C#how Can I Change Form Fields From Write to Read

General-purpose programming language

C
Major implementations
The C Programming Language ^[1] (often referred to as K&R), the seminal book on C
Prototype	Multi-prototype: 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; fifty years ago (1972) ^[ii]

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

Typing bailiwick	Static, weak, manifest, nominal
Os	Cross-platform
Filename extensions	.c, .h
Website	www.iso.org/standard/74528.html world wide web.open up-std.org/jtc1/sc22/wg14/
pcc, GCC, Clang, Intel C, C++Architect, Microsoft Visual C++, Watcom C
Dialects
Whirlwind, Unified Parallel C, Dissever-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, Freeway, Processing, Python, Band,^[5]Rust, Seed7, Vala, Verilog (HDL),^[6] Nim, Zig
C Programming at Wikibooks

C (, every bit in the letter c) is a full general-purpose, procedural computer programming language supporting structured programming, lexical variable telescopic, and recursion, with a static type system. By blueprint, C provides constructs that map efficiently to typical motorcar instructions. It has found lasting use in applications previously coded in associates language. Such applications include operating systems and various awarding software for calculator architectures that range from supercomputers to PLCs and embedded systems.

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

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

Since 2000, C has consistently ranked among the peak two languages in the TIOBE alphabetize, a measure of the popularity of programming languages.^[11]

Overview [edit]

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

The language has a small-scale, fixed number of keywords, including a full fix of control flow primitives: if/else, for, do/while, while, and switch. User-defined names are non distinguished from keywords by whatsoever kind of sigil.
It has a large number of arithmetic, bitwise, and logic operators: +,+=,++,&,||, etc.
More than one assignment may be performed in a single statement.
Functions:
- Function return values tin can be ignored, when not needed.
- Role and data pointers let ad hoc run-time polymorphism.
- Functions may not be defined within the lexical scope of other functions.
Data typing is static, but weakly enforced; all data has a blazon, but implicit conversions are possible.
Proclamation syntax mimics usage context. C has no "define" keyword; instead, a statement showtime with the proper name of a blazon is taken equally a annunciation. At that place is no "part" keyword; instead, a function is indicated past the presence of a parenthesized argument listing.
User-defined (typedef) and compound types are possible.
- Heterogeneous aggregate information types (struct) permit related information elements to be accessed and assigned as a unit of measurement.
- Union is a structure with overlapping members; only the terminal member stored is valid.
- Array indexing is a secondary notation, defined in terms of pointer arithmetics. Dissimilar structs, arrays are non outset-class objects: they cannot be assigned or compared using single congenital-in operators. There is no "array" keyword in employ 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 not a singled-out information type, but are conventionally implemented as null-terminated character arrays.
Low-level admission to calculator retention is possible by converting machine addresses to typed pointers.
Procedures (subroutines not returning values) are a special case of role, with an untyped render blazon void.
A preprocessor performs macro definition, source code file inclusion, and conditional compilation.
There is a basic class of modularity: files tin can be compiled separately and linked together, with command over which functions and data objects are visible to other files via static and extern attributes.
Complex functionality such as I/O, string manipulation, and mathematical functions are consistently delegated to library routines.

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

Relations to other languages [edit]

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

History [edit]

Early developments [edit]

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

The origin of C is closely tied to the development of the Unix operating arrangement, originally implemented in assembly language on a PDP-vii past Dennis Ritchie and Ken Thompson, incorporating several ideas from colleagues. Somewhen, they decided to port the operating organization to a PDP-xi. The original PDP-11 version of Unix was also developed in associates language.^[vii]

Thompson desired a programming language to brand utilities for the new platform. At first, he tried to make a Fortran compiler, but before long gave up the idea. Instead, he created a cut-down version of the recently developed BCPL systems programming linguistic communication. The official description of BCPL was not available at the time,^[12] and Thompson modified the syntax to be less wordy, producing the similar but somewhat simpler B.^[7] Yet, few utilities were ultimately written in B because it was too slow, and B could not take advantage of PDP-11 features such as byte addressability.

