User defined functions in C

User defined functions in C

• ·         The functions which are defined by the user at the time of writing program.
• ·         Functions are made for code reusability and for saving time and space.
• ·         For creating user defined function user can follow the three things.

1. Function prototype

2. Calling function

3. Called function

Function prototype (or) Function Declaration:

General syntax of function declaration is,

return-type function-name (parameter-list) ;

Like variable and an array, a function must also be declared before it is called. A function declaration tells the compiler about a function name and how to call the function. The actual body of the function can be defined separately. A function declaration consist of 4 parts.

·         return-type : It specifies what type of values returned by the function

·         function name: It specifies the name of the function and it follows the identifiers rules.

·         parameter list: The arguments used to perform the function implementation

·         must terminating with semicolon

---

Calling Function:

·                  A function can be accessed by specifying it's name followed by list of arguments enclosed with parenthesis and separated by comma operator. 

·                  If  in function call doesn't require any arguments, Then empty pair of parenthesis must follow the name of the function.

·                  If return type is void in function prototype then syntax to write the calling function is as follows.                  calling_function(var1,var2,var3,......,varn);

                                                                                                Actual arguments (or) actual parameters

·                  If return type is not void in function prototype then syntax to write the calling function is as follows.                            Var_name=calling_function(var1,var2,var3,......,varn);

Ex:          1. message();

                2. add(x,y);

                3. x=add();

                4. x=rfact(n);

Called function (or) Function Definition:

Function definition contains implement code of the function.  The general format of a function definition is:

Syntax: Return_type Function_name (datatype arg1, - - - - - - - , datatype argn)

              {

                        Local Variable Declarations

                        --

                        --          (Implementation Code)

                        --

                        return value;

              }

Where,

• arg1, arg2,  --- , argn arguments are known as “formal arguments”.  List of values passed from the function call are temporarily stored in these formal parameters.
• Variables declared in the function definitions are called local variables.
• After completing implementation of the function, it should return a value to the calling function with a return statement.
• The syntax of a return statement is:

Syntax:           return value;

            If the function should not return any value, then simply placed as:

                        Syntax:           return;