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C (''pronounced'' '' – like the letter c'') is a
general-purpose programming language In computer software, a general-purpose programming language (GPL) is a programming language for building software in a wide variety of application Domain (software engineering), domains. Conversely, a Domain-specific language, domain-specific pro ...
. It was created in the 1970s by Dennis Ritchie and remains very widely used and influential. By design, C's features cleanly reflect the capabilities of the targeted
CPUs A central processing unit (CPU), also called a central processor, main processor, or just processor, is the primary Processor (computing), processor in a given computer. Its electronic circuitry executes Instruction (computing), instructions ...
. It has found lasting use in
operating system An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ...
s code (especially in kernels),
device driver In the context of an operating system, a device driver is a computer program that operates or controls a particular type of device that is attached to a computer or automaton. A driver provides a software interface to hardware devices, enabli ...
s, and protocol stacks, but its use in
application software Application software is any computer program that is intended for end-user use not operating, administering or programming the computer. An application (app, application program, software application) is any program that can be categorized as ...
has been decreasing. C is commonly used on computer architectures that range from the largest
supercomputer A supercomputer is a type of computer with a high level of performance as compared to a general-purpose computer. The performance of a supercomputer is commonly measured in floating-point operations per second (FLOPS) instead of million instruc ...
s to the smallest
microcontroller A microcontroller (MC, uC, or μC) or microcontroller unit (MCU) is a small computer on a single integrated circuit. A microcontroller contains one or more CPUs (processor cores) along with memory and programmable input/output peripherals. Pro ...
s and
embedded system An embedded system is a specialized computer system—a combination of a computer processor, computer memory, and input/output peripheral devices—that has a dedicated function within a larger mechanical or electronic system. It is e ...
s. A successor to the programming language B, C was originally developed at
Bell Labs Nokia Bell Labs, commonly referred to as ''Bell Labs'', is an American industrial research and development company owned by Finnish technology company Nokia. With headquarters located in Murray Hill, New Jersey, Murray Hill, New Jersey, the compa ...
by Ritchie between 1972 and 1973 to construct utilities running on
Unix Unix (, ; trademarked as UNIX) is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, a ...
. It was applied to re-implementing the kernel of the Unix operating system. During the 1980s, C gradually gained popularity. It has become one of the most widely used
programming language A programming language is a system of notation for writing computer programs. Programming languages are described in terms of their Syntax (programming languages), syntax (form) and semantics (computer science), semantics (meaning), usually def ...
s, with C
compiler In computing, a compiler is a computer program that Translator (computing), translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primaril ...
s available for practically all modern computer architectures and
operating system An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ...
s. The book ''
The C Programming Language ''The C Programming Language'' (sometimes termed ''K&R'', after its authors' initials) is a computer programming book written by Brian Kernighan and Dennis Ritchie, the latter of whom originally designed and implemented the C programming langu ...
'', co-authored by the original language designer, served for many years as the ''de facto'' standard for the language. C has been standardized since 1989 by the
American National Standards Institute The American National Standards Institute (ANSI ) is a private nonprofit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organiz ...
(ANSI) and, subsequently, jointly by the
International Organization for Standardization The International Organization for Standardization (ISO ; ; ) is an independent, non-governmental, international standard development organization composed of representatives from the national standards organizations of member countries. M ...
(ISO) and the
International Electrotechnical Commission The International Electrotechnical Commission (IEC; ) is an international standards organization that prepares and publishes international standards for all electrical, electronics, electronic and related technologies. IEC standards cover a va ...
(IEC). C is an imperative procedural language, supporting
structured programming Structured programming is a programming paradigm aimed at improving the clarity, quality, and development time of a computer program by making specific disciplined use of the structured control flow constructs of selection ( if/then/else) and repet ...
,
lexical variable scope In computer programming, the scope of a name binding (an association of a name to an entity, such as a Variable (programming), variable) is the part of a Computer program, program where the name binding is valid; that is, where the name can be use ...
, and
recursion Recursion occurs when the definition of a concept or process depends on a simpler or previous version of itself. Recursion is used in a variety of disciplines ranging from linguistics to logic. The most common application of recursion is in m ...
, with a static type system. It was designed to be compiled to provide low-level access to
memory Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembe ...
and language constructs that map efficiently to machine instructions, all with minimal runtime support. Despite its low-level capabilities, the language was designed to encourage cross-platform programming. A standards-compliant C program written with portability in mind can be compiled for a wide variety of computer platforms and operating systems with few changes to its source code. Since 2000, C has consistently ranked among the top four languages in the TIOBE index, a measure of the popularity of programming languages.


