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Full general-purpose programming linguistic communication

C
Major implementations
The C Programming Language ^[i] (often referred to as 1000&R), the seminal book 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)
First appeared	1972; 50 years ago (1972) ^[2]

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

Typing discipline	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++Architect, Microsoft Visual C++, Watcom C
Dialects
Whirlwind, Unified Parallel C, Split-C, Cilk, C*
Influenced past
B (BCPL, CPL), ALGOL 68,^[4] associates, 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 letter of the alphabet c) is a full general-purpose, procedural calculator programming language supporting structured programming, lexical variable scope, and recursion, with a static blazon system. By design, C provides constructs that map efficiently to typical car instructions. Information technology has institute lasting employ in applications previously coded in associates 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 between 1972 and 1973 to construct utilities running on Unix. Information technology was applied to re-implementing the kernel of the Unix operating organization.^[7] During the 1980s, C gradually gained popularity. It has become i of the virtually widely used programming languages,^[8] ^[9] with C compilers from diverse vendors available for the majority of existing reckoner architectures and operating systems. C has been standardized past ANSI since 1989 (ANSI C) and past the International Organization for Standardization (ISO).

C is an imperative procedural language. It was designed to exist compiled to provide depression-level access to retentiveness and linguistic communication constructs that map efficiently to car instructions, all with minimal runtime back up. Despite its low-level capabilities, the linguistic communication was designed to encourage cantankerous-platform programming. A standards-compliant C plan written with portability in listen tin be compiled for a wide variety of computer platforms and operating systems with few changes to its source lawmaking.^[10]

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

Overview [edit]

Like most procedural languages in the ALGOL tradition, C has facilities for structured programming and allows lexical variable telescopic and recursion. Its static type system prevents unintended operations. In C, all executable code is contained inside subroutines (also chosen "functions", though non strictly in the sense of functional programming). Office parameters are e'er passed by value (except arrays). Laissez passer-by-reference is simulated in C by explicitly passing pointer values. C program source text is gratis-format, using the semicolon equally a statement terminator and curly braces for grouping blocks of statements.

The C language as well exhibits the following characteristics:

The language has a small, stock-still number of keywords, including a full prepare of command period primitives: if/else, for, do/while, while, and switch. User-defined names are not distinguished from keywords by whatsoever kind of sigil.
Information technology has a large number of arithmetic, bitwise, and logic operators: +,+=,++,&,||, etc.
More than one assignment may be performed in a unmarried statement.
Functions:
- Function return values can be ignored, when non needed.
- Function and information pointers permit advertising hoc run-fourth dimension polymorphism.
- Functions may non be divers within the lexical scope of other functions.
Information typing is static, simply 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 kickoff with the proper noun of a type is taken as a declaration. There is no "role" keyword; instead, a function is indicated by the presence of a parenthesized argument list.
User-defined (typedef) and compound types are possible.
- Heterogeneous aggregate data types (struct) let related data elements to be accessed and assigned as a unit.
- Marriage is a structure with overlapping members; only the last fellow member stored is valid.
- Array indexing is a secondary notation, defined in terms of pointer arithmetic. Unlike structs, arrays are not splendid objects: they cannot be assigned or compared using single born operators. There is no "array" keyword in use or definition; instead, square brackets indicate arrays syntactically, for case month[xi].
- Enumerated types are possible with the enum keyword. They are freely interconvertible with integers.
- Strings are non a singled-out data type, simply are conventionally implemented equally naught-terminated character arrays.
Low-level access to computer retentiveness is possible by converting auto addresses to typed pointers.
Procedures (subroutines not returning values) are a special example of function, with an untyped return type void.
A preprocessor performs macro definition, source lawmaking file inclusion, and provisional compilation.
There is a bones form of modularity: files tin can be compiled separately and linked together, with command over which functions and information objects are visible to other files via static and extern attributes.
Circuitous functionality such equally I/O, cord 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, frequently through the use of external libraries (e.g., the GLib Object Organisation or the Boehm garbage collector).

