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C (programming language)

The C programming language is a general-purpose, procedural, imperative computer programming language Programming language

A programming language is an artificial language [i] that can be used to control [i] ... 

 developed in the early 1970s by Dennis Ritchie Dennis Ritchie

Dennis MacAlistair Ritchie is a computer scientist [i] notable for his influence on ALTRAN [i] ... 

 for use on the Unix Unix

Unix or UNIX is a computer [i] operating system [i] originally developed in the 1960s and 1970s by ... 

 operating system Operating system

An operating system is a software program [i] that manages the hardware [i] and software [i] ... 

. It has since spread to many other operating systems, and is now one of the most widely used programming languages. C also has had a great influence on many other popular languages, especially C++ C++

C++ is a general-purpose, high-level [i] programming language [i] with low-level [i] facilities. ... 

 which was originally designed as an enhancement to C. It is the most commonly used programming language for writing system software,

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Encyclopedia

The C programming language is a general-purpose, procedural, imperative computer programming language Programming language

A programming language is an artificial language [i] that can be used to control [i] ... 

 developed in the early 1970s by Dennis Ritchie Dennis Ritchie

Dennis MacAlistair Ritchie is a computer scientist [i] notable for his influence on ALTRAN [i] ... 

 for use on the Unix Unix

Unix or UNIX is a computer [i] operating system [i] originally developed in the 1960s and 1970s by ... 

 operating system Operating system

An operating system is a software program [i] that manages the hardware [i] and software [i] ... 

. It has since spread to many other operating systems, and is now one of the most widely used programming languages. C also has had a great influence on many other popular languages, especially C++ C++

C++ is a general-purpose, high-level [i] programming language [i] with low-level [i] facilities. ... 

 which was originally designed as an enhancement to C. It is the most commonly used programming language for writing system software,
though it is also widely used for writing applications Application software

Application software is a defined subclass of computer software [i] that employs the capabilities of a c ... 

. Though not originally designed as a language for teaching, and despite its somewhat unforgiving character, C is commonly used in computer science education Education

Education is the process by which an individual is encouraged and enabled to develop fully his or her in... 

, in part because the language is so pervasive.

Philosophy

C is a minimalistic programming language Programming language

A programming language is an artificial language [i] that can be used to control [i] ... 

. Among its design goals were that it could be compiled in a straightforward manner using a relatively simple compiler, provide low-level access to memory, generate only a few machine language instructions for each of its core language elements, and not require extensive run-time support. As a result, C code is suitable for many systems-programming applications that had traditionally been implemented in assembly language.

Despite its low-level capabilities, the language was designed to encourage machine-independent programming. A standards-compliant and portably written C program can be compiled for a very wide variety of computer platforms and operating systems with minimal change to its source code. The language has become available on a very wide range of platforms, from embedded microcontrollers to supercomputers.

Characteristics

As an ALGOL Algol

Algol is a bright star [i] in the constellation [i] Perseus [i]. ... 

-based language, C has the following characteristics:

  • A procedural programming paradigm, with facilities for structured programming
  • Lexical variable scope and recursion Recursion

    In mathematics [i] and computer science [i], recursion specifies a class of objects or methods by defi... 

  • A static type system which prevents many meaningless operations
  • Function parameters Parameter

    In mathematics [i], statistics [i], and the mathematical science [i]s, parameters are quantities that d ... 

     are generally passed by value
  • Heterogeneous aggregate data types which allow related data elements to be combined and manipulated as a unit
  • A small set of reserved keywords


C also has the following specific properties:
  • Weak typing — for instance, characters can be used as integers
  • Low-level access to computer memory Computer storage

    Computer storage, computer memory, and often casually memory refer to computer [i] component ... 

     via machine addresses and typed pointers
  • Function pointers allow for a rudimentary form of closures and runtime polymorphism
  • Array Array

    In computer programming [i], a group of [i] element [i]s of a spec ... 

     indexing as a secondary notion, defined in terms of pointer arithmetic
  • A standardized C preprocessor for macro definition, source code Source code

    Source code is any series of statements written in some human-readable [i] computer programming language [i] ... 

     file inclusion, conditional compilation, etc.
  • A simple, small core language, with functionality such as mathematical functions and file handling provided by library routines Library

    In the traditional sense of the word, a library is a collection of book [i]s and periodicals. ... 

