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General-purpose programming language

C
Major implementations
The C Programming Language ^[1] (often referred to as K&R), the seminal book on C
Paradigm	Multi-epitome: imperative (procedural), structured
Designed by	Dennis Ritchie
Developer	Dennis Ritchie & Bell Labs (creators); ANSI X3J11 (ANSI C); ISO/IEC JTC1/SC22/WG14 (ISO C)
First appeared	1972; 50 years ago (1972) ^[2]

Stable release	C17 / June 2018; 3 years ago (2018-06)
Preview release	C2x (N2731) / Oct 18, 2021; 4 months ago (2021-10-18) ^[three]

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

C (, every bit in the letter c) is a full general-purpose, procedural computer programming linguistic communication supporting structured programming, lexical variable scope, and recursion, with a static type system. By pattern, C provides constructs that map efficiently to typical machine instructions. Information technology has found lasting employ in applications previously coded in associates linguistic communication. Such applications include operating systems and diverse awarding software for reckoner architectures that range from supercomputers to PLCs and embedded systems.

A successor to the programming language B, C was originally developed at Bell Labs by Dennis Ritchie between 1972 and 1973 to construct utilities running on Unix. It was applied to re-implementing the kernel of the Unix operating system.^[seven] During the 1980s, C gradually gained popularity. It has become one of the most widely used programming languages,^[8] ^[nine] with C compilers from various vendors available for the majority of existing reckoner architectures and operating systems. C has been standardized by ANSI since 1989 (ANSI C) and by the International Organization for Standardization (ISO).

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

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

Overview [edit]

Like most procedural languages in the ALGOL tradition, C has facilities for structured programming and allows lexical variable scope and recursion. Its static type arrangement prevents unintended operations. In C, all executable code is contained inside subroutines (also called "functions", though non strictly in the sense of functional programming). Part parameters are always passed by value (except arrays). Pass-by-reference is simulated in C by explicitly passing arrow values. C program source text is gratis-format, using the semicolon every bit a statement terminator and curly braces for group blocks of statements.

The C linguistic communication too exhibits the post-obit characteristics:

The linguistic communication has a minor, fixed number of keywords, including a full set of control flow primitives: if/else, for, practice/while, while, and switch. User-defined names are not distinguished from keywords by any kind of sigil.
It has a big 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 non needed.
- Office 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, merely weakly enforced; all data has a type, but implicit conversions are possible.
Annunciation syntax mimics usage context. C has no "define" keyword; instead, a argument showtime 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) and chemical compound types are possible.
- Heterogeneous aggregate data types (struct) allow related data elements to be accessed and assigned as a unit.
- Union is a structure with overlapping members; just the final member stored is valid.
- Array indexing is a secondary notation, defined in terms of pointer arithmetics. Different structs, arrays are not starting time-class objects: they cannot be assigned or compared using single built-in operators. At that place is no "array" keyword in use or definition; instead, square brackets point arrays syntactically, for example month[11].
- Enumerated types are possible with the enum keyword. They are freely interconvertible with integers.
- Strings are not a distinct data type, only are conventionally implemented as null-terminated character arrays.
Low-level access to reckoner retentiveness is possible by converting car addresses to typed pointers.
Procedures (subroutines not returning values) are a special example of office, with an untyped return type void.
A preprocessor performs macro definition, source code file inclusion, and provisional compilation.
There is a basic form of modularity: files can be compiled separately and linked together, with control over which functions and information objects are visible to other files via static and extern attributes.
Complex functionality such as I/O, string manipulation, and mathematical functions are consistently delegated to library routines.

While C does non include certain features constitute in other languages (such as object orientation and garbage drove), these can be implemented or emulated, oftentimes through the utilise of external libraries (due east.grand., the GLib Object System or the Boehm garbage collector).

Relations to other languages [edit]

Many later on languages have borrowed directly or indirectly from C, including C++, C#, Unix'southward C shell, D, Go, Java, JavaScript (including transpilers), Julia, Limbo, LPC, Objective-C, Perl, PHP, Python, Cherry-red, Rust, Swift, Verilog and SystemVerilog (hardware description languages).^[6] These languages take drawn many of their command structures and other basic features from C. Most of them (Python being a dramatic exception) also express highly similar syntax to C, and they tend to combine the recognizable expression and statement syntax of C with underlying blazon systems, data models, and semantics that can exist radically different.