Relations to other languages [edit]

Many afterwards languages have borrowed directly or indirectly from C, including C++, C#, Unix's C shell, D, Get, Coffee, JavaScript (including transpilers), Julia, Limbo, LPC, Objective-C, Perl, PHP, Python, Ruby, Rust, Swift, Verilog and SystemVerilog (hardware clarification languages).^[vi] These languages have drawn many of their control structures and other basic features from C. Most of them (Python being a dramatic exception) also limited highly similar syntax to C, and they tend to combine the recognizable expression and statement syntax of C with underlying type systems, information models, and semantics that can exist radically different.

History [edit]

Early developments [edit]

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

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

Thompson desired a programming linguistic communication to brand utilities for the new platform. At beginning, he tried to make a Fortran compiler, simply soon gave up the thought. Instead, he created a cutting-down version of the recently developed BCPL systems programming language. The official description of BCPL was not bachelor at the time,^[12] and Thompson modified the syntax to be less wordy, producing the similar simply somewhat simpler B.^[7] Nonetheless, few utilities were ultimately written in B because it was too deadening, and B could not have reward 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 language C.^[13] The C compiler and some utilities made with it were included in Version 2 Unix.^[14]

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

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

Unix was one of the first operating system kernels implemented in a language other than assembly. Earlier instances include the Multics system (which was written in PL/I) and Principal Command Program (MCP) for the Burroughs B5000 (which was written in ALGOL) in 1961. In around 1977, Ritchie and Stephen C. Johnson made farther changes to the language to facilitate portability of the Unix operating system. Johnson'due south Portable C Compiler served every bit the basis for several implementations of C on new platforms.^[13]

K&R C [edit]

In 1978, Brian Kernighan and Dennis Ritchie published the starting time edition of The C Programming Language.^[1] This book, known to C programmers as K&R, served for many years as an informal specification of the language. The version of C that it describes is usually referred to as "K&R C". As this was released in 1978, it is likewise referred to as C78.^[15] The second edition of the book^[16] covers the later ANSI C standard, described below.

K&R introduced several language features:

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

Even later the publication of the 1989 ANSI standard, for many years K&R C was still considered the "lowest common denominator" to which C programmers restricted themselves when maximum portability was desired, since many older compilers were all the same in use, and considering carefully written One thousand&R C code can be legal Standard C as well.

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

For example:

                        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 K&R C, simply are required in later standards.

Since K&R function declarations did non include any data virtually function arguments, function parameter type checks were non performed, although some compilers would issue a warning message if a local function was called with the wrong number of arguments, or if multiple calls to an external office used different numbers or types of arguments. Separate tools such as Unix's lint utility were developed that (among other things) could check for consistency of function use 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 item PCC^[17]) and another vendors. These included:

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

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

ANSI C and ISO C [edit]

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

In 1983, the American National Standards Institute (ANSI) formed a 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 as ANSI X3.159-1989 "Programming Language C". This version of the linguistic communication is often referred to equally ANSI C, Standard C, or sometimes C89.

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

ANSI, like other national standards bodies, no longer develops the C standard independently, but defers to the international C standard, maintained past the working group ISO/IEC JTC1/SC22/WG14. National adoption of an update to the international standard typically occurs inside a year of ISO publication.

One of the aims of the C standardization process was to produce a superset of K&R C, incorporating many of the subsequently introduced unofficial features. The standards committee too included several additional features such equally 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 style used in C++, the 1000&R interface connected to be permitted, for compatibility with existing source code.

C89 is supported by current C compilers, and almost mod C code is based on it. Any program written only in Standard C and without whatever hardware-dependent assumptions will run correctly on any platform with a befitting C implementation, within its resource limits. Without such precautions, programs may compile only on a sure platform or with a item compiler, due, for instance, to the use of non-standard libraries, such every bit 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 Grand&R C-based compilers, the __STDC__ macro can exist used to split the code into Standard and K&R sections to foreclose the utilise on a K&R C-based compiler of features bachelor only in Standard C.

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

C99 [edit]

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

C99 introduced several new features, including inline functions, several new data types (including long long int and a complex blazon 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 commencement with //, equally in BCPL or C++. Many of these had already been implemented as extensions in several C compilers.

C99 is for the most role backward compatible with C90, simply is stricter in some means; in detail, a declaration that lacks a type specifier no longer has int implicitly assumed. A standard macro __STDC_VERSION__ is defined with value 199901L to indicate that C99 support is available. GCC, Solaris Studio, and other C compilers now support many or all of the new features of C99. The C compiler in Microsoft Visual C++, however, implements the C89 standard and those parts of C99 that are required for compatibility with C++11.^[20] ^{[ needs update ]}

In addition, support for Unicode identifiers (variable / part names) in the form of escaped characters (e.g. \U0001f431) is at present required. Support for raw Unicode names is optional.

