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General-purpose programming language

C
Major implementations
The C Programming Language ^[1] (frequently referred to as K&R), the seminal volume on C
Image	Multi-prototype: imperative (procedural), structured
Designed by	Dennis Ritchie
Programmer	Dennis Ritchie & Bell Labs (creators); ANSI X3J11 (ANSI C); ISO/IEC JTC1/SC22/WG14 (ISO C)
Outset appeared	1972; fifty years ago (1972) ^[2]

Stable release	C17 / June 2018; iii years ago (2018-06)
Preview release	C2x (N2731) / October xviii, 2021; iv months agone (2021-ten-18) ^[3]

Typing subject	Static, weak, manifest, nominal
Bone	Cross-platform
Filename extensions	.c, .h
Website	world wide web.iso.org/standard/74528.html www.open-std.org/jtc1/sc22/wg14/
pcc, GCC, Clang, Intel C, C++Builder, Microsoft Visual C++, Watcom C
Dialects
Cyclone, Unified Parallel C, Divide-C, Cilk, C*
Influenced by
B (BCPL, CPL), ALGOL 68,^[4] associates, PL/I, FORTRAN
Influenced
Numerous: AMPL, AWK, csh, C++, C--, C#, Objective-C, D, Go, Coffee, JavaScript, JS++, Julia, Limbo, LPC, Perl, PHP, Pike, Processing, Python, Ring,^[5]Rust, Seed7, Vala, Verilog (HDL),^[half-dozen] Nim, Zig
C Programming at Wikibooks

C (, as in the letter c) is a full general-purpose, procedural computer programming language supporting structured programming, lexical variable scope, and recursion, with a static blazon system. By design, C provides constructs that map efficiently to typical automobile instructions. Information technology has institute lasting use in applications previously coded in assembly language. Such applications include operating systems and diverse application software for computer architectures that range from supercomputers to PLCs and embedded systems.

A successor to the programming language B, C was originally adult at Bell Labs past Dennis Ritchie between 1972 and 1973 to construct utilities running on Unix. It was practical to re-implementing the kernel of the Unix operating system.^[7] During the 1980s, C gradually gained popularity. Information technology has become one of the most widely used programming languages,^[eight] ^[9] with C compilers from diverse vendors available for the majority of existing estimator architectures and operating systems. C has been standardized by ANSI since 1989 (ANSI C) and by the International Organisation for Standardization (ISO).

C is an imperative procedural language. Information technology was designed to be compiled to provide low-level access to memory and language constructs that map efficiently to machine instructions, all with minimal runtime support. Despite its low-level capabilities, the linguistic communication was designed to encourage cantankerous-platform programming. A standards-compliant C program written with portability in mind tin can exist compiled for a wide diverseness of figurer platforms and operating systems with few changes to its source lawmaking.^[10]

Since 2000, C has consistently ranked amidst the top two languages in the TIOBE index, a measure of the popularity of programming languages.^[11]

Overview [edit]

Similar nigh procedural languages in the ALGOL tradition, C has facilities for structured programming and allows lexical variable telescopic and recursion. Its static type system prevents unintended operations. In C, all executable code is independent inside subroutines (also chosen "functions", though not strictly in the sense of functional programming). Function parameters are always passed past value (except arrays). Laissez passer-by-reference is simulated in C by explicitly passing arrow values. C program source text is complimentary-format, using the semicolon as a statement terminator and curly braces for group blocks of statements.

The C linguistic communication also exhibits the following characteristics:

The language has 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.
It has a large number of arithmetics, bitwise, and logic operators: +,+=,++,&,||, etc.
More than one assignment may be performed in a single argument.
Functions:
- Role return values can be ignored, when not needed.
- Function and data pointers permit ad hoc run-fourth dimension polymorphism.
- Functions may not exist defined inside the lexical scope of other functions.
Information typing is static, but weakly enforced; all data has a type, but implicit conversions are possible.
Announcement syntax mimics usage context. C has no "define" keyword; instead, a argument beginning with the proper 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 listing.
User-divers (typedef) and chemical compound types are possible.
- Heterogeneous amass information types (struct) allow related information elements to exist accessed and assigned as a unit of measurement.
- Union is a structure with overlapping members; merely the concluding member stored is valid.
- Array indexing is a secondary note, defined in terms of pointer arithmetic. Unlike structs, arrays are not showtime-class objects: they cannot be assigned or compared using unmarried congenital-in operators. There is no "array" keyword in use or definition; instead, square brackets point arrays syntactically, for case calendar month[11].
- Enumerated types are possible with the enum keyword. They are freely interconvertible with integers.
- Strings are not a distinct data type, merely are conventionally implemented as nix-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 instance of function, with an untyped return type void.
A preprocessor performs macro definition, source code file inclusion, and conditional compilation.
In that location is a basic grade of modularity: files tin can exist compiled separately and linked together, with command over which functions and information objects are visible to other files via static and extern attributes.
Circuitous functionality such equally I/O, string manipulation, and mathematical functions are consistently delegated to library routines.

While C does non include certain features plant in other languages (such as object orientation and garbage collection), these tin be implemented or emulated, often through the apply of external libraries (e.one thousand., the GLib Object System or the Boehm garbage collector).

