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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
Paradigm	Multi-paradigm: imperative (procedural), structured
Designed by	Dennis Ritchie
Developer	Dennis Ritchie & Bell Labs (creators); ANSI X3J11 (ANSI C); ISO/IEC JTC1/SC22/WG14 (ISO C)
Outset appeared	1972; 50 years ago (1972) ^[2]

Stable release	C17 / June 2018; 3 years ago (2018-06)
Preview release	C2x (N2731) / October 18, 2021; four months ago (2021-x-18) ^[3]

Typing field of study	Static, weak, manifest, nominal
OS	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, Split-C, Cilk, C*
Influenced by
B (BCPL, CPL), ALGOL 68,^[4] assembly, PL/I, FORTRAN
Influenced
Numerous: AMPL, AWK, csh, C++, C--, C#, Objective-C, D, Go, Java, JavaScript, JS++, Julia, Limbo, LPC, Perl, PHP, Pike, Processing, Python, Ring,^[5]Rust, Seed7, Vala, Verilog (HDL),^[6] Nim, Zig
C Programming at Wikibooks

C (, as in the letterc) is a general-purpose, procedural computer programming language supporting structured programming, lexical variable scope, and recursion, with a static type system. By design, C provides constructs that map efficiently to typical machine instructions. It has constitute lasting employ in applications previously coded in assembly language. Such applications include operating systems and various application software for computer 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 betwixt 1972 and 1973 to construct utilities running on Unix. It was applied to re-implementing the kernel of the Unix operating system.^[7] During the 1980s, C gradually gained popularity. It has become one of the most widely used programming languages,^[eight] ^[9] with C compilers from various vendors available for the majority of existing figurer 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 linguistic communication. It was designed to be compiled to provide low-level access to memory and linguistic communication constructs that map efficiently to machine instructions, all with minimal runtime support. Despite its low-level capabilities, the language was designed to encourage cross-platform programming. A standards-compliant C program written with portability in mind tin be compiled for a wide multifariousness of figurer platforms and operating systems with few changes to its source code.^[10]

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

Overview

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

The C language besides exhibits the post-obit characteristics:

The linguistic communication has a small, fixed number of keywords, including a full set up of control menstruation primitives: if/else, for, do/while, while, and switch. User-defined names are non distinguished from keywords by any kind of sigil.
It has a big number of arithmetic, bitwise, and logic operators: +,+=,++,&,||, etc.
More than ane assignment may be performed in a single statement.
Functions:
- Function return values tin be ignored, when non needed.
- Role and data pointers permit ad hoc run-time polymorphism.
- Functions may not be 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.
Declaration syntax mimics usage context. C has no "define" keyword; instead, a statement beginning with the proper name of a type is taken as a declaration. There is no "function" keyword; instead, a office is indicated past the presence of a parenthesized statement list.
User-defined (typedef) and compound types are possible.
- Heterogeneous amass information types (struct) allow related data elements to be accessed and assigned as a unit of measurement.
- Union is a structure with overlapping members; only the last fellow member stored is valid.
- Assortment indexing is a secondary notation, defined in terms of arrow arithmetic. Unlike structs, arrays are not splendid objects: they cannot exist assigned or compared using unmarried congenital-in operators. There is no "array" keyword in employ or definition; instead, square brackets indicate arrays syntactically, for example calendar month[11].
- Enumerated types are possible with the enum keyword. They are freely interconvertible with integers.
- Strings are not a distinct data type, but are conventionally implemented as null-terminated character arrays.
Depression-level access to computer retention is possible by converting automobile addresses to typed pointers.
Procedures (subroutines not returning values) are a special case of office, with an untyped return type void.
A preprocessor performs macro definition, source code file inclusion, and conditional compilation.
In that location is a bones form of modularity: files can be compiled separately and linked together, with control over which functions and data objects are visible to other files via static and extern attributes.
Complex functionality such as I/O, string manipulation, and mathematical functions are consistently delegated to library routines.

