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General-purpose programming language

C
Major implementations
The C Programming Linguistic communication ^[1] (ofttimes referred to as One thousand&R), the seminal book on C
Paradigm	Multi-paradigm: imperative (procedural), structured
Designed by	Dennis Ritchie
Programmer	Dennis Ritchie & Bell Labs (creators); ANSI X3J11 (ANSI C); ISO/IEC JTC1/SC22/WG14 (ISO C)
First appeared	1972; 50 years ago (1972) ^[ii]

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

Typing discipline	Static, weak, manifest, nominal
OS	Cross-platform
Filename extensions	.c, .h
Website	www.iso.org/standard/74528.html world wide web.open-std.org/jtc1/sc22/wg14/
pcc, GCC, Clang, Intel C, C++Builder, Microsoft Visual C++, Watcom C
Dialects
Cyclone, Unified Parallel C, Split-C, Cilk, C*
Influenced by
B (BCPL, CPL), ALGOL 68,^[4] assembly, PL/I, FORTRAN
Influenced
Numerous: AMPL, AWK, csh, C++, C--, C#, Objective-C, D, Get, Java, JavaScript, JS++, Julia, Limbo, LPC, Perl, PHP, Pike, Processing, Python, Band,^[5]Rust, Seed7, Vala, Verilog (HDL),^[6] Nim, Zig
C Programming at Wikibooks

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

A successor to the programming language B, C was originally developed at Bell Labs by Dennis Ritchie between 1972 and 1973 to construct utilities running on Unix. It was practical to re-implementing the kernel of the Unix operating organisation.^[7] During the 1980s, C gradually gained popularity. Information technology has become one of the almost widely used programming languages,^[8] ^[nine] with C compilers from diverse vendors bachelor for the majority of existing computer architectures and operating systems. C has been standardized by ANSI since 1989 (ANSI C) and by the International Organization for Standardization (ISO).

C is an imperative procedural language. It was designed to be compiled to provide low-level admission to memory and language constructs that map efficiently to auto 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 diverseness of computer platforms and operating systems with few changes to its source code.^[10]

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

Overview [edit]

Similar nearly procedural languages in the ALGOL tradition, C has facilities for structured programming and allows lexical variable scope and recursion. Its static type organization prevents unintended operations. In C, all executable code is independent inside subroutines (also called "functions", though not strictly in the sense of functional programming). Role parameters are always passed past value (except arrays). Laissez passer-by-reference is simulated in C by explicitly passing arrow 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 linguistic communication also exhibits the following characteristics:

The language has a small, fixed number of keywords, including a full set of command flow primitives: if/else, for, do/while, while, and switch. User-defined names are not distinguished from keywords past any kind of sigil.
Information technology has a large number of arithmetic, bitwise, and logic operators: +,+=,++,&,||, etc.
More than than 1 assignment may be performed in a single argument.
Functions:
- Function render values can be ignored, when not needed.
- Function and data pointers permit advert hoc run-time polymorphism.
- Functions may non be defined inside the lexical scope of other functions.
Data typing is static, merely weakly enforced; all data has a blazon, but implicit conversions are possible.
Proclamation syntax mimics usage context. C has no "define" keyword; instead, a statement kickoff with the name of a blazon is taken equally a proclamation. At that place is no "function" keyword; instead, a part is indicated by the presence of a parenthesized statement list.
User-defined (typedef) and compound types are possible.
- Heterogeneous aggregate data types (struct) allow related data elements to be accessed and assigned equally a unit.
- Marriage is a structure with overlapping members; simply the final member stored is valid.
- Assortment indexing is a secondary notation, defined in terms of pointer arithmetic. Dissimilar structs, arrays are non get-go-grade objects: they cannot exist assigned or compared using single congenital-in operators. There is no "array" keyword in utilize 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 non a distinct data type, but are conventionally implemented as cipher-terminated grapheme arrays.
Depression-level access to computer memory is possible by converting machine addresses to typed pointers.
Procedures (subroutines not returning values) are a special instance of role, with an untyped render blazon void.
A preprocessor performs macro definition, source code file inclusion, and conditional compilation.
In that location is a basic form of modularity: files tin can exist compiled separately and linked together, with command over which functions and data objects are visible to other files via static and extern attributes.
Circuitous functionality such as I/O, cord manipulation, and mathematical functions are consistently delegated to library routines.

