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Full general-purpose programming language

C
Major implementations
The C Programming Language ^[1] (often referred to as K&R), the seminal volume on C
Paradigm	Multi-paradigm: imperative (procedural), structured
Designed by	Dennis Ritchie
Developer	Dennis Ritchie & Bong Labs (creators); ANSI X3J11 (ANSI C); ISO/IEC JTC1/SC22/WG14 (ISO C)
First appeared	1972; l years ago (1972) ^[two]

Stable release	C17 / June 2018; 3 years agone (2018-06)
Preview release	C2x (N2731) / Oct 18, 2021; five months agone (2021-ten-18) ^[3]

Typing discipline	Static, weak, manifest, nominal
OS	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,^[4] assembly, PL/I, FORTRAN
Influenced
Numerous: AMPL, AWK, csh, C++, C--, C#, Objective-C, D, Become, Java, JavaScript, JS++, Julia, Limbo, LPC, Perl, PHP, Pike, Processing, Python, Ring,^[5]Rust, Seed7, Vala, Verilog (HDL),^[6] Nim, Zig
C Programming at Wikibooks

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

A successor to the programming language B, C was originally developed at Bell Labs by Dennis Ritchie between 1972 and 1973 to construct utilities running on Unix. Information technology was applied to re-implementing the kernel of the Unix operating arrangement.^[7] During the 1980s, C gradually gained popularity. Information technology has become ane of the nearly widely used programming languages,^[8] ^[ix] with C compilers from various 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 Organization for Standardization (ISO).

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

Since 2000, C has consistently ranked amid the top two languages in the TIOBE index, a mensurate of the popularity of programming languages.^[11]

Overview [edit]

Like most procedural languages in the ALGOL tradition, C has facilities for structured programming and allows lexical variable scope and recursion. Its static blazon system prevents unintended operations. In C, all executable code is contained within subroutines (also called "functions", though non strictly in the sense of functional programming). Function parameters are always passed by value (except arrays). Pass-by-reference is fake in C by explicitly passing pointer values. C program source text is free-format, using the semicolon equally a argument terminator and curly braces for grouping blocks of statements.

The C language likewise exhibits the following characteristics:

The linguistic communication has a small, fixed number of keywords, including a total set of control menses primitives: if/else, for, do/while, while, and switch. User-defined names are non distinguished from keywords by whatever kind of sigil.
It has a large number of arithmetic, bitwise, and logic operators: +,+=,++,&,||, etc.
More than i consignment may exist performed in a single argument.
Functions:
- Function return values can exist ignored, when non needed.
- Function and information pointers let ad hoc run-time polymorphism.
- Functions may not be defined within the lexical scope of other functions.
Information typing is static, but weakly enforced; all information has a type, but implicit conversions are possible.
Declaration syntax mimics usage context. C has no "ascertain" keyword; instead, a statement first with the proper noun of a type is taken as a proclamation. In that location is no "role" keyword; instead, a function is indicated by the presence of a parenthesized argument list.
User-defined (typedef) and compound types are possible.
- Heterogeneous amass data types (struct) allow related information elements to be accessed and assigned every bit a unit.
- Union is a structure with overlapping members; but the final member stored is valid.
- Assortment indexing is a secondary note, defined in terms of arrow arithmetics. Unlike structs, arrays are not outset-course objects: they cannot be assigned or compared using single congenital-in operators. In that location is no "array" keyword in use or definition; instead, square brackets indicate arrays syntactically, for example calendar month[11].
- Enumerated types are possible with the enum keyword. They are freely interconvertible with integers.
- Strings are not a distinct information blazon, but are conventionally implemented as nil-terminated character arrays.
Depression-level admission to reckoner memory is possible by converting auto addresses to typed pointers.
Procedures (subroutines not returning values) are a special example of function, with an untyped render type void.
A preprocessor performs macro definition, source lawmaking file inclusion, and conditional compilation.
At that place is a basic class of modularity: files can exist compiled separately and linked together, with command over which functions and data objects are visible to other files via static and extern attributes.
Complex functionality such as I/O, cord manipulation, and mathematical functions are consistently delegated to library routines.

While C does non include sure features found in other languages (such as object orientation and garbage collection), these can be implemented or emulated, often through the use of external libraries (e.g., the GLib Object System or the Boehm garbage collector).

Relations to other languages [edit]

Many later languages take borrowed direct 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 description languages).^{[half dozen]} These languages take drawn many of their control structures and other bones features from C. Most of them (Python being a dramatic exception) too limited highly similar syntax to C, and they tend to combine the recognizable expression and statement syntax of C with underlying type systems, data models, and semantics that can be radically dissimilar.

History [edit]

Early on developments [edit]

Timeline of language development
Yr	C Standard^[ten]
1972	Birth
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 organisation, originally implemented in assembly language on a PDP-7 past Dennis Ritchie and Ken Thompson, incorporating several ideas from colleagues. Eventually, they decided to port the operating system to a PDP-11. The original PDP-11 version of Unix was too adult in assembly language.^[7]

Thompson desired a programming language to make utilities for the new platform. At first, he tried to make a Fortran compiler, merely soon gave up the idea. Instead, he created a cut-down version of the recently developed BCPL systems programming linguistic communication. The official clarification of BCPL was not available at the time,^[12] and Thompson modified the syntax to exist less wordy, and similar to a simplified ALGOL known every bit SMALGOL.^[xiii] The result was the similar just somewhat simpler language he called B.^[7] Like BCPL, B had a bootstrapping compiler to facilitate porting to new machines.^[thirteen] Thompson described B equally "BCPL semantics with a lot of SMALGOL syntax".^[13] Still, few utilities were ultimately written in B because it was besides deadening, and B could not have reward of PDP-eleven features such as byte addressability.

In 1971, Ritchie started to better B, to utilise the features of the more-powerful PDP-11. A meaning addition was a graphic symbol type. He called this "New B".^[13] Thompson started to use NB to write the Unix kernel, and his requirements shaped the management of the language evolution.^[13] ^[fourteen] Through to 1972, richer types were added to the NB language: NB had arrays of int and char; but then were added pointers, ability to generate pointers to other types, arrays of all of these, types to be returned from functions. Arrays inside expressions became pointers. A new compiler was written, and the language was renamed to C. ^[7]

The C compiler and some utilities fabricated with it were included in Version 2 Unix.^[15]

At Version 4 Unix, released in November 1973, the Unix kernel was extensively re-implemented in C.^[7] Past this fourth dimension, the C language had caused some powerful features such every bit struct types.

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

Unix was one of the get-go operating system kernels implemented in a language other than associates. Earlier instances include the Multics organisation (which was written in PL/I) and Master Command 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 system. Johnson'south Portable C Compiler served as the basis for several implementations of C on new platforms.^[xiv]

K&R C [edit]

In 1978, Brian Kernighan and Dennis Ritchie published the get-go edition of The C Programming Language.^[1] This book, known to C programmers as K&R, served for many years equally an informal specification of the language. The version of C that it describes is unremarkably referred to as "K&R C". As this was released in 1978, it is too referred to as C78.^[sixteen] The second edition of the book^[17] covers the afterward ANSI C standard, described beneath.

Thou&R introduced several language features:

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

Even subsequently the publication of the 1989 ANSI standard, for many years M&R C was still 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 considering carefully written One thousand&R C code can exist legal Standard C also.

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

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

Since Chiliad&R function declarations did not include any information about function arguments, function parameter type checks were not performed, although some compilers would issue a warning bulletin if a local function was chosen with the incorrect number of arguments, or if multiple calls to an external office used different numbers or types of arguments. Split up tools such as Unix's lint utility were developed that (among other things) could check for consistency of office 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^[18]) and some other vendors. These included:

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

The large number of extensions and lack of agreement on a standard library, together with the language popularity and the fact that not fifty-fifty the Unix compilers precisely implemented the One thousand&R specification, led to the necessity of standardization.

ANSI C and ISO C [edit]

During the belatedly 1970s and 1980s, versions of C were implemented for a wide 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; yet, the not-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 frequently 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) equally ISO/IEC 9899:1990, which is sometimes called C90. Therefore, the terms "C89" and "C90" refer to the same programming language.

ANSI, like other national standards bodies, no longer develops the C standard independently, 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.

