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General-purpose programming linguistic communication

C
Major implementations
The C Programming Language ^[1] (oftentimes referred to as K&R), the seminal book on C
Paradigm	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)
First appeared	1972; 50 years agone (1972) ^[2]

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

Typing discipline	Static, weak, manifest, nominal
OS	Cross-platform
Filename extensions	.c, .h
Website	world wide web.iso.org/standard/74528.html www.open-std.org/jtc1/sc22/wg14/
pcc, GCC, Clang, Intel C, C++Builder, Microsoft Visual C++, Watcom C
Dialects
Cyclone, Unified Parallel C, Carve up-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, Coffee, JavaScript, JS++, Julia, Limbo, LPC, Perl, PHP, Pike, Processing, Python, Ring,^[5]Rust, Seed7, Vala, Verilog (HDL),^[6] Nim, Zig
C Programming at Wikibooks

C (, every bit in the letter c) is a general-purpose, procedural computer programming linguistic communication supporting structured programming, lexical variable scope, and recursion, with a static type organisation. By design, C provides constructs that map efficiently to typical car instructions. Information technology has found lasting use in applications previously coded in assembly language. Such applications include operating systems and diverse awarding software for calculator 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.^[7] During the 1980s, C gradually gained popularity. It has become one of the most widely used programming languages,^[8] ^[9] with C compilers from diverse vendors available for the majority of existing computer architectures and operating systems. C has been standardized by ANSI since 1989 (ANSI C) and past the International Organization for Standardization (ISO).

C is an imperative procedural language. It was designed to be compiled to provide low-level admission to memory and language constructs that map efficiently to motorcar instructions, all with minimal runtime support. Despite its low-level capabilities, the language was designed to encourage cantankerous-platform programming. A standards-compliant C program written with portability in mind can be compiled for a wide variety of computer platforms and operating systems with few changes to its source code.^[10]

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

Overview [edit]

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

The C language 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 past whatever kind of sigil.
It has a big number of arithmetics, bitwise, and logic operators: +,+=,++,&,||, etc.
More than ane assignment may be performed in a unmarried statement.
Functions:
- Function return values tin be ignored, when not needed.
- Role and data pointers permit advert hoc run-time polymorphism.
- Functions may non be defined within the lexical scope of other functions.
Data typing is static, merely weakly enforced; all information has a type, but implicit conversions are possible.
Declaration syntax mimics usage context. C has no "define" keyword; instead, a argument outset with the name of a type is taken as a declaration. There is no "function" keyword; instead, a part is indicated by the presence of a parenthesized argument list.
User-divers (typedef) and compound types are possible.
- Heterogeneous amass data types (struct) permit related data elements to be accessed and assigned equally a unit.
- Union is a construction with overlapping members; but the last member stored is valid.
- Assortment indexing is a secondary note, defined in terms of pointer arithmetic. Different structs, arrays are non first-grade objects: they cannot be assigned or compared using single built-in operators. At that place is no "array" keyword in use or definition; instead, square brackets indicate arrays syntactically, for example month[eleven].
- Enumerated types are possible with the enum keyword. They are freely interconvertible with integers.
- Strings are not a distinct data blazon, only are conventionally implemented equally zilch-terminated character arrays.
Low-level access to reckoner memory is possible by converting machine addresses to typed pointers.
Procedures (subroutines not returning values) are a special instance of function, with an untyped return blazon void.
A preprocessor performs macro definition, source lawmaking file inclusion, and conditional compilation.
There is a basic form of modularity: files can exist compiled separately and linked together, with control over which functions and data objects are visible to other files via static and extern attributes.
Circuitous functionality such every bit I/O, string manipulation, and mathematical functions are consistently delegated to library routines.

While C does not include sure features found in other languages (such every bit object orientation and garbage collection), these can be implemented or emulated, ofttimes through the use of external libraries (e.one thousand., the GLib Object Arrangement or the Boehm garbage collector).

Relations to other languages [edit]

Many subsequently languages have borrowed directly or indirectly from C, including C++, C#, Unix'southward C beat, D, Get, Coffee, JavaScript (including transpilers), Julia, Limbo, LPC, Objective-C, Perl, PHP, Python, Ruby, Rust, Swift, Verilog and SystemVerilog (hardware description languages).^[6] These languages take fatigued many of their control structures and other bones features from C. Most of them (Python being a dramatic exception) also limited highly like syntax to C, and they tend to combine the recognizable expression and argument syntax of C with underlying type systems, information models, and semantics that tin be radically different.