C11 [edit]

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

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

C17 [edit]

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

C2x [edit]

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

Embedded C [edit]

Historically, embedded C programming requires nonstandard extensions to the C language in order to support exotic features such as stock-still-point arithmetic, multiple distinct memory banks, and bones I/O operations.

In 2008, the C Standards Committee published a technical report extending the C language^[22] to address these issues past 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-signal 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 meaning in C; however, line boundaries practise have significance during the preprocessing phase. Comments may appear either between the delimiters /* and */, or (since C99) following // until the cease of the line. Comments delimited by /* and */ do non nest, and these sequences of characters are non interpreted as comment delimiters if they announced inside string or character literals.^[24]

C source files contain declarations and function definitions. Function definitions, in turn, contain declarations and statements. Declarations either define new types using keywords such as struct, matrimony, and enum, or assign types to and perhaps reserve storage for new variables, normally by writing the type followed past the variable name. Keywords such as char and int specify built-in types. Sections of code are enclosed in braces ({ and }, sometimes called "curly brackets") to limit the scope of declarations and to deed equally a single statement for control structures.

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

Expressions tin utilise a diverseness of congenital-in operators and may contain function calls. The club in which arguments to functions and operands to most operators are evaluated is unspecified. The evaluations may even exist interleaved. Still, all side effects (including storage to variables) will occur before the adjacent "sequence signal"; sequence points include the cease 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 high caste of object code optimization by the compiler, but requires C programmers to take more care to obtain reliable results than is needed for other programming languages.

Kernighan and Ritchie say in the Introduction of The C Programming Language: "C, like any other language, has its blemishes. Some of the operators accept the wrong precedence; some parts of the syntax could exist better."^[25] The C standard did not attempt to correct many of these blemishes, because of the impact of such changes on already existing software.

Graphic symbol set [edit]

The basic C source character set includes the following characters:

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

Newline indicates the terminate of a text line; it need not correspond to an actual single graphic symbol, although for convenience C treats it as 1.

Boosted multi-byte encoded characters may be used in string literals, but they are non 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 feature is not yet widely implemented.

The bones C execution character fix contains the same characters, forth with representations for alert, backspace, and railroad vehicle return. Run-time support for extended grapheme 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 any purposes other than those for which they are predefined:

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

C99 reserved five more words:

_Bool
_Complex
_Imaginary
inline
restrict

C11 reserved seven more 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 capital letter of the alphabet, considering identifiers of that form were previously reserved by the C standard for use only by implementations. Since existing program source code should not have been using these identifiers, it would not be afflicted when C implementations started supporting these extensions to the programming language. Some standard headers do define more convenient synonyms for underscored identifiers. The language previously included a reserved word called entry, but this was seldom implemented, and has now been removed as a reserved word.^[27]

Operators [edit]

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

arithmetic: +, -, *, /, %
assignment: =
augmented assignment: +=, -=, *=, /=, %=, &=, |=, ^=, <<=, >>=
bitwise logic: ~, &, |, ^
bitwise shifts: <<, >>
boolean logic: !, &&, ||
conditional evaluation: ? :
equality testing: ==, !=
calling functions: ( )
increment and decrement: ++, --
member selection: ., ->
object size: sizeof
gild relations: <, <=, >, >=
reference and dereference: &, *, [ ]
sequencing: ,
subexpression group: ( )
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 unlike ALGOL and its derivatives. C uses the operator == to test for equality. The similarity between these ii operators (assignment and equality) may result in the adventitious use of i in place of the other, and in many cases, the error does not produce an error message (although some compilers produce warnings). For example, the conditional expression if (a == b + one) might mistakenly be written equally if (a = b + 1), which will exist evaluated as true if a is not zero after the assignment.^[28]

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

"Hello, globe" example [edit]

The "hello, world" example, which appeared in the first edition of K&R, has become the model for an introductory plan in most programming textbooks. The program prints "hello, earth" to the standard output, which is usually a terminal or screen display.