While C does not include certain features establish in other languages (such equally object orientation and garbage collection), these tin can be implemented or emulated, oft through the use of external libraries (due east.one thousand., the GLib Object System or the Boehm garbage collector).

Relations to other languages [edit]

Many later 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, Carmine, Rust, Swift, Verilog and SystemVerilog (hardware description languages).^[6] These languages have drawn many of their command structures and other bones features from C. Nearly of them (Python being a dramatic exception) too 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 tin can be radically different.

History [edit]

Early on developments [edit]

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

The origin of C is closely tied to the development of the Unix operating organisation, originally implemented in associates language on a PDP-7 past 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-11 version of Unix was besides adult in assembly language.^[7]

Thompson desired a programming linguistic communication to brand utilities for the new platform. At first, he tried to make a Fortran compiler, but soon gave up the thought. Instead, he created a cut-down version of the recently developed BCPL systems programming language. The official description of BCPL was not bachelor at the time,^[12] and Thompson modified the syntax to exist less wordy, producing the similar but somewhat simpler B.^[7] Nonetheless, few utilities were ultimately written in B considering it was too slow, and B could not take advantage of PDP-xi features such as byte addressability.

In 1972, Ritchie started to better B, virtually notably calculation data typing for variables, which resulted in creating a new language C.^[13] The C compiler and some utilities fabricated with information technology were included in Version 2 Unix.^[14]

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

The preprocessor was introduced 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 unproblematic string replacements: #include and #define of parameterless macros. Soon after that, it was extended, mostly by Mike Lesk and then by John Reiser, to incorporate macros with arguments and conditional compilation.^[7]

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

K&R C [edit]

In 1978, Brian Kernighan and Dennis Ritchie published the outset edition of The C Programming Language.^[1] This book, known to C programmers every bit G&R, served for many years as an informal specification of the linguistic communication. The version of C that it describes is commonly referred to as "K&R C". As this was released in 1978, information technology is also referred to as C78.^[15] The second edition of the book^[16] covers the subsequently ANSI C standard, described beneath.

One thousand&R introduced several language features:

Standard I/O library
long int information blazon
unsigned int data type
Chemical compound assignment operators of the form =op (such equally =-) were inverse to the form op= (that is, -=) to remove the semantic ambivalence created by constructs such every bit i=-10, which had been interpreted equally i =- 10 (decrement i by 10) instead of the possibly intended i = -x (permit i be −10).

Even after the publication of the 1989 ANSI standard, for many years K&R C was however considered the "everyman 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 too.

In early on versions of C, only functions that render types other than int must be declared if used before the function 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                                    >                                    one            )                                                test2                                    =                                    0            ;                                                else                                                test2                                    =                                    other_function            ();                                                return                                    test2            ;                        }

The int type specifiers which are commented out could be omitted in K&R C, but are required in later standards.

Since 1000&R function declarations did not include any information about role arguments, role parameter type checks were non performed, although some compilers would issue a warning bulletin if a local function was called with the wrong number of arguments, or if multiple calls to an external function used different numbers or types of arguments. Separate tools such as Unix's lint utility were developed that (amid other things) could check for consistency of part use beyond 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 render value)
functions returning struct or union types (rather than pointers)
assignment 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 non even the Unix compilers precisely implemented the K&R specification, led to the necessity of standardization.

ANSI C and ISO C [edit]

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

In 1983, the American National Standards Institute (ANSI) formed a committee, X3J11, to establish 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 grouping 1003 to get the ground 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 oftentimes referred to as ANSI C, Standard C, or sometimes C89.

