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General-purpose programming language

C
Major implementations
The C Programming Language ^[1] (ofttimes referred to as Chiliad&R), the seminal volume on C
Prototype	Multi-paradigm: imperative (procedural), structured
Designed past	Dennis Ritchie
Developer	Dennis Ritchie & Bell Labs (creators); ANSI X3J11 (ANSI C); ISO/IEC JTC1/SC22/WG14 (ISO C)
First appeared	1972; l years ago (1972) ^[2]

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

Typing discipline	Static, weak, manifest, nominal
Os	Cantankerous-platform
Filename extensions	.c, .h
Website	www.iso.org/standard/74528.html www.open up-std.org/jtc1/sc22/wg14/
pcc, GCC, Clang, Intel C, C++Builder, Microsoft Visual C++, Watcom C
Dialects
Cyclone, Unified Parallel C, Split-C, Cilk, C*
Influenced by
B (BCPL, CPL), ALGOL 68,^[4] assembly, PL/I, FORTRAN
Influenced
Numerous: AMPL, AWK, csh, C++, C--, C#, Objective-C, D, Go, Java, JavaScript, JS++, Julia, Limbo, LPC, Perl, PHP, Pike, Processing, Python, Ring,^[five]Rust, Seed7, Vala, Verilog (HDL),^[6] Nim, Zig
C Programming at Wikibooks

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

A successor to the programming language B, C was originally developed at Bell Labs by Dennis Ritchie between 1972 and 1973 to construct utilities running on Unix. It was applied to re-implementing the kernel of the Unix operating system.^[seven] During the 1980s, C gradually gained popularity. Information technology has get 1 of the nearly widely used programming languages,^[8] ^[9] 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. Information technology was designed to be compiled to provide low-level access to memory and language constructs that map efficiently to car instructions, all with minimal runtime back up. Despite its low-level capabilities, the language was designed to encourage cross-platform programming. A standards-compliant C program written with portability in heed can be compiled for a wide variety of calculator platforms and operating systems with few changes to its source code.^[10]

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

Overview [edit]

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

The C language also exhibits the following characteristics:

The language has a minor, fixed number of keywords, including a full prepare of control flow primitives: if/else, for, do/while, while, and switch. User-defined names are not distinguished from keywords by whatever kind of sigil.
It has a large number of arithmetic, bitwise, and logic operators: +,+=,++,&,||, etc.
More than one consignment may exist performed in a unmarried statement.
Functions:
- Part render values can be ignored, when not needed.
- Function and data pointers permit ad hoc run-time polymorphism.
- Functions may not exist defined within the lexical scope of other functions.
Information typing is static, but weakly enforced; all data has a type, but implicit conversions are possible.
Declaration syntax mimics usage context. C has no "define" keyword; instead, a argument first with the name of a blazon is taken every bit a declaration. There 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 data elements to be accessed and assigned equally a unit.
- Union is a construction with overlapping members; only the concluding member stored is valid.
- Array indexing is a secondary note, defined in terms of pointer arithmetics. Unlike structs, arrays are not start-course objects: they cannot be assigned or compared using unmarried congenital-in operators. There is no "array" keyword in utilize or definition; instead, square brackets betoken arrays syntactically, for example calendar month[eleven].
- Enumerated types are possible with the enum keyword. They are freely interconvertible with integers.
- Strings are non a distinct information type, merely are conventionally implemented as goose egg-terminated character arrays.
Low-level access to computer memory is possible past converting machine addresses to typed pointers.
Procedures (subroutines non returning values) are a special example of function, with an untyped render type void.
A preprocessor performs macro definition, source lawmaking file inclusion, and conditional compilation.
There is a basic class of modularity: files can exist compiled separately and linked together, with command over which functions and data objects are visible to other files via static and extern attributes.
Complex functionality such every bit I/O, cord manipulation, and mathematical functions are consistently delegated to library routines.

While C does non include certain features found in other languages (such as object orientation and garbage collection), these tin can be implemented or emulated, oftentimes through the use of external libraries (eastward.g., the GLib Object Arrangement or the Boehm garbage collector).

Relations to other languages [edit]

Many later on languages take borrowed directly or indirectly from C, including C++, C#, Unix's C shell, D, Go, Java, JavaScript (including transpilers), Julia, Limbo, LPC, Objective-C, Perl, PHP, Python, Ruby-red, Rust, Swift, Verilog and SystemVerilog (hardware description languages).^[6] These languages accept drawn many of their control structures and other basic features from C. Most of them (Python existence 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, data models, and semantics that tin exist radically dissimilar.

