C# Read Large Object Heap Size at Runtime

General-purpose programming linguistic communication

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

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

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

C (, equally in the letter c) is a general-purpose, procedural computer programming language supporting structured programming, lexical variable scope, and recursion, with a static type organisation. By design, C provides constructs that map efficiently to typical machine instructions. It has constitute lasting utilise in applications previously coded in assembly language. Such applications include operating systems and diverse application software for calculator architectures that range from supercomputers to PLCs and embedded systems.

A successor to the programming language B, C was originally developed at Bell Labs by Dennis Ritchie between 1972 and 1973 to construct utilities running on Unix. It was applied to re-implementing the kernel of the Unix operating system.^[vii] During the 1980s, C gradually gained popularity. Information technology has go one of the most widely used programming languages,^[viii] ^[9] with C compilers from various vendors available for the majority of existing reckoner architectures and operating systems. C has been standardized by ANSI since 1989 (ANSI C) and by the International System for Standardization (ISO).

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

Since 2000, C has consistently ranked amid the top two languages in the TIOBE index, a measure out of the popularity of programming languages.^[11]

Overview [edit]

Similar nigh procedural languages in the ALGOL tradition, C has facilities for structured programming and allows lexical variable scope and recursion. Its static blazon arrangement prevents unintended operations. In C, all executable code is contained inside subroutines (likewise chosen "functions", though not strictly in the sense of functional programming). Function parameters are e'er passed by value (except arrays). Pass-by-reference is simulated in C by explicitly passing pointer values. C program source text is free-format, using the semicolon as a statement terminator and curly braces for group blocks of statements.

The C language also exhibits the post-obit characteristics:

The language has a small, fixed number of keywords, including a full gear up of control flow primitives: if/else, for, do/while, while, and switch. User-defined names are not distinguished from keywords by whatsoever kind of sigil.
It has a large number of arithmetic, bitwise, and logic operators: +,+=,++,&,||, etc.
More than one assignment may be performed in a single argument.
Functions:
- Part return values can be ignored, when non needed.
- Function and data pointers let advertising hoc run-time polymorphism.
- Functions may non be defined inside the lexical scope of other functions.
Data typing is static, but weakly enforced; all data has a blazon, just implicit conversions are possible.
Proclamation syntax mimics usage context. C has no "ascertain" keyword; instead, a statement start with the proper name of a type is taken as a announcement. 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 aggregate data types (struct) allow related information elements to be accessed and assigned equally a unit.
- Marriage is a structure with overlapping members; only the last fellow member stored is valid.
- Array indexing is a secondary note, defined in terms of pointer arithmetic. Dissimilar structs, arrays are not first-class objects: they cannot be assigned or compared using single built-in operators. At that place is no "assortment" keyword in utilise or definition; instead, square brackets point arrays syntactically, for example month[11].
- Enumerated types are possible with the enum keyword. They are freely interconvertible with integers.
- Strings are not a distinct data type, but are conventionally implemented every bit nada-terminated grapheme arrays.
Low-level admission to figurer retention is possible by converting auto addresses to typed pointers.
Procedures (subroutines non 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 bones grade of modularity: files can exist compiled separately and linked together, with control over which functions and data objects are visible to other files via static and extern attributes.
Complex functionality such as I/O, string manipulation, and mathematical functions are consistently delegated to library routines.

While C does non include certain features found in other languages (such as object orientation and garbage collection), these can be implemented or emulated, often through the use of external libraries (eastward.thou., the GLib Object Arrangement or the Boehm garbage collector).

Relations to other languages [edit]

Many later languages have borrowed directly or indirectly from C, including C++, C#, Unix'southward C crush, 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 take drawn many of their control structures and other basic features from C. Nearly of them (Python being a dramatic exception) as well express highly like syntax to C, and they tend to combine the recognizable expression and statement syntax of C with underlying blazon systems, data models, and semantics that tin can exist radically different.

