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

Subject: Re: [Boost-users] Why does async_connect() call listen()?
From: Niklas Angare (li51ckf02_at_[hidden])
Date: 2014-05-20 17:34:14

"Soren Dreijer" <dreijer_at_[hidden]> wrote:
> 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?

The call is made by the class socket_select_interrupter. The socket it is 
calling listen() on is an internal one bound to 127.0.0.1. If a firewall 
interferes with that then the firewall is horribly bugged. I believe the 
socket is set up so that a thread that is blocking on a call to ::select() 
(for example in io_service::run()), can be interrupted from another thread. 
This interruption is probably used when io_service::stop() is called for 
example.

> 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?

It only has to defer work that would block or that the documentation 
explicitly promises it will defer.

Regards

Niklas Angare