Re: [Boost-users] std::future::wait_for() not working in Boost UDP socket async receive operation

Subject: Re: [Boost-users] std::future::wait_for() not working in Boost UDP socket async receive operation
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2016-11-29 19:12:48

On 29/11/2016 23:09, TONGARI J wrote:
> I think we're wrong about whether it's allowed to call
> async_receive_from() multiple times before it completes.
> It was from my impression that we can't do that, but skimming over the
> ASIO docs I found nothing says that.
> Experiments show that it works, so I think my argument in this regard
> was wrong, at least ASIO itself does not enforce the restriction.

It's technically permitted but it's rarely a good idea to have multiple 
pending operations of the same type (eg. more than one read or more than 
one write) on the same socket/whatever, as it becomes undefined what 
order things end up at the other end, which is rarely acceptable.  (You 
can sometimes get away with concurrent writes with UDP, if done 
carefully; you typically can't get away with it with TCP.  It's almost 
never sensible to have concurrent reads with either.)

And also because user code will tend to supply the same buffers for 
multiple such operations of the same type, and that's definitely not 
allowed to use the same buffer in multiple concurrent operations.

It's ok to have a pending read and a pending write on the same 
socket/whatever (as long as they use separate buffers), but then you 
can't selectively cancel() one of them, which may not be what you want.

(It is actually possible to cancel an individual operation in WinAPI, 
but I don't think ASIO exposes this; possibly this isn't supported on 
Linux or because ASIO's own API doesn't really provide operation handles 
to let you specify what you want to be cancelled.)