Overview

C is an imperative, procedural language in the
ALGOL ALGOL (; short for "Algorithmic Language") is a family of imperative computer programming languages originally developed in 1958. ALGOL heavily influenced many other languages and was the standard method for algorithm description used by the ...
tradition. It has a static
type system In computer programming, a type system is a logical system comprising a set of rules that assigns a property called a ''type'' (for example, integer, floating point, string) to every '' term'' (a word, phrase, or other set of symbols). Usu ...
. In C, all executable code is contained within
subroutine In computer programming, a function (also procedure, method, subroutine, routine, or subprogram) is a callable unit of software logic that has a well-defined interface and behavior and can be invoked multiple times. Callable units provide a ...
s (also called "functions", though not in the sense of
functional programming In computer science, functional programming is a programming paradigm where programs are constructed by Function application, applying and Function composition (computer science), composing Function (computer science), functions. It is a declarat ...
). Function parameters are passed by value, although
arrays An array is a systematic arrangement of similar objects, usually in rows and columns. Things called an array include: {{TOC right Music * In twelve-tone and serial composition, the presentation of simultaneous twelve-tone sets such that the ...
are passed as
pointers Pointer may refer to: People with the name * Pointer (surname), a surname (including a list of people with the name) * Pointer Williams (born 1974), American former basketball player Arts, entertainment, and media * ''Pointer'' (journal), the ...
, i.e. the address of the first item in the array. ''Pass-by-reference'' is simulated in C by explicitly passing pointers to the thing being referenced. C program source text is free-form code.
Semicolon The semicolon (or semi-colon) is a symbol commonly used as orthographic punctuation. In the English language, a semicolon is most commonly used to link (in a single sentence) two independent clauses that are closely related in thought, such as ...
s terminate statements, while curly braces are used to group statements into blocks. The C language also exhibits the following characteristics: * The language has a small, fixed number of keywords, including a full set of
control flow In computer science, control flow (or flow of control) is the order in which individual statements, instructions or function calls of an imperative program are executed or evaluated. The emphasis on explicit control flow distinguishes an '' ...
primitives: if/else, for, do/while, while, and
switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type o ...
. 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'' run-time polymorphism. ** Functions may not be defined within the lexical scope of other functions. ** Variables may be defined within a function, with scope. ** A function may call itself, so
recursion Recursion occurs when the definition of a concept or process depends on a simpler or previous version of itself. Recursion is used in a variety of disciplines ranging from linguistics to logic. The most common application of recursion is in m ...
is supported. * Data typing is static, but weakly enforced; all data has a type, but implicit conversions are possible. * User-defined (
typedef typedef is a reserved keyword in the programming languages C, C++, and Objective-C. It is used to create an additional name (''alias'') for another data type, but does not create a new type, except in the obscure case of a qualified typedef of ...
) and compound types are possible. ** Heterogeneous aggregate data types ( struct) allow related data elements to be accessed and assigned as a unit. The contents of whole structs cannot be compared using a single built-in operator (the elements must be compared individually). ** Unions are structures with overlapping members; they allow multiple data types to share the same memory location. ** Array indexing is a secondary notation, defined in terms of pointer arithmetic. Whole arrays cannot be assigned or compared using a single built-in operator. There is no "array" keyword in use or definition; instead, square brackets indicate arrays syntactically, for example month 1/code>. **
Enumerated type In computer programming, an enumerated type (also called enumeration, enum, or factor in the R (programming language), R programming language, a status variable in the JOVIAL programming language, and a categorical variable in statistics) is a data ...
s are possible with the enum keyword. They are freely interconvertible with integers. ** Strings are not a distinct data type, but are conventionally implemented as null-terminated character arrays. * Low-level access to
computer memory Computer memory stores information, such as data and programs, for immediate use in the computer. The term ''memory'' is often synonymous with the terms ''RAM,'' ''main memory,'' or ''primary storage.'' Archaic synonyms for main memory include ...
is possible by converting machine addresses to
pointers Pointer may refer to: People with the name * Pointer (surname), a surname (including a list of people with the name) * Pointer Williams (born 1974), American former basketball player Arts, entertainment, and media * ''Pointer'' (journal), the ...
. * Procedures (subroutines not returning values) are a special case of function, with an empty return type void. * Memory can be allocated to a program with calls to library routines. * A
preprocessor In computer science, a preprocessor (or precompiler) is a Computer program, program that processes its input data to produce output that is used as input in another program. The output is said to be a preprocessed form of the input data, which i ...
performs macro definition,
source code In computing, source code, or simply code or source, is a plain text computer program written in a programming language. A programmer writes the human readable source code to control the behavior of a computer. Since a computer, at base, only ...
file inclusion, and conditional compilation. * There is a basic form of
modularity 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 system into varying ...
: 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 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 system An embedded system is a specialized computer system—a combination of a computer processor, computer memory, and input/output peripheral devices—that has a dedicated function within a larger mechanical or electronic system. It is e ...
s. 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 The C shell (csh or the improved version, tcsh) is a Unix shell created by Bill Joy while he was a graduate student at University of California, Berkeley in the late 1970s. It has been widely distributed, beginning with the 2BSD release of the ...
, D, Go,
Java Java is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea (a part of Pacific Ocean) to the north. With a population of 156.9 million people (including Madura) in mid 2024, proje ...
,
JavaScript JavaScript (), often abbreviated as JS, is a programming language and core technology of the World Wide Web, alongside HTML and CSS. Ninety-nine percent of websites use JavaScript on the client side for webpage behavior. Web browsers have ...
(including transpilers), Julia,
Limbo The unofficial term Limbo (, or , referring to the edge of Hell) is the afterlife condition in medieval Catholic theology, of those who die in original sin without being assigned to the Hell of the Damned. However, it has become the gene ...
, LPC,
Objective-C Objective-C is a high-level general-purpose, object-oriented programming language that adds Smalltalk-style message passing (messaging) to the C programming language. Originally developed by Brad Cox and Tom Love in the early 1980s, it was ...
,
Perl Perl is a high-level, general-purpose, interpreted, dynamic programming language. Though Perl is not officially an acronym, there are various backronyms in use, including "Practical Extraction and Reporting Language". Perl was developed ...
, PHP, Python,
Ruby Ruby is a pinkish-red-to-blood-red-colored gemstone, a variety of the mineral corundum ( aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called sapph ...
,
Rust Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO(OH) ...
,
Swift Swift or SWIFT most commonly refers to: * SWIFT, an international organization facilitating transactions between banks ** SWIFT code * Swift (programming language) * Swift (bird), a family of birds It may also refer to: Organizations * SWIF ...
,
Verilog Verilog, standardized as IEEE 1364, is a hardware description language (HDL) used to model electronic systems. It is most commonly used in the design and verification of digital circuits, with the highest level of abstraction being at the re ...
and SystemVerilog (hardware description languages). These languages have drawn many of their control structures and other basic features from C. Most of them 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 (constituenc ...
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 Unix (, ; trademarked as UNIX) is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, a ...
operating system, originally implemented in
assembly language In computing, assembly language (alternatively 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 bet ...
on a PDP-7 by Dennis Ritchie and
Ken Thompson Kenneth Lane Thompson (born February 4, 1943) is an American pioneer of computer science. Thompson worked at Bell Labs for most of his career where he designed and implemented the original Unix operating system. He also invented the B (programmi ...
, incorporating several ideas from colleagues. Eventually, they decided to port the operating system to a PDP-11. The original PDP-11 version of Unix was also developed in assembly language.


B

Thompson wanted a programming language for developing utilities for the new platform. He first tried writing a Fortran compiler, but he soon gave up the idea and instead created a cut-down version of the recently developed systems programming language called BCPL. 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 ALGOL (; short for "Algorithmic Language") is a family of imperative computer programming languages originally developed in 1958. ALGOL heavily influenced many other languages and was the standard method for algorithm description used by the ...
known as SMALGOL. He called the result ''B'', describing it as "BCPL semantics with a lot of SMALGOL syntax". Like BCPL, B had a bootstrapping compiler to facilitate porting to new machines. Ultimately, few utilities were written in B because it was too slow and could not take advantage of PDP-11 features such as
byte The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable un ...
addressability. Unlike BCPL's // comment marking comments up to the end of the line, B adopted /* comment */ as the comment delimiter, more akin to PL/1, and allowing comments to appear in the middle of lines. (BCPL's comment style would be reintroduced in C++.)


