C# Using Ado to Read Excel Sheet Names

General-purpose programming language

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

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

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

C (, as in the letter of the alphabet c) is a general-purpose, procedural computer programming language supporting structured programming, lexical variable scope, and recursion, with a static type system. By design, C provides constructs that map efficiently to typical machine instructions. It has constitute lasting use 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 adult at Bell Labs by Dennis Ritchie betwixt 1972 and 1973 to construct utilities running on Unix. It was practical to re-implementing the kernel of the Unix operating arrangement.^[seven] During the 1980s, C gradually gained popularity. It has get one of the most widely used programming languages,^[8] ^[9] with C compilers from various vendors bachelor for the majority of existing figurer architectures and operating systems. C has been standardized by ANSI since 1989 (ANSI C) and past the International Organization for Standardization (ISO).

C is an imperative procedural language. It was designed to exist compiled to provide low-level access to memory and linguistic communication 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 heed tin can exist compiled for a broad diverseness of calculator platforms and operating systems with few changes to its source code.^[x]

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

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 arrangement prevents unintended operations. In C, all executable code is contained within subroutines (also 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 past explicitly passing pointer values. C programme source text is free-format, using the semicolon equally a statement terminator and curly braces for grouping blocks of statements.

The C linguistic communication also exhibits the following characteristics:

The language has a pocket-size, fixed number of keywords, including a total fix of control menses primitives: if/else, for, practice/while, while, and switch. User-divers names are not distinguished from keywords past any kind of sigil.
It has a big number of arithmetic, bitwise, and logic operators: +,+=,++,&,||, etc.
More than one assignment may exist performed in a unmarried statement.
Functions:
- Role return values can exist ignored, when not needed.
- Function and data pointers permit advertizing hoc run-fourth dimension polymorphism.
- Functions may not be divers within the lexical scope of other functions.
Information typing is static, but weakly enforced; all data has a type, just implicit conversions are possible.
Annunciation syntax mimics usage context. C has no "define" keyword; instead, a statement beginning with the name of a type is taken as 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) permit related data elements to exist accessed and assigned equally a unit.
- Marriage is a structure with overlapping members; only the concluding member stored is valid.
- Array indexing is a secondary notation, divers in terms of pointer arithmetics. Unlike structs, arrays are non splendid objects: they cannot exist assigned or compared using single congenital-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 singled-out data type, merely are conventionally implemented as null-terminated character arrays.
Low-level access to figurer memory is possible past converting machine addresses to typed pointers.
Procedures (subroutines not returning values) are a special case of role, with an untyped return type void.
A preprocessor performs macro definition, source code file inclusion, and conditional compilation.
There is a basic form of modularity: files can be compiled separately and linked together, with control over which functions and information objects are visible to other files via static and extern attributes.
Complex functionality such as I/O, string manipulation, and mathematical functions are consistently delegated to library routines.

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

Relations to other languages [edit]

Many later languages take borrowed directly or indirectly from C, including C++, C#, Unix'due south C beat out, D, Go, Java, JavaScript (including transpilers), Julia, Limbo, LPC, Objective-C, Perl, PHP, Python, Ruby, Rust, Swift, Verilog and SystemVerilog (hardware description languages).^[6] These languages have fatigued many of their control structures and other bones features from C. Most of them (Python being a dramatic exception) also limited highly similar 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 tin be radically unlike.

History [edit]

Early developments [edit]

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 development of the Unix operating system, originally implemented in associates language on a PDP-seven by Dennis Ritchie and Ken Thompson, incorporating several ideas from colleagues. Eventually, they decided to port the operating organization to a PDP-11. The original PDP-eleven version of Unix was also developed in assembly language.^[vii]

Thompson desired a programming language to make utilities for the new platform. At first, he tried to make a Fortran compiler, merely soon gave up the thought. Instead, he created a cut-downward version of the recently developed BCPL systems programming language. The official description of BCPL was not available at the fourth dimension,^[12] and Thompson modified the syntax to be less wordy, producing the similar just somewhat simpler B.^[seven] Notwithstanding, few utilities were ultimately written in B considering information technology was besides deadening, and B could non take reward 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 language C.^[13] The C compiler and some utilities made with it were included in Version two Unix.^[14]

