C# Sample Program to Read Barcode Scanner

General-purpose programming language

C
Major implementations
The C Programming Linguistic communication ^[ane] (oftentimes referred to equally K&R), the seminal book on C
Paradigm	Multi-paradigm: imperative (procedural), structured
Designed past	Dennis Ritchie
Developer	Dennis Ritchie & Bell Labs (creators); ANSI X3J11 (ANSI C); ISO/IEC JTC1/SC22/WG14 (ISO C)
First appeared	1972; l years ago (1972) ^[2]

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

Typing discipline	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, Band,^[5]Rust, Seed7, Vala, Verilog (HDL),^[vi] Nim, Zig
C Programming at Wikibooks

C (, as in the letterc) is a general-purpose, procedural figurer programming language supporting structured programming, lexical variable scope, and recursion, with a static blazon system. Past pattern, C provides constructs that map efficiently to typical auto instructions. It has found lasting apply in applications previously coded in assembly language. Such applications include operating systems and various application 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 Bell 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 arrangement.^[7] During the 1980s, C gradually gained popularity. It has become one of the about widely used programming languages,^[8] ^[9] 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 System for Standardization (ISO).

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

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

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 lawmaking is independent inside subroutines (as well chosen "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 arrow values. C plan source text is free-format, using the semicolon as a argument terminator and curly braces for grouping blocks of statements.

The C language likewise exhibits the post-obit characteristics:

The language has a pocket-size, fixed number of keywords, including a total set of control flow primitives: if/else, for, do/while, while, and switch. User-divers names are not distinguished from keywords by any kind of sigil.
It has a large number of arithmetic, bitwise, and logic operators: +,+=,++,&,||, etc.
More than than i assignment may exist performed in a single statement.
Functions:
- Role return values tin can be ignored, when not needed.
- Part and data pointers permit advertisement hoc run-fourth dimension polymorphism.
- Functions may not exist defined within the lexical telescopic of other functions.
Data typing is static, but weakly enforced; all data has a blazon, but implicit conversions are possible.
Declaration syntax mimics usage context. C has no "define" keyword; instead, a statement offset with the proper noun of a blazon is taken equally a declaration. There is no "function" keyword; instead, a function is indicated past the presence of a parenthesized statement list.
User-divers (typedef) and compound types are possible.
- Heterogeneous aggregate information types (struct) allow related information elements to be accessed and assigned as a unit.
- Union is a construction with overlapping members; only the last member stored is valid.
- Assortment indexing is a secondary notation, divers in terms of pointer arithmetic. Dissimilar structs, arrays are not commencement-class objects: they cannot exist assigned or compared using single built-in operators. There is no "array" keyword in use or definition; instead, square brackets indicate arrays syntactically, for example month[11].
- Enumerated types are possible with the enum keyword. They are freely interconvertible with integers.
- Strings are non a 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 lawmaking file inclusion, and conditional compilation.
There is a basic grade of modularity: files can be compiled separately and linked together, with command over which functions and information objects are visible to other files via static and extern attributes.
Complex functionality such as I/O, cord manipulation, and mathematical functions are consistently delegated to library routines.

While C does not include certain features found in other languages (such as object orientation and garbage collection), these can be implemented or emulated, often through the use of external libraries (e.m., the GLib Object System or the Boehm garbage collector).

Relations to other languages

Many subsequently languages take 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, Cherry, Rust, Swift, Verilog and SystemVerilog (hardware description languages).^[vi] These languages accept drawn many of their command structures and other basic features from C. Nearly of them (Python being a dramatic exception) also limited highly like syntax to C, and they tend to combine the recognizable expression and statement syntax of C with underlying blazon systems, data models, and semantics that can be radically different.

History

Early developments

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

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

Thompson desired a programming language to make utilities for the new platform. At start, he tried to make a Fortran compiler, but soon gave upwards the idea. Instead, he created a cut-down version of the recently adult BCPL systems programming linguistic communication. The official description of BCPL was not available at the time,^[12] and Thompson modified the syntax to exist less wordy, producing the similar but somewhat simpler B.^[seven] All the same, few utilities were ultimately written in B considering information technology was besides slow, and B could not have advantage of PDP-xi features such as byte addressability.

