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Full general-purpose programming linguistic communication

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

Stable release	C17 / June 2018; 3 years agone (2018-06)
Preview release	C2x (N2731) / October 18, 2021; 4 months ago (2021-10-18) ^[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, Separate-C, Cilk, C*
Influenced by
B (BCPL, CPL), ALGOL 68,^[4] assembly, PL/I, FORTRAN
Influenced
Numerous: AMPL, AWK, csh, C++, C--, C#, Objective-C, D, Go, Coffee, JavaScript, JS++, Julia, Limbo, LPC, Perl, PHP, Pike, Processing, Python, Ring,^[v]Rust, Seed7, Vala, Verilog (HDL),^[six] Nim, Zig
C Programming at Wikibooks

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

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

C is an imperative procedural language. It was designed to be compiled to provide low-level access to retentiveness and language constructs that map efficiently to machine instructions, all with minimal runtime support. Despite its low-level capabilities, the linguistic communication was designed to encourage cross-platform programming. A standards-compliant C program written with portability in mind tin can be compiled for a broad diversity of computer platforms and operating systems with few changes to its source code.^[ten]

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

Overview [edit]

Like most procedural languages in the ALGOL tradition, C has facilities for structured programming and allows lexical variable telescopic and recursion. Its static type system prevents unintended operations. In C, all executable code is independent within subroutines (likewise called "functions", though not strictly in the sense of functional programming). Function parameters are always passed past value (except arrays). Pass-by-reference is simulated in C by explicitly passing pointer values. C program source text is gratuitous-format, using the semicolon every bit a statement terminator and curly braces for grouping blocks of statements.

The C language too exhibits the following characteristics:

The language has a small, fixed number of keywords, including a full set up of control menses primitives: if/else, for, do/while, while, and switch. User-defined names are non distinguished from keywords past any kind of sigil.
It has a big number of arithmetic, bitwise, and logic operators: +,+=,++,&,||, etc.
More than one assignment may be performed in a unmarried statement.
Functions:
- Function render values tin can be ignored, when non needed.
- Part and data pointers permit ad hoc run-time polymorphism.
- Functions may not exist defined within the lexical scope of other functions.
Data typing is static, but weakly enforced; all information has a type, but implicit conversions are possible.
Annunciation syntax mimics usage context. C has no "ascertain" keyword; instead, a statement starting time with the name of a blazon is taken equally a declaration. In that location is no "function" keyword; instead, a function is indicated by the presence of a parenthesized statement list.
User-defined (typedef) and compound types are possible.
- Heterogeneous aggregate data types (struct) allow related data elements to be accessed and assigned every bit a unit.
- Union is a structure with overlapping members; merely the last member stored is valid.
- Array indexing is a secondary notation, defined in terms of arrow arithmetics. Dissimilar structs, arrays are not first-grade objects: they cannot be assigned or compared using unmarried built-in operators. At that place is no "array" keyword in use or definition; instead, foursquare brackets bespeak arrays syntactically, for case calendar month[11].
- Enumerated types are possible with the enum keyword. They are freely interconvertible with integers.
- Strings are not a singled-out data type, but are conventionally implemented as zero-terminated character arrays.
Low-level access to computer memory is possible by converting machine addresses to typed pointers.
Procedures (subroutines not returning values) are a special example of office, with an untyped return type void.
A preprocessor performs macro definition, source code file inclusion, and conditional compilation.
There 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, string 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, oft through the use of external libraries (eastward.m., the GLib Object System or the Boehm garbage collector).

Relations to other languages [edit]

Many afterward languages have borrowed directly or indirectly from C, including C++, C#, Unix'due south C beat out, D, Go, Java, JavaScript (including transpilers), Julia, Limbo, LPC, Objective-C, Perl, PHP, Python, Cherry, Rust, Swift, Verilog and SystemVerilog (hardware description languages).^[six] These languages have drawn many of their control structures and other basic features from C. Near of them (Python existence a dramatic exception) too express highly like 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 developments [edit]

Timeline of language evolution
Twelvemonth	C Standard^[10]
1972	Nascency
1978	K&R C
1989/1990	ANSI C and ISO C
1999	C99
2011	C11
2017	C17
TBD	C2x

