Example of memory addressing in C

data_member_addressing.c

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

struct S {
	// This is complicated by padding, but:

	// NB: sizeof(int32_t) = 4 bytes (32 bits)
	// base_address + 0
	int32_t x;
	// base_address + sizeof(x) = base_address + 4
	int32_t y;
};

int main() {
	struct S s = {
		.x = 0xAAAAAAAA,
		.y = 0xBBBBBBBB,
	};

	// These two accesses are compiled down to:
	//   (int32_t*)(&s + 0)
	//   (int32_t*)(&s + 4)
	// The "constant offsets" here are 0 and 4, which are calculated from
	// the structure of S at compile time -- not determined at runtime.

	printf(
		"%8x\n"
		"%8x\n",
		s.x,
		s.y
	);

	// Or, to use the utilities provided by the language in C99
	// (see http://en.cppreference.com/w/c/types/offsetof)...
	// This is effectively equivalent to:
	// (size_t)(((struct S*)0)->member)

	size_t x_offset = offsetof(struct S, x);
	size_t y_offset = offsetof(struct S, y);

	int32_t* x_ptr = (int32_t*) ( ((uint8_t*)&s) + x_offset );
	int32_t* y_ptr = (int32_t*) ( ((uint8_t*)&s) + y_offset );

	printf(
		// (base_address + offset) = value
		"*(%p + %u) = %8x\n"
		"*(%p + %u) = %8x\n",
		&s, (unsigned)x_offset, *x_ptr,
		&s, (unsigned)y_offset, *y_ptr
	);

	return 0;
}