History [edit]

Early developments [edit]

Timeline of language evolution
Year	C Standard^[x]
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 organization, originally implemented in associates linguistic communication on a PDP-7 by Dennis Ritchie and Ken Thompson, incorporating several ideas from colleagues. Somewhen, they decided to port the operating system to a PDP-11. The original PDP-11 version of Unix was also adult in assembly language.^[7]

Thompson desired a programming language to make utilities for the new platform. At first, he tried to make a Fortran compiler, but shortly gave up the thought. Instead, he created a cut-down version of the recently developed BCPL systems programming language. The official clarification of BCPL was not available at the time,^[12] and Thompson modified the syntax to be less wordy, producing the similar but somewhat simpler B.^[seven] Withal, few utilities were ultimately written in B because it was as well slow, and B could non accept advantage of PDP-11 features such as byte addressability.

In 1972, Ritchie started to improve B, about notably adding data typing for variables, which resulted in creating a new linguistic communication C.^[xiii] The C compiler and some utilities made with it were included in Version 2 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 caused some powerful features such as struct types.

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

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

K&R C [edit]

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

Yard&R introduced several language features:

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

Fifty-fifty after the publication of the 1989 ANSI standard, for many years K&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 still in use, and because carefully written Thou&R C code tin be legal Standard C as well.

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

For case:

                        long                                    some_function            ();                        /* int */                                    other_function            ();                        /* int */                                    calling_function            ()                        {                                                long                                    test1            ;                                                register                                    /* int */                                    test2            ;                                                test1                                    =                                    some_function            ();                                                if                                    (            test1                                    >                                    ane            )                                                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 after standards.

Since Grand&R function declarations did not include any data about role arguments, role parameter type checks were not performed, although some compilers would issue a warning message if a local function was chosen with the wrong number of arguments, or if multiple calls to an external function used dissimilar numbers or types of arguments. Divide tools such every bit Unix's lint utility were adult that (among other things) could check for consistency of role utilise across multiple source files.

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

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

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

ANSI C and ISO C [edit]

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

In 1983, the American National Standards Institute (ANSI) formed a committee, X3J11, to establish a standard specification of C. X3J11 based the C standard on the Unix implementation; notwithstanding, the non-portable portion of the Unix C library was handed off to the IEEE working grouping 1003 to get the basis for the 1988 POSIX standard. In 1989, the C standard was ratified as ANSI X3.159-1989 "Programming Language C". This version of the 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 chosen C90. Therefore, the terms "C89" and "C90" refer to the same programming language.

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

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

C89 is supported by electric current C compilers, and most modern C code is based on it. Any program written only in Standard C and without whatsoever hardware-dependent assumptions volition run correctly on whatever platform with a conforming C implementation, within its resource limits. Without such precautions, programs may compile only on a certain platform or with a particular compiler, due, for example, to the use of non-standard libraries, such every bit 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-befitting or Yard&R C-based compilers, the __STDC__ macro can be used to split the code into Standard and G&R sections to forbid the use on a K&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 Subpoena 1 to the 1990 C standard (ISO/IEC 9899/AMD1:1995, known informally every bit C95) was published, to correct some details and to add more than extensive support for international character sets.^[18]

C99 [edit]

The C standard was farther revised in the tardily 1990s, leading to the publication of ISO/IEC 9899:1999 in 1999, which is commonly referred to as "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 circuitous blazon to represent complex numbers), variable-length arrays and flexible assortment members, improved back up for IEEE 754 floating bespeak, support for variadic macros (macros of variable arity), and support for one-line comments beginning with //, as in BCPL or C++. Many of these had already been implemented equally extensions in several C compilers.

C99 is for the most part backward uniform with C90, but 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 bespeak 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++, nevertheless, implements the C89 standard and those parts of C99 that are required for compatibility with C++11.^[xx] ^{[ needs update ]}

In addition, back up for Unicode identifiers (variable / role names) in the grade of escaped characters (due east.g. \U0001f431) is at present required. Support for raw Unicode names is optional.

