Re: [Boost-users] Thread safe full duplex sockets

Subject: Re: [Boost-users] Thread safe full duplex sockets
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2017-02-08 22:09:07

On 8/02/2017 22:41, Christi258 wrote:
> This question has been asked many times, but I'm still not satisfied with the
> answers.
> I want to use a socket in full duplex:
> 1. asynchronously: with async_read, async_write (multiple threads running
> io_service.run())
> 2. synchronously: with read and write in two distinct threads
>
> I am pretty sure that 1. is thread safe using strands.
> But I can find no information about 2.

ASIO makes no specific guarantees AFAIK, it's down to your OS.  But in 
general sockets are safe to read and write on separate threads as long 
as you only have one concurrent read and one concurrent write.  This is 
also the case with serial ports, but not the case with files.

(The standard pattern is to have reading in an implicit strand by 
kicking off an initial read on connection and subsequent reads whenever 
the first read completes, and then to do writes either with an explicit 
strand or with an application layer that guarantees it won't issue 
concurrent writes.  Depending on your protocol and application structure 
writes might be triggered from the read handlers or they might be 
triggered elsewhere.)

Also, strongly consider using async with coroutines (which highly 
resembles sync code) instead of using actual sync code -- it scales a 
lot better.

Where you have to be a bit more careful with multiple threads are 
operations that mutate the state of the socket itself -- in particular 
if something closes the socket (typically in response to an error). 
Closing the socket on one thread should result in aborting any pending 
operations on the other threads, but you need to be careful to not start 
new operations (since clearing the OS handle might not have propagated 
to the other thread yet, and the OS might have reused the handle id for 
some other connection in the meantime).