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General-purpose programming language

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

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

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

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

A successor to the programming language B, C was originally developed at Bell Labs past Dennis Ritchie betwixt 1972 and 1973 to construct utilities running on Unix. Information technology was applied to re-implementing the kernel of the Unix operating system.^[7] During the 1980s, C gradually gained popularity. It has go i of the near widely used programming languages,^[eight] ^[ix] with C compilers from various vendors bachelor for the majority of existing computer architectures and operating systems. C has been standardized by ANSI since 1989 (ANSI C) and by the International Organization for Standardization (ISO).

C is an imperative procedural language. It was designed to exist compiled to provide low-level access to retention and language constructs that map efficiently to automobile instructions, all with minimal runtime support. Despite its low-level capabilities, the language was designed to encourage cross-platform programming. A standards-compliant C programme written with portability in heed can be compiled for a wide variety of calculator platforms and operating systems with few changes to its source lawmaking.^[10]

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

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 blazon organization prevents unintended operations. In C, all executable code is contained within subroutines (also chosen "functions", though non strictly in the sense of functional programming). Office parameters are always passed by value (except arrays). Pass-by-reference is imitation in C by explicitly passing pointer values. C program source text is gratis-format, using the semicolon as a argument terminator and curly braces for group blocks of statements.

The C linguistic communication also exhibits the following characteristics:

The language has a small, fixed number of keywords, including a full set of control flow primitives: if/else, for, do/while, while, and switch. User-defined names are not distinguished from keywords by whatsoever kind of sigil.
Information technology has a large number of arithmetic, bitwise, and logic operators: +,+=,++,&,||, etc.
More than than one assignment may exist performed in a single statement.
Functions:
- Function return values can be ignored, when not needed.
- Part and data pointers permit ad hoc run-time polymorphism.
- Functions may not be divers within the lexical scope of other functions.
Data typing is static, but weakly enforced; all data has a type, but implicit conversions are possible.
Declaration syntax mimics usage context. C has no "define" keyword; instead, a statement beginning with the name of a blazon is taken as a announcement. In that location is no "function" 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) allow related information elements to be accessed and assigned as a unit of measurement.
- Matrimony is a structure with overlapping members; only the last member stored is valid.
- Array indexing is a secondary notation, defined in terms of pointer arithmetic. Unlike structs, arrays are non excellent objects: they cannot exist assigned or compared using unmarried built-in operators. At that place is no "array" keyword in utilize or definition; instead, foursquare brackets signal arrays syntactically, for example month[11].
- Enumerated types are possible with the enum keyword. They are freely interconvertible with integers.
- Strings are not a distinct data type, simply are conventionally implemented equally nix-terminated character arrays.
Depression-level admission to calculator memory is possible by converting motorcar addresses to typed pointers.
Procedures (subroutines not returning values) are a special instance of office, with an untyped return type void.
A preprocessor performs macro definition, source code file inclusion, and conditional compilation.
In that location is a basic form of modularity: files tin be compiled separately and linked together, with control over which functions and data objects are visible to other files via static and extern attributes.
Complex functionality such equally I/O, cord manipulation, and mathematical functions are consistently delegated to library routines.

While C does not include certain features plant in other languages (such every bit object orientation and garbage collection), these can be implemented or emulated, often through the utilise of external libraries (e.grand., the GLib Object System or the Boehm garbage collector).

Relations to other languages [edit]

Many afterward languages have borrowed straight or indirectly from C, including C++, C#, Unix's C beat out, D, Become, Coffee, JavaScript (including transpilers), Julia, Limbo, LPC, Objective-C, Perl, PHP, Python, Reddish, Rust, Swift, Verilog and SystemVerilog (hardware description languages).^{[half dozen]} These languages take drawn many of their control structures and other bones features from C. Well-nigh of them (Python being a dramatic exception) likewise express 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 tin can be radically unlike.

