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General-purpose programming language

C
Major implementations
The C Programming Language ^[1] (frequently referred to equally K&R), the seminal volume on C
Epitome	Multi-epitome: imperative (procedural), structured
Designed by	Dennis Ritchie
Programmer	Dennis Ritchie & Bell Labs (creators); ANSI X3J11 (ANSI C); ISO/IEC JTC1/SC22/WG14 (ISO C)
Get-go appeared	1972; 50 years agone (1972) ^[2]

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

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

C (, as in the alphabetic characterc) is a general-purpose, procedural reckoner programming language supporting structured programming, lexical variable scope, and recursion, with a static blazon system. Past design, C provides constructs that map efficiently to typical machine instructions. It has plant lasting use in applications previously coded in assembly linguistic communication. Such applications include operating systems and diverse awarding software for figurer architectures that range from supercomputers to PLCs and embedded systems.

A successor to the programming linguistic communication B, C was originally developed at Bong Labs past Dennis Ritchie between 1972 and 1973 to construct utilities running on Unix. It was applied to re-implementing the kernel of the Unix operating organization.^[7] During the 1980s, C gradually gained popularity. It has become i of the well-nigh 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 Arrangement for Standardization (ISO).

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

Since 2000, C has consistently ranked amid the summit 2 languages in the TIOBE index, a measure of the popularity of programming languages.^[11]

Overview

Like virtually procedural languages in the ALGOL tradition, C has facilities for structured programming and allows lexical variable scope and recursion. Its static blazon arrangement prevents unintended operations. In C, all executable lawmaking is contained inside subroutines (also chosen "functions", though not strictly in the sense of functional programming). Function parameters are e'er passed by value (except arrays). Pass-past-reference is false in C by explicitly passing pointer values. C program source text is free-format, using the semicolon as a statement terminator and curly braces for grouping blocks of statements.

The C linguistic communication also exhibits the post-obit characteristics:

The language has a small, fixed number of keywords, including a full fix of control menses primitives: if/else, for, exercise/while, while, and switch. User-defined names are non distinguished from keywords by any kind of sigil.
It has a big number of arithmetic, bitwise, and logic operators: +,+=,++,&,||, etc.
More than one consignment may exist performed in a single statement.
Functions:
- Office return values can exist ignored, when not needed.
- Part and information pointers let ad hoc run-fourth dimension polymorphism.
- Functions may not be defined within the lexical scope of other functions.
Data typing is static, but weakly enforced; all data has a type, but implicit conversions are possible.
Declaration syntax mimics usage context. C has no "define" keyword; instead, a argument beginning with the name of a type is taken as a declaration. There is no "function" keyword; instead, a function is indicated by the presence of a parenthesized argument list.
User-defined (typedef) and compound types are possible.
- Heterogeneous aggregate data types (struct) permit related data elements to be accessed and assigned every bit a unit.
- Union is a structure with overlapping members; just the last member stored is valid.
- Assortment indexing is a secondary notation, defined in terms of pointer arithmetics. Unlike structs, arrays are not splendid objects: they cannot be assigned or compared using single congenital-in operators. There is no "array" keyword in use or definition; instead, foursquare brackets indicate arrays syntactically, for example month[11].
- Enumerated types are possible with the enum keyword. They are freely interconvertible with integers.
- Strings are non a distinct information type, but are conventionally implemented as zilch-terminated character arrays.
Depression-level access to computer memory is possible by converting car addresses to typed pointers.
Procedures (subroutines non returning values) are a special case of function, with an untyped render type void.
A preprocessor performs macro definition, source lawmaking file inclusion, and conditional compilation.
There is a basic form of modularity: files tin be compiled separately and linked together, with control over which functions and data objects are visible to other files via static and extern attributes.
Complex functionality such every bit I/O, string manipulation, and mathematical functions are consistently delegated to library routines.

While C does not include certain features establish in other languages (such equally object orientation and garbage collection), these tin exist implemented or emulated, often through the use of external libraries (eastward.thou., the GLib Object System or the Boehm garbage collector).

