C programming
C (/siː/, as in the letter c) is a general-purpose,imperative computer programming language, supporting structured programming, lexical variable scope and recursion, while a static type system prevents many unintended operations. By design, C provides constructs that map efficiently to typical machine instructions, and therefore it has found lasting use in applications that had formerly been coded in assembly language, includingoperating systems, as well as variousapplication software for computers ranging from supercomputers to embedded systems.
C was originally developed by Dennis Ritchiebetween 1969 and 1973 at Bell Labs,[6] and used to re-implement the Unix operating system.[7] It has since become one of themost widely used programming languages of all time,[8][9] with C compilers from various vendors available for the majority of existingcomputer architectures and operating systems. C has been standardized by theAmerican National Standards Institute (ANSI) since 1989 (see ANSI C) and subsequently by the International Organization for Standardization (ISO).
C is an imperative procedural language. It was designed to be compiled using a relatively straightforward compiler, to provide low-level access to memory, to provide language constructs that map efficiently to machine instructions, and to require minimal run-time support. Despite its low-level capabilities, the language was designed to encourage cross-platform programming. A standards-compliant and portably written C program can be compiled for a very wide variety of computer platforms and operating systems with few changes to its source code. The language has become available on a very wide range of platforms, from embeddedmicrocontrollers to supercomputers.
Overview
Like most imperative languages in the ALGOLtradition, C has facilities for structured programming and allows lexical variable scope and recursion, while a static type system prevents many unintended operations. In C, all executable code is contained withinsubroutines, which are called "functions" (although not in the strict sense of functional programming). Function parameters are always passed by value. Pass-by-reference is simulated in C by explicitly passing pointervalues. C program source text is free-format, using the semicolon as a statementterminator and curly braces for groupingblocks of statements.
The C language also exhibits the following characteristics:
- There is a small, fixed number of keywords, including a full set of control flowprimitives:
for,if/else,while,switch, anddo/while. User-defined names are not distinguished from keywords by any kind of sigil. - There are a large number of arithmetical and logical operators, such as
+,+=,++,&,~, etc. - More than one assignment may be performed in a single statement.
- Function return values can be ignored when not needed.
- Typing is static, but weakly enforced: all data has a type, but implicit conversions may be performed.
- Declaration syntax mimics usage context. C has no "define" keyword; instead, a statement beginning with the name of a type is taken as a declaration. There is no "function" keyword; instead, a function is indicated by the parentheses of an argument list.
- User-defined (
typedef) and compound types are possible.- Heterogeneous aggregate data types (
struct) allow related data elements to be accessed and assigned as a unit. - Union is a variation from structure.A union may have a number of data elements .But the total allocated memory is the size of the largest data member.The lastly assigned data member takes that memory space displacing the previous.
- 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
enumkeyword. They are not tagged, and are freely interconvertible with integers. - Strings are not a separate data type, but are conventionally implemented asnull-terminated arrays of characters.
- Heterogeneous aggregate data types (
- Low-level access to computer memory is possible by converting machine addresses to typed pointers.
- Procedures (subroutines not returning values) are a special case of function, with an untyped return type
void. - Functions may not be defined within the lexical scope of other functions.
- Function and data pointers permit ad hocrun-time polymorphism.
- A preprocessor performs macro definition,source code file inclusion, and conditional compilation.
- There is a basic form of modularity: files can be compiled separately and linkedtogether, with control over which functions and data objects are visible to other files via
staticandexternattributes. - Complex functionality such as I/O, stringmanipulation, and mathematical functions are consistently delegated to library routines.
While C does not include some features found in some other languages, such as object orientation or garbage collection, such features can be implemented or emulated in C, often by way of external libraries (e.g., theBoehm garbage collector or the GLib Object System).
Relations to other languages
Many later languages have borrowed directly or indirectly from C, including C++, C#, Unix'sC shell, D, Go, Java, JavaScript, Limbo, LPC,Objective-C, Perl, PHP, Python, Rust, Swift, andVerilog (hardware description language)[5]. These languages have drawn many of theircontrol structures and other basic features from C. Most of them (with Python being the most dramatic exception) are also verysyntactically similar to C in general, and they tend to combine the recognizable expression and statement syntax of C with underlying type systems, data models, and semantics that can be radically different.
History
Early developments
The origin of C is closely tied to the development of the Unix operating system, originally implemented in assembly languageon 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 developed in assembly language. The developers were considering rewriting the system using the B language, Thompson's simplified version of BCPL.[10] However B's inability to take advantage of some of the PDP-11's features, notably byte addressability, led to C. The name of C was chosen simply as the next after B.[11]
The development of C started in 1972 on the PDP-11 Unix system[12] and first appeared inVersion 2 Unix.[13] The language was not initially designed with portability in mind, but soon ran on different platforms as well: a compiler for the Honeywell 6000 was written within the first year of C's history, while anIBM System/370 port followed soon.[1][12]
Also in 1972, a large part of Unix was rewritten in C.[14] By 1973, with the addition of
struct types, the C language had become powerful enough that most of the Unix kernelwas now in C.