In 1972, Ritchie started to improve B, about notably adding information typing for variables, which resulted in creating a new linguistic communication C.^[13] The C compiler and some utilities made with information technology were included in Version two Unix.^[14]

At Version iv Unix, released in November 1973, the Unix kernel was extensively re-implemented in C.^[7] Past 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 too 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 cord replacements: #include and #ascertain of parameterless macros. Soon after that, it was extended, mostly past Mike Lesk and then past John Reiser, to contain macros with arguments and conditional compilation.^[7]

Unix was one of the first operating system kernels implemented in a linguistic communication other than associates. Earlier instances include the Multics system (which was written in PL/I) and Main Control 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's Portable C Compiler served every bit the basis for several implementations of C on new platforms.^[13]

K&R C [edit]

In 1978, Brian Kernighan and Dennis Ritchie published the commencement edition of The C Programming Linguistic communication.^[1] This volume, known to C programmers as Chiliad&R, served for many years as an informal specification of the linguistic communication. The version of C that it describes is commonly referred to as "One thousand&R C". As this was released in 1978, it is too referred to as C78.^[15] The second edition of the volume^[xvi] covers the later ANSI C standard, described below.

K&R introduced several language features:

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

Even afterward the publication of the 1989 ANSI standard, for many years K&R C was notwithstanding considered the "lowest mutual denominator" to which C programmers restricted themselves when maximum portability was desired, since many older compilers were still in utilise, and considering carefully written Yard&R C code tin can be legal Standard C likewise.

In early versions of C, only functions that return types other than int must be alleged if used before the function definition; functions used without prior announcement 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            ();                                                render                                    test2            ;                        }

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

Since Grand&R part declarations did non include any information virtually role arguments, role parameter blazon checks were not 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 dissimilar numbers or types of arguments. Separate tools such equally Unix'due south lint utility were developed that (among other things) could check for consistency of function employ across multiple source files.

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

void functions (i.eastward., functions with no return value)
functions returning struct or union types (rather than pointers)
consignment 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 fifty-fifty the Unix compilers precisely implemented the G&R specification, led to the necessity of standardization.

ANSI C and ISO C [edit]

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

In 1983, the American National Standards Establish (ANSI) formed a committee, X3J11, to plant 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 basis for the 1988 POSIX standard. In 1989, the C standard was ratified every bit ANSI X3.159-1989 "Programming Language C". This version of the language is often referred to as ANSI C, Standard C, or sometimes C89.

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

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

Ane of the aims of the C standardization process was to produce a superset of Thou&R C, incorporating many of the afterward 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 style used in C++, the K&R interface connected to exist permitted, for compatibility with existing source code.

C89 is supported by electric current C compilers, and most modern C code is based on information technology. Any plan written but in Standard C and without whatsoever hardware-dependent assumptions volition run correctly on whatever platform with a befitting C implementation, within its resource limits. Without such precautions, programs may compile just on a certain platform or with a particular compiler, due, for case, to the employ of non-standard libraries, such as GUI libraries, or to a reliance on compiler- or platform-specific attributes such as the exact size of information types and byte endianness.

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

After the ANSI/ISO standardization procedure, the C linguistic communication 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.^[18]

C99 [edit]

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 as "C99". It has since been amended three times past Technical Corrigenda.^[xix]

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

C99 is for the most office backward 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 betoken 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++, however, implements the C89 standard and those parts of C99 that are required for compatibility with C++eleven.^[20] ^{[ needs update ]}

In improver, support for Unicode identifiers (variable / function names) in the course of escaped characters (e.1000. \U0001f431) is at present required. Support for raw Unicode names is optional.

C11 [edit]

In 2007, piece of work began on some other 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 past existing implementations.

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

C17 [edit]

Published in June 2018, C17 is the 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 every bit 201710L.

