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C (, as in the
letter ''c''
letter ''c''
) is a general-purpose, procedural computer
programming language A programming language is a formal language In logic, mathematics, computer science, and linguistics, a formal language consists of string (computer science), words whose symbol (formal), letters are taken from an alphabet (computer science) ...

programming language
supporting
structured programming Structured programming is a programming paradigm Programming paradigms are a way to classify programming languages based on their features. Languages can be classified into multiple paradigms. Some paradigms are concerned mainly with impli ...
, lexical variable scope, and
recursion Recursion (adjective: ''recursive'') occurs when a thing is defined in terms of itself or of its type. Recursion is used in a variety of disciplines ranging from linguistics Linguistics is the scientific study of language, meaning tha ...
, with a static type system. By design, C provides constructs that map efficiently to typical
machine instructions In computer programming, machine code, consisting of machine language instruction set, instructions, is a low-level programming language used to directly control a computer's central processing unit (CPU). Each instruction causes the CPU to perf ...
. It has found lasting use in applications previously coded in
assembly language In computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a particular task. Programming involves tasks such as analysis, gene ...
. Such applications include
operating system An operating system (OS) is system software System software is software designed to provide a platform for other software. Examples of system software include operating systems (OS) like macOS, Linux, Android (operating system), Android and Mi ...

operating system
s and various
application software Application software (app for short) is computing software designed to carry out a specific task other than one relating to the operation of the computer itself, typically to be used by end-users. Examples of an application Application may refer ...
for computer architectures that range from
supercomputer upright=1.5, Computing power of the top 1 supercomputer each year, measured in FLOPS A supercomputer is a computer with a high level of performance as compared to a general-purpose computer. The performance of a supercomputer is commonly mea ...

supercomputer
s to
PLCs
PLCs
and
embedded system An embedded system is a computer system A computer is a machine that can be programmed to carry out Sequence, sequences of arithmetic or logical operations automatically. Modern computers can perform generic sets of operations known as Co ...
s. A successor to the programming language ''B'', C was originally developed at
Bell Labs Nokia Bell Labs (formerly named Bell Labs Innovations (1996–2007), AT&T Bell Laboratories (1984–1996) and Bell Telephone Laboratories (1925–1984)) is an American industrial research and scientific development company A company, ab ...
by
Dennis Ritchie Dennis MacAlistair Ritchie (September 9, 1941 – October 12, 2011) was an American . He created the and, with long-time colleague , the and . Ritchie and Thompson were awarded the from the in 1983, the from the in 1990 and the from in ...

Dennis Ritchie
between 1972 and 1973 to construct utilities running on
Unix Unix (; trademarked as UNIX) is a family of multitasking, multiuser Multi-user software is computer software Software is a collection of Instruction (computer science), instructions that tell a computer how to work. This is in contrast t ...

Unix
. It was applied to re-implementing the kernel of the Unix operating system. During the 1980s, C gradually gained popularity. It has become one of the most widely used programming languages, with C
compiler In computing, a compiler is a computer program that Translator (computing), translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily ...

compiler
s from various vendors available for the majority of existing
computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implementation of computer systems. The architecture of a system refers to its structure in terms of separately specifi ...
s and operating systems. C has been standardized by
ANSI The American National Standards Institute (ANSI ) is a private non-profit organization A nonprofit organization (NPO), also known as a non-business entity, not-for-profit organization, or nonprofit institution, is a legal entity organiz ...
since 1989 (
ANSI C ANSI C, ISO C and Standard C are successive standards for the C programming language C (, as in the letter ''c'') is a general-purpose, procedural computer programming language A programming language is a formal language comprising a ...
) and by the
International Organization for Standardization The International Organization for Standardization (ISO ) is an international standard An international standard is a technical standard A technical standard is an established norm Norm, the Norm or NORM may refer to: In academic discipline ...
(ISO). C is an
imperative Imperative may refer to: *Imperative mood, a grammatical mood (or mode) expressing commands, direct requests, and prohibitions *Imperative programming, a programming paradigm in computer science *Imperative logic *Imperative (film), ''Imperative'' ...
procedural language. It was designed to be
compiled In computing, a compiler is a computer program that Translator (computing), translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily ...

compiled
to provide low-level access to
memory Memory is the faculty of the brain A brain is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exis ...
and language constructs that map efficiently to
machine instructions In computer programming, machine code, consisting of machine language instruction set, instructions, is a low-level programming language used to directly control a computer's central processing unit (CPU). Each instruction causes the CPU to perf ...
, all with minimal runtime support. Despite its low-level capabilities, the language was designed to encourage cross-platform programming. A
standards Standard may refer to: Flags * Colours, standards and guidons * Standard (flag), a type of flag used for personal identification Norm, convention or requirement * Standard (metrology), an object that bears a defined relationship to a unit of ...
-compliant C program 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. Since 2000, C has consistently ranked among the top two languages in the
TIOBE index The TIOBE programming community index is a measure of popularity of programming languages, created and maintained by the TIOBE Software BV based in Eindhoven, the Netherlands The Netherlands ( nl, Nederland ), informally referred to as Hollan ...
, a measure of the popularity of programming languages.