Relations to other languages

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

History

Early developments

Timeline of language development
Year	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 assembly linguistic communication on a PDP-7 by Dennis Ritchie and Ken Thompson, incorporating several ideas from colleagues. Eventually, they decided to port the operating organisation to a PDP-xi. The original PDP-11 version of Unix was also developed in assembly language.^[7]

Thompson desired a programming language to make utilities for the new platform. At first, he tried to brand a Fortran compiler, but before long gave up the idea. Instead, he created a cutting-down version of the recently developed BCPL systems programming language. The official description of BCPL was not bachelor at the time,^[12] and Thompson modified the syntax to be less wordy, producing the like but somewhat simpler B.^[7] However, few utilities were ultimately written in B because it was as well deadening, and B could not take advantage of PDP-xi features such as byte addressability.

In 1972, Ritchie started to improve B, near notably calculation information typing for variables, which resulted in creating a new language C.^[xiii] The C compiler and some utilities made with it were included in Version 2 Unix.^[14]

At Version 4 Unix, released in November 1973, the Unix kernel was extensively re-implemented in C.^[7] By this fourth dimension, 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 as well in recognition of the usefulness of the file-inclusion mechanisms available in BCPL and PL/I. Its original version provided merely included files and simple string replacements: #include and #define of parameterless macros. Soon after that, it was extended, more often than not past Mike Lesk and then by John Reiser, to incorporate macros with arguments and provisional compilation.^[vii]

Unix was one of the first operating arrangement kernels implemented in a language other than assembly. Earlier instances include the Multics system (which was written in PL/I) and Primary Control Program (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 every bit the basis for several implementations of C on new platforms.^[thirteen]

K&R C

In 1978, Brian Kernighan and Dennis Ritchie published the first edition of The C Programming Language.^[1] This book, known to C programmers as K&R, served for many years as an informal specification of the language. The version of C that it describes is commonly referred to as "K&R C". As this was released in 1978, it is also referred to as C78.^[fifteen] The second edition of the book^[xvi] covers the later ANSI C standard, described below.

K&R introduced several language features:

Standard I/O library
long int information type
unsigned int data type
Compound consignment operators of the form =op (such as =-) were changed 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 = -ten (let i be −x).

Even after the publication of the 1989 ANSI standard, for many years K&R C was still considered the "everyman common denominator" to which C programmers restricted themselves when maximum portability was desired, since many older compilers were notwithstanding in apply, and because carefully written K&R C code tin be legal Standard C equally well.

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

For example:

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

The int type specifiers which are commented out could be omitted in 1000&R C, but are required in later standards.

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

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

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

The large number of extensions and lack of agreement on a standard library, together with the linguistic communication popularity and the fact that non fifty-fifty the Unix compilers precisely implemented the K&R specification, led to the necessity of standardization.

ANSI C and ISO C

During the tardily 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 Plant (ANSI) formed a committee, X3J11, to establish a standard specification of C. X3J11 based the C standard on the Unix implementation; however, the non-portable portion of the Unix C library was handed off to the IEEE working group 1003 to become the ground for the 1988 POSIX standard. In 1989, the C standard was ratified as ANSI X3.159-1989 "Programming Language C". This version of the language is often referred to as ANSI C, Standard C, or sometimes C89.

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

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

Ane of the aims of the C standardization procedure was to produce a superset of M&R C, incorporating many of the afterward introduced unofficial features. The standards committee also included several boosted features such equally 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 style used in C++, the Thou&R interface connected to exist permitted, for compatibility with existing source code.

C89 is supported by electric current C compilers, and nigh modern C lawmaking is based on it. Whatever program written merely in Standard C and without whatsoever hardware-dependent assumptions volition run correctly on whatsoever platform with a befitting C implementation, within its resources limits. Without such precautions, programs may compile only on a sure platform or with a item compiler, due, for example, to the use of non-standard libraries, such as GUI libraries, or to a reliance on compiler- or platform-specific attributes such as the verbal size of data types and byte endianness.

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

After 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 together more than extensive support for international character sets.^[18]

C99

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

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

C99 is for the almost part astern uniform with C90, just is stricter in some ways; in detail, a proclamation that lacks a type specifier no longer has int implicitly causeless. A standard macro __STDC_VERSION__ is defined with value 199901L to indicate that C99 back up is bachelor. GCC, Solaris Studio, and other C compilers now support many or all of the new features of C99. The C compiler in Microsoft Visual C++, nevertheless, implements the C89 standard and those parts of C99 that are required for compatibility with C++xi.^[xx] ^{[ needs update ]}

In improver, 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

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 past existing implementations.