C11 [edit]

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

The C11 standard adds numerous new features to C and the library, including type generic macros, anonymous 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 indicate that C11 support is bachelor.

C17 [edit]

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

C2x [edit]

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

Embedded C [edit]

Historically, embedded C programming requires nonstandard extensions to the C language in order to support exotic features such every bit fixed-point arithmetics, multiple distinct memory banks, and basic 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. It includes a number of features not available in normal C, such as fixed-betoken arithmetic, named accost spaces, and basic I/O hardware addressing.

Syntax [edit]

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

C source files contain declarations and function definitions. Role definitions, in turn, contain declarations and statements. Declarations either define new types using keywords such as struct, spousal relationship, and enum, or assign types to and maybe reserve storage for new variables, unremarkably by writing the type followed past the variable proper noun. Keywords such every bit char and int specify congenital-in types. Sections of lawmaking are enclosed in braces ({ and }, sometimes called "curly brackets") to limit the scope of declarations and to human action as a unmarried argument for control structures.

Equally an imperative language, C uses statements to specify deportment. 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 modify the normal sequential execution of statements, C provides several control-menses statements identified past reserved keywords. Structured programming is supported by if … [else] conditional execution and past do … while, while, and for iterative execution (looping). The for statement has carve up initialization, testing, and reinitialization expressions, any or all of which can be omitted. break and continue tin can be used to leave the innermost enclosing loop argument or skip to its reinitialization. There is also a non-structured goto statement which branches directly to the designated characterization within the function. switch selects a instance to be executed based on the value of an integer expression.

Expressions can use a variety of built-in operators and may contain role calls. The guild in which arguments to functions and operands to about operators are evaluated is unspecified. The evaluations may even be interleaved. However, all side effects (including storage to variables) will occur before the adjacent "sequence point"; sequence points include the end of each expression statement, and the entry to and return from each function call. Sequence points too occur during evaluation of expressions containing sure operators (&&, ||, ?: and the comma operator). This permits a loftier degree of object code optimization by the compiler, merely requires C programmers to take more intendance to obtain reliable results than is needed for other programming languages.

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

Grapheme fix [edit]

The basic C source character set includes the following characters:

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

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

Additional multi-byte encoded characters may be used in string literals, merely they are non 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 yet widely implemented.

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

Reserved words [edit]

C89 has 32 reserved words, as well known as keywords, which are the words that cannot be used for whatever purposes other than those for which they are predefined:

car
break
example
char
const
proceed
default
do
double
else
enum
extern
bladder
for
goto
if
int
long
annals
render
brusk
signed
sizeof
static
struct
switch
typedef
wedlock
unsigned
void
volatile
while

C99 reserved 5 more words:

_Bool
_Complex
_Imaginary
inline
restrict

C11 reserved seven more than words:^[26]

_Alignas
_Alignof
_Atomic
_Generic
_Noreturn
_Static_assert
_Thread_local

Most of the recently reserved words brainstorm with an underscore followed by a capital letter, because identifiers of that form were previously reserved by the C standard for use but by implementations. Since existing program source lawmaking should not accept been using these identifiers, it would not be affected when C implementations started supporting these extensions to the programming linguistic communication. Some standard headers do define more user-friendly synonyms for underscored identifiers. The language previously included a reserved discussion called entry, but this was seldom implemented, and has now been removed as a reserved give-and-take.^[27]

Operators [edit]

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

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

C uses the operator = (used in mathematics to express equality) to indicate assignment, following the precedent of Fortran and PL/I, only unlike ALGOL and its derivatives. C uses the operator == to test for equality. The similarity between these two operators (consignment and equality) may upshot in the accidental use of one in place of the other, and in many cases, the mistake does not produce an error bulletin (although some compilers produce warnings). For example, the conditional expression if (a == b + ane) might mistakenly be written every bit if (a = b + ane), which will exist evaluated as true if a is non naught after the consignment.^[28]

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

"How-do-you-do, globe" example [edit]

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

The original version was:^[xxx]

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

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

                        #include                                    <stdio.h>                        int                                    chief            (            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 entire text of the stdio.h standard header, which contains declarations for standard input and output functions such equally printf and scanf. The bending brackets surrounding stdio.h indicate that stdio.h is located using a search strategy that prefers headers provided with the compiler to other headers having the same proper name, every bit opposed to double quotes which typically include local or project-specific header files.

