Basic Structure of a C Program

Basic Structure of a C Program

C language is very popular language among all the languages. Sometimes I think that had there been no "C" language there would have no C++ and even Java. So let me explain you the basic structure of a C language.

The structure of a C program is a protocol (rules) to the programmer, while writing a C program. The general basic structure of C program is shown in the figure below. The whole program is controlled within main ( ) along with left brace denoted by “{” and right braces denoted by “}”. If you need to declare local variables and executable program structures are enclosed within “{” and “}” is called the body of the main function. The main ( ) function can be preceded by documentation, preprocessor statements and global declarations. 

Documentation Section

Pre-Process or Link Section

Definition Section

Global Declaration Section

Main() Function Section { Declaration Part Executable Part }

Subprogram Section Function 1 Function 2 .  . . Function n ( User–Defined functions)

·         Documentations - The documentation section consist of a set of comment lines giving the name of the program, another name and other details, which the programmer would like to use later.

·         Link Section - The link section provides instructions to the compiler to link function from the system library.

·         Preprocessor Statements - The preprocessor statement begin with # symbol and are also called the preprocessor directive. These statements instruct the compiler to include C preprocessors such as header files and symbolic constants before compiling the C program. Some of the preprocessor statements are listed below.

Example:

Header Files:

#include<stdio.h>

#include <math.h>

#include<stdlib.h>

#include <conio.h>

Symbolic Constants:

#define PI 3.1412

#define TRUE 1

#define FALSE 0

·         Global Declarations - The variables are declared before the main ( ) function as well as user defined functions are called global variables. These global variables can be accessed by all the user defined functions including main ( ) function.

·         The main ( ) function - Each and Every C program should contain only one main ( ). The C program execution starts with main ( ) function. No C program is executed without the main function. The main ( ) function should be written in small (lowercase) letters and it should not be terminated by semicolon. Main ( ) executes user defined program statements, library functions and user defined functions and all these statements should be enclosed within left and right braces.

·         Braces - Every C program should have a pair of curly braces ({, }). The left braces indicates the beginning of the main ( ) function and the right braces indicates the end of the main ( ) function. These braces can also be used to indicate the user-defined functions beginning and ending. These two braces can also be used in compound statements.

·         Local Declarations - The variable declaration is a part of C program and all the variables are used in main ( ) function should be declared in the local declaration section is called local variables. Not only variables, we can also declare arrays, functions, pointers etc. These variables can also be initialized with basic data types. For examples-

Main ( )

{

            int sum = 0;

            int x;

            float y;

}

Here, the variable sum is declared as integer variable and it is initialized to zero. Other variables declared as int and float and these variables inside any function are called local variables.

·         Program statements

These statements are building blocks of a program. They represent instructions to the computer to perform a specific task (operations). An instruction may contain an input-output statements, arithmetic statements, control statements, simple assignment statements and any other statements and it also includes comments that are enclosed within /* and */ . The comment statements are not compiled and executed and each executable statement should be terminated with semicolon.

·         User defined functions

These are subprograms, generally, a subprogram is a function and these functions are written by the user are called user ; defined functions. These functions are performed by user specific tasks and this also contains set of program statements. They may be written before or after a main () function and called within main () function. This is an optional to the programmer.

Now, let us write a small program to display some message shown below.

Example:

/*Program to add and subtract two no.

Name of program Prog1.c

Author name is Ramesh

Date of creation 4/4/05 */

#include<stdio.h>

#define PI 3.1416

int Add(int x, int y);

int main()

{ int a,b,c;

a=10, b=5;

c=Add(a,b);

printf("Sum is : %d",c);

getchar();

return 0;

}

/* Function for Adding two numbers */

int Add(int x, int y)

{ int z;

z=x+y;

return z;

}

Note the followings

·         C is a case sensitive programming language. It means in C printf and Printf will have different meanings.

·         C has a free-form line structure. End of each C statement must be marked with a semicolon.

·         Multiple statements can be one the same line.

·         White Spaces (i.e. tab space and space bar) are ignored.

·         Statements can continue over multiple lines.