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Function Declaration or Prototype in C programming

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The function declaration or prototype is model or blueprint of the function. If functions are used before they are defined, then function declaration or prototype is necessary. Many programmers prefer a “top-down” approach in which main appears ahead of the programmer defined function definition. Function prototypes are usually written at the beginning of a program, ahead of any user-defined function including main(). Function prototypes provide the following information to the compiler.
− The name of the function
− The type of the value returned by the function
− The number and the type of arguments that must be supplied while calling the function.
In “bottom-up” approach where user-defined functions are defined ahead of main() function, there is no need of function prototypes. The general syntax of function prototype is
 return_type function_name (type1, type2, ...., typen) ;
where, return_type specifies the data type of the value returned by the function. A function can return value of any data type. If there is no return value, the keyword void is used. type1, type2, ....., typen are type of arguments. Arguments are optional.
 For example:
 int add (int, int) ; /* int add (int a, int b) ;

 void display (int a) ; /* void display (int); */
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