Unix was one of the first operating system kernels implemented in a language other thanassembly. Earlier instances include theMultics system (which was written in PL/I) and Master Control Program (MCP) for theBurroughs B5000 (which was written inALGOL) in 1961. In around 1977, Ritchie andStephen 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]
K&R C
The cover of the book, The C Programming Language, first edition by Brian Kernighanand Dennis Ritchie
In 1978, Brian Kernighan and Dennis Ritchiepublished 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. The second edition of the book[15] covers the later ANSI Cstandard, described below.
K&R introduced several language features:
- Standard I/O library
long intdata typeunsigned intdata type- Compound assignment operators of the form
=op(such as=-) were changed to the formop=(that is,-=) to remove the semantic ambiguity created by constructs such asi=-10, which had been interpreted asi =- 10(decrementiby 10) instead of the possibly intendedi = -10(letibe -10).
Even after the publication of the 1989 ANSI standard, for many years K&R C was still considered the "lowest common denominator" to which C programmers restricted themselves when maximum portability was desired, since many older compilers were still in use, and because carefully written K&R C code can be legal Standard C as well.
In early versions of C, only functions that return types other than
int must be declared if used before the function definition; functions used without prior declaration were presumed to return type int.
For example:
long some_function();
/* int */ other_function();
/* int */ calling_function()
{
long test1;
register /* int */ test2;
test1 = some_function();
if (test1 > 0)
test2 = 0;
else
test2 = other_function();
return test2;
}
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 checkswere not performed, although some compilers would issue a warning message if a local function was called with the wrong number of arguments, or if multiple calls to an external function used different numbers or types of arguments. Separate tools such as Unix's lintutility 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[16]) and some other vendors. These included:
voidfunctions (i.e., functions with no return value)- functions returning
structoruniontypes (rather than pointers) - assignment for
structdata types - enumerated types
The large number of extensions and lack of agreement on a standard library, together with the language popularity and the fact that not even the Unix compilers precisely implemented the K&R specification, led to the necessity of standardization.
ANSI C and ISO C
During the late 1970s and 1980s, versions of C were implemented for a wide variety ofmainframe computers, minicomputers, andmicrocomputers, including the IBM PC, as its popularity began to increase significantly.
In 1983, the American National Standards Institute (ANSI) formed a committee, X3J11, to establish a standard specification of C. X3J11 based the C standard on the Unix implementation; however, the non-portable portion of the Unix C library was handed off to the IEEE working group 1003 to become the basis for the 1988 POSIX standard. In 1989, the C standard was ratified as ANSI X3.159-1989 "Programming Language C". This version of the language is often referred to asANSI C, Standard C, or sometimes C89.
In 1990, the ANSI C standard (with formatting changes) was adopted by the International Organization for Standardization (ISO) as ISO/IEC 9899:1990, which is sometimes called C90. Therefore, the terms "C89" and "C90" refer to the same programming language.
ANSI, like other national standards bodies, no longer develops the C standard independently, but defers to the international C standard, maintained by the working group ISO/IEC JTC1/SC22/WG14. National adoption of an update to the international standard typically occurs within a year of ISO publication.
One of the aims of the C standardization process was to produce a superset of K&R C, incorporating many of the subsequently introduced unofficial features. The standards committee also included several additional features such as function prototypes(borrowed from C++),
void pointers, support for international character sets andlocales, and preprocessor enhancements. Although the syntax 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 C code being written today is based on it. Any program written only in Standard C and without any hardware-dependent assumptions will run correctly on anyplatform 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 byteendianness.
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.[citation needed]
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.[17]
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.
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.[18]C11
In 2007, work began on another revision of the C standard, informally called "C1X" until its official publication on 2011-12-08. The C standards committee adopted guidelines to limit the adoption of new features that had not been tested 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.Embedded C
Historically, embedded C programming requires nonstandard extensions to the C language in order to support exotic features such as fixed-point arithmetic, multiple distinct memory banks, and basic I/O operations.
