C# Read From Xml Don't Know Data Type

General-purpose programming language

C
Major implementations
The C Programming Linguistic communication ^[i] (oft referred to as One thousand&R), the seminal book on C
Paradigm	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)
Get-go appeared	1972; fifty years ago (1972) ^[ii]

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

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

C (, as in the letter of the alphabetc) is a general-purpose, procedural computer programming language supporting structured programming, lexical variable telescopic, and recursion, with a static blazon arrangement. By pattern, C provides constructs that map efficiently to typical machine instructions. It has plant lasting utilize in applications previously coded in associates language. Such applications include operating systems and various awarding software for reckoner architectures that range from supercomputers to PLCs and embedded systems.

A successor to the programming language B, C was originally developed at Bong Labs by Dennis Ritchie between 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 i of the near widely used programming languages,^[8] ^[9] with C compilers from various vendors bachelor for the majority of existing computer architectures and operating systems. C has been standardized by ANSI since 1989 (ANSI C) and by the International Organization for Standardization (ISO).

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

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

Overview

Similar nigh procedural languages in the ALGOL tradition, C has facilities for structured programming and allows lexical variable telescopic and recursion. Its static type system prevents unintended operations. In C, all executable lawmaking is independent inside subroutines (as well called "functions", though not strictly in the sense of functional programming). Office parameters are always passed by value (except arrays). Pass-past-reference is simulated in C by explicitly passing pointer values. C program source text is free-format, using the semicolon every bit a statement terminator and curly braces for grouping blocks of statements.

The C language as well exhibits the post-obit characteristics:

The language has a small, fixed number of keywords, including a total fix of control menses primitives: if/else, for, do/while, while, and switch. User-defined names are not distinguished from keywords by whatever kind of sigil.
It has a large number of arithmetic, bitwise, and logic operators: +,+=,++,&,||, etc.
More than one assignment may be performed in a unmarried statement.
Functions:
- Function return values can exist ignored, when not needed.
- Function and data pointers let ad hoc run-time polymorphism.
- Functions may non be defined within the lexical scope of other functions.
Data typing is static, but weakly enforced; all data has a type, but implicit conversions are possible.
Declaration syntax mimics usage context. C has no "define" keyword; instead, a statement beginning with the proper name of a type is taken as a declaration. There is no "function" keyword; instead, a function is indicated by the presence of a parenthesized argument list.
User-defined (typedef) and compound types are possible.
- Heterogeneous amass information types (struct) permit related data elements to be accessed and assigned as a unit.
- Union is a structure with overlapping members; merely the terminal member stored is valid.
- Array indexing is a secondary notation, defined in terms of pointer arithmetics. Dissimilar structs, arrays are not first-grade objects: they cannot be assigned or compared using unmarried built-in operators. In that location is no "assortment" keyword in use or definition; instead, square brackets bespeak arrays syntactically, for instance month[11].
- Enumerated types are possible with the enum keyword. They are freely interconvertible with integers.
- Strings are not a distinct data type, simply are conventionally implemented as null-terminated character arrays.
Depression-level access to computer retention is possible past converting car addresses to typed pointers.
Procedures (subroutines not returning values) are a special case of function, with an untyped return type void.
A preprocessor performs macro definition, source code file inclusion, and conditional compilation.
There is a basic grade of modularity: files tin be compiled separately and linked together, with command over which functions and information objects are visible to other files via static and extern attributes.
Complex functionality such as I/O, string manipulation, and mathematical functions are consistently delegated to library routines.

While C does non include certain features found in other languages (such every bit object orientation and garbage collection), these can be implemented or emulated, oftentimes through the use of external libraries (e.g., the GLib Object System or the Boehm garbage collector).

Relations to other languages

Many later languages have borrowed directly or indirectly from C, including C++, C#, Unix's C shell, D, Go, Java, JavaScript (including transpilers), Julia, Limbo, LPC, Objective-C, Perl, PHP, Python, Cerise, Rust, Swift, Verilog and SystemVerilog (hardware description languages).^[vi] These languages have drawn many of their control structures and other basic features from C. Most of them (Python existence 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 type systems, data models, and semantics that tin be radically different.

