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

Subject: Re: [Boost-users] Why does async_connect() call listen()?
From: Soren Dreijer (dreijer+list_at_[hidden])
Date: 2014-05-20 21:49:20

Thanks for the clarification Niklas. I'll have to dig deeper and figure out
what's blocking the listen() call on these machines.

On Tue, May 20, 2014 at 2:34 PM, Niklas Angare <li51ckf02_at_[hidden]>wrote:

> "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
>
>
>
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