In 2008, the C Standards Committee published a technical report extending the C language[19] 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 specified by the C standard.[20] 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.[21]
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 intspecify built-in types. Sections of code are enclosed in braces ({ and }, 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 statementsto specify actions. The most common statement is an expression statement, consisting of an expression to be evaluated, followed by a semicolon; as a side effect of the evaluation, functions may be called and variables may be assigned new values. To modify the normal sequential execution of statements, C provides several control-flow statements identified by reserved keywords.Structured programming is supported by
if(-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 continuecan be used to leave the innermost enclosing loop statement or skip to its reinitialization. There is also a non-structured gotostatement which branches directly to the designated label within the function. switch selects a case to be executed based on the value of an integer expression.
Expressions can use a variety of built-in operators and may contain function calls. The order in which arguments to functions and operands to most operators are evaluated is unspecified. The evaluations may even be interleaved. However, all side effects (including storage to variables) will occur before the next "sequence point"; sequence points include the end of each expression statement, and the entry to and return from each function call. Sequence points also occur during evaluation of expressions containing certain operators (
&&, ||, ?: and the comma operator). This permits a high degree of object code optimization by the compiler, but requires C programmers to take more care to obtain reliable results than is needed for other programming languages.
Kernighan and Ritchie say in the Introduction of The C Programming Language: "C, like any other language, has its blemishes. Some of the operators have the wrong precedence; some parts of the syntax could be better."[22]The C standard did not attempt to correct many of these blemishes, because of the impact of such changes on already existing software.
Character set
The basic C source character set includes the following characters:
- Lowercase and uppercase letters of ISO Basic Latin Alphabet:
a–zA–Z - Decimal digits:
0–9 - Graphic characters:
! " # % & ' ( ) * + , - . / : ; < = > ? [ \ ] ^ _ { | } ~ - Whitespace characters: space, horizontal tab, vertical tab, form feed, newline
Newline indicates the end of a text line; it need not correspond to an actual single character, although for convenience C treats it as one.
Additional multi-byte encoded characters may be used in string literals, but they are not entirely portable. The latest C standard (C11) allows multi-national Unicode characters to be embedded portably within C source text by using
\uXXXX or \UXXXXXXXX encoding (where the X denotes a hexadecimal character), although this feature is not yet widely implemented.
The basic C execution character set contains the same characters, along with representations for alert, backspace, andcarriage return. Run-time support for extended character sets has increased with each revision of the C standard.
Reserved words
C89 has 32 reserved words, also known 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:[23]
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.[24]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:
+,-,*,/,% - assignment:
= - augmented assignment:
+=,-=,*=,/=,%=,&=,|=,^=,<<=,>>= - bitwise logic:
~,&,|,^ - bitwise shifts:
<<,>> - boolean logic:
!,&&,|| - conditional evaluation:
? : - equality testing:
==,!= - calling functions:
( ) - increment and decrement:
++,-- - member selection:
.,-> - object size:
sizeof - order relations:
<,<=,>,>= - reference and dereference:
&,*,[ ] - sequencing:
, - subexpression grouping:
( ) - type conversion:
(typename)
C uses the operator
= (used in mathematics to express equality) to indicate assignment, following the precedent ofFortran 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.[25]
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.[26]"Hello, world" example
The "hello, world" example, which appeared in the first edition of K&R, has become the model for an introductory program in most programming textbooks, regardless of programming language. The program prints "hello, world" to the standard output, which is usually a terminal or screen display.
The original version was:[27]
main()
{
printf("hello, world\n");
}
A standard-conforming "hello, world" program is:[a]
#include <stdio.h>
int main(void)
{
printf("hello, world\n");
}
The first line of the program contains apreprocessing 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. 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 next line indicates that a function named
main is being defined. The mainfunction serves a special purpose in C programs; the run-time environment calls the main function to begin program execution. The type specifier int indicates that the value that is returned to the invoker (in this case the run-time environment) as a result of evaluating the main function, is an integer. The keyword void as a parameter list indicates that this function takes no arguments.[b]
The opening curly brace indicates the beginning of the definition of the
mainfunction.
The next line calls (diverts execution to) a function named
printf, which in this case is supplied from a system library. In this call, the printf function is passed (provided with) a single argument, the address of the first character in the string literal "hello, world\n". The string literal is an unnamedarray with elements of type char, set up automatically by the compiler with a final 0-valued character to mark the end of the array (printf needs to know this). The \n is an escape sequence that C translates to anewline character, which on output signifies the end of the current line. The return value of the printf function is of type int, but it is silently discarded since it is not used. (A more careful program might test the return value to determine whether or not the printf function succeeded.) The semicolon ; terminates the statement.
The closing curly brace indicates the end of the code for the
main function. According to the C99 specification and newer, the main function, unlike any other function, will implicitly return a value of 0 upon reaching the } that terminates the function. (Formerly an explicit return 0;statement was required.) This is interpreted by the run-time system as an exit code indicating successful execution.[28]
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