關於指標間加減運算

c-pointer-1.c

#include "string.h"
#include "stdlib.h"
#include "stdio.h"
 
 
/*
20150608 關於指標間加減運算
by CantBelieve W
*/ 
 
int main(void)
{
    short *a, *b;
    a = (short *)1;
    b = (int *)1;
    printf("%d %d %d %d %d\n", a, b ,a + (int)b, (int)(a + (int)b), (int)a + (int)b); 
    /*
      a+(int)b = *a + sizeof(*a)*(*b) = 1 + 2*1 = 3
      (int)(a + (int)b) = (int)3 = 3
      (int)a + (int)b = 1 + 1 = 2   
    */
    
    a = (int *)0x5566;
    b = (int *)0x5570; 
    printf("%d %d %d %d %d\n", a, b, (int)a+(int)b, a+(int)b, sizeof(int *));
    /*
      a = 21862, b = 21872 (transfer base 16 to base 10)
      (int)a+(int)b = 21862 + 21872 = 43734
      a+(int)b = *a + sizeof(*a)*b = 21862 + 2*21872 = 65606
    */
    
    printf("%d\n", a - b);
    printf("%d\n", ((int)b - (int)a)/sizeof(int *));
    
    /*
      a - b = (21862 - 21872)/sizeof(*a) = -10/2 = -5
      (在這為一個 offset 的概念，意即-5為位移了5 bits，位址只能相減不能相加) <=重要!
      ((int)b - (int)a)/sizeof(int *) = 10/4 = 2 
    */
        
    a = (int*)0x1000;
    b = a + 3;
    printf("%d %d %p %p \n", a, b, a, b);
     
    /* 
       a = 4096
       b = a + 3 = 4096 + 3*sizeof(*a) = 4096 + (3*2) = 4102   
    */ 
     
    system("pause");
    return 0;
}