Communication between applications using socket through UDP making Multicast

Question

I’m trying to communicate between applications where one of them transmits a certain message and the others just receive, I don’t know the IP address of the ones that will receive and even if they won’t be there to receive the message, I just need to do a multicast on the net.

I’m using Ararat Synapse with the class TUDPBlockSocket and found the following example on the component website...

Receiving the message

procedure TForm1.btnReceberClick(Sender: TObject);
var
   rcvsock:TUDPBlockSocket;
   buf:string;
begin
  rcvsock:=TUDPBlockSocket.Create;
  try
    rcvsock.createsocket;
    rcvsock.Bind('0.0.0.0','22401');
    rcvsock.AddMulticast('234.5.6.7');
    buf:=rcvsock.RecvPacket(60000);
    showmessage(buf);
  finally
    rcvsock.free;
  end;
end;

Sending the message

procedure TForm1.btnEnviarClick(Sender: TObject);
var
  sndsock:TUDPBlockSocket;
begin
  sndsock:=TUDPBlockSocket.Create;
  try
    sndsock.createsocket;
    sndsock.Bind('0.0.0.0','0');
    sndsock.MulticastTTL := 1;
    sndsock.connect('234.5.6.7','22401');
    sndsock.SendString('Ahoy!'+CRLF);
  finally
    sndsock.free;
  end;
end;

This all works fine if opening only 2 instances of the application and at first click bo button btnReceber and on the second click the button btnEnviar. However, I open 3 instances, one sending and the other two receiving only one receives and the other is standing there waiting for the 60000 milliseconds. The test with only two instances works only if both are running on the same micro, I even checked if my router was enabled to allow the multicast, but it still didn’t work through the network.

I’m using Delphi XE and did tests on Windows XP on a VM with Virtual Box and also on a micro with Windows 7.

Answer 1

Hello,

I don’t know much about Lphi but I’ve worked a lot with multicast in C so I’ll give my pinch...

Are you testing with 3 instances on the same machine? How did you get the two client instances to do the "bind" on the same port? The second should give some error in the bind (something like "address already in use") since normally you do not have 2 processes "listening" the same port.

It would be useful to test the return of the bind() function to see if it actually worked.

Answer 2

Try using Indy components like Tidudpserver and Tidudpclient

on receipt you treat so:

procedure TMainHost.UDPServer2UDPRead(AThread: TIdUDPListenerThread;
  AData: TBytes; ABinding: TIdSocketHandle);
begin
//

//onde AData é o recebimento em bytes... 
//Você pode tratar os caracteres e montar sua mensagem com o Chr...

end;

in shipping you treat so for example:

function TMainHost.DoSendModificarCamera(
  aCamera: TCameraJSONRecognized): Integer;
var
  strTemp: String;
begin


  strTemp := 'string de envio';

  try

    IdUDPClient1.Host := '255.255.255.255';
    IdUDPClient1.Port := 34567;
    IdUDPServer2.Active := False;
    IdUDPServer2.DefaultPort := 34567;
    IdUDPClient1.Active := True;
    IdUDPClient1.Send(strTemp);
    IdUDPClient1.Active := False;
    IdUDPServer2.Active := True;
    Result := Length(x);
  except
    Result := -Length(x);
  end;

end;