Overview
C is an imperative, procedural language in the ALGOL tradition. It has a static type system. In C, all executable code is contained within if/else
, for
For or FOR may refer to:
English language
*For, a preposition
*For, a complementizer
*For, a grammatical conjunction
Science and technology
* Fornax, a constellation
* for loop, a programming language statement
* Frame of reference, in physic ...
, do/while
, while
, and switch
. User-defined names are not distinguished from keywords by any kind of sigil.
* It has a large number of arithmetic, bitwise, and logic operators: , etc.
* More than one assignment may be performed in a single statement.
* Functions:
** Function return values can be ignored, when not needed.
** Function and data pointers permit ''ad hoc'' struct
In computer science, a record (also called a structure, struct, or compound data) is a basic data structure. Records in a database or spreadsheet are usually called "rows".
A record is a collection of '' fields'', possibly of different data typ ...
) allow related data elements to be accessed and assigned as a unit.
** Union is a structure with overlapping members; only the last member stored is valid.
** Array indexing is a secondary notation, defined in terms of pointer arithmetic. Unlike structs, arrays are not first-class objects: they cannot be assigned or compared using single built-in operators. There is no "array" keyword in use or definition; instead, square brackets indicate arrays syntactically, for example month 1/code>.
** Enumerated types are possible with the enum
keyword. They are freely interconvertible with integers.
** Strings
String or strings may refer to:
*String (structure), a long flexible structure made from threads twisted together, which is used to tie, bind, or hang other objects
Arts, entertainment, and media Films
* ''Strings'' (1991 film), a Canadian anim ...
are not a distinct data type, but are conventionally implemented as null-terminated character arrays.
* Low-level access to computer memory is possible by converting machine addresses to pointers.
* Procedures
Procedure may refer to:
* Medical procedure
* Instructions or recipes, a set of commands that show how to achieve some result, such as to prepare or make something
* Procedure (business), specifying parts of a business process
* Standard operati ...
(subroutines not returning values) are a special case of function, with an untyped return type void
.
* Memory can be allocated to a program with calls to library routines.
* A preprocessor performs macro
Macro (or MACRO) may refer to:
Science and technology
* Macroscopic, subjects visible to the eye
* Macro photography, a type of close-up photography
* Image macro, a picture with text superimposed
* Monopole, Astrophysics and Cosmic Ray Observat ...
definition, source code file inclusion, and conditional compilation
In computer programming, conditional compilation is a compilation technique which results in an executable program that is able to be altered by changing specified parameters. This technique is commonly used when these alterations to the program a ...
.
* There is a basic form of modularity
Broadly speaking, modularity is the degree to which a system's components may be separated and recombined, often with the benefit of flexibility and variety in use. The concept of modularity is used primarily to reduce complexity by breaking a sy ...
: files can be compiled separately and linked together, with control over which functions and data objects are visible to other files via static
and extern
attributes.
* Complex functionality such as I/O, string
String or strings may refer to:
*String (structure), a long flexible structure made from threads twisted together, which is used to tie, bind, or hang other objects
Arts, entertainment, and media Films
* ''Strings'' (1991 film), a Canadian anim ...
manipulation, and mathematical functions are consistently delegated to library routines.
* The generated code after compilation has relatively straightforward needs on the underlying platform, which makes it suitable for creating operating systems and for use in embedded systems.
While C does not include certain features found in other languages (such as object orientation and garbage collection), these can be implemented or emulated, often through the use of external libraries (e.g., the GLib Object System or the Boehm garbage collector).
Relations to other languages
Many later languages have borrowed directly or indirectly from C, including C++, C#, Unix's C shell, D, Go, Java, JavaScript (including transpilers), Julia, Limbo, LPC
LPC may refer to:
Science and technology
* Linear predictive coding, a method used in audio signal processing and speech processing
* Leaf protein concentrate, a concentrated form of the proteins found in the leaves of plants
* Long period comet, ...
, Objective-C, Perl, PHP, Python, Ruby, Rust, Swift, Verilog and SystemVerilog (hardware description languages). These languages have drawn many of their control structures and other basic features from C. Most of them (Python being a dramatic exception) also express highly similar syntax
In linguistics, syntax () is the study of how words and morphemes combine to form larger units such as phrases and sentences. Central concerns of syntax include word order, grammatical relations, hierarchical sentence structure ( constituency) ...
to C, and they tend to combine the recognizable expression and statement syntax of C with underlying type systems, data models, and semantics that can be radically different.
History
Early developments
The origin of C is closely tied to the development of the Unix operating system, originally implemented in assembly language
In computer programming, assembly language (or assembler language, or symbolic machine code), often referred to simply as Assembly and commonly abbreviated as ASM or asm, is any low-level programming language with a very strong correspondence be ...
on a PDP-7 by Dennis Ritchie and Ken Thompson, incorporating several ideas from colleagues. Eventually, they decided to port the operating system to a PDP-11
The PDP-11 is a series of 16-bit minicomputers sold by Digital Equipment Corporation (DEC) from 1970 into the 1990s, one of a set of products in the Programmed Data Processor (PDP) series. In total, around 600,000 PDP-11s of all models were sold, ...
. The original PDP-11 version of Unix was also developed in assembly language.
B
Thompson desired a programming language to make utilities for the new platform. At first, he tried to make a Fortran compiler, but soon gave up the idea. Instead, he created a cut-down version of the recently developed BCPL
BCPL ("Basic Combined Programming Language") is a procedural, imperative, and structured programming language. Originally intended for writing compilers for other languages, BCPL is no longer in common use. However, its influence is still ...
systems programming language. The official description of BCPL was not available at the time, and Thompson modified the syntax to be less wordy, and similar to a simplified ALGOL known as SMALGOL. The result was what Thompson called B. He described B as "BCPL semantics with a lot of SMALGOL syntax". Like BCPL
BCPL ("Basic Combined Programming Language") is a procedural, imperative, and structured programming language. Originally intended for writing compilers for other languages, BCPL is no longer in common use. However, its influence is still ...
, B had a bootstrapping compiler to facilitate porting to new machines. However, few utilities were ultimately written in B because it was too slow, and could not take advantage of PDP-11 features such as byte addressability.
New B and first C release
In 1971, Ritchie started to improve B, to utilise the features of the more-powerful PDP-11. A significant addition was a character type. He called this ''New B''. Thompson started to use NB to write the Unix kernel, and his requirements shaped the direction of the language development. Through to 1972, richer types were added to the NB language: NB had arrays of int
and char
. Pointers, the ability to generate pointers to other types, arrays of all types, and types to be returned from functions were all also added. Arrays within expressions became pointers. A new compiler was written, and the language was renamed C.
