] Basic example of object-oriented programming as described above: Class Person { public $first; public $last; public function __construct($f,$l) { $this->first = $f; $this->last = $l; } public function greeting() { return 'Hello, my name is ' . $this->first . ' ' . $this->last . '.'; // return 'Hello, my name is ' . $this->first . ' ' . $this->last . '.'; also works when not called from Person::greeting() } } $him = new Person('John','Smith'); $her = new Person('Sally','Davis'); echo $him->greeting(); // prints "Hello, my name is John Smith." echo '<br>'; echo $her->greeting(); // prints "Hello, my name is Sally Davis."

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236

Speed optimization PHP source code is compiled on-the-fly to an internal format that can be executed by the PHP engine.[67] [68] In order to speed up execution time and not have to compile the PHP source code every time the webpage is accessed, PHP scripts can also be deployed in executable format using a PHP compiler. Code optimizers aim to reduce the runtime of the compiled code by reducing its size and making other changes that can reduce the execution time with the goal of improving performance. The nature of the PHP compiler is such that there are often opportunities for code optimization,[69] and an example of a code optimizer is the eAccelerator PHP extension.[70] Another approach for reducing overhead for high load PHP servers is using an Opcode cache. Opcode caches work by caching the compiled form of a PHP script (opcodes) in shared memory to avoid the overhead of parsing and compiling the code every time the script runs. An opcode cache, APC, will be built into an upcoming release of PHP.[71] Opcode caching is also available in Zend Server Community Edition.

Compilers The PHP language was originally implemented using a PHP interpreter. Several compilers now exist, which decouple the PHP language from the interpreter: • phc [72] - a C++ based compiler for PHP, using the Zend run-time for maximum compatibility • Roadsend - achieves native compilation by compiling to bigloo scheme, which in turn is compiled to C, then to machine code • Raven [73] - a rewrite of Roadsend PHP (rphp), based on LLVM and a new C++ runtime • Phalanger - compiles source code written in the PHP scripting language into CIL byte-code • Caucho Resin/Quercus [74] - compiles PHP to Java bytecode • HipHop - developed at Facebook and now available as open source, transforms the PHP Script into C++, then compiles it. Advantages of compilation include not only better execution speed, but also obfuscation, static analysis, and improved interoperability with code written in other languages.[75]

Resources PHP includes free and open source libraries with the core build. PHP is a fundamentally Internet-aware system with modules built in for accessing FTP servers, many database servers, embedded SQL libraries such as embedded PostgreSQL, MySQL and SQLite, LDAP servers, and others. Many functions familiar to C programmers such as those in the stdio family are available in the standard PHP build.[76] PHP allows developers to write extensions in C to add functionality to the PHP language. These can then be compiled into PHP or loaded dynamically at runtime. Extensions have been written to add support for the Windows API, process management on Unix-like operating systems, multibyte strings (Unicode), cURL, and several popular compression formats. Some more unusual features include integration with Internet Relay Chat, dynamic generation of images and Adobe Flash content, and even speech synthesis. The PHP Extension Community Library (PECL) project is a repository for extensions to the PHP language.[77] Zend provides a certification exam for programmers to become certified PHP developers.

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237

See also • • • • • • • • • • • •

Active Server Pages Comparison of programming languages Comparison of web application frameworks EasyPHP (The first WebServer for PHP) LAMP (software bundle) List of PHP editors PHP accelerator Template processor XAMPP (WebServer for PHP) Zend Certified Engineer Zend Framework Zend Server Community Edition

External links • The PHP Group [78] • PHP [79] at the Open Directory Project • PHP Reference Manual [80] • PHP CLI (Command Line Interface) web site [81]

References [1] Rasmus Lerdorf began assembling C code originally written for CGI scripts into a library and accessing the library's functions, including SQL queries, through HTML-embedded commands in 1994; by 1995 the commands had taken the shape of PHP code that would be familiar of users of the language today. Lerdorf, Rasmus (2007-04-26). "PHP on Hormones - history of PHP presentation by Rasmus Lerdorf given at the MySQL Conference in Santa Clara, California" (http:/ / itc. conversationsnetwork. org/ shows/ detail3298. html#) (mp3). The Conversations Network. . Retrieved 2009-12-11. "Every day I would change the language drastically, and it didn't take very long, so by 1995, mid-1995 or so, PHP looked like this (http:/ / talks. php. net/ show/ mysql07key/ 4). This isn't that far from what PHP looks like today, actually." [2] http:/ / quercus. caucho. com/ [3] "History of PHP and related projects" (http:/ / www. php. net/ history). The PHP Group. . Retrieved 2008-02-25. [4] Lerdorf, Rasmus (2007-04-26). "PHP on Hormones" (http:/ / itc. conversationsnetwork. org/ shows/ detail3298. html) (mp3). The Conversations Network. . Retrieved 2009-06-22. [5] Lerdorf, Rasmus (2007). "Slide 3" (http:/ / talks. php. net/ show/ mysql07key/ 3). slides for 'PHP on Hormones' talk. The PHP Group. . Retrieved 2009-06-22. [6] Lerdorf, Rasmus (1995-06-08). "Announce: Personal Home Page Tools (PHP Tools)". [news:comp.infosystems.www.authoring.cgi comp.infosystems.www.authoring.cgi]. (Web link) (http:/ / groups. google. com/ group/ comp. infosystems. www. authoring. cgi/ msg/ cc7d43454d64d133). Retrieved on 2006-09-17. [7] "[[Zend Engine (http:/ / www. zend. com/ zend/ zend-engine-summary. php)] version 2.0: Feature Overview and Design"]. Zend Technologies Ltd.. . Retrieved 2006-09-17. [8] "php.net 2007 news archive" (http:/ / www. php. net/ archive/ 2007. php). The PHP Group. 2007-07-13. . Retrieved 2008-02-22. [9] Kerner, Sean Michael (2008-02-01). "PHP 4 is Dead—Long Live PHP 5" (http:/ / www. internetnews. com/ dev-news/ article. php/ 3725291). InternetNews. . Retrieved 2008-03-16. [10] Trachtenberg, Adam (2004-07-15). "Why PHP 5 Rocks!" (http:/ / www. onlamp. com/ pub/ a/ php/ 2004/ 07/ 15/ UpgradePHP5. html). O'Reilly. . Retrieved 2008-02-22. [11] "Late Static Binding in PHP" (http:/ / www. digitalsandwich. com/ archives/ 53-Late-Static-Binding-in-PHP. html). Digital Sandwich. 2006-02-23. . Retrieved 2008-03-25. [12] "Static Keyword" (http:/ / www. php. net/ language. oop5. static). The PHP Group. . Retrieved 2008-03-25. [13] "PHP 6" (http:/ / news. php. net/ php. internals/ 47120). The PHP project. . Retrieved 2010-03-27. [14] "Using Register Globals" (http:/ / www. php. net/ register_globals). PHP. . Retrieved 2008-04-04. [15] "Prepare for PHP 6" (http:/ / www. corephp. co. uk/ archives/ 19-Prepare-for-PHP-6. html). CorePHP. 2005-11-23. . Retrieved 2008-03-24. [16] "PHP 5.3 migration guide" (http:/ / www. php. net/ migration53). The PHP project. . Retrieved 2009-07-03. [17] "GoPHP5" (http:/ / www. gophp5. org/ projects). .