New B and first C release

In 1971 Ritchie started to improve B, to use the features of the more-powerful PDP-11. A significant addition was a character data type. He called this ''New B'' (NB). Thompson started to use NB to write the
Unix Unix (, ; trademarked as UNIX) is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, a ...
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, and to these types were added pointers, the ability to generate pointers to other types, arrays of all types, and types to be returned from functions. Arrays within expressions were effectively treated as 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 Unix kernel re-write

At
Version 4 Unix Research Unix refers to the early versions of the Unix operating system for PDP-7, DEC PDP-7, PDP-11, VAX and Interdata 7/32 and 8/32 computers, developed in the Bell Labs Computing Sciences Research Center (CSRC). The term ''Research Unix'' first ...
, released in November 1973, the
Unix Unix (, ; trademarked as UNIX) is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, a ...
kernel was extensively re-implemented in C. By this time, the C language had acquired some powerful features such as struct types. The
preprocessor In computer science, a preprocessor (or precompiler) is a Computer program, program that processes its input data to produce output that is used as input in another program. The output is said to be a preprocessed form of the input data, which i ...
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 PL/I (Programming Language One, pronounced and sometimes written PL/1) is a procedural, imperative computer programming language initially developed by IBM. It is designed for scientific, engineering, business and system programming. It has b ...
. 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. Unix was one of the first operating system kernels implemented in a language other than assembly. Earlier instances include the
Multics Multics ("MULTiplexed Information and Computing Service") is an influential early time-sharing operating system based on the concept of a single-level memory.Dennis M. Ritchie, "The Evolution of the Unix Time-sharing System", Communications of t ...
system (which was written in
PL/I PL/I (Programming Language One, pronounced and sometimes written PL/1) is a procedural, imperative computer programming language initially developed by IBM. It is designed for scientific, engineering, business and system programming. It has b ...
) and Master Control Program (MCP) for the Burroughs B5000 (which was written in
ALGOL ALGOL (; short for "Algorithmic Language") is a family of imperative computer programming languages originally developed in 1958. ALGOL heavily influenced many other languages and was the standard method for algorithm description used by the ...
) in 1961. In around 1977, Ritchie and Stephen C. Johnson made further changes to the language to facilitate portability of the Unix operating system. Johnson's Portable C Compiler served as the basis for several implementations of C on new platforms. (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 Brian Wilson Kernighan (; born January 30, 1942) is a Canadian computer scientist. He worked at Bell Labs and contributed to the development of Unix alongside Unix creators Ken Thompson and Dennis Ritchie. Kernighan's name became widely known ...
and Dennis Ritchie published the first edition of ''
The C Programming Language ''The C Programming Language'' (sometimes termed ''K&R'', after its authors' initials) is a computer programming book written by Brian Kernighan and Dennis Ritchie, the latter of whom originally designed and implemented the C programming langu ...
''. Known as ''K&R'' from the initials of its authors, the book served for many years as an informal
specification A specification often refers to a set of documented requirements to be satisfied by a material, design, product, or service. A specification is often a type of technical standard. There are different types of technical or engineering specificati ...
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 now 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: * Standard I/O library * long int data type * unsigned int data type * Compound assignment operators of the form =''op'' (such as =-) were changed to the form ''op''= (that is, -=) to remove the semantic ambiguity created by constructs such as i=-10, which had been interpreted as i =- 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 In mathematics, the lowest common denominator or least common denominator (abbreviated LCD) is the lowest common multiple of the denominators of a set of fractions. It simplifies adding, subtracting, and comparing fractions. Description The l ...
" 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. Because this is an early version of C, there is an implicit 'int' type here. A comment shows where the explicit 'int' type specifier would be required in later versions. */ /* int */ calling_function() /* This is a function definition, including the body of the code following in the . Because no return type is specified, the function implicitly returns an 'int' in 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 different calls to an external function used different numbers or types of arguments. Separate tools such as Unix's lint utility were developed that (among other things) could check for consistency of function use across multiple source files. In the years following the publication of K&R C, several features were added to the language, supported by compilers from AT&T (in particular PCC) and some other vendors. These included: * void functions (i.e., functions with no return value) * functions returning struct or union types (previously only a single pointer, integer or float could be returned) * assignment for struct data types *
enumerated type In computer programming, an enumerated type (also called enumeration, enum, or factor in the R (programming language), R programming language, a status variable in the JOVIAL programming language, and a categorical variable in statistics) is a data ...
s (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 (computing), library made available across Programming language implementation, implementations of a programming language. Often, a standard library is specified by its associated program ...
, 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, enterprise ...
s,
minicomputer A minicomputer, or colloquially mini, is a type of general-purpose computer mostly developed from the mid-1960s, built significantly smaller and sold at a much lower price than mainframe computers . By 21st century-standards however, a mini is ...
s, and microcomputers, including the
IBM PC The IBM Personal Computer (model 5150, commonly known as the IBM PC) is the first microcomputer released in the List of IBM Personal Computer models, IBM PC model line and the basis for the IBM PC compatible ''de facto'' standard. Released on ...
, as its popularity began to increase significantly. In 1983 the
American National Standards Institute The American National Standards Institute (ANSI ) is a private nonprofit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organiz ...
(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 The Institute of Electrical and Electronics Engineers (IEEE) is an American 501(c)(3) organization, 501(c)(3) public charity professional organization for electrical engineering, electronics engineering, and other related disciplines. The IEEE ...
working group A working group is a group of experts working together to achieve specified goals. Such groups are domain-specific and focus on discussion or activity around a specific subject area. The term can sometimes refer to an interdisciplinary collab ...
1003 to become the basis for the 1988
POSIX The Portable Operating System Interface (POSIX; ) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines application programming interfaces (APIs), along with comm ...
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 The International Organization for Standardization (ISO ; ; ) is an independent, non-governmental, international standard development organization composed of representatives from the national standards organizations of member countries. M ...
(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/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 In mathematics, a set ''A'' is a subset of a set ''B'' if all elements of ''A'' are also elements of ''B''; ''B'' is then a superset of ''A''. It is possible for ''A'' and ''B'' to be equal; if they are unequal, then ''A'' is a proper subset ...
of K&R C, incorporating many of the subsequently introduced unofficial features. The standards committee also included several additional features such as
function prototype In computer programming, a function prototype is a declaration of a function that specifies the function's name and type signature (arity, data types of parameters, and return type), but omits the function body. While a function definition ...
s (borrowed from C++), void pointers, support for international
character sets Character encoding is the process of assigning numbers to graphical character (computing), characters, especially the written characters of human language, allowing them to be stored, transmitted, and transformed using computers. The numerical v ...
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 (constituenc ...
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 libraries, or to a reliance on compiler- or platform-specific attributes such as the exact size of data types and byte
endianness file:Gullivers_travels.jpg, ''Gulliver's Travels'' by Jonathan Swift, the novel from which the term was coined In computing, endianness is the order in which bytes within a word (data type), word of digital data are transmitted over a data comm ...
. 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 function In the C (programming language), C and C++ programming languages, an inline function is one qualified with the Keyword (computer programming), keyword inline; this serves two purposes: # It serves as a compiler directive that suggests (but doe ...
s, several new
data type In computer science and computer programming, a data type (or simply type) is a collection or grouping of data values, usually specified by a set of possible values, a set of allowed operations on these values, and/or a representation of these ...
s (including long long int and a complex type to represent
complex number In mathematics, a complex number is an element of a number system that extends the real numbers with a specific element denoted , called the imaginary unit and satisfying the equation i^= -1; every complex number can be expressed in the for ...
s), variable-length arrays and
flexible array member C struct data types may end with a flexible array member with no specified size: struct vectord ; Typically, such structures serve as the header in a larger, variable memory allocation: struct vectord *vector = malloc(...); vector->len ...
s, improved support for
IEEE 754 The IEEE Standard for Floating-Point Arithmetic (IEEE 754) is a technical standard for floating-point arithmetic originally established in 1985 by the Institute of Electrical and Electronics Engineers (IEEE). The standard #Design rationale, add ...
floating point, support for variadic macros (macros of variable
arity In logic, mathematics, and computer science, arity () is the number of arguments or operands taken by a function, operation or relation. In mathematics, arity may also be called rank, but this word can have many other meanings. In logic and ...
), and support for one-line comments beginning with //, as in BCPL 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 C++11 is a version of a joint technical standard, ISO/IEC 14882, by the International Organization for Standardization (ISO) and International Electrotechnical Commission (IEC), for the C++ programming language. C++11 replaced the prior vers ...
. In addition, the C99 standard requires support for
identifiers An identifier is a name that identifies (that is, labels the identity of) either a unique object or a unique ''class'' of objects, where the "object" or class may be an idea, person, physical countable object (or class thereof), or physical mass ...
using
Unicode Unicode or ''The Unicode Standard'' or TUS is a character encoding standard maintained by the Unicode Consortium designed to support the use of text in all of the world's writing systems that can be digitized. Version 16.0 defines 154,998 Char ...
in the form of escaped characters (e.g. or ) and suggests support for raw Unicode names.