The C compiler and some utilities made with it were included in Version 2 Unix, which is also known as Research Unix.
Structures and the Unix kernel re-write
At Version 4 Unix, released in November 1973, the Unix kernel was extensively re-implemented in C. By this time, the C language had acquired some powerful features such as struct
types.
The preprocessor was introduced around 1973 at the urging of Alan Snyder and also in recognition of the usefulness of the file-inclusion mechanisms available in BCPL and PL/I. Its original version provided only included files and simple string replacements: #include
and #define
of parameterless macros. Soon after that, it was extended, mostly by Mike Lesk and then by John Reiser, to incorporate macros with arguments and conditional compilation
In computer programming, conditional compilation is a compilation technique which results in an executable program that is able to be altered by changing specified parameters. This technique is commonly used when these alterations to the program a ...
.
Unix was one of the first operating system kernels implemented in a language other than assembly
Assembly may refer to:
Organisations and meetings
* Deliberative assembly, a gathering of members who use parliamentary procedure for making decisions
* General assembly, an official meeting of the members of an organization or of their representa ...
. Earlier instances include the Multics system (which was written in PL/I) and Master Control Program
The MCP (Master Control Program) is the operating system of the Burroughs small, medium and large systems, including the Unisys Clearpath/MCP systems.
MCP was originally written in 1961 in ESPOL (Executive Systems Problem Oriented Language). In ...
(MCP) for the Burroughs B5000 (which was written in ALGOL) in 1961. In around 1977, Ritchie and Stephen C. Johnson
Stephen Curtis Johnson (b. 1944; known as Steve Johnson) is a computer scientist who worked at Bell Labs and Old AT&T, AT&T for nearly 20 years. He is best known for Yacc, Lint (software), Lint, spell (Unix), spell, and the Portable C Compiler, w ...
made further changes to the language to facilitate portability
Portability may refer to:
*Portability (social security), the portability of social security benefits
* Porting, the ability of a computer program to be ported from one system to another in computer science
** Software portability, the portability ...
of the Unix operating system. Johnson's Portable C Compiler served as the basis for several implementations of C on new platforms. (Note: The PDF is an OCR scan of the original, and contains a rendering of "IBM 370" as "IBM 310".)
K&R C
In 1978, Brian Kernighan and Dennis Ritchie
Dennis MacAlistair Ritchie (September 9, 1941 – October 12, 2011) was an American computer scientist. He is most well-known for creating the C programming language and, with long-time colleague Ken Thompson, the Unix operating system and B p ...
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". As this was released in 1978, it is also referred to as ''C78''.
The second edition of the book covers the later ANSI C standard, described below.
''K&R'' introduced several language features:
* C file input/output, Standard I/O library
* long int
In computer science, an integer is a data, datum of integral data type, a data type that represents some interval (mathematics), range of mathematical integers. Integral data types may be of different sizes and may or may not be allowed to contain ...
data type
* unsigned int
data type
* Compound assignment operators of the form =''op''
(such as =-
) were changed to the form ''op''=
(that is, -=
) to remove the semantic ambiguity created by constructs such as i=-10
, which had been interpreted as i =- 10
(decrement i
by 10) instead of the possibly intended i = -10
(let i
be −10).
Even after the publication of the 1989 ANSI standard, for many years K&R C was still considered the " lowest common denominator" to which C programmers restricted themselves when maximum portability was desired, since many older compilers were still in use, and because carefully written K&R C code can be legal Standard C as well.
In early versions of C, only functions that return types other than int
must be declared if used before the function definition; functions used without prior declaration were presumed to return type int
.
For example:
long some_function(); /* This is a function declaration, so the compiler can know the name and return type of this function. */
/* int */ other_function(); /* Another function declaration. There is an implicit 'int' type here since we're talking about early version of C. It's commented out here to show where it could go in later variants. */
/* int */ calling_function() /* this is a function definition, including the body of the code following in the the return type is 'int', but this is implicit so no need to state 'int' when using this early version of C */
The int
type specifiers which are commented out could be omitted in K&R C, but are required in later standards.
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 or types of arguments. Separate tools such as Unix's lint
Lint may refer to:
* Fibrous coat of thick hairs covering the seeds of the cotton plant
* Lint (material), an accumulation of fluffy fibers that collect on fabric
Places
* Lint, Belgium, a municipality located in Antwerp, Belgium
* Linț, a vill ...
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 language, supported by compilers from AT&T (in particular PCC
PCC may refer to:
Science and technology
* Pearson correlation coefficient (''r''), in statistics
* Periodic counter-current chromatography, a type of affinity chromatography
* Portable C Compiler, an early compiler for the C programming language
...
) and some other vendors. These included:
* void
functions (i.e., functions with no return value)
* functions returning struct
In computer science, a record (also called a structure, struct, or compound data) is a basic data structure. Records in a database or spreadsheet are usually called "rows".
A record is a collection of '' fields'', possibly of different data typ ...
or union
types (previously only a single pointer, integer or float could be returned)
* assignment for struct
data types
* enumerated types (previously, preprocessor definitions for integer fixed values were used, e.g. #define GREEN 3
)
The large number of extensions and lack of agreement on a standard library
In computer programming, a standard library is the library made available across implementations of a programming language. These libraries are conventionally described in programming language specifications; however, contents of a language's as ...
, 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 and 1980s, versions of C were implemented for a wide variety of mainframe computer
A mainframe computer, informally called a mainframe or big iron, is a computer used primarily by large organizations for critical applications like bulk data processing for tasks such as censuses, industry and consumer statistics, enterpris ...
s, minicomputer
A minicomputer, or colloquially mini, is a class of smaller general purpose computers that developed in the mid-1960s and sold at a much lower price than mainframe and mid-size computers from IBM and its direct competitors. In a 1970 survey, ...
s, and microcomputer
A microcomputer is a small, relatively inexpensive computer having a central processing unit (CPU) made out of a microprocessor. The computer also includes memory and input/output (I/O) circuitry together mounted on a printed circuit board (PC ...
s, including the IBM PC
The IBM Personal Computer (model 5150, commonly known as the IBM PC) is the first microcomputer released in the IBM PC model line and the basis for the IBM PC compatible de facto standard. Released on August 12, 1981, it was created by a team ...
, as its popularity began to increase significantly.
In 1983, the American National Standards Institute
The American National Standards Institute (ANSI ) is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organi ...
(ANSI) formed a committee, X3J11, to establish 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 basis for the 1988 POSIX standard. In 1989, the C standard was ratified as ANSI X3.159-1989 "Programming Language C". This version of the language is often referred to as ANSI C, Standard C, or sometimes C89.
