pthread_join

pthread_join - wait for thread termination
(pthread_tryjoin_np pthread_timedjoin_np)非阻塞版本和超时版本
超时通过futex控制

• 检查是否detached

/* Is the thread joinable?.  */
  if (IS_DETACHED (pd))
    /* We cannot wait for the thread.  */
    return EINVAL;

每个线程都有一个joinid， 表示谁正在join该线程，如果pd->joinid == pd自身, 就是detached了
detached的线程在退出时会自己释放资源

pthread_detach 函数会设置该值
另外，如果设置了DETACHSTATE标志，那么pthread_create返回之前就会设置该值
pd->joinid = iattr->flags & ATTR_FLAG_DETACHSTATE ? pd : NULL;

• 设置自身可以被异步取消

struct pthread *self = THREAD_SELF;
  int result = 0;

  /* During the wait we change to asynchronous cancellation.  If we
     are canceled the thread we are waiting for must be marked as
     un-wait-ed for again.  */
  pthread_cleanup_push (cleanup, &pd->joinid);

  /* Switch to asynchronous cancellation.  */
  int oldtype = CANCEL_ASYNC ();

默认情况，线程是同步取消,  这意味这只有在某个遇到某个取消点才允许被取消， 换句话说
如果线程已经阻塞了，或者while(1); 线程是不会退出的

如果线程a试图join线程b，那么a被设置为异步取消，这样，只要有人试图pthread_cancel(a)
a将会退出
cleanup 函数在被取消的情况下会调用，其参数为&pd->joinid
它再次允许别的线程取消

static void
cleanup (void *arg)
{//
  /* If we already changed the waiter ID, reset it.  The call cannot
     fail for any reason but the thread not having done that yet so
     there is no reason for a loop.  */
  (void) atomic_compare_and_exchange_bool_acq ((struct pthread **) arg, NULL,
                           THREAD_SELF);
}

• 原子的设置joinid为self，不允许别的线程再join该线程

/* Wait for the thread to finish.  If it is already locked something
     is wrong.  There can only be one waiter.  */
if (atomic_compare_and_exchange_bool_acq (&pd->joinid, self, NULL))
    return EINVAL;

即如果joinid == NULL, 则设置其值为self， 如果 joinid != NULL则不做改动，标示它被别人join了

• 等待线程退出

/* Wait for the 
child.  */
    lll_wait_tid (pd->tid);

该函数使用futex系统调用，直到pd->tid等于0
(内核会在thread退出的时候唤醒等待线程,  因为clone的时候使用了CLONE_CHILD_CLEARTID)

• 修改取消模式

  /* Remove the handler.  */
  pthread_cleanup_pop (0);
不执行cleanup函数

• 释放内存并返回

      /* Store the return value if the caller is interested.  */
      if (thread_return != NULL)
    *thread_return = pd->result;


      /* Free the TCB.  */
      __free_tcb (pd);