General Socket Question - Transferring C++ Structs from Java to C++

Question

I have a general socket programming question for you.

I have a C struct called Data:

struct data {
      double speed;
      double length; 
      char carName[32];
      struct Attribs;
}

struct Attribs {
      int color;
}

I would like to be able to create a similar structure in Java, create a socket, create the data packet with the above struct, and send it to a C++ socket listener.

What can you tell me about having serialized data (basically, the 1's and 0's that are transferred in the packet). How does C++ "read" these packets and recreate the struct? How are structs like this stored in the packet?

Generally, anything you can tell me to give me ideas on how to solve such a matter.

Thanks!

Answer 1

• Be weary of endianness if you use binary serialization. Sun's JVM is Big Endian, and if you are on an Intel x86 you are on a little endian machine.
• I would use Java's ByteBuffer for fast native serialization. ByteBuffers are part of the NIO library, thus supposedly higher performance than the ol' DataInput/OutputStreams.
• Be especially weary of serializing floats! As suggested above, its safer to transfer all your data to character strings across the wire.
• On the C++ side, regardless of the the networking, you will have a filled buffer of data at some point. Thus your deserialization code will look something like:

---

size_t amount_read = 0;
data my_data;
memcpy(buffer+amount_read, &my_data.speed, sizeof(my_data.speed))
amount_read += sizeof(my_data.speed)
memcpy(buffer+amount_read, &my_data.length, sizeof(my_data.length))
amount_read += sizeof(my_data.length)

• Note that the sizes of basic C++ types is implementation defined, so you primitive types in Java and C++ don't directly translate.
• You could use Google Protocol buffers. My preferred solution if dealing with a variety of data structures.
• You could use JSON for serialization too.

Answer 2

The basic process is:

• java app creates some portable version of the structs in the java app, for example XML
• java app sends XML to C++ app via a socket
• C++ app receives XML from java app
• C++ app creates instances of structs using the data in the XML message