UDP DatagramPacket sent between C (client) and Java (server)

Question

Hi,
I have a problem sending a struct from C to Java UDP server.
In the C (client) program, the struct is defined as
typedef strurct dataType_s {
char name[52];
char add[52];
long x;
long y;
} dataType;
dataType data;
strcpy(data.name, "sun");
strcpy(data.add, "com");
data.x = 100;
data.y = 50;
sendto(sock, (struct dataType*)&dataType, sizeof(dataType), (struct sockaddr*)&dest, sizeof(dest))
In the Java server side, I have the following code.
byte[] buffer = new Byte[1024];
DatagramPacket packet = new DatagramPacket(buffer, 1024);
udpSock.receive(buffer);
byte[] data = buffer.getData();
String name = new String(data, 0, 52);
String add = new String(data, 52, 52);
String _xx = new String(data, 104, 4);
String _yy = new String(data, 108, 4);
Byte x = new Byte(xx.getBytes());
long x = _x.longValue();
Byte y = new Byte(yy.getBytes());
long y = _y.longValue();
I am having problem to read the contents of x and y above. The string name and add received by the server
are correct. However, I receive garbage value
for x (long) and y (long). In fact, I get expection
that I cannot create Byte x and Byte y as shown
above. Any idea of how to send a mixture of types
from C/C++ to Java server?
Thanks a lot.
Kelvin ([email protected])

Answer 1

sendto(sock, (struct dataType*)&dataType,
sizeof(dataType), (struct sockaddr*)&dest,
sizeof(dest))You have to check the alignment of your structs. You can't be sure that a long takes up four bytes. It can very well be 8 bytes or something else depending on your settings at compilation.
udpSock.receive(buffer);
byte[] data = buffer.getData();I suppose this is a typo, but of course you should use packet not buffer here.
>
Byte x = new Byte(xx.getBytes());
long x = _x.longValue();
Byte y = new Byte(yy.getBytes());
long y = _y.longValue();
in my opinion its easier to use a DataInputStream instead:
DataInputStream in = new DataInputStream(new ByteArrayInputStream(data));
int y = in.readInt();
Notice that a long in Java is 8 bytes. A C long corresponds to a Java int.
/Per-Arne