History

Early developments

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

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

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

In 1972, Ritchie started to improve B, most notably adding data typing for variables, which resulted in creating a new linguistic communication C.^[thirteen] The C compiler and some utilities made with it were included in Version 2 Unix.^[14]

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

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

Unix was one of the first operating system kernels implemented in a language other than associates. Before instances include the Multics arrangement (which was written in PL/I) and Master Control Plan (MCP) for the Burroughs B5000 (which was written in ALGOL) in 1961. In around 1977, Ritchie and Stephen C. Johnson made further changes to the linguistic communication to facilitate portability of the Unix operating system. Johnson'southward Portable C Compiler served as the basis for several implementations of C on new platforms.^[13]

G&R C

In 1978, Brian Kernighan and Dennis Ritchie published the first edition of The C Programming Language.^[1] This book, known to C programmers every bit G&R, served for many years every bit an informal specification of the language. The version of C that it describes is ordinarily referred to as "K&R C". Every bit this was released in 1978, it is as well referred to every bit C78.^[15] The second edition of the book^[16] covers the later on ANSI C standard, described beneath.

Yard&R introduced several language features:

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

Even after the publication of the 1989 ANSI standard, for many years G&R C was still considered the "lowest common denominator" to which C programmers restricted themselves when maximum portability was desired, since many older compilers were however in utilize, and because carefully written Grand&R C lawmaking can be legal Standard C also.

In early versions of C, merely functions that return types other than int must exist 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            ();                                                render                                    test2            ;                        }

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

Since Thousand&R function declarations did not include any data almost function arguments, function parameter type checks were not performed, although some compilers would effect a warning bulletin 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. Split up tools such as Unix'due south lint utility were adult that (among other things) could bank check for consistency of function use across multiple source files.

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

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

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

ANSI C and ISO C

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

In 1983, the American National Standards Institute (ANSI) formed a committee, X3J11, to establish a standard specification of C. X3J11 based the C standard on the Unix implementation; still, 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 as ANSI X3.159-1989 "Programming Linguistic communication C". This version of the language is oft referred to every bit ANSI C, Standard C, or sometimes C89.

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

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

One of the aims of the C standardization process was to produce a superset of G&R C, incorporating many of the later introduced unofficial features. The standards committee also included several additional features such every bit office 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 way used in C++, the K&R interface continued to exist permitted, for compatibility with existing source code.

C89 is supported past electric current C compilers, and nearly modern C code is based on it. Whatever program written just in Standard C and without whatsoever hardware-dependent assumptions volition run correctly on any platform with a conforming C implementation, inside its resource limits. Without such precautions, programs may compile only on a certain platform or with a detail compiler, due, for instance, to the apply of not-standard libraries, such as GUI libraries, or to a reliance on compiler- or platform-specific attributes such every bit the exact size of data types and byte endianness.

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

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

C99

The C standard was further revised in the late 1990s, leading to the publication of ISO/IEC 9899:1999 in 1999, which is usually referred to every bit "C99". It has since been amended 3 times past Technical Corrigenda.^[19]

C99 introduced several new features, including inline functions, several new data types (including long long int and a complex blazon to correspond 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 ane-line comments starting time with //, equally in BCPL or C++. Many of these had already been implemented as extensions in several C compilers.

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

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

C11

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 by existing implementations.

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

C17

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

C2x

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

Embedded C

Historically, embedded C programming requires nonstandard extensions to the C language in society to support exotic features such as fixed-bespeak arithmetic, multiple singled-out memory banks, and bones I/O operations.

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

Syntax

C has a formal grammar specified by the C standard.^[23] Line endings are generally not significant in C; even so, line boundaries do have significance during the preprocessing phase. Comments may appear either betwixt the delimiters /* and */, or (since C99) post-obit // until the terminate of the line. Comments delimited past /* and */ exercise not nest, and these sequences of characters are non interpreted as comment delimiters if they appear inside string or character literals.^[24]

C source files comprise declarations and function definitions. Part definitions, in plough, comprise declarations and statements. Declarations either ascertain new types using keywords such as struct, union, and enum, or assign types to and perchance reserve storage for new variables, normally by writing the blazon followed past the variable proper noun. Keywords such as char and int specify built-in types. Sections of code are enclosed in braces ({ and }, sometimes chosen "curly brackets") to limit the scope of declarations and to human activity as a single statement for control structures.

