struct packet {
  char type;
};

#define TAILQ_HEAD(name, type)            \
struct name {               \
  struct type *tqh_first; /* first element */     \
  struct type **tqh_last; /* addr of last next element */   \
}

void test()
{
    TAILQ_HEAD(, packet) outgoing; //结构体的标签为NULL
    TAILQ_HEAD(xxx, packet) outgoing1; //结构体的标签为xxx
}