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

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

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

C89 is supported by current C compilers, and nigh modern C code is based on it. Whatsoever plan written only in Standard C and without any hardware-dependent assumptions will run correctly on whatsoever platform with a befitting C implementation, within its resources limits. Without such precautions, programs may compile only on a certain platform or with a particular compiler, due, for example, to the use of non-standard libraries, such as GUI libraries, or to a reliance on compiler- or platform-specific attributes such as the exact size of data types and byte endianness.

In cases where lawmaking must be compilable by either standard-befitting or M&R C-based compilers, the __STDC__ macro can be used to split the lawmaking into Standard and K&R sections to prevent the use on a Chiliad&R C-based compiler of features bachelor only in Standard C.

Subsequently the ANSI/ISO standardization process, the C linguistic communication specification remained relatively static for several years. In 1995, Normative Amendment i to the 1990 C standard (ISO/IEC 9899/AMD1:1995, known informally every bit C95) was published, to right some details and to add more extensive back up for international grapheme sets.^[eighteen]

C99 [edit]

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

C99 introduced several new features, including inline functions, several new data types (including long long int and a complex blazon to correspond complex numbers), variable-length arrays and flexible array members, improved back up for IEEE 754 floating point, support for variadic macros (macros of variable arity), and back up for one-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 near part backward uniform with C90, but is stricter in some ways; in particular, a declaration that lacks a type specifier no longer has int implicitly causeless. A standard macro __STDC_VERSION__ is divers 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.^[20] ^{[ needs update ]}

In addition, support for Unicode identifiers (variable / function names) in the form of escaped characters (due east.one thousand. \U0001f431) is now required. Back up for raw Unicode names is optional.

C11 [edit]

In 2007, piece of 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, bearding structures, improved Unicode back up, diminutive operations, multi-threading, and bounds-checked functions. It also makes some portions of the existing C99 library optional, and improves compatibility with C++. The standard macro __STDC_VERSION__ is defined as 201112L to indicate that C11 back up is bachelor.

C17 [edit]

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

C2x [edit]

C2x is an breezy name for the next (later on C17) major C language standard revision. Information technology is expected to exist voted on in 2023 and would therefore be called C23.^[21] ^{[ better source needed ]}

Embedded C [edit]

Historically, embedded C programming requires nonstandard extensions to the C language in guild to support exotic features such as fixed-bespeak arithmetic, multiple distinct memory banks, and basic I/O operations.

In 2008, the C Standards Commission published a technical report extending the C linguistic communication^[22] to address these problems past providing a common standard for all implementations to adhere to. It includes a number of features not available in normal C, such equally fixed-point 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 not significant in C; however, line boundaries practise have significance during the preprocessing stage. Comments may announced either betwixt the delimiters /* and */, or (since C99) following // until the end of the line. Comments delimited by /* and */ do not nest, and these sequences of characters are not interpreted as comment delimiters if they appear inside cord or graphic symbol literals.^[24]

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

As an imperative linguistic communication, C uses statements to specify actions. The about mutual statement is an expression statement, consisting of an expression to be evaluated, followed past a semicolon; as a side effect of the evaluation, functions may be chosen and variables may be assigned new values. To change the normal sequential execution of statements, C provides several control-period statements identified by reserved keywords. Structured programming is supported past if … [else] conditional execution and by practise … while, while, and for iterative execution (looping). The for statement has split up initialization, testing, and reinitialization expressions, whatever or all of which can exist omitted. break and continue tin be used to leave the innermost enclosing loop argument or skip to its reinitialization. At that place is also a not-structured goto argument which branches directly to the designated characterization inside the function. switch selects a instance to be executed based on the value of an integer expression.

Expressions can apply a variety of built-in operators and may comprise function calls. The order in which arguments to functions and operands to most operators are evaluated is unspecified. The evaluations may even exist interleaved. However, all side furnishings (including storage to variables) will occur before the side by side "sequence point"; sequence points include the end of each expression statement, and the entry to and render from each function call. Sequence points also occur during evaluation of expressions containing certain operators (&&, ||, ?: and the comma operator). This permits a high degree of object code optimization by the compiler, just requires C programmers to have 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 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 non try to correct many of these blemishes, because of the impact of such changes on already existing software.

