when does Boost.Asio call async_read_some callbacks?

Question

Lets say I have a Boos.Asio TCP client, that can send a string, like this:

client.sendStr("1111");

I also have a asynchronous Boost.Asio TCP server, that will handle incoming packets and print them, like this:

received string: "1111", size: 5.

Problem is, that when I call client.sendStr() multiple times, like this

client.sendStr("1111");
client.sendStr("2222");
client.sendStr("3333");

I might get various results on the server side, for example:

received string: "1111", size: 5.
received string: "2222", size: 5.
received string: "2222", size: 5.

or

received string: "111122223333", size: 13.

or

received string: "11112222", size: 9.
received string: "3333", size: 5.

I am a networking newbie, but I guess that the reason for this situation is that TCP is a "stream based protocol".

My questions are:

1) Can I prevent Boost.Asio from doing this? (lets say "1111" and "2222" are control packets of my application, and I don't what them to be connected).

2) Is there a possibility that I will get callbacks like this:

received string: "111122", size: 7.
received string: "22333", size: 6.
received string: "3", size: 2.

Answer 1

The documentation for async_read_some is very clear:

Remarks

The read operation may not read all of the requested number of bytes. Consider using the async_read function if you need to ensure that the requested amount of data is read before the asynchronous operation completes.

TCP is a stream based protocol, you will need to handle the message framing at the application layer. This largely depends on your protocol, one possible solution in your scenario may be to null terminate each message. Alternatively, include a fixed byte header before each message indicating the length of the payload.