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

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

Unix was one of the commencement operating organisation kernels implemented in a language other than assembly. Earlier instances include the Multics system (which was written in PL/I) and Primary Control Program (MCP) for the Burroughs B5000 (which was written in ALGOL) in 1961. In effectually 1977, Ritchie and Stephen C. Johnson made further changes to the language to facilitate portability of the Unix operating arrangement. Johnson's Portable C Compiler served as the ground for several implementations of C on new platforms.^[13]

K&R C [edit]

In 1978, Brian Kernighan and Dennis Ritchie published the first edition of The C Programming Language.^[i] This book, known to C programmers equally 1000&R, served for many years equally an breezy specification of the linguistic communication. The version of C that it describes is ordinarily referred to as "G&R C". Every bit this was released in 1978, information technology is also referred to as C78.^[15] The second edition of the volume^[16] covers the later ANSI C standard, described below.

Thou&R introduced several language features:

Standard I/O library
long int data type
unsigned int data type
Compound consignment operators of the form =op (such as =-) were changed to the form op= (that is, -=) to remove the semantic ambiguity created past constructs such every bit i=-x, which had been interpreted equally i =- x (decrement i by x) instead of the peradventure intended i = -10 (let i be −x).

Fifty-fifty after the publication of the 1989 ANSI standard, for many years Grand&R C was still considered the "lowest mutual denominator" to which C programmers restricted themselves when maximum portability was desired, since many older compilers were still in use, and considering carefully written K&R C lawmaking tin exist legal Standard C as well.

In early versions of C, only functions that return types other than int must be declared if used before 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                                    >                                    1            )                                                test2                                    =                                    0            ;                                                else                                                test2                                    =                                    other_function            ();                                                return                                    test2            ;                        }

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

Since M&R function declarations did not include any information well-nigh function arguments, function parameter type checks were not performed, although some compilers would event a warning message if a local function was called with the wrong number of arguments, or if multiple calls to an external function used different numbers or types of arguments. Separate tools such as Unix's lint utility were developed that (among other things) could check for consistency of function apply across multiple source files.

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

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

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

ANSI C and ISO C [edit]

During the belatedly 1970s and 1980s, versions of C were implemented for a wide multifariousness of mainframe computers, minicomputers, and microcomputers, including the IBM PC, equally 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; nonetheless, the non-portable portion of the Unix C library was handed off to the IEEE working group 1003 to become the basis for the 1988 POSIX standard. In 1989, the C standard was ratified 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 Organisation 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, similar other national standards bodies, no longer develops the C standard independently, only defers to the international C standard, maintained past the working group ISO/IEC JTC1/SC22/WG14. National adoption of an update to the international standard typically occurs within a twelvemonth of ISO publication.

One of the aims of the C standardization process was to produce a superset of G&R C, incorporating many of the subsequently introduced unofficial features. The standards commission also included several additional features such as function prototypes (borrowed from C++), void pointers, back up 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 continued to be permitted, for compatibility with existing source lawmaking.

C89 is supported by electric current C compilers, and about modern C code is based on information technology. Any program written but 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 sure platform or with a detail compiler, due, for instance, to the utilize of non-standard libraries, such as GUI libraries, or to a reliance on compiler- or platform-specific attributes such as the exact size of data types and byte endianness.

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

After the ANSI/ISO standardization process, the C linguistic communication specification remained relatively static for several years. In 1995, Normative Subpoena 1 to the 1990 C standard (ISO/IEC 9899/AMD1:1995, known informally as C95) was published, to correct some details and to add more than extensive support for international character sets.^[18]

C99 [edit]

The C standard was further revised in the belatedly 1990s, leading to the publication of ISO/IEC 9899:1999 in 1999, which is unremarkably referred to every bit "C99". It has since been amended three 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 array members, improved support for IEEE 754 floating point, support for variadic macros (macros of variable arity), and support for one-line comments commencement with //, as in BCPL or C++. Many of these had already been implemented as extensions in several C compilers.

C99 is for the about part backward compatible with C90, just is stricter in some ways; in particular, a announcement that lacks a type specifier no longer has int implicitly assumed. A standard macro __STDC_VERSION__ is defined with value 199901L to point that C99 back up 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++, nonetheless, implements the C89 standard and those parts of C99 that are required for compatibility with C++11.^[xx] ^{[ needs update ]}

In improver, back up for Unicode identifiers (variable / function names) in the form of escaped characters (e.g. \U0001f431) is now required. Support for raw Unicode names is optional.