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

C89 is supported past current C compilers, and about modern C code is based on it. Any program written only in Standard C and without any hardware-dependent assumptions volition run correctly on any platform with a conforming C implementation, within its resource limits. Without such precautions, programs may compile but on a sure platform or with a particular compiler, due, for example, to the utilize of not-standard libraries, such as GUI libraries, or to a reliance on compiler- or platform-specific attributes such as the exact size of data types and byte endianness.

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

Afterward the ANSI/ISO standardization process, the C linguistic communication 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 all-encompassing support for international character sets.^[19]

C99 [edit]

The C standard was farther revised in the belatedly 1990s, leading to the publication of ISO/IEC 9899:1999 in 1999, which is commonly referred to as "C99". It has since been amended three times by Technical Corrigenda.^[20]

C99 introduced several new features, including inline functions, several new information types (including long long int and a complex type to stand for complex 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 i-line comments commencement with //, as in BCPL or C++. Many of these had already been implemented as extensions in several C compilers.

C99 is for the most part backward compatible with C90, but is stricter in some ways; in particular, a declaration that lacks a type specifier no longer has int implicitly causeless. A standard macro __STDC_VERSION__ is defined with value 199901L to signal that C99 support is available. GCC, Solaris Studio, and other C compilers at present support many or all of the new features of C99. The C compiler in Microsoft Visual C++, however, implements the C89 standard and those parts of C99 that are required for compatibility with C++11.^[21] ^{[ needs update ]}

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

C11 [edit]

In 2007, work began on some other revision of the C standard, informally chosen "C1X" until its official publication on 2011-12-08. The C standards commission 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 equally 201112L to betoken that C11 back up is available.

C17 [edit]

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

C2x [edit]

C2x is an informal proper noun for the next (after C17) major C linguistic communication standard revision. It is expected to be voted on in 2023 and would therefore be called C23.^[22] ^{[ better source needed ]}

Embedded C [edit]

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

In 2008, the C Standards Committee published a technical study extending the C language^[23] to accost these issues by providing a common standard for all implementations to adhere to. It includes a number of features non available in normal C, such as stock-still-betoken arithmetic, named address spaces, and basic I/O hardware addressing.

Syntax [edit]

C has a formal grammar specified by the C standard.^[24] Line endings are by and large non significant in C; even so, line boundaries do have significance during the preprocessing phase. Comments may appear either betwixt the delimiters /* and */, or (since C99) following // until the end of the line. Comments delimited by /* and */ do not nest, and these sequences of characters are not interpreted as annotate delimiters if they appear within string or grapheme literals.^[25]

C source files contain declarations and function definitions. Role definitions, in plough, contain declarations and statements. Declarations either define new types using keywords such every bit struct, union, and enum, or assign types to and perhaps reserve storage for new variables, usually by writing the type followed by the variable name. Keywords such every bit 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 human activity as a unmarried statement for command structures.

As an imperative linguistic communication, C uses statements to specify actions. The most common statement is an expression statement, consisting of an expression to be evaluated, followed by a semicolon; equally a side issue of the evaluation, functions may be called and variables may be assigned new values. To modify the normal sequential execution of statements, C provides several command-flow statements identified by reserved keywords. Structured programming is supported by if … [else] provisional execution and past do … while, while, and for iterative execution (looping). The for argument has divide initialization, testing, and reinitialization expressions, whatsoever or all of which can be omitted. intermission and go on can be used to get out the innermost enclosing loop statement or skip to its reinitialization. There is also a not-structured goto argument which branches directly to the designated label within the office. switch selects a case to be executed based on the value of an integer expression.

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

Kernighan and Ritchie say in the Introduction of The C Programming Language: "C, similar whatsoever other language, has its blemishes. Some of the operators have the wrong precedence; some parts of the syntax could be better."^[26] The C standard did non try to correct many of these blemishes, considering of the impact of such changes on already existing software.

Character ready [edit]

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

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

Newline indicates the terminate of a text line; it demand not correspond to an actual single character, although for convenience C treats information technology every bit 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 be embedded portably within C source text past using \uXXXX or \UXXXXXXXX encoding (where the 10 denotes a hexadecimal grapheme), although this feature is non all the same widely implemented.