  • C discarded the well established logical connectives and and or of most other algol derivatives and replaced them with && and ||, which
    • Were invented in order to make bit-wise operations syntactically distinct — C's predecessor B used & and | for both meanings
    • Never evaluate the right operand if the result can be determined from the left alone
  • C popularized the controversial decision to free the equal-sign for assignment use by replacing = with

.

C lacks features found in some other systems implementation languages:
  • No non-scalar operations such as copying of arrays or strings
  • No automatic garbage collection
  • No bounds checking of arrays
  • No semi-dynamic arrays until the C99 standard
  • No syntax for ranges, such as the A..B notation used in both newer and older languages
  • No nested function definitions
  • No formal closures or functions as parameters
  • No generators or coroutines; intra-thread control flow consists of nested function calls, except for the use of the longjmp or setcontext library functions
  • No exception handling; standard library functions signify error conditions with the global errno variable and/or special return values
  • Rudimentary support for modular programming
  • No compile-time polymorphism in the form of function or operator overloading; only rudimentary support for generic programming
  • No support for object-oriented programming; in particular, no support for polymorphism, inheritance and limited support for encapsulation, even though there are libraries offering object systems for C, and many object-oriented languages are themselves written in C
  • No native support for multithreading and networking Computer networking

    Computer networking is the scientific [i] and engineering [i] discipline concerned with communic ... 

    , though these facilities are provided by popular libraries
  • No standard libraries for graphics and several other application programming needs


Although the list of built-in features C lacks is long, this has contributed significantly to its acceptance, as new C compilers Compiler

A compiler is a computer program [i] that translates text written in a computer language [i] into ano ... 

 can be developed quickly for new platforms. The relatively low-level nature of the language affords the programmer close control over what the program is doing, while allowing solutions that can be specially tailored and aggressively optimized for a particular platform. This allows the code to run efficiently on very limited hardware, such as mass-produced consumer embedded systems Embedded system

An embedded system is a special-purpose system in which the computer [i] is completely encapsulated by t ... 

, which today are as capable as the general-purpose machines originally used to implement C.

A number of the above missing features are available through the use of third party libraries. In some cases, a missing feature can be approximated within C. For example, the original implementation of C++ consisted of a preprocessor that translated the C++ syntax into C source code. Most object oriented functions include a special "this" pointer, which refers to the current object. By passing this pointer as a function argument in C, the same functionality can be performed in C. For example, in C++ one might write:
stack->push;
while in C, one would write:
push;
where the stack argument of C is a pointer to a struct which is equivalent to the this pointer of C++, which is a pointer to an object.

History


Early developments

The initial development of C occurred at AT&T AT&T

AT&T Inc. is the largest provider of both local and long distance telephone services, wireless service, ... 

 Bell Labs Bell Labs

[i] [[Bell System]... 

 between 1969 and 1973; according to Ritchie, the most creative period occurred in 1972. It was named "C" because many of its features were derived from an earlier language called "B," which according to Ken Thompson Ken Thompson

Kenneth Lane Thompson , commonly referred to as Ken Thompson, is a pioneer of computer science [i] ... 

 was a stripped down version of the BCPL programming language.

There are many legends as to the origin of C and the closely related Unix Unix

Unix or UNIX is a computer [i] operating system [i] originally developed in the 1960s and 1970s by ... 

 operating system, including these:
  • The development of Unix was the result of programmers' desire to play the video-game Computer and video games

    A computer game is a computer [i]-controlled game. ... 

    . They had been playing it on their company's mainframe Mainframe computer

    For the electro band comprising Murray Munro & John Molloy see Mainframe [i]

... 

, but as it was underpowered and had to support about 100 users, Thompson and Ritchie found they did not have sufficient control over the spaceship to avoid collisions with the wandering space rocks Asteroid

Asteroid, minor planet, and planetoid are synonyms, and are used to indicate a diverse group of small ce... 

. This led to the decision to port the game to an idle PDP-7 PDP-7

The DEC [i] PDP-7 is a minicomputer [i] produced by Digital Equipment Corporation [i] ... 

 in the office. As this machine lacked an operating system Operating system

An operating system is a software program [i] that manages the hardware [i] and software [i] ... 

, the two set out to develop one, based on several ideas from colleagues. Eventually it was decided to port the operating system to the office's PDP-11 PDP-11

The PDP-11 was a series of 16-bit [i] minicomputer [i]s sold by Digital Equipment Corp. [i] ... 

