The first C programme.

C

An Overview of C

The purpose of this chapter is to present an overview of the C programming language, its origins, its

uses, and its underlying philosophy. This chapter is mainly for newcomers to C.

A Brief History of C

            C was invented and first implemented by Dennis Ritchie on a DEC PDP-11 that used the Unix

operating system. C is the result of a development process that started with an older language called

BCPL. BCPL was developed by Martin Richards, and it influenced a language called B, which was

invented by Ken Thompson. B led to the development of C in the 1970s.

           For many years, the de facto standard for C was the version supplied with the Unix operating

system. It was first described in The C Programming Language by Brian Kernighan and Dennis

Ritchie (Englewood Cliffs, N.J.: Prentice-Hall, 1978). In the summer of 1983 a committee was

established to create an ANSI (American National Standards Institute) standard that would define

the C language. The standardization process took six years (much longer than anyone reasonably

expected).

The ANSI C standard was finally adopted in December 1989, with the first copies becoming

available in early 1990. The standard was also adopted by ISO (International Standards

Organization), and the resulting standard was typically referred to as ANSI/ISO Standard C. In

1995, Amendment 1 to the C standard was adopted, which, among other things, added several new

library functions. The 1989 standard for C, along with Amendment 1, became a base document for

Standard C++, defining the C subset of C++. The version of C defined by the 1989 standard is

commonly referred to as C89.

During the 1990s, the development of the C++ standard consumed most programmers' attention.

However, work on C continued quietly along, with a new standard for C being developed. The end

result was the 1999 standard for C, usually referred to as C99. In general, C99 retained nearly all of

the features of C89. Thus, C is still C! The C99 standardization committee focused on two main

areas: the addition of several numeric libraries and the development of some special-use, but highly

innovative, new features, such as variable-length arrays and the restrict pointer qualifier. These

innovations have once again put C at the forefront of computer language development.

As explained in the part opener, Part One of this book describes the foundation of C, which is the

version defined by the 1989 standard. This is the version of C in widest use, it is currently accepted

by all C compilers, and it forms the basis for C++. Thus, if you want to write C code that can be

compiled by a legacy compiler, for example, you will want to restrict that code to the features

described in Part One. Part Two will examine the features added by C99. C Is a Middle-Level Language

C is often called a middle-level computer language. This does not mean that C is less powerful,

harder to use, or less developed than a high-level language such as BASIC or Pascal, nor does it

imply that C has the cumbersome nature of assembly language (and its associated troubles). Rather,

C is thought of as a middle-level language because it combines the best elements of high-level

languages with the control and flexibility of assembly language. Table 1-1 shows how C fits into the

spectrum of computer languages.

As a middle-level language, C allows the manipulation of bits, bytes, and addresses— the basic

elements with which the computer functions. Despite this fact, C code is also very portable.

Portability means that it is easy to adapt software written for one type of computer or operating

system to another type. For example, if you can easily convert a program written for DOS so that it

runs under Windows 2000, that program is portable.

High level             Ada

Modula-2

Pascal

COBOL

FORTRAN

BASIC

Middle level         Java

C++

C

FORTH

Macro-assembler

Low level              Assembler

A Tutorial Introduction

Let us begin with a quick introduction in C. Our aim is to show the essential elements of the language in real

programs, but without getting bogged down in details, rules, and exceptions. At this point, we are not trying to be

complete or even precise (examples are meant to be correct). We want to get you as quickly as

possible to the point where you can write useful programs, and to do that we have to concentrate on the basics:

variables and constants, arithmetic, control flow, functions, and the rudiments of input and output. We are

intentionally leaving out of this chapter features of C that are important for writing bigger programs. These include

pointers, structures, most of C's rich set of operators, several control-flow statements, and the standard library.

1.1 Getting Started

The only way to learn a new programming language is by writing programs in it. The first program to write is the

same for all languages:

Print the words

hello, world

This is a big hurdle; to leap over it you have to be able to create the program text somewhere, compile it

successfully, load it, run it, and find out where your output went. With these mechanical details mastered,

everything else is comparatively easy.

In C, the program to print ``hello, world'' is

#include <stdio.h>

main()

{

printf("hello, world\n");

}

Just how to run this program depends on the system you are using. As a specific example, on the UNIX operating

system you must create the program in a file whose name ends in ``.c'', such as hello.c, then compile it with

the command

cc hello.c

If you haven't botched anything, such as omitting a character or misspelling something, the compilation will

proceed silently, and make an executable file called a.out. If you run a.out by typing the command

a.out

it will print

hello, world

Now, for some explanations about the program itself. A C program, whatever its size, consists of functions and

variables. A function contains statements that specify the computing operations to be done, and variables store

values used during the computation. C functions are like the subroutines and functions in Fortran or the procedures

and functions of Pascal. Our example is a function named main. Normally you are at liberty to give functions

whatever names you like, but ``main'' is special - your program begins executing at the beginning of main. This

means that every program must have a main somewhere.

main will usually call other functions to help perform its job, some that you wrote, and others from libraries that

are provided for you. The first line of the program,

#include <stdio.h>

tells the compiler to include information about the standard input/output library; the line appears at the beginning

of many C source files.

One method of communicating data between functions is for the calling function to provide a list of values, called

arguments, to the function it calls. The parentheses after the function name surround the argument list. In this

example, main is defined to be a function that expects no arguments, which is indicated by the empty list ( ).

#include <stdio.h>           include information about standard library

main()                       define a function called main

that received no argument values

{                            statements of main are enclosed in braces

printf("hello, world\n");    main calls library function printf

to print this sequence of characters

}                            \n represents the newline character

The statements of a function are enclosed in braces { }. The function main contains only one statement,

printf("hello, world\n");

A function is called by naming it, followed by a parenthesized list of arguments, so this calls the function printf

with the argument "hello, world\n". printf is a library function that prints output, in this case the string

of characters between the quotes.

A sequence of characters in double quotes, like "hello, world\n", is called a character string or string

constant. For the moment our only use of character strings will be as arguments for printf and other functions.

The sequence \n in the string is C notation for the newline character, which when printed advances the output to

the left margin on the next line. If you leave out the \n (a worthwhile experiment), you will find that there is no

line advance after the output is printed. You must use \n to include a newline character in the printf argument;

if you try something like

printf("hello, world

");

the C compiler will produce an error message.

printf never supplies a newline character automatically, so several calls may be used to build up an output line

in stages. Our first program could just as well have been written

#include <stdio.h>

main()

{

printf("hello, ");

printf("world");

printf("\n");

}

to produce identical output.

                                                                                                          Author: Danish Beigh

                                                                                                          Email id: beigh.danish@gmail.com