The basic C execution grapheme prepare contains the same characters, along with representations for alert, backspace, and carriage 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, besides known as keywords, which are the words that cannot be used for whatsoever purposes other than those for which they are predefined:

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

C99 reserved v more words:

_Bool
_Complex
_Imaginary
inline
restrict

C11 reserved seven more words:^[27]

_Alignas
_Alignof
_Atomic
_Generic
_Noreturn
_Static_assert
_Thread_local

Near of the recently reserved words begin with an underscore followed past a capital letter letter, because identifiers of that form were previously reserved past the C standard for utilise merely by implementations. Since existing program source code should not accept been using these identifiers, it would not exist affected when C implementations started supporting these extensions to the programming language. Some standard headers do define more convenient synonyms for underscored identifiers. The language previously included a reserved word called entry, but this was seldom implemented, and has now been removed as a reserved give-and-take.^[28]

Operators [edit]

C supports a rich set of operators, which are symbols used inside 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: ++, --
member selection: ., ->
object size: sizeof
society relations: <, <=, >, >=
reference and dereference: &, *, [ ]
sequencing: ,
subexpression grouping: ( )
type conversion: (typename)

C uses the operator = (used in mathematics to express equality) to indicate assignment, following the precedent of Fortran and PL/I, but unlike ALGOL and its derivatives. C uses the operator == to test for equality. The similarity between these two operators (assignment and equality) may effect in the accidental apply of one in place of the other, and in many cases, the mistake does not produce an fault message (although some compilers produce warnings). For example, the provisional expression if (a == b + one) might mistakenly exist written equally if (a = b + i), which will be evaluated every bit true if a is not zippo after the assignment.^[29]

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

"Hello, earth" example [edit]

The "hello, earth" instance, which appeared in the first edition of K&R, has become the model for an introductory plan in almost programming textbooks. The program prints "hello, world" to the standard output, which is unremarkably a terminal or screen display.

The original version was:^[31]

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

A standard-conforming "howdy, world" programme is:^[a]

                        #include                                    <stdio.h>                        int                                    main            (            void            )                        {                                                printf            (            "hi, earth            \n            "            );                        }

The first line of the programme contains a preprocessing directive, indicated by #include. This causes the compiler to replace that line with the unabridged 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 point that stdio.h is located using a search strategy that prefers headers provided with the compiler to other headers having the same proper noun, as opposed to double quotes which typically include local or project-specific header files.

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

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

The next line calls (diverts execution to) a function named printf, which in this instance is supplied from a system library. In this call, the printf part is passed (provided with) a single argument, the address of the first character in the string literal "hello, world\n". The string literal is an unnamed array with elements of blazon char, fix up automatically by the compiler with a concluding 0-valued character to marking the terminate of the array (printf needs to know this). The \n is an escape sequence that C translates to a newline character, which on output signifies the stop of the electric current line. The return value of the printf function is of type int, but information technology is silently discarded since it is non used. (A more careful programme might test the render value to decide whether or not the printf function succeeded.) The semicolon ; terminates the statement.

The closing curly brace indicates the cease of the code for the master function. According to the C99 specification and newer, the principal function, unlike any other function, will implicitly return a value of 0 upon reaching the } that terminates the function. (Formerly an explicit render 0; statement was required.) This is interpreted by the run-time system as an get out lawmaking indicating successful execution.^[32]

Information types [edit]

The blazon system in C is static and weakly typed, which makes it similar to the type arrangement of ALGOL descendants such as Pascal.^[33] There are congenital-in types for integers of various sizes, both signed and unsigned, floating-point numbers, and enumerated types (enum). Integer blazon char is oft used for single-byte characters. C99 added a boolean datatype. There are likewise 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 exist necessary. The compiler attempts to ensure type definiteness of nearly expressions, but the developer can override the checks in diverse ways, 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 another way.

Some find C's annunciation syntax unintuitive, especially for function pointers. (Ritchie's idea was to declare identifiers in contexts resembling their use: "declaration reflects utilise".)^[34]

C's usual arithmetic conversions allow for efficient lawmaking to exist generated, just can sometimes produce unexpected results. For example, a comparing of signed and unsigned integers of equal width requires a conversion of the signed value to unsigned. This can generate unexpected results if the signed value is negative.

