我正在尝试将 Boost.Asio strand 用于我正在编写的服务器,并想澄清一些事情。

假设我们有这样的 SomeClass:

void SomeClass::foo()
{
    strand_.dispatch(boost::bind(&SomeClass::bar, this));
}

此外, strand 有一个 io_service ,有多个线程调用 run()

在关于 strand::dispatch() 的文档中,我读到它保证通过链发布或分派(dispatch)的处理程序不会同时执行,如果遇到这种情况,处理程序 可能会在此函数中执行 。什么决定处理程序是否立即执行?

多线程的情况下,如果多个io_service线程同时调用SomeClass::foo会dispatch block吗? 即除了被执行的那个之外的所有阻塞。如果是这样,处理程序是否按顺序执行(它们称为 dispatch() 的顺序)?

最佳答案

从逻辑上讲,如果没有(b)锁定的可能性,就不可能满足这种保证。 dispatch 的代码(来自 1.46.1 的 detail/impl/strand_service.hpp)证实了这一点。这就是 Boost 的美妙之处,您可以一目了然。文档 here 中有关于处理程序调用顺序的信息,包括有时不提供排序保证的注释。

template <typename Handler>
void strand_service::dispatch(strand_service::implementation_type& impl,
    Handler handler)
{
  // If we are already in the strand then the handler can run immediately.
  if (call_stack<strand_impl>::contains(impl))
  {
    boost::asio::detail::fenced_block b;
    boost_asio_handler_invoke_helpers::invoke(handler, handler);
    return;
  }

  // Allocate and construct an operation to wrap the handler.
  typedef completion_handler<Handler> op;
  typename op::ptr p = { boost::addressof(handler),
    boost_asio_handler_alloc_helpers::allocate(
      sizeof(op), handler), 0 };
  p.p = new (p.v) op(handler);

  // If we are running inside the io_service, and no other handler is queued
  // or running, then the handler can run immediately.
  bool can_dispatch = call_stack<io_service_impl>::contains(&io_service_);
  impl->mutex_.lock();
  bool first = (++impl->count_ == 1);
  if (can_dispatch && first)
  {
    // Immediate invocation is allowed.
    impl->mutex_.unlock();

    // Memory must be releaesed before any upcall is made.
    p.reset();

    // Indicate that this strand is executing on the current thread.
    call_stack<strand_impl>::context ctx(impl);

    // Ensure the next handler, if any, is scheduled on block exit.
    on_dispatch_exit on_exit = { &io_service_, impl };
    (void)on_exit;

    boost::asio::detail::fenced_block b;
    boost_asio_handler_invoke_helpers::invoke(handler, handler);
    return;
  }

  // Immediate invocation is not allowed, so enqueue for later.
  impl->queue_.push(p.p);
  impl->mutex_.unlock();
  p.v = p.p = 0;

  // The first handler to be enqueued is responsible for scheduling the
  // strand.
  if (first)
    io_service_.post_immediate_completion(impl);
}

关于c++ - Boost.Asio strand.dispatch() 可以阻塞吗?,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/5795704/

10-11 19:37