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

Relations to other languages

Many afterward languages have borrowed directly or indirectly from C, including C++, C#, Unix's C crush, D, Go, Java, JavaScript (including transpilers), Julia, Limbo, LPC, Objective-C, Perl, PHP, Python, Blood-red, Rust, Swift, Verilog and SystemVerilog (hardware clarification languages).^[6] These languages have drawn many of their control structures and other basic features from C. Almost of them (Python being a dramatic exception) too limited highly similar syntax to C, and they tend to combine the recognizable expression and argument syntax of C with underlying type systems, data models, and semantics that tin can be radically unlike.

History

Early developments

Timeline of language development
Yr	C Standard^[10]
1972	Birth
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 system, originally implemented in assembly linguistic communication on a PDP-vii by Dennis Ritchie and Ken Thompson, incorporating several ideas from colleagues. Eventually, they decided to port the operating arrangement to a PDP-11. The original PDP-xi version of Unix was as well developed in assembly language.^[7]

Thompson desired a programming language to make utilities for the new platform. At first, he tried to make a Fortran compiler, merely soon gave upwardly the idea. Instead, he created a cut-down version of the recently developed BCPL systems programming linguistic communication. The official description of BCPL was not available at the time,^[12] and Thompson modified the syntax to be less wordy, producing the similar only somewhat simpler B.^[7] However, few utilities were ultimately written in B considering information technology was too dull, and B could non take advantage of PDP-11 features such as byte addressability.

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

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

The preprocessor was introduced effectually 1973 at the urging of Alan Snyder and also in recognition of the usefulness of the file-inclusion mechanisms available in BCPL and PL/I. Its original version provided only included files and elementary cord replacements: #include and #define of parameterless macros. Soon after that, it was extended, mostly by Mike Lesk and and so by John Reiser, to incorporate macros with arguments and conditional compilation.^[seven]

Unix was 1 of the first operating system kernels implemented in a language other than assembly. Earlier instances include the Multics organisation (which was written in PL/I) and Master Control Program (MCP) for the Burroughs B5000 (which was written in ALGOL) in 1961. In effectually 1977, Ritchie and Stephen C. Johnson fabricated farther changes to the language to facilitate portability of the Unix operating system. Johnson'southward Portable C Compiler served as the ground for several implementations of C on new platforms.^[xiii]

One thousand&R C

In 1978, Brian Kernighan and Dennis Ritchie published the first edition of The C Programming Language.^[1] This book, known to C programmers every bit Grand&R, served for many years as an informal specification of the language. The version of C that it describes is normally referred to as "K&R C". As this was released in 1978, it is besides referred to every bit C78.^[15] The second edition of the volume^[sixteen] covers the later ANSI C standard, described beneath.

M&R introduced several linguistic communication features:

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

Even after the publication of the 1989 ANSI standard, for many years K&R C was still considered the "lowest mutual denominator" to which C programmers restricted themselves when maximum portability was desired, since many older compilers were withal in employ, and considering carefully written Chiliad&R C lawmaking can be legal Standard C every bit well.

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

For case:

                        long                                    some_function            ();                        /* int */                                    other_function            ();                        /* int */                                    calling_function            ()                        {                                                long                                    test1            ;                                                register                                    /* int */                                    test2            ;                                                test1                                    =                                    some_function            ();                                                if                                    (            test1                                    >                                    1            )                                                test2                                    =                                    0            ;                                                else                                                test2                                    =                                    other_function            ();                                                return                                    test2            ;                        }

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

Since K&R function declarations did not include any information about role arguments, function parameter type checks were not performed, although some compilers would event a alert bulletin if a local function was called with the incorrect number of arguments, or if multiple calls to an external function used unlike numbers or types of arguments. Separate tools such as Unix's lint utility were developed that (among other things) could check for consistency of function utilize 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 detail PCC^[17]) and another vendors. These included:

void functions (i.e., functions with no return 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 understanding on a standard library, together with the linguistic communication popularity and the fact that not even the Unix compilers precisely implemented the K&R specification, led to the necessity of standardization.

