uv_default_loop() 函数做了什么？

[huenchao]

static uv_loop_t default_loop_struct;
static uv_loop_t* default_loop_ptr;

uv_loop_t* uv_default_loop(void) {
  if (default_loop_ptr != NULL)
    return default_loop_ptr;

  if (uv_loop_init(&default_loop_struct))
    return NULL;

  default_loop_ptr = &default_loop_struct;
  return default_loop_ptr;
}

上面的代码可以看出来，就是一个单例的静态初始化，真正初始化的事情是uv_loop_init做的。

[huenchao]

int uv_loop_init(uv_loop_t* loop) {
  void* saved_data;
  int err;


  saved_data = loop->data;
  memset(loop, 0, sizeof(*loop));
  loop->data = saved_data;

  heap_init((struct heap*) &loop->timer_heap);
  QUEUE_INIT(&loop->wq);
  QUEUE_INIT(&loop->idle_handles);
  QUEUE_INIT(&loop->async_handles);
  QUEUE_INIT(&loop->check_handles);
  QUEUE_INIT(&loop->prepare_handles);
  QUEUE_INIT(&loop->handle_queue);

  loop->active_handles = 0;
  loop->active_reqs.count = 0;
  loop->nfds = 0;
  loop->watchers = NULL;
  loop->nwatchers = 0;
  QUEUE_INIT(&loop->pending_queue);
  QUEUE_INIT(&loop->watcher_queue);

  loop->closing_handles = NULL;
  uv__update_time(loop);
  loop->async_io_watcher.fd = -1;
  loop->async_wfd = -1;
  loop->signal_pipefd[0] = -1;
  loop->signal_pipefd[1] = -1;
  loop->backend_fd = -1;
  loop->emfile_fd = -1;

  loop->timer_counter = 0;
  loop->stop_flag = 0;

  err = uv__platform_loop_init(loop);
  if (err)
    return err;

  uv__signal_global_once_init();
  err = uv_signal_init(loop, &loop->child_watcher);
  if (err)
    goto fail_signal_init;

  uv__handle_unref(&loop->child_watcher);
  loop->child_watcher.flags |= UV_HANDLE_INTERNAL;
  QUEUE_INIT(&loop->process_handles);

  err = uv_rwlock_init(&loop->cloexec_lock);
  if (err)
    goto fail_rwlock_init;

  err = uv_mutex_init(&loop->wq_mutex);
  if (err)
    goto fail_mutex_init;

  err = uv_async_init(loop, &loop->wq_async, uv__work_done);
  if (err)
    goto fail_async_init;

  uv__handle_unref(&loop->wq_async);
  loop->wq_async.flags |= UV_HANDLE_INTERNAL;

  return 0;

fail_async_init:
  uv_mutex_destroy(&loop->wq_mutex);

fail_mutex_init:
  uv_rwlock_destroy(&loop->cloexec_lock);

fail_rwlock_init:
  uv__signal_loop_cleanup(loop);

fail_signal_init:
  uv__platform_loop_delete(loop);

  return err;
}

上面代码量很大，但其实都是初始化一些基本数据，本文不关心这些0还是-1代表什么，具体可以参考下面给的文章去阅读具体含义。但是上面的步骤里至少用到了handle_queue，我在debug的时候发现，起码QUEUE_INSERT_TAIL了，child_watcher的handle ，还有async_cb的handle。大家可以自行debug去证明我说的。
所以整个初始化后，handle_queue这个双向循环链表里起码有3个handle了。

成员变量的含义，可以参考:https://blog.butonly.com/posts/node.js/libuv/2-libuv-event-loop/