In 1972, Ritchie started to better B, most notably calculation data typing for variables, which resulted in creating a new language C.^[thirteen] The C compiler and some utilities fabricated with it were included in Version 2 Unix.^[xiv]

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

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

Unix was one of the start operating system kernels implemented in a language other than assembly. Earlier instances include the Multics system (which was written in PL/I) and Main 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 organization. Johnson'due south Portable C Compiler served as the basis for several implementations of C on new platforms.^[thirteen]

M&R C

In 1978, Brian Kernighan and Dennis Ritchie published the first edition of The C Programming Language.^[i] This volume, known to C programmers as Chiliad&R, served for many years as an breezy specification of the language. The version of C that it describes is commonly referred to every bit "Thousand&R C". Every bit this was released in 1978, it is as well referred to as C78.^[15] The second edition of the book^[sixteen] covers the afterward ANSI C standard, described below.

K&R introduced several language features:

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

Fifty-fifty 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 still in use, and considering carefully written 1000&R C code can exist legal Standard C too.

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

For example:

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

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

Since One thousand&R function declarations did non include whatsoever information about function arguments, part parameter blazon checks were not performed, although some compilers would issue a alert message if a local office was called with the wrong number of arguments, or if multiple calls to an external office used different numbers or types of arguments. Dissever tools such every bit Unix'south lint utility were developed that (among other things) could check for consistency of function utilize beyond multiple source files.

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

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

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

ANSI C and ISO C

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

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

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

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

One of the aims of the C standardization process was to produce a superset of Thou&R C, incorporating many of the subsequently introduced unofficial features. The standards committee too included several additional features such every bit function prototypes (borrowed from C++), void pointers, support for international character sets and locales, and preprocessor enhancements. Although the syntax for parameter declarations was augmented to include the mode used in C++, the G&R interface connected to be permitted, for compatibility with existing source code.

C89 is supported by current C compilers, and about modern C code is based on information technology. Any program written merely in Standard C and without any hardware-dependent assumptions will run correctly on any platform with a conforming C implementation, within its resource limits. Without such precautions, programs may compile only on a certain platform or with a particular compiler, due, for example, to the use of non-standard libraries, such as GUI libraries, or to a reliance on compiler- or platform-specific attributes such as the verbal 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 can exist used to divide the code into Standard and K&R sections to foreclose the employ on a 1000&R C-based compiler of features available only in Standard C.

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

C99

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

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

C99 is for the most part backward uniform with C90, only is stricter in some ways; in item, a announcement that lacks a type specifier no longer has int implicitly assumed. A standard macro __STDC_VERSION__ is divers with value 199901L to point that C99 support is available. GCC, Solaris Studio, and other C compilers now support many or all of the new features of C99. The C compiler in Microsoft Visual C++, notwithstanding, implements the C89 standard and those parts of C99 that are required for compatibility with C++11.^[20] ^{[ needs update ]}

In add-on, support for Unicode identifiers (variable / role names) in the course of escaped characters (e.one thousand. \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 commission adopted guidelines to limit the adoption of new features that had not been tested past 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. Information technology also makes some portions of the existing C99 library optional, and improves compatibility with C++. The standard macro __STDC_VERSION__ is defined equally 201112L to indicate that C11 support 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 name for the next (later on C17) major C linguistic communication standard revision. It is expected to be voted on in 2023 and would therefore be called C23.^[21] ^{[ better source needed ]}

Embedded C

Historically, embedded C programming requires nonstandard extensions to the C language in order to support exotic features such every bit fixed-indicate arithmetic, multiple distinct retention banks, and basic I/O operations.

In 2008, the C Standards Committee published a technical written report extending the C language^[22] to accost these issues by providing a common standard for all implementations to adhere to. It includes a number of features non bachelor in normal C, such as fixed-point arithmetics, named address spaces, and bones I/O hardware addressing.

Syntax

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

C source files contain declarations and role definitions. Function definitions, in plough, contain declarations and statements. Declarations either ascertain new types using keywords such as struct, wedlock, and enum, or assign types to and perhaps reserve storage for new variables, ordinarily by writing the type followed by the variable name. Keywords such equally char and int specify congenital-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 single argument for command structures.

