A TO Z Programing In C


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 origin of C is closely tied to the development of the Unix operating system, originally implemented in assembly language on 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.[11] 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.[12]

The development of C started in 1972 on the PDP-11 Unix system[13] and first appeared in Version 2 Unix.[14] 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.[1][13]

Also in 1972, a large part of Unix was rewritten in C.[15] 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) and Master Control Program (MCP) for the Burroughs B5000 (which was 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.[13]


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