Example of user-defined function

/*Program to demonstrate the working of user defined function*/
#include <stdio.h>
int add(int a, int b);           //function prototype(declaration)
int main(){
     int num1,num2,sum;
     printf("Enters two number to add\n");
     scanf("%d %d",&num1,&num2);
     sum=add(num1,num2);         //function call 
     printf("sum=%d",sum); 
     return 0;
}
int add(int a,int b)            //function declarator
{             
/* Start of function definition. */
     int add;
     add=a+b;
     return add;                  //return statement of function 
/* End of function definition. */   
}                                  

Function prototype(declaration):

Every function in C programming should be declared before they are used. These type of declaration are also called function prototype. Function prototype gives compiler information about function name, type of arguments to be passed and return type.

Syntax of function prototype

return_type function_name(type(1) argument(1),....,type(n) argument(n));

In the above example,int add(int a, int b); is a function prototype which provides following information to the compiler:

1. name of the function is add()
2. return type of the function is int.
3. two arguments of type int are passed to function.

Function prototype are not needed if user-definition function is written before main() function.

Function call

Control of the program cannot be transferred to user-defined function unless it is called invoked.

Syntax of function call

function_name(argument(1),....argument(n));

In the above example, function call is made using statement add(num1,num2); from main(). This make the control of program jump from that statement to function definition and executes the codes inside that function.

Function definition

Function definition contains programming codes to perform specific task.

Syntax of function definition

return_type function_name(type(1) argument(1),..,type(n) argument(n))
{
                //body of function
}

Function definition has two major components: