offset_hack.c

typedef union {
	struct { char pad_x[31]; int x; } __attribute__((packed));
	struct { char pad_y[512]; int y; } __attribute__((packed));
	// ...
} __attribute__((packed)) overlay;

// Macrofied:

#define JOIN1(x, y) x##y
#define JOIN(x, y) JOIN1(x, y)
#define FIELD_OFFSET(field, offset) struct { char JOIN(pad, __LINE__)[offset]; field; } __attribute__((packed))

typedef union {
	FIELD_OFFSET(int x, 31);
	FIELD_OFFSET(int y, 500);
	// ...
} __attribute__((packed)) overlay;