Question

I define a struct for the move

typedef struct {
    MsgType msgType;
    int newFallenStonesSize;
    char *newFallenStones;
} MsgMove;

And send the data like this:

MsgMove message;
message.msgType = MsgTypeMove;     
message.newFallenStones = (char *)malloc(nrNewFallenStones*sizeof(char));
for (int i=0; i<nrNewFallenStones; i++) {
    message.newFallenStones[i]=newFallenStones[i];
}
message.newFallenStonesSize = nrNewFallenStones;

NSData *data = [NSData dataWithBytes:&message length:(2*sizeof(int)+message.newFallenStonesSize*sizeof(char))];          

[[KKGameKitHelper sharedGameKitHelper] sendDataToAllPlayers:data reliable:YES]; 

the data is correct at the moment of sending, but when I receive it like this:

else if (msg->msgType == MsgTypeMove)
{
    MsgMove *msgMove = (MsgMove *) [data bytes]; 
    for (int i=0; i<msgMove->newFallenStonesSize; i++) {
        NSLog(@"New Stone received:%i",msgMove->newFallenStones[i]);
    }
}

The values have changed. For example 1, 6, 3 and I receive 76, 105 98.

Anyone knows why this is happening?

Answer 1

The main issue is that you are sending a pointer to your data over the network rather than the actual data. When you malloc space for message.newFallenStones it will be set to some apparently random location in memory that is not adjacent to your MsgMove structure. You then write your data in this other memory location. What you package up to transfer is the MsgMove structure (with a pointer to somewhere else in memory) plus whatever random bytes happens to immediately follow it in memory.

The typical way this is handled is to instead have your entire message be malloc’ed together and write the data into the end of it. More like:

typedef struct {
    MsgType msgType;
    int newFallenStonesSize;
    char newFallenStones;        // The first newFallenStones value
} MsgMove;

and then send with

MsgMove *message;
message = (MsgMove *)malloc(sizeof(MsgMove)+nrNewFallenStones-1);
message->msgType = MsgTypeMove;     
message->newFallenStonesSize = nrNewFallenStones;
char *newStones = &MsgMove->newFallenStones;
for (int i=0; i<nrNewFallenStones; i++) {
    newStones[i]=newFallenStones[i];
}

NSData *data = [NSData dataWithBytes:message length:(sizeof(MsgMove)+nrNewFallenStones-1)];          

[[KKGameKitHelper sharedGameKitHelper] sendDataToAllPlayers:data reliable:YES];