History [edit]

Early developments [edit]

Timeline of language development
Year	C Standard^[10]
1972	Birth
1978	1000&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 organization, originally implemented in associates linguistic communication on a PDP-7 by Dennis Ritchie and Ken Thompson, incorporating several ideas from colleagues. Eventually, they decided to port the operating system to a PDP-11. The original PDP-11 version of Unix was also developed in assembly linguistic communication.^[7]

Thompson desired a programming linguistic communication to brand utilities for the new platform. At first, he tried to make a Fortran compiler, just soon gave up the thought. Instead, he created a cut-down version of the recently developed BCPL systems programming language. The official description of BCPL was not available at the time,^[12] and Thompson modified the syntax to be less wordy, producing the like simply somewhat simpler B.^[seven] However, few utilities were ultimately written in B because information technology was too boring, and B could not take reward of PDP-11 features such as byte addressability.

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

At Version 4 Unix, released in Nov 1973, the Unix kernel was extensively re-implemented in C.^[7] By this fourth dimension, the C language had acquired some powerful features such equally struct types.

The preprocessor was introduced around 1973 at the urging of Alan Snyder and also 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 #ascertain of parameterless macros. Presently later on that, it was extended, more often than not by Mike Lesk and then by John Reiser, to incorporate macros with arguments and conditional compilation.^[7]

Unix was one of the first operating system kernels implemented in a linguistic communication other than associates. Earlier instances include the Multics system (which was written in PL/I) and Principal Control Program (MCP) for the Burroughs B5000 (which was written in ALGOL) in 1961. In effectually 1977, Ritchie and Stephen C. Johnson made further changes to the language to facilitate portability of the Unix operating system. Johnson's Portable C Compiler served as the footing for several implementations of C on new platforms.^[13]

K&R C [edit]

In 1978, Brian Kernighan and Dennis Ritchie published the offset edition of The C Programming Linguistic communication.^[1] This volume, known to C programmers as K&R, served for many years as an informal specification of the language. The version of C that information technology describes is commonly referred to as "K&R C". As this was released in 1978, it is as well referred to as C78.^[xv] The second edition of the book^[xvi] covers the afterwards ANSI C standard, described below.

Chiliad&R introduced several language features:

Standard I/O library
long int information type
unsigned int data type
Compound assignment operators of the form =op (such every bit =-) were changed to the form op= (that is, -=) to remove the semantic ambivalence created by constructs such every bit i=-10, which had been interpreted as i =- x (decrement i by 10) instead of the possibly intended i = -10 (let i be −10).

Fifty-fifty after the publication of the 1989 ANSI standard, for many years Thousand&R C was notwithstanding considered the "everyman mutual denominator" to which C programmers restricted themselves when maximum portability was desired, since many older compilers were still in apply, and because carefully written 1000&R C code can be legal Standard C too.

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

For case:

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

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

Since M&R part declarations did not include any information near office arguments, role parameter type checks were not performed, although some compilers would effect a alert message if a local office 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 bank check for consistency of function use beyond multiple source files.

In the years post-obit the publication of One thousand&R C, several features were added to the language, supported by compilers from AT&T (in detail PCC^[17]) and some other vendors. These included:

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

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

ANSI C and ISO C [edit]

During the late 1970s and 1980s, versions of C were implemented for a broad multifariousness of mainframe computers, minicomputers, and microcomputers, including the IBM PC, as its popularity began to increase significantly.

In 1983, the American National Standards Institute (ANSI) formed a commission, X3J11, to establish a standard specification of C. X3J11 based the C standard on the Unix implementation; however, the not-portable portion of the Unix C library was handed off to the IEEE working group 1003 to become the ground for the 1988 POSIX standard. In 1989, the C standard was ratified every bit ANSI X3.159-1989 "Programming Language C". This version of the language is often referred to equally 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, like other national standards bodies, no longer develops the C standard independently, simply defers to the international C standard, maintained past 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 process was to produce a superset of 1000&R C, incorporating many of the afterwards introduced unofficial features. The standards commission as well included several additional features such as function prototypes (borrowed from C++), void pointers, support for international character sets and locales, and preprocessor enhancements. Although the syntax for parameter declarations was augmented to include the mode used in C++, the Yard&R interface connected to be permitted, for compatibility with existing source lawmaking.

C89 is supported by current C compilers, and nigh modern C code is based on information technology. Whatsoever program written only in Standard C and without any hardware-dependent assumptions will run correctly on whatsoever platform with a conforming C implementation, inside its resource limits. Without such precautions, programs may compile but on a certain platform or with a particular compiler, due, for case, to the use of non-standard libraries, such as GUI libraries, or to a reliance on compiler- or platform-specific attributes such every bit the exact size of data types and byte endianness.

