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General-purpose programming linguistic communication

C
Major implementations
The C Programming Language ^[1] (often referred to as K&R), the seminal book on C
Paradigm	Multi-paradigm: imperative (procedural), structured
Designed by	Dennis Ritchie
Developer	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-eighteen) ^[3]

Typing subject area	Static, weak, manifest, nominal
Bone	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, Split-C, Cilk, C*
Influenced by
B (BCPL, CPL), ALGOL 68,^[4] assembly, PL/I, FORTRAN
Influenced
Numerous: AMPL, AWK, csh, C++, C--, C#, Objective-C, D, Go, Java, JavaScript, JS++, Julia, Limbo, LPC, Perl, PHP, Pike, Processing, Python, Band,^[5]Rust, Seed7, Vala, Verilog (HDL),^[six] Nim, Zig
C Programming at Wikibooks

C (, as in the letter of the alphabet 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 constitute lasting use in applications previously coded in assembly language. Such applications include operating systems and various application software for calculator architectures that range from supercomputers to PLCs and embedded systems.

A successor to the programming linguistic communication B, C was originally adult at Bell Labs past Dennis Ritchie between 1972 and 1973 to construct utilities running on Unix. Information technology was applied to re-implementing the kernel of the Unix operating system.^[7] During the 1980s, C gradually gained popularity. Information technology has become one of the most widely used programming languages,^[8] ^[9] with C compilers from various vendors available for the majority of existing computer architectures and operating systems. C has been standardized past ANSI since 1989 (ANSI C) and by the International Arrangement for Standardization (ISO).

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

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

Overview [edit]

Like almost procedural languages in the ALGOL tradition, C has facilities for structured programming and allows lexical variable telescopic and recursion. Its static type system prevents unintended operations. In C, all executable code is contained inside subroutines (too called "functions", though not strictly in the sense of functional programming). Function parameters are always passed by value (except arrays). Laissez passer-past-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 language also exhibits the following characteristics:

The linguistic communication has a small, fixed number of keywords, including a full set of control flow primitives: if/else, for, do/while, while, and switch. User-defined names are not distinguished from keywords by any kind of sigil.
Information technology has a large number of arithmetics, bitwise, and logic operators: +,+=,++,&,||, etc.
More than one consignment may be performed in a single argument.
Functions:
- Function return values can be ignored, when not needed.
- Function and data pointers permit advertisement hoc run-time polymorphism.
- Functions may non be defined within the lexical telescopic of other functions.
Data typing is static, but 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 beginning with the proper name of a type is taken as a declaration. There is no "function" keyword; instead, a function is indicated by the presence of a parenthesized argument list.
User-defined (typedef) and chemical compound types are possible.
- Heterogeneous aggregate data types (struct) allow related information elements to be accessed and assigned as a unit of measurement.
- Union is a structure with overlapping members; just the last fellow member stored is valid.
- Array indexing is a secondary note, defined in terms of pointer arithmetic. Unlike structs, arrays are not kickoff-grade objects: they cannot be assigned or compared using single built-in operators. There is no "array" keyword in use or definition; instead, square brackets indicate arrays syntactically, for case month[11].
- Enumerated types are possible with the enum keyword. They are freely interconvertible with integers.
- Strings are not a distinct data type, just are conventionally implemented every bit null-terminated character arrays.
Depression-level access to computer memory is possible by converting automobile addresses to typed pointers.
Procedures (subroutines non returning values) are a special instance of part, with an untyped return blazon void.
A preprocessor performs macro definition, source code file inclusion, and conditional compilation.
There is a basic form of modularity: files can be compiled separately and linked together, with control over which functions and data objects are visible to other files via static and extern attributes.
Circuitous functionality such as I/O, string manipulation, and mathematical functions are consistently delegated to library routines.

While C does not include certain features constitute in other languages (such as object orientation and garbage collection), these can exist implemented or emulated, frequently through the use of external libraries (e.thousand., the GLib Object System or the Boehm garbage collector).

