<C Primer Plus >2 Altering Variables in the Calling Function

#include <stdio.h>
int diff(int x, int y);//the first form
void interchange(int *u, int *v);//the second form
int main(void){
    int x = 9;
    int y = 5;
    int z;
    printf("The Originally x = %d and y = %d
", x, y);//alter variables in the calling function
    interchange(&x, &y);
    z = diff(x, y);//need a value for some calculation or action
    printf("Now  x = %d and y = %d
", x, y);
    printf("z = %d
", z);
    return 0;
}

int diff(int x, int y){
    return(x - y);
}

void interchange(int *u,int *v){
    int temp;

    temp = *u;
    *u = *v;
    *v = temp;
}

int function1(int x);

int function2(int *ptr);

Remeber:

1 Use the first form if the function needs a value for some calculation or action;

2 Use the second form if the function needs to alter variables in the calling function.

　　One function does not have direct access to variables declared in another function. If you do need one function to acess another function's data,you can use pointer function arguments.