Overview

Like most procedural languages in the
ALGOL Algol , designated Beta Persei (β Persei, abbreviated Beta Per, β Per), known colloquially as the Demon Star, is a bright multiple star in the constellation A constellation is an area on the celestial s ...
tradition, C has facilities for
structured programming Structured programming is a programming paradigm Programming paradigms are a way to classify programming languages based on their features. Languages can be classified into multiple paradigms. Some paradigms are concerned mainly with impli ...
and allows lexical variable scope and recursion. Its static
type system In programming language A programming language is a formal language comprising a Instruction set architecture, set of instructions that produce various kinds of Input/output, output. Programming languages are used in computer programming to ...
prevents unintended operations. In C, all
executable code In computing, executable code, an executable file, or an executable program, sometimes simply referred to as an executable or binary, causes a computer "to perform indicated tasks according to encoded instruction (computer science), instructions" ...
is contained within
subroutine In computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a particular task. Programming involves tasks such as analysis, generat ...
s (also called "functions", though not strictly in the sense of
functional programming In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of , ...
). Function parameters are always passed by value (except
arrays ARRAY, also known as ARRAY Now, is an independent distribution company launched by film maker and former publicist Ava DuVernay Ava Marie DuVernay (; born August 24, 1972) is an American filmmaker. She won the directing award in the U.S. drama ...
). Pass-by-reference is simulated in C by explicitly passing pointer values. C program source text is free-format, using the
semicolon The semicolon or semi-colon is a symbol commonly used as orthographic punctuation Punctuation (or sometimes interpunction) is the use of spacing, conventional signs (called punctuation marks), and certain typographical devices as aids to ...

semicolon
as a
statement Statement or statements may refer to: Common uses *Statement (computer science), the smallest standalone element of an imperative programming language *Statement (logic), declarative sentence that is either true or false *Statement, a Sentence_(lin ...
terminator and curly braces for grouping
blocks of statements In computer programming, a block or code block is a lexical structure of source code which is grouped together. Blocks consist of one or more Declaration (computer programming), declarations and Statement (computer science), statements. A program ...
. The C language also exhibits the following characteristics: * The language has a small, fixed number of keywords, including a full set of
control flow In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of Algor ...
primitives: if/else, , do/while,
while ''While'' is a word in the English language that functions both as a noun and as a Grammatical conjunction, subordinating conjunction. Its meaning varies largely based on its intended function, position in the phrase and even the writer or speake ...
, and
switch In electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics The field of electronics is a branch of p ...
. User-defined names are not distinguished from keywords by any kind of
sigil A sigil (; pl. ''sigilla'' or ''sigils'') is a type of symbol used in magic Magic or Magick may refer to: * Ceremonial magic, encompasses a wide variety of rituals of magic * Chaos magic#REDIRECT Chaos magic {{Redirect category shell, 1= { ...
. * It has a large number of arithmetic, bitwise, and logic operators: , etc. * More than one assignment may be performed in a single statement. * Functions: ** Function return values can be ignored, when not needed. ** Function and data pointers permit ''ad hoc'' run-time polymorphism. ** Functions may not be defined within the lexical scope of other functions. * Data typing is static, but weakly enforced; all data has a type, but
implicit conversion In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of Algor ...
s are possible. *
Declaration Declaration may refer to: Arts, entertainment, and media Literature * ''Declaration'' (book), a self-published electronic pamphlet by Michael Hardt and Antonio Negri * ''The Declaration'' (novel), a 2008 children's novel by Gemma Malley Music ...
syntax In linguistics Linguistics is the scientific study of language, meaning that it is a comprehensive, systematic, objective, and precise study of language. Linguistics encompasses the analysis of every aspect of language, as well as the ...
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 presence of a parenthesized argument list. * User-defined (
typedef typedef is a reserved keyword in the programming language A programming language is a formal language In logic, mathematics, computer science, and linguistics, a formal language consists of string (computer science), words whose symbol (fo ...
) and compound types are possible. ** Heterogeneous aggregate data types (
struct In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of Algori ...
) 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 ARRAY, also known as ARRAY Now, is an independent distribution company launched by film maker and former publicist Ava DuVernay Ava Marie DuVernay (; born August 24, 1972) is an American filmmaker. She won the directing award in the U.S. dram ...
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/code>. **
Enumerated type In computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a particular task. Programming involves tasks such as analysis, generati ...
s are possible with the enum keyword. They are freely interconvertible with integers. **
Strings String or strings may refer to: *String (structure), a long flexible structure made from threads twisted together, which is used to tie, bind, or hang other objects Arts, entertainment, and media Films * Strings (1991 film), ''Strings'' (1991 fil ...
are not a distinct data type, but are conventionally implemented as null-terminated character arrays. * Low-level access to
computer memory In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and soft ...
is possible by converting machine addresses to typed
pointers Pointer may refer to: Places * Pointer, Kentucky * Pointers, New Jersey * Pointers Airport, Wasco County, Oregon, United States * The Pointers, a pair of rocks off Antarctica People with the name * Pointer (surname), a surname (including a list of ...
. *
Procedures Procedure may refer to: * Medical procedure * Instructions or recipes, a set of commands that show how to achieve some result, such as to prepare or make something * Procedure (business), specifying parts of a business process * Standard operatin ...
(subroutines not returning values) are a special case of function, with an untyped return type void. * A
preprocessor In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of Algori ...
performs
macro Macro (or MACRO) may refer to: Science and technology * Macroscopic The macroscopic scale is the length scale on which objects or phenomena are large enough to be visible with the naked eye, without magnifying optical instruments. It is the o ...
definition,
source code In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and ...