In cases where code must be compilable past either standard-conforming or One thousand&R C-based compilers, the __STDC__ macro tin exist used to split the code into Standard and One thousand&R sections to preclude the utilise on a Thou&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 equally C95) was published, to correct some details and to add more all-encompassing support for international grapheme sets.^[18]

C99 [edit]

The C standard was further revised in the belatedly 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 past Technical Corrigenda.^[nineteen]

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 back up for 1-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 well-nigh office backward compatible with C90, but is stricter in some ways; in particular, a announcement that lacks a type specifier no longer has int implicitly assumed. A standard macro __STDC_VERSION__ is defined with value 199901L to point that C99 support is available. GCC, Solaris Studio, and other C compilers now support many or all of the new features of C99. The C compiler in Microsoft Visual C++, however, implements the C89 standard and those parts of C99 that are required for compatibility with C++11.^[20] ^{[ needs update ]}

In addition, support for Unicode identifiers (variable / function names) in the form of escaped characters (e.k. \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 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 blazon generic macros, anonymous structures, improved Unicode back up, diminutive operations, multi-threading, and premises-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 point that C11 support is available.

C17 [edit]

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

C2x [edit]

C2x is an informal proper name for the next (later on C17) major C language standard revision. It is expected to be voted on in 2023 and would therefore be called C23.^[21] ^{[ better source needed ]}

Embedded C [edit]

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

In 2008, the C Standards Committee published a technical report extending the C linguistic communication^[22] to address these problems past providing a common standard for all implementations to adhere to. Information technology includes a number of features not available in normal C, such as stock-still-point arithmetic, 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 significant in C; however, line boundaries practice 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 */ practice not nest, and these sequences of characters are not interpreted as comment delimiters if they appear within string or character literals.^[24]

C source files comprise declarations and function definitions. Part definitions, in turn, contain declarations and statements. Declarations either ascertain new types using keywords such as struct, union, and enum, or assign types to and perhaps reserve storage for new variables, ordinarily past writing the blazon followed past the variable proper name. Keywords such as char and int specify built-in types. Sections of code are enclosed in braces ({ and }, sometimes called "curly brackets") to limit the telescopic of declarations and to act as a single statement for command structures.

Equally an imperative language, C uses statements to specify actions. The most common argument is an expression argument, consisting of an expression to be evaluated, followed by a semicolon; as a side effect of the evaluation, functions may be called and variables may exist assigned new values. To change 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 exercise … while, while, and for iterative execution (looping). The for statement has divide initialization, testing, and reinitialization expressions, whatever or all of which can exist omitted. break and continue can be used to leave the innermost enclosing loop statement or skip to its reinitialization. There is besides a not-structured goto statement which branches directly to the designated label within the function. switch selects a case to exist 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 exist interleaved. All the same, all side effects (including storage to variables) will occur before the next "sequence betoken"; sequence points include the end of each expression argument, and the entry to and render from each function call. Sequence points also occur during evaluation of expressions containing certain operators (&&, ||, ?: and the comma operator). This permits a high caste of object code optimization by the compiler, just requires C programmers to have 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 exist better."^[25] The C standard did not attempt to correct many of these blemishes, because of the impact of such changes on already existing software.

Character set [edit]

The basic C source graphic symbol set includes the following characters:

Lowercase and upper-case letter letters of ISO Basic Latin Alphabet: a–z A–Z
Decimal digits: 0–9
Graphic characters: ! " # % & ' ( ) * + , - . / : ; < = > ? [ \ ] ^ _ { | } ~
Whitespace characters: space, horizontal tab, vertical tab, course feed, newline

Newline indicates the end of a text line; it need not correspond to an bodily single graphic symbol, although for convenience C treats information technology as one.

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

The basic C execution character set contains the aforementioned characters, along with representations for alert, backspace, and railroad vehicle return. Run-time back up for extended character sets has increased with each revision of the C standard.