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

C17

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 defined as 201710L.

C2x

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

Embedded C

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

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

Syntax

C has a formal grammer specified by the C standard.^[23] Line endings are more often than not not significant in C; however, line boundaries do have significance during the preprocessing phase. Comments may announced either betwixt the delimiters /* and */, or (since C99) following // until the end of the line. Comments delimited by /* and */ do not nest, and these sequences of characters are not interpreted as comment delimiters if they appear within cord or character literals.^[24]

C source files contain declarations and part definitions. Function definitions, in plow, contain declarations and statements. Declarations either define new types using keywords such equally struct, union, and enum, or assign types to and peradventure reserve storage for new variables, usually by writing the type followed past the variable name. Keywords such as char and int specify built-in types. Sections of code are enclosed in braces ({ and }, sometimes called "curly brackets") to limit the 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 nearly common statement is an expression statement, consisting of an expression to exist evaluated, followed by a semicolon; as a side event 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-flow statements identified by reserved keywords. Structured programming is supported by if … [else] conditional execution and by do … while, while, and for iterative execution (looping). The for argument has dissever initialization, testing, and reinitialization expressions, any or all of which can exist omitted. break and keep can exist used to leave the innermost enclosing loop statement or skip to its reinitialization. There is also a non-structured goto argument which branches directly to the designated label within the function. switch selects a case to be executed based on the value of an integer expression.

Expressions can use a multifariousness of congenital-in operators and may contain function calls. The society in which arguments to functions and operands to most operators are evaluated is unspecified. The evaluations may fifty-fifty exist interleaved. Even so, all side effects (including storage to variables) will occur before the next "sequence point"; sequence points include the end of each expression statement, and the entry to and return from each part call. Sequence points also occur during evaluation of expressions containing certain operators (&&, ||, ?: and the comma operator). This permits a high degree of object lawmaking optimization past the compiler, but requires C programmers to accept 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, 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 non endeavour to correct many of these blemishes, considering of the impact of such changes on already existing software.

Character set up

The bones C source graphic symbol set includes the following characters:

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

Newline indicates the end of a text line; it need not correspond to an bodily single character, although for convenience C treats it as i.

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

The basic C execution graphic symbol set contains the aforementioned characters, forth with representations for alarm, backspace, and carriage return. Run-time support for extended grapheme sets has increased with each revision of the C standard.

Reserved words

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

motorcar
break
case
char
const
continue
default
do
double
else
enum
extern
float
for
goto
if
int
long
register
render
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 than words:^[26]

_Alignas
_Alignof
_Atomic
_Generic
_Noreturn
_Static_assert
_Thread_local

Most of the recently reserved words begin with an underscore followed past a uppercase, considering identifiers of that form were previously reserved by the C standard for use just by implementations. Since existing plan source code should not accept been using these identifiers, it would non be affected when C implementations started supporting these extensions to the programming language. Some standard headers exercise define more than user-friendly synonyms for underscored identifiers. The language previously included a reserved word called entry, just this was seldom implemented, and has at present been removed equally a reserved word.^[27]

Operators

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

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

C uses the operator = (used in mathematics to express equality) to indicate assignment, post-obit the precedent of Fortran and PL/I, just unlike ALGOL and its derivatives. C uses the operator == to test for equality. The similarity betwixt these two operators (assignment and equality) may event in the adventitious utilize 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 conditional expression if (a == b + 1) might mistakenly exist written as if (a = b + 1), which will be evaluated as true if a is not nil subsequently the assignment.^[28]

The C operator precedence is non always intuitive. For instance, the operator == binds more than tightly than (is executed prior to) the operators & (bitwise AND) and | (bitwise OR) in expressions such equally x & 1 == 0, which must be written as (x & one) == 0 if that is the coder's intent.^[29]

"Hello, world" example

The "hello, world" case, which appeared in the showtime edition of K&R, has go the model for an introductory plan in nearly programming textbooks. The program prints "hello, world" to the standard output, which is usually a final or screen display.