Relations to other languages [edit]

Many afterward languages have borrowed direct or indirectly from C, including C++, C#, Unix's C shell, D, Go, Java, JavaScript (including transpilers), Julia, Limbo, LPC, Objective-C, Perl, PHP, Python, Cerise, Rust, Swift, Verilog and SystemVerilog (hardware clarification languages).^{[half dozen]} These languages have drawn many of their command structures and other basic features from C. Well-nigh of them (Python being a dramatic exception) also limited 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 tin be radically different.

History [edit]

Early developments [edit]

Timeline of language development
Year	C Standard^[10]
1972	Nativity
1978	K&R C
1989/1990	ANSI C and ISO C
1999	C99
2011	C11
2017	C17
TBD	C2x

The origin of C is closely tied to the development of the Unix operating organisation, originally implemented in associates language on a PDP-7 past Dennis Ritchie and Ken Thompson, incorporating several ideas from colleagues. Somewhen, they decided to port the operating arrangement to a PDP-11. The original PDP-11 version of Unix was also developed in assembly language.^[7]

Thompson desired a programming language to make utilities for the new platform. At offset, he tried to brand a Fortran compiler, only soon gave upward the idea. Instead, he created a cut-down version of the recently adult BCPL systems programming language. The official description of BCPL was not available at the time,^[12] and Thompson modified the syntax to exist less wordy, producing the similar but somewhat simpler B.^[7] However, few utilities were ultimately written in B because it was also dull, and B could not have reward of PDP-11 features such as byte addressability.

In 1972, Ritchie started to amend B, most notably calculation data typing for variables, which resulted in creating a new language C.^[13] The C compiler and some utilities made with it were included in Version 2 Unix.^[14]

At Version 4 Unix, released in November 1973, the Unix kernel was extensively re-implemented in C.^[7] Past this time, the C language had caused some powerful features such as struct types.

The preprocessor was introduced around 1973 at the urging of Alan Snyder and likewise in recognition of the usefulness of the file-inclusion mechanisms bachelor in BCPL and PL/I. Its original version provided only included files and simple string replacements: #include and #define of parameterless macros. Soon later that, it was extended, mostly by Mike Lesk then by John Reiser, to comprise macros with arguments and conditional compilation.^[7]

Unix was one of the first operating organisation kernels implemented in a linguistic communication 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 fabricated 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.^[thirteen]

K&R C [edit]

In 1978, Brian Kernighan and Dennis Ritchie published the first edition of The C Programming Linguistic communication.^[i] This book, known to C programmers equally 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 "M&R C". As this was released in 1978, it is besides referred to as C78.^[15] The 2nd edition of the book^[16] covers the later ANSI C standard, described below.

Chiliad&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 inverse to the form op= (that is, -=) to remove the semantic ambivalence created by constructs such as i=-10, which had been interpreted as i =- 10 (decrement i by 10) instead of the peradventure intended i = -ten (let i be −10).

Even after the publication of the 1989 ANSI standard, for many years K&R C was still considered the "everyman common denominator" to which C programmers restricted themselves when maximum portability was desired, since many older compilers were withal in use, and because carefully written K&R C code tin can be legal Standard C as well.

In early versions of C, only functions that return types other than int must exist alleged if used before the role definition; functions used without prior declaration were presumed to return type int.

For case:

                        long                                    some_function            ();                        /* int */                                    other_function            ();                        /* int */                                    calling_function            ()                        {                                                long                                    test1            ;                                                annals                                    /* int */                                    test2            ;                                                test1                                    =                                    some_function            ();                                                if                                    (            test1                                    >                                    ane            )                                                test2                                    =                                    0            ;                                                else                                                test2                                    =                                    other_function            ();                                                return                                    test2            ;                        }

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

Since Thousand&R role declarations did not include any information about office arguments, office parameter type checks were non performed, although some compilers would outcome a warning message if a local function was called with the wrong number of arguments, or if multiple calls to an external role used unlike numbers or types of arguments. Divide tools such as Unix's lint utility were developed that (among other things) could check for consistency of role employ beyond 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^[17]) and some other vendors. These included:

void functions (i.e., functions with no render value)
functions returning struct or matrimony types (rather than pointers)
assignment for struct data types
enumerated types

The large number of extensions and lack of agreement on a standard library, together with the language popularity and the fact that not fifty-fifty the Unix compilers precisely implemented the G&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 diversity of mainframe computers, minicomputers, and microcomputers, including the IBM PC, equally its popularity began to increment significantly.

In 1983, the American National Standards Establish (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 IEEE working group 1003 to get 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 linguistic communication.

ANSI, similar 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 inside a year of ISO publication.

One of the aims of the C standardization procedure was to produce a superset of Thousand&R C, incorporating many of the afterwards introduced unofficial features. The standards committee also included several boosted features such every bit role prototypes (borrowed from C++), void pointers, back up 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 connected to be permitted, for compatibility with existing source lawmaking.

C89 is supported by current C compilers, and most modernistic C lawmaking is based on it. Whatever 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 item compiler, due, for example, to the use of not-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 past either standard-conforming or K&R C-based compilers, the __STDC__ macro can exist used to split the code into Standard and K&R sections to prevent the utilize on a K&R C-based compiler of features available only in Standard C.