Reserved words [edit]

C89 has 32 reserved words, likewise known equally keywords, which are the words that cannot be used for whatever purposes other than those for which they are predefined:

auto
break
case
char
const
proceed
default
practice
double
else
enum
extern
bladder
for
goto
if
int
long
register
render
short
signed
sizeof
static
struct
switch
typedef
union
unsigned
void
volatile
while

C99 reserved five more words:

_Bool
_Complex
_Imaginary
inline
restrict

C11 reserved vii more words:^[26]

_Alignas
_Alignof
_Atomic
_Generic
_Noreturn
_Static_assert
_Thread_local

Most of the recently reserved words begin with an underscore followed by a majuscule alphabetic character, considering identifiers of that course were previously reserved by the C standard for use merely by implementations. Since existing programme source lawmaking should not have been using these identifiers, information technology would non exist affected when C implementations started supporting these extensions to the programming language. Some standard headers exercise define more convenient synonyms for underscored identifiers. The language previously included a reserved word chosen entry, but this was seldom implemented, and has now been removed every bit 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:

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

C uses the operator = (used in mathematics to express equality) to indicate assignment, following the precedent of Fortran and PL/I, only unlike ALGOL and its derivatives. C uses the operator == to examination for equality. The similarity between these two operators (assignment and equality) may result in the adventitious employ of one in place of the other, and in many cases, the mistake does not produce an mistake bulletin (although some compilers produce warnings). For instance, the conditional expression if (a == b + i) might mistakenly be written equally if (a = b + one), which volition exist evaluated as true if a is not zip subsequently the consignment.^[28]

The C operator precedence is not e'er intuitive. For case, the operator == binds more tightly than (is executed prior to) the operators & (bitwise AND) and | (bitwise OR) in expressions such every bit x & one == 0, which must exist written equally (x & 1) == 0 if that is the coder'due south intent.^[29]

"Howdy, world" example [edit]

The "hello, globe" example, which appeared in the first edition of Thou&R, has go the model for an introductory programme in about programming textbooks. The program prints "hello, world" to the standard output, which is usually a terminal or screen display.

The original version was:^[thirty]

                        main            ()                        {                                                printf            (            "hello, world            \n            "            );                        }

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

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

The first line of the program contains a preprocessing directive, indicated past #include. This causes the compiler to supercede 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 angle brackets surrounding stdio.h betoken that stdio.h is located using a search strategy that prefers headers provided with the compiler to other headers having the same name, equally opposed to double quotes which typically include local or project-specific header files.

The next line indicates that a function named main is existence defined. The main function serves a special purpose in C programs; the run-fourth dimension surroundings 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 surroundings) as a result of evaluating the chief function, is an integer. The keyword void equally a parameter list indicates that this function takes no arguments.^[b]

The opening curly brace indicates the get-go of the definition of the chief function.

The next line calls (diverts execution to) a part named printf, which in this case is supplied from a arrangement library. In this call, the printf function is passed (provided with) a unmarried argument, the accost of the first grapheme in the string literal "hello, world\north". The string literal is an unnamed array with elements of type char, fix up automatically past the compiler with a terminal 0-valued character to mark the terminate of the assortment (printf needs to know this). The \n is an escape sequence that C translates to a newline character, which on output signifies the end of the current line. The return value of the printf function is of type int, merely it is silently discarded since it is not used. (A more careful program might examination the render value to determine whether or not the printf function succeeded.) The semicolon ; terminates the statement.

The closing curly brace indicates the terminate of the lawmaking for the master function. According to the C99 specification and newer, the main role, unlike any other function, volition 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-fourth dimension organization equally an exit code indicating successful execution.^[31]

Data types [edit]

The type system in C is static and weakly typed, which makes it like to the blazon system of ALGOL descendants such as Pascal.^[32] There are born types for integers of diverse sizes, both signed and unsigned, floating-indicate numbers, and enumerated types (enum). Integer blazon 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 (spousal relationship).

C is often used in low-level systems programming where escapes from the type system may exist necessary. The compiler attempts to ensure blazon correctness of well-nigh expressions, but the developer can override the checks in various means, either by using a blazon bandage to explicitly convert a value from 1 type to another, or past using pointers or unions to reinterpret the underlying bits of a information object in some other fashion.

