C# Read Color Palete From Act File

General-purpose programming language

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

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

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++Builder, Microsoft Visual C++, Watcom C
Dialects
Cyclone, Unified Parallel C, Split-C, Cilk, C*
Influenced past
B (BCPL, CPL), ALGOL 68,^[four] associates, PL/I, FORTRAN
Influenced
Numerous: AMPL, AWK, csh, C++, C--, C#, Objective-C, D, Become, Coffee, JavaScript, JS++, Julia, Limbo, LPC, Perl, PHP, Pike, Processing, Python, Ring,^[5]Rust, Seed7, Vala, Verilog (HDL),^[vi] Nim, Zig
C Programming at Wikibooks

C (, every bit in the letterc) is a general-purpose, procedural calculator programming language supporting structured programming, lexical variable telescopic, and recursion, with a static type system. By pattern, C provides constructs that map efficiently to typical machine instructions. It has found lasting apply in applications previously coded in assembly language. Such applications include operating systems and various application 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 Bell Labs by Dennis Ritchie between 1972 and 1973 to construct utilities running on Unix. It was practical to re-implementing the kernel of the Unix operating organisation.^[7] During the 1980s, C gradually gained popularity. It has become one of the most widely used programming languages,^[8] ^[9] with C compilers from various vendors bachelor for the majority of existing reckoner architectures and operating systems. C has been standardized past ANSI since 1989 (ANSI C) and by the International Organization for Standardization (ISO).

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

Since 2000, C has consistently ranked amidst the top 2 languages in the TIOBE index, a measure of the popularity of programming languages.^[xi]

Overview

Like almost 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 code is independent within subroutines (likewise called "functions", though not strictly in the sense of functional programming). Office parameters are always passed past value (except arrays). Pass-by-reference is simulated in C past explicitly passing pointer values. C program source text is costless-format, using the semicolon as a argument terminator and curly braces for grouping blocks of statements.

The C language also exhibits the post-obit characteristics:

The language has a small, fixed number of keywords, including a full fix of control flow primitives: if/else, for, do/while, while, and switch. User-defined names are not distinguished from keywords by any kind of sigil.
It has a large number of arithmetics, bitwise, and logic operators: +,+=,++,&,||, etc.
More than than one assignment may be performed in a unmarried argument.
Functions:
- Function return values can exist ignored, when non needed.
- Function and data pointers permit advertising hoc run-time polymorphism.
- Functions may not be divers within the lexical telescopic of other functions.
Information typing is static, merely weakly enforced; all information has a blazon, but implicit conversions are possible.
Declaration syntax mimics usage context. C has no "define" keyword; instead, a statement beginning with the name of a type is taken as a declaration. There is no "function" keyword; instead, a function is indicated past the presence of a parenthesized argument list.
User-defined (typedef) and compound types are possible.
- Heterogeneous aggregate data types (struct) allow related data elements to be accessed and assigned as a unit.
- Union is a construction with overlapping members; just the last member stored is valid.
- Array indexing is a secondary notation, divers in terms of pointer arithmetic. Unlike structs, arrays are not first-class objects: they cannot be assigned or compared using single built-in operators. In that location is no "array" keyword in use or definition; instead, square brackets signal 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 as null-terminated graphic symbol arrays.
Low-level access to reckoner memory is possible by converting machine addresses to typed pointers.
Procedures (subroutines not returning values) are a special example of part, with an untyped render type void.
A preprocessor performs macro definition, source code file inclusion, and conditional compilation.
There is a bones form of modularity: files can exist compiled separately and linked together, with control over which functions and information objects are visible to other files via static and extern attributes.
Complex functionality such as I/O, cord manipulation, and mathematical functions are consistently delegated to library routines.

While C does non include sure features found in other languages (such as object orientation and garbage drove), these tin can be implemented or emulated, ofttimes through the use of external libraries (eastward.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, Get, Java, JavaScript (including transpilers), Julia, Limbo, LPC, Objective-C, Perl, PHP, Python, Ruby, Rust, Swift, Verilog and SystemVerilog (hardware description languages).^[6] These languages have drawn many of their control structures and other basic features from C. Most of them (Python being a dramatic exception) likewise limited highly similar syntax to C, and they tend to combine the recognizable expression and statement syntax of C with underlying type systems, information models, and semantics that can be radically different.