After the ANSI/ISO standardization process, the C linguistic communication 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 together more extensive support for international grapheme sets.^[xviii]

C99 [edit]

The C standard was further revised in the tardily 1990s, leading to the publication of ISO/IEC 9899:1999 in 1999, which is commonly referred to every bit "C99". Information technology has since been amended iii times past Technical Corrigenda.^[19]

C99 introduced several new features, including inline functions, several new data types (including long long int and a circuitous type to represent circuitous numbers), variable-length arrays and flexible assortment members, improved support for IEEE 754 floating indicate, support for variadic macros (macros of variable arity), and support for ane-line comments beginning with //, as in BCPL or C++. Many of these had already been implemented every bit extensions in several C compilers.

C99 is for the nearly function backward compatible with C90, just is stricter in some ways; in particular, a declaration that lacks a blazon specifier no longer has int implicitly assumed. A standard macro __STDC_VERSION__ is defined with value 199901L to indicate that C99 back up 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++, notwithstanding, implements the C89 standard and those parts of C99 that are required for compatibility with C++11.^[twenty] ^{[ needs update ]}

In addition, support for Unicode identifiers (variable / function names) in the form of escaped characters (e.thou. \U0001f431) is now required. Support for raw Unicode names is optional.

C11 [edit]

In 2007, work began on some other revision of the C standard, informally chosen "C1X" until its official publication on 2011-12-08. The C standards commission adopted guidelines to limit the adoption of new features that had non been tested past 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 likewise makes some portions of the existing C99 library optional, and improves compatibility with C++. The standard macro __STDC_VERSION__ is defined equally 201112L to indicate that C11 support is available.

C17 [edit]

Published in June 2018, C17 is the current standard for the C programming language. It introduces no new linguistic communication features, just technical corrections, and clarifications to defects in C11. The standard macro __STDC_VERSION__ is defined as 201710L.

C2x [edit]

C2x is an breezy name for the adjacent (after C17) major C linguistic communication standard revision. It is expected to be voted on in 2023 and would therefore be called C23.^[21] ^{[ improve source needed ]}

Embedded C [edit]

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

In 2008, the C Standards Commission published a technical written report extending the C language^[22] to accost these problems by providing a mutual standard for all implementations to attach to. It includes a number of features non available in normal C, such as fixed-bespeak arithmetics, named address spaces, and basic I/O hardware addressing.

Syntax [edit]

C has a formal grammar specified by the C standard.^[23] Line endings are generally non meaning in C; notwithstanding, line boundaries do accept significance during the preprocessing phase. Comments may announced either betwixt the delimiters /* and */, or (since C99) following // until the terminate of the line. Comments delimited by /* and */ practice not nest, and these sequences of characters are not interpreted as comment delimiters if they announced inside string or character literals.^[24]

C source files contain declarations and function definitions. Part definitions, in turn, contain declarations and statements. Declarations either define new types using keywords such as struct, wedlock, and enum, or assign types to and perchance reserve storage for new variables, usually by writing the type followed by the variable name. Keywords such as char and int specify congenital-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 unmarried statement for control structures.

As an imperative linguistic communication, C uses statements to specify actions. The almost mutual statement is an expression statement, consisting of an expression to exist 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 past if … [else] conditional execution and by do … while, while, and for iterative execution (looping). The for statement has split up initialization, testing, and reinitialization expressions, any or all of which can be omitted. break and continue can be used to go out the innermost enclosing loop argument or skip to its reinitialization. There is also a non-structured goto statement which branches directly to the designated label inside the function. switch selects a case to be executed based on the value of an integer expression.

Expressions tin 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 fifty-fifty be interleaved. Nevertheless, all side effects (including storage to variables) volition occur before the next "sequence point"; sequence points include the end of each expression statement, and the entry to and return from each role call. Sequence points as well occur during evaluation of expressions containing certain operators (&&, ||, ?: and the comma operator). This permits a high caste of object lawmaking optimization by the compiler, but requires C programmers to take more intendance to obtain reliable results than is needed for other programming languages.

Kernighan and Ritchie say in the Introduction of The C Programming Language: "C, similar whatsoever other linguistic communication, has its blemishes. Some of the operators have the wrong precedence; some parts of the syntax could be meliorate."^[25] The C standard did not attempt to correct many of these blemishes, because of the impact of such changes on already existing software.

Graphic symbol set up [edit]

The basic C source character ready includes the following characters:

Lowercase and upper-case letter messages of ISO Bones Latin Alphabet: a–z A–Z
Decimal digits: 0–nine
Graphic characters: ! " # % & ' ( ) * + , - . / : ; < = > ? [ \ ] ^ _ { | } ~
Whitespace characters: infinite, horizontal tab, vertical tab, grade feed, newline

Newline indicates the end of a text line; it need not represent to an actual single character, although for convenience C treats information technology equally 1.

Additional multi-byte encoded characters may exist used in string literals, merely 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 characteristic is non yet widely implemented.

The bones C execution character set contains the same characters, along with representations for alert, backspace, and carriage render. Run-fourth dimension support for extended graphic symbol sets has increased with each revision of the C standard.

