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General-purpose programming language

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

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

Typing discipline	Static, weak, manifest, nominal
OS	Cross-platform
Filename extensions	.c, .h
Website	www.iso.org/standard/74528.html world wide web.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 past
B (BCPL, CPL), ALGOL 68,^[4] assembly, PL/I, FORTRAN
Influenced
Numerous: AMPL, AWK, csh, C++, C--, C#, Objective-C, D, Go, Coffee, JavaScript, JS++, Julia, Limbo, LPC, Perl, PHP, Pike, Processing, Python, Band,^[v]Rust, Seed7, Vala, Verilog (HDL),^[6] Nim, Zig
C Programming at Wikibooks

C (, as in the letter of the alphabet c) 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 plant lasting utilise in applications previously coded in assembly language. Such applications include operating systems and various awarding 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 past 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.^[7] During the 1980s, C gradually gained popularity. It has become i of the most widely used programming languages,^[eight] ^[nine] with C compilers from various vendors available for the majority of existing computer architectures and operating systems. C has been standardized by ANSI since 1989 (ANSI C) and past the International Organization for Standardization (ISO).

C is an imperative procedural language. It was designed to be compiled to provide low-level admission to memory and language constructs that map efficiently to automobile instructions, all with minimal runtime back up. Despite its low-level capabilities, the linguistic communication was designed to encourage cross-platform programming. A standards-compliant C program written with portability in mind can be compiled for a broad variety of computer platforms and operating systems with few changes to its source code.^[ten]

Since 2000, C has consistently ranked amongst the top 2 languages in the TIOBE alphabetize, a mensurate of the popularity of programming languages.^[eleven]

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 system prevents unintended operations. In C, all executable lawmaking is contained inside subroutines (as well chosen "functions", though non strictly in the sense of functional programming). Function parameters are ever passed past value (except arrays). Pass-by-reference is simulated in C by explicitly passing pointer values. C program source text is gratuitous-format, using the semicolon as a statement terminator and curly braces for grouping blocks of statements.

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

The language has a pocket-size, fixed number of keywords, including a full set of control catamenia primitives: if/else, for, do/while, while, and switch. User-divers names are non distinguished from keywords past whatever kind of sigil.
Information technology has a large number of arithmetic, bitwise, and logic operators: +,+=,++,&,||, etc.
More than 1 assignment may be performed in a unmarried statement.
Functions:
- Function return values can be ignored, when not needed.
- Function and information pointers permit advertising hoc run-time polymorphism.
- Functions may not be defined inside the lexical scope of other functions.
Data typing is static, but weakly enforced; all data has a type, but implicit conversions are possible.
Announcement syntax mimics usage context. C has no "define" keyword; instead, a statement beginning with the name of a type is taken as a proclamation. In that location is no "function" keyword; instead, a part is indicated by the presence of a parenthesized argument listing.
User-divers (typedef) and chemical compound types are possible.
- Heterogeneous aggregate data types (struct) permit related data elements to exist accessed and assigned as a unit.
- Union is a structure with overlapping members; merely the concluding fellow member stored is valid.
- Array indexing is a secondary notation, defined in terms of pointer arithmetic. Different structs, arrays are not fantabulous objects: they cannot be assigned or compared using unmarried built-in operators. There is no "array" keyword in use or definition; instead, square brackets betoken arrays syntactically, for case month[11].
- Enumerated types are possible with the enum keyword. They are freely interconvertible with integers.
- Strings are not a distinct data type, simply are conventionally implemented as goose egg-terminated character arrays.
Low-level access to computer retentiveness is possible by converting machine addresses to typed pointers.
Procedures (subroutines not returning values) are a special instance of function, with an untyped return blazon void.
A preprocessor performs macro definition, source code file inclusion, and conditional compilation.
There is a basic form of modularity: files tin can be compiled separately and linked together, with command over which functions and data objects are visible to other files via static and extern attributes.
Complex functionality such as I/O, string manipulation, and mathematical functions are consistently delegated to library routines.

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

Relations to other languages [edit]

Many later languages have borrowed directly or indirectly from C, including C++, C#, Unix's C beat out, D, Go, Java, JavaScript (including transpilers), Julia, Limbo, LPC, Objective-C, Perl, PHP, Python, Blood-red, Rust, Swift, Verilog and SystemVerilog (hardware clarification languages).^{[half dozen]} These languages have drawn many of their control structures and other basic features from C. About 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, information models, and semantics that tin be radically different.

