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A programming language is a formal language, which comprises a set of instructions that produce various kinds of output. Programming languages are used in computer programming to implement algorithms.

Most programming languages consist of instructions for computers. There are programmable machines that use a set of specific instructions, rather than general programming languages. Early ones preceded the invention of the digital computer, the first probably being the automatic flute player described in the 9th century by the brothers Musa in Baghdad, during the Islamic Golden Age.^[1] Since the early 1800s, programs have been used to direct the behavior of machines such as Jacquard looms, music boxes and player pianos.^[2] The programs for these machines (such as a player piano's scrolls) did not produce different behavior in response to different inputs or conditions.

Thousands of different programming languages have been created, and more are being created every year. Many programming languages are written in an imperative form (i.e., as a sequence of operations to perform) while other languages use the declarative form (i.e. the desired result is specified, not how to achieve it).

The description of a programming language is usually split into the two components of 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) while other languages (such as Perl) have a dominant implementation that is treated as a reference. Some languages have both, with the basic language defined by a standard and extensions taken from the dominant implementation being common.

• 2History
• 3Elements
  • 3.2Semantics
  • 3.3Type system
• 4Design and implementation
• 6Use

Definitions[edit]

A programming language is a notation for writing programs, which are specifications of a computation or algorithm.^[3] Some authors restrict the term 'programming language' to those languages that can express all possible algorithms.^[3][4] Traits often considered important for what constitutes a programming language include:

Function and target

A computer programming language is a language used to write computer programs, which involves a computer performing some kind of computation^[5] or algorithm and possibly control external devices such as printers, disk drives, robots,^[6] 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.^[7] In most practical contexts, a programming language involves a computer; consequently, programming languages are usually defined and studied this way.^[8] 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.

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.^[9] This principle is sometimes formulated as a recommendation to the programmer to make proper use of such abstractions.^[10]

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-92 and Charity are examples of languages that are not Turing complete, yet often called programming languages.^[11][12]

Markup languages like XML, HTML, or troff, which define structured data, are not usually considered programming languages.^[13][14][15] Programming languages may, however, share the syntax with markup languages if a computational semantics is defined. XSLT, for example, is a Turing complete language entirely using XML syntax.^[16][17][18] Moreover, LaTeX, which is mostly used for structuring documents, also contains a Turing complete subset.^[19][20]

The term computer language is sometimes used interchangeably with programming language.^[21] 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.^[22] Similarly, 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.^[23]

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.^[24]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.^[25]

History[edit]

Early developments[edit]

Very early computers, such as Colossus, were programmed without the help of a stored program, by modifying their circuitry or setting banks of physical controls.

Slightly later, programs could be written in machine language, where the programmer writes each instruction in a numeric form the hardware can execute directly. For example, the instruction to add the value in two memory location might consist of 3 numbers: an 'opcode' that selects the 'add' operation, and two memory locations. The programs, in decimal or binary form, were read in from punched cards, paper tape, magnetic tape or toggled in on switches on the front panel of the computer. Machine languages were later termed first-generation programming languages (1GL).

The next step was development of so-called second-generation programming languages (2GL) or assembly languages, which were still closely tied to the instruction set architecture of the specific computer. These served to make the program much more human-readable and relieved the programmer of tedious and error-prone address calculations.

The first high-level programming languages, or third-generation programming languages (3GL), were written in the 1950s. An early 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.^[26]

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John Mauchly's Short Code, proposed in 1949, was one of the first high-level languages ever developed for an electronic computer.^[27] Unlike machine code, Short Code statements represented mathematical expressions in understandable form. However, the program had to be translated into machine code every time it ran, making the process much slower than running the equivalent machine code.

At the University of Manchester, Alick Glennie developed Autocode in the early 1950s. As a programming language, it used a compiler to automatically convert the language into machine code. The first code and compiler was developed in 1952 for the Mark 1 computer at the University of Manchester and is considered to be the first compiled high-level programming language.^[28][29]

The second autocode was developed for the Mark 1 by R. A. Brooker in 1954 and was called the 'Mark 1 Autocode'. Brooker also developed an autocode for the Ferranti Mercury in the 1950s in conjunction with the University of Manchester. The version for the EDSAC 2 was devised by D. F. Hartley of University of Cambridge Mathematical Laboratory in 1961. Known as EDSAC 2 Autocode, it was a straight development from Mercury Autocode adapted for local circumstances and was noted for its object code optimisation and source-language diagnostics which were advanced for the time. A contemporary but separate thread of development, Atlas Autocode was developed for the University of Manchester Atlas 1 machine.

In 1954, FORTRAN was invented at IBM by John Backus. It was the first widely used high-level general purpose programming language to have a functional implementation, as opposed to just a design on paper.^[30][31] It is still a popular language for high-performance computing^[32] and is used for programs that benchmark and rank the world's fastest supercomputers.^[33]

Another early programming language was devised by Grace Hopper in the US, called FLOW-MATIC. It was developed for the UNIVAC I at Remington Rand during the period from 1955 until 1959. Hopper found that business data processing customers were uncomfortable with mathematical notation, and in early 1955, she and her team wrote a specification for an English programming language and implemented a prototype.^[34] The FLOW-MATIC compiler became publicly available in early 1958 and was substantially complete in 1959.^[35] FLOW-MATIC was a major influence in the design of COBOL, since only it and its direct descendant AIMACO were in actual use at the time.^[36]

Refinement[edit]

The increased use of high-level languages introduced a requirement for low-level programming languages or system programming languages. These languages, to varying degrees, provide facilities between assembly languages and high-level languages. They can be used to perform tasks which require direct access to hardware facilities but still provide higher-level control structures and error-checking.

The period from the 1960s to the late 1970s brought the development of the major language paradigms now in use:

• APL introduced array programming and influenced functional programming.^[37]
• ALGOL refined both structured procedural programming and the discipline of language specification; the 'Revised Report on the Algorithmic Language ALGOL 60' became a model for how later language specifications were written.
• Lisp, implemented in 1958, was the first dynamically typed functional programming language.
• 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 for the Unix operating system and remains popular.^[38]
• Prolog, designed in 1972, was the first logic programming language.
• In 1978, ML built a polymorphic type system on top of Lisp, pioneering statically typedfunctional programming languages.

Each of these languages spawned descendants, and most modern programming 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.^[39]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.^[40]

Consolidation and growth[edit]

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.^[41] 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 decades.

One important trend in language design for programming large-scale systems during the 1980s was an increased focus on the use of modules or large-scale organizational units of code. Modula-2, Ada, and ML all developed notable module systems in the 1980s, which were often wedded to generic programming constructs.^[42]

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, and bytecode virtual machines became popular again in commercial settings with their promise of 'Write once, run anywhere' (UCSD Pascal had been popular for a time in the early 1980s). These developments were not fundamentally novel, rather they were refinements of many existing languages and paradigms (although their syntax was often 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.

Fourth-generation programming languages (4GL) are computer programming languages which aim to provide a higher level of abstraction of the internal computer hardware details than 3GLs. Fifth-generation programming languages (5GL) are programming languages based on solving problems using constraints given to the program, rather than using an algorithm written by a programmer.

Elements[edit]

All programming languages have some primitive building blocks for the description of data and the processes or transformations applied to them (like the addition of two numbers or the selection of an item from a collection). These primitives are defined by syntactic and semantic rules which describe their structure and meaning respectively.

Syntax[edit]

A programming language's surface form is known as its syntax. Most programming languages are purely textual; they use sequences of text including words, numbers, and punctuation, much like written natural languages. On the other hand, there are some programming languages which are more graphical in nature, using visual relationships between symbols to specify a program.

