[Boost-users] Why does async_connect() call listen()?

Subject: [Boost-users] Why does async_connect() call listen()?
From: Soren Dreijer (dreijer_at_[hidden])
Date: 2014-05-17 22:06:15

Hi there,

By coincidence, I ended up looking at a stack trace where my application
calls async_connect() on a asio::ip::tcp::socket. Two things took me by
surprise:

 1. Asio calls listen() under the hood. Why does it do that when I'm just
trying to make an outbound TCP connection?
 2. If the listen() call above is indeed required, why does it happen on
the same thread where I call async_connect()? I thought the point of the
method was to defer the work to the reactor? (For what it's worth, I've had
a case on a customer's machine where the call to listen() would hang for 30
seconds before returning -- probably because of a bad firewall -- causing
my entire app to hang since the code assumes async_connect() is indeed
async.)

I've included the full stack trace below.

Thanks,
Soren

0:000> k
ChildEBP RetAddr
0117db50 0f681c65 WS2_32!listen
0117db70 0f691013 EvoClr!boost::asio::detail::socket_ops::listen+0x35
[c:\private\src\vendor\boost_1_55_0_sdk\include\boost\asio\detail\impl\socket_ops.ipp
@ 658]
0117dbd8 0f690e9f
EvoClr!boost::asio::detail::socket_select_interrupter::open_descriptors+0x163
[c:\private\src\vendor\boost_1_55_0_sdk\include\boost\asio\detail\impl\socket_select_interrupter.ipp
@ 78]
0117dbe4 0f690cc0
EvoClr!boost::asio::detail::socket_select_interrupter::socket_select_interrupter+0xf
[c:\private\src\vendor\boost_1_55_0_sdk\include\boost\asio\detail\impl\socket_select_interrupter.ipp
@ 42]
0117dc1c 0f690c1e
EvoClr!boost::asio::detail::select_reactor::select_reactor+0x70
[c:\private\src\vendor\boost_1_55_0_sdk\include\boost\asio\detail\impl\select_reactor.ipp
@ 48]
0117dc44 0f17837e
EvoClr!boost::asio::detail::service_registry::create<boost::asio::detail::select_reactor>+0x4e
[c:\private\src\vendor\boost_1_55_0_sdk\include\boost\asio\detail\impl\service_registry.hpp
@ 81]
0117dc80 0f6909e9
EvoClr!boost::asio::detail::service_registry::do_use_service+0x9e
[c:\private\src\vendor\boost_1_55_0_sdk\include\boost\asio\detail\impl\service_registry.ipp
@ 123]
0117dca0 0f6908ee
EvoClr!boost::asio::detail::service_registry::use_service<boost::asio::detail::select_reactor>+0x39
[c:\private\src\vendor\boost_1_55_0_sdk\include\boost\asio\detail\impl\service_registry.hpp
@ 49]
0117dca8 0f68fab0
EvoClr!boost::asio::use_service<boost::asio::detail::select_reactor>+0xe
[c:\private\src\vendor\boost_1_55_0_sdk\include\boost\asio\impl\io_service.hpp
@ 34]
0117dcbc 0f68f731
EvoClr!boost::asio::detail::win_iocp_socket_service_base::get_reactor+0x30
[c:\private\src\vendor\boost_1_55_0_sdk\include\boost\asio\detail\impl\win_iocp_socket_service_base.ipp
@ 620]
0117dce4 0f68f6f1
EvoClr!boost::asio::detail::win_iocp_socket_service_base::start_connect_op+0x11
[c:\private\src\vendor\boost_1_55_0_sdk\include\boost\asio\detail\impl\win_iocp_socket_service_base.ipp
@ 550]
0117dd28 0f68ed22
EvoClr!boost::asio::detail::win_iocp_socket_service<boost::asio::ip::tcp>::async_connect<boost::_bi::bind_t<void,boost::_mfi::mf1<void,Swift::BoostConnection,boost::system::error_code
const
&>,boost::_bi::list2<boost::_bi::value<boost::shared_ptr<Swift::BoostConnection>
>,boost::arg<1> > > >+0xc1
[c:\private\src\vendor\boost_1_55_0_sdk\include\boost\asio\detail\win_iocp_socket_service.hpp
@ 515]
0117dd68 0f68c41a
EvoClr!boost::asio::stream_socket_service<boost::asio::ip::tcp>::async_connect<boost::_bi::bind_t<void,boost::_mfi::mf1<void,Swift::BoostConnection,boost::system::error_code
const
&>,boost::_bi::list2<boost::_bi::value<boost::shared_ptr<Swift::BoostConnection>
>,boost::arg<1> > > >+0x52
[c:\private\src\vendor\boost_1_55_0_sdk\include\boost\asio\stream_socket_service.hpp
@ 234]
0117ddec 0f68a32e
EvoClr!boost::asio::basic_socket<boost::asio::ip::tcp,boost::asio::stream_socket_service<boost::asio::ip::tcp>
>::async_connect<boost::_bi::bind_t<void,boost::_mfi::mf1<void,Swift::BoostConnection,boost::system::error_code
const
&>,boost::_bi::list2<boost::_bi::value<boost::shared_ptr<Swift::BoostConnection>
>,boost::arg<1> > > >+0x11a
[c:\private\src\vendor\boost_1_55_0_sdk\include\boost\asio\basic_socket.hpp
@ 779]
0117de9c 0f210146 EvoClr!Swift::BoostConnection::connect+0x11e
[c:\private\src\vendor\swift\swiften\network\boostconnection.cpp @ 70]