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/