Re: [Boost-users] Modify an asio echo server so that it does not send message back to client

Subject: Re: [Boost-users] Modify an asio echo server so that it does not send message back to client
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2015-06-10 20:16:06

On 11/06/2015 11:56, Nicholas Yue wrote:
>      I now have a process() method with the bytes_transferred parameter.
>
>      Newbie question, how do I know the last chunk has been sent, so
> that I can now process the data (which will be binary) ?
>
>      Is there some simple example which illustrate this I can refer to ?

There isn't any inherent way to know.  TCP is a stream protocol -- you 
just receive a continuous stream of bytes with no demarcation of any kind.

You will need to define something at the application layer of the 
protocol to decide when a logical unit is complete, typically either by 
inserting some delimiter that you know can never occur mid-message or by 
sending the length of the following data early on.

Note that ASIO does provide some helper methods for these scenarios -- 
instead of using async_read_some, which just returns data as it becomes 
available, you can use asio::async_read (which you can tell to not 
return until it has read a specific number of bytes) or 
asio::async_read_until (read until a specific delimiter).  (It's tricky, 
though not impossible, to do a combination of the two -- but it's 
something you should avoid if you have a choice.)

So for a line-based protocol you could async_read_until a CRLF (or just 
LF, depending on the other end), or for a binary protocol that sends a 
4-byte length followed by a payload you could async_read 4 bytes to get 
the length, then async_read that many bytes to get the payload.