Question

I want to create a function which gets a void pointer. This pointer points to an arbitrary user data. This is not relevant now. Which is more important a descriptor, which describe this user data. It is under this data. I want to get it’s address, there are two different solution:

struct data_desc {
     size_t size;
     data_type_t type;
     /* And so on... */
};

/* Operate on the descriptor */
void operate_on_data(void *ptr)
{
    struct data_desc *desc;
    /* Now I want to get the desc /*

    /* This is the first approach, simply fails */
    desc = ((struct data_desc *)ptr) - sizeof(struct data_desc);
    /* This is the second, it works...*/
    desc = (struct data_desc *)ptr;
    desc--; 
    /* Do something with desc */
}

As you can see I use two diffrent approach to get the descriptor’s address. I thought the first is more natural, but it’s just not works. I used more parentheses to avoid the precedence problems.

I know that these methods are not safe. The first does not work, why? What is the reason behind this behavior?

Thanks in advance!

Answer 1

The problem is pointer arithmetic. When you take a pointer and subtract 1, you are really subtracting (1*sizeof(struct)). The first equation with the “-sizeof”, you are really subtracting (sizeof(struct) * sizeof(struct)) bytes from the pointer. Make sense?