History [edit]

Early developments [edit]

Timeline of linguistic communication evolution
Twelvemonth	C Standard^[10]
1972	Nascency
1978	G&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 evolution of the Unix operating organisation, originally implemented in assembly linguistic communication on a PDP-7 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 also developed in associates linguistic communication.^[7]

Thompson desired a programming language to make utilities for the new platform. At starting time, he tried to make a Fortran compiler, but soon gave up the idea. Instead, he created a cut-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 exist less wordy, producing the similar but somewhat simpler B.^[7] Nonetheless, few utilities were ultimately written in B considering it was too tedious, and B could not take advantage of PDP-11 features such equally byte addressability.

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

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

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

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

Grand&R C [edit]

In 1978, Brian Kernighan and Dennis Ritchie published the first edition of The C Programming Linguistic communication.^[1] This book, known to C programmers equally Chiliad&R, served for many years equally an informal specification of the language. The version of C that it describes is ordinarily referred to as "K&R C". As this was released in 1978, it is also referred to as C78.^[xv] 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 blazon
Compound assignment operators of the form =op (such every bit =-) were inverse to the grade op= (that is, -=) to remove the semantic ambiguity created past constructs such every bit i=-10, which had been interpreted as i =- 10 (decrement i by 10) instead of the possibly intended i = -10 (allow i exist −x).

Even after the publication of the 1989 ANSI standard, for many years K&R C was nonetheless considered the "lowest common denominator" to which C programmers restricted themselves when maximum portability was desired, since many older compilers were still in employ, and because carefully written 1000&R C code tin can be legal Standard C as well.

In early versions of C, only functions that return types other than int must exist declared if used before the role definition; functions used without prior declaration were presumed to return 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 office declarations did not include any information nigh function arguments, role parameter type checks were not performed, although some compilers would consequence a warning message if a local function was called with the wrong number of arguments, or if multiple calls to an external function used different numbers or types of arguments. Separate tools such as Unix's lint utility were developed that (amidst other things) could check for consistency of function apply across multiple source files.

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

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

The big number of extensions and lack of agreement on a standard library, together with the linguistic communication popularity and the fact that not even the Unix compilers precisely implemented the Grand&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 variety of mainframe computers, minicomputers, and microcomputers, including the IBM PC, as its popularity began to increase significantly.

In 1983, the American National Standards Institute (ANSI) formed a commission, X3J11, to plant a standard specification of C. X3J11 based the C standard on the Unix implementation; however, the not-portable portion of the Unix C library was handed off to the IEEE working group 1003 to become the basis for the 1988 POSIX standard. In 1989, the C standard was ratified every bit ANSI X3.159-1989 "Programming Linguistic communication C". This version of the linguistic communication is often referred to as 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) as ISO/IEC 9899:1990, which is sometimes called C90. Therefore, the terms "C89" and "C90" refer to the same programming language.

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

One of the aims of the C standardization process was to produce a superset of Thousand&R C, incorporating many of the later introduced unofficial features. The standards committee also included several boosted features such as role prototypes (borrowed from C++), void pointers, support for international grapheme sets and locales, and preprocessor enhancements. Although the syntax for parameter declarations was augmented to include the style used in C++, the K&R interface continued to exist permitted, for compatibility with existing source code.

C89 is supported by current C compilers, and well-nigh modern C lawmaking 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 conforming C implementation, within its resource limits. Without such precautions, programs may compile only on a certain platform or with a particular compiler, due, for example, 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 data types and byte endianness.

In cases where code must be compilable by either standard-conforming or K&R C-based compilers, the __STDC__ macro tin be used to carve up the code into Standard and K&R sections to forestall the utilise on a Chiliad&R C-based compiler of features available only in Standard C.

Afterward the ANSI/ISO standardization process, the C language specification remained relatively static for several years. In 1995, Normative Amendment 1 to the 1990 C standard (ISO/IEC 9899/AMD1:1995, known informally as C95) was published, to correct some details and to add together more extensive support for international grapheme 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 equally "C99". It has since been amended iii times past Technical Corrigenda.^[19]

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

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

In addition, support for Unicode identifiers (variable / function names) in the form of escaped characters (e.thousand. \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 non been tested by existing implementations.

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

C17 [edit]

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

C2x [edit]

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

Embedded C [edit]

Historically, embedded C programming requires nonstandard extensions to the C language in social club to support exotic features such equally fixed-point arithmetic, multiple distinct retention banks, and basic I/O operations.

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

Syntax [edit]

C has a formal grammar specified past the C standard.^[23] Line endings are generally not significant in C; however, line boundaries do take 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 */ do non nest, and these sequences of characters are not interpreted equally annotate delimiters if they announced inside string or character literals.^[24]

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

As an imperative language, C uses statements to specify actions. The well-nigh common statement is an expression argument, consisting of an expression to be evaluated, followed by a semicolon; as a side issue of the evaluation, functions may be chosen and variables may exist assigned new values. To modify the normal sequential execution of statements, C provides several control-period statements identified by reserved keywords. Structured programming is supported by if … [else] conditional execution and past exercise … while, while, and for iterative execution (looping). The for statement has split initialization, testing, and reinitialization expressions, any or all of which can be omitted. suspension and go along can be used to exit the innermost enclosing loop statement or skip to its reinitialization. There is as well a not-structured goto argument which branches directly to the designated characterization inside the function. switch selects a example to be executed based on the value of an integer expression.

Expressions can employ a diversity of congenital-in operators and may contain part calls. The club in which arguments to functions and operands to most operators are evaluated is unspecified. The evaluations may even exist interleaved. Even so, all side effects (including storage to variables) volition occur before the next "sequence bespeak"; sequence points include the end of each expression statement, and the entry to and render from each role phone call. Sequence points also occur during evaluation of expressions containing sure 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 whatsoever other language, has its blemishes. Some of the operators have the wrong precedence; some parts of the syntax could be amend."^[25] The C standard did non endeavour to right many of these blemishes, considering of the impact of such changes on already existing software.

Character set up [edit]

The basic C source graphic symbol set includes the post-obit characters:

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

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

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

The basic C execution character gear up contains the aforementioned characters, forth with representations for alarm, backspace, and carriage render. Run-time support for extended character sets has increased with each revision of the C standard.

Reserved words [edit]

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

auto
break
case
char
const
continue
default
exercise
double
else
enum
extern
float
for
goto
if
int
long
register
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 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, because identifiers of that class were previously reserved by the C standard for employ merely past implementations. Since existing program source code should not have been using these identifiers, information technology would not exist affected when C implementations started supporting these extensions to the programming linguistic communication. Some standard headers practice define more than convenient synonyms for underscored identifiers. The language previously included a reserved give-and-take called entry, simply this was seldom implemented, and has now been removed every bit a reserved word.^[27]

Operators [edit]

C supports a rich set of operators, which are symbols used within an expression to specify the manipulations to exist 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
club 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 dissimilar ALGOL and its derivatives. C uses the operator == to test for equality. The similarity between these two operators (assignment and equality) may result in the adventitious employ of one in place of the other, and in many cases, the mistake does non produce an fault bulletin (although some compilers produce warnings). For example, the conditional expression if (a == b + i) might mistakenly be written every bit if (a = b + 1), which will be evaluated as true if a is not zero after the assignment.^[28]

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

"Hello, world" instance [edit]

The "hello, globe" case, which appeared in the start edition of Thousand&R, has get the model for an introductory programme in most programming textbooks. The program prints "hello, world" to the standard output, which is usually a terminal or screen display.

The original version was:^[30]

                        main            ()                        {                                                printf            (            "hello, globe            \northward            "            );                        }

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

                        #include                                    <stdio.h>                        int                                    principal            (            void            )                        {                                                printf            (            "howdy, 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 unabridged text of the stdio.h standard header, which contains declarations for standard input and output functions such as 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 office named primary is existence divers. The main office serves a special purpose in C programs; the run-time environment calls the chief role to begin plan execution. The blazon specifier int indicates that the value that is returned to the invoker (in this case the run-time surround) as a result of evaluating the main role, is an integer. The keyword void every bit a parameter list indicates that this function takes no arguments.^[b]

The opening curly brace indicates the starting time of the definition of the main function.

The next line calls (diverts execution to) a role named printf, which in this case is supplied from a organisation library. In this call, the printf function is passed (provided with) a single argument, the address of the starting time grapheme in the string literal "hello, globe\n". The cord literal is an unnamed array with elements of blazon char, set up automatically by the compiler with a final 0-valued character to mark the end of the assortment (printf needs to know this). The \n is an escape sequence that C translates to a newline graphic symbol, which on output signifies the end of the current line. The return value of the printf function is of type int, but it is silently discarded since it is non used. (A more conscientious program might examination the render value to decide whether or non the printf office succeeded.) The semicolon ; terminates the statement.

The closing curly brace indicates the stop of the code for the main role. Co-ordinate to the C99 specification and newer, the main part, unlike any other function, will implicitly render a value of 0 upon reaching the } that terminates the role. (Formerly an explicit render 0; statement was required.) This is interpreted past the run-time system equally an leave code indicating successful execution.^[31]

Information types [edit]

The type system in C is static and weakly typed, which makes it like to the blazon system of ALGOL descendants such as Pascal.^[32] There are built-in types for integers of various sizes, both signed and unsigned, floating-bespeak numbers, and enumerated types (enum). Integer blazon char is often used for single-byte characters. C99 added a boolean datatype. There are likewise derived types including arrays, pointers, records (struct), and unions (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 definiteness of near expressions, but the programmer can override the checks in various ways, either by using a blazon cast to explicitly catechumen a value from 1 type to another, or past using pointers or unions to reinterpret the underlying bits of a data object in some other fashion.

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

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

Pointers [edit]

C supports the employ of pointers, a type of reference that records the address or location of an object or function in memory. Pointers can be dereferenced to access data stored at the address pointed to, or to invoke a pointed-to office. Pointers can be manipulated using consignment or pointer arithmetics. The run-time representation of a pointer value is typically a raw memory accost (perhaps augmented by an offset-inside-word field), but since a arrow'south type includes the type of the thing pointed to, expressions including pointers can be blazon-checked at compile time. Pointer arithmetic is automatically scaled by the size of the pointed-to data blazon. Pointers are used for many purposes in C. Text strings are commonly manipulated using pointers into arrays of characters. Dynamic memory resource allotment is performed using pointers. Many data 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 as arguments to higher-society functions (such as qsort or bsearch) or as callbacks to be invoked past issue handlers.^[31]

A aught arrow value explicitly points to no valid location. Dereferencing a cypher pointer value is undefined, often resulting in a partitioning fault. Zippo pointer values are useful for indicating special cases such as no "next" pointer in the last node of a linked list, or as an error indication from functions returning pointers. In appropriate contexts in source code, such as for assigning to a pointer variable, a zip pointer 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 conditional contexts, aught arrow values evaluate to false, while all other arrow values evaluate to true.

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

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

Arrays [edit]

Array types in C are traditionally of a fixed, static size specified at compile time. The more than recent C99 standard as well allows a grade of variable-length arrays. However, it is too possible to allocate a cake of memory (of arbitrary size) at run-time, using the standard library'southward malloc function, and care for it as an array.

Since arrays are always accessed (in effect) via pointers, array accesses are typically not checked confronting the underlying array size, although some compilers may provide premises checking as an option.^[34] ^[35] Array bounds violations are therefore possible and tin lead to diverse repercussions, including illegal memory accesses, corruption of information, buffer overruns, and run-time exceptions.

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

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

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

Array–pointer interchangeability [edit]

The subscript notation ten[i] (where 10 designates a arrow) is syntactic saccharide for *(ten+i).^[36] Taking advantage of the compiler's noesis of the pointer type, the accost that 10 + i points to is not the base address (pointed to by x) incremented by i bytes, but rather is divers to be the base of operations address incremented by i multiplied past the size of an element that 10 points to. Thus, ten[i] designates the i+ithursday chemical element of the array.

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

The total size of an array x can be determined by applying sizeof to an expression of assortment type. The size of an chemical element can be determined past applying the operator sizeof to whatsoever dereferenced element of an assortment A, equally in n = sizeof A[0]. This, the number of elements in a declared array A can be determined every bit sizeof A / sizeof A[0]. Notation, that if only a pointer to the first element is available every bit it is frequently the example in C code considering of the automatic conversion described higher up, the information nigh the full blazon of the array and its length are lost.

Retentiveness direction [edit]

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

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

These three approaches are appropriate in unlike situations and have diverse trade-offs. For example, static retentivity allocation has trivial allocation overhead, automatic allocation may involve slightly more overhead, and dynamic memory resource allotment can potentially accept a dandy deal of overhead for both allocation and deallocation. The persistent nature of static objects is useful for maintaining state information across part calls, automatic allocation is piece of cake to use but stack infinite is typically much more express and transient than either static memory or heap space, and dynamic memory allocation allows convenient allocation of objects whose size is known only at run-time. Most C programs make all-encompassing employ of all three.

Where possible, automated or static allocation is usually simplest because the storage is managed by the compiler, freeing the programmer of the potentially error-decumbent job of manually allocating and releasing storage. Even so, many data structures can alter in size at runtime, and since static allocations (and automatic allocations before C99) must have a fixed size at compile-time, there are many situations in which dynamic allocation is necessary.^[31] Prior to the C99 standard, variable-sized arrays were a common case of this. (See the article on malloc for an case of dynamically allocated arrays.) Unlike automatic resource allotment, which can fail at run time with uncontrolled consequences, the dynamic allocation functions return an indication (in the form of a nothing arrow value) when the required storage cannot be allocated. (Static allocation that is too big is usually detected by the linker or loader, before the program tin fifty-fifty brainstorm execution.)

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

Heap memory allocation has to be synchronized with its bodily usage in whatever program to be reused every bit much as possible. For case, if the just arrow to a heap retentivity allocation goes out of scope or has its value overwritten earlier it is deallocated explicitly, and so that retentiveness cannot be recovered for afterward reuse and is essentially lost to the program, a phenomenon known as a retention leak. Conversely, information technology 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 program unrelated to the code that causes the fault, making it difficult to diagnose the failure. Such problems 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 gear up of functions contained within a single "annal" file. Each library typically has a header file, which contains the prototypes of the functions independent within the library that may be used by a program, and declarations of special data types and macro symbols used with these functions. In order for a program to use a library, it must include the library's header file, and the library must be linked with the plan, which in many cases requires compiler flags (e.g., -lm, shorthand for "link the math library").^[31]

The nearly common C library is the C standard library, which is specified past 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 allotment, mathematics, grapheme strings, and fourth dimension values. Several split up 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-similar systems, especially functions which provide an interface to the kernel. These functions are detailed in various standards such equally POSIX and the Single UNIX Specification.

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

File treatment and streams [edit]

File input and output (I/O) is not part of the C language itself merely instead is handled by libraries (such as the C standard library) and their associated header files (e.k. stdio.h). File handling is by and large implemented through loftier-level I/O which works through streams. A stream is from this perspective a data menses 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 retentivity area or queue) is temporarily used to store data before it'south sent to the terminal destination. This reduces the time spent waiting for slower devices, for example a hard drive or solid land drive. Low-level I/O functions are not part of the standard C library^{[ description needed ]} but are mostly function of "blank metal" programming (programming that's independent of any operating organisation such as most embedded programming). With few exceptions, implementations include depression-level I/O.

Language tools [edit]

A number of tools have been developed to help C programmers find and prepare statements with undefined behavior or maybe erroneous expressions, with greater rigor than that provided by the compiler. The tool lint was the outset such, leading to many others.

Automatic source code checking and auditing are benign in any language, and for C many such tools exist, such every bit Lint. A common practice is to use Lint to detect questionable code when a plan is kickoff written. Once a program passes Lint, it is and then compiled using the C compiler. Besides, many compilers can optionally warn about syntactically valid constructs that are likely to actually be errors. MISRA C is a proprietary gear up of guidelines to avoid such questionable code, adult for embedded systems.^[37]

There are also compilers, libraries, and operating organization level mechanisms for performing deportment that are not a standard office of C, such equally bounds checking for arrays, detection of buffer overflow, serialization, dynamic memory tracking, and automatic garbage collection.

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

Uses [edit]

The C Programming Linguistic communication

C is widely used for systems programming in implementing operating systems and embedded system applications,^[38] because C code, when written for portability, tin be used for most purposes, yet when needed, organization-specific lawmaking can be used to access specific hardware addresses and to perform type punning to lucifer externally imposed interface requirements, with a depression run-time demand on arrangement resource.

C tin 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 often chosen over interpreted languages because of its speed, stability, and nearly-universal availability.^[40]

A issue of C's broad availability and efficiency is that compilers, libraries and interpreters of other programming languages are oft 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 thin, and its overhead is depression, 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 as an intermediate language by implementations of other languages. This arroyo may exist used for portability or convenience; by using C as an intermediate language, additional auto-specific code generators are non necessary. C has some features, such equally line-number preprocessor directives and optional superfluous commas at the end of initializer lists, that back up compilation of generated lawmaking. However, some of C's shortcomings accept prompted the development of other C-based languages specifically designed for utilize as intermediate languages, such as C--.

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

[edit]

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

C has both directly and indirectly influenced many later languages such every bit C#, D, Get, Java, JavaScript, Limbo, LPC, Perl, PHP, Python, and Unix'south 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, information models, and/or large-calibration plan structures that differ from those of C, sometimes radically.

Several C or near-C interpreters be, 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 and so compiled with a C compiler.^[43]

The C++ programming linguistic communication (originally named "C with Classes") was devised by Bjarne Stroustrup as an approach to providing object-oriented functionality with a C-like syntax.^[44] C++ adds greater typing force, 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 "sparse" layer on top of C, and remains a strict superset of C that permits object-oriented programming using a hybrid dynamic/static typing paradigm. Objective-C derives its syntax from both C and Smalltalk: syntax that involves preprocessing, expressions, function declarations, and function calls is inherited from C, while the syntax for object-oriented features was originally taken from Smalltalk.

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

Notes [edit]

^ The original example lawmaking will compile on most modern compilers that are not in strict standard compliance manner, but it does non fully conform to the requirements of either C89 or C99. In fact, C99 requires that a diagnostic message exist produced.
^ The main function actually has two arguments, int argc and char *argv[], respectively, which can be used to handle command line arguments. The ISO C standard (section v.one.2.2.1) requires both forms of primary to be supported, which is special treatment not afforded to whatsoever other function.

References [edit]