, but faced with the daunting task of translating a large body of custom-written assembly language code, the programmers began considering using a portable, high-level language so that the OS could be ported easily from one computer to another. They looked at using B, but it lacked functionality to take advantage of some of the PDP-11's advanced features. This led to the development of an early version of the C programming language.
  • The justification for obtaining the original computer to be used in developing the Unix operating system was to create a system to automate the filing of patents. The original version of the Unix system was developed in assembly language. Later, the entire operating system was rewritten in C, an unprecedented move at a time when nearly all operating systems were written in assembly.


By 1973, the C language had become powerful enough that most of the Unix Unix

Unix or UNIX is a computer [i] operating system [i] originally developed in the 1960s and 1970s by ... 

 kernel, originally written in PDP-11 PDP-11

The PDP-11 was a series of 16-bit [i] minicomputer [i]s sold by Digital Equipment Corp. [i] ... 

 assembly language, was rewritten in C. This was one of the first operating system kernels implemented in a language other than assembly.

K&R C

In 1978, Dennis Ritchie Dennis Ritchie

Dennis MacAlistair Ritchie is a computer scientist [i] notable for his influence on ALTRAN [i] ... 

 and Brian Kernighan published the first edition of The C Programming Language. This book, known to C programmers as "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." The second edition of the book covers the later ANSI C ANSI C

ANSI C is the standard published by the American National Standards Institute [i] for the C programming language [i] ... 

 standard.

K&R introduced several language features:
  • struct data types
  • long int data type
  • unsigned int data type
  • The =- operator was changed to -= to remove the semantic ambiguity created by the construct i=-10, which could be interpreted as either i =- 10 or i = -10


For many years after the introduction of ANSI C, 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 K&R C code can be legal ANSI C as well.

In early versions of C, only functions that returned a non-integer value needed to be declared if used before the function definition; a function used without any previous declaration was assumed to return an integer.

For example:

long int SomeFunction;
int OtherFunction;

int CallingFunction


In the example, both SomeFunction and OtherFunction were declared before use. In K&R, OtherFunction declaration could be omitted.

Since K&R function declarations did not include any information about function arguments, function parameter type checks were not performed, although some compilers would issue a warning message if a local function was called with the wrong number of arguments, or if multiple calls to an external function used different numbers of arguments. Separate tools such as Unix's lint utility were developed that could check for consistency of function use across multiple source files.

In the years following the publication of K&R C, several unofficial features were added to the language , supported by compilers from AT&T and some other vendors. These included:

  • void functions
  • Functions returning struct Object composition

    In computer science [i], composition is a way and practice to combine simple object [i]s or data type [i] ... 

    or union types
  • Assignment for struct data types
  • const qualifier to make an object read-only
  • Enumerated types


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

ANSI C and ISO C


During the late 1970s, C began to replace BASIC BASIC

In computer programming [i], BASIC refers to a family of high-level programming language [i]s.... 

 as the leading microcomputer Microcomputer

Although there is no rigid definition, a microcomputer is most often taken to mean a computer [i] with ... 

 programming language. During the 1980s, it was adopted for use with the IBM PC IBM PC

The IBM PC , was the original version and progenitor of the IBM PC compatible [i] hardware platform [i] ... 

, and its popularity began to increase significantly.
At the same time, Bjarne Stroustrup Bjarne Stroustrup

Bjarne Stroustrup is a computer scientist [i] and the College of Engineering [i] Chair Professor [i]... 

 and others at Bell Labs began work on adding object-oriented programming language constructs to C, resulting in the language now called C++ C++

C++ is a general-purpose, high-level [i] programming language [i] with low-level [i] facilities. ... 

.

In 1983, the American National Standards Institute  formed a committee, X3J11, to establish a standard specification of C. In 1989, the standard was ratified as ANSI X3.159-1989 "Programming Language C." This version of the language is often referred to as ANSI C ANSI C

ANSI C is the standard published by the American National Standards Institute [i] for the C programming language [i] ... 

, Standard C, or sometimes C89.

In 1990, the ANSI C standard was adopted by the International Organization for Standardization International Organization for Standardization

The International Organization for Standardization is an international standard-setting body composed o... 

  as ISO/IEC 9899:1990. This version is sometimes called C90. Therefore, the terms "C89" and "C90" refer to essentially the same language.

