C¹

It has been suggested that ANSI C be merged into this article or section. (Discuss)

C
Paradigm	imperative (procedural) systems implementation language
Appeared in	1972
Designed by	Dennis Ritchie
Developer	Dennis Ritchie & Bell Labs
Turing-complete	Yes
Typing discipline	static, weak
Major implementations	GCC, MSVC, Borland C, Watcom C
Influenced by	B (BCPL,CPL), ALGOL 68[1] , Assembly, PL/I, FORTRAN
Influenced	awk, csh, C++, C#, Objective-C, BitC, D, Java, JavaScript, Limbo, Perl, PHP, Python

C is a general-purpose, block structured, procedural, imperative computer programming language developed in 1972 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system.^[2] It has since spread to many other platforms. Although C was designed as a system implementation language,^[3][4] it is also widely used for applications. C has also greatly influenced many other popular languages,^[5] especially C++, which was originally designed as an extension to C.

Contents 1 Philosophy 2 Characteristics 3 History 3.1 Early developments 3.2 K&R C 3.3 ANSI C and ISO C 3.4 C99 3.4.1 New features 3.4.2 Backward compatibility with C90 3.4.3 Support by major compilers 3.4.4 Version detection 3.4.5 Future standards directions 4 Uses 5 Syntax 6 C Operators 6.1 Operator precedence and associativity 7 "Hello, world" example 8 Data structures 8.1 Pointers 8.2 Arrays 8.3 Array-pointer interchangeability 9 Memory management 10 Libraries 11 Criticism 12 Tools for mitigating problems with C 13 Related languages 13.1 C++ 13.2 D 13.3 Objective-C 13.4 Other influences 14 See also 15 References 16 Further information 17 External links 17.1 History 17.2 Tutorials 17.3 Resources

Philosophy

C is an imperative (procedural) systems implementation language. Its design goals were for it to be compiled using a relatively straightforward compiler, provide low-level access to memory, provide language constructs that map efficiently to machine instructions, and require minimal run-time support. C was therefore useful for many applications that had formerly been coded in assembly language.

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

Characteristics

Like most imperative languages in the ALGOL tradition, 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 within functions. Function parameters are always passed by value. Pass-by-reference is achieved in C by explicitly passing pointer values. Heterogeneous aggregate data types (struct) allow related data elements to be combined and manipulated as a unit. C program source text is free-format, using the semicolon as a statement terminator (not a delimiter).

C also exhibits the following more specific characteristics:

• non-nestable function definitions, although variables may be hidden in nested blocks
• partially weak typing; for instance, characters can be used as integers
• low-level access to computer memory by converting machine addresses to typed pointers
• function pointers allowing for a rudimentary form of closures and runtime polymorphism
• array indexing as a secondary notion, defined in terms of pointer arithmetic
• a preprocessor for macro definition, source code file inclusion, and conditional compilation
• complex functionality such as I/O, string manipulation, and mathematical functions consistently delegated to library routines
• around 30 reserved keywords
• syntax divergent from ALGOL, often following the lead of C's predecessor B, for example using
  • { ... } rather than ALGOL's begin ... end
  • the equal-sign for assignment (copying), much like Fortran
  • two consecutive equal-signs to test for equality (compare to .EQ. in Fortran or the equal-sign in BASIC)
  • && and || in place of ALGOL's and and or, which
    • are syntactically distinct from the bit-wise operators & and | (used by B for both meanings)
    • never evaluate the right operand if the result can be determined from the left alone (short-circuit evaluation)
  • a large number of compound operators, such as +=, ++, etc.

History

Early developments

The initial development of C occurred at AT&T Bell Labs between 1969 and 1973; according to Ritchie, the most creative period occurred in 1972. It was named "C" because many of its features were derived from an earlier language called "B", which according to Ken Thompson was a stripped down version of the BCPL programming language.

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 Ritchie and Thompson, incorporating several ideas from colleagues. Eventually they decided to port the operating system to a PDP-11. B's lack of functionality to take advantage of some of the PDP-11's features, notably byte addressability, led to the development of an early version of the C programming language.

The original PDP-11 version of the Unix system was developed in assembly language. By 1973, with the addition of struct types, the C language had become powerful enough that most of the Unix kernel was rewritten in C. This was one of the first operating system kernels implemented in a language other than assembly. (Earlier instances include the Multics system (written in PL/I), and MCP (Master Control Program) for the Burroughs B5000 written in ALGOL in 1961.)

