The arguments passed to function can be of two types

1. Values passed
2. Address passed

The first type refers to call by value and the second type refers to call by reference.

For instance consider program1

main()
{
int x=50, y=70;
interchange(x,y);
printf(“x=%d y=%d”,x,y);
}

interchange(x1,y1)
int x1,y1;
{
int z1;
z1=x1;
x1=y1;
y1=z1;
printf(“x1=%d y1=%d”,x1,y1);
}

Here the value to function interchange is passed by value.

Consider program2

main()
{
int x=50, y=70;
interchange(&x,&y);
printf(“x=%d y=%d”,x,y);
}

interchange(x1,y1)
int *x1,*y1;
{
int z1;
z1=*x1;
*x1=*y1;
*y1=z1;
printf(“*x=%d *y=%d”,x1,y1);
}

Here the function is called by reference. In other words address is passed by using symbol & and the value is accessed by using symbol *.

The main difference between them can be seen by analyzing the output of program1 and program2.

The output of program1 that is call by value is

x1=70 y1=50
x=50 y=70

But the output of program2 that is call by reference is

*x=70 *y=50
x=70 y=50

This is because in case of call by value the value is passed to function named as interchange and there the value got interchanged and got printed as

x1=70 y1=50

and again since no values are returned back and therefore original values of x and y as in main function namely

x=50 y=70 got printed.

But in case of call by reference address of the variable got passed and therefore what ever changes that happened in function interchange got reflected in the address location and therefore the got reflected in original function call in main also without explicit return value. So value got printed as *x=70 *y=50 and x=70 y=50