History [edit]

Early on developments [edit]

Timeline of language development
Year	C Standard^[10]
1972	Birth
1978	One 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 system, originally implemented in assembly linguistic communication on a PDP-vii by Dennis Ritchie and Ken Thompson, incorporating several ideas from colleagues. Somewhen, they decided to port the operating system to a PDP-xi. The original PDP-11 version of Unix was likewise developed in associates linguistic communication.^[seven]

Thompson desired a programming language to brand utilities for the new platform. At get-go, he tried to make a Fortran compiler, but shortly 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 non available at the time,^[12] and Thompson modified the syntax to be less wordy, producing the like but somewhat simpler B.^[7] Nonetheless, few utilities were ultimately written in B because it was as well slow, and B could not take advantage of PDP-11 features such as byte addressability.

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

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

The preprocessor was introduced around 1973 at the urging of Alan Snyder and also in recognition of the usefulness of the file-inclusion mechanisms bachelor in BCPL and PL/I. Its original version provided only included files and simple string replacements: #include and #define of parameterless macros. Before long afterward that, it was extended, generally past Mike Lesk and so past John Reiser, to comprise macros with arguments and conditional compilation.^[7]

Unix was 1 of the starting time operating system kernels implemented in a language other than associates. Earlier instances include the Multics system (which was written in PL/I) and Main Control Program (MCP) for the Burroughs B5000 (which was written in ALGOL) in 1961. In around 1977, Ritchie and Stephen C. Johnson made further changes to the linguistic communication 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.^[xiii]

Grand&R C [edit]

In 1978, Brian Kernighan and Dennis Ritchie published the outset edition of The C Programming Language.^[1] This volume, known to C programmers as Thou&R, served for many years as an informal specification of the language. The version of C that information technology describes is unremarkably referred to equally "G&R C". As this was released in 1978, it is also referred to every bit C78.^[fifteen] The second edition of the book^[sixteen] covers the later ANSI C standard, described below.

Grand&R introduced several language features:

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

Even after the publication of the 1989 ANSI standard, for many years Chiliad&R C was yet 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 as well.

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

For example:

                        long                                    some_function            ();                        /* int */                                    other_function            ();                        /* int */                                    calling_function            ()                        {                                                long                                    test1            ;                                                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 be omitted in Thou&R C, just are required in later standards.

Since K&R function declarations did non include any information about function arguments, part parameter type checks were non performed, although some compilers would effect a alarm message if a local office was called with the wrong number of arguments, or if multiple calls to an external function used different numbers or types of arguments. Split tools such as Unix'due south lint utility were adult that (among other things) could check for consistency of function use across multiple source files.

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

void functions (i.e., functions with no return value)
functions returning struct or union types (rather than pointers)
consignment 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 even the Unix compilers precisely implemented the 1000&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 multifariousness of mainframe computers, minicomputers, and microcomputers, including the IBM PC, as its popularity began to increase significantly.

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

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

ANSI, similar other national standards bodies, no longer develops the C standard independently, simply 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 inside a twelvemonth of ISO publication.

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

C89 is supported by current C compilers, and about modern C code is based on information technology. Whatsoever program written only in Standard C and without any hardware-dependent assumptions will run correctly on whatever platform with a conforming C implementation, within its resource limits. Without such precautions, programs may compile merely on a certain platform or with a particular compiler, due, for instance, to the use of non-standard libraries, such equally GUI libraries, or to a reliance on compiler- or platform-specific attributes such equally the verbal 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 can exist used to split the lawmaking into Standard and G&R sections to preclude the employ on a K&R C-based compiler of features available only in Standard C.

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

C99 [edit]

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

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

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

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

C11 [edit]

In 2007, work began on another revision of the C standard, informally called "C1X" until its official publication on 2011-12-08. The C standards 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, atomic operations, multi-threading, and premises-checked functions. Information technology 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, simply technical corrections, and clarifications to defects in C11. The standard macro __STDC_VERSION__ is divers equally 201710L.