The original version was:^[thirty]

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

A standard-befitting "hello, earth" program is:^[a]

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

The first line of the program contains a preprocessing directive, indicated by #include. This causes the compiler to supervene upon that line with the entire text of the stdio.h standard header, which contains declarations for standard input and output functions such equally 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 side by side line indicates that a function named primary is beingness defined. The master office serves a special purpose in C programs; the run-time surroundings calls the chief function to brainstorm plan execution. The type specifier int indicates that the value that is returned to the invoker (in this example the run-time surroundings) as a result of evaluating the main part, is an integer. The keyword void equally a parameter listing indicates that this role takes no arguments.^[b]

The opening curly caryatid indicates the offset of the definition of the main office.

The next line calls (diverts execution to) a function named printf, which in this case is supplied from a system library. In this call, the printf function is passed (provided with) a single statement, the address of the first character in the string literal "how-do-you-do, world\n". The string literal is an unnamed array with elements of blazon char, prepare automatically past the compiler with a final 0-valued graphic symbol to mark the end of the array (printf needs to know this). The \n is an escape sequence that C translates to a newline character, which on output signifies the end of the electric current line. The return value of the printf function is of blazon int, but it is silently discarded since information technology is not used. (A more than careful programme might exam the render value to decide whether or non the printf part succeeded.) The semicolon ; terminates the statement.

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

Data types [edit]

The type organisation in C is static and weakly typed, which makes information technology like to the type organisation of ALGOL descendants such every bit Pascal.^[32] At that place are congenital-in types for integers of various sizes, both signed and unsigned, floating-indicate numbers, and enumerated types (enum). Integer blazon char is oft 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 frequently used in low-level systems programming where escapes from the type organization may be necessary. The compiler attempts to ensure type definiteness of most expressions, but the programmer can override the checks in various means, either by using a type cast to explicitly catechumen a value from one type to another, or by using pointers or unions to reinterpret the underlying bits of a data object in some other way.

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

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

Pointers [edit]

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

A null pointer value explicitly points to no valid location. Dereferencing a nix pointer value is undefined, oft resulting in a partitioning fault. Zip pointer values are useful for indicating special cases such every bit no "side by side" pointer in the final node of a linked list, or equally an error indication from functions returning pointers. In appropriate contexts in source code, such every bit for assigning to a pointer variable, a null arrow constant can be written as 0, with or without explicit casting to a pointer blazon, or every bit the NULL macro defined by several standard headers. In provisional contexts, naught pointer values evaluate to false, while all other pointer values evaluate to true.

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

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

Arrays [edit]

Assortment types in C are traditionally of a stock-still, static size specified at compile fourth dimension. The more recent C99 standard too allows a form of variable-length arrays. Yet, information technology is also possible to allocate a block of retentivity (of arbitrary size) at run-fourth dimension, using the standard library's malloc part, and treat information technology as an array.

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

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

The post-obit instance using modern C (C99 or later) shows allocation of a 2-dimensional array on the heap and the use of multi-dimensional array indexing for accesses (which can employ bounds-checking on many C compilers):

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

Array–pointer interchangeability [edit]

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

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

The total size of an assortment x tin be determined past applying sizeof to an expression of array blazon. The size of an element can be determined past applying the operator sizeof to whatever dereferenced element of an array A, as in north = sizeof A[0]. This, the number of elements in a declared array A can be determined as sizeof A / sizeof A[0]. Annotation, that if only a pointer to the first element is bachelor as it is oft the case in C lawmaking because of the automatic conversion described above, the information nearly the full type of the array and its length are lost.

Memory management [edit]

One of the most important functions of a programming language is to provide facilities for managing retentiveness and the objects that are stored in retentivity. C provides three distinct ways to allocate retentiveness for objects:^[31]

Static retentivity allocation: infinite for the object is provided in the binary at compile-time; these objects have an extent (or lifetime) as long as the binary which contains them is loaded into retentivity.
Automated memory allocation: temporary objects tin can exist 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 retentiveness of arbitrary size tin be requested at run-fourth dimension using library functions such as malloc from a region of memory chosen the heap; these blocks persist until after freed for reuse by calling the library role realloc or gratuitous

These three approaches are advisable in dissimilar situations and have various merchandise-offs. For example, static memory resource allotment has little allocation overhead, automatic allocation may involve slightly more overhead, and dynamic memory allocation tin potentially have a great deal of overhead for both allocation and deallocation. The persistent nature of static objects is useful for maintaining state information beyond function calls, automatic allocation is easy to employ only stack space is typically much more than limited and transient than either static memory or heap infinite, and dynamic memory allocation allows convenient allocation of objects whose size is known only at run-time. Most C programs make extensive apply of all three.