The origin of C is closely tied to the development of the Unix operating system, originally implemented in associates linguistic communication on a PDP-7 by Dennis Ritchie and Ken Thompson, incorporating several ideas from colleagues. Eventually, they decided to port the operating arrangement to a PDP-11. The original PDP-11 version of Unix was likewise developed in assembly language.^[7]

Thompson desired a programming language to make utilities for the new platform. At showtime, he tried to make a Fortran compiler, just presently gave up the idea. Instead, he created a cut-down version of the recently developed BCPL systems programming language. The official clarification of BCPL was not available at the time,^[12] and Thompson modified the syntax to exist less wordy, producing the like only somewhat simpler B.^[7] However, few utilities were ultimately written in B because it was too slow, and B could non have reward of PDP-eleven features such every bit byte addressability.

In 1972, Ritchie started to improve B, most notably adding data typing for variables, which resulted in creating a new language C.^[13] The C compiler and some utilities made with it were included in Version 2 Unix.^[fourteen]

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

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

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

K&R C [edit]

In 1978, Brian Kernighan and Dennis Ritchie published the start edition of The C Programming Linguistic communication.^[1] This book, known to C programmers equally Thousand&R, served for many years as an informal specification of the language. The version of C that information technology describes is unremarkably referred to equally "Thou&R C". As this was released in 1978, information technology is also referred to as C78.^[15] The second edition of the book^[16] covers the later ANSI C standard, described below.

M&R introduced several linguistic communication features:

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

Even later the publication of the 1989 ANSI standard, for many years Chiliad&R C was even so considered the "lowest mutual denominator" to which C programmers restricted themselves when maximum portability was desired, since many older compilers were all the same in use, and because carefully written K&R C code can be legal Standard C every bit well.

In early versions of C, but functions that render types other than int must be declared if used before the function definition; functions used without prior proclamation 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 type specifiers which are commented out could be omitted in Thou&R C, but are required in later on standards.

Since K&R function declarations did not include any data almost function arguments, role parameter type checks were not performed, although some compilers would issue a warning bulletin if a local function was called with the incorrect number of arguments, or if multiple calls to an external part used dissimilar numbers or types of arguments. Separate tools such every bit Unix's lint utility were adult that (amid 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 item PCC^[17]) and some other vendors. These included:

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

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

ANSI C and ISO C [edit]

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

In 1983, the American National Standards Establish (ANSI) formed a committee, X3J11, to establish a standard specification of C. X3J11 based the C standard on the Unix implementation; yet, the non-portable portion of the Unix C library was handed off to the IEEE working group 1003 to become the basis for the 1988 POSIX standard. In 1989, the C standard was ratified as ANSI X3.159-1989 "Programming Language C". This version of the linguistic communication is often referred to as ANSI C, Standard C, or sometimes C89.

In 1990, the ANSI C standard (with formatting changes) was adopted past the International Organization for Standardization (ISO) every bit 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 twelvemonth of ISO publication.

One of the aims of the C standardization process was to produce a superset of K&R C, incorporating many of the subsequently introduced unofficial features. The standards commission 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 style used in C++, the One thousand&R interface connected to be permitted, for compatibility with existing source code.

C89 is supported by current C compilers, and most modern C lawmaking is based on information technology. Whatever plan written simply in Standard C and without any hardware-dependent assumptions will run correctly on whatever 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 use of non-standard libraries, such as GUI libraries, or to a reliance on compiler- or platform-specific attributes such equally the exact size of data types and byte endianness.

In cases where code must exist compilable by either standard-conforming or K&R C-based compilers, the __STDC__ macro can be used to separate the lawmaking into Standard and Thou&R sections to prevent the utilise on a One thousand&R C-based compiler of features available just in Standard C.

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

C99 [edit]

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

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

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

In add-on, back up for Unicode identifiers (variable / role names) in the class of escaped characters (e.g. \U0001f431) is now required. Back up for raw Unicode names is optional.

C11 [edit]

In 2007, work began on another revision of the C standard, informally 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 non been tested by existing implementations.