Some find C's annunciation syntax unintuitive, particularly for function pointers. (Ritchie's idea was to declare identifiers in contexts resembling their use: "declaration reflects utilise".)^[33]

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

Pointers [edit]

C supports the apply of pointers, a blazon of reference that records the address or location of an object or part in retentiveness. Pointers tin exist dereferenced to access data stored at the accost pointed to, or to invoke a pointed-to part. Pointers tin be manipulated using assignment or arrow arithmetics. The run-time representation of a arrow value is typically a raw retentiveness address (mayhap augmented past an offset-within-word field), but since a pointer's blazon includes the blazon of the thing pointed to, expressions including pointers can be type-checked at compile time. Arrow arithmetics is automatically scaled by the size of the pointed-to data type. Pointers are used for many purposes in C. Text strings are commonly manipulated using pointers into arrays of characters. Dynamic memory allocation is performed using pointers. Many data types, such as copse, are commonly implemented equally dynamically allocated struct objects linked together using pointers. Pointers to functions are useful for passing functions equally arguments to college-social club functions (such as qsort or bsearch) or as callbacks to be invoked by event handlers.^[31]

A nix pointer value explicitly points to no valid location. Dereferencing a cypher pointer value is undefined, often resulting in a segmentation fault. Aught pointer values are useful for indicating special cases such equally no "adjacent" arrow in the last 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 nothing pointer constant tin be written as 0, with or without explicit casting to a pointer type, or equally the NULL macro defined past 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 immune, although they can easily be (and in many contexts implicitly are) converted to and from any other object pointer type.^[31]

Careless utilize of pointers is potentially dangerous. Because they are typically unchecked, a pointer variable tin be made to point to any arbitrary location, which tin can crusade undesirable effects. Although properly used pointers point to prophylactic places, they can be made to betoken to unsafe places past using invalid arrow 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 direct assigned an unsafe value using a cast, union, or through another decadent pointer. In full general, C is permissive in allowing manipulation of and conversion between arrow types, although compilers typically provide options for various levels of checking. Some other programming languages address these problems by using more restrictive reference types.

Arrays [edit]

Assortment types in C are traditionally of a fixed, static size specified at compile fourth dimension. The more than recent C99 standard besides allows a form of variable-length arrays. Withal, it is also possible to allocate a cake of memory (of capricious size) at run-fourth dimension, using the standard library'south malloc function, and treat it equally an array.

Since arrays are ever accessed (in issue) via pointers, array accesses are typically non checked against the underlying array size, although some compilers may provide bounds checking every bit an option.^[34] ^[35] Assortment bounds violations are therefore possible and can lead to various repercussions, including illegal memory accesses, corruption of information, buffer overruns, and run-time exceptions.

C does non have a special provision for declaring multi-dimensional arrays, but rather relies on recursion within the type system to declare arrays of arrays, which effectively accomplishes the same matter. 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 detail task. Yet, in early on versions of C the premises of the array must exist 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 classify the assortment with an additional "row vector" of pointers to the columns.) C99 introduced "variable-length arrays" which address this consequence.

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

                        int                                    func            (            int                                    Due north            ,                                    int                                    Yard            )                        {                                                float                                    (            *            p            )[            N            ][            G            ]                                    =                                    malloc            (            sizeof                                    *            p            );                                                if                                    (            !            p            )                                                render                                    -i            ;                                                for                                    (            int                                    i                                    =                                    0            ;                                    i                                    <                                    Due north            ;                                    i            ++            )                                                for                                    (            int                                    j                                    =                                    0            ;                                    j                                    <                                    K            ;                                    j            ++            )                                                (            *            p            )[            i            ][            j            ]                                    =                                    i                                    +                                    j            ;                                                print_array            (            N            ,                                    Grand            ,                                    p            );                                                gratuitous            (            p            );                                                return                                    ane            ;                        }

Array–pointer interchangeability [edit]

The subscript notation x[i] (where 10 designates a pointer) is syntactic sugar for *(ten+i).^[36] Taking advantage of the compiler'due south knowledge of the pointer type, the address that x + i points to is non the base address (pointed to past x) incremented past i bytes, but rather is divers to be the base of operations accost incremented by i multiplied past the size of an chemical element that x points to. Thus, 10[i] designates the i+1th chemical element of the array.

Furthermore, in nigh expression contexts (a notable exception is as operand of sizeof), an expression of array blazon is automatically converted to a pointer to the assortment's first element. This implies that an array is never copied as a whole when named equally an argument to a role, but rather only the address of its showtime chemical element is passed. Therefore, although role calls in C use pass-by-value semantics, arrays are in effect passed by reference.

The total size of an array x tin be determined by applying sizeof to an expression of array type. The size of an element can exist adamant 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 tin can be adamant as sizeof A / sizeof A[0]. Notation, that if only a arrow to the first chemical element is available every bit information technology is often the case in C lawmaking because of the automatic conversion described in a higher place, the information most the full blazon of the array and its length are lost.

