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General-purpose programming linguistic communication

C
Major implementations
The C Programming Linguistic communication ^[1] (often referred to as K&R), the seminal book on C
Paradigm	Multi-prototype: 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) ^[2]

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

Typing discipline	Static, weak, manifest, nominal
Bone	Cross-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,^[iv] associates, PL/I, FORTRAN
Influenced
Numerous: AMPL, AWK, csh, C++, C--, C#, Objective-C, D, Go, Coffee, JavaScript, JS++, Julia, Limbo, LPC, Perl, PHP, Pike, Processing, Python, Band,^[5]Rust, Seed7, Vala, Verilog (HDL),^[6] 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 organisation. By design, C provides constructs that map efficiently to typical machine instructions. It has found lasting use in applications previously coded in assembly language. Such applications include operating systems and various application software for computer architectures that range from supercomputers to PLCs and embedded systems.

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

C is an imperative procedural language. Information technology was designed to be compiled to provide low-level access to retentivity and language constructs that map efficiently to machine 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 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 acme two languages in the TIOBE index, a mensurate of the popularity of programming languages.^[eleven]

Overview [edit]

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

The C language besides exhibits the following characteristics:

The language has a small, fixed number of keywords, including a full gear up of control catamenia primitives: if/else, for, do/while, while, and switch. User-divers names are not distinguished from keywords by any kind of sigil.
Information technology has a large number of arithmetic, bitwise, and logic operators: +,+=,++,&,||, etc.
More than than one assignment may be performed in a single argument.
Functions:
- Function return values can be ignored, when not needed.
- Function and data pointers permit advert hoc run-fourth dimension polymorphism.
- Functions may not exist defined within the lexical telescopic of other functions.
Data typing is static, but weakly enforced; all data has a type, but implicit conversions are possible.
Declaration syntax mimics usage context. C has no "define" keyword; instead, a statement beginning with the name of a type is taken as a declaration. There is no "function" keyword; instead, a role is indicated by the presence of a parenthesized argument list.
User-defined (typedef) and compound types are possible.
- Heterogeneous aggregate information types (struct) allow related data elements to exist accessed and assigned every bit a unit.
- Matrimony is a structure with overlapping members; merely the concluding member stored is valid.
- Array indexing is a secondary note, defined in terms of pointer arithmetic. Unlike structs, arrays are not outset-form 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 example month[11].
- Enumerated types are possible with the enum keyword. They are freely interconvertible with integers.
- Strings are non a singled-out information type, but are conventionally implemented as zip-terminated character arrays.
Low-level admission to calculator memory is possible by converting motorcar addresses to typed pointers.
Procedures (subroutines not returning values) are a special case of office, with an untyped return blazon void.
A preprocessor performs macro definition, source code file inclusion, and provisional compilation.
At that place is a bones form of modularity: files can be compiled separately and linked together, with command over which functions and data objects are visible to other files via static and extern attributes.
Complex functionality such as I/O, cord manipulation, and mathematical functions are consistently delegated to library routines.

While C does not include certain features found in other languages (such equally object orientation and garbage collection), these tin exist implemented or emulated, frequently through the use of external libraries (e.m., the GLib Object Organization or the Boehm garbage collector).

Relations to other languages [edit]

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

History [edit]

Early developments [edit]

Timeline of language development
Year	C Standard^[x]
1972	Nascency
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 evolution of the Unix operating organization, originally implemented in assembly language on a PDP-seven 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 also developed in assembly linguistic communication.^[7]

Thompson desired a programming linguistic communication to make utilities for the new platform. At outset, he tried to make a Fortran compiler, just presently gave up the idea. Instead, he created a cut-downwards version of the recently developed BCPL systems programming linguistic communication. The official description of BCPL was not bachelor at the time,^[12] and Thompson modified the syntax to be less wordy, producing the similar but somewhat simpler B.^[7] Yet, few utilities were ultimately written in B because it was besides slow, and B could not take advantage of PDP-11 features such equally byte addressability.

