Structure Of a C Program

A typical c program has the following structure:

1. Pre Processor Directives
2. Declaration Of Global Variables
3. Function Prototype
4. Main Function
5. User Defined Function definition.

Example:

//pre processor directive
#include<stdio.h>
//declaration of global variables
int k=0;
//function prototype
void add(int,int);
//main function
void main()
{
int a=10;
add(a,k);
}
//User defined Function definition
void add(int value1,int value2)
{
printf("sum:%d",value1+value2);
}

Pre Processor Directives:

These are the directives to the compiler, they are followed while compiling the code.

ex:

1.#include<****.h>:directive to include the header file during 

compilation

2.#define PI 3.1419  directive defining the value of constant PI,

wherever PI is used it will be replaced by 3.1419

Function Prototype:

A function prototype is nothing but signature of function.(for more info read article on functions) 

ex:

int add(int,int);

It informs the compiler about the existence of a function add in the program.(whose body may be defined after the main function.)

Declaration Of Global Variables:

Global variables are those variables which can be used/ whose scope is throughout the program, ie these variables are common to main function and user defined functions.These variables have to be declared before the main function.

Main Function:

Every C program should have a main function, the execution of a program starts from main function.

However the we can call all other functions from a main function.(Read article on functions for more info on calling a function.).

User Defined Function Definition:

user defined function definition contains the logic for user defined functions.

ex:

void add(int var1,int var2)

{

printf("Sum:%d",var1+var2);

}

this function adds the two variables passed and prints the results on the computer screen.

similarly we can define custom functions to perform tasks desired by us.