source code
file inclusion, and
conditional compilationIn computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a specific task. Programming involves tasks such as: analysis, generatin ...
. * There is a basic form of
modularity Broadly speaking, modularity is the degree to which a system A system is a group of Interaction, interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its en ...
: 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 In computing, input/output (I/O, or informally io or IO) is the communication between an information processing system, such as a computer, and the outside world, possibly a human or another information processing system. Information, Inputs are t ...
,
string String or strings may refer to: *String (structure), a long flexible structure made from threads twisted together, which is used to tie, bind, or hang other objects Arts, entertainment, and media Films * Strings (1991 film), ''Strings'' (1991 fil ...
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 Waste collection is a part of the process of waste management. It is the transfer of solid waste from the point of use and disposal to the point of list of waste treatment technologies, treatment or landfill. Waste collection also includes the ...
), these can be implemented or emulated, often through the use of external libraries (e.g., the GLib Object System or the
Boehm garbage collector The Boehm–Demers–Weiser garbage collector, often simply known as Boehm GC, is a conservative Conservatism is a Political philosophy, political and social philosophy promoting traditional social institutions. The central tenets of conserv ...
).


Relations to other languages

Many later languages have borrowed directly or indirectly from C, including
C++ C++ () is a general-purpose programming language In computer software, a general-purpose programming language is a programming language dedicated to a general-purpose, designed to be used for writing software in a wide variety of application ...

C++
, C#, Unix's
C shell The C shell (csh or the improved version, tcsh) is a Unix shell created by Bill Joy while he was a graduate student at University of California, Berkeley in the late 1970s. It has been widely distributed, beginning with the 2BSD release of the B ...
, D, Go,
Java Java ( id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 147.7 million people, Java is the world's List of ...
,
JavaScript JavaScript (), often abbreviated JS, is a programming language A programming language is a formal language In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), ma ...

JavaScript
(including transpilers),
Julia Julia is usually a feminine given name. It is a Latinate feminine form of the name Julio (given name), Julio and Julius. (For further details on etymology, see wikt:Iulius#Latin, Wiktionary entry “Julius”.) The given name ''Julia'' had been ...
,
Limbo In Catholic theology Catholic theology is the understanding of Catholic doctrine or teachings, and results from the studies of theologians. It is based on Biblical canon, canonical Catholic Bible, scripture, and sacred tradition, as interp ...
, LPC,
Objective-C Objective-C is a general-purpose, object-oriented Object-oriented programming (OOP) is a programming paradigm Program, programme, programmer, or programming may refer to: Business and management * Program management, the process of m ...
,
Perl Perl is a family of two high-level High-level and low-level, as technical terms, are used to classify, describe and point to specific Objective (goal), goals of a systematic operation; and are applied in a wide range of contexts, such as, for ...
,
PHP PHP is a general-purpose scripting language A scripting language or script language is a programming language A programming language is a formal language comprising a Instruction set architecture, set of instructions that produce various k ...

PHP
,
Python PYTHON was a Cold War contingency plan of the Government of the United Kingdom, British Government for the continuity of government in the event of Nuclear warfare, nuclear war. Background Following the report of the Strath Committee in 1955, the ...
,
Ruby A ruby is a pink-ish red to blood-red colored gemstone A gemstone (also called a fine gem, jewel, precious stone, or semi-precious stone) is a piece of mineral In geology and mineralogy, a mineral or mineral species is, broadly spea ...
,
Rust Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous ferric oxides, hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxi ...
,
Swift The Society for Worldwide Interbank Financial Telecommunication (SWIFT), legally S.W.I.F.T. SCRL, is a Belgian Belgian may refer to: * Something of, or related to, Belgium Belgium, ; french: Belgique ; german: Belgien officially the ...
,
Verilog Verilog, standardized as IEEE 1364, is a hardware description language In computer engineering, a hardware description language (HDL) is a specialized computer language used to describe the structure and behavior of electronic circuits, and m ...
and
SystemVerilog SystemVerilog, standardized as IEEE 1800, is a hardware description and hardware verification language A hardware verification language, or HVL, is a programming language used to verify the designs of electronic circuits Electronic may refer to ...
(hardware description languages). These languages have drawn many of their
control structures In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of Algor ...
and other basic features from C. Most of them (Python being a dramatic exception) also express highly similar
syntax In linguistics Linguistics is the scientific study of language, meaning that it is a comprehensive, systematic, objective, and precise study of language. Linguistics encompasses the analysis of every aspect of language, as well as the ...
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