The syntax of a language describes the possible combinations of symbols that form a syntactically correct program. The meaning given to a combination of symbols is handled by semantics (either formal or hard-coded in a reference implementation). Since most languages are textual, this article discusses textual syntax.

Programming language syntax is usually defined using a combination of regular expressions (for lexical structure) and Backus–Naur form (for grammatical structure). Below is a simple grammar, based on Lisp:

This grammar specifies the following:

• an expression is either an atom or a list;
• an atom is either a number or a symbol;
• a number is an unbroken sequence of one or more decimal digits, optionally preceded by a plus or minus sign;
• a symbol is a letter followed by zero or more of any characters (excluding whitespace); and
• a list is a matched pair of parentheses, with zero or more expressions inside it.

The following are examples of well-formed token sequences in this grammar: 12345, () and (a b c232 (1)).

Not all syntactically correct programs are semantically correct. Many syntactically correct programs are nonetheless ill-formed, per the language's rules; and may (depending on the language specification and the soundness of the implementation) result in an error on translation or execution. In some cases, such programs may exhibit undefined behavior. Even when a program is well-defined within a language, it may still have a meaning that is not intended by the person who wrote it.

Using natural language as an example, it may not be possible to assign a meaning to a grammatically correct sentence or the sentence may be false:

• 'Colorless green ideas sleep furiously.' is grammatically well-formed but has no generally accepted meaning.
• 'John is a married bachelor.' is grammatically well-formed but expresses a meaning that cannot be true.

The following C language fragment is syntactically correct, but performs operations that are not semantically defined (the operation *p >> 4 has no meaning for a value having a complex type and p->im is not defined because the value of p is the null pointer):

If the type declaration on the first line were omitted, the program would trigger an error on undefined variable 'p' during compilation. However, the program would still be syntactically correct since type declarations provide only semantic information.

The grammar needed to specify a programming language can be classified by its position in the Chomsky hierarchy. The syntax of most programming languages can be specified using a Type-2 grammar, i.e., they are context-free grammars.^[43] Some languages, including Perl and Lisp, contain constructs that allow execution during the parsing phase. Languages that have constructs that allow the programmer to alter the behavior of the parser make syntax analysis an undecidable problem, and generally blur the distinction between parsing and execution.^[44] In contrast to Lisp's macro system and Perl's BEGIN blocks, which may contain general computations, C macros are merely string replacements and do not require code execution.^[45]

Semantics[edit]

The term semantics refers to the meaning of languages, as opposed to their form (syntax).

Static semantics[edit]

The static semantics defines restrictions on the structure of valid texts that are hard or impossible to express in standard syntactic formalisms.^[3] For compiled languages, static semantics essentially include those semantic rules that can be checked at compile time. Examples include checking that every identifier is declared before it is used (in languages that require such declarations) or that the labels on the arms of a case statement are distinct.^[46] Many important restrictions of this type, like checking that identifiers are used in the appropriate context (e.g. not adding an integer to a function name), or that subroutine calls have the appropriate number and type of arguments, can be enforced by defining them as rules in a logic called a type system. Other forms of static analyses like data flow analysis may also be part of static semantics. Newer programming languages like Java and C# have definite assignment analysis, a form of data flow analysis, as part of their static semantics.

Dynamic semantics[edit]

Once data has been specified, the machine must be instructed to perform operations on the data. For example, the semantics may define the strategy by which expressions are evaluated to values, or the manner in which control structures conditionally execute statements. The dynamic semantics (also known as execution semantics) of a language defines how and when the various constructs of a language should produce a program behavior. There are many ways of defining execution semantics. Natural language is often used to specify the execution semantics of languages commonly used in practice. A significant amount of academic research went into formal semantics of programming languages, which allow execution semantics to be specified in a formal manner. Results from this field of research have seen limited application to programming language design and implementation outside academia.

Type system[edit]

A type system defines how a programming language classifies values and expressions into types, how it can manipulate those types and how they interact. The goal of a type system is to verify and usually enforce a certain level of correctness in programs written in that language by detecting certain incorrect operations. Any decidable type system involves a trade-off: while it rejects many incorrect programs, it can also prohibit some correct, albeit unusual programs. In order to bypass this downside, a number of languages have type loopholes, usually unchecked casts that may be used by the programmer to explicitly allow a normally disallowed operation between different types. In most typed languages, the type system is used only to type check programs, but a number of languages, usually functional ones, infer types, relieving the programmer from the need to write type annotations. The formal design and study of type systems is known as type theory.

Typed versus untyped languages[edit]

A language is typed if the specification of every operation defines types of data to which the operation is applicable.^[47] For example, the data represented by 'this text between the quotes' is a string, and in many programming languages dividing a number by a string has no meaning and will not be executed. The invalid operation may be detected when the program is compiled ('static' type checking) and will be rejected by the compiler with a compilation error message, or it may be detected while the program is running ('dynamic' type checking), resulting in a run-time exception. Many languages allow a function called an exception handler to handle this exception and, for example, always return '-1' as the result.

A special case of typed languages are the single-typed languages. These are often scripting or markup languages, such as REXX or SGML, and have only one data type^[dubious]–—most commonly character strings which are used for both symbolic and numeric data.

In contrast, an untyped language, such as most assembly languages, allows any operation to be performed on any data, generally sequences of bits of various lengths.^[47] High-level untyped languages include BCPL, Tcl, and some varieties of Forth.

In practice, while few languages are considered typed from the type theory (verifying or rejecting all operations), most modern languages offer a degree of typing.^[47] Many production languages provide means to bypass or subvert the type system, trading type-safety for finer control over the program's execution (see casting).

Static versus dynamic typing[edit]

In static typing, all expressions have their types determined prior to when the program is executed, typically at compile-time. For example, 1 and (2+2) are integer expressions; they cannot be passed to a function that expects a string, or stored in a variable that is defined to hold dates.^[47]

Statically typed languages can be either manifestly typed or type-inferred. In the first case, the programmer must explicitly write types at certain textual positions (for example, at variable declarations). In the second case, the compiler infers the types of expressions and declarations based on context. Most mainstream statically typed languages, such as C++, C# and Java, are manifestly typed. Complete type inference has traditionally been associated with less mainstream languages, such as Haskell and ML. However, many manifestly typed languages support partial type inference; for example, C++, Java and C# all infer types in certain limited cases.^[48] Additionally, some programming languages allow for some types to be automatically converted to other types; for example, an int can be used where the program expects a float.

Dynamic typing, also called latent typing, determines the type-safety of operations at run time; in other words, types are associated with run-time values rather than textual expressions.^[47] As with type-inferred languages, dynamically typed languages do not require the programmer to write explicit type annotations on expressions. Among other things, this may permit a single variable to refer to values of different types at different points in the program execution. However, type errors cannot be automatically detected until a piece of code is actually executed, potentially making debugging more difficult. Lisp, Smalltalk, Perl, Python, JavaScript, and Ruby are all examples of dynamically typed languages.

Weak and strong typing[edit]

Weak typing allows a value of one type to be treated as another, for example treating a string as a number.^[47] This can occasionally be useful, but it can also allow some kinds of program faults to go undetected at compile time and even at run time.

Strong typing prevents these program faults. An attempt to perform an operation on the wrong type of value raises an error.^[47] Strongly typed languages are often termed type-safe or safe.