C11 [edit]

In 2007, work began on another revision of the C standard, informally chosen "C1X" until its official publication on 2011-12-08. The C standards committee adopted guidelines to limit the adoption of new features that had not been tested by existing implementations.

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

C17 [edit]

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

C2x [edit]

C2x is an breezy name for the next (later C17) major C language standard revision. It is expected to be voted on in 2023 and would therefore be chosen C23.^[21] ^{[ amend 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 arithmetic, multiple distinct memory banks, and bones I/O operations.

In 2008, the C Standards Committee published a technical report extending the C linguistic communication^[22] to address these issues by providing a common standard for all implementations to adhere to. Information technology includes a number of features not available in normal C, such every bit fixed-betoken arithmetic, named address spaces, and basic I/O hardware addressing.

Syntax [edit]

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

C source files contain declarations and function definitions. Office definitions, in turn, incorporate declarations and statements. Declarations either ascertain new types using keywords such as struct, matrimony, and enum, or assign types to and perhaps reserve storage for new variables, usually by writing the type followed by the variable name. Keywords such every bit char and int specify congenital-in types. Sections of code are enclosed in braces ({ and }, sometimes called "curly brackets") to limit the telescopic of declarations and to act every bit a single argument for control structures.

As an imperative linguistic communication, C uses statements to specify actions. The most 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 called and variables may be assigned new values. To change the normal sequential execution of statements, C provides several control-flow statements identified by reserved keywords. Structured programming is supported by if … [else] provisional execution and by practise … while, while, and for iterative execution (looping). The for statement has dissever initialization, testing, and reinitialization expressions, whatever or all of which can exist omitted. break and go along can be used to leave the innermost enclosing loop statement or skip to its reinitialization. In that location is also a not-structured goto argument which branches directly to the designated label within the function. switch selects a case to be executed based on the value of an integer expression.

Expressions tin use a variety of built-in operators and may contain function calls. The order in which arguments to functions and operands to nearly operators are evaluated is unspecified. The evaluations may fifty-fifty be interleaved. However, all side furnishings (including storage to variables) will occur before the next "sequence betoken"; sequence points include the end of each expression statement, and the entry to and return from each function call. Sequence points likewise occur during evaluation of expressions containing certain operators (&&, ||, ?: and the comma operator). This permits a high caste of object code optimization past the compiler, merely 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 amend."^[25] The C standard did not attempt to correct many of these blemishes, because of the impact of such changes on already existing software.

Character set [edit]

The basic C source character set includes the post-obit characters:

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

Newline indicates the end of a text line; it need not stand for to an actual single graphic symbol, although for convenience C treats it as 1.

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

The basic C execution character set contains the aforementioned characters, along with representations for alert, backspace, and carriage 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, besides known equally keywords, which are the words that cannot be used for any purposes other than those for which they are predefined:

automobile
break
example
char
const
continue
default
practice
double
else
enum
extern
bladder
for
goto
if
int
long
register
return
brusk
signed
sizeof
static
struct
switch
typedef
spousal relationship
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

Almost of the recently reserved words brainstorm with an underscore followed past a uppercase, because identifiers of that form were previously reserved by the C standard for use but by implementations. Since existing program source code should not have been using these identifiers, information technology would non be afflicted when C implementations started supporting these extensions to the programming language. Some standard headers do define more than convenient synonyms for underscored identifiers. The language previously included a reserved give-and-take called entry, but this was seldom implemented, and has now been removed as a reserved word.^[27]

Operators [edit]

C supports a rich fix 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: !, &&, ||
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 betoken assignment, following the precedent of Fortran and PL/I, only different ALGOL and its derivatives. C uses the operator == to test for equality. The similarity between these two operators (assignment and equality) may issue in the accidental use of one in place of the other, and in many cases, the error does not produce an error bulletin (although some compilers produce warnings). For example, the conditional expression if (a == b + 1) might mistakenly exist written as if (a = b + one), which will be evaluated every bit true if a is not zero after the assignment.^[28]

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

"Hello, globe" example [edit]

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

The original version was:^[30]

                        chief            ()                        {                                                printf            (            "hullo, world            \n            "            );                        }

A standard-conforming "hello, world" plan is:^[a]

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

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

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

The opening curly brace indicates the beginning of the definition of the principal 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 accost of the showtime character in the string literal "howdy, world\n". The string literal is an unnamed array with elements of type char, set upwards automatically past the compiler with a final 0-valued character to mark the end of the assortment (printf needs to know this). The \north is an escape sequence that C translates to a newline character, which on output signifies the finish of the current line. The return value of the printf function is of type int, but information technology is silently discarded since it is non used. (A more careful plan might test the render value to decide whether or not the printf function succeeded.) The semicolon ; terminates the statement.