Early developments

The origin of C is closely tied to the development of the
Unix Unix (; trademarked as UNIX) is a family of multitasking, multiuser Multi-user software is computer software Software is a collection of Instruction (computer science), instructions that tell a computer how to work. This is in contrast t ...

Unix
operating system, originally implemented in
assembly language In computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a particular task. Programming involves tasks such as analysis, gene ...
on a
PDP-7 The PDP-7 was a minicomputer A minicomputer, or colloquially mini, is a class of smaller general purpose computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. ...
by Dennis Ritchie and Ken Thompson, incorporating several ideas from colleagues. Eventually, they decided to port the operating system to a
PDP-11 The PDP-11 is a series of 16-bit 16-bit microcomputer A microcomputer is a small, relatively inexpensive computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically ...
. The original PDP-11 version of Unix was also developed in assembly language. Thompson desired a programming language to make utilities for the new platform. At first, he tried to make a
Fortran Fortran (; formerly FORTRAN) is a general-purpose, compiled language, compiled imperative programming, imperative programming language that is especially suited to numerical analysis, numeric computation and computational science, scientific com ...

Fortran
compiler, but soon gave up the idea. Instead, he created a cut-down version of the recently developed
BCPL BCPL ("Basic Combined Programming Language") is a procedural, imperative Imperative may refer to: *Imperative mood, a grammatical mood (or mode) expressing commands, direct requests, and prohibitions *Imperative programming, a programming pa ...
systems programming language A system programming language is a programming language A programming language is a formal language comprising a Instruction set architecture, set of instructions that produce various kinds of Input/output, output. Programming languages are use ...
. The official description of BCPL was not available at the time, and Thompson modified the syntax to be less wordy, producing the similar but somewhat simpler B. However, few utilities were ultimately written in B because it was too slow, and B could not take advantage of PDP-11 features such as
byte The byte is a unit of digital information that most commonly consists of eight bit The bit is a basic unit of information in computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It ...
addressability. In 1972, Ritchie started to improve B, most notably adding data typing for variables, which resulted in creating a new language C. The C compiler and some utilities made with it were included in
Version 2 UnixResearch Unix refers to early versions of the Unix operating system for PDP-7, DEC PDP-7, PDP-11, VAX and Interdata 7/32 and 8/32 computers, developed in the Bell Labs Computing Sciences Research Center (CSRC). History The term ''Research Unix'' ...
. At
Version 4 UnixResearch Unix refers to early versions of the Unix Unix (; trademarked as UNIX) is a family of Computer multitasking, multitasking, multiuser computer operating systems that derive from the original AT&T Corporation, AT&T Unix, whose development ...
, released in November 1973, the
Unix Unix (; trademarked as UNIX) is a family of multitasking, multiuser Multi-user software is computer software Software is a collection of Instruction (computer science), instructions that tell a computer how to work. This is in contrast t ...

Unix
kernel Kernel may refer to: Computing * Kernel (operating system), the central component of most operating systems * Kernel (image processing), a matrix used for image convolution * Compute kernel, in GPGPU programming * Kernel method, in machine learnin ...
was extensively re-implemented in C. By this time, the C language had acquired some powerful features such as struct types. The
preprocessor In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of Algori ...
was introduced around 1973 at the urging of Alan Snyder and also in recognition of the usefulness of the file-inclusion mechanisms available in BCPL and PL/I. Its original version provided only included files and simple string replacements: #include and #define of parameterless macros. Soon after that, it was extended, mostly by
Mike Lesk Michael E. Lesk (born 1945) is an American computer scientist A computer scientist is a person who has acquired the knowledge of computer science Computer science deals with the theoretical foundations of information, algorithms and the a ...
and then by John Reiser, to incorporate macros with arguments and conditional compilation. Unix was one of the first operating system kernels implemented in a language other than
assembly Assembly may refer to: Organisations and meetings * Deliberative assembly A deliberative assembly is a gathering of members (of any kind of collective) who use parliamentary procedure Parliamentary procedure is the body of ethics, Procedural l ...
. Earlier instances include the
Multics Multics ("Multiplexed Information and Computing Service") is an influential early time-sharing In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experi ...

Multics
system (which was written in
PL/I PL/I (Programming Language One, pronounced and sometimes written PL/1) is a procedural, imperative Imperative may refer to: *Imperative mood, a grammatical mood (or mode) expressing commands, direct requests, and prohibitions *Imperative prog ...
) and Master Control Program (MCP) for the
Burroughs B5000Burroughs may refer to: * Former spelling of boroughs *Burroughs, Georgia, a historically African American community now a neighborhood of Savannah, Georgia * Burroughs Corporation, a maker of adding machines and computers * Burroughs (surname), peo ...
(which was written in
ALGOL Algol , designated Beta Persei (β Persei, abbreviated Beta Per, β Per), known colloquially as the Demon Star, is a bright multiple star in the constellation A constellation is an area on the celestial s ...
) 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 The Portable C Compiler (also known as pcc or sometimes pccm - portable C compiler machine) is an early compiler In computing, a compiler is a computer program that Translator (computing), translates computer code written in one programming lan ...
served as the basis for several implementations of C on new platforms. (Note: The PDF is an OCR scan of the original, and contains a rendering of "IBM 370" as "IBM 310".)


K&R C

In 1978,
Brian Kernighan Brian Wilson Kernighan (; born 1942) is a Canadian computer scientist A computer scientist is a person A person (plural people or persons) is a being that has certain capacities or attributes such as reason, morality, consciousness or self- ...
and
Dennis Ritchie Dennis MacAlistair Ritchie (September 9, 1941 – October 12, 2011) was an American . He created the and, with long-time colleague , the and . Ritchie and Thompson were awarded the from the in 1983, the from the in 1990 and the from in ...

Dennis Ritchie
published the first edition of ''
The C Programming Language ''The C Programming Language'' (sometimes termed ''K&R'', after its authors' initials) is a computer programming book written by Brian Kernighan and Dennis Ritchie, the latter of whom originally designed and implemented the language, as well as ...

The C Programming Language
''. This book, known to C programmers as ''K&R'', served for many years as an informal
specification A specification often refers to a set of documented requirements to be satisfied by a material, design, product, or service. A specification is often a type of technical standard A technical standard is an established norm (social), norm or require ...
of the language. The version of C that it describes is commonly referred to as "K&R C". As this was released in 1978, it is also referred to as ''C78''.
The second edition of the book covers the later
ANSI C ANSI C, ISO C and Standard C are successive standards for the C programming language C (, as in the letter ''c'') is a general-purpose, procedural computer programming language A programming language is a formal language comprising a ...
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 (computers), 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: long some_function(); /* int */ other_function(); /* int */ calling_function() 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) and some other vendors. These included: *
void Void may refer to: Science, engineering, and technology * Void (astronomy) Cosmic voids are vast spaces between filaments (the largest-scale structures in the universe The universe ( la, universus) is all of space and time and their c ...
functions (i.e., functions with no return value) * functions returning
struct In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of Algori ...
or union types (rather than pointers) * assignment for struct data types *
enumerated type In computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a particular task. Programming involves tasks such as analysis, generati ...
s The large number of extensions and lack of agreement on a
standard library A standard library in computer programming is the library (computing), library made available across implementations of a programming language. These libraries are conventionally described in programming language specifications; however, contents o ...
, together with the language popularity and the fact that not even the Unix compilers precisely implemented the K&R specification, led to the necessity of standardization.


ANSI C and ISO C

During the late 1970s and 1980s, versions of C were implemented for a wide variety of
mainframe computer A mainframe computer, informally called a mainframe or big iron, is a computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern computers can perform gener ...
s,
minicomputer A minicomputer, or colloquially mini, is a class of smaller general purpose computers that developed in the mid-1960s and sold for much less than Mainframe computer, mainframe and mid-size computers from IBM and BUNCH, its direct competitors. In ...
s, and
microcomputer A microcomputer is a small, relatively inexpensive computer with a microprocessor as its central processing unit (CPU). It includes a microprocessor, Computer memory, memory and minimal input/output (I/O) circuitry mounted on a single printed ci ...
s, including the
IBM PC The IBM Personal Computer (model 5150, commonly known as the IBM PC) is the first computer released in the IBM PC model line and the basis for the IBM PC compatible IBM PC compatible computers are similar to the original IBM PC The IBM ...

IBM PC
, as its popularity began to increase significantly. In 1983, the
American National Standards Institute The American National Standards Institute (ANSI ) is a private non-profit organization that oversees the development of Standardization, voluntary consensus standards for products, services, processes, systems, and personnel in the United Sta ...
(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 Institute of Electrical and Electronics Engineers, IEEE working 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 ANSI C, ISO C and Standard C are successive standards for the C programming language C (, as in the letter ''c'') is a general-purpose, procedural computer programming language A programming language is a formal language comprising a ...
, Standard C, or sometimes C89. In 1990, the ANSI C standard (with formatting changes) was adopted by the
International Organization for Standardization The International Organization for Standardization (ISO ) is an international standard An international standard is a technical standard A technical standard is an established norm Norm, the Norm or NORM may refer to: In academic discipline ...
(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 encoding, character sets and Locale (computer software), locales, and preprocessor enhancements. Although the
syntax In linguistics Linguistics is the scientific study of language, meaning that it is a comprehensive, systematic, objective, and precise study of language. Linguistics encompasses the analysis of every aspect of language, as well as the ...
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 modern C code is based on it. Any program written only in Standard C and without any hardware-dependent assumptions will run correctly on any Computing platform, 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 Graphical user interface, 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.