Grapheme set [edit]

The basic C source graphic symbol gear up includes the following characters:

Lowercase and majuscule 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 demand not correspond to an actual single grapheme, although for convenience C treats information technology as i.

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

The basic C execution graphic symbol set contains the same characters, along with representations for alarm, backspace, and carriage render. Run-time back up 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 exist used for any purposes other than those for which they are predefined:

motorcar
break
example
char
const
keep
default
do
double
else
enum
extern
float
for
goto
if
int
long
annals
return
brusque
signed
sizeof
static
struct
switch
typedef
union
unsigned
void
volatile
while

C99 reserved five more words:

_Bool
_Complex
_Imaginary
inline
restrict

C11 reserved seven more words:^[26]

_Alignas
_Alignof
_Atomic
_Generic
_Noreturn
_Static_assert
_Thread_local

About of the recently reserved words begin with an underscore followed by a capital letter, because identifiers of that form were previously reserved past the C standard for use only by implementations. Since existing program source lawmaking should not accept been using these identifiers, it would not exist affected when C implementations started supporting these extensions to the programming linguistic communication. Some standard headers do define more than convenient synonyms for underscored identifiers. The language previously included a reserved word called entry, only this was seldom implemented, and has now been removed equally a reserved word.^[27]

Operators [edit]

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

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

C uses the operator = (used in mathematics to express equality) to signal consignment, following the precedent of Fortran and PL/I, but unlike ALGOL and its derivatives. C uses the operator == to test for equality. The similarity between these two operators (assignment and equality) may result in the accidental use of one in place of the other, and in many cases, the fault does not produce an error bulletin (although some compilers produce warnings). For example, the conditional expression if (a == b + ane) might mistakenly be written equally if (a = b + i), which will be evaluated as true if a is not nada afterward the assignment.^[28]

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

"Hello, world" case [edit]

The "howdy, world" case, which appeared in the beginning edition of 1000&R, has get the model for an introductory programme in most programming textbooks. The program prints "hello, world" to the standard output, which is usually a concluding or screen display.

The original version was:^[30]

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

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

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

The first line of the program contains a preprocessing directive, indicated past #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 every bit 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, every bit opposed to double quotes which typically include local or project-specific header files.

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

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

The adjacent line calls (diverts execution to) a function named printf, which in this case is supplied from a arrangement library. In this call, the printf function is passed (provided with) a unmarried argument, the accost of the first character in the string literal "hi, world\n". The string literal is an unnamed array with elements of type char, fix up automatically by the compiler with a final 0-valued character to marker the terminate of the array (printf needs to know this). The \north is an escape sequence that C translates to a newline character, which on output signifies the terminate of the current line. The return value of the printf function is of type int, but it is silently discarded since information technology is non used. (A more conscientious program might test the return value to decide whether or non the printf function succeeded.) The semicolon ; terminates the statement.

The closing curly brace indicates the terminate of the code for the main role. Co-ordinate to the C99 specification and newer, the main role, unlike any other function, will implicitly return a value of 0 upon reaching the } that terminates the function. (Formerly an explicit return 0; argument was required.) This is interpreted by the run-time arrangement every bit an leave lawmaking indicating successful execution.^[31]

Data types [edit]

The type system in C is static and weakly typed, which makes it similar to the type system of ALGOL descendants such as Pascal.^[32] There are congenital-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 (matrimony).

C is often used in low-level systems programming where escapes from the type system may exist necessary. The compiler attempts to ensure blazon correctness of near expressions, but the developer can override the checks in various ways, either by using a blazon cast to explicitly convert a value from one type to another, or by using pointers or unions to reinterpret the underlying bits of a data object in another way.

Some notice C'south announcement syntax unintuitive, especially for role pointers. (Ritchie's thought was to declare identifiers in contexts resembling their apply: "declaration reflects apply".)^[33]

C's usual arithmetic conversions allow for efficient code to exist generated, merely 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 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 address or location of an object or function in memory. Pointers can be dereferenced to access data stored at the address pointed to, or to invoke a pointed-to role. Pointers tin be manipulated using consignment or pointer arithmetic. The run-fourth dimension representation of a pointer value is typically a raw retentivity address (perhaps augmented past an offset-within-word field), but since a pointer's type includes the type of the thing pointed to, expressions including pointers tin can be 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 retention allocation is performed using pointers. Many data types, such as trees, are commonly implemented as dynamically allocated struct objects linked together using pointers. Pointers to functions are useful for passing functions as arguments to higher-order functions (such equally qsort or bsearch) or as callbacks to be invoked past consequence handlers.^[31]

A cipher pointer value explicitly points to no valid location. Dereferencing a nix pointer value is undefined, oftentimes resulting in a segmentation error. Null arrow values are useful for indicating special cases such as no "next" pointer in the concluding node of a linked list, or as an fault indication from functions returning pointers. In advisable contexts in source code, such as for assigning to a pointer variable, a null pointer constant can be written equally 0, with or without explicit casting to a pointer type, or as the NULL macro defined by several standard headers. In provisional contexts, cipher pointer values evaluate to simulated, 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 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 any other object pointer type.^[31]

Careless employ of pointers is potentially dangerous. Because they are typically unchecked, a pointer variable can exist made to point to whatsoever capricious location, which can cause undesirable furnishings. Although properly used pointers point to safe places, they can be made to signal to unsafe places past using invalid pointer arithmetic; the objects they point to may keep to be used after deallocation (dangling pointers); they may exist used without having been initialized (wild pointers); or they may be directly assigned an unsafe value using a cast, union, or through another corrupt pointer. In general, C is permissive in assuasive manipulation of and conversion betwixt pointer types, although compilers typically provide options for various levels of checking. Some other programming languages address these bug by using more restrictive reference types.

Arrays [edit]

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

Since arrays are always accessed (in issue) via pointers, array accesses are typically non checked against the underlying assortment size, although some compilers may provide bounds checking as an option.^[34] ^[35] Assortment premises violations are therefore possible and can lead to various repercussions, including illegal retentiveness accesses, abuse of information, buffer overruns, and run-fourth dimension exceptions.

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

The following example using mod C (C99 or later) shows allocation of a two-dimensional assortment on the heap and the use of multi-dimensional array indexing for accesses (which can utilise bounds-checking on many C compilers):

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

Array–pointer interchangeability [edit]

The subscript note x[i] (where 10 designates a pointer) is syntactic saccharide for *(x+i).^[36] Taking reward of the compiler's knowledge of the pointer type, the address that x + i points to is not the base accost (pointed to past x) incremented by i bytes, but rather is defined to exist the base of operations accost incremented by i multiplied by the size of an element that 10 points to. Thus, x[i] designates the i+1thursday chemical element of the array.

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

The full size of an array 10 tin be determined by applying sizeof to an expression of array blazon. The size of an chemical element can be 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]. Annotation, that if just a pointer to the first element is available as information technology is oftentimes the case in C code considering of the automatic conversion described higher up, the information most the full type of the assortment and its length are lost.

Memory management [edit]

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

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

These three approaches are advisable in different situations and have various trade-offs. For example, static retentivity allotment has piffling allocation overhead, automatic allotment may involve slightly more overhead, and dynamic retentivity allocation can potentially have a great bargain of overhead for both resource allotment and deallocation. The persistent nature of static objects is useful for maintaining country information across function calls, automated allocation is easy to use just stack space is typically much more express and transient than either static memory or heap space, and dynamic retentiveness allocation allows user-friendly allocation of objects whose size is known only at run-fourth dimension. Nigh C programs make extensive apply of all three.

Where possible, automated or static allocation is usually simplest considering the storage is managed by the compiler, freeing the programmer of the potentially error-prone 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 resource allotment is necessary.^[31] Prior to the C99 standard, variable-sized arrays were a common example of this. (See the article on malloc for an example of dynamically allocated arrays.) Different automatic resource allotment, which tin can neglect at run time with uncontrolled consequences, the dynamic allotment functions render an indication (in the form of a null arrow value) when the required storage cannot exist allocated. (Static allocation that is as well large is usually detected by the linker or loader, before the plan can even begin execution.)

Unless otherwise specified, static objects contain nil or null pointer values upon programme startup. Automatically and dynamically allocated objects are initialized only if an initial value is explicitly specified; otherwise they initially accept indeterminate values (typically, whatever flake blueprint happens to exist present in the storage, which might not even correspond a valid value for that type). If the program attempts to admission an uninitialized value, the results are undefined. Many modern compilers try to detect and warn most this problem, but both false positives and simulated negatives can occur.

Heap memory allocation has to be synchronized with its actual usage in any plan to be reused equally much every bit possible. For instance, if the only arrow to a heap memory allotment 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 essentially lost to the programme, a phenomenon known every bit a retention leak. Conversely, it is possible for memory to be freed, but is referenced subsequently, leading to unpredictable results. Typically, the failure symptoms appear in a portion of the program unrelated to the lawmaking that causes the error, making it hard to diagnose the failure. Such issues are ameliorated in languages with automatic garbage collection.

Libraries [edit]

The C programming language uses libraries as its main method of extension. In C, a library is a set of functions independent within a unmarried "archive" file. Each library typically has a header file, which contains the prototypes of the functions contained within the library that may be used by a program, and declarations of special data types and macro symbols used with these functions. In gild for a programme to use a library, information technology must include the library'due south header file, and the library must be linked with the program, which in many cases requires compiler flags (e.chiliad., -lm, shorthand for "link the math library").^[31]

The near common C library is the C standard library, which is specified by the ISO and ANSI C standards and comes with every C implementation (implementations which target limited environments such equally embedded systems may provide just a subset of the standard library). This library supports stream input and output, memory resource allotment, mathematics, character strings, and time values. Several divide standard headers (for instance, stdio.h) specify the interfaces for these and other standard library facilities.

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

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

File handling and streams [edit]

File input and output (I/O) is not part of the C language itself but instead is handled by libraries (such as the C standard library) and their associated header files (e.g. stdio.h). File handling is by and large implemented through loftier-level I/O which works through streams. A stream is from this perspective a data flow that is independent of devices, while a file is a concrete device. The high-level I/O is done through the clan of a stream to a file. In the C standard library, a buffer (a retentivity surface area or queue) is temporarily used to store information before information technology's sent to the final destination. This reduces the time spent waiting for slower devices, for example a difficult bulldoze or solid state drive. Low-level I/O functions are not part of the standard C library^{[ clarification needed ]} but are more often than not part of "bare metal" programming (programming that's independent of any operating system such as nearly embedded programming). With few exceptions, implementations include depression-level I/O.

Language tools [edit]

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

Automated source lawmaking checking and auditing are beneficial in any language, and for C many such tools be, such as Lint. A common do is to use Lint to discover questionable code when a program is first written. One time a plan passes Lint, it is and so compiled using the C compiler. Also, many compilers tin can optionally warn about syntactically valid constructs that are probable to really be errors. MISRA C is a proprietary fix of guidelines to avoid such questionable lawmaking, developed for embedded systems.^[37]

There are likewise compilers, libraries, and operating system level mechanisms for performing actions that are non a standard part of C, such as bounds checking for arrays, detection of buffer overflow, serialization, dynamic retentiveness tracking, and automatic garbage collection.

Tools such as Purify or Valgrind and linking with libraries containing special versions of the memory resource allotment functions can help uncover runtime errors in retentivity 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, can be used for virtually purposes, yet when needed, system-specific code can be used to admission specific hardware addresses and to perform blazon punning to friction match externally imposed interface requirements, with a low run-fourth dimension need on system resource.

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

A result of C's broad availability and efficiency is that compilers, libraries and interpreters of other programming languages are ofttimes implemented in C. For example, the reference implementations of Python, Perl, Ruby-red, and PHP are written in C.

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

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

C has also been widely used to implement end-user applications. However, such applications can 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 as C#, D, Go, Java, 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 argument and (more or less recognizably) expression syntax of C with blazon systems, information models, and/or big-scale programme structures that differ from those of C, sometimes radically.

Several C or about-C interpreters exist, including Ch and CINT, which tin can also exist used for scripting.

When object-oriented programming languages became popular, C++ and Objective-C were 2 dissimilar extensions of C that provided object-oriented capabilities. Both languages were originally implemented equally source-to-source compilers; source code was translated into C, and and so compiled with a C compiler.^[43]

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

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

In add-on to C++ and Objective-C, Ch, Cilk, and Unified Parallel C are well-nigh supersets of C.

See besides [edit]

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

Notes [edit]

^ The original example code will compile on most modern compilers that are not in strict standard compliance mode, but information technology does not fully conform to the requirements of either C89 or C99. In fact, C99 requires that a diagnostic bulletin be produced.
^ The main office actually has ii arguments, int argc and char *argv[], respectively, which tin can exist used to handle control line arguments. The ISO C standard (section 5.ane.ii.2.i) requires both forms of main to be supported, which is special treatment not afforded to any other part.

References [edit]