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

C17 [edit]

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

C2x [edit]

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

Embedded C [edit]

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

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

Syntax [edit]

C has a formal grammer specified past the C standard.^[23] Line endings are by and large not pregnant in C; nevertheless, line boundaries do take significance during the preprocessing phase. Comments may appear either between 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 appear inside string or character literals.^[24]

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

As an imperative language, C uses statements to specify actions. The near common statement is an expression statement, consisting of an expression to exist evaluated, followed by a semicolon; as a side effect of the evaluation, functions may be called and variables may be assigned new values. To modify the normal sequential execution of statements, C provides several control-menstruum statements identified by reserved keywords. Structured programming is supported by if … [else] conditional execution and past do … while, while, and for iterative execution (looping). The for statement has separate initialization, testing, and reinitialization expressions, any or all of which tin can be omitted. suspension and keep tin be used to leave the innermost enclosing loop statement or skip to its reinitialization. At that place is also a not-structured goto statement which branches directly to the designated label within the function. switch selects a instance to exist executed based on the value of an integer expression.

Expressions can employ a variety of built-in operators and may contain role calls. The order in which arguments to functions and operands to most operators are evaluated is unspecified. The evaluations may even be interleaved. However, all side effects (including storage to variables) volition occur before the adjacent "sequence indicate"; sequence points include the end of each expression statement, and the entry to and return from each function telephone call. Sequence points also occur during evaluation of expressions containing certain operators (&&, ||, ?: and the comma operator). This permits a high caste of object code optimization by the compiler, just requires C programmers to have more intendance to obtain reliable results than is needed for other programming languages.

Kernighan and Ritchie say in the Introduction of The C Programming Language: "C, like whatever other language, has its blemishes. Some of the operators have the wrong precedence; some parts of the syntax could be better."^[25] The C standard did not endeavor to right many of these blemishes, because of the touch on of such changes on already existing software.

Character set [edit]

The bones C source character fix includes the post-obit 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 need not correspond to an actual single character, although for convenience C treats it as ane.

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

The bones C execution character fix contains the same characters, along with representations for alert, backspace, and carriage render. 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 any purposes other than those for which they are predefined:

automobile
pause
case
char
const
proceed
default
exercise
double
else
enum
extern
bladder
for
goto
if
int
long
register
return
curt
signed
sizeof
static
struct
switch
typedef
marriage
unsigned
void
volatile
while

C99 reserved five more than words:

_Bool
_Complex
_Imaginary
inline
restrict

C11 reserved seven more words:^[26]

_Alignas
_Alignof
_Atomic
_Generic
_Noreturn
_Static_assert
_Thread_local

Most of the recently reserved words begin with an underscore followed past a capital letter, because identifiers of that form were previously reserved by the C standard for utilise simply by implementations. Since existing program source lawmaking should not take been using these identifiers, it would non be affected when C implementations started supporting these extensions to the programming language. Some standard headers do define more user-friendly synonyms for underscored identifiers. The language previously included a reserved discussion called entry, just this was seldom implemented, and has now been removed equally a reserved word.^[27]

Operators [edit]

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

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

C uses the operator = (used in mathematics to express equality) to point assignment, following the precedent of Fortran and PL/I, but dissimilar ALGOL and its derivatives. C uses the operator == to test for equality. The similarity between these 2 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 error message (although some compilers produce warnings). For case, the provisional expression if (a == b + ane) might mistakenly be written as if (a = b + 1), which will be evaluated as true if a is not zero after the consignment.^[28]

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

"Hello, world" case [edit]

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

The original version was:^[30]

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

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

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

The first line of the program contains a preprocessing directive, indicated past #include. This causes the compiler to supervene upon 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 aforementioned name, as opposed to double quotes which typically include local or project-specific header files.

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

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

The next line calls (diverts execution to) a function named printf, which in this case is supplied from a system library. In this phone call, the printf function 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, set up automatically past the compiler with a final 0-valued character to marker the end of the array (printf needs to know this). The \due north is an escape sequence that C translates to a newline character, which on output signifies the end of the current line. The render value of the printf function is of blazon int, merely it is silently discarded since it is not used. (A more than careful program might test the return value to determine whether or not the printf function succeeded.) The semicolon ; terminates the statement.