Reserved words [edit]

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

auto
break
instance
char
const
proceed
default
practise
double
else
enum
extern
float
for
goto
if
int
long
register
return
brusque
signed
sizeof
static
struct
switch
typedef
union
unsigned
void
volatile
while

C99 reserved five more words:

_Bool
_Complex
_Imaginary
inline
restrict

C11 reserved seven more words:^[26]

_Alignas
_Alignof
_Atomic
_Generic
_Noreturn
_Static_assert
_Thread_local

Most of the recently reserved words begin with an underscore followed past a capital letter of the alphabet, because identifiers of that course were previously reserved past the C standard for employ only by implementations. Since existing program source lawmaking should non 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 ascertain more convenient synonyms for underscored identifiers. The linguistic communication previously included a reserved word chosen entry, just this was seldom implemented, and has now been removed equally a reserved word.^[27]

Operators [edit]

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

arithmetics: +, -, *, /, %
consignment: =
augmented assignment: +=, -=, *=, /=, %=, &=, |=, ^=, <<=, >>=
bitwise logic: ~, &, |, ^
bitwise shifts: <<, >>
boolean logic: !, &&, ||
provisional evaluation: ? :
equality testing: ==, !=
calling functions: ( )
increment and decrement: ++, --
member pick: ., ->
object size: sizeof
social club relations: <, <=, >, >=
reference and dereference: &, *, [ ]
sequencing: ,
subexpression grouping: ( )
blazon conversion: (typename)

C uses the operator = (used in mathematics to express equality) to bespeak assignment, post-obit the precedent of Fortran and PL/I, but unlike ALGOL and its derivatives. C uses the operator == to examination for equality. The similarity between these two operators (assignment and equality) may event in the adventitious use of one in place of the other, and in many cases, the error does not produce an error message (although some compilers produce warnings). For case, the conditional expression if (a == b + i) might mistakenly be written every bit if (a = b + 1), which will exist evaluated as true if a is not zero after the assignment.^[28]

The C operator precedence is not ever 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 & ane == 0, which must be written every bit (x & i) == 0 if that is the coder'due south intent.^[29]

"Hello, earth" case [edit]

The "hello, earth" case, which appeared in the get-go edition of One thousand&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:^[30]

                        main            ()                        {                                                printf            (            "how-do-you-do, world            \n            "            );                        }

A standard-conforming "hi, world" program is:^[a]

                        #include                                    <stdio.h>                        int                                    main            (            void            )                        {                                                printf            (            "hello, globe            \n            "            );                        }

The get-go line of the plan contains a preprocessing directive, indicated by #include. This causes the compiler to supervene upon that line with the entire text of the stdio.h standard header, which contains declarations for standard input and output functions such every bit printf and scanf. The bending brackets surrounding stdio.h point that stdio.h is located using a search strategy that prefers headers provided with the compiler to other headers having the same proper name, as opposed to double quotes which typically include local or project-specific header files.

The adjacent line indicates that a part named main is being divers. The chief office serves a special purpose in C programs; the run-fourth dimension environs calls the principal function to begin programme execution. The type specifier int indicates that the value that is returned to the invoker (in this case the run-time surroundings) 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 caryatid indicates the starting time of the definition of the main function.

The next line calls (diverts execution to) a office named printf, which in this case is supplied from a organisation library. In this call, the printf function is passed (provided with) a single statement, the address of the first character in the string literal "howdy, earth\due north". The cord literal is an unnamed array with elements of blazon char, set upwards 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 graphic symbol, which on output signifies the terminate of the electric current line. The render value of the printf function is of type int, but it is silently discarded since it is not used. (A more careful programme might test the render value to decide whether or not the printf function succeeded.) The semicolon ; terminates the statement.

The closing curly brace indicates the end of the code for the master role. According to the C99 specification and newer, the primary function, unlike any other part, will implicitly render a value of 0 upon reaching the } that terminates the office. (Formerly an explicit return 0; statement was required.) This is interpreted by the run-fourth dimension organization as an exit lawmaking indicating successful execution.^[31]

Information types [edit]

The type system in C is static and weakly typed, which makes it like to the type system of ALGOL descendants such as Pascal.^[32] There are born types for integers of various sizes, both signed and unsigned, floating-signal numbers, and enumerated types (enum). Integer type char is often used for single-byte characters. C99 added a boolean datatype. There are likewise derived types including arrays, pointers, records (struct), and unions (marriage).

C is often used in low-level systems programming where escapes from the type system may be necessary. The compiler attempts to ensure blazon definiteness of most expressions, but the programmer tin override the checks in various means, either past using a type cast to explicitly catechumen a value from 1 type to another, or by using pointers or unions to reinterpret the underlying bits of a information object in some other way.

Some discover C'southward declaration syntax unintuitive, particularly for office pointers. (Ritchie'southward idea was to declare identifiers in contexts resembling their utilize: "annunciation reflects utilize".)^[33]

C's usual arithmetic conversions allow for efficient lawmaking to be generated, only tin sometimes produce unexpected results. For instance, a comparing 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 part in memory. Pointers can be dereferenced to access information stored at the address pointed to, or to invoke a pointed-to role. Pointers can exist manipulated using assignment or pointer arithmetic. The run-time representation of a pointer value is typically a raw retentivity address (perhaps augmented by an offset-inside-word field), only since a pointer's type includes the type of the affair pointed to, expressions including pointers tin be type-checked at compile time. Arrow arithmetic is automatically scaled by the size of the pointed-to data type. Pointers are used for many purposes in C. Text strings are unremarkably manipulated using pointers into arrays of characters. Dynamic retentivity allotment is performed using pointers. Many information types, such equally trees, are commonly implemented as dynamically allocated struct objects linked together using pointers. Pointers to functions are useful for passing functions as arguments to college-society functions (such equally qsort or bsearch) or as callbacks to be invoked by event handlers.^[31]

A null arrow value explicitly points to no valid location. Dereferencing a null arrow value is undefined, often resulting in a sectionalization mistake. Zippo pointer values are useful for indicating special cases such as no "next" arrow in the terminal node of a linked list, or as an fault indication from functions returning pointers. In appropriate contexts in source code, such as for assigning to a arrow variable, a null pointer abiding can be written every bit 0, with or without explicit casting to a pointer type, or as the Zilch macro divers past several standard headers. In conditional contexts, naught pointer values evaluate to false, while all other pointer values evaluate to true.