An alternative definition for 'weakly typed' refers to languages, such as Perl and JavaScript, which permit a large number of implicit type conversions. In JavaScript, for example, the expression 2 * x implicitly converts x to a number, and this conversion succeeds even if x is null, undefined, an Array, or a string of letters. Such implicit conversions are often useful, but they can mask programming errors.Strong and static are now generally considered orthogonal concepts, but usage in the literature differs. Some use the term strongly typed to mean strongly, statically typed, or, even more confusingly, to mean simply statically typed. Thus C has been called both strongly typed and weakly, statically typed.^[49][50]

It may seem odd to some professional programmers that C could be 'weakly, statically typed'. However, notice that the use of the generic pointer, the void* pointer, does allow for casting of pointers to other pointers without needing to do an explicit cast. This is extremely similar to somehow casting an array of bytes to any kind of datatype in C without using an explicit cast, such as (int) or (char).

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Standard library and run-time system[edit]

Most programming languages have an associated core library (sometimes known as the 'standard library', especially if it is included as part of the published language standard), which is conventionally made available by all implementations of the language. Core libraries typically include definitions for commonly used algorithms, data structures, and mechanisms for input and output.

The line between a language and its core library differs from language to language. In some cases, the language designers may treat the library as a separate entity from the language. However, a language's core library is often treated as part of the language by its users, and some language specifications even require that this library be made available in all implementations. Indeed, some languages are designed so that the meanings of certain syntactic constructs cannot even be described without referring to the core library. For example, in Java, a string literal is defined as an instance of the java.lang.String class; similarly, in Smalltalk, an anonymous function expression (a 'block') constructs an instance of the library's BlockContext class. Conversely, Scheme contains multiple coherent subsets that suffice to construct the rest of the language as library macros, and so the language designers do not even bother to say which portions of the language must be implemented as language constructs, and which must be implemented as parts of a library.

Design and implementation[edit]

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.^[51][52] 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.^[53] 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 programming languages have been designed from scratch, altered to meet new needs, and combined with other languages. Many have 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.^[54] 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.
• 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.^[55]

Natural language programming has 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'.^[56]Alan Perlis was similarly dismissive of the idea.^[57] Hybrid approaches have been taken in Structured English and SQL.

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.

Specification[edit]

The specification of a programming language is an artifact that the language users and the implementors can use to agree upon whether a piece of source code is a valid program in that language, and if so what its behavior shall be.

A programming language specification can take several forms, including the following:

• 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^[58] and Scheme^[59] specifications).
• 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.
• A reference or model implementation, sometimes written in the language being specified (e.g., Prolog or ANSI REXX^[60]). The syntax and semantics of the language are explicit in the behavior of the reference implementation.

Implementation[edit]

An implementation of a programming language provides a way to write programs in that language and execute them 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 much faster than those that are interpreted in software.^{[61][better source needed]}

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.

Proprietary languages[edit]

Although most of the most commonly used programming languages have fully open specifications and implementations, many programming languages exist only as proprietary programming languages with the implementation available only from a single vendor, which may claim that such a proprietary language is their intellectual property. Proprietary programming languages are commonly domain specific languages or internal scripting languages for a single product; some proprietary languages are used only internally within a vendor, while others are available to external users.

Some programming languages exist on the border between proprietary and open; for example, Oracle Corporation asserts proprietary rights to some aspects of the Java programming language,^[62] and Microsoft's C# programming language, which has open implementations of most parts of the system, also has Common Language Runtime (CLR) as a closed environment.^[63]

Many proprietary languages are widely used, in spite of their proprietary nature; examples include MATLAB, VBScript, and Wolfram Language. Some languages may make the transition from closed to open; for example, Erlang was originally an Ericsson's internal programming language.^[64]

Use[edit]

Thousands of different programming languages have been created, mainly in the computing field.^[65]Software is commonly built with 5 programming languages or more.^[66]

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. On the other hand, ideas about an algorithm can be communicated to humans without the precision required for execution by using pseudocode, which interleaves natural language with code written in a programming language.

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).^[67]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 can run its commands through an interpreter (such as a Unix shell or other command-line interface), without compiling, it is called a scripting language.^[68]

Measuring language usage[edit]

Determining which is the most widely used programming language is difficult since the definition of usage varies by context. One language may occupy the greater number of programmer hours, a different one has more lines of code, and a third may consume 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;^[69][70]Fortran in scientific and engineering applications; Ada in aerospace, transportation, military, real-time and embedded applications; and C in embedded applications and operating systems. 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^[71]
• the number of books sold that teach or describe the language^[72]
• estimates of the number of existing lines of code written in the language – which may underestimate languages not often found in public searches^[73]
• 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, stackify.com reported the ten most popular programming languages as (in descending order by overall popularity): Java, C, C++, Python, C#, JavaScript, VB .NET, R, PHP, and MATLAB.^[74]

Dialects, flavors and implementations[edit]

A dialect of a programming language or a data exchange language is a (relatively small) variation or extension of the language that does not change its intrinsic nature. With languages such as Scheme and Forth, standards may be considered insufficient, inadequate or illegitimate by implementors, so often they will deviate from the standard, making a new dialect. In other cases, a dialect is created for use in a domain-specific language, often a subset. In the Lisp world, most languages that use basic S-expression syntax and Lisp-like semantics are considered Lisp dialects, although they vary wildly, as do, say, Racket and Clojure. As it is common for one language to have several dialects, it can become quite difficult for an inexperienced programmer to find the right documentation. The BASIC programming language has many dialects.

The explosion of Forth dialects led to the saying 'If you've seen one Forth.. you've seen one Forth.'

Taxonomies[edit]

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, with general-purpose programming languages distinguished from domain-specific programming languages. 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.^[75] 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).^[76] Some general purpose languages were designed largely with educational goals.^[77]

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 deliberately esoteric or not.

See also[edit]

• Computer science and Outline of computer science
• Metaprogramming
• Software engineering and List of software engineering topics

References[edit]