C2x [edit]

C2x is an informal proper name for the side by side (later C17) major C language standard revision. It is expected to be 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 guild to back up exotic features such equally fixed-point arithmetic, multiple singled-out memory 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 bachelor in normal C, such as fixed-point arithmetics, 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 generally not significant in C; however, line boundaries do have significance during the preprocessing phase. Comments may appear either betwixt the delimiters /* and */, or (since C99) following // until the terminate of the line. Comments delimited by /* and */ practise non nest, and these sequences of characters are non interpreted as comment delimiters if they appear inside string or character literals.^[24]

C source files contain declarations and part definitions. Office definitions, in turn, comprise declarations and statements. Declarations either define new types using keywords such equally struct, union, and enum, or assign types to and mayhap reserve storage for new variables, usually by writing the blazon followed by the variable proper noun. Keywords such equally char and int specify congenital-in types. Sections of code are enclosed in braces ({ and }, sometimes called "curly brackets") to limit the scope of declarations and to act equally a single statement for control structures.

Equally an imperative language, C uses statements to specify actions. The most common statement is an expression statement, consisting of an expression to be evaluated, followed by a semicolon; every bit a side outcome 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-menses statements identified by reserved keywords. Structured programming is supported by if … [else] conditional execution and by practice … while, while, and for iterative execution (looping). The for argument has separate initialization, testing, and reinitialization expressions, whatsoever or all of which can be omitted. break and continue can be used to get out the innermost enclosing loop statement or skip to its reinitialization. There is also a non-structured goto argument which branches directly to the designated characterization inside the role. switch selects a case to be executed based on the value of an integer expression.

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

Grapheme set up [edit]

The basic C source character gear up includes the following characters:

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

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

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

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

Reserved words [edit]

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

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

C99 reserved 5 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 majuscule letter, because identifiers of that grade were previously reserved by the C standard for use only past implementations. Since existing program source lawmaking should not have been using these identifiers, it would not be affected when C implementations started supporting these extensions to the programming language. Some standard headers do ascertain more convenient synonyms for underscored identifiers. The language previously included a reserved give-and-take called entry, merely this was seldom implemented, and has now been removed as a reserved word.^[27]

Operators [edit]

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

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

C uses the operator = (used in mathematics to express equality) to point assignment, following the precedent of Fortran and PL/I, but dissimilar ALGOL and its derivatives. C uses the operator == to examination for equality. The similarity between these two operators (assignment and equality) may upshot in the accidental use of ane in place of the other, and in many cases, the error does not produce an error message (although some compilers produce warnings). For example, the provisional expression if (a == b + 1) might mistakenly exist written every bit if (a = b + 1), which will be evaluated as true if a is not zilch after the assignment.^[28]

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

"Hello, world" example [edit]

The "hello, globe" example, which appeared in the first edition of K&R, has become the model for an introductory program in most programming textbooks. The program prints "hullo, world" to the standard output, which is unremarkably a last or screen brandish.

The original version was:^[30]

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

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

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

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

The next line indicates that a function named main is beingness divers. The main function serves a special purpose in C programs; the run-time environment calls the main role to begin plan execution. The type specifier int indicates that the value that is returned to the invoker (in this instance the run-fourth dimension environment) as a result of evaluating the primary part, is an integer. The keyword void every bit a parameter list indicates that this part takes no arguments.^[b]

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

The next line calls (diverts execution to) a function named printf, which in this instance is supplied from a system library. In this telephone call, the printf function is passed (provided with) a single argument, the accost of the outset grapheme in the string literal "how-do-you-do, world\due north". The string literal is an unnamed array with elements of blazon char, prepare up automatically by the compiler with a last 0-valued character to mark the end of the array (printf needs to know this). The \due north is an escape sequence that C translates to a newline character, which on output signifies the finish of the electric current line. The return value of the printf function is of type int, but information technology is silently discarded since information technology is not used. (A more careful program might test the render value to decide whether or non the printf role succeeded.) The semicolon ; terminates the statement.

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

Data types [edit]

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

C is frequently used in low-level systems programming where escapes from the blazon system may be necessary. The compiler attempts to ensure type correctness of most expressions, only the programmer can override the checks in diverse ways, either by using a type bandage to explicitly catechumen a value from one type to another, or past using pointers or unions to reinterpret the underlying bits of a data object in some other mode.

