Question

Here is the code in question

#include <stdio.h>

struct test {
    unsigned char t;
    unsigned short u;
    unsigned char v;
};


int main ()
{
    struct test  * a = (void *) 0x1000;

    printf("%x %p %p\n",
           sizeof(struct test),
           a + sizeof(struct test),
           a - sizeof(struct test));

    return 0;
}

The sizeof(struct test) prints 6, so I would expect to see:

6 0xffa 0x1006

Instead I get

6 0x1024 0xfdc

Last time I checked, 0x24, or 36, was not equal to 6. It’s not even aligned to anything that I can tell. I am at a complete loss.

Can someone please explain to me why I’m getting these values?

Answer 1

The problem is that when you do pointer arithmetic, it increments by a multiple of the size of the datatype.

So what you’re effectively doing is adding by the square of sizeof(struct test).

Since sizeof(struct test) = 6, you are incrementing the address by 6 * 6 = 36. Hence why you get 0x1024 and 0xfdc instead of 0x1006 and 0xffa. (You also switched the + and -, but that’s a small thing.)

Instead, just do this:

printf("%x %p %p\n",
       sizeof(struct test),
       a + 1,
       a - 1);