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General-purpose programming language

C
Major implementations
The C Programming Language ^[1] (often referred to as One thousand&R), the seminal book on C
Paradigm	Multi-paradigm: imperative (procedural), structured
Designed by	Dennis Ritchie
Developer	Dennis Ritchie & Bell Labs (creators); ANSI X3J11 (ANSI C); ISO/IEC JTC1/SC22/WG14 (ISO C)
First appeared	1972; 50 years ago (1972) ^[2]

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

Typing discipline	Static, weak, manifest, nominal
OS	Cross-platform
Filename extensions	.c, .h
Website	www.iso.org/standard/74528.html www.open-std.org/jtc1/sc22/wg14/
pcc, GCC, Clang, Intel C, C++Builder, Microsoft Visual C++, Watcom C
Dialects
Cyclone, Unified Parallel C, Divide-C, Cilk, C*
Influenced past
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, Superhighway, Processing, Python, Ring,^[5]Rust, Seed7, Vala, Verilog (HDL),^[6] Nim, Zig
C Programming at Wikibooks

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

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

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

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

Overview

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

The linguistic communication has a small-scale, fixed number of keywords, including a full set of command menstruation primitives: if/else, for, do/while, while, and switch. User-divers names are not distinguished from keywords by any kind of sigil.
Information technology has a large number of arithmetics, bitwise, and logic operators: +,+=,++,&,||, etc.
More than one assignment may be performed in a single statement.
Functions:
- Part return values can be ignored, when not needed.
- Function and data pointers allow advertisement hoc run-time polymorphism.
- Functions may non be defined within the lexical scope of other functions.
Data typing is static, simply weakly enforced; all information has a type, simply implicit conversions are possible.
Declaration syntax mimics usage context. C has no "define" keyword; instead, a statement beginning with the proper name of a type is taken every bit a declaration. There is no "role" keyword; instead, a function is indicated past the presence of a parenthesized argument 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 of measurement.
- Union is a structure with overlapping members; only the last member stored is valid.
- Array indexing is a secondary annotation, defined in terms of pointer arithmetic. Unlike structs, arrays are not first-class objects: they cannot be assigned or compared using single congenital-in operators. At that place is no "assortment" keyword in use or definition; instead, square brackets indicate arrays syntactically, for example month[11].
- Enumerated types are possible with the enum keyword. They are freely interconvertible with integers.
- Strings are non a distinct data type, but are conventionally implemented as null-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 instance of function, with an untyped return type void.
A preprocessor performs macro definition, source code file inclusion, and conditional compilation.
There is a basic class of modularity: files can be compiled separately and linked together, with control over which functions and data objects are visible to other files via static and extern attributes.
Complex functionality such as I/O, string manipulation, and mathematical functions are consistently delegated to library routines.

While C does not include sure features found in other languages (such as object orientation and garbage drove), these can exist implemented or emulated, ofttimes through the use of external libraries (eastward.g., the GLib Object System or the Boehm garbage collector).

Relations to other languages

Many later languages accept borrowed direct or indirectly from C, including C++, C#, Unix's C shell, D, Become, Coffee, JavaScript (including transpilers), Julia, Limbo, LPC, Objective-C, Perl, PHP, Python, Ruby, Rust, Swift, Verilog and SystemVerilog (hardware description languages).^[six] These languages take drawn many of their control structures and other bones features from C. Almost of them (Python beingness 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 different.

History

Early on developments

Timeline of linguistic communication 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 organisation, originally implemented in assembly language 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-11. The original PDP-eleven version of Unix was also developed in assembly language.^[seven]

Thompson desired a programming language to brand utilities for the new platform. At first, he tried to brand a Fortran compiler, simply soon gave upward the idea. Instead, he created a cutting-downwards version of the recently developed BCPL systems programming language. The official description of BCPL was non available at the fourth dimension,^[12] and Thompson modified the syntax to be less wordy, producing the similar merely somewhat simpler B.^[7] However, few utilities were ultimately written in B considering it was too slow, and B could non take advantage of PDP-11 features such equally byte addressability.

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

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

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

Unix was i of the first operating system kernels implemented in a language other than assembly. Earlier instances include the Multics organization (which was written in PL/I) and Master Command Program (MCP) for the Burroughs B5000 (which was written in ALGOL) in 1961. In around 1977, Ritchie and Stephen C. Johnson fabricated further changes to the language to facilitate portability of the Unix operating organisation. Johnson'due south Portable C Compiler served as the ground for several implementations of C on new platforms.^[13]

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 breezy specification of the linguistic communication. The version of C that it describes is normally referred to equally "K&R C". As this was released in 1978, it is likewise referred to as C78.^[15] The second edition of the book^[sixteen] covers the afterward ANSI C standard, described below.