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

Information types [edit]

The type arrangement in C is static and weakly typed, which makes it similar to the type 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 unmarried-byte characters. C99 added a boolean datatype. In that location are too derived types including arrays, pointers, records (struct), and unions (matrimony).

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

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

C'south usual arithmetic conversions allow for efficient code to be 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 [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 tin be dereferenced to admission data stored at the address pointed to, or to invoke a pointed-to role. Pointers can be manipulated using consignment or pointer arithmetic. The run-time representation of a pointer value is typically a raw memory accost (perhaps augmented by an offset-within-word field), but since a pointer's blazon includes the type of the affair pointed to, expressions including pointers can exist type-checked at compile time. Arrow 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 retentiveness allocation is performed using pointers. Many data types, such as trees, are ordinarily implemented as dynamically allocated struct objects linked together using pointers. Pointers to functions are useful for passing functions as arguments to higher-guild functions (such as qsort or bsearch) or as callbacks to exist invoked by event handlers.^[31]

A nix pointer value explicitly points to no valid location. Dereferencing a zilch pointer value is undefined, frequently resulting in a segmentation mistake. Nothing arrow values are useful for indicating special cases such as no "next" pointer in the final node of a linked list, or equally an mistake indication from functions returning pointers. In appropriate contexts in source code, such as for assigning to a pointer variable, a null pointer constant tin can exist written as 0, with or without explicit casting to a arrow blazon, or equally the Zippo macro defined by several standard headers. In conditional contexts, cypher pointer values evaluate to false, while all other arrow values evaluate to true.

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

Devil-may-care use of pointers is potentially unsafe. Considering they are typically unchecked, a pointer variable can be fabricated to point to whatsoever capricious location, which tin cause undesirable furnishings. Although properly used pointers point to safe places, they can be made to betoken to unsafe places past using invalid pointer arithmetic; the objects they bespeak to may continue to be used subsequently deallocation (dangling pointers); they may be used without having been initialized (wild pointers); or they may be directly assigned an unsafe value using a cast, matrimony, or through some other corrupt pointer. In general, C is permissive in allowing manipulation of and conversion between pointer types, although compilers typically provide options for various levels of checking. Some other programming languages address these problems past using more restrictive reference types.

Arrays [edit]

Assortment types in C are traditionally of a fixed, static size specified at compile time. The more recent C99 standard also allows a course of variable-length arrays. All the same, it is as well possible to allocate a cake of memory (of arbitrary size) at run-time, using the standard library's malloc function, and treat it every bit 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 not have a special provision for declaring multi-dimensional arrays, just rather relies on recursion within the blazon organization to declare arrays of arrays, which effectively accomplishes the same thing. The index values of the resulting "multi-dimensional array" can be thought of as increasing in row-major order. Multi-dimensional arrays are commonly used in numerical algorithms (mainly from applied linear algebra) to shop matrices. The structure of the C array is well suited to this particular task. Even so, in early versions of C the bounds of the array must be known stock-still values or else explicitly passed to any subroutine that requires them, and dynamically sized arrays of arrays cannot be accessed using double indexing. (A workaround for this was to allocate the array with an additional "row vector" of pointers to the columns.) C99 introduced "variable-length arrays" which address this issue.

The following example using mod C (C99 or later) shows allotment of a two-dimensional assortment on the heap and the use of multi-dimensional array indexing for accesses (which can use bounds-checking on many C compilers):

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

Array–pointer interchangeability [edit]

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

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

The total size of an array x can be adamant by applying sizeof to an expression of array type. The size of an element tin can be determined by applying the operator sizeof to whatsoever dereferenced element of an array A, as in northward = sizeof A[0]. This, the number of elements in a declared array A tin can be determined equally sizeof A / sizeof A[0]. Annotation, that if only a pointer to the showtime chemical element is available as it is often the case in C code considering of the automatic conversion described above, the data well-nigh the full blazon of the array and its length are lost.

