Socket Data issue between Android and iOS

Question

I'm facing a bit of an issue trying to communicate with 2 devices through a socket (UDP, but not the issue here...) One device is Android, the other is iOS.

Here's the Android code to create the Data I'm sending :

int part = 1;
int num = -1;

ByteBuffer bb = ByteBuffer.allocate(20);
bb.order(ByteOrder.nativeOrder());
bb.putChar('R');
bb.putInt(part);
bb.putInt(num);
bb.flip();

byte[] toSend = new byte[bb.remaining()]; // bb.remaining() = 10
bb.get(toSend);

send(toSend);

On iOS, here's the code I use to parse the data I get from the socket

char *sData = (char*)[data bytes];
if (sData[0] == 'R') {
        sData += sizeof(char);
        int part = 0;
        memcpy(&part, sData, sizeof(int));
        sData += sizeof(int);
        int num = 0;
        memcpy(&num, sData, sizeof(int));
}

The data I get is :

part = 256
num = -256

The weird thing is if I change value from the Android device I get :

part = 0 & num = -1 ---> part = 0 & num = -256
part = 1 & num = -1 ---> part = 256 & num = -256
part = 2 & num = -1 ---> part = 512 & num = -256
part = 3 & num = -1 ---> part = 768 & num = -256

As you can see, the value are correct, they are just multiplied by 256, and I can't understand why...

To avoid compatibility issue between Android and iOS (32/64) I avoided the long type, because on

Android long = 8
iOS 32b long = 4
iOS 64b long = 8

Does anybody can see why it is multiplying my value by 256 ?

EDIT 1 :

From iOS to iOS data transfer, it is working fine, so my guess is that the bug is coming from Android.

Answer 1

In Java, your buffer contains this:

char 2 bytes | int 4 bytes | int 4 bytes

Since iOS stores char in one byte, you are reading it as if it were:

char 1 byte | int 4 bytes | int 4 bytes

This also explains why iOS -> iOS works.

[Edit]

To fix, you could use byte in Java.

byte b = (byte) 'R';

Like this:

ByteBuffer bb = ByteBuffer.allocate(20);
bb.order(ByteOrder.nativeOrder());
bb.putByte((byte)'R');
bb.putInt(part);
bb.putInt(num);
bb.flip();

Note though that casting char to byte "loses" the high 8 bits so this will only work for ASCII chars.