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

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

The preprocessor was introduced around 1973 at the urging of Alan Snyder and also in recognition of the usefulness of the file-inclusion mechanisms bachelor in BCPL and PL/I. Its original version provided simply included files and simple string replacements: #include and #ascertain of parameterless macros. Soon after that, it was extended, mostly by Mike Lesk and so by John Reiser, to incorporate macros with arguments and conditional compilation.^[7]

Unix was 1 of the first operating system kernels implemented in a language other than associates. Earlier instances include the Multics organisation (which was written in PL/I) and Master Control Programme (MCP) for the Burroughs B5000 (which was written in ALGOL) in 1961. In effectually 1977, Ritchie and Stephen C. Johnson made farther changes to the linguistic communication to facilitate portability of the Unix operating organisation. Johnson'southward Portable C Compiler served as the basis for several implementations of C on new platforms.^[13]

Grand&R C [edit]

In 1978, Brian Kernighan and Dennis Ritchie published the get-go edition of The C Programming Language.^[i] This book, known to C programmers as Yard&R, served for many years equally 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 likewise referred to equally C78.^[15] The 2nd edition of the book^[16] covers the later ANSI C standard, described below.

Thousand&R introduced several linguistic communication features:

Standard I/O library
long int data type
unsigned int data type
Compound assignment operators of the class =op (such as =-) were inverse to the grade op= (that is, -=) to remove the semantic ambiguity created by constructs such as i=-10, which had been interpreted equally i =- 10 (decrement i by 10) instead of the possibly intended i = -10 (allow i be −10).

Fifty-fifty afterward the publication of the 1989 ANSI standard, for many years Grand&R C was however considered the "lowest common denominator" to which C programmers restricted themselves when maximum portability was desired, since many older compilers were still in use, and because advisedly written K&R C code can be legal Standard C as well.

In early versions of C, only functions that return types other than int must be alleged if used before the office definition; functions used without prior announcement were presumed to return type int.

For example:

                        long                                    some_function            ();                        /* int */                                    other_function            ();                        /* int */                                    calling_function            ()                        {                                                long                                    test1            ;                                                annals                                    /* int */                                    test2            ;                                                test1                                    =                                    some_function            ();                                                if                                    (            test1                                    >                                    1            )                                                test2                                    =                                    0            ;                                                else                                                test2                                    =                                    other_function            ();                                                return                                    test2            ;                        }

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

Since K&R office declarations did not include whatever information nigh office 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. Carve up tools such as Unix's lint utility were developed that (amidst other things) could check for consistency of function use across multiple source files.

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

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

The large number of extensions and lack of agreement on a standard library, together with the linguistic communication 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 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; all the same, the not-portable portion of the Unix C library was handed off to the IEEE working group 1003 to go 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 linguistic communication is oft 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) as ISO/IEC 9899:1990, which is sometimes called C90. Therefore, the terms "C89" and "C90" refer to the aforementioned programming language.

ANSI, similar 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 twelvemonth of ISO publication.

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

C89 is supported past current C compilers, and most mod C code is based on it. Any program written just in Standard C and without whatsoever hardware-dependent assumptions will run correctly on whatsoever platform with a conforming C implementation, within its resource limits. Without such precautions, programs may compile only on a certain platform or with a particular compiler, due, for example, to the apply of non-standard libraries, such every bit GUI libraries, or to a reliance on compiler- or platform-specific attributes such as the verbal size of data types and byte endianness.

In cases where code must exist compilable past either standard-conforming or G&R C-based compilers, the __STDC__ macro can be used to separate the code into Standard and K&R sections to prevent the use on a K&R C-based compiler of features available but in Standard C.

