Question

I need to get the source IP for a datagram in a UDP server I’m writing using boost ASIO.
In the example udp datagram server the line:

Note: my current code is identical to the existing udp async server example in the boost asio documentation.

 socket_.async_receive_from(
        boost::asio::buffer(data_, max_length), sender_endpoint_,
        boost::bind(&server::handle_receive_from, this,
          boost::asio::placeholders::error,
          boost::asio::placeholders::bytes_transferred));

Is passing error and bytes_transferred. I also need the address of the message.

Now I know I can use sender_endpoint_.address().to_string() directly, but I
I would like to pass this in as a parameter.

So I tried placing sender_endpoint_.address() in as a parameter.
i.e.

socket_.async_receive_from(
        boost::asio::buffer(data_, MAX_LENGTH), sender_endpoint_,
        boost::bind(&server::handle_receive_from, this,
          boost::asio::placeholders::error,
                    boost::asio::placeholders::bytes_transferred,
                    sender_endpoint_.address()));

But the result is 0.0.0.0 when read from handle_receive_from.
However, if I simply access the sender_endpoint_.address() inside handle_receive_from
I get 127.0.0.1 which is correct.

How do pass this correctly? I need this because there are going to be multiple threads calling io_service.run() so a packet from another source may overwrite sender_endpoint and I need to be able to reply to that source packet.

Answer 1

I think that if you access sender_endpoint_.address() in your handler before you set up another async handler, it will be fine and not overridden. You do call async_receive_from again in your handler, right, to be ready to receive another packet?

In your second code block, where you try sender_endpoint_.address() as a parameter, sender_endpoint_.address() is getting called when you set up the handler, not being executed by the handler when a packet is received. That’s why it’ll always be 0.0.0.0.