C (programming language)

C (/ˈsiː/, 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, including operating systems, as well as various application software for computers ranging from supercomputers to embedded systems.

C was originally developed by Dennis Ritchie between 1969 and 1973 at Bell Labs, and used to re-implement the Unix operating system. It has since become one of the most widely used programming languages of all time, with C compilers from various vendors available for the majority of existing computer 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).

Design

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. Therefore, C was useful for many applications that had formerly been coded in assembly language, for example in system programming.

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.

Overview

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 systemprevents 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.

Relations to other languages

Many later languages have borrowed directly or indirectly from C, including C++, D, Go, Rust, Java, JavaScript, Limbo, LPC, C#, Objective-C, Perl, PHP, Python, Swift, Verilog(hardware description language),^[4] and Unix's C shell. These languages have drawn many of their control structures and other basic features from C. Most of them (with Python being the most dramatic exception) are also very syntactically 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.

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

Early developments

The origin of C is closely tied to the development of the Unix operating system, originally implemented in assembly language on a PDP-7by 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. 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.

The development of C started in 1972 on the PDP-11 Unix system and first appeared in Version 2 Unix. 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 an IBM System/370 port followed soon.

Also in 1972, a large part of Unix was rewritten in C. By 1973, with the addition of struct types, the C language had become powerful enough that most of the Unix kernel was now in C.

Unix was one of the first operating system kernels implemented in a language other than assembly. Earlier instances include the Multics system which was written in PL/I), andMaster Control Program (MCP) for the Burroughs B5000 written in ALGOL in 1961. In around 1977, Ritchie and Stephen C. Johnson made further changes to the language to facilitate portability of the Unix operating system. Johnson's Portable C Compiler served as the basis for several implementations of C on new platforms.

This is Dennis Richtie's book Dennis Richtie C book

This is an introduction to C programming C programming Tutorial