The endmost curly brace indicates the end of the code for the main function. According to the C99 specification and newer, the master office, unlike any other part, volition implicitly return a value of 0 upon reaching the } that terminates the office. (Formerly an explicit return 0; argument was required.) This is interpreted by the run-time system every bit an exit lawmaking indicating successful execution.^[31]

Data types [edit]

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

C is often used in depression-level systems programming where escapes from the type arrangement may be necessary. The compiler attempts to ensure type definiteness of most expressions, just 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 $.25 of a information object in some other style.

Some find C'southward declaration syntax unintuitive, particularly for part pointers. (Ritchie's thought was to declare identifiers in contexts resembling their use: "declaration reflects use".)^[33]

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

Pointers [edit]

C supports the employ of pointers, a type of reference that records the accost or location of an object or function in retentivity. Pointers can be dereferenced to access data stored at the address pointed to, or to invoke a pointed-to function. Pointers tin can be manipulated using assignment or pointer arithmetic. The run-time representation of a pointer value is typically a raw retentivity address (perhaps augmented by an showtime-inside-discussion field), just since a pointer's type includes the type of the matter pointed to, expressions including pointers tin exist type-checked at compile time. Pointer arithmetic is automatically scaled by the size of the pointed-to data blazon. Pointers are used for many purposes in C. Text strings are ordinarily manipulated using pointers into arrays of characters. Dynamic memory allocation is performed using pointers. Many data types, such as copse, are ordinarily implemented equally dynamically allocated struct objects linked together using pointers. Pointers to functions are useful for passing functions as arguments to higher-order functions (such every bit qsort or bsearch) or as callbacks to be invoked past upshot handlers.^[31]

A zero pointer value explicitly points to no valid location. Dereferencing a null pointer value is undefined, often resulting in a division fault. Null pointer values are useful for indicating special cases such as no "next" pointer in the final node of a linked list, or equally an error indication from functions returning pointers. In advisable contexts in source code, such as for assigning to a pointer variable, a cipher pointer constant tin be written as 0, with or without explicit casting to a pointer type, or as the NULL macro defined by several standard headers. In conditional contexts, null pointer values evaluate to fake, while all other pointer values evaluate to true.

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

Devil-may-care use of pointers is potentially dangerous. Considering they are typically unchecked, a pointer variable tin 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 dangerous places by using invalid pointer arithmetics; the objects they point to may keep to be used afterwards deallocation (dangling pointers); they may be used without having been initialized (wild pointers); or they may be direct assigned an dangerous value using a cast, marriage, or through another corrupt pointer. In general, C is permissive in assuasive manipulation of and conversion between pointer types, although compilers typically provide options for various levels of checking. Some other programming languages address these problems past using more restrictive reference types.

Arrays [edit]

Array types in C are traditionally of a fixed, static size specified at compile time. The more recent C99 standard besides allows a form of variable-length arrays. Notwithstanding, information technology is also possible to allocate a block of memory (of arbitrary size) at run-fourth dimension, using the standard library'due south malloc office, and treat it as an array.

Since arrays are always accessed (in event) via pointers, assortment accesses are typically not checked against the underlying assortment size, although some compilers may provide premises checking every bit an option.^[34] ^[35] Array bounds violations are therefore possible and can lead to diverse repercussions, including illegal memory accesses, corruption of information, buffer overruns, and run-time exceptions.

C does not have a special provision for declaring multi-dimensional arrays, but rather relies on recursion within the type system to declare arrays of arrays, which effectively accomplishes the same thing. The index values of the resulting "multi-dimensional array" can be thought of as increasing in row-major social club. Multi-dimensional arrays are commonly used in numerical algorithms (mainly from practical linear algebra) to store matrices. The structure of the C array is well suited to this particular task. Even so, in early versions of C the premises of the array 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 issue.