One of the aims of the C standardization process was to produce a superset of K&R C, incorporating many of the unofficial features subsequently introduced. However, the standards committee also included several new features, such as function prototypes , void pointers, support for international character sets and locales, and a more capable preprocessor. The syntax for parameter declarations was also augmented to include the C++ style:


int main



although the K&R interface


int main
int argc;
char **argv;



continued to be permitted, for compatibility with existing source code.

C89 is supported by current C compilers, and most C code being written nowadays is based on it. Any 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 certain platform or with a particular compiler, due, for example, to the use of non-standard libraries, such as GUI Graphical user interface

A graphical user interface , is a particular case of user interface [i] for interacting with a computer [i] ... 

 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 code must be compilable by either standard-conforming or K&R C-based compilers, the __STDC__ macro can be used to split the code into Standard and K&R sections,
in order to take advantage of features available only in Standard C.

  1. ifdef __STDC__

extern int getopt;
  1. else

extern int getopt;
  1. endif



In the above example, a compiler which has defined the __STDC__ macro only interprets the line following the ifdef command. In other, nonstandard compilers which don't define the macro, only the line following the else command is interpreted.

C99


Note: C99 is also the name of a C compiler for the Texas Instruments Texas Instruments

|

homepage =
}}
Texas Instruments , better known in the electronics industry as TI, is an... 

 TI-99/4A Texas Instruments TI-99/4A

The Texas Instruments [i] TI-99/4A was an early home computer [i], released in June of 1981 [i], origina ... 

 home computer. Aside from being a C compiler, it is otherwise unrelated.


After the ANSI standardization process, the C language specification remained relatively static for some time, whereas C++ C++

C++ is a general-purpose, high-level [i] programming language [i] with low-level [i] facilities. ... 

 continued to evolve, largely during its own standardization effort. Normative Amendment 1 created a new standard for the C language in 1995, but only to correct some details of the C89 standard and to add more extensive support for international character sets. However, the standard underwent further revision in the late 1990s, leading to the publication of ISO 9899:1999 in 1999.
This standard is commonly referred to as "C99." It was adopted as an ANSI standard in March 2000.

C99 introduced several new features, many of which had already been implemented as extensions in several compilers:

  • Inline functions
  • Variables can be declared anywhere , rather than only after another declaration or the start of a compound statement
  • Several new data types, including long long int, an explicit boolean data type, and a complex type to represent complex number Complex number

    In mathematics [i], a complex number is a number [i] of the form

... 

s
  • Variable-length array Array

    In computer programming [i], a group of [i] element [i]s of a spec ... 

    s
  • Support for one-line comments beginning with //, as in BCPL or C++
  • New library functions, such as snprintf
  • New header files, such as stdbool.h and inttypes.h
  • Type-generic math functions
  • Improved support for IEEE floating point IEEE floating-point standard

    The IEEE [i] Standard for Binary Floating-Point Arithmetic is the most widely-used standard for floating-point [i]... 

  • Designated initializers
  • Compound literals
  • Support for variadic macros
  • restrict qualification to allow more aggressive code optimization


C99 is for the most part upward-compatible with C90, but is stricter in some ways; in particular, a declaration that lacks a type specifier no longer has int implicitly assumed. The C standards committee decided that it was of more value for compilers to diagnose inadvertent omission of the type specifier than to silently process legacy code that relied on implicit int. In practice, compilers are likely to diagnose the omission but also assume int and continue translating the program.

GCC GNU Compiler Collection

The GNU Compiler Collection is a set of programming language [i] compiler [i]s produced by the GNU Project [i] ... 

 and other C compilers now support many of the new features of C99. However, there has been less support from vendors such as Microsoft Microsoft

company_name = Microsoft Corporation
... 

 and Borland Borland

[i] company headquartered in [[California]... 

 that have mainly focused on C++, since C++ provides similar functionality in sometimes incompatible ways . Microsoft's Brandon Bray said "In general, we have seen little demand for many C99 features. Some features have more demand than others, and we will consider them in future releases provided they are compatible with C++."

GCC, despite its extensive C99 support, is still not a completely compliant implementation; several key features are missing or don't work correctly.