C99

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. 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 BCPL ("Basic Combined Programming Language") is a procedural, imperative Imperative may refer to: *Imperative mood, a grammatical mood (or mode) expressing commands, direct requests, and prohibitions *Imperative programming, a programming pa ...
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. GNU Compiler Collection, GCC, Sun Studio (software), 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. In addition, support for Unicode identifiers (variable / function names) in the form of escaped characters (e.g. ) is now required. Support for raw Unicode names is optional.


C11

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.


C17

Published in June 2018, C17 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.


C2x

C2x is an informal name for the next (after C17) major C language standard revision. It is expected to be voted on in 2023 and would therefore be called C23.


Embedded C

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 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

C has a formal grammar specified by the C standard. Contains a Backus-Naur form, BNF grammar for C. 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 literal, string or character literals. C source files contain declarations and function definitions. Function definitions, in turn, contain declarations and Statement (computer science), 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 (, 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 (computer science), side effect of the evaluation, functions may be procedure call, called and variables may be assignment (computer science), 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 dowhile, 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 (computer science), 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." 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

The basic C source character set includes the following characters: * Lowercase and uppercase letters of ISO Basic Latin Alphabet: az AZ * Decimal digits: 09 * Graphic characters: ! " # % & ' ( ) * + , - . / : ; < = > ? [ \ ] ^ _ ~ * Whitespace characters: ''space (punctuation), space'', ''Tab character, horizontal tab'', ''Tab character, vertical tab'', ''Page break, 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 software portability, portable. The latest C standard (C11 (C standard revision), 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 Bell character, alert, backspace, and carriage return. Run time (program lifecycle phase), Run-time support for extended character sets has increased with each revision of the C standard.


Reserved words

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: * auto * break * case * char * const (computer programming), const * continue * default * do * double precision, double * else * enumerated type, enum * External variable, extern * floating point, float * for loop, for * goto * if * int * long integer, long * register (C programming language), register * return * short integer, short * signed * sizeof * Static (keyword), static *
struct In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of Algori ...
* switch statement, switch * typedef * union * unsigned *
void Void may refer to: Science, engineering, and technology * Void (astronomy) Cosmic voids are vast spaces between filaments (the largest-scale structures in the universe The universe ( la, universus) is all of space and time and their c ...
* volatile variable, volatile * while loop, while C99 reserved five more words: * _Bool * _Complex * _Imaginary * Inline function, inline * restrict C11 reserved seven more words: * _Alignas * _Alignof * _Atomic * _Generic * _Noreturn * _Static_assert * _Thread_local 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.


Operators

C supports a rich set of Operator (computer programming), operators, which are symbols used within an Expression (computer science), expression to specify the manipulations to be performed while evaluating that expression. C has operators for: * arithmetic: addition, +, subtraction, -, multiplication, *, division (mathematics), /, modulo operation, % * assignment: = * augmented assignment: * Bitwise operation, bitwise logic: ~, &, , , ^ * Bit shift, bitwise shifts: <<, >> * Boolean algebra, boolean logic: !, &&, , , * ?:, conditional evaluation: ?:, ? : * equality testing: equality (mathematics),

, inequality (mathematics), != * Subroutine, calling functions: ( ) * increment and decrement operators, increment and decrement: ++, -- * record (computer science), member selection: ., -> * object size: sizeof * Order theory, order relations: <, <=, >, >= * pointer (computer programming), reference and dereference: &, *, [ ] * sequencing: comma operator, , * order of operations#Programming languages, subexpression grouping: ( ) * type conversion: (''typename'') C uses the operator = (used in mathematics to express equality) to indicate assignment, following the precedent of
Fortran Fortran (; formerly FORTRAN) is a general-purpose, compiled language, compiled imperative programming, imperative programming language that is especially suited to numerical analysis, numeric computation and computational science, scientific com ...

Fortran
and
PL/I PL/I (Programming Language One, pronounced and sometimes written PL/1) is a procedural, imperative Imperative may refer to: *Imperative mood, a grammatical mood (or mode) expressing commands, direct requests, and prohibitions *Imperative prog ...
, but unlike
ALGOL Algol , designated Beta Persei (β Persei, abbreviated Beta Per, β Per), known colloquially as the Demon Star, is a bright multiple star in the constellation A constellation is an area on the celestial s ...
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 (a

b + 1)
might mistakenly be written as if (a = b + 1), which will be evaluated as true if a is not zero after the assignment. The C Order of operations, 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.