K&R C

In 1978, Brian Kernighan and Dennis Ritchie published the first edition of The C Programming Language. This book, known to C programmers as "K&R", served for many years as an informal specification of the language. The version of C that it describes is commonly referred to as "K&R C". The second edition of the book covers the later ANSI C standard.

K&R introduced several language features:

• standard I/O library
• long int data type
• unsigned int data type
• compound assignment operators =op were changed to op= to remove the semantic ambiguity created by the construct i=-10, which had been interpreted as i =- 10 instead of the possibly intended i = -10

Even after the publication of the 1989 C 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 returned a non-integer value needed to be declared if used before the function definition; a function used without any previous declaration was assumed to return an integer, if its value was used.

For example:

long int SomeFunction();
/* int OtherFunction(); */

/* int */ CallingFunction()
{
    long int test1;
    register /* int */ test2;

    test1 = SomeFunction();
    if (test1 > 0) 
          test2 = 0;
    else 
          test2 = OtherFunction();

    return test2;
}

All the above commented-out int declarations could be omitted in K&R C.

Since K&R function declarations did not include any information about function arguments, function parameter type checks were not performed, although some compilers would issue a warning message if a local function was called with the wrong number of arguments, or if multiple calls to an external function used different numbers or types of arguments. Separate tools such as Unix's lint 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 unofficial features were added to the language, supported by compilers from AT&T and some other vendors. These included:

• void functions
• functions returning struct or union types (rather than pointers)
• assignment for struct data 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 of mainframe computers, minicomputers, and microcomputers, 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. In 1989, the standard was ratified as ANSI X3.159-1989 "Programming Language C." This version of the language is often referred to as ANSI C, Standard C, or sometimes C89.

In 1990, the ANSI C standard (with a few minor modifications) was adopted by the International Organization for Standardization (ISO) as ISO/IEC 9899:1990. This version is sometimes called C90. Therefore, the terms "C89" and "C90" refer to essentially the same language.

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

int main(int argc, char **argv)
{
...
}

although the K&R interface

int main(argc, argv)
    int argc;
    char **argv;
{
...
}

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

C89 is supported by current C compilers, and most C code being written nowadays is based on it. Any program written only in Standard C and without any hardware-dependent assumptions will run correctly on any platform with a conforming C implementation, within its resource limits. Without such precautions, programs may compile only on a certain platform or with a particular compiler, due, for example, to the use of non-standard libraries, such as GUI libraries, or to a reliance on compiler- or platform-specific attributes such as the exact size of data types and byte endianness.

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

#ifdef __STDC__
extern int getopt(int,char * const *,const char *);
#else
extern int getopt();
#endif

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

C99

Note: C99 is also the name of a C compiler for the Texas Instruments TI-99/4A home computer. Aside from being a C compiler, it is otherwise unrelated.

After the ANSI standardization process, the C language specification remained relatively static for some time, whereas C++ continued to evolve, largely during its own standardization effort. Normative Amendment 1 created a new standard for the C language in 1995, but only to correct some details of the C89 standard and to add more extensive support for international character sets. However, the standard underwent further revision in the late 1990s, leading to the publication of ISO/IEC 9899:1999 in 1999. This standard is commonly referred to as "C99." It was adopted as an ANSI standard in May 2000. The international C standard is maintained by the working group ISO/IEC JTC1/SC22/WG14.

New features

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

• inline functions
• variable declaration no longer restricted to file scope or the start of a compound statement
• several new data types, including long long int, optional extended integer types, an explicit boolean data type, and a complex type to represent complex numbers
• variable-length arrays
• support for one-line comments beginning with //, as in BCPL or C++
• new library functions, such as snprintf
• new header files, such as stdbool.h and inttypes.h
• type-generic math functions (tgmath.h)
• improved support for IEEE floating point
• designated initializers
• compound literals
• support for variadic macros (macros of variable arity)
• restrict qualification to allow more aggressive code optimization

Backward compatibility with C90

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. The C standards committee decided that it was of more value for compilers to diagnose inadvertent omission of the type specifier than to silently process legacy code that relied on implicit int. In practice, compilers are likely to diagnose the omission but also assume int and continue translating the program.

Support by major compilers

GCC and other C compilers now support many of the new features of C99. However, there has been less support from vendors such as Microsoft and Borland that have mainly focused on C++, since C++ provides similar functionality improvement.