Where possible, automatic or static allocation is unremarkably simplest considering the storage is managed by the compiler, freeing the developer of the potentially error-prone chore of manually allocating and releasing storage. However, many information structures can change in size at runtime, and since static allocations (and automatic allocations before C99) must have a fixed size at compile-time, at that place are many situations in which dynamic allocation is necessary.^[31] Prior to the C99 standard, variable-sized arrays were a common example of this. (See the article on malloc for an example of dynamically allocated arrays.) Unlike automatic allocation, which can fail at run time with uncontrolled consequences, the dynamic allocation functions return an indication (in the class of a null arrow value) when the required storage cannot be allocated. (Static resource allotment that is too large is usually detected by the linker or loader, before the program can even begin execution.)

Unless otherwise specified, static objects contain zero or null arrow values upon program startup. Automatically and dynamically allocated objects are initialized only if an initial value is explicitly specified; otherwise they initially have indeterminate values (typically, whatsoever bit pattern happens to be present in the storage, which might not even represent a valid value for that type). If the program attempts to admission an uninitialized value, the results are undefined. Many modernistic compilers effort to detect and warn nigh this problem, only both simulated positives and false negatives can occur.

Heap retentivity allocation has to be synchronized with its actual usage in any program to be reused equally much every bit possible. For case, if the only pointer to a heap retention allocation goes out of telescopic or has its value overwritten before information technology is deallocated explicitly, then that retention cannot be recovered for later reuse and is substantially lost to the programme, a phenomenon known every bit a memory leak. Conversely, it is possible for retentivity to be freed, just is referenced subsequently, leading to unpredictable results. Typically, the failure symptoms announced in a portion of the program unrelated to the code that causes the error, making it difficult to diagnose the failure. Such issues are ameliorated in languages with automatic garbage collection.

Libraries [edit]

The C programming language uses libraries as its principal method of extension. In C, a library is a set of functions contained within a single "archive" file. Each library typically has a header file, which contains the prototypes of the functions contained 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 programme to apply a library, information technology must include the library's header file, and the library must be linked with the program, which in many cases requires compiler flags (e.m., -lm, shorthand for "link the math library").^[31]

The most common C library is the C standard library, which is specified by the ISO and ANSI C standards and comes with every C implementation (implementations which target 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, graphic symbol strings, and time values. Several separate standard headers (for example, stdio.h) specify the interfaces for these and other standard library facilities.

Some other mutual fix 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 as POSIX and the Unmarried UNIX Specification.

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

File handling and streams [edit]

File input and output (I/O) is not part of the C language itself but instead is handled by libraries (such as the C standard library) and their associated header files (due east.g. stdio.h). File handling is generally 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 loftier-level I/O is done through the association of a stream to a file. In the C standard library, a buffer (a memory surface area or queue) is temporarily used to store information before it'due south sent to the terminal destination. This reduces the time spent waiting for slower devices, for example a hard drive or solid country bulldoze. Low-level I/O functions are not part of the standard C library^{[ description needed ]} but are generally part of "bare metal" programming (programming that's contained of any operating system such as most embedded programming). With few exceptions, implementations include low-level I/O.

Language tools [edit]

A number of tools have been developed to assistance C programmers find and ready statements with undefined behavior 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 lawmaking checking and auditing are beneficial in whatever language, and for C many such tools exist, such as Lint. A mutual practise is to use Lint to observe questionable lawmaking when a program is first written. In one case a program passes Lint, information technology is then compiled using the C compiler. Too, many compilers tin optionally warn about syntactically valid constructs that are probable to really be errors. MISRA C is a proprietary ready of guidelines to avoid such questionable code, developed for embedded systems.^[37]

In that location are also compilers, libraries, and operating system level mechanisms for performing actions that are not a standard part of C, such as bounds checking for arrays, detection of buffer overflow, serialization, dynamic memory tracking, and automatic garbage collection.