History [edit]

Early on developments [edit]

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

The origin of C is closely tied to the development of the Unix operating organization, originally implemented in assembly language on a PDP-7 by Dennis Ritchie and Ken Thompson, incorporating several ideas from colleagues. Eventually, they decided to port the operating system to a PDP-11. The original PDP-11 version of Unix was also developed in assembly linguistic communication.^[seven]

Thompson desired a programming language to brand utilities for the new platform. At first, he tried to make a Fortran compiler, but presently gave up the thought. Instead, he created a cut-down version of the recently developed BCPL systems programming language. The official clarification of BCPL was non available at the fourth dimension,^[12] and Thompson modified the syntax to exist less wordy, producing the similar but somewhat simpler B.^[7] However, few utilities were ultimately written in B considering it was besides slow, and B could not have advantage of PDP-11 features such as byte addressability.

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

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

The preprocessor was introduced effectually 1973 at the urging of Alan Snyder and also in recognition of the usefulness of the file-inclusion mechanisms available in BCPL and PL/I. Its original version provided only included files and uncomplicated string replacements: #include and #define of parameterless macros. Before long after that, it was extended, mostly by Mike Lesk and and then by John Reiser, to incorporate macros with arguments and conditional compilation.^[7]

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

K&R C [edit]

In 1978, Brian Kernighan and Dennis Ritchie published the beginning edition of The C Programming Linguistic communication.^[1] This book, known to C programmers every bit K&R, served for many years every bit an informal specification of the language. The version of C that it describes is unremarkably referred to as "M&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 beneath.

Grand&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 every bit =-) were inverse to the class op= (that is, -=) to remove the semantic ambiguity created by constructs such as i=-ten, which had been interpreted every bit i =- 10 (decrement i by 10) instead of the mayhap intended i = -ten (let i be −ten).

Fifty-fifty subsequently 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 still in use, and because carefully written K&R C code can be legal Standard C likewise.

In early versions of C, merely functions that return types other than int must exist alleged if used earlier the part 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            ;                                                annals                                    /* 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 G&R C, simply are required in later standards.

Since One thousand&R function declarations did not include any information well-nigh function arguments, role parameter type checks were not performed, although some compilers would issue a alarm message if a local office was called with the wrong number of arguments, or if multiple calls to an external function used dissimilar numbers or types of arguments. Dissever tools such as Unix's lint utility were adult that (amid other things) could check for consistency of role use beyond multiple source files.

In the years post-obit the publication of M&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.e., functions with no return value)
functions returning struct or union types (rather than pointers)
assignment for struct data types
enumerated types

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

ANSI C and ISO C [edit]

During the belatedly 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 Establish (ANSI) formed a committee, X3J11, to establish a standard specification of C. X3J11 based the C standard on the Unix implementation; nonetheless, the non-portable portion of the Unix C library was handed off to the IEEE working group 1003 to get the basis for the 1988 POSIX standard. In 1989, the C standard was ratified as ANSI X3.159-1989 "Programming Linguistic communication C". This version of the language is often referred to as ANSI C, Standard C, or sometimes C89.

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

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

One of the aims of the C standardization procedure was to produce a superset of Thousand&R C, incorporating many of the subsequently introduced unofficial features. The standards commission also included several additional features such equally part 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 way used in C++, the Thousand&R interface continued to exist permitted, for compatibility with existing source code.

C89 is supported by current C compilers, and most modern C code is based on it. Any program written only in Standard C and without any hardware-dependent assumptions will run correctly on any platform with a conforming C implementation, within its resources limits. Without such precautions, programs may compile only on a certain platform or with a particular compiler, due, for instance, to the use of not-standard libraries, such as GUI libraries, or to a reliance on compiler- or platform-specific attributes such every bit the exact size of data types and byte endianness.

In cases where code must be compilable by either standard-conforming or Yard&R C-based compilers, the __STDC__ macro can be used to divide the code into Standard and Chiliad&R sections to prevent the utilize on a G&R C-based compiler of features available only in Standard C.

