Asynchronously waiting until a socket is available for reading/writing in Asio

Question

I want to do the following with Boost Asio. I have a socket and I want to register a callback to be called when data is available for reading/writing on the socket, but I don't want it to actually do the reading/writing. Basically, what I need is similar to async_read_some/async_write_some, except that the actual reading and writing is not done.

I need this because I'm using an external library with its own read and write function that require a socket descriptor as an input parameter and I want to use this library in an asynchronous way.

Answer 1

You are looking for reactor-style operations. These can be obtained by providing boost::asio::null_buffers to the asynchronous operations. Reactor-style operations can be useful for integrating with third party libraries, using shared memory pools, etc. The Boost.Asio documentation provides some information and the following example code:

ip::tcp::socket socket(my_io_service);
...
socket.non_blocking(true);
...
socket.async_read_some(null_buffers(), read_handler);
...
void read_handler(boost::system::error_code ec)
{
  if (!ec)
  {
    std::vector<char> buf(socket.available());
    socket.read_some(buffer(buf));
  }
}

Boost.Asio also provides an official nonblocking example, illustrating how to integrate with libraries that want to perform the read and write operations directly on a socket.