int pointer 依照記憶體對齊的特性，一定會排在記憶體位置可被 4 整除的位置上（binary 最後兩位為 0)，所以可以在最後兩位儲存不大於 4 的資料。
#include <stdio.h>
#include <assert.h>
int main() {
assert(sizeof(int) == 4);
int x = 701;
int x2 = 702;
int x3 = 123;
int *p = &x;
int *p3 = &x3;
int *p2 = &x2;
printf("pointer 1: %p \n", p);
printf("pointer 2: %p \n", p2);
printf("pointer 3: %p \n", p3);
// pointer 1: 0x16cfcaf5c
// pointer 2: 0x16cfcaf58
// pointer 3: 0x16cfcaf54
// c, 8, 4(hexidecimal) in binary all ends with at least two 0.
}
#include <assert.h>
#include <stdio.h>
void put_data(int* ptr, unsigned int data) {
assert(data < 4);
*ptr |= data;
}
unsigned int get_data(int* ptr) {
return (*ptr & 3);
}
void cleanse_pointer(int* ptr) {
*ptr &= ~3;
}
int main() {
int x = 701;
int* x_addr = &x;
printf("x's address: %p\n", x_addr);
put_data(&x_addr, 3);
printf("x's address after putting data in: %p\n", x_addr);
printf("data stored in x's address: %d\n", get_data(&x_addr));
cleanse_pointer(&x_addr);
printf("Cleansed ptr: %p\n", x_addr);
printf("Dereferencing cleansed ptr: %d\n", *x_addr);
return 0;
}
/*
x's address: 0x16ba9b0e8
x's address after putting data in: 0x16ba9b0eb
data stored in x's address: 3
Cleansed ptr: 0x16ba9b0e8
Dereferencing cleansed ptr: 701
*/struct rb_node {
unsigned long __rb_parent_color;
struct rb_node *rb_right;
struct rb_node *rb_left;
} __attribute__((aligned(sizeof(long))));
/* The alignment might seem pointless, but allegedly CRIS needs it */
struct rb_root {
struct rb_node *rb_node;
};__rb_parent_color stores the parent’s address.
#define rb_parent(r) ((struct rb_node *)((r)->__rb_parent_color & ~3))
技巧同上述的 cleanse_pointer，先將顏色去除，就可得到真正的 address。