A standard macro __STDC_VERSION__ is defined with value 199901L to indicate that C99 support is available. As with the __STDC__ macro for C90, __STDC_VERSION__ can be used to write code that will compile differently for C90 and C99 compilers, as in this example that ensures that inline is available in either case.
#if __STDC_VERSION__ >= 199901L
/* "inline" is a keyword */
  1. else
  2. define inline /* nothing */
  3. endif



Usage


One consequence of C's wide acceptance and efficiency is that the compilers, libraries, and interpreters of other higher-level languages are often implemented in C.

C is used as an intermediate language by some higher-level languages. This is implemented in one of two ways, as languages which:
  • Can output object code, machine code, or another representation , and C source code. Examples: some Lisp dialects, Squeak Squeak

    The Squeak programming language [i] is a Smalltalk [i] implementation, derived directly from Smalltalk-8... 

    's C-subset Slang.
  • Do not output object code, machine code, or another representation, but output C source code only. Examples: Eiffel, Sather; Esterel.


C source code is then input to a C compiler, which then outputs finished object or machine code. This is done to gain portability and to avoid having to develop machine-specific code generators.

Unfortunately, C was designed as a programming language, not as a compiler target language, and is thus less than ideal for use as an intermediate language. This has led to development of C-based intermediate languages such as C--.

Syntax


Main article: C syntax C syntax

The syntax of the C programming language [i] is a set of rules that defines how a C [i] ... 




Unlike languages such as FORTRAN 77 Fortran

FORTRAN is a general-purpose [i], procedural [i] ... 

, C source code is free-form which allows arbitrary use of whitespace to format code, rather than column-based or text-line-based restrictions. Comments may appear either between the delimiters /* and */, or following // until the end of the line.

Each source file contains declarations and function definitions. Function definitions, in turn, contain declarations and statements. Declarations either define new types using keywords such as struct, union, and enum, or assign types to and perhaps reserve storage for new variables, usually by writing the type followed by the variable name. Keywords such as char and int, as well as the pointer-to symbol *, specify built-in types. Sections of code are enclosed in braces Bracket

Brackets are punctuation [i] marks used in pairs to set apart or interject text within other text. ... 

  to indicate the extent to which declarations and control structures apply.

As an imperative language, C depends on statements to do most of the work. Most statements are expression statements which simply evaluate an expression; as a side effect, variables may receive new values. Control-flow statements are also available for conditional or iterative execution, constructed with reserved keywords such as if, else, switch, do, while, and for. Arbitrary jumps are possible with goto. A variety of built-in operators perform primitive arithmetic, Boolean logical, comparative, bitwise logical, and array indexing operations and assignment. Expressions can also invoke functions, including a large number of standard library functions, for performing many common tasks.

"hello, world" example

The following simple application appeared in the first edition of K&R, and has become the model for an introductory program in most programming textbooks, regardless of programming language. The program prints out "hello, world" to the standard output Standard streams

The standard streams are preconnected input or output channels between a computer program and its enviro... 

, which is usually a terminal or screen display. However, it might be a file or some other hardware device, depending on how standard output is mapped at the time the program is executed.

main


The above program will compile correctly on most modern compilers that are not in compliance mode. However, it produces several warning messages when compiled with a compiler that conforms to the ANSI C ANSI C

ANSI C is the standard published by the American National Standards Institute [i] for the C programming language [i] ... 

 standard, and won't compile at all if the compiler strictly conforms to the C99 standard. The current, ANSI C or C99, "hello world" program is written as follows:

#include 

int main


What follows is a line-by-line analysis of the above program:

#include 

This first line of the program is a preprocessing directive, #include. This causes the preprocessor — the first tool to examine source code when it is compiled — to substitute for that line the entire text of the file to which it refers. In this case, the header stdio.h, which contains declarations for standard input and output functions such as printf, will replace that line. The angle brackets surrounding stdio.h indicate that stdio.h can be found using an implementation-defined search strategy. Double quotes may also be used for headers, thus allowing the implementation to supply two strategies. Typically, angle brackets are reserved for headers supplied by the implementation, and double quotes for local or installation-specific headers.

int main

This next line indicates that a function named main is being defined. The main function serves a special purpose in C programs: When the program is executed, main is the function called by the run-time environment—otherwise it acts like any other function in the program. The type specifier int indicates that the return value, the value of evaluating the main function that is returned to its invoker , is an integer. The keyword in between the parentheses indicates that the main function takes no arguments. See also void.