In 1990, the ANSI C standard (with formatting changes) was adopted by the International Organization for Standardization (ISO) as ISO/IEC 9899:1990, which is sometimes called C90. Therefore, the terms "C89" and "C90" refer to the same programming language.
ANSI, like other national standards bodies, no longer develops the C standard independently, but defers to the international C standard, maintained by the working group ISO/IEC JTC1/SC22 ISO/IEC JTC 1/SC 22 Programming languages, their environments and system software interfaces is a standardization subcommittee of the Joint Technical Committee ISO/IEC JTC 1 of the International Organization for Standardization (ISO) and the Inte ...
/WG14. National adoption of an update to the international standard typically occurs within a year of ISO publication.
One of the aims of the C standardization process was to produce a superset of K&R C, incorporating many of the subsequently introduced unofficial features. The standards committee also included several additional features such as function prototypes (borrowed from C++), void
pointers, support for international character sets and locales, and preprocessor enhancements. Although the syntax
In linguistics, syntax () is the study of how words and morphemes combine to form larger units such as phrases and sentences. Central concerns of syntax include word order, grammatical relations, hierarchical sentence structure ( constituency) ...
for parameter declarations was augmented to include the style used in C++, the K&R interface continued to be permitted, for compatibility with existing source code.
C89 is supported by current C compilers, and most modern C code 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
The GUI ( "UI" by itself is still usually pronounced . or ), graphical user interface, is a form of user interface that allows users to interact with electronic devices through graphical icons and audio indicator such as primary notation, inste ...
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 to prevent the use on a K&R C-based compiler of features available only in Standard C.
After the ANSI/ISO standardization process, the C language specification remained relatively static for several years. In 1995, Normative Amendment 1 to the 1990 C standard (ISO/IEC 9899/AMD1:1995, known informally as C95) was published, to correct some details and to add more extensive support for international character sets.
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.
C99 introduced several new features, including inline functions, several new data type
In computer science and computer programming, a data type (or simply type) is a set of possible values and a set of allowed operations on it. A data type tells the compiler or interpreter how the programmer intends to use the data. Most progra ...
s (including long long int
and a complex
type to represent complex numbers), variable-length arrays and flexible array members, improved support for IEEE 754
The IEEE Standard for Floating-Point Arithmetic (IEEE 754) is a technical standard for floating-point arithmetic established in 1985 by the Institute of Electrical and Electronics Engineers (IEEE). The standard addressed many problems found i ...
floating point, support for variadic macros (macros of variable arity
Arity () is the number of arguments or operands taken by a function, operation or relation in logic, mathematics, and computer science. In mathematics, arity may also be named ''rank'', but this word can have many other meanings in mathematics. In ...
), and support for one-line comments beginning with //
, as in BCPL
BCPL ("Basic Combined Programming Language") is a procedural, imperative, and structured programming language. Originally intended for writing compilers for other languages, BCPL is no longer in common use. However, its influence is still ...
or C++. Many of these had already been implemented as extensions in several C compilers.
C99 is for the most part backward compatible with C90, but is stricter in some ways; in particular, a declaration that lacks a type specifier no longer has int
implicitly assumed. A standard macro __STDC_VERSION__
is defined with value 199901L
to indicate that C99 support is available. GCC, Solaris Studio, and other C compilers now support many or all of the new features of C99. The C compiler in Microsoft Visual C++, however, implements the C89 standard and those parts of C99 that are required for compatibility with C++11.
In addition, the standard requires support for Unicode identifiers (variable / function names) in the form of escaped characters (e.g. ) and suggests support for raw Unicode names.
C11
In 2007, work began on another revision of the C standard, informally called "C1X" until its official publication of ISO/IEC 9899:2011 on 2011-12-08. The C standards committee adopted guidelines to limit the adoption of new features that had not been tested by existing implementations.
The C11 standard adds numerous new features to C and the library, including type generic macros, anonymous structures, improved Unicode support, atomic operations, multi-threading, and bounds-checked functions. It also makes some portions of the existing C99 library optional, and improves compatibility with C++. The standard macro __STDC_VERSION__
is defined as 201112L
to indicate that C11 support is available.
C17
Published in June 2018 as ISO/IEC 9899:2018, 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 (after C17) major C language standard revision. It is expected to be voted on in 2023 and would therefore be called C23.
Embedded C
Historically, embedded C programming requires nonstandard extensions to the C language in order to support exotic features such as fixed-point arithmetic
In computing, fixed-point is a method of representing fractional (non-integer) numbers by storing a fixed number of digits of their fractional part. Dollar amounts, for example, are often stored with exactly two fractional digits, representi ...
, multiple distinct memory banks, and basic I/O operations.
In 2008, the C Standards Committee published a technical report
A technical report (also scientific report) is a document that describes the process, progress, or results of technical or scientific research or the state of a technical or scientific research problem. It might also include recommendations and co ...
extending the C language to address these issues by providing a common standard for all implementations to adhere to. It includes a number of features not available in normal C, such as fixed-point arithmetic
In computing, fixed-point is a method of representing fractional (non-integer) numbers by storing a fixed number of digits of their fractional part. Dollar amounts, for example, are often stored with exactly two fractional digits, representi ...
, named address spaces, and basic I/O hardware addressing.
Syntax
C has a formal grammar specified by the C standard. Contains a BNF grammar for C. Line endings are generally not significant in C; however, line boundaries do have significance during the preprocessing phase. Comments may appear either between the delimiters /*
and */
, or (since C99) following //
until the end of the line. Comments delimited by /*
and */
do not nest, and these sequences of characters are not interpreted as comment delimiters if they appear inside string
String or strings may refer to:
*String (structure), a long flexible structure made from threads twisted together, which is used to tie, bind, or hang other objects
Arts, entertainment, and media Films
* ''Strings'' (1991 film), a Canadian anim ...
or character literals.
C source files contain 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
specify built-in types. Sections of code are enclosed in braces (
, sometimes called "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 actions. The most common statement is an ''expression statement'', consisting of an expression to be evaluated, followed by a semicolon; as a side effect of the evaluation, functions may be called and variables may be assigned new values. To modify the normal sequential execution of statements, C provides several control-flow statements identified by reserved keywords. Structured programming
Structured programming is a programming paradigm aimed at improving the clarity, quality, and development time of a computer program by making extensive use of the structured control flow constructs of selection ( if/then/else) and repetition ( ...
is supported by if
... else
">code>elseconditional execution and by do
... 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 be used to leave the innermost enclosing loop statement or skip to its reinitialization. There is also a non-structured goto
GoTo (goto, GOTO, GO TO or other case combinations, depending on the programming language) is a statement found in many computer programming languages. It performs a one-way transfer of control to another line of code; in contrast a function ca ...
statement which branches directly to the designated label within the function. switch
selects a case
to be executed based on the value of an integer expression.