Memory management [edit]

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

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

These three approaches are appropriate in different situations and have diverse trade-offs. For example, static memory allocation has little allocation overhead, automatic allocation may involve slightly more overhead, and dynamic memory allocation tin potentially have a swell deal of overhead for both allocation and deallocation. The persistent nature of static objects is useful for maintaining land information across role calls, automatic allotment is piece of cake to use but stack space is typically much more express and transient than either static retentiveness or heap space, and dynamic memory allocation allows user-friendly resource allotment of objects whose size is known only at run-time. Almost C programs make all-encompassing use of all iii.

Where possible, automated or static allocation is usually simplest because 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 tin can change in size at runtime, and since static allocations (and automatic allocations before C99) must have a fixed size at compile-fourth dimension, at that place are many situations in which dynamic resource allotment is necessary.^[31] Prior to the C99 standard, variable-sized arrays were a mutual instance of this. (See the article on malloc for an case of dynamically allocated arrays.) Unlike automatic allocation, which tin can fail at run fourth dimension with uncontrolled consequences, the dynamic allotment functions return an indication (in the form of a null pointer value) when the required storage cannot be allocated. (Static allotment that is too large is commonly detected by the linker or loader, before the program tin can even brainstorm execution.)

Unless otherwise specified, static objects contain naught or null arrow values upon program startup. Automatically and dynamically allocated objects are initialized simply if an initial value is explicitly specified; otherwise they initially have indeterminate values (typically, whatever bit pattern happens to be present in the storage, which might not even correspond a valid value for that type). If the plan attempts to access an uninitialized value, the results are undefined. Many modern compilers try to find and warn about this problem, but both fake positives and simulated negatives tin can occur.

Heap memory allocation has to be synchronized with its bodily usage in any plan to be reused as much every bit possible. For case, if the only pointer to a heap memory allocation goes out of scope or has its value overwritten before it is deallocated explicitly, then that retentiveness cannot exist recovered for later reuse and is essentially lost to the programme, a miracle known as a retentiveness leak. Conversely, it is possible for memory to be freed, merely is referenced subsequently, leading to unpredictable results. Typically, the failure symptoms appear in a portion of the program unrelated to the code that causes the error, making it hard to diagnose the failure. Such issues are ameliorated in languages with automated garbage drove.

Libraries [edit]

The C programming linguistic communication uses libraries as its primary method of extension. In C, a library is a set of functions contained within a unmarried "annal" 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 information types and macro symbols used with these functions. In order for a program to utilize a library, it must include the library's header file, and the library must exist linked with the programme, which in many cases requires compiler flags (eastward.g., -lm, shorthand for "link the math library").^[31]

The most mutual C library is the C standard library, which is specified by the ISO and ANSI C standards and comes with every C implementation (implementations which target express environments such equally embedded systems may provide only a subset of the standard library). This library supports stream input and output, retention allocation, mathematics, character strings, and fourth dimension values. Several separate standard headers (for instance, stdio.h) specify the interfaces for these and other standard library facilities.

Another mutual prepare of C library functions are those used by applications specifically targeted for Unix and Unix-like systems, particularly functions which provide an interface to the kernel. These functions are detailed in diverse standards such 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 and so that the routines can exist used from higher-level languages like Coffee, Perl, and Python.^[31]

File handling and streams [edit]

File input and output (I/O) is not role of the C linguistic communication itself but instead is handled by libraries (such as the C standard library) and their associated header files (e.g. stdio.h). File handling is generally implemented through high-level I/O which works through streams. A stream is from this perspective a data menstruum that is independent 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 store data before it'due south sent to the final destination. This reduces the time spent waiting for slower devices, for instance a hard drive or solid state drive. Low-level I/O functions are not part of the standard C library^{[ clarification needed ]} but are by and large part of "bare metal" programming (programming that'southward independent of any operating system such as well-nigh embedded programming). With few exceptions, implementations include low-level I/O.

Language tools [edit]

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

Automatic source code checking and auditing are beneficial in whatever language, and for C many such tools be, such as Lint. A common practice is to apply Lint to detect questionable code when a programme is get-go written. In one case a program passes Lint, it is so compiled using the C compiler. Besides, many compilers can optionally warn about syntactically valid constructs that are likely to actually exist errors. MISRA C is a proprietary set of guidelines to avert such questionable code, developed for embedded systems.^[37]

There are likewise compilers, libraries, and operating system level mechanisms for performing actions that are not a standard part of C, such as bounds checking for arrays, detection of buffer overflow, serialization, dynamic retention tracking, and automatic garbage collection.

