求结构体的字段的偏移量

linux内核中求偏移量的宏定义如下
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)

------------------------------宏测试小程序---------------------------------------------------------------------

#include
#include

#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)

struct test_struct{
     char   ch;
     int    it;
     int    ul;
};

int main()
{
     size_t off=-1;
      struct test_struct *st,*rt;

     st = (struct test_struct *) malloc (sizeof(struct test_struct));

     st->ch='a';
     st->it=1;
     st->ul=1ul;

/*    off =(size_t)&(((struct test_struct *)0)->it);*/
      off=offsetof(struct test_struct,it);
//    rt=(struct test_struct *)((char *)(&st->it)-off);
      rt=(struct test_struct *)(((char *)&(st->it)-(char *)off));

      printf("%d %d %d",rt->it,off,sizeof(struct test_struct));


      return 0;
}
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由于4字节补齐的原因，sizeof(struct test_struct)=12,而不是等于9