ANSI C and ISO C

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

In 1983, the American National Standards Constitute (ANSI) formed a committee, X3J11, to establish a standard specification of C. X3J11 based the C standard on the Unix implementation; still, the non-portable portion of the Unix C library was handed off to the IEEE working group 1003 to become the basis for the 1988 POSIX standard. In 1989, the C standard was ratified equally ANSI X3.159-1989 "Programming Language C". This version of the language is ofttimes 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 aforementioned 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 within a yr of ISO publication.

One of the aims of the C standardization process was to produce a superset of K&R C, incorporating many of the later introduced unofficial features. The standards committee also included several additional features such equally function 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 way used in C++, the Thousand&R interface continued to be permitted, for compatibility with existing source code.

C89 is supported by current C compilers, and virtually modernistic C code is based on it. Any program written only in Standard C and without whatever hardware-dependent assumptions will run correctly on whatever platform with a conforming C implementation, inside its resource limits. Without such precautions, programs may compile just on a sure platform or with a item compiler, due, for example, to the use of non-standard libraries, such equally GUI libraries, or to a reliance on compiler- or platform-specific attributes such as the exact size of information types and byte endianness.

In cases where code must be compilable by either standard-conforming or K&R C-based compilers, the __STDC__ macro tin can be used to split the lawmaking into Standard and 1000&R sections to prevent the use on a K&R C-based compiler of features available only in Standard C.

After the ANSI/ISO standardization process, the C language specification remained relatively static for several years. In 1995, Normative Amendment 1 to the 1990 C standard (ISO/IEC 9899/AMD1:1995, known informally as C95) was published, to right some details and to add more extensive support for international character sets.^[18]

C99

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

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 back up for IEEE 754 floating point, back up for variadic macros (macros of variable arity), and support for one-line comments beginning with //, as in BCPL or C++. Many of these had already been implemented as extensions in several C compilers.

C99 is for the most function backward compatible with C90, but is stricter in some ways; in detail, a declaration that lacks a type specifier no longer has int implicitly assumed. A standard macro __STDC_VERSION__ is divers with value 199901L to point that C99 support is available. GCC, Solaris Studio, and other C compilers at present 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.^[xx] ^{[ needs update ]}

In add-on, back up for Unicode identifiers (variable / function names) in the class of escaped characters (e.one thousand. \U0001f431) is now required. Support for raw Unicode names is optional.

C11

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

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

C17

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

C2x

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

Embedded C

Historically, embedded C programming requires nonstandard extensions to the C linguistic communication in club to support exotic features such as fixed-bespeak arithmetic, multiple singled-out retentiveness banks, and basic I/O operations.

In 2008, the C Standards Committee published a technical study extending the C linguistic communication^[22] to address these issues by providing a common standard for all implementations to adhere to. It includes a number of features not available in normal C, such as stock-still-point arithmetic, named accost spaces, and basic I/O hardware addressing.

Syntax

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

C source files comprise declarations and function definitions. Function definitions, in plough, contain declarations and statements. Declarations either define new types using keywords such as struct, matrimony, and enum, or assign types to and peradventure 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 human action as a unmarried statement for control structures.

Equally an imperative language, C uses statements to specify actions. The nearly common statement is an expression argument, consisting of an expression to be evaluated, followed past a semicolon; as a side effect of the evaluation, functions may exist called and variables may be 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 by if … [else] conditional execution and by exercise … while, while, and for iterative execution (looping). The for argument has separate initialization, testing, and reinitialization expressions, any or all of which tin can be omitted. break and go on tin can be used to leave the innermost enclosing loop statement or skip to its reinitialization. There is besides a non-structured goto argument which branches direct to the designated label within the part. switch selects a case to be executed based on the value of an integer expression.

Expressions can utilise a diverseness of built-in operators and may contain role calls. The gild in which arguments to functions and operands to most operators are evaluated is unspecified. The evaluations may fifty-fifty be interleaved. Notwithstanding, all side furnishings (including storage to variables) volition occur earlier the side by side "sequence point"; sequence points include the cease of each expression argument, and the entry to and return from each function call. Sequence points also occur during evaluation of expressions containing certain operators (&&, ||, ?: and the comma operator). This permits a loftier degree of object lawmaking optimization by the compiler, simply requires C programmers to take more care to obtain reliable results than is needed for other programming languages.

