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General-purpose programming language

C
Major implementations
The C Programming Language ^[1] (often referred to as One thousand&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; l years ago (1972) ^[2]

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

Typing subject field	Static, weak, manifest, nominal
OS	Cantankerous-platform
Filename extensions	.c, .h
Website	www.iso.org/standard/74528.html www.open-std.org/jtc1/sc22/wg14/
pcc, GCC, Clang, Intel C, C++Builder, Microsoft Visual C++, Watcom C
Dialects
Cyclone, Unified Parallel C, Divide-C, Cilk, C*
Influenced past
B (BCPL, CPL), ALGOL 68,^[4] assembly, PL/I, FORTRAN
Influenced
Numerous: AMPL, AWK, csh, C++, C--, C#, Objective-C, D, Become, Coffee, JavaScript, JS++, Julia, Limbo, LPC, Perl, PHP, Motorway, Processing, Python, Ring,^[5]Rust, Seed7, Vala, Verilog (HDL),^[vi] Nim, Zig
C Programming at Wikibooks

C (, as in the letter c) is a general-purpose, procedural computer programming language supporting structured programming, lexical variable scope, and recursion, with a static type system. By design, C provides constructs that map efficiently to typical motorcar instructions. It has found lasting use 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 Bong Labs by Dennis Ritchie betwixt 1972 and 1973 to construct utilities running on Unix. Information technology was applied to re-implementing the kernel of the Unix operating organization.^[7] During the 1980s, C gradually gained popularity. It has become one of the virtually widely used programming languages,^[8] ^[ix] with C compilers from various vendors available for the bulk of existing computer architectures and operating systems. C has been standardized by ANSI since 1989 (ANSI C) and by the International Organization for Standardization (ISO).

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

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

Overview [edit]

Like most procedural languages in the ALGOL tradition, C has facilities for structured programming and allows lexical variable scope and recursion. Its static blazon system prevents unintended operations. In C, all executable code is independent within subroutines (besides called "functions", though not strictly in the sense of functional programming). Part parameters are e'er passed by value (except arrays). Pass-by-reference is simulated in C by explicitly passing pointer values. C programme source text is free-format, using the semicolon as a statement terminator and curly braces for grouping blocks of statements.

The C language besides exhibits the post-obit characteristics:

The language has a small, fixed number of keywords, including a full set of control period primitives: if/else, for, practise/while, while, and switch. User-defined names are non distinguished from keywords by whatever kind of sigil.
Information technology has a large number of arithmetic, bitwise, and logic operators: +,+=,++,&,||, etc.
More one assignment may be performed in a unmarried statement.
Functions:
- Function return values can be ignored, when not needed.
- Role and data pointers permit ad hoc run-fourth dimension polymorphism.
- Functions may not be defined within the lexical scope of other functions.
Data typing is static, simply weakly enforced; all data has a type, just implicit conversions are possible.
Declaration syntax mimics usage context. C has no "define" keyword; instead, a statement kickoff with the proper name of a type is taken as a annunciation. 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) let related data elements to exist accessed and assigned as a unit.
- Union is a construction with overlapping members; simply the last fellow member stored is valid.
- Array indexing is a secondary notation, defined in terms of pointer arithmetic. Unlike structs, arrays are not first-course objects: they cannot be assigned or compared using unmarried congenital-in operators. There is no "assortment" keyword in apply or definition; instead, square brackets indicate arrays syntactically, for example month[11].
- Enumerated types are possible with the enum keyword. They are freely interconvertible with integers.
- Strings are not a distinct data type, simply are conventionally implemented every bit null-terminated character arrays.
Depression-level access to computer memory is possible by converting machine addresses to typed pointers.
Procedures (subroutines not returning values) are a special case of function, with an untyped return type void.
A preprocessor performs macro definition, source code file inclusion, and conditional compilation.
At that place is a bones form of modularity: files can be compiled separately and linked together, with control over which functions and information 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 certain features plant in other languages (such every bit object orientation and garbage collection), these tin be implemented or emulated, oftentimes through the use of external libraries (e.grand., the GLib Object Arrangement or the Boehm garbage collector).