History [edit]

Early developments [edit]

Timeline of language evolution
Year	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 associates language on a PDP-seven past 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 too developed in associates language.^[7]

Thompson desired a programming language to brand utilities for the new platform. At starting time, he tried to brand a Fortran compiler, but soon gave up the thought. Instead, he created a cut-downward 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 similar only somewhat simpler B.^[7] Yet, few utilities were ultimately written in B because it was too slow, and B could not take advantage of PDP-11 features such as byte addressability.

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

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

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

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

Thou&R C [edit]

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

Thou&R introduced several language features:

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

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

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

For example:

                        long                                    some_function            ();                        /* int */                                    other_function            ();                        /* int */                                    calling_function            ()                        {                                                long                                    test1            ;                                                annals                                    /* 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 K&R C, only are required in later standards.

Since K&R role declarations did non include whatever data most function arguments, part parameter type checks were not performed, although some compilers would issue a alarm message if a local part was called with the wrong number of arguments, or if multiple calls to an external role used different numbers or types of arguments. Carve up tools such as Unix's lint utility were developed that (among other things) could bank check for consistency of role utilise across multiple source files.

In the years following the publication of Grand&R C, several features were added to the linguistic communication, 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 union types (rather than pointers)
assignment for struct data types
enumerated types

The large number of extensions and lack of agreement on a standard library, together with the language popularity and the fact that non even the Unix compilers precisely implemented the Chiliad&R specification, led to the necessity of standardization.

ANSI C and ISO C [edit]

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

In 1983, the American National Standards Institute (ANSI) formed a committee, X3J11, to establish a standard specification of C. X3J11 based the C standard on the Unix implementation; however, the non-portable portion of the Unix C library was handed off to the IEEE working grouping 1003 to go the basis for the 1988 POSIX standard. In 1989, the C standard was ratified as ANSI X3.159-1989 "Programming Language C". This version of the language is often referred to as ANSI C, Standard C, or sometimes C89.

In 1990, the ANSI C standard (with formatting changes) was adopted by the International Organization for Standardization (ISO) as ISO/IEC 9899:1990, which is sometimes called C90. Therefore, the terms "C89" and "C90" refer to the same programming language.

ANSI, similar other national standards bodies, no longer develops the C standard independently, merely 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 within a year of ISO publication.

Ane of the aims of the C standardization process was to produce a superset of K&R C, incorporating many of the subsequently introduced unofficial features. The standards committee also included several additional features such as function prototypes (borrowed from C++), void pointers, support for international character sets and locales, and preprocessor enhancements. Although the syntax for parameter declarations was augmented to include the manner used in C++, the 1000&R interface continued to exist permitted, for compatibility with existing source lawmaking.

C89 is supported by current C compilers, and near modern C code is based on it. Any program written only in Standard C and without any hardware-dependent assumptions will run correctly on any platform with a conforming C implementation, within its resources limits. Without such precautions, programs may compile but on a certain platform or with a particular compiler, due, for example, to the apply of not-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 by either standard-conforming or Yard&R C-based compilers, the __STDC__ macro tin be used to split the code into Standard and 1000&R sections to prevent the use on a M&R C-based compiler of features available only in Standard C.

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

C99 [edit]

The C standard was farther revised in the late 1990s, leading to the publication of ISO/IEC 9899:1999 in 1999, which is usually 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 information types (including long long int and a complex blazon to represent complex numbers), variable-length arrays and flexible array members, improved support for IEEE 754 floating point, support for variadic macros (macros of variable arity), and support for one-line comments beginning with //, as in BCPL or C++. Many of these had already been implemented as extensions in several C compilers.

C99 is for the nearly office backward compatible with C90, only 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 indicate that C99 support is available. GCC, Solaris Studio, and other C compilers now support many or all of the new features of C99. The C compiler in Microsoft Visual C++, however, implements the C89 standard and those parts of C99 that are required for compatibility with C++11.^[twenty] ^{[ needs update ]}

In improver, back up for Unicode identifiers (variable / part names) in the form of escaped characters (e.yard. \U0001f431) is at present required. Back up for raw Unicode names is optional.