Expressions can use a variety of built-in operators and may contain function calls. The order in which arguments to functions and operands to most operators are evaluated is unspecified. The evaluations may even be interleaved. However, all side effects (including storage to variables) will occur before the next " sequence point"; sequence points include the end of each expression statement, and the entry to and return from each function call. Sequence points also occur during evaluation of expressions containing certain operators (&&
, , ,
, ?:
and the comma operator). This permits a high degree of object code optimization by the compiler, but requires C programmers to take more 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." The C standard did not attempt to correct many of these blemishes, because of the impact of such changes on already existing software.
Character set
The basic C source character set includes the following characters: * Lowercase and uppercase letters of ISO Basic Latin Alphabet:a
–z
A
–Z
* Decimal digits: 0
–9
* Graphic characters: ! " # % & ' ( ) * + , - . / : ; < = > ? \ ^ _ ~
* Whitespace characters: '' space'', '' horizontal tab'', ''\uXXXX
or \UXXXXXXXX
encoding (where the X
denotes a hexadecimal character), although this feature is not yet widely implemented.
The basic C execution character set contains the same characters, along with representations for alert, Reserved words
C89 has 32 reserved words, also known as 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
* do
* double
A double is a look-alike or doppelgänger; one person or being that resembles another.
Double, The Double or Dubble may also refer to:
Film and television
* Double (filmmaking), someone who substitutes for the credited actor of a character
* Th ...
* else
* enum
* extern
* float
* for
For or FOR may refer to:
English language
*For, a preposition
*For, a complementizer
*For, a grammatical conjunction
Science and technology
* Fornax, a constellation
* for loop, a programming language statement
* Frame of reference, in physic ...
* goto
GoTo (goto, GOTO, GO TO or other case combinations, depending on the programming language) is a statement found in many computer programming languages. It performs a one-way transfer of control to another line of code; in contrast a function ca ...
* if
* int
* long
Long may refer to:
Measurement
* Long, characteristic of something of great duration
* Long, characteristic of something of great length
* Longitude (abbreviation: long.), a geographic coordinate
* Longa (music), note value in early music mensur ...
* register
* return
* short
Short may refer to:
Places
* Short (crater), a lunar impact crater on the near side of the Moon
* Short, Mississippi, an unincorporated community
* Short, Oklahoma, a census-designated place
People
* Short (surname)
* List of people known as ...
* signed
* sizeof
* static
* struct
In computer science, a record (also called a structure, struct, or compound data) is a basic data structure. Records in a database or spreadsheet are usually called "rows".
A record is a collection of '' fields'', possibly of different data typ ...
* switch
* typedef
* union
* unsigned
Unsigned can refer to:
* An unsigned artist is a musical artist or group not attached or signed to a record label
** Unsigned Music Awards, ceremony noting achievements of unsigned artists
** Unsigned band web, online community
* Similarly, the c ...
* void
* volatile
* while
C99 reserved five more words:
* _Bool
* _Complex
Complex commonly refers to:
* Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe
** Complex system, a system composed of many components which may interact with each ...
* _Imaginary
* inline
* restrict
C11 reserved seven more words:
* _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 language. Some standard headers do define more convenient synonyms for underscored identifiers. The language previously included a reserved word called entry
, but this was seldom implemented, and has now been removed as a reserved word.
Operators
C supports a rich set of operators, which are symbols used within an expression to specify the manipulations to be performed while evaluating that expression. C has operators for: *+
, -
, *
, /
, %
* assignment: =
* augmented assignment:
* ~
, &
, ,
, ^
* bitwise shifts: <<
, >>
* !
, &&
, , ,
* conditional evaluation: ? :
* equality testing:
, !=
* calling functions: ( )
* increment and decrement: ++
, --
* member selection: .
, ->
* object size: sizeof
* order relations: <
, <=
, >
, >=
* reference and dereference: &
, *
, /code>
* sequencing: ,
* subexpression grouping: ( )
* type conversion: (''typename'')
C uses the operator =
(used in mathematics to express equality) to indicate assignment, following the precedent of Fortran and PL/I, but unlike ALGOL and its derivatives. C uses the operator
to test for equality. The similarity between these two operators (assignment and equality) may result in the accidental use of one in place of the other, and in many cases, the mistake does not produce an error message (although some compilers produce warnings). For example, 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 zero after the assignment.
The C operator precedence is not always intuitive. For example, the operator
binds more tightly than (is executed prior to) the operators &
(bitwise AND) and ,
(bitwise OR) in expressions such as x & 1 0
, which must be written as (x & 1) 0
if that is the coder's intent.
"Hello, world" example
The " hello, world" example, which appeared in the first edition of '' K&R'', has become the model for an introductory program 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:
main()
A standard-conforming "hello, world" program is:
#include
int main(void)
The first line of the program contains a preprocessing directive, indicated by #include
. This causes the compiler to replace that line with the entire text of the stdio.h
The C programming language provides many standard library functions for file input and output. These functions make up the bulk of the C standard library header . The functionality descends from a "portable I/O package" written by Mike Lesk ...
standard header, which contains declarations for standard input and output functions such as printf
and scanf
. The angle brackets surrounding stdio.h
indicate that stdio.h
can be 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 next line indicates that a function named main
is being defined. The main
function serves a special purpose in C programs; the run-time environment calls the main
function to begin program execution. The type specifier int
indicates that the value that is returned to the invoker (in this case the run-time environment) as a result of evaluating the main
function, is an integer. The keyword void
as a parameter list indicates that this function takes no arguments.
The opening curly brace indicates the beginning 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 call, the printf
function is ''passed'' (provided with) a single argument, the address of the first character in the string literal "hello, world\n"
. The string literal is an unnamed array with elements of type char
, set up automatically by the compiler with a final 0-valued character to mark the end of the array (printf
needs to know this). The \n
is an '' escape sequence'' that C translates to a ''newline
Newline (frequently called line ending, end of line (EOL), next line (NEL) or line break) is a control character or sequence of control characters in character encoding specifications such as ASCII, EBCDIC, Unicode, etc. This character, or a ...
'' character, which on output signifies the end of the current line. The return value of the printf
function is of type int
, but it is silently discarded since it is not used. (A more careful program might test the return value to determine whether or not the printf
function succeeded.) The semicolon ;
terminates the statement.