The original version was:^[30]

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

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

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

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

The adjacent line indicates that a function named main is being divers. The chief function serves a special purpose in C programs; the run-fourth dimension environment calls the main function to brainstorm program execution. The type specifier int indicates that the value that is returned to the invoker (in this instance the run-time surround) equally a result of evaluating the main function, is an integer. The keyword void every bit 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 instance is supplied from a system library. In this call, the printf role is passed (provided with) a unmarried argument, the address of the first character in the string literal "hello, world\north". The string literal is an unnamed array with elements of type char, prepare automatically by the compiler with a final 0-valued grapheme to marker the end of the array (printf needs to know this). The \north is an escape sequence that C translates to a newline character, which on output signifies the end of the electric current line. The return value of the printf function is of blazon int, merely it is silently discarded since it is non used. (A more careful program might test the return value to decide whether or non the printf function succeeded.) The semicolon ; terminates the argument.

The closing curly brace indicates the end of the code for the primary office. According to the C99 specification and newer, the main role, unlike whatever other office, will implicitly return a value of 0 upon reaching the } that terminates the function. (Formerly an explicit return 0; argument was required.) This is interpreted past the run-time organization as an leave code indicating successful execution.^[31]

Data types

The type system in C is static and weakly typed, which makes it like to the type organization of ALGOL descendants such equally Pascal.^[32] There are built-in types for integers of diverse sizes, both signed and unsigned, floating-point numbers, and enumerated types (enum). Integer blazon char is often used for unmarried-byte characters. C99 added a boolean datatype. In that location are too derived types including arrays, pointers, records (struct), and unions (union).

C is often used in low-level systems programming where escapes from the type organisation may be necessary. The compiler attempts to ensure type correctness of most expressions, just the developer can override the checks in various ways, either by using a type cast to explicitly convert a value from i blazon to some other, or by using pointers or unions to reinterpret the underlying bits of a data object in some other way.

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

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

Pointers

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

A null pointer value explicitly points to no valid location. Dereferencing a nada arrow value is undefined, ofttimes resulting in a partitioning fault. Zero pointer values are useful for indicating special cases such as no "next" pointer in the final node of a linked listing, or as an fault indication from functions returning pointers. In appropriate contexts in source code, such as for assigning to a arrow variable, a null arrow constant can be written every bit 0, with or without explicit casting to a pointer type, or as the NULL macro defined by several standard headers. In conditional contexts, zilch pointer values evaluate to false, while all other pointer values evaluate to true.

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

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

Arrays

Array types in C are traditionally of a fixed, static size specified at compile fourth dimension. The more contempo C99 standard also allows a grade of variable-length arrays. Withal, it is too possible to classify a block of memory (of arbitrary size) at run-fourth dimension, using the standard library's malloc function, and treat it as an array.

Since arrays are always accessed (in event) via pointers, array accesses are typically non checked confronting the underlying assortment size, although some compilers may provide premises checking as an selection.^[34] ^[35] Array bounds violations are therefore possible and can pb to diverse repercussions, including illegal memory accesses, corruption of data, 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 finer accomplishes the aforementioned thing. The index values of the resulting "multi-dimensional assortment" can be thought of as increasing in row-major society. Multi-dimensional arrays are commonly used in numerical algorithms (mainly from applied linear algebra) to store matrices. The structure of the C assortment is well suited to this particular task. However, 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 boosted "row vector" of pointers to the columns.) C99 introduced "variable-length arrays" which address this issue.

The following instance using modernistic C (C99 or subsequently) shows allocation of a two-dimensional assortment on the heap and the apply of multi-dimensional array indexing for accesses (which tin employ premises-checking on many C compilers):

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

Array–pointer interchangeability

The subscript notation x[i] (where x designates a arrow) is syntactic carbohydrate for *(x+i).^[36] Taking reward of the compiler'south noesis of the arrow type, the accost that x + i points to is not the base address (pointed to by x) incremented by i bytes, but rather is defined to be the base of operations accost incremented by i multiplied past the size of an element that 10 points to. Thus, x[i] designates the i+ith element of the array.

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

The total size of an array 10 can be determined by applying sizeof to an expression of assortment type. The size of an element can be adamant by applying the operator sizeof to whatsoever dereferenced element of an array A, as in northward = sizeof A[0]. This, the number of elements in a declared array A can be determined as sizeof A / sizeof A[0]. Note, that if merely a arrow to the first element is available as it is oftentimes the case in C code because of the automatic conversion described to a higher place, the information about the full blazon of the array and its length are lost.

