如何使用UDP从C应用程序向Java应用程序发送结构

Question

I actually want to send a structure over UDP from a C application to a java application.

The struct looking like that :

typedef struct {
type1 liste1;
type2 liste2;
type3 liste3;
type4 liste4;
}liste;

And type1,2,3,4 are also structure which include themselves others structures too.

For now, I just send the structure like that (liste is my structure) :

sendto(socketOut, &(liste), sizeof(liste), 0, (SOCKADDR *) &(recvAddrAck),sizeof(recvAddrAck));

And I get the UDP like that on the java application :

DatagramSocket socket = new DatagramSocket(SERVERPORT);
byte[] buf = new byte[1500];
DatagramPacket packet = new DatagramPacket(buf, buf.length);
while (true) {
socket.receive(packet);
data = packet.getData();
}

So, my question is, how to decode data ? I'm quite a newbe with UDP protocols, but I've made some research with my friend Google, and I found some things like htons/htonl/... for the C side, and ByteArrayInputStream/DataInputStream ... for the java side. But I don't really understand how and when I have to use this things, especially for the java side.

Hoping my question is understandable

Thanks

Answer 1

You will need to serialize in some way. Either manually, via some text format or XML, or if you want something more robust, look into ASN.1 / BER libraries on both sides.

Answer 2

Not sure to understand, but did you take a look at DatagramPacket ?

Something like

DatagramPacket answerPacket = null;
  try { 
    byte[] buffer = new byte[REC_BUFFER_SIZE];
    answerPacket = new DatagramPacket(buffer, REC_BUFFER_SIZE);
    yourSocket.receive(answerPacket);
    log.debug("Received UDP Packet from " + answerPacket.getAddress().getHostAddress());
  } catch (SocketTimeoutException e) {
    log.debug("No Answer - Connection to DatagramSocket timed out");
  }

if (answerPacket != null && answerPacket.getLength() != 0) {
    String result = new String(answerPacket.getData());
    result = result.trim(); // remove leading and trailing whitespaces...
}

might do what you need, but it means the usage of a custom format.