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

C's usual arithmetic conversions allow for efficient code to be generated, but 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 accost or location of an object or office in memory. Pointers can be dereferenced to access data stored at the address pointed to, or to invoke a pointed-to function. Pointers can exist manipulated using assignment or pointer arithmetic. The run-time representation of a pointer value is typically a raw retentivity address (perhaps augmented past an offset-within-word field), only since a arrow'south type includes the type of the thing pointed to, expressions including pointers tin can exist type-checked at compile time. Pointer arithmetic is automatically scaled by the size of the pointed-to information type. Pointers are used for many purposes in C. Text strings are commonly manipulated using pointers into arrays of characters. Dynamic memory allocation is performed using pointers. Many information types, such as trees, are unremarkably implemented as dynamically allocated struct objects linked together using pointers. Pointers to functions are useful for passing functions as arguments to college-gild functions (such equally qsort or bsearch) or equally callbacks to exist invoked by event handlers.^[31]

A nada arrow value explicitly points to no valid location. Dereferencing a cipher arrow value is undefined, often resulting in a segmentation mistake. Goose egg pointer values are useful for indicating special cases such as no "next" pointer in the final node of a linked list, or as an mistake indication from functions returning pointers. In advisable contexts in source code, such as for assigning to a pointer variable, a goose egg arrow constant can be written as 0, with or without explicit casting to a pointer type, or equally the Naught macro defined by several standard headers. In conditional contexts, null pointer values evaluate to false, while all other pointer values evaluate to true.

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

Careless use of pointers is potentially dangerous. Considering they are typically unchecked, a pointer variable tin can be made to point to any capricious location, which can cause undesirable furnishings. Although properly used pointers bespeak to safe places, they can be made to indicate to unsafe places past using invalid pointer arithmetic; the objects they point to may keep to be used subsequently deallocation (dangling pointers); they may be used without having been initialized (wild pointers); or they may be straight assigned an unsafe value using a cast, wedlock, or through some other decadent pointer. In general, C is permissive in allowing manipulation of and conversion between pointer types, although compilers typically provide options for diverse levels of checking. Some other programming languages address these issues by using more restrictive reference types.

Arrays [edit]

Array types in C are traditionally of a fixed, static size specified at compile fourth dimension. The more recent C99 standard as well allows a grade of variable-length arrays. However, it is besides possible to allocate a block of memory (of arbitrary size) at run-fourth dimension, using the standard library'due south malloc function, and treat it as an array.

Since arrays are ever accessed (in effect) via pointers, array accesses are typically not checked against the underlying array size, although some compilers may provide premises checking equally an selection.^[34] ^[35] Array premises violations are therefore possible and can pb to various repercussions, including illegal retentiveness accesses, abuse of data, buffer overruns, and run-fourth dimension exceptions.

C does non accept a special provision for declaring multi-dimensional arrays, just rather relies on recursion within the blazon system to declare arrays of arrays, which effectively accomplishes the aforementioned thing. The index values of the resulting "multi-dimensional array" can exist thought of every bit increasing in row-major social club. Multi-dimensional arrays are unremarkably used in numerical algorithms (mainly from applied linear algebra) to shop 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 exist 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 following example using modern C (C99 or later) shows allocation of a two-dimensional array on the heap and the utilise 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            ][            M            ]                                    =                                    malloc            (            sizeof                                    *            p            );                                                if                                    (            !            p            )                                                return                                    -1            ;                                                for                                    (            int                                    i                                    =                                    0            ;                                    i                                    <                                    N            ;                                    i            ++            )                                                for                                    (            int                                    j                                    =                                    0            ;                                    j                                    <                                    M            ;                                    j            ++            )                                                (            *            p            )[            i            ][            j            ]                                    =                                    i                                    +                                    j            ;                                                print_array            (            N            ,                                    M            ,                                    p            );                                                free            (            p            );                                                return                                    ane            ;                        }

Array–arrow interchangeability [edit]

The subscript notation ten[i] (where x designates a pointer) is syntactic sugar for *(x+i).^[36] Taking advantage of the compiler's noesis of the pointer type, the accost that 10 + i points to is not the base address (pointed to by ten) incremented by i bytes, simply rather is defined to be the base of operations address incremented by i multiplied past the size of an element that 10 points to. Thus, x[i] designates the i+1th element of the array.

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

The full size of an array x can be determined by applying sizeof to an expression of array type. The size of an element can exist adamant past 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 declared array A can be determined as sizeof A / sizeof A[0]. Note, that if only a pointer to the first element is bachelor as it is oft the instance in C code because of the automatic conversion described above, the information nigh the full type of the array and its length are lost.