The next line indicates that a function named principal is being defined. The main function serves a special purpose in C programs; the run-fourth dimension 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 case the run-time surround) every bit a result of evaluating the main office, is an integer. The keyword void as a parameter list indicates that this function takes no arguments.^[b]

The opening curly brace indicates the beginning of the definition of the main function.

The next line calls (diverts execution to) a office named printf, which in this instance is supplied from a arrangement library. In this call, the printf function is passed (provided with) a single argument, the address of the first graphic symbol in the string literal "hello, world\northward". The cord literal is an unnamed array with elements of blazon char, set upwards automatically by the compiler with a terminal 0-valued character to mark the end of the assortment (printf needs to know this). The \due north is an escape sequence that C translates to a newline graphic symbol, which on output signifies the end of the current line. The render value of the printf function is of blazon int, but information technology is silently discarded since it is not used. (A more than careful plan might test the return value to decide whether or not the printf role succeeded.) The semicolon ; terminates the statement.

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

Data types [edit]

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

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

Some find C's declaration syntax unintuitive, peculiarly for function pointers. (Ritchie's idea was to declare identifiers in contexts resembling their use: "annunciation reflects employ".)^[33]

C's usual arithmetic conversions allow for efficient code to be generated, simply tin sometimes produce unexpected results. For case, a comparison of signed and unsigned integers of equal width requires a conversion of the signed value to unsigned. This can generate unexpected results if the signed value is negative.

Pointers [edit]

C supports the utilize of pointers, a blazon of reference that records the address or location of an object or role in retention. Pointers tin can exist dereferenced to access data stored at the address pointed to, or to invoke a pointed-to part. Pointers can be manipulated using consignment or pointer arithmetic. The run-time representation of a pointer value is typically a raw memory address (perhaps augmented by an offset-within-discussion field), only since a pointer'due south type includes the type of the matter pointed to, expressions including pointers can be type-checked at compile time. Pointer arithmetic is automatically scaled by the size of the pointed-to data type. Pointers are used for many purposes in C. Text strings are commonly manipulated using pointers into arrays of characters. Dynamic memory allocation is performed using pointers. Many information types, such as copse, are ordinarily implemented as dynamically allocated struct objects linked together using pointers. Pointers to functions are useful for passing functions equally arguments to higher-social club functions (such every bit qsort or bsearch) or every bit callbacks to exist invoked by issue handlers.^[31]

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

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

Careless use of pointers is potentially unsafe. Because they are typically unchecked, a pointer variable can exist made to point to whatever arbitrary location, which can cause undesirable effects. Although properly used pointers point to safe places, they can be made to point to dangerous places by using invalid pointer arithmetic; the objects they indicate to may go on to exist used later deallocation (dangling pointers); they may be used without having been initialized (wild pointers); or they may exist directly assigned an unsafe value using a cast, union, or through another decadent arrow. In full general, C is permissive in assuasive manipulation of and conversion between arrow types, although compilers typically provide options for various levels of checking. Some other programming languages address these problems by using more than restrictive reference types.