Equally an imperative linguistic communication, C uses statements to specify actions. The well-nigh common statement is an expression statement, consisting of an expression to be evaluated, followed by a semicolon; as a side effect of the evaluation, functions may be chosen and variables may be assigned new values. To alter the normal sequential execution of statements, C provides several command-flow statements identified by reserved keywords. Structured programming is supported by if … [else] conditional execution and by practice … while, while, and for iterative execution (looping). The for statement has divide initialization, testing, and reinitialization expressions, any or all of which tin can be omitted. interruption and go along tin be used to leave the innermost enclosing loop statement or skip to its reinitialization. There is too a non-structured goto argument which branches directly to the designated label within the part. switch selects a case to be executed based on the value of an integer expression.

Expressions tin can utilise a multifariousness of congenital-in operators and may contain function calls. The order in which arguments to functions and operands to most operators are evaluated is unspecified. The evaluations may even be interleaved. However, all side effects (including storage to variables) will occur earlier the next "sequence bespeak"; sequence points include the end of each expression argument, and the entry to and return from each function call. Sequence points too occur during evaluation of expressions containing certain operators (&&, ||, ?: and the comma operator). This permits a high degree of object lawmaking optimization by the compiler, but requires C programmers to take more than 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 any other language, has its blemishes. Some of the operators have the wrong precedence; some parts of the syntax could be ameliorate."^[25] The C standard did non try to correct many of these blemishes, because of the impact of such changes on already existing software.

Character set

The basic C source character prepare includes the post-obit 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, form feed, newline

Newline indicates the end of a text line; it need not correspond to an actual single character, although for convenience C treats it every bit one.

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

The bones C execution character gear up contains the same characters, forth with representations for warning, backspace, and railroad vehicle return. Run-fourth dimension support for extended character sets has increased with each revision of the C standard.

Reserved words

C89 has 32 reserved words, besides known every bit keywords, which are the words that cannot exist used for whatever purposes other than those for which they are predefined:

auto
break
instance
char
const
proceed
default
do
double
else
enum
extern
bladder
for
goto
if
int
long
register
return
short
signed
sizeof
static
struct
switch
typedef
union
unsigned
void
volatile
while

C99 reserved v more words:

_Bool
_Complex
_Imaginary
inline
restrict

C11 reserved seven more words:^[26]

_Alignas
_Alignof
_Atomic
_Generic
_Noreturn
_Static_assert
_Thread_local

Most of the recently reserved words brainstorm with an underscore followed by a majuscule letter, considering identifiers of that course were previously reserved by the C standard for use just past implementations. Since existing programme source code should not have been using these identifiers, it would not exist affected when C implementations started supporting these extensions to the programming language. Some standard headers practice ascertain more convenient synonyms for underscored identifiers. The linguistic communication previously included a reserved word chosen entry, but this was seldom implemented, and has now been removed every bit a reserved word.^[27]

Operators

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

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

C uses the operator = (used in mathematics to limited equality) to indicate assignment, following the precedent of Fortran and PL/I, just dissimilar ALGOL and its derivatives. C uses the operator == to examination for equality. The similarity between these two operators (assignment and equality) may result in the adventitious utilise of 1 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 + one) might mistakenly be written as if (a = b + one), which will exist evaluated as true if a is not nothing after the consignment.^[28]

The C operator precedence is not always intuitive. For case, the operator == binds more tightly than (is executed prior to) the operators & (bitwise AND) and | (bitwise OR) in expressions such equally x & one == 0, which must be written as (10 & 1) == 0 if that is the coder's intent.^[29]

"Hi, earth" example

The "howdy, world" example, which appeared in the starting time edition of K&R, has become the model for an introductory program in virtually programming textbooks. The plan prints "hullo, world" to the standard output, which is ordinarily a terminal or screen display.

The original version was:^[30]

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

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

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

The start 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 equally printf and scanf. The angle 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, equally opposed to double quotes which typically include local or projection-specific header files.