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238 [18] GoPHP5. "PHP projects join forces to Go PHP 5" (http:/ / gophp5. org/ sites/ gophp5. org/ files/ press_release. pdf) (PDF). GoPHP5 Press Release. . Retrieved 2008-02-23. [19] "GoPHP5" (http:/ / gophp5. org/ ). GoPHP5. . Retrieved 2008-02-22. [20] "Unicode" (http:/ / www. php. net/ ~derick/ meeting-notes. html#unicode). The PHP Group. . Retrieved 2008-03-25. [21] Byfield, Bruce (February 28, 2007). "Upcoming PHP release will offer Unicode support" (http:/ / www. linux. com/ archive/ feature/ 60386). linux.com. . Retrieved 2009-06-23. [22] The PHP Group. "PHP For Windows snapshots" (http:/ / windows. php. net/ snapshots/ ). PHP Windows Development Team. . Retrieved 2009-05-25. [23] The PHP License, version 3.01 (http:/ / www. php. net/ license/ 3_01. txt) [24] "GPL-Incompatible, Free Software Licenses" (http:/ / www. fsf. org/ licensing/ education/ licenses/ index_html/ #GPLIncompatibleLicenses). Various Licenses and Comments about Them. Free Software Foundation. . Retrieved 2008-02-22. [25] "PHP: PHP 4 ChangeLog" (http:/ / www. php. net/ ChangeLog-4. php). The PHP Group. 2008-01-03. . Retrieved 2008-02-22. [26] "PHP: Using PHP from the command line - Manual:" (http:/ / us3. php. net/ manual/ en/ features. commandline. php). The PHP Group. . Retrieved 2009-09-11. [27] "4.4.8 Release Announcement" (http:/ / www. php. net/ releases/ 4_4_8. php). PHP. 2008-08-08. . Retrieved 2009-07-29. [28] "Downloads" (http:/ / www. php. net/ downloads. php#v4). PHP. . Retrieved 2009-07-29. [29] "4.4.9 Release Announcement" (http:/ / www. php. net/ releases/ 4_4_9. php). PHP. . Retrieved 2009-07-29. [30] "PHP: PHP 5 ChangeLog" (http:/ / www. php. net/ ChangeLog-5. php). The PHP Group. 2007-11-08. . Retrieved 2008-02-22. [31] http:/ / www. php. net/ ChangeLog-5. php#5. 3. 1 [32] "Embedding PHP in HTML" (http:/ / www. onlamp. com/ pub/ a/ php/ 2001/ 05/ 03/ php_foundations. html). O'Reilly. 2001-05-03. . Retrieved 2008-02-25. [33] (http:/ / web. archive. org/ web/ 20080611231433/ http:/ / web. archive. org/ web/ 20080611231433/ http:/ / gtk. php. net/ manual1/ it/ html/ intro. whatis. php. whatdoes. html) at the Wayback Machine [34] "PHP and MySQL" (http:/ / cs. ua. edu/ 457/ Notes/ PHP and MySQL. ppt). University of Alabama. . Retrieved 2008-02-25. [35] "PHP Server-Side Scripting Language" (http:/ / webmaster. iu. edu/ PHPlanguage/ index. shtml). Indiana University. 2007-04-04. . Retrieved 2008-02-25. [36] "JavaServer Pages Technology — JavaServer Pages Comparing Methods for Server-Side Dynamic Content White Paper" (http:/ / java. sun. com/ products/ jsp/ jspservlet. html). Sun Microsystems. . Retrieved 2008-02-25. [37] "PHP: PHP Usage Stats" (http:/ / www. php. net/ usage. php). SecuritySpace. 2007-04-01. . Retrieved 2008-02-24. [38] "PHP and Facebook | Facebook" (http:/ / blog. facebook. com/ blog. php?post=2356432130). Blog.facebook.com. . Retrieved 2009-07-29. [39] "Manual:Installation requirements#PHP" (http:/ / www. mediawiki. org/ w/ index. php?title=Manual:Installation_requirements& oldid=299556#PHP). MediaWiki. 2010-01-25. . Retrieved 2010-02-26. "PHP is the programming language in which MediaWiki is written [...]" [40] "About WordPress" (http:/ / wordpress. org/ about/ ). . Retrieved 2010-02-26. "WordPress was [...] built on PHP" [41] "Moodle - About" (http:/ / docs. moodle. org/ en/ About_Moodle). Moodle.org. . Retrieved 2009-12-20. [42] "PHP-related vulnerabilities on the National Vulnerability Database" (http:/ / www. coelho. net/ php_cve. html). 2008-03-01. . [43] "Security and... Driving? (and Hiring) - Sean Coates: PHP, Web (+Beer)" (http:/ / seancoates. com/ security-and-driving-and-hiring). Sean Coates. . Retrieved 2009-07-29. [44] Computerworlduk.com (http:/ / www. computerworlduk. com/ toolbox/ open-source/ blogs/ index. cfm?entryid=533& blogid=14), Interview: Ivo Jansch, February 26, 2008 [45] "PHP Taint Mode RFC" (http:/ / wiki. php. net/ rfc/ taint). . [46] "Developer Meeting Notes, Nov. 2005" (http:/ / www. php. net/ ~derick/ meeting-notes. html#sand-boxing-or-taint-mode). . [47] "Taint mode decision, Nov 2007" (http:/ / devzone. zend. com/ article/ 2798-Zend-Weekly-Summaries-Issue-368#Heading1). . [48] "The Power of PHP, both Good and Evil" (http:/ / www. cwihosting. com/ php_security. php). 2009-02-28. . [49] "Hardened-PHP Project" (http:/ / www. hardened-php. net). 2008-08-15. . [50] http:/ / wiki. php. net/ rfc/ shortags [51] "PHP: Basic syntax" (http:/ / www. php. net/ manual/ en/ language. basic-syntax. php). The PHP Group. . Retrieved 2008-02-22. [52] "Your first PHP-enabled page" (http:/ / www. php. net/ manual/ en/ tutorial. firstpage. php). The PHP Group. . Retrieved 2008-02-25. [53] Bray, Tim; et al (26 November 2008). "Processing Instructions" (http:/ / www. w3. org/ TR/ 2008/ REC-xml-20081126/ #sec-pi). Extensible Markup Language (XML) 1.0 (Fifth Edition). W3C. . Retrieved 2009-06-18. [54] "Variables" (http:/ / www. php. net/ manual/ en/ language. variables. php). The PHP Group. . Retrieved 2008-03-16. [55] "Instruction separation" (http:/ / www. php. net/ basic-syntax. instruction-separation). The PHP Group. . Retrieved 2008-03-16. [56] "Comments" (http:/ / www. php. net/ manual/ en/ language. basic-syntax. comments. php). The PHP Group. . Retrieved 2008-03-16. [57] "Integers in PHP, running with scissors, and portability" (http:/ / www. mysqlperformanceblog. com/ 2007/ 03/ 27/ integers-in-php-running-with-scissors-and-portability/ ). MySQL Performance Blog. March 27, 2007. . Retrieved 2007-03-28. [58] "Types" (http:/ / www. php. net/ manual/ en/ language. types. php). The PHP Group. . Retrieved 2008-03-16. [59] "Strings" (http:/ / www. php. net/ manual/ en/ language. types. string. php). The PHP Group. . Retrieved 2008-03-21. [60] "SPL — StandardPHPLibrary" (http:/ / www. php. net/ spl). PHP.net. March 16, 2009. . Retrieved 2009-03-16. [61] "PHP.NET: Process Control" (http:/ / nz. php. net/ manual/ en/ book. pcntl. php). . Retrieved 2009-08-06.