History

Early developments

Timeline of language development
Twelvemonth	C Standard^[10]
1972	Nascence
1978	Chiliad&R C
1989/1990	ANSI C and ISO C
1999	C99
2011	C11
2017	C17
TBD	C2x

The origin of C is closely tied to the evolution of the Unix operating 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-eleven version of Unix was also developed in associates linguistic communication.^[7]

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

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

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

The preprocessor was introduced around 1973 at the urging of Alan Snyder and also in recognition of the usefulness of the file-inclusion mechanisms available 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, more often than not past Mike Lesk then by John Reiser, to comprise macros with arguments and conditional compilation.^[7]

Unix was one of the first operating system kernels implemented in a language other than assembly. Earlier instances include the Multics arrangement (which was written in PL/I) and Chief Command Program (MCP) for the Burroughs B5000 (which was written in ALGOL) in 1961. In effectually 1977, Ritchie and Stephen C. Johnson made further changes to the linguistic communication to facilitate portability of the Unix operating system. Johnson's Portable C Compiler served equally the ground 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.^[i] This volume, known to C programmers every bit 1000&R, served for many years as an breezy specification of the language. The version of C that information technology describes is commonly referred to as "Yard&R C". As this was released in 1978, it is also referred to as C78.^[15] The 2d edition of the book^[sixteen] covers the later ANSI C standard, described beneath.

K&R introduced several language features:

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

Fifty-fifty later the publication of the 1989 ANSI standard, for many years K&R C was nonetheless considered the "everyman common denominator" to which C programmers restricted themselves when maximum portability was desired, since many older compilers were however in utilize, and considering advisedly written Thousand&R C code tin be legal Standard C as well.

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

For example:

                        long                                    some_function            ();                        /* int */                                    other_function            ();                        /* int */                                    calling_function            ()                        {                                                long                                    test1            ;                                                register                                    /* int */                                    test2            ;                                                test1                                    =                                    some_function            ();                                                if                                    (            test1                                    >                                    1            )                                                test2                                    =                                    0            ;                                                else                                                test2                                    =                                    other_function            ();                                                return                                    test2            ;                        }

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

Since Chiliad&R function declarations did non include any information virtually function arguments, part parameter blazon checks were not performed, although some compilers would result a warning message if a local function was called with the incorrect number of arguments, or if multiple calls to an external function used different numbers or types of arguments. Separate tools such equally Unix's lint utility were developed that (among other things) could 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 linguistic communication, 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 spousal relationship types (rather than pointers)
assignment for struct data types
enumerated types

The big number of extensions and lack of agreement on a standard library, together with the language popularity and the fact that non even the Unix compilers precisely implemented the Yard&R specification, led to the necessity of standardization.

ANSI C and ISO C

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

In 1983, the American National Standards Institute (ANSI) formed a committee, X3J11, to found a standard specification of C. X3J11 based the C standard on the Unix implementation; however, the non-portable portion of the Unix C library was handed off to the IEEE working group 1003 to become the ground 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 ofttimes referred to as ANSI C, Standard C, or sometimes C89.

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

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

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

C89 is supported by current C compilers, and virtually modern C code is based on it. Whatever program written only in Standard C and without any hardware-dependent assumptions will run correctly on any platform with a conforming C implementation, within its resource limits. Without such precautions, programs may compile only on a sure platform or with a particular compiler, due, for instance, to the use of non-standard libraries, such as GUI libraries, or to a reliance on compiler- or platform-specific attributes such equally the exact size of data types and byte endianness.

In cases where lawmaking must exist compilable past either standard-conforming or One thousand&R C-based compilers, the __STDC__ macro can exist used to split the code into Standard and K&R sections to prevent the use on a K&R C-based compiler of features available only in Standard C.

After the ANSI/ISO standardization procedure, the C language specification remained relatively static for several years. In 1995, Normative Subpoena one to the 1990 C standard (ISO/IEC 9899/AMD1:1995, known informally as C95) was published, to right some details and to add more extensive support for international graphic symbol 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 commonly referred to as "C99". It has since been amended three times by Technical Corrigenda.^[xix]

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

C99 is for the about role astern compatible with C90, but 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 divers 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++, however, implements the C89 standard and those parts of C99 that are required for compatibility with C++11.^[20] ^{[ needs update ]}

In addition, support for Unicode identifiers (variable / function names) in the form of escaped characters (e.g. \U0001f431) is now 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 past existing implementations.