Relations to other languages [edit]

Many afterward languages take borrowed directly or indirectly from C, including C++, C#, Unix'due south 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 take fatigued many of their control structures and other bones features from C. Most of them (Python being a dramatic exception) as well express 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 can exist radically different.

History [edit]

Early developments [edit]

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

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

Thompson desired a programming language to make utilities for the new platform. At first, he tried to make a Fortran compiler, but before long gave upwardly the idea. Instead, he created a cut-down version of the recently developed BCPL systems programming language. The official description of BCPL was not available at the time,^[12] and Thompson modified the syntax to be less wordy, producing the similar but somewhat simpler B.^[seven] However, few utilities were ultimately written in B because it was too 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 data typing for variables, which resulted in creating a new language C.^[13] The C compiler and some utilities fabricated 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 language had acquired some powerful features such as struct types.

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

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

Thou&R C [edit]

In 1978, Brian Kernighan and Dennis Ritchie published the first edition of The C Programming Linguistic communication.^[1] This volume, known to C programmers as Grand&R, served for many years every bit an informal specification of the language. The version of C that it describes is commonly referred to equally "Yard&R C". As this was released in 1978, it is as well referred to equally C78.^[fifteen] The second edition of the book^[sixteen] covers the after ANSI C standard, described below.

K&R introduced several language features:

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

Even after the publication of the 1989 ANSI standard, for many years K&R C was nevertheless 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 also.

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

For example:

                        long                                    some_function            ();                        /* int */                                    other_function            ();                        /* int */                                    calling_function            ()                        {                                                long                                    test1            ;                                                register                                    /* int */                                    test2            ;                                                test1                                    =                                    some_function            ();                                                if                                    (            test1                                    >                                    one            )                                                test2                                    =                                    0            ;                                                else                                                test2                                    =                                    other_function            ();                                                return                                    test2            ;                        }

The int type specifiers which are commented out could exist omitted in Chiliad&R C, simply are required in later standards.

Since K&R part declarations did not include any information nearly function arguments, function parameter blazon checks were non performed, although some compilers would effect a warning message if a local function was chosen with the wrong number of arguments, or if multiple calls to an external office used different numbers or types of arguments. Separate tools such as Unix's lint utility were developed that (among other things) could check for consistency of function use across multiple source files.

In the years following the publication of 1000&R C, several features were added to the language, supported past compilers from AT&T (in particular PCC^[17]) and some other vendors. These included:

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

The large number of extensions and lack of agreement on a standard library, together with the language popularity and the fact that 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 late 1970s and 1980s, versions of C were implemented for a wide variety of mainframe computers, minicomputers, and microcomputers, including the IBM PC, equally its popularity began to increase significantly.

In 1983, the American National Standards Plant (ANSI) formed a committee, X3J11, to institute a standard specification of C. X3J11 based the C standard on the Unix implementation; however, the not-portable portion of the Unix C library was handed off to the IEEE working grouping 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 ofttimes referred to equally 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) equally ISO/IEC 9899:1990, which is sometimes called C90. Therefore, the terms "C89" and "C90" refer to the aforementioned programming language.

ANSI, like other national standards bodies, no longer develops the C standard independently, 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 within a yr of ISO publication.

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

C89 is supported by current C compilers, and well-nigh modern C lawmaking is based on it. Any program written only in Standard C and without any hardware-dependent assumptions volition run correctly on whatsoever platform with a conforming C implementation, within its resource limits. Without such precautions, programs may compile only on a sure platform or with a detail compiler, due, for example, to the use of non-standard libraries, such as GUI libraries, or to a reliance on compiler- or platform-specific attributes such as the verbal size of data types and byte endianness.

In cases where lawmaking must exist compilable by either standard-conforming or Thousand&R C-based compilers, the __STDC__ macro can be used to dissever the code into Standard and Yard&R sections to foreclose the apply on a K&R C-based compiler of features available only in Standard C.

