Question

I’m trying to send a Java UUID to C++, where it will be used as a GUID, then send it back and see it as a UUID, and I’m hoping to send it across as just 16 bytes.

Any suggestions on an easy way to do this?

I’ve got a complicated way of doing it, sending from Java to C++, where I ask the UUID for its least and most significant bits, write this into a ByteBuffer, and then read it out as bytes.

Here is my silly-complicated way of getting 2 longs out of a UUID, sending them to C++:

Java

public static byte[] asByteArray(UUID uuid) 
 {
    long msb = uuid.getMostSignificantBits();
    long lsb = uuid.getLeastSignificantBits();
    byte[] buffer = new byte[16];

    for (int i = 0; i < 8; i++) {
            buffer[i] = (byte) (msb >>> 8 * (7 - i));
    }
    for (int i = 8; i < 16; i++) {
            buffer[i] = (byte) (lsb >>> 8 * (7 - i));
    }

    return buffer;

}




    byte[] bytesOriginal = asByteArray(uuid);
    byte[] bytes = new byte[16];

    // Reverse the first 4 bytes
    bytes[0] = bytesOriginal[3];
    bytes[1] = bytesOriginal[2];
    bytes[2] = bytesOriginal[1];
    bytes[3] = bytesOriginal[0];
    // Reverse 6th and 7th
    bytes[4] = bytesOriginal[5];
    bytes[5] = bytesOriginal[4];
    // Reverse 8th and 9th
    bytes[6] = bytesOriginal[7];
    bytes[7] = bytesOriginal[6];                                 
    // Copy the rest straight up        
    for ( int i = 8; i < 16; i++ )
    {
        bytes[i] = bytesOriginal[i];
    }    

    // Use a ByteBuffer to switch our ENDIAN-ness
    java.nio.ByteBuffer buffer = java.nio.ByteBuffer.allocate(16);
    buffer.order(java.nio.ByteOrder.BIG_ENDIAN);
    buffer.put(bytes);
    buffer.order(java.nio.ByteOrder.LITTLE_ENDIAN);
    buffer.position(0);

    UUIDComponents x = new UUIDComponents();

    x.id1 = buffer.getLong();
    x.id2 = buffer.getLong();

C++

    google::protobuf::int64 id1 = id.id1();
    google::protobuf::int64 id2 = id.id2();

    char* pGuid = (char*) &guid;
    char* pGuidLast8Bytes = pGuid + 8;
    memcpy(pGuid, &id1, 8);
    memcpy(pGuidLast8Bytes, &id2, 8);

This works, but seems way too complex, and I can’t yet get it working in the other direction.

(I’m using google protocol buffers to send the two longs back and forth)

• Alex

Answer 1

I got something working.

Instead of sending it across as two longs, I send it across as bytes, here is the Java code:

public static UUID fromBytes( ByteString byteString)
{
    byte[] bytesOriginal = byteString.toByteArray();
    byte[] bytes = new byte[16];

    // Reverse the first 4 bytes
    bytes[0] = bytesOriginal[3];
    bytes[1] = bytesOriginal[2];
    bytes[2] = bytesOriginal[1];
    bytes[3] = bytesOriginal[0];
    // Reverse 6th and 7th
    bytes[4] = bytesOriginal[5];
    bytes[5] = bytesOriginal[4];
    // Reverse 8th and 9th
    bytes[6] = bytesOriginal[7];
    bytes[7] = bytesOriginal[6];                                 
    // Copy the rest straight up        
    for ( int i = 8; i < 16; i++ )
    {
        bytes[i] = bytesOriginal[i];
    }    

    return toUUID(bytes);
}

public static ByteString toBytes( UUID uuid )
{
    byte[] bytesOriginal = asByteArray(uuid);
    byte[] bytes = new byte[16];

    // Reverse the first 4 bytes
    bytes[0] = bytesOriginal[3];
    bytes[1] = bytesOriginal[2];
    bytes[2] = bytesOriginal[1];
    bytes[3] = bytesOriginal[0];
    // Reverse 6th and 7th
    bytes[4] = bytesOriginal[5];
    bytes[5] = bytesOriginal[4];
    // Reverse 8th and 9th
    bytes[6] = bytesOriginal[7];
    bytes[7] = bytesOriginal[6];                                 
    // Copy the rest straight up        
    for ( int i = 8; i < 16; i++ )
    {
        bytes[i] = bytesOriginal[i];
    }    

    return ByteString.copyFrom(bytes);
}


private static byte[] asByteArray(UUID uuid) 
 {
    long msb = uuid.getMostSignificantBits();
    long lsb = uuid.getLeastSignificantBits();
    byte[] buffer = new byte[16];

    for (int i = 0; i < 8; i++) {
            buffer[i] = (byte) (msb >>> 8 * (7 - i));
    }
    for (int i = 8; i < 16; i++) {
            buffer[i] = (byte) (lsb >>> 8 * (7 - i));
    }

    return buffer;

}

private static UUID toUUID(byte[] byteArray) {

    long msb = 0;
    long lsb = 0;
    for (int i = 0; i < 8; i++)
            msb = (msb << 8) | (byteArray[i] & 0xff);
    for (int i = 8; i < 16; i++)
            lsb = (lsb << 8) | (byteArray[i] & 0xff);
    UUID result = new UUID(msb, lsb);

    return result;
}

Doing it this way, the bytes can be used straight up on the C++ side. I suppose the switching around of the order of the bytes could be done on either end.

C++

    memcpy(&guid, data, 16);