The C11 standard adds numerous new features to C and the library, including blazon generic macros, bearding structures, improved Unicode support, atomic operations, multi-threading, and bounds-checked functions. It as well makes some portions of the existing C99 library optional, and improves compatibility with C++. The standard macro __STDC_VERSION__ is defined every bit 201112L to indicate 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, only technical corrections, and clarifications to defects in C11. The standard macro __STDC_VERSION__ is defined as 201710L.

C2x

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

Embedded C

Historically, embedded C programming requires nonstandard extensions to the C language in order to back up exotic features such equally fixed-signal arithmetics, multiple distinct memory banks, and basic I/O operations.

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

Syntax

C has a formal grammer specified by the C standard.^[23] Line endings are generally non pregnant 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 end of the line. Comments delimited by /* and */ exercise not nest, and these sequences of characters are not interpreted equally comment delimiters if they appear inside string or grapheme literals.^[24]

C source files comprise declarations and part definitions. Function definitions, in turn, contain declarations and statements. Declarations either define new types using keywords such every bit struct, wedlock, and enum, or assign types to and perhaps reserve storage for new variables, ordinarily by writing the type followed by the variable proper noun. Keywords such as char and int specify congenital-in types. Sections of code are enclosed in braces ({ and }, sometimes chosen "curly brackets") to limit the scope of declarations and to act as a single statement for control structures.

As an imperative language, C uses statements to specify deportment. The well-nigh common statement is an expression argument, consisting of an expression to exist evaluated, followed by a semicolon; as a side upshot of the evaluation, functions may exist called and variables may be assigned new values. To modify the normal sequential execution of statements, C provides several command-flow statements identified by reserved keywords. Structured programming is supported by if … [else] conditional execution and by practice … while, while, and for iterative execution (looping). The for statement has separate initialization, testing, and reinitialization expressions, any or all of which can be omitted. break and continue can exist used to leave the innermost enclosing loop argument or skip to its reinitialization. There is also a non-structured goto argument which branches directly to the designated label inside the role. switch selects a case to be executed based on the value of an integer expression.

Expressions tin can apply a variety of built-in operators and may incorporate function calls. The guild in which arguments to functions and operands to most operators are evaluated is unspecified. The evaluations may even exist interleaved. Even so, all side furnishings (including storage to variables) will occur before the side by side "sequence indicate"; sequence points include the terminate of each expression statement, and the entry to and render from each part phone call. Sequence points also occur during evaluation of expressions containing certain operators (&&, ||, ?: and the comma operator). This permits a loftier degree of object code optimization by the compiler, just requires C programmers to accept more care to obtain reliable results than is needed for other programming languages.

Kernighan and Ritchie say in the Introduction of The C Programming Language: "C, like any other language, has its blemishes. Some of the operators have the wrong precedence; some parts of the syntax could be better."^[25] The C standard did non endeavor to correct many of these blemishes, because of the bear upon of such changes on already existing software.

Graphic symbol ready

The basic C source character set up includes the following characters:

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

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

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

The basic C execution graphic symbol prepare contains the aforementioned 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 every bit keywords, which are the words that cannot be used for any purposes other than those for which they are predefined:

auto
break
case
char
const
continue
default
practise
double
else
enum
extern
bladder
for
goto
if
int
long
annals
render
brusk
signed
sizeof
static
struct
switch
typedef
marriage
unsigned
void
volatile
while

C99 reserved 5 more words:

_Bool
_Complex
_Imaginary
inline
restrict

C11 reserved 7 more than words:^[26]

_Alignas
_Alignof
_Atomic
_Generic
_Noreturn
_Static_assert
_Thread_local

Most of the recently reserved words begin with an underscore followed by a capital letter, because identifiers of that form were previously reserved by the C standard for use only by implementations. Since existing program source code should not have 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 convenient synonyms for underscored identifiers. The language previously included a reserved discussion chosen entry, merely this was seldom implemented, and has now been removed as a reserved word.^[27]