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239 [62] "Functions" (http:/ / www. php. net/ manual/ en/ language. functions. php). The PHP Group. . Retrieved 2008-03-16. [63] http:/ / wiki. php. net/ rfc/ closures [64] "PHP 5 Object References" (http:/ / mjtsai. com/ blog/ 2004/ 07/ 15/ php-5-object-references/ ). mjtsai. . Retrieved 2008-03-16. [65] "Classes and Objects (PHP 5)" (http:/ / www. php. net/ zend-engine-2. php). The PHP Group. . Retrieved 2008-03-16. [66] "Object cloning" (http:/ / www. php. net/ language. oop5. cloning). The PHP Group. . Retrieved 2008-03-16. [67] "How do computer languages work?" (http:/ / www. linux-tutorial. info/ modules. php?name=Howto& pagename=Unix-and-Internet-Fundamentals-HOWTO/ languages. html). . Retrieved 2009-11-04. [68] (Gilmore 2006, p.Â 43) [69] "PHP Accelerator 1.2 (page 3, Code Optimisation)" (http:/ / www. php-accelerator. co. uk/ PHPA_Article. pdf) (PDF). Nick Lindridge. . Retrieved 2008-03-28. [70] "eAccelerator" (http:/ / eaccelerator. net/ ). . Retrieved 2009-09-18. [71] "Upcoming PHP6 Additions & Changes" (http:/ / davidwalsh. name/ php6). . Retrieved 2009-09-18. [72] http:/ / www. phpcompiler. org/ [73] http:/ / code. roadsend. com/ rphp/ [74] http:/ / www. theserverside. com/ news/ thread. tss?thread_id=38144 [75] Owlient.eu (http:/ / technow. owlient. eu/ index. php?post/ 2010/ 02/ 20/ php-compilers) [76] "PHP Function List" (http:/ / www. php. net/ quickref. php). The PHP Group. . Retrieved 2008-02-25. [77] "Developing Custom PHP Extensions" (http:/ / www. devnewz. com/ 090902b. html). devnewz. 2002-09-09. . Retrieved 2008-02-25. [78] http:/ / www. php. net/ [79] http:/ / www. dmoz. org/ Computers/ Programming/ Languages/ PHP/ / [80] http:/ / www. php. net/ manual [81] http:/ / www. php-cli. com/

Active Server Pages

240

Active Server Pages Developer(s)

Microsoft

Stable release

3.0 (no further versions planned)

Type

Web application framework

License

Proprietary

Active Server Pages (ASP), also known as Classic ASP or ASP Classic, was Microsoft's first server-side script engine for dynamically-generated web pages. Initially released as an add-on to Internet Information Services (IIS) via the Windows NT 4.0 Option Pack, it was subsequently included as a free component of Windows Server (since the initial release of Windows 2000 Server). It has now been superseded by ASP.NET. Developing functionality in ASP websites is enabled by the active scripting engine's support of the Component Object Model (COM), with each object providing a related group of frequently-used functions and data attributes. In ASP 2.0 there were six built-in objects: Application, ASPError, Request, Response, Server, and Session. Session, for example, is a cookie-based session object that maintains the state of variables from page to page. Functionality is further extended by objects which, when instantiated, provide access to the environment of the web server; as an example FileSystemObject (FSO) is used to create, read, update and delete files. Web pages with the .asp file extension use ASP, although some Web sites disguise their choice of scripting language for security purposes (e.g. still using the more common .htm or .html extension). Pages with the .aspx extension are ASP.NET (based on Microsoft's .NET Framework) and compiled, which makes them faster and more robust than server-side scripting in ASP which is interpreted at run-time; however, many ASP.NET pages still include some ASP scripting. Such marked differences between ASP and ASP.NET have led to the terms Classic ASP or ASP Classic being used, which also implies some nostalgia for the simpler platform. Most ASP pages are written in VBScript, but any other Active Scripting engine can be selected instead by using the @Language directive or the <script language="language" runat="server"> syntax. JScript (Microsoft's implementation of ECMAScript) is the other language that is usually available. PerlScript (a derivative of Perl) and others are available as third-party installable Active Scripting engines.

History Based on the dbWeb and iBasic tools, created by Aspect Software Engineering, ASP was one of the first web application development environments that integrated web application execution directly into the web server, 9 months after the release of NeXT's (now Apple) WebObjects. This was done in order to achieve high performance compared to calling external executable programs or CGI scripts which were the most popular method for writing web applications at the time. Prior to Microsoft's release of ASP for IIS 3, web programmers working in IIS relied on IDC and HTX files combined with ODBC drivers to display and manipulate dynamic data and pages. The basics of these file formats and structures were used, at least in part, in the implementation of the early versions of ASP. Halcyon InstantASP (iASP) and Chili!Soft ASP are third-party products that run ASP on platforms other than the Microsoft Windows operating systems. Neither alternative to real ASP fully emulates every feature, and may require additional components with which traditional ASP has no issues, such as database connectivity. MS access database support is a particular issue on non-Windows systems. iASP is able to use the VBScript and JScript languages unlike Chili!Soft ASP which uses JScript. Microsoft's ASP can use both and has the potential to have other languages make use of the scripting engine. iASP was written in

Active Server Pages Java, and as such will run on almost any operating system. iASP appears to be no longer available or at least hard to find. Examples of other languages available are Perl and TCL, although they are not as widely known or used for ASP scripting. There is an Apache Webserver mod that runs an ASP-like Perl script language.[1] Chili!Soft, initially released in 1997, was acquired by Cobalt Networks on May 24, 2000. Cobalt Networks subsequently was purchased by Sun Microsystems on December 7, 2000. Chili!Soft was renamed "Sun ONE Active Server Pages", then later renamed to "Sun Java System Active Server Pages". Chilisoft ASP was written in C/C++ and is tied rather tightly to specific web server versions. According to Sun "Sun Java System Active Server Pages has entered its End Of Life".[2]

Versions ASP has gone through three major releases: • ASP version 1.0 (distributed with IIS 3.0) in December 1996 • ASP version 2.0 (distributed with IIS 4.0) in September 1997 • ASP version 3.0 (distributed with IIS 5.0) in November 2000 ASP 3.0 is currently available in IIS 6.0 on Windows Server 2003 and IIS 7.0 on Windows Server 2008. ASP.NET is often confused as the newest release of ASP, but the technologies are very different. ASP.NET relies on the .NET Framework and is a compiled language, whereas ASP is strictly an interpreted scripting language. The move from ASP 2.0 to ASP 3.0 was a relatively modest one. One of the most important additions was the Server.Execute [3] methods, as well as the ASPError object [4].[5] Microsoft's What's New in IIS 5.0 [6] lists some additional changes. There are solutions to run "Classic ASP" sites as standalone applications, such as ASPexplore, a software package that runs Microsoft Active Server Pages offline.

Sample usage Any scripting languages compatible with Microsoft's Active Scripting standard may be used in ASP. The default scripting language (in classic ASP) is VBScript: <html> <body> <% Response.Write "Hello World!" %> </body> </html> Or in a simpler format <html> <body> <%= "Hello World!" %> </body> </html> The examples above print "Hello World!" into the body of an HTML document. Here's an example of how to connect to an Access Database

241

Active Server Pages <% Set oConn = Server.CreateObject("ADODB.Connection") oConn.Open "DRIVER={Microsoft Access Driver (*.mdb)}; DBQ=" & Server.MapPath("DB.mdb") Set rsUsers = Server.CreateObject("ADODB.Recordset") rsUsers.Open "SELECT UserID FROM Users", oConn,1,3 %>

See also • Template processor • ASP.NET

External links • ASP on MSDN [7]

References [1] [2] [3] [4] [5] [6] [7]

"Apache::ASP" (http:/ / www. apache-asp. org/ ). Chamas Enterprises Inc. . Retrieved 2009-01-08. "Sun Java System Active Server Pages" (http:/ / www. sun. com/ software/ chilisoft/ ). Sun Microsystems. . Retrieved 2008-12-31. http:/ / msdn. microsoft. com/ library/ default. asp?url=/ library/ en-us/ iissdk/ html/ db562da1-d49d-4fe5-9747-64ef530de23f. asp http:/ / msdn. microsoft. com/ library/ default. asp?url=/ library/ en-us/ iissdk/ html/ 541697df-5fb9-40a4-8fa1-380b4717cbf1. asp 4 Guys From Rolla's A Look at ASP 3.0 (http:/ / www. 4guysfromrolla. com/ webtech/ 010700-1. shtml) http:/ / www. microsoft. com/ technet/ prodtechnol/ windows2000serv/ reskit/ iisbook/ c01_programmability. mspx?mfr=true http:/ / msdn. microsoft. com/ en-us/ library/ aa286483. aspx

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Article Sources and Contributors