Kernighan and Ritchie say in the Introduction of The C Programming Linguistic communication: "C, like whatever other linguistic communication, has its blemishes. Some of the operators have the wrong precedence; some parts of the syntax could exist better."^[25] The C standard did non endeavour to correct many of these blemishes, because of the bear upon of such changes on already existing software.

Graphic symbol set

The basic C source character set includes the following characters:

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

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

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

The basic C execution grapheme prepare contains the aforementioned characters, along with representations for alert, backspace, and railroad vehicle return. Run-time support for extended graphic symbol sets has increased with each revision of the C standard.

Reserved words

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

auto
break
case
char
const
go on
default
do
double
else
enum
extern
float
for
goto
if
int
long
register
return
curt
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 than words:^[26]

_Alignas
_Alignof
_Atomic
_Generic
_Noreturn
_Static_assert
_Thread_local

Almost of the recently reserved words begin with an underscore followed by a capital letter of the alphabet, considering identifiers of that grade were previously reserved by the C standard for employ only by implementations. Since existing program source code should not accept been using these identifiers, it would not be afflicted when C implementations started supporting these extensions to the programming language. Some standard headers do define more than user-friendly synonyms for underscored identifiers. The language previously included a reserved word chosen entry, but this was seldom implemented, and has now been removed as a reserved word.^[27]

Operators

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

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

C uses the operator = (used in mathematics to express equality) to indicate assignment, following the precedent of Fortran and PL/I, but different ALGOL and its derivatives. C uses the operator == to examination for equality. The similarity between these two operators (consignment and equality) may consequence in the accidental employ of one in place of the other, and in many cases, the error does not produce an fault message (although some compilers produce warnings). For instance, the conditional expression if (a == b + 1) might mistakenly be written as if (a = b + 1), which will be evaluated equally truthful if a is not zero afterwards the assignment.^[28]

The C operator precedence is not always intuitive. For example, the operator == binds more tightly than (is executed prior to) the operators & (bitwise AND) and | (bitwise OR) in expressions such equally 10 & 1 == 0, which must be written as (x & 1) == 0 if that is the coder's intent.^[29]

"Hello, earth" example

The "howdy, earth" case, which appeared in the commencement edition of G&R, has go the model for an introductory program in well-nigh 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, earth            \n            "            );                        }

A standard-conforming "hello, globe" programme is:^[a]

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

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

The adjacent line indicates that a function named primary is being divers. The main function serves a special purpose in C programs; the run-time environment calls the main part to begin program execution. The type specifier int indicates that the value that is returned to the invoker (in this example the run-time environment) as a effect of evaluating the chief function, is an integer. The keyword void as a parameter listing indicates that this function takes no arguments.^[b]

The opening curly brace indicates the beginning of the definition of the principal office.

The adjacent line calls (diverts execution to) a office named printf, which in this case is supplied from a system library. In this call, the printf role is passed (provided with) a unmarried argument, the address of the first character in the string literal "hello, world\n". The string literal is an unnamed array with elements of type char, set up automatically by the compiler with a final 0-valued character to mark the stop of the array (printf needs to know this). The \north is an escape sequence that C translates to a newline graphic symbol, which on output signifies the end of the current line. The return value of the printf function is of type int, but information technology is silently discarded since it is not used. (A more conscientious plan might test the return value to determine whether or non the printf office succeeded.) The semicolon ; terminates the statement.

The endmost curly caryatid indicates the end of the code for the main function. Co-ordinate to the C99 specification and newer, the main function, different any other role, will implicitly render a value of 0 upon reaching the } that terminates the role. (Formerly an explicit return 0; statement was required.) This is interpreted by the run-fourth dimension system equally an get out lawmaking indicating successful execution.^[31]

Information types

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

C is often used in low-level systems programming where escapes from the type organization may be necessary. The compiler attempts to ensure type correctness of well-nigh expressions, but the programmer can override the checks in various ways, either by using a type bandage to explicitly convert a value from i type to some other, or by using pointers or unions to reinterpret the underlying bits of a data object in some other mode.

