原来是pthread_create后没有调pthread_join或者pthread_detach, 导致memory leak.

A  thread may either be joinable or detached.  If a thread is joinable,
       then another thread can call pthread_join(3) to wait for the thread  to
       terminate  and  fetch its exit status.  Only when a terminated joinable
       thread has been joined are the last of its resources released  back  to
       the system.  When a detached thread terminates, its resources are auto-
       matically released back to the system: it is not possible to join  with
       the  thread  in  order  to  obtain  its  exit  status.  Making a thread
       detached is useful for some types of daemon threads whose  exit  status
       the  application does not need to care about.  By default, a new thread
       is created in a joinable state, unless  attr  was  set  to  create  the
       thread in a detached state (using pthread_attr_setdetachstate(3)).