Relations to other languages [edit]

Many later languages have borrowed directly or indirectly from C, including C++, C#, Unix's C shell, D, Go, Java, JavaScript (including transpilers), Julia, Limbo, LPC, Objective-C, Perl, PHP, Python, Red, Rust, Swift, Verilog and SystemVerilog (hardware description languages).^[6] These languages accept fatigued many of their command structures and other bones features from C. About of them (Python being a dramatic exception) likewise express highly like syntax to C, and they tend to combine the recognizable expression and argument syntax of C with underlying type systems, data models, and semantics that can be radically different.

History [edit]

Early developments [edit]

Timeline of language development
Year	C Standard^[10]
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 evolution of the Unix operating organization, originally implemented in assembly linguistic communication on a PDP-7 by Dennis Ritchie and Ken Thompson, incorporating several ideas from colleagues. Eventually, they decided to port the operating system to a PDP-xi. The original PDP-11 version of Unix was too adult in associates linguistic communication.^[7]

Thompson desired a programming linguistic communication to make utilities for the new platform. At first, he tried to brand a Fortran compiler, but soon gave up the idea. Instead, he created a cutting-downwardly version of the recently developed BCPL systems programming linguistic communication. 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 besides wearisome, and B could non take advantage of PDP-11 features such as byte addressability.

In 1972, Ritchie started to better B, about notably adding data typing for variables, which resulted in creating a new language C.^[13] The C compiler and some utilities made with it were included in Version ii Unix.^[14]

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

The preprocessor was introduced effectually 1973 at the urging of Alan Snyder and as well in recognition of the usefulness of the file-inclusion mechanisms available in BCPL and PL/I. Its original version provided just included files and simple string replacements: #include and #define of parameterless macros. Soon after that, it was extended, more often than not by Mike Lesk and and then by John Reiser, to incorporate macros with arguments and conditional compilation.^[7]

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

K&R C [edit]

In 1978, Brian Kernighan and Dennis Ritchie published the outset edition of The C Programming Linguistic communication.^[1] This book, known to C programmers equally 1000&R, served for many years as an informal specification of the language. The version of C that it describes is commonly referred to as "K&R C". As this was released in 1978, it is besides referred to as C78.^[15] The second edition of the book^[16] covers the after ANSI C standard, described below.

G&R introduced several linguistic communication features:

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

Fifty-fifty afterward the publication of the 1989 ANSI standard, for many years Thou&R C was all the same considered the "lowest common denominator" to which C programmers restricted themselves when maximum portability was desired, since many older compilers were all the same in use, and because carefully written K&R C lawmaking can be legal Standard C also.

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

For example:

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

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

Since K&R function declarations did non include whatever information about role arguments, function parameter type checks were not performed, although some compilers would issue a warning message if a local function was called with the wrong number of arguments, or if multiple calls to an external part used different numbers or types of arguments. Divide tools such as Unix'southward lint utility were adult that (among other things) could check for consistency of part use across multiple source files.

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

void functions (i.e., functions with no render value)
functions returning struct or union types (rather than pointers)
consignment for struct information 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 fifty-fifty 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 increment 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; yet, the non-portable portion of the Unix C library was handed off to the IEEE working group 1003 to become the basis for the 1988 POSIX standard. In 1989, the C standard was ratified as ANSI X3.159-1989 "Programming Language C". This version of the language is often referred to as ANSI C, Standard C, or sometimes C89.

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

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

1 of the aims of the C standardization process was to produce a superset of K&R C, incorporating many of the after introduced unofficial features. The standards commission too included several additional features such as office prototypes (borrowed from C++), void pointers, back up 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 be permitted, for compatibility with existing source lawmaking.

C89 is supported past electric current C compilers, and most modern C lawmaking is based on it. Any program written only in Standard C and without any hardware-dependent assumptions will run correctly on any platform with a conforming C implementation, within its resources limits. Without such precautions, programs may compile merely on a certain platform or with a particular compiler, due, for example, to the use of non-standard libraries, such equally GUI libraries, or to a reliance on compiler- or platform-specific attributes such every bit the verbal size of information types and byte endianness.