Pointers [edit]

C supports the utilise of pointers, a type of reference that records the address or location of an object or function in memory. Pointers can be dereferenced to admission data stored at the address pointed to, or to invoke a pointed-to part. Pointers can be manipulated using consignment or pointer arithmetic. The run-time representation of a arrow value is typically a raw memory address (perhaps augmented by an offset-within-word field), only since a pointer's type includes the type of the thing pointed to, expressions including pointers tin can be type-checked at compile fourth dimension. Pointer arithmetic is automatically scaled by the size of the pointed-to information type. Pointers are used for many purposes in C. Text strings are ordinarily manipulated using pointers into arrays of characters. Dynamic retentiveness allocation is performed using pointers. Many data types, such equally copse, are usually implemented as dynamically allocated struct objects linked together using pointers. Pointers to functions are useful for passing functions as arguments to college-order functions (such equally qsort or bsearch) or as callbacks to be invoked by effect handlers.^[32]

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

Void pointers (void *) indicate to objects of unspecified blazon, and can therefore exist used as "generic" information pointers. Since the size and type of the pointed-to object is not known, void pointers cannot be dereferenced, nor is pointer arithmetic on them allowed, although they can easily be (and in many contexts implicitly are) converted to and from any other object pointer type.^[32]

Careless use of pointers is potentially dangerous. Considering they are typically unchecked, a arrow variable can be fabricated to point to any arbitrary location, which can cause undesirable effects. Although properly used pointers point to safe places, they tin can be made to point to unsafe places by using invalid pointer arithmetic; the objects they point to may continue to exist used after deallocation (dangling pointers); they may be used without having been initialized (wild pointers); or they may exist direct assigned an unsafe value using a cast, spousal relationship, or through another corrupt pointer. In general, C is permissive in assuasive manipulation of and conversion between arrow types, although compilers typically provide options for various levels of checking. Another programming languages address these issues by 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 also allows a form of variable-length arrays. However, information technology is likewise possible to allocate a cake of retention (of capricious size) at run-time, using the standard library's malloc role, and care for it as an assortment.

Since arrays are always accessed (in effect) via pointers, array accesses are typically not checked against the underlying array size, although some compilers may provide bounds checking as an option.^[35] ^[36] Assortment bounds violations are therefore possible and can lead to various repercussions, including illegal memory accesses, corruption of data, buffer overruns, and run-fourth dimension exceptions.

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

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

                        int                                    func            (            int                                    Northward            ,                                    int                                    M            )                        {                                                bladder                                    (            *            p            )[            N            ][            Thousand            ]                                    =                                    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            ,                                    One thousand            ,                                    p            );                                                free            (            p            );                                                render                                    1            ;                        }

Array–pointer interchangeability [edit]

The subscript notation ten[i] (where ten designates a arrow) is syntactic carbohydrate for *(x+i).^[37] Taking reward of the compiler'due south knowledge of the pointer type, the accost that 10 + i points to is not the base address (pointed to past ten) incremented past i bytes, but rather is divers to be the base of operations accost incremented by i multiplied by the size of an chemical element that x points to. Thus, x[i] designates the i+1th element of the array.

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

The total size of an array ten tin can be determined by applying sizeof to an expression of array blazon. The size of an chemical element can exist determined by applying the operator sizeof to any dereferenced element of an assortment A, as in n = sizeof A[0]. This, the number of elements in a alleged array A tin can be adamant as sizeof A / sizeof A[0]. Note, that if simply a arrow to the beginning element is available equally information technology is frequently the case in C code because of the automatic conversion described above, the data about the full type of the assortment and its length are lost.

Memory management [edit]

One of the nearly important functions of a programming language is to provide facilities for managing retentivity and the objects that are stored in retention. C provides three distinct means to allocate retention for objects:^[32]

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

These three approaches are appropriate in dissimilar situations and accept diverse trade-offs. For case, static memory allocation has little allocation overhead, automated allocation may involve slightly more overhead, and dynamic memory allocation can potentially accept a great deal of overhead for both allocation and deallocation. The persistent nature of static objects is useful for maintaining state information across role calls, automatic allocation is easy to utilise merely stack space is typically much more express and transient than either static memory or heap space, and dynamic memory resource allotment allows convenient allocation of objects whose size is known only at run-time. Most C programs brand extensive use of all three.