After 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 equally C95) was published, to correct some details and to add more extensive support for international character sets.^[eighteen]

C99 [edit]

The C standard was farther 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 3 times by 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 bespeak, support for variadic macros (macros of variable arity), and support for one-line comments starting time with //, as in BCPL or C++. Many of these had already been implemented as extensions in several C compilers.

C99 is for the most part backward uniform with C90, only is stricter in some ways; in particular, a annunciation that lacks a blazon specifier no longer has int implicitly assumed. A standard macro __STDC_VERSION__ is defined with value 199901L to bespeak that C99 back up is available. GCC, Solaris Studio, and other C compilers at present support many or all of the new features of C99. The C compiler in Microsoft Visual C++, however, implements the C89 standard and those parts of C99 that are required for compatibility with C++11.^[20] ^{[ needs update ]}

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

C11 [edit]

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

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

C17 [edit]

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

C2x [edit]

C2x is an informal proper name for the side by side (subsequently C17) major C language standard revision. It is expected to exist voted on in 2023 and would therefore exist called 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 fixed-point arithmetics, multiple distinct memory banks, and bones I/O operations.

In 2008, the C Standards Commission published a technical study extending the C linguistic communication^[22] to address these bug by providing a common standard for all implementations to adhere to. Information technology includes a number of features not bachelor in normal C, such every bit stock-still-indicate arithmetic, named accost spaces, and basic I/O hardware addressing.

Syntax [edit]

C has a formal grammar specified by the C standard.^[23] Line endings are more often than not not pregnant in C; however, line boundaries exercise have significance during the preprocessing phase. Comments may appear either between the delimiters /* and */, or (since C99) post-obit // until the end of the line. Comments delimited past /* and */ do non nest, and these sequences of characters are non interpreted as comment delimiters if they appear within cord or graphic symbol literals.^[24]

C source files incorporate declarations and function definitions. Function definitions, in plough, contain declarations and statements. Declarations either define new types using keywords such as struct, marriage, and enum, or assign types to and perchance reserve storage for new variables, usually by writing the blazon 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 act equally a single statement for command structures.

Every bit an imperative linguistic communication, C uses statements to specify actions. The near 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-flow 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 statement has carve up initialization, testing, and reinitialization expressions, any or all of which can be omitted. break and continue can exist used to leave the innermost enclosing loop statement or skip to its reinitialization. There is also a non-structured goto argument which branches directly to the designated label inside the function. switch selects a case to be executed based on the value of an integer expression.

Expressions can use a multifariousness of born operators and may contain function calls. The order in which arguments to functions and operands to most operators are evaluated is unspecified. The evaluations may even be interleaved. Notwithstanding, all side effects (including storage to variables) will occur before the next "sequence point"; sequence points include the terminate of each expression statement, and the entry to and return from each function call. Sequence points besides 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 whatsoever other language, has its blemishes. Some of the operators have the wrong precedence; some parts of the syntax could be meliorate."^[25] The C standard did not attempt to correct many of these blemishes, because of the impact of such changes on already existing software.

Character set [edit]

The bones C source character set includes the following characters:

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

Newline indicates the end of a text line; it demand not correspond to an actual single grapheme, although for convenience C treats it as i.

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

The basic C execution character set contains the same characters, along with representations for alert, backspace, and carriage return. Run-time support for extended graphic symbol sets has increased with each revision of the C standard.

Reserved words [edit]

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

motorcar
break
instance
char
const
keep
default
do
double
else
enum
extern
float
for
goto
if
int
long
register
render
short
signed
sizeof
static
struct
switch
typedef
union
unsigned
void
volatile
while

C99 reserved five more words:

_Bool
_Complex
_Imaginary
inline
restrict

C11 reserved 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 letter, considering identifiers of that grade were previously reserved by the C standard for utilise but by implementations. Since existing program source lawmaking should not have been using these identifiers, information technology would not exist affected when C implementations started supporting these extensions to the programming language. Some standard headers do define more user-friendly synonyms for underscored identifiers. The language previously included a reserved word chosen entry, but this was seldom implemented, and has now been removed as a reserved word.^[27]

Operators [edit]

C supports a rich set 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:

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

C uses the operator = (used in mathematics to express equality) to indicate assignment, following the precedent of Fortran and PL/I, but unlike 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 accidental use of one in identify of the other, and in many cases, the mistake does not produce an error message (although some compilers produce warnings). For case, the conditional expression if (a == b + 1) might mistakenly be written equally if (a = b + 1), which will be evaluated every bit true if a is not zero afterwards the assignment.^[28]

