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Subject: Re: [Boost-users] [iostreams] Devices and WOULD_BLOCK
From: David Hawkins (dwh_at_[hidden])
Date: 2015-01-26 17:22:26

Hi Gavin,

> I wasn't talking about mixing async with non-async (although you can do
> that as long as you're careful to not have concurrent operations, it's
> rarely-to-never useful to do so in practice). I was talking about
> mixing the two types of read calls ("read", which reads up to a
> specified number of bytes, and "read_until", which reads up to a
> specified termination value/sequence).

Thanks for correcting me :)

>> Do you see anything wrong with using a shared_ptr<streambuf>
>> member variable? I wanted to use a streambuf so that I could
>> pass it directly to the async_read_until and async_write functions
>> (saving the buffer() conversions used in the original code).
> There are negative performance consequences to copying a shared_ptr (ie.
> incrementing or decrementing its refcount). *Most* applications don't
> need to care about this (it's very small) but sometimes it's worthy of
> note, and there's no harm in avoiding copies in silly places (which is
> why I thwack people that pass a shared_ptr as a value parameter).

In the case of the original chat example, the chat message class
contained a char array. I added a std::cout message in the constructor
copy constructor and destructor, and could see that the message was
copied numerous times inside the bind command sequence, so figured
a shared_ptr<> was a lighter-weight way to do. I then made sure to
pass a reference to a shared_ptr<> where ever I could to avoid
a head "thwack" :)

> Personally, though, I use a streambuf only in the connection-management
> class, which isn't copyable, so I've never had that problem. Messages
> themselves are copied out of the streambuf into a std::string or some
> dedicated message data type, and then copies of these can be made
> reasonably freely.
> Of course, copying data is also a negative performance consequence, so
> it's a matter of picking the right tradeoff for your particular
> application and workload. :)

It turns out that my use of a streambuf was not suitable for
use within the message class, since the message can be sent to
multiple connected clients - sending to the first client "consumes"
the streambuf within the shared message, so the next client sees
an empty streambuf - oops! The solution was as you point out, to pass
around a message object (containing a char array or std::string),
and copy the message into or out of a streambuf as needed.

My modified chat client/server now works with the modified protocol.
I've modified the client code to work with a serial port, so
that I can connect two clients using two USB-to-Serial ports
and a cross-over cable. For some strange reason the
async_read_until calls work fine under Cygwin but not Linux,
so I'm in the process of tracking that down :)

Thanks again.


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