Page 85 - Programming Microcontrollers in C

70 Chapter 2 Advanced C Topics

Pointers and Function Arguments
Values passed to functions as arguments are copies of the real
values. The data to be passed to a function are pushed on the stack or
saved in registers prior to the function call. Therefore, if the function
should modify any of the arguments, this modification would not
propagate to the calling function. Therefore, a function like
void swap(int x, int y)
{
int temp;

temp = x;
x = y;
y = temp;
}
does nothing but swap two passed values, and those values are never
returned to the calling program. This performance is good as well as
bad. The bad situation is shown above when the function simply
does not work as expected. The good side is that it is not easy to
inadvertently change variable values in the calling program. The use
of pointers permits this problem to be avoided. The technique is called
passing parameters by reference. Consider the following function:

void swap(int* px, int* py)
{
int temp;

temp = *px;
*px = *py;
*py = temp;
}
Here the integers pointed to by px and py are swapped. These
integers are the values in the calling program. The pointer values in
the calling program are unaltered.
C makes extensive use of passing parameters by reference. Re­
call the first program in Chapter 1. That program has the line

printf(“Microcontrollers run the world!\n”);