C11 [edit]

In 2007, work began on another revision of the C standard, informally called "C1X" until its official publication on 2011-12-08. The C standards commission adopted guidelines to limit the adoption of new features that had non been tested by existing implementations.

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

C17 [edit]

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

C2x [edit]

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

Embedded C [edit]

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

In 2008, the C Standards Committee published a technical written report extending the C language^[22] to address these issues 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 fixed-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 not significant in C; even so, line boundaries practise accept significance during the preprocessing phase. Comments may appear either between the delimiters /* and */, or (since C99) following // until the terminate of the line. Comments delimited by /* and */ practice non nest, and these sequences of characters are not interpreted every bit comment delimiters if they appear inside string or character literals.^[24]

C source files contain declarations and part definitions. Role definitions, in plough, incorporate declarations and statements. Declarations either define new types using keywords such equally struct, union, and enum, or assign types to and maybe reserve storage for new variables, usually by writing the type followed by the variable name. Keywords such equally char and int specify built-in types. Sections of lawmaking 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.

As an imperative linguistic communication, C uses statements to specify actions. The most mutual statement is an expression statement, consisting of an expression to be evaluated, followed past a semicolon; equally a side effect of the evaluation, functions may be called and variables may be assigned new values. To modify the normal sequential execution of statements, C provides several control-flow statements identified by reserved keywords. Structured programming is supported by if … [else] provisional execution and by do … while, while, and for iterative execution (looping). The for statement has separate initialization, testing, and reinitialization expressions, any or all of which tin can exist omitted. pause and keep tin can exist used to leave the innermost enclosing loop argument or skip to its reinitialization. There is as well a non-structured goto statement which branches straight to the designated label within the office. switch selects a case to exist executed based on the value of an integer expression.

Expressions tin can use a diverseness of congenital-in operators and may contain office calls. The guild in which arguments to functions and operands to most operators are evaluated is unspecified. The evaluations may fifty-fifty be interleaved. However, all side effects (including storage to variables) volition occur before the adjacent "sequence bespeak"; sequence points include the terminate of each expression argument, and the entry to and return from each part 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 past the compiler, but requires C programmers to accept 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 whatever other language, has its blemishes. Some of the operators have the incorrect precedence; some parts of the syntax could exist improve."^[25] The C standard did not attempt to correct many of these blemishes, because of the touch on of such changes on already existing software.

Graphic symbol ready [edit]

The bones C source character set includes the following characters:

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

Newline indicates the end of a text line; it demand not correspond to an actual single graphic symbol, although for convenience C treats it every bit one.

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

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

Reserved words [edit]

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

auto
break
instance
char
const
continue
default
exercise
double
else
enum
extern
float
for
goto
if
int
long
annals
return
short
signed
sizeof
static
struct
switch
typedef
union
unsigned
void
volatile
while

C99 reserved five more than words:

_Bool
_Complex
_Imaginary
inline
restrict

C11 reserved seven more than words:^[26]

_Alignas
_Alignof
_Atomic
_Generic
_Noreturn
_Static_assert
_Thread_local

Most of the recently reserved words begin with an underscore followed by a upper-case letter alphabetic character, considering identifiers of that form were previously reserved by the C standard for use only by implementations. Since existing program source code should non take been using these identifiers, it would not be affected when C implementations started supporting these extensions to the programming language. Some standard headers exercise define more convenient synonyms for underscored identifiers. The linguistic communication previously included a reserved discussion called entry, only this was seldom implemented, and has now been removed every bit a reserved discussion.^[27]

Operators [edit]

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

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

C uses the operator = (used in mathematics to express equality) to indicate consignment, following the precedent of Fortran and PL/I, but unlike ALGOL and its derivatives. C uses the operator == to test for equality. The similarity between these two operators (consignment and equality) may effect in the adventitious use of one in place of the other, and in many cases, the mistake does not produce an fault message (although some compilers produce warnings). For example, the conditional expression if (a == b + 1) might mistakenly exist written as if (a = b + 1), which volition be evaluated as true if a is not zilch afterwards the consignment.^[28]

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

"Howdy, world" case [edit]

The "hello, globe" case, which appeared in the first edition of M&R, has become the model for an introductory program in most programming textbooks. The program prints "hullo, world" to the standard output, which is commonly a terminal or screen display.

The original version was:^[30]

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

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

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

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 every bit printf and scanf. The angle brackets surrounding stdio.h indicate that stdio.h is located using a search strategy that prefers headers provided with the compiler to other headers having the same name, as opposed to double quotes which typically include local or project-specific header files.

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

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

The next line calls (diverts execution to) a role named printf, which in this instance is supplied from a system library. In this call, the printf role is passed (provided with) a single argument, the address of the kickoff character in the string literal "hullo, earth\n". The string literal is an unnamed assortment with elements of type char, gear up up automatically past the compiler with a final 0-valued character to marking the finish of the array (printf needs to know this). The \due north is an escape sequence that C translates to a newline character, which on output signifies the end of the current line. The return value of the printf function is of type int, but it is silently discarded since it is not used. (A more careful program might test the return value to make up one's mind whether or non the printf function succeeded.) The semicolon ; terminates the argument.

The endmost curly brace indicates the cease of the code for the principal function. According to the C99 specification and newer, the primary 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 past the run-time system as an exit code indicating successful execution.^[31]

Information types [edit]

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

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

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

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

Pointers [edit]

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

A aught pointer value explicitly points to no valid location. Dereferencing a null pointer value is undefined, often resulting in a segmentation fault. Zero pointer values are useful for indicating special cases such equally no "side by side" pointer in the final node of a linked listing, or as an error indication from functions returning pointers. In appropriate contexts in source code, such as for assigning to a arrow variable, a goose egg arrow constant can be written every bit 0, with or without explicit casting to a arrow type, or as the NULL macro defined by several standard headers. In conditional contexts, null arrow values evaluate to simulated, while all other pointer values evaluate to true.

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

Careless use of pointers is potentially dangerous. Because they are typically unchecked, a pointer variable can exist made to point to whatsoever arbitrary location, which can crusade undesirable effects. Although properly used pointers point to rubber places, they tin be fabricated to indicate to unsafe places by using invalid pointer arithmetics; the objects they betoken to may continue to exist used afterward deallocation (dangling pointers); they may be used without having been initialized (wild pointers); or they may exist direct assigned an unsafe value using a cast, wedlock, or through another corrupt pointer. In 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 past 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 contempo C99 standard likewise allows a form of variable-length arrays. However, information technology is as well possible to allocate a block of retentivity (of arbitrary size) at run-fourth dimension, using the standard library'south malloc function, and treat it every bit an array.

Since arrays are always accessed (in event) via pointers, array accesses are typically non checked against the underlying array size, although some compilers may provide premises checking as an option.^[34] ^[35] Array bounds violations are therefore possible and tin pb to various repercussions, including illegal memory accesses, abuse of data, buffer overruns, and run-time exceptions.

C does not have a special provision for declaring multi-dimensional arrays, simply rather relies on recursion within the type system to declare arrays of arrays, which effectively accomplishes the aforementioned matter. The index values of the resulting "multi-dimensional array" tin can be thought of as increasing in row-major order. Multi-dimensional arrays are normally used in numerical algorithms (mainly from applied linear algebra) to store matrices. The structure of the C array is well suited to this detail chore. However, in early versions of C the bounds of the array must be known fixed values or else explicitly passed to whatsoever subroutine that requires them, and dynamically sized arrays of arrays cannot be accessed using double indexing. (A workaround for this was to classify the array with an boosted "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 ii-dimensional array on the heap and the utilise of multi-dimensional array indexing for accesses (which tin employ bounds-checking on many C compilers):

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

Array–pointer interchangeability [edit]

The subscript annotation ten[i] (where x designates a pointer) is syntactic sugar for *(x+i).^[36] Taking advantage of the compiler's noesis of the pointer type, the address that x + i points to is not the base of operations address (pointed to by x) incremented by i bytes, but rather is defined to be the base address incremented past i multiplied past the size of an element that x points to. Thus, x[i] designates the i+1th chemical element of the array.

Furthermore, in most expression contexts (a notable exception is as operand of sizeof), an expression of array type is automatically converted to a pointer to the array's first element. This implies that an array is never copied as a whole when named every bit an argument to a function, but rather merely the accost of its offset element is passed. Therefore, although part calls in C use pass-by-value semantics, arrays are in effect passed by reference.