Arrays [edit]

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

Since arrays are e'er accessed (in effect) via pointers, array accesses are typically not checked against the underlying assortment size, although some compilers may provide bounds checking as an pick.^[34] ^[35] Array bounds violations are therefore possible and can lead to diverse 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, but rather relies on recursion inside the type organization to declare arrays of arrays, which effectively accomplishes the aforementioned affair. 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 store matrices. The structure of the C array is well suited to this particular job. However, in early versions of C the bounds of the array must be known fixed values or else explicitly passed to any subroutine that requires them, and dynamically sized arrays of arrays cannot be accessed using double indexing. (A workaround for this was to allocate the assortment 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 afterward) shows allocation 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                                    Chiliad            )                        {                                                float                                    (            *            p            )[            North            ][            Chiliad            ]                                    =                                    malloc            (            sizeof                                    *            p            );                                                if                                    (            !            p            )                                                return                                    -one            ;                                                for                                    (            int                                    i                                    =                                    0            ;                                    i                                    <                                    Northward            ;                                    i            ++            )                                                for                                    (            int                                    j                                    =                                    0            ;                                    j                                    <                                    M            ;                                    j            ++            )                                                (            *            p            )[            i            ][            j            ]                                    =                                    i                                    +                                    j            ;                                                print_array            (            N            ,                                    M            ,                                    p            );                                                free            (            p            );                                                return                                    1            ;                        }

Array–pointer interchangeability [edit]

The subscript notation 10[i] (where ten designates a pointer) is syntactic sugar for *(x+i).^[36] Taking advantage of the compiler'due south cognition of the pointer blazon, the address that x + i points to is not the base of operations address (pointed to by 10) incremented by i bytes, but rather is defined to exist the base address incremented past i multiplied by the size of an element that x points to. Thus, ten[i] designates the i+1th element of the assortment.

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

The total size of an array x tin can exist determined past applying sizeof to an expression of array blazon. The size of an element can be adamant past applying the operator sizeof to any dereferenced element of an array A, as in n = sizeof A[0]. This, the number of elements in a declared array A tin exist adamant as sizeof A / sizeof A[0]. Annotation, that if just a arrow to the start chemical element is available as it is oftentimes the case in C code because of the automated conversion described above, the information about the full type of the array and its length are lost.

Memory management [edit]

One of the nearly important functions of a programming language is to provide facilities for managing retentiveness and the objects that are stored in memory. C provides 3 singled-out ways to allocate retentiveness for objects:^[31]

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

These three approaches are appropriate in unlike situations and have various merchandise-offs. For example, static memory resource allotment has niggling allocation overhead, automated allocation may involve slightly more overhead, and dynamic memory allocation tin can potentially take a great deal of overhead for both allocation and deallocation. The persistent nature of static objects is useful for maintaining state information across function calls, automatic allocation is easy to utilize but stack infinite is typically much more than limited and transient than either static memory or heap space, and dynamic retention allocation allows convenient allocation of objects whose size is known only at run-time. Most C programs make extensive use of all iii.

Where possible, automatic or static allocation is normally simplest considering the storage is managed by the compiler, freeing the programmer of the potentially error-prone chore of manually allocating and releasing storage. However, many data structures tin change in size at runtime, and since static allocations (and automated allocations before C99) must have a fixed size at compile-fourth dimension, there are many situations in which dynamic resource allotment is necessary.^[31] Prior to the C99 standard, variable-sized arrays were a common case of this. (See the article on malloc for an example of dynamically allocated arrays.) Dissimilar automatic allocation, which tin neglect at run time with uncontrolled consequences, the dynamic allotment functions render an indication (in the form of a null pointer value) when the required storage cannot be allocated. (Static allotment that is too large is normally detected by the linker or loader, earlier the plan tin fifty-fifty begin execution.)

Unless otherwise specified, static objects contain zero or zippo pointer values upon programme startup. Automatically and dynamically allocated objects are initialized only if an initial value is explicitly specified; otherwise they initially have indeterminate values (typically, whatever flake blueprint happens to be present in the storage, which might not even correspond a valid value for that blazon). If the program attempts to access an uninitialized value, the results are undefined. Many modern compilers effort to notice and warn about this problem, but both false positives and false negatives tin can occur.

