C (programming Language)
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The C Programming Language[1] (often referred to as K&R), the seminal book on C
ParadigmImperative (procedural), structured
Designed byDennis Ritchie
DeveloperDennis Ritchie & Bell Labs (creators); ANSI X3J11 (ANSI C); (ISO C)
First appeared1972; 49 years ago (1972)[2]
Stable release
C17 / June 2018; 2 years ago (2018-06)
Preview release
C2x (N2596) / December 11, 2020; 2 months ago (2020-12-11)[3]
Typing disciplineStatic, weak, manifest, nominal
Filename extensions.c, .h
Major implementations
K&R C, GCC, Clang, C++Builder, ,
Influenced by
B (BCPL, CPL), ALGOL 68,[4]Assembly, PL/I, FORTRAN
Numerous: AMPL, AWK, csh, C++, C--, C#, Objective-C, D, Go, Java, JavaScript, Julia, Limbo, LPC, Perl, PHP, Pike, Processing, Python, Rust, Seed7, Vala, Verilog (HDL),[5]Nim, Zig


A successor to the programming language B, C was originally developed at Bell Labs by Dennis Ritchie between 1972 and 1973 to construct utilities running on Unix. It was applied to re-implementing the kernel of the Unix operating system.[6] During the 1980s, C gradually gained popularity. It has become one of the most widely used programming languages,[7][8] with C compilers from various vendors available for the majority of existing computer architectures and operating systems. C has been standardized by the ANSI since 1989 (ANSI C) and by the International Organization for Standardization (ISO).

C is an imperative procedural language. It was designed to be compiled to provide low-level access to memory 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.[]

As of January 2021, C was ranked first in the TIOBE index, a measure of the popularity of programming languages, moving up from the no. 2 spot the previous year.[9]


Dennis Ritchie (right), the inventor of the C programming language, with Ken Thompson

Like most procedural languages in the ALGOL tradition, C has facilities for structured programming and allows lexical variable scope and recursion. Its static type system prevents unintended operations. In C, all executable code is contained within subroutines (also called "functions", though not strictly in the sense of functional programming). Function parameters are always passed by value (except arrays). Pass-by-reference is simulated in C by explicitly passing pointer values. C program source text is free-format, using the semicolon as a statement terminator and curly braces for grouping blocks of statements.

The C language also exhibits the following characteristics:

  • The language has a small, fixed number of keywords, including a full set of control flow primitives: if/else, for, do/while, while, and switch. User-defined names are not distinguished from keywords by any kind of sigil.
  • It has a large number of arithmetic, bitwise, and logic operators: +, +=, ++, &, ||, etc.
  • Functions:
    • Function return values can be ignored, when not needed.
  • Data typing is static, but weakly enforced; all data has a type, but implicit conversions are possible.
    • Union is a structure with overlapping members; only the last member stored is valid.
    • Array indexing is a secondary notation, defined in terms of pointer arithmetic. Unlike structs, arrays are not first-class objects: they cannot be assigned or compared using single built-in operators. There is no "array" keyword in use or definition; instead, square brackets indicate arrays syntactically, for example month[11].
    • Enumerated types are possible with the enum keyword. They are freely interconvertible with integers.
    • Strings are not a distinct data type, but are conventionally implemented as null-terminated character arrays.

While C does not include certain features found in other languages (such as object orientation and garbage collection), these can be implemented or emulated, often through the use of external libraries (e.g., the GLib Object System or the Boehm garbage collector).

Relations to other languages

Many later languages have borrowed directly or indirectly from C, including C++, C#, Unix's C shell, D, Go, Java, JavaScript (including transpilers), Julia, Limbo, LPC, Objective-C, Perl, PHP, Python, Ruby, Rust, Swift, Verilog and SystemVerilog (hardware description languages).[5] These languages have drawn many of their control structures and other basic features from C. Most of them (Python being a dramatic exception) also express highly similar syntax 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.


Early developments

Timeline of language development
Year C Standard[10]
1972 Birth
1978 K&R C
1989/1990 ANSI C and ISO C
1999 C99
2011 C11
2017 C17

The origin of C is closely tied to the development of the Unix operating system, originally implemented in assembly language on a PDP-7 by Dennis Ritchie and Ken Thompson, incorporating several ideas from colleagues. Eventually, they decided to port the operating system to a PDP-11. The original PDP-11 version of Unix was also developed in assembly language.[6]

Thompson desired a programming language to make utilities for the new platform. At first, he tried to make a Fortran compiler, but soon gave up the idea. Instead, he created a cut-down version of the recently developed BCPL systems programming language. The official description of BCPL was not available at the time,[11] and Thompson modified the syntax to be less wordy, producing the similar but somewhat simpler B.[6] However, few utilities were ultimately written in B because it was too slow, and B could not take advantage of PDP-11 features such as byte addressability.