C11

Work began in 2007 on another revision of the C standard, informally called "C1X" until its official publication of ISO/IEC 9899:2011 on December 8, 2011. 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

C17 is an informal name for ISO/IEC 9899:2018, a standard for the C programming language published in June 2018. It introduces no new language features, only technical corrections, and clarifications to defects in C11. The standard macro __STDC_VERSION__ is defined as 201710L to indicate that C17 support is available.


C23

C23 is an informal name for the current major C language standard revision. It was informally known as "C2X" through most of its development. C23 was published in October 2024 as ISO/IEC 9899:2024. The standard macro __STDC_VERSION__ is defined as 202311L to indicate that C23 support is available.


C2Y

C2Y is an informal name for the next major C language standard revision, after C23 (C2X), that is hoped to be released later in the 2020s, hence the '2' in "C2Y". An early working draft of C2Y was released in February 2024 as N3220 by the working group ISO/IEC JTC1/SC22/WG14.


Embedded C

Historically, embedded C programming requires non-standard extensions to the C language 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, represen ...
, 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 ...
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, named address spaces, and basic I/O hardware addressing.


Syntax

C has a
formal grammar A formal grammar is a set of Terminal and nonterminal symbols, symbols and the Production (computer science), production rules for rewriting some of them into every possible string of a formal language over an Alphabet (formal languages), alphabe ...
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 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 In medicine, a side effect is an effect of the use of a medicinal drug or other treatment, usually adverse but sometimes beneficial, that is unintended. Herbal and traditional medicines also have side effects. A drug or procedure usually use ...
of the evaluation, functions may be called and variables 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 specific disciplined use of the structured control flow constructs of selection ( if/then/else) and repet ...
is supported by if ... code>else
conditional 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 within the loop. Break is used to leave the innermost enclosing loop statement and continue is used to skip to its reinitialisation. There is also a non-structured goto statement which branches directly to the designated
label A label (as distinct from signage) is a piece of paper, plastic film, cloth, metal, or other material affixed to a container or product. Labels are most often affixed to packaging and containers using an adhesive, or sewing when affix ...
within the function.
switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type o ...
selects a case to be executed based on the value of an integer expression. Different from many other languages, control-flow will fall through to the next case unless terminated by a break. 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 the
ISO basic Latin alphabet The ISO basic Latin alphabet is an international standard (beginning with ISO/IEC 646) for a Latin-script alphabet that consists of two sets (uppercase and lowercase) of 26 letters, codified in various national and international standards and u ...
: az, AZ * Decimal digits: 09 * Graphic characters: ! " # % & ' ( ) * + , - . / : ; < = > ? \ ^ _ ~ * Whitespace characters: ''
space Space is a three-dimensional continuum containing positions and directions. In classical physics, physical space is often conceived in three linear dimensions. Modern physicists usually consider it, with time, to be part of a boundless ...
'', ''
horizontal tab The tab key (abbreviation of tabulator key or tabular key) on a keyboard is used to advance the cursor to the next tab stop. History The word ''tab'' derives from the word ''tabulate'', which means "to arrange data in a tabular, or table, ...
'', '' vertical tab'', '' form feed'', ''
newline A 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 ...
'' The ''newline'' character indicates the end of a text line; it need not correspond to an actual single character, although for convenience C treats it as such. Additional multi-byte encoded characters may be used in
string literal string literal or anonymous string is a literal for a string value in the source code of a computer program. Modern programming languages commonly use a quoted sequence of characters, formally "bracketed delimiters", as in x = "foo", where , "foo ...
s, but they are not entirely portable. Since C99 multi-national Unicode characters can be embedded portably within C source text by using \uXXXX or \UXXXXXXXX encoding (where X denotes a hexadecimal character). The basic C execution character set contains the same characters, along with representations for alert,
backspace Backspace (, ⌫) is the keyboard key that in typewriters originally pushed the carriage one position backwards, and in modern computer systems typically moves the display cursor one position backwards,The meaning of "backwards" depends on the dir ...
, and carriage return. Run-time support for extended character sets has increased with each revision of the C standard.