As an imperative language, C uses statements to specify actions. The virtually common statement is an expression argument, consisting of an expression to be evaluated, followed by a semicolon; as a side issue of the evaluation, functions may be called and variables may be assigned new values. To modify the normal sequential execution of statements, C provides several control-catamenia statements identified past reserved keywords. Structured programming is supported past if … [else] conditional execution and by exercise … while, while, and for iterative execution (looping). The for argument has split up initialization, testing, and reinitialization expressions, any or all of which can exist omitted. break and proceed tin can be used to leave the innermost enclosing loop statement or skip to its reinitialization. There is besides a not-structured goto statement which branches directly to the designated label inside the part. switch selects a case to be executed based on the value of an integer expression.

Expressions can utilise a variety of built-in operators and may comprise function calls. The order in which arguments to functions and operands to nearly operators are evaluated is unspecified. The evaluations may even be interleaved. Withal, all side effects (including storage to variables) will occur before the next "sequence point"; sequence points include the end of each expression argument, and the entry to and return from each part telephone call. Sequence points as well occur during evaluation of expressions containing certain operators (&&, ||, ?: and the comma operator). This permits a high degree of object code optimization by the compiler, but requires C programmers to take more than care to obtain reliable results than is needed for other programming languages.

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

Character set

The bones C source grapheme set includes the post-obit 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; information technology need non correspond to an actual single character, although for convenience C treats it equally ane.

Additional 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 be embedded portably within C source text by using \uXXXX or \UXXXXXXXX encoding (where the Ten denotes a hexadecimal graphic symbol), although this feature is not yet widely implemented.

The basic C execution character fix contains the same characters, along with representations for alert, backspace, and carriage return. Run-time support for extended character sets has increased with each revision of the C standard.

Reserved words

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

motorcar
suspension
case
char
const
continue
default
do
double
else
enum
extern
float
for
goto
if
int
long
register
return
short
signed
sizeof
static
struct
switch
typedef
union
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

Most of the recently reserved words begin with an underscore followed past a capital letter, because identifiers of that grade were previously reserved by the C standard for use only by implementations. Since existing plan source code should not accept been using these identifiers, it would not be afflicted when C implementations started supporting these extensions to the programming language. Some standard headers do define more convenient synonyms for underscored identifiers. The language previously included a reserved word called entry, just this was seldom implemented, and has now been removed every bit a reserved word.^[27]

Operators

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

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

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

"Hello, world" instance

The "howdy, earth" example, which appeared in the first edition of K&R, has go the model for an introductory program in most programming textbooks. The program prints "hullo, world" to the standard output, which is usually a terminal or screen brandish.

The original version was:^[thirty]

                        principal            ()                        {                                                printf            (            "hello, world            \n            "            );                        }

A standard-conforming "how-do-you-do, earth" program is:^[a]

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

The first line of the program contains a preprocessing directive, indicated by #include. This causes the compiler to supervene upon that line with the unabridged 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 signal that stdio.h is located using a search strategy that prefers headers provided with the compiler to other headers having the same proper name, equally 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 function serves a special purpose in C programs; the run-time environment calls the main function to begin program execution. The type specifier int indicates that the value that is returned to the invoker (in this example the run-fourth dimension environment) as a result of evaluating the main role, is an integer. The keyword void as a parameter listing indicates that this function takes no arguments.^[b]

The opening curly caryatid indicates the first of the definition of the master function.

The adjacent line calls (diverts execution to) a office named printf, which in this instance is supplied from a system library. In this call, the printf role is passed (provided with) a single argument, the address of the kickoff character in the string literal "hi, world\northward". The string literal is an unnamed array with elements of blazon char, set up automatically past the compiler with a final 0-valued character to marker the end of the array (printf needs to know this). The \n is an escape sequence that C translates to a newline character, which on output signifies the end of the current line. The render value of the printf office is of type int, but information technology is silently discarded since it is not used. (A more than careful programme might test the render value to make up one's mind whether or non the printf function succeeded.) The semicolon ; terminates the statement.

