c/c++ 에서 구조체 선언에 관해 질문드려요~~~

# vim include/osip2/osip_mt.h
 
struct osip_thread;
 
struct osip_thread *osip_thread_create (int stacksize, void *(*func) (void *), void *arg);

# vim src/osip2/port_thread.c
 
struct osip_thread *osip_thread_create (int stacksize, void *(*func) (void *), void *arg)
{
  int i;
  osip_thread_t *thread = (osip_thread_t *) osip_malloc (sizeof (osip_thread_t));
 
  if (thread == NULL)
    return NULL;
 
  i = pthread_create (thread, NULL, func, (void *) arg);
  if (i != 0) {
    OSIP_TRACE (osip_trace (__FILE__, __LINE__, OSIP_ERROR, NULL, "Error while creating a new thread\n"));
    osip_free (thread);
    return NULL;
  }
  return (struct osip_thread *) thread;
}