send_bytes in receive callback

[reinzor]

Hi,

I am trying to call the send_bytes method in a registered receive_callback; however, the data is never sent. Am I doing something wrong or what could be the issue here?

Setup:

void callback(const uint8_t* buf, size_t len)
{
  char message[] = "hello world!";
  udp.send_bytes((uint8_t*) message, std::strlen(message));
}

// open UDP ports
async_comm::UDP udp("localhost", 14620, "localhost", 14625);
udp.register_receive_callback(&callback);

Thanks,

Rein

[dpkoch]

Hi Rein, a couple of thoughts come to mind:

• In this snippet, udp is not defined in the callback() scope
• Are you sure that you're actually receiving the data and that the callback is being called?

For reference, making the following changes to examples/udp_hello_world.cpp works for me:

void ack_callback(async_comm::UDP &udp, const uint8_t* buf, size_t len)
{
  char ack_message[] = "Acknowledge received";
  udp.send_bytes(reinterpret_cast<uint8_t*>(ack_message), std::strlen(ack_message));
  for (size_t i = 0; i < len; i++)
  {
    std::cout << buf[i];
  }
}

int main()
{
  // open UDP ports
  async_comm::UDP udp1("localhost", 14620, "localhost", 14625);
  udp1.register_receive_callback(std::bind(ack_callback, std::ref(udp1), std::placeholders::_1, std::placeholders::_2));

  // ...
}

[dpkoch]

You could also do it with a lambda capture, like this:

async_comm::UDP udp1("localhost", 14620, "localhost", 14625);

std::function<void(const uint8_t*, size_t)> ack_callback = [&udp1](const uint8_t* buf, size_t len) {
  char ack_message[] = "Acknowledge received\n";
  udp1.send_bytes(reinterpret_cast<uint8_t*>(ack_message), std::strlen(ack_message));
  for (size_t i = 0; i < len; i++)
  {
      std::cout << buf[i];
  }
};

udp1.register_receive_callback(ack_callback);

Or if the UDP object and the callback were both members of some containing class, then obviously you'd have access to the UDP object from within the callback

[reinzor]

Hi @dpkoch , thanks for your quick response. It was a simple snippet so don't bother about scoped etc. I looked a little further and tried your example and it gives me the following output:

$ nc -u localhost 14620
dsa
Acknowledge received

So the Acknowledge received message is sent back to the netcat client, how does this work? Why isn't it sent to the remote_port as specified (14625). When I listen to the 14625 port, it is not sending any data:

$ nc -kul 14625

No message is coming in here. I thought data was being received on the bind address/port and we are sending data to the remote address/port. Maybe I am totally misunderstandings things here ..

Thanks again for your help!

[reinzor]

I think I've found the problem here:

async_comm/src/udp.cpp

Line 107 in 12e172d

socket_.async_receive_from(buffer, remote_endpoint_, handler);

Is it logical to update the remote_endpoint_ here?