(type *) ((unsigned char *) q -

#include <stddef.h>
#include <stdio.h>
#include <iostream>
using namespace std;

#define q_data(q, type, link) \
    (type *) ((unsigned char *) q - offsetof(type, link))
    
    
typedef struct b{
    int c;
}b_t;
typedef struct a{
    b_t b;
}a_t;

int main() {

    a_t aa;
    b_t* bb= &(aa.b);
    bb->c=1;

    printf("%p\n",&aa);
    printf("%p\n",&bb);
    
    a_t* aa_test=nullptr;
    b_t* bb_test;
    bb_test=bb;

    aa_test = (a_t *)((unsigned char *)bb_test - offsetof(a_t, b));
   
    printf("%p\n",aa_test);
    
    printf("%d",aa_test->b.c);
    return 0;
}