The closing curly brace indicates the end of the code for the main
function. According to the C99 specification and newer, the main
function, unlike any other function, will implicitly return a value of 0
upon reaching the }
that terminates the function. (Formerly an explicit return 0;
statement was required.) This is interpreted by the run-time system as an exit code indicating successful execution.
Data types
The type system in C is static and weakly typed
In computer programming, one of the many ways that programming languages are colloquially classified is whether the language's type system makes it strongly typed or weakly typed (loosely typed). However, there is no precise technical definition o ...
, which makes it similar to the type system of ALGOL descendants such as Pascal
Pascal, Pascal's or PASCAL may refer to:
People and fictional characters
* Pascal (given name), including a list of people with the name
* Pascal (surname), including a list of people and fictional characters with the name
** Blaise Pascal, Fren ...
. There are built-in types for integers of various sizes, both signed and unsigned, floating-point numbers, and enumerated types (enum
). Integer type char
is often used for single-byte characters. C99 added a boolean datatype. There are also derived types including arrays, pointers, records (struct
In computer science, a record (also called a structure, struct, or compound data) is a basic data structure. Records in a database or spreadsheet are usually called "rows".
A record is a collection of '' fields'', possibly of different data typ ...
), and unions (union
).
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 data object in some other way.
Some find C's declaration syntax unintuitive, particularly for function pointers. (Ritchie's idea was to declare identifiers in contexts resembling their use: "declaration reflects use
In the C programming language, data types constitute the semantics and characteristics of storage of data elements. They are expressed in the language syntax in form of declarations for memory locations or variables. Data types also determin ...
".)
C's ''usual arithmetic conversions'' allow for efficient code to be generated, but 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 use of pointers, a type of reference that records the address or location of an object or function in memory. Pointers can be ''dereferenced'' to access data stored at the address pointed to, or to invoke a pointed-to function. Pointers can be manipulated using assignment or pointer arithmetic. The run-time representation of a pointer value is typically a raw memory address (perhaps augmented by an offset-within-word field), 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 purposes in C. Text strings are commonly manipulated using pointers into arrays of characters. Dynamic memory allocation is performed using pointers; the result of a malloc
is usually cast to the data type of the data to be stored. Many data types, such as trees, are commonly implemented as dynamically allocated struct
objects linked together using pointers. Pointers to other pointers are often used in multi-dimensional arrays and arrays of struct
objects. Pointers to functions ('' function pointers'') are useful for passing functions as arguments to higher-order function
In mathematics and computer science, a higher-order function (HOF) is a function that does at least one of the following:
* takes one or more functions as arguments (i.e. a procedural parameter, which is a parameter of a procedure that is itself ...
s (such as qsort or bsearch
In computer science, binary search, also known as half-interval search, logarithmic search, or binary chop, is a search algorithm that finds the position of a target value within a sorted array. Binary search compares the target value to the ...
), in dispatch tables, or as callbacks
In computer programming, a callback or callback function is any reference to executable code that is passed as an argument to another piece of code; that code is expected to ''call back'' (execute) the callback function as part of its job. Thi ...
to event handlers .
A ''null pointer
In computing, a null pointer or null reference is a value saved for indicating that the pointer or reference does not refer to a valid object. Programs routinely use null pointers to represent conditions such as the end of a list of unknown lengt ...
value'' explicitly points to no valid location. Dereferencing a null pointer value is undefined, often resulting in a segmentation fault. Null pointer values are useful for indicating special cases such as no "next" pointer in the final node of a linked list
In computer science, a linked list is a linear collection of data elements whose order is not given by their physical placement in memory. Instead, each element points to the next. It is a data structure consisting of a collection of nodes whic ...
, or as an error indication from functions returning pointers. In appropriate contexts in source code, such as for assigning to a pointer variable, a ''null pointer constant'' can be written as 0
, with or without explicit casting to a pointer type, or as the NULL
macro defined by several standard headers. In conditional contexts, null pointer values evaluate to false, while all other pointer values evaluate to true.
Void pointers (void *
) point to objects of unspecified type, and can therefore be used 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 allowed, although they can easily be (and in many contexts implicitly are) converted to and from any other object pointer type.
Careless use of pointers is potentially dangerous. Because they are typically unchecked, a pointer variable can be made to point to any arbitrary location, which can cause undesirable effects. Although properly used pointers point to safe 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 after deallocation ( dangling pointers); they may be used without having been initialized ( wild pointers); or they may be directly assigned an unsafe value using a cast, union, or through another corrupt pointer. In general, C is permissive in allowing manipulation of and conversion between pointer types, although compilers typically provide options for various levels of checking. Some other programming languages address these problems by using more restrictive reference types.
Arrays
Array types in C are traditionally of a fixed, static size specified at compile time. The more recent C99 standard also allows a form of variable-length arrays. However, it is also possible to allocate a block of memory (of arbitrary size) at run-time, using the standard library's malloc
function, and treat it as an array.
Since arrays are always accessed (in effect) via pointers, array accesses are typically ''not'' checked against the underlying array size, although some compilers may provide bounds checking as an option.For example, gcc provides _FORTIFY_SOURCE. Array bounds violations are therefore possible and can lead to various repercussions, including illegal memory accesses, corruption of data, buffer overruns
In information security and programming, a buffer overflow, or buffer overrun, is an anomaly whereby a program, while writing data to a buffer, overruns the buffer's boundary and overwrites adjacent memory locations.
Buffers are areas of memory ...
, and run-time exceptions.
C does not have a special provision for declaring multi-dimensional array
In computer science, array is a data type that represents a collection of ''elements'' (values or variables), each selected by one or more indices (identifying keys) that can be computed at run time during program execution. Such a collection i ...
s, 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 "multi-dimensional array" can be thought of as increasing in row-major order
In computing, row-major order and column-major order are methods for storing multidimensional arrays in linear storage such as random access memory.
The difference between the orders lies in which elements of an array are contiguous in memory. In ...
. Multi-dimensional arrays are commonly used in numerical algorithms (mainly from applied linear algebra) to store matrices. The structure of the C array is well suited to this particular task. However, in early versions of C the bounds of the array must be known fixed values or else explicitly passed to any subroutine that requires them, and dynamically sized arrays of arrays cannot be accessed using double indexing. (A workaround for this was to allocate the array with an additional "row vector" of pointers to the columns.) C99 introduced "variable-length arrays" which address this issue.