Void pointers (void *) betoken to objects of unspecified type, and tin can therefore be used as "generic" data pointers. Since the size and type of the pointed-to object is non known, void pointers cannot exist dereferenced, nor is pointer arithmetic on them allowed, although they can easily be (and in many contexts implicitly are) converted to and from whatsoever other object arrow type.^[31]

Careless utilise of pointers is potentially unsafe. Because they are typically unchecked, a pointer variable can be made to point to any capricious location, which tin can cause undesirable effects. Although properly used pointers point to safe places, they tin be fabricated to point to unsafe places by using invalid arrow arithmetic; the objects they indicate to may go on to be used after deallocation (dangling pointers); they may be used without having been initialized (wild pointers); or they may be direct assigned an unsafe value using a cast, union, or through another corrupt pointer. In general, C is permissive in allowing manipulation of and conversion betwixt pointer types, although compilers typically provide options for various levels of checking. Another programming languages address these problems by using more than restrictive reference types.

Arrays [edit]

Array types in C are traditionally of a fixed, static size specified at compile time. The more contempo C99 standard also allows a course of variable-length arrays. Even so, it is besides possible to allocate a block of retention (of arbitrary size) at run-fourth dimension, using the standard library'southward malloc role, and care for it as an assortment.

Since arrays are ever accessed (in effect) via pointers, array accesses are typically not checked confronting the underlying assortment size, although some compilers may provide bounds checking as an option.^[34] ^[35] Array bounds violations are therefore possible and tin atomic number 82 to various repercussions, including illegal memory accesses, abuse of data, buffer overruns, and run-time exceptions.

C does non have a special provision for declaring multi-dimensional arrays, but rather relies on recursion inside the type system to declare arrays of arrays, which effectively accomplishes the aforementioned thing. The index values of the resulting "multi-dimensional array" can be thought of as increasing in row-major order. Multi-dimensional arrays are unremarkably used in numerical algorithms (mainly from practical linear algebra) to store matrices. The construction of the C array is well suited to this particular task. However, in early on versions of C the bounds of the array must be known fixed values or else explicitly passed to whatever subroutine that requires them, and dynamically sized arrays of arrays cannot exist accessed using double indexing. (A workaround for this was to allocate the assortment with an additional "row vector" of pointers to the columns.) C99 introduced "variable-length arrays" which address this issue.

The following example using mod C (C99 or afterward) shows allocation of a two-dimensional array on the heap and the utilise of multi-dimensional array indexing for accesses (which tin can use premises-checking on many C compilers):

                        int                                    func            (            int                                    N            ,                                    int                                    G            )                        {                                                float                                    (            *            p            )[            N            ][            K            ]                                    =                                    malloc            (            sizeof                                    *            p            );                                                if                                    (            !            p            )                                                return                                    -1            ;                                                for                                    (            int                                    i                                    =                                    0            ;                                    i                                    <                                    N            ;                                    i            ++            )                                                for                                    (            int                                    j                                    =                                    0            ;                                    j                                    <                                    M            ;                                    j            ++            )                                                (            *            p            )[            i            ][            j            ]                                    =                                    i                                    +                                    j            ;                                                print_array            (            Northward            ,                                    Thou            ,                                    p            );                                                gratis            (            p            );                                                return                                    1            ;                        }

Array–arrow interchangeability [edit]

The subscript notation x[i] (where 10 designates a pointer) is syntactic sugar for *(x+i).^[36] Taking advantage of the compiler's knowledge of the pointer type, the address that x + i points to is non the base address (pointed to by ten) incremented by i bytes, simply rather is defined to be the base address incremented past i multiplied by the size of an element that x points to. Thus, x[i] designates the i+1thursday element of the assortment.

Furthermore, in about 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 starting time element. This implies that an array is never copied as a whole when named as an statement to a function, simply rather only the address of its first chemical element is passed. Therefore, although role calls in C employ pass-past-value semantics, arrays are in effect passed by reference.

The total size of an array x can be adamant by applying sizeof to an expression of array type. The size of an element can exist determined past applying the operator sizeof to any dereferenced element of an array A, every bit in n = sizeof A[0]. This, the number of elements in a declared assortment A tin be adamant as sizeof A / sizeof A[0]. Note, that if merely a pointer to the first element is bachelor as it is often the instance in C code because of the automatic conversion described to a higher place, the data almost the total blazon of the array and its length are lost.