The closing curly brace indicates the end of the code for the main part. According to the C99 specification and newer, the primary function, unlike any other office, 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 by the run-time system equally an get out code indicating successful execution.^[31]

Data types

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

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

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

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

Pointers

C supports the employ of pointers, a type of reference that records the address or location of an object or role in memory. Pointers can be dereferenced to admission data stored at the address pointed to, or to invoke a pointed-to office. Pointers tin can be manipulated using assignment or pointer arithmetics. The run-time representation of a pointer value is typically a raw memory accost (mayhap augmented by an first-within-word field), but since a pointer's blazon 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 usually manipulated using pointers into arrays of characters. Dynamic memory allocation is performed using pointers. Many data types, such as trees, are commonly implemented every bit dynamically allocated struct objects linked together using pointers. Pointers to functions are useful for passing functions as arguments to higher-order functions (such as qsort or bsearch) or every bit callbacks to exist invoked past event handlers.^[31]

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

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

Careless use of pointers is potentially unsafe. Considering they are typically unchecked, a pointer variable can exist made to point to any arbitrary location, which can crusade undesirable effects. Although properly used pointers bespeak to safe places, they can be made to point to unsafe places by using invalid pointer arithmetic; the objects they bespeak to may continue to be used after deallocation (dangling pointers); they may be used without having been initialized (wild pointers); or they may be directly assigned an dangerous value using a cast, union, or through another decadent 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 bug by using more restrictive reference types.

Arrays

Assortment types in C are traditionally of a fixed, static size specified at compile time. The more recent C99 standard as well allows a form of variable-length arrays. Nonetheless, it is also possible to allocate a block of memory (of arbitrary size) at run-fourth dimension, using the standard library's malloc part, and treat it as an array.

Since arrays are always accessed (in effect) via pointers, array accesses are typically non checked confronting the underlying array size, although some compilers may provide bounds checking as an option.^[34] ^[35] Array bounds violations are therefore possible and can lead to various repercussions, including illegal retentivity accesses, corruption of information, buffer overruns, and run-fourth dimension exceptions.

C does not have a special provision for declaring multi-dimensional arrays, only rather relies on recursion within the type organization to declare arrays of arrays, which effectively accomplishes the same thing. The index values of the resulting "multi-dimensional array" tin be thought of as increasing in row-major gild. Multi-dimensional arrays are commonly used in numerical algorithms (mainly from applied linear algebra) to store matrices. The construction of the C array is well suited to this detail task. Withal, in early versions of C the bounds of the assortment must exist known fixed values or else explicitly passed to any subroutine that requires them, and dynamically sized arrays of arrays cannot exist 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 outcome.

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

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

Array–pointer interchangeability

The subscript note 10[i] (where ten designates a arrow) is syntactic sugar for *(x+i).^[36] Taking advantage of the compiler'south knowledge of the pointer type, the address that x + i points to is non the base accost (pointed to past ten) incremented by i bytes, but rather is divers to exist the base address incremented by i multiplied by the size of an chemical element that x points to. Thus, x[i] designates the i+oneth element of the assortment.

Furthermore, in near expression contexts (a notable exception is as operand of sizeof), an expression of array type is automatically converted to a pointer to the assortment's first element. This implies that an array is never copied as a whole when named equally an argument to a function, merely rather only the accost of its start element is passed. Therefore, although part calls in C use laissez passer-past-value semantics, arrays are in effect passed past reference.

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

Retention management

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

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

These three approaches are appropriate in different situations and have various merchandise-offs. For example, static retentivity allocation has little allocation overhead, automated allocation may involve slightly more overhead, and dynamic retention allocation can potentially take a keen bargain of overhead for both allocation and deallocation. The persistent nature of static objects is useful for maintaining state data across role calls, automatic allotment is easy to use but stack infinite is typically much more than limited and transient than either static memory or heap space, and dynamic retentiveness allocation allows convenient allotment of objects whose size is known merely at run-time. Most C programs brand all-encompassing apply of all three.