C2x [edit]

C2x is an informal proper name for the side by side (afterward C17) major C language standard revision. Information technology is expected to exist voted on in 2023 and would therefore exist called C23.^[21] ^{[ better source needed ]}

Embedded C [edit]

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

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

Syntax [edit]

C has a formal grammar specified past the C standard.^[23] Line endings are generally not pregnant in C; however, line boundaries do take significance during the preprocessing phase. Comments may appear either between 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 not interpreted as annotate delimiters if they appear inside string or character literals.^[24]

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

As an imperative language, C uses statements to specify actions. The about mutual argument is an expression argument, consisting of an expression to exist evaluated, followed by a semicolon; every bit a side result of the evaluation, functions may exist called and variables may be assigned new values. To change the normal sequential execution of statements, C provides several command-menses statements identified by reserved keywords. Structured programming is supported past if … [else] conditional execution and by exercise … while, while, and for iterative execution (looping). The for statement has dissever initialization, testing, and reinitialization expressions, whatever or all of which can be omitted. interruption and keep tin exist used to go out the innermost enclosing loop statement or skip to its reinitialization. There is too a non-structured goto statement which branches directly to the designated label within the function. switch selects a case to exist executed based on the value of an integer expression.

Expressions can use a variety of congenital-in operators and may incorporate office calls. The order in which arguments to functions and operands to almost operators are evaluated is unspecified. The evaluations may even be interleaved. Nonetheless, all side effects (including storage to variables) will occur before the next "sequence point"; sequence points include the terminate of each expression statement, and the entry to and render from each function phone call. Sequence points also occur during evaluation of expressions containing certain operators (&&, ||, ?: and the comma operator). This permits a high degree of object code optimization by the compiler, just requires C programmers to take more care to obtain reliable results than is needed for other programming languages.

Kernighan and Ritchie say in the Introduction of The C Programming Language: "C, similar any other linguistic communication, has its blemishes. Some of the operators have the wrong precedence; some parts of the syntax could be better."^[25] The C standard did not attempt to correct many of these blemishes, because of the impact of such changes on already existing software.

Grapheme set [edit]

The basic C source character set includes the following characters:

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

Newline indicates the end of a text line; it need non stand for to an bodily single character, although for convenience C treats it as one.

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

The basic C execution character ready contains the same characters, along with representations for warning, backspace, and railroad vehicle return. Run-time back up for extended graphic symbol sets has increased with each revision of the C standard.

Reserved words [edit]

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

automobile
break
case
char
const
continue
default
do
double
else
enum
extern
float
for
goto
if
int
long
register
return
curt
signed
sizeof
static
struct
switch
typedef
marriage
unsigned
void
volatile
while

C99 reserved five more words:

_Bool
_Complex
_Imaginary
inline
restrict

C11 reserved seven more words:^[26]

_Alignas
_Alignof
_Atomic
_Generic
_Noreturn
_Static_assert
_Thread_local

Well-nigh of the recently reserved words begin with an underscore followed past a capital letter, because identifiers of that form were previously reserved past the C standard for use only past implementations. Since existing program source code should non have been using these identifiers, it would not exist affected when C implementations started supporting these extensions to the programming language. Some standard headers do define more convenient synonyms for underscored identifiers. The linguistic communication previously included a reserved give-and-take chosen entry, just this was seldom implemented, and has now been removed equally a reserved discussion.^[27]

Operators [edit]

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

arithmetic: +, -, *, /, %
assignment: =
augmented assignment: +=, -=, *=, /=, %=, &=, |=, ^=, <<=, >>=
bitwise logic: ~, &, |, ^
bitwise shifts: <<, >>
boolean logic: !, &&, ||
conditional evaluation: ? :
equality testing: ==, !=
calling functions: ( )
increment and decrement: ++, --
member selection: ., ->
object size: sizeof
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 unlike ALGOL and its derivatives. C uses the operator == to test for equality. The similarity betwixt these two operators (assignment and equality) may result in the accidental utilise of i in place of the other, and in many cases, the mistake does non produce an error message (although some compilers produce warnings). For example, the provisional expression if (a == b + i) might mistakenly exist written equally if (a = b + 1), which will exist evaluated equally true if a is not zero afterwards the assignment.^[28]

The C operator precedence is not ever 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 [edit]

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

The original version was:^[thirty]

                        chief            ()                        {                                                printf            (            "howdy, earth            \n            "            );                        }

A standard-befitting "hello, world" program is:^[a]

                        #include                                    <stdio.h>                        int                                    main            (            void            )                        {                                                printf            (            "hello, 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 angle brackets surrounding stdio.h point that stdio.h is located using a search strategy that prefers headers provided with the compiler to other headers having the same name, every bit opposed to double quotes which typically include local or project-specific header files.