Retentiveness management

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

Static retentiveness resource allotment: space for the object is provided in the binary at compile-fourth dimension; these objects have an extent (or lifetime) as long as the binary which contains them is loaded into retentivity.
Automatic memory resource allotment: 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 retention allocation: blocks of memory of arbitrary size can be requested at run-time using library functions such as malloc from a region of memory called the heap; these blocks persist until later on freed for reuse by calling the library function realloc or free

These 3 approaches are appropriate in different situations and have diverse trade-offs. For example, static memory allotment has little allocation overhead, automatic resource allotment may involve slightly more overhead, and dynamic retentivity allocation can potentially have a bang-up deal of overhead for both resource allotment and deallocation. The persistent nature of static objects is useful for maintaining land data beyond function calls, automated allocation is easy to use but stack infinite is typically much more than express and transient than either static memory or heap space, and dynamic memory allocation allows user-friendly allocation of objects whose size is known only at run-time. Near C programs make extensive use of all iii.

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

Unless otherwise specified, static objects contain zero or zip pointer values upon programme startup. Automatically and dynamically allocated objects are initialized but if an initial value is explicitly specified; otherwise they initially have indeterminate values (typically, whatever scrap blueprint happens to be nowadays in the storage, which might not even represent a valid value for that type). If the plan attempts to access an uninitialized value, the results are undefined. Many modern compilers try to detect and warn virtually this trouble, merely both fake positives and simulated negatives can occur.

Heap memory allocation has to exist synchronized with its actual usage in any programme to be reused as much equally possible. For example, if the but arrow to a heap memory allocation goes out of scope or has its value overwritten before information technology is deallocated explicitly, and so that memory cannot be recovered for later reuse and is essentially lost to the program, a phenomenon known as a memory leak. Conversely, it is possible for memory to be freed, but is referenced subsequently, leading to unpredictable results. Typically, the failure symptoms appear in a portion of the programme unrelated to the code that causes the error, making information technology difficult to diagnose the failure. Such issues are ameliorated in languages with automatic garbage collection.

Libraries

The C programming language uses libraries as its primary method of extension. In C, a library is a fix of functions contained inside 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 plan, and declarations of special data types and macro symbols used with these functions. In order for a plan to utilize a library, it must include the library'southward header file, and the library must exist linked with the program, which in many cases requires compiler flags (e.g., -lm, autograph for "link the math library").^[31]

The most common C library is the C standard library, which is specified by the ISO and ANSI C standards and comes with every C implementation (implementations which target limited environments such equally embedded systems may provide merely a subset of the standard library). This library supports stream input and output, memory resource allotment, 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 gear up of C library functions are those used by applications specifically targeted for Unix and Unix-like systems, peculiarly functions which provide an interface to the kernel. These functions are detailed in various standards such as POSIX and the Single UNIX Specification.

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

File handling and streams

File input and output (I/O) is not part of the C linguistic communication itself just instead is handled by libraries (such as the C standard library) and their associated header files (east.g. 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 loftier-level I/O is washed through the clan of a stream to a file. In the C standard library, a buffer (a retentivity area or queue) is temporarily used to store data earlier information technology's sent to the terminal destination. This reduces the time spent waiting for slower devices, for example a hard drive or solid state drive. Depression-level I/O functions are not function of the standard C library^{[ description needed ]} but are mostly part of "bare metal" programming (programming that's independent of any operating system such every bit most embedded programming). With few exceptions, implementations include low-level I/O.

Language tools

A number of tools have been adult to help C programmers find and set up statements with undefined behavior or possibly erroneous expressions, with greater rigor than that provided by the compiler. The tool lint was the commencement such, leading to many others.

Automated source code checking and auditing are beneficial in any linguistic communication, and for C many such tools exist, such equally Lint. A mutual practise is to use Lint to detect questionable code when a program is first written. Once a plan passes Lint, it is so compiled using the C compiler. Also, many compilers can optionally warn about syntactically valid constructs that are likely to actually be errors. MISRA C is a proprietary set of guidelines to avert such questionable code, developed for embedded systems.^[37]

There are besides compilers, libraries, and operating organisation level mechanisms for performing deportment that are non a standard part of C, such every bit bounds checking for arrays, detection of buffer overflow, serialization, dynamic retention tracking, and automatic garbage collection.