The total size of an array x tin can be determined by applying sizeof to an expression of assortment blazon. The size of an element tin be determined by applying the operator sizeof to any dereferenced element of an array A, as in n = sizeof A[0]. This, the number of elements in a alleged array A tin be adamant as sizeof A / sizeof A[0]. Note, that if only a pointer to the first chemical element is available equally it is often the example in C code because of the automatic conversion described above, the data about the total blazon of the array and its length are lost.

Retentiveness management [edit]

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

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

These three approaches are appropriate in different situations and have various trade-offs. For instance, static memory allocation has little allocation overhead, automatic allocation may involve slightly more overhead, and dynamic memory resource allotment can potentially take a corking bargain of overhead for both allocation and deallocation. The persistent nature of static objects is useful for maintaining land information across function calls, automated allocation is easy to utilise but stack infinite is typically much more limited and transient than either static memory or heap space, and dynamic memory allotment allows convenient allocation of objects whose size is known only at run-fourth dimension. Most C programs make extensive use of all three.

Where possible, automated or static allotment is usually simplest because the storage is managed by the compiler, freeing the programmer of the potentially fault-prone chore of manually allocating and releasing storage. Still, many data structures tin can alter in size at runtime, and since static allocations (and automated allocations before C99) must have a fixed size at compile-time, there are many situations in which dynamic resource allotment is necessary.^[31] Prior to the C99 standard, variable-sized arrays were a mutual example of this. (See the article on malloc for an example of dynamically allocated arrays.) Unlike automatic allocation, which can fail at run time with uncontrolled consequences, the dynamic resource allotment functions render an indication (in the form of a null pointer value) when the required storage cannot exist allocated. (Static resource allotment that is too large is usually detected by the linker or loader, earlier the program can fifty-fifty begin execution.)

Unless otherwise specified, static objects contain aught or zippo pointer values upon program startup. Automatically and dynamically allocated objects are initialized simply if an initial value is explicitly specified; otherwise they initially accept indeterminate values (typically, whatever chip pattern happens to be present in the storage, which might not fifty-fifty represent a valid value for that type). If the program attempts to admission an uninitialized value, the results are undefined. Many modernistic compilers try to find and warn about this trouble, but both fake positives and false negatives can occur.

Heap memory allocation has to be synchronized with its actual usage in any plan to exist reused as much as possible. For example, if the just pointer to a heap memory allocation goes out of scope or has its value overwritten before it is deallocated explicitly, and then that memory cannot be recovered for later reuse and is essentially lost to the program, a miracle known equally a memory leak. Conversely, it is possible for memory to be freed, but is referenced later on, leading to unpredictable results. Typically, the failure symptoms appear in a portion of the programme 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 [edit]

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

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

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

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

File handling and streams [edit]

File input and output (I/O) is non part of the C linguistic communication itself but instead is handled by libraries (such as the C standard library) and their associated header files (east.thou. 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 menstruum that is independent of devices, while a file is a physical device. The high-level I/O is washed through the association of a stream to a file. In the C standard library, a buffer (a memory area or queue) is temporarily used to shop data before it's sent to the final destination. This reduces the time spent waiting for slower devices, for instance a hard drive or solid state drive. Low-level I/O functions are non part of the standard C library^{[ clarification needed ]} merely are generally part of "blank metallic" programming (programming that's contained of any operating system such as near embedded programming). With few exceptions, implementations include depression-level I/O.

Language tools [edit]

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

Automated source code checking and auditing are beneficial in any language, and for C many such tools exist, such equally Lint. A common practise is to use Lint to detect questionable code when a programme is get-go written. One time a plan passes Lint, it is then compiled using the C compiler. Too, many compilers tin can optionally warn about syntactically valid constructs that are likely to actually be errors. MISRA C is a proprietary set of guidelines to avoid such questionable code, developed for embedded systems.^[37]

At that place are also compilers, libraries, and operating organization level mechanisms for performing actions 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 as Purify or Valgrind and linking with libraries containing special versions of the retentivity allocation functions can assistance uncover runtime errors in memory usage.

Uses [edit]

The C Programming Language

C is widely used for systems programming in implementing operating systems and embedded system applications,^[38] because C code, when written for portability, can be used for nearly purposes, yet when needed, organization-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-time demand on system resource.