Later on the ANSI/ISO standardization process, the C language specification remained relatively static for several years. In 1995, Normative Amendment 1 to the 1990 C standard (ISO/IEC 9899/AMD1:1995, known informally as C95) was published, to correct some details and to add more than extensive support for international character sets.^[18]

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 normally referred to as "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 circuitous type to stand for circuitous 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 support for 1-line comments beginning with //, every bit in BCPL or C++. Many of these had already been implemented as extensions in several C compilers.

C99 is for the about part backward compatible with C90, but is stricter in some ways; in particular, a proclamation that lacks a type specifier no longer has int implicitly causeless. A standard macro __STDC_VERSION__ is defined with value 199901L to indicate that C99 back up is available. 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++, notwithstanding, implements the C89 standard and those parts of C99 that are required for compatibility with C++eleven.^[20] ^{[ needs update ]}

In add-on, support for Unicode identifiers (variable / function names) in the grade of escaped characters (e.thou. \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 called "C1X" until its official publication on 2011-12-08. The C standards committee adopted guidelines to limit the adoption of new features that had not been tested by existing implementations.

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

C17 [edit]

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

C2x [edit]

C2x is an informal proper name for the adjacent (after C17) major C linguistic communication standard revision. It is expected to exist voted on in 2023 and would therefore exist called C23.^[21] ^{[ better source needed ]}

Embedded C [edit]

Historically, embedded C programming requires nonstandard extensions to the C linguistic communication in gild to support exotic features such every bit fixed-bespeak arithmetics, multiple distinct retentiveness banks, and bones I/O operations.

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

Syntax [edit]

C has a formal grammar specified past the C standard.^[23] Line endings are generally not significant in C; nonetheless, line boundaries do take significance during the preprocessing phase. Comments may appear either betwixt the delimiters /* and */, or (since C99) following // until the stop of the line. Comments delimited by /* and */ practise not nest, and these sequences of characters are not interpreted as comment delimiters if they announced within string or grapheme literals.^[24]

C source files contain declarations and office definitions. Part definitions, in plough, contain declarations and statements. Declarations either ascertain new types using keywords such equally struct, union, and enum, or assign types to and perchance reserve storage for new variables, usually by writing the type followed by the variable name. Keywords such as char and int specify born types. Sections of lawmaking are enclosed in braces ({ and }, sometimes called "curly brackets") to limit the telescopic of declarations and to act as a single argument for command structures.

As an imperative language, 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; every bit a side effect of the evaluation, functions may be called and variables may exist assigned new values. To modify the normal sequential execution of statements, C provides several command-flow statements identified by reserved keywords. Structured programming is supported past if … [else] provisional execution and by do … while, while, and for iterative execution (looping). The for statement has split up initialization, testing, and reinitialization expressions, any or all of which can exist omitted. break and continue tin can exist used to leave the innermost enclosing loop statement or skip to its reinitialization. There is also a non-structured goto statement which branches directly to the designated label within the function. switch selects a example to be executed based on the value of an integer expression.

Expressions can utilize 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. Nevertheless, all side effects (including storage to variables) will occur before the adjacent "sequence point"; sequence points include the finish of each expression statement, and the entry to and return from each function call. Sequence points likewise occur during evaluation of expressions containing certain operators (&&, ||, ?: and the comma operator). This permits a high degree of object code optimization past the compiler, but requires C programmers to take more care to obtain reliable results than is needed for other programming languages.

Kernighan and Ritchie say in the Introduction of The C Programming Linguistic communication: "C, like any other linguistic communication, has its blemishes. Some of the operators take the wrong 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 set [edit]

The basic C source grapheme set includes the following characters:

Lowercase and uppercase 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 graphic symbol, although for convenience C treats it as one.

Additional multi-byte encoded characters may be used in string 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 by using \uXXXX or \UXXXXXXXX encoding (where the X denotes a hexadecimal grapheme), although this characteristic is not all the same widely implemented.

The bones C execution character set contains the aforementioned characters, along with representations for warning, backspace, and railroad vehicle return. Run-time support for extended character sets has increased with each revision of the C standard.