This closing curly brace indicates the end of the code for the main function.

If the above code were compiled and executed, it would do the following:

  • Print the string "hello, world" onto the standard output device ,
  • Move the current position indicator to the beginning of the next line, then
  • Return a "successful" exit status to the calling process .

Data structures

C has a static Weak typing type system that shares some similarities with that of other ALGOL Algol

Algol is a bright star [i] in the constellation [i] Perseus [i]. ... 

 descendants such as Pascal. There are built-in types for integers of various sizes, both signed and unsigned, floating-point numbers, characters, and enumerated types . There are also derived types including array Array

In computer programming [i], a group of [i] element [i]s of a spec ... 

s, pointers, records , and untagged unions .

C is often used in low-level systems programming where "escapes" from the type system may be necessary. The compiler attempts to ensure type correctness of most expressions, but the programmer can override the checks in various ways, either by using a type cast to explicitly convert a value from one type to another, or by using pointers or unions to reinterpret the underlying bits of a value in some other way.
Pointers
C allows the use of pointers, a very simple type of reference that records, in effect, the address or location of an object or function in memory. Pointers can be dereferenced to access the data stored at the address pointed to, or to invoke the pointed-to function. Pointers can be manipulated using normal assignments and also pointer arithmetic. The run-time representation of a pointer value is typically a raw memory address, but since a pointer's type includes the type of the thing pointed to, expressions including pointers can be type-checked at compile time. Pointer arithmetic is automatically scaled by the size of the pointed-to data type. Pointers are used for many different purposes in C. Text strings are commonly manipulated using pointers into arrays of characters. Dynamic memory allocation, which is described below, is performed using pointers. Pointers to functions are useful for callbacks from event handlers.

A null pointer is a pointer value that points to no valid location . Dereferencing a null pointer is therefore meaningless, typically resulting in a run-time error. Null pointers are useful for indicating special cases such as no next pointer in the final node of a linked list Linked list

In computer science [i], a linked list is one of the fundamental data structure [i]s used in computer programming [i] ... 

, or as an error indication from functions returning pointers. Void pointers also exist and point to objects of unknown type, 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 possible, although they can easily be converted to and from any other object pointer type.
Arrays
Array Array

In computer programming [i], a group of [i] element [i]s of a spec ... 

 types in C are always one-dimensional and, traditionally, of a fixed, static size specified at compile time. However, it is also possible to allocate a block of memory at run-time, using the standard library's malloc function, and treat it as an array. C's unification of arrays and pointers means that true arrays and these dynamically-allocated, simulated arrays are virtually interchangeable. Since arrays are always accessed via pointers, array accesses are typically not checked against the underlying array size, although the compiler may provide bounds checking as an option. Array bounds violations are therefore possible and rather common in carelessly written code , and can lead to various repercussions: illegal memory accesses, corruption of data, buffer overrun, run-time exceptions, etc.

C does not have a special provision for declaring multidimensional arrays, but rather relies on recursion within the type system to declare arrays of arrays, which effectively accomplishes the same thing. The index values of the resulting "multidimensional array" can be thought of as increasing in row-major order.
Array?pointer interchangeability
A unique feature of C is its treatment of arrays and pointers. The array-subscript notation x[i] can also be used when x is a pointer; the interpretation is to access the th of several adjacent data objects pointed to by x, counting the object that x points to as the first element of the array.

Formally, x[i] is equivalent to *. Since the type of the pointer involved is known to the compiler at compile time, the address that x + i points to is not the address pointed to by x incremented by i bytes, but rather incremented by i multiplied by the size of an element that x points to. The size of these elements can be determined with the operator sizeof by applying it to any dereferenced element of x, as in n = sizeof *x or n = sizeof x[0].

Furthermore, in most contexts , the name of an array is automatically converted to a pointer to the array's first element; this implies that an array is never copied as a whole when named as an argument to a function, but rather only the address of its first element is passed. Therefore, although C's function calls use pass-by-value semantics, arrays are in effect passed by reference.

The number of elements in an array a can be determined as sizeof a / sizeof a[0], provided that the name is "in scope" .