The C operator precedence is not always 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 ten & 1 == 0, which must be written as (x & 1) == 0 if that is the coder's intent.^[29]

"Hello, world" instance [edit]

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

The original version was:^[30]

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

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

                        #include                                    <stdio.h>                        int                                    main            (            void            )                        {                                                printf            (            "how-do-you-do, world            \north            "            );                        }

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

The side by side line indicates that a role named chief is beingness defined. The chief office serves a special purpose in C programs; the run-time surroundings calls the chief role to brainstorm program execution. The type 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 primary function, is an integer. The keyword void equally a parameter list indicates that this role takes no arguments.^[b]

The opening curly brace indicates the first of the definition of the main function.

The adjacent 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 argument, the address of the first grapheme in the cord literal "hello, world\n". The string literal is an unnamed array with elements of type char, fix 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 character, which on output signifies the end of the electric current line. The return value of the printf function is of blazon int, merely it is silently discarded since information technology is not used. (A more careful program might exam the return value to determine whether or not the printf function succeeded.) The semicolon ; terminates the statement.

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

Data types [edit]

The type system in C is static and weakly typed, which makes it similar to the type system of ALGOL descendants such as Pascal.^[32] In that location are built-in types for integers of diverse sizes, both signed and unsigned, floating-betoken numbers, and enumerated types (enum). Integer type char is ofttimes used for single-byte characters. C99 added a boolean datatype. There are as well derived types including arrays, pointers, records (struct), and unions (matrimony).

C is oft used in depression-level systems programming where escapes from the blazon system may be necessary. The compiler attempts to ensure blazon correctness of most expressions, simply the programmer tin override the checks in various ways, either by using a type bandage 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 mode.

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

C's usual arithmetic conversions allow for efficient code to be generated, simply 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 use of pointers, a type of reference that records the address or location of an object or function in retentiveness. Pointers tin can exist dereferenced to access data stored at the address pointed to, or to invoke a pointed-to part. Pointers can be manipulated using assignment or pointer arithmetics. The run-time representation of a pointer value is typically a raw memory address (perhaps augmented by an offset-within-discussion field), but since a pointer's type includes the type of the thing pointed to, expressions including pointers can exist type-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 retentivity allocation is performed using pointers. Many data types, such every bit trees, are commonly implemented as dynamically allocated struct objects linked together using pointers. Pointers to functions are useful for passing functions every bit arguments to higher-order functions (such as qsort or bsearch) or as callbacks to exist invoked by event handlers.^[31]

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

Void pointers (void *) point to objects of unspecified type, and can therefore be used as "generic" data pointers. Since the size and type of the pointed-to object is not known, void pointers cannot be dereferenced, nor is pointer arithmetic on them 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 employ of pointers is potentially dangerous. Because they are typically unchecked, a pointer variable can be made to point to any arbitrary location, which can cause undesirable effects. Although properly used pointers betoken to condom places, they tin can exist made to point to unsafe places by using invalid arrow arithmetic; the objects they point to may continue to be used after deallocation (dangling pointers); they may exist used without having been initialized (wild pointers); or they may be directly assigned an unsafe value using a cast, matrimony, or through another corrupt arrow. In general, C is permissive in assuasive manipulation of and conversion between pointer types, although compilers typically provide options for various levels of checking. Another programming languages address these bug by using more 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 also allows a grade of variable-length arrays. However, information technology is likewise possible to allocate a block of memory (of arbitrary size) at run-time, using the standard library'southward malloc function, and treat 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 bounds checking as an option.^[34] ^[35] Assortment bounds violations are therefore possible and tin can lead to various repercussions, including illegal retention accesses, corruption of data, buffer overruns, and run-fourth dimension exceptions.

C does not have a special provision for declaring multi-dimensional arrays, but rather relies on recursion within the type organization to declare arrays of arrays, which effectively accomplishes the same thing. The index values of the resulting "multi-dimensional array" tin can be thought of as increasing in row-major order. Multi-dimensional arrays are unremarkably used in numerical algorithms (mainly from applied linear algebra) to shop matrices. The structure of the C assortment is well suited to this particular task. However, in early on versions of C the premises of the array must be known fixed values or else explicitly passed to whatever 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 boosted "row vector" of pointers to the columns.) C99 introduced "variable-length arrays" which address this consequence.