Where possible, automatic or static allocation is usually simplest considering the storage is managed by the compiler, freeing the developer of the potentially mistake-decumbent chore of manually allocating and releasing storage. However, many data structures can change in size at runtime, and since static allocations (and automated allocations before C99) must take a stock-still size at compile-time, there are many situations in which dynamic allocation is necessary.^[31] Prior to the C99 standard, variable-sized arrays were a mutual instance of this. (See the article on malloc for an example of dynamically allocated arrays.) Unlike automatic allocation, which can fail at run time with uncontrolled consequences, the dynamic allotment functions render an indication (in the form of a null arrow value) when the required storage cannot be allocated. (Static allocation that is besides large is unremarkably detected by the linker or loader, before the program can fifty-fifty begin execution.)

Unless otherwise specified, static objects comprise zero or null pointer values upon program startup. Automatically and dynamically allocated objects are initialized simply if an initial value is explicitly specified; otherwise they initially take indeterminate values (typically, whatever bit blueprint happens to be nowadays in the storage, which might not even correspond a valid value for that type). If the programme attempts to admission an uninitialized value, the results are undefined. Many mod compilers attempt to detect and warn near this problem, simply both false positives and faux negatives tin can occur.

Heap retentivity allotment has to exist synchronized with its actual usage in any programme to be reused equally much as possible. For case, if the only arrow to a heap memory allotment goes out of scope or has its value overwritten before information technology is deallocated explicitly, then that retentiveness cannot be recovered for later reuse and is substantially lost to the plan, a phenomenon known every bit a retentivity leak. Conversely, it is possible for memory to be freed, merely is referenced afterward, leading to unpredictable results. Typically, the failure symptoms appear in a portion of the program unrelated to the code that causes the error, making information technology hard to diagnose the failure. Such issues are ameliorated in languages with automatic garbage collection.

Libraries

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 single "archive" file. Each library typically has a header file, which contains the prototypes of the functions independent inside the library that may be used by a program, and declarations of special data types and macro symbols used with these functions. In lodge for a program to use a library, it must include the library'south header file, and the library must be linked with the plan, which in many cases requires compiler flags (eastward.grand., -lm, autograph for "link the math library").^[31]

The most common C library is the C standard library, which is specified by the ISO and ANSI C standards and comes with every C implementation (implementations which target limited environments such as embedded systems may provide only a subset of the standard library). This library supports stream input and output, memory resource allotment, mathematics, character strings, and time values. Several separate standard headers (for example, stdio.h) specify the interfaces for these and other standard library facilities.

Another common 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 Unmarried 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 be used from higher-level languages like Coffee, Perl, and Python.^[31]

File handling and streams

File input and output (I/O) is not part of the C language itself just instead is handled by libraries (such as the C standard library) and their associated header files (e.yard. stdio.h). File handling is more often than not implemented through high-level I/O which works through streams. A stream is from this perspective a data flow that is 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 retentiveness expanse or queue) is temporarily used to store data before it'southward sent to the final destination. This reduces the time spent waiting for slower devices, for instance a difficult drive or solid state drive. Depression-level I/O functions are not part of the standard C library^{[ clarification needed ]} but are generally part of "blank metal" programming (programming that'due south independent of any operating system such equally most embedded programming). With few exceptions, implementations include low-level I/O.

Language tools

A number of tools have been developed to aid C programmers find and gear up 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 code checking and auditing are benign in whatsoever language, and for C many such tools exist, such as Lint. A mutual practice is to use Lint to detect questionable lawmaking when a plan is starting time written. Once a plan passes Lint, it is then compiled using the C compiler. Also, many compilers can optionally warn about syntactically valid constructs that are probable to actually be errors. MISRA C is a proprietary set of guidelines to avoid such questionable lawmaking, developed for embedded systems.^[37]

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

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

Uses

The C Programming Language

C is widely used for systems programming in implementing operating systems and embedded system applications,^[38] considering C lawmaking, when written for portability, tin be used for most purposes, nevertheless when needed, arrangement-specific code can be used to access specific hardware addresses and to perform type punning to lucifer externally imposed interface requirements, with a low run-time demand on organisation resource.