Operators

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: +, -, *, /, %
assignment: =
augmented assignment: +=, -=, *=, /=, %=, &=, |=, ^=, <<=, >>=
bitwise logic: ~, &, |, ^
bitwise shifts: <<, >>
boolean logic: !, &&, ||
conditional evaluation: ? :
equality testing: ==, !=
calling functions: ( )
increment and decrement: ++, --
fellow member choice: ., ->
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 bespeak assignment, post-obit 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 2 operators (assignment and equality) may upshot in the accidental utilize of 1 in place of the other, and in many cases, the mistake does not produce an error bulletin (although some compilers produce warnings). For instance, the conditional expression if (a == b + 1) might mistakenly be written as if (a = b + 1), which will be evaluated as true if a is not zilch afterwards the assignment.^[28]

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

"Hello, world" instance

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

The original version was:^[30]

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

A standard-conforming "how-do-you-do, globe" 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 entire text of the stdio.h standard header, which contains declarations for standard input and output functions such equally printf and scanf. The angle brackets surrounding stdio.h betoken that stdio.h is located using a search strategy that prefers headers provided with the compiler to other headers having the same name, as opposed to double quotes which typically include local or project-specific header files.

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

The opening curly brace indicates the starting time of the definition of the main function.

The next line calls (diverts execution to) a function named printf, which in this case is supplied from a system library. In this telephone call, the printf office is passed (provided with) a single argument, the address of the offset character in the cord literal "hello, earth\n". The cord literal is an unnamed array with elements of type char, ready automatically past the compiler with a last 0-valued graphic symbol to marker the stop of the assortment (printf needs to know this). The \n is an escape sequence that C translates to a newline character, which on output signifies the terminate of the electric current line. The return value of the printf role is of type int, simply it is silently discarded since it is not used. (A more than careful program might exam the return value to decide whether or not the printf role succeeded.) The semicolon ; terminates the statement.

The endmost curly brace indicates the cease of the lawmaking for the chief function. According to the C99 specification and newer, the main function, unlike any other function, will implicitly render 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-fourth dimension arrangement equally an exit lawmaking indicating successful execution.^[31]

Information types

The type organisation in C is static and weakly typed, which makes it similar to the blazon organization 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 type char is frequently used for unmarried-byte characters. C99 added a boolean datatype. There are besides derived types including arrays, pointers, records (struct), and unions (union).

C is often used in low-level systems programming where escapes from the blazon system may exist necessary. The compiler attempts to ensure type correctness of most expressions, just the programmer can override the checks in various ways, either by using a type bandage 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 find C'southward annunciation syntax unintuitive, particularly for function pointers. (Ritchie's idea was to declare identifiers in contexts resembling their use: "declaration reflects employ".)^[33]

C's usual arithmetic conversions allow for efficient lawmaking to be generated, but tin can sometimes produce unexpected results. For example, 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

C supports the utilize of pointers, a blazon of reference that records the address or location of an object or function in retentiveness. Pointers tin be dereferenced to access data stored at the address pointed to, or to invoke a pointed-to function. Pointers can be manipulated using consignment or pointer arithmetic. The run-fourth dimension representation of a arrow value is typically a raw retentivity address (perhaps augmented by an get-go-within-word field), but since a arrow'south type includes the type of the thing pointed to, expressions including pointers can exist blazon-checked at compile fourth dimension. Pointer arithmetics is automatically scaled by the size of the pointed-to information type. Pointers are used for many purposes in C. Text strings are commonly manipulated using pointers into arrays of characters. Dynamic retentivity allocation is performed using pointers. Many information types, such as trees, are commonly implemented as dynamically allocated struct objects linked together using pointers. Pointers to functions are useful for passing functions as arguments to higher-order functions (such as qsort or bsearch) or every bit callbacks to be 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 equally no "next" pointer in the terminal node of a linked list, or every bit an fault indication from functions returning pointers. In appropriate contexts in source code, such equally for assigning to a pointer variable, a zip pointer constant can be written every bit 0, with or without explicit casting to a arrow type, or equally the Naught macro defined by several standard headers. In conditional contexts, cypher pointer values evaluate to faux, while all other arrow values evaluate to true.