Where possible, automatic or static allotment is usually simplest considering the storage is managed past the compiler, freeing the programmer of the potentially error-prone chore of manually allocating and releasing storage. Yet, many information structures can change in size at runtime, and since static allocations (and automatic allocations before C99) must accept a fixed size at compile-time, there are many situations in which dynamic allotment is necessary.^[32] Prior to the C99 standard, variable-sized arrays were a common instance of this. (See the article on malloc for an example of dynamically allocated arrays.) Unlike automatic allocation, which can fail at run time with uncontrolled consequences, the dynamic resource allotment functions return an indication (in the course of a naught pointer value) when the required storage cannot be allocated. (Static resource allotment that is likewise big is normally detected by the linker or loader, before the plan can even brainstorm execution.)

Unless otherwise specified, static objects contain zilch or nil arrow values upon program startup. Automatically and dynamically allocated objects are initialized only if an initial value is explicitly specified; otherwise they initially have indeterminate values (typically, whatever bit design happens to be present in the storage, which might not even represent a valid value for that blazon). If the program attempts to access an uninitialized value, the results are undefined. Many modern compilers endeavor to detect and warn about this problem, but both simulated positives and false negatives can occur.

Heap memory resource allotment has to be synchronized with its actual usage in any program to be reused as much as possible. For instance, 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 memory cannot be recovered for afterward reuse and is essentially lost to the programme, a phenomenon known as a memory leak. Conversely, information technology is possible for memory to be freed, but is referenced subsequently, leading to unpredictable results. Typically, the failure symptoms appear in a portion of the program unrelated to the lawmaking that causes the 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 as its master method of extension. In C, a library is a fix of functions contained within a single "archive" file. Each library typically has a header file, which contains the prototypes of the functions contained within the library that may be used by a programme, and declarations of special data types and macro symbols used with these functions. In guild for a programme to apply a library, it must include the library's header file, and the library must be linked with the programme, which in many cases requires compiler flags (due east.k., -lm, shorthand for "link the math library").^[32]

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 only a subset of the standard library). This library supports stream input and output, memory 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.

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

Since many programs have been written in C, there are a broad variety of other libraries available. Libraries are often written in C because C compilers generate efficient object code; programmers then create interfaces to the library so that the routines can exist used from college-level languages similar Java, Perl, and Python.^[32]

File handling and streams [edit]

File input and output (I/O) is not part of the C linguistic communication itself but instead is handled past libraries (such as the C standard library) and their associated header files (due east.g. stdio.h). File handling is mostly implemented through high-level I/O which works through streams. A stream is from this perspective a data menses that is independent of devices, while a file is a concrete 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 memory area or queue) is temporarily used to store data before it'southward sent to the final destination. This reduces the time spent waiting for slower devices, for instance a hard bulldoze or solid country bulldoze. Low-level I/O functions are not function of the standard C library^{[ clarification needed ]} simply are generally role of "blank metal" programming (programming that's contained of any operating system such as most embedded programming). With few exceptions, implementations include low-level I/O.

Language tools [edit]

A number of tools have been developed to aid C programmers observe and ready 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 lawmaking checking and auditing are beneficial in whatsoever language, and for C many such tools exist, such as Lint. A mutual exercise is to apply Lint to notice questionable code when a program is beginning written. In one case a plan passes Lint, it is then compiled using the C compiler. Also, many compilers can optionally warn near syntactically valid constructs that are likely to actually be errors. MISRA C is a proprietary fix of guidelines to avoid such questionable code, developed for embedded systems.^[38]

At that place are as well compilers, libraries, and operating arrangement level mechanisms for performing actions that are not a standard part of C, such as premises checking for arrays, detection of buffer overflow, serialization, dynamic retentiveness tracking, and automatic garbage collection.

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

Uses [edit]

The C Programming Language

C is widely used for systems programming in implementing operating systems and embedded organization applications,^[39] because C lawmaking, when written for portability, can be used for well-nigh purposes, all the same when needed, system-specific code tin can exist used to access specific hardware addresses and to perform type punning to match externally imposed interface requirements, with a low run-time demand on system resources.