K&R introduced several linguistic communication features:

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

Even after the publication of the 1989 ANSI standard, for many years Yard&R C was all the same considered the "lowest common denominator" to which C programmers restricted themselves when maximum portability was desired, since many older compilers were still in utilise, and because advisedly written G&R C code can be legal Standard C too.

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

For example:

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

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

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

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

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

The big 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 Grand&R specification, led to the necessity of standardization.

ANSI C and ISO C

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, every bit its popularity began to increase significantly.

In 1983, the American National Standards Institute (ANSI) formed a committee, X3J11, to establish a standard specification of C. X3J11 based the C standard on the Unix implementation; however, the not-portable portion of the Unix C library was handed off to the IEEE working group 1003 to become the footing 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) as ISO/IEC 9899:1990, which is sometimes chosen C90. Therefore, the terms "C89" and "C90" refer to the aforementioned programming linguistic communication.

ANSI, similar other national standards bodies, no longer develops the C standard independently, but defers to the international C standard, maintained by the working group ISO/IEC JTC1/SC22/WG14. National adoption of an update to the international standard typically occurs within a year 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 after introduced unofficial features. The standards committee too included several additional features such every bit 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 K&R interface continued to be permitted, for compatibility with existing source code.

C89 is supported past current C compilers, and most modern C code is based on it. Whatever plan written only in Standard C and without whatever hardware-dependent assumptions will run correctly on whatever platform with a conforming C implementation, within its resources 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 every bit the exact size of data types and byte endianness.

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

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

C99

The C standard was further revised in the late 1990s, leading to the publication of ISO/IEC 9899:1999 in 1999, which is unremarkably referred to every bit "C99". It has since been amended 3 times by Technical Corrigenda.^[xix]

C99 introduced several new features, including inline functions, several new information types (including long long int and a circuitous blazon to correspond complex numbers), variable-length arrays and flexible assortment members, improved support for IEEE 754 floating betoken, support for variadic macros (macros of variable arity), and support for ane-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 almost office astern compatible with C90, but is stricter in some ways; in item, a declaration that lacks a type specifier no longer has int implicitly assumed. A standard macro __STDC_VERSION__ is defined with value 199901L to point that C99 support is available. GCC, Solaris Studio, and other C compilers at present support many or all of the new features of C99. The C compiler in Microsoft Visual C++, yet, implements the C89 standard and those parts of C99 that are required for compatibility with C++xi.^[xx] ^{[ needs update ]}

In add-on, support for Unicode identifiers (variable / role 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 not 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 likewise makes some portions of the existing C99 library optional, and improves compatibility with C++. The standard macro __STDC_VERSION__ is divers as 201112L to indicate that C11 support is available.

C17

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 every bit 201710L.

C2x

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

Embedded C

Historically, embedded C programming requires nonstandard extensions to the C linguistic communication in order to back up 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 report extending the C language^[22] to accost these issues past providing a common standard for all implementations to adhere to. It includes a number of features not available in normal C, such as fixed-point arithmetic, named address spaces, and bones I/O hardware addressing.

Syntax

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

C source files contain declarations and function definitions. Function definitions, in turn, comprise declarations and statements. Declarations either define new types using keywords such as struct, marriage, and enum, or assign types to and maybe reserve storage for new variables, usually past writing the blazon 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 as a unmarried statement for control structures.

As an imperative language, C uses statements to specify actions. The about common statement is an expression statement, consisting of an expression to exist evaluated, followed by a semicolon; every bit a side event of the evaluation, functions may be chosen 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 past if … [else] conditional execution and by do … while, while, and for iterative execution (looping). The for statement has separate initialization, testing, and reinitialization expressions, any or all of which can be omitted. break and continue can be used to leave the innermost enclosing loop argument or skip to its reinitialization. There is also a non-structured goto statement which branches directly to the designated label within the function. switch selects a case to be executed based on the value of an integer expression.