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

Careless use of pointers is potentially dangerous. Because they are typically unchecked, a pointer variable can be made to bespeak to any arbitrary location, which tin can crusade undesirable furnishings. Although properly used pointers point to rubber places, they can be made to point to unsafe places by using invalid pointer arithmetic; the objects they point to may continue to be used afterwards 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 corrupt pointer. In general, C is permissive in assuasive manipulation of and conversion between pointer types, although compilers typically provide options for various levels of checking. Some other programming languages address these issues by using more restrictive reference types.

Arrays

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

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

C does not have a special provision for declaring multi-dimensional arrays, but rather relies on recursion inside the type organization to declare arrays of arrays, which effectively accomplishes the aforementioned affair. The alphabetize 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 structure of the C assortment is well suited to this particular chore. However, in early versions of C the bounds of the array must be known stock-still values or else explicitly passed to any subroutine that requires them, and dynamically sized arrays of arrays cannot be accessed using double indexing. (A workaround for this was to allocate the assortment with an additional "row vector" of pointers to the columns.) C99 introduced "variable-length arrays" which address this result.

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

                        int                                    func            (            int                                    N            ,                                    int                                    M            )                        {                                                bladder                                    (            *            p            )[            Northward            ][            M            ]                                    =                                    malloc            (            sizeof                                    *            p            );                                                if                                    (            !            p            )                                                return                                    -ane            ;                                                for                                    (            int                                    i                                    =                                    0            ;                                    i                                    <                                    Northward            ;                                    i            ++            )                                                for                                    (            int                                    j                                    =                                    0            ;                                    j                                    <                                    M            ;                                    j            ++            )                                                (            *            p            )[            i            ][            j            ]                                    =                                    i                                    +                                    j            ;                                                print_array            (            Northward            ,                                    M            ,                                    p            );                                                gratuitous            (            p            );                                                return                                    one            ;                        }

Assortment–pointer interchangeability

The subscript note x[i] (where x designates a pointer) is syntactic sugar for *(x+i).^[36] Taking reward of the compiler's knowledge of the pointer type, the accost that x + i points to is non the base address (pointed to past x) incremented by i bytes, but rather is defined to be the base accost incremented by i multiplied past the size of an element that x points to. Thus, x[i] designates the i+onethursday element of the array.

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

The total size of an array x can exist determined by applying sizeof to an expression of assortment type. The size of an element can be adamant by applying the operator sizeof to any dereferenced element of an array A, as in northward = sizeof A[0]. This, the number of elements in a alleged array A can be determined every bit sizeof A / sizeof A[0]. Note, that if simply a arrow to the starting time element is bachelor every bit information technology is often the case in C code considering of the automatic conversion described above, the information near the full type of the array and its length are lost.

Retentivity management

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

Static retentiveness allocation: space for the object is provided in the binary at compile-time; these objects take an extent (or lifetime) as 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 subsequently the cake in which they are declared is exited.
Dynamic memory allotment: blocks of memory of arbitrary size can be requested at run-fourth dimension using library functions such as malloc from a region of memory called the heap; these blocks persist until afterward freed for reuse by calling the library part realloc or free

These iii approaches are advisable in different situations and accept various trade-offs. For case, static retention allocation has little allocation overhead, automated resource allotment may involve slightly more overhead, and dynamic retentiveness allocation can potentially have a cracking bargain of overhead for both allocation and deallocation. The persistent nature of static objects is useful for maintaining land information across office calls, automatic allocation is easy to use simply stack space is typically much more limited and transient than either static retentiveness or heap infinite, and dynamic memory resource allotment allows convenient allocation of objects whose size is known only at run-time. Nearly C programs brand extensive use of all three.

Where possible, automated or static allocation is normally simplest considering the storage is managed by the compiler, freeing the programmer of the potentially mistake-prone chore of manually allocating and releasing storage. Yet, many information structures tin change in size at runtime, and since static allocations (and automatic allocations earlier C99) must take a fixed size at compile-time, in that location are many situations in which dynamic resource allotment is necessary.^[31] Prior to the C99 standard, variable-sized arrays were a common example of this. (Run into the commodity on malloc for an example of dynamically allocated arrays.) Unlike automated allocation, which can fail at run fourth dimension with uncontrolled consequences, the dynamic allocation functions return an indication (in the grade of a zero pointer value) when the required storage cannot exist allocated. (Static allocation that is too large is usually detected by the linker or loader, before the program can fifty-fifty begin execution.)