Article Sources and Contributors Programming language Source: http://en.wikipedia.org/w/index.php?oldid=358748749 Contributors: -Barry-, 10.7, 151.203.224.xxx, 16@r, 198.97.55.xxx, 199.196.144.xxx, 203.37.81.xxx, 212.188.19.xxx, 2988, 96.186, AJim, Abednigo, Abeliavsky, Abram.carolan, Acacix, Acaciz, AccurateOne, Addicted2Sanity, Ahoerstemeier, Ahy1, Akadruid, Alansohn, Alex, AlexPlank, Alhoori, Alksub, Allan McInnes, Alliswellthen, Altenmann, Amire80, Ancheta Wis, AndonicO, Andre Engels, Andres, Andylmurphy, Angela, Angusmclellan, Antonielly, Ap, Apwestern, ArmadilloFromHell, AstroNomer, Autocratique, Avono, B4hand, Behnam, Beland, Ben Standeven, Benjaminct, Bevo, Bh3u4m, BigSmoke, Bill122, BirgitteSB, Blaisorblade, Blanchardb, Bobblewik, Bobo192, Bonaovox, Booyabazooka, Borislav, Brandon, Brentdax, Brianjd, Brick Thrower, Brion VIBBER, Bubba73, Burkedavis, CSProfBill, Calltech, Can't sleep, clown will eat me, CanisRufus, Capricorn42, CarlHewitt, Catgut, Centrx, Charlesriver, Charlie Huggard, Chillum, Chinabuffalo, Chun-hian, Cireshoe, Ckatz, Closedmouth, Cmichael, Cobaltbluetony, ColdFeet, Conor123777, Conversion script, Cp15, Cybercobra, DMacks, DVD R W, Damieng, Dan128, Danakil, Dave Bell, David.Monniaux, DavidHOzAu, Davidfstr, Davidpdx, Dcoetzee, DeadEyeArrow, DenisMoskowitz, DennisDaniels, DerHexer, Derek Ross, Derek farn, Diego Moya, Dolfrog, Dominator09, Don't Copy That Floppy, Donhalcon, Doradus, DouglasGreen, Dtaylor1984, Duke Ganote, Dysprosia, EJF, ESkog, EagleOne, Edward301, Eivind F Øyangen, ElAmericano, Elembis, EncMstr, EngineerScotty, Epbr123, Esap, Evercat, Everyking, Ewlyahoocom, Ezrakilty, Fantom, Faradayplank, Fayt82, Fieldday-sunday, Finlay McWalter, Fl, Foobah, Four Dog Night, Fplay, Fredrik, Friedo, Fubar Obfusco, Funandtrvl, FvdP, Gaius Cornelius, Galoubet, Gazpacho, Giftlite, Giorgios, Gioto, Goodgerster, Gploc, Green caterpillar, GregAsche, Grin, Gurch, Gutza, Hadal, Hairy Dude, Hammer1980, Hans Adler, HarisM, Harmil, Hayabusa future, Headbomb, HeikoEvermann, HenryLi, Hfastedge, HopeChrist, Hoziron, Hut 8.5, Hyad, INkubusse, IanOsgood, Icey, Ideogram, Ilario, Imran, Indon, Infinoid, Iwantitalllllllll, Ixfd64, J.delanoy, JMSwtlk, JPINFV, JaK81600, Jason5ayers, Jaxad0127, Jaxl, Jeffrey Mall, Jeltz, Jeronimo, Jerryobject, Jitse Niesen, Jj137, Johann Wolfgang, John254, JohnLai, JohnWittle, Jonik, Jorend, Jossi, Joyous!, Jpbowen, Jpk, JulesH, Juliancolton, Jusjih, Jwissick, K.lee, K12308025, KHaskell, KSmrq, KTC, Karingo, Kbdank71, Kbh3rd, Kedearian, Ketiltrout, Kickstart70, Kiminatheguardian, Kinema, Klasbricks, KnowledgeOfSelf, Knyf, Koyaanis Qatsi, Krauss, Krawi, Kris Schnee, Krischik, Kuciwalker, Kungfuadam, Kwertii, KymFarnik, L Gottschalk, L33tminion, LC, Lagalag, Leibniz, Liao, Lightmouse, Ligulem, LindsayH, LinguistAtLarge, LordCo Centre, Lradrama, Lulu of the Lotus-Eaters, Luna Santin, Lupo, MER-C, MK8, Mac c, Macaldo, Macrakis, Magnus Manske, Mahanga, Malcolm Farmer, Malleus Fatuorum, Mangojuice, Manpreett, Marcoscramer, Mark Renier, MartinHarper, MartyMcGowan, Marudubshinki, Matthew Woodcraft, Mattisse, Mav, Maxis ftw, McSly, Mccready, MearsMan, MegaHasher, Mellum, Mendaliv, Merbabu, Merphant, Michael Hardy, Michael Zimmermann, Mike Rosoft, Minesweeper, MisterCharlie, Miym, Mkdw, Monz, Mpils, Mpradeep, Mrjeff, Ms2ger, Mschel, Murray Langton, Mwaisberg, Mxn, N5iln, Naderi 8189, Nameneko, Nanshu, Napi, Natalie Erin, Natkeeran, NawlinWiki, Necklace, NewbieDoo, Nick125, Nikai, Ningauble, Nixdorf, Noisy, Noldoaran, Noosentaal, NotQuiteEXPComplete, Nottsadol, Novasource, Ntalamai, Nuggetboy, Oblivious, Ohms law, Ohnoitsjamie, Oldadamml, Oleg Alexandrov, Omphaloscope, Orderud, OrgasGirl, Ossmann, Papercutbiology, Paul August, PaulFord, Peter, Phil Sandifer, PhilKnight, Phyzome, Pieguy48, Piet Delport, PlayStation 69, Pohta ce-am pohtit, Poor Yorick, Pooryorick, Positron, Prolog, Ptk, Pumpie, Pwv1, Quagmire, Quiddity, Quota, Quuxplusone, RainerBlome, Raise exception, Ranafon, RedWolf, Reddi, Reelrt, Reinis, RenamedUser2, Revived, RexNL, Rich Farmbrough, Rjstott, Rjwilmsi, Rlee0001, Robbe, Robert A West, Robert Skyhawk, Roland2, Romanm, Ronhjones, Roux, Royboycrashfan, Rrburke, Rursus, Rushyo, Russell Joseph McCann, Ruud Koot, S.Örvarr.S, Saccade, Sam Korn, Science History, SeeAnd, Sekelsenmat, Sgbirch, Shadowjams, Shane A. Bender, Shanes, ShelfSkewed, SimonP, Simplyanil, Sjakkalle, Skytreader, Slaad, Slakr, Slashem, SmartBee, Specs112, Speed Air Man, SpeedyGonsales, Speuler, SpuriousQ, Stephen B Streater, Stephenb, Stevertigo, SubSeven, Suffusion of Yellow, Suruena, Swirsky, Switchercat, Systemetsys, TakuyaMurata, Tarret, Taxman, Techman224, Tedickey, Template namespace initialisation script, Teval, Tewy, Tgr, The Thing That Should Not Be, Thniels, Thv, Tiddly Tom, Tim Starling, Timhowardriley, Tizio, Tobias Bergemann, Tony1, TonyClarke, Torc2, Toussaint, TuukkaH, Tysto, Ubiq, Ulric1313, Ultra two, Undeference, Useight, Vadmium, Vahid83, Vaibhavkanwal, Vald, VampWillow, VictorAnyakin, Vivin, Vkhaitan, Vriullop, Vsion, WAS 4.250, Waterfles, Wavelength, Wiki alf, WikiTome, Wikibob, Wikibofh, Wikisedia, Wimt, Windharp, Wlievens, Wmahan, Woohookitty, Ww, Xaosflux, Xavier Combelle, Yana209, Yath, Yk Yk Yk, Yoric, Zaheen, Zarniwoot, Zawersh, ZeWrestler, Zero1328, Zoicon5, Zondor, ²¹², ‫یرون سارائ‬, 775 anonymous edits Computer software Source: http://en.wikipedia.org/w/index.php?oldid=357698170 Contributors: 12dstring, 16@r, 194.237.150.xxx, A Stop at Willoughby, A.K.R., ABF, AFriedman, ALargeElk, Aatayyab, Abby, Abufaisal65, Acroterion, Addshore, Ademsaykin, AdjustShift, Ah2190, Ahadrt, Ahoerstemeier, AlMac, Alan012, Alansohn, Alasdair, Aleemqureshi, Ali K, AlisonW, AlistairMcMillan, Allen4names, Alphachimp, Amalhotra124, Anabus, Andre Engels, Andrea105, Andrejj, Andrewspencer, Angela, Angela 2502, Antandrus, Aoratos, Ap, Aqeelbilal, ArchonMagnus, Arkrishna, Ashawley, Asiananimal, AuburnPilot, AussieLegend, Autocratique, Avono, Aweiredguy, Backslash Forwardslash, Bact, BalderV, Barek, Bean2thousand, Bearly541, Ben-Zin, Betterusername, Bevo, Big Bird, Big milsy, BigDunc, Binary TSO, Blarrrgy, Bobo192, Bonadea, Bongwarrior, Booyabazooka, Boriszex, Boxplot, Brat32, Brent Gulanowski, Brinnington, Brucevdk, Brusegadi, Bryan Derksen, Bubba73, Burnisk, Butros, CIreland, Can't sleep, clown will eat me, CanadianLinuxUser, Canaima, Canterbury Tail, Cargils02, Carl007, Casperdog2227, Cassandra 73, Catgut, Causa sui, Cburress, Centrx, Christian List, Chun-hian, Chuunen Baka, Chzz, Citypanther, CommonsDelinker, Conan, Conversion script, CoolD, Coolcaesar, Cpl Syx, Crossmr, Cst17, Cunya, Cyberstrike3000X, Cyrius, DBHunter, DJ Clayworth, Damian Yerrick, Dan100, Danakil, Dandaman32, Daniel5127, DanielEng, Darkevilfairy, Darrendeng, Darth Panda, Dave6, Davennmarr, Daveydweeb, DavidWBrooks, Davie4125, Dayewalker, Dbstommy, DeadEyeArrow, Deggalega, Delirium, Denny, DerHexer, Derek farn, Dethme0w, Dibcom, Dina, Discospinster, Dispenser, DivineAlpha, Dj stone, DmitTrix, Dominator09, Dougofborg, Drawnman247, Dreamatalana, Drewster1829, Drmies, Dugmn, Dycedarg, E. Sn0 =31337=, EdBever, Edcolins, Edivorce, Edlin, Eeekster, Egbsystem, Ehheh, ElBenevolente, Ellenaz, Emana, Emvn, Epbr123, Equendil, Erdal Ronahi, Esoteric Rogue, Everyking, Excirial, Ezubaric, FJPB, Fang Aili, Favonian, Fazilati, Fctk, Feedmecereal, Felyza, Fenice, Fern80, Flarn2006, Flash man999, Flipjargendy, Flubeca, FlyingToaster, Frap, Fratrep, Fred Bradstadt, Fredrik, Fredtheflyingfrog, Freeeeeesoft, Friday, Frip1000, Frosted14, Fubar Obfusco, Fyyer, GDallimore, GSCC, GTBacchus, Gabecuevas, Gadfium, Gaius Cornelius, Galoubet, Gardar Rurak, Gary King, Gazpacho, Geneb1955, Georgia guy, Giftlite, GilbertoSilvaFan, Gilliam, Ginsengbomb, Gogo Dodo, Gordmoo, Greenrd, GregorB, Gronky, Gtg204y, Guanaco, Gueldenberg, Guppy, Gurch, Gwernol, Hadal, Harvester, Hauberk, Haunting The Better, Hayabusa future, Hectorthebat, Hello32020, Hellosandimas, HenryLi, Hmains, Hmcnally, Hno3, Howdoesthiswo, Hu12, Hydrogen Iodide, Iknowyourider, Imnotminkus, Imroy, IngerAlHaosului, Instinct, Intelligentsium, Into The Fray, Iridescent, Ishara665g, Itsmine, Ixfd64, J Di, J. Spencer, J.delanoy, JLaTondre, JMetzler, Jaan513, Jackaranga, Jacob.jose, Jacqueline7894y, Jam2k, Jamesiemiller, Japonca, Jarda-wien, Jay, Jcuadros, Jcw69, Jeff G., Jehochman, Jeremy Visser, JeremyA, Jester7777, Jh51681, Jiang, Jiddisch, Jiy, Jmabel, JoanneB, Johan1298, JohnCD, JonHarder, Jonnabuz, Jop, Joy, Jpbowen, Jpgordon, Jreferee, Jsheadixon, Jtalledo, Jtbatalla, Jusdafax, Jusjih, Justin Eiler, Jóna Þórunn, K.Nevelsteen, K.lee, KFP, Kajasudhakarababu, Karol Langner, Kathmandu2007, Kc03, Keilana, Kelldall, Kelly Martin, Kenny sh, KevinCuddeback, Kingpin13, Kinneyboy90, Knownot, Knucmo2, Kornxi, Kostisl, Kozuch, KrakatoaKatie, Krawi, Ktanzer, Kuldeep06march, Kumioko, Kurowoofwoof111, Kuru, La Pianista, Lagalag, LaggedOnUser, Lajpatdhingra, Landroo, Lareina3656y, LauriO, Leandrod, LeoNomis, Liao, Liberty Miller, Liftarn, Ligulem, LinDrug, Lindberg G Williams Jr, Listlist, LittleOldMe, Livajo, Lookingchris, Lucianob, Luna Santin, Lunaumbrax, Luntrasul, Lupo, MC10, MD66, MER-C, Macy, Majorbrainy, Malcolm Farmer, Malcolma, Mandarax, Manik762007, Manionc, Maralia, Marek69, Markaci, Martarius, Master&Expert, Matt Britt, Matusz, Mav, Maximaximax, Maximus Rex, Mcdonald.ross5, Mdd, Mdebets, MeekSaffron, Meelar, Mendalus, Mentifisto, Mermaid from the Baltic Sea, Message From Xenu, Mgrand, Michael Drüing, Michael Hardy, Michal Jurosz, Miguel.mateo, Mild Bill Hiccup, Mindspillage, Minesweeper, Minghong, Mini-Geek, Miquonranger03, Mjchonoles, Mkdw, Monobi, Mpete510, Mr Barndoor, MrDolomite, MrOllie, Muffhen, Murray Langton, Mwanner, Mwilso24, Mxn, Myanw, N n abc123, Nabeth, Naik5abhi, Nanshu, Narayana vvss, Nathanielrichards, Navstar, NawlinWiki, Neil916, Neurophyre, Nharipra, Nickels360, Nikai, Nike8, NinjaCross, Nixdorf, Nixeagle, Nk, Nmrd, Normxxx, Nosferatus2007, Nourybouraqadi, Nsaa, NuclearWarfare, NurAzije, Nurg, Oda Mari, Ohnoitsjamie, Oicumayberight, Old Moonraker, Oldwes, Oleg Alexandrov, Oliver Lineham, Omicronpersei8, One more night, OrgasGirl, Oxymoron83, P99am, PEH2, PIrish, Page Up, Paranoid, Party, Passargea, Paul August, Paul Niquette, PeaceAnywhere, Perkinsleslie, Peruvianllama, Peter Winnberg, Pgk, Phatom87, PhilHibbs, PhilKnight, Philip Howard, Philip Trueman, Phillip Ca, Piano non troppo, Pilotguy, Pip2andahalf, Poccil, Pohatu771, Pohta ce-am pohtit, Pointillist, Poo1000, Pooryorick, Priyadarshi.pratyush, Programmer13, Prohlep, Psantora, Puchiko, Purgatory Fubar, Pyfan, Quinobi, Quinsareth, QuintusQuill, R. S. Shaw, RHaworth, Ragha joshi, Ragib, Rainier3, RainierHa, Rajeshmagic, Random89, RandorXeus, Raul654, Ray Van De Walker, Raylu, Rbreen, Recnilgiarc, RedWolf, Reedy, Rehnn83, Remember the dot, Rettetast, RevRagnarok, Revised, Rgill, Rholton, Rhye123, Riana, Rich Farmbrough, Richard cocks, RickK, Ripogenus77, Rl, Rogger.Ferguson, Roland2, Ronhjones, RossPatterson, Roybb95, Rurik, Rwwww, Ryulong, S.K., SC979, SF007, ST47, Sadeq, Sadi Carnot, Safalra, Safarj, Saleem110, Salsa Shark, Sango123, SchfiftyThree, Scientus, Scrool, Seb az86556, Seidenstud, Senator Palpatine, Setveen, Shakya ind, Siggy28, Sigondronggondrong, Siliconov, SimonP, Sintonak.X, Sitarherophil, Sivaguru411, Sjö, Skarebo, SkyBon, Slingerjansen, Slowking Man, Slowmo1993, Smack, Smalljim, Snowmanradio, Sokrato, Soler97, South Bay, Speaksleft, Spectrogram, SpuriousQ, Starkiller88, Stephenb, Steven Zhang, Stewartadcock, StuffOfInterest, Supadawg, Super-Magician, Supersteve04038, Suruena, SusanLesch, SwirlBoy39, Syrthiss, T4tarzan, THEN WHO WAS PHONE?, TPK, TaintedMustard, Tangent747, Tasja, TastyPoutine, TechOutsider, Tejas81, Teryan2006, Th1rt3en, Tharcore, The Anome, The Earwig, The Rambling Man, The Thing That Should Not Be, The Transhumanist, The Transhumanist (AWB), TheBiaatch, Thelb4, Thingg, This acccount is 4 vandalism, Thumperward, Thurak13, Tide rolls, Tigershrike, Tim Q. Wells, Timhowardriley, Timothy Neilen, Topbanana, Torc2, Torinor, Traroth, Trevor MacInnis, Triona, Triwbe, True Genius, Trueshow111, Trusilver, Turlo Lomon, Tuxide, Twsx, Uncle Milty, Unyoyega, Uriyan, Vbigdeli, Versageek, Versus22, Virtualsfera, Viskonsas, WLU, Wavelength, Wayward, Werdna, Weregerbil, Wernher, Wexhammer, Weyes, Whitew123, Wiki alf, Wikicrazier2011, Wikidudeman, Wikieditor06, Wimt, Wireless friend, Wm, WojPob, Wshun, Wysprgr2005, Xdenizen, Xevious, Yamamoto Ichiro, Yidisheryid, Yoooder, Yworo, Zarkos, Zedlik, ZeroOne, Zerokewl, Zhang He, ZimZalaBim, Zybez, Александър, Евгени Симеонов, Петър Петров, 1463 anonymous edits History of programming languages Source: http://en.wikipedia.org/w/index.php?oldid=358397472 Contributors: Abcarter, Acaciz, Aljullu, Altenmann, Andrzejsliwa, Antic-Hay, Arch dude, Ashawley, B3tamike, Banus, Beland, Ben Standeven, Capricorn42, Cassowary, CharlesGillingham, Chris the speller, Cleme263, Danakil, DiarmuidPigott, Diego Moya, Djsasso, Duncandewar, Dylan Lake, Firsfron, Funandtrvl, Fuzheado, Gerweck, Ghettoblaster, GraemeL, Greenrd, Hairy Dude, Hotcrocodile, Huffers, IanOsgood, Icey, Indeterminate, JLaTondre, JMK, John Vandenberg, Jost Riedel, Jpbowen, KoshVorlon, Lakinekaki, Lambiam, Lulu of the Lotus-Eaters, Mahanga, Malcolmxl5, Marc Mongenet, Mdd, Mr700, Oxymoron83, Pgan002, Pgr94, PhilKnight, R. S. Shaw, RedWolf, Reeve, Rheostatik, Rp, Rursus, Rwwww, Sabre23t, Sceptre, Sietse Snel, Sinbadbuddha, Skäpperöd, Sligocki, Szyslak, T-bonham, Tablizer, The Thing That Should Not Be, Tony Sidaway, Torc2, Valodzka, Yuriz, Zawersh, Zoicon5, 101 anonymous edits Low- level programming language Source: http://en.wikipedia.org/w/index.php?oldid=348809741 Contributors: 16@r, AbsolutDan, AdmN, Alksentrs, Amcbride, Andreas Kaufmann, Antonielly, Awg1010, Brianga, Caknuck, CanisRufus, DJ LoPaTa, DanielCristofani, Diego Moya, Doktorspin, FatalError, Fox2030, Fram, Func, Greenrd, Haakon, Haiviet, J M Rice, Jackbaird, Jesin, Just Another Dan, Kate, Krawi, Lerdsuwa, Macrakis, Mecanismo, Medmrt2008, Michael Hardy, Mindmatrix, Minotaur2, Mr.Z-man, Nacefe, NewEnglandYankee, Peng, Pinkadelica, R. S. Shaw, RedWolf, RockMFR, Ruud Koot, Sarg, SoWhy, TakuyaMurata, Tgok, Thehotelambush, Theresa knott, Toxee, Traal, Trylks, Valodzka, Vkhaitan, Zx-man, 今古庸龍, 69 anonymous edits