GCC, despite its extensive C99 support, is still not a completely compliant implementation; several key features are missing or do not work correctly.^[6]

According to Sun Microsystems, Sun Studio (which is freely downloadable) now supports the full C99 standard.^[7]

Version detection

A standard macro __STDC_VERSION__ is defined with value 199901L to indicate that C99 support is available. As with the __STDC__ macro for C90, __STDC_VERSION__ can be used to write code that will compile differently for C90 and C99 compilers, as in this example that ensures that inline is available in either case.

#if __STDC_VERSION__ >= 199901L
  /* "inline" is a keyword */
#else
# define inline /* nothing */
#endif

Future standards directions

Since ratification of the 1999 C standard, the standards working group has prepared technical reports specifying improved support for embedded processing, additional character data types (Unicode support), and library functions with improved bounds checking. Work continues on technical reports addressing decimal floating point, additional mathematical special functions, and additional dynamic memory allocation functions. The C and C++ standards committees have been collaborating on specifications for threaded programming.

As of 2007, work has begun in anticipation of another revision of the C standard, informally called "C1x". The C standards committee has adopted guidelines that should limit the adoption of new features that have not been not tested by existing implementations.

It is likely that the standard gets function, which was officially deprecated in response to a defect report concerning its unsafe interface design, will not be specified in the next revision of the C standard.

Uses

C's primary use is for "system programming", including implementing operating systems and embedded system applications, due to a combination of desirable characteristics such as code portability and efficiency, ability to access specific hardware addresses, ability to "pun" types to match externally imposed data access requirements, and low runtime demand on system resources.

C has also been widely used to implement end-user applications, although as applications became larger much of that development shifted to other, higher-level languages.

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

C is used as an intermediate language by some implementations of higher-level languages, which translate the input language to C source code, perhaps along with other object representations. The C source code is compiled by a C compiler to produce object code. This approach may be used to gain portability (C compilers exist for nearly all platforms) or for convenience (it avoids having to develop machine-specific code generators). Some programming languages which use C this way are Eiffel, Esterel, Gambit, the Glasgow Haskell Compiler, Lisp dialects, Lush, Sather, Squeak, and Vala.

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

Syntax

Main article: C syntax

Unlike languages such as FORTRAN 77, C source code is free-form which allows arbitrary use of whitespace to format code, rather than column-based or text-line-based restrictions. Comments may appear either between the delimiters /* and */, or (in C99) following // until the end of the line.

Each source file contains declarations and function definitions. Function definitions, in turn, contain declarations and statements. Declarations either define new types using keywords such as struct, union, and enum, or assign types to and perhaps reserve storage for new variables, usually by writing the type followed by the variable name. Keywords such as char and int specify built-in types. Sections of code are enclosed in braces ({ and }) to limit the scope of declarations and to act as a single statement for control structures.

As an imperative language, C uses statements to specify actions. The most common statement is an expression statement, consisting of an expression to be evaluated, followed by a semicolon; as a side effect of the evaluation, functions may be called and variables may be assigned new values. To modify the normal sequential execution of statements, C provides several control-flow statements identified by reserved keywords. Structured programming 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 continue can be used to leave the innermost enclosing loop statement or skip to its reinitialization. There is also a non-structured goto statement which branches directly to the designated label within the function. switch selects a case to be executed based on the value of an integer expression.

Expressions can use a variety of built-in operators (see below) and may contain function calls. The order in which operands to most operators, as well as the arguments to functions, 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. This permits a high degree of object code optimization by the compiler, but requires C programmers to exert more care to obtain reliable results than is needed for other programming languages.

C Operators

Main article: Operators in C and C++

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
• equality testing
• order relations
• boolean logic
• bitwise logic
• assignment
• increment and decrement
• reference and dereference
• conditional evaluation
• member selection
• type conversion
• object size
• function argument collection
• sequencing
• subexpression grouping

Operator precedence and associativity

What follows is the list of C operators sorted from highest to lowest priority (precedence). Operators of same priority are presented on the same line. (That the post-increment operator (++) has higher priority than the dereference operator (*) means that an expression *p++ is grouped as *(p++) and not (*p)++. That the subtraction operator (-) has left-to-right associativity means that an expression a-b-c is grouped as (a-b)-c and not a-(b-c).)