The adjacent line indicates that a function named primary is existence defined. The principal function serves a special purpose in C programs; the run-time surround calls the principal office to begin programme execution. The type specifier int indicates that the value that is returned to the invoker (in this case the run-fourth dimension surround) as a consequence of evaluating the main function, is an integer. The keyword void every bit a parameter list indicates that this role takes no arguments.^[b]

The opening curly brace indicates the beginning of the definition of the chief 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 function is passed (provided with) a single argument, the address of the first character in the string literal "hello, world\n". The string literal is an unnamed array with elements of blazon char, set up automatically by the compiler with a final 0-valued character to mark the stop of the assortment (printf needs to know this). The \n is an escape sequence that C translates to a newline character, which on output signifies the end of the electric current line. The render value of the printf function is of type int, only it is silently discarded since it is not used. (A more careful program might test the render value to make up one's mind whether or not the printf function succeeded.) The semicolon ; terminates the statement.

The closing curly brace indicates the end of the lawmaking for the main office. Co-ordinate to the C99 specification and newer, the main 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]

Information types

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

C is frequently 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, merely the programmer can override the checks in various ways, either past using a type bandage to explicitly convert a value from one type to some other, or by using pointers or unions to reinterpret the underlying bits of a data object in another fashion.

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

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

Pointers

C supports the use of pointers, a type of reference that records the address or location of an object or part in memory. Pointers can be dereferenced to admission data stored at the accost pointed to, or to invoke a pointed-to role. Pointers can be manipulated using assignment or pointer arithmetic. The run-fourth dimension representation of a arrow value is typically a raw memory address (peradventure augmented by an offset-within-word field), but since a pointer'south type includes the type of the thing pointed to, expressions including pointers tin can be type-checked at compile time. Pointer arithmetic is automatically scaled past the size of the pointed-to information type. Pointers are used for many purposes in C. Text strings are ordinarily manipulated using pointers into arrays of characters. Dynamic memory allocation is performed using pointers. Many data types, such as trees, are commonly implemented as dynamically allocated struct objects linked together using pointers. Pointers to functions are useful for passing functions as arguments to higher-order functions (such as qsort or bsearch) or as callbacks to be invoked by event handlers.^[31]

A null pointer value explicitly points to no valid location. Dereferencing a cypher pointer value is undefined, often resulting in a segmentation fault. Nix pointer values are useful for indicating special cases such as no "next" pointer in the terminal node of a linked listing, or as an mistake indication from functions returning pointers. In appropriate contexts in source code, such equally for assigning to a arrow variable, a zilch arrow abiding tin can be written as 0, with or without explicit casting to a pointer type, or as the NULL macro defined past several standard headers. In provisional contexts, null pointer values evaluate to false, while all other pointer values evaluate to true.

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

Careless use of pointers is potentially dangerous. Considering they are typically unchecked, a pointer variable tin can be fabricated to point to any capricious location, which tin can cause undesirable furnishings. Although properly used pointers point to safe places, they can be made to point to unsafe places by using invalid pointer arithmetic; the objects they betoken to may continue to exist used after deallocation (dangling pointers); they may exist used without having been initialized (wild pointers); or they may be direct assigned an dangerous value using a cast, union, or through another corrupt pointer. In general, C is permissive in assuasive manipulation of and conversion betwixt pointer types, although compilers typically provide options for various levels of checking. Some other programming languages address these issues past using more than restrictive reference types.

Arrays

Array types in C are traditionally of a fixed, static size specified at compile time. The more than recent C99 standard also allows a class of variable-length arrays. However, it is also possible to allocate a block of memory (of arbitrary size) at run-time, using the standard library's malloc function, and treat it equally an array.