The adjacent line indicates that a function named main is existence defined. The chief function serves a special purpose in C programs; the run-time surround calls the main office to brainstorm program execution. The type specifier int indicates that the value that is returned to the invoker (in this case the run-time environs) as a result of evaluating the principal function, is an integer. The keyword void as a parameter list indicates that this part takes no arguments.^[b]

The opening curly brace indicates the beginning of the definition of the primary part.

The side by side line calls (diverts execution to) a role named printf, which in this example 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 "hi, world\northward". The string literal is an unnamed assortment with elements of type char, prepare upward automatically by the compiler with a final 0-valued character to marker the end of the assortment (printf needs to know this). The \n is an escape sequence that C translates to a newline grapheme, which on output signifies the end of the current line. The return value of the printf function is of type int, just it is silently discarded since it is not used. (A more than 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 cease of the lawmaking for the master function. According to the C99 specification and newer, the main office, dissimilar 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-fourth dimension system equally an exit code indicating successful execution.^[31]

Data types [edit]

The type 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 frequently used for single-byte characters. C99 added a boolean datatype. There are also derived types including arrays, pointers, records (struct), and unions (marriage).

C is frequently used in low-level systems programming where escapes from the type system may be necessary. The compiler attempts to ensure type correctness of most expressions, just the programmer tin override the checks in various ways, either by using a type bandage to explicitly convert a value from 1 type to another, or by using pointers or unions to reinterpret the underlying $.25 of a data object in some other way.

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

C'due south usual arithmetic conversions let for efficient code to be generated, merely can 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 [edit]

C supports the use of pointers, a type of reference that records the address or location of an object or function in memory. Pointers can exist dereferenced to admission data stored at the accost pointed to, or to invoke a pointed-to function. Pointers tin can be manipulated using assignment or pointer arithmetic. The run-time representation of a arrow value is typically a raw retention accost (perhaps augmented by an offset-within-word field), but since a pointer's type includes the type of the thing pointed to, expressions including pointers can be type-checked at compile time. Pointer arithmetic is automatically scaled by the size of the pointed-to information type. Pointers are used for many purposes in C. Text strings are commonly manipulated using pointers into arrays of characters. Dynamic retentivity resource allotment 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 equally qsort or bsearch) or as callbacks to be invoked past event handlers.^[31]

A null pointer value explicitly points to no valid location. Dereferencing a null arrow value is undefined, often resulting in a partition error. Nothing arrow values are useful for indicating special cases such as no "next" pointer in the last node of a linked listing, or as an error indication from functions returning pointers. In advisable contexts in source code, such as for assigning to a arrow variable, a naught pointer constant can be written as 0, with or without explicit casting to a pointer blazon, or as the Nix macro defined past several standard headers. In provisional contexts, null pointer values evaluate to false, while all other pointer values evaluate to truthful.

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

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

Arrays [edit]

Array types in C are traditionally of a stock-still, static size specified at compile time. The more recent C99 standard likewise allows a form 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'due south malloc office, and treat it as an array.

Since arrays are e'er accessed (in effect) via pointers, array accesses are typically not checked against the underlying array size, although some compilers may provide bounds checking every bit an option.^[34] ^[35] Array bounds violations are therefore possible and can pb to diverse repercussions, including illegal retentiveness accesses, corruption of data, buffer overruns, and run-fourth dimension exceptions.

C does non have a special provision for declaring multi-dimensional arrays, merely rather relies on recursion within the type system to declare arrays of arrays, which finer accomplishes the aforementioned affair. The index values of the resulting "multi-dimensional array" 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 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 allocate the array with an additional "row vector" of pointers to the columns.) C99 introduced "variable-length arrays" which accost this issue.