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

Afterwards the ANSI/ISO standardization process, the C language specification remained relatively static for several years. In 1995, Normative Subpoena 1 to the 1990 C standard (ISO/IEC 9899/AMD1:1995, known informally as C95) was published, to correct some details and to add more extensive support for international character sets.^[eighteen]

C99 [edit]

The C standard was further revised in the tardily 1990s, leading to the publication of ISO/IEC 9899:1999 in 1999, which is commonly referred to every bit "C99". Information technology 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 type to correspond circuitous numbers), variable-length arrays and flexible array members, improved support for IEEE 754 floating point, support for variadic macros (macros of variable arity), and support for one-line comments offset with //, as in BCPL or C++. Many of these had already been implemented every bit extensions in several C compilers.

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

In addition, support for Unicode identifiers (variable / function names) in the form of escaped characters (e.g. \U0001f431) is now required. Back up 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 non been tested by existing implementations.

The C11 standard adds numerous new features to C and the library, including type generic macros, anonymous structures, improved Unicode support, diminutive operations, multi-threading, and 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 every bit 201112L to bespeak that C11 back up is bachelor.

C17 [edit]

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

C2x [edit]

C2x is an informal name for the adjacent (after 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 support exotic features such as fixed-point arithmetic, multiple distinct retentivity banks, and basic I/O operations.

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

Syntax [edit]

C has a formal grammer specified by the C standard.^[23] Line endings are generally non meaning in C; however, line boundaries do have significance during the preprocessing phase. Comments may appear either between the delimiters /* and */, or (since C99) following // until the finish of the line. Comments delimited by /* and */ practice not nest, and these sequences of characters are non interpreted as annotate delimiters if they appear inside string or character literals.^[24]

C source files comprise declarations and office definitions. Function definitions, in turn, comprise declarations and statements. Declarations either define new types using keywords such as struct, union, and enum, or assign types to and peradventure reserve storage for new variables, unremarkably by writing the type followed past the variable name. Keywords such every bit char and int specify built-in types. Sections of code are enclosed in braces ({ and }, sometimes called "curly brackets") to limit the scope of declarations and to deed as a unmarried argument for command structures.

As an imperative linguistic communication, C uses statements to specify deportment. The most common statement is an expression statement, consisting of an expression to be evaluated, followed by a semicolon; as a side effect of the evaluation, functions may exist called and variables may exist assigned new values. To modify the normal sequential execution of statements, C provides several control-flow statements identified by reserved keywords. Structured programming is supported by if … [else] conditional execution and by practise … while, while, and for iterative execution (looping). The for statement has separate initialization, testing, and reinitialization expressions, any or all of which can be omitted. pause and continue tin can be used to leave the innermost enclosing loop statement or skip to its reinitialization. There is also a non-structured goto argument which branches directly to the designated label within the function. switch selects a instance to exist executed based on the value of an integer expression.

Expressions can use a variety of born operators and may contain function calls. The order in which arguments to functions and operands to most operators are evaluated is unspecified. The evaluations may even exist interleaved. However, all side effects (including storage to variables) volition occur before the side by side "sequence point"; sequence points include the end of each expression statement, and the entry to and return from each function call. Sequence points also occur during evaluation of expressions containing certain operators (&&, ||, ?: and the comma operator). This permits a loftier degree of object lawmaking optimization past the compiler, but requires C programmers to accept more intendance to obtain reliable results than is needed for other programming languages.

Kernighan and Ritchie say in the Introduction of The C Programming Language: "C, like any other language, has its blemishes. Some of the operators have the incorrect precedence; some parts of the syntax could be improve."^[25] The C standard did non attempt to correct many of these blemishes, because of the impact of such changes on already existing software.