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

C does non take a special provision for declaring multi-dimensional arrays, just rather relies on recursion within the type system to declare arrays of arrays, which effectively accomplishes the same thing. The alphabetize values of the resulting "multi-dimensional array" tin be thought of as increasing in row-major lodge. Multi-dimensional arrays are commonly used in numerical algorithms (mainly from applied linear algebra) to store matrices. The construction of the C array is well suited to this particular task. Notwithstanding, in early versions of C the bounds of the assortment must be known fixed values or else explicitly passed to whatsoever subroutine that requires them, and dynamically sized arrays of arrays cannot be accessed using double indexing. (A workaround for this was to classify 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 later) shows resource allotment of a two-dimensional assortment on the heap and the use of multi-dimensional array indexing for accesses (which can apply bounds-checking on many C compilers):

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

Array–pointer interchangeability

The subscript notation ten[i] (where x designates a pointer) is syntactic carbohydrate for *(x+i).^[36] Taking advantage of the compiler's cognition of the pointer type, the address that 10 + i points to is not the base address (pointed to past 10) incremented past i bytes, simply rather is defined to be the base address incremented by i multiplied by the size of an element that x points to. Thus, x[i] designates the i+onethursday element of the array.

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

The total size of an array x can be determined past applying sizeof to an expression of array type. The size of an element can be determined by applying the operator sizeof to any dereferenced element of an array A, every bit in due north = sizeof A[0]. This, the number of elements in a alleged array A can be determined as sizeof A / sizeof A[0]. Notation, that if only a pointer to the outset element is available as it is oftentimes the case in C code because of the automatic conversion described above, the information about the full type of the assortment and its length are lost.

Retentiveness management

One of the nigh important functions of a programming linguistic communication is to provide facilities for managing retentiveness and the objects that are stored in retentivity. C provides three singled-out means to allocate retentiveness for objects:^[31]

Static memory resource allotment: space 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 be stored on the stack, and this space is automatically freed and reusable subsequently the block in which they are declared is exited.
Dynamic memory allocation: blocks of memory of capricious size tin can exist requested at run-time using library functions such as malloc from a region of memory chosen the heap; these blocks persist until later freed for reuse by calling the library function realloc or costless

These three approaches are appropriate in different situations and take various merchandise-offs. For instance, static retention allocation has little allocation overhead, automatic allocation may involve slightly more overhead, and dynamic retentiveness allotment tin can potentially take a bang-up deal of overhead for both allocation and deallocation. The persistent nature of static objects is useful for maintaining state information across function calls, automatic allocation is easy to apply but stack space is typically much more limited and transient than either static memory or heap infinite, and dynamic retention allotment allows convenient allotment of objects whose size is known just at run-fourth dimension. Most C programs make all-encompassing use of all three.

Where possible, automatic or static allocation is usually simplest considering the storage is managed by the compiler, freeing the programmer of the potentially fault-prone chore of manually allocating and releasing storage. However, many data structures can change in size at runtime, and since static allocations (and automatic allocations before C99) must have a stock-still size at compile-time, at that place are many situations in which dynamic allocation is necessary.^[31] Prior to the C99 standard, variable-sized arrays were a common instance of this. (See the article on malloc for an case of dynamically allocated arrays.) Unlike automatic allocation, which can fail at run fourth dimension with uncontrolled consequences, the dynamic resource allotment functions render an indication (in the class of a null pointer value) when the required storage cannot exist allocated. (Static allocation that is too large is usually detected by the linker or loader, earlier the plan can even brainstorm execution.)

Unless otherwise specified, static objects contain zippo or null pointer values upon plan startup. Automatically and dynamically allocated objects are initialized only if an initial value is explicitly specified; otherwise they initially have indeterminate values (typically, whatsoever fleck pattern happens to be present in the storage, which might not even represent a valid value for that blazon). If the program attempts to admission an uninitialized value, the results are undefined. Many modernistic compilers try to discover and warn almost this trouble, but both imitation positives and false negatives can occur.

Heap retentivity allocation has to be synchronized with its actual usage in any program to exist reused equally much every bit possible. For example, if the only arrow 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 later reuse and is substantially lost to the programme, a phenomenon known every bit a memory leak. Conversely, information technology is possible for memory to be freed, but is referenced later, leading to unpredictable results. Typically, the failure symptoms announced in a portion of the program unrelated to the lawmaking that causes the error, making it difficult to diagnose the failure. Such issues are ameliorated in languages with automatic garbage collection.