An interesting demonstration of the remarkable interchangeability of pointers and arrays is shown below. These four lines are equivalent and each is valid C code. Note how the last line contains the strange code i[x] = 1;, which has the index variable i apparently interchanged with the array variable x. This last line might be found in obfuscated C code.

x[i] = 1;
  • = 1;
  • = 1;

i[x] = 1; /* strange, but correct */


There is, however, a distinction to be made between arrays and pointer variables. Even though the name of an array is in most contexts converted to a pointer , this pointer does not itself occupy any storage. Consequently, you cannot change what an array "points to", and it is impossible to assign to an array.

Memory management

One of the most important functions of a programming language is to provide facilities for managing memory Computer storage

Computer storage, computer memory, and often casually memory refer to computer [i] component ... 

 and the objects that are stored in memory. C provides three distinct ways to allocate memory for objects:
  • Static memory allocation: space for the object is provided in the binary at compile-time; these objects have an extent  as long as the binary which contains them is loaded into memory
  • Automatic memory allocation: temporary objects can be stored on the stack Call stack

    In computer science [i], a call stack is a special stack [i] which stores information about the ac ... 

    , and this space is automatically freed and reusable after the block in which they are declared is exited
  • Dynamic memory allocation: blocks of memory of arbitrary size can be requested at run-time using library functions such as malloc from a region of memory called the heap; these blocks can be subsequently freed for reuse by calling the library function free


These three approaches are appropriate in different situations and have various tradeoffs. For example, static memory allocation has no allocation overhead, automatic allocation has a small amount of overhead during initialization, and dynamic memory allocation can potentially have a great deal of overhead for both allocation and deallocation. On the other hand, stack space is typically much more limited and transient than either static memory or heap space, and dynamic memory allocation allows allocation of objects whose size is known only at run-time. Most C programs make extensive use of all three.

Where possible, automatic or static allocation is usually preferred because the storage is managed by the compiler, freeing the programmer of the potentially error-prone hassle of manually allocating and releasing storage. Unfortunately, many data structures can grow in size at runtime; since automatic and static allocations must have a fixed size at compile-time, there are many situations in which dynamic allocation must be used. Variable-sized arrays are a common example of this .

Libraries


The C programming language uses libraries Library

In the traditional sense of the word, a library is a collection of book [i]s and periodicals. ... 

 as its primary method of extension. In C, a library is a collection of functions contained within a single file. Each library typically has a header file, which contains the prototypes of the functions contained within 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 use a library, the header file from that library must be declared at the top of a source file, and the library must be linked to the program, which in many cases requires compiler flags .

The most common C library is the C standard library, which is specified by the ISO and ANSI C standard C (programming language)

The C programming language is a general-purpose, procedural [i], imperative [i] ... 

 and comes standard with every modern C compiler. This library supports stream input and output, memory allocation, mathematics, character strings, and time values.

Another common set of C library functions are those used by applications specifically targeted for Unix Unix

Unix or UNIX is a computer [i] operating system [i] originally developed in the 1960s and 1970s by ... 

 and Unix-like Unix-like

A "Unix-like" operating system [i] is one that behaves in a manner similar to a Unix [i] system, while n... 

 systems, especially functions which provide an interface to the kernel. These functions are detailed in various standards such as POSIX and the Single UNIX Specification.

Since many programs have been written in C, there are a wide variety of other libraries available. Libraries are often written in C because C generates efficient object code; programmers then create interfaces to the library so that the routines can be used from higher-level languages like Java Java (programming language)

Java is an object-oriented [i] programming language [i] developed by James Gosling [i] ... 

, Perl Perl

Perl, also Practical Extraction and Report Language is a dynamic [i] ... 

, and Python Python (programming language)

Python is a programming language [i] created by Guido van Rossum [i] in 1990 [i]. ... 

.

Criticism



Despite its popularity, C has been widely criticized. Such criticisms fall into two broad classes: desirable operations that are too hard to achieve using unadorned C, and undesirable operations that are too easy to accidentally achieve while using C. Putting this another way, the safe, effective use of C requires more programmer skill, experience, effort, and care than is required for some other programming languages.

Tools for mitigating issues with C


Tools have been created to help C programmers avoid some of the problems inherent in the language.

Automated source code checking and auditing are beneficial in any language, and for C many such tools exist, such as Lint. A common practice is to use Lint to detect questionable code when a program is first written. Once a program passes Lint, it is then compiled using the C compiler.

