quick guide

It is one of the most important feature of ANSI C.
A function prototype tells the compiler the type of data
returned by the function, the number of arguments the
function expects to receive, the types of arguments,and the
order in which these arguments are expected.The compiler
uses function prototype to validate function calls.
The function prototype is written as follows:
Data-type function-name(type1,type2 ..........typen);
Where data-type is the data-type returned by the function
type1,type2,.......typen are the type of data fed as
arguments.
Any C function by default returns an int value. More
specifically whenever a call is made to a function,the
compiler assumes that this function would return a value of
the type int.If we desire that a function should return a
value other than an int, the it is necessary to explicitly
mention so in the calling function as well as in the called
function. Suppose we want to find out square of a number
using a function,

1) main()
{
float a,b;
printf("\n Enter any number");
scanf("%f",&a);
b=square(a);
printf("\n square of %f is %f", a,b);
}
square(float x)
{
float y;
y=x*x;
return(y);
}
And here are the sample runs of this program
Enter any number 3
square of 3 is 9.000000
Enter any number 1.5
square of 1.5 is 2.000000
Enter any number 2.5
square of 2.5 is 6.000000

The first of these answers is correct. But square of 1.5
is definitely not 2. This is happened because any C function,
by default always returns an integer value. Therefore, even
though the function square() calculates the square of 1.5 as
2.25,it is not capable of returning a float value. To
overcome this problem, the function square() must be declared
as float in main() as well.The statement float square() means
that it is a function which will return a float value.

Return Statement:
The return statement has two important uses:

1) It causes an important exit from the function it is in.
That is, it causes program execution to return to the
calling code.
2) It can be used to return the value.
The syntax of the return statement is:
return;
return(var);
return(expression);
Where var is any variable and expression is any arithmetic
or logical expression.When the return statement is encount-
ered in the called function, control is passed on to the
calling function and a variable var or the value obtained by
the arithmetic or logical expression is returned by the
called function to the calling function.