Character fix [edit]

The basic C source character set includes the following characters:

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

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

Boosted multi-byte encoded characters may be used in string literals, but they are non 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 X denotes a hexadecimal character), although this feature is not yet widely implemented.

The basic C execution grapheme set contains the aforementioned characters, along with representations for warning, backspace, and carriage return. Run-time support for extended grapheme 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 whatsoever purposes other than those for which they are predefined:

motorcar
intermission
case
char
const
continue
default
do
double
else
enum
extern
float
for
goto
if
int
long
register
return
short
signed
sizeof
static
struct
switch
typedef
matrimony
unsigned
void
volatile
while

C99 reserved 5 more words:

_Bool
_Complex
_Imaginary
inline
restrict

C11 reserved seven more than 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, considering identifiers of that form were previously reserved by the C standard for employ merely by implementations. Since existing program source code should non have been using these identifiers, it would non be afflicted when C implementations started supporting these extensions to the programming language. Some standard headers do ascertain more user-friendly synonyms for underscored identifiers. The language previously included a reserved word chosen entry, but this was seldom implemented, and has now been removed as a reserved word.^[27]

Operators [edit]

C supports a rich 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:

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

C uses the operator = (used in mathematics to express equality) to indicate consignment, following the precedent of Fortran and PL/I, just unlike ALGOL and its derivatives. C uses the operator == to examination for equality. The similarity between these two operators (consignment and equality) may result in the accidental utilize of one in place of the other, and in many cases, the mistake does not produce an error message (although some compilers produce warnings). For example, the provisional expression if (a == b + 1) might mistakenly be written as if (a = b + 1), which volition be evaluated as truthful if a is not zip 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 every bit x & 1 == 0, which must be written as (x & 1) == 0 if that is the coder's intent.^[29]

"Hello, world" example [edit]

The "hi, earth" instance, which appeared in the showtime edition of K&R, has become the model for an introductory programme in nigh programming textbooks. The program prints "hello, world" to the standard output, which is usually a concluding or screen display.

The original version was:^[30]

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

A standard-befitting "hello, world" plan is:^[a]

                        #include                                    <stdio.h>                        int                                    main            (            void            )                        {                                                printf            (            "howdy, world            \northward            "            );                        }

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

The next line indicates that a part named main is being divers. The main role serves a special purpose in C programs; the run-fourth dimension environment calls the main function to brainstorm program execution. The blazon specifier int indicates that the value that is returned to the invoker (in this case the run-time surroundings) equally a result of evaluating the main function, is an integer. The keyword void equally a parameter listing indicates that this function takes no arguments.^[b]

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

The side by side line calls (diverts execution to) a office named printf, which in this example is supplied from a system library. In this call, the printf function is passed (provided with) a unmarried argument, the address of the first graphic symbol in the string literal "hello, world\n". The string literal is an unnamed assortment with elements of type char, fix automatically by the compiler with a last 0-valued graphic symbol to mark the end of the assortment (printf needs to know this). The \n is an escape sequence that C translates to a newline character, which on output signifies the end of the current line. The return value of the printf role is of type int, merely it is silently discarded since it is not used. (A more than careful program might exam the return value to determine whether or not the printf function succeeded.) The semicolon ; terminates the argument.

The endmost curly caryatid indicates the end of the code for the chief function. According to the C99 specification and newer, the master function, different any other office, 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-fourth dimension system as an exit code indicating successful execution.^[31]

Information types [edit]

The blazon organisation in C is static and weakly typed, which makes information technology similar to the type arrangement of ALGOL descendants such as Pascal.^[32] There are congenital-in types for integers of various sizes, both signed and unsigned, floating-betoken numbers, and enumerated types (enum). Integer blazon char is often used for single-byte characters. C99 added a boolean datatype. There are also derived types including arrays, pointers, records (struct), and unions (union).

C is often used in low-level systems programming where escapes from the type system may be necessary. The compiler attempts to ensure type correctness of virtually expressions, only the programmer can override the checks in diverse means, either past 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 data object in some other style.