In 1972, Ritchie started to improve B, which resulted in creating a new language C.[12] The C compiler and some utilities made with it were included in Version 2 Unix.[13]

At Version 4 Unix, released in November 1973, the Unix kernel was extensively re-implemented in C.[6] By this time, the C language had acquired some powerful features such as struct types.

Preprocessor was introduced around 1973 at the urging of Alan Snyder and also in recognition of the usefulness of the file-inclusion mechanisms available in BCPL and PL/I. Its original version provided only included files and simple string replacements: #include and #define of parameterless macros. Soon after that, it was extended, mostly by Mike Lesk and then by John Reiser, to incorporate macros with arguments and conditional compilation.[14]

Unix was one of the first operating system kernels implemented in a language other than assembly. Earlier instances include the Multics system (which was written in PL/I) and Master Control Program (MCP) for the Burroughs B5000 (which was written in ALGOL) 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.[12]


The cover of the book The C Programming Language, first edition, by Brian Kernighan and Dennis Ritchie

In 1978, Brian Kernighan and Dennis Ritchie published the first edition of The C Programming Language.[1] This book, known to C programmers as K&R, served for many years as an informal specification of the language. The version of C that it describes is commonly referred to as "K&R C". As this was released in 1978, it is also referred to as C78.[15] The second edition of the book[16] 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).


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;
/* int */ other_function;

/* int */ calling_function
    long test1;
    register /* int */ test2;

    test1 = some_function;
    if (test1 > 0)
          test2 = 0;
          test2 = other_function;
    return test2;



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[17]) and some other vendors. These included:










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.[18]



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


In addition, support for Unicode identifiers (variable / function names) in the form of escaped characters (e.g. \U0001f431) is now required. Support for raw Unicode names like ? is optional.



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.


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


C2x is an informal name for the next (after C17) major C language standard revision. It is not expected to be voted on until December 2021.

Embedded C


In 2008, the C Standards Committee published a technical report extending the C language[21] 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.


C has a formal grammar specified by the C standard.[22] 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.[23]




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."[24] 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:


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


Reserved words

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

C99 reserved five more words:

C11 reserved seven more words:[25]

  • _Alignas
  • _Alignof
  • _Atomic
  • _Generic
  • _Noreturn
  • _Static_assert
  • _Thread_local

Most of the recently reserved words begin with an underscore followed by a capital letter, because identifiers of that form were previously reserved by the C standard for use only by implementations. Since existing program source code should not have been using these identifiers, it would not be affected when C implementations started supporting these extensions to the programming language. Some standard headers do define more convenient synonyms for underscored identifiers. The language previously included a reserved word called entry, but this was seldom implemented, and has now been removed as a reserved word.[26]



C uses the operator = (used in mathematics to express equality) to indicate assignment, following the precedent of Fortran and PL/I, but unlike ALGOL and its derivatives. C uses the operator == to test for equality. The similarity between these two operators (assignment and equality) may result in the accidental use of one in place of the other, and in many cases, the mistake does not produce an error message (although some compilers produce warnings). For example, the conditional expression if (a == b + 1) might mistakenly be written as if (a = b + 1), which will be evaluated as true if a is not zero after the assignment.[27]

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.[28]

"Hello, world" example

"Hello, World!" program by Brian Kernighan (1978)

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

The original version was:[29]

    printf("hello, world\n");


#include <stdio.h>

int main(void)
    printf("hello, world\n");

The first line of the program contains a preprocessing directive, indicated by #include. This causes the compiler to replace that line with the entire text of the stdio.h standard header, which contains declarations for standard input and output functions such as printf and scanf. The angle brackets surrounding stdio.h indicate that stdio.h is located using a search strategy that prefers headers provided with the compiler to other headers having the same name, as opposed to double quotes which typically include local or project-specific header files.




The closing curly brace indicates the end of the code for the main function. According to the C99 specification and newer, the main function, unlike any other function, will implicitly return a value of 0 upon reaching the } that terminates the function. (Formerly an explicit return 0; statement was required.) This is interpreted by the run-time system as an exit code indicating successful execution.[30]

Data types

The type system in C is static and weakly typed, which makes it similar to the type system of ALGOL descendants such as Pascal.[31] There are built-in types for integers of various sizes, both signed and unsigned, floating-point numbers, and enumerated types (enum). Integer type char is often used for single-byte characters. C99 added a boolean datatype. There are also derived types including arrays, pointers, records (struct), and unions (union).











Since arrays are always accessed (in effect) via pointers, array accesses are typically not checked against the underlying array size, although some compilers may provide bounds checking as an option.[33][34] Array bounds violations are therefore possible and rather common in carelessly written code, and can lead to various repercussions, including illegal memory accesses, corruption of data, buffer overruns, and run-time exceptions. If bounds checking is desired, it must be done manually.