Reserved words [edit]

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

automobile
break
instance
char
const
continue
default
do
double
else
enum
extern
float
for
goto
if
int
long
register
return
short
signed
sizeof
static
struct
switch
typedef
marriage
unsigned
void
volatile
while

C99 reserved five more words:

_Bool
_Complex
_Imaginary
inline
restrict

C11 reserved seven more words:^[26]

_Alignas
_Alignof
_Atomic
_Generic
_Noreturn
_Static_assert
_Thread_local

Most of the recently reserved words brainstorm with an underscore followed past a capital letter alphabetic character, because identifiers of that form were previously reserved past the C standard for use simply by implementations. Since existing program source lawmaking should not have been using these identifiers, it would not exist affected when C implementations started supporting these extensions to the programming linguistic communication. Some standard headers practise ascertain more convenient synonyms for underscored identifiers. The language previously included a reserved give-and-take called entry, but this was seldom implemented, and has now been removed as a reserved word.^[27]

Operators [edit]

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

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

C uses the operator = (used in mathematics to limited equality) to indicate assignment, post-obit the precedent of Fortran and PL/I, but dissimilar ALGOL and its derivatives. C uses the operator == to test for equality. The similarity betwixt these two operators (assignment and equality) may event in the accidental apply of one in place of the other, and in many cases, the fault does not produce an error message (although some compilers produce warnings). For example, the conditional expression if (a == b + ane) might mistakenly exist written as if (a = b + i), which will be evaluated every bit truthful if a is not zero after the assignment.^[28]

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

"Hi, world" instance [edit]

The "hi, world" example, which appeared in the starting time edition of 1000&R, has go the model for an introductory program in most programming textbooks. The plan prints "hello, world" to the standard output, which is usually a terminal or screen display.

The original version was:^[30]

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

A standard-conforming "hullo, earth" program is:^[a]

                        #include                                    <stdio.h>                        int                                    chief            (            void            )                        {                                                printf            (            "hello, world            \n            "            );                        }

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

The side by side line indicates that a function named principal is being defined. The main function serves a special purpose in C programs; the run-time environment calls the main role to begin program execution. The type specifier int indicates that the value that is returned to the invoker (in this case the run-time environment) as a result of evaluating the main part, is an integer. The keyword void as a parameter listing indicates that this part takes no arguments.^[b]

The opening curly caryatid indicates the beginning of the definition of the master office.

The adjacent line calls (diverts execution to) a role named printf, which in this case is supplied from a arrangement library. In this call, the printf function is passed (provided with) a single argument, the address of the showtime character in the string literal "hello, world\due north". The cord literal is an unnamed array with elements of blazon char, set up automatically by the compiler with a final 0-valued character to marking the cease of the assortment (printf needs to know this). The \n is an escape sequence that C translates to a newline grapheme, which on output signifies the end of the current line. The return value of the printf part is of blazon int, just it is silently discarded since information technology is not used. (A more conscientious program might test the return value to determine whether or not the printf function succeeded.) The semicolon ; terminates the statement.

The closing curly brace indicates the cease of the lawmaking for the master function. According to the C99 specification and newer, the main function, unlike whatsoever 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-fourth dimension system as an go out code indicating successful execution.^[31]

Information types [edit]

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

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

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

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

A null pointer value explicitly points to no valid location. Dereferencing a nil pointer value is undefined, often resulting in a segmentation fault. Null pointer values are useful for indicating special cases such equally no "next" arrow in the final node of a linked list, or equally 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 as 0, with or without explicit casting to a pointer type, or equally the Nothing macro defined past several standard headers. In conditional contexts, zilch pointer values evaluate to false, while all other pointer values evaluate to true.

