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: 2014-07-01 18:33:52

"Nathaniel Fries" <nfries88_at_[hidden]> wrote:
>>
>> Niklas Angare <li51ckf02_at_[hidden]> wrote:
>>
>>> "Soren Dreijer" <dreijer_at_[hidden]> wrote:
>>>> 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. 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.

> I don't see why it is necessary in this case, since the calling thread is
> not doing work that needs to be interrupted in case of calling
> io_service::stop.

Soren said only that he calls async_connect(). Imagine he then calls 
io_service::run(). That thread is now blocked in a call to ::select() 
waiting for the connect to finish. If another thread then did anything with 
that io_service, such as another async call or indeed stop(), the original 
thread would have to be interrupted somehow.

Regards,

Niklas Angare