Libraries

The C programming language uses libraries every bit its primary method of extension. In C, a library is a prepare of functions contained within a single "annal" 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 program, and declarations of special data types and macro symbols used with these functions. In order for a program to apply a library, it must include the library's header file, and the library must be 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 as embedded systems may provide only a subset of the standard library). This library supports stream input and output, retentivity resource allotment, mathematics, grapheme strings, and fourth dimension values. Several separate standard headers (for case, stdio.h) specify the interfaces for these and other standard library facilities.

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

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

File handling and streams

File input and output (I/O) is not part of the C linguistic communication itself merely instead is handled past libraries (such as the C standard library) and their associated header files (due east.thousand. stdio.h). File handling is generally 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 concrete device. The high-level I/O is washed through the association of a stream to a file. In the C standard library, a buffer (a memory expanse or queue) is temporarily used to store data before it'south sent to the final destination. This reduces the fourth dimension spent waiting for slower devices, for instance a hard drive or solid state drive. Low-level I/O functions are not office of the standard C library^{[ clarification needed ]} but are generally part of "bare metallic" programming (programming that's independent of any operating system such as nearly embedded programming). With few exceptions, implementations include low-level I/O.

Linguistic communication 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 benign in any language, and for C many such tools exist, such as Lint. A common practice is to use Lint to detect questionable lawmaking when a programme is outset written. In one case a programme passes Lint, it is and then compiled using the C compiler. Also, many compilers can optionally warn well-nigh syntactically valid constructs that are likely to really exist errors. MISRA C is a proprietary set of guidelines to avoid such questionable code, developed for embedded systems.^[37]

In that location are likewise compilers, libraries, and operating arrangement level mechanisms for performing deportment that are not a standard part of C, such as bounds checking for arrays, detection of buffer overflow, serialization, dynamic retentiveness tracking, and automatic garbage collection.

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

Uses

The C Programming Linguistic communication

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

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

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

C is sometimes used as an intermediate linguistic communication by implementations of other languages. This approach may exist used for portability or convenience; by using C as an intermediate linguistic communication, additional machine-specific code generators are not necessary. C has some features, such every bit line-number preprocessor directives and optional superfluous commas at the cease of initializer lists, that support compilation of generated code. Withal, some of C's shortcomings have prompted the development of other C-based languages specifically designed for use as intermediate languages, such as C--.

C has also been widely used to implement terminate-user applications. All the same, such applications can also be written in newer, higher-level languages.

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

C has both directly and indirectly influenced many afterwards languages such equally C#, D, Go, Coffee, 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, information models, and/or large-calibration program structures that differ from those of C, sometimes radically.

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

When object-oriented programming languages became pop, C++ and Objective-C were 2 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 and so compiled with a C compiler.^[43]

The C++ programming linguistic communication (originally named "C with Classes") was devised past 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. Nearly a superset of C, C++ at present supports most of C, with a few exceptions.

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

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

See as well

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

Notes

^ The original example lawmaking will compile on about modernistic compilers that are not in strict standard compliance way, 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 primary function actually has two arguments, int argc and char *argv[], respectively, which tin can be used to handle command line arguments. The ISO C standard (department 5.1.2.two.ane) requires both forms of main to be supported, which is special treatment not afforded to any other part.