Retentivity management [edit]

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

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

These iii approaches are advisable in unlike situations and have various merchandise-offs. For example, static retentiveness resource allotment has picayune allocation overhead, automatic allocation may involve slightly more overhead, and dynamic retention allocation can potentially take a great deal of overhead for both resource allotment and deallocation. The persistent nature of static objects is useful for maintaining state information across role calls, automatic allocation is piece of cake to use simply stack space is typically much more than limited and transient than either static memory or heap space, and dynamic retentivity allocation allows convenient resource allotment of objects whose size is known just at run-time. Most C programs make extensive use of all three.

Where possible, automatic or static allotment is unremarkably simplest because the storage is managed past the compiler, freeing the programmer of the potentially error-decumbent chore of manually allocating and releasing storage. However, many data structures can change in size at runtime, and since static allocations (and automatic allocations before C99) must have a fixed size at compile-time, at that place are many situations in which dynamic allocation is necessary.^[31] Prior to the C99 standard, variable-sized arrays were a mutual instance 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 form of a aught pointer value) when the required storage cannot exist allocated. (Static allocation that is too large is usually detected past the linker or loader, earlier the programme can even brainstorm execution.)

Unless otherwise specified, static objects comprise 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 fleck pattern happens to be nowadays in the storage, which might non even represent a valid value for that type). If the plan attempts to access an uninitialized value, the results are undefined. Many modern compilers try to detect and warn well-nigh this trouble, only both faux positives and fake negatives can occur.

Heap retentiveness allotment has to be synchronized with its actual usage in any program to be reused as much as possible. For example, if the only pointer to a heap memory allocation goes out of scope or has its value overwritten before it is deallocated explicitly, then that memory cannot be recovered for later reuse and is substantially lost to the program, a phenomenon known every bit a memory leak. Conversely, it is possible for memory to exist freed, but is referenced afterwards, leading to unpredictable results. Typically, the failure symptoms appear in a portion of the program unrelated to the code that causes the error, making information technology difficult to diagnose the failure. Such issues are ameliorated in languages with automated garbage drove.

Libraries [edit]

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

The nigh mutual C library is the C standard library, which is specified by the ISO and ANSI C standards and comes with every C implementation (implementations which target express environments such as embedded systems may provide only a subset of the standard library). This library supports stream input and output, memory allocation, mathematics, character strings, and fourth dimension values. Several separate standard headers (for example, stdio.h) specify the interfaces for these and other standard library facilities.

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

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

File treatment and streams [edit]

File input and output (I/O) is not part of the C linguistic communication itself but instead is handled by libraries (such as the C standard library) and their associated header files (east.g. stdio.h). File handling is generally implemented through high-level I/O which works through streams. A stream is from this perspective a data catamenia that is contained of devices, while a file is a concrete device. The loftier-level I/O is done through the association of a stream to a file. In the C standard library, a buffer (a memory area or queue) is temporarily used to shop data before it's sent to the concluding destination. This reduces the time spent waiting for slower devices, for instance a hard drive or solid state drive. Low-level I/O functions are not part of the standard C library^{[ clarification needed ]} but are by and large part of "blank metal" programming (programming that's independent of any operating system such every bit near embedded programming). With few exceptions, implementations include low-level I/O.

Language tools [edit]

A number of tools have been developed to aid C programmers find and set up statements with undefined beliefs or possibly erroneous expressions, with greater rigor than that provided past the compiler. The tool lint was the first such, leading to many others.

Automated source code checking and auditing are beneficial in any language, and for C many such tools exist, such as Lint. A common exercise is to use Lint to notice questionable code when a plan is outset written. Once a programme passes Lint, it is and then compiled using the C compiler. Likewise, many compilers tin optionally warn almost syntactically valid constructs that are probable to really be errors. MISRA C is a proprietary set of guidelines to avoid such questionable lawmaking, developed for embedded systems.^[37]

In that location are also compilers, libraries, and operating organisation level mechanisms for performing actions that are not a standard function of C, such as bounds checking for arrays, detection of buffer overflow, serialization, dynamic retentiveness tracking, and automated 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 memory usage.

Uses [edit]

The C Programming Linguistic communication

C is widely used for systems programming in implementing operating systems and embedded arrangement applications,^[38] because C code, when written for portability, tin be used for most purposes, yet when needed, organisation-specific code can be used to access specific hardware addresses and to perform type punning to friction match externally imposed interface requirements, with a low run-fourth dimension demand on system resources.