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

Uses [edit]

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 near purposes, yet when needed, system-specific code can be used to admission specific hardware addresses and to perform type punning to match externally imposed interface requirements, with a low run-time demand on organization resource.

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

A consequence of C's broad availability and efficiency is that compilers, libraries and interpreters of other programming languages are often implemented in C. For instance, the reference implementations of Python, Perl, Ruby, and PHP are written in C.

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

C is sometimes used as an intermediate language past implementations of other languages. This approach may be used for portability or convenience; by using C as an intermediate language, additional machine-specific code generators are not necessary. C has some features, such as line-number preprocessor directives and optional superfluous commas at the stop of initializer lists, that support compilation of generated code. All the same, some of C's shortcomings take prompted the evolution of other C-based languages specifically designed for use as intermediate languages, such as C--.

C has besides been widely used to implement stop-user applications. However, such applications tin as well be written in newer, college-level languages.

[edit]

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

C has both directly and indirectly influenced many later languages such equally C#, D, Go, Java, JavaScript, Limbo, LPC, Perl, PHP, Python, and Unix's C shell.^[42] The most pervasive influence has been syntactical; all of the languages mentioned combine the statement and (more or less recognizably) expression syntax of C with blazon systems, data 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 tin 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 equally source-to-source compilers; source lawmaking was translated into C, 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-like syntax.^[44] C++ adds greater typing forcefulness, scoping, and other tools useful in object-oriented programming, and permits generic programming via templates. Nearly a superset of C, C++ at present supports almost of C, with a few exceptions.

Objective-C was originally a very "sparse" layer on meridian of C, and remains a strict superset of C that permits object-oriented programming using a hybrid dynamic/static typing image. Objective-C derives its syntax from both C and Smalltalk: syntax that involves preprocessing, expressions, function declarations, and part 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.

Notes [edit]

^ The original example code volition compile on most modern compilers that are not in strict standard compliance mode, but it does not fully suit to the requirements of either C89 or C99. In fact, C99 requires that a diagnostic bulletin exist produced.
^ The main function actually has two arguments, int argc and char *argv[], respectively, which can be used to handle command line arguments. The ISO C standard (section v.1.2.two.1) requires both forms of main to be supported, which is special handling not afforded to whatsoever other function.

References [edit]

