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

Subject: Re: [Boost-users] Why does async_connect() call listen()?
From: Nathaniel Fries (nfries88_at_[hidden])
Date: 2014-06-12 19:19:07

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.

On Tue, May 20, 2014 at 9:49 PM, Soren Dreijer <dreijer+list_at_[hidden]>
wrote:

> 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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