Retention management [edit]

I of the nearly of import functions of a programming language is to provide facilities for managing retention and the objects that are stored in memory. C provides three distinct ways to allocate memory for objects:^[31]

Static memory resource allotment: space for the object is provided in the binary at compile-fourth dimension; these objects accept an extent (or lifetime) as long as the binary which contains them is loaded into memory.
Automated retentivity 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 retentiveness allocation: blocks of memory of arbitrary size tin be requested at run-time using library functions such equally malloc from a region of memory called the heap; these blocks persist until subsequently freed for reuse by calling the library function realloc or complimentary

These 3 approaches are appropriate in different situations and take various trade-offs. For example, static memory allocation has piddling allotment overhead, automatic resource allotment may involve slightly more than overhead, and dynamic retention 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 retention or heap space, and dynamic retentiveness allocation allows user-friendly allocation of objects whose size is known only at run-time. Most C programs make extensive utilise of all iii.

Where possible, automatic or static allocation is usually simplest because the storage is managed past the compiler, freeing the programmer of the potentially error-prone chore of manually allocating and releasing storage. Nonetheless, many information structures can alter in size at runtime, and since static allocations (and automatic allocations before C99) must take a stock-still size at compile-fourth dimension, there are many situations in which dynamic allocation is necessary.^[31] 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.) Dissimilar automated allotment, which can fail at run time with uncontrolled consequences, the dynamic allocation functions return an indication (in the class of a null arrow value) when the required storage cannot be allocated. (Static allocation that is as well large is usually detected by the linker or loader, earlier the program can fifty-fifty begin execution.)

Unless otherwise specified, static objects comprise nix or null arrow 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 exist present in the storage, which might not even represent a valid value for that blazon). If the program attempts to access an uninitialized value, the results are undefined. Many modern compilers endeavour to notice and warn about this problem, simply both false positives and faux negatives can occur.

Heap retention allocation has to be synchronized with its actual usage in whatsoever program to be reused as much as possible. For example, if the but 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 after reuse and is essentially lost to the program, a phenomenon known as a memory leak. Conversely, it is possible for retentiveness to be freed, but is referenced afterwards, leading to unpredictable results. Typically, the failure symptoms appear in a portion of the program unrelated to the lawmaking that causes the fault, making information technology difficult to diagnose the failure. Such issues are ameliorated in languages with automatic garbage collection.

Libraries [edit]

The C programming linguistic communication uses libraries equally its primary method of extension. In C, a library is a set of functions contained within a unmarried "archive" file. Each library typically has a header file, which contains the prototypes of the functions independent within the library that may be used by a plan, and declarations of special data types and macro symbols used with these functions. In order for a plan to use a library, information technology must include the library'due south header file, and the library must exist linked with the program, which in many cases requires compiler flags (e.g., -lm, shorthand for "link the math library").^[31]

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 equally embedded systems may provide only a subset of the standard library). This library supports stream input and output, memory allotment, 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 up of C library functions are those used by applications specifically targeted for Unix and Unix-similar systems, especially functions which provide an interface to the kernel. These functions are detailed in diverse standards such as POSIX and the Single UNIX Specification.

Since many programs have been written in C, in that location are a wide variety of other libraries available. Libraries are often written in C considering C compilers generate efficient object code; programmers then create interfaces to the library so that the routines can exist used from higher-level languages similar Java, Perl, and Python.^[31]

File treatment and streams [edit]

File input and output (I/O) is not part of the C language itself merely instead is handled by libraries (such equally the C standard library) and their associated header files (e.grand. stdio.h). File handling is generally implemented through high-level I/O which works through streams. A stream is from this perspective a data period that is independent of devices, while a file is a physical device. The loftier-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 shop data before it'south sent to the final destination. This reduces the time spent waiting for slower devices, for instance a hard drive or solid country drive. Low-level I/O functions are not part of the standard C library^{[ description needed ]} but are generally part of "bare metal" programming (programming that's contained of any operating system such equally virtually embedded programming). With few exceptions, implementations include low-level I/O.

Language tools [edit]

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

Automated source code checking and auditing are beneficial in whatsoever language, and for C many such tools be, such every bit Lint. A common practice is to utilise Lint to detect questionable code when a program is first written. One time a plan passes Lint, information technology is then compiled using the C compiler. Also, many compilers tin can optionally warn almost syntactically valid constructs that are likely to actually be errors. MISRA C is a proprietary gear up of guidelines to avoid such questionable code, developed for embedded systems.^[37]

There are also compilers, libraries, and operating organisation level mechanisms for performing actions that are non a standard function of C, such equally 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 retentiveness allocation functions can help uncover runtime errors in memory usage.

Uses [edit]

The C Programming Language

C is widely used for systems programming in implementing operating systems and embedded system applications,^[38] because C lawmaking, when written for portability, can be used for most purposes, nonetheless when needed, system-specific code can be used to admission specific hardware addresses and to perform blazon punning to friction match externally imposed interface requirements, with a low run-time demand on system resource.

C tin be used for website programming using the Common Gateway Interface (CGI) as a "gateway" for data between the Spider web awarding, the server, and the browser.^[39] C is ofttimes called over interpreted languages considering of its speed, stability, and most-universal availability.^[40]

A consequence of C's wide availability and efficiency is that compilers, libraries and interpreters of other programming languages are often implemented in C. For instance, the reference implementations of Python, Perl, Cherry-red, and PHP 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 of import 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 exist used for portability or convenience; past using C as an intermediate language, additional automobile-specific code generators are not necessary. C has some features, such as line-number preprocessor directives and optional superfluous commas at the terminate of initializer lists, that support compilation of generated lawmaking. However, some of C's shortcomings have prompted the development of other C-based languages specifically designed for use every bit intermediate languages, such as C--.