The following example using modern C (C99 or later) shows allocation of a two-dimensional array on the heap and the use of multi-dimensional array indexing for accesses (which can use bounds-checking on many C compilers):
int func(int N, int M)
And here is a similar implementation using C99's ''Auto VLA'' feature:
int func(int N, int M)
Array–pointer interchangeability
The subscript notation x /code> (where x
designates a pointer) is syntactic sugar for *(x+i)
. Taking advantage of the compiler's knowledge of the pointer type, the address that x + i
points to is not the base address (pointed to by x
) incremented by i
bytes, but rather is defined to be the base address incremented by i
multiplied by the size of an element that x
points to. Thus, x /code> designates the i+1
th 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 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 function calls in C use pass-by-value semantics, arrays are in effect passed by reference.
The total size of an array x
can be determined by applying sizeof
to an expression of array type. The size of an element can be determined by applying the operator sizeof
to any dereferenced element of an array A
, as in n = sizeof A /code>. Thus, the number of elements in a declared array A
can be determined as sizeof A / sizeof A /code>. Note, that if only a pointer to the first element is available as it is often the case in C code because of the automatic conversion described above, the information about the full type of the array and its length are lost.
Memory management
One of the most important functions of a programming language is to provide facilities for managing memory and the objects that are stored in memory. C provides three principal 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 (or lifetime) as long as the binary which contains them is loaded into memory.
* Automatic memory allocation
__NOTOC__
In computer programming, an automatic variable is a local variable which is allocated and deallocated automatically when program flow enters and leaves the variable's scope. The scope is the lexical context, particularly the function or b ...
: temporary objects can be stored on the stack
Stack may refer to:
Places
* Stack Island, an island game reserve in Bass Strait, south-eastern Australia, in Tasmania’s Hunter Island Group
* Blue Stack Mountains, in Co. Donegal, Ireland
People
* Stack (surname) (including a list of people ...
, 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
C dynamic memory allocation refers to performing manual memory management for dynamic memory allocation in the C programming language via a group of functions in the C standard library, namely , , , and .
The C++ programming language includes ...
from a region of memory called the heap
Heap or HEAP may refer to:
Computing and mathematics
* Heap (data structure), a data structure commonly used to implement a priority queue
* Heap (mathematics), a generalization of a group
* Heap (programming) (or free store), an area of memory f ...
; these blocks persist until subsequently freed for reuse by calling the library function realloc
C dynamic memory allocation refers to performing manual memory management for dynamic memory allocation in the C programming language via a group of functions in the C standard library, namely , , , and .
The C++ programming language includes ...
or free
Free may refer to:
Concept
* Freedom, having the ability to do something, without having to obey anyone/anything
* Freethought, a position that beliefs should be formed only on the basis of logic, reason, and empiricism
* Emancipate, to procur ...
These three approaches are appropriate in different situations and have various trade-offs. For example, static memory allocation has little allocation overhead, automatic allocation may involve slightly more overhead, and dynamic memory allocation can potentially have a great deal of overhead for both allocation and deallocation. The persistent nature of static objects is useful for maintaining state information across function calls, automatic allocation is easy to use but stack space is typically much more limited and transient than either static memory or heap space, and dynamic memory allocation allows convenient 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 simplest because the storage is managed by the compiler, freeing the programmer of the potentially error-prone chore of manually allocating and releasing storage. However, many data structures can change in size at runtime, and since static allocations (and automatic allocations before C99) must have a fixed size at compile-time, there are many situations in which dynamic allocation is necessary. Prior to the C99 standard, variable-sized arrays were a common example of this. (See the article on malloc
C dynamic memory allocation refers to performing manual memory management for dynamic memory allocation in the C programming language via a group of functions in the C standard library, namely , , , and .
The C++ programming language includes ...
for an example of dynamically allocated arrays.) Unlike automatic allocation, which can fail at run time with uncontrolled consequences, the dynamic allocation functions return an indication (in the form of a null pointer value) when the required storage cannot be allocated. (Static allocation that is too large is usually detected by the linker or loader
Loader can refer to:
* Loader (equipment)
* Loader (computing)
** LOADER.EXE, an auto-start program loader optionally used in the startup process of Microsoft Windows ME
* Loader (surname)
* Fast loader
* Speedloader
* Boot loader
** LOADER.COM ...
, before the program can even begin execution.)
Unless otherwise specified, static objects contain zero or null pointer values upon program startup. Automatically and dynamically allocated objects are initialized only if an initial value is explicitly specified; otherwise they initially have indeterminate values (typically, whatever bit pattern
The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represented a ...
happens to be present in the storage
Storage may refer to:
Goods Containers
* Dry cask storage, for storing high-level radioactive waste
* Food storage
* Intermodal container, cargo shipping
* Storage tank
Facilities
* Garage (residential), a storage space normally used to store car ...
, which might not even represent a valid value for that type). If the program attempts to access an uninitialized value, the results are undefined. Many modern compilers try to detect and warn about this problem, but both false positives and false negatives can occur.
Heap memory allocation has to be synchronized with its actual usage in any program to be reused as much as possible. For example, if the only pointer to a heap memory allocation goes out of scope or has its value overwritten before it is deallocated explicitly, then that 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 memory to be freed, but is referenced subsequently, leading to unpredictable results. Typically, the failure symptoms appear in a portion of the program unrelated to the code that causes the error, making it difficult to diagnose the failure. Such issues are ameliorated in languages with automatic garbage collection
In computer science, garbage collection (GC) is a form of automatic memory management. The ''garbage collector'' attempts to reclaim memory which was allocated by the program, but is no longer referenced; such memory is called ''garbage''. G ...
.
Libraries
The C programming language uses libraries as its primary method of extension. In C, a library is a set of functions contained within a single "archive" file. Each library typically has a header file, which contains the prototypes of the functions 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, it must include the library's header file, and the library must be linked with the program, which in many cases requires compiler flag
A command-line interpreter or command-line processor uses a command-line interface (CLI) to receive commands from a user in the form of lines of text. This provides a means of setting parameters for the environment, invoking executables and pro ...
s (e.g., -lm
, shorthand for "link the math library").
The most common C library is the C standard library
The C standard library or libc is the standard library for the C programming language, as specified in the ISO C standard.ISO/IEC (2018). '' ISO/IEC 9899:2018(E): Programming Languages - C §7'' Starting from the original ANSI C standard, it wa ...
, which is specified by the ISO and ANSI C standards and comes with every C implementation (implementations which target limited environments such as embedded systems may provide only a subset of the standard library). This library supports stream input and output, memory 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.
Another common set of C library functions are those used by applications specifically targeted for Unix and Unix-like systems, especially functions which provide an interface to the kernel. These functions are detailed in various standards such as POSIX and the Single UNIX Specification.
Since many programs have been written in C, there are a wide variety of other libraries available. Libraries are often written in C because C compilers generate efficient object code; programmers then create interfaces to the library so that the routines can be used from higher-level languages like Java, Perl, and Python.
