void *ptr = ((char*)l_p - (unsigned int)&((struct node_s*)0)->list);

struct node_s{
    char a;
    char b;
    char c;
    char d;
    struct list_head list;
};

(unsigned int)&((struct node_s*)0)->list

(struct node_s*)0

((struct node_s*)0)->list

&((struct node_s*)0)->list;

(unsigned int)&((struct node_s*)0)->list

((char*)l_p - (unsigned int)&((struct node_s*)0)->list)

void *ptr = (char*)l_p - offsetof(struct node_s, list);

void *ptr = container_of(l_p, struct node_s, list);