There are also compilers, libraries and operating system level mechanisms for performing array bounds checking, buffer overflow detection, and automatic garbage collection, that are not a standard part of C.

Many compilers, most notably Visual C++, deal with the long compilation times inflicted by header file inclusion using precompiled headers, a system where declarations are stored in an intermediate format that is quick to parse. Building the precompiled header files in the first place is expensive, but this is generally done only for system header files, which are larger and more numerous than most application header files and also change much less often.

is a program that will read a C source file and output prototypes of all the functions within the source file. This program can be used in conjunction with the "make" command to create new files containing prototypes each time the source file has been changed. These prototype files can be included by the original source file , which reduces the problems of keeping function definitions and source files in agreement.

It should be recognized that these tools are not a panacea. Because of C's flexibility, some types of errors involving misuse of variadic functions, out-of-bounds array indexing, and incorrect memory management cannot be detected on some architectures without incurring a significant performance penalty. However, some common cases can be recognized and accounted for.

Related languages


When object-oriented languages became popular, C++ C++

C++ is a general-purpose, high-level [i] programming language [i] with low-level [i] facilities. ... 

 and Objective-C were two different extensions of C that provided object-oriented capabilities. Both languages were originally implemented as preprocessors -- source code was translated into C, and then compiled with a C compiler.

C++

The C++ C++

C++ is a general-purpose, high-level [i] programming language [i] with low-level [i] facilities. ... 

 programming language was derived from C and is Bjarne Stroustrup Bjarne Stroustrup

Bjarne Stroustrup is a computer scientist [i] and the College of Engineering [i] Chair Professor [i]... 

's answer to adding object-oriented functionality with C-like syntax. C++ adds greater typing strength, scoping and other tools useful in object-oriented programming and permits generic programming via templates. Considered nearly a superset of C, C++ supports most of C, with a few relevant exceptions .

Objective-C

Objective-C is a very "thin" layer on top of and 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 is taken from Smalltalk. Objective-C and C++ differ in philosophies -- see the Objective-C article for details.

See also


  • C preprocessor
  • C standard library
  • C string
  • C syntax C syntax

    The syntax of the C programming language [i] is a set of rules that defines how a C [i] ... 

  • C variable types and declarations
  • Comparison of programming languages
  • C++ C++

    C++ is a general-purpose, high-level [i] programming language [i] with low-level [i] facilities. ... 

  • C# C Sharp

    C# is an object-oriented [i] programming language [i] developed by Microsoft [i] ... 

  • International Obfuscated C Code Contest
  • List of articles with C programs List of Articles With C Programs

    Sorry, no overview for this topic 

  • Objective-C
  • Operators in C and C++
  • Programming tools: Cygwin Cygwin

    Cygwin // - is a collection of free software [i] tools originally developed by Cygnus Solutions [i] to a ... 

    , Dev-C/C++ Dev-C++

    Dev-C++ is a free [i] integrated development environment [i] distributed under the GNU General Public License [i] ... 

    , DJGPP DJGPP

    DJGPP is a 32-bit [i] C [i]/C++ [i] development system for 386 [i] and above ... 

    , GNU Compiler Collection GNU Compiler Collection

    The GNU Compiler Collection is a set of programming language [i] compiler [i]s produced by the GNU Project [i] ... 

    , LCC, Linker Linker

    In computer science [i], a linker or link editor is a program [i] that takes one ... 

    , make, lint, Small-C, C--
  • Pascal and C
  • C to Java Virtual Machine compilers


Footnotes



References



External links


Tutorials

at Wikibooks Wikibooks

[i], previously called Wikimedia Free Textbook Project and Wikime... 


  • by M.Banahan-D.Brady-M.Doran — A very interesting and complete book for beginners/intermediate, now off-print and free.
  • by Richard Heathfield and others contributors in comp.lang.c — An interesting book about advanced ISO-C.
  • — Short C guide, great for programmers new to C.
  • - A series of ANSI C progamming tutorials for the beginner.

Resources

  • — An unpublished book about "detailed analysis of the International Standard for the C language. "

Optimization techniques



C99

  • - in PDF
  • by Peter Seebach


Support

  • at Cprogramming.com
  • at Daniweb
  • at ProgrammingForums.org

History

  • by Dennis M. Ritchie Dennis Ritchie

    Dennis MacAlistair Ritchie is a computer scientist [i] notable for his influence on ALTRAN [i] ...