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

A effect of C's wide availability and efficiency is that compilers, libraries and interpreters of other programming languages are often implemented in C. For example, the reference implementations of Python, Perl, Ruddy, and PHP are written in C.

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

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

C has also been widely used to implement end-user applications. Yet, such applications can too be 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 direct and indirectly influenced many after languages such equally 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 type systems, data models, and/or large-scale program structures that differ from those of C, sometimes radically.

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

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

The C++ programming language (originally named "C with Classes") was devised by Bjarne Stroustrup as an approach to providing object-oriented functionality with a C-similar 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 most of C, with a few exceptions.

Objective-C was originally a very "thin" layer on top of C, and remains a strict superset of C that permits object-oriented programming using a hybrid dynamic/static typing paradigm. Objective-C derives its syntax from both C and Smalltalk: syntax that involves preprocessing, expressions, function declarations, and function calls is inherited from C, while the syntax for object-oriented features was originally taken from Smalltalk.

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

See as well [edit]

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

Notes [edit]

^ The original example code will compile on nigh modernistic compilers that are not in strict standard compliance way, just it does not fully conform to the requirements of either C89 or C99. In fact, C99 requires that a diagnostic bulletin be produced.
^ The main function really has two arguments, int argc and char *argv[], respectively, which tin can exist used to handle command line arguments. The ISO C standard (section 5.1.two.two.1) requires both forms of primary to be supported, which is special treatment non afforded to any other role.

References [edit]