Later on the ANSI/ISO standardization process, the C language specification remained relatively static for several years. In 1995, Normative Amendment 1 to the 1990 C standard (ISO/IEC 9899/AMD1:1995, known informally as C95) was published, to correct some details and to add together more extensive support for international graphic symbol 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 commonly referred to every bit "C99". It has since been amended 3 times past Technical Corrigenda.^[nineteen]

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

C99 is for the about part backward uniform with C90, but 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 indicate 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++11.^[twenty] ^{[ needs update ]}

In addition, support for Unicode identifiers (variable / office names) in the class of escaped characters (e.k. \U0001f431) is now required. Support for raw Unicode names is optional.

C11 [edit]

In 2007, work began on another revision of the C standard, informally chosen "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 blazon generic macros, anonymous structures, improved Unicode support, atomic operations, multi-threading, and bounds-checked functions. It also makes some portions of the existing C99 library optional, and improves compatibility with C++. The standard macro __STDC_VERSION__ is defined as 201112L to indicate that C11 support is bachelor.

C17 [edit]

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

C2x [edit]

C2x is an informal name for the next (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 order to back up exotic features such as fixed-point arithmetic, multiple singled-out memory banks, and basic I/O operations.

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

Syntax [edit]

C has a formal grammar specified by the C standard.^[23] Line endings are generally non significant in C; still, line boundaries practice accept significance during the preprocessing stage. Comments may announced either between the delimiters /* and */, or (since C99) following // until the end of the line. Comments delimited by /* and */ do non nest, and these sequences of characters are not interpreted equally comment delimiters if they appear inside string or character literals.^[24]

C source files contain declarations and office definitions. Office definitions, in turn, contain declarations and statements. Declarations either define new types using keywords such as struct, union, and enum, or assign types to and possibly reserve storage for new variables, usually by writing the type followed by the variable proper name. Keywords such every bit char and int specify congenital-in types. Sections of code are enclosed in braces ({ and }, sometimes chosen "curly brackets") to limit the scope of declarations and to act as a unmarried statement for control structures.

As an imperative language, C uses statements to specify deportment. The most mutual statement is an expression statement, consisting of an expression to be evaluated, followed by a semicolon; every bit 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 command-menstruum 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 separate initialization, testing, and reinitialization expressions, any or all of which tin be omitted. break and continue can be used to leave the innermost enclosing loop statement or skip to its reinitialization. There is too a not-structured goto statement which branches direct to the designated label within the office. switch selects a case to be executed based on the value of an integer expression.

Expressions tin use a variety of built-in operators and may contain function calls. The society in which arguments to functions and operands to nigh operators are evaluated is unspecified. The evaluations may even be interleaved. However, all side effects (including storage to variables) will occur before the next "sequence betoken"; sequence points include the terminate of each expression argument, and the entry to and return from each role call. Sequence points besides occur during evaluation of expressions containing sure operators (&&, ||, ?: and the comma operator). This permits a loftier caste of object code optimization past the compiler, only 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 Linguistic communication: "C, like whatsoever 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, because of the impact of such changes on already existing software.

Character ready [edit]

The basic C source character ready includes the following characters:

Lowercase and uppercase messages 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; information technology need non stand for to an actual single character, although for convenience C treats it as i.

Additional multi-byte encoded characters may exist used in string literals, only they are not entirely portable. The latest C standard (C11) allows multi-national Unicode characters to be embedded portably within C source text by using \uXXXX or \UXXXXXXXX encoding (where the 10 denotes a hexadecimal character), although this feature is not yet widely implemented.

The basic C execution character gear up contains the same characters, along with representations for alarm, backspace, and carriage render. 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 whatever purposes other than those for which they are predefined:

car
break
example
char
const
continue
default
practice
double
else
enum
extern
float
for
goto
if
int
long
annals
render
short
signed
sizeof
static
struct
switch
typedef
marriage
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, because identifiers of that form were previously reserved by the C standard for use just past implementations. Since existing program source code should not have been using these identifiers, it would not be affected when C implementations started supporting these extensions to the programming linguistic communication. Some standard headers exercise define more convenient synonyms for underscored identifiers. The linguistic communication previously included a reserved discussion called entry, just this was seldom implemented, and has now been removed every bit a reserved word.^[27]

Operators [edit]

