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I'm new to networking so hopefully I will be able to explain my issue.

My boss recently made a dummy server for me to test my application's networking and I ran into issue regarding java to c++. His server was written in c++ and when run, it waits and listens for a particular c++ data struct named "RemoteMsgStruct" to be passed:

typedef struct
{
unsigned int Length;
unsigned int BoardType;
unsigned int MsgType;
unsigned char Msg[MAX_MSG_SIZE];
} RemoteMsgStruct;

The problems lies in the fact that my program is written in java. How would I go about passing a similar message through java for the server to process?

Thanks ahead of time for the help. If anything isn't explained well enough or appears to be missing let me know. Like I said, I'm new to both networking and stackoverflow :)

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  • You need to have a look how data from TCP socket is being handled by the socket. The way you encode the data in your application must also be decoded at the server side. So first settle on a protocol and move ahead with whatever encoding decoding techniques..eg raw byte format/XML/JSON/Protocol Buffer etc Commented Aug 1, 2011 at 14:46

3 Answers 3

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You can pass exactly the data expected by your boss's test program, without asking your boss to modify it.

On the Java side, you can write data in the layout defined by the C++ struct via several methods, including: * java.io.DataOutputStream * java.nio.ByteBuffer and java.nio.channels.SocketChannel.

You may encounter issues of field alignment and byte order. * Some fields in a C struct may start after padding to align with word boundaries. * Java will write network byte order (big-endian) by default. On the Java side, ByteBuffer allows choice of a different order.

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Welcome to SO.

As to your question, there's a reason why messaging formats like XML and JSON are so popular, they solve problems like this. As it is you are going to have to battle with sending over unsigned ints and worrying about character encoding and endian-ness. If you cannot change the format on the server side then I will recommend you take a look at ByteBuffer. You will still have to do some massaging but at least it will relieve some of the pain.

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Andy's answer is correct. However it should be said that your boss's program needs to be rewritten. Passing C structs directly over a network is never a good idea even when the other end is written in C. That technique relies on the following being the same at both ends:

  1. The hardware byte order
  2. The compiler's filling and packing rules
  3. The surrounding #pragmas

and therefore

  1. The compiler vendor
  2. The compiler version

This (incomplete) list is far too long already, and we are still only considering C to C.

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2 Comments

"Needs to be rewritten" is rather strong, given the limited information we have. Note that #1 can be addressed via ntohl etc. #2/#3 are important if multiple compilers will be a requirement.
@Andy Thomas-Cramer I disagree. I don't think it's strong enough. This kind of thing is to be deprecated in the strongest possible terms. Wire protocols should be defined independently of the host language, and this thread is sufficient proof.

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