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2. ^Ettinger, James (2004) Jacquard's Web, Oxford University Press
3. ^ ^abcAaby, Anthony (2004). Introduction to Programming Languages. Archived from the original on 8 November 2012. Retrieved 29 September 2012.
4. ^In mathematical terms, this means the programming language is Turing-completeMacLennan, Bruce J. (1987). Principles of Programming Languages. Oxford University Press. p. 1. ISBN978-0-19-511306-8.
5. ^ACM SIGPLAN (2003). 'Bylaws of the Special Interest Group on Programming Languages of the Association for Computing Machinery'. Archived from the original on 22 June 2006. Retrieved 19 June 2006., '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.'
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7. ^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, ISBN0-12-012108-5, 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]
8. ^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.
9. ^David A. Schmidt, The structure of typed programming languages, MIT Press, 1994, ISBN0-262-19349-3, p. 32
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11. ^Digital Equipment Corporation. 'Information Technology – Database Language SQL (Proposed revised text of DIS 9075)'. ISO/IEC 9075:1992, Database Language SQL. Archived from the original on 21 June 2006. Retrieved 29 June 2006.
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18. ^Scott, Michael (2006). Programming Language Pragmatics. Morgan Kaufmann. p. 802. ISBN978-0-12-633951-2. XSLT, though highly specialized to the transformation of XML, is a Turing-complete programming language.
19. ^Oetiker, Tobias; Partl, Hubert; Hyna, Irene; Schlegl, Elisabeth (20 June 2016). 'The Not So Short Introduction to LATEX 2ε'(Version 5.06). tobi.oetiker.ch. pp. 1–157. Archived(PDF) from the original on 14 March 2017. Retrieved 16 April 2017.
20. ^Syropoulos, Apostolos; Antonis Tsolomitis; Nick Sofroniou (2003). Digital typography using LaTeX. Springer-Verlag. p. 213. ISBN978-0-387-95217-8. TeX is not only an excellent typesetting engine but also a real programming language.
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23. ^S.K. Bajpai, Introduction To Computers And C Programming, New Age International, 2007, ISBN81-224-1379-X, p. 346
24. ^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, ISBN0-12-012108-5, 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.'
25. ^John C. Reynolds, 'Some thoughts on teaching programming and programming languages', SIGPLAN Notices, Volume 43, Issue 11, November 2008, p.109
26. ^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)Archived 18 October 2014 at the Wayback Machine
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34. ^Hopper (1978) p. 16.
35. ^Sammet (1969) p. 316
36. ^Sammet (1978) p. 204.
37. ^Richard L. Wexelblat: History of Programming Languages, Academic Press, 1981, chapter XIV.
38. ^François Labelle. 'Programming Language Usage Graph'. SourceForge. Archived from the original on 17 June 2006. Retrieved 21 June 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.
39. ^Hayes, Brian (2006). 'The Semicolon Wars'. American Scientist. 94 (4): 299–303. doi:10.1511/2006.60.299.
40. ^Dijkstra, Edsger W. (March 1968). 'Go To Statement Considered Harmful'(PDF). Communications of the ACM. 11 (3): 147–148. doi:10.1145/362929.362947. Archived(PDF) from the original on 13 May 2014. Retrieved 22 May 2014.
41. ^Tetsuro Fujise, Takashi Chikayama, Kazuaki Rokusawa, Akihiko Nakase (December 1994). 'KLIC: A Portable Implementation of KL1' Proc. of FGCS '94, ICOT Tokyo, December 1994. 'Archived copy'. Archived from the original on 25 September 2006. Retrieved 9 October 2006.CS1 maint: Archived copy as title (link) KLIC is a portable implementation of a concurrent logic programming language KL1.
42. ^Jim Bender (15 March 2004). 'Mini-Bibliography on Modules for Functional Programming Languages'. ReadScheme.org. Archived from the original on 24 September 2006. Retrieved 27 September 2006.
43. ^Michael Sipser (1996). Introduction to the Theory of Computation. PWS Publishing. ISBN978-0-534-94728-6. Section 2.2: Pushdown Automata, pp.101–114.
44. ^Jeffrey Kegler, 'Perl and UndecidabilityArchived 17 August 2009 at the Wayback Machine', The Perl Review. Papers 2 and 3 prove, using respectively Rice's theorem and direct reduction to the halting problem, that the parsing of Perl programs is in general undecidable.
45. ^Marty Hall, 1995, Lecture Notes: MacrosArchived 6 August 2013 at the Wayback Machine, PostScriptversionArchived 17 August 2000 at the Wayback Machine
46. ^Michael Lee Scott, Programming language pragmatics, Edition 2, Morgan Kaufmann, 2006, ISBN0-12-633951-1, p. 18–19
47. ^ ^abcdefgAndrew Cooke. 'Introduction To Computer Languages'. Archived from the original on 15 August 2012. Retrieved 13 July 2012.
48. ^Specifically, instantiations of generic types are inferred for certain expression forms. Type inference in Generic Java—the research language that provided the basis for Java 1.5's bounded parametric polymorphism extensions—is discussed in two informal manuscripts from the Types mailing list: Generic Java type inference is unsoundArchived 29 January 2007 at the Wayback Machine (Alan Jeffrey, 17 December 2001) and Sound Generic Java type inferenceArchived 29 January 2007 at the Wayback Machine (Martin Odersky, 15 January 2002). C#'s type system is similar to Java's, and uses a similar partial type inference scheme.
49. ^'Revised Report on the Algorithmic Language Scheme'. 20 February 1998. Archived from the original on 14 July 2006. Retrieved 9 June 2006.
50. ^Luca Cardelli and Peter Wegner. 'On Understanding Types, Data Abstraction, and Polymorphism'. Manuscript (1985). Archived from the original on 19 June 2006. Retrieved 9 June 2006.
51. ^Steven R. Fischer, A history of language, Reaktion Books, 2003, ISBN1-86189-080-X, p. 205
52. ^Éric Lévénez (2011). 'Computer Languages History'. Archived from the original on 7 January 2006.
53. ^Jing Huang. 'Artificial Language vs. Natural Language'. Archived from the original on 3 September 2009.
54. ^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.' ('PL/I'. Encyclopedia of Mathematics. Archived from the original on 26 April 2012. Retrieved 29 June 2006.). Ada and UNCOL had similar early goals.
55. ^Frederick P. Brooks, Jr.: The Mythical Man-Month, Addison-Wesley, 1982, pp. 93–94
56. ^Dijkstra, Edsger W. On the foolishness of 'natural language programming.'Archived 20 January 2008 at the Wayback Machine EWD667.
57. ^Perlis, Alan (September 1982). 'Epigrams on Programming'. SIGPLAN Notices Vol. 17, No. 9. pp. 7–13. Archived from the original on 17 January 1999.
58. ^Milner, R.; M. Tofte; R. Harper; D. MacQueen (1997). The Definition of Standard ML (Revised). MIT Press. ISBN978-0-262-63181-5.
59. ^Kelsey, Richard; William Clinger; Jonathan Rees (February 1998). 'Section 7.2 Formal semantics'. Revised⁵ Report on the Algorithmic Language Scheme. Archived from the original on 6 July 2006. Retrieved 9 June 2006.
60. ^ANSI – Programming Language Rexx, X3-274.1996
61. ^Steve, McConnell. Code complete (Second ed.). Redmond, Washington. pp. 590, 600. ISBN0735619670. OCLC54974573.
62. ^See: Oracle America, Inc. v. Google, Inc.
63. ^'Guide to Programming Languages ComputerScience.org'. ComputerScience.org. Retrieved 13 May 2018.
64. ^'The basics'. www.ibm.com. 10 May 2011. Retrieved 13 May 2018.
65. ^'HOPL: an interactive Roster of Programming Languages'. Australia: Murdoch University. Archived from the original on 20 February 2011. Retrieved 1 June 2009. This site lists 8512 languages.
66. ^Mayer, Philip; Bauer, Alexander (2015). An empirical analysis of the utilization of multiple programming languages in open source projects. Proceedings of the 19th International Conference on Evaluation and Assessment in Software Engineering – EASE '15. New York, NY, USA: ACM. pp. 4:1–4:10. doi:10.1145/2745802.2745805. ISBN978-1-4503-3350-4. Results: We found (a) a mean number of 5 languages per project with a clearly dominant main general-purpose language and 5 often-used DSL types, (b) a significant influence of the size, number of commits, and the main language on the number of languages as well as no significant influence of age and number of contributors, and (c) three language ecosystems grouped around XML, Shell/Make, and HTML/CSS. Conclusions: Multi-language programming seems to be common in open-source projects and is a factor which must be dealt with in tooling and when assessing development and maintenance of such software systems.
67. ^Abelson, Sussman, and Sussman. 'Structure and Interpretation of Computer Programs'. Archived from the original on 26 February 2009. Retrieved 3 March 2009.CS1 maint: Multiple names: authors list (link)
68. ^Brown Vicki (1999). 'Scripting Languages'. mactech.com. Archived from the original on 2 December 2017. Retrieved 17 November 2014.
69. ^Georgina Swan (21 September 2009). 'COBOL turns 50'. computerworld.com.au. Archived from the original on 19 October 2013. Retrieved 19 October 2013.
70. ^Ed Airey (3 May 2012). '7 Myths of COBOL Debunked'. developer.com. Archived from the original on 19 October 2013. Retrieved 19 October 2013.
71. ^Nicholas Enticknap. 'SSL/Computer Weekly IT salary survey: finance boom drives IT job growth'. Computerweekly.com. Archived from the original on 26 October 2011. Retrieved 14 June 2013.
72. ^'Counting programming languages by book sales'. Radar.oreilly.com. 2 August 2006. Archived from the original on 17 May 2008. Retrieved 3 December 2010.
73. ^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
74. ^'Most Popular and Influential Programming Languages of 2018'. stackify.com. 18 December 2017. Retrieved 29 August 2018.
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76. ^'TUNES: Programming Languages'. Archived from the original on 20 October 2007.
77. ^Wirth, Niklaus (1993). Recollections about the development of Pascal. Proc. 2nd ACM SIGPLAN Conference on History of Programming Languages. 28. pp. 333–342. CiteSeerX10.1.1.475.6989. doi:10.1145/154766.155378. ISBN978-0-89791-570-0. Retrieved 30 June 2006.