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

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

Array–pointer interchangeability [edit]

The subscript notation x[i] (where x designates a arrow) is syntactic sugar for *(x+i).^[36] Taking reward of the compiler's knowledge of the pointer type, the address that x + i points to is not the base address (pointed to by ten) incremented by i bytes, but rather is defined to be the base address incremented past i multiplied by the size of an element that x points to. Thus, x[i] designates the i+1th element of the assortment.

Furthermore, in most expression contexts (a notable exception is equally 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 equally a whole when named as an statement to a function, merely rather merely the address of its outset element is passed. Therefore, although function calls in C use laissez passer-by-value semantics, arrays are in issue passed by reference.

The total size of an array 10 tin be determined past applying sizeof to an expression of array type. The size of an chemical element can be determined by applying the operator sizeof to any dereferenced element of an array A, equally in n = sizeof A[0]. This, the number of elements in a declared assortment A tin can be determined as sizeof A / sizeof A[0]. Note, that if only a pointer to the first element is available every bit it is often the case in C code considering of the automatic conversion described above, the information almost the full type of the array and its length are lost.

Memory management [edit]

Ane of the most important functions of a programming language is to provide facilities for managing retentiveness and the objects that are stored in retentivity. C provides three singled-out ways to allocate retentiveness for objects:^[31]

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

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

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

Unless otherwise specified, static objects contain nix or aught arrow values upon program startup. Automatically and dynamically allocated objects are initialized just if an initial value is explicitly specified; otherwise they initially have indeterminate values (typically, any bit pattern happens to be present in the storage, which might not 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 about this problem, only both false positives and fake negatives can occur.

Heap memory allocation has to be synchronized with its actual usage in any program to exist reused as much as possible. For case, if the but arrow to a heap memory allocation goes out of scope or has its value overwritten before it is deallocated explicitly, then that retentivity cannot exist recovered for afterwards reuse and is essentially lost to the program, a miracle known as a memory leak. Conversely, it is possible for retentivity to exist freed, only 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 bug are ameliorated in languages with automated garbage collection.

Libraries [edit]

The C programming language uses libraries equally its master 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 be used past a program, and declarations of special data types and macro symbols used with these functions. In gild for a program to employ a library, it must include the library'due south header file, and the library must be linked with the program, which in many cases requires compiler flags (eastward.thousand., -lm, shorthand for "link the math library").^[31]

The virtually common C library is the C standard library, which is specified by the ISO and ANSI C standards and comes with every C implementation (implementations which target limited environments such every bit embedded systems may provide but a subset of the standard library). This library supports stream input and output, memory allotment, mathematics, character strings, and fourth dimension values. Several divide standard headers (for instance, stdio.h) specify the interfaces for these and other standard library facilities.

Some other common set 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 can be used from higher-level languages similar Java, Perl, and Python.^[31]

File handling and streams [edit]

File input and output (I/O) is not part of the C linguistic communication itself just instead is handled past libraries (such equally the C standard library) and their associated header files (e.g. stdio.h). File treatment is generally implemented through high-level I/O which works through streams. A stream is from this perspective a data menses 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 memory area or queue) is temporarily used to shop data before information technology'south sent to the last destination. This reduces the time spent waiting for slower devices, for case a hard bulldoze or solid state drive. Low-level I/O functions are not role of the standard C library^{[ clarification needed ]} but are mostly office of "blank metal" programming (programming that's independent of any operating system such equally nearly embedded programming). With few exceptions, implementations include depression-level I/O.

Linguistic communication tools [edit]

A number of tools take been developed to help C programmers observe and fix statements with undefined behavior or possibly erroneous expressions, with greater rigor than that provided by the compiler. The tool lint was the first such, leading to many others.

Automatic source lawmaking checking and auditing are beneficial in any language, and for C many such tools be, such as Lint. A common practice is to use Lint to discover questionable code when a programme is first written. Once a program passes Lint, it is then compiled using the C compiler. Also, many compilers can optionally warn about syntactically valid constructs that are probable to really be errors. MISRA C is a proprietary prepare of guidelines to avert such questionable code, adult for embedded systems.^[37]

There are likewise compilers, libraries, and operating system level mechanisms for performing actions that are not a standard office of C, such every bit bounds checking for arrays, detection of buffer overflow, serialization, dynamic retentiveness tracking, and automatic garbage drove.