Expressions can use a variety of built-in operators and may contain office calls. The club 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) will occur before the next "sequence bespeak"; sequence points include the stop of each expression statement, and the entry to and render from each function call. Sequence points also occur during evaluation of expressions containing certain operators (&&, ||, ?: and the comma operator). This permits a high caste of object lawmaking optimization by the compiler, but requires C programmers to accept more intendance to obtain reliable results than is needed for other programming languages.

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

Character ready

The basic C source grapheme prepare includes the following characters:

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

Newline indicates the end of a text line; it need non represent to an actual unmarried graphic symbol, although for convenience C treats it as i.

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

The bones C execution character set contains the aforementioned characters, along with representations for alarm, backspace, and carriage return. Run-time back up for extended character 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 whatsoever purposes other than those for which they are predefined:

car
break
case
char
const
continue
default
do
double
else
enum
extern
bladder
for
goto
if
int
long
register
return
short
signed
sizeof
static
struct
switch
typedef
wedlock
unsigned
void
volatile
while

C99 reserved five 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 capital letter, because identifiers of that form were previously reserved by the C standard for employ only by implementations. Since existing program source code should not have been using these identifiers, information technology would non exist affected when C implementations started supporting these extensions to the programming language. Some standard headers do ascertain more than convenient synonyms for underscored identifiers. The language previously included a reserved word chosen entry, but this was seldom implemented, and has now been removed as a reserved discussion.^[27]

Operators

C supports a rich set 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:

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

C uses the operator = (used in mathematics to express equality) to indicate assignment, following the precedent of Fortran and PL/I, but dissimilar ALGOL and its derivatives. C uses the operator == to test for equality. The similarity between these two operators (assignment and equality) may result in the accidental utilise of one in place of the other, and in many cases, the error does non produce an fault message (although some compilers produce warnings). For example, the conditional expression if (a == b + ane) might mistakenly exist written as if (a = b + one), which will be evaluated as truthful if a is non zero subsequently the assignment.^[28]

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

"Hello, world" example

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

The original version was:^[30]

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

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

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

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

The next line indicates that a function named main is being divers. The primary role serves a special purpose in C programs; the run-time environment calls the chief function to begin program execution. The blazon specifier int indicates that the value that is returned to the invoker (in this case the run-time environs) 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 outset 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 organisation library. In this call, the printf role is passed (provided with) a single argument, the address of the starting time grapheme in the cord literal "how-do-you-do, world\due north". The string literal is an unnamed assortment with elements of blazon char, set up upward automatically by the compiler with a final 0-valued character to mark 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 terminate of the current line. The render value of the printf function is of blazon int, just it is silently discarded since it is not used. (A more conscientious plan might examination the return value to make up one's mind whether or non the printf role succeeded.) The semicolon ; terminates the argument.

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

Data types

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

C is oftentimes used in depression-level systems programming where escapes from the type organisation may be necessary. The compiler attempts to ensure type correctness of almost expressions, but the programmer can override the checks in various means, either past using a blazon cast to explicitly convert a value from 1 type to another, or by using pointers or unions to reinterpret the underlying bits of a data object in another way.

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

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

A null pointer value explicitly points to no valid location. Dereferencing a null arrow value is undefined, oft resulting in a sectionalization mistake. Zilch pointer values are useful for indicating special cases such every bit no "next" pointer in the final node of a linked list, or as an error indication from functions returning pointers. In appropriate contexts in source code, such equally for assigning to a pointer variable, a nothing arrow constant can be written as 0, with or without explicit casting to a pointer type, or every bit the NULL macro divers by several standard headers. In conditional contexts, null pointer values evaluate to fake, while all other pointer values evaluate to true.

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

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

Arrays

Array types in C are traditionally of a fixed, static size specified at compile time. The more recent C99 standard also allows a form of variable-length arrays. However, information technology is also possible to classify a block of retentiveness (of arbitrary size) at run-fourth dimension, using the standard library's malloc function, and treat information technology as an array.

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

C does non have a special provision for declaring multi-dimensional arrays, only rather relies on recursion within the blazon arrangement to declare arrays of arrays, which effectively accomplishes the same affair. The alphabetize values of the resulting "multi-dimensional array" tin can be idea of every bit 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. However, in early on versions of C the premises of the array must be known fixed 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 allotment of a two-dimensional assortment on the heap and the use of multi-dimensional array indexing for accesses (which can use bounds-checking on many C compilers):

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

Array–arrow interchangeability

The subscript notation x[i] (where ten designates a pointer) is syntactic saccharide for *(10+i).^[36] Taking advantage of the compiler's noesis of the pointer type, the accost that ten + i points to is not the base of operations accost (pointed to by x) incremented by i bytes, but rather is defined to exist the base address incremented by i multiplied by the size of an element that x points to. Thus, x[i] designates the i+aneth 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 get-go element. This implies that an assortment is never copied every bit a whole when named as an argument to a part, but rather but the accost of its showtime element is passed. Therefore, although function calls in C utilise pass-by-value semantics, arrays are in issue passed by reference.

