我最近读了一本关于如何在Unix环境下编写代码的书。其中有一个示例代码，让我非常困惑。
示例代码如下：

# include "apue.h"

# include <pthread.h>

void *
thr_fn1(void *arg) {
  printf("thread 1 returning\n");
  return ((void *)1);
}

void *
thr_fn2(void *arg) {
  printf("thread 2 exiting\n");
  pthread_exit((void *)2);
}

int 
main(void) {
  int err;
  pthread_t tid1, tid2;
  void *tret;

  err = pthread_create(&tid1, NULL, thr_fn1, NULL);
  if (err != 0) {
    err_exit(err, "can't create thread 1");
  }
  err = pthread_create(&tid2, NULL, thr_fn2, NULL);
  if (err != 0) {
    err_exit(err, "can't create thread 2");
  }
  err = pthread_join(tid1, &tret);
  if (err != 0) {
    err_exit(err, "can't join with thread 1");
  }
  printf("thread 1 exit code %ld\n", (long)tret);
  err = pthread_join(tid2, &tret);
  if (err != 0) {
    err_exit(err, "can't join with thread 2");
  }
  printf("thread 2 exit code %ld\n", (long)tret);
  exit(0);
}

如何算出“(空*)1”或“(空*)2”？是否可以将“void*”类型转换为“long”类型？在我看来，“(void*)1”告诉我1是一个地址，但当我使用deref获取存储在地址1处的值时，它显然是错误的。所以，我认为这个代码太疯狂了。