• Hello everyone!. You might remember about my last post on “Introduction to Function” in which I had discussed about the what function is and what advantages it can bring. If you haven’t read my previous post yet, I strongly suggest you to read it first as this post is a supplementary for the same.
  • In this post as promised I will try to explain basic elements of functions in as easy way as possible.

Elements of Functions

  • Every function has the following elements:
  1. Function Declaration (Function Prototype)
  2. Function Definition
  3. Function Call
  1. Function Declaration (Function prototype)

  • Similar to variables, in C, every function needs to be declared before it can be used.
  • Functions can be declared either globally (inside the main() ) or locally (outside the main() ) as per the program requirement. (You can read more on this at Structure of C Program)


return_type function_name( argument list ) ;
  • Function prototype has the following three parts:
Return type :
  • It specifies return type of the function that means type of the data that a function will return such as int, float, char etc.
  • Function that does not return any value will have return type as void.
Function Name:
  • It is the name of the function and it should be any meaningful name.
  • It should follow all naming convention rules.
  • Some examples of function names are display, show, calculate etc.
Argument List:
  •  Argument list specifies list of all arguments required for the function to perform the desired task.
  • Arguments are separated by comma. It may include both data type and argument name but normally only data type is provided.
  • The following is an example of function prototype for doing sum of two float values:
Function Declaration Example
float sum ( float, float ) ;  OR  float sum (float x, float y) ;
  • Where, int is a return type, sum is the name of the function and int x, int y are the argument list.
  1. Function Definition

  • Function definition contains executable block of statements.
  • It can be written either before or after the main() but normally it is written after the main().


return_type function_name( argument list )
    // local variable declaration ;

    // executable statements ;


    // return statement ;
  • The function definition contains the following two parts:
  1. Function Header
  2. Function Body
  1. Function Header:

  • The first line of function definition is known as function header.
  • It consists of three parts: return type, function name and argument list.
  1. Function Body:

  • It consists of three parts: local variable declaration part, executable part and return statement.
  • All the required variables are declared at the local variable declaration part.
  • Executable part contains all the executable statements that performs actual task.
  • return statement should be the last statement inside the function body. It is optional.
  • If return type of function is void then it can be omitted but function having return type other than void must include return statement.
  • Example of function definition is given below for above function declaration.
Function Definition Example
float sum ( float x, float y)
    float result ;          // local variable
    result = x + y ;       // executable statement

    return result ;       // return statement
  • Example of function without return type is as follow:
void sum ( float x, float y)
    float result ;         // local variable
    result = x + y ;      // executable statement

    printf(“\n Sum is %.2f” , result);

    return ;        // return statement. its optional
  • Note : We can omit return statement if function return type is void.
  1. Function Call

  • A function can be called simply by using name of the function followed by a list of Actual parameters (Arguments) if any.
  • The following example calls sum() function that we defined above.
void main ( )
    float result ;                   // receiver local variable

    result = sum (10.0, 20.0 );     // function call

    printf(“\n Sum is %.2f” , result);

  • When the compiler encounters a function call, the control is transferred to the function definition and executes the function line by line and a computed value is returned with the help of return statement.
  • The returned value is assigned to the receiver variable. This is illustrated as below:

Function Call

  • Arguments that passed in function call and arguments receiving in function definition should have the same data type.
  • For example in above function call sum (10.0, 20.0) we have passed arguments of float type and inside function definition those float type arguments are received by float type variable x and y respectively. In short, type of arguments in function call and function definition should be same.
  • The argument names used in function definition is known as formal parameters.