Heap retentiveness allocation has to be synchronized with its actual usage in whatsoever program to be reused as much as possible. For example, if the merely pointer to a heap memory allotment goes out of scope or has its value overwritten earlier information technology is deallocated explicitly, and so that retentiveness cannot be recovered for afterwards reuse and is essentially lost to the program, a phenomenon known equally a memory leak. Conversely, it is possible for memory to be freed, only is referenced subsequently, leading to unpredictable results. Typically, the failure symptoms appear in a portion of the program unrelated to the lawmaking that causes the error, making information technology difficult to diagnose the failure. Such problems are ameliorated in languages with automatic garbage collection.

Libraries [edit]

The C programming language uses libraries as its primary method of extension. In C, a library is a gear up of functions independent within a unmarried "archive" file. Each library typically has a header file, which contains the prototypes of the functions contained inside the library that may be used by a program, and declarations of special data types and macro symbols used with these functions. In order for a program to utilize a library, it must include the library's header file, and the library must be linked with the program, which in many cases requires compiler flags (eastward.1000., -lm, autograph for "link the math library").^[31]

The most common C library is the C standard library, which is specified past 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, retention allocation, mathematics, character strings, and fourth dimension values. Several carve up 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 past applications specifically targeted for Unix and Unix-like systems, especially functions which provide an interface to the kernel. These functions are detailed in various standards such as POSIX and the Single UNIX Specification.

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

File treatment and streams [edit]

File input and output (I/O) is not part of the C language itself only instead is handled by libraries (such as the C standard library) and their associated header files (e.k. stdio.h). File handling is generally implemented through high-level I/O which works through streams. A stream is from this perspective a data flow that is independent of devices, while a file is a concrete device. The high-level I/O is done through the association of a stream to a file. In the C standard library, a buffer (a memory expanse or queue) is temporarily used to store information before it's sent to the final destination. This reduces the time spent waiting for slower devices, for example a hard bulldoze or solid country bulldoze. Low-level I/O functions are not part of the standard C library^{[ clarification needed ]} but are mostly function of "bare metal" programming (programming that's independent of any operating system such as most embedded programming). With few exceptions, implementations include low-level I/O.

Language tools [edit]

A number of tools have been developed to help C programmers observe and fix statements with undefined beliefs 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 any language, and for C many such tools be, such as Lint. A common practice is to utilise Lint to observe questionable code when a plan is first written. Once a programme passes Lint, information technology is and then compiled using the C compiler. Also, many compilers tin can optionally warn about syntactically valid constructs that are likely to actually be errors. MISRA C is a proprietary set of guidelines to avoid such questionable code, developed for embedded systems.^[37]

In that location are also 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 memory tracking, and automatic garbage collection.

Tools such as Purify or Valgrind and linking with libraries containing special versions of the memory allotment 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 well-nigh purposes, yet when needed, system-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-fourth dimension demand on system resources.

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

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

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

C is sometimes used as an intermediate language by implementations of other languages. This approach may be used for portability or convenience; 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 end of initializer lists, that support compilation of generated code. However, some of C's shortcomings have prompted the development of other C-based languages specifically designed for apply as intermediate languages, such every bit C--.

C has also been widely used to implement end-user applications. Even so, such applications can besides exist written in newer, higher-level languages.

[edit]

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

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

Several C or near-C interpreters exist, including Ch and CINT, which can besides 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 code 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++ now supports well-nigh of C, with a few exceptions.

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

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

Notes [edit]

^ The original instance code will compile on near modern compilers that are not in strict standard compliance mode, simply 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 two arguments, int argc and char *argv[], respectively, which can exist used to handle control line arguments. The ISO C standard (section 5.1.ii.2.1) requires both forms of master to exist supported, which is special treatment not afforded to any other part.