Further reading[edit]

• Abelson, Harold; Sussman, Gerald Jay (1996). Structure and Interpretation of Computer Programs (2nd ed.). MIT Press. Archived from the original on 9 March 2018. Retrieved 22 October 2011.
• Raphael Finkel: Advanced Programming Language Design, Addison Wesley 1995.
• Daniel P. Friedman, Mitchell Wand, Christopher T. Haynes: Essentials of Programming Languages, The MIT Press 2001.
• Maurizio Gabbrielli and Simone Martini: 'Programming Languages: Principles and Paradigms', Springer, 2010.
• David Gelernter, Suresh Jagannathan: Programming Linguistics, The MIT Press 1990.
• Ellis Horowitz (ed.): Programming Languages, a Grand Tour (3rd ed.), 1987.
• Ellis Horowitz: Fundamentals of Programming Languages, 1989.
• Shriram Krishnamurthi: Programming Languages: Application and Interpretation, online publication.
• 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.
• Terrence W. Pratt and Marvin V. Zelkowitz: Programming Languages: Design and Implementation (4th ed.), Prentice Hall 2000.
• Peter H. Salus. Handbook of Programming Languages (4 vols.). Macmillan 1998.
• Ravi Sethi: Programming Languages: Concepts and Constructs, 2nd ed., Addison-Wesley 1996.
• Michael L. Scott: Programming Language Pragmatics, Morgan Kaufmann Publishers 2005.
• Robert W. Sebesta: Concepts of Programming Languages, 9th ed., Addison Wesley 2009.
• Franklyn Turbak and David Gifford with Mark Sheldon: Design Concepts in Programming Languages, The MIT Press 2009.
• Peter Van Roy and Seif Haridi. Concepts, Techniques, and Models of Computer Programming, The MIT Press 2004.
• David A. Watt. Programming Language Concepts and Paradigms. Prentice Hall 1990.
• David A. Watt and Muffy Thomas. Programming Language Syntax and Semantics. Prentice Hall 1991.
• David A. Watt. Programming Language Processors. Prentice Hall 1993.
• David A. Watt. Programming Language Design Concepts. John Wiley & Sons 2004.

External links[edit]

C (/siː/, as in the letter c) is a general-purpose, imperative computer programming language, supporting structured programming, lexical variable scope and recursion, while a static type system prevents many unintended operations. By design, C provides constructs that map efficiently to typical machine instructions, and it has therefore found lasting use in applications that were previously coded in assembly language. Such applications include operating systems, as well as various application software for computers ranging from supercomputers to embedded systems.

C was originally developed at Bell Labs by Dennis Ritchie, between 1972 and 1973. It was created to make utilities running on Unix. Later, it was applied to re-implementing the kernel of the Unix operating system^[6]. During the 1980s, C gradually gained popularity. Nowadays, it is one of the most widely used programming languages^[7][8], with C compilers from various vendors available for the majority of existing computer architectures and operating systems. C has been standardized by the American National Standards Institute (ANSI) since 1989 (see ANSI C) and subsequently by the International Organization for Standardization (ISO).

C is an imperativeprocedural 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 runtime support. Despite its low-level capabilities, the language was designed to encourage cross-platform programming. A standards-compliant C program that is written with portability in mind can be compiled for a wide variety of computer platforms and operating systems with few changes to its source code; the language has become available on various platforms, from embedded microcontrollers to supercomputers.

• 1Overview
• 2History
• 3Syntax
• 5Data types
• 7Libraries

Overview[edit]

Like most imperative languages in the ALGOL tradition, C has facilities for structured programming and allows lexical variable scope and recursion. Its static type system prevents unintended operations. In C, all executable code is contained within subroutines (also called 'functions', though not strictly in the sense of functional programming). Function parameters are always passed by value. Pass-by-reference is simulated in C by explicitly passing pointer values. C program source text is free-format, using the semicolon as a statement terminator and curly braces for grouping blocks of statements.

The C language also exhibits the following characteristics:

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• There is a small, fixed number of keywords, including a full set of control flow primitives: if/else, for, do/while, while, and switch. User-defined names are not distinguished from keywords by any kind of sigil.
• There are a large number of arithmetic, bitwise and logic operators: +, +=, ++, &, , etc.
• More than one assignment may be performed in a single statement.
• Function return values can be ignored when not needed.
• Typing is static, but weakly enforced; all data has a type, but implicit conversions are possible.
• Declarationsyntax mimics usage context. C has no 'define' keyword; instead, a statement beginning with the name of a type is taken as a declaration. There is no 'function' keyword; instead, a function is indicated by the parentheses of an argument list.
• User-defined (typedef) and compound types are possible.
  • Heterogeneous aggregate data types (struct) allow related data elements to be accessed and assigned as a unit.
  • Union is a structure with overlapping members; only the last member stored is valid.
  • Array indexing is a secondary notation, defined in terms of pointer arithmetic. Unlike structs, arrays are not first-class objects: they cannot be assigned or compared using single built-in operators. There is no 'array' keyword in use or definition; instead, square brackets indicate arrays syntactically, for example month[11].
  • Enumerated types are possible with the enum keyword. They are freely interconvertible with integers.
  • Strings are not a distinct data type, but are conventionally implemented as null-terminated character arrays.
• Low-level access to computer memory is possible by converting machine addresses to typed pointers.
• Procedures (subroutines not returning values) are a special case of function, with an untyped return type void.
• Functions may not be defined within the lexical scope of other functions.
• Function and data pointers permit ad hocrun-time polymorphism.
• A preprocessor performs macro definition, source code file inclusion, and conditional compilation.
• There is a basic form of modularity: files can be compiled separately and linked together, with control over which functions and data objects are visible to other files via static and extern attributes.
• Complex functionality such as I/O, string manipulation, and mathematical functions are consistently delegated to library routines.

While C does not include certain features found in other languages (such as object orientation and garbage collection), these can be implemented or emulated, often through the use of external libraries (e.g., the GLib Object System or the Boehm garbage collector).

Relations to other languages[edit]

Many later languages have borrowed directly or indirectly from C, including C++, C#, Unix's C shell, D, Go, Java, JavaScript, Limbo, LPC, Objective-C, Perl, PHP, Python, Rust, Swift, Verilog and SystemVerilog (hardware description languages).^[5] These languages have drawn many of their control structures and other basic features from C. Most of them (Python being a dramatic exception) also express highly similar syntax to C, and they tend to combine the recognizable expression and statement syntax of C with underlying type systems, data models, and semantics that can be radically different.

