How to send c++ doubles from one system to another Sender: Winsock Receiver: Java

Question

The sending C-side

double tmp = htonl(anydouble);
send(socket, (char *) &tmp, sizeof(tmp), 0);

On the Java side, im reading the network data into a char[8]

What is the proper way of performing the conversion back to double? Is it the way to go to simply send a string and parse it back to double?

Sotirios Delimanolis · Answer 1 · 2013-02-15T19:49:31.187

3

This should work.

char[] sentDouble = readBytes(); // get your bytes char[8]
String asString = new String(sentDouble); // make a string out of it
double myDouble = Double.parseDouble(asString); // parse it into a double

With a byte[] you can do

import java.nio.ByteBuffer;

public static double toDouble(byte[] bytes) {
    return ByteBuffer.wrap(bytes).getDouble();
}

edited Feb 15 '13 at 19:49

answered Feb 15 '13 at 19:28

Sotirios Delimanolis

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This is pseudo code obviously. I don't have a `readbytes()` method. `Doube.parseDouble()` throws exception, catch it. – Sotirios Delimanolis Feb 15 '13 at 19:33
This would probably work if i sent the double in a human readable form as a string, but not sending the "plain" 64Bit. – Simbi Feb 15 '13 at 19:41
A Double is 8 bytes, so you would need byte[8] or char[4]. With byte[8] you can use what I edited in my answer. – Sotirios Delimanolis Feb 15 '13 at 19:51
+1 for a non-String solution that doesn't reinvent the wheel like I just did. – rgettman Feb 15 '13 at 19:58
With the ByteBuffer you need to make sure you match the byte order used for sending the double or make sure the server sends it in network byte order. – Michael Krussel Feb 15 '13 at 20:14
You would need that for char[] as well. So, yes, order and size of sent and received must match. – Sotirios Delimanolis Feb 15 '13 at 20:15

score 1 · Accepted Answer · answered Feb 15 '13 at 19:30

I would advice to use Google Protocol Buffers: https://developers.google.com/protocol-buffers/docs/overview It has libs for C and Java and many other languages, like for example Python, which you may find useful later. It is efficient, robust, will take care of endianness, etc.

score 1 · Answer 3 · answered Feb 15 '13 at 20:05

You're reinterpreting a double as a long. This won't be portable. Here are some alternatives:

do you really need to use floating point values? Can you represent your values as integers (e.g. milliseconds instead of seconds)?
encode the double as a string (e.g. sprintf and Double.parseDouble)
extract mantissa and exponent with frexp and send as integers, then convert back in Java with Double.longBitsToDouble

score 0 · Answer 4 · answered Feb 15 '13 at 19:46

There's no need for String overhead. Assemble the bytes into a long first, then use Double's static longBitsToDouble method.

// The bit pattern for the double "1.0", assuming most signficant bit first in array.
char[] charArray = new char[] {0x3F, 0xF0, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00};
long bytesAsLong = 0L;
for (int i = 0; i < 8; i++)
{
   bytesAsLong |= ((long) charArray[i] << (56 - 8*i));
}
double value = Double.longBitsToDouble(bytesAsLong);
System.out.println(value);  // prints "1.0"

How to send c++ doubles from one system to another Sender: Winsock Receiver: Java

4 Answers4