Some detect C'due south annunciation syntax unintuitive, peculiarly for role pointers. (Ritchie's idea was to declare identifiers in contexts resembling their use: "annunciation reflects use".)^[33]

C's usual arithmetic conversions permit for efficient lawmaking to be generated, but can sometimes produce unexpected results. For case, 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

C supports the use of pointers, a type of reference that records the address or location of an object or function in memory. Pointers tin be dereferenced to access data stored at the accost pointed to, or to invoke a pointed-to office. Pointers can be manipulated using consignment or pointer arithmetic. The run-fourth dimension representation of a pointer value is typically a raw memory accost (perhaps augmented by an get-go-inside-word field), but since a arrow'southward type includes the blazon of the thing pointed to, expressions including pointers can be blazon-checked at compile time. Pointer arithmetics is automatically scaled past 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 retentiveness allocation is performed using pointers. Many information types, such every bit trees, are unremarkably implemented as dynamically allocated struct objects linked together using pointers. Pointers to functions are useful for passing functions as arguments to college-order functions (such equally qsort or bsearch) or as callbacks to be invoked by consequence handlers.^[31]

A null arrow value explicitly points to no valid location. Dereferencing a null pointer value is undefined, frequently resulting in a segmentation fault. Null pointer values are useful for indicating special cases such as no "next" arrow in the final node of a linked list, or every bit an error indication from functions returning pointers. In appropriate contexts in source code, such equally for assigning to a pointer variable, a null pointer constant can be written as 0, with or without explicit casting to a pointer blazon, or as the Nothing macro defined by several standard headers. In conditional contexts, null pointer values evaluate to false, while all other arrow values evaluate to truthful.

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

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

Arrays

Assortment types in C are traditionally of a fixed, static size specified at compile time. The more recent C99 standard as well allows a form of variable-length arrays. Nonetheless, it is likewise possible to allocate a block of memory (of arbitrary size) at run-time, using the standard library's malloc function, and care for it as an assortment.

Since arrays are always accessed (in result) via pointers, array accesses are typically not checked against the underlying array size, although some compilers may provide bounds checking as an option.^[34] ^[35] Array premises violations are therefore possible and can atomic number 82 to various repercussions, including illegal retention accesses, corruption of data, buffer overruns, and run-fourth dimension exceptions.

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

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

                        int                                    func            (            int                                    N            ,                                    int                                    M            )                        {                                                float                                    (            *            p            )[            N            ][            M            ]                                    =                                    malloc            (            sizeof                                    *            p            );                                                if                                    (            !            p            )                                                return                                    -ane            ;                                                for                                    (            int                                    i                                    =                                    0            ;                                    i                                    <                                    North            ;                                    i            ++            )                                                for                                    (            int                                    j                                    =                                    0            ;                                    j                                    <                                    M            ;                                    j            ++            )                                                (            *            p            )[            i            ][            j            ]                                    =                                    i                                    +                                    j            ;                                                print_array            (            N            ,                                    One thousand            ,                                    p            );                                                gratuitous            (            p            );                                                return                                    1            ;                        }

Array–pointer interchangeability

The subscript annotation ten[i] (where ten 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 x) incremented by i bytes, but rather is defined to exist the base of operations address incremented by i multiplied by the size of an element that x points to. Thus, x[i] designates the i+1th element of the array.

Furthermore, in virtually 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 offset element. This implies that an assortment is never copied as a whole when named as an argument to a part, but rather only the address of its first chemical element is passed. Therefore, although function calls in C use pass-by-value semantics, arrays are in effect passed by reference.

The full size of an array 10 tin can exist adamant by applying sizeof to an expression of assortment type. The size of an element can exist adamant by applying the operator sizeof to whatever dereferenced element of an array A, as in northward = sizeof A[0]. This, the number of elements in a declared assortment A can be determined as sizeof A / sizeof A[0]. Note, that if only a pointer to the commencement element is available as it is often the example in C code because of the automatic conversion described above, the data near the total type of the array and its length are lost.