C can exist used for website programming using the Common Gateway Interface (CGI) every bit a "gateway" for information betwixt the Spider web application, the server, and the browser.^[xl] C is often called over interpreted languages because of its speed, stability, and virtually-universal availability.^[41]

A result of C's broad 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, Ruddy, and PHP are written in C.

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

C is sometimes used as an intermediate linguistic communication by implementations of other languages. This arroyo may exist used for portability or convenience; by using C as an intermediate linguistic communication, additional 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. Withal, some of C's shortcomings have prompted the development of other C-based languages specifically designed for use as intermediate languages, such as C--.

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

[edit]

The TIOBE index graph, showing a comparing of the popularity of diverse programming languages^[42]

C has both directly and indirectly influenced many afterward languages such as C#, D, Go, Coffee, JavaScript, Limbo, LPC, Perl, PHP, Python, and Unix's C shell.^[43] The about pervasive influence has been syntactical; all of the languages mentioned combine the statement and (more than or less recognizably) expression syntax of C with type systems, data models, and/or large-scale program structures that differ from those of C, sometimes radically.

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

When object-oriented programming languages became pop, C++ and Objective-C were ii different extensions of C that provided object-oriented capabilities. Both languages were originally implemented as source-to-source compilers; source lawmaking was translated into C, and so compiled with a C compiler.^[44]

The C++ programming language (originally named "C with Classes") was devised by Bjarne Stroustrup equally an approach to providing object-oriented functionality with a C-similar syntax.^[45] C++ adds greater typing force, scoping, and other tools useful in object-oriented programming, and permits generic programming via templates. Virtually a superset of C, C++ now supports about 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, function 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 nigh supersets of C.

See too [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 lawmaking will compile on most modern compilers that are not in strict standard compliance style, simply it does not fully adapt to the requirements of either C89 or C99. In fact, C99 requires that a diagnostic message exist produced.
^ The primary function actually has 2 arguments, int argc and char *argv[], respectively, which can be used to handle control line arguments. The ISO C standard (department 5.1.2.ii.one) requires both forms of primary to be supported, which is special treatment non afforded to whatever other office.

References [edit]

