This chapter is the continuation to the function Introduction chapter.
Example of user-defined function
Write a C program to add two integers. Make a function
add to add integers and display sum in
/*Program to demonstrate the working of user defined function*/
int add(int a, int b); //function prototype(declaration)
printf("Enters two number to add\n");
sum=add(num1,num2); //function call
int add(int a,int b) //function declarator
/* Start of function definition. */
return add; //return statement of function
/* End of function definition. */
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:
- name of the function is
- return type of the function is
- two arguments of type
int are passed to function.
Function prototype are not needed if user-definition function is written before
Control of the program cannot be transferred to user-defined function unless it is called invoked.
Syntax of function call
In the above example, function call is made using statement
main(). This make the control of program jump from that statement to function definition and executes the codes inside that function.
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:
1. Function declarator
Function declarator is the first line of function definition. When a function is called, control of the program is transferred to function declarator.
Syntax of function declarator
return_type function_name(type(1) argument(1),....,type(n) argument(n))
Syntax of function declaration and declarator are almost same except, there is no semicolon at the end of declarator and function declarator is followed by function body.
In above example,
int add(int a,int b) in line 12 is a function declarator.
2. Function body
Function declarator is followed by body of function inside braces.
Passing arguments to functions
In programming, argument(parameter) refers to data this is passed to function(function definition) while calling function.
In above example two variable, num1 and num2 are passed to function during function call and these arguments are accepted by arguments a and b in function definition.
Arguments that are passed in function call and arguments that are accepted in function definition should have same data type. For example:
If argument num1 was of int type and num2 was of float type then, argument variable a should be of type int and b should be of type float,i.e., type of argument during function call and function definition should be same.
A function can be called with or without an argument.
Return statement is used for returning a value from function definition to calling function.
Syntax of return statement
In above example, value of variable add in
add() function is returned and that value is stored in variable sum in
main() function. The data type of expression in return statement should also match the return type of function.