References [edit]

^ ^a ^b Kernighan, Brian W.; Ritchie, Dennis Grand. (Feb 1978). The C Programming Language (1st ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-110163-0.
^ Ritchie (1993): "Thompson had made a brief attempt to produce a system coded in an early version of C—before structures—in 1972, just gave upwards the endeavour."
^ Fruderica (December 13, 2020). "History of C". The cppreference.com. Archived from the original on October 24, 2020. Retrieved October 24, 2020.
^ Ritchie (1993): "The scheme of type composition adopted by C owes considerable debt to Algol 68, although it did non, perhaps, sally in a course that Algol'southward adherents would approve of."
^ Ring Team (October 23, 2021). "The Ring programming language and other languages". band-lang.cyberspace.
^ ^a ^b "Verilog HDL (and C)" (PDF). The Research School of Information science at the Australian National University. June three, 2010. Archived from the original (PDF) on November half-dozen, 2013. Retrieved August 19, 2013. 1980s: ; Verilog first introduced ; Verilog inspired by the C programming linguistic communication
^ ^a ^b ^c ^d ^e Ritchie (1993)
^ "Programming Language Popularity". 2009. Archived from the original on January 16, 2009. Retrieved January 16, 2009.
^ "TIOBE Programming Community Alphabetize". 2009. Archived from the original on May 4, 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 October 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. 1000. (1978). "Portability of C Programs and the UNIX Organization". Bell Organization Tech. J. 57 (vi): 2021–2048. CiteSeerXten.1.1.138.35. doi:ten.1002/j.1538-7305.1978.tb02141.x. S2CID 17510065. (Note: The PDF is an OCR scan of the original, and contains a rendering of "IBM 370" as "IBM 310".)
^ McIlroy, Grand. D. (1987). A Enquiry Unix reader: annotated excerpts from the Programmer's Manual, 1971–1986 (PDF) (Technical report). CSTR. Bong Labs. p. 10. 139. Archived (PDF) from the original on Nov 11, 2017. Retrieved Feb 1, 2015.
^ "C manual pages". FreeBSD Miscellaneous Information Manual (FreeBSD 13.0 ed.). May 30, 2011. Archived from the original on January 21, 2021. Retrieved January 15, 2021. [1] Archived January 21, 2021, at the Wayback Machine
^ Kernighan, Brian W.; Ritchie, Dennis M. (March 1988). The C Programming Language (2nd ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-110362-seven.
^ Stroustrup, Bjarne (2002). Sibling rivalry: C and C++ (PDF) (Study). AT&T Labs. Archived (PDF) from the original on August 24, 2014. Retrieved Apr xiv, 2014.
^ C Integrity. International Organisation for Standardization. March 30, 1995. Archived from the original on July 25, 2018. Retrieved July 24, 2018.
^ "JTC1/SC22/WG14 – C". Home page. ISO/IEC. Archived from the original on February 12, 2018. Retrieved June two, 2011.
^ Andrew Binstock (Oct 12, 2011). "Interview with Herb Sutter". Dr. Dobbs. Archived from the original on August 2, 2013. Retrieved September 7, 2013.
^ "Revised C23 Schedule WG xiv N 2759" (PDF). www.open-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 Feb 25, 2021. Retrieved July 26, 2011.
^ Harbison, Samuel P.; Steele, Guy L. (2002). C: A Reference Manual (5th ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-13-089592-9. Contains a BNF grammar for C.
^ Kernighan & Ritchie (1996), p. 192.
^ Kernighan & Ritchie (1978), p. 3.
^ "ISO/IEC 9899:201x (ISO C11) Commission Draft" (PDF). Archived (PDF) from the original on Dec 22, 2017. Retrieved September 16, 2011.
^ Kernighan & Ritchie (1996), pp. 192, 259.
^ "ten Common Programming Mistakes in C++". Cs.ucr.edu. Archived from the original on Oct 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. vi.
^ ^a ^b ^c ^d ^e ^f ^g Klemens, Ben (2013). 21st Century C. O'Reilly Media. ISBN978-1-4493-2714-9.
^ Feuer, Alan R.; Gehani, Narain H. (March 1982). "Comparison of the Programming Languages C and Pascal". ACM Computing Surveys. fourteen (1): 73–92. doi:ten.1145/356869.356872. S2CID 3136859.
^ Kernighan & Ritchie (1996), p. 122.
^ For case, gcc provides _FORTIFY_SOURCE. "Security Features: Compile Time Buffer Checks (FORTIFY_SOURCE)". fedoraproject.org. Archived from the original on January 7, 2007. Retrieved August v, 2012.
^ เอี่ยมสิริวงศ์, โอภาศ (2016). Programming with C. Bangkok, Thailand: SE-EDUCATION PUBLIC Visitor LIMITED. pp. 225–230. ISBN978-616-08-2740-iv.
^ Raymond, Eric S. (October eleven, 1996). The New Hacker's Dictionary (third ed.). MIT Press. p. 432. ISBN978-0-262-68092-9. Archived from the original on November 12, 2012. Retrieved August 5, 2012.
^ "Man Page for lint (freebsd Department one)". unix.com. May 24, 2001. Retrieved July 15, 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. November–Dec 1995.
^ "Using C for CGI Programming". linuxjournal.com. March 1, 2005. Archived from the original on February thirteen, 2010. Retrieved January iv, 2010.
^ McMillan, Robert (August ane, 2013). "Is Java Losing Its Mojo?". Wired. Archived from the original on February 15, 2017. Retrieved March 5, 2017.
^ O'Regan, Gerard (September 24, 2015). Pillars of computing : a compendium of select, pivotal technology firms. ISBN978-3319214641. OCLC 922324121.
^ Rauchwerger, Lawrence (2004). Languages and compilers for parallel calculating : 16th international workshop, LCPC 2003, College Station, TX, USA, Oct 2-4, 2003 : revised papers. Springer. ISBN978-3540246442. OCLC 57965544.
^ Stroustrup, Bjarne (1993). "A History of C++: 1979−1991" (PDF). Archived (PDF) from the original on February 2, 2019. Retrieved June 9, 2011.