C does not have a special provision for declaring multi-dimensional arrays, but rather relies on recursion within the type system to declare arrays of arrays, which effectively accomplishes the same thing. The index values of the resulting "multi-dimensional array" can be thought of as increasing in row-major order.



Array-pointer interchangeability

The subscript notation x[i] (where x designates a pointer) is syntactic sugar for *(x+i).[35] 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[i] designates the i+1th element of the array.


The size of an element can be determined by applying the operator sizeof to any dereferenced element of x, as in n = sizeof *x or n = sizeof x[0], and the number of elements in a declared array A can be determined as sizeof A / sizeof A[0]. The latter only applies to array names: variables declared with subscripts (int A[20]). Due to the semantics of C, it is not possible to determine the entire size of arrays through pointers to arrays, such as arrays created by dynamic allocation (malloc) or array function parameters; code such as sizeof arr / sizeof arr[0] (where arr designates a pointer) will not work since the compiler assumes the size of the pointer itself is being requested.[36][37] Since array name arguments to sizeof are not converted to pointers, they do not exhibit such ambiguity. However, arrays created by dynamic allocation are accessed by pointers rather than true array variables, so they suffer from the same sizeof issues as array pointers.


Memory management






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.



The most common C library is the C standard library, which is specified by the ISO and ANSI C standards and comes with every C implementation (implementations which target limited environments such as embedded systems may provide 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.



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 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's sent to the final destination. This reduces the time spent waiting for slower devices, for example a hard drive 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's independent of any operating system such as most but not all 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. The tool lint was the first such, leading to many others.


There are also compilers, libraries, and operating system level mechanisms for performing actions that are not a standard part of C, such as bounds checking for arrays, detection of buffer overflow, serialization, dynamic memory tracking, and automatic garbage collection.



The TIOBE index graph, showing a comparison of the popularity of various programming languages[39]

C is widely used for systems programming in implementing operating systems and embedded system applications,[40] because C code, when written for portability, can be used for most purposes, yet when needed, system-specific code can be used to access specific hardware addresses and to perform type punning to match externally imposed interface requirements, with a low run-time demand on system resources.

C can be used for website programming using the Common Gateway Interface (CGI) as a "gateway" for information between the Web application, the server, and the browser.[41] C is often chosen over interpreted languages because of its speed, stability, and near-universal availability.[42]

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

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

C is sometimes used as an intermediate language by implementations of other languages. This approach may be used for portability or convenience; by using C as an intermediate language, additional machine-specific code generators are not necessary. C has some features, such as line-number preprocessor directives and optional superfluous commas at the end of initializer lists, that support compilation of generated code. However, some of C's shortcomings have prompted the development of other C-based languages specifically designed for use as intermediate languages, such as C--.


Related languages

C has both directly and indirectly influenced many later languages such as C#, D, Go, Java, JavaScript, Limbo, LPC, Perl, PHP, Python, and Unix's C shell.[43] The most pervasive influence has been syntactical; all of the languages mentioned combine the statement and (more or less recognizably) expression syntax of C with type systems, data models, and/or large-scale program structures that differ from those of C, sometimes radically.



The C++ programming language was devised by Bjarne Stroustrup as an approach to providing object-oriented functionality with a C-like syntax.[45] 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.


In addition to C++ and Objective-C, Ch, Cilk, and Unified Parallel C are nearly supersets of C.