Void pointers (void *) point to objects of unspecified type, and can therefore be used 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 tin hands be (and in many contexts implicitly are) converted to and from any other object pointer type.^[31]

Careless use of pointers is potentially unsafe. Because they are typically unchecked, a pointer variable can be made to betoken to any arbitrary location, which can cause undesirable furnishings. Although properly used pointers indicate to safe places, they can be made to bespeak to unsafe places past 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 directly assigned an unsafe value using a cast, union, or through another corrupt arrow. In general, C is permissive in allowing manipulation of and conversion between pointer types, although compilers typically provide options for diverse levels of checking. Another programming languages address these problems by using more restrictive reference types.

Arrays [edit]

Array types in C are traditionally of a stock-still, static size specified at compile fourth dimension. The more contempo C99 standard also allows a form of variable-length arrays. However, it is also possible to allocate a block of memory (of arbitrary size) at run-fourth dimension, using the standard library'due south malloc function, and treat it every bit an array.

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

C does not take a special provision for declaring multi-dimensional arrays, but rather relies on recursion inside the type system to declare arrays of arrays, which effectively accomplishes the aforementioned thing. The alphabetize values of the resulting "multi-dimensional array" can be idea of every bit increasing in row-major order. Multi-dimensional arrays are commonly used in numerical algorithms (mainly from applied linear algebra) to store matrices. The construction of the C array is well suited to this particular job. Even so, in early on 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 array with an additional "row vector" of pointers to the columns.) C99 introduced "variable-length arrays" which address this effect.

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

                        int                                    func            (            int                                    N            ,                                    int                                    M            )                        {                                                bladder                                    (            *            p            )[            N            ][            M            ]                                    =                                    malloc            (            sizeof                                    *            p            );                                                if                                    (            !            p            )                                                return                                    -i            ;                                                for                                    (            int                                    i                                    =                                    0            ;                                    i                                    <                                    Due north            ;                                    i            ++            )                                                for                                    (            int                                    j                                    =                                    0            ;                                    j                                    <                                    One thousand            ;                                    j            ++            )                                                (            *            p            )[            i            ][            j            ]                                    =                                    i                                    +                                    j            ;                                                print_array            (            N            ,                                    One thousand            ,                                    p            );                                                free            (            p            );                                                render                                    one            ;                        }

Array–arrow interchangeability [edit]

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

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

The total size of an assortment 10 can be determined by applying sizeof to an expression of array blazon. The size of an element can be determined by applying the operator sizeof to whatsoever dereferenced element of an array A, every bit in n = sizeof A[0]. This, the number of elements in a declared array A can be determined as sizeof A / sizeof A[0]. Note, that if only a pointer to the start element is available as information technology is often the instance in C code because of the automatic conversion described higher up, the data about the full type of the array and its length are lost.

Memory direction [edit]

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

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

These 3 approaches are appropriate in different situations and have diverse trade-offs. For instance, static memory allotment has piffling allotment overhead, automatic resource allotment may involve slightly more than overhead, and dynamic retention allotment tin 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 office calls, automatic allocation is easy to utilize but stack space is typically much more limited and transient than either static retention or heap space, and dynamic memory allocation allows convenient allocation of objects whose size is known simply at run-time. Almost C programs make extensive use of all 3.

Where possible, automated or static allotment is normally simplest considering the storage is managed by the compiler, freeing the programmer of the potentially error-prone job of manually allocating and releasing storage. However, many data structures can alter in size at runtime, and since static allocations (and automatic allocations before C99) must have a fixed size at compile-fourth dimension, there are many situations in which dynamic allocation is necessary.^[31] Prior to the C99 standard, variable-sized arrays were a mutual case of this. (See the commodity on malloc for an example of dynamically allocated arrays.) Dissimilar automatic resource allotment, which tin neglect at run time with uncontrolled consequences, the dynamic allotment functions return an indication (in the grade of a null pointer value) when the required storage cannot be allocated. (Static allocation that is also large is usually detected by the linker or loader, before the program tin even begin execution.)