Some discover C's declaration syntax unintuitive, particularly for role pointers. (Ritchie'southward idea was to declare identifiers in contexts resembling their apply: "declaration reflects utilize".)^[33]

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

Pointers [edit]

C supports the use of pointers, a type of reference that records the accost or location of an object or function in memory. Pointers tin be dereferenced to access data stored at the accost pointed to, or to invoke a pointed-to role. Pointers can exist manipulated using consignment or pointer arithmetic. The run-time representation of a pointer value is typically a raw retention accost (mayhap augmented by an offset-inside-give-and-take field), but since a arrow's blazon includes the type of the affair pointed to, expressions including pointers tin can be type-checked at compile time. Arrow arithmetic is automatically scaled by the size of the pointed-to data type. Pointers are used for many purposes in C. Text strings are unremarkably manipulated using pointers into arrays of characters. Dynamic retentiveness allocation is performed using pointers. Many data types, such as copse, are commonly implemented as dynamically allocated struct objects linked together using pointers. Pointers to functions are useful for passing functions equally arguments to college-social club functions (such as qsort or bsearch) or as callbacks to be invoked by consequence handlers.^[31]

A nothing pointer value explicitly points to no valid location. Dereferencing a null pointer value is undefined, often resulting in a partition fault. Zero pointer values are useful for indicating special cases such as no "next" pointer in the final node of a linked list, or every bit an error indication from functions returning pointers. In advisable contexts in source code, such as for assigning to a pointer variable, a zilch pointer constant can be written as 0, with or without explicit casting to a pointer type, or equally the Zip macro defined past several standard headers. In conditional contexts, zip pointer values evaluate to false, while all other pointer values evaluate to true.

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

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

Arrays [edit]

Array types in C are traditionally of a stock-still, static size specified at compile time. The more recent C99 standard besides allows a form of variable-length arrays. However, it is also possible to classify a block of retentivity (of arbitrary size) at run-time, using the standard library's malloc function, and treat it equally an array.

Since arrays are e'er accessed (in result) via pointers, array accesses are typically non checked against the underlying array size, although some compilers may provide bounds checking as an pick.^[34] ^[35] Array bounds violations are therefore possible and can lead to various repercussions, including illegal memory accesses, abuse of data, buffer overruns, and run-fourth dimension exceptions.

C does non accept a special provision for declaring multi-dimensional arrays, but rather relies on recursion within the blazon system to declare arrays of arrays, which effectively accomplishes the aforementioned thing. The index values of the resulting "multi-dimensional array" can be thought of as increasing in row-major order. Multi-dimensional arrays are commonly used in numerical algorithms (mainly from applied linear algebra) to shop matrices. The construction of the C assortment is well suited to this particular task. However, in early versions of C the premises of the assortment must be known fixed values or else explicitly passed to any subroutine that requires them, and dynamically sized arrays of arrays cannot exist accessed using double indexing. (A workaround for this was to allocate the assortment with an boosted "row vector" of pointers to the columns.) C99 introduced "variable-length arrays" which address this consequence.

The following example using mod C (C99 or after) shows allocation of a 2-dimensional array on the heap and the utilize of multi-dimensional array indexing for accesses (which can use bounds-checking on many C compilers):

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

Array–arrow interchangeability [edit]

The subscript notation x[i] (where x designates a pointer) is syntactic saccharide for *(ten+i).^[36] Taking advantage of the compiler's knowledge of the pointer blazon, the address that ten + i points to is non the base address (pointed to by x) incremented by i bytes, but rather is divers to be the base address incremented by i multiplied by the size of an element that x points to. Thus, 10[i] designates the i+1thursday chemical element of the array.

Furthermore, in virtually expression contexts (a notable exception is equally operand of sizeof), an expression of assortment type is automatically converted to a arrow to the array'south offset element. This implies that an array is never copied as a whole when named as an statement to a part, but rather only the address of its offset element is passed. Therefore, although part calls in C use pass-past-value semantics, arrays are in result passed by reference.