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

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

C enables programmers to create efficient implementations of algorithms and data structures, considering the layer of abstraction from hardware is thin, and its overhead is low, an of import criterion for computationally intensive programs. For case, 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 approach may be used for portability or convenience; by using C as an intermediate language, additional car-specific code generators are not necessary. C has some features, such equally line-number preprocessor directives and optional superfluous commas at the stop of initializer lists, that back up compilation of generated code. Notwithstanding, some of C's shortcomings have prompted the development of other C-based languages specifically designed for use as intermediate languages, such as C--.

C has besides been widely used to implement terminate-user applications. However, such applications can also exist written in newer, higher-level languages.

[edit]

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

C has both directly and indirectly influenced many later languages such as C#, D, Get, Java, JavaScript, Limbo, LPC, Perl, PHP, Python, and Unix's C shell.^[42] The nearly 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 programme 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 lawmaking was translated into C, and so compiled with a C compiler.^[43]

The C++ programming language (originally named "C with Classes") was devised past Bjarne Stroustrup every bit 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 "sparse" layer on acme of C, and remains a strict superset of C that permits object-oriented programming using a hybrid dynamic/static typing epitome. Objective-C derives its syntax from both C and Smalltalk: syntax that involves preprocessing, expressions, function declarations, and function calls is inherited from C, while the syntax for object-oriented features was originally taken from Smalltalk.

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

See besides [edit]

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

Notes [edit]

^ The original example lawmaking will compile on most modern compilers that are non in strict standard compliance mode, only it does non fully arrange to the requirements of either C89 or C99. In fact, C99 requires that a diagnostic bulletin be produced.
^ The chief function really has two arguments, int argc and char *argv[], respectively, which can be used to handle command line arguments. The ISO C standard (department 5.1.ii.2.ane) requires both forms of chief to be supported, which is special treatment not afforded to any other office.

References [edit]