History[edit]

Early developments[edit]

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 Dennis Ritchie and Ken Thompson, incorporating several ideas from colleagues. Eventually, they decided to port the operating system to a PDP-11. The original PDP-11 version of Unix was developed in assembly language. Thompson needed a programming language to make utilities. At first, he tried to make a Fortran compiler, but soon gave up the idea and made a new language, B, Thompson's simplified version of BCPL.^[10] However, few utilities were written in B because B was too slow, and B could not take advantage of PDP-11 features such as byte addressability.

In 1972, Ritchie started to improve B, which resulted in creating a new language C^[11]. C compiler and some utilities made by C were included in Version 2 Unix.^[12] At Version 4 Unix released at Nov. 1973, the Unixkernel was extensively re-implemented by C.^[10] By this time, the C language had acquired some powerful features such as struct types.

Unix was one of the first operating system kernels implemented in a language other than assembly. Earlier instances include the Multics system (which was written in PL/I) and Master Control Program (MCP) for the Burroughs B5000 (which was written in ALGOL) in 1961. In around 1977, Ritchie and Stephen C. Johnson made further changes to the language to facilitate portability of the Unix operating system. Johnson's Portable C Compiler served as the basis for several implementations of C on new platforms.^[11]

K&R C[edit]

In 1978, Brian Kernighan and Dennis Ritchie published the first edition of The C Programming Language.^[1] 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^[13] covers the later ANSI C standard, described below.

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= (that is, -=) to remove the semantic ambiguity created by constructs such as i=-10, which had been interpreted as i =- 10 (decrement i by 10) instead of the possibly intended i = -10 (let i be -10).

Even after the publication of the 1989 ANSI 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 return types other than int must be declared if used before the function definition; functions used without prior declaration were presumed to return type int.

For example:

The int type specifiers which are commented out could be omitted in K&R C, but are required in later standards.

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 features were added to the language, supported by compilers from AT&T (in particular PCC^[14]) and some other vendors. These included:

• void functions (i.e., functions with no return value)
• functions returning struct or union types (rather than pointers)
• assignment for struct data 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[edit]

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. X3J11 based the C standard on the Unix implementation; however, the non-portable portion of the Unix C library was handed off to the IEEEworking group 1003 to become the basis for the 1988 POSIX standard. In 1989, the C 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 international C standard, maintained by the working group ISO/IEC JTC1/SC22/WG14. National adoption of an update 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 subsequently introduced unofficial features. The standards committee also included several additional features such as function prototypes (borrowed from C++), void pointers, support for international character sets and locales, and preprocessor enhancements. Although the syntax for parameter declarations was augmented to include the style used in C++, 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 today 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 prevent the use on a K&R C-based compiler of features available only in Standard C.

After the ANSI/ISO standardization process, the C language specification remained relatively static for several years. In 1995, Normative Amendment 1 to the 1990 C standard (ISO/IEC 9899/AMD1:1995, known informally as C95) was published, to correct some details and to add more extensive support for international character sets.^[15]

C99[edit]

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.^[16]

Programming In Objective C 6th Edition

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 and flexible array members, improved support for IEEE 754 floating point, 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, Solaris Studio, and other C compilers now support many or all of the new features of C99. The C compiler in Microsoft Visual C++, however, implements the C89 standard and those parts of C99 that are required for compatibility with C++11.^[17]

C11[edit]

In 2007, work began on another revision of the C standard, informally called 'C1X' until its official publication on 2011-12-08. The C standards committee adopted guidelines to limit the adoption of new features that had not been tested by existing implementations.

The C11 standard adds numerous new features to C and the library, including type generic macros, anonymous structures, improved Unicode support, atomic operations, multi-threading, and bounds-checked functions. It also makes some portions of the existing C99 library optional, and improves compatibility with C++. The standard macro __STDC_VERSION__ is defined as 201112L to indicate that C11 support is available.

C18[edit]

Published in June 2018, C18 is the current standard for the C programming language. It introduces no new language features, only technical corrections and clarifications to defects in C11. The standard macro __STDC_VERSION__ is defined as 201710L.

Embedded C[edit]

Historically, embedded C programming requires nonstandard extensions to the C language in order to support exotic features such as fixed-point arithmetic, multiple distinct memory banks, and basic I/O operations.

In 2008, the C Standards Committee published a technical report extending the C language^[18] to address these issues by providing a common standard for all implementations to adhere to. It includes a number of features not available in normal C, such as fixed-point arithmetic, named address spaces, and basic I/O hardware addressing.

Syntax[edit]

C has a formal grammar specified by the C standard.^[19] Line endings are generally not significant in C; however, line boundaries do have significance during the preprocessing phase. Comments may appear either between the delimiters /* and */, or (since C99) following // until the end of the line. Comments delimited by /* and */ do not nest, and these sequences of characters are not interpreted as comment delimiters if they appear inside string or character literals.^[20]

C source files contain 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 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.

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.'^[21] The C standard did not attempt to correct many of these blemishes, because of the impact of such changes on already existing software.

Character set[edit]

The basic C source character set includes the following characters:

• Lowercase and uppercase letters of ISO Basic Latin Alphabet: a–zA–Z
• Decimal digits: 0–9
• Graphic characters: ! ' # % & ' ( ) * + , - . / : ; < = > ? [ ] ^ _ { } ~
• Whitespace characters: space, horizontal tab, vertical tab, form feed, newline

Newline indicates the end of a text line; it need not correspond to an actual single character, although for convenience C treats it as one.

Additional multi-byte encoded characters may be used in string literals, but they are not entirely portable. The latest C standard (C11) allows multi-national Unicode characters to be embedded portably within C source text by using uXXXX or UXXXXXXXX encoding (where the X denotes a hexadecimal character), although this feature is not yet widely implemented.

The basic C execution character set contains the same characters, along with representations for alert, backspace, and carriage return. Run-time support for extended character sets has increased with each revision of the C standard.

Reserved words[edit]

C89 has 32 reserved words, also known as keywords, which are the words that cannot be used for any purposes other than those for which they are predefined:

C99 reserved five more words:

C11 reserved seven more words:^[22]

Most of the recently reserved words begin with an underscore followed by a capital letter, because identifiers of that form were previously reserved by the C standard for use only by implementations. Since existing program source code should not have been using these identifiers, it would not be affected when C implementations started supporting these extensions to the programming language. Some standard headers do define more convenient synonyms for underscored identifiers. The language previously included a reserved word called entry, but this was seldom implemented, and has now been removed as a reserved word.^[23]

Operators[edit]

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: +, -, *, /, %
• assignment: =
• augmented assignment: +=, -=, *=, /=, %=, &=, =, ^=, <<=, >>=
• bitwise logic: ~, &, , ^
• bitwise shifts: <<, >>
• boolean logic: !, &&,
• conditional evaluation: ? :
• equality testing: , !=
• calling functions: ( )
• increment and decrement: ++, --
• member selection: ., ->
• object size: sizeof
• order relations: <, <=, >, >=
• reference and dereference: &, *, [ ]
• sequencing: ,
• subexpression grouping: ( )
• type conversion: (typename)

C uses the operator = (used in mathematics to express equality) to indicate assignment, following the precedent of Fortran and PL/I, but unlike ALGOL and its derivatives. C uses the operator to test for equality. The similarity between these two operators (assignment and equality) may result in the accidental use of one in place of the other, and in many cases, the mistake does not produce an error message (although some compilers produce warnings). For example, the conditional expression if(ab+1) might mistakenly be written as if(a=b+1), which will be evaluated as true if a is not zero after the assignment.^[24]

The C operator precedence is not always intuitive. For example, the operator binds more tightly than (is executed prior to) the operators & (bitwise AND) and (bitwise OR) in expressions such as x & 1 0, which must be written as (x & 1) 0 if that is the coder's intent.^[25]

'Hello, world' example[edit]

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:^[26]

A standard-conforming 'hello, world' program is:^[a]

The first line of the program contains a preprocessing directive, indicated by #include. This causes the compiler to replace that 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 headers provided with the compiler to other headers having the same name, as opposed to double quotes which typically 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 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 this function takes no arguments.^[b]

The opening curly brace indicates the beginning of the definition of the main function.

