Here's another example:

#include <stdio.h>

void add_one_to(int *n)
{
        *n = *n + 1;

        printf("%d\n", i);
        add_one_to(&i);
        printf("%d\n", i);

        return 0;

Use this code as an example, if you don't understand something, read the
book again.

--- Begin Code ---
#include <stdio.h>

void foo(int);
void foo2(int *);

int main(void)
{
  int num = 23;

  printf("Before: %d\n", num);
  foo(num);
  printf("After: %d\n", num);

  printf("Before: %d\n", num);
  foo2(&num);
  printf("After: %d\n", num);

*Output:*
Before: 23
After: 23
Before: 23
After: 100

Bill Cunningham wrote

| On pg 27 1.8 of k&r2, Brian Kernighan writes, in the last sentence, "When
| necessary, it is possible to arrange for a function to modify a variable
in
| a calling routine. The caller must provide the address of the variable to
be
| set..." What's he saying?

He's saying that "In C, all function arguments are passed ``by value.'' This
means that the called function is given the values of its arguments in
temporary variables rather than the originals." However, "When necessary, it
is possible to arrange for a function to modify a variable in a calling
routine" by passing in not the value but the *address* of the variable to be
set. This address, naturally, is still passed by value. But that doesn't
matter, because a pointer is a pointer, and dereferencing the value will
have exactly the same effect as dereferencing the original.

 I've seen prototypes with the * operator, is
| scanf() an example of what he is saying? Is he talking about calling
another
| function or a function calling on a variable in itself?

He's talking about a function dereferencing a pointer to an object in the
caller.