C tin can exist used for website programming using the Common Gateway Interface (CGI) as a "gateway" for information between the Web application, the server, and the browser.^[39] C is often chosen over interpreted languages because of its speed, stability, and near-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 sparse, and its overhead is low, an important criterion for computationally intensive programs. For example, the GNU Multiple Precision Arithmetic Library, the GNU Scientific Library, Mathematica, and MATLAB are completely or partially written in C.

C is sometimes used as an intermediate language by implementations of other languages. This approach may exist used for portability or convenience; by using C as an intermediate language, boosted motorcar-specific code generators are non necessary. C has some features, such equally line-number preprocessor directives and optional superfluous commas at the end of initializer lists, that back up compilation of generated code. However, some of C'south shortcomings take prompted the development of other C-based languages specifically designed for use as intermediate languages, such as C--.

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

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

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

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

When object-oriented programming languages became pop, C++ and Objective-C were two dissimilar extensions of C that provided object-oriented capabilities. Both languages were originally implemented as source-to-source compilers; source code was translated into C, and and then compiled with a C compiler.^[43]

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

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

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

Notes

^ The original example code will compile on most mod compilers that are not in strict standard compliance mode, but it does non fully conform to the requirements of either C89 or C99. In fact, C99 requires that a diagnostic message be produced.
^ The main function actually has ii arguments, int argc and char *argv[], respectively, which tin be used to handle command line arguments. The ISO C standard (section 5.1.2.2.1) requires both forms of main to be supported, which is special treatment not afforded to any other function.