The following example using modern C (C99 or afterwards) shows allocation of a two-dimensional array on the heap and the use of multi-dimensional array indexing for accesses (which can employ premises-checking on many C compilers):

                        int                                    func            (            int                                    N            ,                                    int                                    M            )                        {                                                float                                    (            *            p            )[            N            ][            Yard            ]                                    =                                    malloc            (            sizeof                                    *            p            );                                                if                                    (            !            p            )                                                return                                    -1            ;                                                for                                    (            int                                    i                                    =                                    0            ;                                    i                                    <                                    Due north            ;                                    i            ++            )                                                for                                    (            int                                    j                                    =                                    0            ;                                    j                                    <                                    M            ;                                    j            ++            )                                                (            *            p            )[            i            ][            j            ]                                    =                                    i                                    +                                    j            ;                                                print_array            (            Due north            ,                                    Grand            ,                                    p            );                                                gratis            (            p            );                                                render                                    1            ;                        }

Assortment–pointer interchangeability [edit]

The subscript annotation x[i] (where ten designates a pointer) is syntactic sugar for *(x+i).^[36] Taking advantage of the compiler's knowledge of the pointer type, the address that ten + i points to is not the base of operations address (pointed to by ten) incremented by i bytes, but rather is defined to be the base of operations address incremented by i multiplied by the size of an element that x points to. Thus, x[i] designates the i+ith element of the array.

Furthermore, in most expression contexts (a notable exception is every bit operand of sizeof), an expression of array blazon is automatically converted to a pointer to the array'due south first element. This implies that an array is never copied as a whole when named as an argument to a office, merely rather merely the address of its kickoff chemical element is passed. Therefore, although function calls in C employ pass-by-value semantics, arrays are in effect passed by reference.

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

Memory direction [edit]

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

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

These 3 approaches are appropriate in different situations and have various merchandise-offs. For example, static memory allocation has little resource allotment overhead, automated resource allotment may involve slightly more than overhead, and dynamic retentiveness allocation can potentially have a swell deal of overhead for both resource allotment and deallocation. The persistent nature of static objects is useful for maintaining state data across function calls, automated allotment is easy to utilise but stack space is typically much more limited and transient than either static memory or heap space, and dynamic memory allocation allows convenient allocation of objects whose size is known just at run-time. Most C programs make all-encompassing use of all three.

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

Unless otherwise specified, static objects comprise cypher or null arrow values upon program startup. Automatically and dynamically allocated objects are initialized merely if an initial value is explicitly specified; otherwise they initially accept indeterminate values (typically, whatever bit design happens to exist present in the storage, which might non even stand for a valid value for that type). If the programme attempts to admission an uninitialized value, the results are undefined. Many mod compilers endeavor to detect and warn about this problem, but both false positives and false negatives can occur.

Heap retentiveness resource allotment has to be synchronized with its bodily usage in whatsoever program to be reused equally much equally possible. For example, if the merely pointer to a heap memory allocation goes out of scope or has its value overwritten before it is deallocated explicitly, then that memory cannot exist recovered for later reuse and is substantially lost to the program, a phenomenon known every bit a retentivity leak. Conversely, it is possible for retentivity to be freed, merely is referenced after, leading to unpredictable results. Typically, the failure symptoms appear in a portion of the program unrelated to the code that causes the fault, making it difficult to diagnose the failure. Such issues are ameliorated in languages with automatic garbage collection.

Libraries [edit]

The C programming language uses libraries every bit its primary method of extension. In C, a library is a set of functions contained 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 be used past a programme, and declarations of special information types and macro symbols used with these functions. In order for a plan to apply 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 well-nigh 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 merely a subset of the standard library). This library supports stream input and output, retention allocation, mathematics, character strings, and fourth dimension values. Several separate standard headers (for example, stdio.h) specify the interfaces for these and other standard library facilities.

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

Since many programs take been written in C, there are a wide diversity 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 can exist used from college-level languages similar Coffee, Perl, and Python.^[31]

File handling and streams [edit]

File input and output (I/O) is not office of the C linguistic communication itself but instead is handled by libraries (such as the C standard library) and their associated header files (east.yard. stdio.h). File treatment is more often than not implemented through high-level I/O which works through streams. A stream is from this perspective a data flow that is independent of devices, while a file is a 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 expanse or queue) is temporarily used to store data before it's sent to the concluding destination. This reduces the time spent waiting for slower devices, for example a hard drive or solid state drive. Low-level I/O functions are not function of the standard C library^{[ clarification needed ]} but are mostly part of "bare metal" programming (programming that's independent 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 assist C programmers find and gear up statements with undefined behavior or possibly erroneous expressions, with greater rigor than that provided past the compiler. The tool lint was the kickoff such, leading to many others.