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

[edit]

The TIOBE index graph, showing a comparison of the popularity of various programming languages^[41]

C has both directly and indirectly influenced many later on languages such as C#, D, Become, Java, JavaScript, Limbo, LPC, Perl, PHP, Python, and Unix'due south C shell.^[42] The most pervasive influence has been syntactical; all of the languages mentioned combine the statement and (more than or less recognizably) expression syntax of C with type systems, data models, and/or large-scale plan structures that differ from those of C, sometimes radically.

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

When object-oriented programming languages became popular, C++ and Objective-C were two unlike extensions of C that provided object-oriented capabilities. Both languages were originally implemented as source-to-source compilers; source lawmaking was translated into C, and and so compiled with a C compiler.^[43]

The C++ programming language (originally named "C with Classes") was devised by Bjarne Stroustrup as an approach to providing object-oriented functionality with a C-like syntax.^[44] C++ adds greater typing strength, scoping, and other tools useful in object-oriented programming, and permits generic programming via templates. Almost a superset of C, C++ now supports nearly of C, with a few exceptions.

Objective-C was originally a very "thin" layer on acme of C, and remains a strict superset of C that permits object-oriented programming using a hybrid dynamic/static typing epitome. 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 about supersets of C.

Come across too [edit]

Compatibility of C and C++
Comparison of Pascal and C
Comparison of programming languages
International Obfuscated C Lawmaking Competition
List of C-based programming languages
Listing of C compilers

Notes [edit]

^ The original example code will compile on near modern compilers that are not in strict standard compliance mode, simply it does not fully conform to the requirements of either C89 or C99. In fact, C99 requires that a diagnostic message be produced.
^ The main function really has 2 arguments, int argc and char *argv[], respectively, which can be used to handle command line arguments. The ISO C standard (section 5.1.ii.ii.ane) requires both forms of chief to be supported, which is special treatment non afforded to whatever other function.

References [edit]