^ ^a ^b Kernighan, Brian Westward.; Ritchie, Dennis One thousand. (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 endeavor to produce a system coded in an early version of C—before structures—in 1972, but gave upward the effort."
^ Fruderica (Dec thirteen, 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 composition adopted by C owes considerable debt to Algol 68, although it did non, perhaps, emerge in a form that Algol's adherents would approve of."
^ Ring Team (October 23, 2021). "The Ring programming linguistic communication and other languages". ring-lang.net.
^ ^a ^b "Verilog HDL (and C)" (PDF). The Research School of Computer Scientific discipline at the Australian National University. June 3, 2010. Archived from the original (PDF) on Nov half dozen, 2013. Retrieved August 19, 2013. 1980s: ; Verilog commencement introduced ; Verilog inspired by the C programming language
^ ^a ^b ^c ^d ^eastward Ritchie (1993)
^ "Programming Linguistic communication 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 iv, 2009. Retrieved May 6, 2009.
^ ^a ^b "History of C". en.cppreference.com. Archived from the original on May 29, 2018. Retrieved May 28, 2018.
^ "TIOBE Index for October 2021". Retrieved October 7, 2021.
^ Ritchie, Dennis. "BCPL to B to C". Archived from the original on December 12, 2019. Retrieved September 10, 2019.
^ ^a ^b Johnson, S. C.; Ritchie, D. M. (1978). "Portability of C Programs and the UNIX System". Bell System Tech. J. 57 (6): 2021–2048. CiteSeerX10.1.1.138.35. doi:10.1002/j.1538-7305.1978.tb02141.10. S2CID 17510065. (Notation: The PDF is an OCR scan of the original, and contains a rendering of "IBM 370" every bit "IBM 310".)
^ McIlroy, M. D. (1987). A Research Unix reader: annotated excerpts from the Programmer's Manual, 1971–1986 (PDF) (Technical report). CSTR. Bong Labs. p. 10. 139. Archived (PDF) from the original on Nov 11, 2017. Retrieved February one, 2015.
^ "C manual pages". FreeBSD Miscellaneous Information Transmission (FreeBSD 13.0 ed.). May 30, 2011. Archived from the original on Jan 21, 2021. Retrieved January xv, 2021. [ane] Archived January 21, 2021, at the Wayback Machine
^ Kernighan, Brian W.; Ritchie, Dennis M. (March 1988). The C Programming Language (2nd ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-thirteen-110362-7.
^ Stroustrup, Bjarne (2002). Sibling rivalry: C and C++ (PDF) (Report). AT&T Labs. Archived (PDF) from the original on Baronial 24, 2014. Retrieved April xiv, 2014.
^ C Integrity. International Organization for Standardization. March 30, 1995. Archived from the original on July 25, 2018. Retrieved July 24, 2018.
^ "JTC1/SC22/WG14 – C". Home page. ISO/IEC. Archived from the original on February 12, 2018. Retrieved June two, 2011.
^ Andrew Binstock (October 12, 2011). "Interview with Herb Sutter". Dr. Dobbs. Archived from the original on Baronial 2, 2013. Retrieved September 7, 2013.
^ "Revised C23 Schedule WG 14 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 Feb 25, 2021. Retrieved July 26, 2011.
^ Harbison, Samuel P.; Steele, Guy L. (2002). C: A Reference Transmission (5th ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-xiii-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 Draft" (PDF). Archived (PDF) from the original on December 22, 2017. Retrieved September sixteen, 2011.
^ Kernighan & Ritchie (1996), pp. 192, 259.
^ "x Common Programming Mistakes in C++". Cs.ucr.edu. Archived from the original on October 21, 2008. Retrieved June 26, 2009.
^ Schultz, Thomas (2004). C and the 8051 (3rd ed.). Otsego, MI: PageFree Publishing Inc. p. twenty. ISBN978-1-58961-237-ii. Archived from the original on July 29, 2020. Retrieved Feb 10, 2012.
^ Kernighan & Ritchie (1978), p. 6.
^ ^a ^b ^c ^d ^e ^f ^g Klemens, Ben (2013). 21st Century C. O'Reilly Media. ISBN978-1-4493-2714-9.
^ Feuer, Alan R.; Gehani, Narain H. (March 1982). "Comparison of the Programming Languages C and Pascal". ACM Computing Surveys. 14 (1): 73–92. doi:10.1145/356869.356872. S2CID 3136859.
^ Kernighan & Ritchie (1996), p. 122.
^ For case, gcc provides _FORTIFY_SOURCE. "Security Features: Compile Fourth dimension Buffer Checks (FORTIFY_SOURCE)". fedoraproject.org. Archived from the original on January 7, 2007. Retrieved Baronial 5, 2012.
^ เอี่ยมสิริวงศ์, โอภาศ (2016). Programming with C. Bangkok, Thailand: SE-EDUCATION PUBLIC COMPANY Limited. pp. 225–230. ISBN978-616-08-2740-iv.
^ Raymond, Eric S. (October xi, 1996). The New Hacker'southward Dictionary (third ed.). MIT Press. p. 432. ISBN978-0-262-68092-ix. Archived from the original on November 12, 2012. Retrieved August 5, 2012.
^ "Man Page for lint (freebsd Section 1)". unix.com. May 24, 2001. Retrieved July 15, 2014.
^ Dale, Nell B.; Weems, Chip (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–Dec 1995.
^ "Using C for CGI Programming". linuxjournal.com. March 1, 2005. Archived from the original on February 13, 2010. Retrieved January four, 2010.
^ McMillan, Robert (August 1, 2013). "Is Java Losing Its Mojo?". Wired. Archived from the original on February xv, 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, Us, Oct 2-4, 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 Development of the C Language". ACM SIGPLAN Notices. ACM. 28 (3): 201–208. doi:10.1145/155360.155580.
Ritchie, Dennis M. (1993). "The Evolution of the C Language". The Second ACM SIGPLAN Conference on History of Programming Languages (HOPL-II). ACM. pp. 201–208. doi:10.1145/154766.155580. ISBN0-89791-570-4 . Retrieved November 4, 2014.
Kernighan, Brian W.; Ritchie, Dennis One thousand. (1996). The C Programming Language (2nd ed.). Prentice Hall. ISBN7-302-02412-10.

External links [edit]

ISO C Working Group official website
- ISO/IEC 9899, publicly available official C documents, including the C99 Rationale
- "C99 with Technical corrigenda TC1, TC2, and TC3 included" (PDF). (iii.61 MB)
comp.lang.c Often Asked Questions
A History of C, by Dennis Ritchie

shullthavier73.blogspot.com

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