Types of functions
While calling a function, there are two ways in which arguments can be passed to a function:
Call by value
The call by value method of passing arguments to a function copies the actual value of an argument into the formal parameter of the function. In this case, changes made to the parameter inside the function have no effect on the argument.
By default, C programming uses call by value to pass arguments. In general, it means the code within a function cannot alter the arguments used to call the function. Consider the function swap() definition as follows.
/* function definition to swap the values */
void swap(int x, int y)
{
int temp; temp = x; /* save the value of x */
x = y; /* put y into x */
y = temp; /* put temp into y */
return;
}
Call by reference
The call by reference method of passing arguments to a function copies the address of an argument into the formal parameter. Inside the function, the address is used to access the actual argument used in the call. It means the changes made to the parameter affect the passed argument.
To pass a value by reference, argument pointers are passed to the functions just like any other value. So accordingly you need to declare the function parameters as pointer types as in the following function swap(), which exchanges the values of the two integer variables pointed to, by their arguments.
/* function definition to swap the values */
void swap(int *x, int *y) {
int temp;
temp = *x; /* save the value at address x */
*x = *y; /* put y into x */
*y = temp; /* put temp into y */
return;
}