^ ^a ^b Kernighan, Brian W.; Ritchie, Dennis Grand. (February 1978). The C Programming Linguistic communication (1st ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-110163-0.
^ Ritchie (1993): "Thompson had made a brief try to produce a system coded in an early on version of C—before structures—in 1972, just gave upward the effort."
^ Fruderica (December thirteen, 2020). "History of C". The cppreference.com. Archived from the original on October 24, 2020. Retrieved October 24, 2020.
^ Ritchie (1993): "The scheme of type composition adopted past C owes considerable debt to Algol 68, although information technology did not, possibly, emerge in a form that Algol's adherents would approve of."
^ Ring Squad (October 23, 2021). "The Ring programming language and other languages". band-lang.internet.
^ ^a ^b "Verilog HDL (and C)" (PDF). The Inquiry School of Computer Science at the Australian National Academy. June 3, 2010. Archived from the original (PDF) on November half-dozen, 2013. Retrieved August xix, 2013. 1980s: ; Verilog kickoff introduced ; Verilog inspired by the C programming linguistic communication
^ ^a ^b ^c ^d ^e Ritchie (1993)
^ "Programming Language Popularity". 2009. Archived from the original on January 16, 2009. Retrieved January 16, 2009.
^ "TIOBE Programming Customs Index". 2009. Archived from the original on May iv, 2009. Retrieved May half-dozen, 2009.
^ ^a ^b "History of C". en.cppreference.com. Archived from the original on May 29, 2018. Retrieved May 28, 2018.
^ "TIOBE Index for October 2021". Retrieved October 7, 2021.
^ Ritchie, Dennis. "BCPL to B to C". Archived from the original on December 12, 2019. Retrieved September 10, 2019.
^ ^a ^b Johnson, Southward. C.; Ritchie, D. M. (1978). "Portability of C Programs and the UNIX Organisation". Bell System Tech. J. 57 (half dozen): 2021–2048. CiteSeerXx.1.1.138.35. doi:x.1002/j.1538-7305.1978.tb02141.x. S2CID 17510065. (Annotation: The PDF is an OCR browse of the original, and contains a rendering of "IBM 370" every bit "IBM 310".)
^ McIlroy, M. D. (1987). A Enquiry Unix reader: annotated excerpts from the Programmer's Manual, 1971–1986 (PDF) (Technical report). CSTR. Bell Labs. p. 10. 139. Archived (PDF) from the original on November 11, 2017. Retrieved February 1, 2015.
^ "C manual pages". FreeBSD Miscellaneous Information Transmission (FreeBSD 13.0 ed.). May 30, 2011. Archived from the original on January 21, 2021. Retrieved Jan 15, 2021. [1] Archived Jan 21, 2021, at the Wayback Motorcar
^ Kernighan, Brian Westward.; Ritchie, Dennis One thousand. (March 1988). The C Programming Linguistic communication (2nd ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-xiii-110362-7.
^ Stroustrup, Bjarne (2002). Sibling rivalry: C and C++ (PDF) (Report). AT&T Labs. Archived (PDF) from the original on August 24, 2014. Retrieved April 14, 2014.
^ C Integrity. International System for Standardization. March thirty, 1995. Archived from the original on July 25, 2018. Retrieved July 24, 2018.
^ "JTC1/SC22/WG14 – C". Domicile page. ISO/IEC. Archived from the original on February 12, 2018. Retrieved June 2, 2011.
^ Andrew Binstock (October 12, 2011). "Interview with Herb Sutter". Dr. Dobbs. Archived from the original on Baronial ii, 2013. Retrieved September 7, 2013.
^ "Revised C23 Schedule WG 14 N 2759" (PDF). www.open-std.org. Archived (PDF) from the original on June 24, 2021. Retrieved Oct 10, 2021.
^ "TR 18037: Embedded C" (PDF). ISO / IEC. Archived (PDF) from the original on February 25, 2021. Retrieved July 26, 2011.
^ Harbison, Samuel P.; Steele, Guy L. (2002). C: A Reference Transmission (fifth ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-089592-nine. Contains a BNF grammar for C.
^ Kernighan & Ritchie (1996), p. 192.
^ Kernighan & Ritchie (1978), p. 3.
^ "ISO/IEC 9899:201x (ISO C11) Commission Draft" (PDF). Archived (PDF) from the original on Dec 22, 2017. Retrieved September sixteen, 2011.
^ Kernighan & Ritchie (1996), pp. 192, 259.
^ "10 Common Programming Mistakes in C++". Cs.ucr.edu. Archived from the original on October 21, 2008. Retrieved June 26, 2009.
^ Schultz, Thomas (2004). C and the 8051 (3rd ed.). Otsego, MI: PageFree Publishing Inc. p. twenty. ISBN978-ane-58961-237-two. Archived from the original on July 29, 2020. Retrieved February ten, 2012.
^ Kernighan & Ritchie (1978), p. 6.
^ ^a ^b ^c ^d ^e ^f ^g Klemens, Ben (2013). 21st Century C. O'Reilly Media. ISBN978-one-4493-2714-9.
^ Feuer, Alan R.; Gehani, Narain H. (March 1982). "Comparison of the Programming Languages C and Pascal". ACM Calculating Surveys. 14 (1): 73–92. doi:10.1145/356869.356872. S2CID 3136859.
^ Kernighan & Ritchie (1996), p. 122.
^ For example, gcc provides _FORTIFY_SOURCE. "Security Features: Compile Fourth dimension Buffer Checks (FORTIFY_SOURCE)". fedoraproject.org. Archived from the original on January 7, 2007. Retrieved Baronial 5, 2012.
^ เอี่ยมสิริวงศ์, โอภาศ (2016). Programming with C. Bangkok, Thailand: SE-Teaching PUBLIC Visitor Limited. pp. 225–230. ISBN978-616-08-2740-4.
^ Raymond, Eric S. (October 11, 1996). The New Hacker's Dictionary (3rd ed.). MIT Press. p. 432. ISBN978-0-262-68092-9. Archived from the original on November 12, 2012. Retrieved August 5, 2012.
^ "Man Page for lint (freebsd Section 1)". unix.com. May 24, 2001. Retrieved July 15, 2014.
^ Dale, Nell B.; Weems, Bit (2014). Programming and problem solving with C++ (sixth ed.). Burlington, MA: Jones & Bartlett Learning. ISBN978-1449694289. OCLC 894992484.
^ Dr. Dobb'southward Sourcebook. U.South.A.: Miller Freeman, Inc. Nov–December 1995.
^ "Using C for CGI Programming". linuxjournal.com. March 1, 2005. Archived from the original on February xiii, 2010. Retrieved January 4, 2010.
^ McMillan, Robert (August 1, 2013). "Is Java Losing Its Mojo?". Wired. Archived from the original on February 15, 2017. Retrieved March 5, 2017.
^ O'Regan, Gerard (September 24, 2015). Pillars of computing : a compendium of select, pivotal technology firms. ISBN978-3319214641. OCLC 922324121.
^ Rauchwerger, Lawrence (2004). Languages and compilers for parallel computing : 16th international workshop, LCPC 2003, College Station, TX, USA, October two-iv, 2003 : revised papers. Springer. ISBN978-3540246442. OCLC 57965544.
^ Stroustrup, Bjarne (1993). "A History of C++: 1979−1991" (PDF). Archived (PDF) from the original on February 2, 2019. Retrieved June nine, 2011.

Sources [edit]

Ritchie, Dennis M. (March 1993). "The Development of the C Language". ACM SIGPLAN Notices. ACM. 28 (3): 201–208. doi:10.1145/155360.155580.
Ritchie, Dennis M. (1993). "The Development of the C Language". The Second ACM SIGPLAN Briefing on History of Programming Languages (HOPL-2). ACM. pp. 201–208. doi:10.1145/154766.155580. ISBN0-89791-570-4 . Retrieved November iv, 2014.
Kernighan, Brian West.; Ritchie, Dennis M. (1996). The C Programming Linguistic communication (2nd ed.). Prentice Hall. ISBNseven-302-02412-X.

External links [edit]

ISO C Working Group official website
- ISO/IEC 9899, publicly bachelor official C documents, including the C99 Rationale
- "C99 with Technical corrigenda TC1, TC2, and TC3 included" (PDF). (iii.61 MB)
comp.lang.c Oft Asked Questions
A History of C, by Dennis Ritchie

paytonfrousess.blogspot.com

Source: https://en.wikipedia.org/wiki/C_(programming_language)