Unless otherwise specified, static objects contain zero or nix pointer values upon program startup. Automatically and dynamically allocated objects are initialized only if an initial value is explicitly specified; otherwise they initially have indeterminate values (typically, any scrap pattern happens to be present in the storage, which might not even represent a valid value for that type). If the plan attempts to admission an uninitialized value, the results are undefined. Many modernistic compilers effort to detect and warn near this problem, but both false positives and false negatives can occur.

Heap memory allotment has to be synchronized with its actual usage in whatsoever program to be reused as much every bit possible. For example, if the only pointer to a heap retentiveness allocation goes out of telescopic or has its value overwritten before it is deallocated explicitly, so that retentiveness cannot be recovered for subsequently reuse and is substantially lost to the programme, a miracle known equally a memory leak. Conversely, it is possible for memory to exist freed, but is referenced subsequently, leading to unpredictable results. Typically, the failure symptoms announced in a portion of the plan unrelated to the code that causes the mistake, making it difficult to diagnose the failure. Such issues are ameliorated in languages with automatic garbage collection.

Libraries [edit]

The C programming language uses libraries equally its chief method of extension. In C, a library is a set of functions independent within a single "archive" file. Each library typically has a header file, which contains the prototypes of the functions independent within the library that may exist used past a program, and declarations of special information types and macro symbols used with these functions. In society for a programme to employ a library, it must include the library'south header file, and the library must be linked with the program, which in many cases requires compiler flags (e.g., -lm, shorthand for "link the math library").^[31]

The nearly 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 only a subset of the standard library). This library supports stream input and output, memory resource allotment, mathematics, character strings, and fourth dimension values. Several separate standard headers (for instance, stdio.h) specify the interfaces for these and other standard library facilities.

Another common set of C library functions are those used by applications specifically targeted for Unix and Unix-like systems, particularly 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 variety of other libraries available. Libraries are often written in C because C compilers generate efficient object code; programmers then create interfaces to the library so that the routines tin exist used from higher-level languages similar Java, Perl, and Python.^[31]

File handling and streams [edit]

File input and output (I/O) is not office of the C language itself but instead is handled by libraries (such as the C standard library) and their associated header files (e.thou. stdio.h). File handling is generally implemented through high-level I/O which works through streams. A stream is from this perspective a data menses that is contained of devices, while a file is a physical device. The high-level I/O is done through the association of a stream to a file. In the C standard library, a buffer (a retentivity area or queue) is temporarily used to shop data before it'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 ]} just are by and large part of "bare metal" programming (programming that's independent of whatever operating system such as most embedded programming). With few exceptions, implementations include low-level I/O.

Linguistic communication tools [edit]

A number of tools accept been adult to assist C programmers notice and fix statements with undefined behavior 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 code checking and auditing are beneficial in any language, and for C many such tools be, such as Lint. A common practice is to use Lint to find questionable code when a program is first written. Once a program passes Lint, information technology is so compiled using the C compiler. Also, many compilers tin optionally warn about syntactically valid constructs that are likely to actually be errors. MISRA C is a proprietary set of guidelines to avert such questionable code, developed for embedded systems.^[37]

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

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

Uses [edit]

The C Programming Language

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

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

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

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

C is sometimes used as an intermediate language by implementations of other languages. This approach may be used for portability or convenience; past using C as an intermediate language, additional car-specific lawmaking generators are not necessary. C has some features, such as line-number preprocessor directives and optional superfluous commas at the stop of initializer lists, that back up compilation of generated code. However, some of C'southward shortcomings accept prompted the evolution of other C-based languages specifically designed for utilise as intermediate languages, such as C--.

C has also been widely used to implement end-user applications. Nonetheless, such applications can likewise 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 afterward languages such as C#, D, Go, Java, JavaScript, Limbo, LPC, Perl, PHP, Python, and Unix's C shell.^[42] The near pervasive influence has been syntactical; all of the languages mentioned combine the argument and (more or less recognizably) expression syntax of C with type systems, data models, and/or large-scale program structures that differ from those of C, sometimes radically.