Unless otherwise specified, static objects incorporate zero or goose egg arrow values upon plan startup. Automatically and dynamically allocated objects are initialized only if an initial value is explicitly specified; otherwise they initially have indeterminate values (typically, whatsoever bit pattern happens to be present in the storage, which might not even stand for a valid value for that blazon). If the program attempts to access an uninitialized value, the results are undefined. Many modern compilers try to observe and warn well-nigh this problem, just both faux positives and false negatives can occur.

Heap memory allocation has to be synchronized with its actual usage in any program to be reused every bit much as possible. For instance, if the only pointer to a heap retentiveness allocation goes out of scope or has its value overwritten earlier information technology is deallocated explicitly, then that memory cannot be recovered for later reuse and is essentially lost to the program, a phenomenon known as a memory leak. Conversely, it is possible for retentiveness to be freed, simply is referenced subsequently, leading to unpredictable results. Typically, the failure symptoms appear in a portion of the plan 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

The C programming linguistic communication uses libraries equally its main method of extension. In C, a library is a set of functions contained 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 past a plan, and declarations of special information types and macro symbols used with these functions. In order for a program to employ a library, information technology must include the library'due south header file, and the library must be linked with the program, which in many cases requires compiler flags (e.g., -lm, shorthand 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 but a subset of the standard library). This library supports stream input and output, memory allocation, mathematics, character strings, and time values. Several separate standard headers (for example, stdio.h) specify the interfaces for these and other standard library facilities.

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

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

File handling and streams

File input and output (I/O) is not part of the C linguistic communication itself simply instead is handled past libraries (such as the C standard library) and their associated header files (due east.thousand. stdio.h). File handling is by and large implemented through high-level I/O which works through streams. A stream is from this perspective a data menstruum that is independent of devices, while a file is a physical device. The high-level I/O is done through the clan of a stream to a file. In the C standard library, a buffer (a retention area or queue) is temporarily used to store data before it's sent to the final destination. This reduces the time spent waiting for slower devices, for example a difficult drive or solid state drive. Depression-level I/O functions are not part of the standard C library^{[ description needed ]} just are more often than not role of "bare metal" programming (programming that'south independent of any operating system such as almost embedded programming). With few exceptions, implementations include low-level I/O.

Linguistic communication tools

A number of tools accept been developed to help C programmers observe 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.

Automated source code checking and auditing are beneficial in any language, and for C many such tools exist, such equally Lint. A common practice is to use Lint to detect questionable code when a program is first written. One time a programme passes Lint, it is and so compiled using the C compiler. As well, many compilers can optionally warn near syntactically valid constructs that are likely to really be errors. MISRA C is a proprietary fix of guidelines to avoid such questionable lawmaking, adult for embedded systems.^[37]

There are likewise compilers, libraries, and operating organisation level mechanisms for performing actions that are not 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 retention allocation functions can assistance 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] because C code, when written for portability, can exist used for well-nigh purposes, yet when needed, arrangement-specific code can be used to access specific hardware addresses and to perform type punning to friction match externally imposed interface requirements, with a depression run-fourth dimension demand on system resource.

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

A effect of C'south wide availability and efficiency is that compilers, libraries and interpreters of other programming languages are often implemented in C. For example, the reference implementations of Python, Perl, Cerise, and PHP are written in C.

C enables programmers to create efficient implementations of algorithms and information structures, because the layer of abstraction from hardware is thin, and its overhead is low, an important benchmark for computationally intensive programs. For instance, the GNU Multiple Precision Arithmetics 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 equally 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 lawmaking. Even so, some of C'due south shortcomings have prompted the development of other C-based languages specifically designed for apply as intermediate languages, such every bit C--.

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

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

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

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

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

The C++ programming language (originally named "C with Classes") was devised by Bjarne Stroustrup as an approach to providing object-oriented functionality with a C-similar syntax.^[44] C++ adds greater typing strength, scoping, and other tools useful in object-oriented programming, and permits generic programming via templates. Most a superset of C, C++ now supports 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 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 addition to C++ and Objective-C, Ch, Cilk, and Unified Parallel C are nearly supersets of C.