Class	Associativity	Operators
Grouping	Nesting	(expr)
Postfix	Left-to-Right	(args)  []  −>  .  expr++  expr−−
Unary	Right-to-Left	!  ~  +  −  *  &  (typecast)  sizeof  ++expr  −−expr
Multiplicative	Left-to-Right	*  /  %
Additive	Left-to-Right	+  −
Shift	Left-to-Right	<<  >>
Relational	Left-to-Right	<  <=  >  >=
Equality	Left-to-Right	==  !=
Bitwise AND	Left-to-Right	&
Bitwise XOR	Left-to-Right	^
Bitwise OR	Left-to-Right	|
Logical AND	Left-to-Right	&&
Logical OR	Left-to-Right	||
Conditional	Right-to-Left	?:
Assignment	Right-to-Left	=  +=  −=  *=  /=  %=  &=  |=  ^=  <<=  >>=
Sequence	Left-to-Right	,

"Hello, world" example

The following simple application appeared in the first edition of K&R, and has become the model for an introductory program in most programming textbooks, regardless of programming language. The program prints out "hello, world" to the standard output, which is usually a terminal or screen display. Standard output might also be a file or some other hardware device, depending on how standard output is mapped at the time the program is executed.

main()
{
    printf("hello, world\n");
}

The above program will compile on most modern compilers that are not in compliance mode, but does not meet the requirements of either C89 or C99. Compiling this program in C99 compliance mode will result in warning or error messages.^[8] A compliant version of the above program follows:

#include <stdio.h>

int main(void)
{
    printf("hello, world\n");
    return 0;
}

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

#include <stdio.h>

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

int main(void)

This next line indicates that a function named main is being defined. The main function serves a special purpose in C programs: The run-time environment calls the main function to begin program execution. The type specifier int indicates that the return value, the value of evaluating the main function that is returned to its invoker (in this case the run-time environment), is an integer. The keyword void as a parameter list indicates that the main function takes no arguments.^[9]

{

This opening curly brace indicates the beginning of the definition of the main function.

    printf("hello, world\n");

This line calls (executes the code for) a function named printf, which is declared in the included header stdio.h and supplied from a system library. In this call, the printf function is passed (provided with) a single argument, the address of the first character in the string literal "hello, world\n". The string literal is an unnamed array with elements of type char, set up automatically by the compiler with a final 0-valued character to mark the end of the array (printf needs to know this). The \n is an escape sequence that C translates to the newline 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 by the caller. (A more careful program might test the return value to determine whether or not the printf function succeeded.) The semicolon ; terminates the statement.

    return 0;

This line terminates the execution of the main function and causes it to return the integer value 0, which is interpreted by the run-time system as an exit code (indicating successful execution).

}

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

Data structures

C has a static weak typing type system that shares some similarities with that of other ALGOL descendants such as Pascal. There are built-in types for integers of various sizes, both signed and unsigned, floating-point numbers, characters, and enumerated types (enum). C99 added a boolean datatype. There are also derived types including arrays, pointers, records (struct), and untagged 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 value in some other way.

Pointers

C supports the use of pointers, a very simple type of reference that records, in effect, the address or location of an object or function in memory. Pointers can be dereferenced to access data stored at the address pointed to, or to invoke a pointed-to function. Pointers can be manipulated using assignment and also 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. (See Array-pointer interchangeability below.) Pointers are used for many different purposes in C. Text strings are commonly manipulated using pointers into arrays of characters. Dynamic memory allocation, which is described below, is performed using pointers. Many data types, such as trees, are commonly implemented as dynamically allocated struct objects linked together using pointers. Pointers to functions are useful for callbacks from event handlers.

A null pointer is a pointer value that points to no valid location (it is often represented by address zero). Dereferencing a null pointer is therefore meaningless, typically resulting in a run-time error. Null pointers are useful for indicating special cases such as no next pointer in the final node of a linked list, or as an error indication from functions returning pointers.

Void pointers (void *) point to objects of unknown type, and can therefore be used as "generic" data pointers. Since the size and type of the pointed-to object is not known, void pointers cannot be dereferenced, nor is pointer arithmetic on them allowed, although they can easily be (and in many contexts implicitly are) converted to and from any other object pointer type.

Arrays

Array types in C are always one-dimensional and, 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. C's unification of arrays and pointers (see below) means that true arrays and these dynamically-allocated, simulated arrays are virtually interchangeable. Since arrays are always accessed (in effect) via pointers, array accesses are typically not checked against the underlying array size, although the compiler may provide bounds checking as an option. Array bounds violations are therefore possible and rather common in carelessly written code (see the "Criticism" article), and can lead to various repercussions: illegal memory accesses, corruption of data, buffer overrun, run-time exceptions, etc.

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

Array-pointer interchangeability

A distinctive (but potentially confusing) feature of C is its treatment of arrays and pointers. The array-subscript notation x[i] can also be used when x is a pointer; the interpretation (using pointer arithmetic) is to access the (i+1)th of several adjacent data objects pointed to by x, counting the object that x points to (which is x[0]) as the first element of the array.