Retention management [edit]

One of the near important functions of a programming linguistic communication is to provide facilities for managing memory and the objects that are stored in retentiveness. C provides three distinct ways to classify retentiveness for objects:^[31]

Static memory allotment: infinite for the object is provided in the binary at compile-fourth dimension; these objects have an extent (or lifetime) as long as the binary which contains them is loaded into retention.
Automated retention allocation: temporary objects can be stored on the stack, and this space is automatically freed and reusable later on the cake in which they are declared is exited.
Dynamic memory allocation: blocks of retentivity of arbitrary size can be requested at run-time using library functions such as malloc from a region of retention called the heap; these blocks persist until subsequently freed for reuse by calling the library function realloc or free

These three approaches are appropriate in different situations and have various trade-offs. For example, static retentivity resource allotment has piddling allocation overhead, automatic allocation may involve slightly more overhead, and dynamic memory allocation can potentially take a great deal of overhead for both allocation and deallocation. The persistent nature of static objects is useful for maintaining state information beyond office calls, automatic allocation is easy to employ only stack space is typically much more than limited and transient than either static retentiveness or heap infinite, and dynamic retention resource allotment allows convenient allocation of objects whose size is known just at run-time. Most C programs make all-encompassing 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-decumbent job of manually allocating and releasing storage. Nonetheless, many data structures tin can change in size at runtime, and since static allocations (and automatic allocations earlier C99) must take a fixed size at compile-time, 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. (Run into the article on malloc for an instance of dynamically allocated arrays.) Unlike automated allocation, which tin fail at run time with uncontrolled consequences, the dynamic allocation functions return an indication (in the grade of a null pointer value) when the required storage cannot be allocated. (Static allocation that is too large is usually detected by the linker or loader, earlier the plan can fifty-fifty begin execution.)

Unless otherwise specified, static objects contain zero or null arrow values upon program startup. Automatically and dynamically allocated objects are initialized simply if an initial value is explicitly specified; otherwise they initially have indeterminate values (typically, whatever bit pattern happens to be present in the storage, which might not fifty-fifty represent a valid value for that type). If the plan attempts to access an uninitialized value, the results are undefined. Many modern compilers try to detect and warn about this problem, only both false positives and faux negatives tin occur.

Heap retentivity allocation has to exist synchronized with its actual usage in any program to be reused as much as possible. For example, if the simply pointer to a heap retentiveness resource allotment goes out of scope or has its value overwritten before it is deallocated explicitly, then that retention cannot be recovered for later reuse and is essentially lost to the program, a miracle known as a memory leak. Conversely, it is possible for retentivity to be freed, just 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 automated garbage collection.

Libraries [edit]

The C programming language uses libraries as its main method of extension. In C, a library is a set of functions independent 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 exist used past a program, and declarations of special data types and macro symbols used with these functions. In order for a program to utilise 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.m., -lm, autograph for "link the math library").^[31]

The nearly mutual 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 every bit embedded systems may provide only a subset of the standard library). This library supports stream input and output, retention allocation, mathematics, character strings, and fourth dimension values. Several carve up standard headers (for instance, stdio.h) specify the interfaces for these and other standard library facilities.

Another common set of C library functions are those used by applications specifically targeted for Unix and Unix-like systems, especially functions which provide an interface to the kernel. These functions are detailed in various standards such as POSIX and the Single UNIX Specification.

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

File handling and streams [edit]

File input and output (I/O) is not part of the C language itself but instead is handled past libraries (such as the C standard library) and their associated header files (east.chiliad. stdio.h). File treatment is more often than not implemented through high-level I/O which works through streams. A stream is from this perspective a data flow that is independent of devices, while a file is a concrete device. The loftier-level I/O is done through the clan of a stream to a file. In the C standard library, a buffer (a retentivity expanse or queue) is temporarily used to store information before it's sent to the concluding destination. This reduces the time spent waiting for slower devices, for example a hard drive or solid country drive. Low-level I/O functions are non part of the standard C library^{[ clarification needed ]} but are by and large office of "bare metallic" programming (programming that'southward independent of any operating system such as most embedded programming). With few exceptions, implementations include low-level I/O.

Language tools [edit]

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

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

At that place are also compilers, libraries, and operating system level mechanisms for performing deportment that are non a standard part of C, such as bounds checking for arrays, detection of buffer overflow, serialization, dynamic memory tracking, and automatic garbage collection.

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

Uses [edit]

The C Programming Linguistic communication

C is widely used for systems programming in implementing operating systems and embedded system applications,^[38] because C code, when written for portability, tin can be used for well-nigh purposes, yet when needed, organisation-specific code can exist used to access specific hardware addresses and to perform type punning to match externally imposed interface requirements, with a low run-fourth dimension need on arrangement resources.