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

arithmetics: +, -, *, /, %
assignment: =
augmented consignment: +=, -=, *=, /=, %=, &=, |=, ^=, <<=, >>=
bitwise logic: ~, &, |, ^
bitwise shifts: <<, >>
boolean logic: !, &&, ||
conditional evaluation: ? :
equality testing: ==, !=
calling functions: ( )
increase and decrement: ++, --
member selection: ., ->
object size: sizeof
social club 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 dissimilar ALGOL and its derivatives. C uses the operator == to test for equality. The similarity betwixt these ii operators (assignment and equality) may consequence in the adventitious utilize of ane in place of the other, and in many cases, the error does not produce an error bulletin (although some compilers produce warnings). For example, the conditional expression if (a == b + 1) might mistakenly be written every bit if (a = b + 1), which will be evaluated as true if a is not cipher after the assignment.^[28]

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

"Hi, world" instance [edit]

The "howdy, globe" case, which appeared in the first edition of K&R, has get the model for an introductory programme in nigh programming textbooks. The programme prints "hello, world" to the standard output, which is usually a terminal or screen brandish.

The original version was:^[thirty]

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

A standard-befitting "hello, world" 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 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 angle brackets surrounding stdio.h betoken that stdio.h is located using a search strategy that prefers headers provided with the compiler to other headers having the same proper name, every bit opposed to double quotes which typically include local or projection-specific header files.

The next line indicates that a function named main is being defined. The main office serves a special purpose in C programs; the run-time surround calls the main function to brainstorm program execution. The type specifier int indicates that the value that is returned to the invoker (in this case the run-time surroundings) as a event of evaluating the master office, is an integer. The keyword void every bit a parameter list indicates that this function takes no arguments.^[b]

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

The side by side line calls (diverts execution to) a function named printf, which in this example is supplied from a arrangement library. In this call, the printf part is passed (provided with) a single argument, the address of the first graphic symbol in the string literal "howdy, world\n". The string literal is an unnamed array with elements of type char, set upward automatically by the compiler with a last 0-valued grapheme to mark the cease of the array (printf needs to know this). The \n is an escape sequence that C translates to a newline grapheme, which on output signifies the end of the electric current line. The return value of the printf part is of blazon int, only it is silently discarded since information technology is not used. (A more than careful plan might test the return value to determine whether or non the printf part succeeded.) The semicolon ; terminates the argument.

The closing curly brace indicates the terminate of the code for the primary function. Co-ordinate to the C99 specification and newer, the main role, dissimilar whatsoever other function, volition implicitly return a value of 0 upon reaching the } that terminates the function. (Formerly an explicit return 0; statement was required.) This is interpreted by the run-fourth dimension arrangement as an go out lawmaking indicating successful execution.^[31]

Data types [edit]

The type system in C is static and weakly typed, which makes it like to the type organization of ALGOL descendants such every bit Pascal.^[32] There 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 unmarried-byte characters. C99 added a boolean datatype. There are besides derived types including arrays, pointers, records (struct), and unions (wedlock).

C is often used in low-level systems programming where escapes from the blazon organization may be necessary. The compiler attempts to ensure type correctness of near expressions, simply the programmer tin override the checks in various ways, either by using a type cast to explicitly convert a value from one type to another, or by using pointers or unions to reinterpret the underlying bits of a information object in some other mode.

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

C's usual arithmetic conversions let for efficient lawmaking to be generated, but can sometimes produce unexpected results. For example, a comparing of signed and unsigned integers of equal width requires a conversion of the signed value to unsigned. This tin 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 function in retentivity. Pointers tin can be dereferenced to admission information stored at the accost pointed to, or to invoke a pointed-to function. Pointers tin be manipulated using assignment or pointer arithmetic. The run-time representation of a arrow value is typically a raw memory address (peradventure augmented by an kickoff-within-give-and-take field), but since a pointer's type includes the type of the affair pointed to, expressions including pointers can be type-checked at compile time. Pointer arithmetic is automatically scaled by the size of the pointed-to data type. Pointers are used for many purposes in C. Text strings are commonly manipulated using pointers into arrays of characters. Dynamic memory resource allotment is performed using pointers. Many information types, such as trees, are commonly implemented every bit dynamically allocated struct objects linked together using pointers. Pointers to functions are useful for passing functions as arguments to higher-order functions (such as qsort or bsearch) or as callbacks to be invoked by effect handlers.^[31]

A null pointer value explicitly points to no valid location. Dereferencing a null pointer value is undefined, often resulting in a segmentation mistake. Null pointer values are useful for indicating special cases such as no "next" pointer in the final node of a linked listing, or as an error indication from functions returning pointers. In appropriate contexts in source lawmaking, 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 blazon, or as the NULL macro defined past several standard headers. In conditional contexts, nada pointer values evaluate to false, while all other pointer values evaluate to true.

Void pointers (void *) point to objects of unspecified blazon, and tin 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 easily be (and in many contexts implicitly are) converted to and from whatsoever other object pointer blazon.^[31]

Careless utilise of pointers is potentially dangerous. Because they are typically unchecked, a arrow variable can be made to point to whatsoever arbitrary location, which can crusade undesirable effects. Although properly used pointers indicate to prophylactic places, they can be made to betoken to dangerous places by using invalid pointer arithmetic; the objects they bespeak to may proceed to be used afterwards deallocation (dangling pointers); they may be used without having been initialized (wild pointers); or they may exist directly assigned an unsafe value using a cast, union, or through some other corrupt pointer. In full general, C is permissive in allowing manipulation of and conversion between pointer types, although compilers typically provide options for various levels of checking. Another programming languages address these problems by using more restrictive reference types.

Arrays [edit]

Assortment types in C are traditionally of a fixed, static size specified at compile time. The more contempo C99 standard as well allows a course of variable-length arrays. However, it is also possible to allocate a block of retention (of capricious size) at run-time, using the standard library'southward malloc function, and treat it as an assortment.

Since arrays are ever accessed (in issue) via pointers, array accesses are typically not checked against the underlying array size, although some compilers may provide bounds checking as an choice.^[34] ^[35] Array bounds violations are therefore possible and tin can lead to various repercussions, including illegal memory accesses, corruption of data, buffer overruns, and run-time exceptions.

C does not have a special provision for declaring multi-dimensional arrays, but rather relies on recursion within the type system to declare arrays of arrays, which effectively accomplishes the same affair. The alphabetize values of the resulting "multi-dimensional array" can be thought of equally increasing in row-major order. Multi-dimensional arrays are normally used in numerical algorithms (mainly from applied linear algebra) to store matrices. The construction of the C array is well suited to this particular task. Still, 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 classify the array with an additional "row vector" of pointers to the columns.) C99 introduced "variable-length arrays" which address this issue.

The post-obit example using mod C (C99 or later) shows allotment of a two-dimensional array 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            )[            Northward            ][            M            ]                                    =                                    malloc            (            sizeof                                    *            p            );                                                if                                    (            !            p            )                                                return                                    -1            ;                                                for                                    (            int                                    i                                    =                                    0            ;                                    i                                    <                                    Northward            ;                                    i            ++            )                                                for                                    (            int                                    j                                    =                                    0            ;                                    j                                    <                                    M            ;                                    j            ++            )                                                (            *            p            )[            i            ][            j            ]                                    =                                    i                                    +                                    j            ;                                                print_array            (            North            ,                                    M            ,                                    p            );                                                gratuitous            (            p            );                                                render                                    ane            ;                        }

Array–arrow interchangeability [edit]

The subscript notation x[i] (where x designates a arrow) is syntactic carbohydrate for *(10+i).^[36] Taking advantage of the compiler'south knowledge of the pointer type, the address that ten + i points to is not the base address (pointed to by 10) incremented by i bytes, but rather is defined to be the base address incremented by i multiplied by the size of an element that x points to. Thus, x[i] designates the i+aneth element of the assortment.

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

The total size of an array x tin can be adamant by applying sizeof to an expression of array type. The size of an chemical element tin be determined past applying the operator sizeof to whatever dereferenced element of an array A, as in north = sizeof A[0]. This, the number of elements in a declared array A tin can be determined as sizeof A / sizeof A[0]. Note, that if just a pointer to the outset element is bachelor as it is often the case in C lawmaking because of the automatic conversion described above, the information about the total type of the array and its length are lost.