Retentiveness direction

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 singled-out ways to allocate retentiveness for objects:^[31]

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

These three approaches are advisable in different situations and have various trade-offs. For example, static memory allocation has little allotment overhead, automatic resource allotment may involve slightly more overhead, and dynamic retentivity allocation can potentially take a great deal of overhead for both allotment 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 express and transient than either static retentivity or heap space, and dynamic memory allocation allows convenient allocation of objects whose size is known only at run-fourth dimension. Well-nigh C programs brand extensive utilise of all three.

Where possible, automated or static allotment is unremarkably simplest because the storage is managed by the compiler, freeing the programmer of the potentially error-prone chore of manually allocating and releasing storage. Yet, many data structures can change in size at runtime, and since static allocations (and automated allocations earlier C99) must have 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 across the article on malloc for an instance of dynamically allocated arrays.) Unlike automated resource allotment, which tin can fail at run time with uncontrolled consequences, the dynamic allotment functions return an indication (in the form of a nil pointer value) when the required storage cannot exist allocated. (Static allotment that is too large is commonly detected by the linker or loader, before the plan tin can even begin execution.)

Unless otherwise specified, static objects incorporate nothing or null arrow values upon programme startup. Automatically and dynamically allocated objects are initialized simply if an initial value is explicitly specified; otherwise they initially have indeterminate values (typically, any bit pattern happens to be nowadays in the storage, which might not even represent a valid value for that blazon). If the plan attempts to access an uninitialized value, the results are undefined. Many modern compilers endeavour to detect and warn most this trouble, but both simulated positives and simulated negatives can occur.

Heap memory allocation has to be synchronized with its actual usage in any programme to be reused equally much as possible. For example, if the only arrow to a heap memory allotment goes out of scope or has its value overwritten before information technology is deallocated explicitly, then that memory cannot be recovered for afterward reuse and is essentially lost to the plan, a phenomenon known every bit a memory leak. Conversely, information technology is possible for retentivity to exist freed, simply is referenced afterward, leading to unpredictable results. Typically, the failure symptoms appear in a portion of the program unrelated to the code that causes the fault, making information technology difficult to diagnose the failure. Such issues are ameliorated in languages with automatic garbage collection.

Libraries

The C programming language uses libraries every bit its primary 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 inside the library that may be used by a program, and declarations of special data types and macro symbols used with these functions. In order for a program to use a library, it must include the library'due south header file, and the library must exist linked with the program, which in many cases requires compiler flags (eastward.thousand., -lm, shorthand for "link the math library").^[31]

The well-nigh 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 every bit embedded systems may provide only a subset of the standard library). This library supports stream input and output, memory allocation, mathematics, character strings, and fourth dimension values. Several dissever standard headers (for example, stdio.h) specify the interfaces for these and other standard library facilities.

Some other mutual set of C library functions are those used past applications specifically targeted for Unix and Unix-similar systems, especially functions which provide an interface to the kernel. These functions are detailed in various standards such equally POSIX and the Single UNIX Specification.

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

File handling and streams

File input and output (I/O) is non part of the C language itself but instead is handled by libraries (such equally the C standard library) and their associated header files (e.1000. stdio.h). File handling 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 physical device. The high-level I/O is done through the association of a stream to a file. In the C standard library, a buffer (a memory area or queue) is temporarily used to store information earlier it's sent to the final destination. This reduces the fourth dimension spent waiting for slower devices, for instance a hard drive or solid state drive. Low-level I/O functions are not part of the standard C library^{[ clarification needed ]} but are mostly part of "blank metal" programming (programming that's contained of whatever operating system such as almost embedded programming). With few exceptions, implementations include low-level I/O.

Language tools

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

Automated source code checking and auditing are beneficial in any language, and for C many such tools exist, such every bit Lint. A mutual practice is to utilise Lint to find questionable code when a program is start written. One time a program passes Lint, it is so compiled using the C compiler. Likewise, many compilers can optionally warn about syntactically valid constructs that are probable to really be errors. MISRA C is a proprietary set 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 not a standard part of C, such as bounds checking for arrays, detection of buffer overflow, serialization, dynamic retentivity tracking, and automatic garbage collection.

