As we all know, before using a variable we need to declare it. Similarly, before using a function we need to declare the function. This declaration of the function is called as function prototyping.

Consider the below program.

float fnGetAverage(int, int);

void main()
{
    int x,y;
    float z;
    printf(“\nEnter the values for x and y”);
    scanf(“%d %d”, &x, &y);
    z = fnGetAverage(x, y); 			//function call
    printf(“\nThe average is %d”, z);
}

float fnGetAverage(int a, int b)
{ 					        //function definition
    float c;
    c = (a+b)/2; //calculating average
    return c; //returning the value
}

There is one statement before the main() function.

The statement is: float fnGetAverage(int, int);
This statement is called function prototype. By doing this we are telling the compiler that we will use the function having following features:

1. The function name would be fnGetAverage
2. The function will return float value
3. The function will contain two int arguments

If we do not wish to use function prototyping then we need to define the function before calling it

This can be done as follows:

float fnGetAverage(int a, int b)
{ 					        //function definition
    float c;
    c = (a+b)/2; //calculating average
    return c; //returning the value
}
void main()
{
    int x,y;
    float z;
    printf(“\nEnter the values for x and y”);
    scanf(“%d %d”, &x, &y);
    z = fnGetAverage(x, y); 			//function call
    printf(“\nThe average is %d”, z);
}