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

                        int                                    func            (            int                                    N            ,                                    int                                    M            )                        {                                                float                                    (            *            p            )[            N            ][            M            ]                                    =                                    malloc            (            sizeof                                    *            p            );                                                if                                    (            !            p            )                                                return                                    -i            ;                                                for                                    (            int                                    i                                    =                                    0            ;                                    i                                    <                                    North            ;                                    i            ++            )                                                for                                    (            int                                    j                                    =                                    0            ;                                    j                                    <                                    1000            ;                                    j            ++            )                                                (            *            p            )[            i            ][            j            ]                                    =                                    i                                    +                                    j            ;                                                print_array            (            N            ,                                    K            ,                                    p            );                                                free            (            p            );                                                return                                    1            ;                        }

Array–arrow interchangeability [edit]

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

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

The full size of an array 10 tin can exist determined by applying sizeof to an expression of array type. The size of an element can be adamant by applying the operator sizeof to any dereferenced chemical element of an array A, as in n = sizeof A[0]. This, the number of elements in a alleged array A can exist determined every bit sizeof A / sizeof A[0]. Note, that if only a pointer to the first chemical element is bachelor as it is oftentimes the case in C code because of the automated conversion described to a higher place, the data about the full type of the array and its length are lost.

Retention direction [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 memory. C provides three distinct ways to allocate memory for objects:^[31]

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

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

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

Unless otherwise specified, static objects contain zero or cypher 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, any bit pattern happens to be present in the storage, which might not even represent a valid value for that blazon). If the plan attempts to access an uninitialized value, the results are undefined. Many modern compilers effort to detect and warn about this problem, simply both false positives and simulated negatives can occur.

Heap memory allocation has to be synchronized with its bodily usage in whatsoever program to be reused as much as possible. For case, if the merely pointer to a heap memory allocation goes out of scope or has its value overwritten before it is deallocated explicitly, and so that retentiveness cannot be recovered for later reuse and is essentially lost to the plan, a phenomenon known as a memory leak. Conversely, it is possible for memory to exist freed, merely is referenced after, leading to unpredictable results. Typically, the failure symptoms appear in a portion of the program unrelated to the code that causes the error, making it difficult to diagnose the failure. Such issues are ameliorated in languages with automated garbage collection.

Libraries [edit]

The C programming language uses libraries as its primary method of extension. In C, a library is a ready 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 plan, and declarations of special data types and macro symbols used with these functions. In club for a program to use 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 (eastward.g., -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 limited environments such as embedded systems may provide but a subset of the standard library). This library supports stream input and output, retention resource allotment, mathematics, grapheme strings, and time values. Several carve up standard headers (for example, stdio.h) specify the interfaces for these and other standard library facilities.

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

Since many programs have been written in C, there are a broad variety of other libraries available. Libraries are ofttimes written in C because 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 Java, Perl, and Python.^[31]

File treatment and streams [edit]

File input and output (I/O) is non office of the C linguistic communication itself but instead is handled by libraries (such as the C standard library) and their associated header files (eastward.grand. stdio.h). File handling is generally implemented through high-level I/O which works through streams. A stream is from this perspective a information catamenia that is independent of devices, while a file is a concrete device. The high-level I/O is done through the clan of a stream to a file. In the C standard library, a buffer (a memory surface area or queue) is temporarily used to shop data before it's sent to the final destination. This reduces the fourth dimension spent waiting for slower devices, for case a hard drive or solid land drive. Low-level I/O functions are not part of the standard C library^{[ clarification needed ]} but are generally part of "blank 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 adult to help C programmers find and ready statements with undefined beliefs or possibly erroneous expressions, with greater rigor than that provided by the compiler. The tool lint was the start such, leading to many others.

Automated source code checking and auditing are beneficial in whatsoever language, and for C many such tools exist, such as Lint. A common practise is to utilise Lint to detect questionable code when a program is start written. Once a program passes Lint, it is then compiled using the C compiler. Also, many compilers can optionally warn about syntactically valid constructs that are likely to really be errors. MISRA C is a proprietary fix of guidelines to avoid such questionable code, developed for embedded systems.^[37]

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

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

Uses [edit]

The C Programming Language

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

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

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

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

C is sometimes used as an intermediate language by implementations of other languages. This arroyo may be used for portability or convenience; by using C as an intermediate language, additional machine-specific code generators are not necessary. C has some features, such as line-number preprocessor directives and optional superfluous commas at the end of initializer lists, that support compilation of generated code. Even so, some of C's shortcomings take prompted the development of other C-based languages specifically designed for use as intermediate languages, such as C--.