Memory management [edit]

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

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

These iii approaches are appropriate in different situations and take various trade-offs. For case, static retentivity allotment has piddling allocation overhead, automatic resource allotment may involve slightly more overhead, and dynamic retentivity allocation can potentially have a bully deal of overhead for both allocation and deallocation. The persistent nature of static objects is useful for maintaining state data across function calls, automatic allocation is like shooting fish in a barrel to use but stack space is typically much more limited and transient than either static memory or heap space, and dynamic retention allocation allows user-friendly resource allotment of objects whose size is known simply at run-time. Most C programs make all-encompassing utilize of all three.

Where possible, automatic or static allotment is normally simplest because the storage is managed by the compiler, freeing the programmer of the potentially error-decumbent job 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, in that location are many situations in which dynamic allocation is necessary.^[31] Prior to the C99 standard, variable-sized arrays were a common example of this. (See the commodity 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 null pointer value) when the required storage cannot be allocated. (Static allotment that is likewise large is commonly detected by the linker or loader, before the program can even begin execution.)

Unless otherwise specified, static objects contain zero or zippo pointer values upon program startup. Automatically and dynamically allocated objects are initialized merely if an initial value is explicitly specified; otherwise they initially have indeterminate values (typically, whatever bit design happens to be nowadays in the storage, which might not even correspond a valid value for that blazon). If the programme attempts to access an uninitialized value, the results are undefined. Many modern compilers try to detect and warn about this problem, but both false positives and false negatives tin occur.

Heap memory resource allotment has to be synchronized with its actual usage in any program to be reused as much as possible. For case, if the only pointer to a heap memory resource allotment goes out of scope or has its value overwritten before it is deallocated explicitly, then that memory cannot exist recovered for later on reuse and is essentially lost to the programme, a phenomenon known as a retentiveness leak. Conversely, it is possible for retentivity to exist freed, simply is referenced subsequently, leading to unpredictable results. Typically, the failure symptoms appear in a portion of the plan unrelated to the code that causes the fault, making it difficult to diagnose the failure. Such issues are ameliorated in languages with automatic garbage collection.

Libraries [edit]

The C programming language uses libraries equally its primary method of extension. In C, a library is a set of functions contained inside a single "archive" file. Each library typically has a header file, which contains the prototypes of the functions contained within the library that may be used past a plan, and declarations of special information types and macro symbols used with these functions. In order for a program to utilise a library, it must include the library's header file, and the library must be linked with the program, which in many cases requires compiler flags (eastward.m., -lm, shorthand for "link the math library").^[31]

The most 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 limited environments such every bit embedded systems may provide only a subset of the standard library). This library supports stream input and output, memory resource allotment, mathematics, character strings, and fourth dimension values. Several carve up standard headers (for instance, stdio.h) specify the interfaces for these and other standard library facilities.

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

Since many programs take been written in C, there 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 can be used from higher-level languages similar Java, Perl, and Python.^[31]

File handling and streams [edit]

File input and output (I/O) is non part of the C linguistic communication itself but instead is handled by libraries (such as the C standard library) and their associated header files (eastward.thousand. stdio.h). File handling is generally implemented through high-level I/O which works through streams. A stream is from this perspective a information flow that is contained of devices, while a file is a physical device. The high-level I/O is washed through the association of a stream to a file. In the C standard library, a buffer (a memory surface area or queue) is temporarily used to store data before it's sent to the final destination. This reduces the time spent waiting for slower devices, for instance a hard bulldoze or solid land drive. Low-level I/O functions are not part of the standard C library^{[ clarification needed ]} but are generally function of "bare metal" programming (programming that'southward independent of whatever operating system such as about embedded programming). With few exceptions, implementations include depression-level I/O.

Linguistic communication tools [edit]

A number of tools have been adult to help C programmers detect and fix statements with undefined behavior or perhaps erroneous expressions, with greater rigor than that provided by the compiler. The tool lint was the commencement such, leading to many others.