Several C or near-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 different extensions of C that provided object-oriented capabilities. Both languages were originally implemented every bit 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 past Bjarne Stroustrup every bit an arroyo to providing object-oriented functionality with a C-similar syntax.^[44] C++ adds greater typing strength, scoping, and other tools useful in object-oriented programming, and permits generic programming via templates. Most a superset of C, C++ now supports most of C, with a few exceptions.

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

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

See besides [edit]

Compatibility of C and C++
Comparing of Pascal and C
Comparison of programming languages
International Obfuscated C Code Competition
Listing 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 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 2 arguments, int argc and char *argv[], respectively, which can be used to handle command line arguments. The ISO C standard (department five.i.2.ii.1) requires both forms of main to be supported, which is special treatment non afforded to any other part.

References [edit]

^ ^a ^b Kernighan, Brian W.; Ritchie, Dennis Thou. (February 1978). The C Programming Language (1st ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-thirteen-110163-0.
^ Ritchie (1993): "Thompson had made a brief attempt to produce a system coded in an early version of C—earlier structures—in 1972, only gave upwardly the effort."
^ Fruderica (December 13, 2020). "History of C". The cppreference.com. Archived from the original on October 24, 2020. Retrieved October 24, 2020.
^ Ritchie (1993): "The scheme of type composition adopted by C owes considerable debt to Algol 68, although information technology did not, perhaps, emerge in a form that Algol'southward adherents would approve of."
^ Ring Team (October 23, 2021). "The Ring programming language and other languages". ring-lang.cyberspace.
^ ^a ^b "Verilog HDL (and C)" (PDF). The Research School of Computer Science at the Australian National Academy. June 3, 2010. Archived from the original (PDF) on November 6, 2013. Retrieved August 19, 2013. 1980s: ; Verilog first introduced ; Verilog inspired by the C programming language
^ ^a ^b ^c ^d ^eastward Ritchie (1993)
^ "Programming Language Popularity". 2009. Archived from the original on January xvi, 2009. Retrieved Jan 16, 2009.
^ "TIOBE Programming Customs Index". 2009. Archived from the original on May 4, 2009. Retrieved May 6, 2009.
^ ^a ^b "History of C". en.cppreference.com. Archived from the original on May 29, 2018. Retrieved May 28, 2018.
^ "TIOBE Index for October 2021". Retrieved October vii, 2021.
^ Ritchie, Dennis. "BCPL to B to C". Archived from the original on December 12, 2019. Retrieved September x, 2019.
^ ^a ^b Johnson, Due south. C.; Ritchie, D. M. (1978). "Portability of C Programs and the UNIX System". Bell Arrangement Tech. J. 57 (6): 2021–2048. CiteSeerX10.1.1.138.35. doi:10.1002/j.1538-7305.1978.tb02141.10. S2CID 17510065. (Note: The PDF is an OCR scan of the original, and contains a rendering of "IBM 370" every bit "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. x. 139. Archived (PDF) from the original on November 11, 2017. Retrieved February 1, 2015.
^ "C manual pages". FreeBSD Miscellaneous Information Manual (FreeBSD thirteen.0 ed.). May thirty, 2011. Archived from the original on January 21, 2021. Retrieved Jan 15, 2021. [1] Archived January 21, 2021, at the Wayback Motorcar
^ Kernighan, Brian Due west.; Ritchie, Dennis M. (March 1988). The C Programming Language (2nd ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-thirteen-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 fourteen, 2014.