Sources [edit]

Ritchie, Dennis M. (March 1993). "The Development of the C Linguistic communication". ACM SIGPLAN Notices. ACM. 28 (3): 201–208. doi:x.1145/155360.155580.
Ritchie, Dennis M. (1993). "The Evolution of the C Language". The Second ACM SIGPLAN Conference on History of Programming Languages (HOPL-Ii). ACM. pp. 201–208. doi:ten.1145/154766.155580. ISBN0-89791-570-4 . Retrieved November iv, 2014.
Kernighan, Brian Due west.; Ritchie, Dennis M. (1996). The C Programming Language (2nd ed.). Prentice Hall. ISBN7-302-02412-10.

Farther reading [edit]

Kernighan, Brian; Ritchie, Dennis (1988). The C Programming Language (ii ed.). Prentice Hall. ISBN978-0131103627. (annal)
Plauger, P.J. (1992). The Standard C Library (1 ed.). Prentice Hall. ISBN978-0131315099. (source)
Banahan, 1000.; Brady, D.; Doran, M. (1991). The C Book: Featuring the ANSI C Standard (2 ed.). Addison-Wesley. ISBN978-0201544336. (free)
Harbison, Samuel; Steele Jr, Guy (2002). C: A Reference Manual (5 ed.). Pearson. ISBN978-0130895929. (archive)
King, Thou.N. (2008). C Programming: A Modern Approach (2 ed.). Due west. W. Norton. ISBN978-0393979503. (archive)
Griffiths, David; Griffiths, Dawn (2012). Head Commencement C (1 ed.). O'Reilly. ISBN978-1449399917.
Perry, Greg; Miller, Dean (2013). C Programming: Absolute Beginner'southward Guide (3 ed.). Que. ISBN978-0789751980.
Deitel, Paul; Deitel, Harvey (2015). C: How to Program (eight ed.). Pearson. ISBN978-0133976892.
Gustedt, Jens (2019). Modern C (2 ed.). Manning. ISBN978-1617295812. (gratis)

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 Frequently Asked Questions
A History of C, by Dennis Ritchie

cummingsonely1942.blogspot.com

Source: https://en.wikipedia.org/wiki/C_(programming_language)