C Programming Language

C (pronounced like the letter C) is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system.

Although C was designed for implementing system software, it is also widely used for developing portable application software.

C is one of the most widely used programming languages of all time and there are very few computer architectures for which a C compiler does not exist. C has greatly influenced many other popular programming languages, most notably C++, which began as an extension to C.

Design

C is an imperative (procedural) systems implementation 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. 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 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 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 subroutines, 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 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 more specific characteristics:

• There are a small, fixed number of keywords, including a full set of flow of control primitives: for , if , while , switch , and do..while . There is basically one namespace, and 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 statement.
• Function return values can be ignored when not needed.
• Typing is static, but weakly-enforced: all data has a type, but implicit conversions can be performed; for instance, characters can be used as integers.
• 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, for example assigned, as a unit.
  • Array indexing is a secondary notion, 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, e.g. month .
  • Enumerated types are possible with the enum keyword. They are not tagged, and are freely interconvertible with integers.
  • Strings are not a separate data type, but are conventionally implemented as null-terminated arrays of characters.
• 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, returning the dummy type void .
• Functions may not be defined within the lexical scope of other functions.
• Function and data pointers support ad hoc run-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 linked together, with control over which functions and data objects are visible to other files via static and extern attributes.
• Complex functionality such as I/O, string manipulation, and mathematical functions are consistently delegated to library routines.

C does not include some features found in newer, more modern high-level languages, including:

• Object orientation
• Garbage collection

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 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 inability to take advantage of some of the PDP-11's features, notably byte addressability, led to the development of an early version of C.

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 of the form = op (such as =- ) were changed to the form op = to remove the semantic ambiguity created by such constructs as 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- int value needed to be declared if used before the function definition; a function used without any previous declaration was assumed to return type int , if its value was used.

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
                              
                                ;
                              
                              
                                }
                              
                            

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 (i.e. functions with no return value)
• 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 standa