The total size of an array x can be determined by applying sizeof to an expression of array type. The size of an element can be determined past applying the operator sizeof to whatever dereferenced element of an array A, as in n = sizeof A[0]. This, the number of elements in a declared array A can be determined equally sizeof A / sizeof A[0]. Annotation, that if only a arrow to the showtime element is available as it is oft the case in C code because of the automated conversion described higher up, the information well-nigh the full blazon of the array and its length are lost.

Memory management [edit]

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

Static retentivity allocation: infinite for the object is provided in the binary at compile-fourth dimension; these objects have an extent (or lifetime) every bit long equally 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 after the cake in which they are declared is exited.
Dynamic memory allocation: blocks of memory of arbitrary size can be requested at run-time using library functions such as malloc from a region of memory called the heap; these blocks persist until subsequently freed for reuse past calling the library function realloc or complimentary

These iii approaches are appropriate in different situations and have diverse merchandise-offs. For example, static memory allotment has little resource allotment overhead, automatic allocation may involve slightly more overhead, and dynamic retentivity allocation can potentially have a great deal of overhead for both allocation and deallocation. The persistent nature of static objects is useful for maintaining state information across function calls, automatic allocation is piece of cake to use merely stack infinite is typically much more than limited and transient than either static memory or heap space, and dynamic memory allocation allows convenient allocation of objects whose size is known simply at run-fourth dimension. Almost C programs brand 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-decumbent job of manually allocating and releasing storage. Still, many data structures can change in size at runtime, and since static allocations (and automatic allocations earlier C99) must have a fixed size at compile-time, there are many situations in which dynamic allocation is necessary.^[31] Prior to the C99 standard, variable-sized arrays were a common example of this. (Encounter the commodity on malloc for an case of dynamically allocated arrays.) Unlike automatic resource allotment, which tin neglect at run time with uncontrolled consequences, the dynamic allocation functions return an indication (in the course of a null pointer value) when the required storage cannot be allocated. (Static allocation that is too large is usually detected by the linker or loader, before the plan tin even begin execution.)

Unless otherwise specified, static objects contain zero or nil pointer values upon program startup. Automatically and dynamically allocated objects are initialized simply if an initial value is explicitly specified; otherwise they initially have indeterminate values (typically, whatever chip pattern happens to exist present in the storage, which might not even correspond a valid value for that type). If the program attempts to access an uninitialized value, the results are undefined. Many modern compilers effort to detect and warn nigh this trouble, merely both false positives and false negatives tin can occur.

Heap memory allotment has to exist synchronized with its bodily usage in any program to be reused as much as possible. For case, if the only pointer to a heap memory allocation goes out of telescopic or has its value overwritten before it is deallocated explicitly, then that retention cannot be recovered for subsequently reuse and is essentially lost to the program, a phenomenon known equally a retentivity leak. Conversely, it is possible for memory to be freed, but is referenced subsequently, leading to unpredictable results. Typically, the failure symptoms announced in a portion of the program unrelated to the code that causes the error, making it difficult to diagnose the failure. Such problems are ameliorated in languages with automated garbage collection.

Libraries [edit]

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

The most common C library is the C standard library, which is specified by the ISO and ANSI C standards and comes with every C implementation (implementations which target limited environments such every bit embedded systems may provide simply a subset of the standard library). This library supports stream input and output, retention allocation, mathematics, character strings, and time values. Several separate standard headers (for example, stdio.h) specify the interfaces for these and other standard library facilities.

