Call by value vs.
call by reference

Concept

"Call by Value" is the normal method by which C passes arguments to a function. A copy of the value of the argument is sent; therefore it is impossible to modify the original argument using the associated parameter in the function.

"Call by Reference" is the other method supported by C, where the argument to be communicated is passed via an address. Again, the address of the argument (variable) and not the (variable's) value is passed. The function can modify the argument since it has its address!

The SetToZero example

This won't work:

     x = 1;
     SetToZero(x);
     ...

     void SetToZero(int x) 
     {
        x = 0;
     }

This will:

     x = 1;
     SetToZero(&x);
     ...

     void SetToZero(int *varPtr) 
     {
        *varPtr = 0;
     }

The SwapIntegers example

     void SwapIntegers (int *p1, int *p2) 
     {

        int temp;
     
        temp = *p1;
        *p1  = *p2;
        *p2  = temp;
     }

Returning multiple values

Call by reference is one way to write a function that "returns" more than one value. See the hours.c example.