Automated source code checking and auditing are benign in any language, and for C many such tools be, such as Lint. A common practice is to apply Lint to detect questionable code when a program is get-go written. One time a program passes Lint, it is then compiled using the C compiler. Also, many compilers can optionally warn about syntactically valid constructs that are probable to actually exist errors. MISRA C is a proprietary set up of guidelines to avoid such questionable code, adult for embedded systems.^[37]

In that location are also compilers, libraries, and operating organization 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 retentiveness allocation functions can assist uncover runtime errors in retentivity usage.

Uses [edit]

The C Programming Language

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

C can be used for website programming using the Mutual Gateway Interface (CGI) equally a "gateway" for data between the Web application, the server, and the browser.^[39] C is oft 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 often implemented in C. For case, the reference implementations of Python, Perl, Blood-red, and PHP are written in C.

C enables programmers to create efficient implementations of algorithms and data structures, considering the layer of brainchild from hardware is sparse, and its overhead is depression, 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 past implementations of other languages. This approach may be used for portability or convenience; by using C every bit an intermediate language, additional machine-specific lawmaking 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. However, some of C'due south shortcomings take prompted the development of other C-based languages specifically designed for use as intermediate languages, such as C--.

C has likewise been widely used to implement end-user applications. However, such applications can also exist written in newer, higher-level languages.

[edit]

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

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

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

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

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

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

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

Notes [edit]

^ The original example code will compile on most mod compilers that are not in strict standard compliance mode, but it does not fully accommodate to the requirements of either C89 or C99. In fact, C99 requires that a diagnostic message exist produced.
^ The main function really has ii arguments, int argc and char *argv[], respectively, which can be used to handle command line arguments. The ISO C standard (section v.ane.2.2.1) requires both forms of main to be supported, which is special treatment not afforded to any other function.

References [edit]