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

Uses

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 nigh purposes, yet when needed, system-specific code can be used to access specific hardware addresses and to perform type punning to friction match externally imposed interface requirements, with a low run-time demand on system resources.

C can be used for website programming using the Common Gateway Interface (CGI) as a "gateway" for information between the Spider web application, the server, and the browser.^[39] C is ofttimes chosen over interpreted languages because 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 often implemented in C. For case, 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, because the layer of abstraction from hardware is thin, and its overhead is depression, 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 past implementations of other languages. This arroyo may be used for portability or convenience; by using C as an intermediate language, additional car-specific code generators are not necessary. C has some features, such equally line-number preprocessor directives and optional superfluous commas at the finish of initializer lists, that back up compilation of generated code. Still, some of C's shortcomings have prompted the development of other C-based languages specifically designed for employ every bit intermediate languages, such as C--.

C has as well been widely used to implement stop-user applications. Yet, such applications tin likewise be written in newer, higher-level languages.

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

C has both straight and indirectly influenced many later languages such every bit C#, D, Go, Coffee, JavaScript, Limbo, LPC, Perl, PHP, Python, and Unix's C shell.^[42] The most pervasive influence has been syntactical; all of the languages mentioned combine the statement and (more or less recognizably) expression syntax of C with blazon systems, information models, and/or big-calibration program structures that differ from those of C, sometimes radically.

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

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

The C++ programming language (originally named "C with Classes") was devised past Bjarne Stroustrup equally an approach to providing object-oriented functionality with a C-like syntax.^[44] C++ adds greater typing forcefulness, scoping, and other tools useful in object-oriented programming, and permits generic programming via templates. Nearly a superset of C, C++ at present supports most of C, with a few exceptions.

Objective-C was originally a very "sparse" layer on meridian 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 office 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 well-nigh supersets of C.

See as well

Compatibility of C and C++
Comparing of Pascal and C
Comparing of programming languages
International Obfuscated C Code Contest
Listing of C-based programming languages
Listing of C compilers

Notes

^ The original example code will compile on most modern compilers that are non in strict standard compliance manner, but it does not fully conform to the requirements of either C89 or C99. In fact, C99 requires that a diagnostic message exist produced.
^ The main role really has ii arguments, int argc and char *argv[], respectively, which tin exist used to handle command line arguments. The ISO C standard (department 5.ane.two.two.1) requires both forms of main to be supported, which is special treatment non afforded to any other role.

