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C is a general-purpose, procedural, imperative computer programming language developed in 1972 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system.[1] It has since spread to many other platforms, and is now one of the most widely used programming languages. C has also greatly influenced many other popular languages,[2] especially C++, which was originally designed as an enhancement to C. It is the most commonly used programming language for writing system software,[3][4] though it is also widely used for writing applications.
PhilosophyC is an imperative (procedural) systems implementation language. Among its minimalistic design goals were that it could be compiled in a straightforward manner using a relatively simple compiler, provide low-level access to memory, generate only a few machine language instructions for each of its core language elements, and not require extensive run-time support. C is therefore suitable for many applications that had traditionally been implemented in assembly language.
CharacteristicsAs 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. Parameters of C functions are always passed by value. Pass-by-reference is achieved in C by explicitly passing pointer values. Heterogeneous aggregate data types (the struct in C) allow related data elements to be combined and manipulated as a unit. C has around 30 reserved keywords and the source text is free-format, using semicolon as a statement terminator (not a delimiter). C also exhibits the following more specific characteristics:
HistoryEarly developmentsThe 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.
By 1973, the C language had become powerful enough that most of the Unix kernel, originally written in PDP-11 assembly language, 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 CIn 1978, Dennis Ritchie and Brian Kernighan 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:
For many years after the introduction of ANSI C, 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 ANSI 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. For example:
long int SomeFunction();
int OtherFunction();
int CallingFunction()
{
long int test1;
int test2;
test1 = SomeFunction();
if (test1 > 0)
test2 = 0;
else
test2 = OtherFunction();
return test2;
}
In the example, both 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 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 (since there was no standard), supported by compilers from AT&T and some other vendors. These included:
The large number of extensions and lack of 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 CImage:Kr c prog lang.jpg The C Programming Language, 2nd edition, is a widely used reference on ANSI C. During the late 1970s, C began to replace BASIC as the leading microcomputer programming language. During the 1980s, it was adopted for use with the IBM PC, and its popularity began to increase significantly. At the same time, Bjarne Stroustrup and others at Bell Labs began work on adding object-oriented programming language constructs to C, resulting in the language now called C++. 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++),
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 #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 C99
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 9899:1999 in 1999. This standard is commonly referred to as "C99." It was adopted as an ANSI standard in March 2000. C99 introduced several new features, many of which had already been implemented as extensions in several compilers:
C99 is for the most part upward-compatible with C90, but is stricter in some ways; in particular, a declaration that lacks a type specifier no longer has 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 don't work correctly.[2] A standard macro #if __STDC_VERSION__ >= 199901L /* "inline" is a keyword */ #else # define inline /* nothing */ #endif UsageC'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 higher-level languages. This is implemented in one of two ways, as languages which:
C source code is then input to a C compiler, which then outputs finished machine or object code. This is done to gain portability (C compilers exist for nearly all platforms) and to avoid having to develop machine-specific code generators. 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
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 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 As an imperative language, C depends on statements to do most of the work. Most statements are expression statements which simply evaluate an expression; as a side effect, variables may receive new values. Control-flow statements are also available for conditional or iterative execution, constructed with reserved keywords such as Operator precedence
What follows is the list of C operators sorted from highest to lowest priority. Operators of same priority are presented on the same line. "R→L" associativity means that adjacent operators of the same priority are executed from right to left, and conversely for "L→R".
"hello, world" exampleThe 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.[5] 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, int main(void) This next line indicates that a function named {
This opening curly brace indicates the beginning of the definition of the printf("hello, world\n");
This line calls (executes the code for) a function named return 0; This line terminates the execution of the } This closing curly brace indicates the end of the code for the If the above code were compiled and executed, it would do the following:
Data structuresC 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 ( 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. (The use of type casts obviously sacrifices some of the safety normally provided by the type system.) PointersC also allows 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 the data stored at the address pointed to, or to invoke the pointed-to function. Pointers can be manipulated using normal assignments and also pointer arithmetic. The run-time representation of a pointer value is typically a raw memory address, 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. 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 ( ArraysArray 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 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 interchangeabilityA distinctive (but potentially confusing) feature of C is its treatment of arrays and pointers. The array-subscript notation Formally, Furthermore, in most expression contexts (a notable exception is The number of elements in an array 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 /* x designates an array */ x[i] = 1; *(x + i) = 1; *(i + x) = 1; i[x] = 1; /* strange, but correct */ 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 Memory managementOne 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:
These three approaches are appropriate in different situations and have various tradeoffs. For example, static memory allocation has no allocation overhead, automatic allocation has a small amount of overhead during initialization, 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 hassle of manually allocating and releasing storage. Unfortunately, many data structures can grow in size at runtime; since automatic and static allocations must have a fixed size at compile-time, there are many situations in which dynamic allocation must be used. Variable-sized arrays are a common example of this (see "malloc" for an example of dynamically allocated arrays). LibrariesThe 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., 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. CriticismDespite 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 achieve 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 issues with CTools 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, 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 languagesWhen 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++The C++ programming language was derived from C and is Bjarne Stroustrup's answer to adding 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 relevant exceptions (mostly of stronger typing restriction; see Compatibility of C and C++ for an exhaustive list of differences). Objective-CObjective-C is a very "thin" layer on top of, and is 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 is taken from Smalltalk. Objective-C and C++ differ in philosophies -- see the Objective-C article for details. Other influencesC has directly or indirectly influenced many later languages such as Java, C#, Perl, PHP, JavaScript, 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. Notes
References
See also
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