References

^ ^a ^b Kernighan, Brian Due west.; Ritchie, Dennis M. (February 1978). The C Programming Language (1st ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-110163-0.
^ Ritchie (1993): "Thompson had fabricated a brief effort to produce a system coded in an early on version of C—before structures—in 1972, but gave up the effort."
^ Fruderica (Dec thirteen, 2020). "History of C". The cppreference.com. Archived from the original on October 24, 2020. Retrieved October 24, 2020.
^ Ritchie (1993): "The scheme of blazon composition adopted by C owes considerable debt to Algol 68, although it did non, perhaps, emerge in a class that Algol'southward adherents would corroborate of."
^ Band Team (Oct 23, 2021). "The Ring programming language and other languages". ring-lang.internet.
^ ^a ^b "Verilog HDL (and C)" (PDF). The Research Schoolhouse of Estimator Science at the Australian National University. June iii, 2010. Archived from the original (PDF) on November half-dozen, 2013. Retrieved Baronial 19, 2013. 1980s: ; Verilog showtime introduced ; Verilog inspired by the C programming language
^ ^a ^b ^c ^d ^e Ritchie (1993)
^ "Programming Linguistic communication Popularity". 2009. Archived from the original on January 16, 2009. Retrieved January sixteen, 2009.
^ "TIOBE Programming Customs Index". 2009. Archived from the original on May 4, 2009. Retrieved May 6, 2009.
^ ^a ^b "History of C". en.cppreference.com. Archived from the original on May 29, 2018. Retrieved May 28, 2018.
^ "TIOBE Index for October 2021". Retrieved Oct 7, 2021.
^ Ritchie, Dennis. "BCPL to B to C". Archived from the original on Dec 12, 2019. Retrieved September 10, 2019.
^ ^a ^b Johnson, S. C.; Ritchie, D. M. (1978). "Portability of C Programs and the UNIX Arrangement". Bong Arrangement Tech. J. 57 (6): 2021–2048. CiteSeerX10.one.one.138.35. doi:10.1002/j.1538-7305.1978.tb02141.10. S2CID 17510065. (Note: The PDF is an OCR browse of the original, and contains a rendering of "IBM 370" as "IBM 310".)
^ McIlroy, M. D. (1987). A Research Unix reader: annotated excerpts from the Developer's Manual, 1971–1986 (PDF) (Technical report). CSTR. Bell Labs. p. 10. 139. Archived (PDF) from the original on November eleven, 2017. Retrieved February 1, 2015.
^ "C transmission pages". FreeBSD Miscellaneous Data Manual (FreeBSD 13.0 ed.). May 30, 2011. Archived from the original on Jan 21, 2021. Retrieved January 15, 2021. [1] Archived January 21, 2021, at the Wayback Motorcar
^ Kernighan, Brian W.; Ritchie, Dennis G. (March 1988). The C Programming Language (2nd ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-thirteen-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 fourteen, 2014.
^ C Integrity. International System 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 2, 2013. Retrieved September vii, 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 ten, 2021.
^ "TR 18037: Embedded C" (PDF). ISO / IEC. Archived (PDF) from the original on February 25, 2021. Retrieved July 26, 2011.
^ Harbison, Samuel P.; Steele, Guy L. (2002). C: A Reference Transmission (5th ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-089592-ix. Contains a BNF grammar for C.
^ Kernighan & Ritchie (1996), p. 192.
^ Kernighan & Ritchie (1978), p. three.
^ "ISO/IEC 9899:201x (ISO C11) Committee 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. 20. ISBN978-1-58961-237-2. Archived from the original on July 29, 2020. Retrieved February 10, 2012.
^ Kernighan & Ritchie (1978), p. 6.
^ ^a ^b ^c ^d ^{due east} ^f ^grand Klemens, Ben (2013). 21st Century C. O'Reilly Media. ISBN978-one-4493-2714-9.
^ Feuer, Alan R.; Gehani, Narain H. (March 1982). "Comparison of the Programming Languages C and Pascal". ACM Computing Surveys. fourteen (1): 73–92. doi:ten.1145/356869.356872. S2CID 3136859.
^ Kernighan & Ritchie (1996), p. 122.
^ For example, gcc provides _FORTIFY_SOURCE. "Security Features: Compile Time Buffer Checks (FORTIFY_SOURCE)". fedoraproject.org. Archived from the original on January 7, 2007. Retrieved August 5, 2012.
^ เอี่ยมสิริวงศ์, โอภาศ (2016). Programming with C. Bangkok, Thailand: SE-EDUCATION PUBLIC Visitor Express. pp. 225–230. ISBN978-616-08-2740-four.
^ Raymond, Eric Due south. (October 11, 1996). The New Hacker's Lexicon (3rd ed.). MIT Printing. p. 432. ISBN978-0-262-68092-9. Archived from the original on November 12, 2012. Retrieved August 5, 2012.
^ "Homo Page for lint (freebsd Department 1)". unix.com. May 24, 2001. Retrieved July 15, 2014.
^ Dale, Nell B.; Weems, Bit (2014). Programming and problem solving with C++ (6th ed.). Burlington, MA: Jones & Bartlett Learning. ISBN978-1449694289. OCLC 894992484.
^ Dr. Dobb's Sourcebook. U.South.A.: Miller Freeman, Inc. November–December 1995.
^ "Using C for CGI Programming". linuxjournal.com. March 1, 2005. Archived from the original on Feb xiii, 2010. Retrieved January four, 2010.
^ McMillan, Robert (August 1, 2013). "Is Java Losing Its Mojo?". Wired. Archived from the original on February 15, 2017. Retrieved March five, 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 computing : 16th international workshop, LCPC 2003, Higher Station, TX, Usa, Oct 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 ix, 2011.

Sources

Ritchie, Dennis M. (March 1993). "The Evolution of the C Language". ACM SIGPLAN Notices. ACM. 28 (three): 201–208. doi:10.1145/155360.155580.
Ritchie, Dennis M. (1993). "The Development of the C Language". The Second ACM SIGPLAN Briefing on History of Programming Languages (HOPL-Two). ACM. pp. 201–208. doi:10.1145/154766.155580. ISBN0-89791-570-iv . Retrieved November 4, 2014.
Kernighan, Brian Due west.; Ritchie, Dennis M. (1996). The C Programming Language (second ed.). Prentice Hall. ISBN7-302-02412-X.

External links

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

This page was final edited on 1 March 2022, at 08:47

C# Read From Xml Don't Know Data Type

Source: https://wiki2.org/en/C_(programming_language)

Doyne Allace