Automated source code checking and auditing are beneficial in whatever language, and for C many such tools exist, such every bit Lint. A common practice is to use Lint to observe questionable lawmaking when a program is kickoff written. One time 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 actually be errors. MISRA C is a proprietary set of guidelines to avoid such questionable code, adult for embedded systems.^[37]

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

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

Uses [edit]

The C Programming Language

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

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

A consequence of C'south broad 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, Blood-red, 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 every bit an intermediate language by implementations of other languages. This arroyo may be used for portability or convenience; past using C as an intermediate linguistic communication, additional machine-specific code generators are not necessary. C has some features, such every bit line-number preprocessor directives and optional superfluous commas at the stop of initializer lists, that back up compilation of generated code. However, some of C'due south shortcomings accept prompted the evolution of other C-based languages specifically designed for use as intermediate languages, such as C--.

C has besides been widely used to implement stop-user applications. Nonetheless, such applications tin also be written in newer, higher-level languages.

[edit]

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

C has both directly and indirectly influenced many later languages such equally C#, D, Go, Coffee, JavaScript, Limbo, LPC, Perl, PHP, Python, and Unix's C shell.^[42] The most pervasive influence has been syntactical; all of the languages mentioned combine the statement and (more or less recognizably) expression syntax of C with type systems, data models, and/or large-calibration program structures that differ from those of C, sometimes radically.

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

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

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

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

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

See too [edit]

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

Notes [edit]

^ The original example code volition compile on most mod compilers that are not in strict standard compliance mode, but it does not fully conform to the requirements of either C89 or C99. In fact, C99 requires that a diagnostic message be produced.
^ The main function really has two arguments, int argc and char *argv[], respectively, which can exist used to handle command line arguments. The ISO C standard (department 5.1.two.2.one) requires both forms of master to be supported, which is special treatment not afforded to whatsoever other function.

References [edit]