^ ^a ^b Kernighan, Brian W.; Ritchie, Dennis M. (February 1978). The C Programming Language (1st ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-110163-0.
^ Ritchie (1993): "Thompson had made a brief attempt to produce a system coded in an early version of C—before structures—in 1972, but gave up the endeavor."
^ Fruderica (Dec 13, 2020). "History of C". The cppreference.com. Archived from the original on October 24, 2020. Retrieved October 24, 2020.
^ Ritchie (1993): "The scheme of type composition adopted past C owes considerable debt to Algol 68, although it did not, perhaps, emerge in a grade that Algol'southward adherents would corroborate of."
^ Ring Team (October 23, 2021). "The Ring programming language and other languages". ring-lang.net.
^ ^a ^b "Verilog HDL (and C)" (PDF). The Research Schoolhouse of Computer Science at the Australian National University. June 3, 2010. Archived from the original (PDF) on November 6, 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 January 16, 2009.
^ "TIOBE Programming Community Index". 2009. Archived from the original on May iv, 2009. Retrieved May six, 2009.
^ ^a ^b "History of C". en.cppreference.com. Archived from the original on May 29, 2018. Retrieved May 28, 2018.
^ "TIOBE Index for Oct 2021". Retrieved October vii, 2021.
^ Ritchie, Dennis. "BCPL to B to C". Archived from the original on Dec 12, 2019. Retrieved September 10, 2019.
^ ^a ^b Johnson, South. C.; Ritchie, D. M. (1978). "Portability of C Programs and the UNIX System". Bell System Tech. J. 57 (6): 2021–2048. CiteSeerX10.ane.one.138.35. doi:10.1002/j.1538-7305.1978.tb02141.x. S2CID 17510065. (Note: The PDF is an OCR scan of the original, and contains a rendering of "IBM 370" as "IBM 310".)
^ McIlroy, Chiliad. D. (1987). A Research Unix reader: annotated excerpts from the Developer's Manual, 1971–1986 (PDF) (Technical report). CSTR. Bell Labs. p. 10. 139. Archived (PDF) from the original on November eleven, 2017. Retrieved February 1, 2015.
^ "C manual pages". FreeBSD Miscellaneous Data Transmission (FreeBSD 13.0 ed.). May thirty, 2011. Archived from the original on January 21, 2021. Retrieved January xv, 2021. [1] Archived January 21, 2021, at the Wayback Machine
^ Kernighan, Brian West.; Ritchie, Dennis M. (March 1988). The C Programming Language (2nd ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-110362-7.
^ Stroustrup, Bjarne (2002). Sibling rivalry: C and C++ (PDF) (Written report). AT&T Labs. Archived (PDF) from the original on Baronial 24, 2014. Retrieved April 14, 2014.
^ C Integrity. International Organisation for Standardization. March 30, 1995. Archived from the original on July 25, 2018. Retrieved July 24, 2018.
^ "JTC1/SC22/WG14 – C". Home page. ISO/IEC. Archived from the original on February 12, 2018. Retrieved June 2, 2011.
^ Andrew Binstock (October 12, 2011). "Interview with Herb Sutter". Dr. Dobbs. Archived from the original on Baronial ii, 2013. Retrieved September 7, 2013.
^ "Revised C23 Schedule WG 14 N 2759" (PDF). www.open up-std.org. Archived (PDF) from the original on June 24, 2021. Retrieved October x, 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 50. (2002). C: A Reference Transmission (5th ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-089592-nine. Contains a BNF grammar for C.
^ Kernighan & Ritchie (1996), p. 192.
^ Kernighan & Ritchie (1978), p. iii.
^ "ISO/IEC 9899:201x (ISO C11) Committee Draft" (PDF). Archived (PDF) from the original on December 22, 2017. Retrieved September xvi, 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 (third ed.). Otsego, MI: PageFree Publishing Inc. p. 20. ISBN978-one-58961-237-2. Archived from the original on July 29, 2020. Retrieved February 10, 2012.
^ Kernighan & Ritchie (1978), p. 6.
^ ^a ^b ^c ^d ^e ^f ^g Klemens, Ben (2013). 21st Century C. O'Reilly Media. ISBN978-1-4493-2714-9.
^ Feuer, Alan R.; Gehani, Narain H. (March 1982). "Comparison of the Programming Languages C and Pascal". ACM Calculating Surveys. fourteen (1): 73–92. doi:x.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 7, 2007. Retrieved August 5, 2012.
^ เอี่ยมสิริวงศ์, โอภาศ (2016). Programming with C. Bangkok, Thailand: SE-Didactics PUBLIC Visitor LIMITED. pp. 225–230. ISBN978-616-08-2740-four.
^ Raymond, Eric S. (October xi, 1996). The New Hacker'due south Dictionary (tertiary ed.). MIT Press. p. 432. ISBN978-0-262-68092-9. Archived from the original on November 12, 2012. Retrieved Baronial five, 2012.
^ "Man Page for lint (freebsd Section 1)". unix.com. May 24, 2001. Retrieved July 15, 2014.
^ Dale, Nell B.; Weems, Flake (2014). Programming and problem solving with C++ (sixth ed.). Burlington, MA: Jones & Bartlett Learning. ISBN978-1449694289. OCLC 894992484.
^ Dr. Dobb'south Sourcebook. U.S.A.: Miller Freeman, Inc. November–December 1995.
^ "Using C for CGI Programming". linuxjournal.com. March 1, 2005. Archived from the original on February thirteen, 2010. Retrieved January 4, 2010.
^ McMillan, Robert (August 1, 2013). "Is Java Losing Its Mojo?". Wired. Archived from the original on February xv, 2017. Retrieved March five, 2017.
^ O'Regan, Gerard (September 24, 2015). Pillars of computing : a compendium of select, pivotal engineering science firms. ISBN978-3319214641. OCLC 922324121.
^ Rauchwerger, Lawrence (2004). Languages and compilers for parallel computing : 16th international workshop, LCPC 2003, College Station, TX, United states of america, October 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 G. (1993). "The Development of the C Language". The Second ACM SIGPLAN Conference on History of Programming Languages (HOPL-II). ACM. pp. 201–208. doi:10.1145/154766.155580. ISBN0-89791-570-4 . Retrieved November 4, 2014.
Kernighan, Brian W.; Ritchie, Dennis Chiliad. (1996). The C Programming Language (2d ed.). Prentice Hall. ISBN7-302-02412-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). (three.61 MB)
comp.lang.c Frequently Asked Questions
A History of C, by Dennis Ritchie