C has likewise been widely used to implement stop-user applications. However, such applications tin also exist 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 straight and indirectly influenced many later languages such every bit C#, D, Go, Java, JavaScript, Limbo, LPC, Perl, PHP, Python, and Unix's C vanquish.^[42] The most pervasive influence has been syntactical; all of the languages mentioned combine the statement and (more than or less recognizably) expression syntax of C with type systems, data models, and/or large-scale program structures that differ from those of C, sometimes radically.

Several C or about-C interpreters be, including Ch and CINT, which can likewise be used for scripting.

When object-oriented programming languages became popular, C++ and Objective-C were 2 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 so compiled with a C compiler.^[43]

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

Objective-C was originally a very "thin" layer on top of C, and remains a strict superset of C that permits object-oriented programming using a hybrid dynamic/static typing image. Objective-C derives its syntax from both C and Smalltalk: syntax that involves preprocessing, expressions, function declarations, and role 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 almost supersets of C.

See besides [edit]

Compatibility of C and C++
Comparison of Pascal and C
Comparing of programming languages
International Obfuscated C Lawmaking Contest
List of C-based programming languages
List of C compilers

Notes [edit]

^ The original example code will compile on nigh modern compilers that are not in strict standard compliance mode, but it does not fully adapt to the requirements of either C89 or C99. In fact, C99 requires that a diagnostic message be produced.
^ The main office actually has ii arguments, int argc and char *argv[], respectively, which can be used to handle command line arguments. The ISO C standard (section 5.one.two.ii.i) requires both forms of main to exist supported, which is special treatment not afforded to whatever other office.

References [edit]

