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: Phil Sean (philsean_at_[hidden])
Date: 2017-11-05 16:46:39

(In reply to Vinnie Falco <vinnie.falco_at_[hidden]>)

Thanks for your reply!
It addressed most of my problems. Just a few points to make clear:

> I believe you can have one outstanding read and one outstanding write
> operation pending simultaneously for ssl::stream and tcp::socket.

What about two outstanding read operations or two outstanding write operations? I think they're safe for tcp::socket but not ssl::stream.

Moreover, because the documentation has an extra clause in thread safety for ssl::stream which tcp::socket does not have:
"The application must also ensure that all asynchronous operations are performed within the same implicit or explicit strand,"
there should be a difference in the calling requirements for the asynchronous operations in ssl::stream and tcp::socket. Specifically, is there some example code/model that can be used with tcp::socket but not ssl::stream, just because of the extra clause?

> Note the careful wording of "implicit or explicit strand." This means: 
> * implicit strand: Only one thread calling io_service::run 
> * explicit strand: Completion handlers are wrapped using 
> io_service::strand::wrap

Apart from handlers called within strands, what other effects does wrapping completion handlers with io_service::strand::wrap have? For example:

void handler(
  const boost::system::error_code& error,
  std::size_t bytes_transferred
) { ... }

void wrapped_handler(
  const boost::system::error_code& error,
  std::size_t bytes_transferred
) {
  strand_.dispatch(boost::bind(&handler, error, bytes_transferred));
}

// How does the following differ?

ssl_stream.async_read_some(buffer, &wrapped_handler);

ssl_stream.async_read_some(buffer, strand_.wrap(&handler));

Since Boost uses argument dependent lookup (ADL) for composed operations [1], I guess it could be similar here.

Thanks!

[1] https://stackoverflow.com/questions/12794107/why-do-i-need-strand-per-connection-when-using-boostasio#12801042