Reserved words

The following reserved words are case sensitive. C89 has 32 reserved words, also known as 'keywords', which cannot be used for any purposes other than those for which they are predefined: * auto * break * case * char * const * continue * default * do *
double Double, The Double or Dubble may refer to: Mathematics and computing * Multiplication by 2 * Double precision, a floating-point representation of numbers that is typically 64 bits in length * A double number of the form x+yj, where j^2=+1 * A ...
* else * enum * extern * float * for * goto * if * int * long * register *
return Return may refer to: In business, economics, and finance * Return on investment (ROI), the financial gain after an expense. * Rate of return, the financial term for the profit or loss derived from an investment * Tax return, a blank document or t ...
* short * signed *
sizeof sizeof is a unary operator in the C and C++ programming languages that evaluates to the storage size of an expression or a data type, measured in units sized as char. Consequently, the expression sizeof(char) evaluates to 1. The number of b ...
* static * struct *
switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type o ...
*
typedef typedef is a reserved keyword in the programming languages C, C++, and Objective-C. It is used to create an additional name (''alias'') for another data type, but does not create a new type, except in the obscure case of a qualified typedef of ...
* union * unsigned * void * volatile * while C99 added five more reserved words: (‡ indicates an alternative spelling alias for a C23 keyword) * inline * restrict * _Bool ‡ * _Complex * _Imaginary C11 added seven more reserved words: (‡ indicates an alternative spelling alias for a C23 keyword) * _Alignas ‡ * _Alignof ‡ * _Atomic * _Generic * _Noreturn * _Static_assert ‡ * _Thread_local ‡ C23 reserved fifteen more words: * alignas * alignof * bool * constexpr * false * nullptr * static_assert * thread_local * true * typeof * typeof_unqual * _BitInt * _Decimal32 * _Decimal64 * _Decimal128 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. Some of those words were added as keywords with their conventional spelling in C23 and the corresponding macros were removed. Prior to C89, entry was reserved as a keyword. In the second edition of their book ''
The C Programming Language ''The C Programming Language'' (sometimes termed ''K&R'', after its authors' initials) is a computer programming book written by Brian Kernighan and Dennis Ritchie, the latter of whom originally designed and implemented the C programming langu ...
'', which describes what became known as C89, Kernighan and Ritchie wrote, "The ... eywordentry, formerly reserved but never used, is no longer reserved." and "The stillborn entry keyword is withdrawn."


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: *
arithmetic Arithmetic is an elementary branch of mathematics that deals with numerical operations like addition, subtraction, multiplication, and division. In a wider sense, it also includes exponentiation, extraction of roots, and taking logarithms. ...
: +, -, *, /, % * assignment: = * augmented assignment: * bitwise logic: ~, &, , , ^ * bitwise shifts: <<, >> *
Boolean logic In mathematics and mathematical logic, Boolean algebra is a branch of algebra. It differs from elementary algebra in two ways. First, the values of the variable (mathematics), variables are the truth values ''true'' and ''false'', usually denot ...
: !, &&, , , * conditional evaluation: ? : * equality testing:

, != * calling functions: ( ) * increment and decrement: ++, -- * member selection: ., -> * object size:
sizeof sizeof is a unary operator in the C and C++ programming languages that evaluates to the storage size of an expression or a data type, measured in units sized as char. Consequently, the expression sizeof(char) evaluates to 1. The number of b ...
* type: typeof, typeof_unqual ''since C23'' * order relations: <, <=, >, >= * reference and dereference: &, *, /code> * sequencing: , * subexpression grouping: ( ) *
type conversion In computer science, type conversion, type casting, type coercion, and type juggling are different ways of changing an expression from one data type to another. An example would be the conversion of an integer value into a floating point val ...
: (''typename'') C uses the operator = (used in mathematics to express equality) to indicate assignment, following the precedent of Fortran and
PL/I PL/I (Programming Language One, pronounced and sometimes written PL/1) is a procedural, imperative computer programming language initially developed by IBM. It is designed for scientific, engineering, business and system programming. It has b ...
, but unlike
ALGOL ALGOL (; short for "Algorithmic Language") is a family of imperative computer programming languages originally developed in 1958. ALGOL heavily influenced many other languages and was the standard method for algorithm description used by the ...
and its derivatives. C uses the operator

to test for equality. The similarity between the operators for 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 unless the value of a is 0 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 that 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 Standard may refer to: Symbols * Colours, standards and guidons, kinds of military signs * Standard (emblem), a type of a large symbol or emblem used for identification Norms, conventions or requirements * Standard (metrology), an object t ...
, 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 of code with the entire text of the stdio.h header file, which contains declarations for standard input and output functions such as printf and scanf. The angle brackets surrounding stdio.h indicate that the header file 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 second 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 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 the main function takes no arguments. The opening curly brace indicates the beginning of the code that defines the main function. The next line of the program is a statement that ''calls'' (i.e. diverts execution to) a function named printf, which in this case is supplied from a system
library A library is a collection of Book, books, and possibly other Document, materials and Media (communication), media, that is accessible for use by its members and members of allied institutions. Libraries provide physical (hard copies) or electron ...
. In this call, the printf function is ''passed'' (i.e. provided with) a single argument, which is the
address An address is a collection of information, presented in a mostly fixed format, used to give the location of a building, apartment, or other structure or a plot of land, generally using border, political boundaries and street names as references, ...
of the first character in the
string literal string literal or anonymous string is a literal for a string value in the source code of a computer program. Modern programming languages commonly use a quoted sequence of characters, formally "bracketed delimiters", as in x = "foo", where , "foo ...
"hello, world\n". The string literal is an unnamed array set up automatically by the compiler, with elements of type char and a final
NULL character The null character is a control character with the value zero. Many character sets include a code point for a null character including Unicode (Universal Coded Character Set), ASCII (ISO/IEC 646), Baudot, ITA2 codes, the C0 control code, and EB ...
(ASCII value 0) marking the end of the array (to allow printf to determine the length of the string). The NULL character can also be written as the
escape sequence In computer science, an escape sequence is a combination of characters that has a meaning other than the literal characters contained therein; it is marked by one or more preceding (and possibly terminating) characters. Examples * In C and ma ...
\0. The \n is a standard escape sequence that C translates to a ''
newline A 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 ...
'' 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 check that 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. The return value of 0 is interpreted by the run-time system as an exit code indicating successful execution of the function.