Notes

^ The original example code will compile on most modern compilers that are not in strict standard compliance mode, but it does not fully conform to the requirements of either C89 or C99. In fact, C99 requires that a diagnostic message be produced.
^ The main office actually has two arguments, int argc and char *argv[], respectively, which can exist used to handle command line arguments. The ISO C standard (section 5.one.2.ii.i) requires both forms of chief to be supported, which is special treatment non afforded to whatsoever other office.

References

^ ^a ^b Kernighan, Brian W.; Ritchie, Dennis M. (February 1978). The C Programming Language (1st ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-xiii-110163-0.
^ Ritchie (1993): "Thompson had made a cursory attempt to produce a arrangement coded in an early on version of C—before structures—in 1972, merely gave upward the endeavor."
^ Fruderica (Dec 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 blazon composition adopted by C owes considerable debt to Algol 68, although it did not, perhaps, emerge in a form that Algol's adherents would corroborate of."
^ Ring Team (October 23, 2021). "The Ring programming linguistic communication and other languages". ring-lang.net.
^ ^a ^b "Verilog HDL (and C)" (PDF). The Inquiry School of Information science at the Australian National University. June 3, 2010. Archived from the original (PDF) on Nov six, 2013. Retrieved August 19, 2013. 1980s: ; Verilog beginning introduced ; Verilog inspired by the C programming linguistic communication
^ ^a ^b ^c ^d ^e Ritchie (1993)
^ "Programming Linguistic communication Popularity". 2009. Archived from the original on Jan xvi, 2009. Retrieved Jan 16, 2009.
^ "TIOBE Programming Customs Alphabetize". 2009. Archived from the original on May 4, 2009. Retrieved May half-dozen, 2009.
^ ^a ^b "History of C". en.cppreference.com. Archived from the original on May 29, 2018. Retrieved May 28, 2018.
^ "TIOBE Index for Oct 2021". Retrieved October 7, 2021.
^ Ritchie, Dennis. "BCPL to B to C". Archived from the original on Dec 12, 2019. Retrieved September 10, 2019.
^ ^a ^b Johnson, South. C.; Ritchie, D. M. (1978). "Portability of C Programs and the UNIX System". Bell System Tech. J. 57 (half dozen): 2021–2048. CiteSeerX10.1.i.138.35. doi:ten.1002/j.1538-7305.1978.tb02141.ten. S2CID 17510065. (Note: The PDF is an OCR browse of the original, and contains a rendering of "IBM 370" every bit "IBM 310".)
^ McIlroy, M. D. (1987). A Research Unix reader: annotated excerpts from the Programmer's Transmission, 1971–1986 (PDF) (Technical study). CSTR. Bell Labs. p. ten. 139. Archived (PDF) from the original on November 11, 2017. Retrieved February 1, 2015.
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^ Andrew Binstock (October 12, 2011). "Interview with Herb Sutter". Dr. Dobbs. Archived from the original on Baronial 2, 2013. Retrieved September seven, 2013.
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^ "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 (fifth ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN978-0-xiii-089592-ix. Contains a BNF grammer for C.
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^ Kernighan & Ritchie (1978), p. 3.
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^ "10 Common Programming Mistakes in C++". Cs.ucr.edu. Archived from the original on October 21, 2008. Retrieved June 26, 2009.
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Sources

Ritchie, Dennis Thousand. (March 1993). "The Development of the C Language". ACM SIGPLAN Notices. ACM. 28 (three): 201–208. doi:10.1145/155360.155580.
Ritchie, Dennis Chiliad. (1993). "The Development of the C Language". The Second ACM SIGPLAN Conference on History of Programming Languages (HOPL-Ii). ACM. pp. 201–208. doi:10.1145/154766.155580. ISBN0-89791-570-4 . Retrieved November 4, 2014.
Kernighan, Brian West.; Ritchie, Dennis M. (1996). The C Programming Linguistic communication (2nd ed.). Prentice Hall. ISBN7-302-02412-X.

External links

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 Often Asked Questions
A History of C, by Dennis Ritchie

This page was last edited on i March 2022, at 08:47

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