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

Uses [edit]

The C Programming Language

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

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

A upshot 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 information structures, because the layer of abstraction from hardware is thin, and its overhead is depression, an important criterion for computationally intensive programs. For example, the GNU Multiple Precision Arithmetic Library, the GNU Scientific Library, Mathematica, and MATLAB are completely or partially written in C.

C is sometimes used as an intermediate language by implementations of other languages. This approach may be used for portability or convenience; past using C equally an intermediate language, additional machine-specific code generators are not necessary. C has some features, such equally line-number preprocessor directives and optional superfluous commas at the finish of initializer lists, that support compilation of generated code. However, some of C'due south shortcomings have prompted the development of other C-based languages specifically designed for apply as intermediate languages, such every bit C--.

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

[edit]

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

C has both directly and indirectly influenced many later languages such equally C#, D, Go, Coffee, JavaScript, Limbo, LPC, Perl, PHP, Python, and Unix's C shell.^[42] The nigh 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-scale plan structures that differ from those of C, sometimes radically.

Several C or virtually-C interpreters exist, including Ch and CINT, which can likewise 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 every bit source-to-source compilers; source code was translated into C, and and so 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-similar syntax.^[44] C++ adds greater typing strength, scoping, and other tools useful in object-oriented programming, and permits generic programming via templates. Nearly a superset of C, C++ now supports most of C, with a few exceptions.

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

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

Notes [edit]

^ The original example code will compile on near modern compilers that are not in strict standard compliance style, but it does not fully conform to the requirements of either C89 or C99. In fact, C99 requires that a diagnostic message be produced.
^ The master function actually has two arguments, int argc and char *argv[], respectively, which tin be used to handle command line arguments. The ISO C standard (section 5.1.two.2.i) requires both forms of main to be supported, which is special treatment not afforded to any other part.

References [edit]