The next line calls (diverts execution to) a function named printf, which in this case 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, worldn'. 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 closing curly brace indicates the end of the code for the main function. According to the C99 specification and newer, the main function, unlike any other function, will implicitly return a value of 0 upon reaching the } that terminates the function. (Formerly an explicit return 0; statement was required.) This is interpreted by the run-time system as an exit code indicating successful execution.^[27]

Data types[edit]

The type system in C is static and weakly typed, which makes it similar to the type system of ALGOL descendants such as Pascal.^[28] There are built-in types for integers of various sizes, both signed and unsigned, floating-point numbers, and enumerated types (enum). Integer type char is often used for single-byte characters. C99 added a boolean datatype. There are also derived types including arrays, pointers, records (struct), and 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 data object in some other way.

Some find C's declaration syntax unintuitive, particularly for function pointers. (Ritchie's idea was to declare identifiers in contexts resembling their use: 'declaration reflects use'.)^[29]

C's usual arithmetic conversions allow for efficient code to be generated, but can sometimes produce unexpected results. For example, a comparison of signed and unsigned integers of equal width requires a conversion of the signed value to unsigned. This can generate unexpected results if the signed value is negative.

Pointers[edit]

C supports the use of pointers, a type of reference that records 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 or 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. Pointers are used for many purposes in C. Text strings are commonly manipulated using pointers into arrays of characters. Dynamic memory allocation 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 passing functions as arguments to higher-order functions (such as qsort or bsearch) or as callbacks to be invoked by event handlers.^[27]

A null pointer value explicitly points to no valid location. Dereferencing a null pointer value is undefined, often resulting in a segmentation fault. Null pointer values 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. In appropriate contexts in source code, such as for assigning to a pointer variable, a null pointer constant can be written as 0, with or without explicit casting to a pointer type, or as the NULL macro defined by several standard headers. In conditional contexts, null pointer values evaluate to false, while all other pointer values evaluate to true.

Void pointers (void *) point to objects of unspecified 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.^[27]

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 continue to be used after deallocation (dangling pointers); they may be used without having been initialized (wild 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[edit]

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 means that declared arrays and these dynamically allocated simulated arrays are virtually interchangeable.

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Since arrays are always accessed (in effect) via pointers, array accesses are typically not checked against the underlying array size, although some compilers may provide bounds checking as an option.^[30][31] 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. If bounds checking is desired, it must be done manually.

C does not have a special provision for declaring multi-dimensional 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 'multi-dimensional array' can be thought of as increasing in row-major order.

Multi-dimensional 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[edit]

The subscript notation x[i] (where x designates a pointer) is syntactic sugar for *(x+i).^[32] Taking advantage of the compiler's knowledge of the pointer type, the address that x + i points to is not the base address (pointed to by x) incremented by i bytes, but rather is defined to be the base address incremented by i multiplied by the size of an element that x points to. Thus, x[i] designates the i+1th element of the array.

Furthermore, in most expression contexts (a notable exception is as operand of sizeof), 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 size of an element can be determined by applying the operator sizeof to any dereferenced element of x, as in n = sizeof *x or n = sizeof x[0], and the number of elements in a declared array A can be determined as sizeof A / sizeof A[0]. The latter only applies to array names: variables declared with subscripts (int A[20]). Due to the semantics of C, it is not possible to determine the entire size of arrays through pointers to arrays or those created by dynamic allocation (malloc); code such as sizeof arr / sizeof arr[0] (where arr designates a pointer) will not work since the compiler assumes the size of the pointer itself is being requested.^[33][34] Since array name arguments to sizeof are not converted to pointers, they do not exhibit such ambiguity. However, arrays created by dynamic allocation are accessed by pointers rather than true array variables, so they suffer from the same sizeof issues as array pointers.

Thus, 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; the array name is not an l-value, and its address is a constant, unlike a pointer variable. Consequently, what an array 'points to' cannot be changed, and it is impossible to assign a new address to an array name. Array contents may be copied, however, by using the memcpy function, or by accessing the individual elements.

Memory management[edit]

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:^[27]

• 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.
• 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.
• 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 realloc or free

These three approaches are appropriate in different situations and have various trade-offs. For example, static memory allocation has little allocation overhead, automatic allocation may involve slightly more overhead, and dynamic memory allocation can potentially have a great deal of overhead for both allocation and deallocation. The persistent nature of static objects is useful for maintaining state information across function calls, automatic allocation is easy to use but stack space is typically much more limited and transient than either static memory or heap space, and dynamic memory allocation allows convenient 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 simplest 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 change in size at runtime, and since static allocations (and automatic allocations before C99) must have a fixed size at compile-time, there are many situations in which dynamic allocation is necessary.^[27] Prior to the C99 standard, variable-sized arrays were a common example of this. (See the article on malloc for an example of dynamically allocated arrays.) Unlike automatic allocation, which can fail at run time with uncontrolled consequences, the dynamic allocation functions return an indication (in the form of a null pointer value) when the required storage cannot be allocated. (Static allocation that is too large is usually detected by the linker or loader, before the program can even begin execution.)

Unless otherwise specified, static objects contain zero or null pointer values upon program startup. Automatically and dynamically allocated objects are initialized only 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 can occur.

Another issue is that heap memory allocation has to be 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() is 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 for memory to be freed but continue to be referenced, leading to unpredictable results. Typically, the symptoms will appear in a portion of the program far removed from the actual error, making it difficult to track down the problem. (Such issues are ameliorated in languages with automatic garbage collection.)

Libraries[edit]

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 'link the math library').^[27]

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 (implementations which target limited environments such as embedded systems may provide only a subset of the standard library). This library supports stream input and output, memory allocation, mathematics, character strings, and time values. Several separate standard headers (for example, stdio.h) specify the interfaces for these and other standard library facilities.

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.^[27]

File handling and streams[edit]

File input and output (I/O) is not part of the C language itself but instead is handled by libraries (such as the C standard library) and their associated header files (e.g. stdio.h). File handling is generally implemented through high-level I/O which works through streams. A stream is from this perspective a data flow that is independent of devices, while a file is a concrete device. The high level I/O is done through the association of a stream to a file. In the C standard library, a buffer (a memory area or queue) is temporarily used to store data before it's sent to the final destination. This reduces the time spent waiting for slower devices, for example a hard drive or solid state drive. Low-level I/O functions are not part of the standard C library but are generally part of 'bare metal' programming (programming that's independent of any operative system such as most but not all embedded programming). With few exceptions, implementations include low-level I/O.

Language tools[edit]

A number of tools have been developed to help C programmers find and fix statements with undefined behavior or possibly erroneous expressions, with greater rigor than that provided by the compiler. The tool lint was the first such, leading to many others.