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

Uses

The C Programming Language

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

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

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

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

C is sometimes used as an intermediate language by implementations of other languages. This approach may be used for portability or convenience; by using C as an intermediate language, additional machine-specific lawmaking generators are non 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. Still, some of C'south shortcomings have prompted the development of other C-based languages specifically designed for use every bit intermediate languages, such equally C--.

C has too been widely used to implement end-user applications. Even so, such applications tin can likewise exist written in newer, higher-level languages.

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

C has both direct and indirectly influenced many later languages such equally 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 type systems, data models, and/or big-calibration program structures that differ from those of C, sometimes radically.

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

When object-oriented programming languages became popular, C++ and Objective-C were two different extensions of C that provided object-oriented capabilities. Both languages were originally implemented as source-to-source compilers; source code 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 equally an approach to providing object-oriented functionality with a C-like syntax.^[44] C++ adds greater typing force, scoping, and other tools useful in object-oriented programming, and permits generic programming via templates. Nearly a superset of C, C++ at present supports nearly of C, with a few exceptions.

Objective-C was originally a very "sparse" layer on tiptop 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, office declarations, and role calls is inherited from C, while the syntax for object-oriented features was originally taken from Smalltalk.

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

Run into also

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

Notes

^ The original example code will compile on most modern compilers that are non in strict standard compliance fashion, but it does not fully conform to the requirements of either C89 or C99. In fact, C99 requires that a diagnostic bulletin be produced.
^ The master function actually has ii arguments, int argc and char *argv[], respectively, which can exist used to handle command line arguments. The ISO C standard (section v.1.2.two.one) requires both forms of main to be supported, which is special treatment non afforded to any other office.

References

^ ^a ^b Kernighan, Brian Due west.; Ritchie, Dennis M. (February 1978). The C Programming Language (1st ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-xiii-110163-0.
^ Ritchie (1993): "Thompson had made a brief endeavour to produce a system coded in an early version of C—before structures—in 1972, merely gave upwards the effort."
^ Fruderica (December 13, 2020). "History of C". The cppreference.com. Archived from the original on Oct 24, 2020. Retrieved Oct 24, 2020.
^ Ritchie (1993): "The scheme of type composition adopted past C owes considerable debt to Algol 68, although it did non, mayhap, emerge in a form that Algol's adherents would approve of."
^ Ring Team (Oct 23, 2021). "The Band programming language and other languages". ring-lang.internet.
^ ^a ^b "Verilog HDL (and C)" (PDF). The Research School of Informatics at the Australian National University. June three, 2010. Archived from the original (PDF) on November 6, 2013. Retrieved Baronial 19, 2013. 1980s: ; Verilog first introduced ; Verilog inspired by the C programming language
^ ^a ^b ^c ^d ^e Ritchie (1993)
^ "Programming Language Popularity". 2009. Archived from the original on Jan 16, 2009. Retrieved January xvi, 2009.
^ "TIOBE Programming Community Alphabetize". 2009. Archived from the original on May four, 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 7, 2021.
^ Ritchie, Dennis. "BCPL to B to C". Archived from the original on December 12, 2019. Retrieved September 10, 2019.
^ ^a ^b Johnson, S. C.; Ritchie, D. Yard. (1978). "Portability of C Programs and the UNIX System". Bell Organization Tech. J. 57 (6): 2021–2048. CiteSeerX10.ane.1.138.35. doi:x.1002/j.1538-7305.1978.tb02141.x. S2CID 17510065. (Note: The PDF is an OCR scan of the original, and contains a rendering of "IBM 370" as "IBM 310".)
^ McIlroy, Grand. D. (1987). A Research Unix reader: annotated excerpts from the Programmer's Manual, 1971–1986 (PDF) (Technical report). CSTR. Bell Labs. p. 10. 139. Archived (PDF) from the original on November 11, 2017. Retrieved February 1, 2015.
^ "C manual pages". FreeBSD Miscellaneous Data Manual (FreeBSD 13.0 ed.). May 30, 2011. Archived from the original on Jan 21, 2021. Retrieved Jan 15, 2021. [one] Archived January 21, 2021, at the Wayback Motorcar
^ Kernighan, Brian W.; Ritchie, Dennis Thou. (March 1988). The C Programming Linguistic communication (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 Baronial 24, 2014. Retrieved April 14, 2014.
^ C Integrity. International Organization for Standardization. March 30, 1995. Archived from the original on July 25, 2018. Retrieved July 24, 2018.
^ "JTC1/SC22/WG14 – C". Home page. ISO/IEC. Archived from the original on Feb 12, 2018. Retrieved June two, 2011.
^ Andrew Binstock (October 12, 2011). "Interview with Herb Sutter". Dr. Dobbs. Archived from the original on Baronial 2, 2013. Retrieved September seven, 2013.
^ "Revised C23 Schedule WG fourteen N 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 L. (2002). C: A Reference Manual (fifth ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-xiii-089592-9. Contains a BNF grammer 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 16, 2011.
^ Kernighan & Ritchie (1996), pp. 192, 259.
^ "10 Common Programming Mistakes in C++". Cs.ucr.edu. Archived from the original on October 21, 2008. Retrieved June 26, 2009.
^ Schultz, Thomas (2004). C and the 8051 (3rd ed.). Otsego, MI: PageFree Publishing Inc. p. twenty. ISBN978-1-58961-237-2. Archived from the original on July 29, 2020. Retrieved February 10, 2012.
^ Kernighan & Ritchie (1978), p. six.
^ ^a ^b ^c ^d ^e ^f ^chiliad Klemens, Ben (2013). 21st Century C. O'Reilly Media. ISBN978-1-4493-2714-9.
^ Feuer, Alan R.; Gehani, Narain H. (March 1982). "Comparing 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 Time Buffer Checks (FORTIFY_SOURCE)". fedoraproject.org. Archived from the original on January vii, 2007. Retrieved August v, 2012.
^ เอี่ยมสิริวงศ์, โอภาศ (2016). Programming with C. Bangkok, Thailand: SE-EDUCATION PUBLIC COMPANY LIMITED. pp. 225–230. ISBN978-616-08-2740-4.
^ Raymond, Eric S. (October xi, 1996). The New Hacker'south Dictionary (3rd ed.). MIT Press. p. 432. ISBN978-0-262-68092-9. Archived from the original on November 12, 2012. Retrieved Baronial 5, 2012.
^ "Man Folio for lint (freebsd Department i)". unix.com. May 24, 2001. Retrieved July 15, 2014.
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Sources