References

^ ^a ^b Kernighan, Brian W.; Ritchie, Dennis M. (February 1978). The C Programming Language (1st ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-xiii-110163-0.
^ Ritchie (1993): "Thompson had made a cursory endeavour to produce a arrangement coded in an early on version of C—before structures—in 1972, but gave up the try."
^ Fruderica (December 13, 2020). "History of C". The cppreference.com. Archived from the original on October 24, 2020. Retrieved Oct 24, 2020.
^ Ritchie (1993): "The scheme of blazon composition adopted past C owes considerable debt to Algol 68, although information technology did not, perhaps, emerge in a form that Algol'due south adherents would approve of."
^ Ring Squad (Oct 23, 2021). "The Ring programming language and other languages". ring-lang.net.
^ ^a ^b "Verilog HDL (and C)" (PDF). The Enquiry Schoolhouse of Computer Science at the Australian National University. June 3, 2010. Archived from the original (PDF) on November 6, 2013. Retrieved Baronial 19, 2013. 1980s: ; Verilog offset introduced ; Verilog inspired by the C programming language
^ ^a ^b ^c ^d ^e Ritchie (1993)
^ "Programming Linguistic communication Popularity". 2009. Archived from the original on January sixteen, 2009. Retrieved January 16, 2009.
^ "TIOBE Programming Community Alphabetize". 2009. Archived from the original on May 4, 2009. Retrieved May 6, 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 Dec 12, 2019. Retrieved September x, 2019.
^ ^a ^b Johnson, S. C.; Ritchie, D. M. (1978). "Portability of C Programs and the UNIX System". Bell Organization Tech. J. 57 (6): 2021–2048. CiteSeerX10.1.1.138.35. doi:10.1002/j.1538-7305.1978.tb02141.x. S2CID 17510065. (Note: The PDF is an OCR browse of the original, and contains a rendering of "IBM 370" as "IBM 310".)
^ McIlroy, M. D. (1987). A Inquiry Unix reader: annotated excerpts from the Programmer's Manual, 1971–1986 (PDF) (Technical written report). CSTR. Bell Labs. p. 10. 139. Archived (PDF) from the original on Nov eleven, 2017. Retrieved February 1, 2015.
^ "C manual pages". FreeBSD Miscellaneous Information Transmission (FreeBSD 13.0 ed.). May xxx, 2011. Archived from the original on January 21, 2021. Retrieved January fifteen, 2021. [one] Archived January 21, 2021, at the Wayback Automobile
^ Kernighan, Brian W.; Ritchie, Dennis Thou. (March 1988). The C Programming Linguistic communication (2nd ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-110362-vii.
^ Stroustrup, Bjarne (2002). Sibling rivalry: C and C++ (PDF) (Study). AT&T Labs. Archived (PDF) from the original on August 24, 2014. Retrieved Apr 14, 2014.
^ C Integrity. International Organization for Standardization. March 30, 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 2, 2011.
^ Andrew Binstock (Oct 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). 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 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 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. 20. ISBN978-1-58961-237-two. Archived from the original on July 29, 2020. Retrieved February 10, 2012.
^ Kernighan & Ritchie (1978), p. half-dozen.
^ ^a ^b ^c ^d ^eastward ^f ^g Klemens, Ben (2013). 21st Century C. O'Reilly Media. ISBN978-i-4493-2714-nine.
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^ For instance, gcc provides _FORTIFY_SOURCE. "Security Features: Compile Fourth dimension Buffer Checks (FORTIFY_SOURCE)". fedoraproject.org. Archived from the original on Jan 7, 2007. Retrieved August 5, 2012.
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^ 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 Nov 12, 2012. Retrieved Baronial 5, 2012.
^ "Human Page for lint (freebsd Section 1)". unix.com. May 24, 2001. Retrieved July 15, 2014.
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Sources

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 Conference on History of Programming Languages (HOPL-2). ACM. pp. 201–208. doi:x.1145/154766.155580. ISBN0-89791-570-4 . Retrieved November four, 2014.
Kernighan, Brian W.; Ritchie, Dennis Yard. (1996). The C Programming Language (2nd ed.). Prentice Hall. ISBNseven-302-02412-X.

Farther reading

Kernighan, Brian; Ritchie, Dennis (1988). The C Programming Linguistic communication (2 ed.). Prentice Hall. ISBN978-0131103627. (annal)
Plauger, P.J. (1992). The Standard C Library (1 ed.). Prentice Hall. ISBN978-0131315099. (source)
Banahan, M.; Brady, D.; Doran, M. (1991). The C Book: Featuring the ANSI C Standard (ii ed.). Addison-Wesley. ISBN978-0201544336. (free)
Harbison, Samuel; Steele Jr, Guy (2002). C: A Reference Manual (5 ed.). Pearson. ISBN978-0130895929. (archive)
Rex, K.North. (2008). C Programming: A Modern Approach (2 ed.). W. W. Norton. ISBN978-0393979503. (archive)
Griffiths, David; Griffiths, Dawn (2012). Head First C (1 ed.). O'Reilly. ISBN978-1449399917.
Perry, Greg; Miller, Dean (2013). C Programming: Absolute Beginner's Guide (3 ed.). Que. ISBN978-0789751980.
Deitel, Paul; Deitel, Harvey (2015). C: How to Program (eight ed.). Pearson. ISBN978-0133976892.
Gustedt, Jens (2019). Mod C (two ed.). Manning. ISBN978-1617295812. (gratuitous)

External links

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). (3.61 MB)
comp.lang.c Oft Asked Questions
A History of C, by Dennis Ritchie

This page was final edited on 1 March 2022, at 08:47

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Source: https://wiki2.org/en/C_(programming_language)