"Hello, world" example

The ""Hello, World!" program, hello, world" example, which appeared in the first edition of ''The C Programming Language, K&R'', has become the model for an introductory program in most programming textbooks. The program prints "hello, world" to the standard output, which is usually a terminal or screen display. The original version was: main() A standard-conforming "hello, world" program is: #include int main(void) The first line of the program contains a Preprocessor, 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 and scanf. 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 (programming), 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. 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 (computing), library. In this call, the printf function is ''passed'' (provided with) a single argument, the address of the first character in the string literal "hello, world\n". The string literal is an unnamed Array data type, 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 sequences in C, 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.


Data types

The
type system In programming language A programming language is a formal language comprising a Instruction set architecture, set of instructions that produce various kinds of Input/output, output. Programming languages are used in computer programming to ...
in C is static typing, static and weak typing, weakly typed, which makes it similar to the type system of
ALGOL Algol , designated Beta Persei (β Persei, abbreviated Beta Per, β Per), known colloquially as the Demon Star, is a bright multiple star in the constellation A constellation is an area on the celestial s ...
descendants such as Pascal (programming language), Pascal. 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 ARRAY, also known as ARRAY Now, is an independent distribution company launched by film maker and former publicist Ava DuVernay Ava Marie DuVernay (; born August 24, 1972) is an American filmmaker. She won the directing award in the U.S. drama ...
,
pointers Pointer may refer to: Places * Pointer, Kentucky * Pointers, New Jersey * Pointers Airport, Wasco County, Oregon, United States * The Pointers, a pair of rocks off Antarctica People with the name * Pointer (surname), a surname (including a list of ...
, record (computer science), records (
struct In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of Algori ...
), and union (computer science), 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 conversion, 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".) 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

C supports the use of
pointers Pointer may refer to: Places * Pointer, Kentucky * Pointers, New Jersey * Pointers Airport, Wasco County, Oregon, United States * The Pointers, a pair of rocks off Antarctica People with the name * Pointer (surname), a surname (including a list of ...
, a type of reference (computer science), 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. String (computer science), Text strings are commonly manipulated using pointers into arrays of characters. Dynamic memory allocation is performed using pointers. Many data types, such as tree (data structure), 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 callback (computer science), callbacks to be invoked by event handlers. 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. 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 (computer science), reference types.


Arrays

Array ARRAY, also known as ARRAY Now, is an independent distribution company launched by film maker and former publicist Ava DuVernay Ava Marie DuVernay (; born August 24, 1972) is an American filmmaker. She won the directing award in the U.S. dram ...
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. 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.For example, gcc provides _FORTIFY_SOURCE. Array bounds violations are therefore possible and can lead to various repercussions, including illegal memory accesses, corruption of data, buffer overflow, buffer overruns, and run-time exceptions. C does not have a special provision for declaring multi-dimensional arrays, but rather relies on
recursion Recursion (adjective: ''recursive'') occurs when a thing is defined in terms of itself or of its type. Recursion is used in a variety of disciplines ranging from linguistics Linguistics is the scientific study of language, meaning tha ...
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, in early versions of C 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 was to allocate the array with an additional "row vector" of pointers to the columns.) C99 introduced "variable-length arrays" which address this issue. The following example using modern C (C99 or later) shows allocation of a two-dimensional array on the heap and the use of multi-dimensional array indexing for accesses (which can use bounds-checking on many C compilers): int func(int N, int M)


Array–pointer interchangeability

The subscript notation x[i] (where x designates a pointer) is syntactic sugar for *(x+i). 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), an expression of array type 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 call-by-value, pass-by-value semantics, arrays are in effect passed by reference (computer science), reference. The total size of an array x can be determined by applying sizeof to an expression of array type. The size of an element can be determined by applying the operator sizeof to any dereferenced element of an array A, as in n = sizeof A[0]. This, the number of elements in a declared array A can be determined as sizeof A / sizeof A[0]. Note, that if only a pointer to the first element is available as it is often the case in C code because of the automatic conversion described above, the information about the full type of the array and its length are lost.


Memory management

One of the most important functions of a programming language is to provide facilities for managing computer memory, memory and the objects that are stored in memory. C provides three distinct ways to allocate memory for objects: * Static memory allocation: space for the object is provided in the binary at compile-time; these objects have an Variable (programming)#Scope and extent, 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 call stack, 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 dynamic memory allocation, heap; these blocks persist until subsequently freed for reuse by calling the library function malloc, realloc or malloc, 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. 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 (computing), linker or Loader (computing), 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, bit pattern happens to be present in the computer storage, 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 Type I and type II errors, false positives and false negatives can occur. Heap memory allocation has to be synchronized with its actual usage in any program 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 it is deallocated explicitly, 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 is referenced subsequently, leading to unpredictable results. Typically, the failure symptoms appear in a portion of the program unrelated to the code that causes the error, making it difficult to diagnose the failure. Such issues are ameliorated in languages with garbage collection (computer science), automatic garbage collection.