Ritchie, Dennis M. (March 1993). "The Evolution of the C Language". ACM SIGPLAN Notices. ACM. 28 (3): 201–208. doi:x.1145/155360.155580.
Ritchie, Dennis Thou. (1993). "The Development of the C Linguistic communication". The 2d ACM SIGPLAN Conference on History of Programming Languages (HOPL-Two). ACM. pp. 201–208. doi:10.1145/154766.155580. ISBN0-89791-570-4 . Retrieved November iv, 2014.
Kernighan, Brian W.; Ritchie, Dennis M. (1996). The C Programming Language (2nd ed.). Prentice Hall. ISBN7-302-02412-Ten.

Farther reading

Kernighan, Brian; Ritchie, Dennis (1988). The C Programming Language (2 ed.). Prentice Hall. ISBN978-0131103627. (archive)
Plauger, P.J. (1992). The Standard C Library (1 ed.). Prentice Hall. ISBN978-0131315099. (source)
Banahan, Chiliad.; Brady, D.; Doran, K. (1991). The C Book: Featuring the ANSI C Standard (two ed.). Addison-Wesley. ISBN978-0201544336. (free)
Harbison, Samuel; Steele Jr, Guy (2002). C: A Reference Manual (five ed.). Pearson. ISBN978-0130895929. (archive)
Rex, K.N. (2008). C Programming: A Modern Approach (2 ed.). Due west. Westward. Norton. ISBN978-0393979503. (archive)
Griffiths, David; Griffiths, Dawn (2012). Head First C (1 ed.). O'Reilly. ISBN978-1449399917.
Perry, Greg; Miller, Dean (2013). C Programming: Absolute Beginner'south Guide (3 ed.). Que. ISBN978-0789751980.
Deitel, Paul; Deitel, Harvey (2015). C: How to Program (8 ed.). Pearson. ISBN978-0133976892.
Gustedt, Jens (2019). Modern C (2 ed.). Manning. ISBN978-1617295812. (free)

External links

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

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