C can exist used for website programming using the Common Gateway Interface (CGI) as a "gateway" for information between the Web application, the server, and the browser.^[39] C is often chosen over interpreted languages considering of its speed, stability, and near-universal availability.^[40]

A consequence of C's wide availability and efficiency is that compilers, libraries and interpreters of other programming languages are oftentimes implemented in C. For instance, the reference implementations of Python, Perl, Ruby, and PHP are written in C.

C enables programmers to create efficient implementations of algorithms and data structures, considering the layer of brainchild from hardware is thin, and its overhead is low, an of import criterion for computationally intensive programs. For case, the GNU Multiple Precision Arithmetics Library, the GNU Scientific Library, Mathematica, and MATLAB are completely or partially written in C.

C is sometimes used as an intermediate linguistic communication by implementations of other languages. This approach may be used for portability or convenience; past 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 back up compilation of generated code. Still, some of C'south shortcomings have prompted the development of other C-based languages specifically designed for apply as intermediate languages, such as C--.

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

[edit]

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

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

Several C or 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 every bit source-to-source compilers; source code was translated into C, so compiled with a C compiler.^[43]

The C++ programming linguistic communication (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. Nearly a superset of C, C++ now supports almost of C, with a few exceptions.

Objective-C was originally a very "sparse" 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 office calls is inherited from C, while the syntax for object-oriented features was originally taken from Smalltalk.

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

Notes [edit]

^ The original example code will compile on most modernistic compilers that are not in strict standard compliance mode, simply information technology does not fully adjust to the requirements of either C89 or C99. In fact, C99 requires that a diagnostic bulletin be produced.
^ The master function actually has two arguments, int argc and char *argv[], respectively, which can be used to handle command line arguments. The ISO C standard (section five.1.2.2.i) requires both forms of main to be supported, which is special treatment not afforded to whatever other function.

References [edit]