Tools such as Purify or Valgrind and linking with libraries containing special versions of the memory resource allotment functions tin can help uncover runtime errors in retention 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, tin be used for near purposes, even so when needed, system-specific lawmaking can exist used to admission specific hardware addresses and to perform type punning to match externally imposed interface requirements, with a low run-time need on system resources.

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

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

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

C is sometimes used as an intermediate language by implementations of other languages. This approach may be used for portability or convenience; by using C as an intermediate linguistic communication, additional machine-specific code generators are not necessary. C has some features, such as line-number preprocessor directives and optional superfluous commas at the stop of initializer lists, that support compilation of generated code. However, some of C's shortcomings have prompted the development of other C-based languages specifically designed for utilize equally intermediate languages, such every bit C--.

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

[edit]

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

C has both directly and indirectly influenced many after languages such as C#, D, Go, Java, JavaScript, Limbo, LPC, Perl, PHP, Python, and Unix'southward 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 big-scale program structures that differ from those of C, sometimes radically.

Several C or near-C interpreters exist, including Ch and CINT, which can also 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 then compiled with a C compiler.^[43]

The C++ programming language (originally named "C with Classes") was devised by Bjarne Stroustrup as an arroyo to providing object-oriented functionality with a C-like syntax.^[44] C++ adds greater typing force, scoping, and other tools useful in object-oriented programming, and permits generic programming via templates. Most a superset of C, C++ at present 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 epitome. Objective-C derives its syntax from both C and Smalltalk: syntax that involves preprocessing, expressions, function declarations, and function calls is inherited from C, while the syntax for object-oriented features was originally taken from Smalltalk.

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

Notes [edit]

^ The original example lawmaking will compile on most modern compilers that are non in strict standard compliance mode, simply it does non 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 ii arguments, int argc and char *argv[], respectively, which tin be used to handle control line arguments. The ISO C standard (section five.1.2.two.1) requires both forms of master to be supported, which is special treatment not afforded to whatever other function.

References [edit]