^ ^a ^b Kernighan, Brian Due west.; Ritchie, Dennis M. (February 1978). The C Programming Language (1st ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-xiii-110163-0.
^ Ritchie (1993): "Thompson had fabricated a cursory attempt to produce a organization coded in an early version of C—earlier structures—in 1972, only gave up the endeavour."
^ Fruderica (December xiii, 2020). "History of C". The cppreference.com. Archived from the original on October 24, 2020. Retrieved October 24, 2020.
^ Ritchie (1993): "The scheme of type composition adopted by C owes considerable debt to Algol 68, although it did not, possibly, emerge in a form that Algol'due south adherents would approve of."
^ Band Team (October 23, 2021). "The Ring programming linguistic communication and other languages". ring-lang.cyberspace.
^ ^a ^b "Verilog HDL (and C)" (PDF). The Research School of Informatics at the Australian National University. June 3, 2010. Archived from the original (PDF) on Nov 6, 2013. Retrieved Baronial 19, 2013. 1980s: ; Verilog offset introduced ; Verilog inspired by the C programming language
^ ^a ^b ^c ^d ^{due east} Ritchie (1993)
^ "Programming Language Popularity". 2009. Archived from the original on January 16, 2009. Retrieved January 16, 2009.
^ "TIOBE Programming Community Index". 2009. Archived from the original on May four, 2009. Retrieved May six, 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 October seven, 2021.
^ Ritchie, Dennis. "BCPL to B to C". Archived from the original on December 12, 2019. Retrieved September 10, 2019.
^ ^a ^b Johnson, S. C.; Ritchie, D. M. (1978). "Portability of C Programs and the UNIX System". Bong System Tech. J. 57 (six): 2021–2048. CiteSeerX10.1.1.138.35. doi:10.1002/j.1538-7305.1978.tb02141.ten. S2CID 17510065. (Note: The PDF is an OCR scan of the original, and contains a rendering of "IBM 370" as "IBM 310".)
^ McIlroy, K. D. (1987). A Research Unix reader: annotated excerpts from the Programmer'southward Manual, 1971–1986 (PDF) (Technical written report). CSTR. Bell Labs. p. 10. 139. Archived (PDF) from the original on November eleven, 2017. Retrieved February 1, 2015.
^ "C manual pages". FreeBSD Miscellaneous Data Manual (FreeBSD 13.0 ed.). May 30, 2011. Archived from the original on January 21, 2021. Retrieved January fifteen, 2021. [1] Archived Jan 21, 2021, at the Wayback Car
^ Kernighan, Brian W.; Ritchie, Dennis M. (March 1988). The C Programming Language (2nd ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-110362-7.
^ Stroustrup, Bjarne (2002). Sibling rivalry: C and C++ (PDF) (Report). AT&T Labs. Archived (PDF) from the original on Baronial 24, 2014. Retrieved Apr 14, 2014.
^ C Integrity. International Organization for Standardization. March 30, 1995. Archived from the original on July 25, 2018. Retrieved July 24, 2018.
^ "JTC1/SC22/WG14 – C". Home 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 ii, 2013. Retrieved September vii, 2013.
^ "Revised C23 Schedule WG 14 North 2759" (PDF). world wide web.open-std.org. Archived (PDF) from the original on June 24, 2021. Retrieved Oct ten, 2021.
^ "TR 18037: Embedded C" (PDF). ISO / IEC. Archived (PDF) from the original on Feb 25, 2021. Retrieved July 26, 2011.
^ Harbison, Samuel P.; Steele, Guy 50. (2002). C: A Reference Manual (5th ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-089592-9. Contains a BNF grammar for C.
^ Kernighan & Ritchie (1996), p. 192.
^ Kernighan & Ritchie (1978), p. 3.
^ "ISO/IEC 9899:201x (ISO C11) Committee Draft" (PDF). Archived (PDF) from the original on December 22, 2017. Retrieved September 16, 2011.
^ Kernighan & Ritchie (1996), pp. 192, 259.
^ "x Common Programming Mistakes in C++". Cs.ucr.edu. Archived from the original on October 21, 2008. Retrieved June 26, 2009.
^ Schultz, Thomas (2004). C and the 8051 (3rd ed.). Otsego, MI: PageFree Publishing Inc. p. 20. ISBN978-i-58961-237-two. Archived from the original on July 29, 2020. Retrieved February 10, 2012.
^ Kernighan & Ritchie (1978), p. 6.
^ ^a ^b ^c ^d ^eastward ^f ^m Klemens, Ben (2013). 21st Century C. O'Reilly Media. ISBN978-i-4493-2714-9.
^ Feuer, Alan R.; Gehani, Narain H. (March 1982). "Comparing of the Programming Languages C and Pascal". ACM Computing Surveys. 14 (i): 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 vii, 2007. Retrieved August five, 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'due south Lexicon (tertiary ed.). MIT Press. p. 432. ISBN978-0-262-68092-9. Archived from the original on November 12, 2012. Retrieved August five, 2012.
^ "Man Page for lint (freebsd Section ane)". unix.com. May 24, 2001. Retrieved July fifteen, 2014.
^ Dale, Nell B.; Weems, Scrap (2014). Programming and problem solving with C++ (6th ed.). Burlington, MA: Jones & Bartlett Learning. ISBN978-1449694289. OCLC 894992484.
^ Dr. Dobb's Sourcebook. U.S.A.: Miller Freeman, Inc. Nov–December 1995.
^ "Using C for CGI Programming". linuxjournal.com. March one, 2005. Archived from the original on February thirteen, 2010. Retrieved January 4, 2010.
^ McMillan, Robert (Baronial 1, 2013). "Is Java Losing Its Mojo?". Wired. Archived from the original on February xv, 2017. Retrieved March 5, 2017.
^ O'Regan, Gerard (September 24, 2015). Pillars of computing : a compendium of select, pivotal engineering firms. ISBN978-3319214641. OCLC 922324121.
^ Rauchwerger, Lawrence (2004). Languages and compilers for parallel calculating : 16th international workshop, LCPC 2003, College Station, TX, The states, October 2-four, 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 Thousand. (March 1993). "The Development of the C Language". ACM SIGPLAN Notices. ACM. 28 (3): 201–208. doi:10.1145/155360.155580.
Ritchie, Dennis 1000. (1993). "The Development 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 November four, 2014.
Kernighan, Brian W.; Ritchie, Dennis M. (1996). The C Programming Linguistic communication (2nd ed.). Prentice Hall. ISBN7-302-02412-X.

External links [edit]

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

chanalligns1937.blogspot.com

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