See also




  2. ^ Ritchie (1993): "Thompson had made a brief attempt to produce a system coded in an early version of C--before structures--in 1972, but gave up the effort."
  3. ^ Fruderica (December 13, 2020). "History of C". The cppreference.com. Retrieved 2020.
  4. ^ Ritchie (1993): "The scheme of type composition adopted by C owes considerable debt to Algol 68, although it did not, perhaps, emerge in a form that Algol's adherents would approve of."
  5. ^ a b "Verilog HDL (and C)" (PDF). The Research School of Computer Science at the Australian National University. June 3, 2010. Archived from the original (PDF) on November 6, 2013. Retrieved 2013. 1980s: ; Verilog first introduced ; Verilog inspired by the C programming language
  6. ^ a b c d Ritchie (1993)
  7. ^ "Programming Language Popularity". 2009. Archived from the original on January 16, 2009. Retrieved 2009.
  8. ^ "TIOBE Programming Community Index". 2009. Archived from the original on May 4, 2009. Retrieved 2009.
  9. ^ "TIOBE Index for January 2021". Archived from the original on January 12, 2021. Retrieved 2021.
  10. ^ "History of C". en.cppreference.com.
  11. ^ Ritchie, Dennis. "BCPL to B to C".
  12. ^ a b Johnson, S. C.; Ritchie, D. M. (1978). "Portability of C Programs and the UNIX System". Bell System Tech. J. 57 (6): 2021-2048. CiteSeerX doi:10.1002/j.1538-7305.1978.tb02141.x. S2CID 17510065. (Note: The PDF is an OCR scan of the original, and contains a rendering of "IBM 370" as "IBM 310".)
  13. ^ McIlroy, M. D. (1987). A Research Unix reader: annotated excerpts from the Programmer's Manual, 1971-1986 (PDF) (Technical report). CSTR. Bell Labs. p. 10. 139.
  14. ^ Ritchie (1993)
  15. ^ "C manual pages". FreeBSD Miscellaneous Information Manual (FreeBSD 13.0 ed.). May 30, 2011. [1]
  17. ^ Stroustrup, Bjarne (2002). Sibling rivalry: C and C++ (PDF) (Report). AT&T Labs.
  18. ^ C Integrity. International Organization for Standardization. March 30, 1995.
  19. ^ "JTC1/SC22/WG14 - C". Home page. ISO/IEC. Retrieved 2011.
  20. ^ Andrew Binstock (October 12, 2011). "Interview with Herb Sutter". Dr. Dobbs. Retrieved 2013.
  21. ^ "TR 18037: Embedded C" (PDF). ISO / IEC. Retrieved 2011.
  22. ^ Harbison, Samuel P.; Steele, Guy L. (2002). C: A Reference Manual (5th ed.). Englewood Cliffs, NJ: Prentice Hall. ISBN 978-0-13-089592-9. Contains a BNF grammar for C.
  23. ^ Kernighan & Ritchie (1996), p. 192.
  24. ^ Kernighan & Ritchie (1978), p. 3.
  25. ^ "ISO/IEC 9899:201x (ISO C11) Committee Draft" (PDF).
  26. ^ Kernighan & Ritchie (1996), pp. 192, 259.
  27. ^ "10 Common Programming Mistakes in C++". Cs.ucr.edu. Retrieved 2009.
  28. ^ Schultz, Thomas (2004). C and the 8051 (3rd ed.). Otsego, MI: PageFree Publishing Inc. p. 20. ISBN 978-1-58961-237-2. Retrieved 2012.
  29. ^ Kernighan & Ritchie (1978), p. 6.
  30. ^ a b c d e f g Klemens, Ben (2013). 21st Century C. O'Reilly Media. ISBN 978-1-4493-2714-9.
  31. ^ Feuer, Alan R.; Gehani, Narain H. (March 1982). "Comparison of the Programming Languages C and Pascal". ACM Computing Surveys. 14 (1): 73-92. doi:10.1145/356869.356872. S2CID 3136859.
  32. ^ Kernighan & Ritchie (1996), p. 122.
  33. ^ For example, gcc provides _FORTIFY_SOURCE. "Security Features: Compile Time Buffer Checks (FORTIFY_SOURCE)". fedoraproject.org. Retrieved 2012.
  34. ^ , (2016). Programming with C. Bangkok, Thailand: SE-EDUCATION PUBLIC COMPANY LIMITED. pp. 225-230. ISBN 978-616-08-2740-4.
  35. ^ Raymond, Eric S. (October 11, 1996). The New Hacker's Dictionary (3rd ed.). MIT Press. p. 432. ISBN 978-0-262-68092-9. Retrieved 2012.
  36. ^ Summit, Steve. "comp.lang.c Frequently Asked Questions 6.23". Retrieved 2013.
  37. ^ Summit, Steve. "comp.lang.c Frequently Asked Questions 7.28". Retrieved 2013.
  38. ^ "Man Page for lint (freebsd Section 1)". unix.com. May 24, 2001. Retrieved 2014.
  39. ^ McMillan, Robert (August 1, 2013). "Is Java Losing Its Mojo?". Wired.
  40. ^ Dale, Nell B.; Weems, Chip (2014). Programming and problem solving with C++ (6th ed.). Burlington, MA: Jones & Bartlett Learning. ISBN 978-1449694289. OCLC 894992484.
  41. ^ Dr. Dobb's Sourcebook. U.S.A.: Miller Freeman, Inc. November-December 1995.
  42. ^ "Using C for CGI Programming". linuxjournal.com. March 1, 2005. Retrieved 2010.
  43. ^ O'Regan, Gerard (September 24, 2015). Pillars of computing : a compendium of select, pivotal technology firms. ISBN 978-3319214641. OCLC 922324121.
  44. ^ Rauchwerger, Lawrence (2004). Languages and compilers for parallel computing : 16th international workshop, LCPC 2003, College Station, TX, USA, October 2-4, 2003 : revised papers. Springer. ISBN 978-3540246442. OCLC 57965544.
  45. ^ Stroustrup, Bjarne (1993). "A History of C++: 1979-1991" (PDF). Retrieved 2011.


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