^ ^a ^b Kernighan, Brian W.; Ritchie, Dennis Grand. (February 1978). The C Programming Linguistic communication (1st ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-110163-0.
^ Ritchie (1993): "Thompson had made a brief attempt to produce a arrangement coded in an early version of C—earlier structures—in 1972, only gave up the endeavour."
^ Fruderica (December 13, 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 information technology did non, mayhap, emerge in a course that Algol'southward adherents would corroborate of."
^ Ring Team (Oct 23, 2021). "The Ring programming language and other languages". ring-lang.net.
^ ^a ^b "Verilog HDL (and C)" (PDF). The Research School of Figurer Science at the Australian National Academy. June 3, 2010. Archived from the original (PDF) on November 6, 2013. Retrieved Baronial xix, 2013. 1980s: ; Verilog first introduced ; Verilog inspired past the C programming linguistic communication
^ ^a ^b ^c ^d ^e Ritchie (1993)
^ "Programming Linguistic communication Popularity". 2009. Archived from the original on January sixteen, 2009. Retrieved January 16, 2009.
^ "TIOBE Programming Customs 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 Index for Oct 2021". Retrieved October vii, 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. M. (1978). "Portability of C Programs and the UNIX System". Bell Organisation Tech. J. 57 (6): 2021–2048. CiteSeerX10.1.1.138.35. doi:10.1002/j.1538-7305.1978.tb02141.10. S2CID 17510065. (Annotation: The PDF is an OCR scan of the original, and contains a rendering of "IBM 370" as "IBM 310".)
^ McIlroy, M. D. (1987). A Inquiry Unix reader: annotated excerpts from the Programmer's Transmission, 1971–1986 (PDF) (Technical report). CSTR. Bell Labs. p. 10. 139. Archived (PDF) from the original on Nov eleven, 2017. Retrieved Feb 1, 2015.
^ "C manual pages". FreeBSD Miscellaneous Data Transmission (FreeBSD xiii.0 ed.). May 30, 2011. Archived from the original on January 21, 2021. Retrieved Jan 15, 2021. [1] Archived January 21, 2021, at the Wayback Auto
^ Kernighan, Brian W.; Ritchie, Dennis G. (March 1988). The C Programming Linguistic communication (2nd ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-xiii-110362-seven.
^ Stroustrup, Bjarne (2002). Sibling rivalry: C and C++ (PDF) (Written report). AT&T Labs. Archived (PDF) from the original on Baronial 24, 2014. Retrieved April xiv, 2014.
^ C Integrity. International Organization for Standardization. March thirty, 1995. Archived from the original on July 25, 2018. Retrieved July 24, 2018.
^ "JTC1/SC22/WG14 – C". Habitation page. ISO/IEC. Archived from the original on February 12, 2018. Retrieved June 2, 2011.
^ Andrew Binstock (October 12, 2011). "Interview with Herb Sutter". Dr. Dobbs. Archived from the original on August 2, 2013. Retrieved September vii, 2013.
^ "Revised C23 Schedule WG fourteen Due north 2759" (PDF). world wide web.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 (5th ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-089592-ix. Contains a BNF grammar for C.
^ Kernighan & Ritchie (1996), p. 192.
^ Kernighan & Ritchie (1978), p. iii.
^ "ISO/IEC 9899:201x (ISO C11) Committee Draft" (PDF). Archived (PDF) from the original on Dec 22, 2017. Retrieved September 16, 2011.
^ Kernighan & Ritchie (1996), pp. 192, 259.
^ "10 Mutual Programming Mistakes in C++". Cs.ucr.edu. Archived from the original on Oct 21, 2008. Retrieved June 26, 2009.
^ Schultz, Thomas (2004). C and the 8051 (3rd ed.). Otsego, MI: PageFree Publishing Inc. p. 20. ISBN978-1-58961-237-ii. Archived from the original on July 29, 2020. Retrieved February 10, 2012.
^ Kernighan & Ritchie (1978), p. 6.
^ ^a ^b ^c ^d ^east ^f ^grand Klemens, Ben (2013). 21st Century C. O'Reilly Media. ISBN978-1-4493-2714-ix.
^ Feuer, Alan R.; Gehani, Narain H. (March 1982). "Comparing 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-EDUCATION PUBLIC COMPANY Express. pp. 225–230. ISBN978-616-08-2740-4.
^ Raymond, Eric Due south. (October xi, 1996). The New Hacker's Dictionary (3rd ed.). MIT Printing. p. 432. ISBN978-0-262-68092-9. Archived from the original on November 12, 2012. Retrieved August v, 2012.
^ "Human Page for lint (freebsd Section 1)". unix.com. May 24, 2001. Retrieved July 15, 2014.
^ Dale, Nell B.; Weems, Fleck (2014). Programming and problem solving with C++ (sixth ed.). Burlington, MA: Jones & Bartlett Learning. ISBN978-1449694289. OCLC 894992484.
^ Dr. Dobb's Sourcebook. U.Due south.A.: Miller Freeman, Inc. November–December 1995.
^ "Using C for CGI Programming". linuxjournal.com. March 1, 2005. Archived from the original on Feb 13, 2010. Retrieved Jan 4, 2010.
^ McMillan, Robert (August 1, 2013). "Is Java Losing Its Mojo?". Wired. Archived from the original on Feb 15, 2017. Retrieved March 5, 2017.
^ O'Regan, Gerard (September 24, 2015). Pillars of computing : a compendium of select, pivotal engineering science firms. ISBN978-3319214641. OCLC 922324121.
^ Rauchwerger, Lawrence (2004). Languages and compilers for parallel computing : 16th international workshop, LCPC 2003, Higher 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 Feb 2, 2019. Retrieved June 9, 2011.

Sources [edit]

Ritchie, Dennis Thou. (March 1993). "The Development of the C Language". ACM SIGPLAN Notices. ACM. 28 (three): 201–208. doi:10.1145/155360.155580.
Ritchie, Dennis Chiliad. (1993). "The Development 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-four . Retrieved November four, 2014.
Kernighan, Brian W.; Ritchie, Dennis M. (1996). The C Programming Language (2nd ed.). Prentice Hall. ISBN7-302-02412-X.

External links [edit]

ISO C Working Grouping 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

smithfluntence.blogspot.com

Source: https://en.wikipedia.org/wiki/C_%28programming_language%29