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

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

File handling and streams [edit]

File input and output (I/O) is non office of the C linguistic communication itself but instead is handled past libraries (such equally the C standard library) and their associated header files (e.1000. stdio.h). File treatment is mostly implemented through loftier-level I/O which works through streams. A stream is from this perspective a information catamenia that is independent of devices, while a file is a concrete device. The loftier-level I/O is done through the association of a stream to a file. In the C standard library, a buffer (a memory area or queue) is temporarily used to store information before information technology's sent to the final destination. This reduces the time spent waiting for slower devices, for instance a hard drive or solid land drive. Low-level I/O functions are non part of the standard C library^{[ clarification needed ]} but are generally part of "blank metal" programming (programming that'southward independent of any operating arrangement such equally most embedded programming). With few exceptions, implementations include low-level I/O.

Linguistic communication tools [edit]

A number of tools have been developed to help C programmers detect 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 whatever language, and for C many such tools exist, such as Lint. A mutual practise is to apply Lint to detect questionable code when a plan is first written. Once a plan passes Lint, it is then compiled using the C compiler. Besides, many compilers can optionally warn about syntactically valid constructs that are likely to really exist errors. MISRA C is a proprietary prepare of guidelines to avert such questionable lawmaking, adult for embedded systems.^[37]

There are also 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 memory tracking, and automated garbage drove.

Tools such every bit Purify or Valgrind and linking with libraries containing special versions of the memory allocation functions can help 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, can be used for most purposes, all the same when needed, system-specific code can be used to admission specific hardware addresses and to perform type punning to match externally imposed interface requirements, with a low run-time demand on system resources.

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

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

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

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

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

[edit]

The TIOBE alphabetize 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, Become, Java, JavaScript, Limbo, LPC, Perl, PHP, Python, and Unix's C shell.^[42] The virtually pervasive influence has been syntactical; all of the languages mentioned combine the statement and (more than or less recognizably) expression syntax of C with type systems, data models, and/or big-calibration programme structures that differ from those of C, sometimes radically.

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

When object-oriented programming languages became popular, C++ and Objective-C were two unlike 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 as an approach 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. About a superset of C, C++ now supports well-nigh of C, with a few exceptions.

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

Run into also [edit]

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

Notes [edit]

^ The original example code volition compile on nigh modern compilers that are not in strict standard compliance manner, but it does non fully conform to the requirements of either C89 or C99. In fact, C99 requires that a diagnostic message be produced.
^ The main function really has ii arguments, int argc and char *argv[], respectively, which can be used to handle command line arguments. The ISO C standard (department 5.1.2.two.1) requires both forms of primary to exist supported, which is special treatment not afforded to any other part.

References [edit]