The total size of an array x can be determined past applying sizeof to an expression of array blazon. The size of an chemical element can exist determined by applying the operator sizeof to whatsoever dereferenced chemical element of an array A, as in n = sizeof A[0]. This, the number of elements in a declared array A can exist determined as sizeof A / sizeof A[0]. Notation, that if but a pointer to the first element is bachelor equally it is oftentimes the case in C code because of the automatic conversion described above, the information most the full type of the array and its length are lost.

Memory direction

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

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

These three approaches are appropriate in different situations and have various merchandise-offs. For example, static retentiveness resource allotment has little allocation overhead, automatic resource allotment may involve slightly more overhead, and dynamic memory allocation can potentially have a great deal of overhead for both allocation and deallocation. The persistent nature of static objects is useful for maintaining state information across part calls, automatic allocation is like shooting fish in a barrel to utilize but stack space is typically much more express and transient than either static memory or heap space, and dynamic retention allocation allows convenient allotment of objects whose size is known merely at run-fourth dimension. Most C programs make extensive use of all iii.

Where possible, automatic or static resource allotment is unremarkably simplest because the storage is managed by the compiler, freeing the programmer of the potentially error-prone job of manually allocating and releasing storage. Even so, many data structures can 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 mutual case of this. (See the article on malloc for an instance of dynamically allocated arrays.) Unlike automated resource allotment, which can fail at run time with uncontrolled consequences, the dynamic allocation functions return an indication (in the form of a naught arrow value) when the required storage cannot be allocated. (Static allotment that is too large is usually detected by the linker or loader, earlier the program can fifty-fifty begin execution.)

Unless otherwise specified, static objects contain cypher or null pointer values upon program startup. Automatically and dynamically allocated objects are initialized simply if an initial value is explicitly specified; otherwise they initially have indeterminate values (typically, whatever bit pattern happens to be present in the storage, which might non even represent a valid value for that type). If the program attempts to access an uninitialized value, the results are undefined. Many modern compilers try to detect and warn nigh this trouble, just both false positives and faux negatives tin can occur.

Heap retentivity allocation has to be synchronized with its actual usage in any program to be reused equally much as possible. For case, if the only pointer to a heap memory allocation goes out of scope or has its value overwritten before it is deallocated explicitly, and then that memory cannot exist recovered for after 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 it hard 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 set of functions contained within a single "archive" file. Each library typically has a header file, which contains the prototypes of the functions contained within the library that may exist used by a plan, and declarations of special data types and macro symbols used with these functions. In order for a programme to employ a library, information technology must include the library'southward header file, and the library must be linked with the program, which in many cases requires compiler flags (eastward.g., -lm, shorthand for "link the math library").^[31]

The 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 express environments such as embedded systems may provide only a subset of the standard library). This library supports stream input and output, retentiveness resource allotment, mathematics, grapheme strings, and fourth dimension values. Several separate standard headers (for instance, stdio.h) specify the interfaces for these and other standard library facilities.

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

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

File handling and streams

File input and output (I/O) is non part of the C language itself but instead is handled by libraries (such as the C standard library) and their associated header files (e.g. stdio.h). File handling is by and large implemented through high-level I/O which works through streams. A stream is from this perspective a information flow that is contained of devices, while a file is a concrete device. The high-level I/O is done through the association of a stream to a file. In the C standard library, a buffer (a retention area or queue) is temporarily used to store data before information technology'south sent to the final destination. This reduces the time spent waiting for slower devices, for case a hard drive or solid state bulldoze. Depression-level I/O functions are not part of the standard C library^{[ clarification needed ]} but are generally part of "bare metallic" programming (programming that'southward independent of any operating system such as almost embedded programming). With few exceptions, implementations include depression-level I/O.

