Fail to get valid host ip address in Mac OS X 10.11.6

[eeyrw]

I modified the example echoserv.c a bit like this to get ip address:

...
static void onAccept(dyad_Event *e) {
   printf("IP:%s\n",dyad_getAddress(e->remote));
  dyad_addListener(e->remote, DYAD_EVENT_DATA, onData, NULL);
  dyad_writef(e->remote, "echo server\r\n");
}
...

int main(void) {
  dyad_Stream *s;
  dyad_init();

  s = dyad_newStream();
  dyad_addListener(s, DYAD_EVENT_ERROR,  onError,  NULL);
  dyad_addListener(s, DYAD_EVENT_ACCEPT, onAccept, NULL);
  dyad_listen(s, 8000);
  printf("IP:%s\n",dyad_getAddress(s));
   

  while (dyad_getStreamCount() > 0) {
    dyad_update();
  }

  return 0;
}

Finally I got the host ip address 0.0.0.0 which looks like invalid but the remote address is valid. The communication works well.
Is it a bug concerned with OS or my incorrect practice of API?