^ ^a ^b Kernighan, Brian Due west.; Ritchie, Dennis Grand. (February 1978). The C Programming Linguistic communication (1st ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-110163-0.
^ Ritchie (1993): "Thompson had made a brief attempt to produce a organisation coded in an early on version of C—before structures—in 1972, but gave up the effort."
^ Fruderica (December thirteen, 2020). "History of C". The cppreference.com. Archived from the original on Oct 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 not, perhaps, emerge in a form that Algol's adherents would corroborate of."
^ Ring Team (October 23, 2021). "The Ring programming language and other languages". band-lang.net.
^ ^a ^b "Verilog HDL (and C)" (PDF). The Enquiry School of Informatics at the Australian National University. June 3, 2010. Archived from the original (PDF) on November half-dozen, 2013. Retrieved August 19, 2013. 1980s: ; Verilog first introduced ; Verilog inspired by the C programming language
^ ^a ^b ^c ^d ^{due east} Ritchie (1993)
^ "Programming Language Popularity". 2009. Archived from the original on January sixteen, 2009. Retrieved Jan sixteen, 2009.
^ "TIOBE Programming Community Index". 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 Alphabetize for October 2021". Retrieved Oct seven, 2021.
^ Ritchie, Dennis. "BCPL to B to C". Archived from the original on December 12, 2019. Retrieved September 10, 2019.
^ ^a ^b Johnson, Due south. C.; Ritchie, D. Grand. (1978). "Portability of C Programs and the UNIX System". Bell System Tech. J. 57 (6): 2021–2048. CiteSeerX10.1.1.138.35. doi:10.1002/j.1538-7305.1978.tb02141.x. S2CID 17510065. (Note: The PDF is an OCR scan of the original, and contains a rendering of "IBM 370" as "IBM 310".)
^ McIlroy, Chiliad. D. (1987). A Research Unix reader: annotated excerpts from the Programmer'due south Manual, 1971–1986 (PDF) (Technical report). CSTR. Bong Labs. p. x. 139. Archived (PDF) from the original on November xi, 2017. Retrieved February one, 2015.
^ "C transmission pages". FreeBSD Miscellaneous Information Manual (FreeBSD 13.0 ed.). May 30, 2011. Archived from the original on January 21, 2021. Retrieved January 15, 2021. [1] Archived January 21, 2021, at the Wayback Machine
^ Kernighan, Brian West.; Ritchie, Dennis Thousand. (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) (Report). AT&T Labs. Archived (PDF) from the original on Baronial 24, 2014. Retrieved April 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". Abode folio. ISO/IEC. Archived from the original on February 12, 2018. Retrieved June 2, 2011.
^ Andrew Binstock (October 12, 2011). "Interview with Herb Sutter". Dr. Dobbs. Archived from the original on August two, 2013. Retrieved September seven, 2013.
^ "Revised C23 Schedule WG xiv N 2759" (PDF). www.open-std.org. Archived (PDF) from the original on June 24, 2021. Retrieved October x, 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 50. (2002). C: A Reference Manual (fifth ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-089592-9. Contains a BNF grammar for C.
^ Kernighan & Ritchie (1996), p. 192.
^ Kernighan & Ritchie (1978), p. three.
^ "ISO/IEC 9899:201x (ISO C11) Commission Draft" (PDF). Archived (PDF) from the original on Dec 22, 2017. Retrieved September sixteen, 2011.
^ Kernighan & Ritchie (1996), pp. 192, 259.
^ "10 Common Programming Mistakes in C++". Cs.ucr.edu. Archived from the original on October 21, 2008. Retrieved June 26, 2009.
^ Schultz, Thomas (2004). C and the 8051 (3rd ed.). Otsego, MI: PageFree Publishing Inc. p. twenty. ISBN978-1-58961-237-2. Archived from the original on July 29, 2020. Retrieved February 10, 2012.
^ Kernighan & Ritchie (1978), p. 6.
^ ^a ^b ^c ^d ^east ^f ^g Klemens, Ben (2013). 21st Century C. O'Reilly Media. ISBN978-i-4493-2714-9.
^ Feuer, Alan R.; Gehani, Narain H. (March 1982). "Comparison of the Programming Languages C and Pascal". ACM Calculating Surveys. 14 (1): 73–92. doi:10.1145/356869.356872. S2CID 3136859.
^ Kernighan & Ritchie (1996), p. 122.
^ For example, gcc provides _FORTIFY_SOURCE. "Security Features: Compile Time Buffer Checks (FORTIFY_SOURCE)". fedoraproject.org. Archived from the original on January 7, 2007. Retrieved Baronial 5, 2012.
^ เอี่ยมสิริวงศ์, โอภาศ (2016). Programming with C. Bangkok, Thailand: SE-EDUCATION PUBLIC Visitor Limited. pp. 225–230. ISBN978-616-08-2740-4.
^ Raymond, Eric Southward. (October 11, 1996). The New Hacker'southward Dictionary (3rd ed.). MIT Press. p. 432. ISBN978-0-262-68092-9. Archived from the original on November 12, 2012. Retrieved Baronial five, 2012.
^ "Man Page for lint (freebsd Section i)". unix.com. May 24, 2001. Retrieved July fifteen, 2014.
^ Dale, Nell B.; Weems, Chip (2014). Programming and problem solving with C++ (6th ed.). Burlington, MA: Jones & Bartlett Learning. ISBN978-1449694289. OCLC 894992484.
^ Dr. Dobb's Sourcebook. The statesA.: Miller Freeman, Inc. Nov–December 1995.
^ "Using C for CGI Programming". linuxjournal.com. March ane, 2005. Archived from the original on February 13, 2010. Retrieved January 4, 2010.
^ McMillan, Robert (August i, 2013). "Is Java Losing Its Mojo?". Wired. Archived from the original on February xv, 2017. Retrieved March five, 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, Higher Station, TX, USA, October ii-iv, 2003 : revised papers. Springer. ISBN978-3540246442. OCLC 57965544.
^ Stroustrup, Bjarne (1993). "A History of C++: 1979−1991" (PDF). Archived (PDF) from the original on February 2, 2019. Retrieved June 9, 2011.

Sources [edit]

Ritchie, Dennis M. (March 1993). "The Development of the C Language". ACM SIGPLAN Notices. ACM. 28 (3): 201–208. doi:x.1145/155360.155580.
Ritchie, Dennis M. (1993). "The Evolution of the C Language". The Second ACM SIGPLAN Conference on History of Programming Languages (HOPL-II). ACM. pp. 201–208. doi:10.1145/154766.155580. ISBN0-89791-570-iv . Retrieved November iv, 2014.
Kernighan, Brian West.; Ritchie, Dennis M. (1996). The C Programming Language (2nd ed.). Prentice Hall. ISBN7-302-02412-Ten.

External links [edit]

ISO C Working Group official website
- ISO/IEC 9899, publicly bachelor official C documents, including the C99 Rationale
- "C99 with Technical corrigenda TC1, TC2, and TC3 included" (PDF). (3.61 MB)
comp.lang.c Frequently Asked Questions
A History of C, past Dennis Ritchie

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