^ ^a ^b Kernighan, Brian Westward.; Ritchie, Dennis M. (February 1978). The C Programming Linguistic communication (1st ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-thirteen-110163-0.
^ Ritchie (1993): "Thompson had fabricated a brief attempt to produce a system coded in an early on version of C—before structures—in 1972, but gave up the effort."
^ Fruderica (December 13, 2020). "History of C". The cppreference.com. Archived from the original on October 24, 2020. Retrieved October 24, 2020.
^ Ritchie (1993): "The scheme of type composition adopted by C owes considerable debt to Algol 68, although information technology did not, perhaps, sally in a form that Algol's adherents would approve of."
^ Ring Team (October 23, 2021). "The Band programming linguistic communication and other languages". band-lang.internet.
^ ^a ^b "Verilog HDL (and C)" (PDF). The Research School of Information science at the Australian National University. June 3, 2010. Archived from the original (PDF) on Nov 6, 2013. Retrieved August 19, 2013. 1980s: ; Verilog first introduced ; Verilog inspired past the C programming linguistic communication
^ ^a ^b ^c ^d ^e Ritchie (1993)
^ "Programming Linguistic communication Popularity". 2009. Archived from the original on January xvi, 2009. Retrieved January xvi, 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 Oct 7, 2021.
^ Ritchie, Dennis. "BCPL to B to C". Archived from the original on Dec 12, 2019. Retrieved September 10, 2019.
^ ^a ^b Johnson, S. C.; Ritchie, D. K. (1978). "Portability of C Programs and the UNIX System". Bell Organisation Tech. J. 57 (vi): 2021–2048. CiteSeerX10.1.1.138.35. doi:x.1002/j.1538-7305.1978.tb02141.x. S2CID 17510065. (Annotation: The PDF is an OCR scan of the original, and contains a rendering of "IBM 370" equally "IBM 310".)
^ McIlroy, 1000. D. (1987). A Enquiry Unix reader: annotated excerpts from the Programmer'south Manual, 1971–1986 (PDF) (Technical report). CSTR. Bell Labs. p. ten. 139. Archived (PDF) from the original on November 11, 2017. Retrieved February one, 2015.
^ "C manual pages". FreeBSD Miscellaneous Data Manual (FreeBSD 13.0 ed.). May thirty, 2011. Archived from the original on Jan 21, 2021. Retrieved January xv, 2021. [i] Archived January 21, 2021, at the Wayback Machine
^ Kernighan, Brian West.; Ritchie, Dennis M. (March 1988). The C Programming Language (2nd ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-110362-7.
^ Stroustrup, Bjarne (2002). Sibling rivalry: C and C++ (PDF) (Report). AT&T Labs. Archived (PDF) from the original on August 24, 2014. Retrieved April 14, 2014.
^ C Integrity. International Organization for Standardization. March xxx, 1995. Archived from the original on July 25, 2018. Retrieved July 24, 2018.
^ "JTC1/SC22/WG14 – C". Home page. ISO/IEC. Archived from the original on February 12, 2018. Retrieved June 2, 2011.
^ Andrew Binstock (October 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 ten, 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 Fifty. (2002). C: A Reference Manual (5th ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-089592-nine. Contains a BNF grammar for C.
^ Kernighan & Ritchie (1996), p. 192.
^ Kernighan & Ritchie (1978), p. 3.
^ "ISO/IEC 9899:201x (ISO C11) Committee Draft" (PDF). Archived (PDF) from the original on December 22, 2017. Retrieved September sixteen, 2011.
^ Kernighan & Ritchie (1996), pp. 192, 259.
^ "x Mutual 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 (3rd ed.). Otsego, MI: PageFree Publishing Inc. p. twenty. ISBN978-ane-58961-237-2. Archived from the original on July 29, 2020. Retrieved February 10, 2012.
^ Kernighan & Ritchie (1978), p. 6.
^ ^a ^b ^c ^d ^eastward ^f ^g Klemens, Ben (2013). 21st Century C. O'Reilly Media. ISBN978-1-4493-2714-nine.
^ Feuer, Alan R.; Gehani, Narain H. (March 1982). "Comparing of the Programming Languages C and Pascal". ACM Computing Surveys. 14 (1): 73–92. doi:x.1145/356869.356872. S2CID 3136859.
^ Kernighan & Ritchie (1996), p. 122.
^ For instance, gcc provides _FORTIFY_SOURCE. "Security Features: Compile Time Buffer Checks (FORTIFY_SOURCE)". fedoraproject.org. Archived from the original on January vii, 2007. Retrieved August 5, 2012.
^ เอี่ยมสิริวงศ์, โอภาศ (2016). Programming with C. Bangkok, Thailand: SE-Didactics PUBLIC Company Limited. pp. 225–230. ISBN978-616-08-2740-4.
^ Raymond, Eric S. (October 11, 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 Page for lint (freebsd Section 1)". unix.com. May 24, 2001. Retrieved July 15, 2014.
^ Dale, Nell B.; Weems, Chip (2014). Programming and problem solving with C++ (6th ed.). Burlington, MA: Jones & Bartlett Learning. ISBN978-1449694289. OCLC 894992484.
^ Dr. Dobb's Sourcebook. U.Southward.A.: Miller Freeman, Inc. November–December 1995.
^ "Using C for CGI Programming". linuxjournal.com. March 1, 2005. Archived from the original on Feb 13, 2010. Retrieved Jan 4, 2010.
^ McMillan, Robert (Baronial i, 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 calculating : 16th international workshop, LCPC 2003, College Station, TX, USA, October 2-4, 2003 : revised papers. Springer. ISBN978-3540246442. OCLC 57965544.
^ Stroustrup, Bjarne (1993). "A History of C++: 1979−1991" (PDF). Archived (PDF) from the original on Feb two, 2019. Retrieved June 9, 2011.

Sources [edit]

Ritchie, Dennis M. (March 1993). "The Development of the C Linguistic communication". ACM SIGPLAN Notices. ACM. 28 (3): 201–208. doi:x.1145/155360.155580.
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:x.1145/154766.155580. ISBN0-89791-570-4 . Retrieved November 4, 2014.
Kernighan, Brian W.; Ritchie, Dennis K. (1996). The C Programming Linguistic communication (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 Frequently Asked Questions
A History of C, past Dennis Ritchie

kunzeounins.blogspot.com

Source: https://en.wikipedia.org/wiki/C_%28programming_language%29