Data types

The
type system In computer programming, a type system is a logical system comprising a set of rules that assigns a property called a ''type'' (for example, integer, floating point, string) to every '' term'' (a word, phrase, or other set of symbols). Usu ...
in C is static and weakly typed, which makes it similar to the type system of
ALGOL ALGOL (; short for "Algorithmic Language") is a family of imperative computer programming languages originally developed in 1958. ALGOL heavily influenced many other languages and was the standard method for algorithm description used by the ...
descendants such as Pascal. 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 data type In computer science, the Boolean (sometimes shortened to Bool) is a data type that has one of two possible values (usually denoted ''true'' and ''false'') which is intended to represent the two truth values of logic and Boolean algebra. It is na ...
. There are also derived types including
arrays An array is a systematic arrangement of similar objects, usually in rows and columns. Things called an array include: {{TOC right Music * In twelve-tone and serial composition, the presentation of simultaneous twelve-tone sets such that the ...
,
pointers Pointer may refer to: People with the name * Pointer (surname), a surname (including a list of people with the name) * Pointer Williams (born 1974), American former basketball player Arts, entertainment, and media * ''Pointer'' (journal), the ...
, records ( struct), 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 pointer A function pointer, also called a subroutine pointer or procedure pointer, is a pointer referencing executable code, rather than data. Dereferencing the function pointer yields the referenced function, which can be invoked and passed arguments ...
s. (Ritchie's idea was to declare identifiers in contexts resembling their use: " declaration reflects use".) 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 Pointer may refer to: People with the name * Pointer (surname), a surname (including a list of people with the name) * Pointer Williams (born 1974), American former basketball player Arts, entertainment, and media * ''Pointer'' (journal), the ...
, a type of
reference A reference is a relationship between objects in which one object designates, or acts as a means by which to connect to or link to, another object. The first object in this relation is said to ''refer to'' the second object. It is called a ''nam ...
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 Memory management (also dynamic memory management, dynamic storage allocation, or dynamic memory allocation) is a form of resource management applied to computer memory. The essential requirement of memory management is to provide ways to dyna ...
is performed using pointers; the result of a malloc is usually
cast Cast may refer to: Music * Cast (band), an English alternative rock band * Cast (Mexican band), a progressive Mexican rock band * The Cast, a Scottish musical duo: Mairi Campbell and Dave Francis * ''Cast'', a 2012 album by Trespassers William ...
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 pointer A function pointer, also called a subroutine pointer or procedure pointer, is a pointer referencing executable code, rather than data. Dereferencing the function pointer yields the referenced function, which can be invoked and passed arguments ...
s'') 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 dispatch tables, or as callbacks to event handlers. A ''
null pointer In computing, a null pointer (sometimes shortened to nullptr or null) or null reference is a value saved for indicating that the Pointer (computer programming), pointer or reference (computer science), reference does not refer to a valid Object (c ...
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 whi ...
, 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, as the NULL macro defined by several standard headers or, since C23 with the constant nullptr. 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 A reference is a relationship between objects in which one object designates, or acts as a means by which to connect to or link to, another object. The first object in this relation is said to ''refer to'' the second object. It is called a ''nam ...
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 In computer programming, bounds checking is any method of detecting whether a variable is within some bounds before it is used. It is usually used to ensure that a number fits into a given type (range checking), or that a variable being used as ...
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, and run-time exceptions. C does not have a special provision for declaring multi-dimensional arrays, but rather relies on
recursion Recursion occurs when the definition of a concept or process depends on a simpler or previous version of itself. Recursion is used in a variety of disciplines ranging from linguistics to logic. The most common application of recursion is in m ...
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. Multi-dimensional arrays are commonly used in numerical algorithms (mainly from applied
linear algebra Linear algebra is the branch of mathematics concerning linear equations such as :a_1x_1+\cdots +a_nx_n=b, linear maps such as :(x_1, \ldots, x_n) \mapsto a_1x_1+\cdots +a_nx_n, and their representations in vector spaces and through matrix (mathemat ...
) 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 In computer science, syntactic sugar is syntax within a programming language that is designed to make things easier to read or to express. It makes the language "sweeter" for human use: things can be expressed more clearly, more concisely, or in an ...
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+1th element of the array. Furthermore, in most expression contexts (a notable exception is as operand of
sizeof sizeof is a unary operator in the C and C++ programming languages that evaluates to the storage size of an expression or a data type, measured in units sized as char. Consequently, the expression sizeof(char) evaluates to 1. The number of b ...
), 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 A reference is a relationship between objects in which one object designates, or acts as a means by which to connect to or link to, another object. The first object in this relation is said to ''refer to'' the second object. It is called a ''nam ...
. 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 Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembe ...
and the objects that are stored in memory. C provides three principal ways to allocate memory for objects: *
Static memory allocation In computer programming, a static variable is a variable (programming), variable that has been memory allocation, allocated "statically", meaning that its variable lifetime, lifetime (or "extent") is the entire run of the program. This is in contr ...
: 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: temporary objects can be stored on the stack, and this space is automatically freed and reusable after the block in which they are declared is exited. *
Dynamic memory allocation Memory management (also dynamic memory management, dynamic storage allocation, or dynamic memory allocation) is a form of resource management applied to computer memory. The essential requirement of memory management is to provide ways to dyna ...
: 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 persist until subsequently freed for reuse by calling the library function realloc or free. 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
C dynamic memory allocation 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 inclu ...
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 Linker or linkers may refer to: Computing * Linker (computing), a computer program that takes one or more object files generated by a compiler or generated by an assembler and links them with libraries, generating an executable program or shar ...
or loader, 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 happens to be present in the storage, 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.


