How can I get a callback when there is some data to read in the boost.asio stream without reading it into the buffer?

Question

It seems like boost 1.40.0 has changed the way it works async_read_some().

Previously, you could pass null_buffer, and you will receive a callback when there is data to read, but without reading frames into any buffer (because there were none!). This basically allowed you to write code that acted like a call select(), where you would be told when your socket has some data.

In the new code, the behavior has been changed to work as follows:

If the total size of all the buffers in the mb sequence is 0, the asynchronous read operation should immediately end and pass 0 as an argument to the handler, which determines the number of bytes read.

This means that my old (and by the way the method shown in this official example ) method of detecting data in a socket no longer works. The problem for me is that I need a way to detect this, because I imposed my own stream classes on top asio socket streams and as such, I can't just read the socket data that my streams will expect from them. The only workaround I can come up with right now is to read one byte, save it, and when my stream classes request a few bytes, return that byte if it is set: not really.

Does anyone know how best to implement this type of behavior in the latest updated boost.asio code?

Answer 1

My quick test with an official example with boost-1.41 works ... So I think it should work anyway (if you use null_buffers)