Language tools

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

Automated source code checking and auditing are benign in any language, and for C many such tools exist, such as Lint. A common practise is to use Lint to detect questionable lawmaking when a program is offset written. Once a plan passes Lint, it is then compiled using the C compiler. Also, many compilers can optionally warn well-nigh syntactically valid constructs that are probable to actually be errors. MISRA C is a proprietary ready of guidelines to avoid such questionable code, adult for embedded systems.^[37]

At that place are also compilers, libraries, and operating organisation level mechanisms for performing actions that are non a standard part of C, such as bounds checking for arrays, detection of buffer overflow, serialization, dynamic memory tracking, and automatic garbage collection.

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

Uses

The C Programming Language

C is widely used for systems programming in implementing operating systems and embedded system applications,^[38] because C lawmaking, when written for portability, can exist used for most purposes, all the same when needed, arrangement-specific code can be used to access specific hardware addresses and to perform type punning to match externally imposed interface requirements, with a low run-fourth dimension demand on organisation resources.

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

A effect of C's wide availability and efficiency is that compilers, libraries and interpreters of other programming languages are often implemented in C. For example, the reference implementations of Python, Perl, 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 sparse, and its overhead is low, an important criterion for computationally intensive programs. For example, the GNU Multiple Precision Arithmetic Library, the GNU Scientific Library, Mathematica, and MATLAB are completely or partially written in C.

C is sometimes used as an intermediate language by implementations of other languages. This arroyo may be used for portability or convenience; by using C as an intermediate linguistic communication, boosted machine-specific code generators are not necessary. C has some features, such as line-number preprocessor directives and optional superfluous commas at the end of initializer lists, that support compilation of generated code. However, some of C'south shortcomings take prompted the evolution of other C-based languages specifically designed for use every bit intermediate languages, such as C--.

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

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

C has both directly and indirectly influenced many later languages such as C#, D, Go, Java, JavaScript, Limbo, LPC, Perl, PHP, Python, and Unix'southward C beat out.^[42] The virtually pervasive influence has been syntactical; all of the languages mentioned combine the statement and (more than or less recognizably) expression syntax of C with type systems, data models, and/or large-scale programme structures that differ from those of C, sometimes radically.

Several C or most-C interpreters exist, including Ch and CINT, which can too 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 lawmaking was translated into C, and and then compiled with a C compiler.^[43]

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

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

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

See likewise

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

Notes

^ The original example code will compile on most modern compilers that are not 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 main function actually has two arguments, int argc and char *argv[], respectively, which can exist used to handle command line arguments. The ISO C standard (section five.ane.two.ii.ane) requires both forms of main to exist supported, which is special treatment not afforded to whatsoever other office.