Libraries

The C programming language uses
libraries A library is a collection of Book, books, and possibly other Document, materials and Media (communication), media, that is accessible for use by its members and members of allied institutions. Libraries provide physical (hard copies) or electron ...
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 An include directive instructs a text file processor to replace the directive text with the content of a specified file. The act of including may be logical in nature. The processor may simply process the include file content at the location of ...
, 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. 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 interface (CLI) is a means of interacting with software via command (computing), commands each formatted as a line of text. Command-line interfaces emerged in the mid-1960s, on computer terminals, as an interactive and more user ...
s (e.g., -lm, shorthand for "link the math library"). The most common C library is the
C standard library The C standard library, sometimes referred to as libc, is the standard library for the C (programming language), C programming language, as specified in the ISO C standard.International Organization for Standardization, ISO/International Electrote ...
, which is specified by the
ISO The International Organization for Standardization (ISO ; ; ) is an independent, non-governmental, international standard development organization composed of representatives from the national standards organizations of member countries. Me ...
and ANSI C standards and comes with every C implementation (implementations which target limited environments such as
embedded system An embedded system is a specialized computer system—a combination of a computer processor, computer memory, and input/output peripheral devices—that has a dedicated function within a larger mechanical or electronic system. It is e ...
s 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 Unix (, ; trademarked as UNIX) is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, a ...
and
Unix-like A Unix-like (sometimes referred to as UN*X, *nix or *NIX) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Uni ...
systems, especially functions which provide an interface to the kernel. These functions are detailed in various standards such as
POSIX The Portable Operating System Interface (POSIX; ) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines application programming interfaces (APIs), along with comm ...
and the
Single UNIX Specification The Single UNIX Specification (SUS) is a standard for computer operating systems, compliance with which is required to qualify for using the "UNIX" trademark. The standard specifies programming interfaces for the C language, a command-line shell, ...
. 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 In computing, object code or object module is the product of an assembler or compiler In computing, a compiler is a computer program that Translator (computing), translates computer code written in one programming language (the ''source'' ...
; programmers then create interfaces to the library so that the routines can be used from higher-level languages like
Java Java is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea (a part of Pacific Ocean) to the north. With a population of 156.9 million people (including Madura) in mid 2024, proje ...
,
Perl Perl is a high-level, general-purpose, interpreted, dynamic programming language. Though Perl is not officially an acronym, there are various backronyms in use, including "Practical Extraction and Reporting Language". Perl was developed ...
, 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 surface water flowing within the bed and banks of a channel. Depending on its location or certain characteristics, a stream may be referred to by a variety of local or regional names. Long, large 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 A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating hard disk drive platter, pla ...
or solid-state drive. Low-level I/O functions are not part of the standard C library but are generally part of "bare metal" programming (programming that is independent of any
operating system An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ...
such as most embedded programming). 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. Automated source code checking and auditing 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. 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 In computer programming, bounds checking is any method of detecting whether a variable is within some bounds before it is used. It is usually used to ensure that a number fits into a given type (range checking), or that a variable being used as ...
for arrays, detection of buffer overflow,
serialization In computing, serialization (or serialisation, also referred to as pickling in Python (programming language), Python) is the process of translating a data structure or object (computer science), object state into a format that can be stored (e. ...
, dynamic memory tracking, and automatic garbage collection. Memory management checking tools like 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 Systems programming, or system programming, is the activity of programming computer system software. The primary distinguishing characteristic of systems programming when compared to application programming is that application programming aims t ...
in implementing
operating system An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ...
s and
embedded system An embedded system is a specialized computer system—a combination of a computer processor, computer memory, and input/output peripheral devices—that has a dedicated function within a larger mechanical or electronic system. It is e ...
applications. This is for several reasons: * The C language permits platform hardware and memory to be accessed with pointers and type punning, so system-specific features (e.g. Control/Status Registers, I/O registers) can be configured and used with code written in C – it allows fullest control of the platform it is running on. * The code generated after compilation does not demand many system features, and can be invoked from some boot code in a straightforward manner – it is 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 is fast to execute. * With its rich set of operators, the C language can use many of the features of target CPUs. Where a particular CPU has more esoteric instructions, a language variant can be constructed with perhaps
intrinsic function In computer software, in compiler theory, an intrinsic function, also called built-in function or builtin function, is a function ( subroutine) available for use in a given programming language whose implementation is handled specially by the com ...
s 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'' garbage collection events – it has predictable performance. * C permits the use and implementation of different memory allocation schemes, including a typical and ; a more sophisticated mechanism with ''arenas''; or a version for an OS kernel that may suit DMA, use within
interrupt handler In computer systems programming, an interrupt handler, also known as an interrupt service routine (ISR), is a special block of code associated with a specific interrupt condition. Interrupt handlers are initiated by hardware interrupts, software ...
s, or integrated with the
virtual memory In computing, virtual memory, or virtual storage, is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a ver ...
system. * Depending on the linker and environment, C code can also call libraries written in
assembly language In computing, assembly language (alternatively 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 bet ...
, 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 is 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.


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 GNU Multiple Precision Arithmetic Library (GMP) is a free software, free library for arbitrary-precision arithmetic, operating on Sign (mathematics), signed integers, Rational data type, rational numbers, and Floating-point arithmetic, floating-p ...
, 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 (programming language), C; wrappers are available for other programming languages. The GSL is part of ...
,
Mathematica Wolfram (previously known as Mathematica and Wolfram Mathematica) is a software system with built-in libraries for several areas of technical computing that allows machine learning, statistics, symbolic computation, data manipulation, network ...
, and
MATLAB MATLAB (an abbreviation of "MATrix LABoratory") is a proprietary multi-paradigm programming language and numeric computing environment developed by MathWorks. MATLAB allows matrix manipulations, plotting of functions and data, implementat ...
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.


Games

Computer games are often built from a combination of languages. C has featured significantly, especially for those games attempting to obtain best performance from computer platforms. Examples include Doom from 1993.


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 LLVM, also called LLVM Core, is a target-independent optimizer and code generator. It can be used to develop a Compiler#Front end, frontend for any programming language and a Compiler#Back end, backend for any instruction set architecture. LLVM i ...
both feature an
intermediate representation An intermediate representation (IR) is the data structure or code used internally by a compiler or virtual machine to represent source code. An IR is designed to be conducive to further processing, such as optimization and translation. A "good" ...
that is not C, and those compilers support front ends for many languages including C.


Other languages written in C

A consequence of C's wide availability and efficiency is that
compiler In computing, a compiler is a computer program that Translator (computing), translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primaril ...
s, libraries and interpreters of other programming languages are often implemented in C. For example, the
reference implementation In the software development process, a reference implementation (or, less frequently, sample implementation or model implementation) is a program that implements all requirements from a corresponding specification. The reference implementation ...
s of Python,
Perl Perl is a high-level, general-purpose, interpreted, dynamic programming language. Though Perl is not officially an acronym, there are various backronyms in use, including "Practical Extraction and Reporting Language". Perl was developed ...
,
Ruby Ruby is a pinkish-red-to-blood-red-colored gemstone, a variety of the mineral corundum ( aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called sapph ...
, and PHP are written in C.