Libraries

The C programming language uses library (computing), 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"). The most common C library is the C standard library, which is specified by the ISO standard, ISO and
ANSI C ANSI C, ISO C and Standard C are successive standards for the C programming language C (, as in the letter ''c'') is a general-purpose, procedural computer programming language A programming language is a formal language comprising a ...
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 Unix (; trademarked as UNIX) is a family of multitasking, multiuser Multi-user software is computer software Software is a collection of Instruction (computer science), instructions that tell a computer how to work. This is in contrast t ...

Unix
and Unix-like systems, especially functions which provide an interface to the kernel (operating system), 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 Java ( id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 147.7 million people, Java is the world's List of ...
,
Perl Perl is a family of two high-level High-level and low-level, as technical terms, are used to classify, describe and point to specific Objective (goal), goals of a systematic operation; and are applied in a wide range of contexts, such as, for ...
, and
Python PYTHON was a Cold War contingency plan of the Government of the United Kingdom, British Government for the continuity of government in the event of Nuclear warfare, nuclear war. Background Following the report of the Strath Committee in 1955, the ...
.


File handling and streams

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 stream (computing), 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 data buffer, 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
operating system An operating system (OS) is system software System software is software designed to provide a platform for other software. Examples of system software include operating systems (OS) like macOS, Linux, Android (operating system), Android and Mi ...

operating system
such as most embedded programming). With few exceptions, implementations include low-level I/O.


Language tools

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 (software), 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 (software), Lint. A common practice is to use Lint to detect questionable code when a program is first written. Once a program passes Lint, it is then compiled using the C compiler. Also, many compilers can optionally warn about syntactically valid constructs that are likely to actually be errors. MISRA C is a proprietary set of guidelines to avoid such questionable code, developed for embedded systems. There are also compilers, libraries, and operating system level mechanisms for performing actions that are not a standard part of C, such as bounds checking for arrays, detection of buffer overflow, serialization, Memory management, dynamic memory tracking, and garbage collection (computer science), automatic garbage collection. Tools such as IBM Rational Purify, Purify or Valgrind and linking with libraries containing special versions of the malloc, memory allocation functions can help uncover runtime errors in memory usage.


Uses

C is widely used for systems programming in implementing
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applications, 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 (program lifecycle phase), run-time demand on system resources. C can be used for website programming using the Common Gateway Interface (CGI) as a "gateway" for information between the Web application, the server, and the browser. C is often chosen over interpreted languages because of its speed, stability, and near-universal availability. A consequence of C's wide availability and efficiency is that
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compiler
s, libraries and Interpreter (computing), interpreters of other programming languages are often implemented in C. For example, the reference implementations of
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, and
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are written in C. C enables programmers to create efficient implementations of algorithms and data structures, because the layer of abstraction from hardware is thin, and its overhead is low, an important criterion for computationally intensive 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 List of C-family programming languages, C-based languages specifically designed for use as intermediate languages, such as C--. C has also been widely used to implement end-user (computer science), end-user applications. However, such applications can also be written in newer, higher-level languages.


Related languages

C has both directly and indirectly influenced many later languages such as C#, D, Go,
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, LPC,
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,
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, and Unix's
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. 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 (computer programming), Ch and CINT, which can also be used for scripting. When object-oriented programming languages became popular,
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and
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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. The
C++ C++ () is a general-purpose programming language In computer software, a general-purpose programming language is a programming language dedicated to a general-purpose, designed to be used for writing software in a wide variety of application ...

C++
programming language (originally named "C with Class (programming), Classes") was devised by Bjarne Stroustrup as an approach to providing Object-oriented programming, object-oriented functionality with a C-like syntax. C++ adds greater typing strength, scoping, and other tools useful in object-oriented programming, and permits generic programming via templates. Nearly a superset of C, C++ now supports most of C, with Compatibility of C and C++, a few exceptions.
Objective-C Objective-C is a general-purpose, object-oriented Object-oriented programming (OOP) is a programming paradigm Program, programme, programmer, or programming may refer to: Business and management * Program management, the process of m ...
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
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C++
and
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, Ch (computer programming), Ch, Cilk, and Unified Parallel C are nearly supersets of C.


See also

* Compatibility of C and C++ * Comparison of Pascal and C * Comparison of programming languages * International Obfuscated C Code Contest * List of C-based programming languages * List of C compilers


Notes


References


Sources

*
*


Further reading

* ''(archive)''
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/small>


External links


ISO C Working Group official website
*
ISO/IEC 9899
publicly available official C documents, including the C99 Rationale **  
comp.lang.c Frequently Asked Questions


by Dennis Ritchie {{Authority control C (programming language), American inventions Articles with example C code C programming language family Cross-platform software High-level programming languages Procedural programming languages Structured programming languages Programming languages created in 1972 Programming languages with an ISO standard Statically typed programming languages Systems programming languages