^ ^a ^b Kernighan, Brian W.; Ritchie, Dennis Thou. (February 1978). The C Programming Language (1st ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-110163-0.
^ Ritchie (1993): "Thompson had fabricated a brief attempt to produce a system coded in an early version of C—before structures—in 1972, but gave upwardly the effort."
^ Fruderica (December 13, 2020). "History of C". The cppreference.com. Archived from the original on Oct 24, 2020. Retrieved Oct 24, 2020.
^ Ritchie (1993): "The scheme of type composition adopted by C owes considerable debt to Algol 68, although information technology did non, mayhap, emerge in a form that Algol'due south 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 Research School of Computer science at the Australian National University. June 3, 2010. Archived from the original (PDF) on Nov vi, 2013. Retrieved August 19, 2013. 1980s: ; Verilog first introduced ; Verilog inspired past the C programming language
^ ^a ^b ^c ^d ^e ^f Ritchie (1993)
^ "Programming Language Popularity". 2009. Archived from the original on Jan xvi, 2009. Retrieved January sixteen, 2009.
^ "TIOBE Programming Community 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 seven, 2021.
^ Ritchie, Dennis. "BCPL to B to C". Archived from the original on Dec 12, 2019. Retrieved September ten, 2019.
^ ^a ^b ^c ^d ^{due east} Jensen, Richard (December 9, 2020). ""A damn stupid thing to do"—the origins of C". Ars Technica . Retrieved March 28, 2022.
^ ^a ^b Johnson, South. C.; Ritchie, D. Yard. (1978). "Portability of C Programs and the UNIX System". Bell Organization Tech. J. 57 (six): 2021–2048. CiteSeerX10.1.one.138.35. doi:x.1002/j.1538-7305.1978.tb02141.x. S2CID 17510065. (Annotation: The PDF is an OCR browse of the original, and contains a rendering of "IBM 370" as "IBM 310".)
^ McIlroy, G. D. (1987). A Research Unix reader: annotated excerpts from the Developer's Manual, 1971–1986 (PDF) (Technical written report). CSTR. Bong Labs. p. x. 139. Archived (PDF) from the original on November 11, 2017. Retrieved February ane, 2015.
^ "C manual pages". FreeBSD Miscellaneous Information Manual (FreeBSD 13.0 ed.). May 30, 2011. Archived from the original on January 21, 2021. Retrieved January 15, 2021. [one] Archived January 21, 2021, at the Wayback Machine
^ Kernighan, Brian W.; Ritchie, Dennis M. (March 1988). The C Programming Language (2nd ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-110362-vii.
^ Stroustrup, Bjarne (2002). Sibling rivalry: C and C++ (PDF) (Study). AT&T Labs. Archived (PDF) from the original on Baronial 24, 2014. Retrieved April xiv, 2014.
^ C Integrity. International System for Standardization. March 30, 1995. Archived from the original on July 25, 2018. Retrieved July 24, 2018.
^ "JTC1/SC22/WG14 – C". Home folio. ISO/IEC. Archived from the original on February 12, 2018. Retrieved June 2, 2011.
^ Andrew Binstock (October 12, 2011). "Interview with Herb Sutter". Dr. Dobbs. Archived from the original on August ii, 2013. Retrieved September 7, 2013.
^ "Revised C23 Schedule WG fourteen Due north 2759" (PDF). www.open up-std.org. Archived (PDF) from the original on June 24, 2021. Retrieved Oct x, 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. 3.
^ "ISO/IEC 9899:201x (ISO C11) Commission Draft" (PDF). Archived (PDF) from the original on Dec 22, 2017. Retrieved September 16, 2011.
^ Kernighan & Ritchie (1996), pp. 192, 259.
^ "x 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 (third ed.). Otsego, MI: PageFree Publishing Inc. p. 20. ISBN978-ane-58961-237-ii. Archived from the original on July 29, 2020. Retrieved February 10, 2012.
^ Kernighan & Ritchie (1978), p. 6.
^ ^a ^b ^c ^d ^e ^f ^g Klemens, Ben (2013). 21st Century C. O'Reilly Media. ISBN978-ane-4493-2714-9.
^ Feuer, Alan R.; Gehani, Narain H. (March 1982). "Comparison of the Programming Languages C and Pascal". ACM Computing Surveys. fourteen (1): 73–92. doi:10.1145/356869.356872. S2CID 3136859.
^ Kernighan & Ritchie (1996), p. 122.
^ For example, gcc provides _FORTIFY_SOURCE. "Security Features: Compile Fourth dimension Buffer Checks (FORTIFY_SOURCE)". fedoraproject.org. Archived from the original on Jan 7, 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-four.
^ Raymond, Eric S. (October eleven, 1996). The New Hacker's Dictionary (3rd ed.). MIT Press. p. 432. ISBN978-0-262-68092-9. Archived from the original on November 12, 2012. Retrieved August 5, 2012.
^ "Man Folio for lint (freebsd Section 1)". unix.com. May 24, 2001. Retrieved July fifteen, 2014.
^ Dale, Nell B.; Weems, Bit (2014). Programming and problem solving with C++ (6th 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 i, 2005. Archived from the original on Feb 13, 2010. Retrieved January 4, 2010.
^ McMillan, Robert (August 1, 2013). "Is Coffee 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 engineering science firms. ISBN978-3319214641. OCLC 922324121.
^ Rauchwerger, Lawrence (2004). Languages and compilers for parallel computing : 16th international workshop, LCPC 2003, College Station, TX, Usa, Oct 2-iv, 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 Grand. (March 1993). "The Evolution of the C Language". ACM SIGPLAN Notices. ACM. 28 (3): 201–208. doi:10.1145/155360.155580.
- By courtesy of the author, also at Ritchie, Dennis M. "Chistory". www.bell-labs.com . Retrieved March 29, 2022.
Ritchie, Dennis M. (1993). "The Development of the C Language". The Second ACM SIGPLAN Conference on History of Programming Languages (HOPL-II). ACM. pp. 201–208. doi:10.1145/154766.155580. ISBN0-89791-570-4 . Retrieved November 4, 2014.
Kernighan, Brian Westward.; Ritchie, Dennis M. (1996). The C Programming Language (2nd ed.). Prentice Hall. ISBN7-302-02412-X.

External links [edit]

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

meighancoust1957.blogspot.com

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