^ ^a ^b Kernighan, Brian W.; Ritchie, Dennis Chiliad. (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 cursory attempt to produce a organisation coded in an early version of C—before structures—in 1972, merely gave up the effort."
^ Fruderica (December 13, 2020). "History of C". The cppreference.com. Archived from the original on October 24, 2020. Retrieved October 24, 2020.
^ Ritchie (1993): "The scheme of blazon composition adopted by C owes considerable debt to Algol 68, although it did not, perhaps, emerge in a form that Algol's adherents would approve of."
^ Ring Team (October 23, 2021). "The Ring programming language and other languages". band-lang.internet.
^ ^a ^b "Verilog HDL (and C)" (PDF). The Research School of Information science at the Australian National University. June three, 2010. Archived from the original (PDF) on Nov 6, 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 Linguistic communication Popularity". 2009. Archived from the original on January 16, 2009. Retrieved January xvi, 2009.
^ "TIOBE Programming Community Index". 2009. Archived from the original on May 4, 2009. Retrieved May vi, 2009.
^ ^a ^b "History of C". en.cppreference.com. Archived from the original on May 29, 2018. Retrieved May 28, 2018.
^ "TIOBE Index for October 2021". Retrieved Oct 7, 2021.
^ Ritchie, Dennis. "BCPL to B to C". Archived from the original on December 12, 2019. Retrieved September 10, 2019.
^ ^a ^b Johnson, S. C.; Ritchie, D. Thou. (1978). "Portability of C Programs and the UNIX Organization". Bong System Tech. J. 57 (6): 2021–2048. CiteSeerX10.1.1.138.35. doi:ten.1002/j.1538-7305.1978.tb02141.10. S2CID 17510065. (Note: The PDF is an OCR browse of the original, and contains a rendering of "IBM 370" every bit "IBM 310".)
^ McIlroy, M. D. (1987). A Research Unix reader: annotated excerpts from the Programmer's Manual, 1971–1986 (PDF) (Technical written report). CSTR. Bong Labs. p. ten. 139. Archived (PDF) from the original on November 11, 2017. Retrieved February ane, 2015.
^ "C transmission pages". FreeBSD Miscellaneous Information Manual (FreeBSD 13.0 ed.). May xxx, 2011. Archived from the original on January 21, 2021. Retrieved January 15, 2021. [1] Archived January 21, 2021, at the Wayback Car
^ Kernighan, Brian Due west.; Ritchie, Dennis M. (March 1988). The C Programming Linguistic communication (2nd ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-thirteen-110362-7.
^ Stroustrup, Bjarne (2002). Sibling rivalry: C and C++ (PDF) (Written report). AT&T Labs. Archived (PDF) from the original on August 24, 2014. Retrieved April xiv, 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 ii, 2011.
^ Andrew Binstock (October 12, 2011). "Interview with Herb Sutter". Dr. Dobbs. Archived from the original on August 2, 2013. Retrieved September 7, 2013.
^ "Revised C23 Schedule WG 14 N 2759" (PDF). www.open up-std.org. Archived (PDF) from the original on June 24, 2021. Retrieved October 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 50. (2002). C: A Reference Transmission (fifth ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-089592-9. Contains a BNF grammer for C.
^ Kernighan & Ritchie (1996), p. 192.
^ Kernighan & Ritchie (1978), p. iii.
^ "ISO/IEC 9899:201x (ISO C11) Committee Typhoon" (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 Oct 21, 2008. Retrieved June 26, 2009.
^ Schultz, Thomas (2004). C and the 8051 (3rd ed.). Otsego, MI: PageFree Publishing Inc. p. 20. ISBN978-1-58961-237-2. Archived from the original on July 29, 2020. Retrieved February 10, 2012.
^ Kernighan & Ritchie (1978), p. 6.
^ ^a ^b ^c ^d ^e ^f ^g Klemens, Ben (2013). 21st Century C. O'Reilly Media. ISBN978-ane-4493-2714-9.
^ Feuer, Alan R.; Gehani, Narain H. (March 1982). "Comparison of the Programming Languages C and Pascal". ACM Computing Surveys. 14 (1): 73–92. doi:10.1145/356869.356872. S2CID 3136859.
^ Kernighan & Ritchie (1996), p. 122.
^ For instance, gcc provides _FORTIFY_SOURCE. "Security Features: Compile Time Buffer Checks (FORTIFY_SOURCE)". fedoraproject.org. Archived from the original on Jan 7, 2007. Retrieved August 5, 2012.
^ เอี่ยมสิริวงศ์, โอภาศ (2016). Programming with C. Bangkok, Thailand: SE-Didactics PUBLIC Company LIMITED. pp. 225–230. ISBN978-616-08-2740-four.
^ Raymond, Eric S. (October eleven, 1996). The New Hacker's Dictionary (3rd ed.). MIT Printing. p. 432. ISBN978-0-262-68092-nine. Archived from the original on November 12, 2012. Retrieved August 5, 2012.
^ "Homo Page for lint (freebsd Section 1)". unix.com. May 24, 2001. Retrieved July 15, 2014.
^ Dale, Nell B.; Weems, Chip (2014). Programming and problem solving with C++ (6th ed.). Burlington, MA: Jones & Bartlett Learning. ISBN978-1449694289. OCLC 894992484.
^ Dr. Dobb'southward Sourcebook. U.S.A.: Miller Freeman, Inc. Nov–December 1995.
^ "Using C for CGI Programming". linuxjournal.com. March 1, 2005. Archived from the original on February 13, 2010. Retrieved January 4, 2010.
^ McMillan, Robert (Baronial 1, 2013). "Is Java Losing Its Mojo?". Wired. Archived from the original on February 15, 2017. Retrieved March 5, 2017.
^ O'Regan, Gerard (September 24, 2015). Pillars of calculating : a compendium of select, pivotal technology firms. ISBN978-3319214641. OCLC 922324121.
^ Rauchwerger, Lawrence (2004). Languages and compilers for parallel computing : 16th international workshop, LCPC 2003, College Station, TX, USA, October 2-four, 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:10.1145/155360.155580.
Ritchie, Dennis M. (1993). "The Development of the C Linguistic communication". The Second ACM SIGPLAN Conference on History of Programming Languages (HOPL-Two). ACM. pp. 201–208. doi:x.1145/154766.155580. ISBN0-89791-570-iv . Retrieved November 4, 2014.
Kernighan, Brian W.; Ritchie, Dennis M. (1996). The C Programming Language (2nd ed.). Prentice Hall. ISBNvii-302-02412-X.

Farther reading [edit]

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

External links [edit]

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

fabertakented1976.blogspot.com

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