File handling and streams
File input and output (I/O) is not part of the C language itself but instead is handled by libraries (such as the C standard library) and their associated header files (e.g. stdio.h
). File handling is generally implemented through high-level I/O which works through streams
A stream is a continuous body of water, body of surface water Current (stream), flowing within the stream bed, bed and bank (geography), banks of a channel (geography), channel. Depending on its location or certain characteristics, a stream ...
. A stream is from this perspective a data flow that is independent of devices, while a file is a concrete device. The high-level I/O is done through the association of a stream to a file. In the C standard library, a buffer (a memory area or queue) is temporarily used to store data before it is sent to the final destination. This reduces the time spent waiting for slower devices, for example a hard drive or solid state drive
A solid-state drive (SSD) is a solid-state storage device that uses integrated circuit assemblies to store data Persistence (computer science), persistently, typically using flash memory, and functioning as secondary storage in the Computer ...
. Low-level I/O functions are not part of the standard C library but are generally part of "bare metal" programming (programming that's independent of any operating system such as most embedded programming
Embedded software is computer software, written to control machines or devices that are not typically thought of as computers, commonly known as embedded systems. It is typically specialized for the particular hardware that it runs on and has time ...
). With few exceptions, implementations include low-level I/O.
Language tools
A number of tools have been developed to help C programmers find and fix statements with undefined behavior or possibly erroneous expressions, with greater rigor than that provided by the compiler. The tool lint
Lint may refer to:
* Fibrous coat of thick hairs covering the seeds of the cotton plant
* Lint (material), an accumulation of fluffy fibers that collect on fabric
Places
* Lint, Belgium, a municipality located in Antwerp, Belgium
* Linț, a vill ...
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 as Lint
Lint may refer to:
* Fibrous coat of thick hairs covering the seeds of the cotton plant
* Lint (material), an accumulation of fluffy fibers that collect on fabric
Places
* Lint, Belgium, a municipality located in Antwerp, Belgium
* Linț, a vill ...
. 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. Also, many compilers can optionally warn about syntactically valid constructs that are likely to actually be errors. MISRA C is a proprietary set of guidelines to avoid such questionable code, developed for embedded systems.
There are also compilers, libraries, and operating system level mechanisms for performing actions that are not a standard part of C, such as bounds checking for arrays, detection of buffer overflow, serialization
In computing, serialization (or serialisation) is the process of translating a data structure or object state into a format that can be stored (e.g. files in secondary storage devices, data buffers in primary storage devices) or transmitted (e ...
, dynamic memory
Memory management is a form of resource management applied to computer memory. The essential requirement of memory management is to provide ways to dynamically allocate portions of memory to programs at their request, and free it for reuse when ...
tracking, and automatic garbage collection
In computer science, garbage collection (GC) is a form of automatic memory management. The ''garbage collector'' attempts to reclaim memory which was allocated by the program, but is no longer referenced; such memory is called ''garbage''. G ...
.
Tools such as Purify or Valgrind and linking with libraries containing special versions of the memory allocation functions can help uncover runtime errors in memory usage.
Uses
Rationale for use in systems programming
C is widely used for systems programming in implementing operating systems and embedded system applications. This is for several reasons:
* The code generated after compilation doesn't demand many system features, and can be invoked from some boot code in a straightforward manner – it's simple to execute.
* The C language statements and expressions typically map well on to sequences of instructions for the target processor, and consequently there is a low run-time demand on system resources – it's fast to execute.
* With its rich set of operators, the C language can utilise many of the features of target CPUs. Where a particular CPU has more esoteric instructions, a language variant can be constructed with perhaps intrinsic functions to exploit those instructions – it can use practically all the target CPU's features.
* The language makes it easy to overlay structures onto blocks of binary data, allowing the data to be comprehended, navigated and modified – it can write data structures, even file systems.
* The language supports a rich set of operators, including bit manipulation, for integer arithmetic and logic, and perhaps different sizes of floating point numbers – it can process appropriately-structured data effectively.
* C is a fairly small language, with only a handful of statements, and without too many features that generate extensive target code – it is comprehensible.
* C has direct control over memory allocation and deallocation, which gives reasonable efficiency and predictable timing to memory-handling operations, without any concerns for sporadic ''stop-the-world
In computer programming, tracing garbage collection is a form of automatic memory management that consists of determining which objects should be deallocated ("garbage collected") by tracing which objects are ''reachable'' by a chain of references ...
'' garbage collection events – it has predictable performance.
* Platform hardware can be accessed with pointers and type punning
In computer science, a type punning is any programming technique that subverts or circumvents the type system of a programming language in order to achieve an effect that would be difficult or impossible to achieve within the bounds of the formal ...
, so system-specific features (e.g. Control/Status Registers, I/O registers) can be configured and used with code written in C – it interacts well with the platform it's running on.
* Depending on the linker and environment, C code can also call libraries written in assembly language
In computer programming, assembly language (or assembler language, or symbolic machine code), often referred to simply as Assembly and commonly abbreviated as ASM or asm, is any low-level programming language with a very strong correspondence be ...
, and may be called from assembly language – it interoperates well with other lower-level code.
* C and its calling conventions and linker structures are commonly used in conjunction with other high-level languages, with calls both to C and from C supported – it interoperates well with other high-level code.
* C has a very mature and broad ecosystem, including libraries, frameworks, open source compilers, debuggers and utilities, and is the de facto standard. It's likely the drivers already exist in C, or that there is a similar CPU architecture as a back-end of a C compiler, so there is reduced incentive to choose another language.
Once used for web development
Historically, C was sometimes used for web development
Web development is the work involved in developing a website for the Internet (World Wide Web) or an intranet (a private network). Web development can range from developing a simple single static page of plain text to complex web applications ...
using the Common Gateway Interface (CGI) as a "gateway" for information between the web application, the server, and the browser. C may have been chosen over interpreted languages because of its speed, stability, and near-universal availability. It is no longer common practice for web development to be done in C, and many other web development tools
Web development tools (often called devtools or inspect element) allow web developers to test and debug their code. They are different from website builders and integrated development environments (IDEs) in that they do not assist in the direct c ...
exist.
Some other languages are themselves written in C
A consequence of C's 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, Ruby, and PHP are written in C.
Used for computationally-intensive libraries
C enables programmers to create efficient implementations of algorithms and data structures, because the layer of abstraction from hardware is thin, and its overhead is low, an important criterion for computationally intensive programs. For example, the GNU Multiple Precision Arithmetic Library, the GNU Scientific Library
The GNU Scientific Library (or GSL) is a software library for numerical computations in applied mathematics and science. The GSL is written in C; wrappers are available for other programming languages. The GSL is part of the GNU Project and is d ...