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

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

The number of elements in a declared array a can be determined as sizeof a / sizeof a[0].

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

/* x designates an array */
x[i] = 1;
*(x + i) = 1;
*(i + x) = 1;
i[x] = 1; /* strange, but correct: i[x] is equivalent to *(i + x) */

However, there is a distinction to be made between arrays and pointer variables. Even though the name of an array is in most expression contexts converted to a pointer (to its first element), this pointer does not itself occupy any storage. Consequently, you cannot change what an array "points to", and it is impossible to assign to an array. (Arrays may however be copied using the memcpy function, for example.)

Memory management

One of the most important functions of a programming language is to provide facilities for managing memory and the objects that are stored in memory. C provides three distinct ways to allocate memory for objects:

• Static memory allocation: space for the object is provided in the binary at compile-time; these objects have an extent (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: 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 free

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

Where possible, automatic or static allocation is usually preferred because the storage is managed by the compiler, freeing the programmer of the potentially error-prone chore of manually allocating and releasing storage. However, many data structures can grow in size at runtime, and since static allocations (and automatic allocations in C89 and C90) must have a fixed size at compile-time, there are many situations in which dynamic allocation must be used. Prior to the C99 standard, variable-sized arrays were a common example of this (see "malloc" for an example of dynamically allocated arrays).

Libraries

The C programming language uses libraries as its primary method of extension. In C, a library is a set of functions contained within a single "archive" file. Each library typically has a header file, which contains the prototypes of the functions contained within the library that may be used by a program, and declarations of special data types and macro symbols used with these functions. In order for a program to use a library, it must include the library's header file, and the library must be linked with the program, which in many cases requires compiler flags (e.g., -lm, shorthand for "math library").

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. ("Freestanding" [embedded] C implementations may provide only a subset of the standard library.) This library supports stream input and output, memory allocation, mathematics, character strings, and time values.

Another common set of C library functions are those used by applications specifically targeted for Unix and Unix-like systems, especially functions which provide an interface to the kernel. These functions are detailed in various standards such as POSIX and the Single UNIX Specification.

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

Criticism

Main article: Criticism of the C programming language

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

Tools for mitigating problems with C

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

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

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

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

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

Related languages

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

C++

Main article: C++

Bjarne Stroustrup devised the C++ programming language as one approach to providing object-oriented functionality with C-like syntax. C++ adds greater typing strength, scoping and other tools useful in object-oriented programming and permits generic programming via templates. Nearly a superset of C, C++ now supports most of C, with a few exceptions (see Compatibility of C and C++ for an exhaustive list of differences).

D

Main article: D (programming language)

Unlike C++, which maintains nearly complete backwards compatibility with C, D makes a clean break with C while maintaining the same general syntax. It abandons a number of features of C which Walter Bright (the designer of D) considered undesirable, including the C preprocessor and trigraphs. Some, but not all, of D's extensions to C overlap with those of C++.

Objective-C

Main article: Objective-C

Objective-C was originally a very "thin" layer on top of, and remains a strict superset of, C that permits object-oriented programming using a hybrid dynamic/static typing paradigm. Objective-C derives its syntax from both C and Smalltalk: 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.

Other influences

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

See also

References

Further information

External links

• The current Standard (C99 with Technical corrigenda TC1, TC2, and TC3 included)PDF (3.61 MiB)

History

• The Development of the C Language by Dennis M. Ritchie

Tutorials

• "Everything You Ever Wanted to Know about C Types" four part series by Peter Seebach for IBM developerWorks:

What's in a type?, Floating point and derived types, Implementation details and Portability and pitfalls

• C Programming course at University of Strathclyde Computer Centre
• The C Book by M.Banahan-D.Brady-M.Doran (Addison-Wesley, 2nd ed.) — book for beginning and intermediate students, now out of print and free to download.
• Everything you need to know about pointers in C; material available under Creative Commons Attribution 2.5 License
• Howstuffworks C Programming - guide for learning C language at home.
• Notes on K&R2 - companion to K&R2

Resources

• ISO C Working Group (official Web site)
• comp.lang.c Frequently Asked Questions
• comp.lang.c Wiki
• Official GNU C Library documentation - covers the C89 standard library and most of the C99 standard library, and many standard Unix library functions
• Programming in C (document collection; formerly at Lysator)
• The New C Standard: An economic and cultural commentaryPDF (10.0 MiB) — An unpublished book about "detailed analysis of the International Standard for the C language."

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