^ C Integrity. International Organization for Standardization. March 30, 1995. Archived from the original on July 25, 2018. Retrieved July 24, 2018.
^ "JTC1/SC22/WG14 – C". Home page. ISO/IEC. Archived from the original on February 12, 2018. Retrieved June 2, 2011.
^ Andrew Binstock (Oct 12, 2011). "Interview with Herb Sutter". Dr. Dobbs. Archived from the original on Baronial 2, 2013. Retrieved September 7, 2013.
^ "Revised C23 Schedule WG 14 North 2759" (PDF). www.open-std.org. Archived (PDF) from the original on June 24, 2021. Retrieved October 10, 2021.
^ "TR 18037: Embedded C" (PDF). ISO / IEC. Archived (PDF) from the original on February 25, 2021. Retrieved July 26, 2011.
^ Harbison, Samuel P.; Steele, Guy L. (2002). C: A Reference Manual (5th ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-089592-9. Contains a BNF grammar for C.
^ Kernighan & Ritchie (1996), p. 192.
^ Kernighan & Ritchie (1978), p. iii.
^ "ISO/IEC 9899:201x (ISO C11) Commission Draft" (PDF). Archived (PDF) from the original on December 22, 2017. Retrieved September 16, 2011.
^ Kernighan & Ritchie (1996), pp. 192, 259.
^ "10 Common Programming Mistakes in C++". Cs.ucr.edu. Archived from the original on Oct 21, 2008. Retrieved June 26, 2009.
^ Schultz, Thomas (2004). C and the 8051 (tertiary ed.). Otsego, MI: PageFree Publishing Inc. p. twenty. ISBN978-1-58961-237-2. Archived from the original on July 29, 2020. Retrieved February 10, 2012.
^ Kernighan & Ritchie (1978), p. half dozen.
^ ^a ^b ^c ^d ^e ^f ^{one thousand} Klemens, Ben (2013). 21st Century C. O'Reilly Media. ISBN978-i-4493-2714-9.
^ Feuer, Alan R.; Gehani, Narain H. (March 1982). "Comparison of the Programming Languages C and Pascal". ACM Computing Surveys. 14 (i): 73–92. doi:10.1145/356869.356872. S2CID 3136859.
^ Kernighan & Ritchie (1996), p. 122.
^ For example, gcc provides _FORTIFY_SOURCE. "Security Features: Compile Time Buffer Checks (FORTIFY_SOURCE)". fedoraproject.org. Archived from the original on January 7, 2007. Retrieved August v, 2012.
^ เอี่ยมสิริวงศ์, โอภาศ (2016). Programming with C. Bangkok, Thailand: SE-EDUCATION PUBLIC Company Express. pp. 225–230. ISBN978-616-08-2740-4.
^ Raymond, Eric S. (October 11, 1996). The New Hacker's Lexicon (3rd ed.). MIT Printing. p. 432. ISBN978-0-262-68092-9. Archived from the original on November 12, 2012. Retrieved August v, 2012.
^ "Man Page for lint (freebsd Department 1)". unix.com. May 24, 2001. Retrieved July 15, 2014.
^ Dale, Nell B.; Weems, Flake (2014). Programming and problem solving with C++ (6th ed.). Burlington, MA: Jones & Bartlett Learning. ISBN978-1449694289. OCLC 894992484.
^ Dr. Dobb'south Sourcebook. U.S.A.: Miller Freeman, Inc. November–Dec 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 15, 2017. Retrieved March 5, 2017.
^ O'Regan, Gerard (September 24, 2015). Pillars of computing : a compendium of select, pivotal technology firms. ISBN978-3319214641. OCLC 922324121.
^ Rauchwerger, Lawrence (2004). Languages and compilers for parallel computing : 16th international workshop, LCPC 2003, College Station, TX, United states of america, Oct 2-4, 2003 : revised papers. Springer. ISBN978-3540246442. OCLC 57965544.
^ Stroustrup, Bjarne (1993). "A History of C++: 1979−1991" (PDF). Archived (PDF) from the original on Feb 2, 2019. Retrieved June ix, 2011.

Sources [edit]

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

External links [edit]

ISO C Working Group official website
- ISO/IEC 9899, publicly available official C documents, including the C99 Rationale
- "C99 with Technical corrigenda TC1, TC2, and TC3 included" (PDF). (3.61 MB)
comp.lang.c Frequently Asked Questions
A History of C, by Dennis Ritchie

deanliffew61.blogspot.com

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