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Article Sources and Contributors High- level programming language Source: http://en.wikipedia.org/w/index.php?oldid=351787140 Contributors: ARUNKUMAR P.R, Alansohn, Andreas Kaufmann, Antonielly, Anwar saadat, AuburnPilot, Borgx, CambridgeBayWeather, Can't sleep, clown will eat me, CanisRufus, Chad103, Closedmouth, DJ LoPaTa, Danilo.Piazzalunga, Diego Moya, Discospinster, Dougofborg, Dureo, Dysprosia, ENeville, El C, English Lock, Epbr123, FatalError, Fiend666, Firetrap9254, Fox2030, Fubar Obfusco, Func, Furrykef, Gail, Guy Harris, Harmil, HenkeB, Ixfd64, Jclemens, JesseGarrett, Jon Harald Søby, Jordandanford, Kbdank71, LOL, Lod, MER-C, Maelor, Maestrosync, Majorly, Mandarax, Melody, Michael Hardy, Mindmatrix, Miohtama, Munibert, Mushroom, Ningnangnong, Pedant, Pmurph, R. S. Shaw, RandomStringOfCharacters, RedWolf, RockMFR, Rsm99833, Ryankitlitz, SF007, Sarg, Shadow1, Shoeofdeath, SkonesMickLoud, Snaxorb, Solitude, Spikey, Stevage, Stewski, Stormscape, Superextremegamer1, Syndicate, TakuyaMurata, TangentCube, The Thing That Should Not Be, Tobias Bergemann, Traal, Tratten, Travelbird, Turkishbob, Vahid83, VampWillow, Vkhaitan, Vranak, Wootery, Yk Yk Yk, Yugsdrawkcabeht, 今古庸龍, 183 anonymous edits Machine code Source: http://en.wikipedia.org/w/index.php?oldid=354949012 Contributors: .:Ajvol:., 16@r, 192.35.241.xxx, AKismet, AdmN, Aguinaldo, Alfio, Algebra, Altenmann, Andre Engels, BiT, Bnugia, CanisRufus, CharlesC, Cjewell, Cmdrjameson, Conversion script, Cst17, DJ Clayworth, Dav4is, Dawnseeker2000, Derek Andrews, Discospinster, Dori, El C, Eric-Wester, Everyking, Fabrictramp, Feezo, Furrykef, GeorgeLouis, Helix84, HenkeB, Husond, Imjustmatthew, Incnis Mrsi, Instinct, IvanLanin, J.delanoy, Jacob.jose, Jagdeepyadav, Javert, Jeff02, Jesdisciple, Jeshan, Johnny 0, JonHarder, Jpk, Jusjih, Kappa, Karol Langner, Katieh5584, Kbdank71, Kim Bruning, Klaser, LazyEditor, Lee.crabtree, LiDaobing, Lowellian, Lyricmac, Manop, Mark, Matthewsim, Meadowbert, Mega Chrome, Megatronium, Melchoir, Microprofessor, Midzata, Mike Van Emmerik, Mindmatrix, Mirror Vax, MmisNarifAlhoceimi, Modest Genius, MrPrada, Muad, Murray Langton, Mxn, Nanshu, NawlinWiki, Nikai, OrgasGirl, Paul Stansifer, Philip Baird Shearer, Prolog, Punctilius, QofASpiewak, Quuxplusone, Randomguy121, Romanm, RussBlau, Rwpostiii, Salsa Shark, Sam Vimes, Sannse, Shmuel, Slady, Slipstream, Soumyasch, Spiritia, Stmrlbs, Sychen, Tedp, The Magician, Tobias Bergemann, Toussaint, Tximist, UnfriendlyFire, VKokielov, Voyagerfan5761, Waggers, WikiBully, Wikinerd, Wtshymanski, Xihix, Xod, Yanco, రవిచంద్ర, 182 anonymous edits Assembly language Source: http://en.wikipedia.org/w/index.php?oldid=358806331 Contributors: 0, 16@r, 3Nigma, A.R., AMRAN AL-AGHBARI, Abdull, Accatagon, Ahoerstemeier, Ahy1, Aitias, Akamad, Akyprian, Alan_d, Ale jrb, Aleph Infinity, Alex.g, Alfio, An-chan, AndonicO, Andre Engels, Andres, Angela, Anger22, Angusus, Anna Lincoln, AnnaFrance, Another-anomaly, AnthonyQBachler, Anwar saadat, Apokrif, Arbabarehman, Archanamiya, ArchiSchmedes, Ashmoo, Athaenara, Atlant, Audriusa, Autodmc, Backslash Forwardslash, Beefball, Beno1000, Bilbo1507, Binrapt, Bison bloke, Blainster, Blakegripling ph, Blashyrk, Brage, BroodKiller, Bryan Derksen, Bumm13, CRGreathouse, CUTKD, Can't sleep, clown will eat me, CanOfWorms, CanisRufus, Capi crimm, Chasingsol, ChazZeromus, Cheeselet, Chieffancypants, Chopin1810, Chris Chittleborough, Christian List, Conversion script, Coolv, Cquarksnow, Crotalus horridus, CultureDrone, Damian Yerrick, Darktemplar, Darrien, Dasnov, DavidCary, Dcoetzee, DeanHarding, DekuDekuplex, Derek Ross, Derek farn, Dexter Nextnumber, Dmbrunton, Doug, DouglasGreen, Dragon DASH, Dsavi.x4, Dtgm, Eagle246, Eagleal, Easyas12c, Edward, Edward.in.Edmonton, Elving, Emperorbma, Emuguru, Epbr123, EricR, Everyking, FDD, Feezo, Femto, Ferkelparade, Filemon, Flewis, Frap, Fred Bradstadt, Furrykef, Fuzzbox, Gaius Cornelius, Galoubet, Gannimo, Garas, Giddie, Giftlite, Gioto, GoldenMeadows, Gondooley, Goodone121, Greenrd, Gutsul, Gwern, Haiviet, HenkeB, Herzleid, Hirzel, Hotdogger, HumphreyW, IanOsgood, Inzy, ItsProgrammable, Ixfd64, Jack O exiled, Jacob.jose, James086, JavierMC, Jeffrey Mall, Jeh, Jiveshwar, JoeBruno, JohnCJWood, Jorgon, Jpsowin, Jth299, Jukrat, Just Another Dan, KD5TVI, KP-Adhikari, Karada, Kaster, Kbdank71, Kdakin, Keilana, Kglavin, Khunglongcon, Kindall, Kjp993, Koektrommel, Konstable, Kubanczyk, LOL, LeaveSleaves, LizardJr8, Lkopeter, Locke Cole, Lowellian, MER-C, MP64, Manatee0, Marasmusine, Mario Blunk, Martynas Patasius, Masgatotkaca, Matt B., Mdanh2002, Mdwh, Meera123, Mellamokb, Mellum, Mendalus, Michal Jurosz, Michele.alessandrini, Miguelito Vieira, Mike