References

^ ^a ^b Kernighan, Brian West.; Ritchie, Dennis M. (February 1978). The C Programming Linguistic communication (1st ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-110163-0.
^ Ritchie (1993): "Thompson had fabricated a brief try to produce a arrangement coded in an early version of C—before structures—in 1972, but gave up the try."
^ 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 limerick adopted by C owes considerable debt to Algol 68, although it did not, perchance, sally in a form that Algol'due south adherents would corroborate of."
^ Ring Team (October 23, 2021). "The Ring programming language and other languages". band-lang.net.
^ ^a ^b "Verilog HDL (and C)" (PDF). The Research School of Estimator Science at the Australian National University. June 3, 2010. Archived from the original (PDF) on November 6, 2013. Retrieved August 19, 2013. 1980s: ; Verilog beginning introduced ; Verilog inspired past the C programming language
^ ^a ^b ^c ^d ^e Ritchie (1993)
^ "Programming Linguistic communication Popularity". 2009. Archived from the original on January xvi, 2009. Retrieved January 16, 2009.
^ "TIOBE Programming Community Index". 2009. Archived from the original on May iv, 2009. Retrieved May vi, 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. M. (1978). "Portability of C Programs and the UNIX System". Bong Arrangement Tech. J. 57 (6): 2021–2048. CiteSeerX10.one.ane.138.35. doi:10.1002/j.1538-7305.1978.tb02141.x. S2CID 17510065. (Note: The PDF is an OCR scan of the original, and contains a rendering of "IBM 370" as "IBM 310".)
^ McIlroy, M. D. (1987). A Research Unix reader: annotated excerpts from the Programmer'south Manual, 1971–1986 (PDF) (Technical report). CSTR. Bong Labs. p. 10. 139. Archived (PDF) from the original on November xi, 2017. Retrieved February i, 2015.
^ "C transmission pages". FreeBSD Miscellaneous Information Manual (FreeBSD 13.0 ed.). May 30, 2011. Archived from the original on January 21, 2021. Retrieved January 15, 2021. [1] Archived January 21, 2021, at the Wayback Machine
^ Kernighan, Brian W.; Ritchie, Dennis M. (March 1988). The C Programming Language (2nd ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-110362-7.
^ Stroustrup, Bjarne (2002). Sibling rivalry: C and C++ (PDF) (Report). AT&T Labs. Archived (PDF) from the original on Baronial 24, 2014. Retrieved April fourteen, 2014.
^ C Integrity. International Organization for Standardization. March 30, 1995. Archived from the original on July 25, 2018. Retrieved July 24, 2018.
^ "JTC1/SC22/WG14 – C". Home page. ISO/IEC. Archived from the original on Feb 12, 2018. Retrieved June 2, 2011.
^ Andrew Binstock (October 12, 2011). "Interview with Herb Sutter". Dr. Dobbs. Archived from the original on August ii, 2013. Retrieved September 7, 2013.
^ "Revised C23 Schedule WG 14 Due north 2759" (PDF). www.open-std.org. Archived (PDF) from the original on June 24, 2021. Retrieved October 10, 2021.
^ "TR 18037: Embedded C" (PDF). ISO / IEC. Archived (PDF) from the original on February 25, 2021. Retrieved July 26, 2011.
^ Harbison, Samuel P.; Steele, Guy L. (2002). C: A Reference Manual (5th ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-xiii-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 Typhoon" (PDF). Archived (PDF) from the original on December 22, 2017. Retrieved September 16, 2011.
^ Kernighan & Ritchie (1996), pp. 192, 259.
^ "10 Mutual 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. 20. ISBN978-1-58961-237-2. Archived from the original on July 29, 2020. Retrieved February 10, 2012.
^ Kernighan & Ritchie (1978), p. 6.
^ ^a ^b ^c ^d ^e ^f ^k Klemens, Ben (2013). 21st Century C. O'Reilly Media. ISBN978-i-4493-2714-9.
^ Feuer, Alan R.; Gehani, Narain H. (March 1982). "Comparison of the Programming Languages C and Pascal". ACM Computing Surveys. xiv (1): 73–92. doi:ten.1145/356869.356872. S2CID 3136859.
^ Kernighan & Ritchie (1996), p. 122.
^ For example, gcc provides _FORTIFY_SOURCE. "Security Features: Compile Time Buffer Checks (FORTIFY_SOURCE)". fedoraproject.org. Archived from the original on January 7, 2007. Retrieved August 5, 2012.
^ เอี่ยมสิริวงศ์, โอภาศ (2016). Programming with C. Bangkok, Thailand: SE-Education PUBLIC Visitor Limited. pp. 225–230. ISBN978-616-08-2740-4.
^ Raymond, Eric S. (October 11, 1996). The New Hacker'southward Dictionary (3rd ed.). MIT Press. p. 432. ISBN978-0-262-68092-9. Archived from the original on Nov 12, 2012. Retrieved August 5, 2012.
^ "Homo Page for lint (freebsd Section 1)". unix.com. May 24, 2001. Retrieved July fifteen, 2014.
^ Dale, Nell B.; Weems, Chip (2014). Programming and problem solving with C++ (sixth ed.). Burlington, MA: Jones & Bartlett Learning. ISBN978-1449694289. OCLC 894992484.
^ Dr. Dobb's Sourcebook. UsaA.: Miller Freeman, Inc. November–December 1995.
^ "Using C for CGI Programming". linuxjournal.com. March 1, 2005. Archived from the original on February xiii, 2010. Retrieved January 4, 2010.
^ McMillan, Robert (Baronial 1, 2013). "Is Coffee Losing Its Mojo?". Wired. Archived from the original on February 15, 2017. Retrieved March five, 2017.
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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:ten.1145/155360.155580.
Ritchie, Dennis M. (1993). "The Evolution of the C Language". The Second ACM SIGPLAN Conference on History of Programming Languages (HOPL-II). ACM. pp. 201–208. doi:10.1145/154766.155580. ISBN0-89791-570-four . Retrieved November four, 2014.
Kernighan, Brian W.; Ritchie, Dennis Thousand. (1996). The C Programming Language (2nd ed.). Prentice Hall. ISBN7-302-02412-X.

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). (iii.61 MB)
comp.lang.c Frequently Asked Questions
A History of C, by Dennis Ritchie

This page was final edited on 16 March 2022, at 14:43

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