^ ^a ^b Kernighan, Brian Westward.; 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 cursory attempt to produce a system coded in an early on version of C—before structures—in 1972, merely gave up the endeavour."
^ Fruderica (December 13, 2020). "History of C". The cppreference.com. Archived from the original on October 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 form that Algol's adherents would approve of."
^ Ring Team (Oct 23, 2021). "The Ring programming language and other languages". ring-lang.net.
^ ^a ^b "Verilog HDL (and C)" (PDF). The Research School of Computer science at the Australian National University. June 3, 2010. Archived from the original (PDF) on Nov 6, 2013. Retrieved August 19, 2013. 1980s: ; Verilog first introduced ; Verilog inspired by the C programming language
^ ^a ^b ^c ^d ^e Ritchie (1993)
^ "Programming Language Popularity". 2009. Archived from the original on Jan 16, 2009. Retrieved Jan 16, 2009.
^ "TIOBE Programming Community Index". 2009. Archived from the original on May iv, 2009. Retrieved May half dozen, 2009.
^ ^a ^b "History of C". en.cppreference.com. Archived from the original on May 29, 2018. Retrieved May 28, 2018.
^ "TIOBE Index for Oct 2021". Retrieved Oct seven, 2021.
^ Ritchie, Dennis. "BCPL to B to C". Archived from the original on December 12, 2019. Retrieved September 10, 2019.
^ ^a ^b Johnson, Southward. C.; Ritchie, D. M. (1978). "Portability of C Programs and the UNIX System". Bell System Tech. J. 57 (half dozen): 2021–2048. CiteSeerXten.ane.1.138.35. 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 Enquiry Unix reader: annotated excerpts from the Programmer'south Transmission, 1971–1986 (PDF) (Technical report). CSTR. Bell Labs. p. 10. 139. Archived (PDF) from the original on November xi, 2017. Retrieved February one, 2015.
^ "C manual pages". FreeBSD Miscellaneous Information Manual (FreeBSD 13.0 ed.). May 30, 2011. Archived from the original on January 21, 2021. Retrieved January 15, 2021. [one] Archived Jan 21, 2021, at the Wayback Auto
^ 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 August 24, 2014. Retrieved April 14, 2014.
^ C Integrity. International System for Standardization. March thirty, 1995. Archived from the original on July 25, 2018. Retrieved July 24, 2018.
^ "JTC1/SC22/WG14 – C". Dwelling folio. ISO/IEC. Archived from the original on February 12, 2018. Retrieved June 2, 2011.
^ Andrew Binstock (October 12, 2011). "Interview with Herb Sutter". Dr. Dobbs. Archived from the original on August 2, 2013. Retrieved September 7, 2013.
^ "Revised C23 Schedule WG 14 N 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 L. (2002). C: A Reference Manual (fifth ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-089592-nine. Contains a BNF grammar for C.
^ Kernighan & Ritchie (1996), p. 192.
^ Kernighan & Ritchie (1978), p. 3.
^ "ISO/IEC 9899:201x (ISO C11) Commission Draft" (PDF). Archived (PDF) from the original on Dec 22, 2017. Retrieved September 16, 2011.
^ Kernighan & Ritchie (1996), pp. 192, 259.
^ "10 Common Programming Mistakes in C++". Cs.ucr.edu. Archived from the original on October 21, 2008. Retrieved June 26, 2009.
^ Schultz, Thomas (2004). C and the 8051 (tertiary ed.). Otsego, MI: PageFree Publishing Inc. p. twenty. ISBN978-1-58961-237-2. Archived from the original on July 29, 2020. Retrieved Feb x, 2012.
^ Kernighan & Ritchie (1978), p. half-dozen.
^ ^a ^b ^c ^d ^east ^f ^chiliad Klemens, Ben (2013). 21st Century C. O'Reilly Media. ISBN978-1-4493-2714-9.
^ Feuer, Alan R.; Gehani, Narain H. (March 1982). "Comparison of the Programming Languages C and Pascal". ACM Computing Surveys. xiv (1): 73–92. doi:10.1145/356869.356872. S2CID 3136859.
^ Kernighan & Ritchie (1996), p. 122.
^ For example, gcc provides _FORTIFY_SOURCE. "Security Features: Compile Fourth dimension Buffer Checks (FORTIFY_SOURCE)". fedoraproject.org. Archived from the original on January 7, 2007. Retrieved August v, 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'south Dictionary (3rd ed.). MIT Press. p. 432. ISBN978-0-262-68092-9. Archived from the original on November 12, 2012. Retrieved August 5, 2012.
^ "Human Page for lint (freebsd Section one)". unix.com. May 24, 2001. Retrieved July 15, 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'southward Sourcebook. U.S.A.: Miller Freeman, Inc. November–December 1995.
^ "Using C for CGI Programming". linuxjournal.com. March 1, 2005. Archived from the original on February thirteen, 2010. Retrieved January 4, 2010.
^ McMillan, Robert (August 1, 2013). "Is Java Losing Its Mojo?". Wired. Archived from the original on February 15, 2017. Retrieved March 5, 2017.
^ O'Regan, Gerard (September 24, 2015). Pillars of computing : a compendium of select, pivotal technology firms. ISBN978-3319214641. OCLC 922324121.
^ Rauchwerger, Lawrence (2004). Languages and compilers for parallel computing : 16th international workshop, LCPC 2003, Higher Station, TX, Usa, October two-4, 2003 : revised papers. Springer. ISBN978-3540246442. OCLC 57965544.
^ Stroustrup, Bjarne (1993). "A History of C++: 1979−1991" (PDF). Archived (PDF) from the original on February 2, 2019. Retrieved June 9, 2011.

Sources [edit]

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 K. (1993). "The Evolution of the C Language". The 2nd ACM SIGPLAN Conference on History of Programming Languages (HOPL-II). ACM. pp. 201–208. doi:10.1145/154766.155580. ISBN0-89791-570-4 . Retrieved Nov four, 2014.
Kernighan, Brian Westward.; Ritchie, Dennis M. (1996). The C Programming Linguistic communication (2nd ed.). Prentice Hall. ISBNseven-302-02412-10.

External links [edit]

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

barkerwastle.blogspot.com

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