Once used for web development

Historically, C was sometimes used for web development using the
Common Gateway Interface file:Common Gateway Interface logo.svg, The official CGI logo from the spec announcement In computing, Common Gateway Interface (CGI) is an interface specification that enables web servers to execute an external program to process HTTP or HTTPS ...
(CGI) as a "gateway" for information between the web application, the server, and the browser. C may have been chosen over
interpreted language In computer science, an interpreter is a computer program that directly executes instructions written in a programming or scripting language, without requiring them previously to have been compiled into a machine language program. An inter ...
s 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 languages are popular. Applications where C-based web development continues include the
HTTP HTTP (Hypertext Transfer Protocol) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web, wher ...
configuration pages on routers, IoT devices and similar, although even here some projects have parts in higher-level languages e.g. the use of Lua within
OpenWRT OpenWrt (from ''open wireless router'') is an open-source project for embedded operating systems based on Linux kernel, Linux, primarily used on Embedded system, embedded devices to Router (computing), route network traffic. The main components ...
.


Web servers

The two most popular
web server A web server is computer software and underlying Computer hardware, hardware that accepts requests via Hypertext Transfer Protocol, HTTP (the network protocol created to distribute web content) or its secure variant HTTPS. A user agent, co ...
s,
Apache HTTP Server The Apache HTTP Server ( ) is a free and open-source software, free and open-source cross-platform web server, released under the terms of Apache License, Apache License 2.0. It is developed and maintained by a community of developers under the ...
and
Nginx (pronounced "engine x" , stylized as NGINX or nginx) is a web server that can also be used as a reverse proxy, load balancer, mail proxy and HTTP cache. The software was created by Russian developer Igor Sysoev and publicly released in 20 ...
, are both written in C. These web servers interact with the operating system, listen on TCP ports for HTTP requests, and then serve up static web content, or cause the execution of other languages handling to 'render' content such as PHP, which is itself primarily written in C. C's close-to-the-metal approach allows for the construction of these high-performance software systems.


End-user applications

C has also been widely used to implement
end-user In product development, an end user (sometimes end-user) is a person who ultimately uses or is intended to ultimately use a product. The end user stands in contrast to users who support or maintain the product, such as sysops, system administrato ...
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. Improper use can lead to memory leaks and dangling pointers. * 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 In computer programming, a type system is a logical system comprising a set of rules that assigns a property called a ''type'' (for example, integer, floating point, string) to every '' term'' (a word, phrase, or other set of symbols). Usu ...
, but it does not apply to areas like variadic functions, and the type checking can be trivially or inadvertently circumvented. It is weakly typed. * Since the code generated by the compiler contains few checks itself, there is a burden on the programmer to consider all possible outcomes, to protect against buffer overruns, array bounds checking,
stack overflow In software, a stack overflow occurs if the call stack pointer exceeds the stack bound. The call stack may consist of a limited amount of address space, often determined at the start of the program. The size of the call stack depends on many fa ...
s, memory exhaustion, and consider 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 or , can lead to buffer overruns. * There is limited standardisation in support for low-level variants in generated code, for example: different function calling conventions 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. The directive and options are not consistently supported. * String handling using the standard library is code-intensive, with explicit memory management required. * The language does not directly support object orientation,
introspection Introspection is the examination of one's own conscious thoughts and feelings. In psychology, the process of introspection relies on the observation of one's mental state, while in a spiritual context it may refer to the examination of one's s ...
, run-time expression evaluation, generics, etc. * There are few guards against inappropriate use of language features, which may lead to unmaintainable code. In particular, the C preprocessor can hide troubling effects such as double evaluation and worse. 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 Source code, code written in C (programming language), C that is the most creatively obfuscated code, obfuscated. Held semi-annually, it is desc ...
'' and the '' Underhanded C Contest''. * C lacks standard support for
exception handling In computing and computer programming, exception handling is the process of responding to the occurrence of ''exceptions'' – anomalous or exceptional conditions requiring special processing – during the execution of a program. In general, an ...
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 ...
that can mitigate against some of the drawbacks. Contemporary C compilers include checks which may generate warnings to help identify many potential bugs.


Related languages

C has both directly and indirectly influenced many later languages such as C++ and
Java Java is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea (a part of Pacific Ocean) to the north. With a population of 156.9 million people (including Madura) in mid 2024, proje ...
. 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 or large-scale program structures that differ from those of C, sometimes radically. Several C or near-C interpreters exist, including Ch and CINT, which can also be used for scripting. When
object-oriented programming Object-oriented programming (OOP) is a programming paradigm based on the concept of '' objects''. Objects can contain data (called fields, attributes or properties) and have actions they can perform (called procedures or methods and impl ...
languages became popular, C++ and
Objective-C Objective-C is a high-level general-purpose, object-oriented programming language that adds Smalltalk-style message passing (messaging) to the C programming language. Originally developed by Brad Cox and Tom Love in the early 1980s, it was ...
were two different extensions of C that provided object-oriented capabilities. Both languages were originally implemented as
source-to-source compiler A source-to-source translator, source-to-source compiler (S2S compiler), transcompiler, or transpiler is a type of translator that takes the source code of a program written in a programming language as its input and produces an equivalent so ...
s; 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 Objective-C is a high-level general-purpose, object-oriented programming language that adds Smalltalk-style message passing (messaging) to the C programming language. Originally developed by Brad Cox and Tom Love in the early 1980s, it was ...
was originally a very "thin" layer on top of C, and remains a strict
superset In mathematics, a set ''A'' is a subset of a set ''B'' if all elements of ''A'' are also elements of ''B''; ''B'' is then a superset of ''A''. It is possible for ''A'' and ''B'' to be equal; if they are unequal, then ''A'' is a proper subset ...
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 a purely object oriented programming language (OOP) that was originally created in the 1970s for educational use, specifically for constructionist learning, but later found use in business. It was created at Xerox PARC by Learni ...
: 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 Objective-C is a high-level general-purpose, object-oriented programming language that adds Smalltalk-style message passing (messaging) to the C programming language. Originally developed by Brad Cox and Tom Love in the early 1980s, it was ...
, Ch,
Cilk Cilk, Cilk++, Cilk Plus and OpenCilk are general-purpose programming languages designed for multithreaded parallel computing. They are based on the C and C++ programming languages, which they extend with constructs to express parallel loop ...
, and
Unified Parallel C Unified Parallel C (UPC) is an extension of the C programming language designed for high-performance computing on large-scale parallel machines, including those with a common global address space ( SMP and NUMA) and those with distributed me ...
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 Source code, code written in C (programming language), C that is the most creatively obfuscated code, obfuscated. Held semi-annually, it is desc ...
*
List of C-family programming languages The C-family programming languages share significant features of the C programming language. Many of these 70 languages were influenced by C due to its success and ubiquity. The family also includes predecessors that influenced C's design such a ...
*
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

* * * * *


Further reading

* ''(source)''
/small> * ''(free)''
/small> * * ''(archive)''
/small> * ''(archive)''
/small> * * * *
(free)
'


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