^ ^a ^b Kernighan, Brian W.; Ritchie, Dennis Thou. (February 1978). The C Programming Language (1st ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-thirteen-110163-0.
^ Ritchie (1993): "Thompson had made a brief attempt to produce a organisation coded in an early on version of C—before structures—in 1972, but gave up the effort."
^ Fruderica (December 13, 2020). "History of C". The cppreference.com. Archived from the original on October 24, 2020. Retrieved October 24, 2020.
^ Ritchie (1993): "The scheme of blazon composition adopted past C owes considerable debt to Algol 68, although information technology did not, mayhap, emerge in a form that Algol'due south adherents would approve of."
^ Ring Team (October 23, 2021). "The Band programming language and other languages". ring-lang.cyberspace.
^ ^a ^b "Verilog HDL (and C)" (PDF). The Inquiry School of Computer science at the Australian National University. June 3, 2010. Archived from the original (PDF) on November six, 2013. Retrieved August xix, 2013. 1980s: ; Verilog first introduced ; Verilog inspired by the C programming language
^ ^a ^b ^c ^d ^e Ritchie (1993)
^ "Programming Language Popularity". 2009. Archived from the original on Jan 16, 2009. Retrieved January xvi, 2009.
^ "TIOBE Programming Community Alphabetize". 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 ten, 2019.
^ ^a ^b Johnson, Due south. C.; Ritchie, D. M. (1978). "Portability of C Programs and the UNIX Arrangement". Bell System Tech. J. 57 (half dozen): 2021–2048. CiteSeerX10.1.1.138.35. doi:10.1002/j.1538-7305.1978.tb02141.x. S2CID 17510065. (Annotation: The PDF is an OCR scan of the original, and contains a rendering of "IBM 370" as "IBM 310".)
^ McIlroy, Yard. D. (1987). A Research Unix reader: annotated excerpts from the Programmer'due south Manual, 1971–1986 (PDF) (Technical report). CSTR. Bell Labs. p. x. 139. Archived (PDF) from the original on Nov 11, 2017. Retrieved February i, 2015.
^ "C manual pages". FreeBSD Miscellaneous Information Manual (FreeBSD 13.0 ed.). May 30, 2011. Archived from the original on January 21, 2021. Retrieved January xv, 2021. [1] Archived Jan 21, 2021, at the Wayback Machine
^ Kernighan, Brian West.; Ritchie, Dennis M. (March 1988). The C Programming Language (second ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-xiii-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 Apr xiv, 2014.
^ C Integrity. International Arrangement for Standardization. March 30, 1995. Archived from the original on July 25, 2018. Retrieved July 24, 2018.
^ "JTC1/SC22/WG14 – C". Dwelling page. ISO/IEC. Archived from the original on February 12, 2018. Retrieved June 2, 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 14 N 2759" (PDF). www.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 Manual (5th ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-089592-9. Contains a BNF grammar for C.
^ Kernighan & Ritchie (1996), p. 192.
^ Kernighan & Ritchie (1978), p. 3.
^ "ISO/IEC 9899:201x (ISO C11) Commission Draft" (PDF). Archived (PDF) from the original on December 22, 2017. Retrieved September 16, 2011.
^ Kernighan & Ritchie (1996), pp. 192, 259.
^ "10 Common Programming Mistakes in C++". Cs.ucr.edu. Archived from the original on October 21, 2008. Retrieved June 26, 2009.
^ Schultz, Thomas (2004). C and the 8051 (3rd ed.). Otsego, MI: PageFree Publishing Inc. p. 20. ISBN978-1-58961-237-2. Archived from the original on July 29, 2020. Retrieved February x, 2012.
^ Kernighan & Ritchie (1978), p. 6.
^ ^a ^b ^c ^d ^e ^f ^g Klemens, Ben (2013). 21st Century C. O'Reilly Media. ISBN978-ane-4493-2714-9.
^ Feuer, Alan R.; Gehani, Narain H. (March 1982). "Comparison of the Programming Languages C and Pascal". ACM Computing Surveys. 14 (one): 73–92. doi:10.1145/356869.356872. S2CID 3136859.
^ Kernighan & Ritchie (1996), p. 122.
^ For example, gcc provides _FORTIFY_SOURCE. "Security Features: Compile Time Buffer Checks (FORTIFY_SOURCE)". fedoraproject.org. Archived from the original on January seven, 2007. Retrieved Baronial 5, 2012.
^ เอี่ยมสิริวงศ์, โอภาศ (2016). Programming with C. Bangkok, Thailand: SE-EDUCATION PUBLIC Visitor LIMITED. pp. 225–230. ISBN978-616-08-2740-4.
^ Raymond, Eric S. (October 11, 1996). The New Hacker's Dictionary (tertiary ed.). MIT Press. p. 432. ISBN978-0-262-68092-9. Archived from the original on Nov 12, 2012. Retrieved August v, 2012.
^ "Man Page for lint (freebsd Section i)". unix.com. May 24, 2001. Retrieved July xv, 2014.
^ Dale, Nell B.; Weems, Chip (2014). Programming and problem solving with C++ (sixth ed.). Burlington, MA: Jones & Bartlett Learning. ISBN978-1449694289. OCLC 894992484.
^ Dr. Dobb's Sourcebook. UsaA.: Miller Freeman, Inc. Nov–December 1995.
^ "Using C for CGI Programming". linuxjournal.com. March 1, 2005. Archived from the original on February 13, 2010. Retrieved January 4, 2010.
^ McMillan, Robert (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 calculating : a compendium of select, pivotal technology firms. ISBN978-3319214641. OCLC 922324121.
^ Rauchwerger, Lawrence (2004). Languages and compilers for parallel calculating : 16th international workshop, LCPC 2003, College Station, TX, U.s., Oct 2-four, 2003 : revised papers. Springer. ISBN978-3540246442. OCLC 57965544.
^ Stroustrup, Bjarne (1993). "A History of C++: 1979−1991" (PDF). Archived (PDF) from the original on February 2, 2019. Retrieved June 9, 2011.

Sources [edit]

Ritchie, Dennis G. (March 1993). "The Development of the C Language". ACM SIGPLAN Notices. ACM. 28 (3): 201–208. doi:10.1145/155360.155580.
Ritchie, Dennis M. (1993). "The Development of the C Language". The Second ACM SIGPLAN Conference on History of Programming Languages (HOPL-2). ACM. pp. 201–208. doi:10.1145/154766.155580. ISBN0-89791-570-4 . Retrieved November iv, 2014.
Kernighan, Brian Westward.; Ritchie, Dennis M. (1996). The C Programming Linguistic communication (2d ed.). Prentice Hall. ISBNvii-302-02412-X.

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 Frequently Asked Questions
A History of C, by Dennis Ritchie

wardmals1938.blogspot.com

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