References

^ ^a ^b Kernighan, Brian Due west.; Ritchie, Dennis Yard. (February 1978). The C Programming Language (1st ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-110163-0.
^ Ritchie (1993): "Thompson had fabricated a brief attempt to produce a organisation coded in an early version of C—earlier structures—in 1972, but gave up the attempt."
^ 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 not, perhaps, emerge in a class that Algol's 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 Computer Science at the Australian National University. June 3, 2010. Archived from the original (PDF) on November 6, 2013. Retrieved August 19, 2013. 1980s: ; Verilog first introduced ; Verilog inspired past the C programming linguistic communication
^ ^a ^b ^c ^d ^east Ritchie (1993)
^ "Programming Language Popularity". 2009. Archived from the original on January xvi, 2009. Retrieved January 16, 2009.
^ "TIOBE Programming Customs Index". 2009. Archived from the original on May 4, 2009. Retrieved May 6, 2009.
^ ^a ^b "History of C". en.cppreference.com. Archived from the original on May 29, 2018. Retrieved May 28, 2018.
^ "TIOBE Index for October 2021". Retrieved Oct 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. Thousand. (1978). "Portability of C Programs and the UNIX System". Bong System Tech. J. 57 (6): 2021–2048. CiteSeerX10.ane.1.138.35. doi:10.1002/j.1538-7305.1978.tb02141.ten. S2CID 17510065. (Note: The PDF is an OCR browse of the original, and contains a rendering of "IBM 370" every bit "IBM 310".)
^ McIlroy, M. D. (1987). A Enquiry Unix reader: annotated excerpts from the Programmer's Manual, 1971–1986 (PDF) (Technical report). CSTR. Bell Labs. p. x. 139. Archived (PDF) from the original on November 11, 2017. Retrieved February i, 2015.
^ "C transmission pages". FreeBSD Miscellaneous Information Transmission (FreeBSD 13.0 ed.). May 30, 2011. Archived from the original on January 21, 2021. Retrieved January 15, 2021. [one] Archived January 21, 2021, at the Wayback Machine
^ Kernighan, Brian West.; Ritchie, Dennis M. (March 1988). The C Programming Linguistic communication (2nd ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-xiii-110362-7.
^ Stroustrup, Bjarne (2002). Sibling rivalry: C and C++ (PDF) (Report). AT&T Labs. Archived (PDF) from the original on August 24, 2014. Retrieved April 14, 2014.
^ C Integrity. International Organization for Standardization. March 30, 1995. Archived from the original on July 25, 2018. Retrieved July 24, 2018.
^ "JTC1/SC22/WG14 – C". Dwelling page. ISO/IEC. Archived from the original on Feb 12, 2018. Retrieved June 2, 2011.
^ Andrew Binstock (Oct 12, 2011). "Interview with Herb Sutter". Dr. Dobbs. Archived from the original on Baronial two, 2013. Retrieved September 7, 2013.
^ "Revised C23 Schedule WG 14 Due north 2759" (PDF). www.open up-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 Fifty. (2002). C: A Reference Manual (5th ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-thirteen-089592-ix. Contains a BNF grammer 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 xvi, 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 (third ed.). Otsego, MI: PageFree Publishing Inc. p. xx. ISBN978-one-58961-237-ii. Archived from the original on July 29, 2020. Retrieved February x, 2012.
^ Kernighan & Ritchie (1978), p. 6.
^ ^a ^b ^c ^d ^east ^f ^g Klemens, Ben (2013). 21st Century C. O'Reilly Media. ISBN978-one-4493-2714-nine.
^ Feuer, Alan R.; Gehani, Narain H. (March 1982). "Comparing of the Programming Languages C and Pascal". ACM Computing Surveys. fourteen (ane): 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 Jan seven, 2007. Retrieved Baronial 5, 2012.
^ เอี่ยมสิริวงศ์, โอภาศ (2016). Programming with C. Bangkok, Thailand: SE-Pedagogy PUBLIC COMPANY LIMITED. pp. 225–230. ISBN978-616-08-2740-4.
^ Raymond, Eric Due south. (October xi, 1996). The New Hacker's Dictionary (tertiary ed.). MIT Press. p. 432. ISBN978-0-262-68092-ix. Archived from the original on November 12, 2012. Retrieved Baronial 5, 2012.
^ "Man Page for lint (freebsd Section 1)". unix.com. May 24, 2001. Retrieved July 15, 2014.
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Sources

Ritchie, Dennis M. (March 1993). "The Development of the C Language". ACM SIGPLAN Notices. ACM. 28 (3): 201–208. doi:10.1145/155360.155580.
Ritchie, Dennis Yard. (1993). "The Development of the C Language". The Second ACM SIGPLAN Conference on History of Programming Languages (HOPL-Ii). ACM. pp. 201–208. doi:10.1145/154766.155580. ISBN0-89791-570-four . Retrieved November 4, 2014.
Kernighan, Brian W.; Ritchie, Dennis M. (1996). The C Programming Language (2d 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). (3.61 MB)
comp.lang.c Frequently Asked Questions
A History of C, past Dennis Ritchie

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