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.^[35]

There are also compilers, libraries, and operating system level mechanisms for performing actions that are not a standard part of C, such as bounds checking for arrays, detection of buffer overflow, serialization, dynamic memory tracking, and automatic garbage collection.

Tools such as Purify or Valgrind and linking with libraries containing special versions of the memory allocation functions can help uncover runtime errors in memory usage.

Uses[edit]

C is widely used for system programming in implementing operating systems and embedded system applications,^[37] because C code, when written for portability, can be used for most purposes, yet when needed, system-specific code can be used to access specific hardware addresses and to perform type punning to match externally imposed interface requirements, with a low run-time 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.^[38] C is often chosen over interpreted languages because of its speed, stability, and near-universal availability.^[39]

One consequence of C's wide availability and efficiency is that compilers, libraries and interpreters of other programming languages are often implemented in C. The reference implementations of Python, Perl and PHP, for example, are all written in C.

Because the layer of abstraction is thin and the overhead is low, C enables programmers to create efficient implementations of algorithms and data structures, useful for computationally intense programs. For example, the GNU Multiple Precision Arithmetic Library, the GNU Scientific Library, Mathematica, and MATLAB are completely or partially written in C.

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, additional machine-specific code generators are not necessary. C has some features, such as line-number preprocessor directives and optional superfluous commas at the end of initializer lists, that support compilation of generated code. However, some of C's shortcomings have prompted the development of other C-based languages specifically designed for use as intermediate languages, such as C--.

C has also been widely used to implement end-user applications. However, such applications can also be written in newer, higher-level languages.

Related languages[edit]

C has both directly and indirectly influenced many later languages such as C#, D, Go, Java, JavaScript, Limbo, LPC, Perl, PHP, Python, and Unix's C shell.^[40] 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.

Several C or near-C interpreters exist, including Ch and CINT, which can also be used for scripting.

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.^[41]

The C++ programming language was devised by Bjarne Stroustrup as an approach to providing object-oriented functionality with a C-like syntax.^[42] 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.

Objective-C was originally a very 'thin' layer on top of C, 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.

In addition to C++ and Objective-C, Ch, Cilk and Unified Parallel C are nearly supersets of C.

See also[edit]

Notes[edit]

1. ^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.
2. ^The main function actually has two arguments, int argc and char *argv[], respectively, which can be used to handle command line arguments. The ISO C standard (section 5.1.2.2.1) requires both forms of main to be supported, which is special treatment not afforded to any other function.

References[edit]

1. ^ ^abcdKernighan, Brian W.; Ritchie, Dennis M. (February 1978). The C Programming Language (1st ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-110163-0.
2. ^Ritchie (1993): 'Thompson had made a brief attempt to produce a system coded in an early version of C—before structures—in 1972, but gave up the effort.'
3. ^Ritchie (1993): '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.'
4. ^Ring Team (5 December 2017). 'Ring language and other languages'. ring-lang.net. ring-lang.
5. ^ ^ab'Verilog HDL (and C)'(PDF). The Research School of Computer Science at the Australian National University. 2010-06-03. Archived from the original(PDF) on 2013-11-06. Retrieved 2013-08-19. 1980s: ; Verilog first introduced ; Verilog inspired by the C programming language
6. ^Ritchie (1993)
7. ^'Programming Language Popularity'. 2009. Archived from the original on 13 December 2007. Retrieved 16 January 2009.
8. ^'TIOBE Programming Community Index'. 2009. Retrieved 6 May 2009.
9. ^'History of C - cppreference.com'. en.cppreference.com.
10. ^ ^abRitchie, Dennis M. (March 1993). 'The Development of the C Language'. ACM SIGPLAN Notices. 28 (3): 201–208. doi:10.1145/155360.155580.
11. ^ ^abJohnson, S. C.; Ritchie, D. M. (1978). 'Portability of C Programs and the UNIX System'. Bell System Tech. J. 57 (6): 2021–2048. CiteSeerX10.1.1.138.35. doi:10.1002/j.1538-7305.1978.tb02141.x. (Note: this reference is an OCR scan of the original, and contains an OCR glitch rendering 'IBM 370' as 'IBM 310'.)
12. ^McIlroy, M. D. (1987). A Research Unix reader: annotated excerpts from the Programmer's Manual, 1971–1986(PDF) (Technical report). CSTR. Bell Labs. p. 10. 139.
13. ^ ^abKernighan, Brian W.; Ritchie, Dennis M. (March 1988). The C Programming Language (2nd ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-110362-7.
14. ^Stroustrup, Bjarne (2002). Sibling rivalry: C and C++(PDF) (Report). AT&T Labs.
15. ^C Integrity. International Organization for Standardization. 1995-03-30.
16. ^'JTC1/SC22/WG14 – C'. Home page. ISO/IEC. Retrieved 2 June 2011.
17. ^Andrew Binstock (October 12, 2011). 'Interview with Herb Sutter'. Dr. Dobbs. Retrieved September 7, 2013.
18. ^'TR 18037: Embedded C'(PDF). ISO / IEC. Retrieved 26 July 2011.
19. ^Harbison, Samuel P.; Steele, Guy L. (2002). C: A Reference Manual (5th ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-089592-9. Contains a BNF grammar for C.
20. ^Kernighan, Brian W.; Ritchie, Dennis M. (1996). The C Programming Language (2nd ed.). Prentice Hall. p. 192. ISBN7 302 02412 X.
21. ^Page 3 of the original K&R^[1]
22. ^ISO/IEC 9899:201x (ISO C11) Committee Draft
23. ^Kernighan, Brian W.; Ritchie, Dennis M. (1996). The C Programming Language (2nd ed.). Prentice Hall. pp. 192, 259. ISBN7 302 02412 X.
24. ^'10 Common Programming Mistakes in C++'. Cs.ucr.edu. Retrieved 26 June 2009.
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33. ^Summit, Steve. 'comp.lang.c Frequently Asked Questions 6.23'. Retrieved March 6, 2013.
34. ^Summit, Steve. 'comp.lang.c Frequently Asked Questions 7.28'. Retrieved March 6, 2013.
35. ^'Man Page for lint (freebsd Section 1)'. unix.com. 2001-05-24. Retrieved 2014-07-15.
36. ^McMillan, Robert (2013-08-01). 'Is Java Losing Its Mojo?'. Wired.
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Sources[edit]

• Ritchie, Dennis M. (1993). The Development of the C Language. The second ACM SIGPLAN History of Programming Languages Conference (HOPL-II). Cambridge, MA, USA — April 20–23, 1993: ACM. pp. 201–208. doi:10.1145/154766.155580. ISBN0-89791-570-4. Retrieved 2014-11-04.

Further reading[edit]

• Banahan, M.; Brady, D.; Doran, M. (1991). The C Book (2nd ed.). Addison-Wesley.
• King, K. N. (April 2008). C Programming: A Modern Approach (2nd ed.). Norton. ISBN978-0-393-97950-3.
• Thompson, Ken. 'A New C Compiler'(PDF). Murray Hill, New Jersey: AT&T Bell Laboratories.
• Feuer, Alan R. (1998). The C Puzzle Book (1st, revised printing ed.). Addison-Wesley. ISBN978-0-201-60461-0.

External links[edit]

• ISO/IEC 9899, publicly available official C documents, including the C99 Rationale
• 'C99 with Technical corrigenda TC1, TC2, and TC3 included'(PDF).(3.61 MB)
• A History of C, by Dennis Ritchie