^ ^a ^b Kernighan, Brian W.; Ritchie, Dennis 1000. (February 1978). The C Programming Language (1st ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-xiii-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, merely gave upwards the effort."
^ Fruderica (December 13, 2020). "History of C". The cppreference.com. Archived from the original on Oct 24, 2020. Retrieved October 24, 2020.
^ Ritchie (1993): "The scheme of type composition adopted past C owes considerable debt to Algol 68, although it did non, possibly, emerge in a class that Algol's adherents would approve of."
^ Ring Team (October 23, 2021). "The Ring programming language and other languages". ring-lang.net.
^ ^a ^b "Verilog HDL (and C)" (PDF). The Inquiry School of Information science at the Australian National University. June 3, 2010. Archived from the original (PDF) on November 6, 2013. Retrieved August 19, 2013. 1980s: ; Verilog start introduced ; Verilog inspired past the C programming language
^ ^a ^b ^c ^d ^e Ritchie (1993)
^ "Programming Language Popularity". 2009. Archived from the original on January sixteen, 2009. Retrieved January 16, 2009.
^ "TIOBE Programming Community Index". 2009. Archived from the original on May 4, 2009. Retrieved May half dozen, 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 Dec 12, 2019. Retrieved September 10, 2019.
^ ^a ^b Johnson, S. C.; Ritchie, D. M. (1978). "Portability of C Programs and the UNIX System". Bell Arrangement Tech. J. 57 (6): 2021–2048. CiteSeerX10.ane.1.138.35. 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, Grand. D. (1987). A Research Unix reader: annotated excerpts from the Programmer'south Manual, 1971–1986 (PDF) (Technical report). CSTR. Bell Labs. p. 10. 139. Archived (PDF) from the original on Nov 11, 2017. Retrieved February 1, 2015.
^ "C manual pages". FreeBSD Miscellaneous Information Manual (FreeBSD xiii.0 ed.). May thirty, 2011. Archived from the original on Jan 21, 2021. Retrieved January 15, 2021. [one] Archived January 21, 2021, at the Wayback Motorcar
^ Kernighan, Brian W.; Ritchie, Dennis One thousand. (March 1988). The C Programming Language (2nd ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-110362-vii.
^ Stroustrup, Bjarne (2002). Sibling rivalry: C and C++ (PDF) (Report). AT&T Labs. Archived (PDF) from the original on Baronial 24, 2014. Retrieved April xiv, 2014.
^ C Integrity. International System for Standardization. March xxx, 1995. Archived from the original on July 25, 2018. Retrieved July 24, 2018.
^ "JTC1/SC22/WG14 – C". Home page. ISO/IEC. Archived from the original on February 12, 2018. Retrieved June two, 2011.
^ Andrew Binstock (Oct 12, 2011). "Interview with Herb Sutter". Dr. Dobbs. Archived from the original on August two, 2013. Retrieved September vii, 2013.
^ "Revised C23 Schedule WG xiv N 2759" (PDF). world wide web.open-std.org. Archived (PDF) from the original on June 24, 2021. Retrieved October 10, 2021.
^ "TR 18037: Embedded C" (PDF). ISO / IEC. Archived (PDF) from the original on 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) 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 (3rd ed.). Otsego, MI: PageFree Publishing Inc. p. xx. ISBN978-1-58961-237-2. Archived from the original on July 29, 2020. Retrieved February x, 2012.
^ Kernighan & Ritchie (1978), p. six.
^ ^a ^b ^c ^d ^e ^f ^thousand 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. fourteen (i): 73–92. doi:10.1145/356869.356872. S2CID 3136859.
^ Kernighan & Ritchie (1996), p. 122.
^ For instance, gcc provides _FORTIFY_SOURCE. "Security Features: Compile Time Buffer Checks (FORTIFY_SOURCE)". fedoraproject.org. Archived from the original on January vii, 2007. Retrieved August 5, 2012.
^ เอี่ยมสิริวงศ์, โอภาศ (2016). Programming with C. Bangkok, Thailand: SE-Didactics PUBLIC COMPANY LIMITED. pp. 225–230. ISBN978-616-08-2740-4.
^ Raymond, Eric S. (Oct eleven, 1996). The New Hacker's Lexicon (3rd ed.). MIT Press. p. 432. ISBN978-0-262-68092-9. Archived from the original on Nov 12, 2012. Retrieved Baronial five, 2012.
^ "Man Page for lint (freebsd Section 1)". unix.com. May 24, 2001. Retrieved July fifteen, 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. U.S.A.: Miller Freeman, Inc. November–December 1995.
^ "Using C for CGI Programming". linuxjournal.com. March 1, 2005. Archived from the original on Feb 13, 2010. Retrieved Jan four, 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 computing : 16th international workshop, LCPC 2003, Higher Station, TX, Usa, October two-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 K. (March 1993). "The Development of the C Linguistic communication". ACM SIGPLAN Notices. ACM. 28 (three): 201–208. doi:ten.1145/155360.155580.
Ritchie, Dennis Yard. (1993). "The Evolution of the C Linguistic communication". The Second ACM SIGPLAN Conference on History of Programming Languages (HOPL-II). ACM. pp. 201–208. doi:ten.1145/154766.155580. ISBN0-89791-570-4 . Retrieved November 4, 2014.
Kernighan, Brian W.; Ritchie, Dennis K. (1996). The C Programming Language (2nd ed.). Prentice Hall. ISBNseven-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, past Dennis Ritchie

moretzleene1965.blogspot.com

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