^ ^a ^b Kernighan, Brian W.; Ritchie, Dennis M. (February 1978). The C Programming Language (1st ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-110163-0.
^ Ritchie (1993): "Thompson had fabricated a cursory attempt to produce a organisation coded in an early version of C—before structures—in 1972, but gave up the attempt."
^ Fruderica (December xiii, 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 by C owes considerable debt to Algol 68, although it did non, perhaps, sally in a class that Algol's adherents would approve of."
^ Ring Team (Oct 23, 2021). "The Ring programming language and other languages". band-lang.net.
^ ^a ^b "Verilog HDL (and C)" (PDF). The Inquiry School of Computer science at the Australian National University. June 3, 2010. Archived from the original (PDF) on November 6, 2013. Retrieved August nineteen, 2013. 1980s: ; Verilog starting time introduced ; Verilog inspired by the C programming linguistic communication
^ ^a ^b ^c ^d ^east Ritchie (1993)
^ "Programming Language Popularity". 2009. Archived from the original on January xvi, 2009. Retrieved January 16, 2009.
^ "TIOBE Programming Community Index". 2009. Archived from the original on May 4, 2009. Retrieved May vi, 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 Oct vii, 2021.
^ Ritchie, Dennis. "BCPL to B to C". Archived from the original on Dec 12, 2019. Retrieved September ten, 2019.
^ ^a ^b Johnson, Southward. C.; Ritchie, D. M. (1978). "Portability of C Programs and the UNIX System". Bell System Tech. J. 57 (six): 2021–2048. CiteSeerX10.1.1.138.35. doi:x.1002/j.1538-7305.1978.tb02141.ten. S2CID 17510065. (Notation: The PDF is an OCR scan of the original, and contains a rendering of "IBM 370" every bit "IBM 310".)
^ McIlroy, M. D. (1987). A Research Unix reader: annotated excerpts from the Programmer'south Manual, 1971–1986 (PDF) (Technical report). CSTR. Bell Labs. p. x. 139. Archived (PDF) from the original on Nov 11, 2017. Retrieved February ane, 2015.
^ "C manual pages". FreeBSD Miscellaneous Information Manual (FreeBSD xiii.0 ed.). May 30, 2011. Archived from the original on Jan 21, 2021. Retrieved January 15, 2021. [1] Archived Jan 21, 2021, at the Wayback Motorcar
^ Kernighan, Brian W.; Ritchie, Dennis M. (March 1988). The C Programming Linguistic communication (2nd ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-110362-7.
^ 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 Organisation for Standardization. March xxx, 1995. Archived from the original on July 25, 2018. Retrieved July 24, 2018.
^ "JTC1/SC22/WG14 – C". Home page. ISO/IEC. Archived from the original on February 12, 2018. Retrieved June ii, 2011.
^ Andrew Binstock (October 12, 2011). "Interview with Herb Sutter". Dr. Dobbs. Archived from the original on August 2, 2013. Retrieved September 7, 2013.
^ "Revised C23 Schedule WG 14 N 2759" (PDF). world wide web.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 Fifty. (2002). C: A Reference Manual (5th 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) Committee Draft" (PDF). Archived (PDF) from the original on December 22, 2017. Retrieved September 16, 2011.
^ Kernighan & Ritchie (1996), pp. 192, 259.
^ "10 Mutual Programming Mistakes in C++". Cs.ucr.edu. Archived from the original on Oct 21, 2008. Retrieved June 26, 2009.
^ Schultz, Thomas (2004). C and the 8051 (3rd ed.). Otsego, MI: PageFree Publishing Inc. p. 20. ISBN978-ane-58961-237-two. Archived from the original on July 29, 2020. Retrieved February x, 2012.
^ Kernighan & Ritchie (1978), p. half dozen.
^ ^a ^b ^c ^d ^east ^f ^g Klemens, Ben (2013). 21st Century C. O'Reilly Media. ISBN978-ane-4493-2714-9.
^ Feuer, Alan R.; Gehani, Narain H. (March 1982). "Comparison of the Programming Languages C and Pascal". ACM Computing 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 Time Buffer Checks (FORTIFY_SOURCE)". fedoraproject.org. Archived from the original on January vii, 2007. Retrieved August five, 2012.
^ เอี่ยมสิริวงศ์, โอภาศ (2016). Programming with C. Bangkok, Thailand: SE-EDUCATION PUBLIC COMPANY LIMITED. pp. 225–230. ISBN978-616-08-2740-iv.
^ Raymond, Eric S. (Oct 11, 1996). The New Hacker's Dictionary (third 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, Chip (2014). Programming and problem solving with C++ (sixth ed.). Burlington, MA: Jones & Bartlett Learning. ISBN978-1449694289. OCLC 894992484.
^ Dr. Dobb's Sourcebook. U.s.A.: Miller Freeman, Inc. November–December 1995.
^ "Using C for CGI Programming". linuxjournal.com. March one, 2005. Archived from the original on February 13, 2010. Retrieved Jan 4, 2010.
^ McMillan, Robert (August 1, 2013). "Is Coffee Losing Its Mojo?". Wired. Archived from the original on Feb xv, 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 2-4, 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 ix, 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 Evolution of the C Linguistic communication". The Second ACM SIGPLAN Conference on History of Programming Languages (HOPL-Ii). ACM. pp. 201–208. doi:x.1145/154766.155580. ISBN0-89791-570-4 . Retrieved November iv, 2014.
Kernighan, Brian W.; Ritchie, Dennis Yard. (1996). The C Programming Language (2nd ed.). Prentice Hall. ISBN7-302-02412-X.

Farther reading [edit]

Kernighan, Brian; Ritchie, Dennis (1988). The C Programming Language (ii ed.). Prentice Hall. ISBN978-0131103627. (archive)
Plauger, P.J. (1992). The Standard C Library (one ed.). Prentice Hall. ISBN978-0131315099. (source)
Banahan, G.; Brady, D.; Doran, M. (1991). The C Volume: Featuring the ANSI C Standard (two ed.). Addison-Wesley. ISBN978-0201544336. (free)
Harbison, Samuel; Steele Jr, Guy (2002). C: A Reference Manual (v ed.). Pearson. ISBN978-0130895929. (annal)
King, K.N. (2008). C Programming: A Modernistic Approach (2 ed.). W. W. Norton. ISBN978-0393979503. (archive)
Griffiths, David; Griffiths, Dawn (2012). Head Commencement C (1 ed.). O'Reilly. ISBN978-1449399917.
Perry, Greg; Miller, Dean (2013). C Programming: Accented Beginner's Guide (3 ed.). Que. ISBN978-0789751980.
Deitel, Paul; Deitel, Harvey (2015). C: How to Program (8 ed.). Pearson. ISBN978-0133976892.
Gustedt, Jens (2019). Modern C (2 ed.). Manning. ISBN978-1617295812. (gratuitous)

External links [edit]

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

gentryincromment.blogspot.com

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