Re: [Boost-users] [asio] Does ssl::stream allow multiple pending async operations?

Subject: Re: [Boost-users] [asio] Does ssl::stream allow multiple pending async operations?
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2017-11-06 04:53:54

On 6/11/2017 12:00, Bjorn Reese wrote:
> On 11/05/2017 05:46 PM, Phil Sean wrote:
> 
>> What about two outstanding read operations or two outstanding write 
>> operations? I think they're safe for tcp::socket but not ssl::stream.
> 
> You are not guaranteed that multiple asynchronous write operations are
> executed in the order you initiate them, so having more than one
> outstanding operation can lead to unexpected results.

They're also not guaranteed to execute atomically -- ie. you can have 
part of write1 interleaved with part of write2, although all of the 
bytes in each individual write should eventually turn up in the correct 
relative order -- but that's rarely sufficient.

The same applies for reads -- if you have two outstanding reads on the 
same socket/stream then any received bytes could up spread 
non-deterministically between the two completion handlers.

So regardless of whether the objects officially support it or not, I 
cannot think of a situation in which actually doing that would be 
anything other than a mess.  So just don't do it.

UDP is a different story; there are scenarios where multiple pending 
reads or writes makes sense there.  But not for anything stream-oriented.