, Mathematica
Wolfram Mathematica is a software system with built-in libraries for several areas of technical computing that allow machine learning, statistics, symbolic computation, data manipulation, network analysis, time series analysis, NLP, optimizat ...
, and MATLAB are completely or partially written in C. Many languages support calling library functions in C, for example, the Python-based framework NumPy uses C for the high-performance and hardware-interacting aspects.
C as an intermediate language
C is sometimes used as an intermediate language by implementations of other languages. This approach may be used for portability or convenience; by using C as an intermediate language, additional machine-specific code generators are not necessary. C has some features, such as line-number preprocessor directives and optional superfluous commas at the end of initializer lists, that support compilation of generated code. However, some of C's shortcomings have prompted the development of other C-based languages specifically designed for use as intermediate languages, such as C--. Also, contemporary major compilers GCC and LLVM both feature an intermediate representation that is not C, and those compilers support front ends for many languages including C.
End-user applications
C has also been widely used to implement end-user applications. However, such applications can also be written in newer, higher-level languages.
Limitations
While C has been popular, influential and hugely successful, it has drawbacks, including:
* The standard dynamic memory handling with malloc
and free
is error prone. Bugs include: Memory leaks when memory is allocated but not freed; and access to previously freed memory.
* The use of pointers and the direct manipulation of memory means corruption of memory is possible, perhaps due to programmer error, or insufficient checking of bad data.
* There is some type checking, but it does not apply to areas like variadic functions, and the type checking can be trivially or inadvertently circumvented.
* Since the code generated by the compiler contains few checks itself, there is a burden on the programmer to consider all possible outcomes, and protect against buffer overruns, array bounds checking, stack overflows, memory exhaustion, race conditions, thread isolation, etc.
* The use of pointers and the run-time manipulation of these means there may be two ways to access the same data (aliasing), which is not determinable at compile time. This means that some optimisations that may be available to other languages are not possible in C. FORTRAN is considered faster.
* Some of the standard library functions, e.g. scanf
, can lead to buffer overruns.
* There is limited standardisation in support for low-level variants in generated code, for example: different function calling conventions
In computer science, a calling convention is an implementation-level (low-level) scheme for how subroutines or functions receive parameters from their caller and how they return a result. When some code calls a function, design choices have been ...
and ABI; different structure packing conventions; different byte ordering within larger integers (including endianness). In many language implementations, some of these options may be handled with the preprocessor directive #pragma
, and some with additional keywords e.g. use __cdecl
calling convention. But the directive and options are not consistently supported.
* String handling
In computer programming, a string is traditionally a sequence of characters, either as a literal constant or as some kind of variable. The latter may allow its elements to be mutated and the length changed, or it may be fixed (after creation). ...
using the standard library is code-intensive, with explicit memory management required.
* The language does not directly support object orientation, introspection, run-time expression evaluation, generics, etc.
* There are few guards against inappropriate use of language features, which may lead to unmaintainable code. This facility for tricky code has been celebrated with competitions such as the ''International Obfuscated C Code Contest
The International Obfuscated C Code Contest (abbreviated IOCCC) is a computer programming contest for the most creatively obfuscated C code. Held annually, it is described as "celebrating 'ssyntactical opaqueness". The winning code for the 27t ...
'' and the ''Underhanded C Contest
The Underhanded C Contest is a programming contest to turn out code that is malicious, but passes a rigorous inspection, and looks like an honest mistake even if discovered. The contest rules define a task, and a malicious component. Entries m ...
''.
* C lacks standard support for exception handling and only offers return codes for error checking. The setjmp
and longjmp
standard library functions have been used to implement a try-catch mechanism via macros.
For some purposes, restricted styles of C have been adopted, e.g. MISRA C or CERT C, in an attempt to reduce the opportunity for bugs. Databases such as CWE attempt to count the ways C etc. has vulnerabilities, along with recommendations for mitigation.
There are tools
A tool is an object that can extend an individual's ability to modify features of the surrounding environment or help them accomplish a particular task. Although many animals use simple tools, only human beings, whose use of stone tools dates ba ...
that can mitigate against some of the drawbacks. Contemporary C compilers include checks which may generate warnings to help identify many potential bugs.
Some of these drawbacks have prompted the construction of other languages.
Related languages
C has both directly and indirectly influenced many later languages such as C++, C#, D, Go, Java, JavaScript, Perl, PHP, Rust and Unix's C shell. 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, data models, and/or large-scale program structures that differ from those of C, sometimes radically.
Several C or near-C interpreters exist, including Ch and CINT
CINT is a command line C/C++ interpreter that was originally included in the object oriented data analysis package ROOT. Although intended for use with the other faculties of ROOT, CINT can also be used as a standalone addition to another progra ...
, which 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 as source-to-source compilers; source code was translated into C, and then compiled with a C compiler.
The C++ programming language (originally named "C with Classes") was devised by Bjarne Stroustrup as an approach to providing object-oriented functionality with a C-like syntax. C++ adds greater typing strength, scoping, and other tools useful in object-oriented programming, and permits generic programming via templates. Nearly a superset of C, C++ now supports most 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
Smalltalk is an object-oriented, dynamically typed reflective programming language. It was designed and created in part for educational use, specifically for constructionist learning, at the Learning Research Group (LRG) of Xerox PARC by Alan Ka ...
: 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.
See also
* Compatibility of C and C++
* Comparison of Pascal and C
* Comparison of programming languages
* International Obfuscated C Code Contest
The International Obfuscated C Code Contest (abbreviated IOCCC) is a computer programming contest for the most creatively obfuscated C code. Held annually, it is described as "celebrating 'ssyntactical opaqueness". The winning code for the 27t ...
* List of C-based programming languages
* List of C compilers
This page is intended to list all current compilers, compiler generators, interpreters, translators, tool foundations, assemblers, automatable command line interfaces ( shells), etc.
Ada Compilers
ALGOL 60 compilers
ALGOL 68 compilers
cf. ...
Notes
References
Sources
*
** By courtesy of the author, also at
*
*
Further reading
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External links
ISO C Working Group official website
*
ISO/IEC 9899
publicly available official C documents, including the C99 Rationale
**
comp.lang.c Frequently Asked Questions
by Dennis Ritchie
C Library Reference and Examples
{{Authority control
American inventions
Articles with example C code
C programming language family
Cross-platform software
High-level programming languages
Procedural programming languages
Structured programming languages
Programming languages created in 1972
Programming languages with an ISO standard
Statically typed programming languages
Systems programming languages