^ ^a ^b Kernighan, Brian W.; Ritchie, Dennis M. (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 system coded in an early on version of C—earlier structures—in 1972, only 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 type limerick adopted by C owes considerable debt to Algol 68, although it did non, perhaps, sally in a form that Algol'due south adherents would approve of."
^ Band Squad (Oct 23, 2021). "The Band programming language and other languages". ring-lang.net.
^ ^a ^b "Verilog HDL (and C)" (PDF). The Enquiry School of Figurer Scientific discipline at the Australian National Academy. June 3, 2010. Archived from the original (PDF) on November 6, 2013. Retrieved August nineteen, 2013. 1980s: ; Verilog first introduced ; Verilog inspired past the C programming language
^ ^a ^b ^c ^d ^e Ritchie (1993)
^ "Programming Language Popularity". 2009. Archived from the original on January 16, 2009. Retrieved Jan sixteen, 2009.
^ "TIOBE Programming Community Index". 2009. Archived from the original on May 4, 2009. Retrieved May 6, 2009.
^ ^a ^b "History of C". en.cppreference.com. Archived from the original on May 29, 2018. Retrieved May 28, 2018.
^ "TIOBE Alphabetize for Oct 2021". Retrieved October seven, 2021.
^ Ritchie, Dennis. "BCPL to B to C". Archived from the original on December 12, 2019. Retrieved September 10, 2019.
^ ^a ^b Johnson, Due south. C.; Ritchie, D. M. (1978). "Portability of C Programs and the UNIX Organization". Bell Organization 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 scan of the original, and contains a rendering of "IBM 370" as "IBM 310".)
^ McIlroy, M. D. (1987). A Research Unix reader: annotated excerpts from the Developer's Manual, 1971–1986 (PDF) (Technical written report). CSTR. Bell Labs. p. 10. 139. Archived (PDF) from the original on November 11, 2017. Retrieved February 1, 2015.
^ "C manual pages". FreeBSD Miscellaneous Information Manual (FreeBSD thirteen.0 ed.). May 30, 2011. Archived from the original on January 21, 2021. Retrieved January 15, 2021. [1] Archived January 21, 2021, at the Wayback Auto
^ Kernighan, Brian West.; Ritchie, Dennis K. (March 1988). The C Programming Language (2nd ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-110362-7.
^ Stroustrup, Bjarne (2002). Sibling rivalry: C and C++ (PDF) (Report). AT&T Labs. Archived (PDF) from the original on August 24, 2014. Retrieved April xiv, 2014.
^ C Integrity. International Organisation 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 two, 2011.
^ Andrew Binstock (Oct 12, 2011). "Interview with Herb Sutter". Dr. Dobbs. Archived from the original on August 2, 2013. Retrieved September 7, 2013.
^ "Revised C23 Schedule WG xiv N 2759" (PDF). world wide web.open-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 L. (2002). C: A Reference Manual (5th ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-thirteen-089592-9. Contains a BNF grammar for C.
^ Kernighan & Ritchie (1996), p. 192.
^ Kernighan & Ritchie (1978), p. 3.
^ "ISO/IEC 9899:201x (ISO C11) Committee Typhoon" (PDF). Archived (PDF) from the original on December 22, 2017. Retrieved September sixteen, 2011.
^ Kernighan & Ritchie (1996), pp. 192, 259.
^ "10 Common Programming Mistakes in C++". Cs.ucr.edu. Archived from the original on October 21, 2008. Retrieved June 26, 2009.
^ Schultz, Thomas (2004). C and the 8051 (3rd ed.). Otsego, MI: PageFree Publishing Inc. p. xx. 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 ^thousand Klemens, Ben (2013). 21st Century C. O'Reilly Media. ISBN978-1-4493-2714-nine.
^ Feuer, Alan R.; Gehani, Narain H. (March 1982). "Comparison of the Programming Languages C and Pascal". ACM Computing Surveys. 14 (1): 73–92. doi:ten.1145/356869.356872. S2CID 3136859.
^ Kernighan & Ritchie (1996), p. 122.
^ For case, gcc provides _FORTIFY_SOURCE. "Security Features: Compile Time Buffer Checks (FORTIFY_SOURCE)". fedoraproject.org. Archived from the original on January 7, 2007. Retrieved August five, 2012.
^ เอี่ยมสิริวงศ์, โอภาศ (2016). Programming with C. Bangkok, Thailand: SE-EDUCATION PUBLIC COMPANY LIMITED. pp. 225–230. ISBN978-616-08-2740-4.
^ Raymond, Eric South. (October eleven, 1996). The New Hacker'southward Dictionary (3rd ed.). MIT Press. p. 432. ISBN978-0-262-68092-9. Archived from the original on Nov 12, 2012. Retrieved August v, 2012.
^ "Homo Page for lint (freebsd Section ane)". unix.com. May 24, 2001. Retrieved July 15, 2014.
^ Dale, Nell B.; Weems, Bit (2014). Programming and trouble solving with C++ (6th ed.). Burlington, MA: Jones & Bartlett Learning. ISBN978-1449694289. OCLC 894992484.
^ Dr. Dobb's Sourcebook. U.s.a.A.: Miller Freeman, Inc. November–December 1995.
^ "Using C for CGI Programming". linuxjournal.com. March 1, 2005. Archived from the original on February thirteen, 2010. Retrieved Jan iv, 2010.
^ McMillan, Robert (August 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 computing : a compendium of select, pivotal technology firms. ISBN978-3319214641. OCLC 922324121.
^ Rauchwerger, Lawrence (2004). Languages and compilers for parallel computing : 16th international workshop, LCPC 2003, College Station, TX, USA, October ii-4, 2003 : revised papers. Springer. ISBN978-3540246442. OCLC 57965544.
^ Stroustrup, Bjarne (1993). "A History of C++: 1979−1991" (PDF). Archived (PDF) from the original on February 2, 2019. Retrieved June 9, 2011.

Sources [edit]

Ritchie, Dennis M. (March 1993). "The Evolution of the C Language". ACM SIGPLAN Notices. ACM. 28 (3): 201–208. doi:10.1145/155360.155580.
Ritchie, Dennis M. (1993). "The Development of the C Language". The Second ACM SIGPLAN Conference on History of Programming Languages (HOPL-II). ACM. pp. 201–208. doi:10.1145/154766.155580. ISBN0-89791-570-4 . Retrieved Nov 4, 2014.
Kernighan, Brian W.; Ritchie, Dennis M. (1996). The C Programming Language (2nd ed.). Prentice Hall. ISBNvii-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). (iii.61 MB)
comp.lang.c Often Asked Questions
A History of C, by Dennis Ritchie

gonzalezhersh1994.blogspot.com

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