Thread: c offsetof macro. quick question

I'm screwing around with the macro offsetof. Just for the sake of screwing around with it. I can't figure out one piece. Here's my code:

Code:

#include <stdlib.h>
#include <stdio.h>
#include <stddef.h>

struct Object {
	int rc;
};

struct RawPoint {
	int x,y;
};

struct Point {
	struct Object _;
	int x;
	int y;
};

int main(int argc, char ** argv) {
	struct Point * point = malloc(sizeof(struct Point));
	point->x = 10;
	point->y = 10;
	size_t offset = offsetof(struct Point,x);
	
	struct RawPoint * rawPoint = (struct RawPoint *) (point + offset);
	struct RawPoint * rawPoint2 = (struct RawPoint *) &(point->x);
	
	//shouldn't rawPoint and rawPoint2 be the same address?
	
	printf("offset: %ld\n",(long)offset);
	printf("point: %p\n",point);
	printf("rawPoint: %p\n",rawPoint);
	printf("rawPoint2 %p\n",rawPoint2);
	
	
	return 0;
}

I'm trying to figure out why rawPoint and rawPoint2 aren't the same addresses? I'm obviously missing something. Any ideas?

Thanks!

Originally Posted by brewbuck

Remember how pointer arithmetic works. "ptr + n" adds n*sizeof(*ptr) to the address, not just n bytes.

When navigating bytewise in memory, everything needs to be cast to char *

doh! thanks!