Field, Mikellewellyn, Mindmatrix, Mmernex, MonstaPro, Monz, Moonlit Knight, Mozillaman, Mr.Do!, MrOllie, Msikma, Murray Langton, Mustafazamany, Mykk, NERIUM, Nandesuka, Nanshu, NightFalcon90909, Nikai, Nmcclana, Ohnoitsjamie, Oldhamlet, Ospix, Owl3638, PJonDevelopment, Paresthesias, Patrick, Paul August, Pgk, Piano non troppo, Pinethicket, Pingveno, Pneuhaus, Pohta ce-am pohtit, Poolisfun, Popsracer, Praefectorian, Quadell, Quarryman, R. S. Shaw, RCX, RHaworth, Ramu50, Rasmus Faber, Rbakels, Rdnk, Red Prince, Regancy42, Reinderien, RexNL, Rfc1394, Rich Farmbrough, Rivanov, Robbe, Robert Merkel, Ronz, Rotundo, Ruud Koot, Sanbec, Sanoj1234, Sanpnr, Schultkl, Scientus, Scipius, Scott Gall, Shadanan, Shadow demon, Shadowjams, SilentC, Simon80, SimonP, SkyWalker, Slaryn, Slashme, Sleigh, Slightsmile, Soumyasch, Spalding, SpareHeadOne, SpeedyGonsales, Spinality, Sploonie, SpooK, SpuriousQ, Starionwolf, Starnestommy, StealthFox, SteinbDJ, Stewartadcock, Stmrlbs, Stormy Ordos, Struway, Stuart Morrow, Surturz, Suruena, System86, TParis00ap, Tarikes, Tcsetattr, TeaDrinker, Tedickey, Th1rt3en, The Thing That Should Not Be, TheStarman, ThomasHarte, Tide rolls, Tim32, Tobias Bergemann, Toksyuryel, Tomasz Tybulewicz, Toussaint, True Pagan Warrior, Tzarius, Uli, Ultimus, Utvik, VampWillow, Velle, Versus22, Vid512, Vobis132, Vwollan, Wengier, Wereon, Wernher, Wesley, Whitehatnetizen, Wiki alf, Wikiklrsc, Wilky DiFendare, WindowsNT, 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Shaw, Raayen, RainbowOfLight, Randomblue, Raul654, Rdsmith4, Reconsider the static, Rejka, Rettetast, RexNL, Rgoodermote, Rholton, Riana, Rich Farmbrough, Rizzardi, RobertG, RobinK, Rpwikiman, Rror, RussBlau, Ruud Koot, Ryguasu, SJP, Salix alba, Salleman, SamShearman, SarekOfVulcan, Savidan, Scarian, Seb, Sesse, Shadowjams, Shipmaster, Silly rabbit, SilverStar, Sitharama.iyengar1, SlackerMom, Snowolf, Snoyes, Sonjaaa, Sophus Bie, Sopoforic, Spankman, Speck-Made, Spellcast, Spiff, Splang, Sridharinfinity, Stephan Leclercq, Storkk, Sundar, Susurrus, Swerdnaneb, Systemetsys, TakuyaMurata, Tarquin, Taw, The Firewall, The Fish, The Thing That Should Not Be, The ansible, TheGWO, TheNewPhobia, Thecarbanwheel, Theodore7, Tiddly Tom, Tide rolls, Tim Marklew, Timc, Timhowardriley, Timir2, Tizio, Tlesher, Tlork Thunderhead, Toncek, Tony1, Trevor MacInnis, Treyt021, TuukkaH, UberScienceNerd, Urenio, User A1, V31a, Vasileios Zografos, Vikreykja, Vildricianus, Wainkelly, Waltnmi, Wavelength, Wayiran, Waynefan23, Weetoddid, Wellithy, Wexcan, Who, Whosyourjudas, WhyDoIKeepForgetting, WikHead, Willking1979, Winston365, Woohookitty, Wvbailey, Xashaiar, Yamamoto Ichiro, Yintan, Ysindbad, Zfr, Zocky, Zondor, Zoney, Zundark, 846 anonymous edits Computer data processing Source: http://en.wikipedia.org/w/index.php?oldid=349245458 Contributors: AVM, Aaron Brenneman, Alexius08, Altenmann, Animum, Anonymous Dissident, Ap, Arnauldvm, Biblbroks, BigFatBuddha, Buickid, CALR, COMPATT, Capricorn42, CliffC, Crohnie, Dataentryindia, DeadEyeArrow, Dontaskme, Doulos Christos, Dreftymac, E-Celtic, ESkog, Eliz81, EmanWilm, Ewawer, Fredrik, Gavrilis, Glenn Maddox, Graham Chapman, Grubbmeister, Himanshu.manisha, Hoof Hearted, Hydrargyrum, Idleguy, Ischemia, JForget, JHunterJ, Jamelan, Jni, Johnleemk, Julia Rossi, Kenny sh, Kingturtle, Kuru, L7HOMAS, Lcarsdata, Leonariso, Libcub, Malcolm rowe, Mani1, Masgatotkaca, Matthew Yeager, Maxis ftw, Mike Rosoft, MrFish, Nasz, NicholasJones, Nitefood, Noel Streatfield, Nuno Tavares, Oicumayberight, OrgasGirl, Palfrey, Piotrus, Prolog, Public Menace, Pvosta, Qwertyus, Raj.sandia, RandomXYZb, Riana, RichardVeryard, RobertG, Rocafela921, Ronz, RoySmith, SMC, Sherbrooke, SimonP, Skittleys, Slakr, Systemetsys, THEASHMAN69, THEN WHO WAS PHONE?, TMC1982, Think outside the box, Titusmars, Versageek, Vonsche, Vpovilaitis, WazzaMan, Wfeuer, Woohookitty, Youngerleo, Zzuuzz, 140 anonymous edits Thread (computer science) Source: http://en.wikipedia.org/w/index.php?oldid=358795906 Contributors: A5b, Abdull, Aff123a, Ahy1, Aldie, Alfio, AlistairMcMillan, Allan McInnes, Allen4names, Allstarecho, Altenmann, Amarniit, AnOddName, Andreas Kaufmann, AndySimpson, Apantomimehorse, Apostrophe, Arch dude, Aris Katsaris, Asafshelly, Asztal, Azraell, BMF81, Bangaram, Barocco, Bart.smaalders, Basil.bourque, Benglar, Betacommand, Bobo192, Borgx, Brick Thrower, Bryan Derksen, Btball, BudVVeezer, CLAES, CanisRufus, Cburnett, ChrisJ, Cmdrjameson, Coherers, Computer Guru, Crashie, Crossland, Csabo, Cybercobra, Deineka, Dianne Hackborn, Dirkbike, Discospinster, Dkanter, Dododerek, Dungodung, Dyl, Dysprosia, Ed g2s, EdC, Edcolins, Elkman, Emperorbma, EngineerScotty, Esap, EvanYares, Face, FatalError, Feezo, Ferdinand Pienaar, Finlay McWalter, FishSpeaker, Foant, Frap, Frau K, Furrykef,

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Image Sources, Licenses and Contributors Image:Programming language textbooks.jpg Source: http://en.wikipedia.org/w/index.php?title=File:Programming_language_textbooks.jpg License: Public Domain Contributors: User:K.lee Image:Operating system placement.svg Source: http://en.wikipedia.org/w/index.php?title=File:Operating_system_placement.svg License: Creative Commons Attribution-Sharealike 3.0 Contributors: User:Golftheman Image:AtariBasicExample.png Source: http://en.wikipedia.org/w/index.php?title=File:AtariBasicExample.png License: unknown Contributors: Angusmclellan, Rockfang Image:Msxbasic.png Source: http://en.wikipedia.org/w/index.php?title=File:Msxbasic.png License: unknown Contributors: User:Ruud Koot Image:IBM Cassette BASIC.png Source: http://en.wikipedia.org/w/index.php?title=File:IBM_Cassette_BASIC.png License: unknown Contributors: ㍐, 3 anonymous edits Image:IBM Disk BASIC.png Source: http://en.wikipedia.org/w/index.php?title=File:IBM_Disk_BASIC.png License: unknown Contributors: ㍐ Image:IBM BASICA.png Source: http://en.wikipedia.org/w/index.php?title=File:IBM_BASICA.png License: unknown Contributors: ㍐ Image:GW-BASIC 3.23.png Source: http://en.wikipedia.org/w/index.php?title=File:GW-BASIC_3.23.png License: unknown Contributors: Jusdafax, Rat at WikiFur, ㍐, 3 anonymous edits Image:TriBasicExample.png Source: http://en.wikipedia.org/w/index.php?title=File:TriBasicExample.png License: GNU General Public License Contributors: Original uploader was Dojarca at en.wikipedia File:kr c prog lang.jpg Source: http://en.wikipedia.org/w/index.php?title=File:Kr_c_prog_lang.jpg License: unknown Contributors: ConradPino, Cybercobra, Jafet, Melesse, 8 anonymous edits Image:C plus plus book.jpg Source: http://en.wikipedia.org/w/index.php?title=File:C_plus_plus_book.jpg License: unknown Contributors: ICReal, Jusjih, Michaelas10, NAHID, Skier Dude, Storkk, Suffusion of Yellow, Yamla, 4 anonymous edits Image:BjarneStroustrup.jpg Source: http://en.wikipedia.org/w/index.php?title=File:BjarneStroustrup.jpg License: GNU Free Documentation License Contributors: Image:Programming-republic-of-perl.png Source: http://en.wikipedia.org/w/index.php?title=File:Programming-republic-of-perl.png License: unknown Contributors: User:66.92.237.111, User:Andros 1337, User:Cydebot, User:Damian Yerrick, User:Dbenbenn, User:Dominus, User:Jonathan Grynspan, User:Lasix, User:PhilHibbs, User:Raul654, User:Sfan00 IMG, User:Template namespace initialisation script File:Fortran acs cover.jpeg Source: http://en.wikipedia.org/w/index.php?title=File:Fortran_acs_cover.jpeg License: unknown Contributors: User:Muhandis File:Ibm704.gif Source: http://en.wikipedia.org/w/index.php?title=File:Ibm704.gif License: Attribution Contributors: Lawrence Livermore National Laboratory File:FortranCardPROJ039.agr.jpg Source: http://en.wikipedia.org/w/index.php?title=File:FortranCardPROJ039.agr.jpg License: Creative Commons Attribution-Sharealike 2.5 Contributors: Arnold Reinhold File:FortranCodingForm.png Source: http://en.wikipedia.org/w/index.php?title=File:FortranCodingForm.png License: Public Domain Contributors: Agateller, Bryan Derksen, 1 anonymous edits Image:C_47964181354789_sharp.png Source: http://en.wikipedia.org/w/index.php?title=File:C_47964181354789_sharp.png License: unknown Contributors: Kkm010 Image:C sharp music.svg Source: http://en.wikipedia.org/w/index.php?title=File:C_sharp_music.svg License: GNU Free Documentation License Contributors: BazookaJoe, Grover cleveland, Ktims, Materialscientist, 3 anonymous edits File:Java logo.svg Source: http://en.wikipedia.org/w/index.php?title=File:Java_logo.svg License: unknown Contributors: Eastmain, MBisanz, Sophus Bie, The wub, TimTay, Tkgd2007, Webmink, 2 anonymous edits File:Wave.svg Source: http://en.wikipedia.org/w/index.php?title=File:Wave.svg License: unknown Contributors: sbmehta converted to SVG from Sun Microsystems AI version. 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